What Are Engineering Thermoplastics?     DATE: 2017-11-29 16:37

Engineering thermoplastics are plastic materials which have better mechanical properties than the more widely used commodity plastics (such as polystyrene, PVC, polypropylene and polyethylene). They are used in applications where requires higher performance, such as chemical resistance, impact resistance, wear resistance, or mechanical strength.
While commodity plastics tend to be very high volume production, and priced on a supply/demand basis as the name suggests, the delineation between engineering plastics and commodity plastics over time has come to mean differences in heat resistance and flame retardancy and in general an overall higher level of performance for engineering plastics. Additionally we recognize a category called Performance Plastics which are the highest heat materials made today, and they are generally considered a subset of engineering plastics.
Engineering Thermoplastics, as with Commodity Thermoplastics, are in a separate category from Thermosets. Both types of thermoplastics are created by the manufacturer as long polymer chains ready to be molded - the polymerization process, which creates chains of repeating monomer units, is accomplished in a chemical plant. These materials are basically melted and formed into the desired shape by a variety of processes and once processed, can be re-melted and formed again, or ground and worked back into the production process to reduce the use of virgin material.
Thermosets, on the other hand, are not created by the manufacturer as ready to use long polymer chains. Thermosets are polymerized during the conversion process to create a part. Under heat and pressure, the resin and catalyst react in the mold to create long, cross linked polymer chains, with permanent chemical bonds. They cannot be re-melted and therefore the scrap is not able to be worked back into the supply stream (except perhaps as a filler in some cases). This basic difference - thermoplastics consisting of long polymer chains that can be made to flow with heat and thermosets consisting of cross linked long polymer chains that cannot be reprocessed with heat or pressure - gives differing properties to the finished product with unique advantages and challenges.
To further illustrate this difference, consider water, which can be frozen into a solid - ice - and remelted to water and back and forth again, similar to a thermoplastic. Alternatively a cake is mixed up as a batter and baked (heated), creating a different chemistry in the final product that cannot be taken back to cake batter regardless of the amount of heat applied.
Within all types of thermoplastics (engineering, performance and commodity) we also have the distinction between amorphous resins and semi-crystalline resins. This refers to the morphology of the solid material after processing, cooling and changes that occur even after cooling that affect the physical arrangement of the molecules in the solid part. Amorphous resins have no preferred alignment-the molecules are much like a bowl of spaghetti, all intertwined and randomly aligned with each other (in some processes there can be alignment attributed to the flow of the material through a delivery system into the mold).
Semi-crystalline materials actually have a tendency for the molecules to align in crystalline-like structures surrounded by amorphous areas. These "crystals" are really areas of molecular alignment reminiscent of crystals that form in metals..but semi-crystalline thermoplastics are really not crystals themselves..this terminology simply refers to the aligned areas where, due to weak chemical attraction, the long polymer chains align linearly in regions that resemble crystallinity. It should be noted that no practical thermoplastics are completely crystalline. While it is theoretically possible to create a morphology that is completely aligned, this is never the case in practical use. In fact the amount of crystallinity is something that can be modified by processing, annealing or actual chemical makeup of the polymer.
Amorphous vs. Semi-Crystalline are the main groups of thermoplastics but there are other variances in chemistry that change the type of polymer and impart different properties to the final plastic. Thermoplastics are made up of long polymer chains of repeating units of monomers. These chains can be simple repetition of one unit, or they can be repeating patterns of a group of two or more units. There are also co-polymers which are two different monomer types chained together to get the properties of both, and there are ways to branch the chains, so you can have long single strands that pack into a space very densely or branched chains that take up more space and are thus less dense, but have more entanglements that will create lower melt flow and potentially higher physical properties. Low Density Polyethylene and High Density Polyethylene are examples of branched and unbranched (respectively) polymers that may be familiar with most readers.
Different types of engineering plastics can also be created by blending two or more resins, and the types blended could be a mix of amorphous and semi-crystalline, branched and unbranched, depending on the properties needed.

In addition to the chemistry of the polymer and whether or not it is a copolymer or a blend, crystalline or amorphous, linear or branched (and many combinations thereof), properties and thus usage can be adjusted with additional ingredients such as flame retardant additives, stabilizers (heat and UV for example) and various fillers from minerals to fibers that are added for stiffness, strength, impact or some other performance feature like electrostatic dissipation, lubricity or thermal conductivity.