Dynamic properties of PBNA–natural rubber vulcanizates

Some crystalline organic compounds containing a β-naphthyl group stiffen rubbers considerably. This paper discusses the dynamic behavior of these systems when subjected to oscillatory strain of increasing amplitude. Modulus–strain amplitude plots similar to that in carbon black–natural rubber systems are obtained. The implication of these results to reinforcement theory is discussed.     

Step‐strain stress relaxation of carbon black‐loaded natural rubber vulcanizates

Step‐strain stress relaxation experiments were performed on natural rubber vulcanizates of various carbon black (HAF) concentrations by subjecting the samples to a very rapid strain and fixing its length at the deformed state. Time–temperature superposition in the viscoelastic region was evaluated to investigate the effect of temperature on the relaxation times of the rubbery composites. Remarkably, it was observed that, at higher HAF concentrations, increasing the temperature had a lesser effect on decreasing the overall stress values. That was attributed to the lower number of elastomeric chains per unit volume due to the agglomeration of the carbon black particles. The energy barrier resulting from the adsorption of the rubbery chains on the filler particles was insufficient to drastically reduce the diffusion and rearrangement of the polymer chains. The activation energy of the rubber‐like deformation calculated from the time–temperature superposition was shown to be independent of temperature. Interestingly, the viscosity coefficients showed a large increase with a modest addition of the carbon black. This is due to the long‐range nature of the temporary bonds formed between the polymer molecules and the surface‐active carbon black. The stress–strain of the rubbery composites was shown to behave in a Gaussian manner in accordance with the Mooney–Rivlin relationship. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3387–3393, 2004     

New aspects of transversal1H-NMR relaxation in natural rubber vulcanizates

Summary Structure and dynamics of unfilled and carbon black filled natural rubber vulcanizates were investigated by means of transversal¹H-NMR-relaxation. A simple theoretical model including different motions in polymer network was extended by the discussion of an anisotropy parameter of segmental motion not only for the intercrosslink chains but also for the dangling ends. It makes characterization of the filling state of the network possible to approximate. The effect of increasing correlation times _f of the fast anisotropic local motion with reduction of the molecular mass of intercrosslink chains which was observed, but not cleared up in previous investigation is evidently smaller by using this modified model in accordance with the expectation of a local motion. The determined values of M_c are in good agreement with the results of fits by using the unmodified relaxation function.¹³C-NMR-spectra and stress-strain-experiments are leading in most cases to similar results, especially for M_c.     

Green strength of natural rubber: The origin of the stress–strain behavior of natural rubber

The variation of the green strength of natural rubber (NR) with the preparation method of the film was studied to elucidate the origin of the stress–strain behavior characteristics of NR in connection with the structure of branch points in NR. A rubber film prepared via casting from NR latex showed the highest modulus and green strength in comparison with films prepared via casting from a toluene solution and via the hot pressing of dry rubber. The modulus and green strength of the NR latex‐casting film decreased after the hot‐press treatment of the film. On the other hand, no difference was observed for synthetic cis‐1,4‐polyisoprene rubber with the preparation method of the film or the heat treatment. This stress–strain behavior characteristics of NR can be ascribed to the changes in the branch points of the NR film with the casting method, which may result in differences in entanglement and crystallizability on stretching. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Influence of oxygen content in the atmosphere on chemical stress relaxation of natural rubber vulcanizates: 1. Relaxation mechanisms of sulphur-cured systems

Stress relaxation of sulphur-cured natural rubbers of different crosslinked structures was investigated in an atmosphere containing variable amounts of oxygen. At high temperature, even very low contents of oxygen strongly affected the degradation of these vulcanizates. The stress relaxation of each vulcanizate was analysed in relation to the sol fraction and the swelling of sample degraded under the same condition. The scission mechanism of the vulcanizates depended on the quantity of oxygen in the atmosphere. Under nitrogen, the scission occured mainly near the crosslink site in all the samples. However, in the case of their being small amounts of oxygen present it took place near the crosslink site in the initial stages of degradation and then along the main chain at the later stages. In the atmosphere of comparative high oxygen content, the degradation seemed to result from the oxidative scission along the main chain. The degradation mechanisms of sulphur-cured natural rubbers were also discussed.     

