Production, classification and properties of natural rubber--part 2.

(Part 1 appeared in the August issue)

Viscosity and viscosity stabilization of NR

The three properties of natural rubber that are most important for its use in tire or other products manufacturing plants are viscosity, fatty acid bloom and compliance with the technical specification. Of these three parameters, viscosity is the most important. This property is a function of the elastomer's molecular weight, molecular weight distribution and amount of other materials present in the polymer, such as low molecular weight resins, fatty acids and other natural products. Viscosity impacts the initial mixing of the rubber with other compounding ingredients and subsequent processing of the compounded materials to form the final manufactured product (refs. 3 and 4).

Natural rubber viscosity is a function of two major factors:

* Viscosity of the rubber produced by the specific clone; and

* the viscosity stabilization method.

A variety of methods is available to characterize the viscosity of natural rubber. The most popular is Mooney viscosity (Vr) and is obtained by measuring the torque that is required to rotate a disc embedded in the rubber or a compounded sample. This procedure is defined in

ASTM D1646 titled 'Standard test methods for Mooney viscosity, stress relaxation and prevulcanization characteristics (Mooney viscometer)' (ref. 17). The viscosity will typically range from 45 to over 100. The information obtained from a Mooney viscometer can include:

* Viscosity (Vr), typically measured at 100[degrees]C, provides a measure of ease with which the material can be processed. It depends on molecular weight and molecular weight distribution, molecular structure such as stereochemistry and polymer chain branching, and non-rubber constituents. Caution is always required when attempting to establish relationships between Mooney viscosity and molecular weight. It is expressed as ML(1+4) or sometimes ML(1+8) (i.e., Mooney large rotor, with a one minute pause and four or eight minute test duration).

* Stress relaxation, which can provide information on gel (tx95), is defined as the response to a cessation of sudden deformation when the rotor of the Mooney viscometer stops. The stress relaxation of rubber is a combination of both elastic and a viscous response. A slow rate of relaxation indicates a higher elastic component in the overall response, while a rapid rate of relaxation indicates a higher viscous component. The rate of stress relaxation can correlate with molecular structure characteristics such as molecular weight distribution, chain branching and gel content. It can be used to give an indication of polydispersity, [M.sub.n]/[M.sub.w]. It is determined by measuring the time for a 95% (T-95) decay of the torque at the conclusion of the viscosity test.

* Delta Mooney, typically run at 100[degrees]C, is the final viscosity after 15 minutes. This provides another measure of the processing characteristics of the rubber. It will provide a measure of the ease to process compounds initially milled before being extruded or calendered (e.g., hot feed extrusion systems).

* Mooney Peak, which is the initial peak viscosity at the start of the test, is a function of the green strength and can be a measure of compound factory shelf life.

* Compounded polymer pre-vulcanization properties or scorch resistance for the compounded natural rubber, which is conducted at temperatures ranging from 120[degrees]C to 135[degrees]C.

Much work has been done to establish a relationship between Mooney viscosity and molecular weight of natural rubber, as well as the molecular weight distribution. Bonfils and co-workers measured the molecular weight and molecular weight distribution of a number of samples of rubber from a variety of clones of Hevea brasiliensis, and noted the trend shown in table 8 (ref. 18).

Although clearly not linear, there is an empirical relationship between Mooney viscosity and [M.sub.w]. Nair (ref. 19) and Subramaniam (refs. 20 and 21) established relationships between intrinsic viscosity and Mooney viscosity with a correlation coefficient of 0.87. This correlation can be improved by mastication of the test samples, thereby improving the homogeneity. Mastication or milling also narrowed the molecular weight distribution, which is an important factor.

The cure characteristics of natural rubber are highly variable, due to such factors as maturation of the specific trees from which the material was extracted, method of coagulation, pH of the coagulant, preservatives used, dry rubber content and viscosity stabilization agent.

A standardized formulation has been developed to enable a comparative assessment of different natural rubbers, and has been known as the ACS1 (American Chemical Society #1). The formulation consists of natural rubber (100 phr), stearic acid (0.5 phr), zinc oxide (6.0 phr), sulfur (3.5 phr) and 2-mercaptobenzothiazole (MBT, 0.5 phr). This formulation is very sensitive to the presence of contaminants or other materials that may be present in natural rubber such as fatty acids, amines and amino acids, which may influence the vulcanization rate. The formulation has been documented in ASTM standard D3184 (ref. 22).

Natural rubber is susceptible to oxidation. This can affect both the processing qualities of the rubber and also the final compounded rubber mechanical properties. Natural antioxidants will offer protection to degradation of natural rubber, and this can be measured by change in the material's plasticity. The Wallace plasticity test reports two measures:

* Plasticity (Po), which is a measure of the compression of a sample after a load has been applied for a defined time.

