III. OTHER NATURALLY OCCURRING MATERIALS

or are under investigation. Though many of these will be discussed elsewhere

in this text, it is appropriate to list them and note their potential application.

Though all require various degrees of processing prior to use in rubber

compounds, they still represent renewable sources of raw materials available

to the rubber technologist.

Inorganic mineral fillers already find extensive use in rubber compounds.

1.



2.



3.



Talc is used in products such as carpet backing and can be

effective when blended with reinforcing fillers such as silica or

carbon black. Particle sizes can range from 0.5 to 10.0 Am.

Clays such as kaolin and bentonite can also be used in combination with silica or carbon black. Particle sizes tend to range

from 0.5 to 5.0 Am. High surface area chemically modified clays

will improve the tensile strength, abrasion resistance, and tear

strength of the rubber product.

Calcium carbonate can be used as a filler even though its reinforcing properties are negligible. Surface modification by use of

coupling agents can enhance the properties of compounds containing calcium carbonate. However, it is most effective when

blended with carbon black or silica.



Biotechnology fillers offer considerable potential and have attracted

attention in recent scientific literature. At this point they fall into three

primary categories: silica ash derived from rice husk waste, starch, and

bamboo fibers. Burning of rice husks leaves a waste consisting of SiO2

(95%), CaO, MgO, Fe2O, K2O, and Na2O. Rice husks that have been milled,

filtered, and then treated with sodium hydroxide, hydrochloric acid, and

water produce a hydrated silica that when compounded can produce a

material with mechanical properties similar to those of silica and carbon

black. White rice husk silica contains around 95% SiO2, whereas black rice

husk silica is approximately 55% silica and 45% carbon. Residual carbon

cannot be completely eliminated because it is trapped within the amorphous

silica structure or is completely coated with silica so it is impossible to remove

it by thermal processes. The reinforcement properties of black rice husk ash

are comparable to those of calcium carbonate and not as effective as those of

carbon black or silica. White rice husk ash when added up to 20 phr in a

natural rubber based compound did show good compound properties that

were nearly equivalent to those found for silica-loaded compounds (37–39).

Starch has considerable potential when blended with carbon black or

silica to improve the hysteretic properties of compounded rubber. This has

implications for improvement in, for example, tire rolling resistance. Of the

range of materials, biofillers hold the most promise for future increases in



Copyright © 2004 by Taylor & Francis



consumption. However, to achieve the most from such systems, either a resorcinol/hexamethylenetetramine system or silane coupling agent is required.

Bamboo fiber–filled natural rubber has been investigated (40,41). With

the use of a silane coupling agent, workers were reported to obtain good

tensile strength, tear strength, and hardness due to bonding between the

polymer matrix and fiber. Filler loadings up to 50 phr are feasible. Work of

this nature merits further investigation.

Rice bran oil has been evaluated as a substitute for process oils, as a

coactivator, and as an antioxidant for natural rubber. Raw rice bran oil

contains fatty acids, phosphatides, and wax. This material was evaluated in

natural rubber compounds containing a conventional cure system and was

found to be an effective substitute for more expensive antioxidants and

processing aids and as a coactivator in place of stearic acid, with no toxicity

concerns (42).

Other naturally occurring materials that have found application in

rubber-based products include silicates, calcium carbonate, rayon, and

cotton. Also of considerable importance are waxes and fatty acids, which

are discussed elsewhere in this volume.

It is anticipated that there will be a growing emphasis on the use of

naturally occurring materials, particularly in tires where vehicle manufacturers desire their products to have a defined level of recycled or potentially

renewable resource content. Future government regulations may also require

that automotive products and parts contain such materials.



IV. RECYCLING OF RUBBER

Not only is there interest in the use of renewable raw materials such as natural

rubber and fillers such as calcium carbonate, there are both environmental

and economic reasons to recycle and reclaim scrap rubber. The automotive

industry has set targets for recycle content of 25% of post-consumer and industrial scrap in their products with no increase in cost or loss in performance.

Post-consumer scrap recycling is the reuse of products that have completed

their service life. These products can be ground into a powder or returned to

their original state via a devulcanization process. Industrial scrap is the waste

material generated in the original manufacturing process. In this instance the

goal of recycling is to ensure that all this material is used in the production of

high quality goods. The purpose of this discussion is to provide the rubber

technologist with introductory information on how to be compliant with

these new environmental objectives and contain cost while satisfying the endproduct design and performance criteria. The discussion will describe the

various forms and types of rubber recyclates available to the compounder



Copyright © 2004 by Taylor & Francis



and show how they can be incorporated into a rubber compounder. The effect

of these rubber recyclates on the rubber compound will also be demonstrated

in the form of physical and performance data.

