Application of precipitation polymerization in coatings

Precipitation polymerization is similar to solution polymerization, but the solvents they use are soluble to the monomers and insoluble to the resulting polymer. The relative molecular mass at the time of precipitation depends on the solubility of the solvent. Since the monomer is the solvent of the polymer, the polymerization medium (monomer+solvent) has a higher solubility than the solvent. Thus, at the beginning of the reaction, due to the high concentration of the monomer, the solubility of the polymerization medium is also high, so that the resulting polymer can be dissolved therein as if it were a solution polymerization. As the reaction proceeds, the monomer is gradually consumed, and the solubility of the polymerization medium is also reduced until the chain grows to a certain critical point and is insoluble and precipitates. At this point, there is still a surplus of monomer, and these remaining monomers are taken up by precipitation and undergo chain growth in the precipitate until the monomer is depleted. This stage is like emulsion polymerization.

The polymer produced by the solution polymerization has a relatively low molecular weight and the emulsion polymerization produces a relatively high molecular weight. Therefore, adjusting the length of these two stages, that is, adjusting the solubility of the solvent and the monomer agent ratio, can control the relative molecular mass of the resulting polymer, which can be used as a "maeromonomer" for resin synthesis in paints. ". Paine studied this, he used monomer/solvent = 15/85, hydroxypropyl fiber as protective colloid, with 0. Benzoyl peroxide at a concentration of 0131 mol / L was used as an initiator at 71. At 5 °C, styrene was precipitated and polymerized with different ratios of ethylene glycol methyl ether and ethanol as solvents. The relationship between the solubility and the relative molecular mass was shown in Figure 1. Explains the possibility of controlling the relative molecular mass.

Figure 1 Relationship between solubility and relative molecular mass

Diakoumakos et al. changed the batch polymerization method to precipitation. For example they will be methyl methacrylate / styrene / ethyl acrylate / hydroxyethyl methacrylate = 15 / 15 / 40 / 30 mixed monomer 60. 9 g, mercaptoethanol 3. The mixture of 1 g and 15 g of petroleum solvent was added dropwise to 15 g of petroleum solvent (bp 175 °C) at 150 °C for 1 h, giving Mn = 1 349 and Mw / Mn = 1. 87 hydroxy macromers.

Preparation of macromonomer by precipitation polymerization is more advantageous than solution polymerization. First, the relative molecular mass is controllable, and the second is that the product has no solvent, and the latter has more freedom to select solvents for subsequent coating formulation or resin synthesis. When the polymer precipitates, due to its large surface area, it will coalesce with each other, often adding a protective colloid in the formulation, and the protective colloid will be adsorbed on it to prevent or reduce the degree of coalescence, thus also making the particle size of the precipitated particles. The distribution is narrow. This, in turn, can be used for the precipitation polymerization of microgel thickeners and matting agents in coatings.

When a polyvinyl monomer is added to the monomer composition, the resulting crosslinked polymer will be obtained. Those highly cross-linked polymers are insoluble in solvents, light weight, fine particles, and tend to float in the wet film. As a matting agent, the effect is good, and 60° and 85° gloss are very close. Polymers that are lightly cross-linked and contain enough hydrophilic groups are insoluble in solvents (water) but swell, and can act as thickeners. They are suspended in the coating by fine, swollen particles that can deform under shear stress. It also has thixotropy. Those containing polyglycols as aqueous thickeners are insensitive to pH and salinity as well as polyvalent ions. With reactive functional groups, it can also enter when the film is cross-linked, so that the film has a better integrity.