Digidim 505 : Technical information : Page 45

Properties of materials

PMMA (Polymethyl methacrylate)

PMMA, also known under the trade name Plexiglas®, is acrylic

plastic. It oers a very high light transmission and is, therefore, ideal

for optical uses. It has a high degree of sheen on the surface and

rigidity. PMMA is easy to shape, strong and oers limitless opportu-

nities for colouring. PMMA is UV stable and has very good weather

resistance, making it suitable for outdoor use. The electrical proper-

ties are also good. The material is not aected by alkaline solutions,

diluted acids or oils, but is attacked by strong acids, acetone and

alcohols. PMMA has relatively low heat resistance and should be

used in situations where the temperature does not exceed 55 °C. It

is possible to make PMMA more impact-resistant, but this is at the

expense of its good optical properties. In the eld of lighting, PMMA

is normally used for shades and cover plates, as well as for outdoor

signs, windows, contact lenses, etc.

PMMA is brittle, and where microscopic notches are present it can

easily break. Just like PC, it is also sensitive to being under tension,

which can lead to cracks forming over time.

Plexiglas® is a trademark of Röhm.

PS (Polystyrene)

Polystyrene has a high degree of stiness and hardness. It has

excellent optical properties and good dimensional tolerance. Other

good properties are its high surface shine, low water absorption

and excellent electrical properties. Among its disadvantages we can

note poor impact resistance and low chemical resistance, especially

to solvents. Polystyrene does not tolerate strong acids and dissolves

in aromatic solvents. It also has low resistance to UV and turns

rapidly yellow (3–6 months) when used in lighting products or out-

doors. The highest temperature at which it can be used is limited to

approx. 70 °C. The material is not authorised for lighting ttings for

professional use, especially high powered ones or those with long

burning times. The material is often chosen for domestic applica-

tions due to its low material costs.

PP (Polypropene)

Polypropene, also called Polypropylene, is one of the most common

thermoplastics. The advantages of PP are its low density and high

strength. Above all, PP has great fatigue strength, which makes it

very suitable for hinge designs. The material can tolerate relatively

high temperatures without losing its properties. Conversely, it copes

less well at low temperatures, which means that in cold winter

weather it is prone to becoming brittle and breaking. Another draw-

back of PP is that it is broken down by UV radiation unless stabilised

with the help of additives. PP is used, among other places, in the

end caps of luminaires.

PC/ABS

Sometimes, it is necessary to mix dierent plastics in order to

obtain the properties required for the parts of the product. PC/ABS

is one such combination of materials; it is rigid and has high tensile

and bending strength and at the same time is impact resistant. The

material is also dimensionally stable, with good weather resistance.

It is customary to use PC/ABS where high demands are made for

ttings, and at higher temperatures. The material is often used in

electronics boxes, end pieces, instrument panels, etc.

Rubber

Rubber parts are most com-

monly found as packing mate-

rial of various types. Normally,

this is for sealing edges along

the periphery of the luminaire

against a glass disc, for instance.

It is also found in insert seals

for cables. More recently, plastic

qualities (elastomers) which

are reminiscent of rubber have

taken over a number of these applications. They are cheaper to

produce and can be made with greater precision. They also provide a

more aesthetically pleasing surface than rubber.

EPDM rubber

Ozone and weather resistant. Good cold resistance. A certain resist-

ance to oxidising acids and chemicals, animal and vegetable oils.

Not resistant to mineral oils. Temperature range -40 °C to +100 °C.

Chloroprene rubber Neoprene (CR)

This material doesn’t deteriorate over time with very good weather

resistance. Good strength against oils, it expands in mineral oils but

is not destroyed and oers very good mechanical properties. Found

only in black mixtures. It crystallises at cold temperatures. Tempera-

ture range -20 °C to +100 °C.

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