CA1178415A - Packaging materials for articles - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The invention relates to a packaging material having excellent thermal adhesion sealability comprising (A) a base layer formed of a stretched propylene polymer and (B) a stretched surface layer formed of a blend of a propylene-ethylene copolymer and a C4-C10 n-olefln-propylene copolymer in a weight proportion of 20 : 80 to 94 : 6 by weight on at least one surface of said base layer. The resulting composite film has good transparency, gloss, anti-static properties and heat sealability. The film is particularly adapted for use in high speed automatic packaging machines.

Description

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The present invent:lon relates to p~ckacJing materials for various articles. More particularly, it relates to packaging materials having desirable properties such as high transparency, gloss, melt-adhesion at low temperature, lubricity in sliding, separability from a hot plate, antistatic properties, ~tc.
Such materials are useful for repackaging and sealing a single article or a mass of articles of the type formerly packed in a case or the like.
In recent years, improved requirements have been demanded for the external packaging of foodstuffs, tobacco, industrial articles and daily miscellaneous goods. For example, with respect to the packaging of foodstuff, the packaging material should have good appearance characteristics (e.g. high transparency, high gloss), good basic characteristics (e.g. moisture-proof properties, fragrance-keeping properties, insect preventing properties, oxygen intercepting properties) and suitable characteristics for packaging in an automatic packaging system (e.g. sealability of a heat-bonded portion, sufficient adhesion strength, prevention of insufficient air-tightness caused by wrinkles in the packaging, prevention of a poor appearance, minimization of the defective rate of automatic packaging). Furthermore, it is undesirable for the packaging material to have an odor, from a sanitary viewpoint, so that vigorous effort has been made by the industry to reduce the remaining solvents in the packaging material.
On the other hand, automatic packaging machines are nowadays operating at a higher speed with a higher efficiency. For use in such automatic packaging machines, the packaging materials should have the following properties:

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good thermal adhesion at low tempo~ature; ~ood lubricity between the packaging material and the metal guide portions of the automatic packaging machine; good separability and slidability between the packaging material and the hot plate; good flaw preventing properties on sliding between the packaging material and the automatic packaging machine;
good automatic applicability of the packaging material to the automatic packaging machine, etc.
The following have been proposed as packaging materials having good heat sealing properties: (1) a coated film produced by applying a low melting point substance dissolved in an organic solvent on a base film, (2) a single film obtained by mixing polypropylene with a low melting point polymer and shaping the mixture into a film, (3) a laminated film obtained by laminating a low melting point polymer film onto a base film in such a manner that the low melting point polymer film forms a heat-seal surface, and (4) a single film obtained by mixing polypropylene with a low molecular weight thermoplastic resin and shaping the blend into a film. However, these packaging materials have the following problems: the films obtained by the coating method are inferior in seal-strength in the hot state and are unsuitable for seal-packages. In addition, it is difficult to fully eliminate the remaining solvent from the coated surface. The films obtained by mixing polypropylene with a low melting point polymer and shaping the mixture into a film are insufficient in sealability at low temperature and transparency. Besides, the film is soft and its firmness and resilience are small so that its automatic suppliability is unstabilized and continuous packaging in an automatic packaging machine is difficult.

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117~ 5 Various attempts h~ve been macle to laminate a low melt~ng point polymer on a base film, but problems arise such as melt adhesion of the polymer causing rolling during stretching by a heat roll or scratches on the film surf~ce in two-stage biaxial orientation. In practice, therefore, cold stretching is necessitated. Under such conditions, surface scratches may be reduced, but voids are caused by stretching so that a transparent film cannot be obtained.
To overcome such drawbacks, a method has been adopted in which a base film is stretched in the machine direction by a heat roll, the low melting point polymer is laminated on one surface or both surfaces of the base film, and then the laminated film is stretched by a tenter in such a manner as not to be in contact with the surface which is susceptible to melt-adhesion (cf. U.S. patènt 3,671,383).
However, the low melting point polymer layer, which i5 made by monoaxial stretching, is more susceptible to damage both thermally and mechanically and is apt to lose trans-parency. The film incorporated with a low molecular weight thermoplastic resin can have the seal strength necessary for overlapping even under low temperature, but has the disadvantage of loss in strength when the seal portion is kept at high temperature, so that it is difficult to form into a package having good sealability. Since the film is kept at high temperature immediately after the heat-sealing, weakness of seal strength in such conditions means inapplicability to seal packaging.
As the result of an extensive study, it has now been found that the use of certain specific polymers in combination can provide a seal packaging material having high transparency and gloss as well as good antistatic properties and heat sealabillty.
Accord~ng to the present invention, there 18 provided a packaging material compris~ng ~A) a stretched base layer formed of a propylene polymer, and ~) a stretched ~urface layer formed of a blend o a propylene-ethylene copolymer and a C4-C10 ~-olefin-propylene copolymer in a weight proportion of 20 : 80 to 94 : 6 by weight on at least one surface of said base layer; wherein the propylene-ethylene copolymer comprises units of propylene and units of ethy-lene in a weight proportion of 99.5 : 0.5 to 90 : 10; and wherein the C4-C10 o-olefin-propylene copolymer com-prises units of C4-C10 -olefin and units of propylene in a weight proportion of 30 : 70 to 5 : 95.
The propylene polymer for the base layer (A) is desirably a polymer mainly comprising propylene and having a melting point of 140C or higher, preferably 150C or higher. Specific examples thereof are isotactic polypropylene having an isotactic index of 85 % by weight or higher, a copolymer of ethylene and propylene having an ethylene content of 7 ~ by weight or lower, a copolymer of propylene and a C4-C6 ~-olefin having a propylene content of 90 % by weight or higher, etc. The propylene polymer preferably has an intrinsic viscosity of 1.6 to 3.0 dl/g (tetraline solution at 135C), particularly 1.8 to 2.5 dl/g.
On a~ least one surface of the base layer (A), a surface layer (B) is pro~ided, the layer (B) being formed of a blend of a propylene-ethylene copolymer and a C4-C10 ~-olefin-propylene copolymer in a proportion of 20 : 80 to 94 : 6. The surface layer (B) may be either monoaxially or biaxially stretched. The surface layer (B) is laminated on either or both surfaces of the base layer 'A) and firmly bonded. The combination may thus be either A/B or B/A/B.

``" 3 1~84~5 The propylene-ethylene copolymer to be u~ed in the blend for the surface layer ~) has an ethylene content wlthin the range of 0.5 to 10 ~ by weight. Particularly preferred is a polymer having a melt index of 0.5 to 10 ~g/10 min), especially a random copolymer having an ethylene content of 3.6 to 10 ~ by weight, with a melt index of 1.0 to 6 ~g/10 min). When the ethylene content is less than 0~5 ~ by weight, the polymer is difficult to mix evenly with a C4-C10 a-olefin-propylene copolymer and is poor in transparency and gloss. In an extreme case, it becomes a semi-mat, translucent film. When the ethylene content is more than 10 % by weight, the lubricity when heated Is deteriorated, and wrinkles and scratches are formed on the package thus obtained. When a laminated film with the surface layer ~B) is to be kept at an elevated temperature even after being subjected to electron beam treatment, the ethylene content should be from 3.6 to 10 % by weight.
The C4-C10 ~-olefin-propylene copolymer in the blend is a copolymer which comprises units of propylene and units of C4-C10 a-olefin ~e.g. butene-l, pentene, hexene) re-spectively in amounts of 70 to 95 % by weight and 30 to 5 % by weight. When the propylene content is less than 75 %
by weight, the product has poor transparency and gloss and a high coefficient of friction at high temperatures (i.e.
1.4 or more) so that the resulting film or sheet has scrat-ches and wrinkles caused by insufficient sliding in heat sealing, making it impossible to obtain a sealed package in an automatic packaging machine. Further, in the case of seqential biaxial stretching, clinging or melt-adhesion to a heat-stretching roll is apt to occur, thus making it im-possible commercially to obtain a smooth package of a film or sh~eet having an excellent appearance without flaws.

