THE RECENT development of optically clear plastics marks the first basic change in materials used for spectacle lenses since their introduction. Some of the more recent compounds bid well to supersede crown glass.
Beginning with Celluloid, in 1870, the organic plastic industry has crossed many hurdles: (1) optical clarity, (2) water absorption, (3) light stability, (4) thermal expansion, (5) thermoplasticity, (6) shrinkage, and (7) warping. Plastics may be divided into two groups according to their reaction to heat: thermolabile (thermoplastics), or those that soften under heat, and thermosetting, or those which once hardened under heat cannot be softened by it. Methyl methacrylates, such as Plexiglas, Lucite, Perspex, Transpex I, and the Tulca lens (California, 1937-1939), and most contact lenses are thermoplastics with softening points from 122 to 254 F. More complex allyl resins (C. R. 39; allyl diglycol carbonate), as used in present Armorlite lenses, are thermosetting.
Softness has been
KEENEY AH, DUERSON HL. EVALUATION OF PLASTIC SPECTACLE LENSES. AMA Arch Ophthalmol. 1953;49(5):530-535. doi:10.1001/archopht.1953.00920020542004