Abstract:
A solid surface product having a thickness of 3 cm or greater is provided which is substantially free from warping, has a homogeneous surface appearance, and is substantially free from entrapped air bubbles or voids. The solid surface material includes from about 25 to about 95 wt %, i.e. weight percent, of a polymer resin and from about 0 to about 70 wt % of a filler. A method for producing the solid surface material is also provided and includes forming a mixture of from about 25 to about 95 wt % of a polymer resin and from about 0 to about 70 wt % of a filler; casting the mixture into an open mold to a thickness of 3 cm or greater; applying a vacuum to the molded mixture to release entrapped gases therein; and curing the molded mixture to produce a thick solid surface material.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to decorative solid surface products formed from filled or unfilled polymer resins, and more particularly to solid surface products having thicknesses equal to or greater than about 3 cm. 
         [0002]    Decorative solid surface products formed from particle filled resins have been long known and are commonly available for use on kitchen and bathroom surfaces. In general, these materials are formed from particle filled resins, the most common resins being polyesters and acrylates, with the fillers ranging widely, often used to give particular visual effects to the product. 
         [0003]    Such solid surface products are typically available in a wide variety of appearances and colors, with patterns that run throughout the thickness of the products, giving them desirable aesthetics and utilities. These materials are also tough and resistant to heat, stains, mildew and impact, and can be easily repaired if damaged. 
         [0004]    Commonly used pigments in these products are composed of small particles of inorganic compounds, often metallic oxides, which are roughly spherical in shape. Other pigments are also sometimes used, as well as fillers and larger pieces of colored material or “particulates” to provide different visual effects. Alumina trihydrate (ATH) is often included as a preferred filler to provide translucency and whiteness to the products, as well as for its excellent fire-retardant properties. 
         [0005]    Various processes are known and used in the art for producing solid surface materials in sheet form for use as countertops and tabletops in kitchens and bathrooms. A number of manufacturers have developed and introduced their own brands of solid surface materials such as, for example Du Pont (CORIAN®) and Formica Corporation (FOUNTAINHEAD®). The overwhelming majority of these products are ½″ (1.25 cm) in thickness. Because of the expense involved in manufacturing the products, they are distributed in nominal sized sheets that are bonded by an adhesive to a thicker and inexpensive substrate material such as medium density fiberboard, particle board, or waferboard, and then cut to form the desired countertop shape or the like. The cutting and bonding of the sheets to thicker substrates are necessary due to the thin nature of the solid surface material and the need to provide a more robust work surface. Accordingly, the finish fabrication of solid-surface sheets has always been labor intensive. 
         [0006]    Countertops are used in a variety of locations to provide a horizontal work space. Methods for manufacturing conventional solid surface countertop materials provide relatively thin sheets of material having thicknesses of ½″ (1.25 cm) up to about ¾″ (1.90 cm). To provide finished countertops of sufficient thickness, traditionally, the sheets of solid surface material are cut into smaller pieces which are then glued to the edges and/or ends of the solid surface sheets. The glued ends and/or edges then must be sanded or polished to remove any visible glue lines. These pieces are glued together to create a thicker edge and/or end and to hide the underlying fiberboard or particle board substrate bonded to the solid surface material and then sanded down to create a finished product. The need for cutting smaller pieces, fitting and gluing those pieces to solid surface sheets, and sanding and polishing those sheets is labor intensive and adds numerous manufacturing steps which results in an end product that is more expensive for the consumer. 
         [0007]    Attempts have been made in the past to produce solid surface sheet materials having greater thicknesses (i.e., greater than about 1.25″/3.0 cm). For example, Slocum, U.S. Reissue Pat. No. 27,093, teaches the preparation of a thick solid surface material comprising a methyl methacrylate polymer and a filler using as a catalyst a hemi-perester of maleic acid and curing the polymer autogenically in air at low (i.e., 15-40° C.) temperature. However, such products have suffered from a number of deficiencies including incomplete curing of the polymer resins, entrapped air bubbles or voids in the cured material, a surface appearance that was not homogeneous, and warping of the product either as manufactured or after cutting at the job site. Accordingly, there remains a need in the art to provide an improved solid surface material which is thicker than previously commercially available material but which does not have the adverse characteristics of prior thick materials. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    The present invention addresses that need by providing a solid surface product that has a thickness of 3 cm or greater and yet is free from warping, has a homogeneous surface appearance, and is substantially free from entrapped air bubbles or voids. As used herein, references to a “thick” solid surface material means a material having a nominal thickness of 3 cm or greater. 
         [0009]    In accordance with one embodiment of the present invention, a thick solid surface material having a thickness of 3 cm or greater is provided and includes from about 25 to about 95 wt %, i.e. weight percent, of a polymer resin and from about 0 to about 70 wt % of a filler. 
