Today, plastic products come in a wide variety. Within the "plastic family", each member has distinct characteristics. For example, some plastics are as hard as rocks, while others are as soft as rubber. Some plastics are even light and porous, like sponges. How does this happen?
Plastics are high-molecular-weight materials primarily composed of synthetic resins. The differences in the properties of various plastics stem from their unique molecular structures. Many plastics consist of long chain-like molecules, such as the most common polyethylene and polyvinyl chloride plastics, which are made up of numerous chain-like molecules packed together. These plastics typically appear as solid materials, but if heated, the long chain-like molecules within them can slip past each other, making the plastic soft and malleable like plasticine. If the temperature is further increased, they will melt into a liquid state! These types of plastics exhibit characteristics similar to paraffin wax, which also becomes soft when heated. If the temperature exceeds 50°C, the wax will melt and solidify again upon cooling. Plastics that melt and become malleable when heated are called thermoplastic plastics. In daily life, plastic bottles for beverages are made from PET (polyethylene terephthalate) thermoplastic plastics, so these bottles should be encouraged for recycling. After recycling, they can be reused to make new products. Due to the presence of independent chain-like molecules in thermoplastic plastics, they typically exhibit elasticity, malleability, solubility in certain solvents, and the ability to melt at high temperatures, as well as low hardness and brittleness. In contrast, another type of plastic called thermosetting plastics is used. Thermosetting plastics are also initially composed of chain-like molecules, but during molding, cross-linking and other reactions occur between the molecules, resulting in a tightly interconnected three-dimensional network structure, making them strong and hard. For example, colorful amino plastics belong to thermosetting plastics. They are resistant to heat and deformation and are often used to make tableware, electrical appliances, and insulation materials. Thermosetting plastics lack elasticity and malleability, cannot be re-dissolved or melted, but can only be swollen. They have high hardness and brittleness. Therefore, items made from thermosetting plastics such as amino plastics and Bakelite cannot be recycled and reused after damage.
In addition, even if it's the same type of plastic, the performance of the products can vary greatly due to different processing techniques or additives. Just like the same batch of flour dough, it can be made into a hard and crispy pancake, a soft noodle, or a fluffy and porous steamed bun with the addition of baking powder. Polyethylene plastics are like this. They can be used to make colorless, transparent, and hard CD cases, but if some foaming agents are added, they will completely change, becoming white, soft, and full of small holes, widely used in making disposable tableware and for shockproof, insulation, and soundproofing purposes.