Inhalt
Introduction
- INTRODUCTION
1.1 Thermoplastic Polyurethane: Thermoplastic polyurethane is a category of plastic which is synthesized by polyadditon reaction between a diisocyanate and one or more diols (a chemical compound containing 2 hydroxyl groups (–OH groups)). Thermoplastic polyurethane (TPU) was first developed in the year of 1937. Upon heating, the polymer is soft and process able and upon cooling it is hard. This versatile polymer has capability to reprocess without losing its structural integrity. TPU can replace hard rubber and malleable engineering plastic. TPU has high elongation and tensile strength. It has ability to resist grease, oil, chemicals and abrasion (http://www.huntsman.com/polyurethanes/Media%20Library/global/files/guide_tpu.pdf). Above characteristics make TPU extremely popular across wide range of market applications. To create solid component, TPU can be extruded or injected on conventional thermoplastic manufacturing equipment. TPU is usually used for footwear, hose and tube, cable and wire, sheet and many more industrial products.
1.1.1 Chemical Classification of TPU
Polyester TPUs: Polyester TPUs are unaffected by chemical and oils. They are offering value in the form of enhanced properties, provide excellent abrasion resistance. Polyester TPUs are compatible with PVC and other polar plastics; they also offer a good balance of physical properties.
Polyether TPUs
Polycaprolactone TPUs: They are an ideal raw material for pneumatic and hydraulic seals. Polycaprolactone TPUs exhibit inherent toughness and relatively high resistance to hydrolysis.
1.1.2 Classification of TPUs on the basis of aromatic and aliphatic varieties
(a) Aromatic TPUs: They are based on isocyanates like MDI (Methylene diphenyl diisocyanate) which are workhorse products and can be used in the applications where flexibility, toughness and strength are required.
(b) Aliphatic TPUs: They are based on isocyanates like H12 MDI, HDI and IPDI (Iso Phorone Diisocynate) which are light stable and offer excellent optical clarity. They are usually used in automotive interior and exterior applications. They are also used in the areas where there are requirements of attributes like optical clarity, adhesion and surface protection.
1.1.3 Thermoplastic Polyurethane Chemistry: A TPU is multi-phase block copolymer. TPU is synthesized by three basic raw materials, which are combined together in a specific way. Individual component which is required to produce thermoplastic polyurethane are: Polyol or long-chain diol, Chain extender or short- chain diol, Diisocyanate
1.1.4 Performance characteristics of thermoplastic polyurethane: High abrasion resistance, High elasticity across the entire hardness range, Good range of flexibility over a wide range of temperature, Tactile properties, Excellent low temperature and impact strength,. Suitability for bonding and welding, Easy recyclability and ease of coloring, High energy radiation resistance
Physical properties of thermoplastic polyurethane
Shore hardness: It is an empirical measurement used to test the resistance offered by TPUs to penetration and indentation under the defined force. Two types of letters are used to categorize the TPU, these are: ‘A’ denotes flexible type and ‘D’ denote rigid type of TPU. Above categories may is overlap sometime. Both are measured on the scale from 0 to 100. Zero indicates very soft and 100 indicate very hard.
Tensile strength: It indicates TPU behavior when a specimen is placed under short term, uniaxial stress.
Tear strength: Tear strength denotes the ability of TPU to counter break and distortion. Greater numbers of utilization option are present when tear strength is higher.
Compression set: A TPUs compression set can be defined as the permanent deformation that remains after the compressive stress has been released. It is calculated as a percentage of original deflection after the material has been allowed to recover at standard conditions for 30 minutes. TPU materials may have very best compression resistance after heat treatment like annealing.
Abrasion
Coarse paper is applied to a substrate under pressure via rotating cylinder, to measure the abrasion resistance of plastic material like TPU. Before and after the abrasion assessment, the weight of the specimen is measured. The original density of the material is considered alongside the roughness of the paper with results typically expressed in terms of volume loss of the substrate in mm3.
Shrinkage: There are different factors which influenced shrinkage of TPU such as part design, wall thickness, and gate design, processing condition, melt and mold temperature as well as injection and holding pressure.
Chemical Properties: The behavior of TPU is decided by its chemistry. It is very important to choose proper grade of TPU for its exposure to high temperature, water and other outdoor consideration.
Acids and alkaline solutions: TPU show its limited resistance for alkaline and acids. It can only withstand for short period of time with diluted acids and alkaline solutions at room temperature.
Saturated hydrocarbons: TPU expand slightly when exposed to saturated hydrocarbons and when exposed to isooctane and petroleum, moderate swelling will occur.
Aromatic hydrocarbons: When TPU come in contact with aromatic hydrocarbons such as benzene and toluene, it will swell resulting in reduction of mechanical properties. Scale of swelling will directly depend upon one type of hydrocarbon.
Natural Fiber: Natural fiber reinforced composite are the structural composite, in which the polymer matrix is combined with fibers. Natural fibers are used in the composites where high strength and stiffness is not first priority. As the natural fibers are low in density therefore the mass of the composite is reduced. Composite with natural fibers are economical to produce with low cost equipment and can easily be recycled. The properties of the natural fiber depends upon whether the fibers are taken from leafs or stem and the quality of plants. In practical, cellulose is the main component of natural fibers (Bledzki et al. 1996). The properties of the composite directly depend on the individual components and their interfacial compatibility. The methylol group of polymer reacts with the hydroxyl group of the cellulose to form a stable ether linkage. Therefore high compatibility between polymer and cellulose is achieved. There is only polymer cellulose compound
Impressum
Verlag: BookRix GmbH & Co. KG
Tag der Veröffentlichung: 16.09.2019
ISBN: 978-3-7487-1557-3
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