* Plasticity retention index (PRI), which measures recovery after a sample has been compressed, heated and subsequently cooled. PRI % is defined as ([P.sub.30]/Po) x 100 and where Po is the plasticity and [P.sub.30] is the plasticity after aging for 30 minutes at typically 140[degrees]C. During processing in, for example, a tire factory, natural rubber with low PRI values tends to break down more rapidly than those with high values.

Various equations have been proposed which provide an empirical relationship between Mooney viscosity (Vr) and Wallace plasticity, Po. These equations depict a linear relationship between these two parameters and are therefore typically of the form

(4) Vr = X Po + C

The numerical coefficient, X, and constant, C, are a function of the clone and grade of rubber. The coefficient X normally falls between 1.15 to 1.50 and the constant C between 4.0 to 12.5 in equation 4 (ref. 21).

There are additional materials that can be added to assist in improving the processability of natural rubber. These include peptizers such as 2,2' dibenzamidodiphenyl disulfide which, when added at levels of around 0.25 phr, can significantly improve productivity of the mixers, allow lower mixing temperatures, improve mixing uniformity and reduce mixing energy. Synthetic polyisoprene, when added at levels up to 50% of the total polyisoprene content, will also reduce viscosity with little loss in other compound mechanical properties. It also allows for better control of component tack that is important in subsequent product assembly steps such as occurs in tire building.

Natural rubber tends to harden during processing and storage at the plantation processing factory, during shipping, and prior to use in the rubber products manufacturing facility. This hardening phenomenon is manifested as an increase in viscosity. The viscosity increase of natural rubber with storage is due to oxidation of the polymer chain and cleavage to form the ketone group, -C(C[H.sub.3]) = O, and aldehyde, -C-CH = O. The aldehyde can readily react with the -N[H.sub.2] groups in proteins to form a gel, and thereby increase polymer viscosity. This occurs primarily during the latex drying process, which can last for five to seven days, when latex temperatures can reach as high as 60[degrees]C.

Materials may be added to natural rubber to suppress this increase in viscosity, and this is the basis for the development of CV rubbers. Hydroxylamine neutral sulfate (N[H.sub.2]OH.[H.sub.2]S[O.sub.4]), denoted as HNS, or propionic hydrazide (PHZ, figure 5), can be added to natural rubber latex at levels between 0.08 and 0.30 wt. %, and 0.20 to 0.40 wt. %, respectively, to prevent gel formation. An accelerated storage-hardening test can measure the hardening of CV rubber that will occur during normal storage. When hydroxylamine neutral sulfate is added before coagulation, treated rubbers will show a Po change of eight units or less (constant viscosity, CV). However, they will tend to display a darker color due to the HNS addition (ref. 4).

[FIGURE 5 OMITTED]

Both HNS and PHZ block the reaction of the aldehyde groups with -N[H.sub.2] by reacting with the -C(C[H.sub.3]) = O group to form (ref. 23):

(5) R'-C (C[H.sub.3]) = N-NH-CO-R and R'-C (C[H.sub.3]) = CH-N-CO-R

In compounded rubber, the term 'bound rubber' has frequently been used to describe this crosslinking condition, both in natural rubber and also other highly unsaturated elastomers such as polybutadiene (ref. 24). Bound rubber, which can also be found in all synthetic unsaturated elastomers, is thought to be due to a variety of factors such as covalent bonding, hydrogen bonding and strong van der Waals forces. It can be readily measured by solvent extraction to remove polymer, leaving a swollen insoluble gel. Bound rubber formation can result from use of high structure carbon blacks, use of silane coupling agents, or application of fast to ultra-fast accelerators (such as zinc diethyldithiocarbamate, found in low temperature cure vulcanization systems). Mooney peak testing can be used as an indication of bound rubber.

A number of natural rubber production techniques can have an impact on the final viscosity of the rubber:

* Latex dilution: The effect is small, with 1:1 dilutions required to have any measurable effect.

* Ammonia: Increase in the ammonia level added initially for preservation from 0.01% to 0.05% can result in a Mooney viscosity increase of up to 10 Mooney units.

* Coagulation method: This can range from natural or bacterial coagulation methods to addition of formic acid or by heating. Mooney viscosity will range from 65 to 85, with higher Mooney material being obtained through use of natural coagulation techniques.

* Maturation: Storage of latex prior to drying and sheeting can cause an increase in Mooney viscosity due to an increase in gel. This rise in gel content can be due to an increase in pH due to partial hydrolysis of protein and amino acids and subsequent crosslinking, or increase in bacterial action.

* Drying temperature: Above 60[degrees]C there is a slight increase in Mooney viscosity.

Other factors that can affect viscosity are baling temperature, the age of the tapped rubber tree, yield stimulants, and seasonal effects may also play a role. If baled hot, the rubber can take a considerable time to cool. When hot, the elastomer gel content or other crosslinking phenomena may increase.

