Rubber Compounding.pdf

612 RUBBER CHEMICALS

Vol. 11

RUBBER COMPOUNDING

Introduction

Rubber compounding is the complex, multidisiplinary science of selecting and

blending the appropriate combination of elastomers and other ingredients to meet

the performance, manufacturing, environmental, and cost requirements for rub-

ber goods made and used in commerce. There is a wide variety of elastomers

and ingredients that are available in making rubber goods, which include all

of the following types of products: tires, innertubes, retreaded tires, footwear,

rubber rolls, hoses, belts, weatherstripping, O-rings, seals, diaphragms, tubing,

rubber and latex gloves, ball bladders, medical devices, bumpers, and numerous

other products. This is a review of the basic materials used in designing many

common types of rubber products. The development of the combination of ma-

terials contained in a usable rubber compound involve many disciplines that

include chemistry, physics, mathematics, and polymer science. In many cases,

classical scientiﬁc methods can predict the interaction effects and estimate the

properties expected from rubber compounds. However, discovery and develop-

ment of new materials that modify the rubber compound to produce new, useful,

and beneﬁcial properties are appearing regularly. Also, environmental concerns

are creating an interest in utilizing recycled rubber products into new rubber

compounds.

Compounding Hierarchy

Compounding is a term that has evolved within the plastics and rubber industry

and in many respects is a misnomer for the material science of modifying a polymer

or polymer blend through addition of other materials to achieve a set of mechanical

properties for a speciﬁc service application. Compounding is therefore a highly

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RUBBER COMPOUNDING 613

complex science involving many traditional disiplines such as organic chemistry,

polymer chemistry, materials physics, mathematics, and engineering mechanics.

There are some excellent reviews and discussion papers on the science of

rubber compounding (1–8). The purpose of this discussion is to provide an intro-

ductory overview of the range of materials used by the materials scientist in com-

pounding elastomers. In designing or making the selection of ingredients for use

in a rubber article, the technologist relies upon published literature, experimental

work, competitive benchmark information, and various information sources such

as raw material supplier data, technical documents, and reports. Raw materials

for a compound are generally selected in the following order:

(1) polymer (natural or synthetic rubber)

(2) ﬁllers or reinforcing agent

(3) antioxidants and antiozonants

(4) plasticizers or oils

(5) bonding agent or adhesive (if needed)

(6) tackifer (if needed)

(7) vulcanization system [curing agent, accelerator(s), or coagent]

Performance requirements of the ﬁnal product often dictate which speciﬁc

type of elastomer can be used. The expected usable life for the product is con-

trolled by many factors such as end-customer needs, competitive situation in the

marketplace, safety, and reliability. Rubber products are almost always used as a

functional part of another system. For example, tires, hoses, belts, O-rings, and

numerous rubber components are used in manufacturing automobiles and trucks.

The overall life of the vehicle as well as its performance level often relates to the

service life or quality level of the rubber parts.

Elastomers Used in Rubber Compounding

Elastomers can be divided into two general categories, natural rubber and syn-

thetic rubbers. Synthetic elastomers in turn are either termed general pur-

pose rubbers (GPR) or special purpose rubbers . Natural rubber is generally

obtained from southeast Asia or Africa. Synthetic rubbers are produced from

monomers obtained from the cracking and reﬁning of petroleum. The most com-

mon monomers are styrene, butadiene, isoprene, isobutylene, ethylene, propylene,

and acrylonitrile. There are monomers for specialty elastomers which include

acrylics, chlorosulfonated polyethylene, chlorinated polyethylene, epichlorohy-

drin, ethylene–acrylic, ethylene–octene rubber, ethylene–propylene rubber, ﬂu-

oroelastomers, polynorbornene, polysulﬁdes, silicone rubber, thermoplastic elas-

tomers, urethanes, and ethylene–vinyl acetate.

Elastomers can be classiﬁed by their heat resistance and resistance to

swelling (Fig. 1). The Society of Automotive Engineers (SAE) has developed a

speciﬁcation, SAE J200 (9), for classifying special-purpose elastomers by swelling

in a special oil denoted IRM 903, which replaced ASTM (American Society for Test-

ing and Materials) Number 3 (10). Properties for evaluating elastomers include

614 RUBBER COMPOUNDING

Vol. 11

Fig. 1. SAE J200 classiﬁcation systems for IRM 903 (ASTM No. 3) oil, where, for volume

swell, NR

no requirement. Polymer abbreviations are deﬁned in Table 1 (9,10).

hardness, tensile strength, change in tensile strength, elongation, and change in

volume, tensile strength, and hardness after exposure to IRM 903 oil. However,

probably the most effective classiﬁcation system has been published by the In-

ternational Standards Organization in speciﬁcation ISO 1629 (11). Polymers con-

taining residual unsaturation such as polybutadiene have abbreviations ending

in R (eg, BR), methylene containing polymers ending in M (eg, ethylene propy-

lene diene monomer, EPDM), silicone rubbers ending in Q, oxygenates ending

in O, and urethanes ending in U. A list of International Institute of Synthetic

Rubber Producers (IISRP) abbreviations are shown in Table 1 (12). Table 2 dis-

plays typical general physical properties for a selection of the different types of

elastomers including specifc gravity, hardness, tensile strength, and ratings for

resilience, compression set, and permeability.

