EP1191143A1

EP1191143A1 - Process for manufacture of wear resistant paper useful in forming decorative laminates

Info

Publication number: EP1191143A1
Application number: EP00203070A
Authority: EP
Inventors: Mahendra Mehta; Larry Hill; Stephen Canary; Mark Johnson
Original assignee: Mead Corp
Current assignee: Mead Corp
Priority date: 2000-09-05
Filing date: 2000-09-05
Publication date: 2002-03-27

Abstract

A continuous process for treating abrasion resistant particles with a coupling agent and producing a wear resistant paper for use in forming a decorative laminate which comprises the steps of: forming a web of cellulosic fibers on a papermachine (24), admixing an aqueous dispersion of a stabilized organofunctional coupling agent with the abrasion resistant particles and forming a slurry (20) of particles which have reacted with the coupling agent; and applying the slurry to the web on the papermachine (24).

Description

The present invention relates to a process for making high abrasion papers such as overlays or décor sheets useful in decorative laminates wherein the abrasion resistant particles are treated with a coupling agent and applied to the paper in a continuous process.
Decorative laminates are conventionally produced by stacking and curing under heat and pressure a plurality of layers of paper impregnated with a synthetic thermosetting resin. In normal practice the assembly from the bottom up, consists of three to eight core sheets made from phenolic resin impregnated kraft paper, above which lies a pattern or décor sheet impregnated with melamine resin; on top of the décor sheet is provided an overlay sheet which, in the laminate, is almost transparent and provides protection for the pattern sheet.
The overlay sheet is almost invariably used when the décor or pattern sheet has a surface printing in order to protect the printing from abrasive wear. The overlay sheet is usually a high quality alpha cellulose paper of about 20 to 30 pounds ream weight that is also impregnated with melamine-formaldehyde resin in a manner similar to that used for the décor sheet, except that a greater amount of resin per unit weight of paper is used. The individual sheets are stacked in the manner indicated above.
It is well known that the addition small, hard abrasion resistant particles (also referred to as "grit") to the overlay paper, or to resin mixtures which coat the impregnated décor sheet, can enhance the abrasion resistance of high-pressure laminates. Alumina has been used to give wear resistance of 400 to 600 cycles. To improve the clarity and appearance of the overlay paper the abrasion resistant particles have been treated with a coupling agent. Although melamine-formaldehyde cured resin has a refractive index similar to aluminium oxide (1.55 vs 1.57), poor contact between the plastic matrix and grit leads to voids producing light scattering and poor transparency (or clarity) of overlay paper. Good clarity is important because overlay papers containing grit are commonly laminated against printed paper for flooring and similar products. When clarity is poor, the printed surface appearance deteriorates, grit reflects light and the colour of the print is dulled (Khurana, Mehta and Scott, 1995 European Plastic Laminates Forum, pp. 31-43). US 5 456 949, US 5 362 557 and US 5 288 540 assigned to Formica Corporation and US 4 482 656 assigned to Battelle Development Corporation, disclose treating metal oxides with coupling agents such as organofunctional silanes, aluminates, zirconates, titanates or zircoaluminates. Improved transparency is obtained by chemically altering the wettability of the oxide, so as to effect good bonding of the mineral surface to the plastic matrix, thereby preventing voids.
Previously, abrasion resistant particles have been treated with the coupling agent in a separate process not coupled to the papermaking process and this has added significantly to the cost of the laminates. These processes typically involve spraying an aqueous mixture of an organofunctional silane on dry grit in a rotating and inclined drum and using forced air to drive off volatile alcohols which are captured by condensation. Aluminium oxide grit is continuously fed into the drum and dry, treated grit is packaged following exit from the drum. Other commercial processes such as slurrying grit with the an aqueous coupling agent followed by spray drying or filtering and drying are also described in the patent literature.
These methods for treating metal oxides with a coupling agent have several potential disadvantages depending on the process used. Multiple steps such as spraying, heating, packaging and shipping can introduce variability to the product. The treated metal oxides may contain environmentally undesirable volatile organic compounds such as methanol or ethanol. Storage in humid environments can cause reverse hydrolysis which can cleave the coupling agent from the particle. Because the grits are treated at one location and shipped to another, it is necessary to dry the grit and drying can oxidize organofunctional amines and sulfides. High energy and cost are also associated with processing such as drying and shipping.
