Secondary Crystallization

Meaning

Material aging processes alter the molecular structure and mechanical properties of molded polymers over time. This slow rearrangement, known as secondary crystallization, occurs when amorphous regions of the polymer slowly reorganize into a crystalline structure. This process continues for weeks or months after the molding cycle is complete.

Molecular Rearrangement

Polymer chains slowly organize into crystal lattices under ambient temperatures. Secondary crystallization is driven by the thermodynamic tendency of the polymer to reach a lower energy state. This process is faster in semi-crystalline polymers like polyethylene and polypropylene.

It can be accelerated by exposing the parts to elevated temperatures during an annealing step.

Density Shift

Physical measurements of the molded part show an increase in specific gravity over time. As secondary crystallization progresses, the density of the molded part increases. This increase in density is accompanied by shrinkage of the part dimensions.

Moulders must measure part dimensions after a specified period to ensure they remain within the tolerance limits.

Mechanical Impact

Part performance changes as the crystalline structure develops over time. The increase in crystallinity caused by secondary crystallization raises the stiffness and yield strength of the part. However, it also reduces the impact resistance and flexibility of the material.

This shift can cause the molded part to become brittle and fail if it is subjected to impact loads during use.

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