Kinetic and thermodynamic investigation of vegetable oil derived macromonomers for use in low-VOC architectural coatings
The incorporation of vegetable oil derived macromonomers (VOMMs) into emulsion polymers has been proposed for use in low-VOC coatings applications. VOMMs are effective plasticizers that facilitate low temperature coalescence and via an auto-oxidation mechanism, impart mechanical properties such as chemical and block resistance. VOMMs have been incorporated into emulsion polymers and formulated into low-VOC, interior coatings. With the success of these polymers there is a desire to expand this technology into other areas such as semi-gloss and light industrial applications. To achieve this, the crosslink density of the polymers must be improved. This can be accomplished by using VOMMs derived from higher unsaturated oils such as soy or linseed oil, however, this poses a problem during the synthesis of the polymer. The carbon-carbon double bonds are susceptible to chain transfer, causing low conversion and high gel content. This study has determined the relationship between the incidence of the chain transfer and the monomer structure, and has identified key structural characteristics that facilitate the incorporation of high levels of unsaturation while minimizing undesirable side reactions. The kinetics of VOMM polymerizations were studied using photo-DSC, NMR, and reaction calorimetry. It was found that there is a relationship between the proximity of the polymerizable functionality to the oil double bonds and the incidence of chain transfer. When the polymerizing group is positioned away from the backbone of the oil, the number of chain transfer events decreases significantly. The reactivity of the polymerizable functionality also plays an important role in the reaction kinetics. When acrylate groups are used, the reactivity of the active radical is so high that abstraction of allylic protons and polymerization through oil double bonds occurs readily. However when the reactivity of the radical is reduced, i.e., use of a methacrylate radical, the incidence of chain transfer drops. The selectivity of the methacrylate radical lessens the likelihood of undesirable side reactions. The polymerization kinetics of the emulsion polymerization was also studied via reaction calorimetry. It was shown that under certain circumstances, the calorimeter is sensitive to variations in polymer architecture. This finding was exploited during the study of VOMM emulsion kinetics and it was found that by monitoring the heat of reaction, the number of chain transfer events could be inferred. This technique in combination with NMR analysis of the resulting polymers provided a clear understanding of VOMM kinetic behavior during the emulsion polymerization.