Influence of rubber composition on change of crosslink density of rubber vulcanizates with EV cure system by thermal aging

Variation of the crosslink density of a rubber vulcanizate depending on the rubber composition after the thermal aging was studied with single rubber, biblend, and triblend vulcanizates of natural rubber (NR), butadiene rubber (BR), and styrene‐butadiene rubber (SBR). The efficient vulcanization (EV) system was employed to minimize the influence of free sulfur in the vulcanizate on the change of the crosslink density. Thermal aging was performed at 40, 60, and 80°C for 20 days with 5‐day intervals. The crosslink densities of the vulcanizates after the thermal aging increase. For the single rubber vulcanizates, variation of the crosslink density by the thermal aging has the order: SBR > BR > NR. For the biblend vulcanizates, variations of the crosslink densities of the NR/SBR and SBR/BR blends are larger than that of NR/BR blend. Variation of the crosslink density of the vulcanizate increases by increasing the SBR content in the vulcanizate. Variation of the crosslink density of the rubber vulcanizate depending on the rubber composition was explained by miscibility of the blends, combination reaction of the pendent groups, and mobility of the pendent group. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1378–1384, 2000     

Effect of crosslink structures on dynamic mechanical properties of natural rubber vulcanizates under different aging conditions

The effects of crosslink structures on the dynamic mechanical properties (DMPs) of unfilled and carbon black N330‐filled natural rubber (NR) vulcanizates cured with conventional (CV), semiefficient (SEV), and efficient (EV) cure systems and having about the same total crosslink densities were investigated before and after aerobic and anaerobic aging at 100°C. The three unfilled NR vulcanizates cured with the CV, SEV, and EV systems had about the same mechanical loss factor (tan δ) values at about 0°C but showed some apparent differences in the tan δ values in the order EV > SEV > CV at relatively high temperatures of 40–80°C before aging. However, N330‐filled NR vulcanizates gave higher tan δ values than the unfilled vulcanizates and showed little effect of the crosslink types on the tan δ at different temperatures over the glass‐transition temperature (T_g) before aging. Aerobic heat aging increased the T_g and tan δ values of the vulcanizates over a wide range of temperatures from ?80 to 90°C that was mainly due to the changes in the total density and types of crosslinks. The unfilled vulcanizates cured with the CV system showed the greatest change in DMP because of their poor resistance to heat aging. Aerobic heat aging of NR vulcanizates caused a more significant change in the DMP than anaerobic heat aging because of the dominant effect of the oxidative degradation during aerobic heat aging on the main‐chain structure, crosslink structures, and DMPs of the vulcanizates. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 710–718, 2001     

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.     

Influence of the rubber blend ratio on blowout behaviors of carbon black‐reinforced natural rubber/styrene‐butadiene rubber

Blowout behaviors of NR/SBR blend composites reinforced with carbon black were studied using a microwave oven and variation of the blowout time and temperature with the blend ratio was investigated. Morphology of the interior of the sample before exploding was observed and change of the crosslink density was measured. The blowout time became slower and the blowout temperature became higher as the SBR content of the specimen increased. The specimen with higher NR content had more cavities in the interior just before explosion. The crosslink density became lower by coming close to the blowout and crosslink density of the inner part was more reduced than that of the outer part. New organic materials were found in the burst region after blowout and they might be decomposed products of the polymer chains. The SBR specimen showed better blowout properties than the NR one. Principal sources to cause the blowout were found to be formation of the cavities in a rubber article, reduction of the crosslink density, and dissociation of the rubber chains. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Mechanical properties and crosslink density of rare earth‐modified high‐abrasion furnace‐filled powdered natural rubber

High‐abrasion furnace‐filled powdered natural rubber [P(NR/HAF)] has more advantages than traditional HAF‐filled bale NR (NR/HAF) because of its better environmental performance and easier processing quality, but its lower mechanical properties are disadvantageous. To improve the mechanical properties, rare earth‐modified HAF‐filled powdered NR [P(NR/HAF‐Ln)] (Ln = Sm,La,Pr) was prepared by means of coacervation–coprecipitation, using rare earth‐modified HAF as separant and filler. The effect on mechanical properties of P(NR/HAF‐Ln) vulcanizate exerted by the emulsifier/HAF ratio, powdering temperature, Ln/HAF ratio and type of Ln, and HAF content were studied. The results indicated that when optimum formulation, the mechanical properties of P(NR/HAF‐Ln) vulcanizate were better than P(NR/HAF) vulcanizate. In addition, the relationship of the apparent crosslink density and HAF content of P(NR/HAF‐Ln), P(NR/HAF), NR/HAF vulcanizates was also investigated, along with their SEM microphotographs of tensile fracture surface, which indicated that the excellent mechanical properties of P(NR/HAF‐Ln) vulcanizate was attributed to correct amount of Ln that could increase crosslink density and reinforce the interface structure of NR matrix/HAF‐Ln particle. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1755–1762, 2006     

