Published on Nov 15, 2016
Polymeric materials – often called plastics in popular usage – are ubiquitous in modern life. Applications range from film to textile fibers to complex electronic interconnects to structural units in automobiles and airplanes to orthopedic implants. Polymers are giant molecules, consisting of hundreds or thousands of connected monomers, or basic chemical units; a polyethylene molecule, for example, is simply a chain of covalently bonded carbon atoms, each carbon containing two hydrogen atoms to complete the four valence sites.
The polyethylene used to maufacture plastic film typically has an average molecular weight (called the number-average molecular weight, denoted Mn) of about 29,000, or about 2,000 ---CH2--- units, each with a molecular weight of 14. The symbol “---” on each side of the CH2 denotes a single covalent bond with the adjacent carbon atom.
The monomer is actually ethylene, CH2 ------CH2, where “------” denotes a double bond between the carbons that opens during the polymerization process, and a single “mer” is ---CH2---CH2---; hence, the molecular weight of the monomer is 28 and the degree of polymerization is about 1,000.) The ultra-high molecular weight polyethylene used in artificial hips and other prosthetic devices has about 36,000 ---CH2--- units. Polystyrene is also a chain of covalently bonded carbon atoms, but one hydrogen on every second carbon is replaced with a phenyl (benzene) ring. Two or more monomers might be polymerized together to form a copolymer, appearing on the chain in either a regular or random sequence.
Polymers are often blended or contain additives to affect the properties of the solid phase; high-impact polystyrene, for example, is a blend in which particles of a rubbery polymer, typically polybutadiene or a styrene-butadiene copolymer, are dispersed in polystyrene. Many polymer composites used in molding applications contain solid fillers, such as calcium carbonate particles, glass fibers, or even nanoscale fillers like exfoliated clays or carbon nanotubes.
The polymer manufacturer, starting from raw materials like natural gas and other low molecular weight chemicals, produces the polymer – say, polyethylene – as a powder or in the form of chips or flakes, which are often converted (densified) into pellets by extrusion.
This resin must be processed to produce the desired product – a molded part, for example. Most processing takes place in the liquid state. The resin must first be melted (we will use the term melt to denote the change from any form of solid to a liquid state, although technically only a crystal has a true meltingtransition), then conveyed through one or more steps to form an object of the desired shape, and finally solidified again. A polymer pipe, for example, is produced by continuously extruding the molten polymer through an annular die and then cooling it quickly to retain the shape. An injection-molded part is producd by forcing the molten polymer into a mold of the desired shape, where the polymer cools until it has solidified, after which the mold is opened, the part removed, and the process repeated.
Extrusion is the most fundamental and most widely used unit operation in melt processing. An extruder is a device that pressurizes a melt in order to force it through a shaping die or some other unit. A ram extruder, for example, is simply a piston that forces a melt from a cylinder through a die. We are usually concerned with continuous extrusion over long periods of time, in which case a ram, which must operate in a semibatch mode (i.e., the cylinder must be refilled periodically), is not appropriate. The most common device for continuous extrusion is the single-screw extruder.
The single-screw extruder is analogous to the meat grinder that was once a fixture in kitchens. In a meat grinder, chunks of meat are placed in a hopper and fall onto a rotating Archimedes screw. The meat is compressed and carried forward by the screw flights until it is forced through a perforated plate, producing the strands that make up “ground” meat.
The counterintuitive feature here, which we rarely think about in the context of a meat grinder, is that the meat enters at atmospheric pressure and is forced through the perforatedplate to emerge at atmospheric pressure; hence, the pressure must increase from the hopper to the upstream side of the plate n order to provide the force necessary to push the meat through the plate. We are accustomed to thinking in terms of pressure drop in flow situations. Similarly, in a screw extruder the polymer, in the form of flakes, chips, or pellets, is fed through the hopper onto the screw, where melting takes place because of frictional and conductive heating and perhaps also deformation heating of the softening solid. The polymer is conveyed forward by the screw, becoming completely molten by the time it reaches the metering section. Pressure builds up in the flow direction until the end of the screw, where the polymer is forced through a shaping die. The pressure drop through the die must equal the pressure buildup along the screw.