Elastomers rubber-filler interaction

In attempting to predict the direction that future research in carbon black technology will follow, a review of the literature suggests that carbon black-elastomer interactions will provide the most potential to enhance compound performance. Le Bras demonstrated that carboxyl, phenolic, quinone, and other functional groups on the carbon black surface react with the polymer and provided evidence that chemical crosslinks exist between these materials in vul-canizates (LeBras and Papirer, 1979). Ayala et al. (1990, 1990) determined a rubber-filler interaction parameter directly from vulcanizatemeasurements. The authors identified the ratio a jn, where a = slope of the stress-strain curve that relates to the black-polymer interaction, and n = the ratio of dynamic modulus E at 1 and 25% strain amplitude and is a measure of filler-filler interaction. This interaction parameter emphasizes the contribution of carbon black-polymer interactions and reduces the influence of physical phenomena associated with networking. Use of this defined parameter enabled a number of conclusions to be made ... 

The reinforcement of rubber composites by CB and/or silica is greatly affected by the rubber filler interactions, the agglomeration of the filler particles within the rubber matrices, and the occlusion of the rubber into the internal voids of the dispersed aggregates. Furthermore, filler-elastomer interactions play a major role in the filler dispersion achievable during mixing process. 

Fig. 24 Influence of the silica polarity on the rubber filler interaction of an elastomer with 7 mol% of epoxy groups-SBR-2 (ep7)...

Payne has proposed a classical splitting of all the effects involved in rubber reinforcement which can be further refined with respect to contemporary views (Figure 5.40). In addition to the intrinsic properties of the elastomer network, the CB particles bring first mere hydrodynamic effects, which are further enhanced by strong rubber-filler interactions, and interaggregate interactions which are weaker and depending on strain level. 

When used as filler for diene elastomers, chemical modification of silica surface is required firstly to promote mixing through decreased inter-particles interactions and secondly to establish adequate rubber-filler interactions. 

See, J.L. Leblanc, Insight into elastomer—filler interactions and their role in the processing behaviour of mbber compounds, Prog. Rubber Plast. TechnoL, 10/2, 110-129, 1994, for a pictorial representation of such a morphology. 

An important feature of filled elastomers is the stress softening whereby an elastomer exhibits lower tensile properties at extensions less than those previously applied. As a result of this effect, a hysteresis loop on the stress-strain curve is observed. This effect is irreversible it is not connected with relaxation processes but the internal structure changes during stress softening. The reinforcement results from the polymer-filler interaction which include both physical and chemical bonds. Thus, deforma-tional properties and strength of filled rubbers are closely connected with the polymer-particle interactions and the ability of these bonds to become reformed under stress. 

It was shown, on the one hand, that gum-filler interactions are associated with the immobilization of a certain amount of rubber on the surface or inside the carbon black aggregates, and, on the other hand, that the corresponding bound or occluded rubbers play important roles in the reinforcement process due either to a restriction of elastomer chain mobility in the vicinity of the filler or to an increase of the effective volume of the latter. What are now the effects exerted by a filler on the stress-strain behavior and the modulus of cured rubbers ... 

A low-resolution proton NMR method is one of the few techniques that have so far proved to be suitable for studying elastomer-filler interactions in carbon-black-filled conventional rubbers and silica-filled silicon rubbers [20, 62, 79]. It was pointed out by McBrierty and Kenny that Many of the basic characteristics of filled elastomers are revealed by low resolution spectra while the more sophisticated techniques and site specific information refine interpretations and clarify motional dynamics [79]. 

Next 129Xe experiments on an EPDM terpolymer, which is present as the elastomer component in a composite material with carbon black will be discussed. The question investigated for these materials is whether the existence of any polymer-filler interaction can be detected by 129Xe NMR. This interaction influences the mobility of the elastomer chains in a relatively large shell around the filler particles. This fraction is called the bound rubber fraction. It is generally believed that the bound rubber fraction influences the mechanical and frictional properties of the filled elastomer [17, 18]. 