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     

三种微生物脱硫废乳胶及其填充天然橡胶的力学性能比较

利用三种微生物鞘氨醇单胞菌、脂环酸芽孢杆菌和酵母菌对废乳胶(WLR)进行了脱硫再生,考察了脱硫过程中微生物的生长情况以及脱硫前后WLR溶胀和交联密度的变化,并将WLR和脱硫WLR(DWLR)以不同份数填充到天然橡胶(NR)中,对比了DWLR/NR硫化胶和WLR/NR硫化胶的力学性能和交联密度。结果表明,利用鞘氨醇单胞菌脱硫的WLR溶胀值较高,交联密度较低。随着乳胶用量的增加,填充NR硫化胶的拉伸强度和交联密度逐渐降低,扯断生长率逐渐增加。DWLR/NR硫化胶的力学性能明显优于WLR/NR硫化胶,且鞘氨醇单胞菌脱硫的DWLR/NR的力学性能明显优于其它两种微生物脱硫的DWLR/NR硫化胶。     

Mechanical properties of deproteinized natural rubber in comparison with synthetic cis‐1, 4 polyisoprene vulcanizates: Gum and black‐filled vulcanizates

Gum and black‐filled vulcanizates having various crosslink densities were prepared from 2 types of rubber, namely, deproteinized natural rubber (DPNR) and synthetic cis‐1, 4 polyisoprene vulcanizates (IR). Their mechanical properties, such as tensile strength, tear strength, abrasion loss, and heat buildup resistance, at various crosslink densities as well as at similar optimum crosslink density were compared. For both gum and black‐filled systems, IR possessed a higher crosslink density than that of DPNR at a fixed curative content. Tensile and tear strength of all vulcanizates passed through a maximum with increasing crosslink density. For gum vulcanizates, tensile and tear strengths of DPNR and IR below the maximum were not much different. However, IR had a narrower tear strength peak relative to DPNR. At a comparable optimum crosslink density, DPNR exhibited higher tensile strength and crack growth resistance than IR. For black‐filled vulcanizates, tensile and tear strengths, and heat buildup resistance of DPNR and IR at a given crosslink density were similar. The results revealed that the properties of gum samples were more dependent upon crosslink density than the black‐filled ones because the reinforcement by carbon black overshadowed the intrinsic properties of the rubbers. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1139–1144, 2005     

Deformation and toughness of polymeric systems: 3. Influence of crosslink density

In part 1 of this series the phenomenon of a critical ligament thickness (ID_c) below which brittle polymers become ductile was investigated for polystyrene (PS). Using the thermoplastic polystyrene-poly(2,6-dimethyl-1,4-phenylene ether) (PS-PPE) model system, it was demonstrated in part 2 of this series that the absolute value of ID_c as well as the maximum toughness (i.e. maximum strain to break) was dependent on the network density of the polymer used. In this study the toughness and ID_c of crosslinked thermosetting polymers were investigated using epoxides based on the diglycidyl ether of bisphenol A as a model system. The crosslink density (v_c) is varied between values comparable with (v_c = 9 × 10²⁵ chains m⁻³), up to values much higher than (v_c = 235 × 10²⁵ chains m⁻³), the entanglement density in the thermoplastic PS-PPE system. The maximum macroscopic toughness proportional to the strain to break (λ_macr) or given by the slow-speed fracture toughness (G_Ic) and the notched high-speed tensile toughness (G_h) of core-shell rubber-modified epoxides uniquely increases with an increasing molecular weight between crosslinks (M_c). Only by using extreme testing conditions (notched high-speed impact testing), could the ID_c of a limited range of epoxides be determined: 0.21 μm (v_c = 9 × 10²⁵ chains m⁻³) ≤ ID_c ≤ 0.29 μm (v_c = 14 × 10²⁵ chains m⁻³). Both the experimentally determined values of ID_c and the toughness of the epoxides compare well with the values determined for the entangled thermoplastic PS-PPE model system in the same range of network densities, elucidating the principal similarity of the influence of entanglements and crosslinks on the deformation processes. Good agreement was observed between the experimentally determined values of ID_c of the epoxides and the values predicted by the simple model introduced in part 2 of this series.     

