Abstract:
Plant oils are renewable resources that have been used to prepare thermosetting
biopolymers for decades. The incorporation of petroleum-derived aromatic compounds
into the formulations is essential for the final plant oil based thermosets to achieve
necessary mechanical strength for practical use. Tannins are naturally occurring
polyphenolic compounds possessing rigid aromatic structures, and have considerable
potential to substitute petroleum-derived aromatic compounds. This will produce plant oil
based polymers which will be completely based on renewable resources and may have
engineering applications
In this research, quebracho tannin (QT) and pine tannin (PT) have been used as starting
materials for the preparation of tannin-lipid based polymers. Combinations of condensed
tannins with lipids can be achieved by acylation of tannins with fatty acid chlorides.
Tannin stearates with various degree of substitution (DS) were prepared from both
tannins employing an efficient acylation agent, N-methylimidazole. The average DS
increased with greater mole ratio of reactants (stearyl chloride/tannin), and the higher DS
obtained with PT was due to its greater hydroxyl content. FTIR and NMR have been used
to chemically characterise products, confirm purity and determine DS, while thermal
behaviour was analysed by thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC). The partially substituted tannin stearates were converted to fully
substituted tannin mixed esters (stearate/acetates) by acetic anhydride treatment. Tannin
stearates and their corresponding mixed esters showed interesting DSC behaviour with
multiple endothermic features observed in the first heating cycle, which merged into one
broad feature with an intermediate peak temperature in the second heating cycle. While
differing stearate DS was the likely cause of this change, and the various conformations
of the starting tannins was also a possible cause. TGA results indicated that the thermal
stability of tannin esters increased with greater DS.
Fully substituted tannin mixed esters of unsaturated fatty acids, tannin oleate/acetates and
tannin Iinoleate/acetates were synthesised. The polymerisation of resulting tannin
linoleate/acetates was investigated by oxidation polymerisation, thermal treatment and
free radical polymerisation. Polymer films ranging from soft rubbers to rigid thermosets
were prepared from tannin linoleate/acetates with linseed oil and tung oil by oxidation
polymerisation and thermal treatment. However, the oxidation polymerisation of tannin
oleate/acetates with linseed stand oil and tung oil resulted in soft rubbers. The films
obtained were characterised by dynamic mechanical thermal analysis (DMTA),
differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and scanning
electron microscopy (SEM). Tannin linoleate/acetate polymers prepared by oxidation
polymerisation had a single Tg ranging from 32 to 72 0C, ambient storage modulus
between 0.12-1.6 GPa and crosslink densities between 451 and 47000 mol∕m3, which
were comparable to some conventional thermosetting polymers. The Tg, crosslink
density and storage modulus increased with greater tannin linoeate/acetate content due
mainly to the contribution of tannin linoleate/acetate to crosslinking. In contrast, the Tg
of tannin oleate/acetate polymer films decreased when the tannin ester content increased,
resulting from the relatively Iow reactivity of oleate. These polymer flms showed a three
stage degradation process with a major thermal decomposition observed at 250-500oC.
The thermal stability of these films increased with greater tannin ester content.