Radiation induced graft copolymerization of acrylonitrile on natural rubber

Acrylonitrile graft natural rubber was prepared by initiating the polymerization of acrylonitrile in natural rubber field latex using γ-rays. The reaction was carried out at different rubber-monomer concentrations and the properties of the modified rubbers were compared with those of natural rubber and nitrile rubber.     

Structure and properties of natural rubber/butadiene rubber (NR/BR) blend/sodium-montmorillonite nanocomposites prepared via a combined latex/melt intercalation method

75/25 (wt %) NR/BR blend/clay nanocomposites were prepared via a combined latex/melt intercalation method, for the first time. At first, NR latex was mixed with various amounts of the aqueous sodium montmorillomte (Na-MMT) dispersion. Obtained mixtures were co-coagulated by dilute solution of the sulfuric acid, washed several times with the distilled water and dried under vacuum. The NR/ clay compounds were then mixed with given amounts of the BR and vulcanizing ingredients in a 6-inch two-roll mill and then vulcanized at 150°C in a hot press. The nanocomposites have better mechanical properties than the clay-free NR/BR blend vulcanizates. Furthermore, modulus and hardness (Shore A) increased by increase of the clay loading in the range of 0–15 phr while tensile strength and elongation at break increased with increasing the clay content up to 5 phr and then decreased gradually by further increase of the clay loading. It was concluded from results of the XRD and mechanical test that nanocomposites containing less than 10 phr clay may show the fully exfoliated structure. With increasing the clay content to 10 and 15 phr, both non-exfoliated (stacked layers) and exfoliated structures may be observed simultaneously in the nanocomposites. TGA results indicated an improvement in main and end decomposition by increasing the clay loading.     

Electron beam crosslinked gels—Preparation,characterization and their effect on the mechanical,dynamic mechanical and rheological properties of rubbers

Electron beam (EB) crosslinked natural rubber (NR) gels were prepared by curing NR latex with EB irradiation over a range of doses from 2.5 to 20 kGy using butyl acrylate as sensitizer. The NR gels were systematically characterized by solvent swelling, dynamic light scattering, mechanical and dynamic mechanical properties. These gels were introduced in virgin NR and styrene butadiene rubber (SBR) matrices at 2, 4, 8 and 16 phr concentration. Addition of the gels improved the mechanical and dynamic mechanical properties of NR and SBR considerably. For example, 16 phr of 20 kGy EB-irradiated gel-filled NR showed a tensile strength of 3.53 MPa compared to 1.85 MPa of virgin NR. Introduction of gels in NR shifted the glass transition temperature to a higher temperature. A similar effect was observed in the case of NR gel-filled SBR systems. Morphology of the gel-filled systems was studied with atomic force microscopy. The NR gels also improved the processability of the virgin rubbers greatly. Both the shear viscosity and the die swell values of EB-irradiated gel-filled NR and SBR were lower than their virgin counterparts as investigated by capillary rheometer.     

Structure and properties of NR/BR blend/clay nanocomposites prepared by the latex method

Rubber blend/clay nanocomposites based on the 50/50 (wt %) natural rubber/butadiene rubber was prepared by the latex method via mixing the latex of 50/50 NR/BR blend with different amounts of the aqueous sodium montmorillonite (Na-MMT) dispersion and co-coagulating the mixture. XRD and TEM were used to characterize structure of the nanocomposites. It was found that fully exfoliated structure could be obtained by this method only when the low loading of layered silicate (up to 5 phr) is used. With increasing the clay content, both non-exfoliated (stacked layers) and exfoliated structures can be observed simultaneously in the nanocomposites. Nanocomposites showed mechanical properties better than the clay-free volcanizate. Moreover, modulus, tensile strength, elongation at break and tear strength increased significantly by increasing the clay amount up to 5 phr and then remained almost constant by further increasing the clay content. Improvement in the mechanical properties by increasing the clay loading up to 5 phr was attributed to the nano-reinforcement effect of Na-MMT. TGA results indicated an improvement in the main decomposition temperature by increasing the clay amount.     

