CA1170557A - Continuous process for producing reinforced resin laminates - Google Patents

Abstract

Invention: CONTINUOUS PROCESS FOR PRODUCING
REINFORCED RESIN LAMINATES

ABSTRACT OF THE DISCLOSURE

A continuous process for producing reinforced resin Laminates comprising the steps of impregnating a fibrous substrate with a liquid resin which is free of volatile solvent and is capable of curing without generating liquid and gaseous byproducts, laminating a plurality or the resin-impregnated substrates into a unitary member, sandwiching the laminate between a pair of covering sheets, and curing the laminate between said pair of covering sheets without applying appreciable pressure. The improvement comprises adjusting the final resin content in said resin impregnated substrate at 10 to 90% by weight based on the total weight of said impreg-nated substrate.

Description

1 ~ 7~5~7 BACKGROUND OF THE INVENTION

The present invention relates to a process for continuous production of metal foil clad and unclad laminates composed of several layers of thermosetting resin-impregnated sheet substrates. Related inventions are the subject of applicant's copending Canadian applications no. 348,430 and no. 354,823.
The primary purpose of the present invention is to produce laminates for electrical insulation and printed circuit wiring uses.
Such laminates are generally required to have various characteristics including excellent electrical insulation, dielectric property, chemical resistance, surface smoothness, clad peel strength, and dimensional stability under various conditions. Further requirements comprise:
i) to have excellent thermal stability to withstand solder temperatures as high as 260C, ii) to generate neither unpleasant odor~ nor hazardous volatile matter when heated, iii) to exhibit no large degree of warping adversely affecting printing and heating processes, iv) to be easily subjected to punching processes, v) to contain no bubbles impairing thermal conductivity and appearance, and vi) to be as cheap as possible.
These laminates are usually provided with smooth surfaces, about 0.5 mm to 5 mm in thickness and about 1000 mm

-2-' ?~c , 5 ~ 7 by 1000 mm in area.
According to conventional techniques in the art, unclad laminates are generally produced by processes wherein a fibrous substrate is impregnated with a varnish solution of a certain resin composition, dried to ~orm a so-called "prepreg", and cut into a predetermined length, and several cut sheets, in turn, are stacked together and subjected ; batchwise to heat-pressing treatment. In such processes, however, the use of a ~olvent is essential ~or making up the varni~h solution, and the prepreg has to be tack-free because of the restricted conditions of processing and workability.
; Consequently, these lead to the introduction of additional complicated plant, and to a con~iderable reduction in pro-ductivity.
; 15 In oonventional proce~es, metal foil clad laminates also are produced through ~tep~ ~imilar to the above, with the addition oP a ~tep wherein the ~tack i~ adhe~ively covered with metal foil, which has been preliminarily coated with an adhesive and heated to bring out the adhe~ive to its B stage.
Although the~e clad laminate~ are u~ed as printed circuit wiring board~ and the like, they involve, in fact, problems concerning their productive efficiency and economy, because of the complicated batch prooe~ses and the high dependence on hand labor and skill.
Recently, in view of thi~, ~everal continuous pro-duction methods for clad or unclad laminate~ have been , .. j ,, " ., .

7~5~7 proposed (for example, USP 3236714, USP 4012267, and Japanese Patent Publication No. SHo.53-88872).
All of these methods, however, have the following drawbacks preventing full realization of the economical and qualitative advantageg inherent in a continuous production system, and are therefore only in limited use.
a) In the case of using a varnish solution comprising a resin, the resin composition deposited in the substrate layer after drying, i9, a~ a rule, nearly solid or an extremely viscous semi~luid, and can hardly impart mirror-like surfaces to the layer. This surface roughness unavoidably allows voids or air bubbles to exist between the layers while the layers are stacked together. In order to remove completely such void~ or air bubbles, the stack has to be subjected to heat as well as to a considerable degree of compression for pro-I longed periods during the cur~ng step, which consequently ¦ nece~itatee the provision of highly complicated equipment.
In addition, the provision o~ a drying oven and solvent recovery equipment diminishes to a large extent the advantages j 20 over the conventional batch system.
¦ b) When liquid thermosetting resin compounds are used instead, the above drying process is not always necessary.
On the other hand if the liquid resin generates gaseous or li~uid volatile byproducts when heated, the necessity of applying prolonged pres~ure during the curing i step still remains.

~, .

~37~5~

c) Against the above dir~icult problem that pressure has to be applied to the continuously moving stack throughout the curing reaction, it might appear obvious to introduce a series of separate compressing devices, such as a serial combination o~ man~ pairs of heat-pre~ing rolls. Neverthe-less, experiments have shown such a compromise is use~ess in providing laminates of high quality, because large periodical variations in pressure occur along the length o~ the moving stack, allowing partly entrapped inner bubbles to expand.
Furthermore, it has been ~ound that the periodic pressures applied to the resin composition in a hot ~luid or semi~luid uncured state imparted on undue local ~luid mobility to the reein composition, resulting in intolerable undulations o~
the laminate sur~aces. As a remedy, the continuou~ insertion ~ a highlr rigid plate such as an iron plate between the ~tack and rolls has been tested to modi~y the adverse ef~ects o~ localization and variation o~ pre~sure, but ~ound to be le~ e~e¢tive and at the expense o~ complicated equipment-SUMMARY OF THE INYENTION
i In accordance with the in~ention, a continuous method for producing rein~orced resin laminates i9 provided compris-ing the step~ o~ impregnating a ~ibrous substrate with a liquid thermosetting resin which is ~ree of volatile solvent and is capable of curing without generating liquid and gaseous , ~ 3 735~7 byproducts, laminating a plurality of the resin-impregnated substrates into a unitary member, sandwiching the laminate between a pair of film-or sheet-like converings, and curing the laminate while supporting the same between ~aid pair of coverings without applying appreclable pressure.
In accordance with the invention, the degree of impregnation - of said substrate in terms of the resin content in the impreg-nated substrated is adjusted to be from 10 to 90% by weight, preferably from 20 to 80~ by weight. The adjustment may be carried out scraping or sqeezing the impregnated substrate before or after the lamination to remove excessive amounts of resin from the ~ub~trate. Alternatively, the substrate may be replenished with the resin in any sultable stage to accom-pli~h the mentioned resin content.
The term 'lliquid thermosetting resin compo~ition"
used herein re~er~ to one that does not contain any solvent ¢omponent, but only componènts wholly ¢onvertlble into a reeinow solld by a thermosetting curing reaction wlthout ~eneration of water, carbon dioxide or the like as byproducts.
Said compoeition i~ therefore mainly composed of resins of the radical polymerization or addition polymerization type, ~or example, un~aturated polyester resin, vinylester resin(or epoxy-acrylate resin) diallylphthalate resin, and epoxy resin.
It there~ore excludes resins of the condensatlon polymeriza-tion type, for example phenol-formaldehyde re~in and melamine-formaldehyde resin.

In addition, said composition contains other compo-nents which carry out or accelerate its curing in a way which i9 generally known. For example, a liquid unsaturated polyestcr resin may contain components such as cross-linking polymeri able monomers, curing catalysts, curing accelerators and the like. Epoxy resins or the like may for example contain curing agents.
In the present invention, a covering material in continuous ~ilm or sheet ~orm is applied to both sur~aces o~
~ald unitary member, simultaneously with or subsequent to the combination o~ substrates. This is a way of imparting smooth surface characteristics to the final laminate products. This may be espeoially e~fective inthe courge of curing thermo-eetting resins o~ the radical polymerization type with the aid of ¢uring catalysts ~or avoiding the adverse effect of atmo~pheric oxigen on the polymerization reaction.
The above applied coveringe may be stripped ~rom the ~urPaces of the member after curing, and recovered by rewind-ing, i~ necessary. The recovery and reuse o~ coverings may be desirable ~or product cost reduction.
In the case of production of one or two sided metal ~oil-clad laminates, a metal foil is used as the above cover-ing, which e~fectively accelerates the curing reaction and is le~t as a permanent member of:the finished product.
In the present invention, prior to the impregnation .

with the resin composition the substrates may be passed through an appropriate pre-impregnating step and, if necessary, a drying step, according to the use, properties and producing conditions o~ the product. In particular, a step wherein a cellulosic substrate is pre-impregnated with a solution o~
N-methylol compound and dried to remove the solvent, prior to the impregnation with liquid unsaturated polyester resin composition, is very e~ective in obtaining electrical insu-lation koard~ exhibiting excellent properties even under humid conditions.
In the present invention, the substrate is impregnated with a liquid thermosetting resin compo~ition that has been initially expo~ed to condition~ of reduced pre~sure, in order that the impregnation time may be reduced, and the inclu~ion ~ bubbles in the rinal product may be almost completely avoided.
In the pre~ent invention, the unitary stack ~ormed by combining a plurality o~ impregnated substrates i~ aontinuously oured by heat but sub~ected to no pressure at an early ouring ~tage, where the ~tack may become hard enough to be cut mechanically with ea~e, and the covering may be stripped readily. Then the stripped stack i~ cut to a practical ~ize, and sub~ected to a complete curing ~tep. This curing procedure ensures the reduction Or warping and residual ~train in the rinal product within tolerable limits for practical use.
In the pre~ent invention, the use Or the liquid ~ ~ ~J ~

thermosetting resin composition which is ~ree o~ volatile solvent and is capable of curing without generating gaseous or liquid byproducts throughout the curing steps, eliminate the need for equipments for drying resin varni~h and recovering solvent. The fluidity of the liquid resin therefore remains practically unchanged from the impregnating step through the substrate-combining step. This makes it possible to inhibit air bubbles from becoming entrapped in the stack to a minimal extent, and renders unnecessary any extra heating and pressing in said step.
Furthermore, the substantial absence of entrapped air bubbles and volatile byproducts in the body o~ the stack makes it possible to yield laminate products of excellent properties only by heating. This eliminates the need for complicated eguipments ~or applying pressure on the laminate during the containuous curing step.
It can be said to be epooh-making that a method ~or ¢ontinuous production o~ such a laminate o~ excellent proP-ertie~ by impregnating sheet substrates with liquid thermo-~ettin~ resin compo~ition, and by curing the reaulting impregnated stack without pressure application, is realized by the present invention. According tothe method of the present invention, many advantages may be of~ered regarding the product and its produotivity. For example, the creation o~ 9trains cau~ed by an extra compression applied in the curing step may be avoided ~rom the product, and excellent ~ a f ~ 5 ~ 7 thermal dimensional stability, e~pecially in thickness, may be secured. Other advantages include the exclusion of such complicated systems as a series of separate compressing devices mentioned previously, and the reproducibility of excellent smooth surface characteristics without using any extra compressing means. Other advantages may become apparent as described later.
In the present invention, the heat-curing oven is operated under normal atmospheric pressure conditions with-out any artificially exerted pressure. In the stricte~t sense, the covering material may exert slight amount~ o~
pressure upon the stack by virtue of its own gravity, but 9uch slight amounts o~ pre~sure may not actually exceed 0.01 kg/cm2, and negligible because resin composition is not :
squeezed out under such ~light pressure.
The types o~ heating and transPerring mean~ employed in the present invention are largely optional, and they neoes~itate no complex equipment ~or continuous compression procedure. For example, each one of the methods mentioned below may be useful ~or heat-curing and transferring the stack without the application of compre#sion:
(a) The continuous length o~ ~tack iB supported on a series of several rollers at intervals o~, ~or example, 1 m, and blown by hot air on one or both sides.
(b) The ~loating dryer method, well-known in the art, is use~ul; that is, the stack i~ allowed to ~loat in ~ ~ 7~5~

air and trans~erred continuously, being subjected on i~s upper and lower sides to jet stream~ of hot air.
(c) The stack is placed on a continuous hot plate, trans~erred and conductively heated.
(d) The stack is heated b~ radiation heat from hot plates and the like ln a heat-curing oven.
The laminates produced by the method o~ the present invention are superior in thickness uni~ormity to those produced by conventional means. For laminates of l.5 mm thickness, for example, the variation ranges at most within 20~ to 30~, while lt conventionally reaches 70~ to 160~.
In addition, the heat exmpansion rates in thickness are as low as 40 to 60~ of tho~e of the conventional laminates.
Furthermore, the present invention exhibits great advantage~ in aspects such as reduction of production cost, increa~e o~ production speed, and simplirication o~ the manufacturing plant.

BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 represents the apparatus used in the present invention.
FIG. 2 is a schematic representation of cross-sectional view of a product resulting from the use of covering material o~ lower modulus.
FIG. 3 is a schematic representation of cross-sectional 5 ~ '7 view of a products resulting from the use of covering material o~ higher modulus.
FIG. 4, ~IG. 5 and FIG. 6 represent other apparatuses used in the present invention.
DETAILED DESCRIPTION OF THE INVENTION

As illu~trated in FIG. 1, the present invention essentially provide~ that a plurality o~ sheet substrates 6 are supplied ~rom ~toring sections 1, continuously transferred to and sequentialy treated by an apparatus to produce laminates 7,used herein comprises a continuous drying equipment 12, impregnating devices 2, a laminating device 3, a continuous heat-curing device 4, a take-off device 13 and a outting device 5. The continuous heat-curing devi¢e 4 doee not include any pressure me¢hani~m.
In the present invention, eaid sheet substrate 6 implles various web materials, which have been used in conventional laminate production, for example, ~iber glass cloths, fiber gla~ nonwoven fabric~, oellulo~ic papers euoh a~ kraft paper and cotton-linter paper, and asbestoe or other inorganic sheetings. As to papers used as the ~heet #ubstrate, paper malnly compoeed o~ celluloeic flber, euch as kraft paper, having an air-dried bulk deneity of 0.3 to 0.7 g/cm3, is preferable in the view point of impregnation and product quality.