Because of the stereoregular structure of the polymer, natural rubber crystallizes when strained or when stored at low temperatures. This phenomenon is reversible and is very different from storage hardening. The rate of crystallization is temperature-dependent and is most rapid between -20[degrees]C and -30[degrees]C, and varies by grade, with pale crepe rubbers tending to show the greatest degree of crystallization. The rapid crystallization of natural rubber is also due to non-rubber constituents present in the rubber. Fatty acids, and particularly stearic acid, can act as a nucleating agent in strain induced crystallization (refs. 25 and 26). This can influence the end product performance, such as tires, where strain crystallized rubber can display reduced fatigue resistance but demonstrate improved green strength, tensile strength and abrasion resistance when compared to elastomers which do not display this phenomenon. High stearic acid levels have also been reported to improve the wear performance of truck tire tread compounds; the improvement being attributed to improved reversion resistance, though clearly factors such as crystallization and better resistance to thermal and oxidative degradation may also play a role (refs. 1 and 27).

Quality

In many respects, the end-user quality requirements for natural rubber are similar to those for synthetic elastomers such as SBR, polybutadiene (BR) or butyl rubber. Quality parameters of importance can therefore include:

* Consistency. Within a grade, end consumers of natural rubber require more uniformity, less spread in properties such as plasticity retention index, and a desire to eliminate the need for warming (placing in a heated room prior to mixing) of the rubber prior to mixing.

* Uniformity in compounding. In tire and industrial goods manufacturing, natural rubber uniformity (e.g., viscosity) is required for final compound consistency and consistent downstream processing characteristics such as extrusion.

* Packaging. Bales must be wrapped properly to ensure no moisture penetration, and prevent mold growth. Adhesion of bales during shipping and storage must be prevented. Material supply reliability includes correct box weights, labeling and other appropriate identification information such as certificates of analysis and material safety data sheets (MSDS), depending on the regulatory requirements and policies of the receiving plant location. An ISO specification, ISO/DIS 20299-2 is in preparation, which will define a low-density polyethylene bale wrap for natural rubber. It is proposed that the gauge will be between 30 [micro]m and 50 [micro]m, and melting point below 110[degrees]C. This will ensure the wrap is melted and adequately dispersed in the compound during the mixing operation.

* Contamination. Considerable work has been done toward lowering the dirt level in both technically specified and visually inspected rubbers. As noted earlier, the last revision to the Standard Malaysian Rubber, SMR, scheme (ref. 28) introduced the following revisions:

* Dirt level specifications were reduced from 0.10 to 0.08, and from 0.20 to 0.16 for SMR 10CV and SMR 20CV respectively.

* CV grades of SMR5 were defined with viscosities of 50 and 60, each within a [+ or -] 5 range (SMR 50CV, SMR 60CV),. Dirt levels of 0.03% are now typical.

* In addition, avoidance of other foreign material, moisture and degraded polymer is to be ensured.

* Fatty acids. Excessive levels of fatty acids, such as palmitic acid, oleic acid and stearic acid, can bloom to the surface of compounded rubber components prepared for tire building or other engineered products assembly. Tire manufacturing plants may have component tack difficulties when, for example, a TSR 20 with fatty acid levels of 0.30 wt. % is changed to a TSR 20 grade with a fatty acid level of 0.9 to 1.0 wt. %. This may be due to bloom. High levels of fatty acids can also influence vulcanization kinetics. Tack-inducing resins such as Escorez 1102 (table 9) may also be used to correct bloom (ref. 29). Phenolic resins are also used to attain specific tack levels as required by the specific manufacturing plant and product performance requirements.

Fatty acid levels, to a large degree, are a function of the amount of washing the raw materials undergo prior to shipping. Malaysian rubbers are produced to clearly defined dirt levels and thus require little washing. In consequence, fatty acid levels can be relatively high. However, other regional sources such as Indonesia rubber may initially contain much higher dirt levels, require more washing, and as a result have a greater amount of fatty acids removed before baling and shipping. Fatty acid amounts are also important in ensuring consistent curing.

Properties of NR compounds

The molecular weight of natural rubber ([M.sub.w]) is in the order of 750,000 to 900,000 (ref. 3). Consequently, natural rubber compounds tend to have high tensile and tear strength. Two ASTM formulas can be used to screen different grades of natural rubber, and are also useful for fundamental filler system and cure system studies (table 10) (ref. 22). Compound 1 is a gum formula containing the natural rubber grade of interest, 6.0 phr zinc oxide, 0.5 phr stearic acid and a conventional cure system consisting on 0.5 phr mercaptobenzothiazole (MBT) and 3.5 phr sulfur. Compound 2 is a filled compound containing 35 phr of high abrasion furnace (HAF, N330) carbon black. For carbon black screening, the filler loading is increased to 50 phr and the accelerator, MBT, is replaced with the disulfide, mercaptobenzothiazole disulfide (MBTS) (ref. 30). These base formulas have become established as recognized compounds in many of the international standards.

