The present invention relates to a polyurethane resin composition for optical lenses such as transparent lenses, sunglass lenses and polarized lenses, impact strength synthetic resin lenses produced by casting the composition, and a method of manufacturing the same.
Generally, plastic lenses are lightweight and less liable to crack compared to glass lenses and also easy to dye. Therefore, their use has spread from spectacle lenses to optomechanical lenses.
Also, interest of users in safety of spectacles has increased, and requirements for lenses particularly for sport and leisure sunglasses which will not crack are increasing. As representative lens materials, diethylene glycol bis(allyl carbonate) (hereinafter referred to as CR-39), polycarbonate resins, polyMMA resins, polystyrene resins, etc. can be cited.
But CR-39, polyMMA resins and polystyrene resins have poor impact strength and are liable to crack. Polycarbonate resins are superior in impact strength and not liable to crack, but have insufficient optical properties as lenses and are poor in chemical resistance and solvent resistance.
Thus, plastic lenses superior in impact strength and optical properties as well as solvent resistance are desired as materials for spectacle lenses, but no material that satisfies all such properties has been developed.
As molding methods for plastic lenses, hot-press molding, cast-molding, injection molding, etc. can be used. For molding of thermosetting resins such as CR-39 and urethane resins, casting is considered to be suitable.
In cast-molding plastic lenses, if internal sliding of flowing resin becomes poor, so-called “striae” (originally the term in the glass molding technique) tends to develop.
If the adhesion between the mold (of glass in many cases) and the thermosetting resin used is strong, releasability tends to worsen, thus worsening the productivity.
In order to cope with such problems, a method has been tried in which an external releasing agent is applied to a mold. But this worsened the surface accuracy of lenses, so that the production efficiency did not improve very much.
Also, since a urethane resin is a thermosetting resin obtained by reacting an aromatic isocyanate prepolymer with 4,4′-methylene bis(2-chloroaniline) and is superior in physical properties, anti-abrasion and impact resilience, it has widely been used for rolls, tires, belts, etc.
But since urethane resins contain aromatic isocyanate, which tends to change color, they cannot be used for applications in which thin colors are required.
An object of the present invention is to improve the transparency of a material for lenses made of a synthetic resin of a polyurethane resin family, which is superior in impact strength, and improve the releasability and productivity to provide a material from which impact strength and striae-free synthetic resin lenses can be manufactured.
Another object of the present invention is to provide a high-quality impact strength lens made of a synthetic resin and having impact strength and good in transparency and surface accuracy and having no striae, and to manufacture impact strength lenses made of a synthetic resin with stable quality and good efficiency.