Conventional processes also limit the treatment of water sensitive or water absorbing materials. Time, temperature and concentration requirements of conventional processes are not cost-effective. In some cases, it may be necessary to retreat the grit and retreatment may not be easily accomplished due to process/cost and variability. The capital and process for treatment in tumbling drums may vary greatly depending on the type of material, such as wet fiber, filler, mineral or pigment that may be treated. Many organofunctional coupling agents may also require the use of non-aqueous solvents or wetting agents to get good reactivity with particles. Aqueous solutions of silane coupling agent triesters are inherently unstable, requiring a slow prehydrolysis to activate and are subject to self-polymerization if left unused for one or more hours. Self-polymerization consumes the active coupling agent. The tendency of silane triesters to self-polymerize also introduces variability in the treated product.
The present invention and its preferred embodiments seek to overcome or at least mitigate the problems of the prior art.
One aspect of the present invention provides a continuous process for treating abrasion resistant particles with a coupling agent and producing a wear resistant paper for use in forming a decorative laminate which comprises the steps of:
forming a web of cellulosic fibers on a papermachine;
admixing an aqueous dispersion of a stabilized organofunctional coupling agent with the abrasion resistant particles and forming a slurry of particles which have reacted with the coupling agent; and
applying said slurry to said web on said papermachine.
Preferably the step of admixing may include forming a spray of the dispersion of the coupling agent and introducing the particles into said spray. More preferably, the step of admixing may include flooding the particles with water after introducing said mineral particles into the spray.
According to an optional feature of this aspect of the present invention said particles may carry about 0.01 to 1% coupling agent based on the dry weight of the particles. Preferably said particles may have a particle size of about 10 to 100 microns. More preferably said particles may be aluminium oxide.
According to a second optional feature of the present invention said particles may be heated to a temperature of about 50 to 200°C and the particles may be mixed with the aqueous dispersion of the coupling agent while hot.
According to a further optional feature of this aspect of the invention the coupling agent may be a silanol. Preferably the coupling agent may be an aminofunctional silanol.
More preferably the aqueous dispersion of the aminofunctional silanol may include formaldehyde or a reactive aldehyde.
According to another optional feature of the present invention the web may have a basis weight of about 5 to 70 pounds per 1000 sq. ft. Preferably the web may be an overlay paper.
According to a further optional feature of the present invention the spray may be in the form of a fan.
According to a yet further optional feature of the present invention the coupling agent may be prehydrolyzed such that the coupling agent can react directly with the particles. According to another optional feature of this aspect of the present invention the web may be a décor sheet
One advantage is that this aspect of the invention provides a continuous process that reacts a coupling agent with an abrasion resistant particle such as a metal oxide grit (or other mineral particle) in conjunction with the manufacture of wear resistant paper containing the treated abrasive. It has been found that certain coupling agents are capable of reacting with the abrasion resistant particle rapidly such that the particles can be treated with the coupling agent and applied to a paper web on the papermachine in a continuous process which is both cost effective and environmentally sound. In accordance with a preferred embodiment of the invention, a stabilized and pre-hydrolyzed organofunctional silane coupling agent (i.e. a silanol) is used to continuously treat grit at room temperature without serious environmental or operational hazards and at a speed consistent with continuous manufacture or wear resistant paper on a commercial scale.
Exemplary embodiments of the invention will now be described by way of example only, with reference to the accompanying drawing in which:
FIGURE 1 is a diagram of one example of a method in accordance with one aspect of the invention.
The grit employed in the present invention can be a mineral particle such as silica, alumina, alundun, corundum, emery, spinel, as well as other materials such as tungsten carbide, zirconium boride, titanium nitrade, tantalum carbide, beryllium carbide, silicon carbide, aluminium boride, boron carbide, diamond dust; or a nonmineral particle such as clay, and mixtures thereof. The suitability of the particular grit will depend on several factors such as availability, cost, particle size availability and even the colour of the particles. Considering cost availability, hardness, particle size availability and lack of color, aluminium oxide is the preferred grit for most applications. End use performance dictates the basis weight, ash loading, size and type of grit particles. The grit preferably has an average particle size (diameter) of about 10 to 100 microns and a particle size distribution of about 10 to 150 microns. Particularly useful in the present invention are 150, 180, 220 and 240 grade aluminium oxide grits.
