What are Engineering Plastics
Engineering Thermoplastics are a subset of plastic materials that are used in applications generally requiring higher performance in the areas of heat resistance, chemical resistance, impact, fire retardancy or mechanical strength. Engineering Thermoplastics are so named as they have properties in one or more areas that exhibit higher performance than commodity materials and are suitable for applications that require engineering to design parts that perform in their intended use.
In the plastics marketplace today, it is generally accepted that engineering plastics are materials that have heat resistance above 100 deg C and good flame retardant properties. 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.
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.
Co-polymerization is the chemical modification process in which two homopolymers are co-polymerized to make an entirely new polymer. When two homopolymers are co-polymerized, the two sets of polymer chains are actually linked together, resulting in an entirely different polymer, composed of an entirely different polymer chain. When three homopolymers are chemically linked together, you get a ter-polymer. A common ter-polymer is ABS (Acrylnitrile-Butadiene-Stryrene).
Blending is the combining of homopolymers to take advantage of the properties of each polymer. Combining homopolymers to create a resin blend, also referred to as alloys, can result in a new or unique product with the best properties of each homopolymer. An example of a blend would be combining a polycarbonate with a polyester like PBT to a get a unique plastic that offers something more than what each individual polymer would have offered. This PC/PBT blend would offer good impact resistance, better than a PBT homopolymer, and very good chemical resistance, much better than PC homoploymer.
The process of blending significantly expands the variety of materials available to better meet the designers or part manufacturer"s specific performance requirements. To recap, blending is the result of a physical combination of polymers, co-polymerization is the chemical combination of two polymers.
What is the difference between a physical combination and a chemical combination? When two homopolymers are blended, they are physically mixed together. A polycarbonate and a PBT polymer are merely stirred together to make a PC/PBT blend or alloy. Within blends, there are various levels of blend miscibility, which determines the degree of mixing in a blend. Some materials have a greater capacity to dissolve in each other.
An example of a highly miscible blend is salt in water. The salt and water will actually break apart into smaller and smaller particles, eventually particles of salt will dissolve with particles of water creating a single-phase blend. In a single phase system, the materials are mixed together and dissolve, creating a single, continuous phase. Salt and water are soluble, when mixed, they dissolve, creating a single-phase system.
Single phase polymers appear homogeneous when examined under magnification. An example of a single phase blend is the polymer blend of PPO (Polyphenylene Oxide) with polystyrene.
If the level of miscibility is low, then what typically results is discrete phases. This is because the parent homopolymers are not miscible or soluble in each other and each maintains a discrete phase in melt and solid states. Low miscibility results in a multiple-phase system. Fruit in Jello is an example of a multiple phase system. The fruit does not dissolve in the Jello; when mixed they remain separate in the mixture, creating a two phase system.