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Effect of Filler Properties on Elastomer Performance

The influence of the filler physical properties on reinforcement and other important properties is a very complex subject as they interact with one another. The general trends are summarized in very simple form in Table 1 below. This shows that the best reinforcement is achieved by small particle size, high structure, strong filler/polymer interaction, and good dispersion. Only a few filler types are able to achieve this, notably carbon blacks, synthetic silicas when used with coupling agents such as organosilanes, and precipitated calcium carbonates with unsaturated carboxylated polymer coupling agents.

Some examples of the importance of the various effects follow.

Some Effects of Particle Size

Table 2 exemplifies how tensile strength and abrasion resistance vary with carbon black primary particle size. This is a based on data in a sulfur cured SBR compound.

Some Effects of Filler Dispersion

Table 3 shows how various filled elastomer properties develop as a function of dispersion. This study was achieved by measuring dispersion and property profile of a compound as a function of mixing time.

This study did not consider the effect of the actual size or nature of the poorly dispersed material. This was investigated in detail in the classic studies by Boonstra and Medalia (1963) who found that particles become detrimental at a size of about 1 pm. They found little further effect of size above this critical value, but did find that properties deteriorated further as the hardness of the particles increased. The most sensitive property to poor dispersion in both studies was found to be abrasion

Table 1 The direction and magnitude of the effects of the main filler properties on those of filled elastomers (adapted from (Boonstra 1975))

Elastomer

property

Filler property

Particle size (decreasing)

Structure

(increasing)

Dispersion

(increasing)

Interaction

(increasing)

Hardness

+ +

+ +

+

Tensile

strength

+ +

LITTLE

+

+

300%

modulus

+

+ +

LITTLE

+ +

Elongation at break

++

Tear

resistance

+ +

LITTLE

LITTLE

LITTLE

Hysteresis

+ +

+

Abrasion

resistance

+ +

+

++

+ +

Notes: + and ++ mean increase in that property and — and perty and++ mean increasedo not mean that the change is beneficial or detrimental. This depends very much on the application. For instance, high hysteresis is good for sound damping, but bad where heat build-up has to be avoided

Table 2 The effect of carbon black primary particle size on tensile sand laboratory abrasion resistance in a sulfur cured SBR compound (adapted from (Boonstra 1975))

Primary particle size nm

Relative tensile strength

Relative abrasion resistance

20

1.00

1.00

28

0.89

0.74

39

0.72

0.47

49

0.64

0.41

70

0.58

0.35

180

0.50

0.16

Table 3 Development of dispersion and properties during compound mixing (adapted from (Boonstra 1975))

Elastomer property

Mixing time (minutes)

1.5

2.5

4

8

16

Dispersion rating%

24

86

99

100

100

Hardness IRHD

65

64

61

59

59

Tensile strength MPa

16.9

24.0

25.5

26.0

25.0

100% modulus MPa

3.3

2.6

1.7

1.5

1.2

300% modulus MPa

12.7

13.9

12.5

12.0

11.7

Elongation at break

380

490

530

540

530

Tear strength MPa

39.2

39.2

40.1

40.1

38.2

Abrasion loss (Akron)

289

142

136

-

-

resistance. Even trace amounts of grit (a few 100 ppm) was found to cause a detectable loss in performance in fatigue tests and are thus important in some critical applications, notably tires.

 
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