The tensile properties of strain‐crystallizing vulcanizates. II. Stress relaxation and hysteresis

Stress relaxation on cessation of extension and hysteresis were examined in conventional and peroxide vulcanizates over a range of crosslink densities at room temperature and at 90°C. The rate of relaxation decreases with an increase in temperature and is attributed to slower nucleation of strain‐induced crystallites. The decrease in the volume fraction of extendable material as a result of strain‐induced crystallites has only a small effect on the rate at which the slope of the stress–strain curve rises. Crystallites that form on cooling, melt on heating, leaving an unaltered network, but when strain‐induced crystallites are melted by heating, they do not reform on cooling. The relaxed network now extends further before failure than a network in which extension was not interrupted by a heating–cooling cycle. This supports proposals in the preceding article that strain‐induced crystals increase tensile properties by altering the network deformation pattern and not by the deflection of propagating flaws, whose mechanism would require the heated sample to fail earlier due to its lower crystalline content. Hysteresis increases sharply at strains at which crystallization in the network becomes possible. The hysteresis ratio reaches a plateau value at higher strains. Relaxation affects both the extension and retraction stress–strain curves and it is proposed that hysteresis is determined by differences in the crystallinity during extension and retraction. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2423–2430, 2006     

Influence of pendent groups on the tensile properties of polyisoprene vulcanizates

Gum compounds of polyisoprene were vulcanized with a number of different curing systems to give networks with crosslink densities in two different ranges. Stress–strain curves were obtained upon rapid (500 mm/min) and slow (0.5 mm/min) extension. In tetramethylthiuram disulfide (TMTD)/sulfur and zinc dimethyldithiocarbamate/sulfur vulcanizates, which crystallize readily, failure occurred at higher tensile values upon rapid than upon slow extension and this is attributed to a greater contribution to tensile strength by a larger amount of stress‐induced crystallites. X‐ray diffraction showed that 2‐benzothiazole‐2,2′‐disulfide (MBTS)/sulfur vulcanizates did not stress‐crystallize and failure occurred at lower tensile values. Furthermore, samples extended rapidly failed at lower tensile values than did slowly extended samples. These differences, compared to TMTD vulcanizates, are attributed to extensive main‐chain modifications (pendent groups), causing delays in the movement of sections of the chain, leading to the load being unequally distributed between chains. The fewer load‐bearing chains ensure earlier failure. The addition of zinc stearate to TMTD/sulfur and MBTS/sulfur formulations increases the ability of vulcanizates of similar crosslink density to crystallize and enhances tensile properties of vulcanizates with similar crosslink densities, outcomes that are attributed to zinc stearate's promoting crosslinking of pendent groups and reducing impediments to crystallization and chain movement. Dicumyl peroxide–cured networks crystallize readily and exhibit a very rapid upturn in the stress–strain curve. However, failure occurs at lower stress values than apply to accelerated sulfur networks and it is suggested that the distribution of subchain lengths between crosslinks may contribute to their inferior properties. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2587–2596, 2001     

The tensile properties of strain‐crystallising vulcanisates. I. A new theory to explain strengthening

The tensile properties of conventional and peroxide vulcanisates were studied over a range of crosslink densities at room temperature and at 90°C. At 90°C the tensile strength and elongation at break of vulcanisates of lower crosslink density are superior to those at room temperature, while for vulcanisates of higher crosslink density the reverse applies. When strain‐induced crystallites form, they act as crosslinks shortening chains within the network. Shortened chains have lower entropies and a larger force is required for their continued extension, i.e., for a further reduction in their entropy. It is proposed that because these stiffer chains resist deformation, other less stiff chains are preferentially extended. This alters the network deformation pattern, forcing more chains to become load bearing and delaying the development of taut chains or chain sequences. Thus the formation of strain‐induced crystals leads to the slope of the stress–strain curve rising rapidly. At elevated temperatures the rate of nucleation of strain‐induced crystallites is slower but data on stress–strain curves obtained with different temperature programs show that, while strain‐induced crystallization is essential for the development of high tensile strength, delaying their formation to higher elongations is advantageous for high tensile properties. In vulcanisates of higher crosslink density the rate of crystallization at high temperatures becomes too slow. Rupture occurs before strain‐induced crystallites can form and protect the network by altering the network deformation pattern. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1562–1569, 2006     

Stress softening in carbon black-reinforced vulcanizates. Strain rate and temperature effects

