C. Natural Rubber Products and Grades
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Figure 4
4.
5.
6.
Schematic of the natural rubber production process.
Block rubber, technically specified
Preserved latex concentrates
Specialty rubbers that have been mechanically or chemically
modified
Among these six types, the first four are in a dry form and represent nearly
90% of the total NR produced in the world. In the commercial market, these
three types of dry NR 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 and remilled crepes; and TSR in block
form. Among the three major types, crepes are now of minor significance in
the world market, accounting for less than 75,000 tonnes 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
Copyright © 2004 by Taylor & Francis
presents a simplified schematic of the process followed in the production of
natural rubber.
1. Sheet Rubber
Natural rubber in sheet form is the oldest and most popular type. Being the
simplest and easiest to produce on a small scale, smallholders’ rubber in most
countries is processed and marketed as sheet rubber. From the end user’s
perspective, two types of sheet rubbers are produced for the commercial
market: ribbed smoked sheets (RSS) and air-dried sheets (ADS). Of the two,
ribbed smoked sheet is the most popular.
Ribbed smoked sheet rubbers are made from intentionally coagulated
whole field latex. They are classified by visual evaluation. To establish
acceptable grades for commercial purposes, the International Rubber Quality
and Packing Conference prepared a description for grading, and the details
are given in the Green Book (13). 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, the
water is removed, and the material is sheeted with a rough surface to facilitate
drying. Sheets are then suspended on poles for drying in a smokehouse for 2–4
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 the buyer’s inspection is not acceptable (except slightly undercured rubber
as specified for RSS-5). Skim rubber made of skim latex cannot be used in
whole or in part in patches as required under packing specifications defined in
the Green Book. Prior to grading RSS, the sheets are separated and 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. The
premium grade is RSS1, and the lower quality grade is typically RSS4. Airdried sheets are prepared under conditions very similar to those for smoked
sheets but are dried in a shed without smoke or additives, with the exception
of sodium bisulfate. Such rubber therefore lacks the anti-oxidation protection
afforded by drying the rubber in a smokehouse. This material can be
substituted for RSS1 or RSS2 grades in various applications.
2. Crepe Rubber
Crepe is a crinkled lace rubber obtained when coagulated latex is selected
from clones that have a low carotene content. Sodium bisulfite is also added to
maintain color and prevent darkening. After straining, the latex is passed
Copyright © 2004 by Taylor & Francis
Table 2 Grade Classification of Ribbed Smoked Sheet Rubber (RSS)
RSS
Rubber
mold
Wrapping
mold
Opaque
spots
1X
No
No
No
1
V. slight
V. slight
2
Slight
3
Oversmoked
spots
Oxidized
spots
Burned
sheets
No
No
No
No
No
No
No
Slight
No
No
No
No
Slight
Slight
Slight
No
No
No
4
Slight
Slight
Slight
Slight
No
No
5
Slight
Slight
Slight
Slight
N/A
No
Comments
Dry, clean,
no blemishes
Dry, clean,
no blemishes
No sand or
foreign matter
No sand or
foreign matter
No sand or
foreign matter
N/A
several times through heavy rolls called creepers 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 are as follows:
1.
2.
3.
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 grades in this category. All these
grades must be produced from the fresh coagula of natural liquid
latex under conditions where all processes are quality controlled.
The rubber is milled to produce both thin and thick crepes. Pale
crepes are used in pharmaceutical appliances such as stoppers and
adhesives (Table 3).
Estate brown crepes. There are six grades in this category. All six
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 precleaned to separate the rubber from the
bark. Powerwash mills are to be used in milling these grades into
both thick and thin brown crepes (Table 4).
Thin brown crepes (remills). There are four grades in this class
or category. These grades are manufactured on powerwash mills
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Table 3 White and Pale Crepes
Discoloration
Class
1X
1X
1X
1
1
1
2
2
3
3
Grade
Color
Uniformity
Spots,
streaks, bark
Odor
Dust,
sand
Oil
stains
Oxidation
Thin white crepe
Thick pale crepe
Thin pale crepe
Thin white crepe
Thick pale crepe
Thin pale crepe
Thick pale crepe
Thin pale crepe
Thick pale crepe
Thin pale crepe
White
Light
Light
White
Light
Light
Slightly darker
Slightly darker
Yellowish
Yellowish
Uniform
Uniform
Uniform
Slight shade
Slight shade
Slight shade
Slight shade
Slight shade
Variation
Variation
No
No
No
No
No
No
Slight, <
10% of bales
Slight, <
10% of bales
OK if <20% of bales
OK if <20% of bales
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
Copyright © 2004 by Taylor & Francis
Table 4 Estate Brown Crepes
Discoloration
Class
1X
1X
2X
2X
3X
3X
Grade
Color
Uniformity
Spots,
streaks
Odor
Dust, sand,
bark
Oil
stains
Oxidation
Thick brown crepe
Thin brown crepe
Thick brown crepe
Thin brown crepe
Thick brown crepe
Thin brown crepe
Light brown
Light brown
Medium brown
Medium brown
Dark brown
Dark brown
Uniform
Uniform
Uniform
Uniform
Variation
Variation
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
Bark
Bark
No
No
No
No
No
No
No
No
No
No
No
No
Copyright © 2004 by Taylor & Francis
4.
