Advances and developments in NR.

The Rubber Research Institute of Malaysia (RRIM) continues to fulfill its important objectives of researching into all aspects of natural rubber cultivation, production and processing, and also rubber product manufacture. Among these objectives, the emphasis is on reducing the cost of production, increasing productivity, improving the quality and consistency of rubber and increasing the usage of rubber in the local rubber manufacturing industries.

In the biological research areas, much progress has been achieved in breeding and selection of hevea. The RRIM 2000 series clones, in particular, have been recommended for planting of latex timber clones (LTCs). These clones are characterized by high latex yield and high wood volume. Twelve clones of this series have been vigorously promoted for planting in the monitored development projects (MDPs) at various locations throughout Peninsular Malaysia.

In the biotechnological research areas, hevea tissue culture has found a new application in the genetic transformation of the rubber tree. The transgenic tree would serve as a `green' factory for the production of high-value proteins such as pharmaceuticals, e.g., insulin, tumor necrosis factors and blood coagulating factors.

In chemistry and technology, considerable achievements have been attained in the processing and product sectors. Important topics of concern to the rubber products manufacturers are being studied, such as low protein latex, deproteinized NR (DPNR), NR without foul smell, chemical derivatives from NR serum, blends from NR and the use of NR in engineering applications.

Latex timber clones

LTCs are characterized by high latex yield and high wood volume. The RRIM, through breeding and selection, introduced latex timber clones belonging to the RRIM 2000 series in 1995. Eight clones of RRIM 2000 series were launched during the period of 1995 to 1997. Wide scale planting of these clones is found in MDPs throughout Malaysia.

The planting of LTCs will help sustain the future supply of rubberwood, an important component in the furniture industry. From recent selection, an additional four clones derived from the RRIM 2000 series were added to the Planting Recommendations. The availability of more LTCs allows flexibility in choice of clones and suitability of various agro-climatic conditions. LTCs are also the preferred planting materials for rubber forest plantation. Altogether there are 14 LTCs, inclusive of two from the RRIM 900 Series, and these are currently being recommended for planting.

The mean yield from the twelve clones ranged from 2,007 to 2,850 kg/ha/yr. (table 1) (ref. 1). These yields are much higher than the yield of the control clones, RRIM 600 and PB 260. In addition, these clones also produce high timber yield. The estimated mean total wood volume ranged between 0.68 to 1.87 [m.sup.3] per tree at 14 years old.

Table 1 - RRIM 2000 series clones

Clone        Yield(*)           Total wood
            (kg/ha/yr.)   volume ([m.sup.3]/tree)

RRIM 2001      2,850               1.23
RRIM 2002      2,348               1.10
RRIM 2008      2,686               1.32
RRIM 2009      2,277               0.68
RRIM 2014      2,007               1.33
RRIM 2015      2,760               1.30
RRIM 2016      2,582               1.28
RRIM 2020      2,232               1.00
RRIM 2023      2,822               0.81
RRIM 2024      2,685               1.26
RRIM 2025      2,700               1.87
RRIM 2026      2,204               1.11

(*) Extrapolated yield from small scale clone trial

The NR industry has become an increasingly important supplier of raw material for the furniture industry. With the world becoming more environmentally conscious and rubberwood being a renewable resource and environmentally friendly, it is not surprising that spectacular growth in export earnings from rubberwood furniture has been recorded. Rubberwood furniture accounts for about 80% of Malaysian annual exports of wooden furniture.

Low labor input latex exploitation systems

The competing demands for labor among various sectors in Malaysia is an impeding factor in rubber cultivation and production in both the estate and smallholder sectors. Reliance on foreign labor is only a short-term measure, but given the current rubber price, even family labor is hard to come by. The interest in rubber cultivation is slowly fading.

Latex exploitation technologies are now being geared towards approaches that require fewer tappers and that optimize tapper productivity. There are two main approaches that have been shown to be practicable; (a) a reduction in the frequency of tapping, and (b) an increment in task size (i.e., the number of trees that a tapper has to complete tapping). Not only do these schemes require fewer tappers, but significantly higher wages that come with higher productivity per tapper, but the significantly higher wages that come with higher productivity per tapper serve as incentive for the tapper to remain on the job.

