Thermoplastics' Chemical Resistance

Thermoplastics are a class of polymers with extremely strong properties. Due to their unique properties of softening when heated and hardening when cooled, they occupy an important position in modern industry and daily life.

Their chemical resistance is complex, which profoundly affects the application and development of thermoplastics in different fields. This characteristic is closely related to their molecular structure and chemical composition. 

From the molecular structure level, thermoplastics are composed of a large number of straight or branched molecular chains, and the molecular chains only interact with each other through weak van der Waals forces or hydrogen bonds.

This relatively loose intermolecular force enables the molecular chains of thermoplastics to obtain enough energy to overcome the constraints between molecules when heated, resulting in relative sliding and softening of the material; when cooled, the movement speed of the molecular chains slows down, rearranges, and is fixed by intermolecular forces, and the material becomes hard.

This structure gives thermoplastics the advantage of being repeatedly processed and molded, but it also affects their chemical resistance to a certain extent. For example, the molecular chain of polyethylene PE is mainly composed of carbon-carbon single bonds and carbon-hydrogen bonds, with a simple and regular structure and weak intermolecular forces. 

At room temperature, polyethylene has good tolerance to a variety of chemicals such as dilute acids and dilute alkalis because its stable carbon-carbon bonds and carbon-hydrogen bonds are not easy to react with common chemical reagents.

However, when encountering strong oxidizing chemicals such as fuming sulfuric acid and concentrated nitric acid, the carbon-hydrogen bonds in the polyethylene molecular chain may be oxidized, resulting in a decrease in material performance.

The chemical composition has a more significant effect on the chemical resistance of thermoplastics. Thermoplastics polymerized from different monomers have different chemical properties. Polyvinyl chloride (PVC) is polymerized from vinyl chloride monomers and contains chlorine atoms in the molecular chain.

The presence of chlorine atoms increases the polarity of the molecular chain, making polyvinyl chloride resistant to certain polar solvents, but also makes it more sensitive to alkali. In an alkaline environment, the chlorine atoms on the polyvinyl chloride molecular chain are easily removed, producing hydrogen chloride gas, which causes material aging and performance degradation.

Polytetrafluoroethylene (PTFE) is polymerized from tetrafluoroethylene, and the fluorine atoms in its molecular chain completely replace hydrogen atoms. The fluorine atom has an extremely high electronegativity, which makes the carbon-fluorine bond extremely stable and can resist corrosion from almost all chemicals, including highly corrosive reagents such as aqua regia.

Therefore, polytetrafluoroethylene is known as the "King of Plastics" and is widely used in fields such as chemical industry and aerospace that require extremely high chemical resistance.

In addition, various additives added to thermoplastics, such as plasticizers, stabilizers, flame retardants, etc., will also affect their chemical resistance. The addition of plasticizers can reduce the glass transition temperature of thermoplastics and improve their flexibility and processing properties, but plasticizers may interact with certain chemicals, causing plasticizer migration, thereby affecting the performance and chemical resistance of the material.

For example, phthalate plasticizers commonly used in polyvinyl chloride plastics are easily dissolved when in contact with certain organic solvents, making the material hard and brittle, and also causing potential harm to the environment and human health.

The role of stabilizers is to improve the stability of thermoplastics to external factors such as light, heat, and oxygen, thereby indirectly improving their chemical resistance.

For example, adding stabilizers such as lead salts and organic tin during the processing of polyvinyl chloride can effectively inhibit the degradation of polyvinyl chloride caused by heat, oxidation and other factors during processing and use, thereby extending its service life.

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