E. Compounding Application of Recycled Materials

Table 18 Use of Reclaim and Recycled Materials in Tires

Component



Passenger tires



Light truck tires



Commercial tires



Retreads



Treads

Subtread

Casing plies

Bead fillera

Sidewalla

Wedges

Squeegee

Liner



Yes

No

No

Yes

Yes

Yes

Yes

Yes



Yes

No

No

Yes

Yes

No

Yes

Yes



No

No

No

No

No

No

Yes

No



Yes

Yes

No

No

No

No

Yes

No



a

From Ref. 1.

Source: Ref. 44.



4.



Flooring, walkway tiles, and sports surfaces such as running or

jogging tracks constitute a growing market for recycled rubber.

Such surfaces are very effective. However, the most important

technical issue is removal of all steel from the material. Cryogenic

grinding is typically used to produce materials for such

applications.



Tables 18 and 19 list a range of applications for recycled materials in

tires and industrial products. Table 18 displays tire components that have the



Table 19

Products



Target Levels for Use of Reclaim and Recycled Materials in Industrial



Product



Potential application



Potential loading (%)



No

Yes

Yes

Yes

Yes

Yes

Yes

Yes



0

3.0

5.0

5.0

10.0

25.0

25.0

50.0



Belt casing/carcass

Conveyor belt covers

Transmission belts (non-O.E.)

Hose covers and inner tubes

Low operating pressure tubing

Weatherstripping (non-O.E.)

Carpet backing

Railroad crossingsa



O.E. = original equipment.

a

Rubber blocks laid between rails at highway-railroad crossings and junctions.

Source: Ref. 44.



Copyright © 2004 by Taylor & Francis



potential to contain varying levels of recycled material. Conversely, many

components in tires cannot contain recycled material owing to potential

deterioration in performance. To address this, two requirements may need to

be addressed, 1) more finely ground material with better defined particle

dimensions and 2) new compounding ingredients to improve factors such as

dispersion, fatigue resistance, and tear strength. Table 19 similarly shows

potential levels for recycled material in industrial products such as belts and

hoses. These provide target levels for the materials scientist developing

compounds with recycle content.



V. SUMMARY

This chapter has reviewed the classification and major uses of natural rubber.

Of the range of naturally occurring materials used in advanced engineered

products, natural rubber is among the most extensively used.

Four key factors will determine its use in the future:

1.



2.



3.



4.



Availability. Given the growth of the global economies and the

automotive industry specifically, additional sources of materials

will be required to meet the shortages anticipated by the year 2007.

Technical specifications. Specifications will be needed for visually inspected rubbers such as RSS grades to meet the end users’

need for consistency and uniformity in their factories.

Quality. End product specifications and performance requirements will continue to be refined, thereby necessitating continuing

improvement in consistency, absence of foreign materials or other

contaminants, and purity.

Chemical modification. 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.



Research institutes throughout the world are working on these issues,

which will ensure the use of natural rubber products long into the future.

The use of recycled rubber will continue to increase throughout the first

decades of the 21st century. This growth will be driven by regulatory and

economic factors rather than technological factors. However, overcapacity in

global vehicle production is having a detrimental impact on pricing, which in

turn is restricting growth in recycling opportunities. It is anticipated that this

will change, and the materials scientist should have the appropriate technologies in place to take advantage of future demand. This discussion has

therefore attempted to provide a foundation for the rubber technologist to



Copyright © 2004 by Taylor & Francis



take advantage of the selection and use of renewable and recycled materials

available for the range of products produced by today’s rubber industry.



REFERENCES

1.

2.



Barlow F. Rubber Compounding. 2d ed. New York: Marcel Dekker, 1994.

Roberts AD. Natural Rubber Chemistry and Technology. New York: Oxford

Univ Press, 1988.

3. Cyr DS. Rubber, natural. In: Kroschwitz JI, ed. Encyclopedia of Polymer Science and Engineering. Vol. 14. 2d ed. New York: Wiley, 1988:687–716.

4. Baker CSL, Fulton WS. Rubber, natural. In: Kroschwitz JI, Howe-Grant M,

eds. Kirk-Othmer Encyclopedia of Chemical Technology. Vol. 21. 4th ed. New

York: Wiley, 1997:562–591.

5. Mahler HR, Cordes EH. Basic Biological Chemistry. New York: Harper & Row,

1969.

