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Title: Mechanical properties and fracture behaviour of rubber toughened PET blends and short glass fibre reinforced hybrid composites
Other Titles: Xiang jiao zeng ren zhi ju dui ben er jia suan yi er chun zhi ji duan bo li xian wei zeng qiang zhi fu he cai liao de ji xie xing neng ji duan lie xing zhi zhi yan jiu
Authors: Fung, Kin Lun (馮健倫)
Department: Dept. of Physics and Materials Science
Degree: Doctor of Philosophy
Issue Date: 2006
Publisher: City University of Hong Kong
Subjects: Glass fibers
Polyethylene terephthalate -- Mechanical properties
Notes: CityU Call Number: TP1180.P65 F86 2006
Includes bibliographical references (leaves 142-157)
Thesis (Ph.D.)--City University of Hong Kong, 2006
xx, 157 leaves : ill. ; 30 cm.
Type: Thesis
Abstract: The aim of this research is to investigate the properties of poly(ethylene terephthalate) (PET) modified by rubbers and/or short glass fibres. The eventual goal is to successfully develop “value-added recycling blends” using recycled PET bottles. Both virgin and recycled PET pellets have been used in this research study. PET has good thermal and mechanical properties, and is used extensively for the production of soft drink bottles. With the high soft drink consumption rate, it is inevitable that huge quantities of plastic waste are produced and cause serious environmental problems. Recycling should always play an important role in any plastic waste management program in order to reduce the demand on landfills. With the successful development of value added recycling PET blends with superior properties over the original PET, the life-cycle of the PET resins can be extended. Hence the amount of plastic waste can be reduced. In this work, a number of tests have been carried out on the PET blends. The thermal behaviour of the blends was studied by dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The mechanical properties of the blends were studied by fracture toughness measurement, tensile test and Charpy impact test. The micro-structural features, fracture mechanisms, glass fibre length and orientation, etc., were investigated with the help of optical and scanning electron microscopy. Thermal analysis result shows that PET matrix is sensitive to the addition of short glass fibre (SGF) and/or rubber. The PET crystallinity is slightly increased by the addition of SGF or rubber. Pronounced cold crystallization was observed for the PET specimens produced by injection moulding. The onset of cold crystallization is affected by the addition of SGF or rubber. In order to enhance the toughness of the PET matrix, a maleic anhydride grafted styreneethylene- butylene-styrene (MA-g-SEBS) rubber was used to enhance the toughness of PET through melt blending. While the addition of 10wt% of MA-g-SEBS improved the impact strength only slightly, a super-tough blend was produced when the MA-g-SEBS content was increased to 20wt% or higher. Similar to conventional rubber modified polymer blends, the tensile Young’s modulus and tensile yield strength of the PET/MAg- SEBS blend decreased proportionally to the increasing rubber content. The addition of short glass fibres into the PET/MA-g-SEBS blends improved the stiffness significantly, but the ductility and impact strength was dramatically reduced. To determine the fracture toughness characteristics of the PET/MA-g-SEBS and PET/MA-g-SEBS/SGF blends, the essential work of fracture (EWF) analysis was used in this work. For the results of the PET/MA-g-SEBS blends, the specific essential fracture work (we) of PET was successfully enhanced by the addition of MA-g-SEBS. The fracture toughness characteristic for the PET/MA-g-SEBS blends was observed to be sensitive to loading rate and temperature. The fracture toughness characteristics of the PET/MA-g-SEBS/SGF hybrids were studied under both quasi-static and impact loading rates. Under a quasi-static loading rate, the major fracture mechanisms were observed to be matrix plastic deformation and fibre pull-out. Therefore a higher toughness can be obtained for the PET/MA-g-SEBS/SGF hybrids that contain higher rubber content. However, the degree of matrix plastic deformation was constrained by increasing the glass fibre content. At impact loading, fibre debonding and pull-out are dominant. Matrix plastic deformation and localized yielding disappeared. Instead fast crack propagate of the matrix can be identified on the fracture surface. Specimens of PET/MA-g-SEBS/SGF hybrids were prepared using recycled PET obtained from a local supplier. Two different compounding sequences were employed to prepare the specimens, which are labeled as: (i) 1-step compounding; and (ii) 2-step compounding. The impact properties of the specimens prepared by both compounding routes are similar. However the tensile strength for the 1-step compounding specimens is better than the 2-step compounding specimens. Fractographic analysis showed that the SGF in the 2-step compounding specimens were encapsulated by MA-g-SEBS, while the SGF in the 1-step compounding specimens were not encapsulated by MA-g-SEBS. The stress transfer mechanisms for the two types of specimens were therefore different and hence affected the tensile strength.
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