Fillers polymer-filler bonding

In the present paper, which is intended as a review of more recent progress only, emphasis is placed on the physical approach, but not to complete exclusion of the nature of the polymer-filler bond. Because of the overwhelming importance of carbon black as a reinforcing filler, and because most of the pertinent literature on reinforcement concerns carbon black filled rubbers, much of the discussion will be directed to carbon black reinforcement. However, the principles involved are general and apply qualitatively also to other fillers. 

A direct semi-quantitative determination of polymer-filler bonding by electron microscopy has been accomplished by Hess, Lyon and Burgess (32). A modification of the method of Kruse (33), in which a microtome section of the filled rubber is supported on a clear polymeric film, makes it possible to observe de-wetting of the filler as a function of elongation. Another technique designed to estimate rubber-filler ad-... 

It is certain that the relaxation behavior of filled rubbers at large strains involves numerous complications beyond the phenomena of linear viscoelasticity in unfilled amorphous polymers. Breakdown of filler structure, strain amplification, failure of the polymer-filler bond, scission of highly extended network chains and changes in network chain configuration probably all play important roles in certain ranges of time, strain rate, and temperature. A clear understanding of the interplay of these effects is not yet at hand. 

Explanations of the Mullins effect have included failure of weak linkages, failure of network chains extending between adjacent filler particles, and polymer-filler bond failure, a form of dewetting. Brennan et al. (1969) and... 

It is well known that for some filler-polymer pairs, bond mbber content is more than 20% of total polymer [2-8,65]. On the other side, from NMR (as we have seen in Section VI) the hard... 

The reinforcing ability of fillers is influenced by three primary characteristics of the filler particle size, polymer-filler bonding, and particle shape complexity [30]. 

Polymer-filler bonding. The ability of the filler to react with the polymer resulting in... 

The authors concluded that the drop in storage modulus with applied strain is found to be higher for ACM/silica nanocomposites and lower for PVA/silica systems. For all measured nanocomposites, this effect is higher with increasing temperature. The reported observations are consistent with relatively weaker polymer-filler bonding in ACM/silica and stronger interactions (matrix vs filler) for ENR/silica and PVA/silica nanocomposites [18]. 

Since we are dealing with a niunber of different effects superimposed on each other, according to Kraus, it is natural to observe a lack of correlation between the bound rubber content and reinforcement. The nature of polymer-filler bonds and their role in the reinforcement of elastomers was considered by Rehner. On the basis of analysis of the deformation properties, it was estab-hshed that the strength of bonds present in vulcanizates is characterized by a very broad spectnim of forces. From the data on the swelling of filled systems, it is possible to determine the concentration of physical bonds (or attachments) of macromolecides to the surface, which restrict the degree of swelhng and the niunber of chemical crosshnks. 

The influence of the structme of the elastomer on reinforcement is linked with the effects of localization of stresses, because the stress, occmring on the smface of the filler particles, is a fiinction of the elastic properties of the material. This explains the fact that for an equal nrunber of polymer-filler bonds and crosslinks, the reinforcement effects are still different for different rubbers. The predominance of physical interactions between rubber and black corresponds well with the mechanism of equahzing of the stresses on stretching. Stronger in-... 

The use of additives to make the surface of precipitated silica less hydrophilic and more rubberphilic facilitates incorporation, dispersion, and more intimate filler-elastomer contact during compoimding. This provides an improvement in rubber physical properties, as would be expected from a high surface area, high oil absorption filler. However, reinforcement comparable to that obtained with carbon black requires a polymer-filler bonding mechanism comparable to that provided at the carbon... 

Organofunctional silanes are used to promote polymer-to-filler bonding with clay or siHca fillers. Vinyl silanes are used in peroxide-cured wire insulation to promote stronger bonding with calcined clay fillers. Mercapto silanes are used to treat kaolin clay in sulfur-cured compounds. 

An intensity factor—the specific activity of the filler-polymer interface causing chemical and/or physical bonding. 

Thus a strong bond is not always desirable. We can see this from Table 7 and 8. The authors of [100] interpreted their experimental data as follows the rigidity of specimens increases with increasing PVC-filler interaction as a result the rate of relaxation of stresses arising at interphases in the course of deformation decreases. The overstressed states at the interphases may, in the authors opinion, promote separation of the polymer from the filler surface. That is, it is more desirable that the matrix-filler bond is not rigid but labile. 

Using calorimetry to estimate the degree of filler-polymer interaction as described in [99] the authors of [318, 319] determined that the filler reaction with PVC is exothermic, which is indicative of a stronger bond in the polymer-filler system. No thermal effect was noted for mechanical mixtures, except for a few cases where it was endothermal. 

The formation of PPD groups on the polymer backbone provides a mechanism to improve the polymer-filler interactions. The nitrogen-hydrogen bonds are capable of hydrogen bonding with polar groups on the surface of the filler. This enhanced interaction provides for somewhat unique dynamic mechanical properties. Under ideal conditions rolling resistance improves when QDI is used in the mix. Also, abrasion characteristics are maintained and in some cases even modest improvements occur. 

Increasing the bound mbber content increases the effective volume fraction of filler by intimately bonding polymer to the filler. This polymer is no longer available to contribute to viscous flow. As a consequence, the viscosity of the compound increases. 

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