Natural rubber: still going strong.

Natural rubber: Still going strong

Natural rubber is the only rubber which can boast a history longer than that of Rubber World. `Discovered' by Christopher Columbus in the fifteenth century, it remained a curiosity until 150 years ago when the twin discoveries of mastication and vulcanization established the basis of modern rubber technology. With the growth of rubber cultivation in South East Asia at the turn of the century, associated with the growing demand for tires for the first motor cars, natural rubber became a viable and profitable crop.

For its first five decades, the pages of Rubber World would have been filled with developments relating almost exclusively to natural rubber. The second five decades have seen enormous changes, with the advent of commercial synthetic rubbers which now hold 66% of the world market. The turning point came about 40 years ago, when, for various reasons, the share of synthetic rubbers started to climb and the growth of natural rubber production slowed down. In 1962 production of synthetics by the free world overtook that of natural rubber.

That natural rubber is still a vital raw material owes much to considerable advances in the science and technology of both production and end-use over the last 50 years, and to its inherent characteristics which still make it the material of choice in many applications.

Developments in production

The steady growth of natural rubber production from around 1 million tons in the late 1930s to over 5 million tons last year is due not only to the increased hectarage under rubber, but also to agricultural research. The original plantings of Hevea brasiliensis yielded about 250 kg/ha/yr - tree breeding and selection have raised potential yields to well over 3,000 kg/ha/yr. One of the most promising new clones being developed in Malaysia has yielded more than 5,000 kg/ha/yr in the preliminary evaluation stages. The newer clones not only give higher yields but are precocious yielders.

Average yields in the major natural rubber producing countries reflect the different structure of their industries. In the three major producing countries, smallholders predominate, but to different extents. In Malaysia they account for 75% of the industry, in Indonesia 85% and in Thailand 95%. Smallholdings tend to suffer from uneconomic size, low productivity and low capital.

Increased output in Indonesia and Thailand has come mostly from increases in hectarage: in the order of half a million hectares in each country since 1960. Yields have increased from 440 kg/ha/yr in 1970 to 480 kg/ha/yr in 1986 in Indonesia; the equivalent figures in Thailand are 300 kg/ha/yr and 630 kg/ha/yr. In Malaysia, on the other hand, there has been a drop in hectarage, but the average yield has increased from 800 kg/ha/yr in 1970 to over 1,100 kg/ha/yr in 1986, leading to a slight increase in overall production. This difference in productivity between Malaysia and the two other major producers is partly due to the fact that Malaysia was the first to undertake major replanting programs in the 1960s and early 1970s.

Two further important differences exist between Malaysia and the other major producers. Malaysia is the only one of these countries with an integrated system of research, development and advisory services covering both production and consumption aspects of natural rubber. The Malaysian Rubber Research and Development Board (MRRDB) is the parent body which directs and coordinates these activities, including the production research carried out by the Rubber Research Institute of Malaysia in Kuala Lumpur, the consumption orientated work of the Malaysian Rubber Producers' Research Association in the UK, and the Technical Advisory Service to consumers worldwide provided by the Malaysian Rubber Bureau. The second difference, affecting smallholder productivity, is that in Malaysia over 400,000 hectares of smallholders land is within various organized land schemes. This enables smallholders to adopt modern agro-management practices and benefit from research, good infrastructure and social amenities. Yields in such organized schemes approach those of the estates, in Malaysia around 1,500 kg/ha/yr.

Recently efforts have been made to widen the genetic base of Hevea. The entire production industry is based on 22 seedlings derived from the material collected by Wickham over 100 years ago. An expedition in 1981 to Brazil organized by the International Rubber Research and Development Board has yielded about 14,500 seedlings, which may result in trees with superior characteristics and dramatic yield increases.

One of the most important advances in propagation techniques has involved the use of grafting to combine the best characteristics of two or more planting materials. This has resulted in the three-part tree: a high yielding trunk is grafted onto a vigorous root stock, and a disease resistant crown onto the trunk. This is now an established commercial practice which allows fast adoption of breeding innovations. A more recent advance is the use of tissue culture techniques: Hevea seedlings have been produced from somatic cells. Ultimately this could eliminate requirements for large nurseries, selection of seedling stocks and budding processes.

Productivity has also been increased by reducing the non-productive immaturity period of trees and by using chemical yield stimulants. Advanced planting materials, produced by budding and the use of core stumps, grow vigorously and uniformly when planted out and can be tapped after four years.

These young trees give high sustained yields with the use of adequate fertilizers and chemical stimulation. Ethylene-based stimulants were discovered in 1970, and their use increases yields throughout the life of the tree, even extending productive life of old trees in combination with controlled upward tapping.

