Page 545 - Carrahers_Polymer_Chemistry,_Eighth_Edition

508 Carraher’s Polymer Chemistry

TABLE 15.1
Typical Contents of a Modern Tire Trea
Material % (by Weight) Purpose
Natural rubber 30 Elastomer
Styrene–butadiene rubber 30 Elastomer
Carbon black 27 Reinforcing fi ller
Aromatic oil 5 Extender
Stearic acid 2 Accelerator
Aryl diamine 2 Antioxidant
Zinc oxide 2 Accelerator
Sulfur 1 Vulcanizing agent
Antiozonate 0.5 Antioxidant
Parafin wax 0.5 Processing aid

N,N-Diphenyl guanidine 0.1 Delayed accelerator

acetate) is internally plasticized because of the increased flexibility brought about by the change in

structure of the polymer chain. The presence of bulky groups on the polymer chain increases seg-
mental motion and placement of such groups through grafting acts as an internal plasticizer. Internal
plasticization achieves its end goal at least in part through discouraging association between poly-
mer chains. However, grafted linear groups with more than 10 carbon atoms can reduce fl exibility
because of side-chain crystallization when the groups are regularly spaced.
External plasticization is achieved through incorporation of a plasticizing agent into a polymer
through mixing and/or heating. The remainder of this section focuses on external plasticization.
Plasticizers should be relatively nonvolatile, nonmobile, inert, inexpensive, nontoxic, and com-
patible with the system to be plasticized. They can be divided on the basis of their solvating power
and compatibility. Primary plasticizers are used as either the sole plasticizer or the major plasticizer
with the effect of being compatible with some solvating nature. Secondary plasticizers are materials
that are generally blended with a primary plasticizer to improve some performance such as fl ame
resistance, mildew resistance, or to reduce cost. The division between primary and secondary plas-
ticizers is at times arbitrary. Here we will deal with primary plasticizers.
According to the ASTM D-883 definition, a plasticizer is a material incorporated into a plastic to

increase its workability and flexibility or dispensability. The addition of a plasticizer may lower the
melt viscosity, elastic modulus, and T .
g
Waldo Semon patented the use of tricresyl phosphate as a plasticizer for poly(vinyl chloride)
(PVC) in 1933. This was later replaced by the less toxic di-2-ethylhexyl phthalate (DOP), which
is now the most widely used plasticizer. The worldwide production of plasticizer is on the order of
3.2 million tons annually. Volume wise, about 90% of the plasticizers are used with PVC and PVC-
containing systems.
The effect of plasticizers has been explained by the lubricity, gel, and free volume theories. The
lubricity theory states that the plasticizer acts as an internal lubricant and permits the polymers to
slip past one another. The gel theory, which is applicable to amorphous polymers, assumes that a
polymer, such as PVC, has many intermolecular attractions that are weakened by the presence of a
plasticizer. In free volume theories, it is assumed that the addition of a plasticizer increases the free
volume of a polymer and that the free volume is identical for polymers at T . There may be some
g
truth in most of these theories. It is believed that a good plasticizer solubilizes segments allowing
them some degree of mobility creating free volume through Brownian movement. In turn, this
lowers the temperature where segmental mobility can occur making the material more fl exible.
Most plasticizers are classified as to being general purpose, performance, or specialty plasti-

cizers. General purpose plasticizers are those that offer good performance inexpensively. Most

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