Stress softening of carbon black-reinforced butadiene-styrene rubber was studied as a function of the rate and temperature of the original tensile deformation. To a good approximation, stress softening depends on the product of the extension rate and a temperature function which is analytically well represented by the familiar Williams-Landel-Ferry relationship. When the elongation of the original deformation is also varied, a good correlation is obtained between stress softening and the maximum stress attained in the original extension, irrespective of the particular combination of strain, strain rate and temperature used to achieve this stress. Variables which tend to increase the stiffness of the vulcanizate, such as increased degree of crosslinking or carbon black chain structure, also increase stress softening; dilution by plasticizers decreases it. Prestressing at high strain rates and low temperatures affects the stress–strain curve of the softened vulcanizates beyond the elongation of the original extension. Connections are established between stress softening and viscoelastic and failure behavior. The evidence presented favors the contribution of several mechanisms to the general phenomenon of stress softening. These are thixotropy of transient filler structures, network chain rupture, and breakage of “permanent” filler structure. The latter appears to be most important at high strain rates, low temperatures, and with highly reticulated “structure” blacks.     

Unusual stress-strain properties of natural rubber vulcanizates with high primary molecular weight

Summary Natural rubber vulcanizates with high primary molecular weight were prepared by mixing raw rubber and dicumyl peroxide in benzene followed by freeze-drying. The stress-strain properties of these vulcanizates were quite different from those prepared by conventional mastication method. The important characteristics of these vulcanizates is high tensile modulus, high tensile strength, large hysteresis loss, and higher degree of strain-induced crystallization. The difference in the stress-strain behavior between these rubbers and conventional vulcanizates are discussed from the standpoint of the network structure.     

Stress softening in natural rubber vulcanizates. Part III. Carbon black-filled vulcanizates

Stress softening (Mullins effect) occurs in rubber vulcanizates during the first and subsequent deformations. This paper shows the similarity of the degree of stress softening in both unfilled and carbon black-filled vulcanizates of natural rubber when stressed almost to break. This confirms the earlier investigations which were confined to moderate stresses. A simple interpretation of the tensile stress–strain results for filled rubber is that the strain in the rubber is increased by the presence of the filler, so that the ratio of the average strain in the rubber to the measured overall strain is given by a strain amplification factor. The usefulness of this concept is confirmed by showing the similarity of the stress-softened curves after normalizing the strains, provided the vulcanizates of both gum and filled vulcanizates were subjected to the same initial stress.     

Cure characteristics and mechanical properties of hydrogenated natural rubber/natural rubber blends

The cure characteristics and mechanical properties of blends consisting of hydrogenated natural rubber (HNR) and natural rubber (NR) blends were investigated. The HNR/NR blends at 50/50 wt ratio were vulcanized using various cure systems: peroxide vulcanization, conventional vulcanization with peroxide, and efficient vulcanization with peroxide. The HNR/NR vulcanizates cured by the combination of peroxide and sulfur donor (tetramethylthiuram disulfide, TMTD) in the efficient vulcanization with peroxide exhibited the best mechanical properties. It was also found that the hydrogenation level of HNR did not affect the tensile strength of the vulcanizates. The tensile strength of the blends decreased with increasing HNR content because of the higher incompatibility to cause the noncoherency behavior between NR and HNR. However, the HNR/NR vulcanizate at 50/50 wt ratio showed the maximum ultimate elongation corresponding to a co‐continuous morphology as attested to by scanning electron micrographs. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Chemical crosslink densit and network structure of natural rubber vulcanizates modified with phosphorylated cardnol prepolymer

The chemical crosslink density (CLD) and network structure of natural rubber (NR) vulcanizates, modified with phosphorylated cardanol prepolymer (PCP), have been studied by equilibrium swelling and other chemical methods. The PCP-modified NR vulcanizates showed lower CLD, as compared to the unmodified NR samples, the decrease being lesser for the semiefficient vulcanization (SEV) system, as compared to the conventional (CV) and efficient (EV) vulcanization system. The superior tensile characteristics of the PCP-modified vulcanizates of the SEV system is presumed to be partly due to the presence of an entangled network structure between the aliphatic segment of PCP and the isoprene chains, as evident from X-ray diffraction studies. The critical role of Zn⁺⁺ions in the crosslinking reactions, especially at higher concentrations of PCP, was evident from the increase in CLD at higher concentrations of ZnO. The reduction in the IR absorption intensity, in the presence of ZnO, indicated the probable complex formation of Zn⁺⁺ions with the phosphate groups of PCP. © 1994 John Wiley & Sons, Inc.     