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When the propylene content is larger than 95 ~ by weight, the film shows a lower heat-sealing property, and in particular heat-sealing at low temperature and at high speed becomes difficult. As it is necessary to elevate the heat-sealing temperature, an attractive heat seal by thermal shrinkage is not obtainable.
In order to enhance the advantageous physical characteristics of the packaging material of the invention, various optional measures may be adopted. One of such measures is to incorporate into the propylene polymer for the base layer (A) a low molecular weight thermoplastic resin. The low molecular weight thermoplastic resin may be blended into the propylene polymer in an amount of 3 to 25 ~ by weight of the weight of the propylene polymer.
The low molecular weight thermoplastic resin should be compatible with the propylene polymer, should have a softening point (determined according to ASTM D-36-26) of 70 to 150C, should show a thermal stability at a temperature of 150C or higher and preferably possess 20 a melting viscosity of about 20,000 cp or lower at 200C. The term "compatible" as used herein is intended to mean that, when the propylene polymer is blended with the low molecular weight thermoplastic resin, no separation between these materials occurs. The term "thermal stability"
is intended to mean that permanent change is not caused in the properties of the resin even after heating at a designa-ted temperature for 1 hour in the presence of air. The melting viscosity is determined according to the method as described in ~STM D-1824-66 by a Brookfield viscometer with heating to the designated temperature.

Examples o~ suit~ble low mo]ecular weight thermoplastic resins are hydrocarbon resins, rosins, dammers, phenol resins, chlorinated aliphatic hydrocarbon waxes, chlorinated polynuclear aromatic hydrocarbons, etc.
The term "hydrocarbon resin(s)" covers hydrocarbon polymers derived from coke oven gas, coal tar distillates, decomposed or deep~decomposed petroleum materials, substan-tially pure hydrocarbon materials and turpentine oil.
Typical examples of the hydrocarbon resin are cumarone-indene resins, petroleum resins, styrene resins, cyclo-pentadiene resins, terpene resins, etc. These resins are described in Kirk-Othmer's "Encyclopedia of Chemical Technology", Second Edition, Vol. 11, 242 - 255 (1966).
The cumarone-indene resins are hydrocarbon resins recovered from coke oven gas or obtained by polymerization of resin-forming substances present in coal tar distillates, phenol-modified cumarone-indene resins and their derivatives~
These resins are described in the said Encyclopedia, Second Edition, Vol. 11, 243 - 247. The petroleum resins are hydrocarbon resins obtained by polymerization of deep-decomposed petroleum materials in the presence of a catalyst.
These petroleum materials usually contain a mlxture of resin-forming substances such as styrene, methylstyrene, vinyltoluene, indene, methylindene, butadiene, isoprene, piperylene and pentylene. These resins are described in the said Encyclopedia, Second Edition, Vol. 11, 248 - 250.
The styrene polymers are low molecular weight homopolymers of styrene and copolymers of styrene with other monomers such as ~-methylstyrene, vinyltoluene and butadiene.
The cyclo-pentadiene resins are cyclopentadiene homopolymers and copolymers derivated from coal tar distillates and ` :~L1';'13~1~5 separated petroleum gas. These r~sins ar~ prepared by keeping cyclo-pen~adie~ne-containing materials at high temperature for quite a long time. Depending on the reaction temperature, dimers, trimers or hi~h polymers may be obtained. These resins are d~scribed in the said Encyclopedia, Second Edition, Vol. 11, 250 - 251. The terpene resins are polymers of terpenes, i~e. hydrocarbons of the formula CloH16 present in almost all essential oils and oil-containing resins of plants and phenol-modified terpene resins. Specific examples of terpenes are ~-pinene, ~-pinene, dipentene, limonene, myrcene, bornylene, camphene and similar terpenes. These resins are described in the said Encyclopedia, Second Edition, Vol. 11, 252 - 254.
The term "rosin(s)" means natural resinous substances present in oil-containing resins of pine trees, rosin esters, modified rosins (e.g. fractionated rosins, hydrogenated rosins, dehydrogenated rosins) and other similar substances. These substances are described in the said Encyclopedia, Second Edition, Vol. 17, 475 - 505.
The term "dammar(s)" is intended to mean a colorless or yellow substance present in plants such as kanari and any similar substance thereto. These substances are described in "Encyclopedia Chimica" (Kyoritsu Shuppan), Vol. 5,776 (1961).
The term "phenol resin(s)" means the reaction product between a phenol and an aldehyde. Examples of the phenol are phenol, cresol, xylenol, p-tert-butylphenol, p-phenyl-phenol, etc. Examples of the aldehyde are formaldehyde, acetaldehyde, furfuralaldehyde, etc. These resins are described in Kirk-Othmer's "Encyclopedia of Chemical TechnolocJy", Second ~dltion, VQ1~ 15, 176 - 207.
The chlorinated aliphatic hydrocarbon waxes are chlorinated paraffin waxes (usually called "chlorinated waxes"). Typical ones contain about 30 to 70 ~ by weight of chlorine.
The chlorinated polynuclear aromatic hydrocarbons are chlorinated hydrocarbons containing at least two aromatic rings such as chlorinated biphenyl, chlorinated terphenyl and their mixtures. Typical ones contain about 30 to 70 %
by weight of chlorine.
The base layer ~A) may additionally contain other polymers in amounts that do not result in a reduction of quality. It may also contain any other additive(s) such as an antistatic agent, a lubricant or an anti-blocking agent and the like. The effects of these additives is greater when incorporated into base layer (A) than when incorporated into surface layer (B). The antistatic agent may be used in an amount of 0.5 to 3 parts by weight to 100 parts by weight of the total amount of the propylene polymer and the low molecular weight thermoplastic resin. A lubricant or an anti-blocking agent is usually employed in an amount of 0.1 to 3 parts by weight to 100 parts by weight of the said total amount.
Examples of the lubricant are higher aliphatic acid amides, higher aliphatic acid esters, waxes, metallic soaps, etc. Examples of the anti-blocking agents are inorganic additives (e.g. silica, calcium carbonate, magnesium silicate, calcium phosphate), nonionic surfactants, anionic surfactants, incompatible organic polymers ~e.g.
polyamides, polyesters, polycarbonates), etc.

