Crystalline vinyl acetate

Carbon Cha.in Backbone Polymers. These polymers may be represented by (4) and considered derivatives of polyethylene, where n is the degree of polymeriza tion and R is (an alkyl group or) a functional group hydrogen (polyethylene), methyl (polypropylene), carboxyl (poly(acryhc acid)), chlorine (poly(vinyl chloride)), phenyl (polystyrene) hydroxyl (poly(vinyl alcohol)), ester (poly(vinyl acetate)), nitrile (polyacrylonitrile), vinyl (polybutadiene), etc. The functional groups and the molecular weight of the polymers, control thek properties which vary in hydrophobicity, solubiUty characteristics, glass-transition temperature, and crystallinity. 

Ethylene has also been copolymerised with a number of non-olefinic monomers and of the copolymers produced those with vinyl acetate have so far proved the most significant commercially . The presence of vinyl acetate residues in the chain reduces the polymer regularity and hence by the vinyl acetate content the amount of crystallinity may be controlled. Copolymers based on 45% vinyl acetate are rubbery and may be vulcanised with peroxides. They are commercially available (Levapren). Copolymers with about 30% vinyl acetate residues (Elvax-Du Pont) are flexible resins soluble in toluene and benezene at room temperature and with a tensile strength of about lOOOlbf/in (6.9 MPa) and a density of about 0.95 g/cm. Their main uses are as wax additives and as adhesive ingredients. 

A further class of ethylene-vinyl acetate copolymer exists where the vinyl acetate content is of the order of 3 mole %. These materials are best considered as a modification of low-density polyethylene, where the low-cost comonomer introduces additional irregularity into the structure, reducing crystallinity and increasing flexibility, softness and, in the case of film, surface gloss. They have extensive clearance as non-toxic materials. 

This chapter has so far dealt with the major fields of use of vinyl chloride polymers, namely plasticised PVC homopolymer, unplasticised PVC, including impact-modified grades, and copolymers particular based on vinyl acetate. There are, however, five particular special forms of vinyl chloride polymer which merit separate consideration, namely crystalline PVC, after-chlorinated PVC (often known as CPVC) and certain graft copolymers and two vinyl-chloride-based copolymers. 

Poly(vinyl acetate) is an atactic material and is amorphous. Whilst the structure of polyfvinyl alcohol) is also atactic the polymer exhibits crystallinity and has essentially the same crystal lattice as polyethylene. This is because the hydroxyl groups are small enough to fit into the lattice without disrupting it. 

For reasons explained below, the effect of increasing the vinyl alcohol content in EVOH is quite different to that of increasing the vinyl acetate content in EVA. In the case of ethylene-vinyl acetate (EVA) copolymers, increasing the vinyl acetate content up to about 50% makes the materials less crystalline and progressively more flexible and then rubbery. In the range 40-70% vinyl acetate content the materials are amorphous and rubbery, whilst above 70% the copolymers become increasingly rigid and brittle. 

These adhesives differ from normal hot-melt adhesives, such as the standard ethylene vinyl acetate hot melts. Standard hot-melt adhesives like EVA have no curing mechanism. They are heated above the crystalline melting point and applied as a low-viscosity liquid in the same manner as is the curing hot melt. The bond is closed in the same manner and strength is developed upon crystallization. 

An interesting application of this reaction was the use of macro-molecular anhydrides, namely, styrene-maleic anhydride or vinyl acetate-maleic anhydride copolymers in the presence of perchloric acid as catalyst, these copolymers acylate mesityl oxide or d rpnone to macromolecular pyrylium salts which, with aryl substituents, are fluorescent.No crystalline products could be obtained from succinic anhydride because of the solubility and ease of decarboxylation. 

Coran and Patel [33] selected a series of TPEs based on different rubbers and thermoplastics. Three types of rubbers EPDM, ethylene vinyl acetate (EVA), and nitrile (NBR) were selected and the plastics include PP, PS, styrene acrylonitrile (SAN), and PA. It was shown that the ultimate mechanical properties such as stress at break, elongation, and the elastic recovery of these dynamically cured blends increased with the similarity of the rubber and plastic in respect to the critical surface tension for wetting and with the crystallinity of the plastic phase. Critical chain length of the rubber molecule, crystallinity of the hard phase (plastic), and the surface energy are a few of the parameters used in the analysis. Better results are obtained with a crystalline plastic material when the entanglement molecular length of the... 

The crystallinity of the polymer could be varied to some extent by changing the reaction conditions and by adding comonomers such as vinyl acetate or ethyl acrylate. The copolymers have lower crystallinity but better flexibility, and the resulting polymer has higher impact strength. ... 

Other polyolefins A variety of other crystalline polyolefins are available such as polybutene-1 (improved creep resistance over polyethylene), poly-4-methyl pentene-1 (excellent temperature deformation resistance) and ethylene-vinyl acetate (greater flexibility). 

FIGURE 31.2 Plots of crystalline melting point, heat of fusion and percent crystallinity of ethylene-vinyl acetate (EVA) samples versus (a) radiation dose (b) trimethylolpropane trimethacrylate (TMPTMA) level from differential scanning calorimetry (DSC) studies. (From Datta, S.K., Bhowmick, A.K., Chaki, T.K., Majali, A.B., and Deshpande, R.S., Polymer, 37, 45, 1996. With permission.)... 

The properties of ethylene-vinyl acetate copolymers vary widely with their ester content. At the lowest levels of vinyl acetate, they have physical properties that are similar to those of low density polyethylene. As the comonomer content increases, the material becomes less crystalline and more elastic. Copolymers made with the highest comonomer levels contain no measurable crystallinity. The resulting products are tough, flexible, and clear. The ester... 

