High pressure decorative laminates are surfacing materials which meet the requirements of the National Electrical Manufacturers Association (NEMA) and which have been in common usage for more than fifty years, finding utility as surfacing materials for bathroom and kitchen surfaces, table tops, work surfaces, cabinets, panelling, etc.

High pressure decorative laminates are usually made by laminating together a plurality of thermosetting resin impregnated paper sheets under heat and pressure, the lower or core sheets being impregnated with phenolic resin, i. e. phenol-formaldehyde resin, and the upper sheet or sheets being impregnated with melamine resin, i. e. melamine-formaldehyde resin. Sometimes there is only one upper sheet which is decorative and is referred to as the decor sheet, and sometimes a second upper sheet is provided which overlays the decor sheet, i. e. a substantially transparent protective overlay which is formed of alpha-cellulose paper impregnated with melamine resin.

Laminates which do not meet the highly stringent requirements of NEMA cannot be properly called high pressure decorative laminates. Among the NEMA standards is one for chemical resistance; standard high pressure decorative laminates have good chemical resistance. However, there are occasions when enhanced chemical resistance is needed in these laminates. One attempt to provide enhanced chemical resistance has been to provide a top layer made of thermoset polyester resin, but this requires special manufacturing equipment and produces laminate having a lower abrasion resistance. Another attempt has utilized an upper layer made of modified phenolic resin; but laminate so produced can be made only in dark colors. Another approach has been to use a special melamine resin which increases the overall costs. A

fourth option has involved the use of a top layer based on thermoset vinyl ester resin containing hollow thermoplastic microspheres, but this requires special manufacturing equipment and results in undesirably high costs.

In accordance with the method disclosed in USP 5,344,704, an abrasion-resistant, thick aesthetic surface layer laminate is produced by providing an abrasion-resistant protective coating on top of the uppermost layer of the high pressure decorative laminate, such protective coating including pre-cured resin particles having the same or substantially the same index of refraction as the laminating resin in which they are dispersed after such laminating resin has been cured. The pre-cured particles are desirably formed of melamine resin, these being dispersed in the liquid melamine laminating resin for the upper layer of the laminate.

By this process, there is provided a high pressure decorative laminate of excellent physical properties having a protective layer which approximates the thickness of an overlay. The thick transparent protective overlayer coating preferably contains fine particles of an abrasion resistant mineral to enhance abrasion resistance, and an initial binder material, preferably microcrystalline cellulose.

An object of the invention is to provide a high- pressure decorative laminate having enhanced chemical resistance, while overcoming the deficiencies of the prior art decorative laminates having enhanced chemical resistance, such as those deficiencies indicated above.

The above and other objects of the present invention are achieved by proceeding according to a variant of the process of the aforementioned USP 5,344,704. In particular, a highly chemical resistant laminate surface layer is achieved by providing a thick layer coating of melamine laminating resin containing a high concentration of pre-cured melamine particulates on the upper surface of the uppermost sheet of the laminate-to-be. The resultant resin layer, after curing during the laminate manufacturing process, is very clear and provides a substantially unobstructed view of the underlying decor sheet as in USP'704. It also provides good gouge and

scratch resistance, and can be formed with a glossy surface consistent with USP 5,545,476, without causing scratching of the highly polished caul plates which are used during the pressing/lamination of glossy surface laminate.

The above and other objects and the nature and advantages of the present invention will be more apparent from the following detailed description of various embodiments thereof.

As in the aforementioned USP 5,344,704, and also USP 5,545,476, the contents of both of which are hereby incorporated by reference, a key feature of the present invention is the utilization of pre-cured thermoset resin particles formed of resin, in this case most preferably melamine resin, corresponding in refractive index with the liquid melamine laminating resin used in the laminating process after its cure. What is meant by the term"pre-cured" is that the cure or set of the melamine resin particles has been advanced either to the maximum degree possible or at least to a state of cure where the melt viscosity of the pre- cured melamine resin particles is sufficiently high to prevent the particles from dissolving in the liquid laminating resin and/or melting and flowing under the usual laminating conditions.

