What type of plastics are characterized by having a density reduced by numerous small cavities?

Expanded (foamed or cellular) plastics are characterized by their reduced density due to the presence of numerous small cavities or bubbles within their structure. This unique composition allows them to be lightweight while still providing adequate thermal and acoustic insulation properties. The expansion process often involves introducing gas into the plastic material, which creates a foam-like structure. This not only decreases the overall weight of the material but also enhances its cushioning properties, making these types of plastics widely used in packaging, insulation, and other applications where weight reduction is critical.

In contrast, solid plastics have a uniform structure without the cavities that define expanded plastics, making them denser and typically heavier. Thermoplastics refer to a category of plastics that become pliable or moldable at a specific elevated temperature and solidify upon cooling, but they don’t necessarily contain the small cavities that reduce density. Composite plastics involve combining two or more different materials to achieve desired properties, often resulting in a denser and stronger material rather than one characterized by reduced density due to cavities. Thus, the distinguishing feature of expanded plastics is their lightweight nature, stemming from the numerous small cavities throughout their structure.