Nonaqueous-dispersion systems

The development of CRP in nonaqueous dispersed systems was envisioned with the aim of controlling simultaneously the polymer chain characteristics along with the colloidal properties of the so-foimed polymer particles. However, in comparison with CRP in aqueous dispersed systems, the nonaqueous systems were much less smdied, although in the past years the number of articles is in constant progression. Two main types of systems were considered the classical organic solvents and SCCO2. 

Nonaqueous dispersed systems (i.e., organic solvents, SCCO2, and ionic liquids) can be applied to a much broader range of... 

Phenolic Dispersions. These systems are predominantly resin-in-water systems in which the resin exists as discrete particles. Particle size ranges from 0.1 to 2 p.m for stable dispersions and up to 100 p.m for dispersions requiring constant agitation. Some of the earliest nonaqueous dispersions were developed for coatings appHcations. These systems consist of an oil-modified phenoHc resin complexed with a metal oxide and a weak solvent. 

Polymerizations that are carried out in nonaqueous continuous phases instead of water are termed dispersion polymerizations regardless of whether the product consists of filterable particles or of a nonaqueous colloidal system. 

Recently, hot melt PSA systems have been introduced and radiation curable PSA systems are at the commercial development stage. High solids (50%-70% by wt.) nonaqueous dispersion acrylic PSA systems have also been reported(1). Unlike the hot melt and radiation cured systems which require new capital outlay in coating head and/or curing (drying) equipment, BFG has developed PSA systems, based on Hycar 2100R reactive acrylic liquid polymers and isocyanate terminated prepolymer, which can be processed at 80% solids (by wt.) with equipment presently used in the PSA industry, namely, the reverse roll and knife-over-roll coater. 

An excellent article by Bernhardt [69] tabulates dispersion systems for hundreds of ceramic powders. These dispersion systems consist of a solvent and surfactant with a range of useful concentrations listed. The solvents are both aqueous and nonaqueous and the surfactants are ionic, nonionic, and ionic polymers. This is the most extensive table of established dispersion systems available in the literature today. 

Surfactants are employed in emulsion polymerizations to facilitate emulsification and impart electrostatic and steric stabilization to the polymer particles. Sicric stabilization was described earlier in connection with nonaqueous dispersion polymerization the same mechanism applies in aqueous emulsion systems. Electrostatic stabilizers are usually anionic surfactants, i.e., salts of organic acids, which provide colloidal stability by electrostatic repulsion of charges on the particle surfaces and their associated double layers. (Cationic surfactants are not commonly used in emulsion polymerizations.)... 

The description of a colloid should include particle size, mobility, charge and their distributions, charge/mass ratio, electrical conductivity of the media, concentration and mobility of ionic species, the extent of a double layer, particle-particle and particle-substrate interaction forces and complete interfacial analysis. The application of classical characterization methods to nonaqueous colloids is limited and, for this reason, the techniques best suited to these systems will be reviewed. Characteristic results obtained with nonaqueous dispersions will be summarized. Physical aspects, such as space charge effects and electrohydrodynamics, will receive special attention while the relationships between chemical and physical properties will not be addressed. An application of nonaqueous colloids, the electrophoretic development of latent images, will also be discussed. 

Shear-Sensitive Systems. In addition to hydrodynamic effects and simple viscous behavior, the act of pigmentation creates a certain amount of complex behavior (13). If the particles are fine. Brownian movement (14-17) and rotational diffusion (14. 18. 19) are among the phenomena that cause dispersed systems to display complex rheology. The role of van der Waals forces in inducing flocculation (20) and the countervailing role of two electroviscous effects (17. 21. 22) in imparting stability, particularly in aqueous systems, have been noted. Steric repulsions appear to be the responsible factor in nonaqueous systems (23. 24). The adsorbed layer can be quite large (25-28). as detected by diffusion and density measurements of filled systems or by viscometry and normal stress differences (29). 

The results obtained with nonaqueous dispersions parallel those presented for aqueous systems. Table 5.3 displays the results obtained for the CFVs and LCFTs of poly(methyl methacrylate) latices stabilized by poly(12-hydroxysteaiic acid) of molecular weight 1750 in n-heptane (Napper, 1968b). TTie stabilizing moieties were attached to three different anchor polymers, each of which was insoluble in n-heptane. The measured CFVs and LCPTs appear to be indifferent to the chemical nature of the anchor polymer. 