While the aforementioned latex exploitation systems employ the conventional excision method of tapping, new systems of tapping, RRIMflow (ref. 2) and ReactoRRIM (ref. 3) offer the most effective systems in increasing both tapper and land productivity. In these systems the rubber trees are subjected to gaseous ethylene stimulation. Latex is extracted through open puncture, rather than excision tapping in the conventional method. The exuded latex is collected in sealed receptacles that are impervious to rain. Latex flow by this method of exploitation is sustained at a slow but steady rate over two to three days as compared with two or three hours with conventional tapping.

Biotechnology: Foreign protein production by the transgenic rubber tree

Besides planting hevea for its conventional rubber crop, the rubber tree can also be genetically engineered for novel applications. The rubber tree produces voluminous latex which can be extracted non-destructively through the process of tapping. By transforming hevea with genes that control the production of high-value proteins (e.g. pharmaceuticals), transgenic rubber plants could serve as efficient, low cost, low maintenance and environmentally-friendly production lines for the production of the targeted protein. The system would enable continual harvesting of the protein exuded in the latex.

The closest parallel to the transgenic rubber tree concept is the production of pharmaceuticals in the milk of transgenic animals, such as cows and sheep. Progress in this area has taken the research to a status far beyond that of a mere laboratory curiosity. Instead of producing pharmaceuticals in the milk of animals, transgenic rubber trees can serve as living `factories' to produce pharmaceuticals in the latex it exudes. Principally among the advantages of using a plant system over an animal system are the relative cost efficiency of growing plants versus raising animals and the fact that plants do not harbor animal viruses (an important consideration in pharmaceuticals for human use). Using this approach, a recombinant antibody has been successfully produced in the latex of the transgenic rubber plant.

New types of latex concentrate

In view of the health related issues confronting the NR latex industry, the RRIM has developed several new types of latex concentrate to address the protein allergy problems at their source.

Several cost effective processes for the reduction of extractable protein (EP) of gloves have been developed by the RRIM (refs. 4-9). These include the use of two types of low protein latices, namely LPPL (low protein prevulcanized latex) produced by re-centrifugation of a prevulcanized latex (refs. 6) and LOPROL (low protein latex), produced by a partial enzymatic deproteinization of latex (refs. 7 and 8). The use of LOPROL is capable of producing gloves with a much reduced level of residual EP, and the reduction becomes more effective when used with proper leaching protocol during processing. The allergenicity of the gloves produced from these latices was found to be negligible by skin prick testing on sensitized individuals. This is in consonance with the EP contents, which are of the order of 0.1 mg/g as measured by RRIM Lowry against bovine serum albumin (figure 1) (ref. 9).

[GRAPH OMITTED]

Standard Malaysian glove

Malaysia is fully committed to addressing the protein allergy issue. Great effort has been made to ensure glove users are provided with the best and safest protective device available in the market whereby the Standard Malaysian Glove (SMG) Scheme provides a mark of quality assurance (ref. 10).

Table 2 shows some of the technical requirements for NR examination gloves made in Malaysia to be certified as SMG. Tensile properties shall be measured in accordance with ISO 37, taking a minimum of three pieces from each glove and using the median value as the test result. The determination of extractable protein shall be conducted following the modified Lowry method.

Table 2 - tensile properties and extractable
protein

Property                                  Unit      Requirement

Minimum tensile strength before
  aging                                   MPa            21
Minimum elongation at break before
  aging                                    %            700
Minimum tensile strength after aging
  7d/70 [degrees] C                       MPa            16
Minimum elongation at break after
  aging 7d/70 [degrees] C                  %            500
EP (prepowdered glove)                 [micro]g/g       300
EP (powder free glove)                 [micro]g/g        50

Quebrachitol from NR serum

Natural rubber serum (NRS) is the aqueous portion of NR latex which is separated from the rubber after the coagulation process, and is usually discarded by depositing into large ponds for a period of time. The RRIM, in collaboration with Yokohama Rubber, has explored an approach in recovering the non-rubber substances from NRS and developed useful applications from them. The most useful substance extracted from NRS is quebrachitol (or 2-O-methyl-L-inositol). It has attracted much attention because of its optical properties and also because of recent investigations which show that certain inositol derivatives are connected to the mechanisms for the transmission of information between living cells, a process which is referred to as "cell-signaling." For example, it has been disclosed recently that inositol-1,4,5-triphosphate, which is a decomposition product from inositol phospholipid and diacyglycerol, plays an important role as "second messengers" in the cell-signaling mechanism. It is thought, therefore, that by suitable chemical modifications, some inositol derivatives might be able to function as an anti-cancer drag, antibiotic or as an enzyme-inhibitor (ref. 11).