6. Hasma H, Subramanian A. Composition of lipids in latex of Hevea brasiliensis

clone RRIM 501. J Nat Rubber Res 1986; 1:30–40.

7. Hasma H. Lipids associated with rubber particles and their possible role in

mechanical stability of latex concentrates. J Nat Rubber Res 1991; 6:105–114.

8. International Standards Organization. ISO 2004. Specifications for Natural

Rubber Latex Concentrates, 1988.

9. EIU Automotive Rubber Trends. Worldwide Rubber Database. 4th quarter

1999. The Economist Intelligence Unit (London).

10. Barbin W, Rodgers MB. Science of rubber compounding. In: Mark JE, Erman

B, Eirich FR, eds. Science & Technology of Rubber. New York: Wiley, 1994.

Chap 9.

11. International Standards Organization. ISO 2000. Rubber Grades, 1964.

12. Rubber Research Institute of Malaysia. Revisions to Standard Malaysian Rubber Scheme, SMR Bull 11. Kuala Lumpur, Malaysia, 1991.

13. 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, January 1979. Washington, DC: Rubber

Manufacturers Assoc, 1979.

14. The Malaysian Rubber Producers Research Association. Technically classified

(TC) rubber—visually graded natural rubber of classified cure characteristics.

Natural Rubber Tech Info Sheet D100, 1982.

15. American Society for Testing and Materials. ASTM D 3184. Rubber—Evaluation of NR (Natural Rubber). Annual Book of ASTM Standards, Philadelphia,

Vol 09.01, 1999.

16. American Society for Testing and Materials. ASTM D 1646. Standard test methods for Mooney viscosity, stress relaxation, and prevulcanization characteristics

(Mooney viscometer). Annual Book of ASTM Standards, Philadelphia, Vol

09.01, 1999.

17. Bonfils F, Char C, Garnier Y, Sanago A, Sainte Beuve J. Inherent molar mass



Copyright © 2004 by Taylor & Francis



18.

19.

20.

21.

22.



23.

24.

25.

26.

27.

28.

29.

30.



31.

32.



33.

34.

35.

36.

37.



38.



39.



distribution of clones and properties of crumb rubber. J Rubber Res 2001; 3:

164–168.

Nair S. Dependence of bulk viscosities (Mooney and Wallace) on molecular

parameters of natural rubber. J Rubber Res Inst Malaya 1970; 23:76–83.

Nair S. Characterization of natural rubber for greater consistency. Rubber

World, July 1998.

Subramaniam A. Molecular weight and other properties of natural rubber: a

study of clonal variations. Int Rubber Conf, Kuala Lumpur, 1975.

Subramaniam A. Viscosity of natural rubber. Planters Bull 1984; 180:104–112.

Kawahara S, Kakubo T, Sakdapipanich JT, Isono Y, Tanaka Y. Characterization of fatty acids linked to natural rubber—role of linked fatty acids on

crystallization of the rubber. Polymer 2000; 41:7483–7488.

Sharples A. Polymer Crystallization. London: Edward Arnold, 1966.

Element’s Tech Bull. DPR liquid natural rubber, isolene, kalene, kalar. Belleville,

NJ, 2003.

Claramma NM, Nair NR, Mathew NM. Production of liquid natural rubber by

thermal depolymerization. Indian J Nat Rubber Res 1991; 4:1–7.

Aziz A, Kadir SA. Advances and developments in NR. Rubber World,

November 2000:44–50.

Chai LP, Sang STM. Epoxidized natural rubber in tubeless tyre inner liners.

International Rubber Conference, Kuala Lumpur, 1985.

Cyr DR. Rubber, natural. In: Grayson M, ed. Kirk-Othmer Encyclopedia of

Chemical Technology. Vol. 20. 3rd ed. New York: Wiley, 1982:468–491.

Pyne JR. Rubber, hard. In: Kroschwitz JI, ed. Encyclopedia of Polymer Science

and Engineering, Vol. 14. 2d ed. New York: Wiley, 1988:670–686.

Hsieh HL, Wagner PH, Wilder CR. Rubber, synthetic. In: McKetta JJ,

Cunningham WA, eds. Encyclopedia of Chemical Processing and Design. Vol

48. New York: Marcel Dekker, 1994:388–393.

Schoenberg E, Marsh HA, Walters SJ, Saltman WM. Polyisoprene. Rubber

Chem Technol 1979; 52:526–604.

