Insight into vulcanization mechanism of novel binary accelerators for natural rubber

A novel TU derivative, N-phenyl-N′-(у-triethoxysilane)-propyl thiourea(STU), is prepared and its binary accelerator system is investigated in detail. Compared to the control references, the optimum curing time of NR compounds with STU is the shortest, indicating a more nucleophilic reaction occurs. The Py-GC/MS results present that the phenyl isothiocyanate fragment still remains in the NR/STU compounds with or without extracting treatment, but no silane segment can be found in the vulcanizate with extracting treatment. Vibrations of C=S, NH and aromatic ring in FTIR experiments and a new methyne carbon peak, as well as the peaks of phenyl group of STU, in the solid state 13C-NMR experiments are found in the NR/STU vulcanizate with extracting treatment. Moreover, the crosslinking density of vulcanizates with STU evolves to lower level, indicating the sulfur atom of STU does not contribute to the sulfur crosslinking. Therefore, a new vulcanization kinetic mechanism of STU is propounded that the thiourea groups can graft to the rubber main chains as pendant groups by chemical bonds during the vulcanization process, which is in accordance with the experimental observations quite well.     

Effect of montmorillonite intercalant structure on the cure parameters of natural rubber

Two quaternary phosphonium salts (aromatic and aliphatic) have been used as intercalants for Na-montmorillonite and the effect of intercalant structure on clay morphology and natural rubber vulcanization kinetics was investigated. Due to its lower rigid structure the aliphatic salt was easier to intercalate into the clay galleries giving rise to a higher interlayer distance and facilitating the rubber intercalation obtaining an exfoliated structure in the nanocomposite. The vulcanization process was sensibly accelerated by this organoclay and a higher crosslinking degree was observed in the nanocomposite which gave rise to materials with improved processing and physical characteristics.     

Styrenated phenol modified nanosilica for improved thermo-oxidative and mechanical properties of natural rubber

The present work aims to prepare thermal and oxidation resistant Natural Rubber (NR) composites using antioxidant-modified nanosilica (MNS). The thermo-oxidative aging performance of the composites was evaluated by the variations in mechanical properties after aging at 100 °C for 24 h. The performance was further monitored through Scanning Electron Microscopy, Fourier Transform Infrared spectroscopy, Thermogravimetric Analysis, and Dynamic Mechanical Analysis. NR nanocomposite with 1–7.5 phr nanosilica (NS) and 3 phr MNS were prepared and its rheological properties were studied. A comparative study of the theoretical models yielded that modified Guth-Gold equation predicted Young's modulus better than other models. Thermal stability of natural rubber MNS composite was improved by 10 °C with pre-eminent mechanical properties like tensile strength and heat build-up. A linear relationship of compression set with modulus of all composites were also established. Equilibrium swelling test revealed improved crosslink density in NR MNS composite. The strong interaction between antioxidant and nanosilica enabled low migration of antioxidant in NR MNS composite. Hence its protective function after aging showed more effective than NR NS composites. These versatile functional properties of NR MNS composite suggest its potential application in electrical, electronic and high performance rubber products.     

The sensitizing effect of acrylates on radiation vulcanization of natural rubber latex

The sensitizing effect of acrylates on radiation vulcanization of natural rubber latex was studied. The results indicate that Gc value of crosslinking (Gc) will be higher at the same radiation dose when a sensitizer exists, and Gc value decreases with the increase of radiation dose (D) conforming to the formula Gc=KD^-α, where K and α are constants depending on sensitizers. The more sensitizers added, the greater the Gc value. However, the viscosity of the natural rubber latex also increases rapidly along with the increase of sensitizers added.Some sensitizers, such as TMPTA, can decrease the optimum dose from about 200 kGy to approximately 20 kGy according to our experiment. The tensile strength of the film can reach round 20 MPa. Other physical properties are comparable to those of unsensitized.     

Effect of high-temperature curing on the crosslink structures and dynamic mechanical properties of gum and N330-filled natural rubber vulcanizates

The effects of high-temperature curing and overcuring on the cure characteristics, crosslink structure, physical properties and dynamic mechanical properties (DMPs) of gum and carbon black (N330) filled natural rubber (NR) vulcanizates cured with conventional (CV), semi-efficient (SEV) and efficient (EV) cure systems, which have about the same total crosslink densities under a moderate curing temperature of 150°C, were investigated. The gum NR vulcanizates cured with CV, SEV and EV curing systems have about the same glass transition temperature (T_g) and tan δ values below the temperature of about 0°C, but showed some apparent differences in the tan δ values increasing in the order CVG′ and tan δ values above T_g higher than those of the gum NR vulcanizates.

