Crystallinity ethylene-vinyl acetate

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. 

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. 

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. 

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... 

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... 

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. 

At room temperature, PE is a semi-crystalline plastomer (a plastic which on stretching shows elongation like an elastomer), but on heating crystallites melt and the polymer passes through an elastomeric phase. Similarly, by hindering the crystallisation of PE (that is, by incorporating new chain elements), amorphous curable rubbery materials like ethylene propylene copolymer (EPM), ethylene propylene diene terpolymer (EPDM), ethylene-vinyl acetate copolymer (EVA), chlorinated polyethylene (CM), and chlorosulphonated polyethylene (CSM) can be prepared. 

Versatility the copolymers are available in a wide range of molecular weights and ethylene/vinyl acetate ratios. As the ethylene domain is crystalline, an increase in the content of ethylene unit affects the ciystallinity and the solubility parameter of the copolymer. Thus the release rate of a dmg from the device can be tailored as required. 

The compatibility of blends of poly (vinyl chloride) (PVC) and a terpolymer (TP) of ethylene, vinyl acetate, and carbon monoxide was investigated by dynamic mechanical, dielectric, and calorimetric studies. Each technique showed a single glass transition and that transition temperature, as defined by the initial rise in E" at 110 Hz, c" at 100 Hz, and Cp at 20°C/min, agreed to within 5°C. PVC acted as a polymeric diluent which lowered the crystallization temperature, Tc, of the terpolymer such that Tc decreased with increasing PVC content while Tg increased. In this manner, terpolymer crystallization is inhibited in blends whose value of (Tc — Tg) was negative. Thus, all blends which contained 60% or more PVC showed little or no crystallinity unless solvent was added. 

Ethylene-propylene copolymers are useful models for homopolymer blends in that they also are biphasic, and their thermo-oxidative response [Singh et al, 1993] indicates that degradation occurs mainly in the crystalline PP phase and not in the amorphous. Ethylene/vinyl acetate copolymer-rubber blends have also been studied [Koshy et al., 1992],... 

Ethylene-vinyl acetate (EVA) copolymers are used in HMAs. The EVA acts as the binder, contributing cohesive strength to the adhesive formulation. Typically, an EVA used in a ElMA is approximately 18-28 mol% vinyl acetate (VA). In an EVA copolymer, the crystalline polyethylene (PE) region provides strength, compatibility with the wax, and the desired high-temperature properties. The amorphous region containing both VA and PE provides compatibility with the tackifier. 

Typical thermoplastic binders which are found in literature for injection molding of ceramic bodies are, styrene-butadiene, polyethylene, polypropylene, polybutene, ethylene vinyl acetate, polymethylmethacrylate and polyoxymethylene. When selecting one of these binders for thermoplastic extrusion of ceramic bodies, it should be noted that the shrinkage of par-tially-crystalline polymers is higher than for amorphous polymers, and hence warping during cooling is more critical in the former case. This is, however, not the only criterion for selection price and processability at adequate temperatures are also important factors to consider. 

Binder resins for the injection molding method are poly(amide)s and ethylene vinyl acetate copolymers, and more recently poly(ether ether ketone) (PEEK) and PPS, because of their superior properties. Crystalline resins such as PPS or PEEK require a high temperature for fusion molding of 350°C or higher, so that there is a disadvantage in that the magnetic powder of the rare earth is likely to be oxidized by the molding process. 

DuPont, in the late fifties, introduced ethylene vinyl acetate (EVA) copolymers as specialty low-crystallinity copolymers. EVA copolymers are produced by introducing vinyl acetate comonomer in the high-pressure process. 

Vinyl acetate residues in ethylene-vinyl acetate copolymers reduce the regularity of polyethylene. This reduces crystallinity in the polymer. Materials containing 45% vinyl acetate are elastomers and can be crosslinked with peroxide. 

Specific interactions in binary blends of ethylene-vinyl acetate copolymer with various low molecular weight terpene-phenol tackifying resins (TPR) were systematically investigated, as a function of the composition of the blend and of the electron acceptor ability of the resin, by using attenuated total reflection FTIR spectroscopy. Molecular acid-base were evidenced between TPR hydroxyl groups and EVA carbonyl groups. Quantitative information on the fraction of acid-base bonded entities, the enthalpy and equilibrium constant of pair formation were obtained. A crystalline transition of the EVA copolymer was observed and discussed in terms of enthalpy and entropy considerations based on FTIR and calorimetric DSC investigations. Fundamental results are then summarised to predict the interfacial reactivity of such polymer blends towards acid or basic substrates. 16 refs. 

Fig. 108. Degree of crystallinity calculated from the enthalpy of fusion of 9% PE gels vs mole percent of branches hydrogenated polybutadiene ( ) ethylene-vinyl acetate (A). Reproduced from Macromolecules [Ref. 338] by the courtesy of the authors and of The American Chemical Society...

Acrylonitrile Butadiene Styrene Acrylonitrile Styrene Acrylate Cyclic Olefin Copolymer Polyethylene Chlorotrifluoroethylene Polyethylene Tetrafluoroethylene Ethylene Vinyl Acetate Fluorinated Ethylene Propylene High Density Polyethylene High Performance Polyamide Liquid Crystalline Polymer Low Density Polyethylene Linear Low Density Polyethylene Medium Density Polyethylene Polyamide (Nylon)... 

Polar copolymers of ethylene, such as ethylene-vinyl acetate (EVA) and ethylene-ethyl acrylate (EEA), are readily crosslinked upon exposure to high energy irradiation [88]. In fact, the melt index of EVA can be controlled by the use of low doses (<50 kGy) of irradiation [89]. The presence in polar ethylene copolymers of comonomer units such as vinyl acetate or alkyl acrylates (methyl, ethyl and n-butyl) proportionately reduces the level of crystallinity, and since the majority of radiation responses of interest take place in the amorphous phase, the responses are more uniform throughout the polymer mass. When the irradiation is done at room temperature, the physical properties after irradiation follow the same trend as polyethylene [90]. 

PA6/LDPE blends compatibilized using maleated hydrolyzed ethylene-vinyl acetate copolymer (EVALM) Crystallinity was studied using DSC EVALM affected the degree of crystallinity and Tg of PA6 phase Luo et al. 2001... 

Copolymerization, that is, with two or more monomers, is a useM way of obtaining materials with specific properties, and in the case of ethylene vinyl acetate copolymers, the effects of the vinyl acetate units is to reduce the crystallinity of polyethylene, and to introduce polar groups, which improve adhesion. 

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