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Particulate Fillers in Elastomers

6

Roger Rothon

Contents

Definition........................................................................................ 126

Introduction...................................................................................... 127

Elastomeric Compounds and Their Important Properties...................................... 129

The Role of Fillers in Elastomeric Compounds................................................ 130

Important Filler Properties...................................................................... 131

Size and Specific Surface Area................................................................. 131

Shape and Structure............................................................................. 131

Transient Structure.............................................................................. 132

Permanent (Persistent) Structure................................................................ 132

Dispersion....................................................................................... 132

Filler to Elastomer Adhesion.................................................................... 133

Effect of Filler Properties on Elastomer Performance.......................................... 134

Some Effects of Particle Size................................................................... 134

Some Effects of Filler Dispersion............................................................... 134

The Effects of Filler to Elastomer Adhesion................................................... 136

The Effects of Filler Structure................................................................... 136

Dynamic Properties of Filled Elastomers....................................................... 137

Tire Applications................................................................................ 138

Tire Treads, Green Tire, Precipitated Silica Versus Carbon Black,

Payne Effect, Rolling Resistance, etc........................................................... 139

Silica Versus Carbon Black in Tire Treads..................................................... 143

Specialist Elastomers............................................................................ 144

Future Directions................................................................................ 144

Cross-References................................................................................ 145

References....................................................................................... 145

R. Rothon (*)

Rothon Consultants and Manchester Metropolitan University, Guilden Sutton, Chester, UK e-mail: This email address is being protected from spam bots, you need Javascript enabled to view it

© Springer International Publishing Switzerland 2017 125

R. Rothon (ed.), Fillers for Polymer Applications, Polymers and Polymeric Composites:

A Reference Series, DOI 10.1007/978-3-319-28117-9_9

Abstract

Fine particulate fillers are widely used in elastomers (indeed are essential for many applications) and are small hard particles usually of carbon or inorganic origin. The main fillers used in general purpose elastomers are carbon black, precipitated silicas, clays, and natural carbonates. There is also significant use of precipitated calcium carbonates and of synthetic flame retardant fillers such as aluminum hydroxide. Fumed silica plays a key role in silicone elastomers while finely ground crystalline silicas are important in specialist high temperature elastomers.

Filler reinforcing effects depend very much on the type of elastomer. They are most obvious in noncrystallizing elastomers, such as most synthetic types. Here, the fine filler particles are able to act as tiny crystallites and markedly improve the tensile strength, abrasion resistance, and higher extension modulus. In the crystallizing types, such as natural rubber, the elastomer crystallites which develop on stretching often hide filler effects in laboratory tests, but they are still present and important in many applications.

Size, shape, and surface activity are all important factors determining reinforcing ability. Small size, high structure, and high surface activity generally give the best blend of properties. Carbon blacks owe their preeminence to the natural ability of their surface to form strong interaction with hydrocarbon polymers. Other fillers generally require the addition of coupling agents to achieve the same effect.

Keywords

Carbon black • Coupling agent • Dynamic properties • Elastomer filler • Fumed silica • Precipitated silica • Reinforcement • Tires

Definition

Elastomers are a specialized class of polymer made up of long chain, high molecular weight molecules with little crystallinity at rest. These chains are entangled with each other, but are coiled up when unstrained. Uncoiling of the chains allows elastomers to reach high extensions when stressed compared to other polymer types, while the desire of the chains to return to the coiled state allows for rapid recovery when the stress is removed. Cross-linking (often referred to as vulcanization when sulfur is used) introduces a small level of permanent cross-linking and this improves the strength and elasticity.

The polymer coils explain the basic features of elastomers, notably high extensibility and recovery. Some elastomers, such as natural rubber, form tiny crystallites on extension; these naturally reinforce the polymer, giving it high strength. Most synthetic elastomers do not achieve this and are thus of inherently low strength when unfilled.

Particulate fillers are widely used in elastomers (indeed are essential for many applications) and are small hard particles usually of carbon or inorganic origin. Their effects depend very much on the type of elastomer. In the crystallizing types, they cannot increase postcrystallization strength (such as determined in conventional tensile testing) much; although care has to be taken not to interfere with the crystallization and reduce it. In many of the applications of such elastomers, crystallization cannot occur and then fillers show their full potential. Fine fillers can, on the other hand, act as tiny crystallites and markedly improve the tensile strength of noncrystallizing elastomers, providing there is a high degree of surface interaction between the filler and elastomer.