Though achievable levels of recycled materials will most likely be lower

than the initial target of 25%, recycling should be a technologist’s objective.

The first attempt at reusing rubber was through reclaiming. However, in

the 1970s reclaim use declined, due to the growth in the radial tire market.

More recently, the use of finely ground rubber (e.g., 20–80 mesh) produced by

ambient and cryogenic processes emerged (Table 11). This was augmented by

the development of wet process grinding of rubber in a water medium to

produce very fine particle sizes, i.e., 60–200 mesh.

There have been numerous attempts to produce reusable rubber

through devulcanization by using some of the following methods and

techniques. Ultrasonic, microwave, and bacterial degradation; chemical

devulcanization; surface modification; solution swelling in active solvents;

and many other methods have been evaluated or are in various stages of

development and use. Rubber recyclates include ambient ground rubber (Fig.

8), cryogenic ground rubber (Fig. 9), and wet ground rubber the latter being

similar to that produced by the ambient grind process. Three publications

worth noting for rubber compounders trying to utilize recycled rubber are

Myhre and MacKillop’s review in Rubber Chemistry and Technology

Annual Rubber Reviews, 2002, of all facets of rubber recycling (43).

Best Practices in Scrap Tires and Rubber Recycling by Klingensmith

and Baranwal, published in 1997, discusses all aspects of rubber

recycling (44).

The Scrap Tire Users Directory, published yearly by the Recycling

Research Institute lists all grinders and processors of scrap tire and

rubber products (45).



Table 11 Mesh Size

Mesh size

10

20

30

40

60

80

100



Dimension

2.00 mm

850 Am

600 Am

425 Am

250 Am

180 Am

150 Am



Copyright © 2004 by Taylor & Francis



0.0787

0.0331

0.0234

0.0165

0.0098

0.0070

0.0059



in.

in.

in.

in.

in.

in.

in.



Figure 8 Simplified schematic of typical ambient grinding and reclaim system.



Figure 9 Schematic of typical cryogenic grinding system.



Copyright © 2004 by Taylor & Francis



The recycling of rubber products can also be considered to fall into four

basic categories, popularly characterized as reduction, reuse, recycle, and

reclaim. Each of these will be considered in turn.

A. Reduction

Materials reduction efforts have focused on optimum use of materials, the

weight reduction of tires and other engineered products, and gauge optimization of product components. This has largely been facilitated through new

manufacturing systems and new designs.

B. Reuse

Reuse of tires and other industrial rubber products has been directed toward

their use as fuel. Excluding the tires that go to landfills or to stockpiles or other

storage facilities, 61% were used for fuel. The balance were recycled into other

uses. Retreading of aircraft and commercial truck tires is probably the most

ideal use of worn products. In the case of aircraft tires, up to four or five

retreads are possible. For commercial truck tires from size 9.00R20 up to

12.00R24 or 315/80R22.5, two retreads are not unusual.

C. Recycle

The major methods for recycling existing rubber are ambient grinding,

cryogenic grinding, and wet grinding. The resulting products are useful for

controlling compound cost and improve processing when added to newly

compounded rubbers.

In ambient grinding, vulcanized scrap rubber is first reduced to chips on

the order of 1–2 in. in size. For some rubber products such as tires, this is

normally accomplished by shredding. The shredded rubber is then passed

over magnetic, mechanical, and pneumatic separators to remove metals and

fibers. These pieces can be reduced in size by further ambient grinding on mills

or by freezing them with liquid nitrogen and then grinding them into fine

particles. The ambient process uses conventional high-powered mills with

close nips that shear the rubber and grind it into small particles. It is common

to produce 10–40 mesh material using this method, and the material is the

least expensive to produce. The finer the desired particle, the longer the rubber

is kept on the mill. Alternatively, multiple grinds can be used to reduce the

particle size. The lower practical limit for the process is the production of 40

mesh material (Table 11). Any fiber and extraneous material must be removed

using an air separator. Steel wires are removed by using a magnetic separator.