Rodgers MB. Rubber tires. In: Encyclopedia of Materials: Science and Technology. London: Buschow KHJ, Cann RW, Fleming MC, Ilschner B, Kramer

EJ, Mahajan S, eds. Elsevier Science, 2001:8237–8242.

Leblanc JL, Hardy P. Evolution of bound rubber during the storage of uncured

compopunds. Kautsch Gummi Kunstst 1991; 44:1119–1124.

Steichen R. Impact of future tire trends on natural rubber. Indonesia Rubber Res

Inst, Bogar, Indonesia, 2000.

Smithers Scientific Services. Tire Analysis Report, 2000.

Barker CSL. Natural rubber. Rubber Dev 1996; 49(3/4):40–44.

Mowdood S, Locatelli JL, De Putdt Y, Serra A. Future performance needs for

tire fillers. Functional Tire Fillers. Intertech Consulting Conference & Studies,

Fort Lauderdale, FL, Jan 29–31, 2001.

Sae-Oui P, Rakee C, Thanmathron P. Use of rice husk ash in natural rubber

vulcanizates: in comparison with other commercial fillers. J Appl Polym Sci

2002; 83:2485–2493.

Da Costa HM, Visconte LLY, Numes RCR, Furtado CRG. Mechanical and



Copyright © 2004 by Taylor & Francis



40.



41.



42.

43.

44.

45.

46.

47.

48.

49.

50.



dynamic mechanical properties of rice husk ash-filled natural rubber compounds. J Appl Polym Sci 2002; 83:2331–2346.

Ismail H, Edyham MR, Wirosentono B. Dynamic and swelling behavour of

bamboo filled natural rubber composites: effect of bonding agents. Iranian

Polym J 2001; 10:377–383.

Ismail H, Schuhelmy S, Edyham MR. The effect of silane couple agent on curing

characteristics and mechanical properties of bamboo-filled rubber composites.

Eur Polym J 2002; 38:39–47.

Kuriakose AP, Rajendran G. Rice bran oil as a novel compounding ingredient in

sulphur vulcanization of natural rubber. Eur Polym J 1995; 31:596–602.

Myhre M, MacKillop DA. Rubber recycling. Rubber Chem Technol 2002;

72:429–474.

Klingensmith W, Baranwal K. Best Practices in Scrap Tires and Rubber

Recycling. Seattle, WA: ReTAP of the Clean Washington Center, 1997.

Sikora M. Scrap Tire Users Directory. Recycling Research Institute, annual.

Washington, DC.

Cryofine EPDM Handbook. Wapakonneta, OH: Midwest Elastomers, 1985.

Cryofine Butyl Handbook. Wapakonneta, OH: Midwest Elastomers, 1985.

Ball J. Manual of Reclaimed Rubber. Ball J, ed. Washington, DC: Rubber

Manufacturers Association, 1956.

Smith FG. Reclaim Rubber Handbook. Norwalk, CT: RT Vanderbilt, 1978.

Technical Bulletin. Rouse Rubber Technical Industries, 1991.



Copyright © 2004 by Taylor & Francis



2

General-Purpose Elastomers

Howard Colvin

Riba-Fairfield, Decatur, Illinois, U.S.A.



I.



INTRODUCTION



General-purpose elastomers played a critical role in the history of the last half

of the 20th century. In 1942 the Rubber Reserve program developed both the

basic technology and manufacturing capability to make emulsion styrene

butadiene rubber (SBR) just a few years after World War II had interrupted

natural rubber supplies. Historians have noted that the scientific contribution

to that effort is comparable to the nuclear research program at Los Alamos

that occurred at the same time (1). After the petroleum shortages of the 1970s,

fuel economy became a primary driving force in the automotive industry, and

the tire industry was challenged to develop new products that would improve

gas mileage. New elastomers based on solution SBR technology proved to be

part of the answer.

Today the tire industry is challenged to meet new environmental

standards while maintaining or improving the vehicle handling, ride, and

durability that has already been achieved. To meet this challenge, the rubber

technologist must have a thorough understanding of how general-purpose

elastomers (i.e., polybutadiene, styrene/butadiene, and styrene/butadiene/

isoprene) affect compound processability, tire rolling resistance, tire traction,

tire treadwear, and overall cost of tire components. Use of these elastomers

outside of the tire industry requires the same type of understanding of

fundamental polymer characteristics and how they affect the final application. This review will describe the basic structure–property relationships

between general-purpose elastomers and end-use properties, with a focus on



Copyright © 2004 by Taylor & Francis



the tire industry. The processes used to make the general-purpose elastomers

will be described with an emphasis on how the polymerization variables

(mechanism, catalyst, process) affect the macrostructure and microstructure

of the polymer. It is polymer microstructure and macrostructure that

determine whether a polymer is suitable for a particular application, not

the type of process or catalyst used to produce the polymer.