High-temperature curing and overcuring cause decreases to various extents in the cure plateau torque, Shore A hardness, 300% modulus and tensile strength, and lead to apparent changes in the DMPs. Typically, there is an increase in T_g of all three kinds of gum and N330-filled NR vulcanizates because of changes in the total crosslink densities and crosslink types. The CV vulcanizates show the most significant change in cure characteristics, physical properties and DMPs since the highest content of polysulfidic crosslinks appears in the CV vulcanizate, causing the highest level of reversion and having a dominant effect on the properties.     

Cellulose nanofibre enabled natural rubber composites: Microstructure,curing behaviour and dynamic mechanical properties

Traditional rubber industries rely heavily on petroleum-based materials, such as carbon black (CB). The present study aims at mitigating the environmental challenges, through partial replacement of CB, while simultaneously consuming an easily accessible agricultural waste. Accordingly, cellulose nanofibre (CNF) was extracted from wheat-straw using chemo-mechanical process, which in-turn was used for fabrication of CNF enabled rubber nanocomposites. Microstructural observation of CNF confirmed nanometric defibrillation of cellulose. A variety of tests were performed on the nanocomposites towards exploring their structure-property correlations, curing-behaviour, thermal degradability and mechanical (static and dynamic) properties. Overall, considerable enhancement in properties such as tensile strength and strain energy density could be realized, owing to synergistic use of CNF and CB in rubber, allowing for replacement of up to 15 phr CB. These were further augmented by significant improvements in dynamic rolling-resistance, traction and stress-softening behaviour. The results were especially significant, considering that the improvements could be achieved without any modification of CNF surface, thereby establishing its potential for development of environment friendly rubber nanocomposites.     

Natural rubber/styrene-butadiene rubber blends prepared by solution mixing: Influence of vulcanization temperature using a Semi-EV sulfur curing system on the microstructural properties

Blends of natural rubber (NR) and styrene-butadiene rubber (SBR) were prepared by solution mixing and vulcanized with sulfur and accelerator in a Semi-EV system at 433 K and 443 K in order to study the vulcanization kinetic and the influence of vulcanization temperature on final structure of the blends. The vulcanization kinetic studied through the variation in rheometer curves was analyzed using the Ding and Leonov model, which takes into account the reversion effect during the cure process. The average free nanohole volume and the fractional free volume of samples with different NR/SBR ratio were estimated using positron annihilation lifetime spectroscopy (PALS). Also, the crosslink density was determined by means of swelling tests in a solvent. For all the compounds, a correlation between the free nanohole volume and the delta torque obtained from the respective rheometer curves was established.     

Oxidation of natural rubber using a sodium tungstate/acetic acid/hydrogen peroxide catalytic system

A catalytic system based on Na₂WO₄/CH₃COOH/H₂O₂ effectively oxidizes natural rubber (NR) to prepare telechelic epoxidised liquid natural rubber (TELNR). The Na₂WO₄/CH₃COOH/H₂O₂ catalytic system possesses a much higher epoxidation efficiency than the traditional CH₃COOH/H₂O₂ system: the epoxidation degree (X_epoxy) of products increases from merely 5.6% (CH₃COOH/H₂O₂) to values as high as 52.1% (Na₂WO₄/CH₃COOH/H₂O₂) by reacting for 24 h at 60 °C. Moreover, this catalytic system also induces hydrolytic degradation so that the weight average molecular weight of NR decreases, e.g., from 14.10 × 10⁵ Da (NR) to 0.57 × 10⁵ Da (TELNR) after reacting for 30 h.The catalytic process probably proceeds via a mononuclear tungsten peroxo-species with coordinated peracetyl/acetyl group, as suggested by ESI-MS measurements. During oxidation, the tungstic anion [W(CH₃COOO)(O)(O₂)₂]⁻ not only catalyzes NR epoxidation, but also induces a further oxidation of epoxy groups to form ketones and aldehydes.     

