System vinyl acetate

Buffers are frequently added to emulsion recipes and serve two main purposes. The rate of hydrolysis of vinyl acetate and some comonomers is pH-sensitive. Hydrolysis of monomer produces acetic acid, which can affect the initiator, and acetaldehyde which as a chain-transfer agent may lower the molecular weight of the polymer undesirably. The rates of decomposition of some initiators are affected by pH and the buffer is added to stabilize those rates, since decomposition of the initiator frequently changes the pH in an unbuffered system. Vinyl acetate emulsion polymerization recipes are usually buffered to pH 4—5, eg, with phosphate or acetate, but buffering at neutral pH with bicarbonate also gives excellent results. The pH of most commercially available emulsions is 4—6. 

Finally, in the presence of Pd(OAc)2 in chloride free systems vinyl acetate is coupled to 1,4-diacetoxybutadiene (15) (Equation 8), while in the presence of PdCL/NaOAc mixtures it decomposes to acetaldehyde and acetic anhydride (104) (Reaction 28). 

In contrast the polymerization of vinyl acetate in the presence of polymethyl methacrylate gives after selective precipitation appreciable amounts of pure graft copolymer, independently of the nature of the initiator moreover the degree of grafting, evaluated by infrared spectrometry, is about equally important. Similar results were obtained in the system vinyl acetate-polyethyl a-chloroacryl ate. 

Hart and de Pauw 98) used this emulsion technique on the system vinyl acetate-acrylic acid. It is well known that the copolymerization parameters rx and r2 are unfavorable in this system therefore the relative solubility of the two monomers exerces only a small influence on the composition of both sequences. The degree of homogeneity of the sequences has been evaluated, after alkaline hydrolysis, by measuring the tendency to lactonization in acid medium. While 72% of the acetate groups could be lactonized in the case of a random copolymer containing 37% vinyl acetate, only 14% are transformed in a block copolymer with the same initial composition. 

Figure 7.3. shows the ESR spectrum obtained for the polymerization system vinyl acetate-acrylnitrile (VAc-AN), the spectram is due to three kinds of free radicals, the VAc monomer radical, (OH)CH2-(C )—CH—(OCOCH3). VAc-AN copolymer radical, (OH)CH2—CH (OCOCH3) CH2—(C ) H(CN), and the AN... 

Ovejero Escudero, G. De Lucas Martinez, A. Moreno Rodriguez, J. M. Vapor-liquid equilibria at 760 mmHg for the system vinyl acetate - n-decane J. Chem. Eng. Data 1992,37,293-295... 

Kapoor, S Rattan, V. K. Vapor-liquid equilibria and excess properties of the binary system vinyl acetate -l- p-xylene J. Chem. Eng. Data 2005,50, 1897-1901... 

Similar to the SMA system, vinyl acetate-MA may be treated with mesityl oxide and HCIO4 to prepare copolymer with pendent pyrylium perchlorate residues and primary amines to prepare imide materials. 

Wisniak, J. Tamir, A. The systems vinyl acetate - toluene and vinyl acetate - propyl bromide - toluene. J. Chem. Eng. Data 1989, 34, 301-305. 

We might be hard pressed to estimate the individual resonance stabilization energies in Eqs. (7.23) and (7.24), but the qualitative apphcation of these ideas is not difficult. Consider once again the styrene-vinyl acetate system ... 

Reactivity ratios for the 7V-vinylphthalimide (molecule 1)-styrene (molecule 2) system were measured, and foundt to be ri = 0.075 and I2 = 8.3. Use these values to estimate values of Q and e for 7V-vinylphthalimide then estimate the parameters rj and 12 for system in which molecule 2 is vinyl acetate. 

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). 

Heteroatom functionalized terpene resins are also utilized in hot melt adhesive and ink appHcations. Diels-Alder reaction of terpenic dienes or trienes with acrylates, methacrylates, or other a, P-unsaturated esters of polyhydric alcohols has been shown to yield resins with superior pressure sensitive adhesive properties relative to petroleum and unmodified polyterpene resins (107). Limonene—phenol resins, produced by the BF etherate-catalyzed condensation of 1.4—2.0 moles of limonene with 1.0 mole of phenol have been shown to impart improved tack, elongation, and tensile strength to ethylene—vinyl acetate and ethylene—methyl acrylate-based hot melt adhesive systems (108). Terpene polyol ethers have been shown to be particularly effective tackifiers in pressure sensitive adhesive appHcations (109). 

An a priori method for choosing a surfactant was attempted by several researchers (50) using the hydroph i1 e—1 ip oph i1 e balance or HLB system (51). In the HLB system a surfactant soluble in oil has a value of 1 and a surfactant soluble in water has a value of 20. Optimum HLB values have been reported for latices made from styrene, vinyl acetate, methyl methacrylate, ethyl acrylate, acrylonitrile, and their copolymers and range from 11 to 18. The HLB system has been criticized as being imprecise (52). 

