PMMA (Polymethylmethacrylate/acrylic)

Overview

The best known acrylic plastic.

Strengths

Excellent clarity and UV resistance. Good hardness, stiffness and abrasion resistance, reasonably high tensile strength and modulus. Low smoke emission. Good tracking and arc resistance. Thermoformable, can be colored to a high saturation, can be polished, excellent surface finish and gloss. Resistant to alkalis. Low mold shrinkage. Fragments formed on cracking are much less sharp than those of glass.

Limitations

oor solvent resistance, susceptible to stress cracking and crazing, not resistant to mineral acids. Water absorption, while reasonably low, has considerable effect on dimensions. Low long term use temperature, poor fatigue resistance, brittle compared to engineering thermoplastics, notch sensitive, poor scratch resistance (despite high surface hardness, though scratches can be polished out). Burns relatively quickly and produces large flaming drips.

Designation

Acrylic, polymethylmethacrylate (PMMA), Poly-[1-methoxycarbonyl)-1-methylethylene] (IUPAC)

Tradenames

Perspex, Lucite, and Plexiglas are the most famous.

Typical uses

Dispersions with fine grain mineral fillers are used to cast sinks and basins, canopies on subsonic aircraft, light duty bullet/projectile proof screens (not as resistant as PC or PC/Glass laminate), lenses, light covers, glazing, light pipes, meter covers, bathroom fittings, outdoor signs, skylights, toys,packaging,safety spectacles, tool handles, motorcycle windscreens, baby incubators.

Compositional summary

CH2-C(CH3)COOCH3)n
Material family	Plastic (thermoplastic, amorphous)
Base material	PMMA (Polymethyl methacrylate / acrylic)
CAS number	9011-14-7

Effect of composition

High impact PMMA (formed by blending with acrylic rubber) has increased haze and poorer tensile properties though improved toughness. Can be blended with PVC to give tough, durable sheets.

Processing properties

Feedstocks & production

Acetone, hydrogen cyanide, sulfuric acid, methanol. HCN not readily available in Japan, so isobutylene or t-butyl alcohol can be used in a two stage oxidation to methacrylic acid before esterification.

First commercial production

1932

Available forms

Pre cast sheets available. Pellets

Forming

Suitable for injection molding, extrusion, apart from cast sheets. Pre drying required. Excellent for vacuum forming. Rotational molding, compression molding and casting possible, depending on grade. Extruded sheet is of lower optical quality than cast sheet.

Machining

Very easy to machine. Low cutting speeds ideal to prevent sticking (though can be high with sufficient cooling). Cooling required. Thin sheets can be scribed and broken like glass.

Joining

Very well suited to friction, hot gas and hot plate welding. Some grades very suitable for ultrasonic welding. Suitable for solvent bonding (methylene dichloride or DMF suitable) though annealing can be necessary to prevent crazing from residual stresses. Conventional one and two part adhesives such as epoxy, alpha cyanoacrylate, polyester or nitrile phenolic adhesives can be used. Uncured cyanoacrylates can cause crazing. Suitable for staking.

Surface treatment

Any residual stresses in parts and use of an inappropriate color pigment can cause stress cracking when printing on the surface. Can be metalized with and without pre treatment of surface. Very suitable for painting except for casting grades.