To illustrate the properties of compound 1, comparative data are presented in table 11, where MBT has been replaced with diphenylguanidine (DPG) and cylohexylbenzothiazole sulfenamide (CBS) at equal phr. CBS tends to show the highest state of cure (rheometer torque), tensile strength, tear strength and lowest compression set (ref. 31).

Similar representative data were prepared for filled compounds using formula 2 in table 12. Three accelerators, cyclohexylbenzothiazole sulfenamide (CBS), t-butyl-2-benzothiazole sulfenamide (TBBS), and 2-(4-morpholinothio)-benzothiazole (MBS) were evaluated at 1.00 phr (table 12), and basic mechanical properties such as tensile strength, tear strength and aged properties measured.

CBS tends to produce superior mechanical properties, demonstrating why it is the preferred primary accelerator in natural rubber compounds. For example, using CBS, tensile strength of 28 MPa was obtained, which compares with 24 and 23 MPa for MBS and TBBS, respectively. For this reason, CBS tends to be the preferred accelerator for natural rubber based radial truck tire tread compounds. Truck tire tread wear, depending on the specific service, is a function of the compound tensile strength, tear strength, and resistance to thermooxidative degradation. High molecular weight natural rubbers such as RSS2 are, therefore, used; with highly reinforcing SAF (N100) or ISAF (N200) grades of carbon black, minimum levels of oil, and with CBS semi-EV cure system (ref. 32).

Since natural rubber is an unsaturated elastomer, it is readily susceptible to oxidation. This is reflected in the loss in tensile strength with aging at 100[degrees]C for 24 hours. For compounds cured with the accelerators CBS, MBS and TBBS, tensile strength losses of 32%, 15% and 33%, respectively, have been noted. Addition ofantioxidants and antiozonants such as polymerized 2,2,4-trimethyl-1,2-dihydroquinoline (TMQ) and dimethylbutyl-N-phenyl-p-phenylenediamine (6PPD) can correct such losses in mechanical properties.

An example of a natural rubber based conveyor belt compound containing TMQ is illustrated in table 13.

For illustrative purposes, two conveyor belt cover compounds are shown, one based on natural rubber (TSR 5) and one on bromobutyl rubber which is 98% isobutylene and up to 2% isoprene. Mechanical properties of bromobutyl compounds such as tensile strength and hardness, which are important for conveyor belt compounds, can in many instances approach those of natural rubber based compounds. Halobutyl polymers have been used in conveyor belts for heat and chemical resistance, and where these properties dominate over other factors such as abrasion resistance. Bromobutyl is used in place of natural rubber for high temperature applications, and blends of bromobutyl and EPDM can be used for less severe uses (ref. 33).

Natural rubber blends

Other elastomers can be blended with natural rubber when a balance of fatigue resistance, aging resistance or abrasion resistance is required. Two categories of blends may be identified: where a synthetic rubber is added at low (10 to 20 phr) to medium amounts (50 phr); and where NR is added at a low amount (10 to 20 phr) to obtain a specific property. The compatibility of natural rubber with other elastomers can be related to the solubility parameter. Compatibility of two elastomers is defined as when the polymers are immiscible, but if in combination provide properties or show characteristics that are more useful than the properties of the original polymers (ref. 34). When blended, incompatible polymers will show a dual Tg. Miscible blends are obtained when, at the molecular level, the two polymers are compatible. For example, blends of two highly polar polymers such as PVC and NBR are truly miscible. Polymer compatibility is governed by three parameters:

* Thermodynamics: Micro domain formation is achieved by surface energy reduction (i.e., use of processing aids), and vulcanization.

* Viscosity: Oil/filler levels in dissimilar elastomers can be adjusted in mixing.

* Cure rate: NR/EPDM and NPUIIR or NR/HIIR systems, where the respective polymers are significantly different.

Polymer compatibilization can therefore be achieved either by reduction of interfacial energy, improvement of dispersion during mixing, stabilization of the polymers to prevent phase separation after mixing, or improvement of interfacial or interdomain adhesion. This can be achieved as follows:

* Use of grafted polymers to reduce interfacial energy;

* addition of a copolymer;

* crosslinking of blended polymers (dynamic vulcanization); and

* polymer functionalization.

Elastomer blending allows improvement in fatigue life, tire tread groove cracking resistance, reduction in hysteresis and many other improvements in mechanical properties. It may also improve processing such as extrusion and, later, molding of the final tire construction. For example, addition of natural rubber to a largely synthetic polymer will improve tire 'buildability,' component to component adhesion and tear strength.

For two elastomers of different composition to be miscible, the Gibbs free energy of mixing, [DELTA]G, must be negative. Gibbs free energy is a function of the enthalpy of mixing ([DELATA]H), entropy of mixing ([DELTA]S) and the temperature (T) in [degrees]K, i.e.,

(6) [DELTA]G=[DELTA]H- T [DELTA]S

Miscible elastomers will tend to show hydrogen bonding or van der Waals forces between chains. However, phase separation may occur if the temperature rises above the critical solution temperature. Truly miscible elastomers will show a single Tg.

