Influence of the filler type on the rupture behavior of filled elastomers

This work is devoted to the rupture behavior of elastomers filled with carbon black (CB) or silica. Two elastomers have been studied: one which crystallizes under strain, natural rubber (NR), and another one which does not crystallize, styrene butadiene rubber (SBR). The study of the crack propagation of Single Edge Notched specimen (SENT) during stretching at different speeds focuses on the crack initiation and crack deviation phenomenon. This deviation is of main importance in the materials crack resistance as it leads to a large increase in the energy needed for rupture. The deviation in filled or unfilled NR is controlled by crystallization, which is a slow process. In unfilled SBR, deviation is controlled by polymer chain orientation, which is hindered by relaxation mechanisms. The introduction of fillers promotes strain amplification, and strain anisotropy in the crack tip region of the notched samples, and therefore crack deviation. In term of energy density at break of the SBR composites, the SBR filled with silica treated with a covering agent is the most efficient. Thus, a weak interface between the silica and SBR promotes better rupture properties. When comparing Silica and CB filled NR, the highest strain energy to rupture is also obtained with silica. This might be due to the weaker filler‐matrix interface for silica. Thus, these results evidence the kinetic aspect of the rupture, and of the mechanisms it involves: the polymer relaxation, the crystallization (for NR), and the filler‐matrix interaction and decohesion, all of them being strongly interrelated. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Enhancement of stain‐induced crystallization in polylactide via thermal preannealing

Stretching of amorphous polylactide above glass transition temperature can lead to strain‐induced crystallization, which is mainly determined by stretching rate and temperature. This study demonstrates that thermal annealing prior to stretching is alternative to enhance strain‐induced crystallization of a given polylactide at the identical stretching conditions. The local order generated during thermal annealing acts as physical crosslinks to effectively alleviate molecular relaxation and thus facilitates the advent of strain‐induced crystallization. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 39993.     

白炭黑/炭黑复合填充对NR性能的影响

在白炭黑和炭黑填料总量一定的情况下,研究了不同白炭黑/炭黑配比对NR性能的影响。结果表明,白炭黑用量增加,NR硫化速率下降,体系中炭黑填料网络被破坏且白炭黑-橡胶间较弱的相互作用也会对NR硫化胶力学性能产生一系列影响。DMA结果表明,加入20~25份白炭黑对提高硫化胶的抗湿滑性和降低滚动阻力最为有效。     

Improved mechanical properties and special reinforcement mechanism of natural rubber reinforced by in situ polymerization of zinc dimethacrylate

The peroxide‐cured natural rubber (NR) was reinforced by in situ polymerization of zinc dimethacrylate (ZDMA). The experimental results showed NR could be greatly reinforced by ZDMA. The tensile strength and the hardness of NR/ZDMA composites increased with the content of ZDMA. The reinforcement mechanism was studied further. Both high crosslinking density provided by ionic crosslinking and strain‐induced crystallization improved the mechanical properties. The crosslinking density was determined by an equilibrium swelling method and the crystallization index was measured by Wide‐angle X‐ray diffraction (WXRD). When the amount of ZDMA was high, the ability of strain‐induced crystallization decreased, due to the strong interactions between the rubber phase and the hard poly‐ZDMA (PZDMA) nanodispersions. At the moment, the increasing ionic crosslinking density made up for the effect of the drop of the strain‐induced crystallization, and played a more important role in the reinforcement. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Synergistic reinforcement of nanoclay and carbon black in natural rubber

The synergistic reinforcement of nanoclay (NC) and carbon black (CB) in natural rubber (NR) has not been much studied. Therefore, the reinforcement mechanism was probed using synchrotron wide‐angle X‐ray diffraction and transmission electron microscopy (TEM) observation and analyzed in terms of tube model theory. A synergistic effect in reinforcement between NC and CB was proved by the marked enhancement in tensile strength from 11.4 MPa for neat NR to 28.2 MPa for NR nanocomposite with 5 wt% NC and 20 wt% CB. From a study of crystallization under deformation it was found that crystallization plays a less important role in the reinforcement of NR/NC/CB. Analysis using tube model theory provided more evidence for the synergistic effect. NR containing a combination of NC and CB exhibited an increase of topological tube‐like constraints in comparison with NR/CB. That is to say, in NR/NC/CB nanocomposites, a CB–NC local filler network, as indicated by TEM images, induced a more entangled structure in which mobility of rubber chains was hindered for lateral fluctuations by the presence of neighboring chains. The synergistic reinforcement of NC and CB in NR/NC/CB nanocomposites can be reasonably understood as due to the formation CB–NC local filler networks. Copyright © 2010 Society of Chemical Industry     

Role of stearic acid in the strain-induced crystallization of crosslinked natural rubber and synthetic cis-1,4-polyisoprene

