Silica-filled Natural Rubber Compounds Research Articles

Sustainable multi-functional additives: Zinc soaps from vegetable oil and fatty acids in natural rubber compounds

Zinc soaps derived from various vegetable oils and fatty acids were synthesized, and the product with the highest yield was chosen for compounding with silica-filled natural rubber (NR). Specifically, zinc soaps of coconut oil and palm oil, with yields of 72% and 73%, respectively, were prepared through precipitation and metathesis methods. Additionally, zinc soaps from palm fatty acid distillate (PFAD) and stearic acid, both with a yield of 70%, were synthesized using modified fusion and precipitation methods, respectively. Subsequently, these zinc soaps were incorporated into silica-filled NR compounds and compared with the commercial dispersing aid, Ultra flow 700 s. The results indicated that the zinc soaps played a significant role in plasticizing and lubricating the rubber compounds, as evidenced by reductions in Mooney viscosity, Mooney relaxation rate, and minimum torque (ML). Furthermore, the zinc soaps influenced curing properties by accelerating the cure process, as reflected in decreased cure time (tc90) and increased cure rate and torque difference compared to compounds without processing aid. Moreover, NR compounded with zinc soap exhibited superior mechanical properties, including higher tensile strength, elongation at break, tear strength, and hardness.

Characterization and properties of silica-filled natural rubber compounds using poly(vinyl propionate)‐grafted natural rubber as compatibilizer

Characterization and properties of silica-filled natural rubber compounds using poly(vinyl propionate)‐grafted natural rubber as compatibilizer

Dynamic Response and Molecular Chain Modifications Associated with Degradation during Mixing of Silica-Reinforced Natural Rubber Compounds

Mixing silica-reinforced rubber for tire tread compounds involves high shear forces and temperatures to obtain a sufficient degree of silanization. Natural Rubber (NR) is sensitive to mastication and chemical reactions, and thus, silica-NR mixing encounters both mechanical and thermal degradation. The present work investigates the degradation phenomena during the mixing of silica-reinforced NR compounds in-depth. The Mooney stress relaxation rates, the dynamic properties with frequency sweep, a novel characterization of branch formation on NR using Δδ values acc. Booij and van Gurp-Palmen plots, together, indicate two major competitive reactions taking place: chain scission or degradation and preliminary cross-linking or branch formation. For masticated pure NR and gum compounds, the viscous responses increase, and the changes in all parameters indicate the dominance of chain scission with increasing dump temperature. It causes molecular weight decrease, broader molecular weight distribution, and branched structures. Different behavior is observed for silica-filled NR compounds in which both physical and chemical cross-links are promoted by silanization and coupling reactions. At high dump temperatures above 150 °C, the results indicate a significant increase in branching due to preliminary cross-linking. These molecular chain modifications that cause network heterogeneity deteriorate the mechanical properties of resulting vulcanizates.

Effect of the Epoxide Contents of Liquid Isoprene Rubber as a Processing Aid on the Properties of Silica-Filled Natural Rubber Compounds.

In this study, we examined the feasibility of using epoxidized liquid isoprene rubber (E-LqIR) as a processing aid for truck and bus radial (TBR) tire treads and investigated the effects of the epoxide content on the wear resistance, fuel efficiency, and resistance to extraction of the E-LqIRs. The results confirmed that, compared to the treated distillate aromatic extract (TDAE) oil, the E-LqIRs could enhance the filler–rubber interactions and reduce the oil migration. However, the consumption of sulfur by the E-LqIRs resulted in a lower crosslink density compared to that of the TDAE oil, and the higher epoxide content decreased the wear resistance and fuel efficiency because of the increased glass-transition temperature (Tg). In contrast, the E-LqIR with a low epoxide content of 6 mol% had no significant effect on the Tg of the final compound and resulted in superior wear resistance and fuel efficiency, compared to those shown by TDAE oil, because of the higher filler–rubber interactions.

Effect of Epoxide Content on the Vulcanizate Structure of Silica-Filled Epoxidized Natural Rubber (ENR) Compounds.

