Abstract
Alkyd resins are versatile polymers which have applications in inks and various coatings like decorative paints. They are mainly composed of fatty acids, polyols and aromatic diacids. In this work, glutamic acid as well as N-acylated and N-alkylated derivatives there of were evaluated as bio-based substitutes for these aromatic diacid monomers in the synthesis of alkyd resins. The resins were characterised in terms of structure, molecular weight, viscosity, oxidative thermal stability and colour. N-Palmitoylglutamic acid dimethyl ester can be successfully incorporated when the polycondensation is performed in two steps. In this approach, the bio-based diacid monomer is only supplied in the second step, because the removal of water in the first step is essential to avoid hydrolysis of the monomer amide bond and the subsequent formation of pyroglutamate groups. The molecular weight, viscosity and oxidative thermal stability are lower than for conventional alkyd resins. The mechanism of the discolouration of alkyd resins during polymerisation is mediated by free radical species, which were generated easily in the presence of free amino groups and/or unsaturated fatty acids. Light-coloured resins could be obtained by using saturated fatty acids or radical scavengers during polymerisation.
Highlights
Alkyd resins nowadays have major applications as binders in the manufacture of paints and coatings
Dark-coloured resins can still be applied in printing inks, this phenomenon is especially undesired regarding the potential application of these bio-based alkyd resins in the formulation of white or light-coloured decorative paints
The lactamisation of glutamic acid to pyroglutamic acid occurs as a competing side reaction at high temperature and should be avoided, because this compound is incorporated as a chain stopper, which is unfavourable in terms of molecular weight
Summary
Alkyd resins nowadays have major applications as binders in the manufacture of paints and coatings These polymers consist of a polyester backbone prepared from polyols and acid anhydrides or diacids such as maleic anhydride, phthalic anhydride and isophthalic acid.[1] The residual backbone hydroxyl groups are esteri ed by long-chain unsaturated fatty acids. Glutamic acid can be proposed as another bio-based diacid to replace conventional aromatic diacids in polyester synthesis This non-essential amino acid is abundantly available on a million ton scale via fermentation.[9,10] as glutamic acid is a major constituent of plant proteins, agro-industrial waste streams can be considered as an additional resource.[11] In this work the feasibility of incorporating glutamic acid as a monomer in alkyd resin synthesis is studied. Oleic acid (1) 6 h at 210 C, (2) 18 h at 6.3 n.d.g a General composition of the synthesis mixture: fatty acid(s) (72 wt%), pentaerythritol (17 wt%), isophthalic acid (5 wt%), diacid or derivative (6 wt%) and mesitylene (3 wt%). b Determined by acid–base titration. c Measured by gel permeation chromatography. d Measured by rheometry at 23 C. e The diacid was added at the onset of polymerisation. f The diacid was added a er 6 h. g n.d. 1⁄4 not determined
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