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ENR with varying levels of epoxide groups ranging from 10 to 50 mol% was prepared in-house and later dynamically phenolic vulcanized by blending it with poly(ether-block-amide) copolymer (PEBA) at 160°C. The results revealed that the thermoplastic vulcanizates (TPVs) of ENR/PEBA blends exhibited a sea-island morphology. Increasing the epoxide content and PEBA proportion improved the strength properties, including higher Young's modulus (stiffness), toughness, tensile properties, and hardness. Additionally, increasing the epoxide content and ENR proportion in the TPVs resulted in decreased tension set values, indicating improved elastic properties. This is attributed to the interaction between the polar groups present in the phenolic-cured ENR domains and the PEBA molecules. This also caused a decrease in the size of the ENR domains dispersed in PEBA matrix as the content of epoxide groups increased. This effect, in turn, contributed to the enhancement of interfacial adhesion between the ENR domains and PEBA interfaces, leading to the observed improvements in the strength and elastic properties of the TPVs. Additionally, an increase in the epoxide content correlated with a decrease in Tan δ and tension set, further supporting the observed improvements in strength and elasticity. In addition, a single glass transition temperature (Tg) was observed in the ENR/PEBA blends, whereas two Tgs were observed in pure PEBA. The presence of a single Tg in the ENR/PEBA blend is attributed to the overlapping of the Tg of the ENR and PEBA immiscible blend components.

Novel green biodegradable thermoplastic natural rubber (GB-TPNR) based on natural rubber and polyester blends was prepared by dynamic vulcanization. The main objective was to improve mechanical and rheological properties, thermal stability and biodegradability of the natural rubber/polyester thermoplastic vulcanizates (TPVs). Three types of natural rubber: unmodified NR, and epoxidized natural rubbers with 25 and 50 mole % epoxide (i.e., ENR-25 and ENR-50); and two polyester types: polybutylene succinate (PBS) and poly(butylene succinate-co-butylene adipate) (PBSA) were used as blend components. It was found that the dynamically cured ENR-50/PBS blends exhibited superior mechanical, rheological, thermal and stress relaxation properties relative to ENR-25/PBS and unmodified NR/PBS TPVs. This is attributed to stronger chemical interactions between the polar functional groups of ENR-50 and PBS, which caused strong interfacial adhesion and hence the smallest vulcanized rubber domains dispersed in the PBS matrix. Furthermore, stress relaxation, thermal resistance, mechanical strength and degree of crystallinity of PBS phase increased with epoxide content in the modified rubber. Moreover, the dynamically blended ENR-50/PBSA showed larger vulcanized rubber domains along with poorer mechanical, relaxation and thermal properties but easier biodegradation than the ENR-50/PBS blend.

The present study aimed to develop adhesives for bonding natural rubber (NR) to steel based on modified NR bearing grafted poly(acetoacetoxyethyl methacrylate), NR-g-PAAEM. Graft copolymers of NR-g-PAAEMs were prepared by emulsion polymerization at 50 °C. A significant increase in the polar component of NR from 1.62 to 6.84 mN/m was observed after grafting modification, indicating an increase in its hydrophilicity. After that, both one-coat and two-coat adhesives (or adhesive/cover-coat system) were then prepared, using polyisocyanate (poly‐HDI) as a bonding agent. The NR/steel joints were made by vulcanization bonding, using the NR-g-PAAEM adhesive in the presence or absence of cover-coat layer. The results reveal that the NR/steel joint bonded using the two-coat system exhibited higher peel strength than that bonded using the one-coat system. For the two-coat system, the peel strength of 874 N/m was attained when the NR-g-PAAEM adhesive was used in combination with the NR cover-coat. The replacement of the NR cover-coat by the NR-g-PAAEM20 cover-coat led to 49% increase in peel strength of the NR/steel joint. Moreover, the X-ray photoelectron spectroscopy analysis also indicated the formation of urethane-like bonding in this adhesive system, as a result of the reaction between the poly‐HDI and metal oxides on the steel surface.

Chemical cross-linking of rubber is a process to significantly modify the physical properties of the polymer. By this transformation, a highly viscous rubber compound is converted into an elastomer...