Introduction

Elastomers (also known as rubbers) are a specialized but widely used class of polymers and can be of natural or synthetic origin. A key feature of elastomers is their ability to undergo repeated deformation cycles (extension or compression) without significant loss in performance, and this makes them ideal for dynamic applications.

Most elastomers require cross-linking to develop useful properties and this is usually achieved chemically, often using sulfur. Sulfur cross-linking is sometimes referred to as vulcanization. These cross-links are largely permanent and thus it is difficult to recover such cross-linked elastomers for reuse. There is a class of elastomers known as thermoplastic elastomers where cross-linking is achieved by phase separation and is reversible. Hence, they are more readily recycled.

The only natural rubber of any significance is that derived from Hevea brasiliensis (the rubber tree) and commonly referred to as natural rubber (NR). The principal synthetic elastomer in use today is styrene butadiene rubber (SBR). SBR (and many other synthetic elastomers) can be made by two different routes, solution or emulsion polymerization. Emulsion polymerization is the commonest, as it is less expensive, but will leave significant amounts of emulsifying agents in the final product. As will be discussed under tire applications, this can inhibit filler to elastomer adhesion. Other important rubbers include nitrile (NBR), butyl (BR), and ethylene propylene copolymers (EP and EPDM). There are also several specialized elastomers including silicone and fluorocarbon elastomers. The properties of any class of elastomer vary with molecular weight, branching, microstructure, and ability to crystallize when extended. Natural rubber is the classic example of a crystallizing elastomer, while SBR is noncrystallizing. The difference between the two types is illustrated by the generalized stress-strain curves in Fig. 1. The noncrystallizing elastomer is inherently weak unfilled, while the crystallizing one becomes strong when an extension is reached where the crystallites can form.

Three general classes of elastomer fillers are generally recognized: reinforcing, semireinforcing, and nonreinforcing. These definitions are a little vague as the effects of a given filler can vary from polymer to polymer. One way of differentiating them is by their effect on pure natural rubber (Skelhorn 2003). According to this

Fig. 1 The different stress-strain curves for crystallizing and noncrystallizing elastomers

approach, a reinforcing filler increases abrasion resistance and tensile and tear strengths, a semireinforcing one increases tensile and tear strengths but not abrasion resistance, while a nonreinforcing one does not enhance any of the properties. The level of reinforcement achievable is largely determined by particle size, increasing with reducing size, provided that a good level of particle to polymer adhesion is maintained. Precise sizes for the different classes are impossible to specify and there is some considerable overlap, but reinforcing fillers are generally regarded as having primary particle sizes of less than 100 nm, with semireinforcing ones being in the range 100-500 nm, while nonreinforcing ones are greater than 500 nm. Some people recognize a further class of fillers called diluent. These have sizes in excess of 5,000 nm and significantly degrade rubber properties.

Another important aspect of the various types of filler is the effect of volume fraction on strength. Reinforcing fillers show a marked peak in performance at relatively low loadings, followed by a significant decline. Semireinforcing ones have a broader peak at higher loading, while nonreinforcing ones have no peak, and a diluent filler shows a steady decline at all loadings.

This is illustrated in Figs. 2 and 3. Figure 2 shows the effect on strength that one can expect to see as a function of filler loading, for the three types of filler in a noncrystallizing elastomer, such as SBR. Figure 3 shows the same effect in a crystallizing elastomer such as NR.

Typical reinforcing fillers include most carbon blacks, fumed and precipitated silicas, nanoclays, and some special minerals such as halloysite. Semireinforcing ones include some clays (often referred to as hard clays), precipitated calcium carbonates, and the thermal blacks. Nonreinforcing ones include other clays (often called soft clays) and natural calcium carbonates.

Both natural and synthetic elastomers are almost always used filled with particulate materials, particularly those of submicron size, such as carbon black and precipitated or fumed silica. This is because such particles are usually required to obtain the required strength.

Fig. 2 The effect of the three types of filler on the strength of a noncrystallizing elastomer such as SBR

The effect of the three types of filler on the strength of a crystallizing elastomer such as NR

Fig- 3 The effect of the three types of filler on the strength of a crystallizing elastomer such as NR

 
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