, ............................................... .

~ ~ 7~5~7 Preceding the impregnation with liquid thermosetting resin composition by the impregnating device 2, the substrate is passed through a pre-impregnating process, and, 1~ necessary, a drying process, according to properties required ~or ~inal products. Pre-impregnated substrate may be provided ~or the storing sections 1. Alternatively, the gheet substrate 6 from the storing sections 1 may be continuously treated through pre-impregnating devices 14, and, i~ necessary, continuous drying equipment 12, which are arranged directly in front o.~ the impregnating devices 2.
m e continuous drying equipment is only a means to remove solvent that may possibly be contained in used pre-impregnating agent, and may therefore by eliminated i~ the pre-impregnation is carried out either without solvent, or 1~ by means of a ga~eou~ compound adsorPtion technique.
Non-limiting pre-impregnating techniques useful in the present invention are as follows, but these may be diversely modi~ied,.as is the oase in the art, depending upon the purposes of the Pinal product.
(I) Pretreatment o~ substrate with various coupling agent~ and ~ur~ace active agents. For example, pretreatment o~ glaY~ cloth substrate with silane coupling agent.
(II) Pre-impregnation of substrate with various monomers capable o~ polymerizing or capable of copolymerizing with thermosetting resin liquids.
~II) Pre-impregnation o~ substrate with variOUs ~ -~ 7~5~7 thermoplastic resins with the aim of improving properties of ~inal laminate product.
(IV) Pre-impregnation of substrate with various thermosetting resin solutions.
(V) Pre-impregnation of substrate with various unsaturated fatty acids.
(VI) Pre-impregnation of substrate with reactive compounds and subsequent reaction~, for example, acetylation of cellulosic substrate.
(~I) Pre-impregnation of substrate with certain catalysts, accelerators and/or curing agents, followed by impregnation with resin liquids of short pot life.
(UnI) Pre-impregnation of substrate with slurry of inorganic ~iller~.
~or example, ac¢ording to (I), glass cloth substrate i9 pretreated with vinyl alkoxysilane, and then impregnated with liquid unsaturated polrester Fesin composition. As a re~ult, flexural strength of the obtained laminate is increased 1.5 times compared with that without pretreatment.
According to (Ir), Kraft paper substrate is pre-impregnated with 10% by weight of polyethylene glycol, and then impregnated with liquid unsaturated polyester resin composition. As a result, impact stregth of the obtained laminate is increaged about 2 timeg compared with that with-out pre-impregnation.
Furthermore, according to (~I), glass cloth substrate

3 ~ 7 is pre-impregnated with polyamide resin by 30% of the liquid epoxy resin by weight, and then impregnated with liquid epoxy resin. As a result, the troubles from the short pot life o~ the liquid epoxy resin can be prevented in its vessels 8, 9 and impregnating devices 2.
The amount of pre-impregnating agent finally deposited in substrate is preferably less than 50~ to the substrate by weight, and an excessive amount may occasionally affect the succeeding impregnation with the liquid resin adversely.
The above pre-impregnation treatment is important ~or the reason described below.
When paper mainly composed of cellulosic fiber is impregnated with liquid unsaturated polyester resln composi-tion, conventionally obtained laminate i9 suf~iciently good under normal oonditions, in various properties inoluding eleotrioal insulation, solder dip thermal resistanoe, oopper foil peel strength, punching processability and meohanical strengths, while it has the drawback o~ decrease in per~ormance o~ laminate by moisture absorption. This is considered to be because, although the cured unsaturated polyester resin~itsel~
i~ excellent in eleotrical insultaion, thermal resistanoe, and moisture- and water-resistance, it i8 rather poor in aPfinity with cellulosic fiber compositing the paper substrate, and consequently causes inter~acial release between the re~in and the cellulosic ~iber upon humidifying, and ~ubsequently allow~ moisture content o~ :che laminate to increase, resulting 5 ~ 7 in deterioration of various properties o~ the laminate.
As attempts to prevent such imper~ections, several propositions have been made so ~ar. For example, paper substrate was preliminarily treated with methylol melamine or methylol guanamine (Japane~e examined publication SH0.38-13781), or acetalized with ~ormaldehyde (Japanese examined publication SH0.40-29189), or cellulosic substrate was etherized with N-methylol acrylamide, washed and dried, and impregnated with diallyl phthalate resin (JaPanese examined publication SH0.39-24121).
However, both of the above methods with methylol melamine or methylol guanamine and with formaldehyde, have the drawback that they require the use o~ an excessive amount o~ ~uch agents, rendering the laminates too hard to be punched with ea~e.
The treatment with N-methylol acrylamide has a draw-back that the re~ction takes a long tlme and require~ com-plicated proceduré~ such as washing prooess. In addition, the laminate~ obtained are poor in punching processability.
After many experimental studies, we have found a way of protecting the laminate from deterioration o~ properties upon humidifying. In this method, cellulo~ic ~ubstrate is dipped into a solution o~ an N-methylol compound that has an unsaturated bond capable o~ copolymerizing with a monomer, far example, a vinyl monomer contained in the liquid un-~aturated polyester resin composition. Eleotrical laminate 5 ~ 7 board~ obtained in this way exhibit excellent properties under humid conditions as well as under normal condition~.
It should be noted that the drying treatment doe~ not encourage reaction between the N-methylol compound and cel-. .
lulose, but only removes the solvent ~uch as water or alchol.
Unsaturated polyester resin u~eful in the present invention may be liquid or sol1d, preferably liquid at room temperature. It~ molecular oonstitution may be, for example, composed of a well-known recurring unit:

~oC~4-0_~ 11_o C2~

The un~aturated polyester resin is ~ynthesized with ¦ the u~e of diol compound~, saturated polyba~ic acids, and un~aturated polyba~ic acids in the way well-known in the art.
, .
l U~e~ul diol compound~ include ethylene ~lycol, prapylene glycol, diethylene glycol, 1,4-butanediol, and 1,5-pentanediol;
useful ~aturated polybasic a¢id~ include phthalic anhydrlde, i~ophthalic acid, terephthalic acid, adiPic acid, ~ebacic a¢id, and azelaic acid; and u~eful un~aturated polyba~ic acids include maleic anhydride and ~umaric acid. The liquid un-~aturated re~in compo~ition iB prepared by mixing the ~aid un~aturated polyester resin with cro~e-linking monomers.
Aa regard~ the cross-linkin6 monomer, variou~ poly-merizable monomer~ are u~eful. S~ylene i~ the most common, but -methylstyrene, vinyltoluenes, chlorostyrenes, divinyl-.
, - 17 -.~ , , 35~7 benzenes, Cl ~ Clo-alkyl acrylates, Cl ~ Clo-alkyl meth-acrylates, diallyl phthalate, triallyl cyanurate and the like are also useful. These polymerizable monomers are used in an amount of 20 to 50% by weight based on the unsaturated poly-ester resin. It should be specially mentioned that a mixture of styrene and divinylbenzenes is readily copolymerizable and contributes to improvement in the mechanical strength of the final product.
To the liquid thermosetting resin composition, a cur-ing catalyst which is usually an organic peroxide oompound, and, if necessary, a curing accelerator are added. For the purpose of curing unsaturated polyester resin, the curing catalyst is preferably selected from the organic peroxide compounds cited below. It should not however be limited thereto, but may be qelected among known other types of curing catalyst, ~uch as light-sensitive or irradiation-sensitive ones.
~lthough there are a number of known organic peroxide compounds for curing unsaturated polyester re~in, the ~elec-tion therefrom i~ important in securing the novel electrical laminate production by the nonpressure curing process of the present invention.
In general, an organic peroxide compound leaves traces of its decomposition residues, in the body of the laminate.
Such decomposition residues may occasionally evaporate with unpleasant odors at temperatures of from 100~ to 260~, which _ 18 -5 ~ 7 are those usually employed in fabrication processes of electrical insulation laminates or copper clad laminates.
According to experiments by the inventors, the single or combined use of certain organic peroxides selected Prom aliphatic peroxides, especially from aliphatic peroxyesters, has been found suitable for producing such laminate~ with little or no odor.
Said aliphatic peroxides are those having a general formula as follows:
ROOH, RmM(OOH)n, ROOR', RmM(OOR')n, RnMOOMR'n, O O O O O O

R(C02H) , RS0200H, RCOOCRI, RCOOCOR', ROCOOCORI, O O O
Il 11 11 RS0200CR', RS0200S02R', R(C02R') , ROCOOR', C(OOR)2 0 R / OORI' ,N~COOR, RS0200R', or C
R' OORIll wherein R, R~, Rll and Rll'represent aliphatic hydrocarbon groups, and M is a metal or metalloid atom.
Speci~ically, they include di-t-butyl peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, acetyl peroxide, isobutyryl peroxide, t-butylperoxy-2-ethylhexanoate, etc.
Said aliphatic peroxyesters are those havine a general ~ormula as ~ollows:
R(C02R')n, ROCOOR', C(OOR)2, /NCOOR, or O . O O

) 5 ~ '7 RS0200R ', where R and R' represent aliphatic hydrocarbon groups, and n is an integer from 1 to 4.
Specifically, they include t-butylpsroxy acetate, t-butylperoxy isobutyrate, t-butylperoxy 2-ethylhexanoate, 5-butylperoxy laurate and the like.~
The reason whey such aliphatic peroxides and per-oxyesters are preferable is probably because, upon heat decompo~ing, they do not generate volatile aromatic compounds that could be responsible ~or odors.
Conditions of temperature and time for curing liquid re~in compo~ition largely depend upon the type of organic peroxide used. In the present invention, the temperature is prePerably lower than 100~ at the initial curing step, so that generation oP bubbles due to volatilization o~ existing liquid copolymeri~able monomer can be prevented, and there-aPter the temperature prePerably lies in the range 50~ to 150~ to make curing complete at atmospheric pressure.
In electrical laminate boards and copper clad laminate products, propertie~ ~uch as thermal resistance, thermal and humid dimensional stabilities, punching processability, bond-ing ~trength between ~ubstrate and cladding copper, and electri¢al ln8ulation properties are very important. For the purpo~e o~ improving the~e propertie~, rariou~ additivee and Piller~ may be incorporatsd into the above liquid un~aturated re~in co~po~ition, without departing Prom the ~cope oP the .' ~

~ 3 7~5~7 present invention.
As regards impregnating epoxy resin, bisphenol A-type epoxy resins, novolak-type epoxy resins, and their mixtures are useful, and they are mixed with a reactive diluent as necessary, and with a curing agent. The epoxy resin itself however is pre~erably liquid at room temperature.
As regards a curing agent, most of the well-known agents of both the acid-curing type and amine-curing type are applicable in the present invention.
In the present invention, however, the use o~ epoxy resin liquid composed of epoxy resin and acid anhydride-type curing agent is e~pecially suitable, because the viscosity of the epoxy resin liquid at 25~ can be maintained at from 0.5 to 30 polse, preferably from 1 to 15 poise, this being appro-priate ~or impregnation of the substrate.
There are a number o~ various epoxy-curing agent~ oP
the amine type, amide-amine type, dicyandiamide type, imidazole type, etc. on the market. The use of these agents with bisphenol A-type epoxy resin of high quality makes the viscosity of the resin liquid difficult to control within a certain appropriate range. This dif~iculty could be alleviated by adding a large amount o~ diluent, which however might cause, in turn, significant deterioration in the propertie~ of the product.
In addition, curing agent~ of the amine type and amide-amine type generally have a drawback in that they shorten the 5 ~ '7 pot life of resin, whereas curing agents of the dicyandiamide type and imidazole type have the drawback or requiring pro-longed high temperatures for curing.
The use of curing agents of the acid anhydride type has neither o~ these drawbacks, and is therefore most suit-able.
The liquid epoxy resin composition used in the present invention i9 more definitely de~cribed as follows.
Bisphenol A-type liquid epoxy resins are generally suitable, but also others such as bisphenol F-type and novolak-type resins are useful. If necessary, their blends with solid epoxy resin or diluent may be acceptable. Useful acid anhydride-type curing agents include phthalic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, methyl-tetrahydrophthalic anhydrlde, methyl-hexahydrophthalic anhydride, methyl-endic anhydride, and their mixtures.
However, methyl-tetrahydrophthalic anhydride, methyl-hexhydro-phthalic anhydrlde, and methyl-endic anhydride, eaoh o~ whioh is liquid at room,temperature, are especially suitable for the present invention.
As curing acoelerators, commercially available agents such as 2-ethyl-4-methylimidazole, boron tri~luoride complex compounds, tertiary amines and their ~alt~, benzyl dimethyl-amine, and myristyl dimethyl benzyl ammonium chloride are useful.
As the sheet substrate, a long continuou~ cloth of ~ ~ 7~5~7 glass fiber, specially pretreated with silane coupling agent as previously de~cribed, is preferable.
The above N-methylol compounds, which are used as the pre-impregnating agent and have an unsaturated bond copolymer-izable with vinyl monomers, include various types o~ compound and mixture~ as follows:
(I) Modified aminotriazine methylol compounds.
These compouns are derived from methylol compounds of amino-triazines such as guanamines and melamines, whose methylol groups may have optionally been etherified, partially or totally by lower alcohols such as methanol, by introducing un~aturated bond~ copolymerisable with vinyl monomers, and I therefore include aminotriazine methylol compound~ Partially esteri~ied with unsaturated carboxylic acids such as acrylic acid and itaconic acid; aminotriazine methylol compounds partially etherified with unsaturated alcohol~ suoh a~ allyl alcohol; condensates of aminotrlazine methylol compounds with unsaturated carboxylic acid amides such as acrylamide and methacrylamide; and condensates of aminotriazine methylol compounds with unsaturated epoxy compounds such as glycidyl methacrylate.
(~) Amide methylol compounds having a general formula:
Rl H2C=C-CO-NH-CH2-OR2 ~
where Rl is a hydrogen atom or methyl group, and R2 is a hydrogen atom or Cl ~ C3-alkyl group.