Thermoplastic natural rubber (TPNR) nanocomposites based on epoxidized natural rubber (ENR) and polypropylene (PP) blends filled with 2.5 wt% nano-silica (SiO2) were prepared using a two-step compounding process to improve the dispersion of SiO2 in TPNR. The effects of the compounding techniques, i.e., solid pre-compounding and latex pre-compounding processes of SiO2 and ENR, and surface modification of SiO2 on the morphological, mechanical, and thermal properties together with the crystallization behavior of PP phase in ENR/PP blends were demonstrated. Scanning electron microscope (SEM) results showed that latex pre-compounding of SiO2 and ENR gave finer dispersion and distribution of SiO2 particles in the TPNR matrix as compared with the solid pre-compounding technique. Also, the superior mechanical properties and thermal stability of the blends were achieved by using the latex pre-compounding technique. Moreover, surface modification of SiO2 by organosilane was found to enhance crystallization rate and the degree of crystallinity of PP phase in the TPNR, leading to an improvement in the strength and mechanical properties of the ENR/PP blends. These results agree well with the morphological properties that presented a good dispersion and distribution of silane-modified SiO2 particles in the TPNR matrix.

Green biodegradable thermoplastic natural rubber based on epoxidized natural rubber (ENR) and poly(butylene succinate) (PBS) blends was prepared via simple blend (SB) or via dynamic vulcanization (DV). Influence of blend proportions of ENR and PBS on morphological, dynamic, mechanical, dynamic mechanical, thermal properties, and water absorption together with biodegradability were investigated. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were also used to assess phase morphologies in the ENR/PBS blends. It was found that the ENR/PBS simple blend had co-continuous phase structure, while the dynamically cured ENR/PBS blend (DV) has spherical micron-sized crosslinked ENR particles dispersed in the PBS matrix. In addition, the particle size of vulcanized ENR domains of DV and the grain size of ENR and PBS phases in the simple blend decreased with increasing PBS content up to 50 wt%. This might be caused by interfacial adhesion effects between the phases. However, increasing the ENR fraction to 70 wt% caused increasing of elasticity in terms of shear viscosity, shear modulus, storage modulus, together with elongation at break, and tension set. It was also found that mechanical and thermal properties as well as biodegradability of dynamically cured ENR/PBS blends were better than for the simple blend counterparts, That is, the lowest biodegradability, as indicated by the least weight loss, is seen for the 50/50 ENR-25/PBS simple blend (about 1.2%) and dynamic vulcanizate (about 0.6%).

Natural rubber (NR) composites filled with carbon nanotubes (CNT), and carbon black (CB) were prepared. Also, other rubber matrices were tested, namely epoxidized-NR (ENR) and isoprene rubber (IR). The aim was to examine the effects of polarity and non-rubber constituents in rubber on mechanical and piezoresistive sensor properties. Thus, the relative resistances during extension were determined under static (stepwise) and dynamic (cycling) deformation of the composites. It was found that ENR-CNT/CB exhibited mechanical properties superior to NR-CNT/CB and IR-CNT/CB. This is attributed to the chemical ENR-CNT/CB linkages and physical interactions of non-rubber components with CNT/CB surfaces. This also helps the recovery of resistivity to the original value after 20 extension cycles. After 10 000 cycles, the resistivity had changed by 2 orders of magnitude before showing the constant resistivity. Thus, ENR-CNT/CB composites can serve in sensors for health monitoring, motion sensors, and other related products, being cost-effective and easy-to-process materials.

Hybrid filler of carbon nanotubes (CNT) and zinc oxide (ZnO) nanoparticles were prepared and then mixed with natural rubber (NR). The ZnO nanoparticles were first synthesized by the sol–gel method. The NR-CNT/ZnO nanocomposites were prepared by latex mixing. Various proportions of ZnO in the CNT/ZnO hybrid filler were tested by mixing with NR to eventually form NR-CNT/ZnO nanocomposites. It was found that the optimum ZnO content in CNT/ZnO hybrid filler was about 3 phr. These ZnO contents provided the NR hybrid composite with superior mechanical properties in terms of tensile strength and modulus, together with good electrical conductivity and dielectric properties. In addition, stress relaxation tests revealed stronger filler network formation after incorporation of CNT-ZnO hybrid filler into the NR matrix. It might be due to electrostatic interactions between CNT and ZnO, also contributing to high electrical conductivity and dielectric properties of the NR-CNT/ZnO composites, when compared to a solely CNT filled NR composite. Furthermore, the Payne effect and morphological properties were qualitatively analyzed, indicating that CNT dispersion was finer in the NR-CNT/ZnO composites having about 1 to 3 phr ZnO content in CNT/ZnO. Higher contents of ZnO in CNT/ZnO hybrid filler caused large filler aggregates degrading mechanical and electrical properties.

