Gel Troubleshooting

摘要

The term "gel" is used to describe any defect in appearnace in a fabricated film. Gels have been a source of irritation to film fabricators, resin suppliers, and equipment designers since plastics were introduced. A key to understanding how to correct gel problems is in identifying the type of gel and determining the source. This paper discusses the analytical methods employed to identify gel types, review some of the equipment used to measure gel levels, and present some data on how to prevent gels in the extruder. There are many descriptive names for gels including black specs, voids, fisheyes, streaks, fibers, unmelt, crosslinked, slabs, and contamination. Counting gels in a film sample has historically been difficult. Visual examination is very dependent on the skill of the operator, and is limited in the amount of film to be counted. Today there are several automated counting systems that can be used for countign the level of gels. Comparison of results will depend on the how the gels were counted and the reporting of the results. Identifying the source of the gel is the key to correcting the problem. Gels can be produced or introduced at any time from the manufacturer of the polymer through the final extrusion process. It is necessary to group gels into one of four general categories: 1. Fibers / Contamination 2. Unmixed 3. Crosslinked 4. Oxidized One method used to characterize gel types is hot-stage microscopy. This method slowly heats a film sample with a gel on a microscope hot stage to a temperature above the melting point of the polymers. Transmitted light is passed through cross-polarized filters. By observing the melting point of the film/gel and noting any birefringence effects, the different gel types can be identified. Once the type of gel is determined then, troubleshooting guidelines can be applied to the specific source of the gel problem. The presence of gels may be aesthetically unacceptable in some applications, but there are many applications where the presence of gels is unacceptable. Downstream process steps such a printing, sealing, and laminating are dramatically influenced by the presence of gels. Applications such as stretch-=cling film have concerns about gel levels due to poor end-use performance (i.e. lower stretch performance).