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Natural Rubber
This is nature's main "ready-made" contribution
to the elastomer field. Its chief source is the Hevea
Brasiliensis, a commercially grown tree found principally
in the Far East. The bulk of natural rubber used for today's
engineering applications is plantation rubber, smoked sheet
and pale crepe being the best and most important. Natural
rubber, polyisoprene, still offers the optimum balance of
properties necessary for high performance in many demanding
mechanical applications. Quality compounds can be made for
a wide range of stiffness requirements.
It has high resilience,
outperformed only by some of the more recent man-made polyisoprenes
and polybutadienes. It exhibits very good tensile and tear
properties over a wide stiffness range and has excellent
resistance to cold flow. It is possible to make compounds
that exhibit low permanent set at temperatures up to 200
F. Abrasion resistance is good, though inferior to BR (polybutadiene)
and SBR (styrene-butadiene). Natural rubber has better low-temperature
flexibility than most synthetics, but is not as good as silicone
rubber or some of the special butadiene and SBR compounds.
Natural rubber compounds can be made with a wide range of
electrical properties.
Natural rubber does not
age as well as many of the synthetics nor is it as chemically
inert as some. It is inferior to many of the synthetics for
heat aging, resistance to sunlight, oxygen, ozone, solvents
or oils.
It can be bonded satisfactorily
to a wide range of materials and is used in a variety of
application, including tires, gaskets, seals, rolls, hose,
tubing, vibration isolators, shock mounts, electrical components,
bumpers, drive wheels, etc. It is the material used for most
high performance ap plications unless some specific environmental
condition is met.
SBR
SBR is a synthetic copolymer
of styrene and butadiene. Although it is one of the earlier
synthetics, it still represents the largest variety being
made today. The copolymer includes a number of types, each
developed for specific applications. It can be obtained in
the form of latex or as a dry product, similar to natural
rubber.
The general balance of
properties that can be obtained is a little below that obtainable
with natural rubber, but the cost of the base material is
lower and fluctuates less. Certain types give slightly better
wear resistance in tire treads; others provide better low-temperature
flexibility. It has about the same resistance to solvents
and chemicals as natural rubber but has superior water resistance.
On heat aging, it hardens
and becomes brittle instead of softening as does natural
rubber. Pure gum (unreinforced) high-strength compounds cannot
be made since molecular alignment to oppose stress is difficult;
crystallization does not occur. Resistance to sunlight and
ozone is about the same as natural rubber.
It can be bonded to a
wide variety of materials and used in many pro ducts interchangeably
with natural rubber. Broadly then, SBR allows controlled
cost preparation of materials providing good wear resistance,
low-temperature flexibility and good resistance to sunlight
and ozone, with wide bonding latitude.
High Quality Neoprene
Neoprene is a polymer
of chloroprene and has several properties superior to natural
rubber, such as better resistance to gasoline, sunlight,
ozone and oxidation. It is not only flame resistant, but
will not support combustion. It has good resist ance to the
corrosive action of chemicals, and its water resistance is
as good as natural rubber. It has good resistance to heat,
and does not soften as does natural rubber under severe exposure.
Resilience is almost equal to natural rubber, being surpassed
today only by the butadienes.
Compression set and creep
characteristics vary in different forms, from types that
are inferior to natural rubber to other types which are better
- particularly under high-temperature long-time service.
The tear resistance is equal to natural rubber at room temperature;
at elevated temperatures tear resistance is poor but can
be improved to some extent by compounding with reinforcing
materials.
Neoprene is commonly blended with
other polymers for various applications. The term "commercial" neoprene
is used to describe a blended neoprene. Physical properties
of a "commercial" neoprene will vary widely.
EPDM
Ethylene-Propylene-Diene
Modified (EPDM) is a copolymer of ethylene and propylene
which has outstanding resistance to aging, weathering, ozone,
oxygen and many chemicals. High and low temperature stability
as well as steam and water resistance are excellent. Dynamic
and mechanical properties are, in general. between natural
rubber and SBR.
