A thermoplastic (sometimes written as thermo plastic) is a type of plastic made from polymer resins that becomes a homogenized liquid when heated and hard when cooled. When frozen, however, a thermoplastic becomes glass-like and subject to fracture. These characteristics, which lend the material its name, are reversible. That is, it can be reheated, reshaped, and frozen repeatedly. This quality also makes thermoplastics recyclable.
There are dozens of kinds of thermoplastics, with each type varying in crystalline organization and density. Some types that are commonly produced today are polyurethane, polypropylene, polycarbonate, and acrylic. Celluloid, which is considered the first thermoplastic, made its appearance in the mid-1800s and reigned in the industry for approximately 100 years. During its peak production, it was used as a substitute for ivory. Today, it is used to make guitar picks.
Sometimes, thermoplastics are confused with thermosetting plastics. Although they may sound the same, they actually possess very different properties. While thermoplastics can be melted to a liquid and cooled to a solid, thermosetting plastics chemically deteriorate when subjected to heat. Ironically, however, thermosetting plastics tend to be more durable when allowed to cool than many thermoplastics.
Thermoplastics also differ from elastomers, even though some are considered both. While many thermoplastics can be stretched to a point, they generally tend to both resist, and stay in the shape they are stretched to. Elastomers, as the name suggests, bounce back. However, the addition of plasticizers to the melt can render a more pliable thermoplastic. In fact, this is usually the case when a thermoplastic is being used for plastic injection molding or extrusion.
The specific action of a plasticizer is to lower the material’s glass transition temperature (Tg), which is the point it becomes brittle when cooled and soft when heated. Tg varies with each type of thermoplastic and is dictated by its crystallization structure. However, Tg can also be adjusted by introducing a thermoplastic into a copolymer, such as polystyrene. Until the use of plasticizers, some molded thermoplastic parts were prone to crack in cold weather.
Thermoplastics have been around for a long time, but are a huge component of everyday life today. For example, acrylonitrile butadiene styrene (ABS) is a type of thermoplastic used to manufacture sports equipment, toys (i.e., LEGO® blocks), and various automobile parts. Polycarbonate is used to make compact discs (CDs), drinking bottles, food storage containers, and eyeglass lenses, among other things. Polyethylene is likely the most commonly encountered thermoplastic and is used to make shampoo bottles, plastic grocery bags, and even bullet proof vests.
Are Thermoplastics Biodegradable?
Biodegradability is an important measure of any substance's eco-friendliness. Simply put, biodegradability means that a material can be broken down naturally by microorganisms. Bacteria and fungi usually aid in decomposition by feeding on organic materials. Once biodegradable materials are broken down, their basic components remain.
Why Regular Plastic Can't Biodegrade
Traditional plastics are not biodegradable. That's because bacteria can't feed off them and break them down into their original components. Despite the fact that plastics are made from naturally occurring materials such as oil, it's the polymerization process that makes them unable to break down.
When crude oil is used to make plastic, its hydrocarbons are eventually separated into smaller molecules known as monomers. Two key monomers in plastic production are ethylene and propylene. These are chemically combined into new configurations, resulting in long repeating molecule chains we call polymers. They're later combined with additives like flame retardants to create each type of plastic's final form.
So if plastics contain carbon, which can't bacteria break them down? The polymerization process turns the original materials into molecular compounds that don't exist in nature. Bacteria and other microorganisms can't consume carbon in these forms.
Thermoplastics and Biodegradability
Like regular plastic, thermoplastics are also made from polymers. There's also thermoplastic starch, made by combining native starches with a plasticizer. These native starches are also called pure starches, derived from foods like wheat and potatoes. They're made of long-chain carbohydrates that are insoluble in cold water.
The process of creating thermoplastic starches begins with the plasticizer-starch mixture heated at between 70 to 90 degrees Celsius. This range lies above normal starch gelatinization temperatures and weakens existing hydrogen bonds in the starch itself. The resulting mixture turns into thermoplastic starch.
Biodegradable plastics like thermoplastic starch may solve the biodegradability problem. Starch itself is biodegradable and a renewable resource. Depending on which source you consult, TPS takes between 30 to 90 days to break down. In contrast, the average plastic bottle takes about 450 years to decompose.
What Is the Main Difference Between Thermoplastic and Thermosetting Plastic?
At first glance, thermoplastic and thermosetting plastic may seem alike. After all, they're both made from polymers. However, they react differently when exposed to hear. Thermosetting plastic does not melt — in fact, it retains its shape and solidity after it's been cured.
So, if thermosetting plastic doesn't really melt, why is it still in use? That inability to melt becomes an advantage in certain situations. Its unique properties include a variety of colors and finishes plus added strength after proper curing. They also resist corrosion, exhibit high dielectric strength and are cheaper to produce. That's why thermosetting plastic is used in many applications: automotive components, appliances and electrical parts, to name a few.
How Can Thermoplastics and Thermosets Be Recycled?
You may be surprised to learn that thermoplastics are recyclable. Part of the reason is their heat tolerance. Polymers in thermoplastics are strong, but the chemical bonds between them are weak. This means they can be heated and cooled, over and over again, without destroying their structural composition. This translates to an easier ability to reuse them repeatedly.
Around 75% of plastic produced worldwide is in thermoplastic form. Much of this includes single-use plastic for food and drink containers. You'll commonly find polyethylene terephthalate, also called PET, used in water bottles.
Once recyclable plastics reach their final destinations, they're melted down and reused through injection molding. But a major problem with trying to recycle thermoplastics and other plastic materials is that they all have slightly different compositions. That's why it's necessary to separate these different kinds of plastic if you plan to recycle them.
As you gather plastic for recycling, you'll want to keep a few things in mind. Bottles from beverages, personal care products, detergents and food containers can be recycled. You can also look for numbers embossed on the plastic containers themselves. A small "1" means it's made of PET, which is recycled. High-density polyethylene is marked with a "2" and is also highly recyclable. For anything else, check with your local recycling facility or municipal refuse collection authority.
Thermoset Plastics and Recycling
Thermoset plastic products must be durable, so they can't easily be recycled. Scientists are examining new ways to recycle them, including chemical linkers that help these items retain their strength while breaking down more easily. This is an exciting development, but widespread implementation may not occur for some time. Once it does, it may prevent more plastic materials from ending up in landfills.