Fillers platy

One of the principal reasons for the use of mineral fillers in these polymers is to reduce this shrinkage problem. Platy fillers have generally been found to be the most effective in this respect, giving the highest levels of shrinkage reduction consistent with maintaining adequate properties in other areas. 

Silica (silicon dioxide) particles, synthetic or natural based, have commonly been used in various polymer systems. Silica has a low coefficient of thermal expansion and high stiffness, translating into increased modulus of the compounded polymer. However, silica filler particles are not flake or plate like as talc or mica particles are, and typically have low aspect ratios. This means that unless the particles are very small, a silica filler addition provides a relatively low surface area for contacting the polymer, and thus it reinforces the resin system less than platy fillers. Researchers have also noted that mica-filled PP, for instance, contains fewer voids than silica-filled PP, which helps explain the higher strengths of mica compounds at 20% filler loadings. This tendency to form voids or cavities increases as silica filler content increases [7-20, 7-21). 

For any given application, certain mechanical properties will be of more importance than others. It is therefore, essential to identify and rank the most relevant properties and formulate or purchase the least expensive composite material that satisfies the requirements. The key mechanical properties for most applications are modulus (tensile or flexural), yield strength, impact strength and possibly HDT. A distinction is often made between reinforcing and nonreinforcing fillers, but unfortunately, the term reinforcement is rarely defined explicitly. Fibres are usually considered to reinforce and isotropic fillers are not, with platy fillers somewhere in between. As shown later, it is not appropriate to define reinforcement in terms of particle shape, because that definition breaks down with variations in anisotropy and particle size. In agreement with Ram [59], the definition of reinforcement as the simultaneous improvement of both modulus and yield strength will be used in this chapter... 

Conventional platy fillers, such as mica and talc, are well known, but the plates are quite thick and the aspect ratios accessible are limited, if the other dimensions are to be kept to reasonable levels. The plate thickness also means that quite high filler loadings are still required. Certain layer silicates, both natural and synthetic, allow very thin plates, with a high aspect ratio to be produced, while keeping the other dimensions to a reasonable level. 

Platy fillers such as talc, clay, and mica increase the polymer modulus more strongly than do isotropic fillers, such as, calcium carbonate, dolomite, silica, and fly ash. In general, the higher the aspect ratio of the filler, the greater the increase is in modulus. 

Mica - Mica is a crystalline platy filler made by wet or dry grinding of muscovite or phlogopite, minerals consisting mainly of aluminum and... 

Summing up the above results the author would claim that microparticles are far more efficient toughness modifiers than nanoparticles. The nanoeffects reported in numerous works should be linked with changes of the crosslink density in the interphase. The related changes are likely caused by the selective absorption of a given component of the resin by the nanoparticles. Note that this happens also when the nanoparticles are available in masterbatch form. Unfortunately, the related aspects e.g., cure kinetics, morphology development) have not yet been addressed by systematic studies. Nevertheless, platy fillers, present in both micro- and nanoscale at the same time, may be better toughener than spherical or fibrous nanofillers. Func-... 

Some polymers are sensitive to ozone and their resistance is improved by the use of platy fillers. 

The situation can become even more complicated with high aspect ratio fillers and platy fillers. High-stmcture carbon blacks are not, overall, spherical, but some of the short fiber mineral fillers could have very unusual effects on viscosity and flow. Unfortunately, this is an area that does not have a wealth of data accumulated. One would expect some directional nature to properties after flow with short fiber or high aspect ratio fillers. In other words, the properties in the direction of flow should be... 

R. J. Eldred. Effect of oriented platy filler on the fracture mechanism of elastomers. Rubb. Chem. TechnoL, 61,620-629,1988. 

---