Effect of soft segment length on foam structure of expanded thermoplastic polyurethane (ETPU) bead foams

Since the second world war, many types of polymers and their products have been produced. This is because of their broad range of properties compared to other materials such as metals and composites, including toughness, viscoelasticity, and a tendency to form glasses and semicrystalline structures rather than crystals. The most prominent features of polymers are, however, their low density, ease of manufacturing, and hence cost-effective. Thermoplastic polyurethane (TPU) is a thermoplastic based elastomer, which means it is meltable and processable when heated, formable and shapable when cooled, and capable of being reprocessed multiple times. TPU is well-known for its many features including; high abrasion resistance; flexibility over a wide range of temperatures; high tensile strength; good resistance to weathering; good resistance to oil, grease, solvents and chemicals. These properties make TPU extremely useful for hundreds of applications such as automotive instrument, caster wheels, power tools, sporting goods, medical devices, drive belts, footwear, and a variety of extruded film, sheet and profile applications. TPU is a block copolymer including alternating sequences of hard and soft segments formed by the reaction of diisocyanates with short-chain diols and diisocyanates with long-chain diols. By varying the ratio, structure and/or molecular weight of the reaction compounds could be changed, and different TPU with varying properties can hence be produced. This allows to tailor the desired final properties of the material. For instance, a greater ratio of soft segments to hard segments will result in a softer TPU, while the reverse will result in a more rigid TPU. The polyol chain of TPU can be either polyether based or polyester based. Polyester TPUs are compatible with polar plastics, offer a good balance of physical properties and are perfect for use in polyblends. Polyether TPUs are slightly lower in specific gravity than polyester. They offer low temperature flexibility and good abrasion and tear resilience. They provide excellent hydrolysis resistance – making them suitable for applications where water is a consideration. TPUs can also be subdivided into aromatic and aliphatic isocynates. TPUs with aromatic isocynates like Methylene diphenyl diisocyanate (MDI) can be used in applications that require flexibility, strength and toughness. TPUs with aliphatic isocyantes like-Methylene dicyclohexyl diisocyanate (H12 MDI), hexamethylene diisocyanate (HDI), and isophorone diisocyanate IPDI are commonly used in projects where optical clarity, adhesion and surface protection are required. Polymeric foams are made up of a polymer matrix with either closed bubbles (cells) or interconnected tunnels, which are known as either closed-cell or open-cell structures. Open-cell foams are mostly flexible, while closed-cell foams are generally more rigid. The gas that is used in the foam is known as blowing agent, which can be either chemical or physical. Physical blowing agents are gases which do not react chemically in the foaming process, while chemical blowing agents take part in a reaction or decompose, giving off chemicals in the process. Polymer foams can be divided into either thermoplastics or thermosets, which are further divided into rigid or flexible foams. The thermoplastics are usually breakable and able to recycle, while thermosets are harder to recycle because they are usually heavily crosslinked. The first polymer foam was foamed polystyrene in the 1930's. During second World War polyurethane was invented. After the war, the flexible polyurethane foam was produced and used for furniture and automotive industry. In the following years, polymer foams were more widely used as new technologies developed, such as injection molding and several extrusion techniques. Recenlty, bead foaming technologies are developed including several methods, for instance foam extrusion with underwater pelletizer, suspension polymerization with blowing agent, and batch foaming process. Batch foaming is noncontinuous process that is qualified to study the foaming behavior of polymers and also exist for industrial applications. In this process, polymer is firstly saturated with physical blowing agent in a high pressure cell. Later, foaming is initiated by either a pressure drop or increase in temperature which results in a thermodynamic disequilibrium. In pressure drop method, pressure drop rate effect on the cell nucleation can be investigated, while increase in temperature method suits to evaluate the processing window of the polymer. The reason why polymer foams are widely used is that they have a lot of advantageous such as low density, cheaper, tailored properties which could influence the final funactional properties such as insulation properties. Bead foams have an extra advantages compare to foam extrusion and foam injection molding, which is its ablity to form low density three dimensional complex geometry foam products. This thesis focus on foaming behavior of expanded thermoplastic polyurethane (ETPU) through batch foaming process. TPUs samples that have different molecular weights and different polyol types are examined. The effect of process conditions; saturation pressure, time and temperature on foam morphology; also, the effect of pressure drop rate on cell nucleation are investigated. At first, to investigate the onset of degradation and thermal decomposition behavior, TPU samples were analyzed by thermal gravimetric analysis (TGA). To know the materials better, the rheological properties of the TPUs were also measured using a rheometer under oscilatory shear loads and using melt flow indexer. Later, the effect of molecular weight and polyol types of TPU on the crystallization behavior was expolered with non-isothermal melt anaylsis in differantial scanning calorimetry (DSC). The next step was isothermal crystallization analysis in DSC, which simulates foaming process without blowing agent, to understand how isothermal annealing in DSC can modifie the crystals and influence the cell nucleation. After the thermal analysis, samples were foamed at different temperatures under 2000psi for 30min to find the optimum process temperature. When optimum temperatures were determined for all samples, they were also foamed at 1000psi and 1500psi for 30 min to manitor the saturation pressure effect on TPU's foaming behavior, also at 2000psi for 15min and 60 min to investigate the saturation time effect. Furthermore, to understand how the hard segment crystals were modified by isothermal annealing under CO2, all process conditions were repeated with extremly slow pressure drop rate that would not cause foaming for all samples. The foamed TPU samples were characterized in two aspects. Firslty, the density of foamed samples and their expansion ratio were measured in pure water and calculated using archimedes equation. One the other hand, the morphology of cellular stucture in foamed samples were observed using scanning electron microscope (SEM). After the observation of cellular morphology; cell distributions and cell dimensions were examined and the cell density of each of samples was calculated. Thermal analysis performed for foamed samples by DSC to monitor the foaming effect on further modification of hard segment crystals.