5.
6.
from wet slab unsmoked sheet at the estates or smallholdings. Tree
bark scrap, if used, must be precleaned 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). The three grades in this category
are also produced on powerwash mills from wet slab unsmoked
sheets, lump, and other high-grade scrap (Table 5).
Flat bark crepes. The two grades of rubber in this category are
produced on powerwash 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
powerwash 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, and 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).
3. Technical Classification of Visually Inspected Rubbers
The Malaysian Rubber Producers Research Association (MRPRA) has
published a technical information sheet describing sheet rubbers that have
been further tested and classified with respect to cure characteristics (14). The
cure or vulcanization classes are distinguished by a color coding (i.e., blue for
fast cure, yellow for medium cure, and red for slow cure) (Table 6) when the
rubber is compounded using the American Society for Testing and Materials
(ASTM) No. 1A formulation (15). This color coding is limited to RSS1 and
air-dried sheets. Upon cure classification, the rubbers are further tested, and at
0.49 MPa the strain on the sample is measured after 1 min. This classification
scheme has not received wide acceptance, which is clearly unfortunate, for a
more quantitative classification scheme is required for visually inspected
grades of natural rubber. For example, rubber meeting a specific visually
determined grade or classification might display poor mechanical properties
when compounded with carbon black and vulcanizing agents owing to a
broad or lower molecular weight distribution. This may in turn create factory
processing difficulties and product performance deficiencies.
4. Technically Specified Natural Rubber (TSR)
The International Standards Organization (ISO) first published a technical
specification (ISO 2000) for natural rubber in 1964 (11). Based on these
specifications, Malaysia introduced a national Standard Malaysian Rubber
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Table 5 Compo, Thin Brown, Thick Blanket, Flat Bark, Pure Smoked Blanket Crepe
Discoloration
Grade
Color
Spots,
streaks
1
2
3
1
2
3
4
2
3
4
Standard
Hard
Pure smoked
Light brown
Brown
Dark brown
Light brown
Medium brown
Medium brown
Dark brown
Light brown
Medium brown
Dark brown
Very dark brown
Black
Not specified
Yes
Yes
Yes
Slight
Yes
Yes
Yes
Slight
Slight
Slight
No
No
No
Type
Compo crepes
Thin brown crepes
Thick blanket crepes (ambers)
Flat bark crepes
Pure smoked
blanket crepe
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Odor
No
No
No
No
No
No
No
No
No
No
No
No
Smoked
odor
Dust, sand,
bark
Oil
stains
Oxidation
No
No
No
No
No
No
Bark
No
No
No
Fine bark
Fine bark
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
Table 6 Technical Certification of Sheet Rubber
Class limits, % strain
Blue
Production classification
Consumer acceptance
Yellow
55–73
55–79
Red
73–85
61–91
85–103
79–103
(SMR) scheme in 1965, and since then all the natural rubber–producing
countries have 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 the block rubbers are also guaranteed to
conform to certain technical specifications, as defined by the national schemes
or by ISO 2000 (Table 7).
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 hundredths
of 1%. In Malaysia, the TSR is designated as Standard Malaysian Rubber
(SMR). In Indonesia, the designation given is Standard Indonesian Rubber
Table 7 Technically Classified Rubbers Defined in ISO 2000
Grade
Property
Dirt content,
max, wt%
Ash content,
max, wt%
Nitrogen content,
max, wt%
Volatile matter,
max, wt%
Initial Wallace
plasticity P0, min
Plasticity retention
index (min)
Color, max, lovibond
units
Mooney viscosity
TSR CV
TSR L
TSR S
TSR 10
TSR 20
TSR 50
0.05
0.05
0.05
0.1
0.2
0.5
0.6
0.6
0.5
0.75
1
1.5
0.6
0.6
0.5
0.6
0.6
0.6
0.8
0.8
0.8
0.8
0.8
0.8
30
60
30
30
30
30
60
60
50
40
30
6
60 F 5
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(SIR). In Thailand, the TSRs are called Standard Thai Rubber (STR; sometimes denoted as TTR). In India, the TSRs are designated as Indian Standard
Natural Rubber (ISNR). 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 Am sieve, and dried.
Technically specified rubber (TSR) accounts for approximately 60% of
the natural rubber produced worldwide. The advantages claimed for the
technically specified rubbers over the conventional sheet and crepe grades of
rubbers are the following:
1.
2.
3.
4.
5.
They are available in a limited number of well-defined grades,
intended to ensure a uniform, defined quality.
Data on the content of foreign and volatile matter can be provided, again to ensure better uniformity.
They are shipped as compact, polyethylene-wrapped bales of
standard weight.
They can be prepared to prevent degradation of the rubber during
storage, handling, and transportation.
They have a standard bale size to enable ease of transport through
mechanized handling and containerization.