Specialty natural rubber grades

In addition to Technically Specified Rubber, Standard Malaysian Rubber (SMR), the Malaysian Rubber Board has developed several types of specialty NR. These specialty rubbers cater to the needs of specific areas of application. The most recent commercially available grades are deproteinized natural rubber (DPNR), NR without foul smell (SUMAR) and epoxidized natural rubber (ENR), while the most established grades are methacrylate-grafted natural rubber (MG) and superior processing natural rubber (SP). New rubbers currently in the development pipeline are liquid natural rubber (LNR), thermoplastic natural rubber (TPNR) and thermoplastic epoxidized natural rubber (TPENR).

Deproteinized natural rubber

Deproteinized natural rubber has low protein and ash content. It is produced by treating NR latex with an enzyme which breaks down the naturally occurring protein and other nonrubber materials into soluble residues that are subsequently washed away during coagulation and washing (ref. 12). By removing these non-rubber materials, its water sensitivity is reduced, resulting in NR with low affinity to water. Formulating DPNR with rubber soluble curative produces vulcanizate that is low in compression set and creep and has good dynamic properties. These properties inherent in DPNR make it the most suitable rubber for use in automotive engineering applications. For such applications, the rubber components function as vibration, noise and shock absorbers, dampers, flexible transmission couplers, seals and bushes.

The physical properties of DPNR CV are comparable to the properties of SMR CV, as shown in table 3 for vulcanizates based on a typical black-filled formulation. In addition to good dynamic properties, DPNR has low volume swell in water and low compression set. A combination of these properties had made DPNR the rubber suitable for hydromount for automotive engines. In this component, the rubber would isolate vibration at high frequency while the fluid (hydro) enhances damping, thus reducing transmissibility peaks at low frequency.

Table 3 - typical vulcanizates properties of DPNR-CV

Property                                         SMR CV   DPNR-CV

Mooney scorch time, t5 @ 120 [degrees] C, min.    >60.0      46.7
Optimum cure time, t95 @ 150 [degrees] C, min.     18.8      18.3
Tensile strength, MPa                              27.4      27.3
Elongation at break, %                              482       520
Modulus at 100%, MPa                               2.35      2.01
Modulus at 300%, MPa                               13.5     11.34
MR 100, MPa                                        1.41      1.25
Hardness, IRHD                                       55        52
Lupke resilience, 23 [degrees] C, %                66.9      63.0
Compression set, 1d/70 [degrees] C, %              23.7      20.2
Volume swell in water, %
  7d/23 [degrees] C                                1.30      0.94
  3d/100 [degrees] C                                8.0      5.70
Compression stress relaxation, 25% strain
  7d/23 [degrees] C, %                             11.7      8.60
Air aging, 7d/100 [degrees] C, % retention
  Tensile strength                                   84        81
  Elongation at break                                73        72
  Modulus at 300%                                   140       149

Natural rubber without foul smell

This new grade of NR which does not have a foul smell is called SUMAR (standard uniprocess natural rubber) and it has provided further avenues for consumers to select the grade of NR preferred for their process (ref. 13). The development of this new grade is considered timely to address the malodor problem which is one of the global environmental issues. The typical physical properties of SUMAR in comparison to SMR 20 and RSS 1 are shown in table 4. SUMAR exhibits better tear properties and abrasion characteristics than RSS 1. The heat build-up is also lower than RSS 1.