Strain-induced crystallization of crosslinked natural rubber (NR) and its synthetic analogue, cis-1,4-polyisoprene (IR), both mixed with various amounts of stearic acid (SA), were investigated by time-resolved X-ray diffraction using a powerful synchrotron radiation source and simultaneous mechanical (tensile) measurement. No acceleration or retardation was observed on NR in spite of the increase of SA amount. Even the SA-free IR crystallized upon stretching, and the overall crystallization behavior of IR shifted to the larger strain ratio with increasing SA content. No difference due to the SA was detected in the deformation of crystal lattice by stress for both NR and IR. These results suggested that the extended network chains are effective for the initiation of crystallization upon stretching, while the role of SA is trivial. These behaviors are much different from their crystallization at low temperature by standing, where SA acts as a nucleating agent.     

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     

Analysis of variation of molecular parameters of natural rubber during vulcanization in conformational tube model. II. Influence of sulfur/accelerator ratio

A natural rubber (NR) with a conventional sulfur cure system and a ratio of sulfur/accelerator (Ω) equal to 3 was investigated. The network structure of the NR during vulcanization was analyzed using a model of rubber elasticity based on the tube concept, which was applied to the treatment of the stress–strain measurements. The influence of cure time and temperature on the chemical crosslinks density was analyzed. The values were compared with those obtained by means of an equilibrium volume swelling measurement. The differences between samples of NR cured with Ω = 3 and 1.5 were analyzed. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2747–2755, 1999     

Dynamic viscoelasticity behaviors of magnesium dimethacrylate/natural rubber composites with different cure extent

In this article, the dynamic viscoelasticity characteristics of the natural rubber (NR)/magnesium dimethacrylate (MDMA) composites with different cure extent are investigated using a Rubber Process Analyzer 2000 (RPA2000). A weak filler–filler network of MDMA is formed in the uncured compounds. MDMA polymerize during peroxide curing and change to the polymerized MDMA (PMDMA), which can aggregate to form strong filler network in the lightly cured composite. The ionic crosslinks and NR crosslinks influence the dynamic viscoelasticity characteristics of the MDMA/NR composites with different cure extent significantly. Although the PMDMA aggregates have strong attraction with each other to form filler network, the ionic crosslinks are not favorable to rebuild the PMDMA network when the NR crosslink network is mature. Finally, the torque verifies that the ionic crosslinks can be influenced by small strain magnitudes (<0.1°) whereas the NR crosslink network is not influenced. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Mechanical and morphological properties of cellular NR/SBR vulcanizates under thermal and weathering ageing

The effects of addition of two chemical blowing agents in cellular rubber blend of natural rubber (NR) and styrene‐butadiene rubber (SBR) at a fixed blend ratio of 1 : 1 on cure characteristics, and mechanical and morphological properties were invesigated. The chemical blowing agents used in this work were Oxybis (benzene sulfonyl) hydrazide (OBSH) and Azo dicarbonamide (ADC). Three different fillers, fly ash (FA) particles, precipitated silica, carbon black (CB) at their optimum concentrations of 40 phr were used, the FA and silica particles being chemically treated by bis‐(3‐triethoxysilylpropyl) tetrasulphide. The results suggested that the overall cure time decreased with OBSH and ADC contents. The OBSH was more effective in cure‐acceleration of the NR/SBR blend than the ADC. The NR/SBR vulcanized foams produced by OBSH and ADC agents had closed‐cell structures. The specific density and mechanical properties of the blend tended to decrease with increasing blowing agent content. The CB gave NR/SBR foams with smaller cell size, better cell dispersion, and higher mechanical properties than the precipitated silica and FA particles. The heat ageing and weathering resulted in an increase in tensile modulus and hardness, but lowered the tensile strength, ultimate elongation and tear strength. The elastic recovery for cellular NR/SBR vulcanizates with FA was superior to that with CB and silica, the elastic recovery of the blends decreasing with blowing agent content. Resilience property was improved by the presence of gas phases. The optimum concentration of OBSH and ADC to be used for NR/SBR vulcanizates was 4 phr. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Effect of Grafted Carbon Black on Properties of Vulcanized Natural Rubber

Summary To modify carbon black (CB) surface, in situ grafting of natural rubber (NR) onto the CB surface by solid-state method was used to get grafted carbon black (GCB). The vulcanization characters of NR compounds employed CB and GCB were investigated. Crosslinking density of vulcanized NR was measured by equilibrium swelling method. As a result, it was found that GCB could improve both cure rate and crosslinking density. Moreover, the mechanical properties of vulcanized NR filled with GCB were enhanced to a large extent compared with vulcanized NR filled with CB. No Payne Effect was observed in GCB filled NR compounds during dynamic properties testing, indicating that there was no filler network and GCB was dispersed uniformly in NR matrix, which was proved by SEM micrographs. Based on above results, it was considered that the grafting modification weakened filler-filler interaction and enhanced filler-polymer interaction.     