Production can be constrained by labor shortages. Tapping needs skilled labor and less labor-intensive tapping systems have been developed. Half-spiral tapping every four to six days with stimulation gives yields at least as high as the current practice of tapping on alternate days, with a labor saving of up to 70%; an added advantage is low bark consumption leading to longer tree life. Mechanization of tapping and collection systems is also being developed, with techniques such as motorized tapping and polybag collection.

Future concerns may include the development of clones suited to different climatic conditions. Work in China has shown that rubber can be grown as far north as 24 [degrees], beyond the established growing belt which extends just 15 [degrees] either side of the equator. China is now the fifth largest producer, with over half a million tons per year despite relatively cool conditions and occasional typhoon winds and droughts. Recently, the RRIM has developed a computer system, Environmax, which recommends the best clones for different soil types and climatic conditions. However, possible changes in the global climate - the `greenhouse effect' - are a relatively new concern: there are many uncertainties and major climatic changes would lie outside the scope of the current database. What is certain is that natural rubber production is environmentally sound, using renewable, non-polluting natural resources. Any increase in the hectarage of rubber worldwide may help to alleviate problems caused by large-scale destruction of the tropical rain forests.

Market patterns

It is generally true that since the early 1950s all natural rubber produced has been sold. It therefore makes good economic sense to increase production in an orderly fashion, both to meet normal growth related to industrial expansion and to increase the market share of natural rubber vis-a-vis the synthetics. Here the activities of the International Natural Rubber Organization have been an important factor: two agreements in the last decade have ensured a degree of price stability and good returns for rubber growers, producing countries are thus confident enough to plan for increased rubber production and consumers are protected from wild price fluctuations.

It has been argued that on its techno-economic merits, natural rubber has the potential to take 42% of the market. Since 1950 natural rubber has been unable to meet increasing world demand for rubber (post-war demand was growing at 6% p.a., while natural rubber production increased at only a little over 2% p.a.) and has lost its predominant market position to the synthetics. Recently, total demand has slowed, with a consequent decline in synthetic growth rates and natural rubber growth remaining steady at 3.7% p.a. Its share of the market over the period 1980 to 1988 has consequently improved from 30% to 34%.

The geographical distribution of natural rubber consumption is changing: in particular, the newly industrialized and developing countries have become more important in the last two decades. Of special interest are the East Asian countries, which have increased their market share by more than 10% since 1970, from about 255,000 tons to over 500,000 tons. The most significant drop in market share has occurred in Western Europe. Consumption has grown only slightly since 1970 to almost one million tons in 1987, a decline in terms of market share from 30% to about 20%. As well as the obvious growth in Taiwan, South Korea, Hong Kong and Singapore, the natural rubber producing countries are rapidly expanding their domestic rubber consumption. Malaysia, for example, is actively encouraging the growth of resource-based industries, and government policy in this area was laid down in the Industrial Master Plan of 1986. Advantages include purchase of raw materials at source, increased export earnings and added employment.

New materials

It is not enough for the natural rubber producers simply to produce more rubber. Attention must be paid to the needs of the consumers, in terms of quality, processability and presentation of the raw rubber, and the properties required of the final product. In the 1960s, when synthetic rubbers appeared to threaten the viability of natural rubber, the first moves were made in this direction with the development and introduction of the Standard Malaysian Rubber (SMR) scheme in 1965. The other major rubber producing countries have followed suit with their own technically specified rubber schemes, and natural rubber has been transformed from an agricultural commodity to an industrial raw material.

The SMR scheme has not remained static, revisions have been, and will continue to be made in response to consumer requirements and advances in production and processing technologies. Constant viscosity grades were introduced in 1969, and further improvements were made in 1979 when the scheme was revised. This revision took account of demands for consistent processing within grades, by defining the raw materials going into each grade, introducing a cure indicator for all latex grades and tightening the rules for regrading. A new viscosity stabilized grade, SMR GP, was introduced, to provide a good quality easy-process rubber, potentially available in large volumes.

For the future, the technically specified rubbers will continue to evolve to meet the needs of the consumers. The current demand is for still greater processing consistency to maximize throughput in increasingly automated rubber product factories. At present SMR processing characteristics are indicated only by the PRI test and initial viscosity.

Current research at both RRIM and MRPRA aims to develop tests more directly indicative of processability which can be included as parameters in the scheme. Ultimately Malaysia intends to ensure that all its natural rubber, including latex, conforms to specifications relevant to consumer operations.

Developments such as technically specified rubber have ensured natural rubbers place in the field of general-purpose rubbers. Many attempts have been made over the years to chemically modify the natural rubber molecule and produce a new polymer with special properties. Some attempts have failed because processes have been uneconomical or a new synthetic material has been developed which, for one reason or another, has cornered the market. There have, however, been some success stories - rubber hydrochloride was manufactured commercially for over 30 years as a food packaging material; chlorinated rubber was used in paints; both depolymerized rubber and methacrylate graft copolymers are still in use today.

