Filler surface treatments interaction with fillers

Another attempt by Tricas et al. to modify the surface of carbon black was by the plasma polymerization of acrylic acid [34]. Treatment with acrylic acid made carbon black hydrophilic. Plasma-coated carbon black was mixed with natural rubber and showed increased filler-filler interaction. The bound rubber content was reduced after the surface treatment of the filler. The authors also concluded that the surface of the carbon black was completely covered by the plasma polymer film, preventing the carbon black surface from playing any role in the polymer matrix. 

The authors of [99] proposed a calorimetric method for determining the degree of the polymer-filler interaction the exothermal effect manifests itself in the high energy of the polymer-filler adhesion, the endothermal effect is indicative of a poor, if any, adhesion. The method was used to assess the strength of the PVC-Aerosil interaction with Aerosil surface subjected to different pre-treatments... 

If such fillers are to be used, they should have a neutral or slightly alkaline pH, otherwise additives such as ethylene glycol and triethanolamine, which are preferentially adsorbed on the surface of the filler, should be used, preventing any undesirable interference reactions between the filler and the crosslinking peroxide. These additives must, however, always be added to the mix before the peroxide. With some mineral fillers, such as some types of clay, the polymer may be bound to the filler by means of silane treatment, and the surface of the filler becomes completely non-polar. Consequently, the interaction with the polymer matrix increases, while the adsorption of the crosslinking peroxide by the filler is severely suppressed. 

Particle/particle interactions induce aggregation, while matrix/filler interaction leads to the development of an interphase with properties different from those of both components. Both influence composite properties significantly. Secondary, van der Waals forces play a crucial role in the development of these interactions. Their modiflcation is achieved by surface treatment. Occasionally reactive treatment is also used, although its importance is smaller in thermoplastics than in thermoset matrices. In the following sections of this chapter attention is focused on interfacial interactions, their modification and on their effect on composite properties. 

The specific surface area of the filler is an important factor which must be taken into consideration during surface treatment. The proportionally bonded surfactant depends linearly on it [74]. ESCA studies carried out on the surface of a CaC03 filler covered with stearic acid have shown that ionic bonds form between the surfactant molecules and the filler surface and that the stearic acid molecules are oriented vertically to the surface [74]. These experiments have demonstrated the importance of both the type of the interaction and the alignment of sur-... 

Fig. 12. Effect of non-reactive surface treatment of a CaC03 filler with stearic acid on its interaction with PP, r=1.8 am (O) non-treated, (A) 75%, ( ) 100% surface coverage... 

The dynamic response of polydimethylsiloxane (PDMS) reinforced with fused silica with and without surface treatment has been discussed in terms of interactions between the filler and polymer [54]. Since bound rubber measurements showed that PDMS chains were strongly attached to the silica surface, agglomeration due to direct contact between silica aggregates was considered an unlikely explanation for the marked increase in storage modulus seen with increasing filler content at low strains. Instead three types of flller-polymer-flller association were proposed which would cause agglomeration, as depicted in Fig. 15. 

Particle interactions resulting in aggregates of particles will adversely affect dispersion. Special surface treatments are provided to reduce these aggregation forces and achieve higher loadings and better suspension stability with less effect on viscosity. These surface treatments can be applied directly to the filler, and many grades of treated fillers are commercially available. 

Chemical modification of filler surface reduces the surface area available for interaction. This reduces bound rubber (Figure 7.26). The quantity of adsorbing additives on the filler surface must be strictly controlled because these additives compete with the reinforcing effect of the bound rubber. Thermal treatment of rubber increased the quantity of bound rubber but only when rubber was added prior to the addition of low molecular processing additives. " This shows that there was competition between the low molecular additive and the rubber for adsorption sites. 

The application of polymer affects choice of filler. For example, to prepare conductive materials, special fillers must be used to obtain the required properties. Also, the method of processing imposes certain constraints on the choice and treatment of the filler before its use. For example, polymers processed at high temperature require fillers which do not contain moisture. This affects both the choice of the filler and/or its pretreatment. The choice of additives used to improve the incorporation of the filler depends on the application and the properties required from a product but it is also determined by the processing method. For example, the viscosity of a melt is reduced by special lubricating agents whereas the viscosity of filler dispersions is controlled by the surface treatment of filler. In some cases, the order of addition is important or a special filler pretreatment is used to achieve the desired results. These methods are discussed in special section in the table. Some fillers simply caimot be used with some polymers. In other cases, special care must be taken to ensure polymer stability or filler may interact with some vital components of the formulation. This subject is discussed in special considerations of filler choice. 

Still, it is important that fillers interact with the polymer (binder) for various reasons. One is the rheological characteristic of paints. Figure 19.5 shows that many processes may affect how a filler behaves in the system. The simple drying of aluminum hydroxide prior to use contributes to an increased paint viscosity. It should be noted that aluminum hydroxide loses water at 220°C, therefore drying at 80 C may only remove the water adsorbed on the surface of particles. But this is apparently sufficient to increase the interaction with the binder since, when the partially dried filler is added, viscosity almost doubles. Similarly, treatment with 1% triethoxymethacryloylpropylsilane, MPS, contributes to an increased viscosity. This data shows that the same filler can be readily modified to give a variety of different results. 

Wood-filled PVC has inferior mechanical properties because of lack of interaction. Treatment of wood filler with aminosilane improves acid-base interaction between filler and polymer to the extent that impact strength and tensile properties of composite are improved over unfilled PVC. Tensile properties of PVC were deteriorated when leather particles were used as a filler. But, after filler particles were treated with ethylene-vinyl acetate copolymer, a coating was produced on the surface of filler particles that promoted adhesion with PVC and improved mechanical properties.These are some recent examples of many applications of filler preparations to improve its interaction with PVC. 

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