Production, classification and properties of NR.

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

* The chemistry and production of natural rubber;

* industry classification, descriptions and specifications; and

* fundamental technological properties of natural rubber.

Chemistry of natural rubber

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

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

[FIGURES 1-2 OMITTED]

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

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

The isopentyl pyrophosphate starting material is also used in the formation of farnesyl pyrophosphate. Subsequent condensation of trans- farnesyl pyrophosphate yields trans- polyisoprene of gutta percha. Gutta percha is an isomeric polymer in which the double bonds have a trans- configuration. It is obtained from trees of the genus Dichopsis found in Southeast Asia. This polymer is synthesized from isopentenyl pyrophosphate via a pathway similar to that for the biosynthesis of terpenes such as geraniol and farnasol. Gutta percha is more crystalline in its relaxed state, it is much harder and less elastic (refs. 3 and 4).

Production of natural rubber

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

Rubber occurs in the trees in the form of particles suspended in serum that also contains proteins, amino acids and various carbohydrates. The serum constitutes the latex, which in turn is contained in specific latex vessels in the tree. Latex constitutes the protoplasm of the latex vessel, and tapping of cutting of the latex vessel creates a hydrostatic pressure gradient along the vessel with consequent flow of latex through the cut. In this way, a portion of the contents of the interconnected latex vessel system can be drained from the tree. Eventually the flow ceases, turgor is re-established in the vessel and the rubber content of the latex is restored to its initial level in around 48 hours (ref. 2).

The tapped latex composition consists of between 30% to 35% rubber, 60% aqueous serum, and 5% to 10% of other constituents such as fatty acids, amino acids and proteins, starches, sterols, esters and salts. Some of the non-rubber substances such as lipids, carotenoid pigments, sterols, triglycerides, glygolipids and phospholipids can influence the final properties of rubber, such as compounded vulcanization characteristics and classical mechanical properties. Hasma and Subramanian (refs. 11 and 12) have conducted a comprehensive study of these materials to which further reference is recommended. Lipids can also affect the mechanical stability of the latex while in storage, since they are a major component in the membrane formed around the rubber particle.

Natural rubber latex is typically coagulated, washed and then dried either in open air of in a smoke house. The processed material consists of about 93% rubber hydrocarbon, 0.5% moisture, 3% acetone extractable materials such as sterols, esters and fatty acids, 3% proteins and 0.5% ash. Raw natural rubber gel can range from 5 to 30%. High gel content can create processing problems in tire of industrial rubber products factories. Nitrogen content is typically in the range of 0.3% to 0.6%. For further clarity, table 1 presents a number of definitions of terms used in natural rubber production and supply (refs. 3 and 4).

The rubber from a tapped tree is collected in three forms: Latex, cup-lump and lace. This is collected as follows:

* Latex collected in cups is coagulated with formic acid, crumbed of sheeted. The sheeted coagulum can be immediately crumbed, aged and then crumbed of smoke-dried at around 60[degrees]C to produce typically ribbed smoked sheet (RSS) rubber. Air dried rubber is finished in the form of air dried sheets (ADS).

* Cup-lump is produced when the latex is left uncollected and, due to bacterial action, is allowed to coagulate on the side of the collecting cup. Field coagulum or cup-lump is eventually gathered, cut, cleaned, creped and crumbed. Crumb rubber can be dried at temperatures up to 100[degrees]C.

* Lace is the coagulated residue left around the bark of the tree where the cut has been made for tapping. The formation of lace seals the latex vessels and stops the flow of rubber latex. It would typically be processed with cup-lump.

The natural rubber processing factories obtain the raw material collected from trees either in large plantations or from smaller independent holdings, in one of two forms, field coagula or field latex (figure 4). Field coagula consist of cup lump from the collection cups and tree and cup lace obtained, for example, from the rim of the cup (table 1). The lower grades of material are prepared from cup lump, partially dried small holder's rubber, rubber tree lace and earth scrap after cleaning. Iron-free water is necessary to prevent polymer oxidation. Field coagula and latex are the base raw materials for the broad range of natural grades to be described in this review.

