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Uses in Polymers

China and ball clays have limited application in thermoplastic and thermoset applications compared to calcium carbonates and talcs. This is due to a number of factors such as poorer color and heat aging, especially in polypropylene. Dehydrochlorination of PVC is another issue which has already been mentioned. The water of crystallization is an important issue for water-sensitive polymers such as nylon and thermoplastic polyesters, as it can be released during processing. As described later, many of the problems are overcome with the calcined grades, which do find significant applications in many types of polymer. The reactivity of clays is also exploited in biopolymers, where they act as a pro-degradant, speeding up the breakdown of packaging during composting.

On the other hand, china and ball clays are widely used non-black fillers in the rubber industry. In practice there are a wide range of products covering a spectrum of performance; but two distinct types known as hard and soft clays are often referred to, especially in older literature. This terminology is based on their effect on the hardness of the rubber composite, not on the properties of the clays themselves.

Today, it is more common to talk in terms of the effect of fillers on the reinforcement of elastomers, with four levels being recognized: reinforcing, semi-reinforcing, non-reinforcing, and diluent (Skelhorn 2003). In this context, reinforcement is determined by the increases produced in the tensile and tear strengths of non-crystallizing elastomers such as styrene-butadiene rubber (SBR). Reinforcing fillers show a marked peak in performance at relatively low loadings and an improvement in abrasion resistance, followed by a significant decline. Semi-reinforcing ones have a broader peak at higher loading and little effect on abrasion resistance, while non-reinforcing ones have no peak and diluent fillers show a steady decline at all loadings.

Clays used in rubber generally range from non-reinforcing through to semireinforcing, with these classes loosely equalling the old hard and soft types. The main difference between them is in particle size, not mineralogy. Hard or semireinforcing clays typically have specific surface areas (a useful indication of size for anisotropic particles) in the range 10-30 m2/g, while the soft (non-reinforcing) ones are in the range 5-7 m2/g. In practice there is no clear-cut distinction with a continuous range of sizes, and hence reinforcement, available.

The use of just two parameters to quantify reinforcement is a great oversimplification, and rubber applications involve many more considerations. These include the following properties, all of which can be affected by a filler:

Processing ease, including viscosity, mill sticking, and mold release

Cure rate: completeness and nature of the cross-links

Hardness

Tensile and tear strengths Stiffness (modulus) at various strains Elongation to break

Resilience and heat buildup (energy losses on deformation)

Stability of cross-links, as measured by permanent set under various conditions Abrasion resistance under a variety of conditions

Cure effects are important for clays which can absorb and partly deactivate accelerators and curatives. This is especially true for the higher surface area products, with more reinforcing potential. Various approaches are used to overcome this, including using additives such as glycols to block the clay surface.

Table 1 shows how a typical semi-reinforcing clay filler affects some rubber composite properties, compared to a non-reinforcing chalk. From this, it can be seen that the clay is clearly superior in terms of tensile and tear strengths and high extension modulus, but is no better than the chalk for abrasion resistance and compression set. Table 2 shows how properties vary with specific surface area for clays of similar mineralogy. The lowest specific area clay would be classed as non-reinforcing.

As mentioned above, organo-silanes can be used to boost the performance of clays in rubber. They achieve this mainly through increasing the adhesion between the filler surface and the elastomer matrix. The main properties to benefit from this are high extension modulus, tear strength, and abrasion resistance. This is demonstrated in Table 3.

Table 1 A comparison between a fine chalk filler and a semi-reinforcing clay in a sulfur-cured SBR formulation (100 phr filler)

Property

Chalk

Semi-reinforcing clay

Hardness (IHRD)

59

66

Tensile strength MPa

4.4

18.5

Elongation%

610

670

Modulus at 300% MPa

2.1

4.2

Tear strength N/mm

30

61

Abrasion loss mm3

290

282

Compression set%

37

37

Table 2 The effect of particle size, as expressed by specific surface area, on some of the important properties of a sulfur-cured SBR elastomer (100 phr Filler)

Property

Specific surface area 5 m2/g

Specific surface area 11 m2/g

Specific surface area 25 m2/g

Hardness (IHRD)

65

66

69

Tensile strength MPa

10.7

18.5

20.5

Elongation%

600

680

700

Modulus at 300% MPa

3.8

4.2

4.0

Tear strength N/mm

45

56

65

Abrasion loss mm3

330

280

287

Compression set%

35

37

36

Table 3 The benefit of using an organo-silane on the properties of a sulfur-cured clay-filled SBR compound

Property

No

silane

With mercapto-silane

(maximum level and amount of silane required to achieve it, expressed as% on clay)

Hardness (IHRD)

73

77 (1.5%)

Tensile strength MPa

13

14 (1.0%)

Modulus at 300% MPa

4.5

9.0 (1.0%)

Tear strength KNm_

62

90 (1.0%)

Elongation%

620

Steady decrease

The platiness of some clays results in increased gas barrier properties when properly dispersed and aligned in elastomers, and this effect can be put to good use in products where this property is important, such as tire inner tubes and liners.

 
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