Page 255 - Mechanical Engineer's Data Handbook

ENGINEERING MATERIALS 243

Polytetrajluoroethylenes (PTFE) ‘Teflon’ or ‘Fluon’, sulphuric acids. It is used for bearings, tyre reinforce-
these are the best known types of PTFEs. PTFEs resist ment, bottles, car parts, gears, and cams.
all known chemicals, weather and heat, have ex-
tremely low coefficients of friction, and are ‘non-stick’. Polyamides (nylons)
They are inert, with good electrical properties. They
are non-toxic, non-flammable and have a working These are a range of thermoplastics, e.g. Nylon 6,
temperature range of - 270 “C to 260 “C. They may be Nylon 66 and Nylon 610, which are among the
glass filled for increased strength. toughest engineering plastics with high vibration-
Applications include chemical, mechanical and elec- damping capacity, abrasion resistance and high load
trical components, bearings (plain or filled with glass capacity for high-speed bearings. They have low
and/or bronze), tubing, and vessels for ‘aggressive’ coefficient of friction and good flexibility. Pigment-
chemicals. stabilized types are not affected by ultraviolet radi-
ation and chemical resistance is good. Unfilled nylon is
Fluoroethylenepropylene (FEP) Unlike PTFE, this prone to swelling due to moisture absorption. Nylon
can be processed on conventional moulding machines bearings may be filled with molybdenum disulphide or
and extruded, but the thermal and chemical properties graphite. Applications include bearings, electrical in-
are slightly less good. sulators, gears, wheels, screw fasteners, cams, latches,
Ethylenetetrajluoroethylene (ETFE) The properties fuel lines and rotary seals.
are similar to those of PTFE, with a thermoplasticity Polyethylene
similar to that of polyethylene.
Perjluoroulcoxy (PFA) This has the same excellent Low density polyethylene is generally called ‘poly-
properties as FTFE, but is melt processable and, thene’ and is used for films, coatings, pipes, domestic
therefore, suitable for linings for pumps, valves, pipes mouldings, cable sheathing and electrical insulation.
and pipe fittings. The high-density type is used for larger mouldings and
is available in the form of sheet, tube, etc. Polyethylene
Ionomers is limited as an engineering material because of its low
strength and hardness. It is attacked by many chemi-
These thermoplastics based on ethylene have high melt cals.
strength which makes them suitable for deep forming,
blowing, etc. They are used for packaging, bottles, Polyethersulphone
mouldings for small components, tool handles, trim,
etc. They have a high acceptance of fillers. This is a high-temperature engineering plastic - useful
up to 180°C and some grades up to 200°C. It is
Methylpentene (TPX) resistant to most chemicals and may be extruded or
injection moulded to close tolerances. The properties
This is a high clarity resin with excellent chemical and are similar to those of nylons. Applications are as a
electrical properties and the lowest density of all replacement for glass for medical needs and food
thermoplastics. It has the best resistance of all trans- handling, circuit boards, general electrical compo-
parent plastics to distortion at high temperature - it nents, and car parts requiring good mechanical prop-
compares well with acrylic for optical use, but has only erties and dimensional stability.
70% of its density. It is used for light covers, medical
and chemical ware, high frequency electrical insula- Polypropylene oxide (PPO)
tion, cables, microwave-oven parts, and radar compo-
nents. It can withstand soft soldering temperatures. This is a useful engineering plastic with excellent
mechanical, thermal and fatigue properties, low creep,
Polyethylene terephthalate (PE TP) and low moisture absorption. Filled grades can be
used as alternatives to thermosets and some metals.
This has good strength, rigidity, chemical and ab- Applications are light engineering parts, and car,
rasion resistance and a very low coefficient of friction. aircraft and business components (especially for heat
It is attacked by acetic acid and strong nitric and and flame resistance).