The demand for truck–bus radial (TBR) tires with enhanced fuel efficiency has grown in recent years. Many studies have investigated silica-filled natural rubber (NR) compounds to address these needs. However, silica-filled compounds offer inferior abrasion resistance compared to carbon black-filled compounds. Further, the use of NR as a base rubber can hinder silanization and coupling reactions due to interference by proteins and lipids. Improved silica dispersion be achieved without the use of a silane coupling agent by introducing epoxide groups to NR, which serve as silica-affinitive functional groups. Furthermore, the coupling reaction can be promoted by facilitating chemical interaction between the hydroxyl group of silica and the added epoxide groups. Thus, this study evaluated the properties of commercialized NR, ENR-25, and ENR-50 compounds with or without an added silane coupling agent, and the filler–rubber interaction was quantitatively calculated using vulcanizate structure analysis. The increased epoxide content, when the silane coupling agent was not used, improved silica dispersion, abrasion resistance, fuel efficiency, and wet grip. Once a basic level of silica dispersion was secured by using the silane coupling agent, both the abrasion resistance and wet grip improved with increasing epoxide content. Furthermore, the silane coupling agent could be partially replaced by ENR due to the high filler–rubber interaction between the ENR and silica. Therefore, epoxidation shows potential for resolving the issues associated with poor coupling reactions and abrasion resistance in silica-filled NR compounds.

ENHANCING PERFORMANCE OF SILICA-REINFORCED NATURAL RUBBER TIRE TREAD COMPOUNDS BY APPLYING ORGANOCLAY AS SECONDARY FILLER

ABSTRACT Silica-reinforced natural rubber (NR) tire tread compounds are investigated using organoclay (OC) as secondary filler. By varying mixer temperature settings at a silica/OC ratio of 45/10 phr, dump temperatures are reached of approximately 120, 140, 150, and 160 °C. The increased dump temperature leads to a better silanization reaction resulting in lower mixing torque, Mooney viscosity, and Payne effect. The optimum mixing dump temperature was found to be around 150 °C. By varying the loadings of OC in the silica-filled NR compounds from 0 to 36 wt% relative to total filler amount, the increased OC loadings decreased the Payne effect and compound viscosities, significantly shortened scorch and cure times, and raised the tan delta at −20 and 0 °C as indications for ice traction and wet skid resistance of tire treads made therefrom. The optimum loading of OC of 9 wt% relative to total filler content shows better Payne effect, cure rate index, tan delta at −20 and 60 °C indicative for rolling resistance, and DIN (German Institute of Standardization) abrasion resistance index. The results indicate that the use of this hybrid filler may provide tires with better wet traction and lower rolling resistance and wear resistance compared with the pure silica-filled system.

The silica-filled natural rubber in presence of epoxidized natural rubbers: curing, swelling and tensile properties

The two types of epoxidized natural rubbers (ENR-25 and ENR-50), having 25 and 50 moles of epoxidation, were used to study the influences of their incorporations on optimum curing time, swelling and tensile properties of silica-filled natural rubber (NR) compounds. The NR vulcanizates were filled by silica filler at a fixed loading at 30.0 phr and the ENRs were incorporated separately into the filled NR compounds with varied concentration i.e 5, 10, 15, 20 and 25 phr. The filled-NR compounds were prepared based on a semi-efficient cure formulation and cured at 150 °C. From the results, it was revealed that both the ENRs acted as curatives and compatibilizers. They increased the optimum curing time and crosslinks level of the silica-filled NR compounds. The higher the ENRs loadings; the higher was the optimum curing time and crosslinks level. It was also revealed that the tensile moduli and tensile strength were raised; the elongations at break were reduced as the ENRs incorporations were raised to 25 phr of concentration. At a similar concentration, the ENR-25 exhibited a less pronounced curative and compatibilization affections than ENR-50.

The effect of non-rubber components on mechanical properties of TESPD silane coupling agent in silica-filled rubber compounds

Silica is reinforcing filler with high polarity, leading to the difficulty to get the homogenous mixing with a natural rubber (NR) compound, which normally becomes silica agglomeration. A silane coupling agent is applied in a silica-filled NR compound in order to enhance compatibility between silica and rubber. Moreover, non-rubber components, especially proteins, are believed to have competition with silane coupling agent during a silanization [1, 2]. In this work, the interaction between the silane coupling agent and silica-filled NR materials was investigated to show the components influencing in the filler-filler and filler-rubber interaction as well as mechanical properties. The rubber types with different non-rubber components such as fresh NR, deproteinized NR (DPNR) and polyisoprene rubber (IR), absence of non-rubber constituents were mixed with bis-triethoxysilylpropyl disulfide (TESPD) and silica by using an internal mixer at high temperature. Payne effect of the unvulcanised rubber samples were characterized to study silica dispersion, respectively. A swelling test was used to determined crosslink density in the rubber samples after a vulcanisation system. In addition, the effect of non-rubber components on mechanical properties of rubbers was investigated to find the relationship between the crosslink density and filler-rubber network.