Thermoplastic vulcanizates based on epoxidized natural rubber/polypropylene blends: Effect of epoxide levels in ENR molecules

Epoxidized natural rubbers (ENR) with various levels of epoxide groups were prepared. Thermoplastic vulcanizates based on 75/25 ENRs/PP blends with Ph‐PP compatibilizer were later prepared by dynamic vulcanization using sulfur curing system. Influence of various levels of epoxide groups on rheological, mechanical morphological properties, and swelling resistance of the TPVs was investigated. It was found that the mixing torque, apparent shear stress, apparent shear viscosity, tensile strength, and hardness properties increased with increasing levels of epoxide groups in the ENR molecules. This may be attributed to increasing level of chemical interaction between the methylol groups of the Ph‐PP molecules and polar functional groups of the ENR molecules. Also, the PP segments in the Ph‐PP molecules are capable of compatibilizing with the PP molecules used as a blend composition. In SEM micrographs, we observed finer dispersion of vulcanized rubber domains as increasing levels of epoxide contents. This corresponds to increasing trend of strength and hardness properties of the TPVs. An increasing trend of tension set and a decreasing trend of elongation at break were observed as increasing levels of epoxide groups in the ENR molecules. This is because of higher rigidity of the vulcanized ENR phase with higher epoxide groups. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3046–3052, 2006     

Swelling behaviour of rubber vulcanizates: 2. Effects of tensile strain on swelling

The effects of tensile strain on the swelling behaviour of acrylonitrile-butadiene copolymer rubber vulcanizates were studied by real-time pulsed nuclear magnetic resonance (n.m.r.) measurements and volume swelling measurements at equilibrium. It was shown that tensile strain causes an increase of the initial swelling rate evaluated by n.m.r. measurements and of the swelling ratio of the rubber matrix at equilibrium. This behaviour was discussed qualitatively in terms of the molecular mobility of the network chains on the basis of Treloar's theory for swelling under deformation. It was suggested that the presence of reinforcing fillers in the rubber matrix exerts two noticeable effects on swelling under deformation: (1) a transient effect through some oriented structure induced by stretching, which restricts the increase of the swelling rate; (2) a strain amplification effect, which causes the increase of the average local strain in the rubber matrix and enhances the swelling ratio more remarkably proportional to the filler concentration as compared with that of the unfilled system.     

Estimation by mechanical analysis of the molecular parameters of SBR vulcanizates at different cure conditions

The changes in the network structure of SBR-1712 during vulcanization were analyzed by means of a study of the stress–strain behavior at uniaxial extension at room temperature. In order to obtain different degrees of crosslinking, samples were cured at 414 K and 433 K at several times and were characterized by a rheometer. The conformational tube model was applied for the treatment of the stress–strain measurements of vulcanized samples. This theory allows the separation of crosslink and constraint contributions to the stress–strain behavior and relevant network parameters can be estimated. In this article the change with the temperature and time of cure of the average molecular mass of the mobile network chains, the crosslink density, the microscopic lateral tube dimension, and the root-mean-square end-to-end distance of the network chain are evaluated. © 1995 John Wiley & Sons, Inc.     

EPDM/POE动态硫化热塑性硫化胶的性能与结构

采用动态硫化法制备EPDM/聚烯烃弹性体(POE)热塑性硫化胶(TPV),研究共混温度和硫化体系配比对其性能的影响,并对其形态结构进行表征.结果表明:随着共混温度的升高和硫化体系用量的增大,共混体系的硫化速率逐渐加快;TPV的拉伸强度和拉断伸长率在共混温度为150℃左右时达到最大,拉断伸长率随着硫化体系用量的增大而减小,而拉伸强度则先减小后略有增大;随着共混温度的升高和硫化体系用量的增大,TPV撕裂强度呈现先增大后减小趋势;当硫黄用量为0.2份时,体系出现了明显的Payne效应,EPDM交联相以平均粒径为1μm左右的颗粒状态分散于POE连续相中.     

Morphological evolution and property changes of acrylate rubber/polyacetal thermoplastic vulcanizates during dynamic vulcanization process

A new type of thermoplastic vulcanizates (TPV) that is resistant to hot oil, high temperature, and aging was prepared using acrylate elastomer (ACM) and polyoxymethylene (POM) as raw materials. The phase transition process and micromorphology development of the blends with the change of dynamic vulcanization (DV) time, and to illustrate the effects of the phase transition and the degree of elastomer phase crosslinking on the macroscopic properties of TPV were investigated. It was shown that ACM crosslinking occurs continuously during 0–8 min of DV, whereas the phase inversion process occurs predominantly during 0–3 min. In contrast, the cross-section of the blends after phase inversion is flat, The constant elongation stress under small deformation, hardness, and permanent deformation after break are reduced, and the elongation at break, nonrelaxable modulus, and aging resistance are improved. The increase in the degree of vulcanization of the ACM phase resulted in a significant increase in the modulus of the blends under large deformation, an increase in the nonrelaxable modulus and relaxable modulus, an increase in the aging resistance, and a decrease in the elongation at tear were observed.     