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~ xamplcs oL the antistatic a~lent are oncs which are blendable with the propylene polymer, e.g. the following compounds:

(R10) m~l R-N
R20 ) nH
( 1 )m 3 R-N /
(R20) nCR3 10~(R10) mCOR3 R-N

( 2 )n ( R10 ) mH
R-CON /

( 2 )n R-N

R -CO

X
,~3~/ 1 RO-N

l X2 (betaine type) Rl-COO
wherein R and R3 are each a monovalent aliphatic group having 12 to 22 carbon atoms, Rl and R2, which may be the same or different, are each a divalent hydrocarbon group having 2 to 4 carbon atoms, Xl and X2 are each a saturated hydrocarbon group having not more than 22 carbon atoms and optionally bearing hydroxyl or alkoxy or a group of the formula:--~R4-O)pH (in which R4 is a divalent hydrocarbon group having 1 to 3 carbon atoms and p is an integer of not more than 20), or they may be taken togethc!r to make a ring and m ~ n is an integer oE 0 to 8. ~here may be also used monoglycerides of fatty acid esters, poly-oxyethylene alkyl phenyl ether, etc. Co-use of more than two kinds of these antistatic agents is particul;lrly effective.
During automatic packaging, it is disadvantageol~s for obtaining a sealed package not to supply the packaging material to the product to be packed in a constant and straight manner because troubles are otherwise encountered in the packaging material-supplying portion of the automatic packaging machine, such as adsorption of the film due to static electricity, winding of the film round the edge of the cutter and pulling of the film toward the guide plate by static attraction. Thus, consideration must be given to prevention of static electricity both when the film is stationary and moving. In this respect, it has been found that static electricity in the automatic packaging machine can be prevented to a great exten~
by incorporation of an antistatic agent into the base layer (A).
In particular, by incorporation of an antistatic agent and a low molecular weight thermoplastic resin into the base layer (A), the antistatic performance is increased and a remarkable improvement of the antistatic effect is exhibited. The use of an antistatic agent in an excessive amount is, however, not favorable since it causes a reduction of the heat sealability over a long period of time, whitening at the surface by bleeding, stick blocking at high temperature, etc. On the other hand, when the use of the antistatic agent is excessively small in amount, ,~
,~, .P~, 3 ~3L5 a suitable antistatlc effect is not obtained.
~ o prevent the occurrellce of static electricity during automatic packaging, at least one surface or both surfaces of a composite film consisting of the base layer (A) and the surface layer(s) (B) may be subjected to an electron beam treatment, e.g. a corona discharge treatment or a glow discharge treatment, in order to improve the antistatic property and suitable slidability will be attained by the use of the antistatic agent in the minimum necessary amount. When, however, the said electron beam treatment is employed, the heat sealability of the composite film, particularly at low temperature, is much reduced, and an increase in the intensity of the electron beam treatment will often result in loss of the heat,sealability not merely at low temperature but also at high temperature.
In order to improve the antistatic property and the slid-ability without reducing the heat sealability, the use of a polymeric mixture comprising a low molecular weight thermoplastic resin for the surface layer (B) and the application of electron beam treatment thereto are recommended.
Particularly useful low molecular weight thermoplastic resins for this purpose are hydrocarbon resins, rosins, dammars, phenol resins, etc.
The antistatic property may be represented, for instance, by an intrinsic resistance at the surface, and it is usually 1012 13 Q.cm or less. When the thickness is smaller, a lower intrinsic resistance is required.
If it is from 108 to 101 Q.cm, it will be applicable to almost all kinds of automatic packaging machines.
The extent of the electron beam treatment is desirably in the range of 30.5 to 58 dyne~cm when measured, t '7~3'a~ 5 for example, with the we~ tension.
While the composite film of the present invention has high transparency and high gloss, it is desirable, in order to impart gloss to the film and provide a suitable antistatic property by electron beam treatment and yet not to impair the heat-sealability, especially the sealability at low temperature, to incorporate a low molecular weight thermoplastic resin into the surface layer (B) in an amount of 3 to 25 % by weight based on the combined amount of the blend and the low molecular weight thermoplastic resin.
When the amount of the low molecular weight thermoplastic resin is less than 3 % by weight, the transparency and the gloss are not improved. On the other hand, when the amount of the low molecular weight thermo-plastic resin is larger than 25 % by weight, the transparency and the gloss are reduced and the tucking power at high temperature is lowered to reduce the grade of seal packaging.
In order to further prevent wrinkles at the sealing part in an automatic packaging machine by lowering the coefficient of friction at high temperature, it is desirable to use a silicone oil in the following proportions, usually in a proportion of 0.01 to 0.15 part by weight to 100 parts by weight of the blend for the surface layer (B). Examples of the silicone oil are polydimethylsiloxane, polymethylphenylsiloxane, olefin-modified silicone, polyether (e.g. polyethylene glycol, polypropylene glycol)-modified silicone, olefine/polyether-modified silicone, epoxy-modified silicone, amino-modified silicone, alcohol-modified silicone, etc. Among them, olefin-modified silicone, polyether-modi~ied silicone and olefin/polyether-modified silicone are particularly preferable.
The silicone oil improves the coefficient of friction of the composite film in the heated state, reduces the slide resistance caused during hot plate seal by an automatic packaging machine and thus prevents generation of wrinkles, which makes it possible to obtain a composite film having a beautiful appearance, a high sealing ability and an excellent close-fitting property to a product to be packaged. Further, decrease of the gloss due to sliding can be prevented in order to obtain a sealed portion with beautiful appearance. By the use of the silicone oil, the friction coefficient at high temperature during heat sealing with sliding can be decreased to 1.4 or smaller.
In order to obtain a sufficient effect, the silicone oil preferably possesses a viscosity of 50 to 10,000 cs, more preferably S0 to 300 cs.
The effect of the silicone oil can be further increased by the combined use of an ethylene oxide-addition product of castor oil having a softening point of 70 to 140C, an oxidized synthetic wax, a higher fatty acid alkyl ester, a polyalcohol alkylate-ethylene oxide addition product, a fatty acid amide, etc. These compounds are usually used in an amount of 1 to 300 parts by weight, preferably 50 to 300 parts by weight, to 100 parts by weight of the silicone oil. The combined use of these compounds, together with the silicone oil, prevents stick-slip at a temperature of room temperature to 100C, which is apt to occur when the silicone oil is used alone, and improves the lubricity between the film and various metal guide plates of the automatic packaging machine to prevent the formation l~ iS
of a bad pac~a~e. ~urther, ~he lubrlcity at high temperature under elevated pressure can be improved, the friction coef-ficient at high temperature being decreased to 1.~ or smaller, thus the use of such oil and additive is extremely important for obtaining an excellen-t sealed package according to the invention. Although the silicone oil and the said additives can give these advantageous effects, they tend to decrease the heat sealing property of the film or sheet and the transparency, and because of this tendency of heat at low temperature, it is required in practice to effect heat-sealing at relatively high temperatures.
Incorporation of a lubricant or an anti-blocking agent into the surface layer (B) in an amount of 0.1 to 3 parts by weight to 100 parts by weight of the amount of the blend and, when used, the low molecular weight thermo-plastic resin for the surface layer (B), is effective to improve the lubricity and the anti-blocking property of the composite film of the invention. Specific examples of these additives may be the same as hereinbefore stated in connection with their use for the base layer (A). It is also possible to improve the antistatic property by incorporation of the antistatic agent as stated in connection with the base layer (A) in an amount of 0.5 to 3 parts by weight to 100 parts by weight of the amount of the blend and, when used, the low molecular weight thermoplastic resin.
The thickness of the composite film of the invention for use as a packaging material may be decided according to the intended use of the sealed package.
In the usual case, a thickness of 5 to 150 microns, particularly of 15 to 60 microns, is adopted. The total ,~

::IL i ~ ~3 L~L ~1" 5 thickness of the surEace layers (~) may be from 0.2 to 50 ~ of the whole thickness of the composite film used as the packaging material. When preparing a sealed package in an automatic packaging machine, the absolute value of the surface layer (B) becomes important, in addition to the said thickness ratio. The desirable thickness of each surface layer (B) is 0.2 to 10 microns, particularly 0.2 to 3 microns, on application to a cigarette hold type automatic packaging machine.
The following two processes are known for operation of automatic packaging machines: a process of thermal adhesion with sliding under heating and elevated pressure, and a process of thermal adhesion with pushing under heating (the ordinary heat-seal system). In the case of the process of thermal adhesion with sliding under heating, the thickness of each surface layer (B) is preferably 0.2 to 3 microns. In the case of the process of thermal adhesion with pushing under heating, e.g. in automatic packaging machine of the form and fill type, the thickness of each surface layer (B) is preferably 0.7 to 10 microns.
The packaging material of the invention is stretched into at least one direction. Preferably, the film for the base layer (A) is biaxially stretched and the film for the surface layer (B) is uniaxially or biaxially stretched.
For instance, the composite film obtained by either one of the following methods may be stretched at least in one direction, or preferably in two directions, to give the packaging material of the invention: the co-extrusion method in which the base layer (A) and the .