Unlike the embodiments disclosed in USP'704, no significant or meaningful quantity of mineral grit is included in the coating and impregnating composition, and the concentration of cured melamine resin particles suspended in the liquid laminating resin is much greater, e. g. a suitable weight ratio of cured resin particles to resin solids of the liquid impregnating and laminating resin is 1: 1, and preferably lies within the range of 0.5-1.4: 1, more preferably 0.7-1.2: 1. Also, it is unnecessary to use any initial binder material, e. g. microcrystalline cellulose, sodium alginate, carboxy methyl cellulose, cellulose ether, etc., although the incorporation of a small amount of fumed silica to help maintain the pre-cured resin particles in suspension in the liquid laminating resin is desirable.

Except for the uppermost layer of the laminate, the sheets which are used to form the laminate are made in the usual way. The core sheets are preferably impregnated with phenolic resin and dried. The decor sheet is impregnated with liquid laminating resin, preferably melamine resin, and then dried. The uppermost sheet, whether it is the decor sheet or an overlay, is made according to the process of the aforementioned USP'704, except that the liquid laminating resin, used to impregnate the uppermost layer contains a substantially greater concentration of pre-cured resin particles, than is provided in any of the embodiments disclosed in USP'704, and also there is no need to include any initial binder material or abrasive particles. Other conventional components including curing catalyst for the liquid resin, foam suppressors, surfactants and waxes, as known to those skilled in the art, can be included if necessary or desirable.

After impregnation, it being understood that the impregnation step leaves a thick coating of pre-cured melamine resin particles on the surface of the uppermost sheet with a film of liquid melamine laminating resin between and among the pre-cured melamine resin particles, the sheet is then dried leaving a two-phase coating on its surface, namely the pre- cured melamine particles in a matrix of dried melamine solids at a lesser stage of cure. Thus, the dried two-phase coating, which ultimately becomes the thick, transparent, chemically resistant protective overlayer during lamination, comprises predominantly the pre-cured melamine resin particles dispersed in the melamine impregnating resin, and optionally a small amount of fumed silica initially used to adjust viscosity and improve the suspension characteristics of the melamine particulates in the uncured melamine solution.

Except for the transparent, chemically resistant protective layer overlaying the upper sheet of the laminate, the laminate of the present invention is suitably made according to standard practice and suitably has a conventional lay-up, e. g. two to eight dried core sheets of phenolic resin impregnated Kraft paper with a dried melamine resin impregnated decor sheet thereover, plus an optional dried

melamine resin impregnated alpha-cellulose overlay, plus the novel dried two-phase overlayer of the present invention over the top resin impregnated paper layer.

The final laminate is made in the typical way such as by pressing the lay-up in a suitable press between pressing plate dies, otherwise known as caul plates; and subjecting the assembly to sufficient heat and pressure for a time sufficient to produce the desired decorative laminate. The conditions of pressing for the high pressure laminate are standard and well known.

The pre-cured melamine particles, of a mean particle diameter of 50-120 ym, are preferably of generally spherical shape and most preferably made in accordance with the process of WO 97/07152, to ensure that the melamine particles are of generally spherical shape. However, smaller and larger particles can be used. The preferred basis formulation consists essentially of a water solution of uncured melamine resin, preferably approximately 60% solids, blended with 50- 140 phr (parts per hundred) of the pre-cured melamine particles based on resin solids in the uncured melamine resins impregnating solution, i. e. 50-140 parts by weight of pre- cured melamine particles per 100 parts by weight of uncured melamine resin solids, more preferably 80-120 phr. As indicated above, fumed silica can be added, desirably in the ratio of 4-10 phr based on uncured melamine resin solids.

The formulation is applied either to the upper paper sheet in such a quantity so as to provide approximately 75-86% total resin content in the paper after the paper has been dried. This provides dried paper, i. e. either decor paper or overlay paper, ready for pressing with core sheet to provide the desired thick, transparent, chemically resistant protective layer. To obtain this result, the resin blend or mixture should be applied at 130 to 285 g/m2, but could be applied at a rate as low as 106 g/m2 for 42 g/m2 paper.

