Currently, extensive research in using bio‐derived polymers is being done, highlighting the
importance of sustainable, green polymeric materials. Some sustainable alternatives to
synthetic polymers include lignin, starch, cellulose or blends of these with petroleum‐based
polymers.
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, the
processability of most proteins is very difficult, but can be improved by blending with synthetic
polymers, such as polyolefins. To improve, the compatibility between these substances, a
functional monomer could be grafted onto the polyolefin chain. Using an appropriate functional
group, the polyolefin could then react with certain amino acids residues in the protein. Lysine
and cystein are the two most appropriate amino acid residues because of their reactivity and
stability 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 of
reaction parameters on grafting. These were: residence time, temperature, initial monomer
concentration as well as peroxide concentration and type. Grafting was characterized in terms
of 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, monomer
and decomposition temperature of the initiator. It was found that above 160 C polymer
degradation occurred, evident from sample discolouration. A higher degree of grafting can be
achieved by increasing the initial monomer concentration up to a limiting concentration. The
highest 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 in
DOG was achieved. It was found that DCP is much more effective at grafting, compared to DTBP
because DTBP is more prone to lead to side reactions than DCP.
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It was found that a residence time of 168 seconds resulted in the highest DOG, corresponding to
4 extrusions in series. However, it was also found that an increase in residence time resulted in
an increase in polymer degradation. The tensile strength of PE decreased after two extrusions
when using DTBP, and three extrusions, when using DCP. Young's modulus decreased only
slightly, while all samples showed a dramatic decrease in ductility, even after one extrusion. It
was concluded that degradation had a more pronounced effect on mechanical properties than
cross‐linking, and residence time should therefore not exceed three extrusions in series, which
corresponded to about 126 seconds.
It can be concluded that a high reaction temperature and high initiator concentration lead to a
low degree of grafting, accompanied by high cross‐linking and increased degradation. On the
other hand, high monomer concentration and high residence time lead to a high degree of
grafting.
Optimising grafting is therefore a trade off between maximal DOG and minimising side reactions
such as cross‐linking and degradation and optimal conditions do not necessarily correspond to a
maximum DOG. Other factors, such as the use of additives to prevent degradation should also
be investigated and may lead to different optimum conditions.