ISO has specified six grades of TSR. The detailed characteristics of the
different grades of TSR are discussed in the following subsections
TSR CV. TSR CV, the CV designating ‘‘constant viscosity,’’ is
produced from field latex and is stabilized to a specified Mooney viscosity.
The storage hardening of this grade of rubber must be within 8 hardness units.
It is shipped in a 1.2 tonne 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 that is dispersible and
compatible with rubber when mixed in an internal mixer at temperatures
exceeding 110jC, which are, of course, typical in any rubber-mixing facility.
TSR CV rubber is generally softer than conventional technically specified
grades. Coupled with its constant-viscosity feature, it can provide a cost
advantage by eliminating premastication. When used in open mills, the
rubber forms a coherent band almost instantaneously, thus potentially
improving milling throughput. Additional claimed benefits of TSR CV
include
1.
2.
3.
4.
Reduction of mixing times, giving higher throughput
Reduction of scraps and rejects due to better material uniformity
Better resistance to chipping and chunking for off-the-road (OTR)
tires
Better green strength
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TSR CV rubber is available in different viscosities, with 50 and 60 being
the more common. This material can be used for high-quality products such
as mechanical mountings for engines and machinery, railway buffers, bridge
bearings, vehicle suspension systems and general automotive components,
large-truck tire treads, conveyor belt covers, cushion gum for retreading,
masking tapes, injection-molded products including rubber–metal bonded
components, industrial rolls, inner tubes, and cement.
TSR L. TSR L is a light-colored rubber produced from high-quality
latex; it has low ash and dirt content and is packed and presented in the same
way as TSR CV. The advantage of TSR L is its light color together with its
cleanliness and better heat-aging resistance. Technologically, TSR L shows
high tensile strength, modulus, and ultimate elongation at break for both
black and nonblack mix.
This material can be used for light-colored and transparent products
such as surgical or pressure-sensitive tape, textiles, rubber bands, hot water
bottles, surgical and pharmaceutical products, large industrial rollers for the
paper printing industry, sportswear, bicycle tubes, chewing gum, cable
sheaths, gaskets, and adhesive solutions and tapes.
TSR 5. TSR 5 is produced from fresh coagulum, ribbed smoked
sheets, or air-dried sheets. It is packed and shipped to the same specifications as TSR CV and TSR L. TSR 5 is typically used for general-purpose
friction and extruded products, small components in passenger vehicles such
as mountings, sealing rings, cushion gum, and brake seals, bridge bearings,
ebonite battery plates, separators, adhesives, and certain components in
tires.
TSR 10. TSR 10 is produced from clean and fresh field coagulum or
from unsmoked sheets. It is packed and shipped in the same way as TSR CV,
TSR L, and TSR 5. TSR 10 has good technological properties similar to those
of RSS2 and RSS3, but has an advantage over RSS because of its
1.
2.
3.
Lower viscosity
Easier mixing characteristics (more rapid breakdown)
Technical specifications and packaging in 33.3 kg bales
It can be used for tires, inner tubes, cushion gum stocks, joint rings by
injection molding, raincoats, microcellular sheets, upholstery and packing,
conveyor belts, and footwear.
TSR 20. This is a large-volume 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 specifications as TSR
CV, TSR L, TSR 5, and TSR 10. TSR 20 has good processing characteristics
Copyright © 2004 by Taylor & Francis
and physical properties. Its low viscosity and easier mixing characteristics
(compared with the RSS grades) can reduce the mastication and mixing
period considerably. It is used mostly for tires, cushion gum stock, bicycle
tires, raincoats, microcellular sheet for upholstery and packing, conveyor
belts, footwear, 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 other grades of TSR. It should be noted that these specifications
will 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, that of SMR 10 from 0.10 to 0.08, and
that of SMR 20 to 0.016.
In addition, Malaysia has produced grades of rubber outside the specific
scope of ISO 2000. 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 SMR10 in specification.
To illustrate the distribution and consumption of these various grades,
shipments of SMR from Malaysia are typically SMR 20, 60%; SMR 10, 27%;
SMR CV and SMR L, 5%; SMR GP, 7%; and SMR 5, 1.0%.
D. Viscosity and Viscosity Stabilization of Natural Rubber
The properties of natural rubber that are most important regarding its use in
the manufacture of tires or other products include viscosity, fatty acid bloom,
and compliance with the technical specifications. Of these three parameters,
viscosity is probably the most important. This property relates to the
molecular weight, molecular weight distribution, and amounts of other
materials present in the polymer such as low molecular weight resins, fatty
acids, and other natural products. It affects the initial mixing of the rubber
with other compounding ingredients and subsequent processing of the
compounded materials to form the final manufactured product.
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 range of methods are available to characterize the viscosity of
natural rubber. The most popular is Mooney viscosity (Vr), which is obtained
by measuring the torque that is required to rotate a disk embedded in rubber
or a compounded sample. This procedure is defined in ASTM D 1646,
‘‘Standard Test Methods for Mooney Viscosity, Stress Relaxation, and
Prevulcanization Characteristics (Mooney viscometer)’’ (16). The viscosity
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