1 ~ 5 surface layer (B) are ex-truded Erom separate extruding machines to form a compound stream in a melted state which is then subjected to extrusion shaping; the laminating method in which the layers are extruded separately in a melted state and the extruded products are laminated before solidification by cooling; the adhesion method in which one of the layers is shaped into film or sheet form with solidification by cooling and the other layer is piled thereon in a melt-extruded state. Further, for instance, a film(s) for the surface layer (B) may be piled or melt-extruded onto at least one surface of a uniaxially stretched film for the base layer (A), followed by stretching into a direction perpendicular to the direction into which the uniaxially stretched film has been stretched.
The packaging material of the invention may be in the form of a flat film or a ring film.
In the present invention, an effective sealing material can be provided by the one stretched at least uniaxially, preferably biaxially. It is based on the characteristics that the base layer (A) is more thermally stable to biaxial stretching and the surface layer (B) does not show any loss of its sealing property even though it is subjected to biaxial orientation.
A characteristic feature of the packaging material of the invention is that it can be prepared by the two-stage biaxial orientation method in which a heating roll is used in the longitudinal stretching, though the preparation can be effected more easily by the simultaneous biaxial stretching method. In the case of the two-stage biaxial orientation, a strong binding power is obtained between the layers to afford a packaging material with excellent heat s~l str~ntJth. In addition, th~ preparation Of the composite film can be effected economically.
The preferable conditions fGr obtaining the packaging material of the invention as a stretched film are explained in the Eollowing description. In the case of uniaxial stretching, the material is stretched 3.5 to 10 fold in a machine direction or a transverse direction.
The stretching temperature is usually 100 to 160C during roll stretching or 140 to 165C in tenter stretching.
In the present invention, in case of subjecting the film to the two-stage biaxial orientation, stretching may be effected 3.5 to 10 fold, preferably 3.8 to 7.5 fold, into a machine direction and 4 to 12 fold, preferably 6 to 9 fold, into a transverse direction. In the case of the simultaneous biaxial orientation, the temperature is 140 to 165C. In the two-stage biaxial orientation, the temperature at the first step is 100 to 160C, preferably 110 to 130C, and the temperature at the second step is 140 to 165C, preferably 145 to 160C. The heat setting is effected after a temperature higher than the said stretching temperature, usually at 140 to 167C, is maintained for 1 second to 1 minute. The obtained composite films are usually subjected to melt adhesion with heating, the surface layers (B) being opposite to each other.
If necessary, melt adhesion with heating between the base layer (A) and the surface layer (B) may be effected.
The packaging material of the present invention may be used for the packaging of various articles by conventional procedures using heat sealing. An automatic packaging machine as commonly used has a packaging speed of 30 to 500 packages/min. The packaging speed of the most 1.5 popular ona is rom 50 to 300 packagcs/mL~ n ttlosa conventional machines, the heat seal temperature using the packaging material of the invention may be from 120 to 190C, preferably from 130 to 180DC, although it depends on the packaging speed, the film thickness, etc. The heating time is usually from 0.05 to 2.0 seconds, particularly from 0.2 to 1.0 second, though this varies greatly with repetition of heating.
When heat sealing is effected at a high temper-ature and at a high speed by the use of an automatic packaging machine, many conventional films or sheets are apt to be damaged on their surfaces. In addition, a flat seal surface is difficult to obtain due to the production of thermal shrinkage. In contrast to films made of polyvinyl chloride, the biaxially stretched films made of polypropylene can not attain even shrinkage because of their high crystallinity and melting point. In the case of biaxially stretched films made of polypropylene, the part brought into contact with a hot plate is readily shrunk, and, depending on the conditions of contact, only an uneven seal surface may be obtainable, thus making it difficult to effect seal packaging.
The packaging material of the invention has good antistatic properties and heat sealability at low temperature and can be used in conventional automatic packaging machines with ease. When the sealed product is kept at 100C, the retention of the seal strength is more than 60 %, frequently more than 80 %. The friction coefficient at 120C is less than 1.4. Due to these characteristics, packaging can be accomplished with less production of wrinkles and scratches.

The packa~in~ material o~ the invention may be bonded not only with itsel~ but also to any other heat sealable surface such as polypropylene laminated film, a polyethylene laminated film or a polybutene laminated film or sheet. Further, it may be utiliæed as a surace convering such as adhesive tape, sheet and protective film.
Practical and presently preferred embodiments of the invention are illustratively shown in the following Examples wherein part(s) and percentages are by weight unless otherwise indicated. In these Examples, the physical properties were determined as follows:
1) Coefficient of friction:-(A) Friction coefficient at room temperature The determination was effected at 20C ata relative humidity of 65 ~ according to ASTM D-1894.
(B) Friction coefficient at high temperature An adhesive tape made of polyvinylidene fluoride was stuck on the surface of a hot plate heated at 120C, and a test film or sheet was contacted therewith. Sliding was caused under a vertical load of 4.5 g/cm2, with a speed of 2 m/sec, and the friction coefficient was recorded.

2) Heat seal strength:-Heat sealing was effected by means of a thermal inclination heat sealer (manufactured by Toyo Seiki K.K.) under a pressure of 1 kg/cm2 for 0.5 second, and the peeling-off strength was measured under a speed of 200 mm/min.

3) Sealing:-By means of an automatic packing machine, 100 sealings per minute were effected at 140C, and the air-tightness of the sealed portion was judged from the amount . -20-, ~ 2'~ S
of leaking water. Water containiny 0.2 % of a surface active agent (50 ml) was poured into an externally packed product in a box form, and the amount of water leaking in 1 minute was measured. Evaluation was effected accordlng to the following criteria:
ClassAmount_of leaking water (ml/min) E larger than 50

4) Transparency and haze:-Determination was made by the aid of a haze tester (manufactured by Toyo Seiki K.K.) according to JIS-K6714.

5) Gloss:-Determination was made according to JIS-Z8714.

6) Rate of bad packaging:-After effecting packaging at a heat sealtemperature of 140C at the rate of 100 pcs/min, 200 packages were picked up at random from the resulting packages and the number of defective articles, such as defective folding, defective sealing, defective articles which could not be packaged in contact with the article to be packaged, etc., were counted and the values obtained by dividing by 200 pcs. were expressed in percentages.

7) Wrinkles at the heat-sealed part:-Evaluation was made on the following criteria:

A: None B: Slight but beautiful C: Partial D: Much E: Over the whole surface

8) Degree of close-fitting packaging Evaluation was made on the following criteria:

., t~.5 ully ti.gll~-pac1;agecl B: Nearly tiyht-packaged C: Slight spaces D: Many spaces E: Remarkable spaces

9) Automatic supply:-Packaging was efEected at a rate oE 100 packages/min by the use of an automatie packaging maehine W-37 (manufactured by Tokyo Automatic Machinery Co., Ltd.).
The state of continuous automatic supply of a film was ob-served for 1 minute, and evaluation was made on the following criteria:
o: No material problem; smooth supply ~: Occasional problems occur x: Automatic supply was impossible due to winding-up of the film around the cutter and electrostatic adhesion of the film onto the guide surface

- 10) Surface wetting tension:-A test liquid consisting of dimethylformamide and ethyleneglycol monoethylether (manufactured by Wako Pure Chemicals Co., Ltd.) was applied onto the surface of a film. When the cohesion of the test liquid was going to start 2 seconds after the application, the wetting tension was measured and taken as the surface tension of the film.
The abbreviations used in the following Examples have the meanings as follows:

(P-l): Isotactic polypropylene; intrinsic viscosity, 2.0 dl/g (determined in tetraline at 135C).