Elastomer-filler interactions were the subject of many intensive investigations. Kaufmann and co-workers [17] investigated carbon-black-filled EPDM by nuclear spin relaxation time measurements and found three distinct regions in the material. These regions are characterised by different mobility of the elastomer chains a mobile region in which the polymer chains have no interaction with the filler particles, loosely bound rubber in an outer shell around the carbon black particles and an inner shell of tightly bound elastomer chain with limited mobility. 

The study of the elastomer-filler interactions at a macroscopic level is carried out by different types of mechanical testing and by the determination of the content of bound (insoluble) rubber. These methods measure properties which are of interest for commercial applications of the materials and provide indirect information on elastomer-filler interactions. 

The characterization of the elastomer-filler interactions at a molecular level may be cairied out by spectroscopic techniques such as IR and NMR spectroscopy. X-ray and neutron scattering, dynamic mechanical and dielectric spectroscopy, and molecular dynamics simulations [6]. Up to now, the most comprehensive studies of silica filled PDMS [4, 7-22] and carbon black filled conventional rubbers [23] have been carried out by H [4, 7—20, 23], [21], and C NMR relaxation experiments [22],... 

The rabber modulus increases with an increasing volume fraction of Aerosil. The modulus increase can be caused by the elastomer-filler and filler-filler interactions and by an increase of effective filler content. A very sharp peak for the tanZ is observed at 163 K for an unfilled crosslinked sample. This maximum corresponds to the glass transition of the rubber. Furthermore, it is observed that the Tg of the rubber does not change in the presence of filler. However, the second maximum of to 5 can be seen in the vicinity of 200 K for filled samples. The intensity of this maximum becomes more pronounced with increasing Aerosil content. This observation is in agreement with the results of the h and Ty relaxation study, as demonstrated in Fig. 4a and 6, respectively. Therefore, it seems reasonable to assign the maximum for at 200 K to the motion of adsorbed chain units. This maximum is observed at a lower temperature than the H and T, minimum for the adsorbed chain units (at about 280 K) due to difference in frequency of these methods 1.6 Hz and 46-90 MHz, respectively. 

It is generally believed that the nature of elastomer-filler interactions is of major importance for marked improvement in mechanical properties of the filled elastomers [39-44]. Adsorption of elastomer chains at the filler surface has a double effect on the enhancement of mechanical properties of filled elastomers. Firstly, the ability of filler particles to share deformation increases due to adsorption interactions between the filler particles and the host matrix. Secondly, these interactions provide significant amount of adsorption and topological junctions in the elastomer matrix outside the adsorption layer. It appears that less mobile chain units in the adsorption layer do not contribute directly to the rubber modulus, since the fiaction of PDMS chain units in this layer is only a few percent of the Aerosil content used in conunercial rubbers [7,8,12,21]. 

Considerable effort has been spent to explain the effect of reinforcement of elastomers by active fillers. Apparently, several factors contribute to the property improvements for filled elastomers such as, e.g., elastomer-filler and filler-filler interactions, aggregation of filler particles, network structure composed of different types of junctions, an increase of the intrinsic chain deformation in the elastomer matrix compared with that of macroscopic strain and some others factors [39-44]. The author does not pretend to provide a comprehensive explanation of the effect of reinforcement. One way of looking at the reinforcement phenomenon is given below. An attempt is made to find qualitative relations between some mechanical properties of filled PDMS on the one hand and properties of the host matrix, i.e., chain dynamics in the adsorption layer and network structure in the elastomer phase outside the adsorption layer, on the other hand. The influence of filler-filler interactions is also of importance for the improvement of mechanical properties of silicon rubbers (especially at low deformation), but is not included in the present paper. 

