Matrix Moisture

Chemical Reaction

Presence of water at high temperatures triggers a reaction called hydrolysis which breaks the polymer chains into shorter segments. This reduction in molecular weight leads to a significant loss of impact strength and a drop in the viscosity of the melt. In pet, excessive matrix moisture results in the formation of acetaldehyde, which can taint the flavor of bottled contents.

Moulders must use desiccant dryers to pull this internal moisture out before the resin enters the injection unit.

Measurement Techniques

Quantifying this internal water requires more sophisticated methods than simple weight loss analysis. The Karl Fischer titration is the standard technique, as it uses a chemical reaction to specifically target water molecules while ignoring other volatiles. Another method involves heating the resin in a vacuum and measuring the evolved water vapor with a sensor.

These measurements are taken in parts per million to ensure the resin is dry enough for stable processing.

Operational Impact

Resin that has not been properly dried will produce parts with visible defects like splay marks or internal bubbles. Even if the part looks acceptable, the hidden loss of physical properties can lead to premature failure in the field. Maintaining a consistent dew point in the drying system is the only way to hold matrix moisture within the narrow limits required for high-performance engineering plastics.

Records of these drying cycles are a key part of the quality assurance process for critical components.