(P-2): Propylene/ethylene copolymer; ethylene content, 4.5 % by weight; melt index, 2.0 g/10 min.

(P-3): Propylene/butene-l copolymer; butene-l content, 15 % by weight.
(P-4): Isotactic polypropylene; intrinsic viscosity, 2.1 dl/g.

~22-~`" 1 1 7~ X
(P-5): Polybutelle-l; melt Index, 2.0 c~/ln min.
(P-6): rropylono~ y~ e ~ol)c)lym(~r; ~LhyLI~no contcllt, 3.5 ~ ~y weiclht mel~ inclax, ~.n ~ o min.
(P-7): ~thylelle/vinyl ac~t~t~ copolymer; vinyl acetate content, 30 % by weight; melt index, 7.0 g/10 min.
(P-8): Propylene/ethylene copolymer; ethylene content, 4.0 ~ by weight; melt index, 2.5 g~10 min.
(P-9): Propylene/butene-l copolymer; butene-l content, 20 ~ by weight.
(P-10): Isotactic polypropylene; melt index, 4.5 g/10 min.
(P-ll): Propylene/ethylene copolymer; ethylene content, 5.0% by weight.
(P-12): Butene/propylene copolymer; propylene content, 10 %.
(P-13): Isotactic polypropylene; intrinsic viscosity, 1.8 dl/g.
(P-14): Propylene/ethylene copolymer; ethylene content, 4.5 ~ by weight; melt index, 4.0 g/10 min.
(L-l): Petroleum resin (tradename "ALCON P-115" manu-factured by Arakawa Rinsan Co., Ltd.).
(L-2): Polyethylene wax; molecular weight, 2000.
(L-3): Rosin ester.
(L-4): Terpene phenol resin (tradename "YS POLYSTAR"
manufactured by Yasuhara Yushi Co., Ltd.).
(A-l): Stearic acid monoglyceride.
(A-2): Alkylamine-ethylene oxide adduct (tradename "DENON
331" manufactured by Marubishi Yuka Co., Ltd.).
(A-3): Castor oil-ethylene oxide adduct.
(A-4): Hydroxystearoamide (tradename "DIAMID KH" manu-factured by Nihon Kasei Co., Ltd.).
(A-5): Polyoxyethylene monostearate stearylamine.
(A-~): Erucic acid amide.
(S-l): Polypropylene glycol-modified silicone; viscosity, 100 centistokes at 20C.
(S-2): Polyolefine-modified silicone.
(S-3): Polyether-modified silicone.

Moreover, in the Eollowing Examples, reference is made to the accompanying drawings, in which:
Fig. 1 is a graph showing the relationship between the heat sealability at low temperature and the frictional co-efficient at high tem~erature of various films as produced in the Examples;
Fig. 2 is a graph of heat seal strength against surface watting tension for various films;
Fig. 3 is a graph of heat seal strength against mixing ratios of components of various films;
Fig. 4 is a graph similar to Fig. 3 for different films;
Fig. 5 is a graph of haze ratio against mixing ratios of components of various films;
Fig. 6 is a graph of the grade of seal packaging against the mixing ratios of components of various films;
Fig. 7 is a graph of grade of seal packaging against the amounts of a certain component of various films;
and Fig. 8 is a graph of the heat seal strength against the amounts of a certain component of various films.
Example 1 A composition comprising a mixture of 90 parts of (P-l) and 10 parts of (L-l) incorpoxated with 0.5 part of (A-l) and 1.0 part of (A-2) was used as a base layer (A).
A composition comprising a mixture of 40 parts of (P-2) and 60 parts of (P-3) incorporated with 0.04 part of (S-l), 0.02 part of (A-3), 0.1 part of (L-2), 0.3 part of (A-l) and 0.3 part of (A-4) was used as a surface layer (B).

., ~., Sald compositiolls o~ laycr ~A) and laycr ~B) were melt extruded on two extruders to obtain an unstrctched composite film composed of the three layers (B)/(A)/(B) and having a thickness of 960 microns. The film was stretched at 130C 4.0 fold in the machine direction and 8.0 fold in the transverse direction, subjected to heat setting at 155C with a relaxation of 5 ~ and then cooled in a stream of 20C air to obtain a biaxially stretched composite film having a thickness of 30 microns.
The composite film was slit to make a narrow strip, and external packages of square boxes of 70 mm in height, 55 mm in width and 20 mm in thickness was effected by means of an automatic packaging machine of the sliding type at a temperature as shown in Table 1 at a speed of 100 boxes/min.
Comparative Example 1 A mixture comprising poly-vinylidene chloride as the main component and a lubricating agent and an antistatic agent as the additives was applied on each surface of a biaxially stretched polypropylene film comprising solely the composition for the base layer (A) in Examvle 1 to form a coating layer of 1.5 g/m2 so as to obtain a two surface-heat-sealable packaging material. Using the thus obtained packaging material, packages of square boxes were effected as in Example 1 by means of an automatic packaging machine.
Comparative Example 2 A mixture of 90 parts of (P-l), 10 parts of (L-3) and 0.5 part of (A-l) was melt extruded to obtain an un-stretched film having a thickness of 1200 microns. This film was stretched 5.0 fold in the machine direction at 140C and then 8 fold in the transverse direction at 150C
to obtain a biaxially stretched film having a thickness of 30 microns, which was subjected to thermal setting at 160C Eor 10 secollds ancl then to ~ corona discharc3e treatment to make a wet tension of 40 dyne/cm. Using the thus treated film, packages of square boxes were effected as in Example 1 by means of an automatic packaging machine.
Comparative Example 3 Using poly-layer dies having three manifolds, an unstretched film composed of three layers, i.e. the base layer (A) and the surface layers (B) laid on both surfaces of the base layer (A), was prepared by melt-extruding as in Example 1. The base layer (A) comprised 100 parts of (P-4), 0.5 part of (A-l) and 1.0 part of (A-2), and the surface layers (B) comprised 100 parts of (P-3), 0.04 part of (S-l), 0.02 part of (A-3), 0.1 part of (L-2) and 0.3 part of (A-l). The thus obtained film having a thickness of 960 microns was stretched 4 fold in the machine direction at 12gC and 8 fold in the transverse direction at 150C
and then subjected to heat treatment to obtain a biaxially stretched film having a thickness of about 30 microns.
The thickness of the surface layer was about 0.8 micron on one surface. Using the thus treated film, packages of square boxes were effected as in Example 1 by means of All automatic packaging machine.
Comparative Example 4 The preparation of a biaxially stretched composite film was effected in the same manner as in Comparative Example 3 except that (P-2) was employed in place of (P-3) as the polymer for the surface layer (A). Using the thus obtained film, packages of square boxes were effected as in Example 1 by means of an automatic packaging machine.
Comparative Example 5 The preparation of a biaxially stretched fil~
was effected in the same manner as in Example 1 except that .