Effect of acrylonitrile butadiene rubber on the properties of silica-reinforced natural rubber

The effect of acrylonitrile butadiene rubber (NBR) as a compatibilizer in silica-reinforced natural rubber (NR) is studied. NBR, Emulcril 3380 with 32-34%, was various at 5, 10, 15, 20 and 25 part per hundred parts of rubber (phr) to be used as a compatibilizer for silica-filled NR. The use of NBR as a compatiblizer can improve cure time (T90), cure rate index (CRI), and Mooney viscosity of silica-filled NR compounds, but the re-agglomeration of filler still observes. The use of NBR does not show significantly effect on bound rubber contents. However, tensile strength, reinforcement index and tear strength are increased with increasing amount of NBR contents due to a presence of some rubber-filler interaction through acrylonitrile groups of NBR and silanol groups on the silica surface. It leads to observe more surface roughness on tensile fractured surfaces.

Enhancements in cure rate index and mechanical properties of silica-filled natural rubber using octadecanol-fatty alcohol

The enhancements in cure rate index and mechanical/tensile properties of natural rubber (NR) filled with silica because of the addition of octadecanol (OCD)-fatty alcohol (FA) were investigated. The silica as a reinforcing filler was compounded into the NR at a fixed loading, 30 parts per hundred rubber (phr.). The OCD-FA was derived from the oil of palm kernel and compounded into the NR as an additive agent. The loading variations of OCD-FA were 0 to 4 phr. It was found that the OCD-FA functioned as a curative and plasticizing agent for the silica-filled NR compounds. As a co-curing agent, the OCD-FA enhanced the time of scorches and cure rate index-CRI of the compounds of NR. A higher the OCD-FA loading caused in a higher CRI degree. As a plasticizing agent, the OCD-FA decreased tensile moduli but increased elongation at break. It was also found that the OCD-FA increased both tensile strength and abrasion resistance of the compounds of NR up to a 2 phr. of loading. The addition of OCD-FA with a 2 phr. of was an optimum loading for the compounds of NR.

Enhancing the Silanization Reaction of the Silica-Silane System by Different Amines in Model and Practical Silica-Filled Natural Rubber Compounds.

Diphenyl guanidine (DPG) is an essential ingredient in silica-reinforced rubber compounds for low rolling resistance tires, as it not only acts as a secondary accelerator, but also as a catalyst for the silanization reaction. However, because of concern over the toxicity of DPG that liberates aniline during high-temperature processing, safe alternatives are required. The present work studies several amines as potential alternatives for DPG. Different amines (i.e., hexylamine, decylamine, octadecylamine, cyclohexylamine, dicyclohexylamine, and quinuclidine) are investigated in a model system, as well as in a practical rubber compound by taking the ones with DPG and without amine as references. The kinetics of the silanization reaction of the silica/silane mixtures are evaluated using model compounds. The mixtures with amines show up to 3.7 times higher rate constants of the primary silanization reaction compared to the compound without amine. Linear aliphatic amines promote the rate constant of the primary silanization reaction to a greater extent compared to amines with a cyclic structure. The amines with short-alkyl chains that provide better accessibility towards the silica surface, enhance the primary silanization reaction more than the ones with long-alkyl chains. The different amines have no significant influence on the rate constant of the secondary silanization reaction. The amine types that give a higher primary silanization reaction rate constant show a lower flocculation rate in the practical compounds. For the systems with a bit lower primary silanization reaction rate, but higher extent of shielding or physical adsorption that still promotes higher interfacial compatibility between the elastomer and the filler surface, the rubber compounds show a lower Payne effect which would indicate lower filler-filler interaction. However, the flocculation rate constant remained high.

Compatibility enhancement of silica and natural rubber compound using UVA-induced silane-grafted saponified skim natural rubber

Silane coupling agents are potential reagents widely used to improve the compatibility between silica and less polar rubber, especially natural rubber (NR). Nevertheless, high temperature is generally required to generate the interaction between the components during the mixing process. Accordingly, an alternative method by grafting the silane coupling agent onto the rubber molecules would be a desirable approach to develop a compatibilizer for the silica-filled NR compound. In this work, skim NR was used as a starting material due to its linear structure. The optimal conditions of the grafting reaction were found to be 1 phr of an alkoxy silane and 5 phr of benzoyl peroxide under 8 min of UVA irradiation time. These conditions were applied for producing the rubber material used in the mixing process of STR 5L and silica. The cure characteristics, silica dispersion and mechanical properties of the rubber compounds were improved, suggesting that the modified rubber was an efficient material for increasing the compatibility between silica and NR.

