Development of extruded high impact polystyrene foams

Abstract

The use of polymers has been increaseed significantly since the second world war. Currently, hundreds of millions of polymeric products are being manufactured. This is due to the advantages that polymers could provide compared to other metallic or ceramic counterparts. Firstly, polymers possess low density and their manufacturing is cheaper and easier. In addition, polymers are corrosion resistant materials. Polystyrene (PS) is an amorphous polymer that has glass transition temperature of around 105 oC. It is cheap and one of the mostly used polymers in commodity applications. There are many areas for the use of PS such as plastic cutlery, pots, toys, automotive industry, whitegoods industry, electronics etc. General purpose PS (GPPS) and High impact PS (HIPS) are the types of PS that are widerly being used in various areas. GPPS is a transparent and brittle material and it is known as crystal in plastic industry. HIPS is also a blend of PS which contains around 5-15 wt % polybutadiene (PB) rubber. These two polymers form an immiscible blend of PS and PB. Foamed polymers can be defined as polymers include bubbles and pores. Foamed polymers are very light weight and cost-effective materials. In addition, they have comparable mechanical properties and very high strength to weight ratios as it is compared to conventional neat polymers. Because of such advantages, the polymeric foams are used in many applications such as automobile parts, packaging industry, sandwich caps, insulators in construction industry, sport industry etc. The use of blowing agents are required in order to obtain foamed structure in foaming process. Blowing agents are classified into two groups as physical blowing agents (PBAs) and chemical blowing agents (CBAs). PBAs are directly injected into the polymer matrix melt during processing and they are in the form of gas, liquid or supercritical phase. On the other hand, CBAs decompose and generate gases during processing and they are in the form of solid granules or powder. Polymeric foams can be classified as open or closed cell foams based on the morphology; microcellular, fine celled and conventional foams as the cell density; high density, medium density and low density based on void fraction or expansion ratio. Many of traditional polymer processing methods can be performed in order to produce polymeric foams. Batch process and continuous process are the two techniques for foam production. Continious process also classified into two groups as foam extrusion and foam injection molding (semi-continuous). In foam extrusion process, polymer/gas mixture starts to be generated by the directly injection of PBA or decomposition of CBA under higher pressure during processing. The gas dissolve in to polymer matrix when the pressure is under the solubility limit of polymer. When the polymer/gas mixture flew through the extruder die, the sudden pressure drop at the die nozzle creates thermodynamic instability and causes cell nucleation and growth during the foaming step. This thesis studies the extrusion foaming behavior of HIPS through a twin-screw extruder using two various types of CBAs. The filamentary die having 4 mm length and 2 mm diameter was used in order to obtain foamed filament samples for each experiment. The CBAs differences is about their decomposition temperatures. These CBAs were named as CBA-1 and CBA-2 and they were thermally analyzed in order to observe the thermal properties and decomposition temperatures by using differantial scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). Three and five different extruder barrel temperature profiles were determined in order to observe the effect of processing temperature for CBA-1 and CBA-2, respectively. After the determination of optimum processing temperature profile for both CBAs, the five different die temperature profiles (from the highest to the lowest temperature) were tailored during the foaming of HIPS for two different CBAs. The effect of the die temperature on the foaming behavior of HIPS was observed, and the optimum die temperature profile was determined. The next step is to verify the CBA content effect on the foamed HIPS for both CBAs. Three different contents (i.e., low, moderate and high content) of chemical blowing agents were used for each CBAs and content of CBA on foamig was investigated. After the determination of optimum CBA content for both CBAs, effect of screw RPM (revolution per minute) on the foaming behavior of HIPS was, subsequently, illustrated for both CBAs. Four different screw RPM (low, moderate, high, very high RPM) were tailored during the foaming of HIPS for two different CBAs and the most proper RPM conditions were selected for CBA-1 and CBA-2. The HIPS extrusion foaming behavior was further investigated via blending it with GPPS at the various blending cotents (neat HIPS, HIPS with low content GPPS, HIPS with high content GPPS). The effect of GPPS and its blending content was investigated for both CBAs. After that, foaming behavior was also examined via compounding HIPS with three different inorganic fillers (i.e., microlamellar talc, talc, and calcium carbonate) at three different contents (i.e., low, moderate and high content). The effect of the inorganic fillers as a nucleating agent and the effect of inorganic filler type and content were investigated with this study. The foamed HIPS samples were characterized in two aspects. Firstly, the density of foamed samples were measured in pure water and calculated by using archimedes equation. After the determination of the density of foamed HIPS samples, the void fractions of each sample was calculated. On the other hand, the morphology of cellular structure in foamed samples were observed by using scanning electron microscope (SEM). Firstly the foamed HIPS filaments were broken under liquid nitrogen and the samples were coated with gold. After the observation of cellular morphology; cell distributions and cell dimensions were examined and the cell density of each of filamentary samples were calculated.