The evaluation of vegetable oil based macromonomers in emulsion polymerization
A novel class of monomers derived from vegetable oil resources has been proposed by the Thames Research Group (TRG) for application as comonomers in emulsion polymerization and the development of environmentally responsible latex coatings. Vegetable oil macromonomers (VOMMs) are not only derived from renewable resources, but the long hydrocarbon fatty acid moieties promote low temperature film formation, reducing the organic cosolvent requirements. Finally, VOMMs possess internal double bonds which permit oxidative crosslinking at ambient conditions. VOMMs derived from castor oil were incorporated in vinyl-acrylic latexes. The surfactant system selection and process parameters were critical to the success of the polymerization. Staging of the monomer addition to separate the vinyl acetate monomer from the VOMM and pre-emulsifying the VOMM monomer mixture helped to achieve adequate conversion and latex stability. High levels of surfactant were required to pre-emulsify the VOMM monomer mixture. The VOMM structure and the fatty acid unsaturation level proved to be key considerations in the development of VOMM technology. Mechanical properties of cured vinyl-acrylic latex films increased with increasing VOMM unsaturation. VOMMs were also successfully polymerized in single-stage all-acrylic latexes. The high surfactant requirement was avoided by preparing the pre-emulsion under high shear to lower the monomer droplet size. The VOMMs stabilized the monomer droplets and promoted initiation and subsequent polymerization of the droplet itself, as indicated by particle size analysis and AFM imaging of the dried latex film. A final series of VOMMs containing urethane moieties was also evaluated. The urethane-based monomers were synthesized with low free oil content and did not require high shear to produce the pre-emulsion. Particle size analysis and AFM phase imaging indicated that large particles were not formed during the polymerization and resulted in homogeneous films. VOMMs with an improved ability to randomly copolymerize in the emulsion media, such as the urethane VOMMs, showed improved ease of handling and property development. VOMMs with higher concentrations of double bonds offered the highest potential for improvements in mechanical strength.