Page 266 - Handbook of Plastics Technologies

ELASTOMERS

4.58 CHAPTER 4

The rubber is harvested from the tree in the form of a latex, the aqueous emulsion ob-
tained from the tree by tapping into the inner bark and collected in cups attached to the
trees. The latex itself can be used for the fabrication of rubber articles, but most of the NR
is used as a dry raw rubber taken from the coagulated latex. There are many types and
grades of the dry rubber. However, the Malaysian rubber industry produces standard NR
grades that correspond to technical specifications. Their system is being followed by other
producer countries, thus the designations SMR (Standard Malaysian Rubber), SIR (Stan-
dard Indonesian Rubber), SSR (Specified Singapore Rubber), SLR (Standard Lank Rub-
ber), TTR (Thai Tested Rubber), and NSR (Nigerian Standard Rubber). Within a national
standard type, there are grades that differ with respect to color, viscosity, molecular
weight, and other qualities.
NR contains small amounts of highly important nonrubber constituents, e.g., proteins,
sugars, and fatty acids. Some of these nonrubber components are vulcanization activators,
antidegradants, and, unfortunately, allergens.
NR can be vulcanized by using any of a number of vulcanizing systems, e.g, acceler-
ated sulfur, peroxide, phenolic (resole), quinonedioxime, and others. However, by far, ac-
celerated sulfur systems are the most used.
Properties. NR vulcanizates have a range of interesting properties. Individual proper-
ties of NR can be surpassed by those of synthetic rubbers, but the combination of high ten-
sile strength, high resilience, good low-temperature flexibility, and low hysteresis and heat
buildup is unique. In addition, the building tack and green strength of NR are unsurpassed
by synthetic rubbers. Building tack is the ability for unvulcanized pieces of rubber to stick
together during the building process, for example, for a tire, where plies and other compo-
nents must adhere and “become one” before vulcanization. Green strength is the mechani-
cal strength of the uncured polymer. It is high in the case of natural rubber because, even
before vulcanization, natural crystallizes during straining. This property is likely also re-
lated to building tack, wherein there would be crystallization at the autoadhesive interface
due to high local strains as one attempts to pull apart one component from the other.
NR vulcanizates can be produced in a wide hardness range (Shore A 30 to that of hard
rubber or ebonite). Due to its crystallization during strain, NR has high tensile strength
even without reinforcing fillers (e.g., carbon black). Also, because of strain-induced crys-
tallization, the tear resistance of NR vulcanizates is quite high. The ultimate elongation of
a NR vulcanizate is generally between about 500 and 1100 percent. Also, NR vulcanizates
have very good fatigue resistance (resistance to repeated strains, each one alone less than
ultimate). With respect to elastic rebound, NR vulcanizates are surpassed only by those of
BR.
The heat resistance of NR is not good enough for many uses, and it is exceeded by
many synthetic rubbers. It is affected by the choice of vulcanization system, vulcanization
conditions, choice of protective agents, and even choice of filler. To obtain good aging re-
sistance of NR vulcanizates, one must use protective agents in the compound and use rela-
tively short curing cycles at relatively low temperatures.
Because of its main-chain double bonds, unstabilized NR exhibits extremely poor re-
sistance to atmospheric ozone. Its light-colored vulcanizates have poor resistance to

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2006 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.