In accordance with one aspect of the invention, the abrasion resistant particles are treated with a coupling agent. In order to be useful in the process of the present invention, which is characterized in that it is a continuous process in which the treatment of the particles with the coupling agent is integrated or directly coupled with application of the slurry of the treated grit to a web on a papermachine without drying, dewatering or otherwise recovering the treated grit, the coupling agent must be capable of reacting with the particles rapidly such that the coupling reaction rate is fast enough that a slurry of reacted particles can be prepared and delivered to the surface of the web in amounts that are consistent with its application rate to the paper which is typically about 17 pounds per minute. The term "continuous" as used herein means that the particles are not routinely pretreated with the coupling agent and warehoused, but the method of the present invention is not avoided by the occasional storage of treated particle slurries that may be necessitated by web breaks, maintenance and other temporary suspensions of the manufacturing process or by the use of a holding tank which provides processing flexibility such that the treatment of the grit or its application to the web can be temporarily interrupted without necessitating a complete suspension of the process.
The coupling agent, as supplied by the manufacturer, is capable of reacting directly with the grit particles. In the case of a silane coupling agent this means that the coupling agent preferably includes a silanol moieties that can react with the surface of the particles.
Representative examples of useful silanols that are particularly useful in the invention are described in European Patent 0 675 128 A1 which is incorporated herein by reference. This class of coupling agent is pre-hydrolyzed, stripped of volatile organics, and stabilized against self-polymerization by reaction with a hydroxyl containing compound or a water soluble silane such as an aminofunctional silane or a phosphatosilane. Prehydrolyzed and stabilized silane coupling agents are commercially available. For example, the aminosilane, HYDROSIL 2627 (available from Huls America), the diaminosilane, HYDROSIL 2776 (also available from Huls America) and the aminosilane Silquest A-1106 from OSI are useful in the invention.
A comparison of laminates prepared using a commercially treated aluminium oxide and an aluminium oxide treated with HYDROSIL 2627 in accordance with this invention was made using diffuse reflectance Fourier transform infra-red spectroscopy (DRIFT). This comparison showed that there was a direct relationship between absorbance at 2928 cm-1 (assigned to C-H of the methylene groups) and resulting clarity of the laminates in both cases and indicated that the continuous method of the invention is capable of delivering clarity which is at least equivalent to that achieved using the more costly and less efficient conventional methods.
In addition to the aforementioned silanols, other coupling agents such as organofunctional aluminates, zirconates, titanates, zircoaluminates and isocyanates can be used in the present invention provided that they are selected or modified such that they can react directly with the abrasion resistant particle as part of the continuous process described herein. The reactive characteristics of the particle used in the invention will vary depending upon the nature of the particle. A coupling agent can be modified to include moieties which are reactive with the surface of the particle. In the case of the silanols, the silyl hydroxy group is reactive with metal oxides.
The coupling agent includes an organofunctional group which interacts with the laminating resin to reduce the interface which is the source of the reflection which reduces clarity. The organofunctional groups used in coupling agents are well known in the art. For use with melamine type laminating resins, amino functional groups such as alkylamino and alkylaminoalkyl groups are particularly preferred.
Coupling agents such as silanols are often capable of self-polymerization at room temperature, which renders them unstable on storage. For this reason, they are stabilized against self-polymerization by techniques that are known such as salt formation using bulky counterions which sterically prevent polymerisation and other techniques. The term "stabilized" as used herein refers to the fact that the coupling agent does not undergo substantial self polymerisation on storage.
The grit is treated with sufficient coupling agent to impart the desired clarity to the laminate. This amount will vary depending on the nature of the grit and the laminating resin and is easily determined on a case by case basis. In the case of a silane, the grit typically carries about 0.01 to 1% silane.
