G. Use of Natural Rubber in Tires

Figure 7 Natural rubber content by tire line (% of total tire weight) and

relationship to tire performance triangles. (RLT, radial light truck; RMT, radial

medium truck; OTR, off-the-road.) (From Ref. 36.)



Copyright © 2004 by Taylor & Francis



At temperatures below 100jC, natural rubber can be difficult to break

down on mills or internal mixers and subsequently process. However, when

first placed in a hothouse or broken down on warm-up mills, natural rubber–

based compounds can be processed quite easily. Peptizers enable a lowering of

compound mixing temperatures, permit shorter dwell time in internal mixers,

and thus save energy. When natural rubber is compounded with a highly

reinforcing carbon black such as N121 or N134, strong interactions can occur

such as hydrogen bonding, covalent bond formation, and van der Waals

forces. When immersed in a solvent such as toluene, free rubber can be

extracted, leaving a swollen rubber-filler gel (i.e., bound rubber). Uncured

compound that has been stored for long periods will show an increase in

bound rubber content with consequent loss in ease of factory processing (33).

Bound rubber content is not a constant property but will evolve until a fixed

value is attained. The change in bound rubber content can be readily

estimated from Mooney viscosity and Mooney peak data. This provides the

factory rubber technologist with a simple tool to determine factory compound

shelf life and times between compound mixing and subsequent extrusion,

calendering, or other processing step. In the absence of refrigeration, truck

tire tread compounds containing 100 phr natural rubber, 50 phr carbon black,

and 3–5 phr of process oil will have a shelf life of 3–5 days before extrusion due

to the increase in bound rubber.

Radialization has led to significant increases in the use of natural

rubber, and this will continue as the use of radial tires increases in farm

equipment, large earthmovers, and aircraft tires and the size of the bias truck

tire market decreases. A comparative study was undertaken to obtain an

overall assessment of natural rubber use by tire component for both RMT

and automobile tires. The Malaysian Natural Rubber Producers Association

and Smithers Scientific Services have both reported on the use of natural

rubber in the various components of a tire. Typical levels of natural rubber in

tread compound, base compound, sidewall, and wire coat compounds of

three major classes of tires are presented in Table 9 (34–36).

The bulk of natural rubber is compounded with other elastomers to

produce blends and thereby obtain the desired mechanical properties. The

natural rubber content in tread compounds can range from 10 phr, when it

has been added to improve processing qualities, to 100 phr, as when it is used

in commercial radial truck tires for good hysteresis and tear strength. Other

polymers typically blended with natural rubber are polybutadiene (BR) for

resistance to abrasion and fatigue, styrene butadiene rubber (SBR) for

traction and stiffness, butyl rubber (IIR) and halobutyl rubber (CIIR, BIIR)

for enhanced tire traction performance, and synthetic polyisoprene (IR) for

processing qualities. Tire sidewalls are typically 50:50 blends of natural

rubber and polybutadiene for resistance to fatigue, cut growth, and abrasion.



Copyright © 2004 by Taylor & Francis



Table 9 Natural Rubber Levels (phr) in Selected Types of Tires and

Tire Components

Automobile

tires



Component

Tread

Base

Sidewall

Wire coat (breaker coat)



Radial medium

truck tires



Bias truck

tires



45

70

45

100



90

100

55

100



50

—

40

70



Source: Refs. 34–36.



Internal components of tires such as wedges, wire skim or wire coat

compounds, fabric skim compounds, and gum strips typically contain 100

phr natural rubber for component-to-component adhesion, tear strength, and

hysteretic qualities.



III. OTHER NATURALLY OCCURRING MATERIALS

Naturally occurring materials fall into two fundamental classes: organic or

biotechnology products and inorganic materials. Table 10 provides a simple

overview of the range of materials of interest that are either already available



Table 10 Examples of Naturally Ocurring Materials for Use in Rubber

Compounds

Material

Guayule

Rice husks

Starch

Bamboo fillers

Pine tar

Rosin

Coumarone

Indene

Waxes

Fatty acids

Cotton



Compounding

ingredient type



Potential replacement for

synthetic material



Replacement for

natural rubber

Filler

Filler

Filler

Tackifying resin

Tackifying resin

Resin



Silica

Silica

Clay, silica

Synthetic resins

Synthetic resins

Synthetic resins



Processing aid

Vulcanization system

Filler, reinforcement



Synthetic oils

N/A

Carbon black, polyester, nylon



Copyright © 2004 by Taylor & Francis



Synthetic elastomers



or are under investigation. Though many of these will be discussed elsewhere

in this text, it is appropriate to list them and note their potential application.

Though all require various degrees of processing prior to use in rubber

compounds, they still represent renewable sources of raw materials available

to the rubber technologist.

Inorganic mineral fillers already find extensive use in rubber compounds.

1.



2.



3.



Talc is used in products such as carpet backing and can be

effective when blended with reinforcing fillers such as silica or

carbon black. Particle sizes can range from 0.5 to 10.0 Am.

Clays such as kaolin and bentonite can also be used in combination with silica or carbon black. Particle sizes tend to range

from 0.5 to 5.0 Am. High surface area chemically modified clays

will improve the tensile strength, abrasion resistance, and tear

strength of the rubber product.

Calcium carbonate can be used as a filler even though its reinforcing properties are negligible. Surface modification by use of

coupling agents can enhance the properties of compounds containing calcium carbonate. However, it is most effective when

blended with carbon black or silica.



Biotechnology fillers offer considerable potential and have attracted

attention in recent scientific literature. At this point they fall into three

primary categories: silica ash derived from rice husk waste, starch, and

bamboo fibers. Burning of rice husks leaves a waste consisting of SiO2

(95%), CaO, MgO, Fe2O, K2O, and Na2O. Rice husks that have been milled,

filtered, and then treated with sodium hydroxide, hydrochloric acid, and

water produce a hydrated silica that when compounded can produce a

material with mechanical properties similar to those of silica and carbon

black. White rice husk silica contains around 95% SiO2, whereas black rice

husk silica is approximately 55% silica and 45% carbon. Residual carbon

cannot be completely eliminated because it is trapped within the amorphous

silica structure or is completely coated with silica so it is impossible to remove

it by thermal processes. The reinforcement properties of black rice husk ash

are comparable to those of calcium carbonate and not as effective as those of

carbon black or silica. White rice husk ash when added up to 20 phr in a

natural rubber based compound did show good compound properties that

were nearly equivalent to those found for silica-loaded compounds (37–39).

Starch has considerable potential when blended with carbon black or

silica to improve the hysteretic properties of compounded rubber. This has

implications for improvement in, for example, tire rolling resistance. Of the

range of materials, biofillers hold the most promise for future increases in



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