Meaning
Reactive chemical terminations consisting of oxygen and hydrogen atoms on polymer chains influence both the synthesis and the final properties of polyurethanes and polyesters. In polymer sourcing, the concentration of hydroxyl end groups dictates the molecular weight and reactive potential of the resin during compounding and molding.
Resin Reactivity
Moisture absorption by these polar terminations can lead to polymer degradation during melt processing. Resins containing high levels of hydroxyl end groups are highly hygroscopic, requiring extensive drying before injection moulding. If these groups are exposed to moisture at high temperatures, hydrolysis occurs, which reduces the molecular weight and degrades the mechanical properties of the moulded part.
This molecular degradation appears as splay marks on the surface of the component and leads to premature mechanical failure in the field.
Viscosity Control
Melting characteristics and melt flow behavior are directly affected by the chain lengths resulting from these reactive points. Measuring the concentration of hydroxyl end groups allows compounding facilities to adjust the formulation for consistent melt viscosity. This consistency is essential for maintaining uniform filling of multi-cavity moulds, preventing defects like flash or short shots.
Adhesion Promotion
Surface properties of the finished part are enhanced by the presence of these polar groups. The active nature of hydroxyl end groups on the polymer surface improves adhesion to paints, coatings and adhesives. This eliminates the need for aggressive surface pre-treatments like corona or plasma activation, reducing assembly costs.
