دو ماهنامه علوم و تکنولوژی پلیمر
سال بیست و ششم شماره 3 (پیاپی 125، شهریور 1392)

Production of polypropylene is growing for applications in different types of textiles. The main limitations in some of their applications are attributed to its low photostability and related property changes. There are several factors that have considerable effects on the process of photo-oxidation and the resulting photostability. From polymer production up to its usage the recognition of these parameters is necessary in order to control the degradation and to improve the useful life period of the products. In this paper، first the photo-oxidation mechanism of polypropylene in the presence of air is reviewed and then the effects of different factors including additives are examined. The incorporation of additives and light stabilizers in polymers are important in order to preserve their long-term use in the environment. Besides the complex mechanisms of photostabilization، this review presents an in-depth account of the behavior of the additives with respect to the photochemical oxidation of polypropylene and the interactions involved. Stabilization of polypropylene was improved in the presence of a mixture of photostabilizers، exhibiting an enhanced irradiation resistance. It would appear that the origin and composition of the pigment and its interaction with photostabilizers are all important factors. Several analytical techniques were employed to evaluate the stabilization effectiveness of the additives and degradation process; finally the techniques and the experimental methods for evaluation of photo-oxidation level and photo-oxidation products are introduced. Thedifferences in the views are highlighted and the shortcomings are explained.

Different parameters including temperature and amount of titanium tetrachloride (TiCl4) were examined with respect to Ziegler-Natta catalyst morphology and the catalyst performance during its preparation process. The results show that optimum catalyst activity and morphology are achieved when the process is carried out in two stages. In the first stage of the process، the spherical morphology of the catalyst particles of their support (magnesium chloride/ethanol adduct) with TiCl4 were maintained by gradual increases in reaction temperature from subzero (-5ºC) to high temperature (90ºC)، while the sudden increase of temperature resulted in fragmentation of catalyst particles to non-spherical and very small size species. The effect of the highest temperature limit of this step showed that as the temperature went beyond 90ºC the activity of the catalyst decreased. The second step of the reaction showed that by injection of TiCl4 at high temperature (80-90ºC) the catalyst activities were at the highest point. Study on the final washing of the catalyst showed that at high temperature of washing the catalyst performed its best activity. It can be said that at high temperatures، the probable poisonous adducts which originated from the reaction with TiCl4 could be washed effectively. Finally، according to the obtained results، the optimal operation conditions in catalyst preparation process were proposed those which resulted in high activity and spherical morphology.

The aim of this study was to investigate the rate of water vapor permeability (WVP) and moisture sorption of starch-montmorillonite (MMT) nanocomposite films. The nanocomposite films were prepared using the casting method and glycerol was used as a plasticizer. The effect of glycerol concentration on WVP of the films was determined at 25°C with a relative humidity of 0-99% (inside and outside of the vials). The results showed that WVP is increased with higher glycerol content in the nanocomposite samples and there is an exponential relationship between the plasticizer concentration and WVP of the nanocomposite films. Additionally، moisture sorption kinetics of nanocomposite samples was studied by placing the films in humid environments conditioned at 25°C and 75% relative humidity. In addition، the Fick''s second law and four empirical equations were used to predict the films moisture sorption and the results of the modeling demonstrated that the initial stages of moisture sorption were fully supported by Fick''s law. However، by the gradual relaxation of the polymer، its behavior deviated from the law. This result indicated that the concentration gradient was not the only dominant mechanism of mass transfer. The effective moisture diffusion coefficient (D) was increased with increasing MMT concentration. This was attributed to the penetration of clay layers into polymer matrix in the amorphous regions and creation of free spaces in polymer structure. The existence of free spaces in the nanocomposite structure increased the effective moisture diffusion coefficient. Additionally، plasticizer presence facilitated mobility of polymer chains and increased the effective moisture diffusion coefficient.

Nanocomposites based on butadiene rubber (BR)، (0، 3، 5 and 7 phr) organoclay (Cloisite 15A) and (0، 10، 20، 30، 40 phr) powder coating wastes، i. e.، epoxypolyester hybrid (EPH) were prepared using a laboratory-scale internal mixer in order to study the effect of organoclay and EPH content on the mechanical and morphological properties of the nanocomposite samples. Cure characteristics of the prepared compounds including optimum cure time (t90) and scorch time (t5) depicted a decrease in both mentioned factors with increasing nanoclay content and EPH loading. Intercalation of elastomer chains into the silicate layers was determined by d-spacing values calculated according to the results of X-ray diffraction (XRD) patterns. X-ray diffraction (XRD) results reveal the intercalation of elastomer chains into the clay galleries. This phenomenon was also confirmed according to the scanning electron microscopy (SEM) micrographs and mechanical properties of the nanocomposite samples which were observed to be improved with addition of nanoclay and EPH content.

Carbon/phenolic composites are used in the nozzle parts of solid rocket motors due to their heat-resisting، ablation، and high strength characteristics، which are required to endure the high temperature and pressure of combustion gas passing through the nozzle. One of the most important factors on erosion rate is the void content of the ablative composites. Facilities should be designed to simplify the exhaust of volatile components، in order to reduce the void content in the manufactured samples. Accordingly، to reduce the void percentage of carbon/phenolic composites in this study، samples were manufactured by the vacuum bag molding technique and cured in an autoclave. In order to compare the effect of manufacturing process on the ablation characteristics of carbon/phenolic composites، another batch of samples was produced by the acid curing method. The specimens were exposed to plasma torch flame based on ASTM E285-80. The results showed that the void percentage of samples which was manufactured by the autoclave process was 60% lower than the acid-cured samples، and this led to reduce the linear erosion rate of these composites.

A primary adduct of MgCl2. 3. 3EtOH (A0) was prepared by melt quenching and then it was further dealcoholated by thermal and chemical methods. In thermal dealcoholation، two different support samples with MgCl2. 2. 7EtOH (A1) and MgCl2. 1. 8EtOH (A2) formulas were prepared using a controlled temperature-pressure program. XRD spectra of A0، A1 and A2 supports were similar، except with the peaks at 2θ = 8. 9 and 9. 7 which decreased by moving from A0 to A2. After preparation of support samples، final catalysts were prepared by reacting support samples with TiCl4 in the presence of diisobutylphthalate as internal electron donor and examined in slurry phase propylene polymerization using TEA as cocatalyst and diphenyldimethoxysilane as external electron donor. The results showed that in the catalysts prepared using thermal dealcoholated supports، surface area and Ti content increase by moving from A0 to A2 (decreasing ethanol content of the support). In this respect، the variation in dealcoholation level، due to different dealcoholation time، influenced the catalytic activity considerably and maximum activity was observed at A1 sample and then by further dealcoholation the catalyst activity was dropped. Also، chemical dealcoholation was carried out using 1 M triethylaluminum (TEA) at 20 and 80˚C، and the prepared supports were designated as A3 and A4، respectively. In chemical dealcoholation samples، support structure was totally destroyed and no peak was observed at XRD spectra. The final catalysts were prepared likewise، and examined in slurry phase polymerization of propylene using TEA as cocatalyst. By destruction of support structure، Ti content was decreased significantly and hence the catalyst activity was dropped compared to the catalysts prepared by thermal dealcoholation route.