[FIGURE 4 OMITTED]

Fresh Hevea latex has a pH ranging from 6.5 to 7.0 and density of 0.98 (refs. 8 and 9). The traditional preservative is ammonia (in concentrated aqueous solution), which is added in small quantities to the latex when collected from the cup. Tetramethylthiuram disulfide (TMTD) and zinc oxide are also used as a preservative due to their greater effectiveness as a bactericide. Most latex concentrates are produced to meet the International Standard ISO 2004 (ref. 13). This standard defines the minimum content for total solids, dry rubber content, non-rubber solids and alkalinity (as N[H.sub.3]).

Natural rubber products and grades

Natural rubber is available in six basic forms: Sheets; crepes; block rubber, technically specified; technical specification for sheet rubber; preserved latex concentrates; and specialty rubbers which have been mechanically or chemically modified.

Among these six types, the first three represent nearly 90% of the total natural rubber produced in the world. In the commercial market, these three types of dry natural rubber are available in over 40 grades consisting of ribbed smoked sheets, air dried sheets, crepes (which include latex-based, and field coagulum-derived estate brown crepes), remilled crepes and technically specified rubber in block form. Among the three major types, crepes are now of minor significance in the world market, accounting for less than 75,000 tons per year. Field coagulum grade block rubbers have essentially replaced brown crepes, except in India. Only Sri Lanka and India continue to produce latex crepes. Figure 4 presents a simplified schematic of the process followed in the production of natural rubber (ref. 3).

Sheet rubber

From the end-user's perspective, two types of sheet rubbers are produced for the commercial market, namely ribbed smoked sheets (RSS) and air dried sheets (ADS). Among these two types, ribbed smoked sheet is the most popular.

Ribbed smoked sheet rubbers are made from intentionally coagulated whole field latex. They are classified by a visual evaluation. To establish acceptable grades for commercial purposes, the International Rubber Quality and Packing Conference has prepared a description for grading, with details given in the Green Book (ref. 15). Whole field latex used to produce ribbed smoked sheet is first diluted to 15% solids, and then coagulated for around 16 hours with dilute formic acid. The coagulated material is then milled, water removed and sheeted with a rough surface to facilitate drying. Sheets are then suspended on poles for drying in a smokehouse for one to seven days. Only deliberately coagulated rubber latex processed into rubber sheets, properly dried and smoked can be used in making RSS. A number of prohibitions are also applicable to the RSS grades. Wet, bleached, undercured and original rubber and rubber that is not completely visually dry at the time of buyer's inspection are not acceptable (except slightly under-cured rubber as specified for RSS5). Skim rubber made of skim latex cannot be used in whole or in part in the patches as required under packing specifications. Prior to grading RSS, the sheets are separated, inspected and any blemishes are removed by manually cutting and removing defective material.

Table 2 provides a summary of the criteria followed by inspectors in grading ribbed smoked sheet. The darker the rubber, the lower the grade. In practice, the premium grade is RSS1, with the lower quality grade being typically RSS4.

Air dried sheets are prepared under conditions very similar to that for smoked sheets, but dried in a shed without smoke of additives, with the exception of sodium bisulfate. Such rubber, therefore lacks the oxidation protection afforded by drying the rubber in a smokehouse. Regarding the use of this material, it can be substituted for RSS1 or RSS2 grades.

Crepe is a crinkled lace rubber, obtained from coagulated latex of clones which have low carotene content. Sodium bisulfite is also added to maintain color and prevent darkening. After straining, the latex is passed several times through heavy rolls called 'crepers,' and the resultant material is air-dried at ambient temperature. There are different types of crepe rubber, depending upon the type of starting materials from which they are produced. Sri Lanka is the largest producer of pale crepes and the sole producer of thick pale crepe.

