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What are Engineering Plastics...[continued]
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.
Below is a graphical depiction of the different groups of polymer types we have been discussing, with particular materials listed for the engineering and performance materials we’ll cover in this article.
In the following sections we will discuss the properties that these various categories have and how they are typically used and review the different polymer chemistries found in the space of Engineering and Performance Thermoplastics.
Copolymers & Blends
In order to get a clear understanding of copolymers and blends, it's a good idea to understand the homoploymer first. A homopolymer is a polymer formed by the polymerization of a single monomer. In the world of engineering polymers a polycarbonate resin would be considered a homopolymer. Basic homopolymers can be blended or modified with additives. Why would we want to change a basic homopolymer? A basic homopolymer may or may not provide part designers with just the right combination of properties they need. Modification of basic homopolymers by blending or copolymerization increases options in material selection.
There are many basic homopolymers available to meet specific performance requirements. Commodity thermoplastics such as polystyrene and polypropylene are lower performance homopolymers used to manufacture products such as food packaging, and heavy duty containers such as garbage pails. Engineering homopolymers such as polycarbonate and PBT, are higher performance homopolymers. A polycarbonate resin provides excellent stiffness and impact resistance, as well as clarity, which makes it a good candidate for protective eyewear. A PBT resin provides excellent electrical performance, heat resistance, and chemical resistance, and is often used in applications such as electrical connectors. When a homopolymer doesn't meet the requirement needs of an application, a copolymer or blend can be considered. In a copolymer, two polymers are chemically combined to form a new polymer chain, whereas blends are the result of physically mixing the homopolymers, they can be divided into a subset of single phase or multiple phase materials.