A flow chart for an ambient grind process including a side stream for



Copyright © 2004 by Taylor & Francis



reclaiming is shown in Figure 8. The process produces a material with an

irregular jagged particle shape. In addition, the process generates a significant

amount of heat in the rubber during processing. Excess heat can degrade the

rubber, and efficient cooling systems are essential.

Cryogenic grinding uses rubber particles of up to 2 in. and freezes them

with liquid nitrogen. The frozen pellets are passed into a mill for further

grinding. The size of particles typically produced by this method ranges from

60 mesh to 80 mesh. The advantages of this technique are that 1) little heat is

generated so there is no thermal degradation of the material such as is found

with ambient grinding and 2) finer particles are obtained.

The cryogenic process produces fractured surfaces. The most significant

feature of the process is that almost all fiber or steel is liberated from the

rubber, resulting in a high yield of usable product. The cost of liquid nitrogen

has dropped significantly, and cryogenically ground rubber can now compete

on a large scale with ambient ground products. A flow chart for a typical

cryogenic process is shown in Figure 9.

Many manufacturing organizations wish to incorporate their scrap

back into original rubber compound formula. This eliminates scrap disposal,

provides better control over cost, and is an environmentally sound business

practice. However, several practical problems arise in doing this. First, it may

be difficult to accumulate sufficient clean scrap of a given compound or

classification type. This is a significant drawback owing to the desire to attain

consistent properties and performance of the final compound formulation. A

second problem is the need for a recycling organization capable of working

with small quantities of a given lot of waste material and keeping it in suitably

clean condition. A third is that it is necessary to understand the effects of mesh

size and concentration on the rubber properties. The following paragraph on

the effects of concentration and mesh size on rubber properties (46,47) is

based on the Cryofine EPDM Handbook (48).

The effect of variation in recycle content on an SBR-based compound is

illustrated in Table 12. Increasing the amount of 20 mesh crumb rubber from

0% to 50% results in a drop in tensile strength from 10.1 MPa to 3.9 MPa, an

increase in Mooney viscosity from 40.0 to 111.0, and a drop in ODR

rheometer torque from 59 to 34. The important observation from these data

is that a consistent level of recycle material is essential to ensure consistency in

a product’s design specifications. Table 13 presents a simplified comparison of

the two fundamental types of ground rubber, which also must be taken into

consideration when selecting a material for inclusion in a compound formula.

Clearly, cryogenically ground rubber is preferable to ambient ground because

it has no fiber or wire. Some typical properties of an EPDM-based compound

with cryogenically ground crumb added are displayed in Table 14. Mesh size

of the crumb ranged from 40 to 100, and it was added at 10% and 20%



Copyright © 2004 by Taylor & Francis



Table 12 Properties of Ambient Ground Rubber (20 Mesh) SBR 1502

Compounds with 0%, 17%, 33%, and 50 Crumb

Compound

1

Compound Ingredient (phr)

SBR 1502

N660

Aromatic oil

TMQ (polymerized

dihydrotrimethylquinoline)

Stearic acid

Zinc oxide

Sulfur

MBTS

TMTD

Crumb (%)

Property

Mooney viscosity

Rheometer max torque

Tensile strength (MPa)

Ultimate elongation (%)



2



3



4



100.0

90.0

50.0

2.0



100.0

90.0

50.0

2.0



100.0

90.0

50.0

2.0



100.0

90.0

50.0

2.0



1.0

5.0

2.0

1.0

0.5

0.0



1.0

5.0

2.0

1.0

0.5

17.0



1.0

5.0

2.0

1.0

0.5

33.0



1.0

5.0

2.0

1.0

0.5

50.0



40.0

59.0

10.1

330.0



61.0

47.0

7.9

330.0



91.0

33.0

6.0

300.0



111.0

34.0

3.9

270.0



loading. In both cases loss in fundamental mechanical properties such as

tensile strength was negligible. However, increase in loading as noted in the

data displayed in Table 12 did lead to loss in properties.

The data in Table 15 were extracted from the Cryofine Butyl Compounding Handbook (49). They show the effect that an 80 mesh cryogenically

ground butyl rubber has on the mechanical and physical properties of a



Table 13 Characteristics of Ambient vs. Cryogenically Ground

Whole Tire Recycled Rubber

Physical property

Specific gravity

Particle shape

Fiber content

Steel content

Cost



Ambient ground



Cryogenic ground



Same

Irregular

0.5%

0.1%

Comparable



Same

Fractured

nil

nil

Comparable



Source: Ref. 48.