The specifications for the different types of crepe rubbers, for which grade descriptions are given in the Green Book (ref. 15), have been summarized in tabular form as follows:

* Pale latex crepes. Pale crepe is used for light colored products, and therefore commands a premium price. Trees or clones from which the grade is obtained typically have low yellow pigment levels (carotenes) and greater resistance to oxidation and discoloration. There are eight pale grades under this category. All these grades must be produced from the fresh coagula of natural liquid latex under conditions where all processes are subject to quality control. The rubber is milled to produce both thin and thick crepes. Pale crepes are used in pharmaceutical applications, such as stoppers and adhesives, when there is a risk of allergic reaction (table 3).

* Estate brown crepes. There are six grades in this category. All of these grades are made from cup lump and other higher grade rubber scrap (field coagulum) generated on the rubber estates. Tree bark scrap, if used, must be pre-cleaned to separate the rubber from the bark. Power wash mills must be used in milling these grades into a form of both thick and thin brown crepes (table 4).

* Thin brown crepes (remills). There are four different grades in this class or category. They are manufactured on power wash mills from wet slab, un-smoked sheet at the estates or small holdings. Tree bark scrap, if used, must be pre-cleaned to separate the rubber from the bark. Inclusion of earth scrap and smoked scrap is not permissible in these grades (table 5).

* Thick blanket crepes (ambers). There are three grades in this category. These grades are also produced on power wash mills from wet slab, un-smoked sheets, lump and other high-grade scrap (table 5).

* Flat bark crepes. There are two grades of rubber under this category. These grades are produced on power wash mills out of all types of scrap natural rubber in uncompounded form, including earth scrap (table 5).

* Pure smoked blanket crepe. This grade is made by milling (on power wash mills) smoked rubber derived from ribbed smoked sheet (including block sheets), or ribbed smoked sheet cuttings. No other type of rubber can be used. Rubber of this type must be dry, clean, firm, tough, and also must retain an easily detectable smoked sheet odor. Sludge, oil spots, heat spots, sand, dirty packing and foreign matter are not permissible. Color variation from brown to very dark brown is permissible (table 5).

Technically specified natural rubber

The International Standards Organization (ISO) first published a technical specification (ISO 2000) for natural rubber in 1964 (ref. 16). Based on these specifications, Malaysia introduced a national Standard Malaysian Rubber (SMR) scheme in 1965, and since then, all the natural rubber-producing countries started production and marketing of technically specified rubbers based on the ISO 2000 scheme. Technically specified rubbers are shipped in 'blocks' which are generally 33.3 kg bales in the international market and 25.0 kg in India. All block rubbers are also guaranteed to conform to certain technical specifications as defined by the national schemes of by ISO 2000 (table 6).

The nomenclature describing technically specified rubbers consists of a three or four letter country code, followed by a numeral indicating the maximum permissible dirt content for that grade expressed as a hundredth's of one percent. In Malaysia, the TSR is designated as Standard Malaysian Rubber (SMR). In Indonesia, the designation given is Standard Indonesian Rubber (SIR). In Thailand, the TSRs are called Standard Thai Rubber (STR, and sometimes denoted as TTR). In India, the TSRs are designated as Indian Standard Natural Rubber (ISNR) (ref. 4). Grading is based on the dirt content measured as a weight percent. Dirt is considered to be the residue remaining when the rubber is dissolved in a solvent, washed through a 45-micrometer sieve and dried.

Technically specified rubber (TSR) accounts for approximately 60% of the natural rubber produced. The International Standards Organization has specified six grades of technically specified rubber.

