Peroxide crosslinking of ethylene–vinyl acetate copolymer

The work reported here concerns the peroxide crosslinking of ethylene–vinyl acetate rubber. Calculated values for scission-to-crosslinking ratios are higher for EVA than for low-density polyethylene. In the temperature range from 150 to 200°C at a constant peroxide content, a rise in temperature results in a decrease in the obtained gel content. Some tensile and modulus–temperature results on crosslinked EVA samples are also reported on.     

The conversion of ethylene–vinyl acetate copolymers with aluminum alcoholates

If ethylene–vinyl acetate copolymers are converted with aluminum alcoholates, crosslinked products will form. This reaction was studied on the basis of the viscosity variation and the amount of acetic acid ester formed. The constants of the reaction velocity for the conversion with various aluminum alcoholates were determined and a reaction mechanism is discussed.     

Shape memory effect of ethylene–vinyl acetate copolymers

Crosslinked ethylene–vinyl acetate (EVA) copolymers with VA content of 28% by weight were prepared by a two‐step method by evenly dispersing the crosslinking agent (dicumyl peroxide) into the EVA matrix and then crosslinking at elevated temperatures. The crosslinking features of the samples were analyzed by Soxhlet extraction with xylene and dynamic mechanical measurements. All the samples were crystalline at room temperature, and the chemical crosslinks seemed to have little effect on the melting behavior of polyethylene segment crystals in the EVA copolymers. The shape recovery results indicated that only those specimens that had a sufficiently high crosslinking degree (gel content higher than about 30%) were able to show the typical shape memory effect, a large recoverable strain, and a high final recovery rate. The degree of crosslinking can be influenced by the amount of the peroxide and the time and temperature of the reaction. The response temperature of the recovery effect (about 61°C) was related to the melting point of the samples. The EVA shape memory polymer was characterized by its low recovery speed that resulted from the wide melting range of the polyethylene segment crystals. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1063–1070, 1999     

Surface photocrosslinking of ethylene–vinyl acetate copolymer films

Surface photocrosslinking of ethylene–vinyl acetate (EVAc) copolymer films containing benzophenone (BP) was investigated for the purpose of replacing a poly(vinyl chloride) floor. The photogelatin in the EVAc films was effectively observed after UV radiation in the presence of oxygen. The crosslinking reaction was initiated from the surface of the irradiated film, which was mainly due to the dehydrogenation and generation of macroradicals of polymer by the light absorption of BP. The experiments of polyethylene–VAc with BP showed that the VAc‐rich amorphous part in the EVAc copolymer works as a crosslinking site. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1741–1745, 2000     

Rheological properties of partially hydrolyzed ethylene–vinyl acetate copolymers

Studies have been made of steady-shear and dynamic viscosities for melts of two ethylene–vinyl acetate copolymers and their partially hydrolyzed derivatives using a Weissenberg rheogoniometer over the temperature range of 123–150°C with some tests at 160°C. The flow activation energy of all samples studied was essentially independent of shear stress. The introduction of hydroxyl groups in controlled concentrations, however, produced a complicated flow behavior. At low concentrations, there is a marked increase in Newtonian viscosity, flow activation energy, and shear dependence of viscosity. In contrast to previous reports, a further increase in all three functions was not observed with increasing vinyl alcohol concentration. Dynamic viscosities, in contrast, show monotonic increases with increasing hydroxyl group content, as do activation energies derived from the temperature dependence of the dynamic viscosity. These data may result from an increased chain cohesion due to hydrogen bonding of hydroxyl groups.     