         [0010]    While any of a number of different thermosetting polymer resins may be utilized, in a preferred form, the polymer resin is selected from the group consisting of polyester resins, polyacrylate resins, and mixtures thereof. Additionally, the fillers used in the material may also vary widely in composition. In a preferred form, the filler is selected from the group consisting of aluminum trihydrate, calcium carbonate, barium sulfate, magnesium hydroxide, talc, and mixtures thereof. 
         [0011]    In accordance with another aspect of the present invention, a method for producing a thick solid surface material is provided and includes forming a mixture of from about 25 to about 95 wt % of a polymer resin and from about 0 to about 70 wt % of a filler; casting the mixture into an open mold to a thickness of 3 cm or greater; applying a vacuum to the molded mixture to release entrapped gases therein; and curing the molded mixture to produce a thick solid surface material. 
         [0012]    In a preferred form, the vacuum which is applied is less than about 3 pounds per square inch absolute pressure (psia) (2.07×10 4  Pa), or a vacuum greater than about 24 inches of mercury, for about 1 to 3 minutes. Preferably, the mixture is first gelled at ambient temperature and then cured at a temperature of from about 80 to about 100° C. for from about 1 to about 4 hours. 
         [0013]    Accordingly, it is a feature of the present invention to provide solid surface products having thicknesses of 3 cm or greater and to methods of making those products. It is another feature of the present invention to provide nonporous solid surface products with improved dimensional stability and resultant warpage characteristics compared to those of the prior art. It is a further feature of the present invention to provide solid surface products with substantially fewer air bubbles or voids compared to those of the prior art. Still other features and advantages of the present invention will be apparent from the following detailed description and the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0014]    The following detailed description of specific embodiments of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which: 
           [0015]      FIG. 1  is a schematic cut away side view of one embodiment of the mold used in the practice of the present invention; 
           [0016]      FIG. 2  is a schematic top view of the mold of  FIG. 1 ; 
           [0017]      FIG. 3  is a schematic side view of one embodiment of the vacuum chamber used in the practice of the present invention; and 
           [0018]      FIG. 4  is a cutaway side view of one embodiment of the mold loaded into the vacuum chamber. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0019]    Polymer resins which are useful in the practice of embodiments of the present invention are not specifically limited as long as they can be formed into a solid surface material by curing. Preferred free radical, heat curable resins particularly useful in the practice of the present invention are thermosetting unsaturated polyester resins and thermosetting polyacrylate resins, and combinations thereof. The term resin or “syrup” commonly refers to the uncured liquid state of a polymer, where said polymer is present in solution with a suitable crosslinking monomer. Examples of useful unsaturated polyester resins are reaction products of unsaturated dicarboxylic acids, or unsaturated dicarboxylic acids and saturated dicarboxylic acids, with glycols, such as maleic acid, phthalic acid and dipropylene glycol monomers respectively. Commonly, styrene is employed as the monomeric crosslinker for the polyester resins. Examples of useful polyacrylate resins include various kinds of conventional acrylic group monomers, acrylic group partial polymers, vinyl monomers for copolymerization other than acrylic group monomers, or oligomers. As the acrylic group monomer, (meth)acrylic ester is preferable. The use of methyl methacrylate monomer is preferred as the acrylic resin crosslinker. Additional useful thermosettable polymers include epoxies, urethanes, acrylo-urethanes, melamines and combinations thereof. The total amount of fluid polymerizable constituent in the castable composition of the present invention is from about 25 to about 95 wt %, and preferably from about 25 to about 45 wt %. 
         [0020]    Exemplary polymer resins are unsaturated polyester resins, which are the reaction products of glycols and unsaturated dicarboxylic acids, or the reaction products of glycols and unsaturated dicarboxylic acids and saturated dicarboxylic acids. The glycol can comprise one or more of ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, neopentyl glycol, 1,3-propanediol, 1,2-butanediol or 1,4-butanediol. The unsaturated dicarboxylic acid can be one or more of maleic acid, fumaric acid, and their anhydrides. The saturated dicarboxylic acid can be one or more of phthalic acid, isophthalic acid, terephthalic acid, adipic acid, succinic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, and their anhydrides. The above monomers are not considered exhaustive, but are those typically used in the art. Methods of preparing such polymers are well known by those versed in the art, and are also commercially available. The most common monomeric crosslinker, used in the resin syrup, is styrene, which is preferred in the practice of the present invention. The viscosity of the castable syrup composition is related to the molecular weight of the polymer, as well as its concentration in the monomer, which can be varied in such a way as to control the rheology of the syrup composition. 
