Key Engineering Materials Vol. 659

Abstract: Shape memory polymers (SMPs) are polymer materials that can fix the temporary shape and then recover to their original permanent shape by external stimulation, i.e. applied heat. In this research, shape memory polymer composites (SMPCs) from benzoxazine (BA-a)-epoxy binary systems reinforced with adamantine silicon carbide whisker (SiC_w) are investigated. The SiC_w contents are controlled to be in range of 0 to 15% by weight. All specimens were fabricated by compression molding technique. The results revealed that the shape memory polymer composites showed higher glassy state storage modulus with increasing amount of the whisker suggesting substantial reinforcement effect of the whisker used. The glass transition temperature (T_g) was also improved from 102°C of the based polymer to the value about 122°C with the addition of about 15% by weight of the silicon carbide whisker. Finally, shape recovery stress systematically increased from the value about 1.5MPa of the unfilled polymer matrix to the value about 3.2MPa with an addition of 15% by weight of the silicon carbide whisker. The positive effect on thermal stability from SiC_w addition is expected from the modification and will be reported in this work.

Abstract: Artificial leather based on natural rubber (NR) filled with leather fibers was prepared using a hot pressing process. The leather fiber was obtained by shredding chrome-tanned leather into fibers with the sizes of 20-mesh. Three different modifying agents (i.e., 1% aqueous solutions of sodium hydroxide, urea, and sodium bicarbonate) were used in the modification of leather fibers. Mechanical properties of NR composites filled with 100 phr of modified or unmodified leather fibers were then investigated. The results showed that cure characteristics, tensile properties, tear strength and abrasion resistance of the composite combined with the modified leather fibers were clearly improved, compared to those of the simple unmodified leather fiber composite. Furthermore, interfacial adhesion between the leather fiber and natural rubber was found to improve. From this study, the result suggested that the urea solution provided the best property improvement when the other modifying agents were compared with.

Abstract: Natural Rubber (NR) latex obtained from Hevea brasiliensis contains a wide particle size distribution. The aim of this study is to investigate the effect of small rubber particles (SRP) and large rubber particles (LRP) on the characteristics of film formation. The rubber particle with different mean diameters can be separated by centrifugation at various speeds to prepare SRP and LRP latex. The average size of SRP and LRP were characterized by light scattering technique to show that the size of SRP was in the range of 0.20 μm, while that of LRP was larger with the wide distribution. SRP and LRP latex were dried at room temperature to study the film formation behaviors. The results showed that the film compaction time increased with increasing the particle size of NR. Furthermore, the rubber film were aged at room temperature for 3 weeks in order to observe the surface morphology using atomic forced microscopy (AFM) by tapping mode. The AFM images showed that SRP readily formed a coalescence film, while LRP showed individual particles on the surface of film at 24 h of storage time. The surface of both SRP and LRP films was smoother after storage. However, LRP film still showed individual particles on the surface after 3 weeks of storage time.

Abstract: Four types of extracted solvents i.e. mixtures of chloroform:acetone, chloroform:methanol, cyclohexane:acetone and cyclohexane:methanol, in the volume ratio of 4:1 were used to dissolve the natural rubber. Two grades of natural rubber i.e. air dry sheet (ADS) and ribbed smoked sheet No.3 (RSS3) were investigated. The rubber solution was purified by separated the rubber out with methanol. The non-rubber solution was then quantitative analyzed to verify 2,2-diphenyl-1-picryhydrazyl (DPPH) radical scavenging activity using Oxicount-Antioxidant kit test and also calculated the term of half maximal effective concentration (EC50). The plasticity retention index (PRI) was also investigated in order to compare the ageing properties of the rubber and their antioxidant activity. Furthermore, various rubbers i.e. ADS, RSS3 and Standard Thai Rubber 20 (STR20) were collected during early period of yearly tapping seasons to analyze the DPPH radical scavenging activity and EC50. It was found that the mixture of cyclohexane and methanol showed the highest DPPH radical scavenging activity and lowest EC50. RSS3 showed higher DPPH radical scavenging activity than that of ADS which the higher antioxidant activity also displayed the higher PRI values. It also found that during the early period of yearly tapping seasons, DPPH radical scavenging activity of RSS3 was found higher than that of ADS and STR20, respectively when extracted by mixture of cyclohexane and methanol.

Abstract: Polybenzoxazine nanocomposites filled with three different sizes of silica nanoparticles are investigated for their mechanical and thermal properties. In this research, silica nanoparticles with primary particle sizes of 7, 14 and 40 nm were incorporated in polybenzoxazine matrix at a fixed content of 3% by weight. From the experimental results, the storage modulus of the polybenzoxazine nanocomposite was found to systematically increase with decreasing the particle sizes of nanosilica suggesting better reinforcement of the smaller particles. Glass transition temperature was found to slightly increase with the addition of the silica nanoparticles. The uniformity of the composite samples were also evaluated by thermogravimetric analysis to show good dispersion of the silica nanoparticles in the composite samples as a result of high processability of the benzoxazine resin used i.e. low A-stage viscosity with good wetting behaviors. Degradation temperature at 5% weight loss (T_d,5) of polybenzoxazine nanocomposites filled with different particle sizes of silica nanoparticles was found to increase from the value of 325 °C of the neat polybenzoxazine to the maximum value of about 340 °C with an addition of the nanosilica of the smallest particle size used. Finally, the smaller nanosilica particle size was also found to show more pronounced effect on T_d,5 enhancement of the composite samples as a result of greater barrier effect from larger surface area of the smaller particles.