Interphases in ethylene–vinyl acetate copolymer/steel sandwiches

The properties of a polymer near an interface with a substrate can be different from the bulk properties. To characterize the interphasial zone, the influence of the thickness of a polymer inserted between two steel sheets is carried out. The chosen polymer is a semi-crystalline ethylene–vinyl acetate copolymer with different amounts of vinyl acetate. Dynamic mechanical spectroscopy measurements were performed directly on the assemblies using a three-point flexure test in order to characterize the mobility of the amorphous phase. The crystalline properties were analyzed by differential scanning calorimetry. The mechanical transition temperature, T_mech, corresponding to the temperature at which the loss factor goes through a maximum was examined. The results show that at high thicknesses T_mech remains constant. However, when the polymer thickness decreases, T_mech increases greatly, indicating a decrease of mobility of the chains. This effect is seen whatever the vinyl acetate content. The crystalline properties are also modified with a higher proportion of small crystals for thin layers. For interfacial energy-minimization reasons, the vinyl acetate groups of the copolymer chains are oriented toward the polar steel surface. These orientation phenomena probably induce some reorganization of the phases, leading to more crystals that constitute physical ties, reducing the mobility of the amorphous phase. © 1997 John Wiley & Sons, Inc. J Appl Polm Sci 65:347–353, 1997     

Nitric acid digestion and crystallinity of ethylene–vinyl acetate copolymers

Nitric acid digestion studies of ethylene–vinyl acetate copolymers indicated that copolymers containing identical amounts of vinyl acetate but varying in melt index differed in crystallinity. These results were confirmed by x-ray analysis. The differences in crystallinity were interpreted as showing a variation in the degree of short-chain branching in the polyethylene segments of the copolymer chain. This variation was correlated with the conditions of synthesis.     

Study of elastic and thermoelastic properties of ethylene–propylene and ethylene–vinyl acetate copolymers. I. Elastic properties

The elastic properties of ethylene–propylene and ethylene–vinyl acetate copolymers crosslinked to different degrees were studied. A correction of the front factor with respect to temperature has been proposed for calculation of the concentration of network chains from shear modulus G. Deviations from the Gaussian approximation of the dependence of force on deformation were evaluated.     

Study of elastic and thermoelastic properties of ethylene–propylene and ethylene–vinyl acetate copolymers. II. Thermoelastic properties

The thermoelastic properties of ethylene–propylene and ethylene–vinyl acetate copolymers crosslinked to different degrees were studied. An equation was proposed for calculating the relative contribution of the internal energy, f_U/f, from the temperature dependence of shear modulus G. Analysis of a relation for calculating f_U/f derived on the basis of the Mooney-Rivlin equation was made.     

Effect of strain on the properties of an ethylene–octene elastomer with conductive carbon fillers

Composites that incorporate a conductive filler into an ethylene–octene (EO) elastomer matrix were evaluated for DC electrical and mechanical properties. Comparing three types of fillers (carbon fiber, low structure carbon black, and high structure carbon black), it was found that the composite with high structure carbon black exhibited a combination of properties not generally achievable with this type of filler in an elastomeric matrix. A decrease in resistivity at low strains is unusual and has only been reported previously in a few instances. Reversibility in the resistivity upon cyclic deformation is a particularly unusual feature of EO with high structure carbon black. The mechanical and electrical performance of the high structure carbon black composites at high strains was also impressive. Mechanical reinforcement in accordance with the Guth model attested to good particle–matrix adhesion. The EO matrix also produced composites that retained the inherent high elongation of the unfilled elastomer even with the maximum amount of filler (30% by volume). The EO matrix with other conducting fillers did not exhibit the exceptional properties of EO with high structure carbon black. Composites with carbon fiber and low structure carbon black did not maintain good mechanical properties, generally exhibited an increase in resistivity with strain, and exhibited irreversible changes in both mechanical and electrical properties after extension to even low strains. An explanation of the unusual properties of EO with high structure carbon black required unique features of both filler and the matrix. The proposed model incorporates the multifunctional physical crosslinks of the EO matrix and dynamic filler–matrix bonds. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 894–905, 2000     

Characterization of ethylene–propylene rubber and ethylene–propylene–diene rubber networks

