Silica Rubber Research Articles

Using a sulfur-bearing silane to improve rubber formulations for potential use in industrial rubber articles

The availability of the coupling agent bis (3-triethoxysilylpropyl)-tetrasulfide (TESPT) has provided an opportunity for enhancing the reinforcing capabilities of precipitated amorphous white silica in rubber. Styrene-butadiene rubber, synthetic polyisoprene rubber (IR), acrylonitrile-butadiene rubber, and natural rubber (NR) containing the same loading of a precipitated silica filler were prepared. The silica surface was pretreated with TESPT, which is a sulfur-bearing bifunctional organosilane to chemically bond silica to the rubber. The rubber compounds were subsequently cured by reacting the tetrasulfane groups of TESPT with double bonds in the rubber chains and the cure was optimized by adding sulfenamide accelerator and zinc oxide. The IR and NR needed more accelerators for curing. Surprisingly, there was no obvious correlation between the internal double bond content and the accelerator requirement for the optimum cure of the rubbers. Using the TESPT pretreated silanized silica was a very efficient method for cross-linking and reinforcing the rubbers. It reduced the use of the chemical curatives significantly while maintaining excellent mechanical properties of the cured rubbers. Moreover, it improved health and safety at work-place, reduced cost, and minimized damage to the environment because less chemical curatives were used. Therefore, TESPT was classified as “green silane” for use in rubber formulations.

In situ sol–gel obtained silica–rubber nanocomposites: influence of the filler precursors on the improvement of the mechanical properties

Silica–rubber nanocomposites were obtained by in situ sol–gel synthesis, using trialkoxysilanes with different functional groups as precursors. The functionalities were selected in order to favor the formation of differently shaped silica particles and/or to modulate the filler–filler and the filler–rubber interactions. The functional groups included (a) alkyl and alkenyl groups: triethoxy(vinyl) (VTEOS), triethoxy(propyl) (PTEOS), triethoxy (octyl) (OCTEOS); (b) N-containing alkyl groups: triethoxy(3-aminopropyl) (APTEOS), triethoxy(3- cyanopropyl) (CPTEOS), triethoxy(3-propylisocyanate) (ICPTEOS); (c) S-containing alkyl groups: trimethoxy(3-mercaptopropyl) (TMSPM), bis(3-triethoxysilylpropyl) disulfide (TESPD), bis(3-triethoxysilylpropyl) tetrasulfide (TESPT); triethoxy(3-octanoylthio-1-propyl) (NXT). Transmission electron microscopy (TEM) investigation suggested a relationship between the morphology of the filler network and the used trialkoxysilanes, as a function of the particle shape and of the interaction of the particle surface groups between them and with the matrix. The dynamic-mechanical properties of nanocomposites, both uncured and vulcanized, were discussed in relation to the network morphology, suggesting a connection between the used silica precursors and the functional properties. The filler–rubber interaction due to substituents which chemically interact with the polymer, promotes the homogeneous distribution of the silica particles in the matrix, while the filler–filler interaction, favored by the shape induced physical interactions or by the chemical interaction among surface groups, mainly contribute to the filler networking and to the dynamic-mechanical properties of the composites.

Enhancement of rubber–carbon black interaction by amine‐based modifiers and their effect on viscoelastic and mechanical properties

AbstractImprovement in rubber–filler interaction is desirable for rubber technologists due to its influence on numerous properties of rubber compounds and vulcanizates. In practice, there are coupling agents commercially available for the improvement of silica–rubber interaction. Surprisingly, only a limited number of works have been focused on interaction enhancement between carbon black (CB) and rubber. Thus, in the research presented in this article, attempts to improve interaction between ethylene–propylene rubber (EPM) and carbon black (CB) have been made by the use of either p‐phenylene diamine (p‐PDA) or N‐tert‐butyl‐2‐benzothiazole sulfenamide (TBBS) as an interaction modifier. Bound rubber content (BRC), used as an indicator for rubber–filler interaction and viscoelastic behavior of CB masterbatches and CB‐filled EPM compounds were investigated and correlated. Results from the measurement of BRC in the CB masterbatches revealed that p‐PDA was more effective in the enhancement of rubber–CB interaction than TBBS. Such improved interaction led to a decrease in magnitude of CB percolation (Payne effect). In respect of viscoelastic behavior, the interaction modifiers affected G′ only in the small strain region (<1% strain) by slightly raising the value of G′. However, as strain was increased (≥1%), G′ for all compounds was coincident implying a disruption of weak interaction between CB and rubber. In the case of EPM vulcanizates, p‐PDA yielded greater enhancement in mechanical properties than TBBS. The results of BRC, viscoelastic behavior, and mechanical properties were apparently in good agreement. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Silica–rubber microstructure visualised in three dimensions by focused ion beam–scanning electron microscopy