The solubility parameter ([delta]) for a material is related to the enthalpy of mixing. It is defined as the square root of the cohesive energy density, or attractive strength between molecules, i.e.,

(7) [delta] = ([DELTA]E / V )[.sup.1/2]

where [DELTA]E is the energy of vaporization, V is the molar volume of the polymer, in [(MPa).sup.1/2]. As an empirical guide, the solubility parameter difference between two polymers must be less than 1.0 to be compatible. There must also be no change in volume, or reaction between the two elastomers. Solubility parameter can only be determined indirectly by methods such as solvency (in a solvent such as hexane, toluene, dibutyl phthalate or ethanol), although other methods have been used, including swelling, refractive index, dipole moment measurements and intrinsic viscosity. Table 14 shows the solubility parameters for a range of elastomers.

These solubility parameters can be used to estimate the miscibility of the elastomers. For illustrative purposes, natural rubber, with a solubility parameter of 16.6, when mixed with

a NBR with a solubility parameter of 19.4, gives a solubility parameter differential of 2.8. As this is greater than 1.0, the two elastomers would be considered to be immiscible.

Where the solubility parameter rule does not apply is for natural rubber and polybutadiene. The differential in solubility parameters is around 0.6, but the two polymers are immiscible. Polybutadiene grade IISRP 1207 and an oil extended SBR polymer such as IISRP 1712 have a differential of less than 0.1, and in this case, the two elastomers are nearly fully miscible. The blended elastomers' mechanical properties then become a function of the filler type, distribution, vulcanization system and any processing aids present.

Low level addition of natural rubber

Low levels of natural rubber can be added to synthetic elastomers such as halobutyls so as to modify final compound properties. Table 15 illustrates some selected properties of a bromobutyl innerliner compound containing increasing amounts of natural rubber. It can be seen that with an increase in NR level from 10 phr to 40 phr (90 BIIR/10 NR to 60 BIIR/ 40 NR), the oxygen permeability coefficient at 60[degrees]C is increased from 0.92 to 1.74 cc*mm/([m.sup.2]-day-mmHg) (ref. 33). This drop in impermeability is a function of natural rubber content, though tear strength has shown only a minor change.

The rotational restriction of the isobutylene polymer chain causes a high monomeric friction coefficient and Windel-Landel-Ferry constant when compared to other elastomers of similar glass transition temperature (Tg). Isobutylene elastomers are highly damping even though the Tg is below -60[degrees]C. The low permeability is due to restricted polymer chain mobility and low solubility of gases in the saturated polymer. The diffusivity of a number of gases in natural rubber and butyl rubber is presented in table 16.

As shown in figure 6, the diffusion coefficient of nitrogen in various diene rubbers and in butyl rubber increases with increasing differences between the measurement temperature and the corresponding rubber's glass transition temperature. However, although the rate of increase in diffusion coefficient with T-Tg is about the same between diene rubbers and butyl rubber, the absolute values of diffusion coefficient in butyl rubber are significantly less than that of diene rubbers. Considering isobutylene copolymers contain only small amounts of co-monomers, their temperature dependent permeability values follow the same curve as for butyl rubber (reg. 35 and 36). As shown in table 15, the addition of natural rubber to isobutylene polymers will increase the permeability to a significant extent.

It can be readily concluded that natural rubber has many properties that cannot be achieved through use of synthetic elastomers. However, in achieving the technological properties required in many components, for example in a radial tire, blending of natural rubber with other elastomers is necessary. In radial tire sidewalls, if NR exceeds around 30% of the composition, it will tend to display poor fatigue resistance and cracking. In contrast, wire coat or wire skim compounds that contain synthetic elastomers display poor performance. One hundred percent natural rubber is the only feasible polymer for this application, with synthetic polyisoprene being the obvious exception.

Summary

This was a review of the production, classification and properties of natural rubber. Several key factors will determine its continued use in the future and these are:

* Availability: Given the growth of the global economies, and the automotive industry specifically, additional sources of materials will be required, with shortages anticipated in the future.

* Technical specifications: Specifications will be needed for visually inspected robbers, such as RSS grades, to meet the end users' needs for consistency and uniformity in their factories.

* Quality: End product specifications and performance requirements will continue to increase, thereby necessitating continuing improvement in consistency, absence of foreign materials or other contaminants, and purity.

* Chemical modification: Though not discussed in this review, to improve the mechanical properties of current materials and enable their use in novel compounds, new synthetic derivatives of polymers will be required to compete with new functionalized synthetic elastomers.

The greatest challenge in the future, however, may not be with regard to technical properties, but with supply and availability. Global growth in demand of even as low as 3% could create an inadequate supply position, with consequent impact on pricing, demand for synthetic elastomers and redevelopment of compound formulas to ensure performance of the end products.