' ~ .

1 ~7~5~7 Among these compounds, N-methylolacrylamide, N-methoxymethylolacrylamide, N-butoxymethylolacrylamide, N-methylolmethacrylamide, N-methoxymethylolmethacrylamide, and N-butoxymethylolmethacrylamide are pre~erable. Also mixtures o~ condensates of the above two or more compounds are useful.
(Dl) Mixtures of a and b described below, as replacements of modified aminotriazine methylol compounds specified in (I).
(a) N-methylol compounds such as aminotriazine methylol compounds having no unsaturated bonds copolymeriz-able with vinyl monomers.
(b) Modifying agents for N-methylol compounds (a), that is, compounds having both a group condensable with or additionable to N-methylol compounds (a), and an unsaturated bond copolymerizable with vinyl monomers. For example, unsaturated carboxylic acids such as acrylic acid and itaconic acid, unsaturated alcohols such as allyl alcohol, unsaturated carboxylic acid amides ~uch as acrylamide and methacrylamide, and unsaturated epoxy compounds such as glycidyl methacrylate are included.
The pretreatment of paper substrate with a solution of the mixtures specified in (m ) also forms one of the best modes ~or carrying out the present invention, and exhibits Z~ almost similar effects to the pretreatment with compounds ~pecified in (I) and (~). The reason is considered to be - ~4 -~ .~ 7~5~'-because, while the pretreated paper is dried and ~ubsequently impregnated with liquid unsaturated polye~ter resin composi-tion and then cured, reactions take place between the N-meth~lol compounds ~pecified as (a) in (IG), and the modify-ing agents specified as (b) in (m ) .
One o~ the main purpo~es o~ the present invention i~
to improve the combination between paper substrate and un-~aturated polyester resin, and thereby to prevent deteriora-tion of product properties when humid. In order to attain this objective, paper substrate has to be subjected to treat-ment with a compound ~peci~ied in (I) or (~), which ha~ both an N-methylol group capable of combining with cellulose, and an un~aturated bond capable of copolymerizing with a vinyl monomer (i.e, a cross-linking agent ~or unsaturated polyester re~in); or sub~ected to treatment with the mixture ~pecified in (m), which is composed of an N-methylol compound a~ in (a) havlng no un~aturated bond copolymerizable with the said vinyl monomer, and the modifying agent a~ in (b) for the said N-methylol compound. In contrast, the pretreatment with a compound having only either an N-methylol group or an un-~aturated bond copolymerizable with the vinyl monomer is not ~u~ficient in effect. For example, the sole u~e of methylol melamine having only the N-methylol group, and the sole use of acrylamide having only the unsaturated bond, have both been found to yield laminate~ with insufficient humid prop-erties.

1~7~5.~7 The solution o~ the treating agent specified in (I), (~) and (m ~, is prepared of such a concentration that the deposited amount in paper substrate a~ter drying may result in from 3 to 30 parts, pre~erably ~rom 6 to 20 part~ per 100 parts of paper substrate on a dry basis. Deposition o~ less than 3 parts 09 almost inef~ective, whlle that o~ more than 30 parts makes laminates too brittle to be punched with ease.
For a solvent of the above treating agent, water, alcohols, ketones, and esters are useful. In order to promote etheri~ication o~ cellulose with the N-methylol group o~ the treating agent, addition o~ an acld condensation catalyst, - and elevation o~ the curing temperature of pretreated paper may be both e~fective. In thege cageg, although the ether-ification reaction o~ cellulose may possibly partially take place prlor to the ~ubsequent impregnation and curing reaction of unsaturated polyester resin, this hag been found to produce no special ef~ect~.
According to the present invention, in the proce~s of pretreating the paper eubstrate, promotion of the etheri~ica-tion reaction of cellulose by adding a catalyst is not always neceesary, but simply the deposition of the treating agent into paper is e~ficient to improve properties Or products when humid. Adversely, the addition o~ catalysts of certain typee may result in a deterioration in product properties 2g such ae electrical ineulation and punching proceeeability.
Further, such agents ae a polymerization inhibitor, ,, :

1 3 7~ 7 polymerization catalyzer, sur~ace active agent and plasticizer may be effectively added into the solution o~ the above treat-ing agent, i~ necessary.
For example, substrates commonly used in the art, such as kra~t paper, cotton linter paper and ~ibrous cloth, are impregnated with the solution o~ the above treating agent by means o~ a dipping bath, roll coater or spray, and dried to be converted to the pretreated substrate. This drying procedure is only necessary for removing the used solvent, but not for making the cellulose of the substrate react with the treating agent.
Needless to say, such compounds as methylol melamine and methylol guanamine, which had been disclosed in literature as cited above, belog to the group of methylol compounds having no unsaturated bond copolymerizable with vinyl monomer.
In an attempt by the inventors, paper substrate was pretreated with t~ese compounds, and then, according to the present inven-tlon, impregnated with liquid unsaturated polyester resin composition. Laminates thus obtained were ~ound to have con-~iderably improved moisture- or water-resistance, and, in comparison with tho~e obtained without the said pretreatment, to show smaller decreases in humid electrical insultaion and solder thermal resi~tance. These laminates, however, were ~ound to be impractical in respect of punching processability because they readily cracked on impact. On the other hand, punching processability was congidered to largely depend upon , .

~ ~ 7~55'7 the qualities of the impregnating unsaturated polyester resin used. In fact, according to the evaluation of a number o~
grades of commercially available unsaturated polyester resins, there were none among them that could yield excellent punching processability, even though applied to the said pretreated substra~e.
After many elaborate studies from these points of view, the inventors reached the following finding. That is, even when known methylol compounds, such as methylol melamine and methylol guanamine, that have no unsaturated bonds co-polymerizable with vinyl monomers, are employed as pretreating agent, the additional use o~ a higher aliphatic derivative that has one or more functional groups, such as hydroxyl, carboxyl, amino and amide group~, capable of condensing with the said methylol compounds, can give laminate p~oducts both of excellent punching processability and qxcellent humidity resistance. The higher aliphatic derivative may be incorpo-rated with the said methylol compound~ by means of mixing or condensation.
Further detailed descriptions are given below.
The methylol compounds, such as methylol melamine and methy~ol guanamine, that have no unsaturated bonds co-polymerizable with vinyl monomers, more definitely are either early condensates Or melamine, or guanaminè compound~, for 2~ example, ~ormoguanamine, acetoguanamine, propioguanamine, benzoguanamine and adipodiguanamine with ~ormaldehyde, or .~ - 28 -1 J 70~57 their derivatives obtained by partial or total etherification of their methylol groups with lower alcohols such as methanol and butanol.
The above higher aliphatic derivatives, which are mixed or condensed with said methylol compounds for the pur-pose of improving punching processability, include saturated fatty acids such as caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, and stearic acid; unsaturated fatty acids such as oleic acid, erucic acid, linoleic acid, eleostearic acid, and linolenic acid; esters of the above eaturated and unsaturated fatty acids with polyols such as ethylene glycol, polyethylene glycol, propylene glycol, glycerol, pentaerythritol, and sorbitol; amides of the above eaturated and uneaturated fatty acids; saturated and unsatu-rated higher alcohole such ae caprylic alcohol, lauryl alco-hol, myrietyl alcohol, cetyl alcohol, etearyl alcohol, oleyl alcohol, and linoleyl alcohol; ethers of the above higher alcohole with the above polyole; derivatives from the above higher alcohole, euch ae aliphatic amines, and the like.
In addition, hydroxyfatty acids such as rioinoleic acid, and their derivatives are also useful for the same purpose. In ef~ect, the eeeential feature of an agent ~or improving punching proceeeability ie that ite molecule has both a group euch ae hydroxyl, carboxyl, amino and amide capable of condeneation wlth a methylol group of methylol melamine, methylol guanamine, and the like, and a long-chain alkyl ' , group capable of making the intermolecular cohesion ~oderate.
There are a great number of higher aliphatic compounds which meet the above molecular constitutional conditions.
Nevertheless, according to screening studies so far, it has become apParent that the use of compounds having an aliphatic group of 8 or more carbon atoms tends significantly to improve punching processability of the resulting laminates, and that the use of compounds having an aliphatic group of 18 carbon atoms and one unsaturated bond, such as oleic acid, oleyl alcohol, and their derivatives, such as oleic monoglyceride, oleic dlglyceride, oleic amide, and oleyl amine, presents a well-balanced combination of properties o~ laminate products, and thus constitutes a suitable substance ~or use in connec-tion with the present invention.
The optimal amount of the above agent used for im-proving punching processability depends upon the glass tran-sition temperature of the impregnating unsaturated polyester resin, but usually falls in the range of 3 ta 40 parts per 100 parts of methylol melamine or methylol guanamine. The agent can be incorporated in solution or suspen~ion with methylol melamine or methylol guanamine, and alternatively applied in the form of a preliminarily prepared condensate with methylol melamine or methylol guanamine. As solvent~, water, alcohole, ketones and esters are useful.
The whole concentration of the pretreating media, is desirably kept within a range of 3 to 30 parts, preferably 6 ~ 3 -.: :

5 ~ 7 to 20 parts per 100 parts o~ cellulosic substrate on a dry basis, as in the case o~ N-methylol acrylamide mentioned previously. Deposition of less than 3 parts i9 almost ineffective, while that of more than 30 parts makes laminates too brittle to be punched with ease.
Cellulosic paPer substrate such as kra~t paper and cotton linter paper, or cellulosic cloth substrate made ~rom cotton or rayon, is treated with a solution or suspension that has been prepared under the conditions mentioned above, by means of a dipping bath, roll coater or sPray, and dried to remove the solvent. Usually, the drying temperature i9 preferably 70 to 150~, and the drying time is preferably 1 to 60 minutes. The pretreated substrate i~ then subjected to impregnation with the liquid unsaturated polyester resin composition mentioned previously.
Thus, two typeg o~ pretreatment (i.e., preimpregnating treatment) o~ paper ~ubstrate were described as above.
These processes provide the laminate product with excellent punching processability; however, the use of un-saturated polyester resin whose post-curing glas~ transition temperature is 20 to 80~ is also preferable ~or securing low-temperature punching processability.
According to studies by the inventor~, the use o~
unsaturated polyester resin whose post-curing glags transition temperature is 20 to 80~, ig desirable for securing excellent punching processability in common, independently of the above ~ ~ 7~J5~'7 mentioned pretreatment of paper substrate.
In practical use, electrical clad or unclad laminates are often trimmed or provided with holes or slots by punching, and there~ore are required to have excellent punching proc-essability. Recently in accordance with the miniaturization of electronic parts and densification of circuits, such requirements have been increasingly important.
Conventionally, reinforced un~aturated polyester resin laminates have been produced by impregnating the sub-strate with a solution of crystalline or solid unsaturated polyester resin containing a cross-linking agent, drying the substrate to form prepreg, and then curing the stacked pre-preg under heat and pressure. Laminates obtained have had excellent thermal resistance due to the high post-curlng glass transition temperature of the unsaturated polyester re~in u~ed, but have been difficult to be punched out, especially at lower temperatures of 50 to 80~.
After elaborate ~tudies to resolve the problem o~
punching proce~ability, we have found that there i8 a close correlation between the gla~ transition temperature of a cured unsaturated polyester resin composition and the optimal temperature for punching the resulting laminate.
Then, we have found that the temperature for punching should be equal or clo3e to the above ela~ tran~ition tem-perature, within a difference of 20~, preferably 10~. For example, when an unsaturated polye~ter resin composition with ~ ~ 7~5~7 a post-curing glass transition temperature of 20 to 80~, preferably 30 to 70~, is used, the resulting laminate exhibits excellent low-temperature punching procesgability at temper-atures within 20~ of said glass transition temperature, preferably within lO~ of said glass transition temperature.
The punching processability referred to herein may be examined and evaluated in accordance with ASTM D-617-44 "Punching Quality of Phenolic Laminated Sheets". When a laminate produced as herein described i~ ranked fro~ "Excel-lent" to "Fair" in overall evaluation on its edges, surfaces and holes, its punching processability i~ signified as "Good".
Where punching processability at low temperature i9 regarded as important, an un~aturated polyester resin compo-~ition with a po~t-curing glass transition temperature of 20 to 80~, preferably 30 to 70~, is useful. If the ~aid temper-ature ex¢eeds 80~, punching at low temperatures could result ln such seriou~ defects as deficiéncies or bores along edges, cracke or swellings on edge~ or around holes, de~ects in hole walls, expan~ion around holes, and tapering of~ of holes.
When punching i~ per~ormed at temperatures lower than 20~, ~wellings around holes and tapering off of holes becomes much more con~picuous.
In the latter case, improved re~ults can be obtained by cooling the te~ting piece, however thi~ i~ not practical.
When the boards are impregnated with unsaturated polyester re~in composition with a glass transition temperature . .
- 33 ~

