Polymer Degradation Routes and Primary Aromatic Amine Migration Limits
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Isocyanate Hydrolysis
Meaning
Chemical reaction processes where isocyanate groups react with water to form amines and carbon dioxide represent a common degradation pathway for polyurethanes and polyurethane precursors. This isocyanate hydrolysis occurs during the processing and storage of these materials, leading to the formation of gas bubbles in molded parts and a reduction in mechanical properties. It represents a critical processing challenge for molders, as even small amounts of moisture can cause severe defects in the finished product.
This reaction is particularly troublesome in reaction injection molding, where the raw materials must be kept dry to ensure proper polymerization and cell structure in the final molded foam. Preventing moisture contamination is the primary defense against this type of degradation.
Processing Hazard
Moisture exposure in the raw materials or the injection barrel triggers this unwanted reaction during the molding cycle. The carbon dioxide gas generated during the reaction becomes trapped in the polymer melt, creating voids and surface defects that compromise the structural integrity of the part. Molders must use desiccant dryers to ensure that the resin is dry before processing.
Degradation Endpoint
Amine compounds formed during this reaction can further react with remaining isocyanate groups to form polyurea linkages, altering the chemical structure of the material. This alteration reduces the flexibility and tensile strength of the polyurethane, making it more brittle and prone to failure under load. Molders use chemical analysis to monitor the extent of this degradation and adjust processing parameters accordingly.
Moisture Control
Raw material storage conditions must be controlled precisely to prevent moisture absorption and subsequent degradation. Isocyanates are highly hygroscopic and will react with atmospheric moisture if not stored in sealed containers under a dry nitrogen blanket. Ensuring proper storage and handling minimizes the risk of processing defects and ensures consistent part quality.