Copyright © 2004 by Taylor & Francis



Table 14



Properties of EPDM Compounds Containing Cryogenically Rubber

Compound

1



Basic compound

EPDM

N650

N774

Paraffinic oil

Low MW Polyethylene

TMQ (polymerized

dihydrotrimethylquinoline)

Stearic acid

Zinc oxide

Sulfur

TBBS

TMTD

TDEDC (tellurium

diethyldithiocarbamate)

MBT

Properties at 10% loading

Mesh

Tensile strength

Ultimate elongation

300% Modulus

Hardness (Shore A)

Properties at 20% loading

Mesh

Tensile strength

Ultimate elongation

300% Modulus

Hardness (Shore A)



2



3



4



5



100.0

70.0

130.0

70.0

5.0

1.0



100.0

70.0

130.0

70.0

5.0

1.0



100.0

70.0

130.0

70.0

5.0

1.0



100.0

70.0

130.0

70.0

5.0

1.0



100.0

70.0

130.0

70.0

5.0

1.0



1.0

5.0

1.3

8.0

8.0

8.0



1.0

5.0

1.3

8.0

8.0

8.0



1.0

5.0

1.3

8.0

8.0

8.0



1.0

5.0

1.3

8.0

8.0

8.0



1.0

5.0

1.3

8.0

8.0

8.0



1.0



1.0



1.0



1.0



1.0



Control

9.72

410

8.13

73



40

8.89

330

8.4

70



60

9.86

340

8.5

70



80

10.73

400

8.5

70



100

9.93

380

8.4

71



Control

9.72

410

8.13

73



40

8.5

320

8.4

72



60

9.4

390

8.9

70



80

10.1

390

8.8

69



100

9.7

390

7.9

68



typical halobutyl tire innerliner. The effects of 5%, 10%, and 15% loadings

are shown. A 5% level of finely ground butyl scrap is commonly added to

innerliners. Besides reducing compound cost, the ground butyl provides a

path for trapped air to escape from the compound. The number of tires

rejected due to blisters is reduced significantly. This effect of ground rubber is

noted in all elastomers, especially the highly impermeable ones like butyl and

fluoroelastomers.

Wet grinding uses a water suspension of rubber particles and a grinding

mill. The material is finely ground to a mesh size of 60–120. These products

therefore find ready use in tire compounds due to their uniformity and



Copyright © 2004 by Taylor & Francis



Table 15 Properties of Innerline Compounds Loaded with Cryogenically

Ground Butyl Rubber (80 Mesh)

Compound

1

Base compound component (phr)

Chlorobutyl rubber (1066)

Natural rubber (TSR 5)

N650

Mineral rubber

Phenolic resin

Naphthenic oil

Stearic acid

Zinc oxide

Sulfur

MBTS

Ground butyl rubber loading (%)

Rheometer t90

Tensile strength (MPa)

300% Modulus

Air permeability (Qa)



2



3



4



80.0

20.0

65.0

4

4.00

8.00

2.00

3.00

0.50

1.50

0

47.5

9.7

7.7

4.7



80.0

90.0

50.0

4

4.00

8.00

2.00

3.00

0.50

1.50

5

46.3

9.3

7.2

4.7



100.0

90.0

50.0

4

4.00

8.00

2.00

3.00

0.50

1.50

10

47

8.9

6.9

4.5



100.0

90.0

50.0

4

4.00

8.00

2.00

3.00

0.50

1.50

15

46.5

8.8

6.5

4.2



Source: Ref. 49.



minimal contamination. Surface treatment and additives can enhance the

mechanical properties of compounds containing recycled materials. Additives

include materials such as polyurethane precursors, liquid polymers,

oligomers, resin additives, and rubber curatives. In some instances, when

the specific chemical composition of the surface treatment is compatible with

the materials to be reincorporated, retention of the original mechanical

properties of the compound can be achieved. For example, nitrile compounds

should be treated with acrylonitrile butadiene copolymers or block copolymers, which have similar solubility parameters (43).

The surface of crumb rubber can be activated by addition of unsaturated low molecular weight elastomers. Latex is added to the crumb rubber in

an aqueous dispersion. The water is removed, leaving a coating around the

ground rubber. This technique, know as the surface-activated crumb process,

has been commercialized by Vredestein Rubber Recycling Company in

Europe. There is considerable scope for further development in this area. It

is reasonable to state that success of the rubber recycling industry will be

dependent on developing economically effective means by which the surface



Copyright © 2004 by Taylor & Francis



of ground recycled rubber is chemically activated so as to enable attainment

of the original compound’s mechanical properties.

