Claudia Kummerlöwe

Determination of Crosslink Density and Network Structure of NR Vulcanizates by Means of TSSR

Unfilled vulcanizates based on natural rubber (NR) were investigated by temperature scanning stress relaxation (TSSR) measurements. Different sulfur based cure systems, i.e. conventional (CV), semi-efficient (SEV) and efficient (EV) vulcanization system, were used to prepare the vulcanizates. It was found that sulfur/accelerator - ratio has a strong impact on the shape of the relaxation spectrum, deduced from TSSR measurements. By deconvolution of the relaxation spectra, peak separation was performed and 3 different peaks were found in case of CV - cured samples. In contrast, only a single peak was found, in case of the EV-cured sample. After thiolamine treatment the shape of the relaxation spectra altered significantly in case of the CV-cured sample whereas the spectra of the SEV-and EV-cured sample exhibited only slight differences. Additionally, the crosslink density of the samples decreased after thiolamine treatment. This is due to selective cleavage of polysulfidic crosslinks. It has been concluded, that the significant peak in the relaxation spectrum at about 120 °C can be attributed to the cleavage of polysulfidic crosslinks. Furthermore, a linear relationship between the percentage of polysulfidic crosslinks and the sulfur/accelerator - ratio is assumed.

Influence of Filler from a Renewable Resource and Silane Coupling Agent on the Properties of Epoxidized Natural Rubber Vulcanizates

Rice husk ash (RHA) was used as a reinforcing filler in epoxidized natural rubber (ENR) with various loading levels (0, 10, 20, and 30 phr), and silica filled ENR was also studied for comparison. The effects of RHA content on cure characteristics, mechanical properties, dynamic mechanical properties, and thermoelastic behavior of the filled ENR composites were investigated. It was found that the incorporation of RHA significantly affected the cure characteristics and mechanical properties. That is, the incorporation of RHA caused faster curing reactions and increased Young’s modulus and tensile strength relative to the unfilled compound. This might be attributed to the metal oxide impurities in RHA that enhance the crosslinking reactions, thus increasing the crosslink density. Further improvements in the curing behavior and the mechanical properties of the filled composites were achieved by in situ silanization with bis(triethoxysilylpropyl) tetrasulfide (Si69). It was found that the rubber-filler interactions reinforced the composites. This was indicated by the decreased damping characteristic and the other changes in the mechanical properties. Furthermore, the ENR composites with Si69 had improved filler dispersion. Temperature scanning stress relaxation (TSSR) results suggest that the metal oxide impurities in RHA promote degradation of the polymer network at elevated temperatures.

Effect of Modified Natural Rubber and Functionalization of Carbon Nanotubes on Properties of Natural Rubber Composites

The intention of this work was to prepare natural rubber filled with carbon nanotubes (CNT). Various types of natural rubber, including unmodified natural rubber and epoxidized natural rubber with 25 mol% epoxy groups (i.e., ENR-25) were exploited. The state of CNT dispersion in the unmodified natural rubber matrix was improved by the modification of CNT surface with silane coupling agent (Si-69). The modification was carried out by in-situ method, i.e., the silane was directly added during the preparation process of the composites in the internal mixer. The interaction between CNT surface and rubber molecules, with and without silane modification, was characterized by ATR-FTIR measurements. The mechanical properties and crosslink densities were investigated, the electrical conductivity was characterized to determine the percolation threshold.

Thermoplastic natural rubber based on polyamide-12 blended with various types of natural rubber Effect of processing oils and plasticizer

Thermoplastic natural rubber (NR) of dynamically cured 60/40 NR/polyamide (PA) blends has been prepared by using three types of NR: unmodified air dried sheet NR, epoxidized NR (ENR) with 25 and 50 mol% epoxides. Plasticizer (i.e. dioctyl phthalate (DOP)) and processing oils (i.e. treated distillate aromatic extract and paraffinic oils) were incorporated in the blends. Stress relaxation, rheological, mechanical, and thermal properties of the blends were investigated. It was found that mixing torque, shear viscosity, tensile strength, and hardness of the oil-extended blends exhibited lower values compared with the blends without oil. But higher elongation was observed due to the distribution of oil into PA-12 and rubber phases. Furthermore, an increasing trend of stress relaxation behavior and rubber index was observed in the blends with DOP, which revealed an improvement in elastic properties. It was also found that the incorporation of DOP caused an enhancement in thermal behavior in terms of T 10, T 50, T 90, and integral values, but a reduction in the glass transition temperature (T g) of NR phase and crystallization temperature (T c) of PA-12 matrix was also observed. This is attributed to the similar solubility parameter of DOP compared with the blend components. It was also found that the dynamically cured blends showed smaller rubber domains dispersed in PA-12 matrix. However, the addition of DOP resulted in an increase in the size of vulcanized rubber particles.

A Comparative Investigation of Rice Husk Ash and Siliceous Earth as Reinforcing Fillers in Dynamically Cured Blends of Epoxidized Natural Rubber (ENR) and Thermoplastic Polyurethane (TPU)

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.

