Introduction to Miniemulsion Polymerisation

Emulsion polymerisation is a commonly used application in the paint and coatings industry and can be used both on a lab and industrial scale. Unlike suspension polymerisation, where the monomer is insoluble in the polymerisation medium, and the initiator is soluble in the monomer, emulsion polymerisation utilises an initiator which is soluble in the polymerisation medium and not in the monomer. Due to this, initial polymerisation takes place in the aqueous phase. Generally this process utilises water, a surfactant, a water-insoluble monomer, for example styrene, and finally a water-soluble initiator.159,160 An emulsion is then formed by stirring vigorously, followed by heating to produce free radicals, allowing free-radical polymerisation to commence with the help of the initiator. A “latex” is formed which is made up of many polymer particles, each one containing a number of polymer chains. The term latex initially referred to the 'milky' sap of certain plants including rubber trees, the sap of which is a colloidal suspension of rubber particles stabilised by proteins. The term also came to describe polymer colloids, either as 'synthetic latexes' or 'latexes'. These particles are usually between 40 - 800 nm in diameter and are dispersed in the continuous aqueous phase. In order to keep the particles stable

74 and to stop them from coalescing a surfactant is added which keeps them continually dispersed. The surfactant lowers the surface energy between the continuous and dispersed phases.

We chose to investigate miniemulsion polymerisation, as opposed to standard emulsion polymerisation, to incorporate our renewable monomers into polymer latexes. The main differences between the two techniques concern particle nucleation and the transport of the monomer within the system.161 With emulsion polymerisation the emulsion is made up of large monomer droplets, “monomer reservoirs” (>1 μm) and free or micellar surfactant.

Initiation takes place in the aqueous phase and polymerisation occurs, gradually the new polymer is surrounded by dissolved monomer and surfactant, or absorbed by already present monomer micelles.162,163 As the polymerization continues the monomer migrates through the aqueous phase from the “monomer reservoirs” to the site of polymerisation. With miniemulsion however, the monomer is already dispersed into stabilized monomer droplets of between 50-500 nm by pre- emulsification. This is formed by applying high shear to the system, this can be achieved via ultrasound or a high pressure homogeniser. In a good miniemulsion latex the particle size before and after polymerisation should be very similar, unlike with emulsion polymerisation where the particles will grow over the course of the reaction. We chose to investigate miniemulsion polymerisation over standard emulsion due to the highly hydrophobic nature of the intended monomers. Given that these monomers would be derived from vegetable oils, their hydrophobicity would cause slow migration from the monomer reservoir through the aqueous phase to the micelles. With miniemulsion you overcome this problem by having the monomers already dispersed as small droplets, so migration is unnecessary, (Figure 3.1).136

75 Figure 3.1: Schematic of miniemulsion

This approach of forming latexes is useful for our purposes because on drying, the latexes can form cohesive films. The film formation process starts with the latex particles suspended in water; this can be cast onto a flat surface. The polymer particles are able to move closer together as the water evaporates with the particles tending to pack in a hexagonal or a face-centred cubic fashion, (Figure 3.2). However in dispersions containing different particle sizes order is often not achieved, and high ionic strength can also cause disorder when packing, unless a high concentration of polymer is achieved. Gradually more water evaporates until a void free structure can be realised.

76 If the polymer has a low enough Tg the particles are then able to coalesce with interdifusion of the polymer chains allowing for a strong uniform film. Polymers with higher Tgs can be used with the aid of a coalescing agent, however these are generally solvents and thus are VOCs, which would be counterproductive in this instance.

Recent work by Thames and co-workers, has looked into the incorporation of vegetable oil based monomers (VOMMs) into waterborne systems via miniemulsion polymerisation.136 They were looking for ways to successfully incorporate the advantages of oil-modified polyesters with waterborne systems, and to reduce the VOCs in these waterborne coatings. Using conventional emulsion polymerisation would be challenging due to the highly hydrophobic nature of the VOMMs, meaning diffusion through the aqueous phase would be slow. Instead miniemulsion was studied, initially with triglyceride derivatives (82) and (83) derived from soybean, (Figure 3.3).

Figure 3.3 Idealised structures of 2 soybean oil based macromonomer used in miniemulsion studies164

77 Their studies concluded that the VOMM (82) and (83) could be successfully incorporated into the polymer backbone of latexes at 35 wt%. It was postulated that unsaturation in the VOMM backbone was preserved during polymerisation and that these intact double bonds could potentially undergo oxidative cross-linking during the curing process.

During the course of our studies Thames et al. published the use of glycerol free amide functionalised vegetable oil derivatives (58/59) and (60d) in latexes.137 These monomers are closely related in structure to ours (60a-c) described in chapter 2. These monomers were derived from the reaction of vegetable oils such as soybean oil with ethanolamine (62d), substituted ethanolamines (62e) and diamines (60a-c), followed by the addition of methacrylic acid or methacryloyl chloride to give polymerisable functionality, (Figure 3.4). Consequently, in this chapter we investigate the use of related solid diamide derivatives (60a-c) in latex formation and compare them to those liquid biomonomers described by Thames (58/59) and

(60d). It would be difficult to compare the results from our biomonomers (60a-c)

with the Thames group monomers (58/59) and (60d) directly as they only described one latex example with little data given on its properties. Hence we chose to prepare the Thames monomers (58/59) and (60d) and prepare latexes from them ourselves.

78 Figure 3.4: Idealised structure of glycerol ester free VOMMs

They also described a second approach using urethane linked molecules (85), (Figure 3.6). In this approach hydroxyethylmethacrylate (HEMA) was initially reacted with isophorone diisocyanate in hexane, with phenothiazine, methyl hydroquinone and dibutyl tin dilaurate (DBTDL) as additives, to give urethane (84), (Figure, 3.5).

Figure 3.5: Reaction of isophorone diisocyanate with HEMA

The macromonomers (85) were finally synthesised by reacting the hydroxyl functional fatty amide (60d) with the novel isocyanate (84) to afford a urethane fatty amide (85), (Figure 3.6).

79 Figure 3.6: Synthesis of urethane fatty amide monomer

Both of these types of VOMMs (58/59), (60d) and (85) were then incorporated into polymer latexes via miniemulsion polymerisation.