A. Chemistry of Natural Rubber

Figure 1 Cis and trans isomers of natural rubber.



weight fraction contributes to high tensile strength, tear strength, and abrasion resistance. The biosynthesis or polymerization to yield polyisoprene, illustrated in Figure 2, occurs on the surface of the rubber particle(s) (5).

The isopentyl pyrophosphate starting material is also used in the

formation of farnesyl pyrophosphate. Subsequent condensation of transfarnesyl pyrophosphate yields trans-polyisoprene or gutta percha. Gutta

percha is an isomeric polymer in which the double bonds have a trans configuration. It is obtained from trees of the genus Dichopsis found in southeast

Asia. This polymer is synthesized from isopentenyl pyrophosphate via a

pathway similar to that for the biosynthesis of terpenes such as geraniol and

farnasol. Gutta percha is more crystalline in its relaxed state, much harder,

and less elastic.

Natural rubber is obtained by ‘‘tapping’’ the tree Hevea brasiliensis.

Tapping starts when the tree is 5–7 years old and continues until it reaches

around 20–25 years of age. A knife is used to make a downward cut from left

to right and at about a 20–30j angle to the horizontal plane, to a depth

approximately 1.0 mm from the cambium. Latex then exudes from the cut and

can flow from the incision into a collecting cup. Rubber occurs in the trees in

the form of particles suspended in a protein-containing serum, the whole



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Figure 2



Simplified schematic of the biosynthesis of natural rubber.



constituting latex, which in turn is contained in specific latex vessels in the tree

or other plant. Latex constitutes the protoplasm of the latex vessel. Tapping

or cutting of the latex vessel creates a hydrostatic pressure gradient along the

vessel, with consequent flow of latex through the cut. In this way a portion of

the contents of the interconnected latex vessel system can be drained from the

tree. Eventually the flow ceases, turgor is reestablished in the vessel, and the

rubber content of the latex is restored to its initial level in about 48 hr.

The tapped latex consists of 30–35% rubber, 60% aqueous serum, and

5–10% other constituents such as fatty acids, amino acids and proteins,

starches, sterols, esters, and salts. Some of the nonrubber substances such as



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lipids, carotenoid pigments, sterols, triglycerides, glycolipids, and phospholipids can influence the final properties of rubber such as its compounded

vulcanization characteristics and classical mechanical properties. Hasma and

Subramanian (6) conducted a comprehensive study characterizing these

materials to which further reference should be made. Lipids can also affect

the mechanical stability of the latex while it is in storage, because lipids are a

major component of the membrane formed around the rubber particle (7).

Natural rubber latex is typically coagulated, washed, and then dried in either

the open air or a ‘‘smokehouse.’’ The processed material consists of 93%

rubber hydrocarbon; 0.5% moisture; 3% acetone-extractable materials such

as sterols, esters, and fatty acids; 3% proteins; and 0.5% ash. Raw natural

rubber gel can range from 5% to as high as 30%, which in turn can create

processing problems in tire or industrial products factories. Nitrogen content

is typically in the range of 0.3–0.6%. For clarity a number of definitions are

given in Table 1.

The rubber from a tapped tree is collected in three forms: latex, cuplump, and lace. It is collected as follows:

1.



Latex collected in cups is coagulated with formic acid, crumbed,

or sheeted. The sheeted coagulum can be immediately crumbed,

aged and then crumbed, or smoke-dried at around 60jC to

produce typically ribbed smoked sheet (RSS) rubber.



Table 1 Definitions of Natural Rubber Terms

Latex Fluid in the tree obtained by tapping or cutting the tree at a 20–30j angle to

allow the latex to flow into a collecting cup.

Serum Aqueous component of latex that consists of lower molecular weight

materials such as terpenes, fatty acids, proteins, and sterols.

Whole field latex Fresh latex collected from trees.

Cup-lump Bacterially coagulated polymer in the collection cup.

Lace Trim from the edge of collecting vessels and cut on tree.

Earth scrap Collecting vessel overflow material collected from the tree base.

Ribbed smoked sheets (RSS) Sheets produced from whole field latex.

