D. Viscosity and Viscosity Stabilization of Natural Rubber

will typically range from 45 to over 100. The information obtained from a

Mooney viscometer can include

1.



Prevulcanization properties or scorch resistance for the compounded polymer, a test that is conducted at temperatures ranging

from 120jC to 135jC (Fig. 5).

2. Mooney peak, which is the initial peak viscosity at the start of the

test and a function of the green strength and can be a measure of

compound factory shelf life.

3. Viscosity (Vr), typically measured at 100jC, provides a measure of

the ease with which the material can be processed (Fig. 6). It depends on molecular weight and molecular weight distribution,

molecular structure such as stereochemistry and polymer chain

branching, and nonrubber constituents. Caution is always required

when attempting to establish relationships between Mooney viscosity and molecular weight. Mooney viscosity can be expressed as

ML 1 + 4 (i.e., Mooney large rotor, with 1 min pause and 4 min test

duration).

4. Stress relaxation, which can provide information on gel (T-95), 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 viscous response. A

slow rate of relaxation indicates a higher elastic component in the

overall response, whereas a rapid rate of relaxation indicates a

more highly viscous component. The rate of stress relaxation can

correlate with molecular structural characteristics such as molec-



Figure 5 Mooney scorch typically conducted at 121jC and 135jC.



Copyright © 2004 by Taylor & Francis



Figure 6



5.



Mooney plasticity and stress relaxation.



ular weight distribution, chain branching, and gel content. It can

be used to give an indication of polydispersity or Mn/Mw. 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 100jC, is the final viscosity after

15 min. This provides another measure of the processing characteristics of the rubber. It indicates the ease of processing compounds that are milled before being extruded or calendered (e.g.,

hot feed extrusion systems).



Much work has been done to establish a relationship between the

Mooney viscosity (ML) and molecular weight of natural rubber as well as

the molecular weight distribution. Bonfils et al. (17) 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 following trend:



Sample



P0



ML 1 + 4



Mw (kg/mol)



1

2

3

4



32

41

54

62



57

78

92

104



746

739

799

834



Copyright © 2004 by Taylor & Francis



where P0 is initial Wallace plasticity, ML 1 + 4 is Mooney viscosity after 4

min, and Mw is molecular weight.

Though clearly not linear, there is an empirical relationship between

Mooney viscosity and molecular weight. Nair (18,19) explored this, established a relationship between intrinsic viscosity and Mooney viscosity, and

determined a correlation coefficient of 0.87. This correlation can be improved

by mastication of the test samples, which improves the homogeneity. Mastication or milling also narrows the molecular weight distribution, which is an

important factor in this respect (20).

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; it is known as the ACS1

(American Chemical Society No. 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 such as fatty

acids, amines, and amino acids, which may influence the vulcanization rate.

Natural rubber is susceptible to oxidation. This can affect both the

processing qualities of the rubber and the mechanical properties of the final

compounded rubber. Natural antioxidants will offer protection from the degradation of natural rubber, which can be measured by the change in the

material’s plasticity. The Wallace plasticity test reports two measures:

Plasticity, P0, is the initial Wallace plasticity and a measure of the

compression of a sample after a load has been applied for a

defined time.

2. The plasticity retention index (PRI) measures recovery after a

sample has been compressed, heated, and subsequently cooled.

PRI% is defined as ( P30/P0) Â 100, where P0 is the initial

plasticity and P30 is the plasticity after aging for 30 min typically

at 140jC. During processing in, for example, a tire factory,

natural rubbers with low PRI values tend to break down more

rapidly than those with high values.



1.



Various equations have been proposed that provide an empirical

relationship between Mooney viscosity Vr, and Wallace plasticity P0. These



* phr = parts per hundred parts of rubber.



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equations depict a linear relationship between these two parameters and are

therefore typically of the form

Vr ¼ X P0 þ constant C



ð1Þ



The numerical coefficient X and constant C are functions of the clone and

grade of rubber but normally fall between 1.15 and 1.50 for coefficient X and

between 4.0 and 12.5 for C (21).

