Particle charge, fillers

This article addresses the synthesis, properties, and appHcations of redox dopable electronically conducting polymers and presents an overview of the field, drawing on specific examples to illustrate general concepts. There have been a number of excellent review articles (1—13). Metal particle-filled polymers, where electrical conductivity is the result of percolation of conducting filler particles in an insulating matrix (14) and ionically conducting polymers, where charge-transport is the result of the motion of ions and is thus a problem of mass transport (15), are not discussed. 

The charge or zeta ( ) potential of the filler particle (i.e. the charge at the plane of shear between the particle s diffuse double layer and the bulk liquid phase) can be obtained by measuring its mobility in an applied electric field of known magnitude. The mobility is a function of the field gradient and is therefore expressed as a speed per unit potential gradient (/im/s/V/cm). Mobility and therefore zeta potential are both a function of pH (Figure 6.4). 

The presence of dispersed fillers in the polymer material in low amounts may intensify electrization, increase the residual charge and change the friction coefficient. Introduction of the filler in the electret state exerts a still stronger effect on polymer electrization on frictional interaction with metals. Depending on the direction of the field intensity vector formed by the filler particles, the field generated by triboelectrization can be attenuated or intensified. This means that the principle of the electret-triboelectrization superposition is realized [49], which can be used to regulate the parameters of frictional interactions. Thus, by the introduction of the electret filler, e.g. mechanically activated F-3 powder, it is possible to decrease the friction force (Fig. 4.9). 

It is clearly evident that numerous mineralogies are utilised for paper filling applications however, these minerals are all classified in much the same way. Particle size and size distribution, pigment brightness, refractive index, particle shape, and specific surface area are quantifiable characteristics that can be used to predict how the pigment will perform in various paper applications. To a lesser degree, particle charge, or zeta potential, plays a role in how the filler interacts with various paper chemical additives and influences the manner in which the mineral is retained in the paper web. 

Particle size and surface area play a role in chemical demand required to sufficiently retain pigments within the web. Likewise, dispersants often associated with high solids filler slurries tend to interfere with many conventional retention aid chemistries and should be taken into consideration when grade development strategies are formed. Particle charge, or zeta potential, is often an indicator of the relative ease of retention for many fillers. 

Hetero-coagulation This mechanism involves adsorption of oppositely charged particles, e.g., complexes of resin acids and aluminum sulfate, on the surfaces of fibers and filler particles. Hetero-coagulation is sensitive to soluble anionic wood polymers and electrolytes, with which cationic sizing particles, preferentially interact. 

Briefly, paper is made from a very dilute aqueous suspension of anionically charged cellulosic fibres which, after water removal, fam a fibrous network, i.e., a paper sheet. In many cases, filler particles such as ground or precipitated calcium carbonate or clay are added to the fibre suspension to enhance the optical performance or printability of the paper. Also, a variety of other components are added to improve specific properties of the paper sheet (e.g., wet and dry strength agents) or to facilitate the paper production process (e.g., retention aids to minimize the loss of fines and filler material to the process water, dewatering aids or defoaming chemicals). The majority of these chemical additives are polymers. 

Regime three At high reinforcement amounts, the fillers are in close contact so the conduction of charge carriers occurs through the continuous structure of the chain of filler particles in the matrix. The conductivity is mainly determined by the filler and its microscopic contacts to adjacent particles. 

Conductive polymer composites can be defined as insulating polymer matrices which have been blended with filler particles such as carbon black, metal flakes or powders, or other conductive materials to render them conductive. Although the majority of applications of polymers in the electrical and electronic areas are based on their ability to act as electrical insulators, many cases have arisen more recently when electrical conductivity is required. These applications include the dissipation of electrical charge from rubber and plastic parts and the shielding of plastic boxes from the effects of electromagnetic waves. Consequently, materials scientists have sought to combine the versatility of polymers with the electrical properties of metals. The method currently used to increase the electrical conductivity of plastics is to fill them with conductive additives such as metallic powders, metallic fibres, carbon black and intrinsically conducting polymers such as polypyrrole. 

Chemical composition, particle morphology, particle size and particle size distribution, brightness, refractive index, specific surface, particle charge and abrasiveness are commonly used to characterize papermaking fillers. Table 2.5 summarizes some chemical and physical data of fillers and fibers. More detailed information is given in the following paragraphs. 

We have introduced [7] the concept of photovoltaic cells and photodiodes based on the gradient of the concentration of the filler particles within the matrix, as shown in Fig. 1. The existence of this gradient allows optimization of the topology of the conductive particles network, preventing the formation of dead ends and hence decreasing the chance of recombination of the charge carriers during their transfer to the electrodes. 

Electrical properties of filled resin systems are also improved by filler treatment. Filler particles are naturally hydrophilic via their metal hydroxide surfaces, and the particles naturally seek to agglomerate with each other, and so transport electrical charges through resin composite. Treatment with silane-coupling agent alters the chemistry of the filler surface, allow better dispersion of the filler throughout the resin matrix, and imparts improved electrical properties to the composite. Table 15.13 indicates the improved electrical properties of a quartz-fiUed epoxy resin system with 0.3% silane admixed into the formulation. Improved insulation values, including reduced dielectric constant and reduced dissipation factor, are also denoted. 

Consider first the phenomenon, which is called rejection of a filler by rubber. When a silica filler is charged onto premasticated natnral rubber, fine particles of the filler are blown out from the charge hole of the internal mixer like smoke. It is interpreted as a repulsion by the electric charge built in the rubber during mastication. When silica is premixed with carbon black or fine powdered metal carbonates, the repnlsion is significantly reduced [9]. 

---