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Polypropylene
Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications including packaging and labeling, textiles (e.g., ropes, thermal underwear and carpets), stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and polymer banknotes. An addition polymer made from the monomer propylene, it is rugged and unusually resistant to many chemical solvents, bases and acids.
Industrial processes
Hydrocarbon slurry or suspension: Uses a liquid inert hydrocarbon diluent in the reactor to facilitate transfer of propylene to the catalyst, the removal of heat from the system, the deactivation/removal of
the catalyst as well as dissolving the atactic polymer. The range of grades that could be produced was very limited. (The technology has fallen into disuse).
Bulk (or bulk slurry): Uses liquid propylene instead of liquid inert hydrocarbon diluent. The polymer does not dissolve into a diluent, but rather rides on the liquid propylene. The formed polymer is withdrawn and any unreacted monomer is flashed off.
Gas phase: Uses gaseous propylene in contact with the solid catalyst, resulting in a fluidized-bed medium.
Processing Characteristics
Polypropylene is a relatively easy materials to injection mould in spite of its semicrystalline nature. The absence of any real need for high molecular weight, from the mechanical properties view point, leads to low melt viscosity (easy flow). The pseudoplastic nature of polypropylene enhances this effect at high shear rates (fast filling rates).
Typically melt temperatures for injection moulding are between 200 and 250oC. Though they can be as high as 280, or even 300oC, for short periods of time. With Flame Retardant grades it recommended that 220oC is not exceeded. Mould filling rates are generally on the high side, to ensure good surface finish and strong mouldings free of weld lines and flow fronts. Adequate venting of the mould is essential to prevent burn marks.
The melt flow index gives a rough guide to melt flow behaviour, but as a result of the pseudoplastic nature of polypropylene (which is strongly dependant on the molecular weight distribution) this should not be taken too literally. Spiral flow mould data is sometimes provided by manufacturers as a practical means of assessing the flow behaviour, but this is no substitute for the fundamental melt rheological and thermal data which is now becoming more widely available.
Polyethylene
Polyethylene (abbreviated PE) or polythene (IUPAC name polyethene or poly(methylene)) is the most common plastic. Light, versatile synthetic resin made from the polymerization ofethylene. Polyethylene is a member of the important family of polyolefin resins. It is the most widely used plastic in the world, being made into products ranging from clear food wrap and shopping bags to detergent bottles and automobile fuel tanks. It can also be slit or spun into synthetic fibres or modified to take on the elastic properties of a rubber. The annual global production is approximately 80 million tonnes. Its primary use is in packaging (plastic bag, plastic films,geomembranes, containers including bottles, etc.). Many kinds of polyethylene are known, with most having the chemical formula(C2H4)nH2.
Physical properties
Polyethylene is a thermoplastic polymer consisting of long hydrocarbon chains. Depending on the crystallinity and molecular weight, a melting point and glass transition may or may not be observable. The temperature at which these occur varies strongly with the type of polyethylene. For common commercial grades of medium- and high-density polyethylene the melting point is typically in the range 120 to 180 °C (248 to 356 °F). The melting point for average, commercial, low-density polyethylene is typically 105 to 115 °C (221 to 239 °F) The term polyethylene describes a huge family of resins obtained by polymerizing ethylene gas, H2C=CH2, and it is by far the largest volume commercial polymer.
This thermoplastic is available in a range of flexibilities and other properties depending on the production process, with high density materials being the most rigid. Polyethylene can be formed by a wide variety of thermoplastic processing methods and is particularly useful where moisture resistance and low cost are required. Low density polyethylene typically has a density value ranging from 0.91 to 0.925 g/cm³, linear low density polyethylene is in the range of 0.918 to 0.94 g/cm³, while high density polyethylene ranges from 0.935 to 0.96 g/cm³ and above.
Polystyrene
Polystyrene (PS) is a synthetic aromatic polymer made from the monomer styrene. Polystyrene can be solid or foamed. General purpose polystyrene is clear, hard, and rather brittle. It is an inexpensive resin per unit weight. It is a rather poor barrier to oxygen and water vapor and has a relatively low melting point. Polystyrene is one of the most widely used plastics, the scale of its production being several billion kilograms per year. Polystyrene can be naturally transparent, but can be colored with colorants. Uses include protective packaging (such as packing peanuts and CD and DVD cases), containers (such as “clamshells”), lids, bottles, trays, tumblers, and disposable cutlery.
As a thermoplastic polymer, polystyrene is in a solid (glassy) state at room temperature but flows if heated above about 100 °C, its glass transition temperature. It becomes rigid again when cooled. This temperature behavior is exploited for extrusion, and also for molding and vacuum forming, since it can be cast into molds with fine detail.