Novel thermoplastic natural rubber based on polyamide 12 blends: Influence of vulcanization system

Dynamically cured 60/40 epoxidized natural rubber (ENR)/polyamide 12 (PA-12) and unmodified natural rubber (NR)/PA-12 blends with different types of curing systems (i.e. sulfur- and peroxide-cured systems) were prepared. It was found that mixing torque, shear viscosity, tensile strength, hardness, stress relaxation, thermal, and oil resistance properties of the ENR/PA-12 blends were higher than those of the NR/PA-12 blends. This is attributed to the chemical interaction between polar functional groups in ENR and PA-12 molecules which caused the formation of ENR-grafted PA-12. Smaller vulcanized rubber domains dispersed in the PA-12 matrix was observed in the dynamically sulfur-cured ENR/PA-12 blends. This is attributed to higher shear and elongational viscosities during mixing operation at high temperature. However, the peroxide-cured blends exhibited higher relaxation property, oil resistance, and cross-link density than those of the sulfur-cured blends due to strong reversion effect observed in the sulfur-cured system.

Investigation of Structure-Properties Relationship of High Performance TPV Based on ENR/TPU

Thermoplastic natural rubber based on blending of ENR-50/TPU and un-modified NR/TPU was prepared via dynamic vulcanization technique. Influences types of natural rubber (i.e., un-modified NR and ENR-50) on mechanical and morphological properties of the blends were investigated. It was found that the blends with ENR-50 exhibited superior properties than that of the blend with unmodified NR. This is attributed to the interactions between the functional groups of ENR molecules and polar functional groups in TPU molecules which caused higher interfacial adhesion and interaction between both phases. The chemical interactions were confirmed by ATR-FTIR. Furthermore, it was found that the ENR-50/TPU blend showed smaller and finer grain morphology compared with the un-modified NR/TPU.

Effect of Antioxidant on Properties of Thermoplastic Natural Rubber Based on ENR/TPU Blends

Thermoplastic natural rubber based on epoxidized natural rubber (ENR) and thermoplastic polyurethane (TPU) blend was prepared via dynamic vulcanization process. The main objective is to improve thermal properties of the blends. Two types of antioxidant: phenolic antioxidant (Wingstay®L) and N-(1,3-dimethzlbutyl)-N-Phenyl-p-phenylenediamine (6PPD) were used to improve oxidative degradation of the blends. It was found that thermal properties in term of thermal elastic properties and thermal stability can be improved by adding the antioxidants and 6PPD gave the blend with the highest thermal properties. These were measured based on temperature scanning stress relaxation (TSSR) technique. Incorporation of ENR into the TPU caused reduction of the hardness, improved thermal properties, elasticity and oil resistance compared to the neat TPU. These results indicated that the novel high performance TPNRs with high elasticity can be prepared.

Effect of Fillers from Renewable Resources on the Performance of Novel Heat and Oil Resistant Thermoplastic Vulcanizates Based on Epoxidized Natural Rubber/Thermoplastic Polyurethane Blends

Thermoplastic vulcanizates (TPVs) based on thermoplastic polyurethane (TPU) and epoxidized natural rubber (ENR) were prepared by dynamic vulcanization. In this study, the influence of different types and loading levels of filler on the properties of ENR-25/TPU blends was investigated. Furthermore, a filler from renewable resources, namely Rice Husk Ash (RHA), and conventional silica were incorporated in the blends (i.e., premixed with ENR-25). Then, various loading levels of filler were investigated at 10, 20 and 30 phr and compared with the unfilled blend. It was found that the incorporation of filler into the blend shows higher complex viscosity, mechanical properties in terms of Youngs modulus, tensile strength and hardness compared with the TPV without filler. The incorporation of ENR-25 into the blend shows lower hardness than pure TPU. It was also found that TPV filled with RHA revealed almost identical values for Youngs modulus, tensile strength, elongation at break and hardness, than the same TPV filled with conventional silica. This indicates rice husk ash has great potential to be used as filler in polymer composites based on ENR/TPU blends.

Influence of Blend Proportions on Properties of ENR-25/TPU Simple Blends

Thermoplastic natural rubber (TPNR) based on blending of thermoplastic polyurethane (TPU) and epoxidized natural rubber with 25 mol % epoxide (ENR-25) was prepared by simple blend technique. Influence of various blend proportions was investigated. It was found that an incorporation of rubber caused decreasing of mechanical properties in terms of tensile strength, elongation at break and hardness. This is attributed to incorporation of rubber phase may cause chain restriction of the soft segments in TPU and hence lower elongation at break and tensile strength. It was also found that the tension set value decreased with increasing proportion of natural rubber which indicates greater elasticity or tendency to recover to the original shape after prolonged deformation. Additionally, shear stress and shear viscosity of the blends increased with increasing rubber proportions due to higher viscosity of the ENR-25 component which leads to higher force to compress the polymer melt flowing through a capillary channel. Morphological properties of the simple blend with various blend proportions exhibited formation of co-continuous phase structure was observed where larger rubber formation are observed in the blend with higher content of ENR-25 phase.