2,2,4-Trimethyl-1,2-Dihydroquinoline Hydrochloride as Coagulant for Butadiene-Styrene Latices

Coagulating action of 2,2,4-trimethyl-1,2-dihydroquinoline hydrochloride in separating butadienestyrene rubber from latex is studied, and the properties of vulcanizate based on rubbers prepared by this procedure are examined.     

In situ silica reinforcement of methyl methacrylate grafted natural rubber by sol–gel process

In situ silica reinforcement of natural rubber (NR) grafted with methyl methacrylate (MMA) (MMA-GNR) was achieved via the sol–gel reaction of tetraethoxysilane (TEOS) by the use of solid rubber and latex solutions. Silica contents within the MMA-GNR as high as 48 and 19 phr were obtained when using the solid rubber and latex solutions, respectively, under optimum conditions. The conversion efficiency of TEOS to silica was close to 95%. The in situ formed silica MMA-GNR/NR composite vulcanizates were prepared. MMA-GNR/NR composite vulcanizates reinforced with the in situ formed silica prepared by either method had similar mechanical properties to each other, but a shorter cure time and higher mechanical properties than those reinforced with the commercial silica at 9 phr. The TEM micrographs confirmed that the in situ formed silica particles were well dispersed within the MMA-GNR/NR composite matrix, whilst the commercial silica particles showed a significant level of agglomeration and a lower level of dispersion.     

Effect of Deproteinized Methods on the Proteins and Properties of Natural Rubber Latex during Storage

Summary: Three different methods of deproteinization, i.e. saponification, surfactant washing and enzymatic treatment were employed to unravel the effect of deproteinized on the properties of natural rubber (NR) latex. The cleavage of proteins in NR latex was found to proceed with concomitant formation of low molecular weight polypeptides. This results in a lowering in gel formation of the enzyme-treated latex, indicating modification of the remaining proteins at the rubber chain-end. Washing NR latex with surfactant would efficiently reduce and remove proteins from NR latex particles through denaturation and transferring them to the serum phase. The relatively stable gel formed during storage of surfactant-washed NR latex is an indication of the absence of branch formation of proteins at the rubber molecule terminal. Saponification by strong alkali would hydrolyze the proteins and phospholipids adsorbed on the latex particle surface. The reason of the significantly higher gel formed in saponified NR latex is still not clear. The present study shows that deproteinization treatments result in modification of the proteins at the surface of NR latex particles and also those freely-suspended in the serum. The cleavage or the denaturation of the rubber proteins during purification by washing has a profound effect on the properties of the deproteinized NR latex upon storage, in particular the thermal oxidative aging properties of the rubber obtained.     

Morphology dependence of crystallization of natural rubber in blends

Crystallization of natural rubber (NR) was investigated in different morphology for NR/styrene butadiene rubber (SBR) blend and NR/polystyrene-(b)-polyisoprene (SI)/polystyrene (PS) blend. A purified NR (PC-TE) was prepared from pale crape via transesterification. In the blends, PC-TE formed various morphologies; that is, matrix phase, island phase and continuous phase with a nano-scale, respectively, in dependence upon the ratio of the rubbers. The crystallization rate of the blends was also significantly associated with the morphology of the rubbers.     

Prevulcanized natural rubber latex/clay aerogel nanocomposites

Natural rubber latex (NR)/clay aerogel nanocomposites were produced via freeze-drying technique. The pristine clay (sodium montmorillonite) was introduced in 1-3 parts per hundred rubber (phr) in order to study the effect of clay in the NR matrix. The dispersion of the layered clay and the morphology of the nanocomposites were determined by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Cure characteristics, thermal stability, and the crosslink density of thermal and microwave-cured NR and its composites were investigated. XRD patterns indicated that both intercalated and exfoliated structures were observed at loadings of 1-3 phr clay. SEM studies revealed that the clay aerogel structure was formed at 3 phr clay loading. The increment in Shore A hardness of nanocomposites compared with pure NR signified excellent polymer/filler interaction and the reinforcing effect of the clay to rubber matrix. This was supported by an increase in maximum rheometric torque and crosslink density. The crosslink density of clay-filled NR vulcanizate was found to increase with the pristine clay content in both thermal and microwave curing methods. However, microwave-cured 2 and 3 phr-filled NR vulcanizates exhibited higher crosslink density than those which were thermal-cured under the same curing temperature. In addition, thermal stability studies showed that pristine clay accelerated the decomposition of NR by showing a slight decrease in onset and peak decomposition temperatures along with clay content.     