A focused ion beam-scanning electron microscope (FIB-SEM) technique for three-dimensional reconstruction and representation of material microstructures was applied to a silica-filled synthetic rubber for the first time. Backscattered electron imaging allowed differentiation between rubber matrix, silica filler and zinc oxide (used as an activator for the sulphur vulcanisation reaction). Subsequent image processing allowed three-dimensional isosurface model generation of the particulate structure within the rubber composite and separation of zinc oxide from the silica filler. The potential for development and application of this technique using finite element analysis modelling is also highlighted.

Rubber–silica nanocomposites obtained by in situ sol–gel method: particle shape influence on the filler–filler and filler–rubber interactions

Silica–natural rubber composites were prepared by in situ sol–gel synthesis of silica nanoparticles functionalized with alkylthiol or alkylpolysulfide. The functionalizing groups were linked to silica particles by hydrolysis and polycondensation of a mixture of tetraethoxysilane (TEOS) with a suitable amount of (3-mercaptopropyl) trimethoxysilane (TMSPM), bis (3-triethoxysilylpropyl) disulfide (TESPD) or bis (3-triethoxysilylpropyl) tetrasulfide (TESPT). Particles from TEOS are spherical, instead those from TESPD, TESPT and TMSPM have irregular anisotropic shapes. This is due to the presence of isotropic or anisotropic interactions among the particle base units. Silica particles synthesized in the presence of TMSPM can also undergo condensation of the alkylthiol chains with the silanol groups, thus giving rise to a strong preferential direction for the anisotropic shape. Predominant filler–filler interactions and easy self-assembly were detected in particles from TEOS while those from TESPD and TESPT showed high filler–rubber interactions. Both filler–rubber and strong filler–filler interactions are present in silica particles synthesized by TMSPM. The dynamic mechanical properties of the composites, tested with stress-strain measurements, show that the storage modulus increases by increasing the filler–filler interaction, and it is maximum when also the filler–rubber interaction occurs. Strong silica–rubber interaction favors the silica dispersion in rubber, while it makes the filler network less compact and lowers the storage modulus.

Use of Antioxidant-modified Precipitated Silica in Natural Rubber

Fillers play a dominant role in modifying the properties of the base polymer. Precipitated silica is a promising non-black filler for rubber vulcanisates. As silica is hydrophilic and rubber is hydrophobic, uniform dispersion of silica in rubber is difficult and gives rise to problems such as high initial viscosity. Silica has a number of hydroxyl groups, which result in filler–filler particle agglomeration and reagglomeration. Incorporation of silica into rubber is quite difficult compared with the incorporation of carbon black. The main aim of this study was to reduce the hydrophilicity of silica by coating antioxidant onto its surface, and hence achieve easy incorporation of silica into rubber. The preparation of antioxidant-modified precipitated silica and its use as filler in rubber compounds were investigated. The curing and mechanical properties of the compounds were compared with those of conventional compounds containing unmodified silica and antioxidant in the same dosage on natural rubber. The cure and mechanical properties were found to be superior for modified silica composites compared with unmodified silica composites.

AFM Studies on Silica Dispersion in EPDM Rubber

Abstract EPDM rubber formulations filled with precipitated silica and cross-linked by peroxide vulcanization have been investigated with the objective of realizing better processing characteristics and vulcanizate properties. Dispersion of silica in EPDM rubber in the presence of conventional coupling agents and in the presence of an epoxy resin has been investigated using atomic force microscopy (AFM) and scanning electron microscopy (SEM). AFM results of EPDM rubber compounds containing silica fillers in the presence of the epoxy resin have exhibited improved silica dispersion in terms of scanned height. The latter has been found to be in the range of 0–100 nm for EPDM rubber—silica systems with epoxy resin, while in the absence of the resin, scanned height has been found to be in the range of 0–700 nm. The significant reduction in scanned height in the presence of epoxy resin has been attributed to the reduction in surface roughness which is an indication for good dispersion of silica in the rubber. Morphological studies using SEM, mechanical properties, and Mooney-Rivlin constants all have shown significant improvement in performance properties of EPDM rubber—silica compounds containing epoxy resin and vulcanized with dicumyl peroxide.

