Page 315 - Handbook of Plastics Technologies

ELASTOMERS

ELASTOMERS 4.107

conventional thermoset rubber. Key properties for distinguishing between a TPV and a
TPO for a given elastomer-thermoplastic system are as follows:
1. Reduced permanent set
2. Improved ultimate mechanical properties
3. Improved fatigue resistance
4. Greater resistance to attack by fluids, e.g., hot oils
5. Improved high-temperature utility
6. Greater stability of phase morphology in the melt
7. Greater melt strength
8. More reliable thermoplastic fabricability
In earlier literature, these TPEs have been called elastomeric alloys. Most workers in the
TPE field have come to prefer the term TPV, since it conveys more clearly the specific na-
ture of these materials.
TPVs are prepared by dynamic vulcanization, the process of vulcanizing an elastomer
during its intimate molten-state mixing with a nonvulcanizing thermoplastic polymer.
Small elastomer droplets are vulcanized to give a particulate vulcanized elastomer phase
of stable domain morphology during melt processing and subsequently. The process gen-
erates a TPE with properties closer to those of a thermoset rubber than those of a compara-
ble unvulcanized composition if there is a high concentration of rubbery phase.
On melt mixing of the thermoplastic and rubbery polymers under high stress, the less
viscous thermoplastic tends to become the continuous phase with the more viscous rubber
dispersed in it. The dispersed rubber particles are then vulcanized and form a three-dimen-
sional polymer network within each particle; they cannot recombine or agglomerate into
larger particles. The most common polymer system in TPVs is PP/EPDM rubber; how-
ever, a number of other polymer systems have been used commercially. These include
NBR/PP, IIR/PP, and NR/PP.
The morphology of a TPV is best understood as a dispersion of very small, highly
cross-linked elastomer particles in a continuous phase of hard thermoplastic. The size of
the elastomer phase particles is one key to the performance of the TPV. If the particles
have diameters as small as 1 to 5 µm, mechanical properties of the TPV are surprisingly
good, almost reaching those of a corresponding thermoset rubber and vastly exceeding
those of a TPO from the same polymers. TPVs are the TPEs, which are closest to conven-
tional thermoset rubbers with respect to their behavior. A high degree of cross-linking of
the dispersed elastomer phase is necessary for the rubber-like performance of a TPV.
TPVs have mechanical properties such as modulus (compression or tensile), tear
strength, abrasion resistance, and compression set resistance that make them suited for a
broad range of rubber applications. Also, their resistance to fatigue is superior to that of a
thermoset rubber of comparable hardness. The useful long-term (weeks, years) service
temperature range for TPVs is between the T of the soft elastomer phase and the temper-
g
ature at which oxidative degradation of the TPV becomes significant or the material melts.
Because there is no unsaturation in the polymer backbone of EPDM rubber, TPVs based
on EPDM have a higher service temperature limit (approaching T ) than those based on
m
unsaturated elastomers such as NBR and NR. For EPDM/PP TPVs, this range is –60 to
135°C; for NBR/PP TPVs, it is –40 to 125°C; and it is –40 to 100°C for NR/PP TPVs. The
lower temperature of each of these ranges is determined by the brittle point or T of the
g
elastomer. The upper temperature is based on properties retention after continuous aging
in hot air for 1000 hr.

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