Comparison of results from the more informative version of TMA with other methods of polymer testing

Summary Several characteristics of the topological regions determined by the more informative version of TMA for cured rubbers based on NR/CPE and NR/ENR blends as well as PA6/cured rubber blends correlate with static and dynamic mechanical properties. These rubbers differ substantially in the structure of their network and the related topological structure because CPE does not participate in cross-links when ENR can do it. The physical interactions caused by these additives also vary, due to the different polarity that influences the formation of physical networking junctions. In blends of PA6 and cured rubber the networking junctions are assumed as being physical nature only. From these facts and the above-mentioned results it could be preliminarily concluded that the correlation depends on the formulation and related structure of the composite, and on which of the characteristics have been compared. To understand the reasons for the existence, or lack of, correlation observed, further investigations are needed.     

Impact modification of thermoplastics by methyl methacrylate and styrene-grafted natural rubber latexes

The preparation and characterization of polymer blends with structured natural rubber (NR)-based latex particles are presented. By a semicontinuous emulsion polymerization process, a natural rubber latex (prevulcanized or not) was coated with a shell of crosslinked polymethylmethacrylate (PMMA) or polystyrene (PS). Furthermore, core–shell latexes based on a natural rubber/crosslinked PS latex semi-interpenetrating network were synthesized in a batch process. These structured particles were incorporated as impact modifiers into a brittle polymer matrix using a Werner & Pfleiderer twin screw extruder. The mechanical properties of PS and PMMA blends with a series of the prepared latexes were investigated. In the case of PMMA blends, relatively simple core (NR)–shell (crosslinked PMMA) particles improved the mechanical properties of PMMA most effectively. An intermediate PS layer between the core and the shell or a natural rubber core with PS subinclusions allowed the E-modulus to be adjusted. The situation was different with the PS blends. Only core–shell particles based on NR-crosslinked PS latex semi-interpenetrating networks could effectively toughen PS. It appears that microdomains in the rubber phase allowed a modification of the crazing behavior. These inclusions were observed inside the NR particles by transmission electron microscopy. Transmission electron photomicrographs of PS and PMMA blends also revealed intact and well-dispersed particles. Scanning electron microscopy of fracture surfaces allowed us to distinguish PS blends reinforced with latex semi-interpenetrating network-based particles from blends with all other types of particles.     

Effect of chitosan derivatives as rubber antioxidants for increasing durability

A series of chitosan derivatives, namely polydiethylamino-ethylmethacrylate-chitosan-graft-copolymer (chitosan-g-DEAEMA), polycarboxy-chitosan-graft copolymer (chitosan-g-COOH), polyvinyl alcohol chitosan-graft-copolymer (chitosan-g-VOH), and carboxymethyl-chitosan (CM-chitosan), were synthesized and investigated as antioxidants for natural rubber (NR) and acrylonitrile butadiene rubber (NBR) mixes and vulcanizates to increase their durability. The rheometric characteristics of the rubber mixes were determined using an oscillating disc rheometer. The physico-mechanical properties of the rubber vulcanized were measured before and after exposure to thermal oxidative aging. It was found that the CM-chitosan had an accelerating effect on the curing process of NR and NBR. Also, the investigated polymers enhanced the properties of rubbers (NR and NBR) especially after ageing up to 7 days compared with commercial antioxidants, such as phenyl ß-naphthylamine (PßN) and N-isopropel-Nphenyl-p-phenylene diamine (IPPD) which are used in the rubber industry. After ageing, the retained values of tensile strength, modulus at 100 % strain, and elongation at break were improved. The optimum concentration of the investigated compounds used to give good properties was found to be 1–2 parts per 100 of rubber (phr). In addition, these prepared polymers showed a decrease in the equilibrium swelling of rubber in toluene which is the proper solvent and consequently increases the crosslink density for rubbers.     

