Polymer-Particle Filler Systems

In this chapter we consider the characteristics of binary polymer-soHd particle suspensions. Our concern is with polymer-particle interaction and particle-particle interactions, especially in their roles to influence the melt flow and enhance solid mechanical behavior. We discuss the behavior of isotropic- and anisotropic-shaped particle compounds in thermoplastics, including rheological behavior from low loadings to high loadings obtained using various instruments. 

Many of the fillers used in industry are anisotropic in character. Depending on the shape of fillers, they are subdivided into isotropic particles, flakes, and fibers. Anisotropic particles may take on states of orientation because of flow and packing processes. Whether developed during flow or processing, particle orientation influences phenomena ranging from rheological properties to compound processability in industrial processing equipment, electrical characteristics, and mechanical performance. 

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Emulsion Adhesives. The most widely used emulsion-based adhesive is that based upon poly(vinyl acetate)—poly(vinyl alcohol) copolymers formed by free-radical polymerization in an emulsion system. Poly(vinyl alcohol) is typically formed by hydrolysis of the poly(vinyl acetate). The properties of the emulsion are derived from the polymer employed in the polymerization as weU as from the system used to emulsify the polymer in water. The emulsion is stabilized by a combination of a surfactant plus a coUoid protection system. The protective coUoids are similar to those used paint (qv) to stabilize latex. For poly(vinyl acetate), the protective coUoids are isolated from natural gums and ceUulosic resins (carboxymethylceUulose or hydroxyethjdceUulose). The hydroHzed polymer may also be used. The physical properties of the poly(vinyl acetate) polymer can be modified by changing the co-monomer used in the polymerization. Any material which is free-radically active and participates in an emulsion polymerization can be employed. Plasticizers (qv), tackifiers, viscosity modifiers, solvents (added to coalesce the emulsion particles), fillers, humectants, and other materials are often added to the adhesive to meet specifications for the intended appHcation. Because the presence of foam in the bond line could decrease performance of the adhesion joint, agents that control the amount of air entrapped in an adhesive bond must be added. Biocides are also necessary many of the materials that are used to stabilize poly(vinyl acetate) emulsions are natural products. Poly(vinyl acetate) adhesives known as "white glue" or "carpenter s glue" are available under a number of different trade names. AppHcations are found mosdy in the area of adhesion to paper and wood (see Vinyl polymers). 

It should be noted that for polymerization-modified perlite the strength parameters of the composition algo go up with the increasing initial particle size. [164]. In some studies it has been shown that the filler modification effect on the mechanical properties of composites is maximum when only a portion of the filler surface is given the polymerophilic properties (cf., e.g. [166-168]). The reason lies in the specifics of the boundary layer formation in the polymer-filler systems and formation of a secondary filler network . In principle, the patchy polymerophilic behavior of the filler in relation to the matrix should also have place in the failing polymerization-modified perlite. 

Polymer-based multicomponent systems are abundant in many applications. The properties and performance of particulate-filled systems, such as elastomers and impact modified polymers, and also polymer blends, block copolymers, and fiber reinforced systems, depend to a large extent on the distribution of the components. Hence the local analysis of these distributions down to sub-100 nm length scales (dictated, e.g., by the size of primary filler particles) is of considerable significance. Materials contrast in several AFM approaches offers the possibility to address these issues directly at the surface of specimens or on bulk samples that have been prepared correspondingly. 

Figure 1. The normalized radial distribution function of the polymer units with respect to the filler particles for systems Ms.io, Ms,so and Mie.ae, (p, left scale) and the volume fraction of spherical shells not occupied by filler particles in system M8,so (/ti, right scale) as a function of the distance r of the shell from the center of a filler particle (shell thickness = O.lcr).

Figure 8. The normalized density of polymer units in spherical shells of radius r and thickness 0.1a centered on the filler particles in system Mie.ae and in large systems of phantom chains of 100 units with a/ = 16a, tp = 0.36.

Incorporation of fillers into CPEE, however, presents a problem, since the polymer is in the continuous phase and the filler in a dispersed phase. Intermediate layers of 0.001-120 pm thick are formed between filler and polymer particles, with a non-homogeneous structure depending on the properties if the polymer and filler and on the method of production of the system. A chemically active preparation of the filler surfaces is used industrially to increase adhesion and fillers containing an aminosilane or epoxy preparation are recommended. 

The above equation should be used with caution, however, because it does not account for the quality of interfacial contact between the plastic and the filler system. Poor interfacial contact has the same effect as a thermal contact resistance and can result in a significant lowering in the ability of the highly conducting filler particles to transmit heat to the low-conductivity polymer matrix. What complicates the matter further is that these systems may possess good interfacial contact while the polymer matrix is molten but then become lower in thermal conductivity as interfacial contact resistance develops between the filler and the now-solidified polymer. This can be particularly confusing in the case of some filled semicrystalline polymers, where the appearance of the crystalline phase upon solidification should result in increased thermal conductivity, while the actual value appears to decrease. For this reason, it is considered safer to measure the thermal conductivity of filled materials. 

For the polymers containing filler that touch each other, the percolation theory has been developed. This assumes a sharp increase in the effective conductivity of the disordered media, polymer matrix composite, at a critical volume fraction of the reinforcement known as the percolation threshold (( )percoi) which long-range connectivity of the system appears. The model that best expresses these aspects is the one created by Vysotsky (Vysotsky and Roldughin 1999), which presumes a percolation network of nanofiller particles inside the polymer matrix as shown in equation (11.10) ... 

Regime two The fillers are well separated, but their mean distance is below a certain threshold so that electrical field-assisted tunneling can occur between neighboring reinforcement particles. For composite systems near the percolation limit, the conductivity is influenced by the properties of the polymer, the filler material, the interface, and by the dispersion of the filler in the matrix. 

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