Filler-elastomer bonds interactions

In the last part of the paper, filler-elastomer chemical interactions which are able to take place through surface functional groups or surface reactive hydrogens are studied. The effect exerted by the created filler-elastomer bonds in the reinforcement process is then discussed. 

It appears, beyond all doubt, that filler-elastomer interactions result in the formation of chemical bonds between the polymer and the solid surface, which are due to a reaction of the macromolecule either with the surface chemical groups or with the surface hydrogen atoms. Is, however, the formation of covalent filler elastomer bonds a prerequisite for reinforcement to occur ... 

The filler-elastomer chemical interactions take place through its surface functional groups and hydrogen atoms. Coupling agents improve polymer-filler adhesion. From the point of view of dynamic-mechanical properties for low strains, the filler-elastomer bonds have a positive effect in the reinforcement process. 

Zinc salt of maleated EPDM rubber in the presence of stearic acid and zinc stearate behaves as a thermoplastic elastomer, which can be reinforced by the incorporation of precipitated silica filler. It is believed that besides the dispersive type of forces operative in the interaction between the backbone chains and the filler particles, the ionic domains in the polymer interact strongly with the polar sites on the filler surface through formation of hydrogen bonded structures. 

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. 

No such relationship was observed in the case of polyethylene mixtures with butyl rubber (Fig. 6). We came to the conclusion that the activity of polyacrylonitrile as a filler can be connected with EDA interactions between electrons of double bonds and -C E N groups. No such complex type has so far been detected in polymer mixtures. In the given instance an EDA complex could appear only at the interphase boundary and its concentration would be quite low. However, certain symptoms of its existence have been observed. PAN added in the amount of 30 phr raised the cis-1,4-poly-butadiene Tg towards higher temperature region by 4-15 K. This was observed by means of thermomechanical analysis under dynamic as well as under static conditions (Figs. 7 and 8). The presence of the immobilized layer of PB on the PAN domains was also established in studies carried out by the method of pulse NMR. In the mixtures of PB with PAN there appeared additional compliances of the relaxation time T2 "spin-spin" (Fig. 9), as well as relaxation time T "spin-network" (Fig. 10). This indicates that part of the elastomer has... 

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. ... 

Surface Treatment. Carbon black remains the particulate filler of choice for rubber articles since the inherent reinforcing effect of the nonblack fillers in hydrocarbon elastomers is not comparable. This is primarily due to the nonbonded interactions established between the particulate filler and polymer functionality (28). Surface chemistry plays an important role in the interaction of the nonblack fillers and the polymer with contributions ranging from electrostatic interactions to covalent bonding to the polymer backbone. However, surface chemistry also strongly affects the interaction of the nonblack filler with other chemicals in the rubber compound, particularly active metal oxides, curatives, and antidegradants. 

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. 

Vibrational spectroscopy is also widely used for the analysis of filled elastomers [87-89] in order to describe the polymer-filler interaction [87,89], interfacial region [89], sulfur and cross-linking chemistry of elastomers and bonding of BHT fragments to the EPDM matrix [88]. In the rubber industry the FTIR transmission technique is generally accepted... 

From the discussion in Chapter 2, it follows that both physical and chemical bonds play an important role in reinforcement, determining the adhesion at the filler-matrix interface. Kraus made a detailed study of various aspects of interaction between elastomers and reinforcing fillers, in particular, on the influence of the chemical properties of carbon black particles on the reinforcement. It was found that the character of interaction of carbon black with pol5aner differs, depending on chemical properties of the surface of the black particles. In particular, there is a possibility of chemical grafting of the polymer molecules onto the surface. 

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-... 

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