Nonaqueous polymer dispersion

Much work on the preparation of nonaqueous polymer dispersions has involved the radical polymerization of acrylic monomers in the presence of copolymers having the A block the same as the acrylic polymer in the particle core 2). The preparation of polymer dispersions other than polystyrene in the presence of a PS-PDMS diblock copolymer is of interest because effective anchoring of the copolymer may be influenced by the degree of compatibility between the PS anchor block and the polymer molecules in the particle core. The present paper describes the interpretation of experimental studies performed with the aim of determining the mode of anchoring of PS blocks to polystyrene, poly(methyl methacrylate), and poly(vinyl acetate) (PVA) particles. 

Dispersion polymerisation this is usually applied to the preparation of nonaqueous polymer dispersions, and is commonly referred to as nonaqueous dispersion (NAD) polymerisation. In this case, the monomer, initiator, stabiliser (referred as the protective agent) and the solvent initially form a homogeneous solution, and the polymer particles precipitate when the solubility limit of the polymer is exceeded. The particles continue to grow until the monomer is consumed. 

In Procedure 6-1, a remarkably high solids dispersion of poly(vinyl acetate) in cyclohexane is prepared. Since this procedure has been patented, it is given here only to illustrate the techniques used in preparing nonaqueous polymer dispersions. 

The upsurge of interest in this type of colloidal system followed the development in the 1950s of the technique known as dispersion polymerization [3.54]. This process provides a means of preparing nonaqueous polymer dispersions in a controlled manner. A wide range of such dispersions have been made, mainly by free-radical addition polymerization. 

Block and Graft Copolymer Stabilizers in Dispersion Polymerization. A sterically-stabilized, nonaqueous, polymer dispersion is made simply by heating a solution of a free radical initiator (e.g., azobisisobutyronitrile), an appropriate monomer, and a suitable block or graft copolymer in an organic liquid which is a nonsolvent for the polymer product and acts as a diluent for the dispersion. The block or graft copoly-... 

Finally, the preparation of nonaqueous polymer dispersions has been used as a technique for polymer production. Process advantages such as the use of lower temperatures for polyamide and polyester synthesis can be obtained [3.70]. Products such as synthetic elastomers can be obtained in powder form [3.104]. The fine particulate form of dispersions of water-soluble polymers (e.g., polyacrylamide) offers a convenient route to aqueous solutions of polymeric flocculants and thickeners [3.105]. 

The third component, the graft species, acts as a nonaqueous dispersant (NAD), a compound having, in the same structure, polyetheric chains and vinylic polymer chains. This compound assures the stability of the resulting polymer dispersion and prevents sedimentation and coalescence of the vinylic polymer particles [1-5]. The mechanism of this dispersion stabilisation will be discussed later. The median diameter of solid particles for a performance polymer polyol is generally less than 1 [am, usually 0.2-0.5 im [30]. 

The critical concentration of free polymer that must be added to cause flocculation decreased with increasing molecular weight (see Fig. 16.9). The value of Cj depended upon the molecular weight with an exponent of -0-7. This value is quite different from the exponent (- 0-25) originally reported by de Hek and Vrij (1979) for nonaqueous silica dispersions stabilized by stearyl alcohol and flocculated by polystyrene. The value measured by Sperry et al. is... 

Ethylene and tetrafluoroethylene are copolymerized in aqueous, nonaqueous, or mixed medium with free-radical initiators. The polymer is isolated and converted into extmded cubes, powders, and beads, or a dispersion. This family of products is manufactured by Du Pont, Hoechst, Daikin, Asahi Glass, and Ausimont and sold under the trade names of Tefzel, Hostaflon ET, Neoflon EP, Aflon COP, and Halon ET, respectively. 

Nonaqueous Dispersion Polymerization. Nonaqueous dispersion polymers are prepared by polymerizing a methacryhc monomer dissolved in an organic solvent to form an insoluble polymer in the presence of an amphipathic graft or block copolymer. This graft or block copolymer, commonly called a stabilizer, lends coUoidal stabiUty to the insoluble polymer. Particle sizes in the range of 0.1—1.0 pm were typical in earlier studies (70), however particles up to 15 pm have been reported (71). 

Acryhc and methacryhc nonaqueous dispersions (NADs) are primarily utilized by the coatings industry to avoid certain difficulties associated with aqueous dispersion (emulsion) polymers. Water as a suspension medium has numerous practical advantages, but also some inherent difficulties a high heat of evaporation, a low boiling point, and an evaporation rate that depends on the prevailing humidity. Nonaqueous dispersions alleviate these problems, but introduce others such as flammabihty, increased cost, odor, and toxicity. 

Table 11. Examples of Methacrylic Nonaqueous Dispersion Polymers...

Chemical Grafting. Polymer chains which are soluble in the suspending Hquid may be grafted to the particle surface to provide steric stabilization. The most common technique is the reaction of an organic silyl chloride or an organic titanate with surface hydroxyl groups in a nonaqueous solvent. For typical interparticle potentials and a particle diameter of 10 p.m, steric stabilization can be provided by a soluble polymer layer having a thickness of - 10 nm. This can be provided by a polymer tail with a molar mass of 10 kg/mol (25) (see Dispersants). 

Polymerizations conducted in nonaqueous media in which the polymer is insoluble also display the characteristics of emulsion polymerization. When either vinyl acetate or methyl methacrylate is polymerized in a poor solvent for the polymer, for example, the rate accelerates as the polymerization progresses. This acceleration, which has been called the gel effect,probably is associated with the precipitation of minute droplets of polymer highly swollen with monomer. These droplets may provide polymerization loci in which a single chain radical may be isolated from all others. A similar heterophase polymerization is observed even in the polymerization of the pure monomer in those cases in which the polymer is insoluble in its own monomer. Vinyl chloride, vinylidene chloride, acrylonitrile, and methacryloni-trile polymerize with precipitation of the polymer in a finely divided dispersion as rapidly as it is formed. The reaction rate increases as these polymer particles are generated. In the case of vinyl chloride ... 

Other techniques to promote complete polymer hydration include vigorous mixing and slow addition of the polysaccharide. Specially designed mixing devices have been used to promote rapid particle dispersion ( 1). Adding already prepared dispersions of guar, HPG, and HEC in nonaqueous media is another means of promoting rapid... 

Besides temperature and addition of non-solvent, pressure can also be expected to affect the solvency of the dispersion medium for the solvated steric stabilizer. A previous analysis (3) of the effect of an applied pressure indicated that the UCFT should increase as the applied pressure increases, while the LCFT should be relatively insensitive to applied pressure. The purpose of this communication is to examine the UCFT of a nonaqueous dispersion as a function of applied pressure. For dispersions of polymer particles stabilized by polyisobutylene (PIB) and dispersed in 2-methylbutane, it was observed that the UCFT moves to higher temperatures with increasing applied pressure. These results can qualitatively be rationalized by considering the effect of pressure on the free volume dissimilarity contribution to the free energy of close approach of the interacting particles. 

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