Other materials can be added to assist in improving the processability of

natural rubber. These include peptizers such as 2,2V-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 of up to 25% of the total polyisoprene content, will also

reduce the viscosity of the compound with little loss in other mechanical

properties. It also allows for better control of component tack, which is

important in subsequent product assembly steps such as those in tire building.

Natural rubber tends to harden during processing and storage at the

plantation processing factory and also during shipping and prior to use in a

rubber products manufacturing facility. This hardening phenomenon is manifested as an increase in viscosity, which is due to oxidation of the polymer

chain and cleavage to form the functional groups, ketones UC(CH3) = O and

aldehydes UCUCH = O. The aldehyde group can readily react with the –NH2

groups in proteins to form a gel and thereby increase polymer viscosity. This

occurs primarily during the latex drying process, which can last for 5–7 days

at around 60jC. Materials may be added to natural rubber to suppress this

increase in viscosity, and this has been the basis for the development of CV

rubbers. Hydroxylamine neutral sulfate (NH2OHÁH2SO4), denoted as HNS,

or propionic hydrazide (PHZ)

O

k

H2 NÀNHÀCÀEt

Propionic hydrazide (PHZ)

can be added to natural rubber latex at levels of 0.08–0.30 wt % and 0.20–

0.40 wt %, respectively, to prevent gel formation. An accelerated storagehardening test can measure the hardening of CV rubber that will occur

during normal storage. When HNS is added before coagulation, treated

rubbers will show a P0 change of 8 units or less (constant viscosity, CV).

However, they will tend to display a darker color due to the HNS addition.



Copyright © 2004 by Taylor & Francis



Both HNS and PHZ block the reaction of the aldehyde groups with

UNH2 by reacting with the UC(CH3) = O group to form

RVUCðCH3 Þ ¼ N À NHUCOUR

RVUCðCH3 ÞUCH ¼ NUCOUR



and



In compounded rubber, the term ‘‘bound rubber’’ has frequently been used to

describe this cross-linking condition in both natural rubber and polymers

such as polybutadiene. Bound rubber can be found in all synthetic unsaturated elastomers and is 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 and leave a swollen insoluble

gel. The formation of bound rubber can result from the use of high-structure

carbon blacks, the use of silane coupling agents, or the application of fast to

ultrafast accelerators such as zinc diethyldithiocarbamate found in vulcanization systems with low cure temperatures.

A number of production techniques can have an impact on the final

viscosity of the rubber. The field methods are documented as follows.

1.

2.



3.



4.



5.



Latex dilution. The effect is small, with 1:1 dilutions required to

have any measurable effect.

Ammonia. An increase in the ammonia level added initially for

preservation from 0.01% to 0.50% can result in a Mooney viscosity increase of up to 10 Mooney units.

Coagulation method. Coagulation methods can range from

natural or bacterial coagulation to the addition of formic acid

or heating. Mooney viscosity will range from 65 to 85, with higher

Mooney viscosity values being obtained through the 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

content. This rise in gel content can be due to an increase in pH

due to partial hydrolysis of protein and amino acids and subsequent cross-linking or to an increase in bacterial action.

Drying temperature. Above 60jC there is a slight increase in

Mooney viscosity.



Another factor that can affect viscosity is baling temperature. The age of

the tapped rubber tree, yield stimulants, and seasonal effects may also play

some role. If baled hot, the rubber can take a considerable time to cool. When

hot, the polymer gel content or other cross-linking phenomenon may

increase.



Copyright © 2004 by Taylor & Francis



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 at between À20jC

and À30jC. The rate of crystallization 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 nonrubber constituents present in the

rubber. Fatty acids, particularly stearic acid, can act as a nucleating agent in

strain-induced crystallization (22,23). This can influence the end product

performance, for example in tires where strain-crystallized rubber can display

reduced fatigue resistance but improved green strength, tensile strength, and

abrasion resistance compared to elastomers that do not experience this

phenomenon.



