When selecting a plastic material for an application, the first step typically involves characterizing the application requirements that lead to the first “fork in the road decision.” Do I need an amorphous material or a crystalline material? So how does one characterize these two broad groups of materials in terms of performance?
CRYSTALLINE POLYMERS – are characterized by a 3-dimensional lattice structure with uniform intermolecular forces. The structure has a definitive influence on how the polymer will perform.
Crystalline polymers require significant heat energy to sever the intermolecular forces. Once this threshold of temperature is met, the polymer fully melts because of the uniform structure, such as the melting of an ice cube. Secondly, crystalline polymers are characterized as anisotropic.
This means that crystalline polymer properties differ depending on the direction measured. The thermal conductivity, mechanical strength and electrical conductivity are different depending on the direction of measure. Lastly, crystallinity decreases permeability due to the tight lattice structure and thus increases the resistance to chemical attack. Essentially, the fort wall is denser, thus protecting it from attack.
The degree of crystallinity of the polymer determines the polymer’s impact properties, such as notched Izod impact resistance, which is a test that relates to the ability to resist fracture. One interesting feature is that many crystalline polymers in their processed state have matrixes of both amorphous and crystalline zones within the polymer.
Often called semi-crystalline materials, a materials level of crystallinity is controlled in the polymerization process to engineer specific properties in materials such as PET and PEEK. A crystalline phase often dominates machinable PET, thus opaque in color; food packaging is often amorphous-dominated and thus clear.
AMORPHOUS POLYMERS – are characterized by having a disorganized pattern of polymers. The polymer chains are disoriented, random in length, and intertwined like a bowl of pasta. Amorphous comes from the Greek word for “shapeless.” The random structure has definite advantages in terms of properties.
Thermal, unlike crystalline materials that fully melt once the heat reaches a specific temperature, amorphous materials begin to soften as they reach a specific temperature. This is known as Tg or Glass Transition factor.
Pure amorphous materials tend to be clear. In fact, the greater the degree of crystallization in an amorphous material, the more opaque the material becomes. Amorphous materials are less prone to shrinkage as they cool.
They have less chemical resistance due to the loose polymer structure but are easier to bond and weld. Finally, amorphous materials are isotropic and thus have the same properties regardless of the direction tested.
THE MOST IMPORTANT STEP IN SELECTING THE OPTIMAL PLASTIC MATERIAL IS THE DETERMINATION OF CRYSTALLINE VS AMORPHOUS.
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