Table 4 - typical vulcanizate properties of Sumar

Physical properties                   SUMAR   SMR 20   RSS 3

Tensile strength, MPa                    27       28      26
Elongation at break, %                  540      550     510
M100, MPa                               2.4      2.4     2.7
M330, MPa                              10.8     10.9    11.9
Compression set %,                       33       33      34
  22h. @ 70 [degrees] C
DIN abrasion, ARI                       103      110      94
Rebound resilience,                      62       68      65
  % @ 23 [degrees] C
Trouser tear, N/mm                       16       17      11
Hardness, IRHD                           64       63      67
Heat build-up (0.025"/24 lb.)            29       26      34
  100 [degrees] C, temperature rise
  @ 120 min., [degrees] C
Aged 7 days/70 [degrees] C
Tensile strength, MPa                    25       27      26
Elongation at break, %                  500      490     470
M100, MPa                               2.9      2.9     3.0
M300, MPa                              12.5     13.7    14.7
Hardness, IRHD                           70       69      71

Thermoplastic natural rubber

The work on thermoplastic natural rubber (TPNR) has been carried out by various workers (refs. 14-20). The material is essentially prepared by blending NR with polyolefins such as polypropylene. The properties of TPNR depend on the composition of the blend. At high rubber content, the material behaves as a thermoplastic elastomer in which the rubber phase normally contains some crosslinks. For the semi-rigid material in which the rubber content is low, the rubber phase is dispersed in the continuous matrix of the polypropylene phase and hence leads to improved toughness, elongation at break and ductility.

The soft grades of TPNR blends, which can be classified as thermoplastic elastomers, have hardnesses in the range of 55 to 80 Shore A. Typical properties of the blends covering this range are given in table 5. The T-PNR blends are not oil-resistant materials and the oil-resistant types are obtained from the blend of epoxidized NR (ENR) and polypropylene, i.e., TPENR.

Table 5 - typical properties of soft blends of TPNR

Hardness, Shore A                           55    60     70     80
M100(MPa)                                  3.1   3.7    4.8    5.9
Tensile strength (MPa)                     5.4   8.0   10.1   12.8
Elongation at break (%)                    300   300    300    350
Tear strength (N/mm)                        20    21     27     35
Compression set, 22hr./70 [degrees] C, %    42    40     40     50

Thermoplastic epoxidized natural rubber

Epoxidized natural rubber is a Modified form of NR in which a proportion of the double bonds is converted to epoxide via reaction with peracetic acid (ref. 21). One important change is a marked increase in resistance to swelling by oils. As a result of this, the thermoplastic rubber produced by blending ENR with polypropylene is therefore oil resistant (ref. 22). Typical properties of TPENR having hardness in the range of 65 to 85 Shore A are presented in table 6. The tensile and tear strength are typical of dynamically vulcanized blends of elastomers and polypropylene such as Santoprene. The compression set values are good within the context of thermoplastic elastomers, particularly at elevated temperatures. The oil resistance of TPENR is close to that of acrylonitrile butadiene rubber (NBR) containing medium acrylonitrile content.

Table 6 - typical properties of TPENR

Hardness, Shore A                65    75    85
M100, MPa                       3.7   5.0   6.2
Tensile strength, MPa           6.5   8.8   9.6
Elongation at break, %          240   260   255
Tear strength, Die C, N/m        23    31    36
Compression set
  24h./23 [degrees] C, %         24    29    33
  24h./100 [degrees] C, %        36    39    45
  168h./100 [degrees] C, %       44    49    55
  72h./120 [degrees] C, %        49    55    55
Volume swelling, %
  ASTM #2, 3d/125 [degrees] C     0     1     2
  ASTM #3, 3d/125 [degrees] C    14    17    17

The aging resistance of TPENR is excellent, as illustrated by good retention of properties in accelerated aging tests (table 7) and in long term tests (table 8). In addition to this, TPENR has excellent resistance to ozone. There is no cracking after exposure to ozone at levels of up to 200 pphm for three days at 20% strain and 40 [degrees] C. This represents four times the severity, in terms of ozone level, as used in the ISO test.

