Page 273 - Carrahers_Polymer_Chemistry,_Eighth_Edition

236 Carraher’s Polymer Chemistry

TABLE 7.6
Thermoplastic Elastomers Based on Hard-Soft Combinations
Polymer Structure
Hard Polymer Soft/Elastomeric
Blend
Polypropylene Ethylene–propylene Copolymer *
Polypropylene Natural rubber † Dynamic vulcanizate
Dynamic vulcanizate
Polypropylene Ethylene–propylene–diene monomer ‡
Polypropylene Butyl rubber § Dynamic vulcanizate
Dynamic vulcanizate
Polypropylene Nitrile rubber **
Polypropylene Poly(propylene-co-1-hexene) Blend
Blend
Polypropylene Poly(ethylene-covinyl acetate)
Polypropylene Styrene–ethylene–butylene-styrene + oil Blend
Blend
Polystyrene Styrene–butadiene–styrene + oil
Nylon Nitrile rubber Dynamic vulcanizate
††
Poly(vinyl chloride) Nitrile rubber + diluent Blend, dynamic vulcanizate
Chlorinated polyolefi n Ethylene interpolymer ‡‡ Blend
†
*,‡
‡

Sample trade names: Flexothene, Ferroflex, Hifax, Polytrope, Ren-Flex, Telcar; Geolast; Hifax MXL,
**
††
§
Santoprene, Sarlink 3000 and 4000, Uniprene; Sarlink 2000, Trefsin; Geolast, Vyram; Apex N,
‡‡
Chemigum, Sarlink 1000; Alcryn.
EPMs the effect of blending is achieved through polymerizing the finely dispersed elastomer phase

(EPM) simultaneously with the hard polypropylene.
At times, the phases are cross-linked during the mechanical mixing. This process is referred

to as “dynamic vulcanization” and produces a finely dispersed discontinuous cross-linked elasto-
mer phase. The products are referred to as thermoplastic vulcanizates or dynamic vulcanizates.
The products of this process have an insoluble elastomer phase giving the material greater oil and
solvent resistance. The cross-linking also reduces or eliminates the flow of this phase at high tem-

peratures and/or under high stress. This allows the material better resistance to compression set.

Typical thermosetting elastomers are difficult to recycle because their cross-linking prevents them
from being easily solubilized and reformed through application of pressure and heat. Recycling can
be accomplished through the particalizing (grinding into small particles) of the elastomeric mate-

rial followed by a softening-up by application of a suitable liquid and/or heat and, finally addition of
a binder that physically or chemically allows the particles to bind together in the desired shape.
7.7 THERMOPLASTIC ELASTOMERS
A number of thermoplastic elastomers have been developed since the mid-1960s. The initial ther-
moplastic elastomers were derived from plasticized PVC and are called plastisols. Plastisols are
formed from the fusing together of PVC with a compatible plasticizer through heating. The plasti-
cizer acts to lower the T to below room temperature (RT). Conceptually, this can be thought of as
g
the plasticizer acting to put additional distance between the PVC chains thus lowering the inter- and
intrachain forces as well as helping solubilize chain segments. The resulting materials are used in a
number of areas, including construction of boot soles.
The hard/soft segment scenario is utilized in the formation of a number of industrially important
thermoplastic elastomers. Thermoplastic elastomers contain two or more distinct phases and their
properties depend on these phases being intimately mixed and small. These phases may be chem-
ically or physically connected. In order that the material be a thermoplastic elastomer, at least one
phase must be soft or flexible under the operating conditions and at least one phase is hard with the

hard phase(s) becoming soft (or fluid) at higher temperatures. Often the hard segments or phases are

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