References

(1.) A.D. Roberts', Natural Rubber Chemistry and Technology, Oxford University Press, 1988.

(2.) F. Barlow, Rubber Compounding, 2nd Edition, Marcel Dekker, Inc., New York, 1994.

(3.) W. Klingensmith and M.B. Rodgers, "Natural rubber and recycled materials," in Rubber Compounding, Chemistry and Applications, Ed. MB. Rodgers, Marcel Dekker Inc., New York, 2004.

(4.) M.B. Rodgers, D.S. Tracey and W.H. Waddell, "Natural rubber, "presented at a meeting of the Rubber Division, ACS, May, 2004.

(5.) G. Michal, Biochemical Pathways, 3rd Edition, Part 1, Roche Molecular Biochemicals, Boeringer Mannheim GMGH--Biochemica, Germany, 1993.

(6.) Y. Tanaka, "Structural characterization of naturally occurring cis-polyisoprene," ACS Symposium Series 247, (NMR Macromol.), pp. 233-244, 1984.

(7.) Y Tanaka, "Structural characterization of naturally occurring cis--and trans-polyisoprenes by carbon-13 NMR spectroscopy," J. Applied Polymer Science: Applied Polymer Symposium (1989), 44 (Int. Semin. Elastomers) 1989.

(8.) D.S. Cyr, "Rubber, natural, "Encyclopedia of Polymer Science and Engineering, 2nd edition, Vol. 14, pp. 687-716, ed J.I. Kroschwitz, John Wiley & Sons, 1988.

(9.) C.S.L. Baker and W.S. Fulton, "Rubber, natural, " Kirk-Othmer Encyclopedia of Chemical Technology, 4th edition, Vol. 21, pp. 562-591, ed J.L Kroschwitz and M. Howe-Grant, John Wiley & Sons, NY, 1997.

(10.) W. Barbin and M.B. Rodgers, "Science of rubber compounding," in Science & Technology of Rubber, Chapter 9. ed J. Mark, B. Erman and F. Eirich, John Wiley & Sons, New York, 1994.

(11.) H. Hasma and A. Subramaniam, "Composition of lipids in latex of hevea brasiliensis clone RRIM 501, " J. Nat. Rub. Res., Vol. 1, pp. 30-40 (1986).

(12.) H. Hasma, "Lipids associated with rubber particles and their possible role in mechanical stability of latex concentrates," J. Nat. Rub. Res., Vol. 6, pp. 105-114 (1991).

(13.) International Standards Organization, ISO 2004, "Specification Jar natural rubber latex concentrates," 1988.

(14.) The Economist Intelligence Unit. EIU Automotive Rubber Trends, "Worldwide rubber database," 4th Quarter, 1999.

(15.) The Rubber Manufacturers Association. 'The International Standards of Quality and Packaging for Natural Rubber Grades, The Green Book, The International Rubber Quality and Packaging Conference, Office of the Secretariat, Washington, D.C., January 1979.

(16.) International Standards Organization, ISO 2000, 'Rubber Grades' 1964.

(17.) American Society Jar Testing and Materials, ASTM Standard D1646, "Standard Test Methods for Moaner Viscosity, Stress Relaxation and Prevulcanization Characteristics (Mooney Viscometer), "Annual Book of ASTM Standards, vol. 09.01, American National Standards Institute, 1999.

(18.) F. Bonfils, C. Char, Y Garnier, A. Sanage and J. Sainte Beuve, "Inherent molar mass distribution of clones and properties of crumb rubber, "J. Rubb Res., vol. 3, p. 164, 2001.

(19.) S. Nair, "Dependence of bulk viscosities (Mooney and Wallace) on molecular parameters of natural rubber," J. Rubb. Res. Inst. Malaya, vol. 23, p. 76-83, 1970.

(20.) A. Subramaniam, "Molecular weight and other properties of natural rubber: A study of clonal variations, " International Rubber Conference, Kuala Lumpur, 1975.

(21.) A. Subramaniam, "Viscosity of natural rubber, " Planters Bulletin 180, pp. 104-112, 1984.

(22.) American Society for Testing and Materials, ASTM Standard D3184--89, "Natural rubber" Annual Book of ASTM Standards, vol. 09.01.

(23.) T. Hiroshi, E. Nakamura, H. Aoyama and Y. Hiratu, U.S. Patent 5, 710,200, "Natural rubber treated with viscosity, stabilizer and production thereof," Assigned to Bridgestone 1998.

(24.) J.L. Leblanc and P. Hardy, "Evolution of bound rubber during the storage of uncured compounds, "Kautsch. Gummi, Kunstst., vol. 45, 1992.

