Currently, extensive research in using bio‐derived polymers is being done, highlighting theimportance of sustainable, green polymeric materials. Some sustainable alternatives tosynthetic polymers include lignin, starch, cellulose or blends of these with petroleum‐basedpolymers.In New Zealand, large quantities of animal derived proteins are available at very low cost,making it ideal as a sustainable alternative to petroleum‐derived polymers. However, theprocessability of most proteins is very difficult, but can be improved by blending with syntheticpolymers, such as polyolefins. To improve, the compatibility between these substances, afunctional monomer could be grafted onto the polyolefin chain. Using an appropriate functionalgroup, the polyolefin could then react with certain amino acids residues in the protein. Lysineand cystein are the two most appropriate amino acid residues because of their reactivity andstability at a wide pH range.In this study, free radical grafting of itaconic anhydride (IA) onto polyethylene was investigated.IA was selected because it is capable of reacting with polyethylene and amino acid residues,such as lysine. The objective of the research was to identify and investigate the effect ofreaction parameters on grafting. These were: residence time, temperature, initial monomerconcentration as well as peroxide concentration and type. Grafting was characterized in termsof the degree of grafting (DOG), percentage reacted and the extent of side reactions.The reaction temperature was taken above the melting point of the polyethylene, monomerand decomposition temperature of the initiator. It was found that above 160 C polymerdegradation occurred, evident from sample discolouration. A higher degree of grafting can beachieved by increasing the initial monomer concentration up to a limiting concentration. Thehighest DOG achieved was about 1.2 mol IA per mol PE, using 2 wt% DCP. When using 2 wt %peroxide, the limiting concentration was found to be 6 wt% IA, above which no improvement inDOG was achieved. It was found that DCP is much more effective at grafting, compared to DTBPbecause DTBP is more prone to lead to side reactions than DCP.ivIt was found that a residence time of 168 seconds resulted in the highest DOG, corresponding to4 extrusions in series. However, it was also found that an increase in residence time resulted inan increase in polymer degradation. The tensile strength of PE decreased after two extrusionswhen using DTBP, and three extrusions, when using DCP. Young's modulus decreased onlyslightly, while all samples showed a dramatic decrease in ductility, even after one extrusion. Itwas concluded that degradation had a more pronounced effect on mechanical properties thancross‐linking, and residence time should therefore not exceed three extrusions in series, whichcorresponded to about 126 seconds.It can be concluded that a high reaction temperature and high initiator concentration lead to alow degree of grafting, accompanied by high cross‐linking and increased degradation. On theother hand, high monomer concentration and high residence time lead to a high degree ofgrafting.Optimising grafting is therefore a trade off between maximal DOG and minimising side reactionssuch as cross‐linking and degradation and optimal conditions do not necessarily correspond to amaximum DOG. Other factors, such as the use of additives to prevent degradation should alsobe investigated and may lead to different optimum conditions.
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