The copolymerisation of ethylene with vinyl acetate (VA) is another method by which the crystallinity of polyethylene can be reduced and a rubbery polymer obtained. The final properties of the copolymer depend on the VA content at a VA level of 50% the copolymer is entirely amorphous, and elastomeric grades generally contain 40-60% VA by weight. The oil resistance of the copolymer is also dependent on the VA content in general, however, this lies between that of SBR and polychloroprene. It is swollen by most organic solvents and not resistant to animal and vegetable oils, but has some resistance to weak acids and alkalis at ambient temperature. 

EVA and VAE (see Figure 4.24) are copolymerized from ethylene and vinyl acetate that is randomly distributed along the backbone. The backbone is identical to that of the polyolefins but the pendant groups are different, with a polar character and a random structure that decreases crystallinity the more so as the vinyl acetate level rises. 

The properties depend on the vinyl acetate level, the crystallinity (see Figure 4.25), the branching level, the molecular weight and the polarity. 

Figure 4.25 displays examples of crystallinity (%) versus vinyl acetate content (%) for neat EVA. Note the progressive and fast fall in the crystallinity. 

When the vinyl acetate content rises, polarity increases, and crystallinity and hardness decrease. For a 40-50% vinyl acetate content, the copolymer is an amorphous resin that is easily crosslinkable. 

Apart from nitrile-hydrolyzing enzymes, some esterases and cutinases have been used for surface hydrolysis of PAN [74], These enzyme were shown to specifically hydrolyse vinyl acetate moieties present as co-monomer in many commercial PAN materials, with no changes in crystallinity as determined by X-ray diffraction [74],... 

Copolymers of vinyl chloride are less crystalline and may he more flexible than unplastidzed PVC The random copolymer of vinyl chloride (17%) and vinyl acetate (11%) (Vinylite) contains the following repeating units ... 

The EVA polymer emulsions contain crystalline segments resulting from ethylene linkages. In addition to ethylene and vinyl acetate, a carboxylic comonomer is used, such as acrylamide or versa tic acid vinyl ester. The polymers have crystalline melting point of 50-90°C. 

A preferred way to enhance the crystalline domain formation of ethylene in the EVA polymer is to delay the addition of vinyl acetate during the polymerization process such that the unreacted vinyl acetate level present in the reactor is minimal at different stages during the process. Thus, the copolymerization can take place in the initial stage, where most of the ethylene will reside in amorphous regions, and the formation of the majority of crystalline ethylene domains can occur in the later stage of the polymerization process. 

Instead, the distribution of vinyl acetate and ethylene in the copolymer is a major factor. A sufficient level of amorphous ethylene vinyl acetate polymer segments is needed in order to provide adhesion to a substrate. Further, a sufficient level of crystalline ethylene polymer segments is needed to provide the proper balance of heat seal characteristics and non-blocking. 

J.J. Rabasco, C.L. Daniels, D.W. Horwat, M.S. Vratsanos, and R.H. Bott, Semi-crystalline ethylene vinyl acetate emulsion polymers for heat seal applications, US Patent 7189461, assigned to Air Products Polymers, L.P. (Allentown, PA), March 13,2007. 

J.J. Rabasco, G.J. Dearth, C.R. Hegedus, F.R. Pepe, and B.V. Mukku-lainen, Masonry sealing compositions comprising semi-crystalline ethylene-vinyl acetate polymer emulsions, US Patent 7459186, assigned to Wacker Chemical Corporation, December 2, 2008. 

Many dialkyl and diaryl cadmium compounds have found use as polymerization catalysts. For example, the diethyl compound catalyzes polymerization of vinyl chloride, vinyl acetate, and methyl methacrylate (45), and when mixed with TiCl can be used to produce polyethylene and crystalline polypropylene for filaments, textiles, glues, and coatings (45). With >50% TiCl diethyl cadmium polymerizes dienes. Diethyl cadmium maybe used as an intermediate ethylating agent in the production of tetraethyllead. The diaryl compounds such as diphenylcadmium [2674-04-6]> (C H Cd, (mp 174°C) are also polymerization catalysts. These compounds are also prepared using Grignard or arylUthium reagents in tetrahydrofiiran (THF) solvent but may be prepared by direct metal substitution reactions such as ... 

Copolymerization. Vinyl chloride can be copolymerized with a variety of monomers. Vinyl acetate, the most important commercial comonomer, is used to reduce crystallinity, winch aids fusion and allows lower processing temperatures. Copolymers are used in flooring and coatings. This copolymer sometimes contains maleic add or vinyl alcohol (hydrolyzed from the poly(vinyl acetate ) to improve the coating s adhesion to other materials, including metals, Copolymers with vinylidene chloride are used as barrier films and coatings. Copolymers of vinyl chlonde with acrylic esters in latex from are used as film formers in paint, nonwoven fabric binders, adhesives, and coatings. Copolymers with olefins improve thermal stability and melt flow, but at some loss of heat-deflection temperature,... 

Polychloroethene (polyvinyl chloride), as usually prepared, is atactic and not very crystalline. It is relatively brittle and glassy. The properties of polyvinyl chloride can be improved by copolymerization, as with ethenyl ethanoate (vinyl acetate), which produces a softer polymer ( Vinylite ) with better molding properties. Polyvinyl chloride also can be plasticized by blending it with substances of low volatility such as tris-(2-methylphenyl) phosphate (tricresyl phosphate) and dibutyl benzene-1,2-dicarboxylate (dibutyl phthalate) which, when dissolved in the polymer, tend to break down its glasslike structure. Plasticized polyvinyl chloride is reasonably flexible and is widely used as electrical insulation, plastic sheeting, and so on. 

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