~ ~ 7`35~

o~ 30 to 70~, products of good quality ~or low temperature-punching are obtained.
In the cace o~ low-temperature punching, ~or the types of products usually applied in the art concerned, it has been common practice to use a punching temperature ~rom about 50~ to about 80~, while, in accordance with the present invention, the use o~ temperatures ranging from about 30~ to about 80~ is possible ~or punching with ease and with excel-lent results.
Furthermore, when punching processabilit~ at low temperature is regarded as important, various properties of the cured resin composition and the resulting laminate product largely depend upon the type and ratio o~ glyools, saturated and un~aturated dibasic a¢ids, and cross-linking monomerY
used. However, all type~ of unsaturated polyester resins mentioned previously are useful when they are inoorporated with crose-linking monomers 90 that the post-curing glass tran~ition temperature may range ~rom 20 to 80~, preferably rrom 30 to 70~. For example, unsaturated polye~ter re~ins composed of the following components (molar ratio given in parenthe~es) are all applicable:
diethylene glycol, isophthalic acid and maleic an-hydride (3 : 2 : 1);
propylene glycol, isophthalic acid and maleic an-hydride (2 : 1 : 1);
1,3-butanedlol, isophthalic acid and maleic anhydride ! ~ 7 '~5S ~

(2 : l : 1);
1,4-butanediol, isophthalic acid and maleic anhydride (2 ~
dipropylene glycol, isophthalic acid and maleic anhydride (2 : l : l);
diethylene glycol, isophthalic acid and maleic anhydride (2 : 1 : 1);
propylene glycol, phthalic anhydride and maleic anhydride (2 : 1 : l);
propylene glycol, glutaric acid and maleic anhydride (2 : 1 : 1);
propylene glycol, succinic acld and maleic anhydride ( 2 : 1 : 1);
propylene glycol, pimelic acid and maleic anhydride (2 : 1 : 1);
propylene glycol, adipic acid and maleic anhydride (2 : 1 : 1);
propylene glycol, sebacic acid and maleic anhydride (2 ; 1 : 1); and propylene glycol, azelaic acid and maleic anhydride (2 : 1 : 1).
These un~aturated polyester resins are incorporated, ~or example, with styrene at a ratio o~ 65 to 35 by weight to prepare a liquid resin compo~ition.
For example, a liquid re~in compo~ition prepared ~rom the resin composed of propylene glycol, i~ophthalic ~ 35 -5 ~ '~

acid and maleic anhydride (2 : l : 1) has a post-curing glass transition temperature of about 70~, and exhibits excellent low-temperature punching processability when evaluated at 75~.
As for the cross-linking monomer, styrene is useful as is known in the art, but other vinyl compounds such as vinyltoluene, chlorostyrene, dichlorostyrene, divinylbenzene, vinyl acetate, alkyl acrylates, alkyl methacrylate, diallyl phthalate, triallyl cyanurate or their mixtures with styrene are usePul as well. Each o~ these polymerizable monomeric components are incorporated with the above unsaturated poly-ester resin, so that the post-curing glass transition temper-ature of the resulting unsaturated polyester resin composition may range ~rom 20~ to 80~, preferably from 30~ to 70~.
For example, liquid resin compositions may be formu-lated with the use of an un~aturated polyester re~in, which is prepared from diethylene glycol, isophthalic aoid and maleic anhydride (3 : 2 : 1), with styrene and butyl acrylate in three ways as ~hown in Table 1. The reaulting po~t-curing gla~s transition temperatures (Tg) are given in addition.
Table 1 Resin composition~ No. 1 ¦ No. 2 ¦ No. 3 Components (weight ~) Polyester re~in 60 60 gO
Styrene 35 30 3o Butyl acrylate g 10 20 Tg (~) 50 45 35 5 ~ 7 Blending ingredients such as rubbers, plasticizers and fillers with the above resin compositions may be possible, provided that the ~inally cured regin compositions are qual-ified as described herein. In the case o~ rubber, poly-butadienes, butadiene copolymer rubbers, or their maleic-modified products ma~ be used; in the case o~ plasticizers, commercial products ~rom adipic or phthalic acid and glycols, and epoxidized soybean oil may be used; and in the case o~
~illers, certain types of commercial calcium carbonate, silica and titanium dioxide may be use~ul, respectively.
As regards the substrate, various sheet materials well-known in the art are useful as mentioned previously, but paper material is especially desirable for use in connection with the present inventlon.
Unclad laminates and copper clad laminates produced in the above-mentioned ways according to the present invention exhibit de~irable punching processability ~rom 30 to 80~, being ~ree from drawbacks common to conventional un~aturated polyester laminates in this respect. Certain laminates obtained are superior to coventional phenolic laminates in pun¢hing processability.
When a sheet substrate i8 sub~ected to impregnation with a liquid resin compo~ition according to the present invention, it is necessary to take the high viscosity o~ the liquid resin composition into consideration, in contrast with the case o~ conventional impregnation with liquid varnish.

~ 37 -35~7 As an impregnating device 2, both the dipping bath as ~hown in FIG. 4 to FIG. 6, and the curtain flow applicator as shown in FIG. 1 are useful.
In the dipping bath, the substrate is forced through the bath o~ resin liquid. Care should be taken since this method tend~ to leave air bubble~ entrapped within the sub-strate.
In the curtain flow method, the ~ubstrate is fed a curtain flow of the liquid resin onto its upper surface while moving horizontally. This method has the advantage of being able to impregnate~several qheet substrates concurrently under similar conditions, and to exclude entrapped large air bubbles with ease, but has the disadvantage that a number of ~ine bubbles still remain within the substrate, especially the paper sub~trate, even through to the last stage of impregnation from the upper surface through to the lower sur~ace. Most o~ these bubbleg gradually disappear in 7 to 20 minutes. When the sheet substrates are, however, oombined into a unitary stack and cured be~ore the complete disappear-ance of bubbles, the resulting laminate unavoidably includes sustained bubbles.. This inclusion leads to a decrease in the thermal conductivity of the laminate, and consequently causes undeeirable over-heating o~ electronic parts mounted on the laminate. It also adversely affects on the transparency and appearance of the laminate. Even though the impre~nation ability may, needless to say, depend upon such parameters as ~ ~ 7~5~7 applied pressure~ wettability o~ substrate by resin liquid (i.e., contact angle), applied time as well as vlscosity o~
the resin liquid, the general mode o~ the impregnation process is similar to the above.
The long duration of time required for the impregna-tion process necessarily causes the entire line speed to a low level. Needless to say, these e~fects are unde~irable from the view point of securing e~ficient productivity.
It has been generally considered that the number o~
bubbles remaining within the final laminate depends largely upon the impregnating conditions and the heat and pressure conditions during curing, and there~ore that a longer impreg-nation time reduces the number o~ bubbles within the impreg-nated ~ubstrate, and a higher compressing pressure promotes dis~olution o~ remaining bubble~ into the resin material.
Use of prolonged impregnation time and higher pres~ure, however, is apparently accompanied with the disadvantages o~
lowering productivity and expanding àpparàtus.
According to the present invention, it is possible to make the laminate almo~t free from remaining bubbles by exposing the liquid resin composition to a reduced pressure, even with no application of compressing pressure during the curing step.
Further, such a reduced-pressure treatment is effec-tive in reducing impregnation time, for example, cutting this time to 1/3 to 1/10 of the time it would otherwise have taken.

~ 39 -~ :1 7 35S I

The reduced-pressure treatment re~erred to in the present invention requires exposing the liquid resin compo~i-tion to a pressure lower than atmospheric pressure. For example, a liquid resin composition containing a curing agent i9 placed in a pressure vesgel, and the space in the vessel is evacuated. Alternatively, a liquid resin composition i9 intermittently poured, continuously jetted,or otherwise added into a preliminarily evacuated vessel. In either case, the reduced pressure-treated liquid resin composition may be, upon impregnating, sub~ected to contact with the atmosphere, which has, however, no adverse e~ect. Alternatively, a substrate already impregnated with the liquid resin composition may be subjected to reduced pressure treatment in an evacuated veesel .
The reduced pressure-treated liquid resin composition can be exposed, i~ necessary, to atmosphere pressure ~or about 30 to 60 minute~ with no adverse ePfe¢t.
The de~ree o~ reduced pressure applied may depend on the vapor pressure of the solvent and monomer oomponents contained in the liquid resin composition, but is preferably 2 to 100 mmHg. Also the treatment time may dif~er between treating methods, but a resident time for several minutes in the eva¢uated vessel may be su~ficient in the droPwise adding method.
The said reduced-pressure treatment is especially use~ul for application to a liquid resin oompos~tion which - 40 _ is free o~ solvent and is capable of curing without generating gaseous and volatile byproducts. This is because, in such a ca~e, the conditions of reduced-pressure treatment are not restricted by the existence of a volatile solvent and the secondary generation of bubbles during the curing process is effectively prevented without the application of pressure.
It is preferable according to the present invention to use unsaturated polyester resin of liquid state with a viscogity of from about 0.1 to 30 poige at room temperature.
As regards the cross-linking monomer, styrene that is in common u~e for unsaturated polyester resins and has a vapor pressure of about 6 mmHg at room temperature, is preferable al~o in the present invention, and the ratio of styrene to the whole liquid resin composition is preferably from 30 to 5 ~ by weight. Then the reduced-pressure treatment may be e~riciently ¢arried out by supplying the styrene-containing compo~itlon into a ve~sel wherein it is sub~ected to a pressure of from 2 to 30 mmHg.
FIG. 1 show~ an illu~tration of apparatus in which a liguid resin compo~ition is ~uccessively treated under a reduced pre~sure as mentioned abave, and continuously aPPlied onto several sheet ~ubstrates moved in parallel.
The etoring vessel 8 of liquid resin composition is connected to the top o~ a pressure-reducing cyllndrical clo~ed vessel 9 by a pipe 15. The pipe 15 is open at one end at the bottom of the ~toring vessel 8 and, at the other end, ~ointed 1 ~ 7~5~'-with the nozzle, which is arranged in the uppermost part of the pressure-reducing vessel 9 via the valve 16. The liquid resin composition i9 transferred from the storing vessel 8 through the pipe 15, and jetted out of the nozzle by a negative pressure in the pressure-reducing vessel 9. The rate of jetting is adJusted manually by a valve 16 or a supply pump (not shown in the figure).
The uppermost part of the pre~sure-reducing vessel 9 is connected to a væcuum pump 19 of oil-rotation type via leak valve 17 and cold trap 18. The inner pressure of the vessel 9 is maintained negative, preferably lower than 30 mmHg. Thig pres~ure is checked with a manometer 20.
The bottom of the vessel 9 is connected with the impregnating deviceg 2 via a delivery pump 21.
By being ~etted out o~ the nozzle, the liquid resin oompo~ition falls down into the upper evacuated ~pace of the veesel 9 with a falling head of about 50 to 100 cm. By this means, the liquid re~in composition is reduced-pressure treated fully and effectively, and constantly stored in the vessel 9 so as to be steadily supplied to the impregnating devices 2.
When it is necessary to adjust the back pressure in accordance with the capacity o~ the pump 21, it may be effective to place the vessel 9 at a higher level than the pump 21, or to preserve some of the liquid resin composition in a spare vessel (not shown in the ~igure).
As impregnating device 2, either a curtain ~low 5~ ~

