Acrylic and polyurethane-based adhesives are commonly used for adhering PC films to adjacent layers in the ophthalmic lens industry. Although these adhesives have been used for decades, they present manufacturing complexities, as well as health and environmental concerns.
Many acrylic and polyurethane-based adhesives require an organic solvent for proper application. The use of organic solvents for adhesive application presents a variety of health and environmental concerns. Most organic solvents are flammable and must be disposed of under costly, highly-regulated conditions. Many organic solvents are volatile, irritate nasal and esophogeal membranes, and are capable of causing permanent tissue damage. One example of a toxic organic solvent is hexane, which is oxidized by the body to hexane-2,5-dione, a compound which is damaging to nerve cells.
Polyurethane adhesive layers are applied as relatively thick layers, ranging from a minimum thickness of 10 μm to thicknesses as high as 60 μm, depending on the manufacturer. During production processes such as molding, laminates are subjected to high pressures and temperatures. For relatively thick adhesive layers, applied pressure on a laminate layers causes adhesive to be ejected out from between laminate layers. The ejected adhesive contaminates the mold cavity and requires frequent cleaning. An adhesive that can be applied in a relatively thin layer would resist pressure-induced ejection and prevent mold cavity contamination.
In order to address shortcomings associated with acrylic and polyurethane-based adhesives, alternative adhesive strategies have been investigated. U.S. Patent Publication No. 2013/0004775 discloses photochromic laminates using a polyurethane adhesive crosslinked with an isocyanate-active prepolymer via a crosslinking agent. Japanese Patent Application JP2011085900 discloses a PC polarized laminate with particular polyvinyl alcohol (PVOH, also referred to as PVA) film mechanical properties. U.S. Pat. No. 7,198,830 discloses a polycarbonate resin composition comprising a mixture of two polycarbonate resins. U.S. Pat. No. 7,118,806 discloses a plastic molded article comprising a multi-layer laminate adhered to a polycarbonate base material with a urethane adhesive. During lens production, adhesive application and the subsequent thermoforming process must provide a wafer capable of producing a PC polar lens with excellent cosmetics, optics, color, edging results, and crazing results. The laminate must be compatible with currently employed PC polar processes, i.e., thermoforming, injection molding, surfacing, HMC, edging, and mounting. An existing technical challenge is to provide an optical adhesive to bond PC films to adjacent laminate layers that is safe and compatible with current processes.
There is a need in the industry for an improved method for adhering PC layers to adjacent layers in an ophthalmic lens stack which is environmentally-friendly, resistant to processing degradation, and adaptable to current production methods.