Until now, natural rubber has been unable to compete with the special-purpose rubbers in terms of properties such as oil resistance and low gas permeability. To meet these challenges, and that of the fastest growing sector, thermoplastic elastomers, new forms of natural rubber have been developed by MRRDB laboratories over the last decade. Both chemically modified epoxidized natural rubber and thermoplastic natural rubber blends are now commercially available.

With epoxidized natural rubber, the property/cost balance is right, and the competing synthetics have been in existence for some time, so the market area is well documented. In epoxidized natural rubber some of the unsaturation of the natural rubber molecule is converted to epoxide groups randomly distributed along the polymer chain. The process is applied in latex using low-cost ingredients, and is now being carried out commercially by Kumpulan Guthrie Sdn Bhd in Malaysia. Two grades, 25% and 50% epoxidized, are currently available under the trade name Epoxyprene.

Epoxidation raises the glass transition temperature of natural rubber and changes the polarity of the polymer. These factors lead to substantial property changes while maintaining the strain crystallizing nature, and therefore high strength of the base polymer. The result is increased oil resistance, reduced gas permeability, high damping and compatibility with polar synthetic rubbers such as nitrile, polychloroprene and PVC, leading to good adhesion to these polymers.

Added advantages include good reinforcement by silica without the use of coupling agents, and an interesting combination of good wet grip and low rolling resistance. Its unique combination of properties suggest a wide range of potential applications for Epoxyprene, and some of these have already been extensively evaluated. Applications where light colors and high strength are required are of particular interest. Products investigated include milking inflations, conveyors, seals, tubing, vibration mountings, accoustic dampers, inner tubes, shoe soles and adhesives. The good wet grip and low rolling resistance have led to interest in the use of Epoxyprene for specialist pneumatic or solid tires.

Several attempts have been made to produce a viable thermoplastic natural rubber. Work at MRPRA over the last 20 years has resulted in some interesting materials, but these have either been insufficiently thermoplastic or not economically viable. The latest approach is based on the partial dynamic vulcanization of natural rubber blended with polyolefins, and has produced a material with good technical performance and attractive economics. These thermoplastic natural rubber blends are now being produced commercially, by Teknor Apex in the USA (Telcar DVNR) and Vitacom in the UK (Vitacom DVNR). They have attracted considerable interest in the important TPE market, currently growing at 8-10% per year.

The TPNR blends are characterized by excellent stress strain properties, high strength, good recovery and resistance to heat aging and ozone. Depending on the relative proportions of natural rubber and polypropylene, a range from soft thermoplastic elastomers to semi-rigid rubber-toughened plastics can be produced.

The cost/performance profile of the materials lies between the traditional olefinic materials and the `engineering alloys'. Potential applications include replacement of rubber vulcanizates and other thermoplastic elastomers in products such as footwear and sports goods, and seals and hoses in the automotive sector. The latter is also the largest potential market for the hard grades, in applications such as bumpers, spoilers and body protection strips.

Other new materials from natural rubber are under investigation at MRPRA and elsewhere, of particular interest are further thermoplastic blends, an oil-resistant thermoplastic elastomer, and low molecular weight liquid rubbers, either as adhesives or as process aids for rubber.

End use research

Improving the consistency in processing of raw natural rubber and developing new materials from natural rubber are two important aspects of MRPRA's role in protecting and extending the market for Malaysian natural rubber. Other areas of work aim to improve formulations and processes, develop new uses, and generally protect the market from erosion by the synthetics. In the past, significant advances have been made in injection molding with natural rubber, the use of rubber in engineering applications, the development of oil-extended natural rubber (OENR) for winter tire treads and improved vulcanizing systems. All this has been backed by fundamental research which has helped the understanding of vulcanization, aging and properties such as elasticity, strength, abrasion and fatigue.

Past work on engineering applications has led to the acceptance of natural rubber as an engineering material, and the market is now one of the largest and fastest growing areas of nontire natural rubber use. Current research on longevity and dynamic mechanical properties has shown that natural rubber is superior to synthetic engineering polymers in many respects, and this basic work adds to the fund of knowledge used to promote natural rubber in engineering. Some interesting success stories include the world land speed record using natural rubber suspension, and the steel-laminated natural rubber bearings for the solid fuel booster rockets used in the space shuttle.

The most interesting development in civil engineering applications, and one with great market potential, is that of earthquake protection. The idea arose about 15 years ago, following widespread adoption of steel-laminated natural rubber bearings for bridges and buildings. Initial computer studies established that the thinking was sound, and MRPRA then instituted extensive development and testing work in collaboration with the Earthquake Engineering Research Center at the University of California, Berkeley. The resulting system was adopted for the Foothill Communities Law and Justice Center in San Bernardino.