Some important terms used in this chapter are defined in Table 1.



II.



STRUCTURE–PROPERTY RELATIONSHIPS FOR

GENERAL-PURPOSE ELASTOMERS USED

IN TIRE APPLICATIONS



A.



Laboratory Testing Methods



Prediction of tire properties based on laboratory properties has met with

various degrees of success, depending on which property was being predicted.

There is a good correlation between the rolling resistance of tires and the tread

compound tangent delta at 60jC and 40 Hz (2). There is a reasonable



Table 1 Definitions

Polymer microstructure Monomers incorporated into the polymer and the stereochemistry of enchainment (i.e., cis, trans, vinyl).

Polymer macrostructure Polymer molecular weight and molecular weight distribution, molecular geometry (linear, branched, comb), and the order in which monomers are incorporated (block, tapered block, or random).

Number-average molecular weight (Mn) Summation of the number of polymer

chains (N) with a given molecular weight (m) times the molecular weight of each

chain divided by the total number of polymer chains: SmiNi/SNi.

Weight-average molecular weight (Mw) Summation of the number of polymer

chains (N) with a given molecular weight (m) times the square of the molecular

weight of each polymer chain divided by the total number of polymer chains times

the molecular weight of each chain: Sm2Ni/SmiNi.

i

Molecular weight distribution Mw/Mn.

Glass transition temperature (Tg) Temperature at which local molecular motion in

a polymer chain virtually ceases. General-purpose elastomers behave like a glass

below this temperature.

Weight-average Tg Average Tg of a compound:



X  wt: polymer Xn 

ðTg polymer Xn Þ

total polymer wt:



Copyright © 2004 by Taylor & Francis



correlation between tire traction and tangent delta of the tread compound at

0jC and 40 Hz (2). Tire wear is more difficult to predict, with one researcher

observing, ‘‘Despite more than 50 years of effort to devise laboratory abraders

that give a good prediction of the wear resistance in real-world situations, no

abrasion device currently exists that does an acceptable job’’ (3). Typically,

DIN abrasion or some type of blade abrader is used as a general indicator,

however. Rubber processability has been defined in a number of ways (4) but

is usually determined by what type of equipment will be used to process the

rubber. Mooney stress relaxation time to 80% decay (MSR t-80) is a rapid,

effective processability test that works well with both emulsion (5) and

solution SBR (6). Other more sophisticated instruments such as the rubber

processability analyzer (RPA) or capillary rheometer are now becoming more

popular.



B.



Glass Transition Temperature



The most important elastomer variable in determining overall tire performance is the glass transition temperature, Tg. Aggarwal et al. (2) showed that

the tangent delta at 60jC of filled rubber vulcanizates made from ‘‘conventional rubbers’’ correlated with tire rolling resistance and then determined

that the tangent delta values were approximately a linear function of the

compound’s Tg value. This was true whether the polymers were made by a

solution process or an emulsion process. They did not compare solution and

emulsion polymers at the same glass transition temperature.

Oberster et al. (7) showed that traction and wear properties were not

dependent on the way the polymer was manufactured but were functions of

the overall glass transition temperature of the compound, as shown in Figures

1 and 2. In actual tire tests, results are more complicated. The weight-average

Tg of the tread compound is still a major variable, but it is not as dominant as

in laboratory tests. A comprehensive study of tire wear under a variety of

environmental and road conditions showed that tire wear improves linearly as

the ratio of BR to SBR is increased in BR–SBR tread compounds (lower

weight-average Tg). The wear behavior was more complex in BR–NR blends

with low carbon black levels and was shown to be a function of ambient test

temperature (3).

Nordsiek (8) expanded the concept of using the glass transition temperature to using the entire damping curve to predict tire performance. He

divided the damping curve into regions that influenced various tire properties

(Fig. 3). The damping curves for an emulsion SBR, a high-vinyl polybutadiene, and a medium-vinyl SBR at the same Tg were compared and shown to be



Copyright © 2004 by Taylor & Francis