What is the difference between crystalline and amorphous Plastics?

CRYSTALLINE & AMORPHOUS MATERIALS:   Polymers are often described as being either “crystalline” or “amorphous” when it is actually more accurate to describe plastics by their “degree of crystallinity”. Polymers cannot be 100% crystalline, otherwise they would not be able to melt due to the highly organizes structure. Therefore, most polymers are considered semi-crystalline materials with a maximum of 80% crystallinity.

AMORPHOUS MATERIALS have no patterned order between the molecules and can be likened to a bowl of wet spaghetti. Amorphous materials include atactic polymers since the molecular structure does not generally result in crystallization. Examples of these types of plastics are polystyrene, PVC and atactic polypropylene. The presence of polar groups, such as a carbonyl group CO in vinyl type polymers, also restricts crystallization. Polyvinyl acetate, all polyacrylates and polymethylacrylates are examples of carbonyl groups being present and the resulting groups being amorphous. Polyacrylonitrile is an exception to this. Even amorphous materials can have a degree of crystallinity with the formation of crystallites throughout their structure. The degree of crystallinity is an inherent characteristic of each polymer but may also be affected or controlled by processes such as polymerisation and moulding.

CRYSTALLINE MATERIALS exhibit areas of highly organized and tightly packed molecules. These areas of crystallinity are called spherulites and can be varied in shape and size with amorphous areas between the crystallites. The length of polymers contributes to their ability to crystallise as the chains pack closely together, as well as overlapping and aligning the atoms of the molecules in a repeating lattice structure. Polymers with a backbone of carbon and oxygen, such as acetals, readily crystallise. Plastic materials, such as nylon and other polyamides, crystallise due to the parallel chains and strong hydrogen bonds of the carbonyl and amine groups. Polyethylene is crystalline because the chains are highly regular and easily aligned. Polytetrafluoroethylene (PTFE) is also highly symmetric with fluorine atoms replacing all the hydrogens along the carbon backbone. It, too, is highly crystalline. Isomer structures also affect the degree of crystallinity. As the atactic stereochemistry resulted in amorphous polymers, those that are isotactic and syndiotactic result in crystalline structures forming as chains align to form crystallites. These stereospecific forms or propylene are those which are preferable for structural applications due to their degree of crystallinity.

The degree of crystallinity affects many polymeric properties. In turn, other characteristics and processes affect the degree of crystallinity. The higher the molecular weight, the lower the degree of crystallinity and the areas of the crystallites are more imperfect. The degree of crystallinity also depends on the time available for crystallization to occur. Processors can use this time to their advantage by quenching or annealing to control the time for crystallization to occur. Highly branched polymers tend to have lower degrees of crystallinity, as is easily seen in the difference between branched low-density polyethylene (LDPE) and the more crystalline high-density polyethylene (HDPE). LDPE is more flexible, less dense and more transparent than HDPE. This is an excellent example that the same polymer can have varied degrees of crystallinity. Stress can also result in crystallinity as polymer chains align orienting the crystallites. Drawing fibres, the direction of extrusion and gate placements will also affect the orientation of polymers and therefore the crystallites of the material. This allows the processor to maximize the effects and benefits of the inherent crystallinity of the polymer being used in the application.

COMMON CHARACTERISTICS OF CRYSTALLINE AND AMORPHOUS PLASTICS

Higher % Crystalline Higher % Amorphous
Higher heat resistance Lower heat resistance
Sharper melting point Gradual softening / melting point
More opaque More translucent /transparent
Greater shrinkage upon cooling Lower shrinkage upon cooling
Reduced low temperature toughness Greater low temperature toughness
Higher dimensional stability Lower dimensional stability
Lower creep Higher creep