Suspending agent

In a suspension polymerisation monomer is suspended in water as 0.1—5-mm droplets, stabilised by protective coUoids or suspending agents. Polymerisation is initiated by a monomer-soluble initiator and takes place within the monomer droplets. The water serves as both the dispersion medium and a heat-transfer agent. Particle sise is controlled primarily by the rate of agitation and the concentration and type of suspending aids. The polymer is obtained as small beads about 0.1—5 mm in diameter, which are isolated by filtration or centrifugation. 

Suspension polymerization of VDE in water are batch processes in autoclaves designed to limit scale formation (91). Most systems operate from 30 to 100°C and are initiated with monomer-soluble organic free-radical initiators such as diisopropyl peroxydicarbonate (92—96), tert-huty peroxypivalate (97), or / fZ-amyl peroxypivalate (98). Usually water-soluble polymers, eg, cellulose derivatives or poly(vinyl alcohol), are used as suspending agents to reduce coalescence of polymer particles. Organic solvents that may act as a reaction accelerator or chain-transfer agent are often employed. The reactor product is a slurry of suspended polymer particles, usually spheres of 30—100 pm in diameter they are separated from the water phase thoroughly washed and dried. Size and internal stmcture of beads, ie, porosity, and dispersant residues affect how the resin performs in appHcations. 

Solution Polymerization. In solution polymerization, a solvent for the monomer is often used to obtain very uniform copolymers. Polymerization rates ate normally slower than those for suspension or emulsion PVC. Eor example, vinyl chloride, vinyl acetate, and sometimes maleic acid are polymerized in a solvent where the resulting polymer is insoluble in the solvent. This makes a uniform copolymer, free of suspending agents, that is used in solution coatings (99). 

Sodium carboxymethylcellulose is Hsted in the 1990 United States Pharmacopeia under the categories of pharmaceutic aid (suspending agent, tablet binder, viscosity-increasing agent), and cathartic tablets. 

Uses. HEC is used as a thickener, protective coUoid, binder, stabilizer, and suspending agent in a variety of industrial appHcations. A guide to the principal uses is given in Table 6. 

Other Clay Uses. Other appHcations for clays include use as a suspending agent, eg, montmorillonite and attapulgite in Hquid fertilizers and dishwasher detergents in pharmaceuticals (qv), eg, kaoHnite and attapulgite for diarrhea control in cosmetics, montmorillonite and attapulgite and in water impedence where bentonite linings are used for reservoirs and waste disposal areas. 

Most antiperspirant sticks are molded. Sticks dominate in the U.S. market, whereas lotion and cream antiperspirants are preferred in Europe. Stick antiperspirant products may include suspending agents, coupling agents to wet the antiperspirant chemical (about 20—25%), and emoUients. The blend is prepared at about 65°C and poured at about 55°C. Antiperspirant (64) and deodorant (65) compositions have been pubHshed. 

Camphor, dibutyl phthalate [84-74-2], and other Hpidic solvents are common plastici2ers. Nad lacquers requite the presence of a suspending agent because pigments have a tendency to settle. Most tinted lacquers contain a suitable flocculating agent, such as stearalkonium hectorite, a reaction product of hectorite [12173 6-6] and stearalkonium chloride [122-19-0],... 

Bead Polymerization Bulk reaction proceeds in independent droplets of 10 to 1,000 [Lm diameter suspended in water or other medium and insulated from each other by some colloid. A typical suspending agent is polyvinyl alcohol dissolved in water. The polymerization can be done to high conversion. Temperature control is easy because of the moderating thermal effect of the water and its low viscosity. The suspensions sometimes are unstable and agitation may be critical. Only batch reaciors appear to be in industrial use polyvinyl acetate in methanol, copolymers of acrylates and methacrylates, polyacrylonitrile in aqueous ZnCh solution, and others. Bead polymerization of styrene takes 8 to 12 h. 

Other variables in the factorial experiment also have an impact on the character of the final product. The amount of nonsolvent is a very important variable to examine as the pore size of the gel depends on the amount of it present in the formulation. The stabilizer acts as a suspending agent and influences the particle size of the GPC/SEC gel. Lower viscosity suspending agents... 

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