Natural Rubber. Natural Rubber is a biological homopolymer of isoprene

and has the general structure illustrated in equation 1. The principal producing

countries are Malaysia, Indonesia, Thailand, India, China, and Sri Lanka (see

R UBBER ,N ATURAL ). New sources are also being developed. To obtain natural rub-

ber (NR), the Hevea brasiliensis tree is tapped for its sap. An off-white sap is

collected and is coagulated by the addition of a strong acid such as sulfuric acid.

The tapped latex consists of between 30% and 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 nonrubber 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. Natural rubber is predomi-

nantly of the cis structure though the trans isomer, or Gutta Percha , can be found

in latex tapped from the trees of the genus Dichopsis .

There are several systems that deﬁne the quality and uniformity of natural

rubber. One system of grading natural rubber is based on form and visual obser-

vation of color and cleanliness. This is known as the International Natural Rubber

Speciﬁcation. The principal types and grades presented in Table 3 (13).

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RUBBER COMPOUNDING 615

Table 1. IUPAC Nomenclature for Selected Elastomers a

Polyester urethane

Polybutadiene

BIIR

Brominated isobutylene–isoprene (bromobutyl)

CIIR

Chlorinated isobutylene–isoprene (chlorobutyl)

CPE

Chlorinated polyethylene

Chloroprene rubber

CSM

Chlorosulfonyl polyethylene

EAM

Ethylene–vinyl acetate coploymer

EPDM

Terpolymer of ethylene, propylene, and a diene with

a residual unsaturated portion in the chain

EPM

Ethylene–propylene copolymer

Polyether urethane

HNBR

Hydrogenated acrylonitrile–butadiene rubber

(highly saturated nitrile rubber)

IIR

Isobutylene–isoprene rubber

Synthetic polyisoprene

NBR

Acrylonitrile–butadiene rubber

SBR

Styrene–butadiene rubber

E-SBR

Emulsion styrene–butadiene rubber

S-SBR

Solution styrene–butadiene rubber

X-NBR

Carboxylated nitrile–butadiene rubber

X-SBR

Carboxylated styrene–butadiene rubber

YSBR

Block copolymers of styrene and butadiene

a From Ref. 12.

(1)

Malaysian and Indonesian natural rubber growers have established a system

based on technical characteristics. A summary of the standard technical speciﬁca-

tion scheme shown in Table 4 for natural rubber can be found in ISO 2000 (14,15).

In addition to the solid form of natural rubber, it is available as a suspension in

water and is known as latex. Synthetic rubbers are also available in latex form.

Latex has become an important commodity used in the manufacture of dipped

goods for pharmaceutical applications. The principal uses of natural rubber are

as follows: tires and tire retreading, 70%; latex (eg, gloves), 12%; mechanical goods,

9%; load-bearing components, 4%; and other, 5%.

Styrene–Butadiene Rubber. Styrene–butadiene rubber (SBR) is made

by either an emulsion or solution process (eq. 2). The classiﬁcation systems for

emulsion SBR is shown in Table 5. In the emulsion process, it is more difﬁcult to

control polymer microstructure and the ﬁnal product is not as pure as the solution

form. However it tends to show a higher tensile strength and tear strength, and

is easier to process. It is used in applications such as tire treads, sidewalls, bead

Table 2. Physical Properties of Elastomers a

Speciﬁc Hardness Tensile strength, c

Compression Impermeability

Elastomer b

gravity

Shore A

MPa d

Elongation, c % Resilience e

set e

to gases e

0.93

30–100

27.6

750

Polyisoprene

0.92

30–100

24.1

750

SBR

0.94

35–100

20.7

600

Butyl

0.92

30–90

17.2

700

P–F

Polybutadiene

0.91

45–80

17.2

500

EPDM

0.86

30–90

20.7

600

Chloroprene

1.23

35–95

20.7

600

G–E

F–G

Nitrile

1.00

30–100

20.7

600

F–G

Thiokol

1.25–1.35

20–80

10.3

450

P–F

Urethane

1.02–1.25 55–100

55.2

750

F–E

G–E

P–F

Silicone

0.98–1.60

25–90

10.3

800

F–G

G–E

P–F

CSM

1.12–1.28

40–95

20.7

600

F–G

Acrylic

1.09

40–90

13.8

400

F–G

Fluorocarbon

1.85

55–95

20.7

450

G–E

Epichlorohydrin 1.27–1.36

40–90

17.2

400

F–G

Chlorinated PE 1.16–1.25

45–95

24.1

600

F–G

Cross-linked PE

0.92

20.7

500

a Ref. 7.

b NR

natural rubber; SBR

styrene–butadiene rubber; EPDM

ethylene–propylene–diene monomer; CSM

chlorosulfonated polyethy-

polyethylene.

c Maximum value at room temperature.

d To convert MPa to psi, multiply by 145.

e E = excellent; G = good; F = fair, and P = poor.

lene; and PE

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