Diagnosing gels, unmelts and degraded gels.
Gels are often encountered in extrusion. In multilayer coextrusion you have to identify the source layer of the gels.
The causes of gels range from resin contamination to process issues to screw and die design. Depending on the causes of gels, they may have very different characteristics. You first need to deterimine if you have gels or unmelts.
Unmelts are not really gels by definition; rather, they are materials that have not been melted, hence, they are called unmelts. The easiest way to distinguish between gels and unmelts is to use a hot-stage microscope. The idea here is to see whether or not the gel will melt at a reasonable temperature, and if the temperature is within the range of the melting point of a known polymer. This would roughly identify the type of polymer (for example, PE or Polyamide), and therefore the source layer or layers of the unmelts. When the gel will not melt within a reasonable temperature range, then it is most likely a true gel. The use of Fourier Transform Infrared Microscopy (FTIR) will then be required.
If a gel is identified as an unmelt, take a look at your resin sources, conveying system and blending system to rule out any potential resin contamination in these areas.
The next step and the most important; investigate the material’s processing temperature to ensure that all temperature zones are functioning properly and controlling to the preset target temperature on the recipe settings. Often a simple change in the barrel conditions of the extruder will significantly eliminate or reduce the number of unmelts. A basic understanding of the screw design would help with the extruder barrel zone temperature settings. There are basically three functional zones in a screw regardless of whether it is a single-stage compression screw or a barrier screw. They are named feed zone, melting zone and metering zone. All the melting of the polymer should take place before the start of the metering section. Too slow a melting rate results in an incomplete melting process; too fast a melting rate causes a premature solid bed breakup. Both contribute to the generation of unmelts.
Watch for resin blend compatibility. If changing the temperature profiles cannot eliminate unmelts completely, check into each component in the blend and determine how they interact with each other.
Screw design. If the steps described above fail to eliminate the melts you may have to explore a different screw design.
Degraded Gels
When gels are degraded or crosslinked, these gels will not be melted. In severe cases, they show up discolored from yellow to brown, or black.
Again, resin contamination is a possible culprit. Occasionally, foreign fibers in the resin cause a gel-like appearance. Normally, the use of Fourier Transform Infrared Microscopy (FTIR) will identify the foreign substance. Excessive processing temperatures can also contribute to premature degradation of the polymer.
Screw and barrel conditions. If the screw or the barrels are not in good working condition and are worn, the residence time of the materials in the extruder will increase dramatically. This would contribute to unexpectedly high melt temperatures. A variable depth thermocouple can help determine the temperature gradient in the melt stream, and indicate the true melt temperature. A fixed thermocouple only gives the melt temperature at the wall of the adapter, and may not indicate a true melt temperature in the middle of the melt stream.
Screw design. A poor screw design could contribute to potential degradation area.
Die design. Similarly, a poor die design could cause areas of unusually long residence time, which would contribute to polymer degrading in the die.
In summary, to deal with gels you need to find the source layer of the problem; identify the material; determine if problem is a gel, unmelt or degraded gel. You do this by looking at resin blend compatibility, resin contamination, screw and barrel conditions in addition to screw and die design.
In the next issue of Between the Lines watch for our troubleshooting story on interfacial instability.
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