applicator or a dipping bath can be used. But certain types o~ dipping bath are not preferable because of possible stagnation of liquid resin composition therein, whereas the curtain flow applicator is preferable. Then the liquid resin composition is directly applied onto the sheet substrate, and the overflowing portion is recovered, returned to the storing vessel 8 and reused.
A certain number of impregnated substrates 6 are continuously transferred in parallel to the laminating device 3, which is, for example, composed of a pair of rolls, joined together surface-to-surrace, and simultaneously sandwiched between two film coverings or cladding metal foils~10, then guided into the heat curing oven 4 under ~mpressurized condi-tion~, and formed into a laminate. After curing, the laminate is cut into a predetermined length.
The above reduced-pre~sure treatment o~ the liquid resin component is applicable regardless of the types of sub~trates conventionally used in the art, including paper mainly compo~ed of cellulosic fiber, glass cloth, fiber glass nonwoven fabric, asbe~tos cloth, synthetic fabric, and synthetic nonwoven fabric, but it is especially suitable when using cellulosic paper and glass cloth.
It is ~urprising that excellent productivity is ~ecured by introducing the said reduced-pressure treatment.
Although the reason for this fact has not been made fully clear, it may be eupposed that the reduced-pressure treatment ~ 43 -~ ~ ~35~7 makes the amount o~ air dissolved in the liquid resin compo-sition decrease, and there~ore gives the liquid resin compo-sition a reserve capacity to dissolve air, which enable~ the air con~ined in the substrate to dissolve at an effective rate, resulting in the disappearance o~ bubbles within the whole laminate. The reduced-pre~sure treatment i9 probably effective in excluding air bubbles that were entrapped when such agents as catalysts and modifiers were mixed into the composition, but this effect is not a feature of the present invention, because reduction of pressure is a well-known means for deaeration of still V~SCOU9 resin liquids.
The reduced-pressure treatment conventionally emplo~ed for the purpose of deaeration may be considered as different from that in conducting the present invention. The reason may be apparent from the following instance.
If a liquid unsaturated polyester resin composition that has been preliminarily well deaerated by standing still and has a viscosity o~ about 4 poise, is used for impregna-tion oP paper substrate, the impregnation speed may not be increased by this deaeration. On the other hand, if the liquid resin composition that has been reduced-pressure treated according to the present invention and then inten-tionally aerated by agitation, is used for impregnation, then the time for disappearance of bubbles within the impregnated paper substrate is signlficantly shortened.
In effect, by introducing the reduced-pressure Il ~ 7~5S7 treatment according to the present invention, the time for disappearance of bubbles within the impregnated paper sub-strate is shortened usually to less than 7 minutes, at most, to 2 to 5 minutes.
This treatment i~ similarly e~ective when a glass cloth is impregnated with a liquid epoxy resin composition.
.
The degassing treatment o~ the present inven-tion is desirably conducted with means for increasing the total surface area o~ liquid resin composition to be treated.
For example, spraying or jetting the liquid resin composition into a pressure-reduced vessel is much pre~erable to standing it still therein.
The above treatment is invariably effective, even when the liquid resin composition to be treated contains bubbles or entraps bubbles in the course o~ delivery. It is also effective in decreasing the amount of dissolved oxygen therein that adversely a~fects the radical curlng reaction o~
un~aturated polyester resin.
Commercially available liquid unsaturated polyester resins usually contain water in an amount of about 0.03 to 0.1% by weight. The above degassing treatment is also desirable for decreasing this water content to le~g than 0.04%
by weight, pre~erably to less than 0.02% by weight, so as to minimize the generation of bubbles of moisture and the inhibition of curing reaction by moisture.
As described previously, several impregnated sheet I ~ 7~5~'~

substrates are converged and joined together into a unitary member or stack with the aid of a Pair of rolls or a roller coupled with a blade.
In accordance with ~he present invention, the ~inal resin content in ~e impregnated sub~trate, is adiusbed at 10 to 90%, pre~erably from 20 to 80~, more preferably ~rom 30 to 70~ based upon the total weight of impregnated substrate.
This may be effected by one of the following methods or a combination thereof.
I. Each ~ubstrate is fed prior to the lamination with each other, with an excess of liquid resin at the impregnating station 2, and then pas~ed through a pair of scraping blades defining ad~ustable clearance 33 for scraping off the excessive resin, as shown in FIG. 1.
lg ~, After the impregnation each ~ubstrate i~ passed between a pair of rollers 34 to squeeze out excessive llquid re~in a~ in ~IG, 4, and then laminated with each other at the laminating station j.
I~. The nip of clearance o~ laminating rollers 3 i9 made ad~u~table in order that the lamination of plural sub~trates and the removal o~ exce~sive liquid resin may be performed ~imultaneously at the lamination station 3, IV, When the combined substrateg are sandwiched between a pair of covering ~heet~ 10 by a separate pair of rollers 23 as ehown in FIG, 4, the nip or clearance of the rollers 23 ie made ad~ustable to remove exces~ive liquid resin.

' ~ ~ 7~5~'~

V. Before a covering sheets 10 is aPplied onto the both sides of the laminate, the interface between the substrate and the covering sheet is replenished with an additional amount of ~iquid resin from a resin-replenishing means 35 as shown in FIG. 4.
As stated before, the final resin content is adjusted at lO to 90~, preferably from 20 to 8 ~, more preferably from 30 to 70~ based upon the total weight of the impregnated sub~trate. The resin content in a given laminate is an important factor which controlg various propertics of the finished laminate. Since substantially no pressure is aPplied on the laminate in the thickness direction during the curing ~tep thereo~, it is impossible ~or the present invention to ¢ontrDl the re~in content by pressing out excessive resin during the curing step. Accordingly, the ~inal re~in content in the impreenated substrated is ad~u~ted within the ~tated range at least when the laminate o~ impregnated substrates is given a pair of covering sheets on its both ~ides and made ready to cure as such.
The term "resin content" used herein refers to a ratio x represented by the equation: Percent x = A/(A+B) x 100, wherein A is the weight of resin and ~ is the weight o~
~ubstrate.
Insuffioient reYin contentY result in poor quality of the product, while excessive resin contents often cause various disadvantages such as spilling of the liquid resin out from _ ~7~5~1 the edges of the uncured laminate and the like.
The stack of impregnated sheet substrates is, simultaneouslv or subsequently to joining, sandwiched between two ~ilm or sheet coverings. Then it is desirable that the film or sheet coverings extend a little over both edges o~
the stack, so that the possible exudate o~ liquid resin composition onthe edges may be suPported therebetween.
According to the present invention, said film or sheet coverings can be widely selected from various materials, conforming to the requirements of the product, because the curing process is per~ormed substantially without application of continuous compression. For example, various kinds o~
release paper o~ cellophane of 10 to 200~m thickness, syn-thetic film~ of Teflon, polyester and the like, and metal foil~ o~ aluminum, copper, stainless steel, steel, phosphorus bronze and the like, are all applicable as covering.
As illustrated in FIG. 4, the ooverings 10 may be stripped from the stack or laminate a~ter curing, and recovered by rewinding on the recovery rollers 22. The reuse o~ recovered coverings may be desirable from the viewpoint of production cost. For this purpose, it is desirable that the covering can be ea~ily separated ~rom the laminate, and therefore an appropriate combination between the oovering and the laminate is selected, and, if neces~ary, a release agent 2~ is applied between.
In the present invention, a oovering in the form o~

i J~ ~5~7 an endless belt may be reasonably effective, especially in case of using about 1 mm-thick sheet of stainless steel, ~AT ~ phosphorus bronze and Teflon. The release agent, if neces-sary, may preliminarily be aPplied onto the whole area or both edge zones o~ the one surface of the covering due to come in contact with the laminate. The application of release agent on the whole area could occasionally cause the agent to migrate to the laminate, resulting in impairment of print-ability of various types of paste and resiqt. In such cases, it is desirable that the application of the release agent is confined to the edge zones, and, after curing and stripping the covering, the edge zones of the laminate are removed in order to prevent the product from the above impairment. As release agent, agents of silicone type such as Daifree MS
743 (Daikin Koygo) are preferable.
Among the properties of laminate product, surface smoothness is espeoially important ~or printing with resi~t-paste or resist. The trangparency is significant as regards the evaluation of the printed pattern and easy observation of the printed circuit pattern from the reverse side.
As described previously, several impregnated ~heet substrates are ~oined together into a stack with the aid of a pair of rolls or a roller and blade assembly. This aid is reasonable for controlling the oontent of the liquid resin Z5 composition by excluding the excess, and ~or excluding air bubbles which may be entrapped in the stack on ~oining.

,,,, ~ ~ade, rn ~r k 5 ~ ~

That is the stack is sandwiched between two ~ilm or sheet coverings simultaneously with joining (See FIG. 1, at 3), or subsequent to joining (See FIG. 4, at 23) with the aid of a pair of rollers, which exert a certain compression.
The surfaces of the stack prior to being ~andwiched are usually uneven macroscopically and microscopically, and this unevennes~ may be liable to be roughly reproduced on the covering surfaces on sandwiching, especially when flexible coverings are employed, and may be, more or less, imparted to the surfaces of the cured laminate product, according to the non-pressure curing process of the pre~ent invention.
Experiments have shown that the smooth surface char-acteri~tics required in practice can be ~ecured in the product, when the rigidty of the covering uged is more than 3 x 10-3 kg-cm, preferably more than 5 x 10 1 ~g-cm (where the rigidity is expressed as E-d3kg-cm, with E for modulus ln kg/cm2, and d for thickness in cm).
For example, a cotton linter paper and kraft paper of 200 to 300~ thick and having a basis weight of 150 g/m2 may be u~ed as substrates. Such a paper 6 usually has microscop-ically uneven surface~ a~ illustrated in FIG. 2, and conse-quently produces a covered laminate with roughly reproduced uneven surfaces, if a coverin~ of less than 3 x lO 3 kg-cm rigidity, for example, 35 ~ thick polyester film (flexural rigidity: 28.1 x 103 kg/cm2, rigidity: 1.20 x 10-3 kg-cm) is applied. Then, the unevenness on the covered laminate is 5~ ~

much reduced as illustrated in ~IG. 3, i~ a covering o~ more than 3 x 10-3 kg cm rigiditr, for example, 100~ thick poly-ester ~ilm (rigidity: 2.81 x 10-2 kg cm) is applied.
Furthermore, the application of a covering o~ more than 5 x 10 1 kg cm rigidity, for example, 100~ thick aluminum foil (flexural modulus: o.67 x 1o6 kg/cm2, rigidity: 6.7 x 10 1 kg.cm), or 100~ thick stainless steel ~oil (flexural rigidity:
1.86 x 106 kg/cm3, rigidity: 1.86 kg cm) is much more pref-erable for carrying out the present invention.
The above ~ilm or sheet covering may be either o~
single ply type or of composite ply type.
As a general rule, the rigidity of a material decreases with a rise in temperature. Because the covering, however, i~ applied to the ~ur~ace o~ the stack usually at room temp-erature in the present invention, its room-temperature rigidity only need be taken into consideration. Nevertheless, the application o~ ~uch a plastics ~ilm material whose rigidity ~igni~lcantly decrea~es at the curing temperature o~ the ~tack, i~ not de~irable. A film which tends to adhere to cured unsaturated polyester resin or epoxy resin is also not suit~
able. From thi~ point of view, ~ilms made of polyester, polypropylene, Te~lon and polyamide-imide, and foilg of aluminum, press-rolled copper, and stainless steel are all suitable.
Thus, according to the present invention, the applied covering can be removed from the cured laminate with ease, _ 51 -~7~

without the aid of a release agent or release paper. If any intervening material is necessary for removing the covering, it is preferably employed lnthe form of a composite with the covering.
It is desirable that the covering be so lon~ that it may be continuously,unwound from a supplr roller 11, applied to and removed from the laminate, and rewound on a recovery roller 22. Otherwise, the use of a covering in the form of an endless belt may of course be effective, and in this case, the covering is preferably flexible to some extent, with a rigidity of 3 x 10 3 kg-cm to 3 x 101 kg-cm.
A~ is readily understood from FIG. 3, the geometric features of a covered laminate product largely depend upon the roughne~s or geometrio features of the surface of the coverlng used. Thi~ feature i9 one of the most important properties for laminate products for electrical use. For example, when a composite carbon resistor is made by coating a resist paste on an insulation board, the rougher the surPace ofthe lnsulation board, the more pinholes and unusual snags are caused, resulting in noise and short useful li~e of the final product. The desirable surface smoothness o~ a laminate board may be regarded as RmaX (maximum surface roughness) less than about 15~, preferably less than about 9~.
On the other hand, much lower RmaX could occasionally cause a reduction in bonding strength between the resist paste and the insulation board, resulting in pee,ling of the resist .
, - 52 -.

~:~735~'1 layer. This bonding strength mav largely depend upon the polarities o~ the resist paste and insulation board, their mutual compatibility and the surface roughness o~ the insula-tion board. More than about 0.4~ RmaX for the insulation board gives su~ficieni bonding strength and non-repellant property.
The surface smoothness was evaluated according to JIS B 0601 "Surface Roughness". It was determined with a stylus tracing tester under the conditions of 2.5~ radius of curvature of stylus tip and O.lg weight o~ the stylus tip.
The use of a film or sheet covering of sur~ace rough-ness o~ 0.4~ to about 9~ secures such a laminate product as described above.
The above descriptions have been given as ~or a paper substrate, whereas they aPply to other substrates as well.
~or example, the textural sur~ace unevenness o~ a waven glass cloth is not apparently ob~ectionable in carrying o~t the present invention.
The above desoription have been given also as ~or produoing electrical laminates with covering on both sides.
However, the production of electrical laminates with cladding metal foil on one or both sides also can be achieved in similar ways by means o~ the present invention.
As a cladding metal ~oil, the electrolytic copper ~oil that is on the market for printed circuit board use, is suitable for use because o~ its corrosion resistance, etching ~ 53 -.

~ é735~7 quality, and adhe~ive quality.
The description of the production of electrical laminates ~or printed circuit board use, clad on one or both sides with copper foil, electrol~tic iron ~oil or aluminum ~oil, i9 given a~ follows:
According to experiments by the inventors, the metal clad.laminates that wére prepared using commercial electrolytic copper ~oil of 1 oz/ft and paPer substrates, haveloccasionally been in~erior in surface amoothness to conventional ones prepared by press-molding, but have shown no adverse ef~ects as regards their qualities o~ screen printing, etching and the like.
For examPle, practical and use~ul clad laminatej may be produced ~rom unsaturated polyester resin-impregnated S ~ubstrate ~taok.
In convéntional batchwise pres~-molding production proce~e~, for example, phenolic resin-impregnated copper-cl~d paper laminate~ have been made with the u~e o~ electrolytic copper foil that was initially covered with a B-stage adhe~ive of a phenol-modi~ied butyl rubber type. In the contlnuous proces~ according to the present invention, however, it has been ~hown to be much more reasonable from the viewpoint of productivity and product quality if an appropriate adhesive is oontinuously supplied between the foil and the ~oinèd ~ub~trate ~tack. For e~ample, a~ illu~trated in FIG. 6, an adhesive i9 supplied from a reservoir 27 through a delivery ' ' ' ~

.