S
a composition comprisinc~ 50 parts oE (P-l) and 50 ~arls o~
(P-5) was applied as the surface layer (~). UsincJ the thus obtained film, packages of square boxes were effected as in Example 1 by means of an automatic packaging machine.
Comparative Example 6 The preparation of a biaxially stretched film was effected in the same manner as in Example 1 except that a composition comprising 50 parts of (P-6) and 50 parts of (P-5) was applied as the surface layer (B). Using the thus obtained film, packages of square boxes were effected as in Example 1 by the means of an automatic packaging machine.
Comparative Example 7 In the same manner as in Example 1, an unstretched sheet made by laminating the surface layers (B) comprising 100 parts of ~P-7), 0.3 part of (A-l), 0.02 part of (A-3) - and 0.04 part of (S-l) on both surfaces of the base layer lA) according to Example 1 was stretched. However, the sheet adhered to the longitudinal stretching roll and coiled thereon, and the surface layer stuck to the surface of the metal roll to cause peeling, and no laminated film could be produced at all. Therefore, the temperature of the longi-tudinal stretching roll was lowered to 60C, and stretching was effected 4.0 fold, followed by 8.0 fold transverse stretching at 155C, whereupon it was found that the surface layer resin adhered to the clip to reduce the clip holding force, scattering of resin was caused, and the breaking strength during transverse stretching was extremely poor.
The results were evaluated with the small quantity of film which could be produced with difficulty.

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4~5 ~ 5 can be understc)od Eram tho above res~ s, the package material of the invention has extremely good sealability without producing wrinkles.
For attaining a high sealing degree, a large heat-seal strength is desirable, but it is more import~nt to satisfy the following conditions: absence of wrinkles at the sealed portion; absence of the state of bad tucking;
that the surface softened by the hot plate melts in a flat state and slides under adhesion without causing peeling-off of the sealed portion; that a high lubricity is shownat heating without producing wrinkles and the state of bad tucking; and that the sealed portion must be bonded.
The results show that the high heat-seal strength is of course desirable, but that, to obtain a sealed package, a film free from wrinkles, bad tucking and pinholes by fusing of the film is more important.
The film obtained in Comparative Example 1 is excellent in workability during packaging and in appearance, but the degree of close-fitting packaging is extremely low. The use of this film is therefore limited to the packaging of products having a light weight and not requiring moisture-and insect-preventing conditions.
The film obtained in Comparative Example 2 is insufficient in automatic suppliability and can not be used in a packaging machine. By manual packing using this film, the sealed portion is excellent in gloss and appearance, wrinkles being hardly produced. But, the heat-seal strength is small and the seal-packaging degree is extremely low, so that the kinds of products to be packed are limited to a narrow range, as in Comparative Example 1.

~ li7~34~LS
~ .
The film obtain~d in Comparative Example 3 ha~ a high heat-seal st~ength, but the lubricity at high temperature is extremely low, resulting in a lot of wrinkles. In addition, close-fittng to the product to be packaged i9 insufficient thus resulting in a loosely packed state.
The grade of seal-packaging i~ also inferior because of wrinkles and peeling-off of the sealed portion due to the adhesion of the sealed part. Thus, practical use of this film is impossible.
In the film obtained in Comparative Example 4, the amount of wrinkles are som~what decreased, but the heat-sealing ability at low temperature is insufficient. At low temperature at which wrinkles are hardly produced, the seal strength is small, and a sealed package is not obtained. At heat-seal temperatures higher than 150C, at which the seal strength is improved, a lot of wrinkles are produced and the appearance is inferior, so that a sealed package can not be obtained. Thus, the degree of bad package is extremely large as in Comparative Examples 2 and 3.
In the film obtained in Comparative Example 5, the miscibility of polypropylene with polybutene-l is small, and the transparency and the heat-seal strength are de-teriorated. The lubricity at high temperature is also insufficient, so that a sealed package can not be obtained, and the rate of bad packaging is large. Thus, the film can not be employed in practical use.
The results of Comparative Example 6 are similar to those of Comparative Example 5, with only a slight improvement in transparency.

,,".~

.

11'îJ~15 The film obtained in Comparatlve Example 7 has extremely inferior transparency and gloss due mainly to the surface damages caused by adhesion to the longitudinal stretching roll and scratching and to the hollows inside the film caused by the stretching at low temperature only.
Moreover, the film has a high friction coefficient at hi~her temperatures, many wrinkles are formed at the heat-sealed portion and the film has a high friction coefficient at room temperature. It is apt to show stick-slip and has poor automatic suppliability. Though it may be excellent in sealability at high temperature, it is not applicable to automatic packaging machines. Though it may be applicable to heat-sealing when still, it shows the same results as the case of insufficient heat-sealing with an automatic packaging machine. In this process only packaging materials having an inferior degree of close-fitting packaging are obtainable.
For the purpose of giving automatic suppliability to the films of Comparative Examples 2 to 7, (A-2) was incorporated into the composition of the surface layer (B) in an amount of 0.8 ~ to the total weight of the composition in the preparation of each film, and the properties of the thus obtained films are determined. The results (only those in which notable changes are observed in comparison with the results of Table 1) are shown in Table 2.

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O ~0 u a E~ _ .___ --3~--i 117~15 Example ~
The same composition as in Example 1 was employed as the base layer (A). The polymers (P-8) and (P-9) were admixed in varied mixing proportions to obtain a polymer mixture for the surface layer ~). To 100 parts of the resultant polymer mixture, 0.05 part of (S-2), 0.5 part of (A-l), 0.3 part of (A-4) and 0.2 part of ~L-2) were added.
Using these compositions, a composite film was prepared in the same manner as in Example 1. The results are shown in Table 3.

117~34~5 . . __ _ . __-- , ,, .

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Packaging tests were carried out with the films as in Example 2 [(P-8)/(P-9) = 50/50] but changing the thickness of the surface layer, the whole thickness of the composite film being 30 microns. The packaging type was cigarette hold type. The results are shown in Table 4.

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117~4:15 Example 4 __ The preparation of a composite film was carried out using the same composition for the surface layer (B) as in Example 1 but with a varied amount (L-l) mixed in the base layer (A) of Example 1. Examinations were specially carried out on automatic suppliability, grade of seal packaging, close fitting packaging and degree of bad packaging due to poor formation of the tucked portion by the automatic packaging machine. The results are shown in Table 5.
Table 5 L-1(%) 0 2 10 20 30 l50 Propertie \

Automatic suppli-ability o - A o o o x Grade of seal packaging (150C) A A A A C C

Degree of close B A A A B C
fitting package . . .

Bad packaging 2.0 0.2 0.1 0.1 15.5 25 20 Intrinsic resist- 12 8 10 3 10. 9.8 10.2 10.
ance of surface 10 10 10 10 10 10 (Q.cm) When the amount of (L-l) is 0 %, the automatic suppliability is decreased because the antistatic property is somewhat reduced and the film adheres to the guide plate or winds round the automatic cutter. When the content of (L-l) is high, heat generation is caused at the automatic cutter when cutting of the f.ilm takes place for a long time, and the resin softened by heat accumulates on to the cutting edge to cause insufficient cutting or pollution of the cut portion. As to the grade of seal-packaging, thermal contraction of the film is apt to occur when the content of .