Effects of Bio-based Oils on Processing Properties and Cure Characteristics of Silica-filled Natural Rubber Compounds

A new bio-based oil, tea oil (TO), was studied as possible alternative processing oil to the commonly used but potentially carcinogenic petroleum-based oils, viz. heavy naphthenic oil (HNO) and light naphthenic oil (LNO). Comparison was also made with two other bio-based oils, palm oil (PO) and coconut oil (CO). The results revealed that all bio-based oils studied exhibit higher thermal stability than LNO and, in the case of silica-filled natural rubber (NR), provide superior processing properties over the petroleum-based oils. These include higher thermal stability, greater mixing efficiency of NR with silica, improved silica dispersion in NR compound and greater plasticising effect for silica-filled NR compounds when compared under similar mixing condition. For cure characteristics of the rubber compounds, they are not significantly affected by the processing oil type and content. The results show the potential of the bio-based oils studied, TO, PO and CO, as possible alternative processing oils to replace petroleum-based oils.

Silane grafted natural rubber and its compatibilization effect on silica-reinforced rubber tire compounds

Natural Rubber (NR) grafted with 3-octanoylthio-1-propyltriethoxysilane (NXT) was prepared by melt mixing using 1,1′-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane as initiator at 140 °C with NXT contents of 10 and 20 parts per hundred rubber [phr] and initiator 0.1 phr. The silane grafted on NR molecules was confirmed by Fourier transform infrared (FTIR), proton nuclear magnetic resonance (1H-NMR) and scanning electron icroscopy/energy dispersive X-ray spectroscopy (SEM-EDX). Based on1H-NMR, the use of 10 and 20 phr (parts per hundred resin) of silane resulted in grafted NXT onto NR of 0.66 and 1.32 mol%, respectively, or a grafting efficiency of approx.38%. The use of NXT-grafted NR as compatibilizer in silica-filled NR compounds, to give a total amount of NXT in both grafted and non-grafted forms in the range of 0.8-6.1 wt% relative to the silica, decreases the Mooney viscosity and Payne effect of the compounds, improves filler-rubber interaction, and significantly increases the tensile properties of the silica-filled NR-compounds compared to the non-compatibilized one. At the same silane-content, the use of silane-grafted NR gives slightly better properties than the straight use of the same silane. With sulfur compensation, the use of NXT-grafted-NR with about 6 wt% NXT relative to the silica gives technical properties that reach the levels obtained for straight use of bis-(3-triethoxysilyl-propyl) tetrasulfide (TESPT) at 8.6 wt% relative to the silica.

Compatibilization of silica-filled natural rubber compounds by combined effects of functionalized low molecular weight rubber and silane

Epoxidized low molecular weight natural rubber (ELMWNR) with 28 mol% epoxide groups and weight average molecular weight of 49,000 g mol−1 was prepared by oxidative degradation of epoxidized natural rubber (NR) using periodic acid in the latex state. ELMWNR-28 was used at 10 parts per hundred parts of rubber (phr) loading in combination with bis-(triethoxysilylpropyl) tetrasulfide (TESPT) as the silane coupling agent in the range of 0–4.5 phr in silica-reinforced NR compounds. The use of TESPT in combination with ELMWNR-28 gives lower mixing torques and compound viscosities compared with the use of TESPT alone and the system without any compatibilizer. The bound rubber content, modulus, and tensile strength of the compounds with only TESPT strongly depend on the TESPT loading. The use of ELMWNR-28 as a compatibilizer clearly improves such properties compared with the non-compatibilized systems. By adding TESPT into the compound with ELMWNR-28, the properties further improve with increasing TESPT loading. The combined effect of ELMWNR-28 at 10 phr with a small amount of TESPT at 1.5 phr results in compounds with superior processability (i.e. low Mooney viscosity and Payne effect), and only slightly lower modulus and reinforcement index (M300/M100) compared with the use of the optimum content of TESPT. This compatibilizer/TESPT combination has the environmental benefits that the ELMWNR is a naturally based product, and that the reduced amount of TESPT silane coupling agent emits a greatly reduced amount of ethanol during processing.

