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For the first fifty years, glass was the only material used. The lenses were thin, yet reports of injury were rare. In 1938 perspex (polymethylmethacrylate, or PMMA) began to replace glass in contact lens manufacture. PMMA lenses were easier to produce so the production of glass lenses soon ended. Lenses made of PMMA are called hard lenses. [5]
PMMA ignites at 460 °C (860 °F) and burns, forming carbon dioxide, water, carbon monoxide, and low-molecular-weight compounds, including formaldehyde. [17] PMMA transmits up to 92% of visible light (3 mm (0.12 in) thickness), [18] and gives a reflection of about 4% from each of its surfaces due to its refractive index (1.4905 at 589.3 nm). [3]
[2] An alternative use includes a purified version that is used to measure ionising radiation such as alpha particles and neutrons . Although CR-39 is a type of polycarbonate, it should not be confused with the general term " polycarbonate ", a tough homopolymer usually made from bisphenol A .
Methacrylic acid, abbreviated MAA, is an organic compound with the formula CH 2 =C(CH 3)CO 2 H. This colorless, viscous liquid is a carboxylic acid with an acrid unpleasant odor. It is soluble in warm water and miscible with most organic solvents.
It has a low glass-transition temperature about 10 °C (12.5 °C in case of PMA 38). [4] It is soluble in dimethyl sulfoxide (DMSO). [4] PMA is water-sensitive and unlike PMMA, is not stable against alkalies. [2] High-energy radiation leads to cross linking in PMA. However in polymethyl methacrylate (PMMA), a compound similar to PMA ...
Ultrasonic welding can be used to join ASA to PVC, ABS, SAN, PMMA, and some others. [4] ASA can be solvent-welded, using e.g. cyclohexane, 1,2-dichloroethane, methylene chloride, or 2-butanone. Such solvents can also join ASA with ABS and SAN. Solutions of ASA in these solvents can also be used as adhesives. [4]
PMMA was used clinically for the first time in the 1940s in plastic surgery to close gaps in the skull. Comprehensive clinical tests of the compatibility of bone cements with the body were conducted before their use in surgery. The excellent tissue compatibility of PMMA allowed bone cements to be used for anchorage of head prostheses in the 1950s.
The monomer is a viscous liquid with a pungent odour. The first polymeric form of methacrylic acid was described in 1880 by Engelhorn and Fittig. The use of high purity monomers is required for proper polymerization conditions and therefore it is necessary to remove any inhibitors by extraction (phenolic inhibitors) or via distillation. [2]