Preparation of thermoplastic vulcanizates based on waste crosslinked polyethylene and ground tire rubber through dynamic vulcanization

An environmental‐friendly approach called high‐shear mechanical milling was developed to de‐crosslink ground tire rubber (GTR) and waste crosslinked polyethylene (XLPE). The realization of partial devulcanization of GTR and de‐crosslinking of XLPE were confirmed by gel fraction measurements. Fourier transform infrared spectral studies revealed that a new peak at 1723.3 cm^?1 corresponds to the carbonyl group (? C?O) absorption was appeared after milling. The rheological properties showed that the XLPE/GTR blends represent lower apparent viscosity after mechanical milling, which means that the milled blends are easy to process. Thermoplastic vulcanizates (TPVs) could be prepared with these partially de‐crosslinked XLPE/GTR composite powders through dynamic vulcanization. The mechanical properties of the XLPE/GTR composites increased with increasing cycles of milling. The raw XLPE/GTR blends could not be processed to a continuous sheet. After 20 cycles of milling, the tensile strength and elongation at break of XLPE/GTR (50/50) composites increased to 6.0 MPa and 185.3%, respectively. The tensile strength and elongation at break of the composites have been further improved to 9.1 MPa and 201.2% after dynamic vulcanization, respectively. Re‐processability study confirmed the good thermoplastic processability of the TPVs prepared. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Crystallization of vulcanizates. II. Low‐temperature crystallization as a function of extent of cure for polybutadiene vulcanized with dicumyl peroxide,tetramethylthiuram disulfide, 2‐bisbenzothiazole‐2,2′‐disulfide,and zinc–accelerator complexes

Polybutadiene compounds, vulcanized to various degrees of cure, were crystallized in a density column at ?16°C. The percentage crystallinity of vulcanizates was also determined by differential scanning calorimetry where samples, precooled at a programmed rate, were reheated. Curing with peroxides has little effect on either the rate or the extent of crystallization, except at very high crosslink densities, although the induction period prior to crystallization increases progressively with increased crosslink density. Tetramethylthiuram disulfide (TMTD)/sulfur and 2‐bisbenzothiazole‐2,2′‐disulfide (MBTS)/sulfur vulcanizates, cured for progressively longer periods, were found to have lower densities, a result attributed to an increase in free volume occasioned by the formation of accelerator‐terminated pendent groups on the polymer chain. The induction period for crystallization increases and both the rate and the extent of crystallization decrease with extent of cure. These changes are more marked for MBTS vulcanizates that do not crystallize once a gel has formed. Formulations with zinc stearate develop higher crosslink densities and crystallize to a greater extent on cooling, showing the effect of zinc stearate in the crosslinking of pendent groups. The densities of both zinc dimethyldithiocarbamate [Zn₂(dmtc)₄]– and zinc mercaptobenzothiazole [Zn(mbt)₂]–accelerated sulfur vulcanizates increase with cure time, a result attributed to the formation of ZnS in the compounds. Zn₂(dmtc)₄ compounds crystallize extensively on cooling, pointing to limited main‐chain modification. It is suggested that main‐chain modification in these vulcanizates may comprise cyclic sulfide formation. Zn(mbt)₂ compounds crystallize less readily than Zn₂(dmtc)₄ compounds, but to a greater extent than MBTS/sulfur compounds. The crystallization of the vulcanizates is discussed in terms of vulcanization reactions that give rise to crosslinking with the different formulations used. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2573–2586, 2001     

Peroxide vulcanized EPDM rubber/polyhedral oligomeric silsesquioxane nanocomposites: Vulcanization behavior,mechanical properties,and thermal stability