EPDM finds uses in many
static and dynamic applications where the above properties
are important. It can be extruded or molded. It should not
be used where continual contact with petroleum based products
is required.
NBR
NBR or nitrile elastomers
are copolymers of butadiene and acrylonitrile, used primarily
for applications requiring excellent resistance to petroleum
oils and gasoline. Resistance to aromatic hyrocarbons is
better than Neoprene but not as good as polysulfide. NBR
has excellent resistance to mineral and vegetable oils, but
relatively poor resistance to the swelling action of oxygenated
solvents such as acetone, methyl ethyl ketone and other ketones.
It has good resis tance to acids and bases with the exception
of those having strong oxidizing effects. Resistance to heat
aging is good, often a key advantage over natural rubber.
With higher acrylonitrile
contents, the solvent resistance is increased but low-temperature
flex ibility is decreased. Low-temperature resistance is
inferior to natural rubber, and although NBR can be compounded
to give improved perform ance in this area, the gain is normally
at the expense of oil and solvent resistance. As with SBR,
this material does not crystalize on stretching and reinforcing
materials are required to obtain high strength. With compounding
it is possible to get a fairly good balance between low creep,
good resilience, low permanent set and good abrasion resistance.
Tear resistance is inferior
to that of natural rubber and electrical insulation is lower.
NBR is used instead of natural rubber where added resistance
to petroleum oils, gasoline or aromatic hydrocar bons is
required. The properties of this elastomer make it useful
for carburetor and fuelpump diaphragms, aircraft hoses and
gaskets, where it competes with polysulfide and the neoprene
elastomers.
Silicone Rubber
Silicone rubber is one
of the versatile family of semi-organic synthetics known
as silicones that look and feel like organic rubber, yet
have a completely different type of structure than other
elastomers. The backbone of the elastomer is not a chain
of carbon atoms but an arrangement of silicone and oxygen
atoms. This structure gives a very flexible chain with weak
interchain forces. This accounts for the remarkable small
change in dynamic characteristics over a wide range of temperature.
Silicone elastomers show no molecular orientation or crystalization
on stretching and must be strengthened by reinforcing materials.
Silica reinforced elastomers
available today have tensile strengths approaching 2000 psi
compared with peak values of only 600 psi for carlier grades.
In addition, elongations of more than 600% have been achieved
as compared to a maximum of 300% for the earlier materials.
Silicone elastomers can
be made that will with stand temperatures as high as 600
F without serious deterioration, and at the other end of
the temperature scale will retain flexibility 150 F. The
elastomers remain flexible and are serviceable over this
entire temperature range. No plasticizers are needed that
might cause some sacrifice in properties in some temperature
range.
While silicone elastomers
have lower strength than other elastomers, they are amazingly
fatigue and flex resistant, probably as a result of their
chemical inertness. They do not require high tensile and
tear strength to make suitable for dynamic applications.
Fall off in tensile properties at higher temperatures is
less than for other elastomers and these values are retained
on extended exposure. Resistance to chemical deterioration,
oils, oxygen and ozone is also retained under these conditions.
Chemical inertness makes these materials of special interest
for surgical equipment and food processing.
By changing molecular
arrangement, silicone elastomers can be produced with special
characteristics such as: low compression set, low temperature
resistance, high-temperature resistance, or high dielectric
strength. These types provide a tremendous range of property
balance, and still others are under development or in final
stages of field testing.
Flurocarbon Polymer
Fluorocarbons are the
end product of the copolymerization of highly fluorinated
olefins.
Fluorocarbons find unique
usage in hot oil environments where outstanding compression
set is also required. Typical properties at 400 deg. F for
a 70 Shore A hardness compound would be 15% to 30% compression
set and less than 5% volume swell in ASTM #3 oil or automatic
transmission fluid.
Fluorocarbons are resistant
to the effects of ozone, oxygen and sunlight.
Practical compounds are
available in 60-90 Shore A durometer hardness.
Typical examples of trade
names are "Viton" and "Flourel."
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