Vinyl acetate polymerizations

Emulsion Adhesives. The most widely used emulsion-based adhesive is that based upon poly(vinyl acetate)—poly(vinyl alcohol) copolymers formed by free-radical polymerization in an emulsion system. Poly(vinyl alcohol) is typically formed by hydrolysis of the poly(vinyl acetate). The properties of the emulsion are derived from the polymer employed in the polymerization as weU as from the system used to emulsify the polymer in water. The emulsion is stabilized by a combination of a surfactant plus a coUoid protection system. The protective coUoids are similar to those used paint (qv) to stabilize latex. For poly(vinyl acetate), the protective coUoids are isolated from natural gums and ceUulosic resins (carboxymethylceUulose or hydroxyethjdceUulose). The hydroHzed polymer may also be used. The physical properties of the poly(vinyl acetate) polymer can be modified by changing the co-monomer used in the polymerization. Any material which is free-radically active and participates in an emulsion polymerization can be employed. Plasticizers (qv), tackifiers, viscosity modifiers, solvents (added to coalesce the emulsion particles), fillers, humectants, and other materials are often added to the adhesive to meet specifications for the intended appHcation. Because the presence of foam in the bond line could decrease performance of the adhesion joint, agents that control the amount of air entrapped in an adhesive bond must be added. Biocides are also necessary many of the materials that are used to stabilize poly(vinyl acetate) emulsions are natural products. Poly(vinyl acetate) adhesives known as "white glue" or "carpenter s glue" are available under a number of different trade names. AppHcations are found mosdy in the area of adhesion to paper and wood (see Vinyl polymers). 

Vinyl alcohol does not exist as a monomer, but Herrmann and Haehnel (1) were able to obtain the desired product poly(vinyl alcohol) [9002-89-5] (PVA), by polymerizing vinyl acetate and then hydrolyzing the resultant poly(vinyl acetate). This process is employed for the commercial production of PVA even now. The principal concern of the discoverers was development of a suture for surgical operations the fiber then obtained was not suited for clothing use (2). 

Often a chain-transfer agent is added to vinyl acetate polymerizations, whether emulsion, suspension, solution, or bulk, to control the polymer molecular weight. Aldehydes, thiols, carbon tetrachloride, etc, have been added. Some emulsion procedures call for the recipe to include a quantity of preformed PVAc emulsion and sometimes antifoamers must be added (see Foams). 

Solution Polymerization. Solution polymerization of vinyl acetate is carried out mainly as an intermediate step to the manufacture of poly(vinyl alcohol). A small amount of solution-polymerized vinyl acetate is prepared for the merchant market. When solution polymerization is carried out, the solvent acts as a chain-transfer agent, and depending on its transfer constant, has an effect on the molecular weight of the product. The rate of polymerization is also affected by the solvent but not in the same way as the degree of polymerization. The reactivity of the solvent-derived radical plays an important part. Chain-transfer constants for solvents in vinyl acetate polymerizations have been tabulated (13). Continuous solution polymers of poly(vinyl acetate) in tubular reactors have been prepared at high yield and throughput (73,74). 

The physical piopeities of poly(vinyl alcohol) aie highly coiielated with the method of piepaiation. The final piopeities are affected by the polymerization conditions of the parent poly(vinyl acetate), the hydrolysis conditions, drying, and grinding. Further, the term poly(vinyl alcohol) refers to an array of products that can be considered copolymers of vinyl acetate and vinyl alcohol. 

One of the most dramatic examples of a solvent effect on propagation taken from the early literature is for vinyl acetate polymerization.78,79 Kamachi el al.n reported a ca. 80-fold reduction in kp (30aC) on shifting from ethyl acetate to benzonilrile solvent (Table 8.1). Effects on polymer structure were also reported. Hatada ef a m conducted a H NMR study on the structure of the PVAc formed in various solvents. They found that PVAc (M n 20000) produced in ethyl acetate solvent has 0.7 branches/chain while that formed in aromatic solvents is essentially unbranched. 

A typical mixture for vinyl acetate polymerization can be seen below [94] ... 

The density change on polymerization is typically about 20%, and this density gradient can cause significant secondary flows and natural convection effects. The experiments cited above for vinyl acetate polymerization were performed in a helical reactor. The centrifugal force in helical reactors induces secondary flows as well. The effects of helical flow have been analyzed, but were found to be less significant than the effects of natural convection [14]. 

SOLUTION OF KINETIC EQUATIONS FOR LONG CHAIN BRANCHING IN BULK VINYL ACETATE POLYMERIZATION ... 

Lignin, brown coal polymer of methacrylic acid, methacrylamide, hydroxyethyl acrylate, hydroxypropyl acrylate, vinyl acetate, methyl vinyl ether, ethyl vinyl ether, N-methylmeth-acrylamide, N,N-dimethylmethacrylamide, vinyl sulfonate, or 2-acrylamido-N-methylpropane sulfonic acid free radical polymerization of a water-soluble vinyl monomer in an aqueous suspension of coals [705,1847]... 

Loop A continuous process for polymerizing aqueous emulsions of olefinic compounds such as vinyl acetate. Polymerization takes place in a tubular reactor (the loop) with recycle. Invented by Gulf Oil Canada in 1971 and further developed by several United Kingdom paint companies. It is now used for making copolymers of vinyl acetate with ethylene, used in solvent-free paints and adhesives. 

Gustin, J. L., and F. Laganier (1998). "Understanding Vinyl Acetate Polymerization Accidents." IChemE Symposium Series 144, 387-403. 

The next products developed were the thermoplastics made by vinyl polymerization. Vinyl chloride, and vinyl acetate copolymers known as Vinylite, was one of the first produced by Carbide and Carbon. It had been worked on in Europe and had also been worked on by Ostromysslenski, a chemist with the U. S. Rubber Company. The Carbide chemists realized at an early date that to get a uniform composition it was necessary to adjust the feed ratios of the two monomers to get a proper combination in the reactor to give a constant composition to the product. The next major development of a vinyl product was in the development of Lucite or Plexiglas polymers, which are clear, strong plastics. 

Product Identification was by GC/MS, NMR, and IR. Fundamental crosslinking chemistry was explored using swell measurements on simple solution copolymers and swell and tensile measurements with vinyl acetate (VAc), vinyl acetate/butyl acrylate (VAc/BA) or vinyl acetate/ethylene (VAE) emulsion copolymers. Polymer synthesis 1s described In a subsequent paper (6). Homopolymer Tg was measured by DSC on a sample polymerized In Isopropanol. Mechanistic studies were done 1n solution, usually at room temperature, with 1, 2 and the acetyl analogs 1, 2 (R =CH3). 

Vinyl coatings are used primarily on metal surfaces. They provide excellent protection by their strong cohesive forces, although their adhesion to the metal is not good. Used with a phosphoric acid-containing primer to etch the metal surface, this adhesion is markedly improved. The primer also contains poly(vinyl butyral) and is approximately 0.2-0.3 mil thick (1 mil = 1/1000th inch). Poly(vinyl butyral) is made from polymerized vinyl acetate by hydrolysis and reaction with butyraldehyde. 

Anomolous results have been observed in some emulsion polymerizations—inverse dependencies of N, Rp, and Xn on surfactant concentration. Some surfactants act as inhibitors or retarders of polymerization, especially of the more highly reactive radicals from vinyl acetate and vinyl chloride [Okamura and Motoyama, 1962 Stryker et al., 1967]. This is most apparent with surfactants possessing unsaturation (e.g., certain fatty acid soaps). Degradative chain transfer through allyl hydrogens is probably quite extensive. 

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