This building has been subjected to minor earthquakes and has responded well compared to conventionally protected structures, greatly reducing the forces transmitted to the building. The system differs from other proposed base isolation systems in that the rubber is specially designed to eliminate the need for additional mechanical or wind sway prevention devices.

The stability of natural rubber over a wide range of temperatures and frequencies is essential for successful implementation. The system is now being installed in the Los Angeles County Fire Department Control Center and is being considered for several other emergency service buildings in California. Interest is also high in Japan and Europe.

Work has been resumed on OENR in tire treads. Originally developed in the 1970s to provide enhanced grip in ice and snow, OENR is now being assessed in the light of more recent developments: all-season tires and the requirement for low rolling resistance. Compared with standard synthetic compounds, OENR offers improved wear performance at low temperatures, lower rolling resistance and equivalent wet traction, as well as improved snow and ice traction. A slight disadvantage in summer wear rates is more than offset by the low rolling resistance and the low wear in winter.

The wet grip and rolling resistance properties of epoxidized natural rubber have been mentioned already, and MRPRA is assessing its performance in tire treads. Wear performance has been improved by blending with butyl rubber, but at the expense of a slight loss in wet traction. Blends with other polymers and the use of black/silica combinations are now being considered.

A relatively new area of tire research is that of improving natural rubber formulations for truck tire retreads. At one time natural rubber was the preferred material, but its position in this important market has been gradually eroded because of the widespread use of precured retreading where synthetic rubbers are favored. In this process, the good building tack of natural rubber is not a primary consideration, and the profile of the tread reduces the need for natural rubber's low heat-build-up properties. The favored SBR/BR precured retreads give excellent wear performance in long-haul low-severity conditions. New reversion-resistant natural rubber based compounds have been shown, both in the laboratory and in fleet trials, to outperform the best synthetic treads under a range of service conditions. A major UNIDO-funded project is now underway, involving the building and testing of precured truck tire retreads in the UK and five other countries. Technologists from the rubber producing countries are being trained using the latest retreading equipment, thus contributing to technology transfer.

The most important area of use for natural rubber after tires is in latex applications, and consumption is growing at about 5% per year. Malaysia supplies about 75% of the world's natural latex: about 20% of her natural rubber production is in the form of latex concentrate.

The RRIM and MRPRA are actively involved in improving the properties and quality of the raw material and its products, as well as advising manufacturers on latex use. Areas of research include viscosity and stability, prevulcanization, leaching and lubrication. The current surge of commercial interest in the manufacture of dipped disposable examination gloves has led to work aimed specifically at improvements in the production process for examination gloves. Also of topical interest is the bateriocidal nature of natural latex foam: here it shows a distinct advantage over polyurethane foam: this could become a `selling point' to recover some of the market lost to the synthetic material.

Health and safety legislation is now affecting all areas of manufacturing and rubber is no exception. Most of the current interest is in nitrosamines, particularly in baby soothers and teats, in gloves, food contact applications and toys. Raw natural rubber is free of nitrosamines and nitrosatables, and formulations can meet all the current North American and European regulations. Teats and soothers have also been tested for strength properties: natural latex teats have tear, tensile and puncture strengths two to three times greater than silicone teats. The former should therefore meet the requirements of a bite test being developed by the British Standards Institution.

Looking to the future, an area of major importance is likely to be that of polymer blends. Some have said that the development of new rubbers is unnecessary - an almost infinite range of vulcanizate properties may be obtained by blending existing polymers. To be successful, this approach must achieve the correct morphology and optimum crosslink distribution. MRPRA is already working in this area, with a new characterization technique to measure crosslink densities in the separate phases of blends. Once this can be accurately measured, it should be possible to adjust distribution by changing the rubbers and/or curing systems, and ultimately improve blend properties.

Prospects for NR are bright. Research and development has enabled the producing industry to meet demand in terms of both quantity and quality, and enabled consumers to make the best use of top quality materials.

There is no doubt that, with the backing of science and technology, natural rubber will continue to meet the challenges of the competitive world of modern polymers.

PHOTO : Grafting techniques are used for fast, cost-effective propagation of good planting materials. Such techniques are used successfully in commercial estates and have been introduced in the smallholder sector.

PHOTO : The use of yield stimulants painted on the tapping cut allows trees to be opened early for tapping and fine control of latex production.

PHOTO : The recent surge in demand for examination and surgeons gloves created high demand for natural rubber latex.

PHOTO : Nozzle bearings for the solid fuel booster rockets of the space shuttle were manufactured from steel laminated NR.