In this work, studies are carried out to understand the crosslinking reaction of epoxidized natural rubber (50 mol% epoxy, ENR-50) by metal ion namely ferric ion (Fe3+, FeCl3, ferric chloride). It is found that a small amount of FeCl3 can cure ENR to a considerable extent. A direct interaction of the ferric ion with the epoxy group as well as internal polymerization enable the ENR to be cured in an efficient manner. It was also found that with the increased concentration of FeCl3, the crosslinking density of the matrix increased and therefore, the ENR offers higher mechanical properties (i.e., modulus and tensile strength). In addition, the glass transition temperature (tg) of ENR vulcanizate is increased with increasing concentration of FeCl3. Moreover, the thermal degradation temperature (Td) of the ENR-FeCl3 compound was shifted toward higher temperature as increasing concentration FeCl3.

Thermoplastic vulcanizates (TPVs) based on blends of natural rubber (NR) grafted with poly(acetoacetoxyethyl methacrylate), NR-g-PAAEM, and polyamide12 (PA12) were prepared by the dynamic vulcanization technique. NR-g-PAAEM was first prepared and its surface free energy was evaluated by means of contact angles. Blends of NR-g-PAAEM and PA12 were then prepared at a 60/40 blend ratio (wt%) using a dynamic vulcanization technique. The results revealed that TPVs exhibited a dispersed-phase morphology. However, the TPVs filled with carbon black (CB) showed much smaller dispersed rubber particles when compared to the unfilled TPV. TEM analysis also revealed that the CB appeared to be mainly located within the NR-g-PAAEM phase. The presence of CB generally raised the melt viscosity and increased shear during melt-mixing, resulting in finer dispersion of the rubber particles. Additionally, a significant enhancement in the tensile properties of the TPV was also achieved by the incorporation of CB. The optimum tensile properties were attained by the addition 30 phr of CB. The results also showed that the incorporation of CB into the NR-g-PAAEM/PA12 TPVs improved their stress relaxation properties. Moreover, the effects of reprocessing on the mechanical properties of the TPV filled with CB were also investigated. Although the TPV filled with CB exhibited reprocessing capability, reprocessing at high temperature and shear forces was found to negatively affect its tensile properties, especially after the third reprocessing cycle.

Some properties of carbon nanotube (CNT) filled natural rubber (NR) composites were improved by adding an ionic liquid (IL), 1-butyl-3-methyl imidazolium bis (trifluoromethylsulphonyl)mide (BMI). In this work, the CNT and IL (CNT-IL) were mixed with NR by latex mixing method. Cure characteristics, thermo-mechanical properties, Payne effect, electrical conductivity and thermal stability were investigated. It was found that IL (BMI) accelerated vulcanization reactions and reduced scorch time. In addition, Fourier Transform Infrared (FTIR) results confirmed the role of IL in NR composites along with the reaction between CNT and NR molecules. The temperature scanning stress relaxation (TSSR) measurement was used to assess thermo-mechanical properties, and a relaxation peak of IL was found due to interactions of cations and anions in IL (BMI). Furthermore, the Payne effect was used to qualitatively analyze the roles of IL and CNT in three-dimensional CNT networks in the NR matrix. It was found that CNT dispersion was finer in the NR/CNT composites with IL. Furthermore, the NR/CNT-IL composite had higher electrical conductivity and lower percolation threshold concentration than the NR/CNT composite.

