Use of Particulate Fillers in Polymer Composites

A brief description of the choice of fillers in the main polymer types follows. More details can be found in the separate chapters dealing with the use of fillers in elastomers, thermoplastics, and thermosets.

Thermoplastics

The size and shape properties of the main fillers used in thermoplastics are given in Fig. 10.

The principal fillers used are the low-aspect-ratio calcium carbonates and, where flame retardancy is required, aluminum or magnesium hydroxide. For most purposes, average particle size is in the range 1-5 pm, with minimal amounts over 50 pm and below 0.2 pm.

Two distinctions must be made when considering the effects of fillers in this class of polymer. The first is between amorphous and semicrystalline types (e.g., polystyrene and polyolefins, respectively). The second is between conventional- and impact-modified grades (often these contain a dispersed rubber phase or some copolymer as the impact modifier). Polypropylene is a good example of this for semicrystalline types; there are homopolymer, copolymer, and rubber-modified grades with subtle differences in filler effects in these various forms. Where the

Fig. 10 Size and shape of the main filler types used in thermoplastics

Fig. 11 The effect of filler shape on key properties of thermoplastics

polymer has inclusions, such as crystallites and/or rubber phase, the filler is often only found in the amorphous regions of the thermoplastic, and this significantly increases its effective volume fraction. In some special cases, the filler can also be present in the elastomer phase.

Shape, as well as size, is important, and a summary of the effects of filler shape on the key properties of thermoplastic composites is presented in Fig. 11.

Low-aspect-ratio fillers are used when it is necessary to retain good impact resistance. Higher aspect ratio fillers are more effective at reinforcing, that is, increasing strength and modulus. However, higher aspect ratio fillers can be very deleterious to impact resistance. This is a prime example of what composite design is all about, namely, making compromises.

Because they soften with heat, a property known as heat distortion temperature or heat deflection temperature (HDT) is often important for composites based on thermoplastics. This is a measure of the temperature at which the composite deforms under a given load. High-aspect-ratio fillers are preferred for increasing HDT, and this is related to their ability to improve modulus, especially at elevated temperature. It is not widely appreciated that HDT is not meaningfully improved by adding fillers to amorphous plastics but is very much improved in semicrystalline polymers. (HDT can be determined by the methods in ASTM D648, another test used for the same purpose is the Vicat softening point.)

It is often assumed that high-aspect-ratio fillers are intrinsically better than low- aspect-ratio types, but this is because their advantages are overestimated and their disadvantages underrepresented. When one sees data for reinforcement using mica, wollastonite, talc, and kaolin, the modulus and strength improvements are impressive. However, the data from tensile testing are unrealistically positive. When tensile test bars are injection molded, these fillers orientate in the direction of the melt flow. When the bars are tested, the reinforcement is optimal in the test direction. If the same bars were tested perpendicular to the flow direction, then far inferior results would be recorded. In the perpendicular directions, strength and modulus are equivalent to the values obtained using round or blocky shaped particles. In many commercial applications, it would be more realistic to find particles randomly orientated, and in that case the modulus and strength gain from using such fillers may be substantially less than half that for fully aligned particles.

There are other issues as well. For instance, high-aspect-ratio fillers are broken down by handling, extrusion, and injection molding. This is significant and much of the potential reinforcement performance can be lost. Furthermore, they are broken down by recycling so the properties worsen with every recycling pass. In contrast, low-aspect-ratio fillers like calcium carbonate, silica, and dolomite can be recycled multiple times without deterioration of properties. Aligned anisotropic particles induce warpage (differential shrinkage). Lastly, anisotropic fillers lead to weld lines or knit lines in molded parts. When two flow fronts meet, the particles misalign and create a weakened area. This can dramatically reduce the strength of the part.

Barrier properties are important in some applications, especially in packaging films and microporous membranes. Mineral fillers are impermeable so they can be added to polymers in order to reduce the overall permeability of the plastic to gasses and fluids. However, most fillers are not very effective in this regard. For example, adding 10 wt% calcium carbonate equates to only ~3 vol.%. So only 3% of the permeable plastic has been replaced with impermeable material. Even at 60 wt% filler, only ~20% of the plastic has been replaced. The so-called tortuosity effect is far more significant than simple volume replacement. When platy fillers are used and aligned in the same direction, they can provide a very good barrier because the permeant molecules are forced to travel around the long dimension of each plate. So the material behaves as if it were much thicker. This is illustrated in Fig. 12.

Fig. 12 Reduction in permeability due to tortuosity effects of aligned platy particles

The low permeability of nanocomposites made from platy fillers is attributed to tortuosity. That is important but it has been shown to be only one of two important factors. The other is the interphase which is a layer of adsorbed polymer surrounding each filler particle. The polymer in the interphase has constricted mobility and reduced free volume and is thus less permeable. Fine particles have high surface area, meaning that a significant portion of the whole material is interphase with a concomitant reduction in permeability.

Fillers can also help if they nucleate crystal growth in the polymer and thereby increase the overall degree of crystallinity. Polymer crystals are virtually impermeable, whereas the amorphous phase is permeable; so more crystallinity means better barrier properties.

The effect of fillers on barrier properties depends very much upon whether the filler is wetted by the polymer. When properly wetted, particulate fillers decrease permeability; however, if there is a void around each particle, caused by poor wetting, a pathway for fast diffusion is provided, leading to a decrease in barrier properties. This is perfectly illustrated by the data in Table 3. Permeability actually increased with increasing levels of uncoated filler with poor wetting, but decreased when a surface treatment was applied. Factors that encourage wetting are predrying the filler, longer residence time, venting, and using a surface modifier to improve compatibility at the interphase.

It should be noted that improved barrier properties are not always the target. A huge application for fillers is in breathable films. Stearic acid-treated calcium carbonate is added to polypropylene film. Then the film is stretched to intentionally debond the filler from the surrounding polymer to leave voids around the particles

Table 3 How permeability depends upon filler wetting (Steingiser et al. 1978)

allowing easy passage of water vapor. The stearic acid is used because it decreases the adhesion to the matrix polymer and thereby facilitates debonding. It also helps dispersion; it is vital to avoid large particles or agglomerates when making films.