Study of room temperature ionic liquid on miniemulsions polymerization for application in low-emitting waterborne coating products
May 10-15, 2015
As legislations about restrictions on volatile organic compounds (VOCs) become more stringent, there has been a trend moving away from solvent-borne coating towards waterborne coating. For waterborne coating, water is used as solvent for the dispersion of water-reducible resins or latex resins, pigment, filler, and other additives. Though the substitution of organic solvent for water reduces the main VOCs emissions caused by the solvent-borne coating, VOCs derived from organic additives, especially coalescing agent, in waterborne coatings have been a major VOCs problem. For example, Texanol is a coalescing agent wildly used in waterborne coating to reduce the glass transition temperature (Tg), thus enhances film formation at a lower temperature. However，Texanol will be evaporated as VOCs, after film formation. To further reduce the VOCs of waterborne coating, for the first time, the study of replacing conventional coalescing agent by room temperature ionic liquids (RTILs) has been carried out. RTILs with low melting point have been proven to be able to reduce Tg of polymer efficiently, thus they have the same function of coalescing agent. RTILs have negligible vapor pressure, thus the VOCs emitting can be prohibited. In addition, RTILs may play other roles such as enhancing the ionic conductivity of the film. This conductivity widens the application of RTILs in anti-static coating. RTILs also show effectiveness as anti-biofouling agents, which could be used in marine antifouling coatings. These various functions of RTILs enable them to replace various additives required in the conventional coating product formation process, hence the presence of RTILs would be both environmentally and economically beneficial for the new coating products. Considering the water resistance property of coating film, hydrophobic type of RTILs would be suitable. 1-octyl-3-methyl imidazolium hexafluorophosphate (C8mimPF6) is chosen as the target RTILs considering its low melting point (-82oC), and hydrophobicity (2.26g/L). A new method, miniemulsion polymerization which consists of the formation of miniemulsion process and polymerization process, is introduced to encapsulate hydrophobic RTILS inside the latex product. Our investigations are divided into three folds: (1) Preparing stable miniemulsion containing RTILs; (2) miniemulsion polymerization to form latex; (3) Characterisation of the latex product. For step (1), the effect of the concentration of RTILs on miniemulsions stabilities has been reported in our early work. The major focus in this paper is step (2). Systematic investigations on the effect of various factors including initiator type, temperature, concentration of initiator and RTILs on the stability of miniemulsions droplet, product conversion have been conducted. It has been found that different types of initiator significantly affect the stability of miniemusion and the yield of latex. With hydrophilic type redox initiator (e.g. ascorbic acid and hydrogen peroxide), stable latex containing C8mimPF6 with 99% conversion can be achieved. The presence of C8mimPF6 prohibits the micelles nucleation caused by hydrophilic initiator, resulting in more controllable droplets in the products hence better product quality. The reaction rate may be increased with an increase of initiator concentration. The reaction temperature which is related to the decomposing temperature of the initiator being used is another factor that affects the reaction and stability of latex. For example, the hydrophobic initiator, 2,2’-azobis (2-methylpropionitrile) would require reaction to be 60 oC to initiate the polymerisation. However, such high temperature often cause phase conversion. In the presence of C8mimPF6, the reaction temperature can be significantly reduced to as low as 40 oC. The produced latex under this condition exhibited much lower Tg compared to the one without C8mimPF6. The SEM image of the latex product showed that the deformation of latexes has been enhanced. We have established that both lower Tg and deformation of latex are beneficial for film formation, which is essential in the coating process
Binjie Hu, "Study of room temperature ionic liquid on miniemulsions polymerization for application in low-emitting waterborne coating products" in "Polymer Reaction Engineering IX", E. Vivaldo-Lima, UNAM; J. Debling, BASF; F. Zaldo-Garcia, CP-COMEX; J. Tsavalas, Univ. of New Hampshire Eds, ECI Symposium Series, (2015). http://dc.engconfintl.org/polymer_rx_eng_IX/43
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