Alkanolamide as an accelerator, filler-dispersant and a plasticizer in silica-filled natural rubber compounds

A feasibility study was carried out on the utilization of Alkanolamide (ALK) on silica reinforcement of natural rubber (NR) by using a semi-efficient cure system. The ALK was incorporated into the NR compound at 1.0, 3.0, 5.0, 7.0 and 9.0 phr. An investigation was carried out to examine the effect of ALK on the cure characteristics and properties of NR compounds. It was found that ALK gave shorter scorch and cure times for silica-filled NR compounds. ALK also exhibited higher torque differences, tensile modulus, tensile strength, hardness and crosslink density of up to 5.0 phr of ALK loading, and then decreased with further increases of ALK loading. The resilience increased with increased ALK loading. Scanning electron microscopy (SEM) micrographs proved that 5.0 phr of ALK in the silica-filled NR compound exhibited the greatest matrix tearing line and surface roughness due to higher reinforcement level of the silica, as well as better dispersion and cure enhancement.

OPTIMIZATION OF MIXING CONDITIONS FOR SILICA-REINFORCED NATURAL RUBBER TIRE TREAD COMPOUNDS

Abstract The dump temperature and mixing interval between rubber, silica, and silane coupling agent for silica-filled natural rubber (NR) tire tread compounds using bis-triethoxysilylpropyl tetrasulfide (TESPT) as silane were optimized. The dump temperature turns out to be the key parameter governing the properties of the silica-filled NR compounds. The increase in viscosity of the compounds by changing the dump temperature from 100 to 150 °C indicates that inevitably some cross-linking of NR occurs by sulfur contained in TESPT, simultaneous with the silanization reaction between silica and silane. However, the viscosity decreases again when dump temperatures above 150 °C are applied, indicating a dominant occurrence of degradation of the NR molecules. The results are in good agreement with bound rubber contents. The overall properties indicate that a dump temperature in the range of 135–150 °C and a silica–silane–rubber mixing interval of 10 min are the most appropriate mixing conditions for silica-filled NR compounds with TESPT as coupling agent.

Effect of nitrile rubber on properties of silica-filled natural rubber compounds

The effect of nitrile rubber (NBR) on the properties of silica-filled natural rubber (NR) compounds was studied in the presence of a new silane coupling agent, 3-octanoylthio-1-propyltriethoxysilane (NXT). The properties of silica-filled NR compounds were improved by adding NBR. The torque at equilibrium of compounds decreased with increasing NBR content. The dispersion of silica was improved by adding NBR. The scorch time and optimum cure time became shorter with increasing NBR content. The crosslink density of silica-filled NR vulcanizates also increased with increasing NBR content. The modulus and hardness of NR vulcanizates were increased by adding NBR. The wet traction of the NR vulcanizates containing NBR was better than that without NBR, but rolling resistance of the vulcanizates containing NBR was worse than that without NBR.

Efeito do óleo de mamona em composições de borracha natural contendo sílica

O potencial do óleo de mamona como um ativador para a vulcanização de composições de borracha natural (NR) contendo sílica foi investigado. Utilizando-se um planejamento de experimentos adequado, manteve-se a concentração de sílica e outros aditivos constantes enquanto as quantidades de ácido esteárico, óleo de mamona e poli(etileno-glicol) foram variadas. A cura foi conduzida em um reômetro de disco oscilatório TI-100 e a variável de controle escolhida foi a resistência à tração. Os resultados demonstraram que o óleo de mamona sozinho não exerce influência considerável sobre o desempenho mecânico da borracha natural em presença de sílica.

Use of epoxidized rubber seed oil as a coupling agent and a plasticizer in silica-filled natural rubber compounds

To obtain good reinforcement by silica filler in nonpolar rubbers, it is almost essential to use coupling agents, such as bis(triethoxy silyl propyl) tetrasulfane (TESPT). Chemicals that can interact with the silanol groups on the silica particles and reduce their network formation are also expected to enhance reinforcement. We made a comparative evaluation of TESPT, epoxidized rubber seed oil (ERSO), and their combination as a coupling agent and a plasticizer in silica-filled natural rubber compounds. The results indicate that compounds containing ERSO showed physical and mechanical properties between that of TESPT and naphthenic oil. The action of ERSO in the improvement of the properties was expected to be bifunctional and similar to that of TESPT; that is, it caused the hydrophobation of silica, which increased its degree of dispersion and the formation of chemical bonds with the rubber, thereby ensuring strong polymer–filler interactions. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3531–3536, 2004