Investigation into the use of polypropylene for rotomoulding applications

Galindo, Inaki Emaldi (2013) Investigation into the use of polypropylene for rotomoulding applications. Masters by Research thesis (MSc), Manchester Metropolitan University.

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Abstract

Rotational moulding is a technique employed for processing polymers especially thermoplastics. Rotomoulding (RM) has not been one of the key polymer moulding methods, but it is achieving big interest in the recent years. Rotational moulding is based on the rotation of a mould in two axes and the heating of the material inside an oven. This implies that it is a method without any pressure over the material so, the material would have different properties compared with other moulding methods. During the processing high temperatures are required so, the stabilisation of thermoplastics is almost compulsory. The material which was mainly used with this technique was polyethylene (PE), but recently polypropylene (PP) is attracting the attention of rotomoulders. However, polypropylene is more susceptible to thermo-oxidative degradation due to its structure with a pendant methyl group. The research has consisted of the analysis of different properties of different polypropylene grades to determine which is the most suitable for rotomoulding processes. The investigation of the mechanical properties such as tensile strength, flexural strength and impact strength has been carried out. The thermo-oxidative degradation of the grades has been also studied by the carbonyl growth after being exposed to high temperatures (230°C). The determination of oxidation induction time (OIT) and oxidation onset temperature (OOT) are also important and all of them are related with crystalline structure of the polymer which was also measured by the heat-cool-heat method of DSC and X-ray diffraction. The analysis of all these properties concludes in the selection of the most suitable grade. However, due to polypropylene weakness to thermal degradation it has to be stabilised. In an attempt to improve the stabilisation the addition of a primary antioxidant was studied. The antioxidant was part of the hindered phenolic family (Irganox 1010, Penta-erythritoltetrakis( 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate). The combination with secondary antioxidant such as thioesters (Dioctadecyl 3,3’-thiodipropionate), phosphites (Tris(2,4-di-tertbutylphenyl) phosphite) and HALS (Hindered Amine Light Stabilisers) will be studied to see if they can work as synergists. Inorganic controlled release systems were also added because they reduce the mobility of antioxidants due to its high adsorption area. The degradation in PP is mainly caused by oxygen which reacts with the radicals causing chains scissions. The absence of oxygen is supposed to increase polymer stabilization and so the rotomoulding (RM) experiments were also carried out with a N2 flush. In general these modifications led to improve PP behavior against thermo-oxidative degradation.