Structural changes in precipitated silica induced by external forces

The morphology of pure precipitated silica, silica filled in polydimethylsiloxane rubber, and silica filled in styrene butadiene rubber was studied by means of small-angle X-ray scattering experiments. The silica at a length scale of a few nanometers consists of primary particles, which form aggregates, and clusters with aggregates as basic units. It is evidenced that the aggregate branching, represented by the mass fractal dimension, and the aggregate diameter are different if pure silica and silica in rubber are compared. Contrary, the size of the primary particles and their surface are not influenced. It is demonstrated that the change in the aggregate morphology is due to the external mechanical forces appearing during the mixing process. This is achieved by model experiments using a pistil and a mortar and a composite with different silica fractions. By that means, a systematic change in the morphology with grinding time is observed. Then, the experiments on the composite demonstrate that the major contributions to the mass fractal dimensions are due to the external mechanical forces. In order to test reproducibility and universal validity in the case of precipitated silicas, independent experiments on one silica and further silicas are performed. Several important conclusions are obtained from the study. First, it is shown that a comparison of different pure silica samples without knowing their history may be difficult or questionable. Second, it becomes evident that it is not sufficient to provide only a description of the materials, rather than the details of the sample treatment have to be reported. Therefore, solely the characterization of the morphology of the pure silica is not sufficient to be compared to the mechanical properties of the composites.

Modification of isoprene rubber by in situ silica generation

AbstractBACKGROUND: The reinforcement of elastomers by the addition of fillers is one of the most important aspects in rubber science and technology. In order to optimise the filler–polymer interface, innovative in situ generation of silica within isoprene rubber was carried out by means of a bottom‐up approach through a sol–gel process starting from tetraethoxysilane as silica precursor. The main aim was the study of the effect of the silica concentration and of the presence of coupling agent on the morphology and the dynamic mechanical behaviour of the composites.RESULTS: The in situ generated silica particles were homogeneously dispersed in the vulcanised rubber with dimensions from a few nanometres to the submicrometre scale. In the presence of coupling agent a good polymer–filler adhesion was observed. The dynamic mechanical behaviour was nonlinear for silica contents higher than 20 wt%. In this range of compositions silica exerted a marked reinforcement on the low‐amplitude storage modulus, which is related to the silica content according to the Huber–Vilgis model.CONCLUSION: Isoprene rubber can be effectively reinforced by the in situ generation of silica for silica contents higher than 20 wt%, and the interaction at the silica–rubber interface can be optimised by using suitable coupling agents. Copyright © 2009 Society of Chemical Industry

PREPARATION AND CHARACTERIZATION OF THE NATURAL RUBBER COMPOSITES REINFORCED BY EPOXY NATURAL RUBBER GRAFTED SILICA WITH GOOD DISPERSIBILITY

A kind of hydrophobic silica with good dispersibility was prepared first.The silica and epoxy natural rubber (ENR) were blended in Haake internal mixer at high temperature in a powerful shear force filed,and so the grafted silica was made through in-situ grafting reaction between the hydroxyl groups of silica and the epoxy groups of ENR. The grafting processes were performed as follows:first,the silica should be dried about 24 h at 100℃ in an oven to remove the free water molecules that absorbed on the surface of silica.Secondly,the grafting reaction temperature was chosen at 170℃ that confirmed by the test result of a Monsanto MDR-2000 moving die rheometer,and the weight proportions between silica and ENR were chosen to be 1:1,2:1 and 3:1 separately in this study.Thirdly,ENR was mixed in HAAKE internal mixer about 1 min,then silica was added to react with the ENR for another 15 min before discharged,and the outputs named ENR-grafted silica were obtained as we wanted. In order to obtain an appropriate grafting condition,the effect of different weight proportions between silica and ENR on the stress-strain curves of natural rubber composites reinforced by the grafted silica was discussed.Through analyses of the reinforcement effect,the appropriate proportion between silica and ENR was chosen to be 3∶1. FTIR,TGA and TEM analysis results indicated that ENR was successfully grafted onto the silica surface.FTIR result of the grafted silica showed the new absorptions. The peaks at 2966,2929 and 2862 cm~ -1 are assigned to C—H stretching vibrations.The absorption band at 1660 cm~ -1 is characteristic of CC stretching.The stretching vibration of epoxy groups that should be at 870 cm~ -1 was missing,suggesting the epoxy groups were almost used up to react with silanol groups.A new shoulder peak appears at 3640 cm~ -1 ,which is ascribed to —OH stretching of C—OH which is the result of the reaction of silanol groups and ENR.TGA curves revealed ENR grafted silica showed a new step between 348～449℃ due to the decomposition of ENR,and the grafting rate reached 21.3%.TEM images indicated the size of silica agglomerates became small obviously after modification because of the grafting layer preventing the silica particles from conglomerating. Then,compared with the silica without grafting,the properties of NR composites filled with grafted silica were investigated.The "Payne effect" results indicated that the grafted ENR molecules reduced the silica-silica interaction by enhancing the rubber-silica interaction,and so the grafted silica had better dispersibility in natural rubber (NR) composites.Mechanical properties test of NR composites proved that the grafted silica could more effectively improve the reinforcement effect due to the strong interface combination between silica and ENR.DMA results showed that the temperature of glassy transition of ENR bound to the silica surface shifted to higher temperatures.Consequently,the composite contained the grafted silica had high strength and high hysteresis at the same time on the normal temperature.So the composites have potential application in the shocking absorber products,the tire of racing cars,and so on.

