Page 213 - Carrahers_Polymer_Chemistry,_Eighth_Edition

176 Carraher’s Polymer Chemistry

TABLE 5.5
General Physical Properties of Extrusion Grade ABS

o
Heat deflection temperature (1,820 kPa; C) 90
o
Maximum resistance to continuous heat ( C) 90

–5
Coefficient of linear expansion (cm/cm- C, 10 ) 9.5
o
Compressive strength (kPa) 4.8 × 10 4
Flexural strength (kPa) 6.2 × 10 4
Impact strength (Izod: cm-N/cm of notch) 320
Tensile strength (kPa) 3.4 × 10 4
Ultimate elongation (%) 60
Density (g/mL) 1.0
an average energy release of about 20 kcal/mol (80 kJ/mol) with the conversion of one of the double
bonds into lower (potential energy wise; generally more stable) energy single bond. For all of these
products, cross-linking and grafting sites are available through the remaining double bond.
1,4-Butadiene can form three repeat units as described in Equation 5.47, the 1,2; cis-1,4; and
trans-1,4. Commercial polybutadiene is mainly composed of the 1,4-cis isomer and is known as
butadiene rubber (BR). In general, butadiene is polymerized using stereoregulating catalysts. The
composition of the resulting polybutadiene is quite dependent on the nature of the catalyst such that
almost total trans-1,4 units, or cis-1,4 units, or 1,2 units can be formed as well as almost any com-
bination of these units. The most important single application of polybutadiene polymers is its use
in automotive tires where over 10 tons are used yearly in the U.S. manufacture of automobile tires.
7
BR is usually blended with natural rubber, NR, or styrene–butadiene rubber (SBR), to improve tire
tread performance, particularly wear resistance.
A second use of butadiene is in the manufacture of ABS copolymers where the stereogeometry
is also important. A polybutadiene composition of about 60% trans-1,4; 20% cis-1,4; and 20% 1,2

configuration is generally employed in the production of ABS. The good low-temperature impact
strength is achieved in part because of the low T values for the compositions. For instance,
g
the T for trans-1,4-polybutadiene is about −14ºC while the T for cis-1,4-polybutadiene is about
g
g
o
−108 C. Most of the ABS rubber is made employing an emulsion process where the butadiene
is initially polymerized, forming submicron particles. The styrene–acrylonitrile copolymer is
then grafted onto the outside of the BR rubber particles. ABS rubbers are generally tougher than

HIPS rubbers but are more difficult to process. ABS rubbers are used in a number of appliances,
including luggage, power tool housings, vacuum cleaner housings, toys, household piping, and
automotive components such as interior trim. Table 5.5 contains representative data for extrusion
grade ABS.
Another major use of butadiene polymer is in the manufacture of high-impact polystyrene
(HIPS). Most HIPS has about 4%–12% polybutadiene in it so that HIPS is mainly a polystyrene
intense material. Here the polybutadiene polymer is dissolved in a liquid along with styrene mono-
mer. The polymerization process is unusual in that both a matrix composition of polystyrene and
polybutadiene is formed as well as a graft between the growing polystyrene onto the polybutadiene
is formed. The grafting provides the needed compatibility between the matrix phase and the rub-
ber phase. The grafting is also important in determining the structure and size of rubber particles
that are formed. The grafting reaction occurs primarily by hydrogen abstraction from the polyb-
utadiene backbone by growing either polystyrene chains or alkoxy radicals if peroxide initiators
are employed.
Interestingly, isoprene, 2-methyl-1,3-butadiene, exists as an equilibrium mixture of cis and trans
isomers.

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