Usually, the pre-cured melamine particulates portion of the blend or mixture will constitute 50-196 g/m2, preferably 65- 163 g/m2. However, and as is known to those skilled in the art, the application rate is dependent in part on the basis weight of the paper being impregnated and coated, it being

understood that the quantity of laminating resin must be sufficient to thoroughly impregnate the paper sheet so that the final laminate will not de-laminate in use. For any basis weight paper, the desired or optimum application rate can be easily determined by routine testing.

After preparation of the coated/impregnated and dried decor or overlay paper, it is stacked in a typical lay-up as noted above and pressed to a high pressure decorative laminate using standard pressing conditions. A high pressure decorative laminate having a thick layer of clear cured melamine resin on top of the uppermost ply, having an estimated thickness of about 1-3 mils, is obtained which is characterized by excellent chemical resistance, gouge and scratch resistance, and which meets all NEMA requirements for a high pressure decorative laminate.

The use of such a large quantity of pre-cured resin particles dispersed in such a high concentration in the melamine laminating resin liquid results in a thick layer of cured melamine resin on top of the overlay or decor which prevents chemicals, most notably highly concentrated acids, from reaching the paper fibers and damaging the laminate. It is theorized that during the laminating procedure the large amounts of pre-cured melamine resin particulates act as press stoppers during pressing and prevent the top layer of resin from being squeezed out of the uppermost paper layer, thus also providing a higher ratio of resin to fiber in the uppermost layer of the final laminate.

Under ideal conditions, the cured laminating resin and the pre-cured resin particles will form a substantially homogeneous chemically resistant coating on the surface of the laminate. In practice, however, this ideal situation is difficult to achieve. Inspection of the surface of the resultant laminate under a high powered microscope shows that while the smaller pre-cured particles become homogeneously incorporated into the cured matrix, at least a partial interface often exists between the cured matrix and the larger of the pre-cured particles. The continued existence of two phases in the chemically resistant coating of the final

laminate usually presents no problems insofar as chemical resistance is concerned.

The following examples are offered illustratively: Example 1 In a Waring blender were placed 50g of melamine resin solution in water (60% resin solids), 50g of cured melamine resin particles in water (60% solids), and 1.8g of fumed silica (Aerosil 200), and this mixture was stirred in the blender for 45 seconds. The resultant mixture was applied to the top surface of dry 42 g/m2 alpha-cellulose overlay paper, the liquid resin penetrating the overlay and the pre- cured particulate material being deposited on top of overlay.

The resultant wet overlay paper was dried at 129.4°C for three minutes. The total resin content was calculated at 85%-95% of the total weight of the resultant dried overlay.

The resultant dried overlay was assembled on top of a printed decor paper saturated with melamine resin and then dried, and three or four Kraft paper sheets saturated with phenolic resin and then dried to form the core. This assembly was then pressed in a conventional general purpose cycle at 143°C, 1000 psi, for about 30 minutes to produce the laminate.

Such laminate was then tested after 24 hours for chemical resistance by placing three or four drops each of diluted and concentrated acids--i. e. 50% acetic acid, 10% and 37% HCl, 10%, 50% and 95% H2S04, and 10% and 54% nitric acid--and bases--i. e. 10% and 50% NaOH--on top of the laminate and covering them with a watch glass. One such test was run for one hour and another for 16 hours, after which the watch glass was removed and the reagents rinsed off the laminate surface with running tap water. The surface was then dried in air for 24 hours and visually inspected for damage.

There was only slight visible damage noticed from concentrated (95%) sulfuric acid and a minor gloss level reduction from concentrated (54%) nitric acid and concentrated (37%) HC1 after one hour; and clearly visible damage from concentrated (95%) sulfuric acid and minor reductions in gloss from 10% nitric acid, 50% sulfuric acid, 54% nitric acid and 37% HC1 after sixteen hours.