(L-l) is high, and the extent oE cooling o thc sealed portion i9 varied in each part, so that the .ilm is not solidified in a flat form. Therefore, the heat-sealed portion becomes uneven, and the sealing degree is reduced because of the presence of gaps due to the unevenne99.
The degree of close-fitting packaging indicates whether the product to be packaged is tightly packaged. By incorporating 2 to 20 % of (L-l), an adequate power of thermal contraction is produced at the heat sealed portion to obtain a beautiful heat sealed surface. In the heat sealed surface and its environs, a temperature gradient is formed under heating and becomes more notable in a remoter part away from the seal surface, so that an adequate momentary contraction gives a good tension. When the content of (L-l) becomes larger, wave-like slackening of the film is caused - from the sealed portion, and tight packaging is not attained.
Bad package is sometimes caused by insufficient guiding of the film due to static electricity. This is observed particularly when (L-l) is not added. When the 2Q (L-l) content is high, the bendability is improved, but insufficiency of lubricity, probably due to some bleeding-out by the stretching and thermal setting, and deformation of the film due to thermal contraction, becomes notable to pro~uce a bad appearance and to cause unevenness of the sealed portion, so that commercial value is reduced.
Example 5 To 100 parts of a polymer mixture comprising 90 % of (P-l) and 10 % of (L-3), 1.0 part of (A-5) was added to make a composition for the base layer (A). On the other hand, to 100 parts of a mixed resin of (P-ll) and (P-9) in a weight ratio of 1 : 1, 0.01 part of (S-3) and 0.1 part of `` ~178415 (A-6) were incorpor~ted to make a composition for the layer (B). These compositions were co-extruded, the layer (B) being piled on one surface of the layer (A), and the extruded product was stretched 4.5 fold in the machine direction at 130C and 8.5 fold in the transverse direction at 158C.
The obtained film was a biaxially stretched composite film composed of the two layers (A)/(B) and having a thickness of 20 microns. This film was heat-set at 160C and subjected to corona discharge treatment on the surface of the layer (A) to obtain a wet tension of 42 dyne/
cm.
Using the thus obtained composite film, a package of slit-form dried layer was formed in an automatic packaging machine so that the sealability could be examined.
The results are shown in Table 6.
Example 6 Using the same composition for layer (A) as in Example 5, an unstretched film was produced, which was stretched 4.5 fold in the machine direction at 135C. The composition of the surface layer (B) of Example 5 was melt-extruded and laminated on the surface layer of the above uniaxi-ally stretched film to make a composite film, which was stretched in the transverse direction 8.5 fold at 150C. The resulting film comprised the two layers (A) and (B) with biaxial stretching on one surface and uniaxial stretching on the other side, and had a thickness of 20 microns. The film was treated in the same manner as in Example 5, and subjected to the packaging test under the same conditions as in Example 5. The results are shown ~0 in Table 6.

., '115 For compa~ison, the films obtained in the Eollow-ing Comparative Examples were subjected to the same sealability test. The results are also shown in Table 6.
Comparative Example 8 The composition for the base layer (A) was the same as in Example 5. After stretching the base layer ~A) in the machine direction, a surface layer (B) was laminated thereon comprising only the polymer (P-ll) and otherwise the same compositions. The laminated film was transversely stretched and heat-set in the same manner as in Example 5, and corona discharge treatment was applied to the base layer (A) side.
Comparative Example 9 Using the same polymer compositions as those of Comparative Example 8 for both the base layer (A) and the surface layer (B), a biaxially stretched film was obtained in the same manner as in Example S.
Comparative Example 10 The base layer (A), having the same composition as in Example 5, was stretched in the machine direction, on one surface of the resulting film, a surface layer ~B) was laminated comprising only the polymer (P-12) and otherwise ~he same composition as in Example 5. The resulting film was stretched by the same procedure as in Comparative Example 8.

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-When the package thus obtain~d wa9 allow~d to ~tand at qOC in a r~lative humldity of 80 ~ for one month, the layer was moistened when the seal-packaging degree was low, and the lava adhered to the inside of the packaging material to cause a wetting phenomenon, whereby black spots could be seen in the film, and the taste of the laver was deteriorated to reduce the commercial value. In this test, the best results were obtained with the film of Example 5. With the film of Comparative Example 8, packaging at 120 to 135C
afforded bad results, and a sealed package could be obtained only at a high temperature.
The films of Comparative Example 9 show extremely poor sealability at low temperature. The film having the surface layer (B) consisting only of the (P-ll) copolymer shows further inferior results to Comparative Example 8, in consequence of the biaxial stretching. When it is used for packaging at a low temperature, fixing of the seal portion could not be made due to the poor heat seal properties, so that the package formed wrinkles. Naturally, this film showed extremely poor seal packaging.
The film of Comparative Example 10 formed sealed packages. However, the appropriate conditions were limited to a narrow range, and heat-sealing at high temperature was necessitated. Further, the haze value was insufficient, and the apparent color of the laver was changed, only the sealed portion being transparent. Thus, the package obtained had a reduced commercial value.
- Example 7 and Comparative Example 11 The base layer (A) and the surface layer (B) were formed of the polymers as shown in Table 7. In the test, the conditions of extrusion, film-making and stretching . --11 784~5 in Examples 7-1, 7-2, 11-1, 11-2, 11-3, 11~ 5, 11-6 and 11-7 were set respectively as in Examples 1, 6, Compara-tive Examples 3, 4, 5, 7, 6, 6 and 8.

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The relatiollship between "heat-seal temperature at which the heat-seal strength is grea-ter than 100 g/cm"
and "friction coefficient at high temperature" is shown in the accompanying Fig. 1, from which the following character-istics can be observed:
1) Relationship between the heat sealability at low temperature and friction coefficient at high temperature:-A material which is heat sealable at low temper-ature means that is susceptible to softening and melting at a temperature near the temperature at which is heat-sealed. Accordingly, it is apparent from the category on cohesion that generally the friction coefficient of the film becomes high in the range of temperature near the temperature at which the heat-sealing is being effected.
However, as the automatic packaging materials, as explained in the preceding paragraphs, the friction co-efficient at high temperature has a remarkable effect upon the formation of wrinkles at the heat-sealed portion so that a low friction coefficient at high temperature is strongly desired.
On the other hand, in automatic packaging which requires operation at high speed and perfect heat-sealing, heat-sealability at low temperature is the essential factor.
In Fig. 1, a direction (C~Good) is represented, denoting that the polymer compositions of the invention are in extremely good positions. The comparison between Example 7-1 comprising the copolymers only and Example 7-2 shows that the blend and the copolymer have different chemical composi-tions and are different in thermal behavior in respect to friction.

2) Ha~e value and gloss:-Comparative Examples 11-3 and 11-5 and 11-6 are in fairly good regions; but they are inferior in friction co-efficient at high temperature, haze and gloss and are remarkably poor in appearance characteristics as the packag-ing materials. These are the optical behaviors produced from the factors of compatibility between the blend polymers and crystallinity.
3) Results:-The evaluations made on the films comprising only the polymers from which the additives are excluded are that the friction coefficient at high temperature, the haze and the gloss are determined by the essential properties of the polymer. Though the heat seal property is apt to be reduced by the additives, selection of the polymer which essentially has a good sealability is the basic requirement.
As observed, the characteristics which form thekey points in automatic packaging are good appearance characteristics, low friction coefficient at high temper-ature (small adhesion or wrinkles) and the applicabilityof thermal bonding at low temperatures.
Example 8 As the base layer (A~, 0.3 part of (A-l) was added to 100 parts of (P-13). As the surface layer (B), 10 parts of (L-4) were added to 100 parts of a mixture of 30 parts of (P-14) and 70 parts of (P-9). Under the same conditions as in Example 1, these compositions were subjected to extrusion, film-making, stretching and heat setting, followed by the corona discharge treatment. As shown in the accompanying Fig. 2, with various modifications of the degree of corona discharge treatment, the heat-seal properties of the film ,~

