Home Education Fillers for Polymer Applications
The main attraction of nanosized particles is because it has been found that many desirable composite properties improve significantly as filler particle size decreases. This offers two opportunities: better absolute properties, or the same properties at much lower filler levels, and so a lighter product (as fillers generally have a higher specific gravity than polymers). Many proposed applications are in the 5-10% of additive range, which means that the market size for such fillers is going to be much smaller in weight terms than for conventional ones; as a result it is not completely fair to judge their success on this basis.
A number of issues are limiting the commercial success of the recently introduced nanofillers. The first is that reducing size nearly always costs money, especially for nano sizes. Secondly, dispersing particles into polymers and processing the mix becomes more difficult as size reduces and can further add significantly to the cost. Thirdly, surface properties of the filler become more and more significant as size reduces, and this can have unexpected effects on issues such as stabilizer deactivation, as well as adversely affecting the polymer structure (discussed later). The high amount of surface can also significantly increase the amount of expensive coupling agent where this is needed. Next, the maximum addition level attainable decreases with particle size and this can prevent properties above that of conventional composites from being achieved. Finally, very small size can lead to handling issues, and most importantly of all to potential health issues. A discussion of the health issue can be found in NIOSH (2009).
Of all the above, cost is probably the most important factor limiting nanofiller penetration. Thus, nanoclays, one of the least expensive of the new generation of nanofillers, are 10-20 times the cost of good talcs and carbonates and 5-10 times that of other precipitated fillers, such as silica and calcium carbonate. The addition of just 5% of a nanoclay to a commodity polyolefin can thus be expected to increase raw material costs by at least 40%.
Handling and health issues must also not be overlooked and are responsible for much of the difficulty in commercializing some of the technologies. If nanoparticles are generated in the free state, they are extremely light and dusty. Fumed silicas are an excellent example; the bulk density of typical products is in the range 0.05-0.1 which creates very real handling problems. Much of the work on developing nanofillers suitable for general use is thus aimed at producing forms which only release the nanoparticles during composite production, but this is far from easy to do.
Another important consideration that is frequently overlooked is the effect of the filler surface itself on polymer structure and properties. Polymer in the vicinity of a solid surface becomes altered, due to a decrease in the degrees of freedom (see for example, Napper 1983). This modified polymer is usually denser, stiffer, and with a higher Tg than the bulk, but also with less elongation. In thermoplastics, it is also usually more brittle. The thickness of such layers varies with the polymer but is usually regarded as being between 1 and 10 nm. This effect is most noticeable in thermoplastics and elastomers and more detail on the former can be found in ? Chap. 3, “Particulate Fillers in Thermoplastics”. As shown in Tables 1 and 2, the amount of such modified polymer starts to become significant below 1 pm particle size and is dominant once the nanometer range is reached. This modified polymer can be expected to contribute to the effects produced by nanosized fillers. It is probably the cause of many of the effects produced by nanoparticles, but should not be ignored as a contributing factor for nanoplates and fibers as well.
Table 1 The effect of particle size on the amount of immobilized polymer as percentage by weight of the filler (cubic particles)
Table 2 Immobilized polymer produced by a filler as a function of size and addition level (assumes filler specific gravity = 2)
Table 3 Established particulate nanofillers
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