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The easiest way to change interfacial interactions is the surface coating of fillers. Surface modification is often regarded as a magic, which solves all problems of processing technology and product quality, but it works only if the compound used for the treatment (coupling agent, surfactant, etc.) is selected according to the characteristics of the components and the goal of the modification. Surface treatment modifies both particle/particle and matrix/filler interactions, and the properties of the composite are determined by the combined effect of the two. Besides its type, also the amount of the surfactant or coupling agent must be optimized both from the technical and the economical point of view.
The oldest and most often used modification of fillers is the coverage of their surface with a small molecular weight organic compound (Moczo et al. 2002, 2004; Fekete et al. 2004). Usually, amphoteric surfactants are used which have one or more polar groups and a long aliphatic tail. Typical example is the surface treatment of CaCO3 with stearic acid (Pukanszky et al. 1989; Moczo et al. 2002, 2004; Fekete et al. 2004). The principle of the treatment is the preferential adsorption of the surfactant onto the surface of the filler. The high-energy surfaces of inorganic fillers (see Table 2) can often enter into special interactions with the polar group of the surfactant. Preferential adsorption is promoted to a large extent by the formation of ionic bonds between stearic acid and the surface of CaCO3 (Fekete et al. 1990), but in other cases hydrogen or even covalent bonds may also form. Surfactants diffuse to the surface of the filler even from the polymer melt, which is a further proof for preferential adsorption (Marosi et al. 1987).
One of the crucial questions of nonreactive treatment, which, however, is very often neglected, is the amount of surfactant to use. It depends on the type of the interaction, the surface area occupied by the coating molecule, its alignment to the surface, the specific surface area of the filler, and some other factors. The determination of the optimum amount of surfactant is essential for efficient treatment. Insufficient amount does not achieve the desired effect, while excessive quantities lead to processing problems as well as to the deterioration of the mechanical properties and appearance of the product (Fekete et al. 1990). The amount of bonded surfactant can be determined by simple techniques. A dissolution method proved to be very convenient for the optimization of nonreactive surface treatment and for the characterization of the efficiency of the coating technology as well (Fekete et al. 1990). First, the surface of the filler is covered with increasing amounts of surfactant, and then the nonbonded part is dissolved with a solvent. The technique is demonstrated in Fig. 12, which presents a dissolution curve showing the adsorption of stearic acid on CaCO3. Surface coating is preferably carried out with the irreversibly bonded surfactant (C100); at this composition, the total amount of surfactant used for the treatment is bonded to the filler surface. The filler can adsorb more surfactant (Cmax), but during compounding a part of it can be removed from the surface and might deteriorate properties. The specific surface area of the filler is an
Fig. 12 Dissolution curve used for the determination of surfactant adsorption on a filler. CaCO3/stearic acid
important factor which must be taken into consideration during surface treatment. The irreversibly bonded surfactant depends linearly on it (Fekete et al. 1990).
As a result of the treatment, the surface free energy of the filler decreases drastically (Pukanszky et al. 1989; Fekete et al. 2004; Moczo et al. 2004). Smaller surface tension means decreased wetting (see Fig. 11), interfacial tension, and reversible work of adhesion (Fekete et al. 1990). Such changes in the thermodynamic quantities result in a decrease of both particle/particle and matrix/particle interaction. One of the main goals, major reasons, and benefits of nonreactive surface treatment is the first effect, i.e., to change interactions between the particles of fillers and reinforcements. As an effect of nonreactive treatment, not only particle/particle but matrix/filler interaction decreases as well. The consequence of this change is decreased yield stress and strength as well as improved deformability (Pukanszky et al. 1989; Jancar and Kucera 1990a). Strong interaction, however, is not always necessary or advantageous for the preparation of composites with desired properties; the plastic deformation of the matrix is the main energy absorbing process in impact, which increases with decreasing strength of adhesion (Allard et al. 1989; Pukanszky and Maurer 1995).
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