8~5 were evaluated (i.~. Curve 1). Tha results indicate that the heat-seal strength showed little loss. For comparison, the film of Comparative Example 11-1 was oriented, heat-set and provided with the corona discharge treatment, the result of which is shown as Curve 2 in Fig. 2. Usually, the film showed reduction of the heat-seal property when subjected to corona discharge treatment. The gloss of the film was 139 ~ in Comparative Example 11-1 not incorporated with (L-4) but 145 % in Example 8.
Example 9 Except that the compositions forming the base layer (A) and the surface layer (B) in Example 1 were limited to polymers, i.e. (P-l) and (P-2) + (P-3) only, and the mixing proportions between (P-2) and (P-3) were varied, films were prepared under the same conditions as in Example 1. The relationships between the (P-2)/(P-3) mixing ratios of the films and the heat seal strengths at 110C, 120C and 140C
are shown in the accompanying Fig. 3.
Comparative Example 12 Except that the compositions forming the base layer (A) and the surface layer (B) were limited to polymers, i.e. (P-l) and (P-l) + (P-5) only, and the mixing proportions for the surface layer, i.e. (P-l) and (P-5), were changed, films were prepared under the same conditions as in Comparative Example 5. The relationships between the respec-tive mixing ratios of the films and the heat-seal strengths are shown in the accompanying Fig. 4.
Example 10 Except that the mixing proportions between (P-8) and (P-9) in Example 2 were varied, films were obtained under the same conditions as in Example 2. The relationship between the mixin~ ratios and the haze values of the films are shown as Curve 1 in the accompanying Fig. 5.
Except that the polymers used were (P-l) and (P-5) and the mixing proportions between (P-l) and ~P-5) were varied, films were also prepared in the same manner as in Example 2. The relationships between the mixing ratios and the haze values of the films are shown as Curve 2 in Fig. 5.
Excepting that the polymers used were (P-2) and (P-5) and the mixing proportions between (P-2) and (P-5) were varied, films were also prepared in the same manner as in Example 2. The relationships between the mixing ratios and the haze values of the films are shown as Curve 3 in Fig. 5.
Example 11 Except that the mixing proportions between (P-8) and (P-9) were varied, films were prepared in the same manner as in Example 2. The relationships between the mixing ratios and the grade of seal packaging are shown as Curve 1 in the accompanying Fig. 6.
Further, except that the polymers used were limited to (P-l) and (P-5) and the mixing proportions between (P-l) and (P-5) were varied, films were prepared in the same manner as in Example 2. The relationships similar to the above are shown as Curve 2 in Fig. 6.
F~ample 12 With the mixing proportion between (P-8) and (P-9) set at 50/50 and the mixing amounts of (S-2) varied, films were prepared in the same manner as in Example 2. The relationships between the mixing amounts of (S-2) and the grade of seal packaging were inspected, and the results . .

~:~7~ 5 are in the accompanying Fig. 7. ~150, their relationships with the haze vallle were inspected. The results are shown as Curve 1 in Fig. 8. Furthermore, their relationships with the friction coefficient at high temperature were inspected. The results are shown as Curve 2 in Fig. 8.
Still further, their relationships with the heat-seal strengths were also inspected. The results are shown as Curve 3 in Fig. 8.

.. . . . . . _

Claims (18)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A packaging material comprising (A) a stretched base layer formed of a propylene polymer, and (B) a stretched surface layer formed of a blend of a propylane-ethylene copolymer and a C4-C10 .alpha.-olefin-propylene copolymer in a weight proportion of 20 : 80 to 94 : 6 by weight on at least one surface of said base layer; wherein the propylene-ethylene copolymer comprises units of propylene and units of ethylene in a weight proportion of 99.5 : 0.5 to 90 : 10; and wherein the C4-C10 .alpha.-olefin-propylene copolymer comprises units of C4-C10 .alpha.-olefin and units of pro-pylene in a weight proportion of 30 : 70 to 5 : 95.

2. The packaging material according to claim 1, wherein the base layer (A) further comprises a low molecular weight thermoplastic resin in an amount of 80 to 98 parts by weight to 100 parts by weight of the combined amount of the propylene polymer and the low molecular weight thermo-plastic resin.

3. The packaging material according to claim 2, wherein the low molecular weight thermoplastic resin is selected from the group consisting of hydrocarbon resins, rosins, dammars and phenol resins, and their derivatives and modified substances.

4. The packaging material according to claim 1, wherein the base layer (A) further comprises at least one of the following: an antistatic agent, a lubricant and an anti-blocking agent.

5. The packaging material according to claim 4, wherein at least one of the surface layers (B) further comprises at least one of the following: an antistatic agent, a lubricant and an anti-blocking agent.

6. The packaging material according to claim 4, wherein at least one of the surfaces is subjected to an electric discharge treatment and has a wetting tension of 30.5 to 58 dyne/cm.

7. The packaging material according to claim 5, wherein at least one of the surfaces is subjected to electric discharge treatment and has a wetting tension of 30.5 to 58 dyne/cm.

8. The packaging material according to claim 4, wherein the antistatic agent is incorporated in an amount of 0.5 to 3 parts by weight and the lubricant and anti-blocking agent in a combined amount of 0.1 to 3 parts by weight to 100 parts by weight of the propylene polymer.

9. The packaging material according to claim 5, wherein the antistatic agent is incorporated in an amount of 0.5 to 3 parts by weight and the lubricant and anti-blocking agent in a combined amount of 0.1 to 3 parts by weight to 100 parts by weight of the propylene polymer.

10. The packaging material according to any of claims 1 to 3, wherein at least one of the surface layers (B) further comprises a silicone oil in an amount of 0.01 to 0.15 part by weight to 100 parts by weight of the blend.

11. The packaging material according to any of claims 1 to 3, wherein the propylene-ethylene copolymer com-prises units of propylene and units of ethylene in a weight proportion of 96.4 : 3.6 to 90 : 10.

12. The packaging material according to any of claims 1 to 3, wherein the a-olefin is the C4-C10 .alpha.-olefin-propylene copolymer is at least one of butene-1, pentene and hexene.

13. The packaging material according to any of claims 1 to 3, wherein at least one of the surface layers (B) has a thickness of 0.2 to 3 microns.

14. The packaging material according to any of claims 1 to 3, wherein at least one of the surface layers (B) has a thickness of 0.7 to 10 microns.

15. The packaging material according to any of claims 1 to 3, wherein at least one of the surface layers (B) further comprises a low molecular weight thermoplastic resin in an amount of 25 to 3 parts by weight to 100 parts by weight of the combined amount of the blend and the low molecular weight thermoplastic resin.

16. The packaging material according to claim 1, wherein the base layer (A) comprises the propylene polymer and the low molecular weight thermoplastic resin in a weight proportion of 80 : 20 to 98 : 2 and at least one of the surface layers (B) comprises the blend and the low molecular weight thermoplastic resin in a weight proportion of 75 :
25 to 97 : 3.

17. The packaging material according to claim 16, wherein the base layer (A) further comprises 0.5 to 3 parts by weight of an antistatic agent and 0.1 to 3 parts by weight of at least one of the following: a lubricant and an anti-blocking agent, per 100 parts by weight of the combined amount of the propylene polymer and the low molecular weight thermoplastic resin, and at least one of the surface layers (B) further comprises 0.5 to 3 parts by weight of an anti-static agent and 0.1 to 3 parts by weight of at least one of the following: a lubricant and an anti-blocking agent, per 100 parts by weight of the combined amount of the blend and the low molecular weight thermoplastic resin.

18. The packaging material according to claim 17, wherein at least one of the surfaces is subjected to an electric discharge treatment and has a wetting tension of 30.5 to 58 dyne/cm.