~ ~ s'~5~7 device 28 onto the appropriate side of the foil 10, immediately before covering the stack. Further, it is preferable that the foil coated with the adhesive be subsequently moderately heat-treated b~ means of a heat oven 26.
In order to gecure effectively the bond between the metal foil and the resin-impregnated substrate, it is desir-able to employ an adhesive that does not contain any component such as solvent due to be removed, and generate any useless and adverse hyproduct during the curing process but is a liquid or a semiliquid with a preferred viscosit~ of less than 5,000 poise at room temperature. Such an adhe~ive may be selected - from various type~ of unsaturated polyester resin, epoxy resin, polyi~ocyanate re~in and various modifications thereof.
The~e adheslves make it possible to obtain, continu-ously, metal foll-clad laminates exoellent in bonding strength, solder heat resl~tance, and electrical insulation properties.
Alternatively, the application o~ adhe~ive could be ¢arried out by coating the above ~oined ~ubstrate ~tack prior to corering with the foil, or by in~ecting simultaneous to covering. The~e means, however, have been found occasionally to be accompanied by ~uch problems as entrapping of air bubbles, occurrence of unusual curing reaction, and separation of components, re~ulting in a decrea~e in production yield or product quality. Againet the~e problem~ it ha~ been ghown to be more e~fective i~, as illugtrated in FIG. 6, the ~oil 10 is applied with the adhesive with ~e use of a coating device a ~ 7~5~7 25, and then heat treated through a heat oven 26. As the coating device 25, a conventional roll coater, blade coater, wire-bar coater or comma coater can be used.
The first purpose of the heat treatment of adhesive-coated foil is to dry the solvent, if the solvent is used.
However in the present invention the dried adhesive-coated foil need not be tack-free as the conventional process used to require. The second purpose is to precure the thermoset7 ting-type adhesive to a moderate extent, leaving a certain degree of tackines~. The third purpose is to remove air bubbles that may on occasion have been entrapped in the coat-ing, especially when such an adhesive as a two-component epoxy type adhesive of a relatively high viscosity has been employed.
For in~tance, a deqcription of the production of unsaturated polye~ter re~in-impregnated and copper-clad paper laminate with the u~e of epoxy resin adhesive is given a~
folaow~:
As epoxy resin adhe~ive, mixed types of bisphenol A-type epoxy re~in and polyamide resin are preferable.
The paPer 6 i~ unwound from the storage reel 1, and contacted with the liquid un~aturated polyester resin compo-~ition in the impregnating bath 2. Then, ~everal (for example, seven) impregnated papers are ~oined together into a stack by the laminatin~ device 3, and simultaneou~ly 3 or sub~equently 23 covered, on one or both sides, with the eleotrolytic copper ~oil 10, which has been coated with epoxy resin adhe~ive.

735~7 The foil coated with epoxy resin adhe~ive is heat-treated preferably at 100 to 150~ for 2 to 7 minutes, but is allowed to cool to room temperature during the laminating process. Such a heat-treatment is desirable so as to remain a little tack~. Drying to a tack-free ~tate could inhibit adhesion to the stack, while excessive tackiness could allow the adhesive to permeate into the liquid resin component of the stack to a large extent, occasionall~ resulting in a decrease in the bonding strength. The thickness o~ the coated adhesive may be 10 to 150~m, preferably 20 to lOO~m.
In compliance with the product requirement, one side o~ the stack is covered with a plastics film such as a cel-lophane and polyester film, instead o~ copper foil.
Then, the covered stack is transferred to the heating device 4, and cured therein. The ouring conditions depend upon trans~erring ~peed, type of oatalyst, etc., but in most cases range 100-150~ ~or 5-60 minutes.
By the above mean~, the copper-clad laminate, with a ~oll peel ~trength of 1.6 to 2.0 kg/cm con~orming to XPC to XXXPC grades (NEMA standards), can be produced with excellent productivity. Thu~, continuous production o~ metal-clad laminates is made possible by the present invention.
As liquid thermosetting resin composition suitable ~r impregnation o~ the ~ubstrate, those which are in the liquid state at room temperature are desirable. However, those which are in the solid state at room temperature may be u~able, i~

3 ~ I;'a557 they readily turn liquid on heating.
In the present in~ention, adhesive e~ects are further improved for example in the following way.
When an unsaturated polyester resin ~or impregnation o~ the substrate and an epoxy resin as adhesive are used, the latter is desirably o~ an amine-curing type, from the view-point o~ concordance with curing rates of both resins. In addition, the selection o~ the curing catalyst to be added to the un~aturated polyester resin is important. The use o~
single or combined peroxide catalysts selected from peroxy-ketals, dialkyl peroxides, and peroxyesters is remarkably superior to the use o~ those selected from peroxydicarbonates, ketone peroxides, hydroperoxides, and diac~lPeroxides in order to ~ecure qu~ficient solder heat-resistance, electrical in~ultaion propertie~, and bonding strength. The amount added i~ desirably 0.5 to 2.0 parts per 100 parts of the resin.
The reason ~or the above catalyst selectivity has not been fully understood, but 1~ suppo~ed to relate either to permea-tion of the oatalyst into the adhesi~e layer of solubilization between the adhesive and the resin liquid, which may possibly take place throughout the contact and curing processes.
In fact, the u~e of such peroxides as peroxydicarbonates, ketone peroxides, hydroperoxides and diacylperoxides occasion-ally inducey unueual curing of the epoxy re~in, resulting in inadequate product quality.
Accordingly, as peroxide catalyst suitable for u~e 3 ~ 7~5~7 the present invention, are designated the ~ollowing compounds:
peroxyketals such as l,l-bis(t-butylperoxy)-3,3,5-trimethyl-cyclohexane, l,l-bis(t-butylperoxy)cyclohexane, and n-butyl

4,4-bis(t-butylperoxy)valerate; dialkylperoxides such as di-t-butylperoxide, and 2,5-dimethyl-2,5-di(t-butylperoxy)hexine-3; and peroxyesters such as t-butyl peroxyacetate, t-butyl peroxy-2-ethylhexanoate, t-butyl peroxylaurate, and t-butyl peroxybenzoate.
As unsaturated polyester resin, well-known types synthesized from unsaturated dibasic acids, saturated dibasic acids and glycols, are use~ul, but either bisphenol A-type polyester reslns or vinylester-type resins are aPplicable as well. As cross-linking monomer, styrene is common and also suitable for conducting the present invention.
As epoxy resin for adhesive, those of bisphenol A-type are suitable. As amine-type curing agent, any o~ the well-known aliphatic or aromatio amines is applioable, but certain polyamide resins, amino-terminated polybutadienenitrile rubbers and their mixtures may be use~ul as well.
Carrying out the above process with care enables the production o~ metal foil-clad laminates of excellent qualities with high efficiency. However, with the aim of further minimizing the de~ectY o~ peeling of olad foil caused by ~luctuation o~ operating conditiong, the additional use of certain compounds having both an unsaturated double bond and an epoxy group is especially effective. Certainly, if suoh , - 59 -

5 ~, compounds a~ glycidyl methacrylate, glycidy~ allylether, and partially epoxidized soybean oil are applied intermediately between the adhesive and stack layers when they contact with each other, the a~finity between both layer~ improveY much more, and, as a result, the defect9 o~ ~oil peeling can be effectively avoided.
However, if commercially avaiable glass cloth that has already been surface-treated for ePOxy resin impregnation is impregnated with epoxy resin liquid, and clad with com-mercially available electrolytic copper foil for printed circuit use, the above intermediate application may be unnecessary because of sufficient bonding between the epoxy resin and copper foil.
As another means, the application of a surface treat-ing agent, e~pecially a silane coupllng agent upon the surface o~ the copper ~oil brings out ~urther improved bonding e~ects, when unsaturated polyester resin- or epoxy resin-impregnated ~ub~trates are uged. This application iB made prior to the po~sible application of the above adhesi~e upon the ~urface of the copper foil.
A~ a silane coupling agent, any commercial grade for bonding between inorganic and organic materials may be used, althrough A-llO0 and A-187 (Union Carbide) are especially suitable. It is practical to apply continuously and gparingly a 0.1 to 5.0% aLcoholic or aqueous solution of the silane coupling agent upon the surface of the copper ~oil, and to dry 5 ~

it continuously.
In the present invention, it is desirable that the metal foil be pasged through a hot-air oven at 100~200~
for a length of time to remove possible moisture on its surface, even when no surface treating agent is used.
Also the substrate is passed through a hot-air or steam-heating cylinder at 100 200~ for a sufficient length of time immediately before impregnation, in order to remove moi~ture retained therein and to improve its adhesion.
For the purpose of minimizing warping and twisting of the product it has been found that the following means is ef~ective.
In general, cubic contraction of thermosetting resins when cured results in re~idual strain, and causes warping and twiating of the product. When the curing of the re~in i9 not fully ¢ompleted, expo4ure of the produot to heating clrcum-~tan¢e~ produces further warplng and twisting. The incomplete curing also si&~lficantly lower~ the thermal re~i~tance, chemical re~i~tance and mechanical propertie~ of the product.
Experiments have ~hown that making curing complete by a single continuou~ process for laminate production nece~sitate~ very large curing equipment and/or a con~i~erable ~lowdown of line speed.
In the present invention, the stack i9 cut at a partially cured ~tage, trimmed off according to the predeter-mined size, heaped up side by ~ide in multitude, and then i:~7~557 treated in another heating chamber to complete curing.
Thus, in this continuous process, it is rather desirable that curing of the laminate stack proceed to a certain extent, where the laminate becomes hard enough to be cut by a guillo-tine cutter and to be separated from the covering with ea~e.
This means makes it possible to yield laminate products with an economical scale o~ equipment and reasonable line speed of production.
For example, laminates composed of unsaturated poly-ester resin usually take about 10 hour~ at 100~ ~or complete curing, while they take only about 15 minutes at the same temperature to reach su~icient hardness to be cut.
According to a preferred embodiment o~ the invention, the laminate is partially cured in a first curing step, cut into suitable ~ize and then sub~ected to a continued curing etep to complete the curing. The ~irst curing step may be continued until the resln becomes e~sentially tack-free 90 that the oovering sheet may be stripped without sticking.
Al~o the laminate mu~t be sel~-~upporting after the ~ir~t curing 3tep. Experlments have ~hown that the stage of curing reaction at which said ~irst curing ~tep is discontinued may be determined in terms o~ gel content in the ~emicured re~in compo~ition. A gel content o~ at lea~t 80%, pre~erably Prom 90 to 98% based on the weight of re~in pre~ent in the laminate i~ suitable.
In general, the widthwise re~idual strair. resulting 5 ~

from curing of the resin component is relatively easy to remove with resulting warp, while the lengthwise one is ver~
di~ficult to remove because of the long continuation of the laminate. This difference makes residual qtrain anisotroPic, and causes increased warping and twis~ing of product on exposure to elevated temperature.
In the present invention, the continued curing in the second curing step after cutting enables warp and residual strain of the product to decrease to a practically tolerable extent, especially in case of one ~ide clad laminates.
The amount of warp of metal foii-clad laminate largely depends upon the type and composition of the impregnating resin used. Generally it i~ smaller when using epoxy resin, and larger when using unsaturated polyester resin or diallyl phthalate resin, For example, a clad laminate of 1.6mm thick-ne~s, which i~ made of unsaturated polyester re~in-impregnated paper and clad with oopper foil of 35~m thickness, has warp ranging ~rom 0 5 to 30~ according to the designation o~ JIS
C-6481.
2~ In the present invention, the above cut laminates are finally and completely cured by exposing to temperatures preferably higher than t~ose o~ the preceding continuous heat-curing oven, or temperatures which would be encountered by the finished laminate upon use, and then the remaining warp is mechanically modi~ied. Thus the finally obtained laminates are practically flat, and of significantly less warp even 1 ~ 735~7 a~ter exposing to such hot environment.
FIG. 5 contains the illustration of devices used in the above cutting and postcuring processes. A continuous length o~ laminate partially cured inthe heat-curing oven 4 is cut b~ the cutting device 5. Then cut laminates are trans~erred into a second heat-curing oven 29, subjected to heat treatment at a higher temperature such as 150~ ~or such a short time as 15 minutes therein, and ~inally geometricallv modi~ied in two directions which are perpendicular to each other through a warp-modifying device composed of two sets o~ three rollers 31 and a turntable 32. It is pre~erable that the cut laminates are not restrained by a supporting member in the second curing chamber 29, in other words, the cut laminates are not bound or clamped in the chamber allow-ing their ~ree movement.
Furthermore, in order to attain higher productivity, another mean~ ha~ been ~ound effective wherein the coverings 10 are not only applied onthe top and bottom sides o~ one Joined ~taok, but also in~erted among several staoks, which are heat-cured in the oven 4 as one body, and then separated from eaoh other. This means needs little change in drying time, impregnating time and curing time, and makes overall productivity improve dramatically. That is to say, the in~erted coverings take on the role o~ ~eparator, and a thick pile formed by multiple and alternate placing o~ the covering~
and ~oined stacks can be processed as one charge in the same way as described previously.
By this means, both a one-side clad laminate and a two-side clad laminate may be also produced simultaneously from the alternate combination o~ two stacks and three coverings.
Further, as an example, two unsaturated polyester resin-impregnated paper substrates have inserted between them a predried cellophane film, covered on both sides by copper foils, and, a~ter curing, are separated into two one-~ide copper-clad laminates, each being 1.6mm in thickness with a 35~m thick copper cladding. This secures the double production rate o~ the same product.
Furthermore, by this means, two or more dif~erent thick laminates can be concurrently produced in one batch, by the intermediate use of cellophane coverings as separator.
This may contribute to a 9aving o~ time losses in product line shi~ting.
However, when a thick pile of many stacks is sub~ected to the curing process, care should be taken on various conditions such as heating e~ficiency, generation of heat by the curing reaction and its removal, and the like. For use in such a case, a heating device which may be controlled section-wise and stepwise according to prevailing conditions may be necessary.
When using unsaturated polyester resin, it is desir-able that the amountY o~ catalyst and curing agent incorporated 5~'~

in the inner layers of the thick pile be reduced in comparison with those in the outer layers, in consideration of the effects of the inner reaction heat. When the sheet i9 too thick to be cut by a guillotine cutter, the alternative uge of a movable slicer may be effective.
Examples carried out under different conditions in accordance with the present invention are described as follows.
The properties of the product~ obtained are summarized in Table 7.
The gel content of semi-cured resin presenb in the semi-cured laminate may be determined by extracting finely divided particles of the laminate with an affinity solvent and weighing the residue. The gel content may be calculated from the decrease in weight based on the total weight of resin before extraction.

