Page 38 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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Encyclopedia of Physical Science and Technology EN002C-64 May 19, 2001 20:39

242 Biopolymers

FIGURE 23 Synthesis and structure of rubber.

not all the same length, and the chains can vary from a nishes can be obtained from the reaction of chlorine with
hundred to tens of thousands of isoprene units long. rubber.
In rubber molecules, the carbon-carbon bonds of the Raw rubber, however, is limited in use because of its
2
polymer backbone always lie on the same side of the dou- relatively low tensile strength (300 lbs/in. ), its solubil-
ble bond (cis to the double bond) and this configuration ity in some hydrocarbon solvents and its viscoelasticity;
conveys flexibility on the molecular chains. It is believed stretched raw rubber will not return to its original, un-
thatrubbermoleculescoiluprandomlyandthatthisistheir stretched length on release of tension if the tension is
preferred conformation at temperatures around room tem- maintained for some time. This happens because some
perature.Onstretching,however,themoleculesinarubber molecules slip past one another under tension, and do
sample become partially aligned with each other. When not return to their original positions when the tension is
thestretchingforceisreleased,therubbermoleculesreturn released.
to their coiled shapes, and the sample regains its original The properties of raw rubber can be improved by a
length. process called vulcanization, where rubber is heated with
Synthetic rubber, with structure identical to natural rub- sulfur. The result of the treatment is the formation of
ber, can now be synthesized chemically from isoprene, but monosulfide, –S–, and disulfide, –S–S–, cross-links be-
this is of much less economic importance than synthetic tween rubber molecules. Vulcanization is usually speeded
rubbers such as styrene-butadiene rubber, which are chem- up by incorporation of an accelerator, such as a thiazole, in
ically distinct from natural rubber. the presence of an activator like zinc oxide. To increase
The small rubber particles in suspension in latex can the solubility of accelerator and activator in the rubber, the
be coagulated by the addition of acid. The resultant prod- zinc salt of a fatty acid may be added.
uct can be passed through rollers and dried in air to give a Vulcanization decreases the solubility of the rubber and
sheet of raw rubber. If treated with sodium bisulfite before its viscoelasticity. Now rubber molecules can uncoil on
rolling, the product becomes crepe rubber and can be used stretching but are less likely to slip past one another be-
to manufacture shoe soles. Other useful materials can be cause of the cross-links, and so the rubber sample regains
made by milling chemical compounds into the raw rubber its original length when stretching ceases. The tensile
and heating. Thus, addition of sulfuric acid can give adhe- strength of the rubber increases with the amount of sulfur
sives and shellacs, while titanium and iron chlorides can incorporated, but simultaneously the elasticity decreases,
be added to produce a rubber for molding into chemically because too many cross-links prevent molecular uncoiling
resistant dishes and electrical apparatus. when tension is applied to a rubber sample. If the sulfur
Wrapping films can be made from rubber treated with content is less than 5%, the rubber is soft, pliable, and
hydrogen chloride and products used in paints and var- elastic and can be used for rubber tubing, elastic bands,