Reinforcing fillers chemical bonding

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. 

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 ... 

Graphitized carbon blacks, thus undoubtly display reinforcing abilities which become obvious when considering the tensile strength of the unfilled vulcanizate. It follows that the formation of a filler-elastomer chemical bond is not a requirement for reinforcement to occur. It strongly participates, however, in its effectiveness, and determines the good mechanical properties connected with rubber reinforce-... 

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. 

Converting polymers to almost 35,000 plastics includes mechanical mixing/blending one or more polymers with additives, fillers, and/or reinforcement. They do not normally depend on chemical bonds, but do often require special compatibilizers. Mechanical compounding is extensively used (Chapter 5). 

The properties of filled materials are eritieally dependent on the interphase between the filler and the matrix polymer. The type of interphase depends on the character of the interaction which may be either a physical force or a chemical reaction. Both types of interaction contribute to the reinforcement of polymeric materials. Formation of chemical bonds in filled materials generates much of their physical properties. An interfacial bond improves interlaminar adhesion, delamination resistance, fatigue resistance, and corrosion resistance. These properties must be considered in the design of filled materials, composites, and in tailoring the properties of the final product. Other consequences of filler reactivity can be explained based on the properties of monodisperse inorganic materials having small particle sizes. The controlled shape, size and functional group distribution of these materials develop a controlled, ordered structure in the material. The filler surface acts as a template for interface formation which allows the reactivity of the filler surface to come into play. Here are examples ... 

M.P. Wagner The Consequences of Chemical Bonding in Filler Reinforcement of Elastomers, paper presented at the Colloques Intemationaux, Sept. 24-26, 1973, Le Bischenberg-Obemai, France. 

Adhesion promoters include the substances that create close physical and/or chemical bonds between two substrates [108]. The substrates to be cormected are fibrous reinforcing or particulate fillers on the one hand and plastics or metals on the other. The adhesion promoters always form bridges between the interfaces of the two components. For example, adhesion promoter resins based on styrene/butadiene alloys serve as adhesive layers for laminating panels and coextrusion of foils made of styrene polymers with polyolefins, PC, PMMA, and PA [30]. 

Both the modulus-temperature relationships presented in the preceding sections and the tensile data presented above are strikingly similar to those demonstrated for other rubber-plastic combinations, such as the thermoplastic elastomers (see Chapter 4 and the model system presented in Section 10.13) and the impact-resistant plastics (Chapter 3). The IPN s constitute another example of the simple requirement of needing only a hard or plastic phase sufficiently finely dispersed in an elastomer to yield significant reinforcement. Direct covalent chemical bonds between the phases are few in number in both the model system (Section 10.13) and present IPN materials. Also, as indicated in Chapter 10, finely divided carbon black and silicas greatly toughen elastomers, sometimes without the development of many covalent bonds between the polymer and the filler. 

The major conclusion obtained in both of these investigations is that neither primary chemical bonding nor strong secondary physical forces are required for simple reinforcement. It is expected that these results will discomfit those workers whose main theme centers on strong filler interactions, although the latter obviously do play an important role. 

Any one of several factors may be responsible for the reinforcement effect of active fillers. Some fillers can contract chemical bonds with the material to be reinforced. In the case of carbon black, this takes place by means of the radical reactions of the unpaired electrons, which are present in large numbers of carbon black. Carbon black particles cause cross-linking in elastomers. 

Very recently, many studies have been conducted to identify new reinforcing systems. These systems are similar to silica compounds and characterized by the use of a coupling agent to chemically bond elastomer chains to filler surface. Many reinforcing systems have been patented alumina oxyhydroxide and oxide [18,19], titanium oxides [20], and silicon nitride/carbide [21]. 

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