Epoxidized natural rubber with 50 mol% epoxide groups (ENR50) was used as a blend component in the poly (vinylidene fluoride)/epoxidized natural rubber (PVDF/ENR50) blends. The PVDF/ENR50 blends were dynamically cross-linked with phenolic resin in varying amounts from 0 to 11 phr. It was found that 5 phr phenolic resin was optimal for superior mechanical properties of the PVDF/ENR50 blends. Dynamically cured PVDF/ENR50 blends with various blend ratios were investigated. The tensile strength and tension set decreased whereas elongation at break increased with ENR50 content in the dynamically cured PVDF/ENR50 blend. Furthermore, dynamic mechanical analysis (DMA) revealed a shift in the glass transition temperatures of blend components, and differential scanning calorimetry (DSC) revealed nucleation of PVDF crystallization by the rubber particles in the PVDF/ENR50 TPV, indicating molecular interactions between ENR50 and PVDF phases.

Natural rubber (NR) and epoxidized natural rubber (ENR) vulcanizates reinforced by carbon nanotubes (CNT), conductive carbon black (CCB) and CNT/CCB hybrid filler without and with bis(triethoxysilylpropyl)tetrasulfide (TESPT) silane coupling agent were prepared using an internal mixer and a two-roll mill. Attenuated total reflection infrared spectroscopy (ATR-FTIR) was used to determine chemical interactions among rubber molecules, filler surfaces and silane molecules. In addition, the filler-filler interaction in NR and ENR matrices were assessed from wetting ability and Payne effect. Furthermore, the coupling by TESPT of filler surfaces and rubber molecules was clarified by temperature stress scanning relaxation (TSSR) technique. It was found that the rubber bound by physical absorption decreased with addition of TESPT, while the chemically bound amount significantly increased. This correlates well with estimates of physically and chemically bound rubber from swelling method and morphological properties. It was also found that the optimal electrical conductivity, percolation threshold concentration and dielectric constant of the composites were effectively improved by addition of TESPT. The improvement was confirmed by ANOVA. This indicates a great opportunity to manufacture smart materials with superior conductivity and dielectric constant, together with optimal scorch time, cure time and crosslinking properties.

The mesostructure and non-rubber content, including proteins and lipids, in natural rubber from three different Hevea brasiliensis clones, namely RRIT408, PB235 and RRIM600, were investigated by a new technique, temperature scanning stress relaxation (TSSR), along with other relevant methods It was found that the significant relaxation spectrum of TSSR was found as broad peaks in the temperature ranges of 58–60 °C which indicated level of interaction for proteins and lipids at the chain terminals. This directly relates to storage hardening phenomenon. Furthermore, we found that the molecular mass distribution is bimodal for RRIM600 clone while it is unimodal for RRIT408 and PB235. Furthermore, the RRIM600 clone contained high aggregates and non-rubber components that enhanced the mechanical and rheological properties and caused strong storage hardening phenomenon. This might be attributed to the high fraction of short chains providing many chain ends, and to the total content of proteins and lipids. However, the unimodal rubber from PB235 clone exhibited poorer mechanical and physical properties and less storage hardening than those from RRIM600 and RRIT408. This might be attributed to lower content of proteins and lipids and longer molecular chains (low number of chain ends), causing less formation of aggregates or lower gel content.

Effects of addition of raw cashew nut shell liquid (CNSL) or its decarboxylated form (D-CNSL or cardanol) on curing characteristics, physical properties and temperature scanned stress relaxation were investigated. The study revealed that the CNSL containing high anacardic acid concentration acts as an acid activator and strongly affects curing behavior of NR compounds. The effective curing rate and curing efficiency, i.e., torque difference, crosslink density, and activation energy for curing, clearly increased with 2–5 phr of CNSL. Furthermore, physical properties and thermal stability also improved. However, when the acidity was removed by forming D-CNSL, the effects were totally different from those of the CNSL. The D-CNSL behaves like a plasticizer improving chain flexibility and might show dilution effects if used at a high content in the compound formulation. These effects are clearly seen in decreased curing rate and curing efficiency, together with slight losses of tensile properties, tear strength, and thermal stability, with D-CNSL addition.

