Plastic Materials: An Overview of Properties & Applications
by Marjorie Steele, Editor for IQS
Few materials influence our daily lives more than plastic. As consumers, we don’t often stop to think about how a huge percent of the objects we use are made from plastic. In my office alone I count 32 individual plastic objects – and that’s not counting my pencils, pens and binders. Plastic manufacturers, however, must be acutely aware of the vast range of plastic products and applications, as they are constantly designing and fine tuning new products with specific shape, strength, transparency and chemical properties.
With the countless number of products – from disposable packaging to critical automotive parts – that can be made from plastic, it’s no wonder there are such a wide range of plastic materials, all with a very distinct set of properties. Understanding the difference between, say, polyethylene and polyvinyl chloride is crucial for engineering products that will withstand their environments. Acrylic could never provide the strength and structural support of PVC, but polyvinyl chloride would be a clumsy and unattractive replacement to acrylic point of purchase displays.
Let’s take a look at the most important polymer types and their properties.
Polyethylene- By far the most recognizable plastic material, this polymer is commonly found labeled as HDPE (high density polyethylene) or LDPE (low density polyethylene) on a constellation of consumer items. High density polyethylene has a strong yet semi-flexible tensile strength that is desirable in products such as plastic grocery bags, milk jugs, fuel tanks, outdoor structures, beverage bottles, water pipes and many packaging types. Unlike PVC, fiberglass or acrylic, HDPE stretches before it breaks, eliminating brittle shattering. Low density polyethylene has even more flexibility, being used in soft, pliable parts, bags and food or lab containers. LDPE was one of the earliest plastics to be used in mass production, and despite its low density can be fabricated to have great strength and chemical resistance, although it reacts poorly to high heat and oxidation.
Polypropylene – Like nylon, this material is commonly spun into fibers for textiles, ropes, clothing and carpet. Polypropylene is fairly tough with good flexibility and is known for its resistance to fatigue to to wear. Unlike most other polymers, polypropylene may be manufactured to have water-absorbing properties, whether it is a fabric or nonwoven; the combined properties of fatigue resistance and water absorption make polyropylene an excellent material for a number of synthetic cloth, rope and sponge applications, such as diapers and many other sanitary or medical cleaning products. Macrame rope is typically polypropylene, and may be used to manufacture rugs and mats for residences or businesses. It is also manufactured into a number of containers, piping products and electronic insulators.
Polycarbonate – This unique, modern material is “cross-linked” by long chains of carbonate groups, giving polycarbonate an extreme durability that few other plastics can achieve alone. Polycarbonate may be fabricated to be “bullet-proof”, or at the least bullet resistant. Instead of shattering or breaking like acrylic or HDPE would do in similar circumstances, polypropylene’s tensile strength flexes, making it an extremely difficult material to break or scratch. Another unique characteristic is polycarbonate’s transparent clarity, which is better than many types of transparent glass. Given these unique characteristics, polycarbonate is commonly used for optical applications such as eyeglass lenses, as well as a wide range of safety glass, windshield and container applications. Hiker-preferred Nalgene bottles are made from polycarbonate, as well as many high-end drinking bottles, computer and laptop housings and DVDs. Some concern has been raised in recent years over polycarbonate’s tendency to leech bisphenol-A, a health toxic chemical, into food and beverage containers, although this problem has not reached critical levels and is being pursued by manufacturers.
Polyester – Polyethylene terephthalate, or PET is the most common form of polyester, which may be derived from natural chemicals or through synthetic processes. Polyester is best known for its uses in fabric and ropes as a synthetic substitute or supplement to cotton; although polyester offers very little water absorption, it does not shrink like other industrial fibers do, and it self-extinguishes when ignited. Generally considered a cheaper, lower grade material, polyester has less shock absorption than nylon but greater abrasion and UV resistance. Polyester is used as a high resistance, high strength laminate for high-quality products such as guitars, pianos, yachts and auto body fillers.
Acrylic – Commonly known as polymethyl methacrylate (PMMA), this material has been patented and thermoformed under a variety of trade names, including Plexiglass, Perspez and Acrylite. Acrylic’s unique properties come closest to those of polycarbonate; it is a strong, lightweight, impact resistant material with excellent optical properties. Polymer windows, laminates, lacquers, adhesives, transparent covers and various hard plastics are made from acrylic resin, which has various light transmitting capabilities. UV light does not damage, yellow or crack acrylic, combining with acrylic’s crystal clear transparency and high impact resistance to make an excellent material for safety glass laminates, polymer windows and large frames. Hand-held electronic devices such as cell phones use acrylic in display screens and liquid crystal displays.
Polyurethane – Also known as simply “urethane”, this material has perhaps the broadest range of applications, as it may be manufactured to have a wide number of properties. Unlike most other polymers, urethane consists of organic chains joined by carbamate, or urethane links; this allows polyurethane to act similarly to natural elastomers, or rubbers, depending on how the material is cast. Polyurethane’s most common application is in flexible and rigid foam used for seating, insulation panels, soundproofing, seals and gaskets. Urethane cast wheels may be manufactured to various levels of elasticity, providing more or less shock absorption depending on the application. Many types of high performance adhesives, sealants, carpet cushions and elastomeric fibers (such as Spandex) are made from urethane. Memory foam is made from urethane through a catalyzed reaction, as are a variety of hard plastic parts which require elasticity, shock absorption and impact resistance.