Study of the effect of silica fill ratio on some mechanical properties and electrical conductivity of styrene-peptadiene / silica rubber

The materials used in this work are locally produced Styrene Butadiene Rubber , with different silicate filler percent of (20,40,60,80,100) part per hundred rubber . Mechanical properties (Tensile, Hardness, Tear, Density and Elongation) were studied for all samples .The study shows that (Tensile, Hardness, and Density) properties increase , but elongation decreases as the filler percent increases. While the tear property records the best value at 60% of the filler percent . Electrical Conductivity for all samples was studied and it was found that the conductivity decreases as the filler percent increases, and it increases with the applied voltage within the range (1-4) kV .

Synthesis, characterisation and properties of clay and silica based rubber nanocomposites

Rubbers are high molecular weight polymeric materials, which possess very low interchain attraction, low glass transition temperature and predominantly amorphous nature with low green strength. Fillers like carbon black and silica are frequently added to rubbers in order to improve the mechanical properties of the composites. In recent years, more attention has been paid towards polymer nanocomposites, where the dimension of the inorganic fillers is of the order of nanometre (1–100 nm) and which impart better physicomechanical properties as compared with conventional composites. This chapter is a compendium of the authors' work on clay and silica filled nanocomposites based on various rubbers. Rubber–clay nanocomposites have been prepared by modification of the naturally occurring Na montmorillonite clay followed by mixing with rubber in solution. Intercalation and/or exfoliation of the clay layers, which depends on a number of factors, has been observed in styrene–butadiene rubber (SBR), acrylonitrile–butadiene rubber (NBR), polybutadiene rubber (BR), brominated polyisobutylene–co-paramethylstyrene (BIMS), ethylene–octene copolymers and thermoplastic elastomers. Predominant intercalation is observed with modified clays in the case of in situ nanocomposites from acrylic monomers. The average dimension (i.e. the width) of the clay layers ranges from 10 to 60 nm in the hybrid nanocomposites, as observed by transmission electron microscopy, scanning electron microscopy and atomic force microscopy studies. Significant improvement in mechanical, dynamic mechanical and barrier properties has been obtained with these clay filled rubber nanocomposites even at very low filler concentration. Sol–gel technique generating in situ silica from the sequential hydrolysis and condensation reactions of tetraethoxysilane (TEOS) has been adopted to synthesise transparent rubber–silica hybrid nanocomposites from polar rubbers like acrylic rubber (ACM) and epoxidised natural rubber (ENR). The sol–gel reaction parameters such as concentration of TEOS, solvents, TEOS/H2O mole ratio, pH value and temperature have profound effects on the morphology of the dispersed silica phase and hence their interaction with the matrix. These factors are crucial in determining various properties like mechanical, dynamic mechanical, rheological and thermal behaviour of the resultant nanocomposites. Some interesting observations have also been recorded in the case of in situ synthesised acrylic copolymer–terpolymer/silica hybrid nanocomposites.

Basic Approaches to Creating Bonds between Silica Filler and Rubber

Basic Approaches to Creating Bonds between Silica Filler and Rubber

Effect of vulcanization technique on the physical properties of silica‐filled EPDM rubber

AbstractThe present study aims to enhance EPDM rubber–silica interaction by employing a special technique called Two‐Stage Vulcanization, with the help of a multifunctional rubber additive, bis diisopropyl thiophosphoryl disulfide (DIPDIS). In this process EPDM rubber was heated along with rubber additives up to the time just before the commencement of cure and then filler was incorporated to the preheated rubber to get the final mix. The efficiency of this novel technique is evaluated by the enhancement of physical properties of the silica‐filled vulcanizates. This novel technique is also employed to investigate the effect of a silane‐coupling agent, viz., bis (3‐triethoxy silyl propyl) tetrasulphide (TESPT), in addition to DIPDIS, on the rubber–filler interaction. The positive role of this technique in enhancing the rubber–filler interaction is evidenced by the dynamic mechanical analysis and scanning electron microscopy. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 1132–1139, 2006