Efficiency of antioxidants in thick-walled natural rubber samples evaluated by differential scanning calorimetry and compression stress–relaxation

The effect ot two antioxidants, IPPD and DENA, on the aging properties of TMTD-vulcanized natural rubber was studied by DSC. In the case of IPPD, the results were compared with mechanical measurements. The DSC measurements were carried out on specimens taken at different depths from cylindrical samples. The decrease in oxidative induction time was less pronounced for IPPD than for DENA at the edge of the samples. In the interior, the opposite behaviour was observed.     

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     

Prediction of stress–relaxation data of some nylons from stress–strain data

Six commercially available nylons were tested for stress–strain and stress–relaxation properties. Use was made of a recently revealed approach to predicting viscoelastic properties beyond the linear region from an analysis of the stress–strain curve. The equations used were three-dimensional employing the Halsey–Eyring viscoelastic model. The necessary constants were calculated and, when applied to the appropriate equations, reconstituted the stress–strain curve and predicted the stress–relaxation curves quite well.     

Behaviour of the spin-spin relaxation time for natural rubber vulcanizates under large deformation

The spin-spin relaxation time, T₂, for DCP-cured natural rubber with various crosslink densities, v_e, has been measured under various deformation. T₂ is separated into two components: one is the long T₂ component, T_2L, for the mobility of amorphous network chains, the other is the short one, T_2S, for that of the strain-induced crystalline chains. T_2L decreased exponentially with increasing extension ratio,α, and the decreasing rate was more remarkable with increasing v_e. The α and v_e dependence of T_2L has been quantitatively explained by the equation experimentally derived by Nishi et al.T_2L under various extension increased and became almost constant with increasing temperature, while the corrected fraction of T_2S, T_2S (%), gradually decreased. The apparent melting point, T_m, at which the corrected T_2S (%) was zero under various deformation was determined. The α dependence of T_m, has been discussed by using Flory's equation.     

Elastic behavior of natural rubber filled vulcanizates

The reinforcement effect of carbon black and, the effect of accelerator-to-sulfur ratio variation on the elastic behavior of natural rubber vulcanizates have been studied. The Mooney–Rivlin relation was used to describe the behavior of the rubber matrix, and values of constants c₁ and c₂ have been evaluated with the use of the strain-amplification factor. The stress softening of the vulcanizates tested has also been examined.     

Studies on double networks in natural rubber vulcanizates

The concept of double networks is a rather new idea, by which one imparts chain orientation in elastomers. Double networks were made in natural rubber vulcanizates cured with a single and a new binary accelerator system. Double networks with different extensions were prepared and their effects on tensile properties were analyzed. The influence of extent of initial cure on double‐network formation was examined. Thermal stability of the double network formed was analyzed by ageing of the double networks and was found to improve with residual extension. Crosslink density of the networks formed was determined by swelling methods and stress–strain analysis. It was found that crosslink density increased with double‐network formation and residual extension. The stress–strain behavior and moduli were analyzed to study the effect of these properties on double‐network formation. Double networks were hardly affected by the binary accelerator system. Based on the studies it was found that residual extension was the major factor determining the final properties. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1068–1076, 2004     

Influence of the cure level on the monomeric friction coefficient of natural rubber vulcanizates

An experimental investigation was performed in order to correlate the changes observed in dynamic modulus and loss tangent of vulcanized natural rubber with its network structure. A set of samples of natural rubber vulcanized at 413 K and 433 K for different times was prepared to achieve different network structures in the compound material. These networks were analyzed by means of solid‐state NMR techniques in order to distinguish the sulfur bridges formed during vulcanization. The crosslink density was calculated using swelling techniques for each cure condition. Dynamic mechanical tests were performed using a free decay pendulum in the range of temperatures between 213 K and 373 K and oscillating frequencies between 0.1 s^?1 and 20 s^?1. Loss tangent data were analyzed within the frame of the KMF model and the variation of the monomeric friction coefficient with the crosslink density was obtained. This variation is a consequence of the network structure formed in the different stages of the curing process. Copyright © 2004 Society of Chemical Industry     

Stress relaxation in carbon-black-filled rubber vulcanizates at moderate strains

Stress-relaxation measurements have been made on rubber vulcanizates containing carbon black. Evidence has been produced for the existence of a distinct relaxation process which occurs within the first 60 s of relaxation at room temperature. This process becomes less important as the strain is increased and disappears when the temperature is increased or the rubber is swollen. It is suggested that this relaxation is the result of breakdown of structure formed from the carbon-black particles.     