Blending of Natural Rubber/Recycled Ethylene-Propylene-diene Rubber: Promoting the Interfacial Adhesion Between Phases by Natural Rubber Latex

This article deals with blends based on natural rubber (NR) and recycled ethylene-propylene-diene rubber (R-EPDM). Natural rubber latex (NRL) was introduced into the blends to enhance interfacial adhesion between NR and R-EPDM. A new route of compounding was also suggested. The blends were prepared by mixing R-EPDM and other additives in NRL before blending with natural rubber on a two-roll mill. By applying this method, the homogeneity of the blends and cross-linking distribution are significantly improved. The blends exhibited superior state of cure, swelling resistance, mechanical properties and dynamic mechanical properties. The degree of entanglement between NR and R-EPDM also increased after NRL modification.     

Thermal stability and ageing properties of sulphur and gamma radiation vulcanized natural rubber (NR) and carboxylated styrene butadiene rubber (XSBR) latices and their blends

The thermal stability of natural rubber (NR) and carboxylated styrene butadiene rubber (XSBR) latices and their blends was studied by thermogravimetric methods. Ageing characteristics of these latex blends were studied by applying hot air oven thermal ageing for seven days at 70 °C. The mechanical properties of the aged samples were studied. Thermal degradation and ageing properties of these individual latices and their blends were investigated with special reference to blend ratio and vulcanization techniques. As the XSBR content in the blends increased their thermal stability was also found to increase. Among sulphur and radiation-vulcanized samples, radiation cured possesses higher thermal stability due to the higher thermal stability of carbon-carbon crosslinks. DTG curves were used for the determination of different stages involved in the degradation. Activation energy for degradation was determined from Coats-Redfern plot. The properties of aged samples were found to decrease due to chain depletion. However, the moduli of XSBR and NR/XSBR blends were found to increase owing to the formation of crosslinks upon ageing.     

Effects of rubber type on the curing and physical properties of silica filled rubber compounds

As environmental regulations are getting stricter, tire industries for automobiles have shown much interest in substituting silica for conventional carbon black partially or entirely. To take full advantage of silica as fillers for rubbers, it is essential to find a reasonable rubber system that shows an excellent performance with silica reinforcement. Therefore, in this study, several different rubber compounds comprising the same amount of silica were prepared with several different rubber systems, respectively. The processability, curing characteristics, and mechanical and viscoelastic properties of the rubber compounds were investigated to analyze the performance of the rubber compounds as tire tread materials. Among the rubber compounds studied, SBR1721 compound comprising natural rubber (NR) and styrene butadiene rubber (SBR) with high styrene content was considered the most appropriate for application to tire tread materials. Copyright © 2008 John Wiley & Sons, Ltd.     

FTIR Study on the Thermal Ageing of Chlorinated Natural Rubber from Latex

Chlorinated Natural Rubber (CNR) which has good properties of film forming, corrosion-resisting, anti-leakage and anti-combustion and is broadly used in the production of ship paint, container paint,corrosion-resistant paint, adhesive and additive in ink is prepared by the chlorinated modification of NR through solution or latex process. Because of the defects of high equipment investment and cost, environmental hazards and doing harm to workers' health, many countries have prohibited from producing CNR through tetrachloromethane solution process, so the latex process has good prospects in widespread application. There are many reports on the preparation structure and chlorinated mechanism of CNR.But the reports on the thermal ageing of CNR from latex are few. In this paper the dynamic thermal ageing of CNR from latex at 25~360℃ was studied by Fourier Transformed Infrared (FTIR) method.     