(25.) S. Kawahara, Z Kakubo, J. T. Sakdapipanich, Y. Isono and Y. Tanaka, "Characterization of fatty acids' linked to natural rubber--role of linked fatty acids on crystallization of the rubber," Polymer, vol. 41, pp. 7,483-7,488, 2000.

(26.) A. Sharpies, introduction to Polymer Crystallization, Edward Arnold Publishers Ltd., London, 1966.

(27.) ibid.

(28.) Rubber Research Institute of Malaysia, "Revisions to Standard Malaysian Rubber Scheme" SMR Bulletin No. 11. Kuala Lumpur, 1991.

(29.) Web page www.exxonmobilchemical.com

(30.) American Society for Testing and Materials, ASTM Standard D3192--02, "Carbon black evaluation in NR (natural rubber)," Annual Book of A STM Standards, vol. 09.01.

(31.) M.B. Rodgers, "High temperature vulcanization of industrial rubber products, "PhD thesis, The Queen's" University of Belfast, Northern Ireland, 1983.

(32.) D. Bernard, C.S.L. Baker and I.R. Wallace, NR Technology, vol. 16, pp. 19-26, 1985.

(33.) Web page www.butylrubber.com

(34.) M.B. Rodgers, D.S. Tracey and WH. Waddell, "Tire applications of elastomers, part 1--treads, "paper H, presented at meeting of the Rubber Division, ACS, Ma); 2004.

(35.) W.H. Waddell and A.H. Tsou, "Butyl rubber" in Rubber Compounding, Chemistry and Applications, Ed. M.B. Rodgers, Marcel Dekker, Inc., New York, 2004.

(36.) E.N. Kresge, R.H. Schatz and H-C. Wang, "Isobutylene polymers," Encyclopedia of Polymer Science & Engineering, vol 8., 2nd edition, pp. 423-448, John Wiley & Sons, Inc., 1987.

M. Brendan Rodgers, Donald S. Tracey and Walter H. Waddell, ExxonMobil Chemical

Table 8--Wallace plasticity and molecular
weight of natural rubber (ref. 18)

Sample     Po     ML 1+4       MW

  1        32       57       746,000
  2        41       78       739,000
  3        54       92       799,000
  4        62      104       834,000

Table 9--tackifying resins for natural rubber compounding (ref. 29)

Resin       Grade       Type          Softening           Brookfield
                                          point     viscosity (cps @
                                          (R&B)      140[degrees] C)

Escorez     1102        C5/C6             100.0                7,000
                        diolefins
            1304        Aliphatic         100.0                5,500
            1310 lc                        93.0                2,800
            1315                          115.0                5,000
            1580                           80.0                  780

Resin       Grade       Tg     Mn       Mw        Mz        Speck
                                                           gravity

Escorez     1102        50     750     2,400     6,400      0.97
            1304        50     750     1,650     3,000      0.97
            1310 lc     45     750     1,350     2,400      0.97
            1315        65     950     2,400     7,100      0.97
            1580        35     600     1,100     1,800      0.97

Table 10--model compounds for natural rubber evaluations
(refs. 22 and 30)

Compound                          1          2

Natural rubber                  100.00     100.00
N330 carbon black                           35.00
Zinc oxide                        6.00       5.00
Stearic acid                      0.50       0.50
Sulfur                            3.50       2.25
Mercaptobenzothiazole (MBT)       0.50       0.70

Table 11--representative data for model natural rubber gum compound
(ref. 31)

Compound                                       3        4        5

Technically specified rubber 5 (TSR 5)      100.00   100.00   100.00
Zinc oxide                                    6.00     6.00     6.00
Stearic acid                                  0.50     0.50     0.50
Sulfur                                        3.50     3.50     3.50
M BT                                          0.50
DPG                                                    0.50
CBS                                                             0.50
ODR rheometer
Mh-MI (delta torque)                         21.5     14.5     26.5
T90%                                         20.0     30.0     25.25
Tensile strength (MPa)                        6.35    11.0     15.6
Elongation (%)                              630      600      625
300% modulus (MPa)                            1.25     1.40     2.00
Tear die B (kN/m)                            53       47       54.5
Hardness (durometer A)                       52       33       40
Compression set (100[degrees]C, 70 hours)    87.9     90.3     70.6

Table 12--CBS, MBS and TBBS in filled natural rubber compounds
(ref. 31)

Compound                                     6         7         8

Technically specified rubber 5 (TSR 5)    100.00    100.00    100.00
Carbon black (N330)                        35.00     35.00     35.00
Zinc oxide                                  5.00      5.00      5.00
Stearic acid                                2.00      2.00      2.00
Sulfur                                      2.25      2.25      2.25
CBS                                         1.00
MBS                                                   1.00
TBBS                                                            1.00
ODR rheometer
Mh-MI                                      75.0      73.0      77.5
T90%                                        4.85      5.85      5.35
Cure rate index                            42.1      31.7      42.5
Tensile strength (MPa)                     28.6      24.5      22.8
Elongation (%)                            467       435       392
300% modulus (MPa)                         13.65     13.25     14.75
Tear strength (kN/m)                       93.5     103.2      98.3
Hardness (durometer A)                     67        64        62
Aged 100[degrees]C, 24 hours
Tensile strength (MPa)                     19.4      20.8      15.1
Elongation (%)                            336       350       235
200% modulus (MPa)                          9.6      8.85      11.8