Table 7 - percentage retention of properties on
aging TPENR (65 Shore A) in air

Aging condition        7 days at    15 days at    7 days at
                          125          125           135
                      [degrees] C   [degrees] C   [degrees] C

M100                      109           128           130
Tensile strength          104           146           132
Elongation at break       102           117           117
Table 8 - percentage retention of properties on
long aging of TPENR (65 Shore A) at 100 [degrees] C

Aging period, weeks        12            24            48

M100                      112           115           130
Tensile strength          109           102           110
Elongation at break        96            86            76

Liquid natural rubber

Normal NR is of high molecular weight ([M.sub.W]) exceeding one million. The [M.sub.W] of NR can be reduced by means of a depolymerization process. Dry NR in liquid form, termed as liquid NR (LNR), may be produced if the [M.sub.W] is below about 20,000.

Several methods to produce LNR have been developed. The preparation of LNR involves reacting NR latex with an oxidizing agent, as well as a reducing agent to depolymerize the latex to the desired [M.sub.W]. The depolymerized latex can be further processed to LNR by coagulating the latex and drying the latex coagulum or to depolymerize latex concentrate by the normal centrifuging process. Depolymerized latex concentrate is unique as the concentrate may be available up to 80% total solid content. The unconcentrated depolymerized latex can also be blended with normal latex to produce special dry blends of NR and LNR.

LNR is now produced at a pilot plant at RRIM (ref. 23). It is intended for applications in adhesives, binders, sealant, processing aids for polymers, etc. The latex concentrate can be directly used for applications such as in aqueous based adhesives or as [M.sub.W] modifiers. The dry blends of NR and LNR have improved processability characteristics such as ease of mixing during rubber compounding and low mixing energy requirement. At suitable [M.sub.W], the LNR can also co-vulcanize and hence contribute to the physical properties of the rubber vulcanizates. In this case, LNR serves as a co-vulcanizable plasticizer in rubber processing.

NR/EPDM blends

NR vulcanizates have good elasticity and strength properties, but poor resistance to heat and ozone. Ethylene-propylene-terpolymer rubber (EPDM) vulcanizates, on the other hand, have excellent heat aging and ozone resistance. It has been a practice that NR is blended with EPDM in order to obtain the best characteristics of each component or for cost consideration. However, this practice often results in poor physical properties. This has partially been explained by migration of curatives from the EPDM to the NR phase resulting in the EPDM being undercured and the NR overcured.

A recent study, adopting a `reactive mixing' procedure for EPDM masterbatch mix, has shown that improved cocurability and good black distribution throughout the NR/EPDM blends can be obtained, resulting in improved physical properties. The reactive mixing procedure involves the use of BAPD (bisalkylphenol disulfide) with DTDC (dithiodicaprolactam) for effective modification of the EPDM phase in the blend to give improved crosslinking, morphology and good interaction with carbon black (ref. 24). These blends have found useful applications such as automotive weather seals and grommets.

NR impact absorbers

Natural rubber has been successfully used as shock cells to absorb the impact during the installation of gas production platforms in the open sea (ref. 25). The gas production platforms were initially built onshore with the entire auxiliary equipment and substructures installed and commissioned. The integrated platforms were floated-over into the open sea to a designated field and were then mated with preinstalled fixed steel structures. NR absorbers preinstalled in each of the steel structures would absorb the enormous amount of energy generated during the mating process. Each integrated platform weighs between 8,000 to 8,500 metric tons.

Conclusions

The MRB, through its research units, RRIM and TARRC, has undertaken research activities covering a very wide field embracing numerous scientific disciplines from biology to chemistry, physics, rubber technology and manufacturing technology. The skills and expertise acquired over the years have contributed significantly to the progress of the Malaysian rubber industry with respect to innovation, technical advancement, production efficiency and impact on consumers. The R&D activities will continue to be reviewed in order to achieve a high level of productivity in the rubber industry. Some strengthening in the R&D efforts in certain directions is also desirable. It is foreseeable that Malaysia will gradually shift from being primarily a provider of NR as an agricultural material, to that of being a provider of NR as industrial materials and value-added rubber products.

References

(1.) Planters' Bulletin, n. 3, LGM Planting Recommendations 1998-2000 (1998).

(2.) S. Sivakumaran in "Towards greater viability of the natural rubber industry," Abdul Aziz S.A. Kadir, Ed., Rubber Research Institute of Malaysia, 1971, p. 123.

