Safety glass is a term commonly used to describe a glass-plastic laminate designed to reduce the severity of lacerative injuries resulting upon impact sufficient to break the glass. A plastic film is laminated to a glass sheet so that upon impact sufficient to break the glass, the film adheres to the glass fragments, minimizing their dispersion. To be useful as safety glass, a laminate must have high energy absorption to minimize concussive injuries upon impact, high tensile and tear strength to prevent rupture of the film by glass fragments, sufficient adhesion between the layers to reduce lacerative injuries by minimizing the dispersion of glass fragments, and high optical quality. Moreover, the safety glass laminate must retain these properties over a wide range of temperature and humidity conditions.
Commercially employed safety glass, particularly in automobile windshields, is commonly a multiple laminate of two plies of glass with an interlayer of polymeric material, such as plasticized polyvinyl butyral. An alternative to this trilayer type of safety glass laminate is a bilayer laminate having a single ply of glass with a plastic layer. Upon impact sufficient to break the glass in a bilayer windshield, the danger from glass fragments inside the passenger compartment is greatly reduced because there is no interior glass ply. However, since the inner layer is not protected by an interior glass ply, the plastic film must have good weathering properties, chemical stability, and abrasion resistance, to provide durability for the requisite safety and optical quality.
To fulfill such requirements of safety glass, a number of different compositions have been proposed for use a protective coatings. For example, U.S. Pat. No. 3,931,113 discloses polyester urethanes for use in safety glass windshields, which are formed from a one-step bulk polymerization involving the reaction of cycloaliphatic diisocyanate with a low-molecular weight diol, such as butanediol, and with a poly(butylene adipate) or poly(butylene azelate) polyester component or a hydroxy-terminated polycaprolactone polyester. U.S. Pat. No. 4,923,757 discloses an abrasion and solvent resistant protective coating in the form of a polyurethane film which is made through a two-component system, with a first component of an aliphatic isocyanate-terminated prepolymer reacted with a second component of a linear polyol, such as a polyester or a polyether diol. The first prepolymer component is prepared by reacting a symmetrical triol with an isocyanate.
Such traditional coating compositions have demonstrated their usefulness as coatings for certain substrates, such as glass. However, carbonate diols, such as polyester diols are expensive, which increases the costs of the coated product. Also, when used on polymeric substrates, such coating compositions typically require a very high solids content to achieve an effective coating. Coating compositions with higher solids content, however, typically have a higher viscosity, and therefore often require solvents for flow coating processes. Such solvents can have an adverse effect on polymeric substrates during coating thereof. Moreover, certain moieties within the coating composition are believed to be more readily subject to oxidation, such as ethers, esters and carbonates, and therefore have reduced weatherability when used as protective coatings.
Accordingly, while prior art coating compositions have proven useful, there is a need for improved coating compositions that are simple and cost-effective to manufacture, and which increase the durability and weatherability of the coating.