An innovative approach to utilize waste silica fume from zirconia industry to prepare high performance natural rubber composites for multi-functional applications

Silica fume (SF) is silica-rich amorphous waste by-product obtained during zirconium silicate electrofusion process. The key objective of the study was to determine the efficiency of SF as a reinforcing filler in Natural Rubber (NR) compounds vis a vis the conventional filler, high abrasion furnace (HAF) black. Inter-particle distance and particle size distribution analysis from Transmission Electron Microscopy exhibited homogeneous dispersion of filler in hybrid composite (NR SF20/HAF30) with Bis[3-(triethoxysilyl)propyl] tetrasulfide (TESPT). NR composite with 20 phr SF loading improved modulus by 107%, tensile strength by 12%, and tear strength by 28% over gum NR. Hybrid composite showed 111% increase in modulus than NR SF20 composite. Theoretical modelling of Young's modulus with volume fraction of filler quite fit with Guth-Gold equation. Hybrid composite with TESPT showed 72% reduction in heat build-up compared to NR HAF50 composite. Thermal stability improved by 6 °C and rolling resistance reduced by 64% for hybrid TESPT composite compared to NR HAF50 composite. Constrained region in NR composites obtained from dynamic mechanical analysis showed improved rubber-filler interaction in hybrid TESPT composite. Hence, this work not only provides a new approach to utilize industrial waste but also provides for a high performance NR composite at low cost.     

Ternary blends based on poly (ethylene-naphthalate)/glass fibers/nitrile rubber: Preparation,properties and effect of dynamic vulcanization

This work studied the possibility of utilizing nitrile rubber (NBR) to modify the impact properties of poly (ethylene-naphthalate) (PEN). The PEN/NBR ratio used changed from 100/0 to 60/40. At the same time, glass fibers (GF), 40% weight of the PEN component, were used to reinforce the blends to compensate for the loss of mechanical properties of PEN by incorporation of NBR. The results showed that the impact strength of the PEN/GF/NBR blend (PEN/NBR = 60/40) was increased up to 27.6J/m, nearly 5 times higher than that of the neat PEN. Meanwhile, the tensile strength and flexural strength were still maintained at as high as 66.1 MPa and 98.2 MPa, respectively. Dynamic vulcanization further improved the mechanical properties of the PEN/GF/NBR blends, which provided routes to the design of new PEN/elastomer blends. Other properties of the PEN/GF/NBR blends were also investigated in terms of morphology of fractured surface, dynamic mechanical behavior, thermal stability and crystallization, by scanning electron microscopy (SEM), dynamic mechanical analysis (DMA), thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC), respectively.     

Quantitative analysis of isoprene units in natural rubber and synthetic polyisoprene using 1H-NMR spectroscopy with an internal standard

Isoprene units in natural rubber (NR) and its synthetic analogues were quantified by ¹H-NMR spectroscopy using polyethylene glycol (PEG) as an internal standard. The effect of PEG and rubber concentrations, molar ratio of rubber/PEG, measuring temperature and scan number on the quantification was investigated to establish the respective working range. Analysis of commercial grades of NR revealed that the differences in 1,4 isoprene content is caused by the production process and feedstock, in which proteins and lipids were found to be the major impurity in NR. Gel fraction of NR has insignificant effect on the measurement of 1,4 isoprene content. Furthermore, the new method was found to produce good results for the quantification of 1,4 and 3,4 units of synthetic polyisoprenes.     

Influence concentration of modifying agent on properties of natural rubber/organoclay nanocomposites

Sodium-montmorillonite (Na-MMT) nanoclay was modified with different concentrations of octadecylamine organic modifying agent at 0.5, 1.0 and 1.5 times the CEC of Na-MMT. Influence of concentration of modifying agent on properties of the organoclays and natural rubber/organoclay nanocomposites was investigated. It was found that the optimum concentration of modifying agent was 1.5 times the CEC of Na-MMT. That is, at this concentration, larger d-spacing of organoclay particles and higher degree of clay dispersion in natural rubber matrix were observed. Larger interlayer d-spacing also caused enhancement of the mechanical properties of the NR/organoclay nanocomposites. Additionally, the NR/organoclay nanocomposites with higher concentration of modifying agent exhibited faster curing reaction with higher crosslink density. Furthermore, the organoclays with larger d-spacing and higher degree of dispersion in the natural rubber matrix exhibited enhancement of the mechanical and dynamic properties and thermal stability of natural rubber/organoclay nanocomposites.     