Nanocomposites based on carbon nanotube (CNT) filled epoxidized natural rubber (ENR) were prepared by melt mixing method with the main aim to elucidate the critical CNT loading that provides optimal mechanical and electrical properties. The reinforcing effect of CNT in ENR matrix was assessed from torque difference (MH-ML) and 100% moduli that increased with CNT loading. Furthermore, dielectric constant and electrical conductivity increased with CNT loading. In addition, the percolation threshold concentration was about 2.3 phr of CNT with the t-value of fundamental percolation theory at 1.74. This indicates three dimensional (3D) CNT networks dispersed in the ENR matrix. In addition, the dipole-dipole interactions of ENR and CNT increased the polarization effect that caused formation of conductive material with micro-capacitor characteristics.

The concept of using glutaraldehyde (GTA) to crosslink natural rubber (NR) particles bearing diacetone acrylamide (DAAM) functional groups during film formation was investigated in the present work. The primary advantage of this curing system is that it is feasible under ambient conditions, which can lower operating costs of the curing process. Graft copolymers of NR and poly(diacetone acrylamide) prepared with 5 wt% of DAAM (NR–g–PDAAM5) were synthesized by seeded emulsion polymerization at 50 °C. Then, the tensile properties were measured for cast films formed from NR–g–PDAAM5 latex in the absence and presence of GTA. The results revealed increased tensile strength of the NR–g–PDAAM5 film, when GTA was added into the latex prior to film casting. The crosslinking of NR–g–PDAAM5 latex film by reaction with GTA, after film casting, was also investigated using attenuated total reflection Fourier transform infrared (ATR–FTIR) and dynamic mechanical thermal analysis (DMTA). ATR–FTIR analysis demonstrated that crosslinking reactions formed conjugated C=C double bonds between the active carbonyl groups of DAAM and GTA. The complementary use of DMTA also corroborated that crosslinking reactions took place involving the grafted PDAAM chains on the NR particles. This was evidenced by a clear shift towards higher temperatures of the tan δ peak, corresponding to the Tg of NR–g–PDAAM phase, when GTA was incorporated into the NR–g–PDAAM5 latex before film formation. Additionally, a noticeable increase in thermal stability of the NR–g–PDAAM5 film was also observed with added GTA. Hence, it can be concluded that GTA is an efficient room-temperature crosslinker for NR particles functionalized with DAAM. This curing system can also be considered an alternative, simple, and inexpensive method for curing NR latex compounds, as only one component (GTA) is required in the curing process.

Novel biodegradable thermoplastic elastomer based on epoxidized natural rubber (ENR) and poly(butylene succinate) (PBS) blend was prepared by a simple blend technique. Influence of blend ratios of ENR and PBS on morphological, mechanical, thermal and biodegradable properties were investigated. In addition, chemical interaction between ENR and PBS molecules was evaluated by means of the rheological properties and infrared spectroscopy. Furthermore, the phase inversion behavior of ENR/PBS blend was predicted by different empirical and semi-empirical models including Utracki, Paul and Barlow, Steinmann and Gergen models. It was found that the co-continuous phase morphology was observed in the blend with ENR/PBS about 58/42 wt% which is in good agreement with the model of Steinmann. This correlates well to morphological and mechanical properties together with degree of crystallinity of PBS in the blends. In addition, the biodegradability was characterized by soil burial test after 1, 3 and 9 months and found that the biodegradable ENR/PBS blends with optimum mechanical and biodegradability were successfully prepared.

This work aimed to prepare biodegradable thermoplastic elastomers based on NR/LLDPE/TPS ternary simple blends to achieve some exclusive properties, i.e., good biodegradability in terms of water absorption and weight loss after burial, together with reasonable mechanical and thermal properties. A comparative study on biodegradability and other related properties of NR/LLDPE binary and NR/LLDPE/TPS ternary blends was performed. It was found that increasing the TPS proportion decreased storage modulus and complex viscosity. In addition, the size of dispersed TPS domains in the NR/LLDPE co-continuous matrix increased with TPS proportion, while the mechanical properties in terms of 100% moduli, tensile strength, elongation at break, and hardness decreased. This might be attributed to decreased interfacial adhesion with increasing size of TPS domains. Furthermore, increasing the TPS loading in the blend reduced the temperatures for 5 or 50% mass loss (T5 or T50) and the degradation temperature (T d ). However, the biodegradability improved, in terms of increased water absorption and weight loss after burial in soil, with the loading level of TPS.

