High temperature vulcanization of elastomers: 3. Network structure of efficiently vulcanized natural rubber mixes

The effect of vulcanization temperature (140–200°C) and time on the structures of pure gum natural rubber vulcanizates with two different N-cyclohexyl-2-benzothiazylsulphenamide (CBS): sulphur ratios (A, 3·5:1·5; B, 6·0:0·4 CBS/S) has been determined. Analyses of vulcanizates were carried out as reported in Part 2. Results show that both mixes are efficient in crosslinking, resulting in mainly monosulphidic crosslinks and relatively few modifications of the rubber chains. Raising the cure temperature from 140°C reduces the density of chemical crosslinks, particularly those of monosulphidic crosslinks, obtainable in the vulcanizates. This decrease in crosslink density has been shown to be irreversible with respect to cure temperature. The formation of intramolecular sulphidic groups and zinc sulphide increases with rising cure temperature, but this increase is small compared with that reported for the conventional CBS-accelerated system. The main difference between mixes A and B is that mix A yields a higher level of crosslinks and a major proportion of cyclic sulphides as main-chain modification. Negligible chain scission occurs during vulcanization at 140–200°C. These network results are interpreted mechanistically, and essential network features for obtaining good physical properties in high temperature vulcanizates are deduced.     

Effect of vulcanization temperature and vulcanization systems on the structure and properties of natural rubber vulcanizates

The effect of vulcanization temperature (150°–180°C) on the structure and technical properties of gum natural rubber vulcanizates with four different 2-(morpholinodithio)-benzothiazole: sulphur ratios (A, 0.6:2.4; B, 1.5:1.5; C, 2.4:0.6; D, 3.0:0.0) at the respective optimum cure times has been determined. The influence of cure temperature on (a) the chemical crosslink density; (b) the distribution of crosslink types; (c) the extent of sulphidic main chain modifications and (d) the zinc sulphide formation was investigated. Results show that elevated cure temperatures produce a network with lower crosslink density, in particular a lower polysulphidic crosslink density. The formation of intramolecular sulphidic groups and zinc sulphide increase with increasing cure temperatures. The possibility of chain scission during vulcanization, as examined by a quantitative analysis of the sol—gel data, was found to be negligible. The network results have been correlated with the technical properties.     

Structural characterization of vulcanizates. Part IX. Comparison of the vulcanization of natural rubber and cis-trans-isomerized natural rubber with sulfenamide-accelerated sulfur and dicumyl peroxide vulcanization systems

Large variations in the microstructure of 1,4-polyisoprenes, from ca. 100% cis-trialkylethylene groups, as in natural rubber (NR), to ca. 40% cis- and 60% trans-trialkylethylene groups, as in an equilibrium-isomerized NR, have little influence on the overall chemistry of vulcanization of the polyisoprenes by a N-cyclohexylbenzothiazole-2-sulfenamide-accelerated sulfur system or by a dicumyl peroxide system. The peroxide crosslinks the equilibrium-isomerized NR more efficiently than it crosslinks NR; this is attributed to the sulfur dioxide, which is used to isomerize the NR, scavenging some of the nonrubber constituents in the NR, which are known to compete with the rubber hydrocarbon for reaction with free radicals from the peroxide. By comparison with NR vulcanizates, the corresponding equilibrium-isomerized NR vulcanizates have higher values of the C₂ term of the Mooney-Rivlin stress–strain equation and higher χ (polymer–swelling liquid interaction parameter) values of the Flory-Huggins equation.     

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.     

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.     

Stress softening in natural rubber vulcanizates. Part V. The anomalous tensile behavior of natural rubber

Stress-strain cycling of natural rubber to high strains produces greater softening than amorphous rubbers, but only if the force on the sample is relaxed below a certain value during the cycle. This phenomenon, attributed to crystallization shows why nonrelaxing tests give a longer fatigue life than relaxing tests.     

Oxidative aging of natural rubber vulcanizates. Part I. The scission of peroxide vulcanizates

A new method has been developed for measuring the number of molecules of oxygen required (?) to cause a main chain scission in peroxide vulcanizates of natural rubber. This method is based on experimentally derived relationships and is not dependent on statistical theories of rubber elasticity. The value ? is only slightly temperature dependent, ranging from 28 at 65°C. to 22 at 100°C. The effects of several types of antioxidant have been examined, in all cases ? was greatly reduced. The effect of antioxidant 2246 has been studied in more detail and the results indicate that there is a high probability of scission occurring at the termination step in the oxidation chain.     

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.     

天然橡胶硫化动力学研究

采用硫化仪测定了2种不同凝固工艺的天然橡胶（NR）的硫化过程,探讨了制胶工艺、配方和温度对硫化动力学参数的影响。研究结果发现,随着温度的上升,硫化速率常数迅速上升。在纯胶体系中,微生物凝固胶的活化能高于酸凝固胶。当加入炭黑后,酸凝固胶的活化能升高,而微生物凝固胶的活化能下降。     

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     

Radiation vulcanization of natural rubber with polyfunctional monomers

This study presents the effect of the polyfunctional monomers (PFMs) triallylcyanurate (TAC), triallylisocyanurate (TAIC), trimethylolpropane trimethacrylate (TMPT), ethylene glycol dimethacrylate (EDMA) and zinc diacrylate (ZDA) on the mechanical properties of natural rubber (NR) crosslinked by electron beam (EB) processing. Dependence of mechanical properties and crosslink density on irradiation dose was determined from a dose range of 50 to 200 kGy. The control blends, obtained with benzoyl peroxide as curing agent, were prepared by blending on a laboratory roller and the control sample curing was accomplished on hydraulic press at 160 °C. The results showed an increase in mechanical properties and crosslink density due to the introduction of PFMs. Also, based on the comparison between EB irradiation and dibenzoyl peroxide vulcanization efficiency, the experimental results show that EB irradiation gave the best results.     

