دو ماهنامه علوم و تکنولوژی پلیمر
سال بیست و یکم شماره 2 (پیاپی 94، خرداد و تیر 1387)

In this study, the particleboard was made with rice husk and urea formaldehyde resin of 7, 8, 9 and 10 percent (based on dry weight of rice husk). To improve mechanism of bondability a definite amount of isocyanate resin is introduced to replace the same amount of urea formaldehyde resin. The effect of urea formaldehyde resin and diisocyanate content on bondability was evaluated by studying the mechanical properties (bending strength, internal bond strength) and physical properties (thickness swelling and water absorption). Data were statistically analyzed with SPSS software and comparison was made on the mean values employing a Dancan test to identify which groups were significantly different from the rest. Generally the results have shown that all mechanical and physical properties of particleboards improve with increasing urea formaldehyde resin content and its partial replacement with diisocyanate as well. In this study the best mechanical and physical properties of particleboard were obtained with 10 percent resin (the sum of 8 percent urea formaldehyde resin and 2 percent diisocyanate).

Fast fracture is one of the major failure modes in brittle polymers. Generally, crack growth in these polymers may occur under combined tensile-shear loading. However, the fracture test data reported previously for some dental restorative resins can not be predicted by using the available fracture criteria.In this paper, a modified fracture criterion is used for estimating the fracture load for the mentioned resin samples. It is shown that by considering a more accurate description for the crack tip stresses and also by accounting for the craze zone effects in front of the crack tip, the modified criterion presents significantly better predictions for the fracture resistance in such brittle polymers.

Linear low density polyethylene (LLDPE) was blended with esterified styrene-maleic anhydride (ESMA) and its physical and mechanical properties were studied. Styrene-maleic anhydride (SMA) copolymer was synthesized by precipitation reaction. Then, SMA was esterified by 1-decanol in methylethyl ketone solvent. From the FTIR spectrum of ESMA it is evident that the corresponding peaks of anhydride groups have disappeared and on the other hand, the acidic and ester groups are observed. Thermal behavior of SMA and ESMA was studied by DSC and TGA. In DSC micrograph, a single peak at 143oC was observed for SMA, while for ESMA a broad peak appeared, showing the different thermal characteristics of two samples. TGA micrograph showed that SMA was degraded in one stage, while, two stages of weight loss were observed for ESMA. LLDPE was blended with different amounts of ESMA in an internal mixer. Films of various blends were produced by pressure molding. Tensile modulus of the blends shows improvements by increasing the ESMA fractions of the system. On the other hand the tensile strength and elongation-at-break for various blends decreased with increasing ESMA content. Contact angle measurement and incubation in a buffer solution (pH=8) showed that hydrophilicity and water solubility were increased along with ESMA content of the blends.

In the processing of carbon/carbon composites using polymer resin as the matrix precursor, it is inevitable that a porous structure was formed after carbonization. As a result, densification by liquid phase impregnation followed by recarbonization is required to obtain a densified composite. Consequently, the char formability of resin is an important factor in reducing the number of densification cycles and hence the processing cost. In this study, a novel approach is adopted to improve the densification of carbon/carbon composites by using a new phenolic resin modified by pitch. For this purpose, soluble part of pitch was extracted and dispersed in resol type phenolic resin. The polymerization reaction was performed in presence of para-formaldehyde and a resol-pitch (RP) compound was obtained. The second compound was prepared by mixing novolac-furfural (NF) in 55:45 weight ratio containing 9% by weight hexamethylene tetramine. This compound was added to RP compound in 10, 20, 50 and 80 w%. The microstructure of carbonized resin was investigated by X-ray diffraction and char yield, and the linear and volumetric shrinkage were obtained. Results show that in 80:20 ratio of RP to NF, the char yield would be maximized by 71% and volumetric shrinkage would be minimized at 16.4%. At the same time, XRD results indicate that the resin has a strong ability to graphitize carbon/carbon composites matrix as a necessary step for its processing.

