Material Mentor: Thermoplastics

Plastic is a widely used material in manufacturing, both in products and packaging. It is energy efficient and relatively easy to manufacture in comparison to alternatives. However, there are countless different types of plastics that are used in different types of application. In general, plastics can be grouped into two main families: thermoplastics and thermosets. The main difference between these two types of polymers is that thermoplastics soften and become pliable above specific temperatures (upon heating) and solidify again upon cooling, whereas thermosets always remain in a permanent solid state once they have been molded. Today we will look at the thermoplastic group.

Here’s a look at some of the most commonly used types of plastic:

Thermoplastics Infographic

High Density Polyethylene (HDPE): 

HDPE is one of the most commonly used types of plastic. Its simple chemical structure makes it easy to produce and is therefore widely used for various low value applications. There are multiple types of polyethylene and HDPE is a relatively stronger, thicker form. It can be made pigmented or unpigmented. Unpigmented is naturally translucent and has good barrier properties and stiffness, but pigmented has better stress crack resistance. HDPE also has good chemical resistance, permeability to gas, and resistance to moisture.

This stiff plastic is often used for more robust products or packaging such as detergent and shampoo containers, or opaque milk, and juice containers, as well as folding chairs and tables, piping, Tyvek, and storage sheds, for example.


Low Density Polyethylene (LDPE):  
LDPE is generally a thinner, more flexible form of polyethylene. It is known for its toughness, flexibility, resistance to moisture, and relative transparency, making it a good choice in applications that require heat sealing. Due to its molecular makeup, LDPE has less tensile strength but greater ductility. Because of that formability, LDPE can be useful for a wide range of applications. Film applications like grocery bags, frozen food bags, plastic wrap, or coatings for paper cartons are common, but LDPE is can also be used to make rigid products like plastic bottles, buckets, and bowls.

 

Polyethylene Terephthalate (PET, PETE):

PET might be the most widely produced plastic in the world. It is a colorless tough resin that is a great barrier to gases (such as oxygen and carbon dioxide) and liquid (water, etc.). Along with its visually smooth surface, it has strength, high impact capability, and shatter resistance. PET has wide application use. Because of its strong ability as a liquid and gas barrier, it is popular for food and drink packaging. You may have also heard of PET in its fiber form, commonly known as Polyester.


Polypropylene (PP):

This resin is strong, with good chemical resistance and a high melting point. It is semi-transparent with a low-friction surface, and is adaptable to a variety of manufacturing techniques due to its simple chemical structure. PP is used for similar applications as polyethylenes, but is generally stiffer and more heat resistant. Its crystallinity (relating to hardness & density) is quite high, landing somewhere between that of LDPE and HDPE. PP works well for electrical resistance and insulation. It is often used in producing plastic automotive parts, consumer product packaging, textiles, hot food containers, as well as for plastic moldings in which complex shapes can be formed at relatively low cost and high volume.


Polystyrene (PS):
PS is a versatile plastic that in general is clear, hard, and brittle, but can be rigid or foamed. Its melting point is relatively low, but it is an excellent moisture barrier. In its foamed form, commonly known as Styrofoam, PS has low strength, low density, low thermal conductivity, and excellent insulation properties. This is why you will often see Styrofoam used in electronics packaging, coolers, coffee cups, and other insulation materials. In its harder form, common polystyrene applications include disposable cutlery, CD/DVD cases, hangers, test tubes, or petri dishes.


Polyvinyl Chloride (PVC, Vinyl):
PVC is a plastic polymer that in general has stable physical properties. It has good weatherability, chemical resistance, flow characteristics, and stable electrical properties. While pure polyvinyl chloride is a white, brittle solid, it can be blended with numerous chemicals to create resins with various properties from rigid to flimsy, or from soft to leathery. There are two basic forms of PVC: rigid and flexible. The rigid form is often used in construction applications like pipes, plumbing, insulation of electrical wires, vinyl siding, window frames, and decking, among others. The softer, more flexible form is often used in clothing including “pleather” items, or water-resistant pieces such as coats, shoes, or sports bags.


Acrylonitrile Butadiene Styrene (ABS):
ABS is a terpolymer created from, just as its name suggests, styrene, acrylonitrile, and polybutadiene. It Has a shiny, impermeable surface with strong properties including toughness, resistance to physical impacts and strong resistance to corrosive chemicals. Its low melting point makes it simple to use in injection molding or 3D printing. In fact, ABS is one of the top materials used for 3D printing. It is also seen a lot in rapid prototyping for these reasons. The fact that it is simple to manufacture and very easy to machine makes ABS a popular material choice.


Polycarbonate (PC):

Polycarbonates are another group of thermoplastics that contain carbonate groups in their structure. Although it is a thermoplastic that is easily molded, it is stronger than some and will hold up longer to extreme temperatures. It also has relatively high impact strength, however it has low scratch resistance, which makes it necessary to add another hard coating when utilizing the material for things such as eye glass lenses or exterior car parts.  Polycarbonate is a very transparent material, which combined with its strength makes it a popular choice for consumer products needing to be shatter-proof and also for epoxy resins.

Originally the material was developed to be an alternative to die-cast metal or sheet metal to be beneficial for applications where parts that are transparent or don’t conduct electricity are needed. The ability for polycarbonate to be formed or bent without breaking or cracking allows it to work well as an alternative. However, in recent years there have been toxicity concerns revolving around certain applications of PC due to the presence of bisphenol A (BPA), which can leach from polycarbonate. Polycarbonate is still utilized in a variety of product types such as water bottles, eye glass lenses, CDs/DVDs, housing for some electronics, riot gear for police, snowboards, car parts, and more.

Polyamide (PA, Nylon):
While polyamides can occur both naturally and artificially, one of the most well-known artificial polyamide materials is nylon. It is a silky thermoplastic material that can be processed into fibers, shapes, or films. Synthetic polyamides have high strength and durability, are temperature resistant, and have high chemical compatibility. Because of this, they are often used as a substitute for low strength metals in applications such as car engines, or reinforcement in rubber materials like car tires, or injection molded parts for vehicles and mechanical equipment. Additionally, synthetic polyamide is seen in rope, carpet, sportswear, and other textiles.