Chemical groups on the filler surface

Carbon black, because it is used so extensively, is one of the most frequently investigated fillers. However, findings have controversial elements in some of the details which attribute importance to surface groups. 

These differences in the chemical structure of the surface depend not only on the process of manufacture but also on additional treatments or processing conditions. In oxidized carbon fibers, the concentration of carbonyl and, more particularly carboxyl groups, is substantially increased at the expense of hydroxyl groups. In the treatment of carbon fibers, several methods of oxidation are used. Liquid phase oxidation is carried out by the electrochemical and chemical methods whereas gaseous oxidation is carried out in air, oxygen or in the presence of catalysts. Plasma treatment is also used for the surface oxidation of formed fibers. Different methods of oxidation produce different surface characteristics. For example, interlaminar strength is improved by a factor of 10 by electrolytic oxidation over crude oxidation in air. 

Fiber strength cannot be maintained if the oxidation process goes too far. It is therefore important to find a balance between fiber properties and the ability of the fibef s surface to interact with the surrounding materials. Moderate oxidation generally gives the best performance. 

ESCA has been used in the surface analysis of carbon black oxidized by various methods. Again, oxidation in air contributes to the most substantial loss of C-C bonds. Keto-enol groups were detected only in the samples which were oxidized in air. When other oxidative processes were employed, the groups detected were OH, C=0, and COOH. All other analytic methods provided similar information. 

The groups present on the carbon black surface may also come from chemical treatments. In one report,peroxide groups were introduced by radical trapping and then used for radical graft polymerization. In such a method, the entrapped radical plays the role of an initiator. 

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Chemical reaction depends on the presence of reactive substrates and on the probability of their encounters. Thus, the possibilities of reactions can be numerous. The literature describes reactions of OH groups on the surface of kaolin with isocyanates, vulcanization of nitrile rubber by ZnO, reactions of carboxyl groups on the filler surface with amines and epoxy groups, reactions of carboxyl groups with diols," and many others.The presence of a reactant on the surface of a material particle increases the probability of chemical reaction. Other factors include statistical probabilities, surface barriers which affect contact, dilution factors, molecular mobility, and viscosity changes in the system. These are discussed in other sections of this book. 

Prior to the chemical reaction of the silane with the silanol-groups on the sUica surface, the silane molecule has to make contact with the sUica surface by adsorption. Then the chemical reaction of silica with an alkoxy-silyl moiety of the coupling agent takes place in a two-step, endothermic reaction. The primary step is the reaction of alkoxy-groups with silanol-groups on the silica filler surface [4]. Two possible mechanisms are reported ... 

It appears from the evolution of the adhesion index that a distinction has to be made between the interactions carbon blacks are able to have with unsaturated or with saturated (or near-to-saturated) elastomers. Thus, the adhesion index of butyl rubber is enhanced upon oxidation of the black, while the reverse is observed with polybutadiene 38). The improvement of the reinforcing ability of carbon black upon oxidation, in the former case, has been interpreted by Gessler 401 as due to chemical interactions of butyl rubber with active functional groups on the solid surface. Gessler, relating the reinforcing characteristics of the oxidized carbon black for butyl rubber to the presence of carboxyl groups on the surface of the filler, postulated a cationic... 

Any silane surface modification needs to prove its value through a practical test. Proper selection of mineral, silane, and production parameters will lead to optimum properties of the composite [21]. But still, there exists a desire to see the silane on the filler surface, to visualize the silane layers or, at least, to see the difference between a silane-modified and an unmodified mineral surface [22, 23]. In general, surface analysis depends on the type of organofimctional group of the silane. To avoid the analysis of physiosorbed instead of chemically bound silanes, the treated mineral can be eluted with an excess of solvent in which the respective silane is soluble. 

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