D. Reclaim

Reclaim of rubber refers to the recovery of original elastomers in a form in

which they can be used to replace fresh polymer (48). Again, a range of

techniques are available to produce such materials:

Ultrasonic devulcanization. Though it has not been achieved commercially, ultrasonic devulcanization continues to be a potential

method to allow reclamation of the original polymer. Sulfur–sulfur

bonds have lower bond energy than carbon–carbon bonds. Given

this, ultrasonic waves can have enough energy to selectively break

the sulfur bonds, thereby devulcanizing the compound.

Chemical devulcanization. Chemical devulcanization methods involve mixing rubber peelings in a high-swelling solvent with a

catalyst. Heating brings about a significant reduction in cross-link

density. Though other chemical techniques are being investigated,

any future system will most likely involve catalytic degradation in a

solvent at high temperature and pressure.

Thermal devulcanization. This involves the use of microwaves, inducing an increase in temperature with preferential breaking of

sulfur–sulfur bonds. Owing to the cost of operating such systems,

there has been no successful commercial system, but pilot plant

facilities have been in operation.

Chemomechanical and thermomechanical techniques. Such systems

have ranged from the simple addition of vulcanization system ingredients to crumb rubber, and polymer surface modification to add

functionality to the surface of the particles to treatment at higher

temperatures with the intent of activating the surface. No commercially successful systems have been developed, though pilot

plant facilities are in operation.

Through the use of reclaiming agents, steam digestion, and/or mechanical shear, it is possible to convert used tires, tubes, bladders, and other rubber

articles into a form of rubber that can be incorporated into virgin rubber

compounds. There are many processes available to accomplish this. They

include the 1) heater or pan process, 2) dynamic dry digester process, 3) wet

digester process, 4) Reclaimotor process, and 5) Banbury process. The

purpose of this section is not to discuss the manufacturing methods of reclaim

but its benefits and uses, For details on manufacturing, reference should be

made to the RT Vanderbilt Handbook (49).



Copyright © 2004 by Taylor & Francis



Reclaim rubber was used in significant volume up to the early 1970s in

the United States. Then the proliferation of radial tires, environmental

regulations, and the large economy of scale of the new or upgraded SBR

and PBD manufacturing process resulted in low original rubber prices and

significant contraction of the reclaim rubber industry. In the 1960s there were

estimates that as much as 600 million pounds per year of reclaim rubber was

used in the United States. The estimate for the year 2002 was 60 million

pounds, consisting mostly of reclaimed butyl for tire innerliners, reclaimed

NR for mats and low-end static applications, and reclaimed silicone for

automotive and electrical applications.

Reclaim rubber had several distinct potential advantages. These included lower cost than original rubber, improved rheological characteristics

found in manufacture, less shrinkage, and the possible reduction in the need

for curing agents in the compound. However, its lower green strength,

durability, and tear strength led to its removal from radial tire compounds.

In many regions of the world such as India, China, and some southeastern

Asian countries, reclaim is still widely used in footwear, bias tire compounds,

mats, automotive parts, and other goods. An estimated 200–300 reclaimers

are still operating.

Table 16 illustrates the effect of adding reclaim to a radial tire carcass

compound (50). Though caution should be exercised with regard to the impact

on cut growth and fatigue resistance, some classical mechanical properties are

retained when reclaim is added. Static or low performance applications are

therefore still preferred. The definition of low performance product applications largely excludes products such as high-performance tires, high-pressure

hoses, and conveyor belt covers but does include mats, fenders, and flaps.

Table 17 illustrates typical properties achieved when whole tire reclaim is

added to either a natural rubber or SBR automotive mat compound. Basic

mechanical properties quoted may be acceptable for this application.



Table 16 Physical Properties of Radial Tire Casing Compound Containing Wet

Ground and Reclaim Rubber

Control

Modulus at 300% (MPa)

Tensile strength, (MPa)

Elongation (%)

Hardness, Shore A

Tear strength (kNÁm)



Substituting

10 parts GF-80



Substituting 10 parts

whole tire reclaim



3

14.9

875

58

20.7



3.7

15.1

775

58

20.9



3.7

13.6

740

59

20.7



Source: Ref. 50.



Copyright © 2004 by Taylor & Francis