Fillers for the rubber industry

Reinforcement of elastomers by colloidal fillers, like carbon black or silica, plays an important role in the improvement of the mechanical properties of highperformance rubber materials. The reinforcing potential is mainly attributed to two effects: (i) the formation of a physically bonded flexible filler network and (ii) strong polymer–filler couplings. Both of these effects arise from a high surface activity and the specific surface of the filler particles [3, 8, 28, 123]. For a deeper understanding of structure–property relationships of filled rubbers it is necessary to consider the aggregate morphology and surface structure of fillers more closely. The present chapter is devoted to several technological applications of fillers in rubbers. In particular, we demonstrate how the physics of rubber nano-composites facilitates the understanding of how new generations of fillers, like silica (instead of carbon black), boost tire technologies giving simultaneously improved performance in rolling resistance and wet grip behavior.

Since the introduction of the Energy ® tire by Michelin, precipitated silica has proved (through partial or total substitution of carbon black) to be the filler of choice for the manufacture of high-performance pneumatic passenger car tires. The main reason is an improvement in the final compromise between the main interrelated tire performance parameters: it gives a significant improvement in tire performance in regard to rolling resistance, wet grip, and stopping distance for cars equipped with anti-lock braking system (ABS) steering [124]. These improved characteristics mean that silica-filled tread compounds are also the best available materials for winter performance [125].

Why a new book about the science of an apparently old material? This question can be easily posed, when reading the title of this book. Indeed, filled rubbers are well known and well used in daily life. However, it is less known that recipes and the corresponding processing cycles of carbon black or silica filled rubber are extremely complex, which leads to a complex structure of the material in a wide range of length scales. Rubbers are classes of relatively soft materials without which modern technology would be unthinkable, similar to the case of metals, fibres, plastics, glass, etc. No matter where these rubber materials find their application, especially in tires and in a great variety of industrial and consumer products, e.g. motor mounts, fuel hoses, heavy conveyor belts, profiles, etc., the applications make high demands on rubber materials. The requirements are manifold, e.g. high elastic behavior even at large deformation, tailored damping properties during periodic deformations, great toughness under static or dynamic stresses, high abrasion resistance, impermeability to air and water, in many cases a high resistance to swelling in solvents, little damage, and long life.

Their importance for applied sciences and engineering is unquestionable, so why not collect the ideas and facts about these materials in a book? Aren't there many theories and facts around which many could form the basis for a review book? This would be, however, too simple, at least for us and for the completely different backgrounds of the three authors.

The reinforcement of composite materials is far from being a simple problem [1]. Reinforced elastomers, which find application in the car tire industry, are typical and well-known examples of that. Indeed, these materials allow a physical formulation of most of the problems and offer a suggestion for a solution. Complications arise due to the many length and time scales involved and this is one of the issues which will be examined in this book.

The basic aim of filling relatively soft networks, i. e. cross-linked polymer chains, is to achieve a significant reinforcement of the mechanical properties. For this purpose, active fillers like carbon black or silica are of special practical interest as they lead to a stronger modification of the elastic properties of the rubber than adding just hard randomly dispersed particles. The additional reinforcement is essentially caused by the complex structure of the active fillers (see, e.g., [2] and references therein).

The main aim of the present work is to gain further insight into this relationship between disordered filler structure and the reinforcement of elastomers. As a filler type we have chiefly in mind carbon black, which shows “universal” (i. e. carbon-black-type-independent) structural features on different length scales, see Fig. 1.1: carbon black consists of spherical particles with a rough and energetically disordered surface [3, 4]. They form rigid aggregates of about 100 nm across with a fractal structure. Agglomeration of the aggregates on a larger scale leads to the formation of filler clusters and even a filler network at high enough carbon black concentrations. Reinforcement is thus a multiscale problem.

Reminder: Einstein–Smallwood

In the following sections we are going to study the reinforcement obtained by adding particles to the elastic matrix. The mechanisms of the effective enhancement of the elastic modulus cannot be explained by one simple theory, since several interactions and many different length scales are involved [179]. This is because there are different physical levels of reinforcement. The rubber matrix contributes through its rubber elasticity [7], whereas the filler particles contribute in different ways. The most well known of these are volume effects, also called hydrodynamics interactions (due to the analogy with the enhancement of the viscosity of liquids by the addition of particles).

In the context of carbon-black-filled elastomers, the contribution to reinforcement on small scales can be attributed to the complex structure of the branched filler aggregates as well as to a strong surface–polymer interaction, leading to the socalled bound rubber. Thus the filler particles are coated with polymer chains and the binding (physical or chemical) of elastomer chains to the surface of the filler particles changes the elastic properties of the macroscopic material significantly [2]. On larger scales the hydrodynamic aspect of the reinforcement dominates the physical picture. Hydrodynamic reinforcement of elastic systems plays a major role not only in carbon-black-filled elastomers, but also in composite systems with hard and soft inclusions. Finally, at macroscopic length scales filler networking at medium and high filler volume fractions plays a dominant role [179].

