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Al-Azzawi, Farah (2015) Degradation studies on recycled Polyethylene terephthalate. Doctoral thesis, London Metropolitan University.
This project aims to investigate the influence of UV on the properties under natural and artificial weathering. The real-time outdoor weathering exposures provide the most accurate results but, they are very slow and manufacturers cannot afford to wait in order to see if a new or improved product formulation is really an improvement. So, accelerated testing methods are designed to simulate natural weathering with the combined action of the most weathering damaging factors; UV radiation, oxygen, temperature and water. Whatever the application, there is a concern regarding the durability of the products because if its useful lifetime can be estimated in short time, their maintenance and replacement can be planned in advance.
In this study, recycled PET samples were exposed to UV outdoors and to accelerated weathering up to 13,000 hr. The mechanical behaviour (tensile and impact), thermal behaviour (DSC), molecular structure analysis (FTIR), flow characteristics (MFI) and surface properties (colour and gloss) of the samples were investigated. The whole body of the samples was substantially unaffected by degradation for long exposure time and this is confirmed by MFI and DSC results. However, the surface of the samples is affected which is seen in FTIR analysis, colour and gloss change. This explains that the photodegradation is a surface effect.
FTIR analysis shows an increase in the gauche ratio and decrease in the trans ratio for both types of weathering indicating a less ordered structure after the UV radiation due to chain scission by Norrish Type I and II mechanism. Carboxylic acid is formed as a result of degradation and increases with the extent of UV exposure in both types of weathering, the formation of carboxylic acid by Norrish II is dominated over Norrish I. Colour and gloss measurement shows that the effect of degradation takes place strongly in the first 2000 hr of exposure in natural weathering and extended up to 5000 hr in accelerating weathering due to the formation of microcracks.
Mechanical tests show that the UV effect was not significant on bulk properties such as yield stress and elongation at yield for accelerated weathering samples up to 1000 hours, then dropped by 62% and 57% respectively up to 13000hr of exposure and remained unchanged for the whole period of exposure for outdoors samples. The drop in the failure stress and elongation at failure for accelerated weathering samples right from the beginning indicate the formation of microcracks is from the early stages of exposure. For outdoor samples, failure stress remained unchanged for the first 1000 hr of sunlight exposure, then decreases progressively with increasing exposure in the environment. The elongation to failure is unchanged up to 13000 hours.
After 13000 hrs of exposure to sunlight, r-PET samples failed to break while those exposed to UV lamps failed in a brittle manner under impact after 250 hrs and this indicates the transition from ductile to brittle behaviour just after 10 days of accelerated UV exposure because of crack formation. For accelerated weathering samples, the impact strength remained unchanged in the first 1000 hr of exposure, then a decrease up to 5000 hr of exposure when the effects of flaws become significant and dropped sharply by 85% after 5000hr of exposure.
The correlation between both types of weathering shows that one year in natural weathering is equivalent to one and a half months in accelerated weathering according to colour measurements. The effect of accelerated weathering is much bigger than the natural weathering and this is due to higher radiation dosage, temperature and humidity during the test which accelerate chain scission rate that lead to faster crack growth.
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