E. Special-Purpose Natural Rubbers

A considerable amount of work has been directed toward enhancing the

properties of natural rubber through chemical modification. A number of

polymers emerged from this work:

Liquid low molecular weight rubber. Produced by depolymerization

of natural rubber, liquid low molecular weight rubber can be used

as a reactive plasticizer, processing aid, and base polymer. Molecular weights range between 40,000 and 50,000. This rubber is liquid

at room temperature but is also available on a silica carrier (24).

Depolymerized natural rubber finds application in flexible molds for

ceramics, binders for grinding wheels, and sealants. It is susceptible

to oxidation and therefore requires appropriate compounding techniques for adequate aging resistance. Liquid natural rubber can be

produced by a combination of mechanical milling, heat, and the

addition of chemical peptizer. Reference may be made to the work

of Claramma et al. (25) for a discussion on the effect of liquid low

molecular weight natural rubber on compounded classical mechanical properties.

Methyl methacrylate grafting. Three grades of rubber with different

levels of grafted methyl methacrylate are available (Heveaplus MG

30, 40, and 49). These are prepared by polymerizing 30, 40, and 49

parts of methyl methacrylate, respectively, in the latex before

coagulation. They have found application primarily in adhesives

due to the effectiveness of the polar methacrylate group and non-



Copyright © 2004 by Taylor & Francis



polar isoprene bonding dissimilar surfaces. Such polymers tend to

have very high hardness (International Rubber Hardness Degrees,

IRHD), with values up to 96 and have thus had no application in

pneumatic tires (7,8). When blended with regular grades of natural

rubber such as RSS2, vulcanizates with high stiffness are attained

but display Mooney viscosities ranging from 60 to 80 at typical

factory compound processing temperatures.

Oil-extended natural rubber. Oil-extended natural rubber (OENR)

treads are very effective in improving ice grip and snow traction of

tires and have been reported to be used for service in northern

Europe. OENR is produced by one of several methods: 1) cocoagulation of latex with an oil emulsion prior to coagulation or with the

dried field coagulum, 2) Banbury mixing of the oil and rubber, and

3) soaking of the rubber in oilpans followed by milling to facilitate

further incorporation and sheeting. Both aromatic (A) and

naphthenic (N) oils are used at loadings typically around 65 phr.

When compounded, filler loadings can be higher than those typically found in non-oil-extended rubber. The ratio of rubber to oil

and oil type are denoted by a code that would read, for example,

OENR 75/25N for a 75% rubber, 25% naphthenic oil material.

Deproteinized natural rubber. This is produced by treating NR latex

with an enzyme that breaks down naturally occurring protein and

other nonrubber material into water-soluble residues. The residues

are then washed out of the rubber to leave a polymer with low

sensitivity to water. Typically, natural rubber contains around 0.4%

nitrogen as protein; deproteinized rubber contains typically 0.07%.

Deproteinized natural rubber has found application in medical

gloves to protect workers from allergic reactions and in automotive

applications, seals, and bushings. The polymer displays low creep,

exhibits strain relaxation, and enables greater control of product

uniformity and consistency (26).

Epoxidized natural rubber. Compared with natural rubber, epoxidized

NR shows better oil resistance and damping and low gas permeability. However, its tear strength is low, which has prevented its use

in pneumatic tires. Two grades are available, ENR 25 and ENR 50,

i.e., 25 mol% epoxidized and 50 mol% epoxidized. Epoxy groups are

randomly distributed along the polymer chain. Calcium stearate is

required as a stabilizer. These polymers offer a number of advantages

such as improved oil resistance (ENR 50 is comparable to polychloroprene), low gas permeability equivalent to that of butyl rubber,

and compatibility with PVC. When compounded with silica,



Copyright © 2004 by Taylor & Francis



epoxidized NR has reinforcement properties equivalent to those of

carbon black but without the use of silane coupling agents (27).



Thermoplastic natural rubber. Thermoplastic NR materials consist of

blends of natural rubber and polypropylene. No application in tires

or other major elastomer-based products has been developed,

though that remains one area that offers considerable potential for

the future (27).