In the present work, high temperature tensile properties of medium density polyethylene (MDPE) and its nanocomposites are investigated. For this purpose MDPE reinforced with different weight percentages of nano-sized calcium (2, 5, 10) are produced by compression moulding method. Tensile tests have been carried out at different temperatures, i.e. 30, 60, 90C using thermomechanical analysis (TMA) apparatus. Besides, the fracture surface of MDPE and MDPE/CaCO3 nanocomposites are also investigated using scanning electron microscopy (SEM). The TMAresults indicate that the elastic modulus and yield stress have increased by addition of nanosized calcium carbonate as reinforcement for MDPE. At elevated temperature, the tensile strength is shown to be reduced in all materials including MDPE and its nanocomposites. The obtained results confirm that the reinforcing effect of nano-sized calcium carbonate becomes significant particularly at higher temperatures.

In the present work, a titanium-based Ziegler-Natta catalyst was employed to synthesize ultra molecular weight polyethylene in a slurry medium. The effect of different parameters such as ethylene pressure, temperature, catalyst composition, and hydrogen pressure was studied. As the results indicate, increasing either [Al]/[Ti] ratio or temperature causes polymerization rate to go through a maximum. In addition, an increase in either monomer pressure or stirrer speed augments polymer molecular weight. Reducing hydrogen pressure also raises polymer molecular weight. Finally, ultra high molecular weight polyethylene is produced in the absence of hydrogen.

Phenolic resin is among the oldest thermoset resins that have found wide spread applications in several industries, largely due to its high thermal resistance. In this research, room temperature curing of phenolic resin and its reinforced silica composite has been investigated by p-toluene sulphonic acid as a catalyst. Mechanical properties of the composites such as impact strength, flexural properties and hardness are studied. The results show that for achieving an optimum gel time in the order of 20 min an amount of 5.5 wt% acid is required. FTIR studies of the cured resin show no significant difference in chemical structure of both room temperature and hot cured resins. Based on thermal gravimetric analysis the thermal resistance of both systems is similar, but the residual char of the room temperature cured system is higher by 17%. The difference is not related to the carbon content, as revealed by elemental analysis. The room temperature cured composite system shows lower mechanical properties than the heat cured composite. Improved mechanical properties however, can be achieved by its post curing at high temperature.

Thermoplastic honeycombs based on polypropylene are one of the latest types of cores used in sandwich structures. Up to now there have been few experimental and modelling studies on the mechanical behaviour of sandwich structures with thermoplastic cores. In this paper, by considering the similar properties of foams and thermoplastic honeycombs, attempts are made to model the static indentation behaviour of honeycombs based on a crushable foam material model using the ABAQUS finite element programme. Comparison of the experimental results and numerical predictions for indentation values show a good correlation up to 20% depth of the panel thickness. It must be noted that in static indentations it would be irrelevant to study the rupture of the skins or cores and, therefore, the crushable foam model should be suitable for modelling the behaviour of thermoplastic honeycombs.

This research work is devoted to the development of a mathematical model for the simulation of the flow of polymeric melts through the die region of extruders. A set of governing equations are solved using finite element method. Standard Galerkin technique is used in conjunction with the mixed scheme to solve the flow equation. Due to the non-linear nature of global equations, the Picard iteration method is used. The well-known power-law equation was used to describe non-Newtonian rheological behavior of the material and the effect of viscosity on the accuracy of model was investigated. In order to create a finite element mesh for the complex geometries of the die, tetrahedral elements were used. Therefore, the second order tetrahedral elements (10 nodes) and the first order tetrahedral elements (4 nodes) were used for calculation of velocity and pressure, respectively. To show the applicability of the model, the flow of the polyethylene melts with various viscosities through the die of a single screw extruder were simulated. The validity of the present technique under realistic conditions was confirmed by comparing the model simulation results with the experimental data.