Ikram Hussain

Development of polypropylene-based ultraviolet-stabilized formulations for harsh environments

This work reports the outdoor weathering performance of ultraviolet (UV)-stabilized polypropylene (PP) products (using PP resins from Saudi Basic Industries Corporation [SABIC]). Different hindered amine light stabilizers (HALS) were used to stabilize PP-film-based formulations that were exposed for 10 months in harsh outdoor weather of Dhahran, Saudi Arabia. Characterization of the exposed PP film products was done in terms of mechanical and Fourier transform infrared (FTIR) spectroscopic properties. HALS are a very effective light stabilizer for polyolefins.[14–16] They do not act by absorbing UV radiation, but by inhibiting degradation of the polymer, which has started the formation of free radicals. They function by scavenging radicals. HALS has low volatility and high extraction resistance. The effectiveness of HALS is independent of product thickness. Another advantage is that it provides a significant level of stabilization at relatively low concentration.[18] Some important types of high molecular weight HALS are Chimassorb 944 and Tinuvin 622, and a low molecular weight HALS is Tinuvin 770. These HALS, together with other UV stabilizers, are commonly used in polyolefin stabilization. The PP film formulations were divided into five categories based on the type of HALS incorporated. This was done to derive meaningful comparison of the various film formulations. The characterization data are presented for an exposure period of 10 months. The unstabilized PP films were degraded within 2.5 months of the exposure period. The performance in terms of decrease in mechanical properties and FTIR spectroscopic properties was assessed as a result of natural weathering for UV-stabilized samples. Following outdoor weathering trials, the lifetimes of certain formulations were determined. On the basis of the FTIR spectroscopic properties, it was determined that generally, the HALS-stabilized PP film formulations delayed the formation of oxidation products including esters, carbonyls, and trans-vinylenes.

Apparent kinetics of nonisothermal high temperature oxidative degradation of ethylene homopolymers: effects of residual catalyst surface chemistry and structure

The effects of two supported residual catalysts—one Ziegler-Natta and another metallocene—on the nonisothermal thermooxidative degradation of the resulting ethylene homopolymers were investigated using TGA experiments and kinetic modeling. The rigorous constitutive kinetic model (developed in this study), unlike the analytical Horowitz and Metzger model, fitted very well to the entire TGA curve, without distribution of activation energy Ea, for n (overall degradation order) = 1 for both polymers. Neither n nor Ea varied as a function of fractional weight loss of the polymer. Hence, the proposed unified molecular level concept of surface chemistry and structure of the residual catalysts held all through the degradation process. The above feature of n and Ea also indicates the suitability of the model formulation and the effectiveness of the parameter-estimation algorithm. Random polymer chain scission, with the cleavage of the −C−C− and the −O−O− (hydroperoxide) bonds, prevailed. The types of residual catalyst surface chemistry and structure varied the bond cleavage process. The metallocene Zr residual catalyst caused more thermooxidative degradation in MetCat HomoPE than what the Ti one did in Z-N HomoPE. The rigorous constitutive model-predicted apparent kinetic energy Ea, and frequency factor Z also support this finding. The proposed degradation mechanism suggests that the Zr residual catalyst more (i) decreased the activation energy required to decompose the −C−C− and the −O−O− bonds, and (ii) eliminated β-hydrogen (by the carbonyl functionalities) from the polymer chains. These findings were attributed to the differences in surface chemistry and structure of the residual catalysts. Therefore, the current study presents a rigorous constitutive kinetic model that duly illustrates the influence of the characteristic surface chemistry and structure of the residual catalysts on the high temperature oxidative degradation of polyethylenes.

Optimized calibration curve for size exclusion chromatography applied to poly(vinyl chloride)

Abstract The problem of using a polystyrene-standard-based calibration curve for other polymers has been overcome by developing an algorithm based on the concept of parameter estimation and optimization. The algorithm modifies the regression coefficients of the polystyrene-standard-based calibration curve by interactively using the chromatographic output and the absolute number-average molecular weight of the experimental polymer. Analytical expressions have been derived for calculating the directional derivatives, which ensure rapid convergence of the objective function. The algorithm, applied to commercial poly(vinyl chloride), offered a number-average molecular weight of 32037 whereas that measured by membrane osmometry is 32 000. The polymer characterization parameters calculated from the optimized calibration curve closely matched those obtained from the universal calibration curve and Q -factor values. The algorithm needs no narrow standard of the experimental polymer, and holds for homopolymers, copolymers and polymer blends. It can be easily incorporated into commercial size exclusion chromatography data-reduction software packages.