Which of the following is not true of polymethylmethacrylate?
PMMA is a transparent resin of water-like clarity that is transparent to light in the visible and ultraviolet range down to a wavelength of 250 nm. It is a hard resin with a Knoop hardness number of 18 to 20 KHN. It has a tensile strength approximately 60 MPa, a density of 1.19 g/cm3 and a modulus of elasticity of approximately 2.4 GPa. 
It is also extremely stable: it does not discolor in ultraviolet light and it exhibits remarkable aging properties. It is chemically stable to heat below 125°C, softens at 125°C, and can be molded as a thermoplastic material. However, above 125°C, PMMA begins to depolymerize and forms methylmethacrylate monomer, MMA, and at approximately 450°C, 90% of the polymer will have depolymerized.
Reference: PHILLIPS’ SCIENCE OF DENTAL MATERIALS, 12th ed page no 108
Knoop hardness number 18 to 20
Tensile strength of 60 MPa
Density of 1.13 g/cm3
Modulus of elasticity of 3.4 GPa