Mechanistic aspects of the role of coupling agents in silica–rubber composites

Compared to carbon black, the use of silica as reinforcing filler for rubber results in lower hysteretic losses, for tyre applications leading to lower rolling resistance and consequently fuel savings. The compatibility of hydrophilic silica with a hydrophobic rubber polymer matrix is generally poor. Adding bi-functional coupling agents to the compounds, commonly bis(triethoxysilylpropyl) tetrasulphide (TESPT), enhances filler-matrix compatibility. The degree of hydrophobation of silica during rubber mixing then depends on many mutually interacting factors. Irreproducible conditions during mixing and vulcanisation are major causes of irreproducibility of silica-reinforced rubber compounds. Depending on the chemical composition of the coupling agent, the ultimate temperature obtained during the mixing process turns out to be the main factor governing the reactions of the coupling agent: the formation of a proper bond between the silica and the coupling agent, while avoiding a premature reaction with the rubber polymers, leading to premature scorch during mixing. Further, the mechanistic aspects of the reaction of various coupling agents, variants on TESPT, are covered, with silica as well as with the rubber. Of great importance are: the carbon and sulphur chain-lengths within the coupling agents, whether corrections are applied in the compound with elemental sulphur relative to the sulphur contained in the reference TESPT, and the moments the correcting amounts of sulphur are added to the compounds: during the first mixing stage, or together with the curing ingredients later-on in the process. The tensile properties of the vulcanised compounds are most prominently influenced. This is indicative of the dual role of sulphur: on the one hand as direct curative, on the other hand as part of the coupling agent becoming attached to the rubber polymers.

Mica as additional filler in SBR–silica compounds

The mechanical properties of mica–silica–SBR (styrene butadiene rubber) compositions were studied with special emphasis on fatigue resistance, using the Monsanto Fatigue-to-Failure Tester (FTFT) and De Mattia flexing machine. It was found that the addition of mica, in little concentrations, has a positive contribution to modulus, and does not change tensile strength, elongation at break and tear strength. Moreover, the addition of silane coupling agent enhances silica–rubber interactions, whereas the fatigue life of silica filled elastomeric compounds is adversely affected by the addition of mica. These behaviours were supported by scanning electron microscopy observations of the fracture surfaces.

Study on Dispersion Morphology of Silica in Rubber

Abstract A new model of silica's aggregation in rubber, the Hexagon-linking Model, is proposed and tested in this study, using transmitting, scanning and analytical electron microscopy. When wetted in rubber, silica tends to accumulate, by way of hexagonal rings, differently from carbon black; the “wetted” states can be controlled through mixing procedures. The experimental evidence that aggregation of silica (original size of 20 nm) precedes to a final size of 63.8 nm, as compared to a theoretical value of 60 nm, supports the theoretical model. The wetted filler structure can be controlled by the mixing process, and hence desired composite properties can be achieved.

Strain Dependent Dynamic Mechanical Properties of Silica Filled Ethylene Vinyl Acetate Rubber

The dynamic mechanical properties of silica filled ethylene vinyl acetate (EVA) rubber have been studied at room temperature (25°C) at different frequencies (3.5 to 110 Hz) and varying dynamic strain amplitudes (0.07 to 5%). From the dynamic analysis it was found that the trend of variation of in-phase modulus, out-of-phase modulus, and tan δ with dynamic strain for lightly filled EVA vulcanizates is similar to that of the gum EVA vulcanizate. The changes in low strain dynamic properties can be ascribed to the mobility of amorphous chains. For highly filled vulcanizates, the nature of the variation of dynamic mechanical properties follows the pattern of filled conventional rub ber systems. For the systems with intermediate filler loading, the characteris tics follow a transition between those exhibited by the gum and the highly filled systems. The increase of high and low strain moduli with volume fraction of filler could be partly due to the hydrodynamic effect of filler dispersed in the rubber matrix and partly due to the enhanced adsorptive interaction between silica and EVA rubber.

Structural Changes During the Silica Rubber Filler Precipitation Investigated by IR and NIR Spectroscopy

Abstract Different kinds of silicic acids have been proved to have different properties in the infra red and near infra red1–2/. Precipitated white rubber fillers similarly as the pyrogenic Aerosil differ from silica gels by sharper and more intensive Si-O bands in the infra red, which may be explained by a higher periodicity of structure1/. In dry CCl4 Aerosils and precipitated silica rubber fillers form a permanent, more or less transparent suspension, whereas the silica gel particles drop to the bottom. The suspension of silicic acids gives a distinct spectrum of OH bands in the near infra red.