The abrasion resistant particle or grit is preferably treated with the coupling agent by spraying a solution or dispersion of the coupling agent in water on the grit. The concentration of the solution of the aminofunctional silane or other coupling agent may vary from about 0.2% to 2% for optimal effectiveness. In one method according to the invention, the aminosilane solution is pumped through four 0.26 mil (approx 0.7 mm) flat fan spray nozzles to form a symmetrical fan spray about 12 inches in diameter located about 10 inches below a dry grit feeder outlet at the top of a funnel. The solution is sprayed at a rate of about 0.8 g agent/min/pound of grit. The grit is fed into the funnel at a rate of about 17 pounds/min which is consistent with the rate with which it is applied to the web although higher rates could be used. Those skilled in the art will recognize that other techniques can be used to treat the grit with the silane provided that they produce a grit that is uniformly treated with the silane. For example the grit can be immersed in the silane solution, but this method would require a large capital investment for the slurry tanks and is generally undesirable for use in a continuous process.
Several techniques have been found to enhance the grit treatment and may be useful in the overall process to provide the processing times necessary for use in the type of continuous process used in the paper industry. It is desirable to heat the grit to 50 to 200° C immediately prior to exposure to the silane spray. This process can double the amount of aminosilane applied to the grit thereby improving clarity. It is also desirable to add about 0.01 to 1% (e.g. 0.1%) by weight of the grit formaldehyde (37%) or another functional aldehyde to the silane feed. This also increases the efficiency of the silane coupling and improves clarity. Formaldehyde is consumed by aminofunctional groups in condensation reactions. Other functional aldehydes such as glyoxal or glutaraldehyde can also be used. Aldehydes can be added after silanization of the grit as well. These techniques can also be used with coupling agents other than silanes as well.
The wear resistant sheet may be an overlay sheet or a décor sheet. The sheet is formed from fibers conventionally used for such purpose and, preferably, is a bleached kraft pulp. The pulp may consist of hardwoods or softwoods or a mixture of hardwoods and softwoods which is normally preferred. Higher alpha cellulose such as cotton may be added to enhance certain characteristics such as post-formability. The basis weight of the overlay sheet may range from about 10 to 40 pounds per 1000 square feet, and preferably about 15 to 40 pounds per 1000 square feet. The sheet may be a décor sheet or an overlay sheet. Preferably the grit is applied to the sheet at a coat weight of about 2 to 40 pounds per 1000 square feet (dry basis).
The grit-containing slurry is applied to the raw fibrous cellulosic web using the secondary headbox or slot coater which distributes the grit-containing slurry evenly across the surface of the web. Currently a layer of grit and paper fibers is applied to the surface of the overlay sheet using a secondary headbox application on the papermaking machine. However for improved distribution and coverage of grit across the web as well as improved distribution of grit in the Z direction of the sheet, it may be desirable to apply the grit using a slot coater such as a curtain coater. The term "slot coater" as used herein is used in the same manner it is used in the art, namely, to designate a coater having a central cavity which opens on and feeds a slot through which the coating is forced under pressure.
The application of the grit may be positioned anywhere after the primary headbox 23 and before the dryers (not shown), but it is preferably located near and, more preferably, immediately after the dry line i.e. the point at which the deposited fibers begin to exhibit consolidation and there is no layer of surface water. Typically the slurry contains about 0.5 to 2% by weight grit when applied from a secondary headbox and 1 to 40% by weight when applied using a slot coater. The coating operation is conducted at conventional web speeds which can range from 400 to 2000 feet per minute.
The use of a slot coater and more particularly a curtain coater (as contrasted with a secondary headbox) to apply the grit, increases the efficiency of the grit application and reduces waste. The slot coating head applicator is used in conjunction with a positive displacement pump which enables a predetermined amount of the grit composition to be evenly distributed across the surface of the cellulosic sheet. A static mixer is preferably incorporated in the slot coater supply line to prevent or reduce the amount of grit settling out of the slurry.
