High temperature polymers are divided into two categories: amorphous and semi-crystalline.
The difference between the two lies in their molecular structure. In this blog post, well discuss how amorphous and semi-crystalline thermoplastics differ from each other.
Amorphous Characteristics
Amorphous thermoplastics include acrylic, polycarbonate, and ABS. You might recognize these as translucent plastics: most amorphous plastics are clear.
Amorphous polymers have a molecular structure that is randomly ordered, which causes them to have a range of temperatures over which they melt. This also makes them relatively easy to thermoform because the molecules become mobile as heat is applied. The more heat applied, the more mobile the molecules will become.
These thermoplastics are isotropic in flow because of their randomized molecular arrangement. This causes them to possess better dimensional stability and have a lesser tendency to warp. They also typically possess superior impact strength. However, amorphous polymers are reserved for structural applications because they dont perform well as bearings or wear components.
They are more prone to stress cracking than semi-crystalline polymers, and have very poor fatigue resistance. They also tend to have lower chemical resistance and higher friction than their semi-crystalline counterparts.
Amorphous polymers are reserved for structural applications because they dont perform well as bearings or wear components.
High performance amorphous thermoplastics offer excellent resistance to hot water and steam, good chemical resistance, and good stiffness and strength. Materials that would fall into this category include PSU and PEI.
Semi-Crystalline Characteristics
Semi-crystalline thermoplastics include acetal, UHMW-PE, PEEK, fluoropolymers, and nylon. These polymers tend to be opaque, unlike amorphous thermoplastics.
Also unlike the amorphous polymers, semi-crystalline polymers have a very ordered molecular structure that results in a sharply defined melting point. This sharply defined melting point makes it very difficult to thermoform them because the molecules will only become mobile above the materials melting point.
Semi-crystalline thermoplastics are anisotropic in flow. In particular, they shrink more in the direction transverse to flow than they do along the direction of flow. This can result in some dimensional instability when compared to amorphous polymers.
They perform very well in applications involving wear, bearings, and structural loads. Unlike amorphous thermoplastics, they have a lower coefficient of friction and good chemical resistance. Their impact resistance, however, is not as good as that of amorphous plastics.
High performance semi-crystalline thermoplastics provide very good stiffness and strength, good toughness, and a very low coefficient of friction. This includes material such as PTFE and PPS.
Conclusion
Semi-crystalline thermoplastics differ from amorphous thermoplastics because of how their molecular structure is arranged. Because of their ordered structure, semi-crystalline polymers possess characteristics such as a well-defined melting point, better strength, improved fatigue performance, good chemical resistance and improved wear resistence. Amorphous polymers, on the other hand, have a randomized structure that gives them a wider range of temperatures over which they soften and better dimensional stability. Their use, however, is limited to structural applications.
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