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%.     

Structural characterization of vulcanizates part X. Thiol-disulfide interchange for cleaving disulfide crosslinks in natural rubber vulcanizates

The thiol-disulfide exchange reaction is shown to be applicable to cleavage of disulfide crosslinks in accelerated sulfur vulcanizates of natural rubber. The reaction, in conjunction with the previously reported selective cleavage of polysulfide crosslinks, is used to determine the distribution of crosslink types for several accelerated sulfur vulcanization systems as a function of cure time. Discrepancies between the results and published results obtained using the reagent sodium di-n-butyl phosphite for disulfide crosslink cleavage are discussed.     

超声硫化的实验研究

成功地利用超声波对天然橡胶进行了硫化，所得试件展示了与传统硫化方法制得的试件相同的硫化，所得试件展示了与传统硫化方法制得的试件相同的性能。但该工艺表现出许多传统硫化方法所不具备的优点。用滞后能量损失理论及声化学作用，解释了超声硫化的机理。     

天然胶乳橡胶手套硫化问题的探讨

<正>一、引言胶乳硫化能充分改变或改善胶乳胶料性能,通常在胶乳配料时提高温度的情况下进行,硫化后的胶乳称为预硫化胶乳。胶乳硫化一般只适用于天然胶乳,是部分或完     

Long-time sulfenamide-accelerated sulfur vulcanization of natural rubber/chlorobutyl rubber compounds

Sulfenamide-accelerated sulfur vulcanization of natural rubber/chlorobutyl rubber compounds has been investigated at temperatures from 155 to 175°C over 0.1 to 400 min. Continuous measurements in a Cone Rheometer were used to estimate the extent of crosslinking, which was plotted against cure time. On the basis of a kinetic analysis, two first-order vulcanization reactions, crosslinking and degradation, have been evaluated. Over the temperature range studied, there is no significant difference between the values of activation energy for these reactions. The rate of the degradation is slower by a factor of 20 than the rate of crosslinking. The degradation reaction can be limited by increasing the “efficiency” of the vulcanizing system.     

Sulfur vulcanization of natural rubber accelerated with 2-mercaptobenzothiazole plus tetramethylthiuram disulfide

The sulfur vulcanization of NR by a MBT-TMTD mixed accelerator system both in the presence and absence of ZnO and stearic acid with or without DCP has been studied. The rate of decomposition of DCP in the presence of both MBT and TMTD is quite similar to that in the MBT system alone. The reduction in crosslinking due to DCP is dependent mainly on MBT. The decomposition product of TMTD may contribute to it only at the later stage of vulcanization. Though TMTD has no influence on the decomposition rate, it reacts with MBT at least in the initial stage of vulcanization and suppresses the retardation caused by MBT on DCP vulcanization in accordance with the free sulfur decrease, the nature of crosslinking formation both in the presence and absence of DCP, and the methyl iodide treatment of the vulcanizates. The vulcanization process of the MBT-TMTD-S-NR system has been interpreted in terms of both free radical and polar mechanisms. The domination of either of these depends on the dominant amount of either MBT or TMTD in the acclerator ratio. According to the initial high rate of crosslink formation, free sulfur decrease and also the initial additiveness of crosslinking in stocks containing DCP, the vulcanization process of MBT-TMTD-ZnO-St. acid-S-NR system has been explained in terms of an ionic mechanism. The pronounced synergistic nature of such systems has been interpreted by the enhanced activation of MBT-S-ZnO-St. acid complex due to the dithiodicarbamate ion formed in the initial stages of vulcanization, and also by the activation of TMTD accelerated vulcanization due to the mercaptobenzothiazylion.     

On the potential of organoclay with respect to conventional fillers (carbon black,silica) for epoxidized natural rubber compatibilized natural rubber vulcanizates

Onium modified montmorillonite (organoclay) was compounded with natural rubber (NR) in an internal mixer and cured by using a conventional sulfuric system. Epoxidized natural rubber with 50 mol % epoxidation (ENR 50) in 10 parts per hundred rubber (phr) was used as a compatibilizer in this study. For comparison purposes, two commercial fillers: carbon black (grade N330) and silica (grade vulcasil‐S) were used. Cure characteristics were carried out on a Monsanto MDR2000 Rheometer. Organoclay filled vulcanizate showed the lowest values of torque maximum, torque minimum, scorch, and cure times. The kinetics of cure reaction showed organoclay could behave as a cocuring agent. The mechanical testing of the vulcanizates involved the determination of tensile and tear properties. The improvement of tensile strength, elongation at break, and tear properties in organoclay filled vulcanizate were significantly higher compared to silica and carbon black filled vulcanizates. In terms of reinforcing efficiency (RE), organoclay exhibited the highest stiffness followed by silica and carbon black filled vulcanizates. Scanning electron microscopy revealed that incorporation of various types of fillers has transformed the failure mechanism of the resulting NR vulcanizates compared to the gum vulcanizates. Dynamic mechanical thermal analysis (DMTA) revealed that the stiffness and molecular relaxation of NR vulcanizates are strongly affected by the filler–rubber interactions. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 2438–2445, 2004     

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.