1. Field of the Invention
This invention relates to a process for producing highly filled acrylic solid surface sheet material by a direct extrusion process in which the extrudate rheology is controlled to prevent degredation of ingredients.
2. Description of the Related Art
A typical process for producing filled acrylic solid surface sheet material comprises compounding the ingredients in a twin-screw extruder, extruding strands of the mixture and cutting the strands into pellets. The pellets are then extruded using a single screw extruder where sheet is formed by passing the mixture through a die. Optionally, the mixture may be passed through a gear pump and then into a sheet die. After passing through the die, the sheet is passed through calendar rolls to provide a desired finish.
The process requires two extruders and in the process the ingredients are exposed to two heating cycles, which can result in degradation of some of the ingredients. Also, the use of two extruders and the additional processing time adds to the expense of making the sheet material.
Another prior art process comprises melting a resin such as PMMA, adding a plasticizer e.g. a phthalic ester or an aliphatic dibasic ester such as dimethyl phthalate, dioctyl phtalate, dibutyl phthalate, dibutyl sebacate or dioctyl sebacate to lower the viscosity of the resin and then adding a filler such as ATH, aluminum trihyroxide, to a twin screw extuder. The melt then exits the extruder and goes through a die to form a sheet. The plasticizer that is added remains in the sheet and can migrate to the surface. If the sheet is used in applications involving contact with food, the plasticizer based on phthalate chemistry is undesirable due to potential carcenogenic behavior and potential hormone disruptive properties. The plasticizer is needed to lower the melt viscosity so that the melt temperature can be reduced to below the decomposition temperature of the filler, e.g. ATH, which is in the range of 190 to 200xc2x0 C.
There is a need for a direct extrusion process which eliminates the justmentioned disadvantages.
The above-mentioned problems are avoided by a process for producing a highly filled acrylic solid surface sheet material comprising introducing ingredients including an acrylic resin, from 30-62% by weight based on total weight of all ingredients of aluminum trihydroxide as a filler, and a lubricant in an amount from 1-5% by weight of the acrylic resin, into interconnected zones of an apparatus for mixing the ingredients, introducing the acrylic resin into a first zone, introducing the lubricant into the first or a second zone, introducing the filler into a third zone, mixing the ingredients and removing gasses which may develop while advancing the resulting mixture toward an outlet, and forming the mixture into a solid surface sheet or a profiled article, i.e. shaped article, by passing the mixture through a die. The sheets and articles are useful for spa, sanitary ware, countertops, and bathroom and kitchen fixtures.
Particularly desirable products which include an acrylic resin such as poly(methylmethacrylate) (PMMA) highly filled with aluminum trihydroxide (ATH), e.g. 30 to 62% by weight, can be made by a direct extrusion process by controlling the extrudate rheology. The process for extruding a composition containing ATH is limited due to decomposition of the ATH at temperatures above about 200xc2x0 C. In the normal extrusion of PMMA temperatures reach 230xc2x0 C. and upward. At temperatures less than 200xc2x0 C. the PMMA melt has a very high viscosity limiting mixing and extrusion of the ingredients. This problem is overcome in the present process by introducing a lubricant into the PMMA thereby lowering the viscosity of the melt prior to introduction of ATH and permitting the temperature to be maintained below 200xc2x0 C.
The process comprises introducing ingredients including an acrylic resin, from 30-62% by weight based on the total weight of all ingredients of ATH as a filler, and a polymerizable monomer/initiator solution in an amount from 114 5% by weight of the acrylic resin, into an extruder with or without a downstream gear pump, two extuders in series, or a Buss co-Kneader with a downstream gear pump interconnected in which acrylic resin is added at the feed throat, the monomer/initiator solution preferably being added downstream through an injection port in a barrel, and then ATH being added further downstream into a barrel using a stuffer apparatus, and removing gases into a vacuum vent port. The resulting polymer melt may then be fed through a gear pump and the mixture formed into a solid surface sheet by passing the polymer melt mixture through a die.
The preferred range of filler is from 40 to 60% by weight based on total weight of the ingredients. The preferred lubricant is a polymerizable monomer such as 2-ethylhexylacrylate (ETH), sold by ELF Atochem, Inc. which forms a polymer which is incompatible with the acrylic resin. An amount from about 1 to 5% by weight of the ingredients can be used. An initiator to polymerize the acrylate monomer is added. The monomer polymerizes at about the same temperature of the PMMA melt, i.e. about 190-200xc2x0 C. A preferred initiator is a peroxy initiator such as Lupersol(copyright) 101, 2,5-dimethyl-2-5-di(t-butylperoxy)hexane sold by Elf Atochem, Inc. Other acrylate monomers which serve as lubricants may be used. They include but are not limited to methyl acrylate, ethyl acrylate, i-propyl acrylate, n-propyl acrylate, n-butyl acrylate, i-butyl acrylate, and lauryl acrylate. Fatty acid esters such as stearyl acrylate may be used.
In addition to the ingredients noted above, additives such as pigments, dyes, rubbers, flame retardants, antioxidants and the like as know to those skilled in the art may be added.
In the process the melt temperature of the mixture is kept below about 200xc2x0 C. to minimize decomposition of ATH. In the normal extrusion of PMMA, temperatures reach 230xc2x0 C. and upward. At less than about 200xc2x0 C. PMMA melt has a very high viscosity so the torque exceeds the capability of the extrusion machines. In the present process this problem is overcome by introducing the lubricant into the PMMA thereby lowering the viscosity of the melt prior to the introduction of ATH and permitting the temperature to be maintained below about 200xc2x0 C.