Why Simple Thermoplastic Polymers like Polyvinyl Chloride are Generally Less Strong than Thermosets

Thermoplastics are versatile materials used in a wide range of applications due to their ability to be melted and reshaped repeatedly. However, they often lack the strength required for more demanding uses compared to thermosets. This article will explore the reasons behind this difference in strength and how the chemical structure of these polymers influences their properties.

Strength vs. Stiffness: Misconceptions Clarified

It's important to distinguish between strength and stiffness. Strength refers to the maximum load a material can withstand before failing, while stiffness describes the extent of deformation under load before failure occurs. Thermosets are generally stiffer and stronger than thermoplastics like polyvinyl chloride (PVC) due to the nature of their intermolecular bonds and crosslinking.

The Role of Covalent Bonds in Thermosets

Thermosets are characterized by the presence of covalent crosslinking bonds that link polymer molecules throughout the material. These bonds are significantly stronger than the van der Waals forces or dipole interactions that hold individual polymer chains together in thermoplastics. The crosslinking bonds give thermosets their characteristic strength and resistance to deformation and high-temperature deformation.

Characteristics of Thermoplastics: PVC and Beyond

Thermoplastics, such as polyvinyl chloride (PVC), are marked by having only van der Waals forces and dipole bonds between individual polymer chains. While these bonds give PVC its flexibility and ease of molding, they do not provide the same level of strength as the covalent bonds found in thermosets. As a result, PVC is less suitable for applications that require high strength, such as those found in automotive engine compartments.

The Decomposition of Thermoplastics vs. Thermosets

Thermoplastics like PVC can be remelted and reshaped multiple times without significant degradation in their properties. This recycleability is a key advantage over thermosets, which become structurally unstable and lose their strength when remelted. Thermosets, on the other hand, retain their strength and shape even after extended use at high temperatures, making them ideal for applications where thermal resistance and strength are critical.

Applications and Demands

The choice between thermoplastic and thermoset polymers depends heavily on the application requirements. For example, a plastic spoon does not require high strength, as it only needs to hold up while in use. However, engineering components used in high-temperature environments, such as those found in automotive engine compartments, demand polymers with higher strength and melting points. Thermosets excel in these demanding applications due to their heat resistance and structural integrity.

Conclusion

In summary, the lower strength of simple thermoplastic polymers like PVC compared to thermosets is primarily due to the absence of covalent crosslinking bonds. These bonds, which are abundant in thermosets, significantly enhance the material's stiffness and strength. Understanding the differences between these polymers is crucial for selecting the appropriate material for specific applications, ensuring optimal performance and longevity.

Keywords: thermoplastic, thermoset, polyvinyl chloride, bond strength, crosslinking