(3.) Mohd. Raffali Mohd. Nor and Ahmad Zarin Mat Tasi in "Ensuring sustainability and competitiveness of the natural rubber industry," Abdul Aziz S.A. Kadir, Ed., Rubber Research Institute of Malaysia, 1995, p. 113.

(4.) Lai, P.F. and Ng. K.P., "Production of high quality latex products: Malaysian efforts in meeting future requirements," Proc. Edu. Symp. No. 36, ACS Rubber Div. Meeting, Philadelphia, 1995.

(5.) Subramaniam, A., Yip, E., Ng, K.P. and Mok, K.L, "Extractable protein content of gloves from prevulcanized NR latex," Proc. Latex Protein and Glove Industry, 1993, p. 76.

(6.) Hafsah Mohd. Ghazaly, "Properties of NR low protein latex," Proc. Latex Protein and Glove Industry, 1993, p. 81.

(7.) Hafsah Mohd. Ghazaly, "Factory production of examination gloves from low protein latex," J. Nat. Rubb. Res., 1994, 9 (2), p. 96.

(8.) Ng, K.P., Yip, E. and Mok, K.L, "Production of NR latex gloves with low extractable protein content: Some practical recommendations," J. Nat. Rubb. Res., 1994, 9 (2), p. 87.

(9.) Yip E., Turjanmaa, K., Ng, K.P. and Mok, K.L. "Allergic responses and levels of EP in NR latex gloves and dry rubber products," J. Nat. Rubb. Res., 1994, 9(2), p. 78.

(10.) SMG Bulletin No. 1, Malaysian Rubber Board, 1998.

(11.) S. Ozaki and Y. Watanabe, "Synthesis of inositol polyphosphates from myo-inositol," Ehime University.

(12.) T.C. Khoo and Abdul Rahman Rais, "A new process of the production of deproteinized of natural rubber," Nat. Semin. On Intensification of Research in Priority Areas, Industrial Sector, Malaysia. 1991.

(13) Yusof Azia, "New NR to address environmental issues," Malaysian Economic and Technical Mission, 1998, p. 1.

(14.) Campbell, D.S., Elliott, D.J. and Wheelans, M.A., "Thermoplastic natural rubber blends," NR Technology, 1978 9 (2), p. 21.

(15.) Elliot, D.J., "Developments with thermoplastic natural rubber blends, "NR Technology, 1981, 12, 59.

(16.) Elliot, D.J. and Tinker, A.J., "Thermoplastic natural rubber blends," Proc. IRC 1985, Kuala Lumpur.

(17.) Mathew, N.M. and Tinker, A.J., "Impact-resistant polypropylene/natural rubber blends," J. Nat. Rubb. Res., 1986, 1, 240.

(18.) Tinker A.J., "Thermoplastic natural rubber blends - an update, "NR Technology, 1987, 18 (2), 30.

(19.) Gelling I.R. and Tinker A.J., "Thermoplastic natural rubber performance and applications," Proc. IRTC, 1988, Kuala Lumpur.

(20.) Abu bin Amu, "Thermoplastic natural rubber," Plastics and Rubber Inst. Malaysia, Seminar Polym. Blends, 1991, Kuala Lumpur.

(21.) Gelling I.R. and Smith J.J.F., "Controlled viscoelasticity by natural rubber modification," Proc. International Rubber Conference, Venice (1979).

(22.) Patel J., Riddiford C.L. and Tinker A.J., "TPENR, A thermoplastic elastomer combining oil resistance and heat stability," Proc. International Rubber Technology Conference, Kuala Lumpur (1993).

(23.) Sidek Dulngali and Zainul Abidin M, "Liquid natural rubber," Nat. Semin. Intensification of Research in Priority Areas, Industrial Sector, Malaysia, 1991.

(24.) S. Cook, "Solutions to the basic problems of poor physical properties of NR/EPDM blends," Blends of Natural Rubber, Ed. A.J. Tinker and K.P. Jones, Chapman and Hall, p. 169, 1998.

(25.) Kamarul Baharain Basir, "Natural rubber impact absorbers for installation of offshore gas production platforms," Second Regional Conference on Materials Technology, IMM/PRIM, Kuala Lumpur, 1999.