The principal use of the peroxodisulfate salts is as initiators (qv) for olefin polymerisation in aqueous systems, particularly for the manufacture of polyacrylonitrile and its copolymers (see Acrylonitrile polymers). These salts are used in the emulsion polymerisation of vinyl chloride, styrene—butadiene, vinyl acetate, neoprene, and acryhc esters (see Acrylic ester polymers Styrene Vinyl polymers). 

T[[dotb]he nature of the initial attack by the water (eq. 10) is a matter of some controversy (205,206). Stereochemical and kinetic studies of model systems have been reported that support trans addition of external water (207,208) or internal addition of cis-coordinated water (209), depending on the particular model system under study. Other paHadium-cataly2ed oxidations of olefins ia various oxygen donor solvents produce a variety of products including aldehydes (qv), ketones (qv), vinyl acetate, acetals, and vinyl ethers (204). However the product mixtures are complex and very sensitive to conditions. 

Emulsion Polymerization. Poly(vinyl acetate)-based emulsion polymers are produced by the polymerization of an emulsified monomer through free-radicals generated by an initiator system. Descriptions of the technology may be found in several references (35—39). 

Many different combinations of surfactant and protective coUoid are used in emulsion polymerizations of vinyl acetate as stabilizers. The properties of the emulsion and the polymeric film depend to a large extent on the identity and quantity of the stabilizers. The choice of stabilizer affects the mean and distribution of particle size which affects the rheology and film formation. The stabilizer system also impacts the stabiUty of the emulsion to mechanical shear, temperature change, and compounding. Characteristics of the coalesced resin affected by the stabilizer include tack, smoothness, opacity, water resistance, and film strength (41,42). 

Issues to be considered in selecting the best stabilizing system are polymeric chain branching which increases with high temperature and the presence of some stabilizers, polydispersity of the particles produced, and grafting copolymerization, which may occur because of the reaction of vinyl acetate with emulsifiers such as poly(vinyl alcohol) (43,44). 

Continuous polymerization systems offer the possibiUty of several advantages including better heat transfer and cooling capacity, reduction in downtime, more uniform products, and less raw material handling (59,60). In some continuous emulsion homopolymerization processes, materials are added continuously to a first ketde and partially polymerized, then passed into a second reactor where, with additional initiator, the reaction is concluded. Continuous emulsion copolymerizations of vinyl acetate with ethylene have been described (61—64). Recirculating loop reactors which have high heat-transfer rates have found use for the manufacture of latexes for paint appHcations (59). 

Propagation. The rate of emulsion polymerization has been found to depend on initiator, monomer, and emulsifier concentrations. In a system of vinyl acetate, sodium lauryl sulfate, and potassium persulfate, the following relationship for the rate of polymerization has been suggested (85) ... 

Emulsion polymerizations of vinyl acetate in the presence of ethylene oxide- or propylene oxide-based surfactants and protective coUoids also are characterized by the formation of graft copolymers of vinyl acetate on these materials. This was also observed in mixed systems of hydroxyethyl cellulose and nonylphenol ethoxylates. The oxyethylene chain groups supply the specific site of transfer (111). The concentration of insoluble (grafted) polymer decreases with increase in surfactant ratio, and (max) is observed at an ethoxylation degree of 8 (112). 

In the screw conveyor process, solutions of poly(vinyl acetate) and catalyst are mixed in a high intensity mixer and continuously introduced to a screw-type saponification and conveyor system (270). Downstream details are similar to those found in the belt process. 

Tile is based mainly on vinyl chloride and vinyl acetate copolymers. Some polypropylene tile systems have recendy been iatroduced. A petroleum resia is usually employed as an extender and processiag aid conventional vinyl plasticizers and stabilizers also are iacorporated. Reinforcing fibers and limestone constitute the remainder of the tile composition the fibers contribute hot strength for processiag and dimensional stabiHty ia the finished tile, limestone suppHes bulk at an economical cost. Stable pigments are also iacorporated. Siace tile is iastalled oa and below grade level, it is important that the finished product be resistant to the effects of moisture and alkaH. 

Vinyls. Vinyl resins are thermoplastic polymers made principally from vinyl chloride other monomers such as vinyl acetate or maleic anhydride are copolymerized to add solubUity, adhesion, or other desirable properties (see Maleic anhydride, maleic acid, and fumaric acid). Because of the high, from 4,000 to 35,000, molecular weights large proportions of strong solvents are needed to achieve appHcation viscosities. Whereas vinyls are one of the finest high performance systems for steel, many vinyl coatings do not conform to VOC requirements (see Vinyl polymers). 

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