Method for increasing the molecular weight of a polyester granulate by using its residual heat

摘要

The present invention relates to increasing the molecular weight during a thermal treatment of polyester in combination with a latent heat granulation. With the newly-developed method, an SSP (Solid State Postcondensation) can be directly combined with an underwater granulation. The method differs from a conventional solid state postcondensation by an increase in the molecular weight being possible without additional heat input and hence only by using the residual heat and the crystallization heat present. A characterizing element is improved water separation and dehumidification during the granulation. Only in this way is an increase in viscosity possible even with a small granulate of an average particle weight less than 20 mg.

主权项

1. A method for direct increase in the molecular weight using the residual heat during the production of a non-adhering polyester granulate, comprising steps in which
a) a polyester raw material as melt is pressed through a nozzle, granulated in a cooling water flow and, after passing through a cooling water stretch, is separated from the cooling water flow, and b) a bed of the granulate obtained in step a) is subjected, directly following step a), to a thermal drying and post-treatment by supplying the granulate bed with a gas flow of a gas or gas mixture, the thermal drying and post treatment being conducted in a silo wherein the gas flow of a gas or gas mixture is conducted from bottom to top through the granulate bed and the dew point of the gas or gas mixture before being supplied is ≦—0° C., wherein during step b), the dwell time of the granulate is between one and thirty hours and neither the granulate used nor the supplied gas or gas mixture of the gas flow is heated by external energy input, and the gas flow is conducted in counterflow to the conduction direction of the granulate in the silo, and the average temperature of the bed of the granulate before entry in step b) is adjusted to more than 170° C.