         [0021]    Another exemplary polymer resin is poly(methyl methacrylate). As a castable composition, it is typically introduced as a solution or syrup of the polymer preferably dissolved in methyl methacrylate monomer. As for the polyester resins previously described, methods of preparing such a polymer and syrup therefrom are well known and described in the prior art, and are both commercially available as well. The viscosity of the castable syrup composition is related to the molecular weight of the polymer, as well as its concentration in the monomer, which can be varied in such a way as to control the rheology of the syrup composition. 
         [0022]    Other polymerizable polyacrylate monomers useful as fluid polymerizable constituents are alkyl acrylates and methacrylates in which the alkyl groups can vary from 1-18 carbon atoms, but preferably 1-4 carbon atoms. Suitable acrylic monomers are methyl acrylate; ethyl acrylate and methacrylate; n-propyl and i-propyl acrylates and methacrylates; n-butyl, 2-butyl, i-butyl and t-butyl acrylates and methacrylates; 2-ethylhexyl acrylate and methacrylate; cyclohexyl acrylate and methacrylate; omega-hydroxyalkyl acrylates and methacrylates; N,N-dialkylaminoalkyl acrylates and methacrylates; N-[t-butyl]aminoethyl acrylate and methacrylate; styrene; vinyl acetate; acrylonitrile, methacrylonitrile; acrylamide and methacrylamide; allyl acrylate and methacrylate; divinylbenzene and divinyltoluene. The above reactive monomers are not considered exhaustive, but are those typically used in the art. 
         [0023]    While the solid surface material can be unfilled, it is preferred in some embodiments to include a filler. Suitable fillers include, for example, aluminum trihydrate, calcium carbonate, titanium dioxide, barium sulfate, magnesium hydroxide, and talc, which as examples, are not considered to be exhaustive and not intended to limit the scope of the claimed invention. Fillers can be present in amounts of from between about 0 to about 70 wt %. Preferably, fillers are added to the composition in amounts from about 50 to about 70 wt %. 
         [0024]    It is also known to include in solid surface materials other additives such as pigments, dyes, flame retardant agents, release agents, fluidizing agents, viscosity control agents, curing agents, antioxidants, toughening agents, and the like as is well known to those of ordinary skill in the art. For example, additional pigments having differing or contrasting colors may also be added to the composition. The inclusion of minor amounts of any or all of these additives is contemplated. 
         [0025]    In producing the solid surface material, the polymer resin, preferably in syrup form, is mixed optionally with a filler and any other conventional additives deemed beneficial. From about 25 to about 95 wt % of the polymer resin and from about 0 to about 70 wt % of the filler, and preferably from about 25 to about 45 wt % of the polymer resin and from about 50 to about 70 wt % of the filler, are mixed together. In a preferred embodiment of the present invention, a mixture from about 80 to about 90 wt % polyester resin syrup, and from about 10 to about 20 wt % polyacrylate resin syrup is employed as the polymer constituent of the blend, wherein the polyester resin tends to impart toughness and the acrylic resin tends to impart flexibility to the final cured product. Any suitable mixing method may be used, including a dispersing-type blade mixer, kneading mixer, screw mixer, or double planetary mixer. A suitable curing catalyst is also preferably added to the mixture, with the catalyst type and concentration being matched to the particular polymer or polymers, and processing conditions, being used. 
         [0026]    The syrup mixture is dispensed from the mixing vessel and cast into a suitable open mold which is sized to provide a sheet of solid surface material having the desired length, width, and thickness dimensions. In a preferred embodiment as shown in  FIG. 1 , the base  12  of the mold  10  supports a smooth, flat surface  15  such as a large pane of glass or sheet of polished metal. Typical dimensions for the glass are up to 60 inches (152 cm) wide and up to 160 inches (406 cm) in length. The mold  10  includes side walls  14  and  16  and end walls  18  and  20 . Preferably at least one moveable side wall  16  and at least one moveable end wall  18  are provided. This provides flexibility in the sizes of sheets of the solid surface material which can be cast. The side and end walls are of a height which is greater than a typical cast thickness for the solid surface material. The walls may be of metal or plastic and are designed to act as a dam for the liquid resin composition as it is cast into the mold. A suitable mold release agent may be applied to the base and walls of the mold to aid in the removal of the solid surface material once it has been cured. 
         [0027]    As shown in  FIG. 1 , mold  10  can be equipped with means for moving the mold such as caster wheels  22  positioned on rails  24 . The caster wheels  22  are attached to a frame  26  which includes a plurality of leveling bolts  28 . The leveling bolts are used to maintain the mold in a level, horizontal orientation during casting and curing of the resin mixture. 