Stress softening in natural rubber vulcanizates. Part IV. Unfilled vulcanizates

Stress softening (Mullins effect) is observed in gum natural rubber vulcanizates. The magnitude of this softening is similar to that in carbon black-filled vulcanizates. The amount of stress softening is slightly greater in vulcanizates cured to produce predominantly polysulfide crosslinks than in those containing monosulfide or carbon–carbon crosslinks. The total recovery of stress softening in the vulcanizates containing monosulfide or carbon–carbon crosslinks suggests that the phenomenon is attributable to a quasi-irreversible rearrangement of molecular networks due to localized nonaffine deformation resulting from short chains reaching the limit of their extensibility. This non-affine deformation results in a displacement of the network junctions from their initial random state.     

Properties of natural rubber vulcanizates containing mechanochemically devulcanized ground tire rubber

The devulcanization of ground tire rubber (GTR) was carried out with a self-designed pan-mill type mechanochemical reactor. Gel fraction and crosslink density measurements confirmed the occurrence of stress induced mechanochemical devulcanization of GTR. The partially devulcanized GTR (dGTR) was blended with virgin natural rubber (NR) at different ratios. The curing characteristics and mechanical properties of these composites were investigated and compared with those composites of raw ground tire rubber (rGTR) and NR. The results showed that the tensile properties of the dGTR/NR vulcanizates were much better than those of the rGTR/NR vulcanizates, which are comparable to or even better than the virgin vulcanizate, indicating the significant benefit of mechanochemical devulcanization. At the GTR content of 10%, the tensile strength of the dGTR/NR blends increased to 23.2 MPa from 13.7 MPa of the rGTR/NR blends, enhanced by 69% through partial devulcanization of GTR, and the elongation at break increased by 47%.     

A novel grafting‐modified waste rubber powder as filler in natural rubber vulcanizates

Polycardanol was synthesized from cardanol, paraformaldehyde, p‐toluenesulfonic acid, phosphoric acid (H₃PO₄), and phosphorus pentoxide (P₂O₅) via a two‐step process. Results indicated that polycardanol is an acid with high molecular weight and can be self‐crosslinked at high temperature. A modified WRP (MWRP) grafted by long chain can be obtained from the reaction between WRP and polycardanol. The sulfur content of MWRP is 0.27%, which is lower than that of WRP by 0.47%. The oxygen content of MWRP is higher by 13% than that of WRP. The phosphorus content of MWRP reaches 5.25%. The water contact angle of MWRP is 91.5°, whereas that of WRP is 123.7°. The properties of the WRP/NR and MWRP/NR composites were also investigated. MWRP/NR possesses higher tensile strength than WRP/NR because of the enhanced interfacial interaction between MWRP and the NR matrix. Post‐treatment is also conducive for MWRP/NR to improve its tensile strength at high MWRP content. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42993.     

Effect of mica addition on the properties of natural rubber and polybutadiene rubber vulcanizates

The rheometric, mechanical, and dynamic mechanical properties as well as fracture surfaces of natural rubber–mica and polybutadiene rubber–mica vulcanizates were studied. Mica was used in the range of 0–30 phr and the rheometric study was carried out at 160°C. The results indicate that the mechanical properties are improved as filler addition increases. Dynamic mechanical testing was used to analyze the observed mechanical behavior. The two elastomers showed different fracture behaviors. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2156–2162, 2003     

Influence of internal strain on change of crosslink density of natural rubber vulcanizates by thermal ageing

Change of crosslink density of natural rubber (NR) vulcanizates by thermal ageing at 60 °C has been studied under swollen conditions in solvents to investigate the influence of internal strain applied to the vulcanizate on the crosslink density change. The internal strain was controlled by swelling with various solvents such as n‐hexane, toluene, tetrahydrofuran (THF), methanol and water. The order of degree of the swelling is toluene ≈ THF > n‐hexane > methanol > water. The influence of curing system has been investigated with the vulcanizates cured by the conventional and EV cure systems. After thermal ageing, the apparent crosslink densities of the swollen vulcanizates in n‐hexane, toluene and THF decrease irrespective of the cure systems, while that of the swollen vulcanizate in water increases. For the swollen vulcanizates in methanol, the apparent crosslink density of the vulcanizate with the conventional cure system after thermal ageing increases while that of the vulcanizate with the EV cure system decreases. The decrement of the apparent crosslink density of the vulcanizate after thermal ageing becomes larger and larger upon increasing the internal strain. © 2001 Society of Chemical Industry