The stress–strain (S/S) and the swelling equilibrium behavior in a series of ethylene propylene rubber (EPR) and ethylene propylene diene monomer (EPDM) networks were investigated and the results were employed to evaluate the effects of varying the cure conditions on the crosslinking efficiency in these networks. The S/S curve of completely swollen vulcanizates is in agreement with the predictions of rubber elasticity theory, while that of dry or partially swollen vulcanizates is fully described by the Mooney-Rivlin equation. ? values determined in benzene were found to vary linearly with v_r (v_r = equilibrium volume fraction of rubber in swollen sample). Crosslinking efficiency, moles of crosslinks produced per moles of crosslinking agent used, ranges from 3.7 in peroxide-cured EPDM (55% wt ethylene and 2.6% unsaturation) to 0.15 in similarly cured EPR (43% ethylene). Efficiency in the latter system improves to 0.6 by addition of a coagent (sulfur) to the cure formula. Crosslinking efficiency in EPDM (55% ethylene) was found to increase in the order: peroxide- > resin- > sulfur-cured. In the EPDM sulfur vulcanizates, changing the terpolymer in the cure formula resulted in significant changes in the crosslinking efficiency.     

The influence of precure dilation on the stress–strain properties of ethylene–propylene terpolymer networks

A study has been made of the influence of a dilating liquid, present during vulcanization, on the stress–strain behavior of EPT vulcanizates. Stress–strain measurements performed on the vulcanizates after extraction of the dilating liquid, taken over a range of strain rates, indicate the effective absence of time-dependent behavior and suggest that a close approach to equilibrium has been achieved. From the experimental data, values of the C₁ and C₂ constants of the empirical Mooney-Rivlin equation were derived. It was found that both C₁ and C₂ decreased with increased precure dilation, C₂ decreasing more rapidly than C₁. The observed decrease in C₁ is less than would be predicted by simple analogy with the Gaussian modulus; it is suggested that this difference might be explained by changes in network configuration and topology associated with the precure dilation.     

Photodegradation of ethylene–propylene copolymer and ethylene–propylene–ethylidenenorbornene terpolymer

Significant progress has been made in recent years regarding the photooxidation of olefin copolymers, but questions still remain. This paper reviews the progress and probes the photooxidative chemistry of ethylene–propylene (EP) and ethylene/propylene/diene monomer (EPDM) copolymers. Both stabilized and unstabilized polymer plaques were irradiated in a xenon are and the surface chemistry followed using infra-red spectroscopy. Model compounds were used to help elucidate the chemistry caused by unique structural features present in the copolymers. Volatile products evolved during photooxidation were determined giving valuable insight into the degradation chemistry.     

Flow properties of molten ethylene–vinyl acetate copolymer and melt fracture

In an investigation of the behavior and formation mechanism of melt fracture the flow properties of molten ethylene–vinyl acetate (EVA) copolymer in the region of high shear rate were measured with a capillary-type rheometer. EVA copolymer differs slightly in flow curve from low-density polyethylene (LDPE); it seems, however, that the difference is due to the difference in molecular weight distribution (MWD) rather than to the materials themselves. The fluidity of molten EVA copolymer having a narrow MWD is equivalent to that of LDPE having a broad MWD and, generally, EVA copolymer has a higher fluidity than LDPE. It is expected that the fluidity increases with incorporation of vinyl acetate at the same MWD and the same M?_w. The critical shear rate increases with melt index and temperature. It cannot be found that the materials themselves and the MWD directly influence the critical point of melt fracture formation when the melt index is taken as a parameter. The critical viscosity (η_c) at which melt fracture forms decreases in an almost straight line with an increase of melt index. It was found from the studies of end correction and behavior of melt fracture formation that melt fracture occurs at the inlet of the die, and it is supposed that the melt fracture formation is caused by the elastic turbulence in the flow pattern due to a failure of recoverable shear strain at the die inlet.     

