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In nature, starch is found in plants as semicrystalline beads with a size range of 2-100 pm in diameter depending on the plant. The extraction and processing of these for use as conventional polymer fillers is covered in ? Chaps. 16, “Fillers from Organic Sources,” and ? 22, “Sustainable and Recycled Particulate Fillers”. Further processing can produce nanosized particles and this is discussed here.
The production of such nanosized particles proceeds through the isolation of the crystalline phase of the beads. This is generally achieved by acid hydrolysis which dissolves the amorphous and poorly crystalline regions and leaves the water- insoluble highly crystalline regions behind. The structure of the extracted crystallites is mainly determined by the botanical origin of the starch, while the extraction conditions mainly affect the size.
Starch nanocrystals derived from various plants have been shown to have a platelet type morphology with lengths from 20 to 200 nm, widths from 10 to 30 nm, and thicknesses from 5 to 10 nm. They are highly hydroxylated, with up to 14% of the hydroxyls being on the surface and they are thus very hydrophilic. If sulfuric acid is used for the hydrolysis, then many of the surface groups are sulfonated. While this sulfonation helps with some properties, it reduces the thermal stability, unless the sulphonate groups are neutralized by reaction with an alkali. The hydrophilic nature of the crystals can be modified by reaction of the hydroxyls and this is widely practiced, especially for improving compatibility with many organic polymers.
The use of starch nanocrystals in polymer composites is currently limited by cost and processing issues. The “green” and sustainable badge is also compromised by the chemical processing needed to extract the crystals, and by the amount of co-product which needs to be utilized. The processing problems are those common to many solution processed fine particles, that is, the need to isolate them in a redispersible form for most polymer uses. Most progress has been made with direct transfer into a polymer dispersion such as an aqueous latex, examples being natural rubber latex or styrene butadiene rubber latex. This has led to some promising developments in the tire industry (see ? Chap. 16, “Fillers from Organic Sources”). Solution casting techniques have also been used with some success in the laboratory, but are not suited to industrial applications. Producing a dry powder suitable for melt processing or adding to bulk rubbers on a commercial scale is far more difficult and still has not been achieved.
More details can be found in the reviews by Miller and Hobbie (2013) and Lin et al. (2011). Starch fillers in general are also discussed in ? Chap. 22, “Sustainable and Recycled Particulate Fillers”.
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