The presence of filler in the rubber as well as the increase of the surface ability of the Aerosil surface causes an increase in the modulus. The temperature dependence of the modulus is often used to analyze the network density in cured elastomers. According to the simple statistical theory of rubber elasticity, the modulus should increase twice for the double increase of the absolute temperature [35]. This behavior is observed for a cured xmfilled sample as shown in Fig. 15. However, for rubber filled with hydrophilic and hydrophobic Aerosil, the modulus increases by a factor of 1.3 and 1.6, respectively, as a function of temperature in the range of 225-450 K. It appears that less mobile chain units in the adsorption layer do not contribute directly to the rubber modulus, since the fraction of this layer is only a few percent [7, 8, 12, 21]. Since the influence of the secondary structure of fillers and filler-filler interaction is of importance only at moderate strain [43, 47], it is assumed that the change of the modulus with temperature is mainly caused by the properties of the elastomer matrix and the adsorption layer which cause the filler particles to share deformation. Therefore, the moderate decrease of the rubber modulus with increasing temperature, as compared to the value expected from the statistical theory, can be explained by the following reasons a decrease of the density of adsorption junctions as well as their strength, and a decrease of the ability of filler particles to share deformation due to a decrease of elastomer-filler interactions. 

In contrast to the filled samples, the deformation energy for the unfilled ones increases proportionally to the increase in the absolute temperature according to the prediction of the simple statistical theory of rubber elasticity. Thus, it appears that the change of the modulus and the deformation energy with increasing temperature reveals a decrease of the density of adsorption junctions in the elastomer matrix, as well as a decrease of the ability of filler particles to share deformation, resulting from a weakening of elastomer-filler interactions. 

Fillers in Rubber. Carbon black and calcium silicate are able to reinforce rubber. For example, the tensile strength of an SBR vulcanizate can be raised from 350 to 3500 Ib/in. by compounding with 50% of its weight of carbon black (54). The activity of the carbon black depends on particle size and shape, porosity, and number of active sites, which are less than 5% of the total surface. Elastomers of a polar nature, such as chloroprene or nitrile rubber, will interact more strongly with filler surfaces having dipoles, such as -OH and -CCX)H groups or chlorine atoms. 

Both the Japanese Synthetic Rubber Company and Nippon Zeon have reported that anionically prepared elastomers that are functionally terminated by active lithium can be chain terminated with Michler ketone, benzophenone, and a variety of enamide groups. Moreover, these chains can be terminated with silicone or tin metals. Chain end functionalization did not change the viscoelasticity of the polymer chains but rather dramatically improved the elastomer-filler interaction and, therefore, reduced its hysteretic properties. 

For hydrophobic elastomers such as NR and styrene butadiene rubber, carbon black usually has been selected as filler due to the hydrophobic surface characteristics and special particle shapes of carbon black which provide good dispersion. However, the dispersion of polar filler in hydro-phobic rubbers matrix is difficult because of its hydrophilic surface. The hydroxyl groups exist on the surface of polar filler provide strong filler-filler interactions which resulted in poor filler dispersion. The polar surface of filler formed hydrogen bonds with polar materials in a rubber compound. As known, the silica surface is acidic and forms strong hydrogen bonds with basic materials. ... 

In the rubber industry the distribution of particle size is considered to be important as it affects the mechanical properties and performance. Aggregate size also varies with particle size. Aggregates can have any shape or morphology. The fundamental property of the filler used in a filled elastomer is the particle size. This affects the reinforcement of elastomer most strongly. One of the sources of reinforcement between the carbon black surface and the rubber matrix is the van der Waals force attraction. Also, rubber chains are grafted onto the carbon black surface by covalent bonds. The interaction is caused by a reaction between the functional group at the carbon black particle surface and free radicals on polymer chains. Hence, filler-rubber interface is made up of complex physical-chemical interaction. The adhesion at the rubber-filler interface also affects the reinforcement of rubber. When the polymer composites are filled with spherical filler (aspect ratio of the particle is equal to unity), the modulus of the composite depends on the modulus, density, size, shape, volume ratio, and number of the incorporated particles. 

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