Technology: Artificial Transparency (Part 1)
Polymethyl methacrylate (PMMA), also known as acrylic glass or perspex, possesses the best optical properties of all polymers and weighs only half as much as glass. The light transmittance of a 3 mm sheet is 92 per cent. This property applies for the entire light spectrum, which makes the material ideally suited for use in greenhouses and in areas where daylight technology plays a major role. The light transmittance can be modified with coloration down to 0 per cent. PMMA also possesses long-term weatherproof qualities and UV-resistance. The material remains colour-fast even in exposed external situations. Cast sheets are simpler to re-form than extruded material. Standard sheets are obtainable up to 25 mm in thickness and are used for acoustic screening to bridges, for example. Cast monoblock material is used for deep-sea aquaria (with thicknesses of 80 mm to max. 200 mm). PMMA has a standard flame resistance, but can be manufactured in higher fire resistance classes, as for the roof over the Olympic stadium in Munich. When subjected to heat, the sheets shrink and retreat from the source of fire ("memory effect"), thereby allowing smoke and toxic gases to escape. The material can also be cold-bent to a curved form. It is available in flat and corrugated sheets and in webbed, cellular slabs. PMMA is hard and can be easily worked, although there is a danger of tension cracking, so that special tools have to be used to bore it. The high-gloss surface is scratch-proof. The linear thermal expansion of the material is roughly eight times as great as that of glass, which is why clip fixings are preferable to bored, screwed connections. Special guidelines have to be observed where the material is used in conjunction with foodstuffs. PMMA is often used for acoustic reflectors built into concert halls after the completion of construction. Other applications include showcases and model-building, where invisible joints are required. The material is also used in the form of vertical lighting prisms or adjustable horizontal louvres in daylight technology. The latter are usually manufactured by injection moulding, since the surface can be more precisely controlled in this way than in an extrusion process. PMMA is also used as transparent thermal insulation. It can be easily recycled.
