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Abstract

Linear low density polyethylene (LLDPE) samples were exposed to the unique natural environmental conditions of Dhahran, Saudi Arabia. The changes in the significant mechanical, thermal, and chemical properties were monitored using tensile testing system, Fourier transform infrared (FTIR) spectroscopy, and differential scanning calorimeter (DSC), respectively.

Exposure site (Dhahran) selected for study is considered to be a unique laboratory for evaluating the outdoor performance of polymer. Meteorological and radiation environment of test site is influenced by the nearness of the very shallow Arabian Gulf and typical desert surrounding. Resulting in monthly mean temperatures reaching close to 37°C during summer, with daily maxima often approaching the 50°mark. The relative humidity exhibits a large diurnal cycle on the order of 60 percent throughout the year, with daily maxima often rising over the 80 percent level during most months. The solar radiation reaching the ground surface is function of atmospheric turbidity and cloudiness, and because of very little cloud cover throughout most of the year, radiation doses are high in this region.

LLDPE is a relatively new type of polyethylene and unlike low density polyethylene (LDPE) and high density polyethylene (HDPE), which are homopolymers, LLDPE is a comonomer of ethylene and higher-alpha-olefin (e.g. 1-butene). The significant difference between them is of branching with LLDPE having evenly distributed short chains as against HDPE having few short chains, and LDPE containing both short and long chain branches. The unique branching structure of LLDPE imparts certain combination of properties which distinguishes it from conventional polyethylenes.

The weather-induced degradation behavior of LLDPE cannot be predicted from the knowledge obtained for LDPE or HDPE. The FTIR spectroscopic analysis of LLDPE samples exposed in Dhahran, Saudi Arabia, have revealed that using difference spectroscopic technique growth in functional groups (carbonyl, hydroxyl, alkene, vinyl, etc.) is detected at very early stages of degradation. Band indexing and dimensionless number techniques of presenting spectroscopic data of weathered samples have also been used and they present a steady increase in degradation products. DSC technique was used to monitor the percent crystallinity and crystalline melting temperature (T ) of exposed specimens. Percent crystallinity as determined from heat of fusion (H^) shows an increasing trend, whereas Tm was almost constant for most of the exposure duration implying that -CH2 groups in the amorphous region are replaced by the degradation products. Tensile strength and percent elongation have also exhibited a linearly decreasing trend with exposure time, carbonyl groups formation, and percent crystallinity.

Mathematical modelling of degradation in significant properties as a function of weather parameters was accomplished using statistical analysis system (SAS) software package. Tensile strength, carbonyl groups, and percent crystallinity were mathematically shown to be dependent on different weather parameters with UV dose having impact on every property.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science > QD Chemistry T Technology
Departments:	School of Science & Technology School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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