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Oleochemicals and cellulose nanocrystals in solvent-free coatings
- Author / Creator
- Hubmann, Anna M.
Since prehistoric times, unsaturated plant oils, referred to as drying oils, have been used as a natural film forming material as result of oxidation-induced polymerization. Here, we report on the use of oleochemical reactions and photo-chemistry to develop three coating systems with low volatile organic compound (VOC) emissions and up to 100% biobased content targeting improved drying time and durability.
The ozonolysis of canola oil and linseed fatty acid methyl esters (FAME) produced compounds that were shown to be effective in directly adding to conjugated carbon-carbon double bonds resulting in faster polymerization. The drying time reduction effect was found to be strongly dependent on the concentration of ozonized lipids but not greatly influenced by the ozonolysis time or the lipid source. When used as a dual-use drying agent and reactive diluent in tung oil, ozonized linseed FAME demonstrated up to a 90% reduction in curing time and viscosity, which allows the replacement of up to 65% of the tung oil with a lower cost, locally sourced ozonized linseed oil. The addition of 5% to 65% of ozonized linseed FAME reduced the glass transition temperature (Tg) of the dried films by 2°C to 49°C, respectively. However, the Tg of dried films with 25% of ozonized linseed FAME in tung oil reached that of a pure linseed oil film, suggesting a similar degree of cross-linking but with the benefit of faster polymerization and lower viscosity than either of the two pure oils. At the same time, a 100% biorenewable content and air-induced polymerization could be maintained.
While offering a biobased solution to significantly reducing the drying time of tung oil to 1h at 60°C, photo-induced polymerization is still a faster alternative. A drying oil analogue was developed from epoxidized linseed lipids with curing times of about 5min under ultraviolet (UV) or solar irradiation, when photosensitized with curcumin. However, oxygen inhibition of this plant-based photosensitizer was found to limit cross-linking on wood and in thin films unless a tertiary amine was present to bind oxygen. Owing to the formation of ether cross-links at high conversion rates of the epoxy groups (>80%) during photoinduced polymerization, hydrolytic (abiotic and enzymatic) degradation compared to air-dried linseed oil was reduced by 20% whilst maintaining a >93% biorenewable content.
As a result of near-instantaneous polymerization in UV-curable coatings, polymerization stresses can build up and significantly reduce adhesive strength. Using a solvent-exchange approach, unmodified CNC could be kinetically stabilized in a photo-curable urethane-based prepolymer. The hydrogen bonding interactions between the coating components and CNC likely reduced internal stresses during UV-polymerization. At a loading level of only 0.3%, this increased the adhesive strength by more than 150% without sacrificing mechanical properties. The formation of a structured fluid also imparted thixotropic flow behavior that could prevent sagging of the resin prior to UV-curing.
In summary, this thesis demonstrates three new pathways to use plant biomass for performance improvement of solvent-free coatings with regards to drying time, viscosity, hydrolytic stability, and adhesive strength.
- Graduation date
- Spring 2021
- Type of Item
- Doctor of Philosophy
- This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.