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390 Rubber, Natural

TABLE X Some Physical Properties of Natural When heated to about 80 C, it becomes soft and tacky,
◦
Rubber indistinguishable from RSS at that temperature.
X-Ray diffraction studies of rubber in the unstretched
Density 0.92
state show a pattern of two concentric amorphous bands.
◦
Refractive index (20 C) 1.52
When stretched to elongations short of rupture, such as
◦ −1
Coefficient of cubical expansion 0.00062 C
700%, a fiber pattern is revealed. This consists of well-
Cohesive energy density 63.7 cal cm −3
defined interference spots, called “crystallites.” The regu-
Heat of combustion 10,700 cal g −1
lar arrangement indicates a unit cell with a repeat distance
Thermal conductivity 0.00032
˚
along the chain structure of 8.1 A.
C
Dielectric constant 2.37 cal s −1 cm −2 ◦ −1
Raw rubber also shows a property called “racking.” If
15
Volume resistivity 10 cm −3
rubber is stretched repeatedly and rapidly near its ultimate
Dielectric strength 1,000 V mil −1
breaking elongation, followed each time by cooling (dry-
ice bath), a highly fibrous structure develops. These fibers
are very stiff and strong.
Theaveragediameterofthelatexparticlesisabout0.5µm.
Other thermal effects are noted in rubber. When rubber
These particles are constantly in motion due to Brownian
is stretched rapidly, it heats up noticeably, contrary to the
movement. Each particle carries a negative electric charge,
behavior of most materials. Also, if rubber is stretched
hence repelling each other and imparting stability to the
under a load, while held at the other end, it will retract as
latex.
the temperature is raised. These two thermal effects are
Natural rubber has both a sol and gel phase. Differ-
jointly known as the Gough–Joule effect.
ences are illustrated by their behavior to solvation. Highly
Finally, the prime importance of crystallization in nat-
branched and lightly cross-linked gel resists solvation.
ural rubber is its self-reinforcing effect. As elongation
Effective solvents for sol rubber are aliphatic and aromatic
hydrocarbons, chlorinated hydrocarbons, ethers, and car- increases, so does crystallization, contributing to the ulti-
mate tensile strength of natural rubber. It is this factor that
bon disulfide. Nonsolvents include the lower alcohols,
makes natural rubber superior in strength or pure gum or
ketones, and certain esters.
non-black-filled vulcanizates.
The gel phase can be broken down into processible
rubber by mechanical shearing (mills, mixers, plastica-
tors), or oxidation (assisted by heat), or chemically (pep- C. Biosynthesis
tizers). Rubber with high gel content is undesirable for
Natural rubber is structurally a simple example of terpenes
calendering, extrusion, or other fabrication processes.
and terpenoids, one of the most important groups of nat-
ural products. Terpenes are related from the usually reg-
1. Molecular Weight ular union of the isopentane carbon skeleton of isoprene.
This is described in the Ruzicka “Biogenetic Isoprene
Since rubber is a high-molecular-weight substance, ranges
Rule.” The important implication is that all terpenes and
of values exist. Viscosity measurements of the dry rubber
terpenoids have a common precursor. This was found to be
in shear are commonly used for relative assessment of
isopent-3-enyl pyrophosphate (IPP), the addition product
molecular weight. Solution viscosity of the sol rubber in
of pyrophosphoric acid to isoprene.
organic solvents is also valuable.
The search for such precursors only became possible
Fractionation of rubber from solution reveals a range
through isotopic labeling techniques. The conversion of
of polymers of varying molecular weight. Such fractions
IPP into rubber is complex and detailed, best summarized
will range from 200,000 to 400,000 in average molecular
to include the role of acetic acid as a precursor in all rubber
weight. The electron microscope has permitted measure-
plants, and that of IPP as the monomer.
ment of the size of the molecular particles. Size/frequency
data can then be converted into a molecular weight distri-
bution curve. VI. PRODUCT USAGES
Although much blending of raw material is performed
2. Crystallinity
with field latex and/or natural coagula, such as cup lump,
There are two geometric configurations for natural rubber. efforts to attain uniformity are intensive. With standard
These are the cis and trans isomeric forms. The normal rubber grades, such as sheets and crepes, the manufacturer
standard rubber, such as sheets and crepes, has the cis still resorts to blending of various shipment lots. With
form. Balata or gutta-percha has the trans form. The lat- technically specified rubbers, such blending is usually not
ter is hard and horny (crystalline) at room temperature. necessary.