TSR-CV

TSR-CV, the CV designating constant viscosity, is produced from field latex and is viscosity stabilized to a specified Mooney viscosity. The storage hardening of this grade of rubber is also to be within eight hardness units. It is shipped in a 1.2-metric ton pallet, which facilitates handling, transportation and storage space utilization. Each pallet consists of 36 bales of 33.3 kg net weight, and each bale is wrapped in a polyethylene bag which is dispersible and compatible with rubber when mixed in an internal mixer at temperatures exceeding 110[degrees]C, which would be typical in most rubber mixing facilities. TSR-CV rubbers are generally softer than conventional technically specified grades. Coupled with its constant viscosity feature, it can provide a cost advantage in eliminating pre-mastication. When used in open mills, the rubber forms a coherent band almost instantaneously, thus potentially improving milling equipment throughput. Additional claimed benefits of TSR-CV include:

* Reduction of mixing times giving higher mixer throughput and productivity;

* reduction of scrap and rejected material due to better compound uniformity;

* better resistance to chipping and chunking for off-the-road (OTR) or earthmover tires;

* more consistent compound green strength; and

* lower energy consumption in the manufacturing processes.

TSR-CV rubber is available in different Mooney viscosities with grades CV50 and CV60 being the more common. They can be used for high quality products such as mechanical mountings for engines and machinery, bridge bearings, vehicle suspension systems, large truck tire treads, conveyor belt covers, cushion gum for tire retreading, masking tapes, injection molded products including rubber/metal bonded components, industrial rolls and rubber cements. It is frequently blended with other grades of natural rubber to optimize cost and balance of other properties such as viscosity.

TSR-L

This is a light colored rubber produced from high quality latex, and also with low ash and dirt content. TSR-L is packed and presented in the same way as TSR-CV. The benefit of TSR-L is its light color property, along with its cleanliness and superior heat aging resistance. TSR-L shows high tensile strength, modulus and ultimate elongation-at-break properties for both black and non-black mixes.

This natural rubber grade can be used for light colored and transparent products, such as surgical/pressure sensitive tape, textiles, rubber bands, hot water bottles, surgical and pharmaceutical products, large industrial rollers for the paper printing industry, sport wear, bicycle tubes, chewing gum, cable covers, gaskets, and adhesive solutions and tapes.

TSR-5

This is produced from fresh coagulum, air dried sheets or ribbed smoked sheets that have not been through the smoking process. It is packed and shipped to the same specification as that of TSR-CV and TSR-L. TSR-5 is typically used for general-purpose friction and extruded products, small components in passenger vehicles, such as resilient bush/mountings, sealing rings, cushion gum, wire skim compounds in tires and brake seals. Non-automotive applications include bridge bearings, ebonite battery plates, separators, adhesives and certain components in tires.

TSR-10

TSR-10 is produced from clean and fresh field coagulum of from sheets that have not been through the smoking process. It is packed and shipped in the same way as that of TSR-CV, TSR-L and TSR-5. TSR-10 has good technological properties similar to those of RSS-2 and RSS-3, but has an advantage over RSS for:

* Lower viscosity;

* easier mixing characteristics (more rapid breakdown); and

* being technically specified and packed in 33.3 kg bales.

It can be used for tires, cushion gum stocks, joint rings by injection molding, microcellular sheets, for upholstery and packing, conveyor belts and footwear.

TSR-20

In terms of quantities produced, this is a very important grade of technically specified natural rubber. It is produced mostly from field coagulum, lower grades of RSS and unsmoked sheets. It is packed and shipped to the same specification as that of TSR-CV, TSR-L, and TSR-5 and 10. TSR-20 has good processing characteristics and physical properties. Its low viscosity and easier mixing characteristic (compared with the RSS grades) can considerably reduce the mastication and mixing period. It is used mostly for tires, cushion gum stock, bicycle tires, micro-cellular sheet for upholstery and packing, conveyor belts, foot wear and other general products.

TSR-50

This is the lowest grade of TSR and is produced from old dry field coagulum or partly degraded rubber. It is packed and shipped in the same way as that of other grades of TSR.

It should be noted that these specifications would continue to be improved as production methods improve. For example, in 1991 the Rubber Research Institute of Malaysia revised the dirt levels of SMR CV60, CV50 and L from 0.05 to 0.025, SMR 10 from 0.10 to 0.08, and SMR 20 to 0.16. In addition, Malaysia has produced grades of rubber outside the specific scope of the ISO 2000 specification. SMR GP is a standard general purpose (GP) rubber made from a 60:40 mixture of latex-grade sheet rubber and field coagulum. It is viscosity stabilized at 65 Mooney units using hydroxylamine neutral sulfite (HNS). It is similar to SMR 10 in specification (ref. 3).