Effects of two different polyhedral oligomeric silsesquioxane (POSS), an acrylisobutyl POSS (AIBuPOSS) containing an acrylate group along with seven isobutyl group on its cage and an octaisobutyl POSS where the acrylate group is absent, on vulcanization behavior, mechanical properties, and thermal stability of peroxide vulcanized ethylene‐propylene‐diene rubber (EPDM) were investigated. The POSS was incorporated into the EPDM by melt mixing with POSS content of 0–10 part per hundred of rubber (phr). Oscillating disk rheometer analysis revealed that the acrylate group of the POSS are activated by dicumyl peroxide and improves the peroxide crosslinking efficiency of EPDM rubber. Solid state ²⁹Si‐nuclear magnetic resonance spectroscopy analysis and field emission scanning electron microscopy with energy dispersive X‐ray analysis of the EPDM/POSS vulcanizates showed that the AIBuPOSS are covalently grafted onto the EPDM chain during vulcanization and are dispersed uniformly at the nanometer scale in the rubber matrix. The EPDM/AIBuPOSS nanocomposites exhibit great improvement in tensile, tear strength, and modulus with a concurrent increase in elongation‐at‐break. Enhanced thermal stability in the nanocomposite was also observed. POLYM. ENG. SCI., 55:2814–2820, 2015. © 2015 Society of Plastics Engineers     

Mechanical properties and microstructure of acrylonitrile–butadiene rubber vulcanizates reinforced by in situ polymerized phenolic resin

The phenolic resin (PF) was incorporated into acrylonitrile–butadiene rubber (NBR) vulcanizates by in situ polymerization during the vulcanization process. It was found that the tensile strength, tearing strength, and tensile strength (300% elongation) could be considerably enhanced 59.4, 80.2, and 126.4%, respectively, at an optimum PF content of only 15 phr, but the elongation at break and shore A hardness were only slightly affected on the basis of silica‐reinforced NBR vulcanizates. A systematic study of the PF structure formed within the NBR matrix using various experimental schemes and procedures has revealed that the PF resin would form the localized discontinuous three‐dimensional interconnected network structures in the NBR matrix. The substantial reinforcement of PF on the mechanical properties of vulcanized NBR were attributed to the morphology, high flexibility, and moderate stiffness of the PF phases and their excellent bonding with rubbers through “rubber to rubber” and interface layer. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

丁苯橡胶热空气老化性能变化之间的相关性

通过热空气老化试验,建立了丁苯橡胶(SBR)热空气老化拉伸性能变化率与邵尔A硬度变化值之间的相关性。结果表明,在热空气老化过程中,无论是普通硫化体系还是有效硫化体系,SBR硫化胶老化后的邵尔A硬度变化值随老化时间的延长而呈指数趋势增加,且老化后扯断伸长率变化率与邵尔A硬度变化值之间呈线性关系;在普通硫化体系下,硫化胶表现为老化后扯断伸长率变化率和拉伸强度变化率随老化时间的延长而呈指数趋势下降,而老化后拉伸强度变化率与邵尔A硬度变化值之间以及拉伸强度变化率与扯断伸长率变化率之间呈线性关系。     

Influence of 1,2‐polybutadiene on properties of dicumyl peroxide cured brominated butyl rubber

Peroxide curing of brominated butyl rubber (BIIR) is an attractive topic, but the degradation of BIIR during the curing is a drawback needed to be overcome. Coagent assisted peroxide curing system is an attractive and effective choice in order to increase the crosslink density of rubbers. 1,2‐polybutadiene (1,2‐PB) is used as a crosslinking coagent for the curing of BIIR by dicumyl peroxide (DCP), and the effect of 1,2‐PB on the curing characteristics, crosslink density, and mechanical properties is investigated. The addition of 1,2‐PB affects the curing characteristics of BIIR compound and significantly increases the crosslink density of BIIR vulcanizates. With increasing 1,2‐PB content, the tensile strength and stresses at a given extension of BIIR vulcanizates increase, but the elongation at break decreases. A stress‐softening effect of the carbon black filled BIIR vulcanizates is observed and becomes more pronounced with increasing 1,2‐PB content. The addition of 1,2‐PB increases the stress relaxation index of BIIR. GPC and ¹³C‐NMR results indicate 1,2‐PB participates in the crosslinking reaction, and the existence of 1,2‐PB component in the insoluble fraction of BIIR/1,2‐PB vulcanizates is confirmed by solid‐state ¹³C‐NMR. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43280.     

An investigation into the high temperature strength and reinforcement of natural rubber

The failure criterion for amorphous polymers relating hysteresis at break with energy input to failure in tensile stress–strain tests was found to be obeyed only at very high or very low temperatures in natural rubber. Tensile results between about 80°C and 130°C show a high degree of scatter, and this behavior is attributed to the ability of natural rubber to crystallize at high strains. The modification of tensile properties by the addition of carbon black in natural rubber is also discussed and compared with published results from SBR. The effect of changing the degree of crosslinking on the failure properties in both dicumyl peroxide and sulfur-cured vulcanizates of natural rubber is also considered. It is found that differences in failure properties can be accounted for by the use of a crosslinking parameter from simple rubber elasticity theory in some of the failure equations.