高密度聚乙烯/硅橡胶热塑性硫化胶的制备与性能

采用动态硫化法制备了高密度聚乙烯（HDPE）/硅橡胶（MVQ）热塑性硫化胶（TPV）,考察了HDPE与MVQ的质量比及压片工艺对TPV的硫化特性、力学性能及表面亲疏水性能的影响。结果表明,随着MVQ用量的增加,TPV的动态硫化程度逐渐升高。随着压片时间的延长,TPV的拉伸强度、撕裂强度和扯断伸长率均大幅提高。随着MVQ用量的增加,TPV与水的接触角逐渐增加,相应的表面张力逐渐减小。     

Thermoplastic vulcanizates based on epoxidized natural rubber/polypropylene blends: Effect of compatibilizers and reactive blending

Epoxidized natural rubber (ENR) was prepared using the performic epoxidation method. TPVs based on ENR/PP blends were later prepared by melt‐mixing processes via dynamic vulcanization. The effects of blend ratios of ENR/PP, types of compatibilizers, and reactive blending were investigated. Phenolic modified polypropylene (Ph‐PP) and graft copolymer of maleic anhydride on polypropylene molecules (PP‐g‐MA) were prepared and used as blend compatibilizers and reactive blending components of ENR/Ph‐PP and ENR/PP‐g‐MA blends. It was found that the mixing torque, apparent shear stress and apparent shear viscosity increased with increasing levels of ENR. This is attributed to the higher viscosity of the pure ENR than that of the pure PP. Furthermore, there was a higher compatibilizing effect because of the chemical interaction between the polar groups in ENR and PP‐g‐MA or Ph‐PP. Mixing torque, shear flow properties (i.e., shear stress and shear viscosity) and mechanical properties (i.e., tensile strength, elongation at break, and hardness) of the TPVs prepared by reactive blending of ENR/Ph‐PP and ENR/PP‐g‐MA were lower than that of the samples without a compatibilizer. However, the TPVs prepared using Ph‐PP and PP‐g‐MA as compatibilizers exhibited higher values. We observed that the TPVs prepared from ENR/PP with Ph‐PP as a compatibilizer gave the highest rheological and mechanical properties, while the reactive blending of ENR/PP exhibited the lowest values. Trend of the properties corresponds to the morphology of the TPVs. That is, the TPV with Ph‐PP as a blend compatibilizer showed the smallest rubber particles dispersed in the PP matrix, while the reactive blending of ENR/PP‐g‐MA showed the largest particles. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4729–4740, 2006     

Influence of phenolic curative on crosslink density and other related properties of dynamically cured NR/HDPE blends

Dynamically cured 60/40 NR/HDPE blends with various amounts of phenolic curative were prepared in an internal mixer at 160°C. A simple blend (i.e., the blend without curative) was also prepared using the same materials and blend proportion for comparison purposes. Mechanical, dynamic, and morphological properties; swelling resistance and crosslink density of the blends were investigated. It was found that the thermoplastic vulcanizates (TPVs) gave superior mechanical and dynamic properties than the simple blend. Furthermore, the mechanical properties in terms of elongation at break, modulus and tensile strength and elastic response in dynamic test in terms of storage modulus increased with increased loading amount of the curative. The complex viscosity also increased but the tan δ and tension set decreased with increased loading level of the curative. The crosslink density of the TPVs was estimated based on the elastic shear modulus. It was found that the crosslink density of the blends increased with increased loading levels of the curative while the degree of swelling decreased. This correlated well with the trend of mechanical and dynamic properties. SEM micrographs were used to confirm the level of mechanical and dynamic properties. It was found that the simple blend at a given blend ratio exhibited co‐continuous phase morphology. However, the TPVs showed micron scale of vulcanized rubber domains dispersed in a continuous HDPE matrix. The size of vulcanized rubber domains decreased with increasing amounts of the curative. This led to greater interfacial adhesion between the phase and hence superior mechanical and dynamic properties. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

The structure change of dynamically fatigued unfilled natural rubber vulcanizates

The submicrostructure of dynamically fatigued unfilled natural rubber vulcanizates was investigated by using scanning electron microscope (SEM) and atomic force microscope (AFM). AFM photographs showed the sample surface roughness became worse after tensile fatigue and the largest surface undulation was as twice that of the unfatigued sample. SEM photographs showed that many micropores of 10¹–10² nm, a sort of defect, occurred on the cross section of samples after tensile fatigue. The surface roughness became weaker and the size of the micropore was reduced to a few to dozens of nanometers with the addition of antiaging agent N‐(1,3‐dimethyl butyl butyl)‐N′‐phenyl‐p‐phenylene diamine (4020); furthermore, the mechanical properties and dynamic viscoelastic properties in the later period of fatigue changed much. E′ decreased greatly and tan δ increased obviously with the extension of fatigue. It indicated that 4020 was only effective in the early period of tensile fatigue.© 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

指数函数模型拟合NR／顺丁橡胶硫化胶疲劳断裂曲线

采用指数函数模型λ＝ＡｅｘｐＢｌｏｇＮ拟合４种不同配方ＮＲ／顺丁橡胶硫化的疲劳破坏曲线,结果表明,计算值与实测值吻合良好,相对偏差为－２．８３％－２．８９％,且分布较均匀,相关系数为０．９９６６－０．９９９５。     