         [0028]    As best shown in  FIG. 2 , side wall  16  and end wall  18  are moveable to adjust the length and width of the cast composition. For example, different length blocks  30  may be used to secure the moveable side and end walls into proper position. Alternatively, other mechanical devices such as clamps or bolts (not shown) may be used to adjust and then secure the moveable side and end wall as needed. 
         [0029]      FIG. 3  depicts a suitable vacuum chamber  32  which can be used in the practice of the invention. While one form of a vacuum chamber is shown by example only, it will be apparent to those skilled in this art that other forms of vacuum chambers may be utilized. Chamber  32  is fabricated of a suitable material such as steel, including reinforcing ribs  34 . The chamber may be equipped with a hinged door  38  which is gasketed to permit an air-tight seal to be made when the door is closed. The chamber  32  is connected to a source of vacuum such as a vacuum pump  40  through vacuum line  42 . Chamber  32  is also be equipped with a vent valve  44  to control the level of vacuum in the chamber, and a vacuum gauge  46  to permit an operator to monitor and adjust the level of vacuum as needed. As shown, the chamber  32  may also be provided with a sight glass  48  which permits an operator to view the progress of the orientation of the anisotropic pigment particles in the molded resin composition mixture. 
         [0030]    As best shown in  FIG. 4 , the mold  10 , containing the cast polymer mixture  50 , is placed into vacuum chamber  32  that is sized to be able to completely contain the mold. In the exemplary equipment shown in the drawings, the mold may be supported on wheeled casters on rails so that it can be conveniently rolled into and out of chamber  32 . Once mold  10  has been loaded into chamber  32 , a vacuum is applied using pump  40  through vacuum line  42 . As pressure in the chamber is reduced, air and volatile materials (such as monomers) dissolved or entrapped in the liquid polymer resin syrup are “boiled” or stripped off of the cast material as depicted by the vertical arrows (drawn in phantom lines) in  FIG. 4 . Generally, the applied vacuum is less than about 3 psia pressure. Application of the vacuum for from about 1 to about 3 minutes will typically be sufficient to cause any entrapped or dissolved monomer or air to be stripped from the cast material. 
         [0031]    An important benefit of degassing and deaerating the cast liquid resin in situ in the mold by means of a vacuum chamber, rather than vacuum deaeration of the bulk resin composition in its mixing vessel before dispensing the resin to the mold, as taught in the prior art, is that reaeration of the resin during the pouring process is avoided, and the actual deaeration process is considerably more efficient and effective, with at least about 50 times the surface area of the liquid resin exposed to the vacuum atmosphere, analogous to a thin film evaporation process. As a result, the final cast and cured solid surface product contains substantially fewer air bubble voids than sheets produced using prior art methods. 
         [0032]    After sufficient time is allowed for the out-gassing to occur, the vacuum is released by stopping and isolating vacuum pump  40  and opening vent valve  44 , and the mold  10  is then removed from vacuum chamber  32  by opening door  38  and rolling the mold out of the chamber. The cast resin  50  is permitted to gel and harden at ambient temperature for a period of time, typically from about 30 minutes to about 3 hours. Once the resin has substantially completely solidified or set, the solid surface material sheet is de-molded and placed in a “post-cure” oven (not shown) to complete the polymerization and curing process. 
         [0033]    Post-curing is typically carried out at temperatures of from about 80 to about 100° C. for a period of from about 1 to about 4 hours, depending on the size, thickness, and composition of the solid surface material. Preferably, heating of the post-cure oven is accomplished by re-circulation of hot air over the curing material. Cooling then takes place gradually, typically by reducing the temperature of that recirculating oven air at the rate of approximately 1° F. (0.5° C.) per minute. The cast and cured solid surface material sheet is preferably cooled down to at least 115° F. (46° C.) before removal from the oven. The controlled gradual cooling or annealing results in substantially less internal stresses in the final cured sheet, which significantly improves its dimensional stability and reduces the possibility of sheet warpage compared to prior art solid surface sheet products. The resulting solid surface sheets are then sanded and polished and their edges trimmed. 
         [0034]    It is noted that terms like “preferably,” “commonly,” and “typically” are not utilized herein to limit the scope of the claimed invention or to imply that certain features are critical, essential, or even important to the structure or function of the claimed invention. Rather, these terms are merely intended to highlight alternative or additional features that may or may not be utilized in a particular embodiment of the present invention. 
         [0035]    For the purposes of describing and defining the present invention it is noted that the term “substantially” is utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. The term “substantially” is also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue. 
         [0036]    Having described the invention in detail and by reference to specific embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. More specifically, although some aspects of the present invention are identified herein as preferred or particularly advantageous, it is contemplated that the present invention is not necessarily limited to these preferred aspects of the invention.