Dynamic mechanical properties of isotactic polypropylene/nitrile rubber blends: Effects of blend ratio,reactive compatibilization,and dynamic vulcanization

The effect of blend ratio and compatibilization on dynamic mechanical properties of PP/NBR blends was investigated at different temperatures. The storage modulus of the blend decreased with increase in rubber content and shows two T_g's indicating the incompatibility of the system. Various composite models have been used to predict the experimental viscoelastic data. The Takayanagi model fit well with the experimental values. The addition of phenolic modified polypropylene (Ph-PP) and maleic modified polypropylene (MA-PP) improved the storage modulus of the blend at lower temperatures. The enhancement in storage modulus was correlated with the change in domain size of dispersed NBR particles. The effect of dynamic vulcanization using sulfur, peroxide, and mixed system on viscoelastic behavior was also studied. Among these peroxide system shows the highest modulus. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 2309–2327, 1997     

Effects of cellulose nanocrystals on the vulcanization of natural rubber/cellulose nanocrystals nanocomposite and corresponding regulating strategies

Vulcanization is a vital process in rubber processing, it endows rubber with valuable physical and mechanical properties, making rubber a widely used engineering material. In addition to vulcanization agent, reinforcing fillers play a non-ignorable influence on the vulcanization of rubber nanocomposites. Herein, the effects of cellulose nanocrystals (CNCs) on the vulcanization of natural rubber (NR)/CNCs nanocomposite was studied. It was found that even though the addition of CNCs can effectively improve the dispersion of ZnO in NR matrix, the vulcanization of NR was inhibited. This may be attributed to the CNCs' adsorption of vulcanizing agents (DM, ZnO) and the acidic chemical environment on the surface of CNCs. In order to improve the vulcanization properties of NR/CNCs nanocomposite, tetramethyldithiochloram (TMTD) and triethanolamine (TEOA) were used as a combination accelerator and curing activator, respectively, and polyethylene glycol (PEG) was introduced to screen hydroxyl groups on the surface of CNCs to prohibit the CNCs' adsorption of vulcanizing agents. The results indicate that TMTD and TEOA effectively improved the vulcanization rate of NR/CNCs nanocomposite and increased the crosslink density by an order of magnitude. Subsequently, the tensile strength, tear strength, and so forth. of NR/CNCs nanocomposite were significantly improved. However, PEG hardly help to improve the vulcanization properties of NR/CNCs nanocomposite. In addition, the control samples without CNCs were prepared and characterized, the comparation between NR and NR/CNCs nanocomposite shows that the synergistic effect of crosslink density and CNCs' reinforcement more effectively improve mechanical properties of NR. This work not only elucidates the inhibiting mechanisms of CNCs on the vulcanization of NR, but also provides practical strategies for improving the vulcanization and properties of NR/CNCs nanocomposite. It may accelerate the application of CNCs as rubber reinforcing filler.     

Studies of molecular dynamics of carboxylated acrylonitrile-butadiene rubber composites containing in situ synthesized silica particles

The molecular dynamics of carboxylated acrylonitrile-butadiene rubber - silica hybrid materials was investigated. Silica hybrids were formed in situ rubber matrix using varied amounts of N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (DAMS), serving also as a cross-linker. Filler-filler and filler-rubber interactions were present, due to the specific nature of these materials. It was found that the amounts of added aminosilane determined the cross-linking density of obtained materials and was the highest with 20 phr DAMS used. The cross-links had ionic nature. Dielectric relaxation spectroscopy (DRS) revealed β, α and α′ relaxation processes. The β relaxation, correlated with the mobility of polymer side groups, was influenced by the weak interaction between both acrylonitrile and carboxylic groups of the rubber and silanol groups of silica. The activation energy for that relaxation was similar for all materials (∼32 kJ mol⁻¹). Both DRS and dynamical mechanical analysis (DMA) demonstrated that the amount of in situ formed silica filler did not significantly influence either the temperature of the α relaxation (correlated with glass transition) or its activation energy. Therefore, that relaxation was caused by free polymer chains, not attached to the silica particles. Similar values of glass transition temperature (T_g) for all hybrids were confirmed by DSC. It appeared that the amplitude of tangent delta (DMA) within T_g was dependent on silica amount. Detected at higher temperature α′ relaxation resulted from the presence of domains, where polymer chains were affected by silica network, geometrical restrictions and morphology of the silica-rich domains.     