The data displayed in Tables 6.1 and 6.2 illustrate the strong correlation existing between the CFTs and the corresponding 0-temperatures for both aqueous and nonaqueous dispersion media. These results were obtained by five different groups of workers and cover some 13 different stabilizing moiety/dispersion medium systems. It is important to stress that it is not possible in general to equate the CFT with Ae 0-temperature. 

In summary, for nonaqueous dispersions, the combinatorial free energy of interpenetration favours stabilization. Both of the corresponding free energies associated with contact dissimilarity and free volume dissimilarity favour flocculation. These conclusions are represented schematically in Fig. 7.2. Since the combinatorial free energy is purely entropic in origin, it is scarcely surprising that nonaqueous sterically stabilized systems are usually found to be entropically stabilized at room temperature and pressure for it is this term that imparts stability. Anticipating the results of the next section, we stress that this does not necessarily imply that all nonaqueous dispersions are entropically stabilized at room temperature. 

Finally, we stress that the free volume approach is only applicable to nonpolar systems. Aqueous dispersions fall outside its scope. This is vividly illustrated by the data of Evans et al. (1975), who determined experimentally that d(UCFT)/d7 = — 1 x 10 KPa for latex particles sterically stabilized by poly(oxyethylene) in aqueous 0-43 molal magnesium sulphate solutions. Both the sign and magnitude of this quantity is different from that predicted by free volume theory for the UCFT of non aqueous dispersions. Paradoxically, it falls in line with the predictions, both in sign and magnitude, published by Croucher and Hair (1979) for the pressure dependence of the LCFT of poly(a-methylstyrene) in -butyl chloride. This may be merely coincidental, but the very small pressure dependence exhibited by the UCFT of aqueous sterically stabilized dispersions emphasizes the major differences between the origins of flocculation at the UCI T for aqueous and nonaqueous dispersions. The small pressure dependence observed for aqueous systems is scarcely surprising since the UCFT of an aqueous dispersion occurs far from the critical point of water whereas that for nonaqueous dispersions is quite close to the critical point of the dispersion medium. 

Some results obtained in this manner are shown in Fig. 13.5. At low volume fractions the resistance to compression was negligible. Equilibrium was attained very slowly, however, perhaps because the filter displayed a low permeability when in contact with nonaqueous dispersion media. At a volume fraction of ca 0-55, the resistance to compression increased considerably with even small changes of volume fraction. Extremely strong resistance to compression was experienced at a particle voliune fraction of 0-566 indeed, the system virtually became inconipressible (dji /dva oo where Jt =osmotic pressure of the particles at volume fraction V3). Interestingly, no hysteresis was observed in these systems, the compression and decompression cycles behaving quite reversibly. As noted previously, however, equilibrium was achieved only slowly. 

More recently, classification according to the paint or lacquer system has come to be preferred. Here, a distinction is made between solvent paints or lacquers (that is, those with organic solvents), low-solvent systems, water-soluble binders, aqueous dispersions, nonaqueous dispersions, and powder coatings. 

It seems that, unlike gas-in-liquid dispersion systems, interfacial areas do not depend strongly on the various properties of the chemical systems employed and, for a given packing, depend mainly on the liquid flowrate. For instance. Figure 10 shows the data of Sharma and coworkers (47,110,111) which has been checked by several workers over a span of several years with good agreement Table 11 shows the characteristics of these systems. It appears that ionic strength (varied from 1 to 34.5 ion/1) and viscosity (varied from 1 to 9 cP) have little effect on interfacial area (111). Other published information with aqueous solutions (95,96) support this view. Information with nonaqueous solutions is scanty but may well be different (47). 

Hundred percent-active silicone-based antifoam compounds are normally referred to as silicone antifoam compounds. If the silicone antifoam is in water, it is referred to as antifoam emulsions. Mixtures of silicone antifoam compounds with nonaqueous dispersion or delivery systems also exist, to aid their dispersion in aqueous media. 

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