Table 13--model conveyor belt cover compounds (refs. 31 and 33)
comparison of natural rubber with synthetic rubber

Natural rubber conveyor belt cover

Vatural rubber (TSR5)                          100.00
Carbon black (N330)                             50.00
2,2,4-Trimethyl-1,2-dihydroquinoline (TMQ)       2.50
Stearic acid                                     3.00
Zinc oxide                                       5.00
MBTS                                             1.00
DPG                                              0.25
Sulfur                                           3.00
Total phr                                      164.75
Tensile strength (MPa)                          17.30
Elongation (%)                                    531
300% modulus (MPa)                               7.10
Tear strength (Die B kN/m)                        116
Hardness (duro. A)                                 63

Bromobutyl conveyor belt cover

Bromobutyl 2255                                100.00
Carbon black (N220)                             45.00
Naphthenic oil                                   5.00
2,2,4-Trimethyl-1,2-dihydroquinoline (TMQ)       1.00
Magnesium oxide                                  0.15
Stearic acid                                     1.00
Zinc oxide                                       2.00
Sulfur                                           2.00
MBTS                                             1.00
Amyl phenol disulfide (Vultac 7)                 1.00
Total phr                                      158.15
Tensile strength (MPa)                          16.50
Elongation (%)                                    311
300% modulus (MPa)                              16.00
Tear strength (Die B kN/m)                         79
Hardness (duro. A)                                 62

Table 14--summary of solubility parameters for selected elastomers
(ref. 34)

Polymer             Solubility parameter

BR                         17.20           Range 16.20-17.60
IR                         16.40           Range 16.20-16.82
IIR                        16.06           Range 15.70-18.00
NR                         16.60           Range 16.20-17.70
NBR
  ACN content
    16                     19.40
    20                     18.40
    30                     20.26
    40                     21.10
S.SBR
  Styrene content
    10                     17.13
    15                     17.19
    25                     17.29
    30                     17.35
    40                     17.40

Table 15--bromobutyl-NR compound blends and effect on properties
(ref. 33)

Compound                                        1         2         3

Natural rubber (TSR 5)                       0.00     10.00     20.00
Bromobutyl 2222                            100.00     90.00     80.00
Carbon black N660                           60.00     60.00     60.00
Naphthenic oil                               8.00      8.00      8.00
Struktol 40MS                                7.00      7.00      7.00
Phenolic resin                               4.00      4.00      4.00
Stearic acid                                 1.00      1.00      1.00
Zinc oxide                                   1.00      1.00      1.00
MBTS                                         1.20      1.20      1.20
Sulfur                                       0.50      0.50      0.50

Tensile strength (MPa)                       9.60     10.00     10.40
Elongation (%)                                837       796       745
300% modulus (MPa)                           3.30      3.30      3.90
Hardness (duro. A)                             47        47        47
Tear strength (Die B) (kN/m)                54.00     50.00     54.00
Rebound (%) at 23[degrees]C                    10        11        12
Fatigue to failure (kilocycles)               336       225       205
Mocon oxygen permeability (60[degrees])      0.78      0.92      1.20

Compound                                        4         5

Natural rubber (TSR 5)                      30.00     40.00
Bromobutyl 2222                             70.00     60.00
Carbon black N660                           60.00     60.00
Naphthenic oil                               8.00      8.00
Struktol 40MS                                7.00      7.00
Phenolic resin                               4.00      4.00
Stearic acid                                 1.00      1.00
Zinc oxide                                   1.00      1.00
MBTS                                         1.20      1.20
Sulfur                                       0.50      0.50

Tensile strength (MPa)                      10.20     11.80
Elongation (%)                                768       676
300% modulus (MPa)                           3.10      3.80
Hardness (duro. A)                             44        46
Tear strength (Die B) (kN/m)                56.00     58.00
Rebound (%) at 23[degrees]C                    12        15
Fatigue to failure (kilocycles)               207         -
Mocon oxygen permeability (60[degrees])      1.35      1.74

Coefficient (cc*mm/([m.sup.2]-day mmHg)

Table 16--diffusivity of gases in NR and butyl rubber at 25[degrees]C
(refs. 35 and 36)

                            Diffusivity
Gas                 ([cm.sup.2]/sec) x [10.sup.6]

                   Natural rubber     Butyl rubber

Helium                 21.60             5.93
Hydrogen               10.20             1.52
Oxygen                  1.58             0.081
Nitrogen                1.10             0.045
Carbon dioxide          1.10             0.058