Quantification of the distribution of carbon black in natural rubber/polybutadiene blends by differential scanning calorimetry

The distribution of carbon black (CB) in uncured blends of natural rubber (NR) and polybutadiene (PB) can be quantified using DSC. The crystallization temperature of PB containing CB, measured during cooling from the melt, increases with the CB loading. This phenomenon was used to calculate the CB content in PB and conversely in NR in NR/PB/CB mixtures. Two different blends containing N550 CB were studied. A deeper analysis was performed over the entire composition range of NR/PB blends filled with N234 CB. Two different trends were highlighted depending on composition. Below 50 wt% of NR in the elastomer phase, the crystallization temperature of PB is higher than that of pure PB indicating that a part of the CB is dispersed in PB. Above 50 wt% of NR, the crystallization temperature of PB is lower than that of pure PB, indicating that the whole CB is in the NR phase and that fractionated crystallization of PB simultaneously occurs. Both phenomena are consistent as the dispersed phase morphology is favored by a large increase of the matrix viscosity by the filler. For both CBs, the results indicate that CB has larger affinity for NR than for PB. This tendency was confirmed by transmission electron microscopy. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Strain‐induced crystallization behavior of phenolic resin crosslinked natural rubber/clay nanocomposites

Nanocomposites of natural rubber (NR) and unmodified clay were prepared by latex compounding method. Phenolic resin (PhOH) was used to crosslink NR. Crosslinked neat NR was also prepared for comparison. The structure–property relationship of uncrosslinked and crosslinked NR/clay nanocomposites was examined to verify the reinforcement mechanism. Microstructure of NR/clay nanocomposites was studied by using transmission electron microscopic (TEM), X‐ray diffraction (XRD), wide angle X‐ray diffraction (WAXD), and small angle X‐ray scattering (SAXS) analyses. The results showed the evidence of intercalated clay together with clay tactoids for the nanocomposite samples. The highest tensile strength was achieved for the crosslinked NR/clay nanocomposite. The onset strain of deformation induced the crystallization of NR for nanocomposites was found at almost the same strain, and furthermore their crystallization was developed at lower strain than that of the crosslinked neat NR because of the clay orientation and alignment. However, at high strain region, the collaborative crystallization process related to the clay dispersion and conventional crosslink points in the NR was responsible to considerably high tensile strength of the crosslinked NR/clay nanocomposite. Based on these analyses, a mechanistic model for the strain‐induced crystallization and orientational evolution of a network structure of PhOH‐crosslinked NR/clay nanocomposite was proposed. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42580.     

Continuous ultrasonic devulcanization of carbon black‐filled NR vulcanizates

The continuous ultrasonic devulcanization of natural rubber (NR) filled with various concentrations of carbon black (CB) indicated a minimum of crosslink density and gel fraction at an intermediate amplitude, which is independent of CB content. An attempt was made to improve the efficiency of devulcanization by use of various chemicals (1,3 Diphenylguanidine, 2‐Mercaptobenzothiazole, Thianaphthene). However, these experiments did not indicate any improvement in comparison with devulcanization without chemicals. An idea of adding fresh CB into devulcanized compound, which has been shown to improve mechanical properties in the case of styrene–butadiene rubber (SBR), was tested in the present study for CB filled NR compound. The obtained result indicated that an addition of fresh CB to devulcanized CB‐filled NR did not lead to an improvement in mechanical properties upon revulcanization. The revulcanization recipe was optimized to improve the mechanical properties of revulcanized CB‐filled NR vulcanizates. It was found that CB‐filled NR upon revulcanization retained its strain‐induced cystallizability with the tensile strength and elongation at break at about 50 and 70% level of the virgin vulcanizates. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 2340–2348, 2001     

Probing the reinforcing mechanism of graphene and graphene oxide in natural rubber

Natural rubber (NR) containing graphene (GE) and graphene oxide (GO) were prepared by latex mixing. The in situ chemically reduction process in the latex was used to realize the conversion of GO to GE. A noticeable enhancement in tensile strength was achieved for both GO and GE filled NR systems, but GE has a better reinforcing effect than GO. The strain‐induced crystallization was evaluated by synchrotron wide‐angle X‐ray diffraction. Increased crystallinity and special strain amplification effects were observed with the addition of GE. The incorporation of GE produces a faster strain‐induced crystallization rate and a higher crystallinity compared to GO. The entanglement‐bound tube model was used to characterize the chain network structure of composites. It was found that the contribution of entanglement to the conformational constraint increases and the network molecular parameters changes with the addition of GE and GO, while GE has a more evident effect than GO. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Influence of polymer–filler interactions on retraction behaviors of natural rubber vulcanizates reinforced with silica and carbon black

