An investigation of the melt blown web defect known as "shot" using metallocene and ziegler-natta based polypropylene resins

An extensive experimental investigation was undertaken to study the influence of processing parameters and resin type on the production of shot in the melt blowing process. Both early generation commercial metallocene and conventional Ziegler-Natta based polypropylene resins were used in this investigation.

In addition to the base metallocene and Ziegler-Natta resins, the effect of blending these resins together was studied. Finally, the effect of adding nucleating agents to the base metallocene resin on shot production was investigated.

The study showed that the early generation commercial metallocene resin used in this research produced more shot than conventional resins with all other processing conditions being equal. Die air pressure had a strong effect on the shot production; an increase in die air pressure produced more shot in the web sample. Processing temperature produced a similar trend; an increase in process temperature produced more shot in the web sample. Blending Ziegler-Natta polypropylene into a metallocene based polypropylene tended to decrease the shot formation in proportion to the percentage of the conventional resin. Examination of the crystallization kinetics of the base resin indicated that the Ziegler-Natta base resin crystallized faster under quiescent conditions than the metallocene base resin. Similarly, measurements on the blends indicated a gradual increase of crystallization kinetics from that of the metallocene resin to that of the Ziegler-Natta resin.