loss of plasticizer. Unreinforced, they have an extremely high
coefficient of thermal contraction-expansion. However,
reinforcement (woven fiberglass mat or polyester reinforcement)
can vastly improve dimensional stability.
Uncured Elastomers. Uncured elastomers are chemically
intermediate between the thermosetting (i.e., elastomeric) and
thermoplastic polymers. The single-ply membranes containing
these intermediate polymers are CPE (chlorinated polyethylene),
CSPE (chlorosulfonated polyethylene, known commercially as
Hypalon), and PIB (polyisobutylene).
Relative to elastomeric and thermoplastic sheets,
lap seams can be heat-or adhesive-bonded.
easier field-forming of reliable lap seams;
c) may cure or vulcanize over a period of time and
A major disadvantage is that they lack the abrasion
resistance of vulcanized rubber. Another disadvantage of CSPE is
that after it has cured, it is difficult to repair using heat-
Modified Bitumens. Modified bitumens are essentially
rubberized asphalts. The addition of rubber to the asphalt
imparts elastomeric properties to the asphalt and increases the
membrane's breaking strain by a factor of 10 or more compared
with a conventional built-up bituminous membrane's breaking
strain. The polymers used to make these physical improvements in
the modified-bitumen membranes include atactic polypropylene
(APP), styrene-butadiene rubber (SBR), and styrene-butadiene-
styrene block polymer (SBS). Reinforcement is generally
polyester or glass fibers, sometimes in combination.
The Canadian General Standards Board (CGSB) divides
modified bitumen membranes into three categories:
a) By exposure: Type 1, exposed (always anchored);
Type 2, shielded (anchored or loose-laid);
b) By surfacing: Class A, granule-surfaced; Class B,
metallic-surfaced; Class C, plain-surfaced;