Properties and behaviour of starch and rapeseed cake based composites in horticultural applications

Abstract

INTRODUCTION Plastic plant pots, containers, and trays are widely used in industrial greenhouses and in private farms. After use they end up on scrapheaps or are dumped in landfills where they degrade very slowly. In the total flow of agricultural plastic waste, which reaches ca 400 000 tonnes per year, the plastic pots and trays constitute ca 16 000 tonnes (www.greenfacts.org...). Biodegradable items represent a good alternative to plastic articles (Evans and Karcher, 2004; Nechita et al., 2010). Among biodegradable goods, disposable plant pots made from peat or from a mixture of peat and wood fibre are most widely used. They can be either embedded into soil together with plants or digested. However, the peat plant pots have some drawbacks. They are mechanically unstable and have a high permeability to water vapour. The soil often dries out in such pots and the nutrient matter crystallizes in the walls of the pots. The salts concentrated in the walls can later harm the plants. The use of peat compost and other peat products leads to much larger discharge of carbon dioxide than that observed during the cultivation of soil (Maljanen et al., 2010; Verhoeven and Setter, 2010). For these reasons gardeners are not encouraged to use peat products. Moreover, peat digging is an environmentally problematic process. It is usually accompanied by the damage of vegetation and destruction of the fauna habitat. The plant pots prepared from coconut fibre or from bird feathers are mechanically stable and retain moisture well (Evans et al., 2010). However, they can not be embedded into soil together with plants and can only be digested. In addition, the plant pots prepared from coconut fibre or from feathers are rather expensive. Therefore they can only be used when the exploitation time is relatively long. For short-time usages, such as cultivation of plants before planting into soil or into larger plant pots, cheaper biodegradable materials are required. Plastic plant pots are light, cheap, and durable. Their walls are relatively impermeable. Plants cultivated in such pots require less watering; salts do not concentrate in their walls. However, the recycling of the waste plastic plant pots is still an unsolved problem. Therefore development of cheap biodegradable plastic plant pots with properties comparable to those of non-biodegradable plastic pots remains an urgent task. Composites containing components of natural orgin are usually hygroscopic, and their films are permeable to water vapour. Their mechanical properties are dependent on the ambient moisture content. They can change with the migration of the plasticizer from the composites (Orliac et al., 2003; Ma and Yu, 2004). The water vapour permeability rate of the plant pots prepared from such composites and their ability to absorb and retain moisture can influence the temperature and moisture content of the substrate in the pots. These are important factors strongly affecting plant growth. In addition, entry of the components of the pot composites into the soil can change its agrochemical properties. Properties of starch-based biocomposites depend to a large extent on the nature and extent of the plasticizer (Tudorachi et al., 2000; Bourtoom, 2008). Different plasticizers, such as amino acids, polyols, and compounds containing amido moieties are used for the improvement of their elasticity (Stein and Griene, 1997; Pushpadass et al., 2008; Muscat et al., 2012). However, an increase of the concentration of the plasticizer leads to an increase of the hygroscopicity of the composites and to a decrease of their tensile strength. The waterproofness of the composites can be enhanced by the cross-linking of the polymer, or by adding water-resistant fillers (Gaspar et al., 2005; Kumar and Singh, 2008). Thus, by specific variation of fillers, plasticizers, or their mixtures, composites with a desirable combination of mechanical properties, sorption capacity, and water vapour permeability can be obtained. …