Degradation and Crystallisation Behaviours of Reprocessing PP Waste
Reprocessing recycled PP is always accompanied with degradation, crystallisation, and consequent processability problems, which result from molecular chain scission, branching and crosslinking. The degradation behaviour decreases tensile strength and impact strength, while the crystallisation behaviour increases Young’s modulus and yield stress. Therefore, ultimate mechanical properties of the crys- tallisable plastics are determined by both degradation and crystallisation behaviours. In the recycling processes, the degradation behaviour is inevitable, but if the crystallisation behaviour can be taken full advantage of, the recycled plastic products still can maintain high quality and perform well (Andricic et al. 2008). Therefore, a good understanding of the degradation, crystallisation and processability is of scientific and technological importance for recycling PP wastes.
Since PP is an organic polymer, undesirable chemical reactions, mainly caused by photo-oxidation and oxidation, frequently occur during their manufacture processes and service life (da Costa et al. 2007). The chemical reactions result in irreversible changes in the polymer structure, thus affecting the polymer performance. There are four types of distinguished degradation: chemical, thermal, mechanical and biological (Al-Salem et al. 2009). These degradation processes are very complex, and usually several types of degradation occur simultaneously. The phenomena, such as discoloration, loss of volatile components or smoking, and loss of mechanical properties, are frequently observed (Bahlouli et al. 2006).
During the processing stage, PP is subjected to molecular chain damage, including crosslinking, chain scission and formation of double bonds. High shear forces, temperatures and presence of impurities and oxygen sever the polymer chains, producing highly reactive radicals at the end of chains. These radicals cannot recombine, but they can form peroxyl radicals and hydroperoxides with oxygen. Continuous degradation leads to serious molecular chain scission, branching and crosslinking, which may significantly change mechanical properties and processability of the recycled materials.
da Costa et al. (2005) studied PP degradation when it was submitted to multiple extrusion conditions. They found that under the condition of lower temperature (240 °C) and lower extrusion cycles (five cycles), the PP still remained with several entanglement points, and the mechanical chain breaking did not reach a level where extensive degradation occurred. When the PP was under the conditions of higher die zone temperature (270 °C) and higher processing cycles (nineteen cycles), chain scission massively happened, and the material behaved as a liquid-like material of low viscosity, which resulted from the considerable reduction of molar mass, long chains and entanglements. Their further research (da Costa et al. 2007) found that the degradation process considerably reduced the break properties of PP, such as break strain, break stress and energy to break, while the small strain properties, like yield stress and yield modulus, were just slightly affected.
In many operations, the recycled PP is pelletised first in plastic reprocessing plants. The extruded PP pellets are then delivered to plastic manufacturing plants for the production of end products. The double heating and reprocessing in the plastic reprocessing and manufacture plants cause more serious degradation. If this could be undertaken as only one heating cycle, the deterioration would be minimised. Residence time during extrusion or other heating processes would therefore be lowered and the plastics would be subjected to less overall heating. Cold processes, such as shredding and crumbing, are recommended to prevent the degradation from the hot processes, such as pelletisation.