In this chapter we are going to concentrate – on a general basis – on the different mechanisms of elastomer reinforcement in the hydrodynamic regime. To do so, we present two different regimes of reinforcement mechanisms.

General remarks and scaling

Although understanding the behavior of polymers on heterogeneous surfaces is a general problem in theoretical physics it provides deep insight into the problem of reinforcement and contributions. It is well accepted that the filler particles form large clusters which diffuse throughout the mixture to provide the most significant reinforcement effect on large macroscopic scales [179, 181, 197, 198]. Consequently these clusters form large surfaces inside the elastomer and allow significant polymer–filler contacts. Figure 9.1 shows a typical particle aggregate, with its hierarchy of length scales. The aggregate consists of individual particles, each with an irregular rough surface. As the particles form larger aggregates the irregular surfaces become very large. Moreover, the aggregates themselves form large clusters when the filler concentrations are high enough. Therefore we can expect major contributions to the reinforcement from the interaction between the polymer matrix and the irregular, rough surfaces.

However, the filler particles do not have homogeneous surfaces, but are strongly disordered. The disorder can be categorized in two extreme cases. In the first, the filler particles are spatially disordered. The second extreme case arises from the irregularity of the interactions. Imagine the surface to be spatially flat, but with the interaction energy varying randomly at each point on the surface. Such surfaces show non-trivial effects on the surrounding polymers as well. Both cases are driven by typical “disorder effects,” which we will study in Section 9.2. Indeed several studies [135, 156, 199] suggest a strongly heterogeneous surface. Gerspacher and coworkers provided some data which even suggest fractal surface properties for several carbon blacks [135, 199].

General remarks

The main goal of this chapter is to introduce a convenient view of the basic physics and elasticity of the rubber matrix. The easiest way to consider an elastic polymeric solid is as a crosslinked polymer melt. Polymer melts, however, already exhibit some properties of networks, at least on some time scales. This can be seen most beautifully by considering the storage modulus of a polymer melt.

The melt can be made a true solid by adding a reagent which joins each chain to a neighbor. For lightly crosslinked material there will be a few links per chain, but material can also be highly crosslinked [6]. Alternatively irradiation by gamma rays, X-rays, or by electrons will create crosslinks. There is ample evidence that polymers in melts are in random walk configurations, i. e. the molecule has a large choice of configurations and these differ by energies much less than the thermal energy kBT. The kind of picture one has then is as in a computer simulation. The real difficulty is that rubbers are fundamentally three-dimensional and, unlike for crystals, two-dimensional pictures are not comprehensive. However, the reader can imagine a very kinky spaghetti-like mixture with permanent crosslinking bonds along the length. There is ample experimental evidence that perhaps 90% of the free energy of the material is entropic; see [6] for a general discussion and references.

In a network, however, the problem is that all the structural elements that make precise theories for melts difficult become frozen in.

Filler networking in elastomers

Flocculation of fillers during heat treatment

For a deeper understanding of filler networking in elastomers it is useful to monitor structural relaxation phenomena during heat treatment (annealing) of the uncrosslinked composites. This can be achieved by investigations of the time development of the small-strain storage modulus G′0 that provides information about the flocculation dynamics [138,221–224]. Figure 10.1(a) shows the time development of the small-strain storage modulus G′0 at 0.28% strain and 1 Hz of three elastomer composites containing 50 phr carbon black of different grades. The sample with the smallest primary aggregate size (N115) exhibits the most pronounced increase of the storage modulus with annealing time, which levels out after about 10 minutes in this example. The extent of modulus gain reduces with increasing primary aggregate size and the N550 sample shows almost no effect. With increasing dynamic strain amplitude, as depicted in Fig. 10.1(b), the storage modulus decreases by about one order of magnitude (the Payne effect). Thus, it appears that during heat treatment a weakly bonded superstructure develops in the systems which stiffens the polymer matrix, indicating that the increase of the modulus results from flocculation of primary aggregates to form secondary aggregates (clusters) and finally a filler network. The dependence of the effect on the primary aggregate size is in accordance with the picture of a kinetic aggregation process.

Figure 10.2(a) shows the time development of G′0 of S-SBR melts of variable molar mass filled with 50 phr carbon black (N234), when a step-like increase of the temperature from room temperature to 160 °C is applied. Figure 10.2(b) shows a strain sweep of the same systems after 60 minutes annealing time.