Coatings World - September 2015

Novel Bio-based Poly(vinyl ether)s for Coating Applications

To date, plant oil-based poly(vinyl ether) homopolymers have been produced using soybean oil (SBO), hydrogenated SBO (HSBO), corn oil (CO), and palm oil (PO) as the parent plant oil. As expected, the thermal properties of these novel polymers varied as a function of the oil or fatty methyl ester used to produce the monomer. For example, polymers based on SBO and CO were amorphous liquids at room temperature, while the polymer based on HSBO was a waxy solid. The solid nature of the latter can be attributed to the high chain packing ef;ciency of the saturated fatty acid ester pendent chains.

While the polymers based on SBO and CO were liquids at room
temperature, side chain crystallization was observed using dif-
ferential scanning calorimetry (DSC). For example, a weak,
broad endotherm with a peak maximum at - 25 °C was observed
for the SBO-based polymer. This polymer also displayed a Tg
at - 92 °C. 9, 13 Compared to the parent oil, i.e., SBO, the heat
of fusion for the SBO-based polymer was much lower indicat-
ing that the higher viscosity and polymeric nature of the latter
signi;cantly inhibited fatty acid ester chain crystallization. The
polymer based on PO showed a melting temperature just below
room temperature, which is consistent with the relative content
of saturated fatty acid ester chains compared to the polymers
based on SBO and HSBO. 14 The content of saturated fatty acid
esters in PO is intermediate between that of SBO and HSBO.

As a result of the polymeric nature of the plant oil-based polymers, the number of bis-allylic protons, allylic protons, and double bonds per molecule is much higher than that of the parent oil. As a result, the degree of autoxidation needed to produce a crosslinked network is signi;cantly reduced compared to the parent oil. In fact, it was demonstrated that a coating produced by simply blending titanium dioxide with an SBO-based polymer, i.e., poly(2-VOES), became tack-free in less than one tenth the time required for an analogous coating based on linseed oil to become tack-free. 15 Thus, by converting a semi-drying oil, such as SBO, to a polymer, drying properties can be achieved that are far superior to that of a drying oil. This feature of the plant oil-based polymer technology also translates to other curing chemistries. For example, the double bonds in poly(2-VOES) were converted to epoxide groups using perace-tic acid and crosslinked networks produced using an anhydride curing agent. To illustrate the relative difference in the time required to reach the gel-point for this network as compared to an analogous network based on epoxidized SBO (ESBO), rheologi-cal measurements were made as a function of time at 100 °C. For the epoxidized poly(2-VOES)/anhydride mixture, viscosity began to rise after just 20 min, while 2 h was required for the ESBO/anhydride mixture. 13

A similar trend was also observed for a comparison between polyurethane networks based on a polyol derived from poly[(2-vinyloxy)ethyl palmitate] [poly(2-VOEP)] to analogous networks based on a polyol derived from PO. Hydroxy groups were incorporated into poly(2-VOEP) and PO by ;rst epoxidiz-ing the double bonds in the materials and then ring-opening the epoxide groups with methanol. 14 In addition to providing a lower degree of functional group conversion to produce a crosslinked network, the molecular architecture of a plant oil-based polymer enables a signi;cantly higher crosslink density compared to the triglyceride analog. This feature can be attributed to the methine carbon atoms present in the polymer backbone that function as additional crosslinks in the network when the material is cured. 13 Further, cure shrinkage for crosslinked networks derived from a plant oil-based polymer would be expected to be lower than that for an analogous network based on the parent triglyceride simply because of the molecular weight difference between the two materials.