A particular advantage of using a slot coating head applicator is that it enables the delivery of a predetermined amount of the slurry mixture to be applied in an evenly distributed manner to the surface of the overlay sheet. The use of the slot coating head applicator not only increases the efficiency of the operation by evenly distributing the grit slurry mixture across the décor sheet but it reduces the cost of the process significantly by reducing waste while still achieving required or desirable product standards. The use of the slot coating head applicator also enables the introduction of other materials and additives which are typically employed in such overlays to be incorporated directly into the grit slurry. For example, the incorporation of melamine resin in the grit mixture is possible and would allow the application of both resin and grit to the fibrous cellulosic sheet in a single step.
The slot coater can be used to apply slurries containing at least 5% and, more particularly, at least 10% solids. By comparison, a secondary headbox is generally not useful in applying slurries containing more than about 2% or 3% solids. As a result of the higher slurry concentrations that can be applied using a slot coater, higher line speeds and/or lower coating flow rates can be used than are feasible with application of the slurry from a secondary headbox. In particular, using the headbox, it is not unusual when coating a web 10 feet wide to apply the coating at a flow rate of 500-1000 gallons per minute. At these rates, water from the coating slurry cascades through the sheet and carries significant quantities of unretained grit with it. With the slot coater, flow rates on the order of 5-10 gallons per minute are commonly used when coating a web 10 feet wide and the quantities of water and unretained grit are substantially less.
When the grit slurry is applied to the web using a slot coater, it is advantageous to include a thickener in the slurry. Thickeners are particularly recommended when the grit is applied using a slot coater such as a curtain coater. The thickener may be any of the commonly used binders such as melamine resins, polyvinyl alcohol, acrylic latex, starch, casein, styrene-butadiene latex, carboxymethyl cellulose (CMC), microcrystalline cellulose, sodium alginate etc. or mixtures thereof which are used in coating compositions where the coating material is to be bonded to a substrate such as a décor sheet or overlay sheet. Melamine resins such as melamine-formaldehyde are advantageously used as the thickener material because the melamine-formaldehyde resin is also commonly used to saturate the décor sheet. The thickener is usually employed in an amount of about 1 to 10% by weight of coating solids and is added to the silane treated grit in a slurry tank after treatment with the silane.
A slot coater particularly useful in the present invention is a curtain coater sold by Liberty Tool Corp under the tradename Technikote. When using a curtain coater, the slurry can be pumped to a Liberty Tool slot coating head under a pressure of 20 psi and applied to a layer of pulp one foot wide which had been deposited on the papermachine wire. The slurry can be dispensed from the coating head at a flow rate of 0.73 galls per minute (approx 2.75 litres per minute) and a line speed of 67.5 lineal feet per minute (approx 21m per minute). The coating can be applied in a dry coat weight of about 10 pounds per 3000 sq ft.
Figure 1 illustrates a typical process in accordance with the invention. An abrasive particle such as aluminium oxide is fed from a hopper 10 using a screw feeder or equivalent conveyance device 12. The screw feeder 12 meters the rate with which the particles are supplied to fan spray 14. Fan spray 14 is preferably formed from four fan spray nozzles having a 0.26 mil (approximately 0.7mm) horizontally orientaed slot through which the dispersion of the coupling agent in water is fed. As the particles pass through the fan spray, they react with the coupling agent. In the event that the process of the invention is used with higher speed papermachines, e.g. papermachines operating at speeds greater than 1000 feet per minute, additional particle feeds and fan sprays may be employed to keep up with the rate with which the particles are consumed.
After passing through the fan spray 14, the particles are screened with magnetic bars at 18 to remove contaminants. After screening, the particles flow down a chute to a slurry tank 20 where they are drenched with additional water to provide a slurry having the solids content required for the coating operation. As noted above, the slurry concentration will vary depending upon the technique that is used to apply the slurry to the paper. In the illustrated embodiment, the slurry is pumped by pump 19 from tank 20 to a secondary head box 22 on a papermachine 24. The grit stored in a supply tank is transferred through a coating supply line to a positive displacement pump which pumps a predetermined amount of the slurry to the secondary headbox or the slot coating head. An agitating means such as a static mixer located between the pump and the headbox or coating head prevents the grit from settling out of the slurry. The slurry medium, without drying, is pumped to the secondary headbox or coating head. From the head box or curtain coater, the slurry falls on to the web of a laminating paper.
The invention will be illustrated in more detail by the following nonlimiting example.