Superior processing rubber. This consists of a mixture of two types of

natural rubber, one cross-linked and the other not. It is prepared by

blending vulcanized latex with diluted field latex in levels according

to the grade being prepared (SP 20, SP 40, SP 50 with 20%, 40%,

and 50% cross-linked phase, respectively). Two grades are also

available (PA 57 and PA 80) with a processing aid added to further

facilitate factory handling. These two grades contain 80% crosslinked rubber. These two-phase polymer systems display high stiffness with good flow and process qualities.

Guayule. Guayule is a shrub that grows in the southern region of the

United States and northern Mexico. A typical 5–10-year old plant

will grow to about 30 in. in height and have a dry weight of

approximately 20% resinous rubber. Rubber of reasonable quality

can be obtained after extraction. Though work of any significance

has not been conducted in this area for many years, given the

advances in genetic engineering and related fields in biotechnology,

it is an area that merits further exploration. New clones could be

developed that might have improved output and supplement current

supplies of NR extracted from Hevea brasiliensis (28).

Ebonite. Ebonite is a rubber vulcanized with very high levels of

sulfur. True ebonite has a Young’s modulus of 500 MPa and Shore

D hardness of typically 75. The term ‘‘pseudoebonite’’ has been used

to describe rubber with a Shore A hardness, or IRHD (International Rubber Hardness degrees), of 98 or Shore D hardness of 60.

Ebonite has a sulfur content of 25–50 phr, and resins may also be

used to obtain the required hardness or meet any compounding

constraints of concern to the technologist. The principal use of

ebonite materials is in battery boxes, linings, piping valves and



Copyright © 2004 by Taylor & Francis



pumps, and coverings for rollers, where chemical and corrosion

resistance is required (29).

Synthetic polyisoprene. Global production of natural rubber is

expected to stabilize at 8.0 million tonnes/year in 2003–2004, with

a final total capacity of about 8.5 million tonnes within a few years

after that. Demand, however, is projected to grow to 10,500 million

tonnes by the year 2020. Such projected shortages could be met by

the use of synthetic polymers such as polyisoprene, SBR, and PBD.

The isoprene unit not only exists in natural rubber but is also the

building block for terpenes, camphor, and other natural products.

Isoprene for chemical synthesis is typically recovered from the C5

streams obtained in the thermal cracking of naphtha. Three organometallic initiators have attained commercial significance in cispolyisoprene production. These are n-BuLi, TiCl4R3Al anisole

and CS2, and TiCl4 (HAIN-i-C3H7)6, where HAIN is a poly (Nalkylimino alane). The lithium catalyst will produce a polymer with

a microstructure that is 92% cis-1,4-isoprene and 1% trans-1,4isoprene, and 7% vinyl-3,4. Titanium-based catalysts will produce

a polymer typically 96% cis-1,4-isoprene and 4% vinyl or isopropenyl, though there may be trace amounts of trans-1,4-isoprene.

With appropriate levels of modifiers, the level of trans-1, 4-isoprene

can be increased. Such polymers have a much higher glass transition

temperature (Tg) and therefore tend to find application in tire tread

compounds where traction is a required performance parameter.

When the required conversion is complete, a terminator (short stop)

to deactivate the catalyst and a stabilizer are added. The numberaverage molecular weight (Mn) of polyisoprene is 350,000–400,000,

and its Mooney viscosity (ML 1 + 4) ranges from 55 to 95, depending on the commercial grade. The glass transition temperature

is around À70jC. Polyisoprene is more uniform than natural rubber

and thus lends itself better to applications requiring good mixing

efficiency, high-speed extrusions, mix consistency, and component

uniformity as in tires. It is used in applications requiring high

tensile strength, resilience, tear strength, and abrasion resistance

(30,31).



F. Quality

A number of factors can be considered under the broad category of ‘‘quality,’’

such as consistency or uniformity, supply, packaging, and minimum contamination. The following discussion will highlight some general qualities that



Copyright © 2004 by Taylor & Francis



rubber product manufacturers should expect from the raw natural rubber

producers.