The main objective of this study is to develop wood adhesives that can be cured at room temperature, based on modified natural rubber (NR) latex. Two types of modified NR, namely poly(acetoacetoxyethyl methacrylate)-grafted NR, NR-g-PAAEM, and poly(acetoacetoxyethyl methacrylate-co-methyl methacrylate)-grafted NR, NR-g-P(AAEM-co-MMA), were synthesized. Emulsion polymerization technique was exploited to synthesize the graft copolymers. The hydrophilicity of NR after grafting modifications was examined by means of contact angle. These results reveal that the NR become more hydrophilic after being modified with poly(acetoacetoxyethyl methacrylate), PAAEM, and/or poly(methyl methacrylate), PMMA. Adhesion properties in terms of lap shear strength were studied for wood adhesives derived from both types of NR graft copolymers, with glutaraldehyde (GTA) as the crosslinking agent. The results show that adhesives based on the NR graft copolymers gave higher lap shear strength than the NR latex adhesive. For the graft copolymers, the NR-g-P(AAEM-co-MMA) adhesive exhibited much higher lap shear strength than the NR-g-PAAEM adhesive. This indicates that the incorporation of PMMA chains into the NR-g-PAAEM molecules significantly increased the cohesive strength of the resulting adhesive, which is probably due to increased modulus and hardness of the adhesive film. POLYM. ENG. SCI., 2017. © 2017 Society of Plastics Engineers

The present work aimed to develop epoxidized natural rubber/thermoplastic polyurethane thermoplastic vulcanizates (ENR/TPU TPVs) having some specific properties, i.e., good processability and dynamic properties, and excellent heat and oil resistances. A comparative study on the effects of rice husk ash (RHA) and commercial siliceous earth (SE) as reinforcing fillers on alteration and enhancement of TPV properties was performed. It was found that the RHA-filled ENR/TPU TPVs showed an improvement of thermoelastic properties with outstanding thermal resistance relative to the pure TPU. This might be attributed to the synergistic effects of the dispersed vulcanized ENR domains and the RHA particles in the TPVs. Furthermore, the RHA-filled ENR/TPU TPVs had rather similar performance, processability and physical properties to the SE-filled ENR/TPU TPV. This indicates that the RHA has great potential as a filler in thermoplastic vulcanizates, in particular with ENR that is a renewable green resource.

Filled natural rubber (NR) vulcanizates were prepared with three types of carbon based filler, namely carbon nanotubes (CNT), conductive carbon black (CCB), and CNT/CCB hybrid filler. The formations of fillers in the NR matrix were investigated from the morphology during vulcanization, the Payne effect, and filler dispersion. Furthermore, the wetting ability was estimated based on temperature scanning stress relaxation (TSSR). It was found that the NR-CNT composite showed higher interfacial energy than the NR-CCB composite. This improved the arrangement of CNT in the CNT/CCB hybrid filled NR vulcanizates. Also, a significant increase was found in the optimal conductivity of the hybrid composites with 5 phr CNT and 7.5 phr CCB. This might be attributed to high electron tunneling when the CNT encapsulates are bridged by CCB aggregates. The CNT/CCB combination significantly reduced the inter-particle gaps from ∼6 to ∼2 nm, giving very low percolation threshold concentration at which three-dimensional filler networks form. In addition, statistical analysis by ANOVA revealed that the filler type and loading are the principal factors contributing to the relative conductivity of composites, both below and above the percolation threshold.

Conductive thermoplastic vulcanizates based on epoxidized natural rubber (ENR) and copolyamide (COPA) blends were prepared using two alternative mixing stages to incorporate MWCNTs: after or before dynamic vulcanization (i.e., ADV or BDV, respectively). Effects of ionic liquid (IL) loading on properties of the blends were also studied. The results indicated that both ADV and BDV mixing preferentially localized MWCNTs in the COPA phase. However, with BDV mixing some of the MWCNTs also resided in ENR domains. The TPVs made with BDV showed higher electrical conductivity, dielectric properties, and superior stress relaxation behavior relative to those prepared by ADV. This might be due to better dispersion of MWCNTs in both phases of the blends. In addition, the electrical and dielectric properties were improved by the IL in a dose dependent manner with the IL loading. However, the IL reduced stress relaxation of the TPVs due to its plasticizing effect.