Chemically Modified Vegetable Oils

The effects of cross-linking in pre-modified vegetable oils has also been investigated. Gultekin et al have reported co-polymerisation of acrylate modified castor and linseed oils with styrene initiated by benzoyl peroxide,117 whilst Akbas et al undertook similar research but with methyl methacrylate modified soybean and linseed oils with styrene again using benzoyl peroxide.101

Can et al 118,119 have investigated soybean and castor oil based monomers synthesised via alcoholysis with both aliphatic and aromatic alcohols including pentaerythritol and bisphenol A propoxylate. These were then further reacted with maleic anhydride to give (39) and (40) respectively, castor oil was also directly maleinized via reaction of its naturally occurring hydroxyl groups with maleic anhydride to give (41), (Figure 1.22).

24 Figure 1.22: Castor oil pentaerythritol glyceride maleates(39), castor oil bisphenol A

propoxylate maleates (40) and castor oil maleates (41).

These maleinized monomers (39-41) were then copolymerised with styrene118 to produce hard rigid thermosetting polymers.119 They gave a range of moduli from 0.8 to 2.5 GPa with Tg ranging from 72 to 152 °C. Results indicated that the castor oil gave better moduli, strength and Tg in comparison to soybean based derivatives.

The Wool group at the University of Delaware have also synthesised maleinated (42)

25 Figure 1.23: Maleinated soybean oil monoglyceride (42) and acrylated soybean oil (43).

The maleinated soybean oil monoglyceride (42) was synthesised in a two-step process. First monoglycerides were prepared via glycerolysis by reacting soybean oil with glycerol at 230 °C in the presence of calcium hydroxide as a catalyst under a nitrogen atmosphere. Maleinization of these monoglycerides was then carried out with maleic anhydride at 80 °C in the presence of triphenyl antimony and hydroquinone. The modified maleates (42) were then reacted with styrene to make copolymers.99 The acrylated monomers (43) were prepared from epoxidised soybean oil via ring-opening with acrylic acid.

Günduz et al also investigated the maleinization of oils for use in resins and coatings. In this study, waterborne polyurethanes were synthesised using maleinized monoglyceride, hydroxyl-terminated polybutadiene, toluene diisocyanate and ethylene diamine.120

26

1.4.3 Alkyds

Alkyds are a family of coatings which have long since incorporated vegetable oil derivatives, usually in the form of fatty acids. These have been used since the late 1920’s, and are one of the oldest polymer coatings incorporating vegetable oils.12,105,121 Alkyd coatings (44) are comprised of polyhydroxy acids and polybasic alcohols and are a type of polyester. In a lot of cases these coatings can be modified with fatty acids, or mono-glycerides, which have been afforded from the alcoholysis of vegetable oils. The most abundant source of these acids is from triglycerides, hence these oils being a raw material for this type of coatings. Alkyds are predominately synthesised via the condensation polymerisation of acids and alcohols producing a highly branched polymer with a polyester backbone, (Figure 1.24).122 The curing of these applied coatings then takes place via cross-linking of the alkyd chains, as seen with the autoxidation process of the drying oils, (see section, 1.4.1.1, Figure 1.20). Another method of curing includes utilising reactions of the residual hydroxyl groups within the polymer; however this is only efficient at high temperature drying conditions e.g. 80-200 °C. Variation of the oils and fatty acids used in these coatings allows for air drying times to be optimised, highly unsaturated chains would be preferable e.g. linolenic acid.

Figure 1.24: A generic Alkyd resin structure made up from a monoglyceride and phthalic anhydride.122

27 Alkyds can be classified based on the percentage of oil used in their formulation, they are defined as ‘long’, ‘medium’ or ‘short’. Alkyds containing above 60% oil are classed as a long oil alkyds, between 40-60% are medium oils, and less than 40% oil is a short oil alkyd.105 Some of the best drying times and water resistance can be obtained using soybean oil along with maleic anhydride.105

Not only has the effect of variation in the oils and fatty acids used in alkyd paints been investigated but Aydin et al123 determined the effects of various anhydrides on alkyd film properties and viscosities, (Figure 1.25). Two different methods of alkyd preparation were used, the conventional method including alcoholysis of the oil and subsequent reaction with phthalic anhydride and a modified method where partial glycerides were synthesised from sunflower oil and glycerol and washed to remove excess glycerol before reacting with a range of anhydrides. Removing the excess glycerol in the modified method seemed to give alkyds with better film properties.123

Figure 1.25: Glutaric anhydride, maleic anhydride, phthalic anhydride and succinic anhydride

Alkyds are a very popular form of coating as a result to their low cost, good film forming properties, high viscosity, good drying and hardness properties and their ability to blend well with other paint additives.105,124 The main problem surrounding the use of this class of coating is the use of high percentages of organic solvents; because of this, research has looked into high-solid content (e.g lowering of the percentage of organic solvents) and water dispersible alkyd resins. In order to be able to reduce the amount of organic solvent in these resins the viscosity of the polymer itself has to be lower, thus a lower weight or smaller weight distribution needs to be

28 attained. Highly branched alkyds have shown promising results with regards to this, lowering the viscosity and keeping the properties indicative of good quality coatings.125 Water soluble alkyds have also been investigated. Aigbodion et al126 investigated the incorporation of the alkyd resin of maleinized rubber seed oil into an alkyd emulsion. Various samples of maleinized rubber seed oil were synthesised with differing amounts of maleic anhydride incorporated.

Alkyd resins were then prepared using the modified rubber seed oil along with phthalic anhydride, maleic anhydride and glycerol. Initial reaction with glycerol was followed by polycondensation of the resulting alcohols with the anhydrides to produce the polymer. Once acquired the alkyd was mixed with the previously maleinized rubber seed oil and water and then stirred vigorously to give an emulsion. Testing indicated that viable films could be cast which were resistant to water and acid.126

Research on alkyd emulsions has also been reported by Wang et al, this time looking at tung oil modified soybean alkyds.127 Again the alkyd was prepared subsequent to emulsification. In this example, soybean and tung oil were reacted with (45), following the alcoholysis, isophthalic acid was added followed by methanol to afford the alkyd resins, (Figure 1.26). The subsequent emulsion was then prepared using N,N-dimethyl ethanolamine, 2-butoxyethanol and the alkyd resin along with deionised water mixing at a high speed for at least 2 hours. If the ratio of tung oil to soybean was increased the droplet size of the final emulsions rose quite substantially with an increase from 172 to 771 nm. The emulsions were tested for stability over time up to a temperature of 50 °C where the droplet sizes of all samples were considerably reduced with a final droplet size of 40 to 56 nm. In general the film

29 forming properties were shown to be good, with the increase in tung oil improving the properties slightly.

Figure 1.26 Synthesis of alkyds from soybean and tung oil