Kinetics and extent of low-temperature crystallization of ethylene–vinyl acetate copolymers

The enthalpies of fusion of two types of EVA copolymers containing 9 and 16% of vinyl acetate, respectively, were investigated by DSC. After melting, the samples were cooled down and held at 10, 15, 20, and 25°C for different periods of time from 15 min to 2.5 months (only at 20°C). The enthalpy of fusion increased over the 2.5-month period for 9.3 and 11.3 J/g, respectively. There was a new small melting peak on the endotherm of the aged sample whose position and size depended on aging temperature and aging time. During 2.5 months, the peak shifted toward higher temperature for 8°C. The enthalpy of fusion and corresponding degree of crystallinity changed linearly with the logarithm of time, as is the case in high-temperature annealing or secondary crystallization at high temperatures. The rate and the extent of low-temperature crystallization of ethylene copolymers depend on the comonomer content, sequence length distribution, and temperature. © 1996 John Wiley & Sons, Inc.     

Relaxation of stress in polypropylene and ethylene–propylene–diene elastomer blend vulcanizates at moderate and high extensions

Measurements were made of the relaxation of the stress of stretched polypropylene (PP) and ethylene–propylene–diene elastomer blend vulcanizates at various strain levels. It was found that PP-blended vulcanizates showed greater relaxation than that of the gum vulcanizate at all extensions. There was a continual increase in the relaxation rate with the 10% PP-blended vulcanizate but an initial sharp decrease and then a flattening tendency with the above 10% PP-blended vulcanizate at an increasing stain level. An interesting observation of the study was that the rate of stress relaxation decreased linearly in two steps in the case of blend vulcanizates above 10% PP at 100% and above strain levels. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 2155–2162, 1998     

Structure–properties relationships in clay nanocomposites based on PVDF/(ethylene–vinyl acetate) copolymer blends

The relationships between the microscopic structure and the macroscopic properties in two sets of clay nanocomposites based on polymer blends comprised of poly(vinylidene fluoride) (PVDF) and ethylene–vinyl acetate copolymer (EVAc) were examined. In nanocomposites based on a polymer blend matrix with high content of polar groups (VAc) the dispersion of polar nanoclay leads to significant enhancement in toughness and a substantial increase in viscosity. However, in nanocomposite blends based on a less polar matrix (i.e. with fewer VAc groups) it is the hydrophobic organoclay that leads to higher modulus and stronger viscoelasticity. The dispersion of nanoparticles and the mechanical response are discussed in terms of emulsifying efficiency of the clay particles in the immiscible polymer blend, an effect that largely depends on the localized interactions between the polymer groups and the clay surface modifier. The potential of nanoclays to serve as matrix sensitive structure-directing agents in tailor-made materials is demonstrated.     

纤维增强弹性体复合材料膨胀节的工程设计方法

介绍了纤维增强弹性体膨胀节的性能特点与主要应用场合 ,探讨了其中纤维增强弹性体复合材料的选用、连接结构以及补偿量的工程设计方法。     

A paradoxical flame-retardant effect of nitrates in ATH-filled ethylene–vinyl acetate copolymer

In the course of a study of metal salts as flame retardants, it was surprisingly found that metal nitrates reduced the flammability of ATH-filled ethylene–vinyl acetate copolymer (EVA). The limiting oxygen index (LOI) of ATH-filled EVA was increased by the nitrates in the order of Cu(NO₃)₂·3H₂O > (NH₄)₂Ce(NO₃)₆ > Zn(NO₃)₂·6H₂O > Fe(NO₃)₃·9H₂O > Al(NO₃)₃·9H₂O > NaNO₃. The effects were not caused by the water of hydration. All metal nitrates except NaNO₃ reached a UL 94 V-2 rating at 3 phr. Based on TGA, DSC, FTIR, and gas detection, the proposed mechanism of the flame-retardant effect of nitrates is the oxidative degradation of the polymer to produce noncombustible products (CO₂ and nitrogen oxides) at a rate sufficient to interfere with the normal combustion process despite the exothermicity of the oxidative degradation. It is possible that surface carboxylic acid structures also contribute to the flame-retardant effect. © 1995 John Wiley & Sons, Inc.