Page 260 - Mechanical Engineer's Data Handbook

248 MECHANICAL ENGINEER’S DATA HANDBOOK

Relative properties of plastics (continued)

Tensile Compressive Machining Chemical
Material strength strength properties resistance

Thermosetting plastics
Epoxy resin 0
(glass-fibre filled)
Formaldehyde G
(asbestos filled)
Phenol formaldehyde G
(Bakelite)
Polyester E
(glass-fibre filled)
Silicone 0
(asbestos filled)

0 = outstanding, E =excellent, G =good, F =fair, P = poor.
Tensile strength (typical): E=55Nmm-’; P=21 Nmm-’.
Compressive strength (typical): E=210Nmm-’; P=35Nrnm-’.

6.14 Elastomers

Elastomers, or rubbers, are essentially amorphic poly- 100%) under stress. The synthetic rubber styrene
mers with linear chain molecules with some cross- butadiene is the most used elastomer, with natural
linking which ensures elasticity and the return of the rubber a close second. The following describes’ the
material to its original shape when a load is removed. commonly used elastomers and gives some applica-
They are characterized by large strains (typically tions and properties.

6.14. I Natural rubbers 6.14.2 Synthetic rubbers
(polyisoprene, N R)
Styrene butadiene rubbers (SBR, GRS,
These have high strength, flexibility and resilience, but BUNA S)
have poor resistance to fuels, oils, flame and sunlight
ageing. They are more costly than synthetic rubbers These are similar to natural rubbers in application, but
which replace them. ‘Soft rubber’ contains 14%0 are inferior in mechanical properties, although
sulphur. Wear resistance is increased by inclusion of cheaper. They are used in car brake hydraulic systems
fillers such as carbon black, silicon dioxide, clay, and and for hoses, belts, gaskets and anti-vibration
wood flour. ‘Hard rubber’ contains over 25% sulphur. mountings.
Full vulcanization of 45 % produces ebonite. Applica- Shore hardness: 4&80. Temperature range: - 50 “C
tions include vehicle tyres and tubes, seals, anti- to 82°C.
vibration mountings, hoses and belts. Butadiene rubbers (polybutadiene, BR)
Shore hardness: 3&90. Temperature range: -55 “C
to 82°C. These are used as substitutes for natural rubber, but
are generally inferior. They have similar applications
as natural rubber.