Polymers are defined as chemical compounds whose molecules are large and often resembling long chains, which are made up of seemingly endless interconnected link series. The size of these molecules is remarkable, as explained in the Chemistry of Industrial Polymers, ranging from thousands to millions of atomic mass units (as opposed to the tens of atomic mass units generally found in the other chemical compounds). The primary causes of the peculiar properties associated with plastics are the size of the molecules, their physical state, and the structures that they follow.
Thermoplastic and Thermosetting
Polymers, which are classified as plastics, may be divided into two primary categories:
Thermoplastics including polystyrene and polyethylene can be remoulded and moulded several times. Therefore, a foamed-polystyrene cup may be heated and also reshaped into a new form - for suppose, a dish. Individual molecules are separated from one another and flow past one another in the polymer structure that is associated with thermoplastics. The molecules can have either low or extremely high molecular weight, and they can be linear or branched in structure, but the important feature is that of consequent mobility and separability.
On the other hand, thermosets cannot be reprocessed upon reheating. Thermosetting resins undergo a chemical reaction that results in an infusible, insoluble network during their initial processing. Most importantly, the entire heated, finished article becomes a single large molecule. For example, the epoxy polymer, which is used in making a fibre-reinforced laminate for a golf club, undergoes a cross-linking reaction the time it is moulded at a high temperature. The subsequent application of heat, on the other hand, does not soften the material to the point that it can be reworked, and can even break it down.
Physical States and Molecular Morphologies
The morphology of polymers, or the arrangement of molecules on a wide scale, has an effect on their plastic behaviour. Simply stated, polymer morphologies are either crystalline or amorphous. Crystalline molecules can be organised tightly and in a discernible order, whereas amorphous molecules can be arranged arbitrarily and intertwined. The majority of thermosets are amorphous, while thermoplastics may be semicrystalline or amorphous. Crystallites, which are crystalline regions within an amorphous matrix, are visible in semicrystalline materials.
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The figure shows amorphous and semicrystalline polymer morphologies on the left Volume-temperature diagram for the semicrystalline and amorphous polymers, representing the volume increasing with temperature. And the schematic diagram of the semicrystalline morphology, representing amorphous regions and crystallites on the right.
Polymer’s physical state and morphology hold a strong influence on its mechanical properties. A simple measure of the differences, which are produced in mechanical behaviour, is given as the elongation that takes place when plastic is loaded (or stressed) in tension. A glassy polymer like polystyrene is quite stiff, representing a high ratio of initial stress to initial elongation.
Processing and Fabrication of Plastics
The processing of the raw materials into a usable form is referred to as either conversion or fabrication. Plastic pellets can be converted into films or films can be converted into food containers, as an example from the plastics industry. There are also other processing techniques such as forming, mixing, finishing.
In the extrusion process, a melted polymer can be forced through an orifice with a specific cross-section (the die), and a continuous shape can be formed with a constant cross-section as same as that of the orifice. While thermosets can be extruded and cross-linked by heating the extrudate, thermoplastics are more common since they can be extruded and solidified by cooling. Among the products, which are produced by extrusion are film, tubing, sheet, insulation, pipes, and home siding. In every case, the profile can be determined by the die geometry, and solidification can be done by cooling.
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Longitudinal section of the screw extruder of thermoplastic polymers. Plastic pellets can be fed into the extruder barrel from a hopper, where they are melted by mechanical energy provided gradually by a turning screw and distributed along the barrel by heaters. The molten polymer can be forced through a die that shapes the extrudate into a few products.
Most plastic grocery bags and similar items can be made by the continuous extrusion of the tubing. In the blow extrusion, the tube can be expanded prior to being cooled by being made to flow around a massive air bubble. Air can be prevented from escaping from the bubble by collapsing the film on the bubble’s other side.
Laminated structures can be made by extruding several materials at the same time via a single die or several dies for a few applications. Multilayer films can be useful since the outer layers may contribute strength and moisture resistance while the inner layer may control the oxygen permeability, which is an important factor in food packaging.