Page 323 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

P1: GQQ/GLT P2: GRB Final Pages
Encyclopedia of Physical Science and Technology EN014B-670 July 28, 2001 16:50

Rubber, Natural 389

Fully chlorinated rubber is 65% chlorine, with a spe- TABLE VIII Nonrubber Constituents in Latex a
cific gravity of about 1.65. It can be dissolved in aromatic
Percentage
hydrocarbons for use as chemically resistant paints and in
Constituent by weight
bonding rubber to metal.
Fatty acid soaps (e.g., ammonium oleate) 0.5
4. OENR Sterols and sterol esters 0.5
Proteins 0.8
Natural rubber has a tremendous capacity for absorbing
Quebrachitol (a sugar) 0.3
oil. If imbibed without mastication, remarkably little is
Choline 0.1
lost in vulcanized physical properties of the rubber. Such
Glycerophosphate 0.1
rubber has been shown to be superior for traction on dry
Water-soluble carboxylic acid salts 0.3
snow or ice, as a winter tire.
Amino acids and polypeptides 0.2
Inorganic salts (carbonates, phosphates) 0.2
5. DPNR
Total 3.0
Deproteinizednaturalrubberisapremiumrubber,wherein a Trace elements can include potassium, magnesium, cop-
enzymes have broken down the naturally occurring pro- per, manganese, and iron. The proteins and fatty acids are
teins. The resultant rubber has exceptional resistance to highly useful as cure activators.
fatigue failure and to rate of stress relaxation of the vul-
canizate. Its use is aimed at engineering applications, the ratio of C 5 H 8 . Williams discovered in 1860 that de-
especially under dynamic working conditions. structive distillation of rubber produced isoprene as the
building unit. Tilden reported that the probable structure
6. Epoxyprene was 2-methyl-1,3-butadiene. Bouchardat recognized that
isoprene could be polymerized into rubber in 1879.
Epoxyprene is a chemical modification of natural rubber
Analysis of typical samples of natural rubber reveals
resulting in vulcanizates of increased oil resistance,
a small, but important amount of non-rubber constituents
enhanced adhesive properties, high degree of damping,
(Table VIII).
and reduced gas permeation. Functional group interaction
While the nonrubber constituents of latex and dry rub-
results in compatible blends with a range of synthetic
ber are important, the hydrocarbon portion is still the most
polymers.
significant aspect of the total composition (Table IX).
Much evidence for the chemical structure of rubber is
7. Thermoplastic Rubber (TPR)
based on the products formed when the double bond is
TPR is a blend of natural rubber and polypropylene plas- cleavedwithoxidizingagentsorozone.Thestructureises-
tic fluxed at high temperatures. It is flexible at room tem- sentially 1,4-isoprenoid, with units joined “head-to-tail.”
perature, but can be reworked and remilled at elevated
temperatures.
B. Physical Properties
8. Novor The physical properties of rubber may vary slightly, be-
cause of the nonrubbers present, or the degree of crys-
Natural rubber can be vulcanized with isocyanates, result-
tallinity (Table X). Rubber is colloidally dispersed in latex.
ing in high temperature resistance. It is known as Novor.
9. Biological Synthesis TABLE IX Composition of Typical Natural Rubber a
Ingredients Average (%) Range (%)
The ideal goal of forming new genes has progressed as
far as developments of roots and plantlets, through the
Moisture 0.5 0.2–1.0
exploration of tissue culture.
Acetone extract 2.5 1.5–3.5
Protein (calc. from nitrogen) 2.8 2.2–3.4
Ash 0.3 0.2–0.8
V. PROPERTIES
Rubber hydrocarbon 93.0
Total 100.0
A. Chemical Properties
a
The acetone extract contains the fatty acids, sterols, and
The first recorded analysis of natural rubber was by esters. Certain of these are believed to be natural antioxidants
M. Faraday. He reported rubber to be a hydrocarbon in for rubber.