Example 1

Huls America's aqueous stabilized and prehydrolyzed aminosilane Hydrosil 2627 (about 40% active solids) was pumped into a continuous water stream at 34 ml/min having a total flow rate (water plus silane) of 3400 ml/min. The aminosilane solution, now at about 1 % of initial concentration, was forced through four nozzles forming a symmetrical fan spray about 10 inches below a dry grit feeder outlet at the top of a funnel. Commercially available 220 fused aluminium oxide grit was screw fed into the aminosilane spray at a rate of 17 pounds per minute, falling a total of about 20 inches, first being sprayed with aminosilane, then flooded with water, providing a uniform silane/grit slurry and diluting a final concentration of about 1.5% solids. The entire process is done using all materials at ambient temperature.
The grit slurry was continuously pumped to a secondary headbox and on an 80 inch wide web moving at 1,000 feet per minute, producing an overlay grade paper at 28% ash. The final product was laminated felt side (grit side) up against a dark solid color to assist in evaluating overlay clarity. In some cases, clarity was evaluated wire side up but since the grit was facing down, clarity effects were less obvious. Clarity was evaluated subjectively on a 1-10 scale with 10 being excellent or comparable to commercially available grit treated by an external vendor by the conventional method and 1 being poor, equivalent to the worst untreated grit.
Felt side clarity results following treatment with Hydrosil 2627 on paper trials:

Grit Vendor Clarity

A 8

A 11

A 10

A, untreated 2

B 8.5

B 7.5

B* 12

B, untreated 4

C 8.5

D 9

D* 6.5

E* 10

E 13
It will be noted that there is a significant variability in the efficiency of treating 220 grit from different vendors as well as the vendor's own treatment effectiveness but treatment in accordance with the invention always improved the clarity of the laminates. Note that treatment of a previously treated grit yields further improvement (Example E**).
Having described the invention in detail, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims.

Claims

A continuous process for treating abrasion resistant particles with a coupling agent and producing a wear resistant paper for use in forming a decorative laminate which comprises the steps of:

forming a web of cellulosic fibers on a papermachine;

admixing an aqueous dispersion of a stabilized organofunctional coupling agent with the abrasion resistant particles and forming a slurry of particles which have reacted with the coupling agent; and

applying said slurry to said web on said papermachine.
The process of claim 1 wherein said step of admixing includes forming a spray of said dispersion of said coupling agent and introducing said particles into said spray.
The process of claim 2 wherein said step of admixing includes flooding said particles with water after introducing said mineral particles into said spray.
The process of any of claims 2 to 3 wherein said particles carry about 0.01 to 1% coupling agent based on the dry weight of the particles.
The process of claim 4 wherein said particles have a particle size of about 10 to 100 microns.
The process of claim 5 wherein said particles are aluminium oxide.
The process of any of claims 1 to 6 wherein said particles are heated to a temperature of about 50 to 200°C and the particles are mixed with said aqueous dispersion of said coupling agent while hot.
The process of any one of claims 1 to 7 wherein the coupling agent is a silanol.
The process of claim 8 wherein the coupling agent is an aminofunctional silanol.
The process of claim 9 wherein said aqueous dispersion of said aminofunctional silanol includes formaldehyde or a reactive aldehyde.
The process of any preceding claim wherein said web has a basis weight of about 5 to 70 pounds per 1000 sq. ft.
The process of claim 11 wherein said web is an overlay paper.
The process of any preceding claim wherein the spray is in the form of a fan.
The process of any preceding claim wherein the coupling agent is prehydrolyzed such that the coupling agent can react directly with the particles.
The process of claim 11 wherein the web is a décor sheet.