In general, silica‐filled rubber compounds contain a silane coupling agent to improve the filler dispersion and polymer–filler interactions. The silane coupling agent modifies the silica surface and makes crosslinks between the rubber and the silica. Influence of the modification of silica on the retraction behaviors of natural rubber (NR) vulcanizates reinforced with silica and carbon black was studied. Variation of the retraction behaviors of NR vulcanizates with filler composition was also investigated. The vulcanizates containing the silane coupling agent were recovered faster than those without the silane coupling agent. The recovery difference between the vulcanizates without and with the silane coupling agent increased with increased silica content. For the vulcanizates containing the silane coupling agent, the retraction behaviors were nearly the same, irrespective of filler composition. But, for the vulcanizates without the silane coupling agent, the vulcanizate was recovered more and more slowly as the silica content increased. The experimental results are explained with the polymer–filler interactions, modification of silica surface, and formation of crosslinks between silica and rubber. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 691–696, 2006     

Continuous ultrasonic devulcanization of unfilled NR vulcanizates

Sulfur-cured unfilled natural rubber (NR) is successfully devulcanized in a continuous extrusion process under the application of high-power ultrasonic energy. The die characteristics and ultrasonic power consumption are measured. A unique correlation is found between the crosslink density and gel fraction of the devulcanized NR. This correlation is independent of the processing parameters, such as barrel temperature, die gap, flow rate, and amplitude of ultrasound. However, these parameters do influence the degree of devulcanization. In most cases, the degree of devulcanization is found to pass through a maximum at an intermediate level of ultrasonic energy. It is hypothesized that simultaneous breakup and reformation of crosslinks occur during the devulcanization of NR, with the relative contribution of each being determined by the process parameters. The cure curves and mechanical properties of the revulcanized NR are studied. The mechanical properties are found to depend on the revulcanization recipe. On optimizing it, tensile strength as high as 14.2 MPa is achieved, which is about 70% of that of the virgin NR vulcanizate. Ultimate elongation as high as 670% is obtained, which is the same as that of the virgin NR vulcanizate. Such stress–strain behavior is an indication that the devulcanized NR maintains the strain-induced crystallization characteristics inherent to the virgin NR vulcanizates. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 2007–2019, 1998     

填料-橡胶的化学和物理作用及其对补强的影响

研究了炭黑N330和白炭黑与NR／BR的相互作用及其对补强的影响。炭黑N330与NR/BR相互作用的化学交联密度占40％，对拉伸强度的贡献率为82．8％；白炭黑/偶联剂KH—846与NR/BR相互作用的化学交联密度占66．7％，对拉伸强度的贡献率为80．7％；炭黑／白炭黑/偶联剂KH-846与NR/BR相互作用的化学交联密度约占70％，对拉伸强度的贡献率约占94％。3种补强体系对拉伸强度的贡献都是化学作用占优势，贡献率超过80％。     

Tensile properties and reinforcement mechanisms of natural rubber/vapor‐grown carbon nanofiber composite

Natural rubber (NR) composites with different contents of 1, 3, 10, and 20 wt% vapor‐grown carbon nanofibers (VGCFs) were synthesized using a solvent casting method. The initial modulus of composites was improved by 26.5 %/wt% as the VGCFs were added, and the NR/3 wt%VGCF composite had the largest tensile strength. The experiment values of initial moduli agreed well with the values predicted by the Halpin‐Tsai theory. The reinforcement mechanisms of the composites were investigated by scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy and wide‐angle X‐ray diffraction (WAXD). It was found that an efficient stress transfer occurred from NR to VGCFs under the uniaxial stretching. The addition of 10 wt% VGCFs could promote the nucleation process of NR, which resulted in the characteristic of the strain‐induced crystallization (SIC) in NR/10 wt%VGCF composite even for low strain. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

Enhanced mechanical properties of graphene/natural rubber nanocomposites at low content

Graphene/natural rubber (GE/NR) nanocomposites were prepared by a modified latex mixing method combined with in situ chemical reduction. It was found that the GE nanosheets are well dispersed and have strong interfacial interaction with NR. Thus, adding a low content of GE can remarkably increase the tensile strength and the initial tensile modulus of NR. With incorporation of as low as 0.5 phr of GE, a 48% increase in the tensile strength and an 80% increase in the initial tensile modulus are achieved without sacrificing the ultimate strain. But further increasing the GE loading degrades the tensile strength and the ultimate strain. Dynamic mechanical measurement indicates that the storage modulus of the nanocomposites is greatly enhanced with addition of GE, while the loss tangent peak is depressed due to the reduced mobility of the rubber molecules. The reinforcement effect of GE on NR is interpreted as a change in the strain induced crystallization and network structure of the nanocomposites, based on the analysis of Mooney ? Rivlin plots and the tube model.© 2013 Society of Chemical Industry