Gel Reduction
Tek-MixTM: The Anti-Gel Process
Gels are a major quality issue, especially in extrusion of films and fibres. Other than contamination sources, most gels are visual droplets of higher molecular weight material, coming from overheating during crossliking PE, ultra high molecular weight chains in a bimodal resin, breakdown of thermally unstable additives, and unmelted and dusty regrind particles. Gels can be fisheye shaped if from contamination, and more round from molecular weight issues. Polymerization gels (long chains) are called P-Gels, and are a major problem in polyolefins. P-Gels are classified into four categories:
1. Crosslinked and oxidized gels – formed from very long chains and excessive molecular growth
2. Cross-contamination gels – Poor mixing and blending of different density or molecular weight polymers
3. Unmixed gels are created by poorly melted and poorly mixed blends
4. Fiber gels are created by floss created during pneumatic conveying of particles
Extrusion issues can also create gels called E-Gels from seven sources:
1. High extrusion temperatures
2. Long residence times
3. Unmelted particles
4. Worn screws
5. Excess regrind
6. Flow stagnation from dead spots
7. Use of high shear zones (either in shear mixers or between the screw flights and barrel)
To date the solution has been either to fine tune the extruder, or more commonly to use fine mesh screen packs to attempt to filter out the gels.
Tek-MixTM is excellent for reducing gel problems. P- Gels originating from high molecular weight materials which are present in all products, and are shown in molecular weight curves as "Mz" and "Mz+1". Tek-Mix can actually target these long chained molecules, repeatedly stretching and relaxing them, to reduce their length without seriously affecting the remaining chains. E-Gels also can be broken down by the high dispersive effects of elongational flow.
As the following graph shows, the long chains in the Mz and Mz+1 region of a molecular weight distribution (right side of the curve below) were "clipped", resulting in removal of the troublesome gel-producing long chains. This is a valuable benefit for producing fiber, thin films, and profile extrusions. The graph below is a molecular weight distribution curve for virgin and Tek-MixTM treated Polypropylene (PP), showing a reduction of long chain polymeric materials without significantly altering the other chains.
As the following graph shows, the long chains in the Mz and Mz+1 region of a molecular weight distribution (right side of the curve below) were "clipped", resulting in removal of the troublesome gel-producing long chains. This is a valuable benefit for producing fiber, thin films, and profile extrusions. The graph below is a molecular weight distribution curve for virgin and Tek-MixTM treated Polypropylene (PP), showing a reduction of long chain polymeric materials without significantly altering the other chains.
The Tek-MixTM extensional flow system can address issues of long chain molecules. The Polydispersity represents the ratio of molecular weight to molecular number. For virgin PP resin, the Polydispersity Index is 5.9, while for Tek-MixTM treated PP it is 3.8. This reduction represents a reduction in the resin's sensitivity to shear by having significantly reduced long chain molecules. It also illustrates a narrowing of the molecular weight distribution curve, which often improves impact strength, reduces part warpage, and increases melt viscosity.
For more information on Tek-MixTM processing for preventing gel formation, click here.
For more information on Tek-MixTM processing for preventing gel formation, click here.