1 ~ 7~5~7 Apparatus as illustrated in FIG. 4 was used.
An unsaturated polyester resin was synthesized in the usual manner from maleic acid, isophthalic acid, and ethylene glycol in a molar ratio of 82 : 18 : 100, and mixed with styrene in an amount of 37~ by weight. The viscosity of the resulting liquid resin was about 5 poise at 25C. Liquid unsaturated polyester resin composition was prepared by adding one part of cumene hydroperoxide and 0.2 parts of 6% cobalt naphthenate solution to 100 parts of the above resin liquid.
i The properties of cured samples therefrom are shown in Table 2.
Table 2 ... . ... . ... ., Flexural modulus 283 Xg/mm2 Rockwell hardne~s 106 Glas8 transition temperature about 90C
A commercially available kraft paper with the pro-perties shown in Table 3 was used as sheet substrate.
Table 3 Grade MKP-150, Tornoegawa Basis weight about 150 g/m2 Density, air-dried about 0.53 g/cm3 Mean thickness about 280 ~m , 5~7 The polyester film covering 10 were stripped from both sides of the unitary cured member by the use of a pair of rolls 24, and taken up onto rewinding rollers 22.
Main other preparing conditions were as shown in ... .
Table 4.
Table 4 Items Con~ditio.ns Number of paper substrates (1) 2 plies Width of paper substrates 1040 ~n Drying oven (12) no use Impregnating devices ~2) dipping type Impregnating duration for 20 minutes transfering impregnated board from impregnating device (2) to laminating device ~3) Curing duration in heat. 90 minutes curing oven (4) Curing temperature 80C
Film covering~ (10) polyester Thickness 35 ~m Width 1060 mm Rigidity 1.54 xlO 3 kg.cm Flexural modulus 2B100 Kg/cm Surface roughness (RmaX) less than 4 ~
Cut length 1020 mm i The amount of the liquid composition used for the impregnating the paper substrate was adjusted at about 55 based on the total weight of the impregnated paper by 5 ~ 7 adjusting the nip or clearance of rollers which laminate polyester covering sheets lO on the both sides at 23.
Thus, laminates 7 were continuously obtained with 0.50 mm thickness and 1020 x 1020 mm peripheral size.
S The laminate samples showed no changes either in an alkali-resistance test in a 5~ aqueous solution of sodium hydroxide at 40C for 30 minutes, or in a solvent-resistance test in boiling toluene for 2 minutes. All samples from other examples described below did so as regards those alkali- and solvent-resisting tests.

EXAMPLE 1 was repeated except that the paper substrates were continuously passed through a hot-air drying oven 12 at 100C for a duration of 10 minutes.

EXAMPLE 2 was repeated except that the number of paper substrates was increased to 5 plies, and thus lami-nates 7 of 1.5 mm thickness were obtained. The final resin content in the impregnated substrate was adjusted ; at about 60%.

EXAMPLE 3 wa 9 repeated except that the unsaturated polyester resin liquid was replaced for a commercially - 6g -.

~ . :
:

:
. .

~ ~ 7~)5~7 available product RIGOLAC 150 HRN, sold by Showa Kobunshi.
Co. The cured sample had a glass transition temperature of 120C.

S EXAMPLE S
EX~PLE 3 was repeated except that the unsaturated polyester resin was synthesized from maleic acid, iso-phthalic acid, and diethylene glycol in a molar ratio of 32 : 68 : 100, and added with styrene in an amount of 37 by weight. The viscosity of this resin liquid was 4.5 poise at room temperature. The cured sample had a glass transition temperature of about 55C.

EXAMPLES 6, 7 and 8 In these examples, a one-side impregnating tech-nique, in which liquid resin was allowed to flow onto the upper side o the paper substrate by a curtain flow device 2, was used in place of the dipping techni~ue of EXAMPLES
3, 4 and 5. As a result, the number of microscopic bubbles remaining within the product was dramatically reduced, almost to zero, compared with the case of EXAM-PLES 1 to 5. In addition, the solder dip resistance of the product was much improved. Other properties of the product were similar to those in EXAMPLES 3, 4 and 5, respectively.

~ ~rad~ ~a ~ k - 7 I ~ 7~5~7 EXAMPLES 9, 10 and 11 EXAMPLES 6, 7 and 8 were repeated except that the `
resin liquid was initially exposed to a reduced pressure, and that the impregnating time was reduced to 4 minutes.
Using a degassing apparatus as illustrated in FIG.
1, the inner pressure of the airtight cylindrical vessel 9 of 100 cm height and 30 cm inner diameter was maintained ; constant at 20 mmHg, while continuously jetting the resin liquid from the storing vessel 8 to the top of the vessel 9 at a flow rate of 10 liters per minutes. The resin liquid treated under the reduced pressure was continuously pumped out of the bottom of the vessel 9 and up to the impregnating devices 2.
The products obtained in EXAMPLES 9, 10 and 11 had far fewer or no bubbles, and exhibited similar properties, re~pectively, to those in EXAMPLES 3, 4 and 5, regardless of the great reduction in impregnating time.

EXAMPLES 12, 13 and 14 EXAMPLES 3, 4 and 5 were repeated except that t-butylperoxy-2-ethylhexanoate, an aliphatic peroxyester, was used in place of cumene hydroperoxide as curing cata-t.
The laminates obtained were improved in that there 1 25 was a significant reduction in the odor which was emitted when they were heated at 180C for 30 minutes, comparing '~
.

1 ~ 7~7 with those in EXAMPLES 3, 4 and 5 respectively. Further the laminates were cut into a certain length, and treated in a hot-air oven at 100C for 10 hours to be fully cured.
Then they exhibited high quality as regards solder dip resistance, dimensional stability, electrical insulation properties and the like.

A liquid unsaturated polyester resin composition was prepared by mixing 100 parts of the unsaturated poly-ester resin liquid used in EXAMPLE 5, with one part of t-butylperoxy-2-ethylhexanoate used in EX~PLE 14 and 0,2 parts of 6% cobalt naphthenate solution, then subjected to the degassing treatment as described in EXAMPLES 9, 10 lS and 11, and finally used for the impregnation. The impreg-nation took only 5 minutes. The curing temperature was ! 100C, and the curing time was shortened to 22.5 minutes by raising the overall transferring speed by three times.
Other conditions were the same as in EXAMPLE 1.
Cut lamina~e samples were found to be of lower quality due to the insufficient curing treatment; there-fore they were further treated in a hot-air oven at 100C
for 10 hours and at 160C for 10 minutes to be fully cured. The finally obtained laminate~ were exaellent especially in solder dip resistance and thermal dimensional stability. Thus, the overall production rate of the .. ~ , ...... . .

'. ~

-1 ~ 7~5~7 apparatus used in EXAMPLE 1 was increased by about three times simply with the additional provision o~ a second hot-air oven 29 as seen in FIG. 5.

ExAMæLE 16 The paper substrate described in EXAMPLE 1 was preliminarily subjected to pre-impregnating treatment as follows.
Unwound paper was continuously dipped into a bath 14 of 8% methanolic solution of N-methylolacrylamide for 5 minutes, drawn out and air-dried for 30 minutes, and then dried at 100C for 20 minutes in a hot-air dryer 12.
The resulting pre-impregnated paper had a N-methylolacryl-amide pickup of 11.2~ by weight.
Five continuous paper substrates were separately pre-impregnated as above, transferred and subsequently proce~sed in the same way as in EXAMPLE 15. The finished laminates were 1.5 mm thick, and exhibited significantly improved electrical insulation properties and solder dip resistance upon hurllidifying, compared with those in EXAMPLE 15.

As to polyester film-covered laminates obt'ained in EXAMPLE 16, a certain gentle undulation was observed on the surface after stripping the polyester film coverings, I ~ 7`~7 following the unevenness of the covered substrate layer.
In order to prevent this imperfection, EXAMPLE 17 was carried out using long stainless steel foil as covering in place of the polyester film used in EXAMPLE 16. The stain-less steel foil was of SUS 304 grade and so-called "sA
surface finish", 100 ~m in mean thickness, 2.5 ~m in RmaX
of surface roughness, and 1.86 Kg.cm in rigidity.
By this means were obtained laminates that were free of the above undulation, superior in surface smooth-ness or appearance, and satisfactory in printability andinX transition behavior of various types of resists or paste .

The paper substrate described in EXAMPLE l was pre-impregnated in the following way.
Fifty parts of aqueous solution containing 6 parts of methylolmelamine (Nikaresin S-305, Nippon Carbide Industry) was poured into 50 parts of methanolic solution containing 1.5 parts of oleic acid monoglyceride (Rikemal OL-100, Riken Vitamin Oil) under vigorous agitation to make up a suspen3ion.
A continuous length o said paper was continuously dipped into this suspension, drawn up, heated at 120C for 20 minutes, and then processed in the same way as in EXAMPLE 17.

~ trRGle n?Ctrk I ~ 7~5~'~

Finally obtained laminates were 1.5 mm thick, and showed properties similar to those in EXAMPLE 17 as seen in Table 7.

EXAMPLE l9 EXAMPLE 18 was repeated except that 1 oz/ft2 electrolytic copper foil (T-7, Fukuda Metal Foil & Powder) was used as covering in place of stainless steel foil on only one side, and left unstripped after curing, while the stainless steel foil on the other side was stripped.
The properties of the finished one-side copper clad laminate are given in Table 7.

The product of EXAMPLE 19 had the drawback of large warp. EXAMPLE 20 was carried out by supplementing a warp-modifying device 31, 32 illustrated in FIG. 5.
Ad~ustment of the clearances between the three rollers enabled the warp to decrease to a large extent.

EXAMPLE 19 was repeated except that the covering electrolytic copper foil 10 was preliminarily coated with an adhesive using a device 25 as illustrated in F~G. 6.
The composition of the adhesive u~ed in shown in Table 5. The thickness of coating was 60 ~m.

5 ~ 7 AS a result, the peel strength of the copper foil and the solder dip resistance of the clad laminate were much improved, exceeding the JIS requirements.
Table 5 s Components Descriptions Parts .
Epoxy resin Bisphenol A type 70 Viscosity: 120 to 150 poise at 25C
Epoxy equivalent: 184 to 194 Polyamide Polyamide-amine type, dimer 20 resin acid-based Amine value: 330 to 360 Viscosity: 6 to 9 poise at Polybutadiene- Amino-terminated poly- 10 nitrile rubber butadiene-nitrile rubber oligomer Copolymeric ratio of acrylonitrile: 17%
Viscosity: 2250 poise at EXAMPLE 21 was repeated except that the electro-lytic copper foil 10 was heated at 100C for 5 minutes by passing through an oven 26 immediately after coating of the adhesive. Both the solder dip resistance and the foil peel strength of the finished product.were furtherimproved.

I ~ 7~5~7 EXAMPLE 22 was repeated except that l,l-bis(t-butylperoxy)3,3,5-trimethylcyclohexane, a peroxyketal compound, was used instead as the curing catalyst.
Consequently, the solder dip resistance upon humidifying,(conditions: C-96/55/95) was raised up to 10 to 27 seconds. Other properties of the product were similar to those from,EXAMPLE 22.

EXAMPLE 23 was repeated without any curing accel-erator. The properties of the product were similar t~
those from EXAMPLE 23.

EX~PLE 23 was repeated except that the electro-lytic copper foil 10 was preliminarily coated with about 10 ~m thickness of an 0.5% aqueous solution of silane coupling agent (A-187, Union Carbide) by continuously applying the solution with the use of a coating device (not shown in figures), dried at 100C for 2 minutes, and then coated with the adhesive.
The one-side copper-clad laminates obtained were improved, especially as regards solder dip resistance and foil peel strength.

....

