The relative molecular mass (M) of a polymer is one of its structural parameters, which can be characterized by its relative viscosity (η). The larger the M, the larger the η.
M and η affect the spinning properties and fiber quality of the polymer. The quality control of nylon 66 fiber depends on the quality of the nylon 66 salt and the polymerization temperature.
High levels of impurities in the nylon 66 salt will cause the polymer to yellow and reduce M. Therefore, it is crucial to control the refining process conditions of the nylon 66 salt to ensure low color.
Both excessively high and low η will lead to spinning difficulties and problems such as low strength and excessive fuzz in the finished filament. When η is controlled between 77 and 79 during nylon 66 spinning, the spinning process is stable, allowing for the preparation of high-strength nylon 66 filaments. When η increases to 83-85, the filaments become stiff, and spinnability deteriorates; when η is between 62 and 64, it leads to discontinuous filaments. Considering all factors, controlling the viscosity of nylon 66 during spinning between 74 and 79 achieves good spinnability.