Page 258 - Mechanical Engineer's Data Handbook

246 MECHANICAL ENGINEER'S DATA HANDBOOK

Polyester Cellular polyvinyl chlorides (PVC) The low-density
type is closed cell and flexible. It is used for sandwich
This is normally used with glass fabric (the cheapest) structures, thermal insulation, gaskets, trim, buoy-
filler. The mechanical and electrical properties are ancy, and insulating clothing. The moderate .to high
inferior to those of epoxy. It can be rendered in density open-cell type is similar to latex rubber and is
self-colours. used as synthetic leather cloth. The rigid closed-cell
type is used for structural parts, sandwich construc-
Melamine tion, thermal insulation and buoyancy. Rigid open-
cell PVC (microporous PVC) is used for filters and
Fillers used for melamine are paper, cotton fabric, battery separators. In general, cellular PVC has high
asbestos paper fabric, and glass fabric. Melamines strength and good fire resistance and is easy to work.
have a hard non-scratch surface, superior electrical
properties and can be rendered in self-colours. They Polyethylene foams The flexible type is closed cell
are used for insulators, especially in wet and dirty and has low density with good chemical resistance and
conditions, and for decorative and industrial lami- colour availability, but is a poor heat insulator and
nates. costly. The flexible foams are used for vibration
damping, packaging and gaskets. The rigid type has
high density and is used for filters, cable insulation. A
Silicone
structural type has a solid skin and a foam core.
This is used with asbestos paper and fabric and glass Ethylene vinyl acetates (EVA) These are microcellu-
fabric fillers for high-temperature applications (2500C; lar foams similar to microcellular rubber foam, but are
intermittent use 300 "C). It has excellent electrical but much lighter with better chemical resistance and
inferior mechanical properties. colour possibilities.
Other types Other types of thermoplastics include:
Poly imide
cellular acetate which is used as a core material in
constructions; expanded acrylics, which have good
This is used with glass fabric as filler. Polyimides have physical properties, thermal insulation and chemical
superior thermal and electrical properties with a
service temperature as for silicones but with two to resistance; expanded nylon (and expanded ABS) which
three times the strength and flexibility. are low-density, solid-skin constructions; expanded
PVA which has similar properties to expanded poly-
styrene; and expanded polypropylene which gives high-
6.13.4 Foam and cellular plastics
density foams.
Thermoplastics
Thermosets
Polyurethane foams The 'flexible' type is the one Phenolids These can be formed in situ. They have
most used. It is 'open cell' and used for upholstery,
underlays, thermal and vibration insulation, and good rigidity, thermal insulation and high service
temperature. They are brittle.
buoyancy. It can be used in situ. The rigid type has
'closed cells' and is used for'sandwich construction, Urea formaldehyde (UF)foam This is readily formed
insulation, etc. Moulded components are made from in situ and has good thermal insulation. It has open
rigid and semi-rigid types. pores and is used for cavity-wall filling.
Expanded polystyrene This is made only in rigid form Expanded expoxies These have limited use due to
with closed cells. It can be used in situ. The density is their high cost. They give a uniform texture and good
extremely low, as is the cost. Chemical resistance is low dimensional stability, and are used for composite
and the service temperature is only 70 "C. It is used for foams, e.g. with polystyrene beads.
packaging, thermal and acoustic insulation and buoy-
ancy applications. Silicon foams These are rigid and brittle with a high
service temperature (300 "C; 400 "C intermittent use).
High-density polystyrene foam This has a porous Their use is limited to high-temperature-resistant
core with a solid skin. It is used for structural parts. sandwich constructions. The flexible closed-cell type is