Polystyrene (PS) is a kind of natural transparent thermoplastic, which can be used as both a typical solid plastic and a rigid foam material. PS plastics are commonly used in various consumer goods applications and are particularly suitable for commercial packaging. In 1941, Dow Chemical Company invented a proprietary process to manufacture their trademark and the famous polystyrene foam product "polystyrene foam". This material is controversial among environmental protection organizations because of its slow biodegradation rate and its increasing appearance as outdoor garbage (especially in the form of foam floating in waterways and oceans).

The solid plastic form of polystyrene is commonly used for medical equipment applications, such as test tubes or culture media, or daily necessities such as the shell of smoke detectors, the box you use to purchase CDs, often used as a container for yogurt and other foods, or the red "solo" cup you drink at the tailgate of a car, or when you lose beer.

The foam form of polystyrene is most often used as a packaging material. If you buy a new TV or an important new device, such as a herringbone saw, you may have opened a customized foam plastic shell. Similarly, you may be familiar with the polystyrene foam packaging "peanuts", which are used as fillers for various small items in transportation. Polystyrene foam is also used in "take out" containers and disposable tableware in many restaurants.

What are the characteristics of polystyrene?

Now that we know what is polystyrene, let's take a look at some of its key characteristics. Polystyrene is usually (but not always) a homopolymer, which means it is only formed by combining the monomer styrene with itself. According to the type of PS, it can be classified as a "thermoplastic" or "thermosetting" material. This name is related to the way plastic reacts to heat. Thermoplastic materials completely become liquid at the melting point (210-249 degrees Celsius for polystyrene), but begin to flow at the glass transition point (100 degrees Celsius for PS). A major useful characteristic of thermoplastic materials is that they can be heated to the melting point, cooled, and reheated without significant degradation. Thermoplastics are not burned, but liquefied, which makes them easy to inject and recycle. In contrast, thermosetting plastics do not liquefy once they solidify into a solid form.

In contrast, thermosetting plastics can only be heated once (usually during injection molding). The first heating causes the solidification of thermosetting materials (similar to 2-component epoxy resins), resulting in irreversible chemical changes. If you try to heat the thermosetting plastic to a high temperature for the second time, it will burn. This characteristic makes thermosetting materials unsuitable for recycling.