Tribological consequences of rubber composition and structure – case studies

The paper discusses characteristic features of the structure of elastomers and the composition of rubber. Special attention has been paid to differences in structural organization between polymers and metals. Actually existing, mechanistic theory on the friction of elastomers, together with all its drawbacks has been demonstrated. Based on it, another approach to the interpretation of friction phenomenon – from the point of view of material engineering, has been presented on some examples of our own work. The influence of: 1. composition and structure of macromolecules 2. crosslink density and the composition of crosslink constituting a 3-D network, 3. filler loading, the degree of agglomeration and distribution of its particles, as well as 4. the surface migration of low molecular components of rubber mix and segregation in polymer blends, on the friction and wear of vulcanizates have been discussed.

Introduction

Polymers, contrary to metals, exhibit molecular organization, what means that instead of an atom, the basic structural unit is a macromolecule. It manifests itself by the specific behaviour of polymers under stress, which is additionally temperature dependent – Figure 1 [1].

Below glass transition temperature (T_g) the free rotation of macromolecules, even their fragments (backbone fragments or side chains) is blocked (“frozen”), so polymers behave like metals, obeying the Hooke's law. However, above T_g, the “mobility” of macromolecular chains is released and becomes to be limited only by intermolecular interactions, what results in unique, elasto-plastic characteristics of the material [2], which can be described by the equation of high elasticity. In the case of elastomers, which macromolecules are very flexible, it allows for relative elastic deformations exceeding even 1000 %. Further increase of temperature makes eventually these interactions diminished, allowing – above melting temperature (T_m), for totally unlimited free movements of macromolecules, what manifests itself by the flow of material, which can be described by the Newton's law. From the engineering point of view a “window” between T_g and T_m determines conditions for polymer processing and defines frames for their exploitation.

In order to increase mechanical properties of elastomers, as well as to improve their thermal and chemical stability, they have to be crosslinked [3]. Crosslinking is realized by production of intermolecular chemical links. Properties of materials depend on the density of crosslinking, their length and chemical composition. Generally, short C–C bonds make the material stiffer and more thermally resistant, whereas longer S_x links improve its dumping properties and mechanical strength. Apart these two basic groups, there are also a lot of other kinds of specific and unconventional crosslinks.

Very often, again especially in the case of elastomers, even the crosslinking of their macromolecules is not able to meet requirements for mechanical strength. They have to be accomplished filling an elastomer matrix with a solid phase [4]. The preparation of new composite material depends on the kind of filler, being usually characterized by following factors:

• its chemical compatibility with an elastomer matrix,

• the value of specific surface area,

• the degree of agglomeration, and

• the homogeneity of distribution in an elastomer.

The first two factors determine the so-called filler activity, whereas the last two ones are dependent on the quality of mix preparation. The most common fillers used by the rubber industry are carbon black (CB) and silica.

To improve mixing conditions and further processing of rubber mix, apart an elastomer and fillers, it also contains: softeners, processing agents, compatibilizers (adhesion promoters, e.g. silanes) and other ingredients, such as: pigments, ageing protectors (antioxidants and antiozonants) or flame retardants. They are usually low molecular weight substances, influencing the system morphology.

Finally, it can be summarized that rubber is a multicomponent and a multiphase composite material, which morphology is of a key importance to tribological characteristics of the material.

Very first works on mechanisms of rubber friction were published by Shallamach in the late fifties [5]. He proposed deformational model, describing the phenomenon as the propagation of deformations in a material – called “Schallamach's waves” – Figure 2.

Despite a lot of experimental work being performed, a satisfactory explanation on the origin of Schallamach's waves has not been presented so far [6]. However, attempts have been mainly undertaken from the mechanistic point of view, putting less attention to polymer engineering [7]. Nevertheless, some factors influencing the propagation of deformational waves, have been defined and namely: adhesion and the contact geometry of a friction pair, rubber elasticity, sliding speed, normal load and temperature. The critical value of sliding speed required for waves formation decreases with the decrease of normal load, roughness and temperature. Friction force discontinues – after an initial increase up to a maximum value, when being in an adhesive contact (“stick”) with a counterface, the sample slips, what results in the dramatic decrease of a friction force. The situation called “stick-slip” repeats, producing a typical only for this mechanism “saw-like” friction force characteristics. However, the mechanism is valid only for soft (usually unfilled), low loaded elastomers.

From the practical point of view, the friction of rubber has been well described by Moore [8], who proposed to divide a friction force into two components: deformational (hysteretical) and adhesional. It is commonly accepted, that adhesion plays a major role in the friction of materials exhibiting high elasticity, sliding with a low speed over a smooth surface under limited load [9]. Such conditions facilitate the “stick-slip” mechanism of friction. The hysteresis component of friction appears when an elastomer sliding over a rough surface is subjected to deformations, trying to keep contact with it [10]. Despite a fact, that some quantitative relationships between the strength and hysteresis of rubber have already been elaborated by Payne [11], the explanation on its “response” to dynamic loading still remains unsatisfactory [12].

This work is aimed at the description of a role played by rubber morphology in friction. The influence of macromolecular composition and structure, crosslink density and composition, filler loading, its agglomeration and distribution, as well as the surface segregation in polymer blends and blooming of low molecular weight components in rubber, are discussed in terms of their influence on adhesional and deformational components of friction. This knowledge creates a base for tailoring the properties of rubber to dedicated tribological applications.