Polycarbonates are commercially important materials possessing excellent physical and chemical properties which are useful in a wide range application from non-opaque impact resistance sheets to shaped articles. Generally, however, polycarbonates have rather low scratch resistance and are somewhat susceptible to attack by many common solvents and chemicals.
Previous efforts to overcome this low scratch resistance and susceptibility to attack by solvents have included lamination procedures and application onto the polycarbonate of a surface coating. Many of these prior art remedial efforts have been unsuccessful due to the incompatibility of the laminae and coating materials with the polycarbonate substrate. This incompatibility has resulted in stress cracking and crazing of the polycarbonate, crack propagation into the polycarbonate as a result of the brittleness of the coating, and a reduction of the advantageous properties of the polycarbonate such as, for example, impact resistance, tensile strength, non-opacity and elongation.
The prior art coatings for polycarbonates have included organopolysiloxanes, U.S. Pat. No. 3,707,397; polyester-melamines or acrylic-melamines, U.S. Pat. No. 3,843,390; and allyl resins, U.S. Pat. No. 2,332,461. These types of prior art coatings are generally applied from solutions of inert solvents and are cured to final properties by baking at elevated temperatures. The disadvantages of such systems are obvious. The heat curing requires a supply of thermal energy thereby adding to the cost of the system. Further, the thermal curing step is somewhat limited by the heat distortion temperature of the polycarbonate which is to be coated. Thus, in coating of polycarbonates, sheets of 30 mils and less generally cannot be coated and cured economically because of excessive warpage of the sheets during the thermal curing process.
U.S. Pat. No. 3,968,305 describes a synthetic shaped article having a mar-resistant polymer surface layer integrated with the polymer surface body, said polymer surface layer consisting essentially of, in polymerized form, (a) 20 to 100 weight percent of a compound having a total of at least three acryloxy and/or methacryloxy groups linked with a straight chain aliphatic hydrocarbon residue having not more than 20 carbon atoms, (b) 0 to 80 weight percent of at least one copolymerizable mono- or diethylenically unsaturated compound. This type of a surface layer suffers from the fact that it generally has poor durability of adhesion after prolonged exposure to weathering.
U.S. Pat. No. 3,968,309 describes a molded article of plastic having on its surface a cured film of a coating material comprising at least 30% by weight of at least one polyfunctional compound selected from the group consisting of polymethacryloxy compounds having a molecular weight of 250 to 800 and containing at least three methacryloyloxy groups in the molecule and polyacryloyloxy compounds having a molecular weight of 250 to 800 and containing at least three acryloyloxy groups in the molecule. This patent, however, also teaches that this coating must contain from 0.01 to 5% by weight of a fluorine-containing surfactant in order for the coated article to be acceptable. This patent teaches that when the coating material contains less than 0.01% by weight of the fluorine-containing surfactant, it is impossible to obtain a coated article having the requisite degree of surface hardness, surface smoothness, abrasion resistance and optical clarity. If the coating material contains more than 5% by weight of said fluorine-containing surfactant, the adhesion between a cured film of the coating material and a molded substrate of plastic is unsatisfactory.
It has now been found that a coating composition containing certain specific polyfunctional acrylic monomers in combination with resorcinol monobenzoate provides excellent and durable UV cured coatings for polycarbonate. Thus, the present invention provides certain acrylate ester monomer based UV-cured coatings for polycarbonates which adhere tenaciously and durably to the polycarbonate, are compatible with the polycarbonate, are mar, abrasion and solvent resistant, and maintain properties after prolonged exposure to weathering.