Table 7 presents a generalized quality ranking of the major grades of natural rubber (ref. 1). To illustrate consumption, distribution and use of these various grades, shipments of SMR from Malaysia are typically: 60% SMR 20, 25% SMR 10, 5% SMR CV and L, 8% SMR GP and 2% SMR 5.

(Part 2 will appear in next month's issue).

References

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

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

(3.) W. Klingensmith and M.B. Rodgers, "Natural rubber and recycled materials," in Rubber Compounding, Chemistry and Applications, Ed. M.B. 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.I. 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, 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 for 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.

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

Table 1--definitions of natural rubber terms
(refs. 3 and 4)

Cup lump               Bacterially coagulated polymer in
                       the collection cup
Earth scrap            Collecting vessel overflow material
                       collected from the tree base
Hevea brasiliensis     Natural rubber producing tree
ISNR                   Indian Standard Natural Rubber
Lace                   Trim from the edge of collecting vessels
                       or cups and coagulated residue left
                       around the bark of the tree where the
                       cut has been made for tapping
Latex                  Fluid in the tree obtained by tapping or
                       cutting the tree at a 22 degree angle to
                       allow the flow into a collecting cup
LRP                    Large rubber particles
Ribbed smoked sheet    Abbreviated as RSS and is produced
                       from whole field latex
Serum                  Aqueous component of latex which
                       consists of lower molecular weight ma-
                       terials such as terpenes, fatty acids,
                       proteins and sterols
SMR                    Standard Malaysian Rubber
SRP                    Serum rubber particles
TSR                    Technically specified rubber
Whole field latex      Fresh latex collected from trees

Table 2--ribbed smoked sheet classification (ref. 15)

RSS      Rubber    Wrapping    Opaque      Over    Oxidized    Burnt
grade      mold        mold     spots    smoked       spots    sheet
                                          spots

1x           No          No        No        No          No       No
1          Very        Very        No        No          No       No
         slight      slight
2        Slight      Slight        No        No          No       No
3        Slight      Slight    Slight        No          No       No
4        Slight      Slight    Slight    Slight          No       No
5        Slight      Slight    Slight    Slight         N/A       No

RSS         Comments
grade

1x       Clean, dry,
          no blemish
1        Clean, dry,
          no blemish
2         No foreign
            material
3         No foreign
            material
4         No foreign
            material
5                N/A

Table 3--crepe rubber classification (ref. 15)

Class      Grade         Color      Uniformity      Discoloration

                                                   Spots;     Odor
                                                  streaks;
                                                    bark

1 x      Thin white      White      Consistent      None      None
           crepe
1 x      Thick pale      Light      Consistent      None      None
           crepe
1 x      Thin pale       Light      Consistent      None      None
           crepe
1        Thin white      White        Slight        None      None
           crepe                      shade
1        Thick pale      Light        Slight        None      None
           crepe                      shade
1        Thin pale       Light        Slight        None      None
           crepe                      shade
2        Thick pale    Slightly       Slight      <10% of     None
           crepe        darker        shade        bales
2        Thin pale     Slightly       Slight      <10% of     None
           crepe        darker        shade        bales
3        Thick pale    Yellowish     Variable     <20% of     None
           crepe                                   bales
3        Thin pale     Yellowish     Variable     <20% of     None
           crepe                                   bales

Class          Discoloration

         Dust,     Oil       Oxi-
         sand     stains    dation

1 x      None      None      None
1 x      None      None      None
1 x      None      None      None
1        None      None      None
1        None      None      None
1        None      None      None
2        None      None      None
2        None      None      None
3        None      None      None
3        None      None      None