Radiation Graft Copolymerization of n-Butyl Acrylate on Natural Rubber Latex

Abstract

A method of radiation graft copolymerization of n-butyl acrylate (NBA) on natural rubber (NR) latex has been studied. The rate of conversion increases with the increase of NBA in latex. An irradiation dose of about 12 kGy is needed to obtain 90% conversion with 40 phr of NBA in latex. Tensile strength, tear strength, and elongation at break of grafted NR are found to decrease with increasing degree of grafting. The physical strength of a vulcanizate prepared from a mixture of NR and ply-NBA was found to be better than that of NBA-NR graft copolymer vulcanizate. The graft copolymerization reaction takes place in the outer layer of NR particles, and because the secondary bonds between poly-NBA molecules may be weaker than those between NR molecules, the existence of a poly-NBA layer in NR particles will decrease its physical strength.     

Mechanical characteristics of hydrogenated natural rubber vulcanizates

Mechanical properties of partially hydrogenated natural rubber (HNR) vulcanizates were evaluated regarding their chemical structure and crystallizable nature of HNR, and are reported here, to the best of our knowledge, for the first time. HNRs of three levels of hydrogenation (20.6, 29.0, and 40.6 mol%) were successfully prepared by the chemical modification of natural rubber (NR) latex using N₂H₄ and H₂O₂ as reagents, in a sufficient amount for preparing sulfur‐crosslinked samples to be subjected to mechanical and structural measurements. The three HNR vulcanizates were found to be crystallizable upon stretching; it is noted that even 40.6 mol% hydrogenation did not prevent HNR vulcanizates from crystallization upon stretching, while their onset strain of crystallization was higher than that of NR vulcanizate. The hysteresis loss and residual strain up to a stretching ratio of 2 for the HNR vulcanizates tended to become larger with the increase in the degree of the hydrogenation. Tensile and dynamic mechanical properties of 20.6 mol% hydrogenated HNR vulcanizate were comparable to those of NR vulcanizate. From differential scanning calorimetry and temperature dispersion of dynamic modulus or loss, the glass transition temperatures of HNR vulcanizates were found to be almost the same as that of NR vulcanizate, which is also notable. The thermal stability of HNR vulcanizates was better than that of NR vulcanizate. Thus, this chemical modification seems to give a promising NR derivative whose properties can be equivalent or even better than the mother polymer. Copyright © 2008 John Wiley & Sons, Ltd.     

Novel biphasic structured composite prepared by in situ silica filling in natural rubber latex

The sol‐gel reaction of tetraethoxysilane in natural rubber (NR) latex was conducted to produce in situ silica‐filled NR latex, followed by adding sulfur cross‐linking reagents to the latex in a liquid state. The latex was cast and subjected to sulfur curing to result in a unique morphology in the NR composite of a flexible film form. The contents of in situ silica filling were controlled up to 35 parts per one hundred rubber by weight. The silica was locally dispersed around rubber particles to give a filler network. This characteristic morphology brought about the composite of good dynamic mechanical properties. Synchrotron X‐ray absorption near‐edge structure spectroscopy suggested that the sulfidic linkages of the sulfur cross‐linked composites were polysulfidic, S_x (x ≥ 2), and a fraction of shorter polysulfidic linkages became larger with the increase of in situ silica. The present observations will be of use for developing a novel in situ silica‐filled NR composite prepared in NR latex via liquid‐phase soft processing. Copyright © 2011 John Wiley & Sons, Ltd.     

Depolymerization and ionic conductivity of enzymatically deproteinized natural rubber having epoxy group

Preparation of liquid epoxidized natural rubber (ENR) was made by oxidative depolymerization of ENR in latex stage without loss of epoxy group. Epoxidation of fresh natural rubber latex, which was purified by deproteinization with proteolytic enzyme and surfactant, was carried out with freshly prepared peracetic acid. The glass transition temperature (T_g) and gel content of the rubbers increased after the epoxidation, both of which were dependent upon an amount of peracetic acid. The gel content was significantly reduced by oxidative depolymerization of the rubber with (NH₄)₂S₂O₈ in the presence of propanal. The resulting liquid epoxidized rubber (M_n≈10⁴) was found to have well-defined terminal groups, i.e. aldehyde groups and α-β unsaturated carbonyl groups. The novel rubber was applied to transport Li⁺ as an ionic conducting medium, that is, solid polymer electrolyte.