Recently, the substitution of glass glazing with transparent materials which do not shatter or are more resistant to shattering than glass, has become widespread. For example, transparent glazing made from synthetic organic polymers is now utilized in public transportation vehicles, such as trains, buses, taxis and airplanes. Lenses, such as for eye glasses and other optical instruments, as well as glazing for large buildings, also employ shatter-resistant transparent plastics. The lighter weight of these plastics in comparison to glass is a further advantage, especially in the transportation industry where the weight of the vehicle is a major factor in its fuel economy.
While transparent plastics provide the major advantage of being more resistant to shattering and lighter than glass, a serious drawback lies in the ease with which these plastics mar and scratch, due to everyday contact with abrasives, such as dust, cleaning equipment and ordinary weathering. Continuous scratching and marring results in impaired visibility and poor aesthetics, and often requires replacement of the glazing or lens or the like.
One of the most promising and widely used transparent plastics for glazing is polycarbonate, such as that known as Lexan.RTM., sold by General Electric Company. It is a tough material, having high impact strength, high heat deflection temperature, good dimensional stability, as well as being self-extinguishing, and is easily fabricated. Acrylics, such as polymethylmethacrylate, are also widely used transparent plastics for glazing.
Attempts have been made to improve the abrasion-resistance of transparent plastics. For example, scratch-resistant coatings formed from mixtures of silica, such as colloidal silica or silica gel, and hydrolyzable silanes in a hydrolysis medium, such as alcohol and water, are known. U.S. Pat. Nos. 3,708,225, 3,986,997 and 3,976,497, for example, describe such compositions.
In copending U.S. application Ser. No. 964,910, now abandoned, coating compositions having improved resistance to moisture and humidity and ultraviolet light are disclosed. It was discovered therein that, in direct contrast to the teachings of U.S. Pat. No. 3,986,997, compositions having a basic pH, i.e., 7.1-7.8, do not immediately gel but in fact provide excellent abrasion-resistant coatings on solid substrates.
The present invention offers a significant advantage over many of the heretofore known coating compositions in that it does not require heat in order to initiate the cure reaction. The radiation cure system of the present invention expends considerably less thermal energy than conventional heat cure systems.
In copending application Ser. No. 129,297, the present applicant has disclosed a radiation curable hardcoating composition which requires the use of the acid hydrolysis product of an alkoxy functional silane. In another copending application, Ser. No. 167,622, now U.S. Pat. No. 4,348,462 , the present applicant has provided a different radiation curable hardcoating composition which requires the combination of colloidal silica, acryloxy or glycidoxy functional silanes and non-silyl acrylates. However, applicant's disclosure in Ser. No. 167,622 was a precursor of the present invention. The early coating composition required an inert blanketing atmosphere in order to effectuate a proper cure. Ordinarily, free-radical type crosslinking mechanisms are retarded in non-inert atmospheres. Applicant's present invention, however, provides for the use of certain ketone-type and hindered amine photoinitiators which enable the cure of highly abrasion resistant coatings which are acryloxy-siloxane based.
In fact, the present applicant has recently demonstrated the utility of such a UV sensitive, non-inert atmosphere curable photoinitiator system in his copending application, Ser. No. 204,146 filed Nov. 5, 1980, now abandoned for a non-siloxane coating composition. The coating compositions of Ser. No. 204,146 are specific improvements over the coatings and articles disclosed in U.S. Pat. No. 4,198,465 (Moore et al.) both of which are hereby incorporated by reference. The Moore et al. disclosure teaches that certain very useful coatings and coated articles can be provided through the photoreaction of certain polyfunctional acrylate monomers and resorcinol monobenzoate. The Moore et al. coatings use any of several well known UV radiation photosensitizers including ketones such as benzophenone. The Moore et al. disclosure, however, failed to recognize that the improved coatings of the present invention could be provided by combining the ketone-type photoinitiator with a hindered amine-type compound whereupon the photosensitized acrylate-siloxane coating composition could be cured without the necessity of using resorcinol monobenzoate and without the necessity of an inert atmosphere (both of which are required by the Moore et al. disclosure). The savings and convenience provided by the use of a non-inert atmosphere such as air can be substantial, and these savings can be provided by the present invention without derogating the quality of the hard coating composition or coated product.
Since ultraviolet light is one of the most widely used types of radiation because of its relatively low cost, ease of maintenance, and low potential hazard to industrial users, rapid photo-induced polymerizations utilizing UV light rather than thermal energy for the curing of hard coatings offer several other significant advantages. First, faster curing coatings offer substantial economic benefits. Furthermore, heat sensitive materials can be safely coated and cured with UV light without the use of thermal energy which could damage the substrate. Additionally, the essentially solvent free media reduces the necessity for expensive and time consuming pollution abatement procedures.
Thus the advantages provided by the materials of the present invention are particularly useful for a number of purposes. For example, polycarbonates are commercially important materials possessing excellent physical and chemical properties which are useful in a wide range of applications from non-opaque impact resistant 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 applications onto the polycarbonate of a surface coating. Many of these prior art remedial efforts have been unsuccessful due to the incompatibility of the laminate 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 other advantageous properties of the polycarbonate.
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 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, and (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 acryloxy 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 colloidal silica and acryloxy functional alkoxysilanes as well as certain specific polyfunctional acrylic monomers in combination with a blend of ketone and hindered amine photoinitiators provides excellent and durable UV cured coatings, especially for high strength plastic substrates such as polycarbonate, polyester, polymethylmethacrylate, and other polyacrylates, as well as polyamides, nylon and metalized plastic surfaces. These materials may be in films or sheets as well as in the form of molded parts. Thus, the present invention provides certain acrylate-siloxane based UV-cured coatings which adhere tenaciously and durably to the substrate, are compatible with the substrate, are mar, abrasion and solvent resistant, and maintain properties after prolonged exposure to weathering.
It is therefore an object of the present invention to provide ultraviolet light curable coating compositions providing improved mar and abrasion resistance which are curable under a non-inert atmosphere such as air.
It is another object to provide a UV curable coating comprising the photoreaction products of colloidal silica and acryloxy functional silanes in combination with certain polyfunctional acrylate monomers and a blend of ketone-type and hindered amine-type photoinitiators.
It is another object to provide a process for providing abrasion resistant ultraviolet light curable coating compositions which are curable on rigid and flexible substrates, and which do not require an inert atmosphere for proper curing thereon.
It is still another object to provide articles of manufacture which are highly mar and abrasion resistant by virtue of having been coated with the coatings of the present invention which have been cured thereon.