Table 4--estate brown crepes (ref. 15)

Class       Grade       Color     Uniformity      Discoloration

                                                 Spots;     Odor
                                                streaks,
                                                  bark

1 x      Thick brown    Light      Uniform        None      None
            crepe       brown
1 x      Thin brown     Light      Uniform        None      None
            crepe       brown
2x       Thick brown    Medium     Uniform        None      None
            crepe       brown
2x       Thin brown     Medium     Uniform        None      None
            crepe       brown
3x       Thick brown     Dark     Variation       None      None
            crepe       brown
3x       Thin brown      Dark     Variation       None      None
            crepe       brown

Class            Discoloration

           Dust,       Oil       Oxi-
           sand       stains    dation

1 x        None        None      None
1 x        None        None      None
2x         None        None      None
2x         None        None      None
3x         Bark        None      None
         particles
           only
3x         Bark        None      None
         particles
           only

Table 5--combo, thin brown, thick blanket, flat bark,
pure smoked crepe (ref. 15)

Type          Grade          Color           Discoloration

                                           Spots;      Odor
                                          streaks;
                                            bark

Combo           1         Light brown       Yes        None
crepes          2            Brown          Yes        None
                3         Dark brown        Yes        None
Thin            1         Light brown      Slight      None
brown           2        Medium brown       Yes        None
crepes          3        Medium brown       Yes        None
                4         Dark brown        Yes        None
Thick           2         Light brown      Slight      None
blanket         3            Brown         Slight      None
crepe           4         Dark brown       Slight      None
(ambers)
Flat bark    Standard      Very dark        None       None
crepes                       brown
               Hard          Black          None       None
Pure
smoked         Pure      Not specified      None      Smoked
blanket       smoked                                   odor
crepe

Type          Grade              Discoloration

                           Dust,       Oil       Oxi-
                           sand       stains    dation

Combo           1          None        None      None
crepes          2          None        None      None
                3          None        None      None
Thin            1          None        None      None
brown           2          None        None      None
crepes          3          None        None      None
                4          Bark        None      None
                         particles
Thick           2          None        None      None
blanket         3          None        None      None
crepe           4          None        None      None
(ambers)
Flat bark    Standard    Fine bark     None      None
crepes         Hard      Fine bark     None      None
smoked         Pure        None        None      None
blanket       smoked
crepe

Table 6--technically specified rubber (ref. 16)

Grade                     TSR CV       TSR L   TSR 5   TSR 10   TSR 20

Dirt content               0.05         0.05    0.05     0.01     0.2
  (wt% max.)
Ash content                0.6          0.6     0.5      0.8      1.0
  (wt% max.)
Nitrogen content           0.6          0.6     0.6      0.6      0.6
  (wt% max.)
Volatile matter            0.8          0.8     0.8      0.8      0.8
  (wt% max.)
Initial Wallace                        30.0    30.0     30.0     30.0
  plasticity (Po
  minimum)
Plasticity retention      60.0         60.0    60.0     60.0     60.0
  Index (minutes)
Color, max.                             6.0
  (Lovibond units)
Mooney viscosity       60 [+ or -] 5

Grade                  TSR 50

Dirt content             0.5
  (wt% max.)
Ash content              1.5
  (wt% max.)
Nitrogen content         0.6
  (wt% max.)
Volatile matter          0.8
  (wt% max.)
Initial Wallace         30.0
  plasticity (Po
  minimum)
Plasticity retention    60.0
  Index (minutes)
Color, max.
  (Lovibond units)
Mooney viscosity

Table 7--natural rubber quality overview (ref. 1).

Quality      Viscosity stabilized      Latex       Field coagulum
                    grades             grades          grades

Very high           SMR CV           Pale crepe
                    TSR CV             TSR L
                                       SMR L
                                       RSS1,2
High                SMR GP              RSS3           TSR 10
                                                    Estate brown
                                                       crepes
Good                                    RSS4           SMR 20
                                        RSS5          Blanket
                                                       crepes