polyvinyl chloride
An ethenoid polymer produced as a fine white
powder by both emulsion polymerization and
suspension polymerization of the gas, vinyl
chloride, when liquified under nitrogen pressure.
The terms "PVC "
and "vinyl" are commonly used with reference not
only to the polymer, but to all materials of which
polyvinyl chloride is a constituent. PVC compositions are
prepared by hot mixing the polymer with
plasticizers and small proportions of stabilizers,
stearate lubricants, and coloring materials to
give materials of a wide range of hardness, from
rigid (with little or no plasticizer) to very soft
(equal proportions of plasticizers and polymer),
and in a wide range of colors, as well as crystal
clear. Unplasticized flexible PVC foils are
normally produced from vinyl chloride-acetate
copolymers by calendering and stretching. The
product is usually flexible, relatively
nonflammable, hard wearing, and resistant to water
and corrosion.
PVC film can be
heat sealed, either by high frequency vibration
(see: PLASTIC
WELDING ), or by means of a press equipped
with a special barrier to prevent the film from
adhering to the heated jaws. Another method of
sealing is to sew the sheets together, which is
possible because of the remarkably high tear
strength of the material when properly plasticized.
PVC film has an
attractive matte surface; however, in the more
hardwearing grades it is slightly opaque. The
grades used for the protection of archival
materials are readily available in a wide range of
thicknesses and widths.
As with other highly chlorinated materials, the
use of PVC should
be very carefully considered in view of the
possibility of the formation of hydrochloric acid.
(81 , 198 , 235 )
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