LRP Large rubber particles.

NSR Nigerian standard rubber.

SIR Standard Indonesian rubber.

SLR Standard Lanka rubber.

SMR Standard Malaysian rubber.

SRP Serum rubber particles.

SSR Standard Singapore rubber.

TSR Technically specified rubber.

TTR Thai tested rubber.



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2.



3.



Cup-lump is produced when the latex is left uncollected and allowed to coagulate, due to bacterial action, on the side of the

collecting cup. Field coagulum or cup-lump is eventually collected,

cut, cleaned, creped, and crumbed. Crumb rubber can be dried at

temperatures up to 100jC.

Lace is the coagulated residue left around the bark of the tree

where the cut has been made for tapping. The formation of lace

reseals the latex vessels and stops the flow of rubber latex. It is

normally processed with cup-lump.



The processing factories receive natural rubber in one of two forms: field

coagula or field latex. Field coagula consists of cup-lump and tree lace (Table

1). The lower grades of material are prepared from cup-lump, partially dried

small holders of rubber, rubber tree lace, and earth scrap after cleaning. Ironfree water is necessary to minimize rubber oxidation. Field coagula and latex

are the base raw materials for the broad range of natural grades described in

this review. Fresh Hevea latex has a pH of 6.5–7.0 and a density of 0.98 (3,4).

The traditional preservative is ammonia, which in concentrated solution is

added in small quantities to the latex collected from the cup. Tetramethylthiuram disulfide (TMTD) and zinc oxide are also used as preservatives because

of their greater effectiveness as bactericides. Most latex concentrates are

produced to meet the International Standard Organization’s ISO 2004 (8).

This standard defines the minimum content for total solids, dry rubber

content, nonrubber solids, and alkalinity (as NH3).



B. Production of Natural Rubber

Total global rubber consumption in 2001 was approximately 17.5 million

metric tons (tonnes) of which 7.0 million tonnes (40%) was NR and the

remaining was synthetic rubber (9). World production of NR was down by

3% from the same period in 2000, with all the major producing countries

decreasing their output. The major regional consumers of natural rubber are

North America and eastern Asia, led predominantly by China and Japan. For

the period 2002–2007 it is anticipated that Western European and Japanese

consumption will increase due to economic recoveries in both areas, with

sustained economic activity in the United States, Japan, and China having

only limited impact on increased global consumption. The net impact will be

further growth in consumption toward 8.0 million tonnes per year. Natural

rubber consumption will then increase slowly toward 8.5 million tonnes, this

being dependent on global economic conditions (Fig. 3). Globally, natural

rubber consumption is split—with tires consuming around 75%, automotive

mechanical goods at 5%, nonautomotive mechanical goods at 10%, and



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Figure 3 Global natural rubber productions (millions of tonnes).



miscellaneous applications such as medical and health-related products

consuming the remaining 10% (10).

There are around 25 million acres planted with rubber trees, and

production employs nearly 3 million workers, with the majority coming from

smallholdings in Indonesia, Thailand, Malaysia, India, and West Africa.

Many times, the dominance of smallholdings has raised issues regarding

quality and consistency, which will be discussed later. Smallholdings produce

mainly cup-lump, which is used in block rubber. Sheet rubber is generally

regarded to be of higher quality, typically displaying higher tensile and tear

strength.

In 1964 the International Standards Organization published a set of

draft technical specifications that defined contamination, wrapping, and bale

weights and dimensions, with the objectives of improving rubber quality,

uniformity, and consistency and developing additional uses for contaminated

material (11,12).

The three sources leading to crumb rubber (i.e., unsmoked sheet rubber,

aged sheet rubber, and field cup-lump) typically provide different grades of

technically specified rubbers. For example, one grade of technically specified

rubber (TSR L) is produced from coagulated field latex, TSR 5 is produced

from unsmoked sheets, and lower grades such as TSR 10 and 20 are produced

from field coagulum. A simplified schematic of the production process is

presented in Figure 4.

C. Natural Rubber Products and Grades

Natural rubber is available in six basic forms:

1.

2.

3.



Sheets

Crepes

Sheet rubber, technically specified



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