Synergistic effect of graphene oxide and boron-nitrogen structure on flame retardancy of natural rubber/IFR composites

In this paper, GO-BN(graphene oxide grafted boron nitride) was synthesized from graphene oxide and boron nitride by silane coupling agent KH550. Furthermore, GO-BN and intumescent flame retardant (IFR) were added into natural rubber (NR) simultaneously to improve its flame retardancy. The structure of GO-BN was studied by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The analysis showed that GO-BN was successfully synthesized. The enhanced flame retardancy performance of flame retardant natural rubber (FRNR) was evaluated by limiting oxygen index (LOI) and UL-94 tests. Moreover, the combustion action of FRNR in fire was evaluated by cone calorimetry. Notably, the results showed that the sample with a GO-BN content of 12 phr showed the best flame retardancy performance. The heat release rate (HRR) and total heat release rate (THR) were remarkably decreased by 42.8% and 19.4%, respectively. Carbon residues were analyzed by infrared spectroscopy and scanning electron microscopy, which showed that GO-BN and IFR had a synergistic catalytic effect. The formation of compact thermal stable carbon layer after combustion was the key to protect engineering materials from combustion.     

Simultaneous Enhancement of Mechanical and Dynamic Mechanical Properties of Natural Rubber/Recycled Ethylene-Propylene-Diene Rubber Blends by Electron Beam Irradiation

Mechanical and dynamic mechanical properties of natural rubber/recycled ethylene-propylene-diene rubber (NR/R-EPDM) blends were simultanoeusly enhanced by electron beam (EB) irradiation. The cross-linking promoter, trimethylolpropane triacrylate (TMPTA), was also introduced into the blends to induce the cross-linking. By applying EB irradiation, the tensile modulus, hardness, swelling, cross-link density, and storage modulus increased with increase in the irradiation dose; an irradiation dose of 50 kGy was efficient to gain optimum tensile strength. The formation of irradiation-induced cross-links after EB irradiation is a major concern for the enhancement of mechanical, swelling resistance, and dynamic mechanical properties of the blends.     

The effect of mercapto- and thioacetate-modified EPDM on the curing parameters and mechanical properties of natural rubber/EPDM blends

The vulcanization characteristics of natural rubber (NR)/ethylene-propylene-ethylidenenorbornene (EPDM) rubber blends were studied in the presence of thioacetate-(EPDMTA) or mercapto-modified EPDM (EPDMSH), using oscillating disk rheometer. The effect of both functionalized EPDMs was investigated in unaccelerated-sulfur curing system and accelerated-sulfur curing systems containing 0.4 and 0.8 phr of MBTS. Both EPDMTA and EPDMSH act as accelerator agent in the curing process, as indicated by the higher values of cure rate index and lower values of activation energy of vulcanization. A substantial increase of the crosslink density has been also observed in EPDMSH-modified blends. Both EPDMTA and EPDMSH resulted in an increase in tensile strength, but the best performance has been achieved with EPDMSH, probably because of the increase of crosslink density associated to the reactive compatibilization promoted by the reaction between mercapto groups and rubber matrix. The best ageing resistance has been observed in EPDMTA-modified blends.     

钨酸钠/冰乙酸/过氧化氢作用下聚丁二烯橡胶环氧化及降解行为研究

本文研究了在钨酸钠/冰乙酸/过氧化氢催化体系作用下聚丁二烯橡胶环氧化及降解行为.采用红外(FT-IR)、核磁(1H-NMR)和气相凝胶渗透色谱(GPC)对降解产物进行了表征,结果表明60℃反应24h,聚丁二烯橡胶环氧度为21.27％,而其分子量没有明显降低.     

Cut growth and abrasion behaviour,and morphology of natural rubber filled with MWCNT and MWCNT/carbon black

Various amounts of predispersed multi-wall carbon nanotubes (MWCNT) were mixed with natural rubber (NR), with and without carbon black (CB), for preparing MWCNT-filled NR (NC) and MWCNT/CB-filled NR (NH) vulcanizates. All NH vulcanizates contained 30 phr CB and the amount of MWCNT for both NC and NH was varied from 0 to 8 phr. Helium ion microscopy (HIM) and FE-SEM images showed that MWCNT in the NH was dispersed much better than in the NC. Additionally, the well dispersed CB and MWCNT in the NH functioned synergistically in promoting an increase in longitudinal crack growth, leading to enhancement of edge-cut tensile strength (CTS) with increasing MWCNT loading. In contrast, all NC specimens ruptured in a simple lateral direction relating to their lower CTS. Results also revealed that abrasion resistance of the NH was not significantly changed with increasing MWCNT, whereas that of the NC increased. Nevertheless, abrasion resistance of both vulcanizates showed good correlation with the average value of ridge spacing on their abraded surfaces. It was also found that tensile strength of the NH was almost unchanged when the MWCNT loading was increased because the reinforcement by CB predominates over the MWCNT. However, 100% modulus and hardness of both NC and NH increased with increasing MWCNT content.