1. Consistency and Uniformity

Within a grade, end users of natural rubber require uniformity, little spread in

properties such as plasticity retention index, and little or no need to warm the

rubber prior to mixing. In tire and industrial goods manufacturing, natural

rubber uniformity is required for final compound consistency, which in turn

yields consistent processing characteristics. Lack of consistency will result in

variation in mixing specifications, extrudate uniformity, tack, and product

component properties.

The only physical measures that are used to quantify the processing

characteristics of natural rubber are original Wallace rapid plasticity ( P0) and

the plasticity retention index (PRI). P0 tested via the Wallace Plastimeter is

used as a rapid means of measuring plasticity. The level of P0 has been

determined to represent approximately half the level of Mooney viscosity, i.e.,

a P0 of 30 would suggest a Mooney viscosity of about 60. Mooney viscosity

and P0 alter with storage hardening (as a result of the cross-linking of random

functional groups such as aldehyde groups in the polyisoprene chain),

increasing with time (storage between processing at source and use at tire

plant delayed by ocean transit) and unfortunately at an inconsistent rate and

level. In an effort to provide consistency and stability, hydroxylamine neutral

sulfate (HNS) is added to grades such as TSR 10 CV and TSR 20 CV that have

been stabilized. It is also possible to stabilize other grades to a Mooney of 65 F

10 units using HNS if necessary.

2. Packaging

Bales must be wrapped properly to prevent moisture penetration and mold

growth, to maintain the quality levels of the rubber at time of purchase, and to

avoid contamination. Shipping in metal containers avoids wood contamination and is recommended.

3. Contamination

Considerable work has been done to lower the dirt level in both technically

specified and visually inspected rubbers. The last revision to the Standard

Malaysian Rubber (SMR) scheme (12) introduced the following revisions:

1.



Two constant viscosity grades, SMR 10 CV and SMR 20 CV, were

defined whose Mooney viscosities (ML 1 + 4) are 50 F 5 and 60

F 5, respectively.



Copyright © 2004 by Taylor & Francis



2.



Dirt level specifications were reduced from 0.10 to 0.08 for SMR

10 CV and from 0.20 to 0.16 for SMR 20 CV.

3. CV grades of SMR 5 were specified, again with viscosities of 50

and 60 (SMR 50 CV, SMR 60 CV). Dirt levels of 0.03% are now

typical.

In the future, emphasis must be placed on reducing contamination. This

is in recognition that the ‘‘dirt’’ level has improved significantly over the last

few years for all TSR grades. Contaminants include foreign material originating from the field in the form of bark, wood, twigs, leaves, and leaf stems.

These have the potential to cause final product failure, because large foreign

particles do not disperse during compounding and can provide sites for crack

initiation. Although the level of dirt may be measured by the residue weight

and as such can be included in technical specifications, contamination by

foreign light matter such as wood chips and plastic material are not specifiable

at this time. In the washing and cleaning of NR at the processor’s factory the

sedimentation process separates heavy material from the floating light rubber

crumbs, which reduces dirt content, but it does not separate the light, floating

contaminants satisfactorily. The NR industry has focused on reduction of dirt

content by the use of sedimentation within the process, but contamination

with foreign matter is generally caused by material that floats and therefore is

not controlled in the traditional process.

4. 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. Tire plants may have component tack

difficulties when, for example, a TSR 20 with fatty acid levels of 0.25 wt% is

changed to a TSR 20 grade with a fatty acid level of 0.9–1.0 wt%. This is due

to bloom. Such bloom can later cause component separations. High levels of

fatty acids can also affect vulcanization kinetics. Table 8 presents total fatty

acid levels for a variety of natural rubbers and shows that they can vary from

0.3% to 0.8%. Synthetic polyisoprene can be used to control bloom, tack, and

viscosity. Addition of a polymer such as Natsyn 2200 (Goodyear Tire &

Rubber Company) of up to 50% of the final product can be used where there

are concerns regarding excessive fatty acid levels. Tack-inducing resins such

as ExxonMobil Escorez 1102 may also be used to correct bloom.

Fatty acid levels are to a large degree 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, materials from

other regional sources such as Indonesia initially contain much higher dirt



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