In comparison, regular laminate subjected to the same tests showed damage after one hour from concentrated hydrochloric, sulfuric and nitric acids, and reduction in gloss from the other reagents. After 16 hours all acids caused visible damage, with the concentrated acids causing severe damage.

Example 2 Example 1 is repeated using only 1.2g of fumed silica (Aerosil 200). The results are the same.

Example 3 Again, Example 1 is repeated, but this time without any fumed silica. Again, the results are the same.

Example 4 Example 1 is again repeated, but this time using only a 40g slurry of cured melamine resin particles (60% solids). Again the results are the same.

Example 5 Example 1 is again repeated, but this time using a 60g slurry of cured melamine resin particles (60% solids), and again the results are the same.

Example 6 A Kady mill was charged with 76 gallons of standard melamine resin impregnating solution. After starting the mill, 13.6 kg. of Aerosil 200 were added and the mill was run to mix the components for 15 minutes. The resultant mixture was pumped to a large blade mixer to which was added about 364 kg. of a slurry of cured melamine resin particles (60% solids). Mixing was carried out for 10 minutes after which 1.36 kg. of catalyst were added and mixing was continued for an additional two minutes. The mixture was transferred to a coater which was used to coat 42 g/m2 alpha-cellulose paper to provide coated and impregnated overlay. The coating was applied at a rate so as to provide the overlay paper, after drying, with a resin content of 77% and a volatility of 10%- 12%.

The resultant overlay was used to manufacture laminate using an otherwise typical laminate assembly consisting of the so-prepared overlay paper over a melamine resin saturated decor sheet, in turn over a core made of

several several Kraft paper sheets saturated with phenolic resin. The laminating was carried out in a general purpose cycle at 143°C for one hour, including cooling.

Example 7 Example 6 is repeated but the coating is applied to the overlay paper at a slightly greater rate so as to provide dried overlay having 80% resin content.

Example 8 Example 6 is repeated, except the resultant laminate is then successfully postformed at 129°C.

Example 9 Example 7 is repeated but the resultant laminate is then successfully postformed at 129°C.

Example 10 As in Example 6, a Kady mill was charged with 76 gallons of resin. After starting the mill, 9 kg. of Aerosil 200 fumed silica was added and the mill was run for 15 minutes. The resultant mixture was pumped to a large blade mixer to which was added 364 kg. of a slurry of 60% cured melamine resin particles. After mixing for 10 minutes, 1.36 kg. of catalyst was added and mixing was continued for another two minutes. The resultant mixture was transferred to a coater, and overlay paper was coated to provide a resin content of 82% and a volatility of 10-11% after drying in an air oven.

The overlay is placed over an otherwise regular laminate assembly consisting of a melamine resin saturated decor sheet over a core made of phenolic resin impregnated Kraft paper, and pressed in a general purpose cycle at 143°C for one hour including cooling.

Example 11 Example 10 is repeated except that the resin mixed is applied to the overlay paper in a greater amount so as to provide a resin content of 86% after drying.

The invention as described above is disclosed in conjunction with the use of melamine resin to make high pressure decorative laminates meeting NEMA standards.

However, the invention can also be used, and it is within the

scope of the present invention, to make low pressure laminate which does not meet all NEMA standards, but which still has enhanced chemical resistance.

While melamine resin is the preferred resin for both the pre-cured particles and the laminating resin because of its excellent qualities and relatively low price compared with other resins suitable for use in the surface layers of laminates, it will be understood that other resins can also be used such as the condensation resins of other aminotriazines with an aldehyde, e. g. condensation resins based on acetoguanamine, dicyandiamine, benzoguanamine, etc., so long as they cure to a hard and chemically resistant surface. In addition, for most purposes the resin must also be substantially transparent and substantially colorless after curing so that the decor sheet can be seen through the thick transparent, chemically resistant protective overlayer. It is also within the scope of the present invention for the pre- cured particles to be of a different resin than the laminating resin so long as they are compatible and have indices of refraction which are closely similar.

Fumed silica has been mentioned above as a suspension agent, but it will be understood that other suspension agents can be used in its place.