Production, classification and properties of NR

Rubber World, August, 2005 by M. Brendan Rodgers, Donald S. Tracey, Walter H. Waddell

Natural rubber (NR) is among the most important elastomers used in the manufacturing of rubber products such as tires and conveyor belts. Roberts has reviewed natural rubber, covering topics ranging from basic chemistry and physics, production and applications (ref. 1). Barlow (ref. 2) and later, Klingensmith, et al (ref. 3) and Rodgers, et al (ref. 4) prepared comprehensive overviews of natural rubber and compounding. This material, which represents a truly renewable resource, comes primarily from Indonesia, Thailand, Malaysia, India and the Philippines, though many more additional sources of good quality natural rubber are becoming available. It is a material that is capable of rapid deformation and recovery, and it is insoluble in a range of solvents, though it will swell when immersed in organic solvents at elevated temperatures. Some of its many attributes include abrasion resistance, good hysteretic properties, high tear strength, high tensile strength and high green strength. However, due to strain crystallization, it may also display poor fatigue resistance. It may be difficult to process in some factories, and it can show poor tire performance in areas such as traction of wet skid when compared to synthetic elastomers such as styrene butadiene rubber (SBR). Given the importance of NR material, this review will discuss:

* The chemistry and production of natural rubber;

* industry classification, descriptions and specifications; and

* fundamental technological properties of natural rubber.

Chemistry of natural rubber

Natural rubber is cis-1,4-polyisoprene. It is a linear long chain polymer of repeating isoprene (2-methyl-1,3-diene) units with a specific gravity of 0.93 at 20[degrees]C. Natural rubber is synthesized in vivo via enzymatic polymerization of isopentenyl pyrophosphate (IPP). IPP undergoes repeated condensation to yield cis-polyisoprene via the enzyme, rubber transferase. Though bound to the rubber particle, this enzyme is also found in the latex serum. Structurally, cis-polyisoprene is a highly stereoregular polymer with a -OH group at the [alpha]-terminal unit and three to four trans-units at the omega end of the molecule (i.e., the point of synthesis).

The full biosynthesis or polymerization to yield natural polyisoprene is illustrated in figure 1 (refs. 3-5). This biosynthesis begins with the addition of IPP to dimethylallyl pyrophosphate (DMPP) to forro a trans-, trans-, trans-geranylgeranylpyrophosphate. This is believed to be the initiator for polymerization of cis-polyisoprene (figure 2) (refs. 6 and 7).

[FIGURES 1-2 OMITTED]

The resulting presence of the sequence consisting of approximately two to four trans isoprene units linked to the o)terminal unit suggests that the initiator of cis-polyisoprene biosynthesis is a prenylpyrophosphate having ah all-trans configuration. The number of trans units has been reported to be typically three per polymer chain, thus rendering natural rubber from Hevea brasiliensis essentially up to 99.5% cis. The 3,4-isomeric structure has been reported at very low levels. For comparative purposes, synthetic polyisoprene prepared with an Al-Ti catalyst has a microstructure of around 99% cis-1,4, up to 0.7% trans-1,4, and the remainder of up to 0.3%, assuming a vinyl-3,4 structure (refs. 3 and 4).

Molecular weight distribution of Hevea brasiliensis rubber shows considerable variation from clone to clone, ranging from 100,000 to over 1,000,000. Natural rubber has a broad, bimodal, molecular weight distribution. The polydispersity or Mw/Mn can be as high as 9.0 for some clones of natural rubber (refs. 8 and 9). This tends to be of considerable significance in that the lower molecular weight fraction will facilitate ease of processing in end product manufacturing, while the higher molecular weight fraction contributes to high tensile strength, tear strength and abrasion resistance properties (refs. 3 and 10).

The isopentyl pyrophosphate starting material is also used in the formation of farnesyl pyrophosphate. Subsequent condensation of trans- farnesyl pyrophosphate yields trans- polyisoprene of 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, it is much harder and less elastic (refs. 3 and 4).

Production of natural rubber

Natural rubber is obtained by tapping the side of the tree, Hevea brasiliensis. Tapping starts when the tree is from 5 to 7 years old and continues until it reaches around 20 to 25 years, after which time it is usually replaced. A knife is used to make a downward cut from left to right and at around a 20 to 30 degree angle to the horizontal plane, to a depth of approximately 1.0 mm from the cambium. Latex can then exude from the cut and flow from the incision into a collecting cup. Rubber trees are tapped about once every two days, yielding a cupful of latex, each containing approximately 50 grams of solid rubber. With trees cultivated at a density of 375 per hectare (150 per acre), approximately 2,500 kilograms of rubber can be produced per hectare per year, which is approximately one ton per acre per year (ref. 2).