纳米氧化锌对天然胶乳膜干燥及硫化性能影响研究

采用失重法和溶胀法研究了普通氧化锌和纳米氧化锌活化的天然胶乳膜的干燥动力学,以及干燥过程中胶乳膜交联密度变化.结果表明:天然胶乳厚胶膜在成膜后仍含有大量水分并且去除困难.纳米氧化锌活化胶膜的干燥速率明显高于普通氧化锌胶膜.干燥温度对干燥过程影响显著,干燥时间随干燥温度升高明显降低.随着干燥的进行,2种氧化锌活化的胶乳膜的交联密度均迅速上升,在相同条件下,纳米氧化锌活化胶乳膜交联密度均比普通氧化锌的大.纳米氧化锌对天然胶乳膜的干燥及硫化过程均有促进作用.     

微生物凝固天然橡胶交联密度的研究

采用核磁共振交联密度仪,考察了微生物凝固天然橡胶（NR-m）和酸凝固天然橡胶（NR-a）在硫化过程交联密度（XLD）、网链含量（AMc）、自由小分子含量（AT2）以及相邻交联点间的分子质量（Mc）的动态变化规律。研究结果表明,NR-m硫化胶的XLD比NR-a的高,NR-m硫化胶磁化曲线随时间衰减的速率比NR-a快,纵向松弛时间（T1）和横向驰豫时间（T2）比NR-a低,NR-m硫化胶网链部分的质量含量（AMc）明显高于NR-a的,而自由末端数则明显比NR-a的低。这表明微生物凝固法促进了天然橡胶的硫化作用,提高了硫化胶的交联密度。     

Effect of crosslink density on transport of industrial solvents through polyether based polyurethanes

Polyurethanes based on PPG 2000 with variable concentrations of TDI and TMP were prepared and used for sorption studies, employing homologous series of hydrocarbons such as benzene, toluene and xylene. The sorption was observed to be non-Fickian in nature. The solubility parameter of the polyurethane series was observed to be 9·7(calcm^-3)^1/2. The polymer solvent interaction parameter χ was found to be lowest in benzene, suggesting higher interaction with it. The sorption and diffusion coefficients were observed to increase with a decrease in the degree of crosslinking. Molecular weights between crosslinks were calculated using the Flory–Rehner equation and compared with those obtained theoretically. © 1998 Society of Chemical Industry     

Thermo-oxidative ageing and its effect on the network structure and fracture mode of natural rubber vulcanizates

The changes in network structure and fracture mode of unfilled and filled natural rubber vulcanizates, during ageing at 100°C have been studied by chemical analyses and scanning electron microscopy. The fall in properties during ageing is caused mainly by chain scission. The increase in crosslink density during the initial periods of ageing causes an increase in modulus and tensile strength. Carbon black accelerates ageing, probably by surface catalysis. The simple network structure of efficiently vulcanized rubber results in better retention of properties during ageing. Antioxidant retards ageing. The extent of degradation of vulcanizates is reflected in the nature of the fracture surfaces. The roughness of the tensile fracture surface is affected to varying degrees depending upon the ageing resistance of the vulcanizates. However, in tear, the mode of fracture propagation is affected by ageing.     

原位生成甲基丙烯酸锌对天然橡胶补强的研究

在天然橡胶(NR)混炼过程中加入氧化锌与甲基丙烯酸(MAA),在橡胶基体中原位生成甲基丙烯酸锌(ZDMA),并在硫化剂过氧化二异丙苯作用下制备NR/ZnO/MAA硫化胶。用扫描电镜、透射电镜研究了硫化胶的形态,并研究了制备条件对硫化胶力学性能的影响,结果表明,在155℃硫化温度,DCP用量为1.5～2 0质量份(phr)条件下,NR硫化胶具有较佳的力学性能。随着ZnO/MAA用量的增加,硫化胶定伸应力、拉伸强度和撕裂强度显著增加,表现出显著的补强作用。     

老化对NR硫化胶的影响

研究了热空气老化和真空老化对ＮＲ硫化胶结构和性能的影响,采用化学探测剂脱硫的方法测定了老化过程中ＮＲ硫化胶交联密度及交联键分布变化的情况,同时测定了硫化胶的力学性能和动态性能随时间变化的情况。结果显示,热空气老化使硫化胶的交联密度增加,但力学性能损失严重,原因是主链氧化断裂。真空老化使ＮＲ硫化胶交联密度略微降低,但力学性能保持良好,类似于硫化返原。热空气老化使ＮＲ硫化胶的动态性能改善,而真空老化后