Abstract: We develop a simple and competitive fabrication of antireflective (AR) films with high-ordered nanostructure arrays on polycarbonate (PC) substrate by using gas-assisted hot embossing and a self-assembled technique. In this method, a self-assembled monolayer of polystyrene (PS) nanospheres is well-patterned on glass substrates as the first template. Subsequently, we use the plasma sputtering to deposit a conductive layer onto the surface of nanosphere (NS) patterned substrates, and then, electroforming is applied to fabricate a nickel mold with an inverse shape of nanospheres. In the last step, a unique glass transition is utilized to duplicate nanostructures on PC films via gas-assisted hot embossing. Not only in visible light but in near infrared, the optical properties of this AR film are similar or better than for other methods. This fabrication process also has great potential in industry, with its simplicity, large-area but low-cost.

Abstract: Polyhydroxybutyrate-co-hydroxyvalerate (PHBV) is mixed with natural rubber latex to make better mechanical properties of PHBV. The various ratios between PHBV and natural rubber latex are examined to improve their mechanical properties. The PHBV are solid, easily broken, while natural rubber is excessive elastic materials. Concentrations of the employed PHBV solution are 1, 2, and 3 (%w/v). The mixtures of this solution to natural rubber latex are fabricated the biofilms in three different ratios, 4:6, 5:5, and 6:4, respectively. The films are characterized by electron microscope, universal testing machine, and differential scanning calorimetry (DSC). The electron micrographs of the mixed films and unmixed PHBV yield the lowest void distributions in 3%w/v PHBV. For mechanical properties, the averaged elastic moduli of 1, 2, and 3 (%w/v PHBV) mixed films are 773, 955 and 1,008 kPa, respectively. Their tensile strengths increase with increasing the PHBV concentrations. A similar trend is also found in elastic modulus. The crystallization and melting behavior of pure PHBV and the mixed films are examined by DSC. Melting transition temperatures of pure PHBV exhibit two melting peaks at 154°C and 173°C. In addition, the melting peaks of the mixed films remain in the range of 152-156°C and 168-171°C, respectively. According to their morphology, void distributions reduce twice, compared to the unmixed PHBV. Mechanical properties and thermal analysis indicate that the mixed PHBV can be improved their properties with more resilient and wide range temperature than usual.

Abstract: Functionalization of rubber latex is used to improve some weak properties. One method of functionalization is chemical modification by a photo-catalytic reaction. In this work, the functionalization of styrene-butadiene rubber (SBR) and skim latex were carried out under UV irradiation in the presence of TiO₂ film, which was double spin-coated on a glass petri-dish, followed by calcination at 550°C. The structural characterization of functionalized rubber latex was analyzed by FTIR and NMR techniques. In the case of SBR latex, the hydroxyl group was observed after exposure to 80W of UV irradiation in the presence of H₂O₂ at concentration of 20% by weight of dry rubber. However, the gel formation derived from cross-linking as a side reaction obstructed the further characterization of microstructure and limited the applications of latex and solid rubber. In the case of skim latex, the effect of pH, H₂O₂ concentration and UV irradiation time were studied. It was found that the functionalization was successful after exposure to low power of UV irradiation for 1 h in the presence of H₂O₂ at concentration of 5-10% by weight of dry rubber. The weight-average molecular-weight (M_w) slightly decreased from 2x10⁶ to 1x10⁶ g/mol.

Abstract: Silica and carbon black have been widely used as the main reinforcing fillers for improving the properties of natural rubber (NR). In a silica-filled rubber compound, it is known that the low compatibility between NR and silica affects the mechanical properties of rubber products. In order to overcome this drawback, the functionalized saponified NR (FSPNR) was carried out by grafting acrylamide (AM) onto the saponified NR (SPNR) under UV radiation as a continuous process. An increasing in the bound rubber content and Mooney viscosity was found as an increasing AM content. Storage modulus at low strain amplitude of the silica-filled FSPNR was lower than that of the raw NR. In addition, SEM micrographs showed the good dispersion of silica in FSPNR. These confirmatory evidences indicate the improvement of rubber-filler interaction and the reduction of filler-filler interaction by functionalization under UV radiation.

Abstract: Nanocomposite foams made of ethylene-vinyl acetate (EVA), natural rubber (NR) and nanoclay were fabricated by mean of melt mixing in an internal mixer, and later foaming using azodicarbonamide through compression molding. Effect of NR addition (10-40 phr) into the EVA nanocomposite foam was studied. Characterizations by using Oscillating disk rheometer, X-ray diffraction (XRD), scanning electron microscopy (SEM) and mechanical tests were performed. The XRD analysis showed that the clay was mainly intercalated by the rubber. The SEM analysis revealed that the EVA/NR nanocomposite foams had closed-cell structures. Foaming EVA/NR/clay with composition of 100/0/5 (phr/phr/phr) led to low cell density and large cell size. The EVA nanocomposite foam also had low tensile strength, low compressive strength and low elastic recovery. However, the foam cell structure, i.e., greater cell density and smaller cell size, was obtained when combined with NR. Moreover, the strength and elastic recovery of the EVA nanocomposite foam were improved when the NR was added, and the improvement level was increased with increasing NR content.