~ :~ 7~5~7 Eight plies of glass cloth (WE 18K-ZB, Nitto Boseki) were continuously unwound, and moved separately and in parallel. Each ply of the glass cloth was dried at 100C for 10 minutes, and continuously coated by a curtain flow technique with a liquid epoxy resin composition, which had been treated under reduced pressure as described in EXAMPLES 9, 10 and 11.
The liquid resin composition used had a viscosity of 6.5 poise at 25C, and was composed of as shown in Table 6.
Table 6 Components Parts Epikote 828, Shell Chemicals 100 Methyl-tetrahydrophthalic anhydride 80 Benzyldimethylamine 0.5 , Each ply of ~lass cloth was separately and con-tinuously impregnated for 10 minutes, combined into a unitary member and sandwiched between two electrolytic copper foils (T-7, Fukuda Metal Foil and Powder), which had been preliminarily surface-treated with a silane coupling agent (A-llO0, Union Carbide). The final re~in content in the impregnated glass cloth was adjusted at about 58~. Then the sandwiched member was continuously - 1 :11 7~7 heat-cured at 130C for 60 minutes, cut and further heated at 180C for 2 hours.
The laminated products obtained, copper-clad on both sides, were 1.6 mm in thickness, 1020 mm x1020 mm in area and had generally well-balanced properties.

~ ~ s ~557 _ N C _ ~ .C ~ _ ^ ~ ,o ~ _ _ _ ~ _ _ _ _ _ ~ N al al al al a __ ~ l~ . 0 d al ~ _ a ~ _ _ ~ all a _ _ N :1~ 0 O O O _ _ -- N _ _ _ _ _ ~ K K U ~ o O 1~ E ,1 1 1 C D o O _ N D N O N N o O ly l;

~ . . ~o t~) ~ a ~:~ a _ c~ .
I c~ ~ I ~1, I ~Ic~ 1e1~ O

,~ ~ C ~ 0~ ~ u u~ r ~D ~0 ~ r n E D ~ o N E~ t t c ~ ~ O Nl N ~1 N Nl N N N K O N ~ . h ~ t ) ¢
~o ~o ~o ~o ~o ~o ~D o o o o o o o o o ~ t , h t '~
C~ C) C~ ~ ~ C~ t~ ~ I ~ ~oO o I I o ~ ~ ~ r~ al Y

S~

- - ~D ~

3 Jl V 5 ~ 7 ~o o o o o cr a -- o~ t_ o~ _ ~ r------ ~ --~
. ~ ~ 10 l l l . . . l O O O 0 ,~ ,~ _1 ~ ~
1~1 ~ x x ~ t O O ,1 ~1 ~ o _~ ~4 t` O O ~0 h _ _ _ _ _ _ . (d O _ C _ ~ :~ ~
. u~ N N ~ O . ~; D O
e~ ,~ _~ O -~ o C.) O ~1 U~ O 0~ o O
O o o o t~ r~J~ 4Do oo ~ot~ ~ ~ Ir ~O ~ O~ t- 1~ ~D
~_ ~ _~ ~1 . . O O Ir . . . ~d t~l ~/ ~ 0 O O . O O .
N X X ~ ;t O O h ~1 ~1 ~1 h + I + 6 O O ` O O
. ~ ;- _ _ ~d _ _ _ _ _ _ ' :' . ~ ~1 _ _ _ _ _ ~ ~
N o co N ~ ~ O 4o~ co~ r--u~ ~ ~1 ~ u~ ~O ~ r~ ~ o rl . ~O o ,~ ~ . O O U` '. . . ~ C~ ` U~ O O . O O . ,~
1i3 ~ ;~ N ~ ~ O O h ,1 ,1 ,1 h I I + 6 ~r O O :~ h K
CC _ ~ _ ~ ' _ _ _ _ _ _ ~ ~ ~ O

~1 O N O . O . ~ _ _ u~ _ _ _ _ C O O h N u~ o t~ ~ C~ O u~ ~0 ~d N ~I t'` _ u O r~ O C'~ O 4D 111 rl ~1 u ~ X ;i :1 1~ O ~, u _~ ," I h l l l _ o _ o O O 1~ :~ 6 :~
2 ~ _ _ ~ _ ~ _ _ _ _ _ _ _ _ ~ _ ~1 3 li~ o c o o o o o a u c o u L~ o t I ~ Cl O O _ _ O _ C O N
~ ~u ~ ~ _ N ~ ~ ~ ~ . ~ ~ r _ ~ ~ _ ~ ~0 ~3~
. O O O :~ 00 O ~00 N CO rl ~ O N N O ~0 ~ O cr~ O ~D O
t-- 1~ _ ~ X :t :1' ~ O ~0 ~ O ~ h l l I 6 O O t'` O O O h ~ rl ;~ __ , . ~ _ rl _ _ . _ _ _ _ ~ .~
CO O O O :t CO N ~ O ~I N N ~ ~1 ~0 t' C X~ O
K ~ x x :1~ O o ~o I I ~ O o o 0 O O ~ O O o _-.
u, C~l N O O ~ E O o O O ~ o .~
. _ ~ _ . ,1 _ _ _ _ _ _ :> :~ ~ o~
~0 N ~I CO t'l Cl C,) . ~1 u~ lu~ K
O O O ~ ~ rlO ~I N N ~ ;I' ~0 C!) ICO cr . t-~ ~1 ~1 . . O O ,1 0 ~ I I . . . ~ O O . 10 . o o o ~K co N Cx ~ O O K . O O O ~ O O ~ lo O ~D ~ c~

_ _ ~ _. ,1 _ _ _ _ _ _ _ h E
,1 N ,1 ~ ID cr ~ O .~ O N rl ~E~ S
O O O ~ O ~ ~10 ~I N N ~ :t ~0 CO lo~ cr . ~O 4 ~ ~1 . . O O ~ ~ ~ . ~ O O IO . O O
K X X ;t rl . . IU ~D O O O O . . t~ I O O O ....
~1 ~ 00 O O K .6 O O 1 . ~11 1 _ ~ _ _ , ~d _ _ _ _ _ _ _ _ N ~ ~ ~ C~ I~COO r~ c,) ~ ~ u~ O lu~ :1~ ~1 'd K D O O O O O _10 l l l O O O _ O O t~- ¦ ~ ~^1 g oo~
" . ~ ~l X~o 00 ~C _ _ _ O _ _ O _ _ _ _ .i 7~5~

The above has been offered for illustrating purposes only, and it is not for the purpose of limiting the scope of this invention, which is defined in the claims below.

. _ 82 -

Claims (45)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a continuous process for producing rein-forced resin laminates comprising the steps of impregnat-ing a fibrous substrate with a liquid thermosetting resin which is free of volatile solvent and is capable of curing without generating liquid and gaseous byproducts, laminat-ing a plurality of the resin-impregnated substrates into a unitary member, sandwiching the laminate between a pair of covering sheets, and curing the laminate while supporting the same between said pair of covering sheets without applying any appreciable pressure, the improvement comprising the step of adjusting the final resin content in said resin-impregnated substrate at 10 to 90% by weight based upon the total weight of said resin-impregnated substrate.

2. The process of Claim 1, wherein said resin content is adjusted at 20 to 80% by weight.

3. The process of Claim 1, wherein said resin content is adjusted at 30 to 70% by weight.

4. The process of Claim 1, wherein at least one of said pair of covering sheets is a cladding metal foil which is to be retained on the surface of the finished laminate.

5. The process of claim 1, further comprising the step of stripping at least one of said pair of cover-ing sheets after the laminate has become at least self-supporting and tack-free.

6. The process of claim 1, wherein said thermo-setting resin is an unsaturated polyester resin.

7. The process of claim 1, wherein said thermo-setting resin is an epoxy resin.

8. The process of claim 1, wherein said sub-strate is cellulosic.

9. The process of claim 1, wherein said sub-strate is of glass fiber.

10. The process of claim 1, wherein said sub-strate is pre-impregnated with a pre-impregnating liquid and if necessary dried, prior to impregnating with said liquid resin.

11. The process of Claim 10, wherein said sub-strate is cellulosic, said thermosetting resin is an unsaturated polyester resin, and said pre-impregnating liquid contains a N-methylol compound having an unsatu-rated bond capable of copolymerizing with a polymerizable monomer.

12. The process of Claim 11, wherein said N-methylol compound is a modified aminotriazinemethylol compound.

13. The process of Claim 11, wherein said N-methylol compound is a compound of the formula:

where R1 is a hydrogen atom or a methyl group, and R2 is a hydrogen atom or an alkyl group of 1 to 3 carbon atoms.

14. The process of Claim 10, wherein said sheet substrate is cellulosic, said thermosetting resin is an unsaturated polyester resin, and said pre-impregnating liquid contains an N-methylol compound free of unsaturated bond capable of copolymerizing with a polymerizable mono-mer, and a polyfunctional compound having:
a. a functional group capable of a condensation or addition reaction with said N-methylol compound, and b. an unsaturated bond capable of copolymerizing with a polymerizable monomer.

15. The process of Claim 10, wherein said sub-strate is cellulosic, said thermosetting resin is an unsaturated polyester resin, and said pre-impregnating liquid is composed of a mixture or condensate of:
(i) methylol melamine and/or methylol guanamine, and (ii) a higher aliphatic derivative having at least one group capable of condensing with a methylol group.

16. The process of Claim 15, wherein said group capable of condensing with a methylol group is one or more groups selected from hydroxyl, carboxyl, amino and amide groups.

17. The process of Claim 15, wherein said higher aliphatic derivative is selected from oleyl alcohol, oleic acid, oleic monoglyceride, oleic diglyceride, oleic amide, oleyl amine, and a mixture thereof.

18. The process of Claim 15, wherein the depos-ited amount of said mixture or condensate is 3 to 30 parts per 100 parts by weight of the sheet substrate, as impreg-nated and dried.

19. The process of Claim 6, wherein said resin contains an aliphatic peroxide compound as a curing catalyst.

20. The process of Claim 19, wherein said ali-phatic peroxide compound is an aliphatic peroxyester.

21. The process of Claim 6, wherein said resin has a glass transition temperature between 20°C and 80°C.

22. The process of Claim 1, wherein said substrate is impregnated by applying said liquid resin onto one side thereof.

23. The process of Claim 5, wherein the rigidity of said one of covering sheets is more than 3 x10-3 Kg.cm (according to the formula E.d3, where E is flexural modulus in Kg/cm2, and d is thickness in cm).

24. The process as defined in one of Claim 5, wherein said one of covering sheets is between about 0.4 µ
and 9 ? of RmaX in its surface roughness.

25. The process of Claim 4, wherein lamination of the metal foil coverings and the impregnated substrate is carried out with the continuous application of an adhesive therebetween.

26. The process of Claim 25, wherein said adhesive is continously applied to the surface of the said metal foil prior to laminating.

27. The process of Claim 26, wherein said metal foil to which said adhesive has been applied passes through a heating zone prior to laminating.

28. The process of Claim 25, wherein said adhesive neither contains any volatile component nor generates any volatile byproduct during curing, the adhesive being applied to the liquid resin-impregnated laminate andcured there without the application of pressure.

29. The process of Claim 25, wherein said thermo-setting resin is an unsaturated polyester resin, said adhesive is composed of an epoxy resin of amine-curing type, and said curing catalyst for the unsaturated poly-ester resin comprises one or more peroxides selected from peroxyketals, peroxyesters and dialkylperoxides.

30. The process of Claim 25, wherein a compound having a copolymerizable unsaturated bond and an epoxy group is supplied between the liquid resin-impregnated substrate and the adhesive layer coated on said metal foil, and is then cured.

31. The process of Claim 25, wherein said metal foil passes through a drying zone before application of said adhesive thereon.

32. The process of Claim 31, wherein a surface treating agent is continuously supplied to said metal foil before passing through said drying zone.

33. The process of Claim 32, wherein said surface treating agent is a silane coupling agent.

34. The process of Claim 1, wherein said liquid thermosetting resin is subjected to a degassing treatment under a reduced pressure prior to applying to said fibrous substrate.

35. The process of Claim 34, wherein said degass-ing treatment is carried out by jetting said resin liquid into a reduced pressure vessel at less than 30 mmHg.

36. The process of Claim 1, wherein a continuous length of said unitary member is cut after partially cur-ing into a predetermined length, and then fully cured.

37. The process of Claim 5, wherein said one of covering sheets is continuously coated with a releasing agent over its whole area or only at its edge portions.

38. The process of Claim 36, wherein said unitary member is cured in a first curing step until it becomes essentially tack-free and self-supporting, and subjected to a continued curing step after cutting into said pre-determined length.

39. The process of Claim 38, wherein said first curing step is discontinued when the gel content in the semicured resin composition becomes at least 80% based on the weight of resin present in said unitary member.

40. The process of Claim 38, wherein said first curing step is discontinued when the gel content in the semicured resin composition becomes from 90 to 98% based on the weight of resin present in said unitary member.

41. The process of Claim 39, wherein said continued curing step comprises placing the cut segments in a heating chamber for a sufficient length of time for the completion of curing without restraining the same by a supporting member.

42. The process of Claim 36, further comprising a step for mechanically correcting any distortion in the configuration of the finished laminate.

43. The process of Claim 10, wherein said substrate is cellulosic paper, said resin is an unsaturated polyester resin.

44. The process of Claim 43, wherein said pre-impregnating liquid comprises an N-methylol compound.

45. The process of Claim 25, wherein said resin is an unsaturated polyester resin and said adhesive an amine-curing epoxy resin.