What is gel?
A gel is any visible imperfection in a polyethylene film.
What do we know about Gel?
All PE materials contain gel
Gel can be inorganic
Gel can be separated into two basic sizes.
Precursor gel (invisible)
4 Types of Gel
Cross Contamination gels
What situations aide Gel formation?
Increasing the temperature to which the polymer is exposed
Increasing the unsaturation and branch content of the polymer
Subjecting the polymer melt to more mechanical stress
Increasing the oxygen content of the melt
Why care about invisible precursor gel?
Gel needs certain conditions to “grow” to visible size.
For the molecules to reach the critical size to become visible as a gel we need to meet 4 requirements:
Sufficient free radicals
A mechanism for molecular growth
Precursor gel material
to grow molecules to the critical size.
Variables that influence gel growth
The screw design
Wear on screw
Dead spots in die/extruder
These 3 factors relate mainly to the time the polymer molecules will spend inside the extruder, called the residence time
Shutdown and startup procedures
Use of recycle/regrind in a formulation
Temperature control/profile on extruder
Order of production runs
Precursor gels from supplier
During extrusion, gels are produced by any combination of the causes above. Reducing the severity of any one of these factors (more is better), then gel will be reduced.
Extrusion conditions influence the growth speed and size of gel. Size and quantity will depend on the presence of precursor gel material. Precursor gels are larger than the average molecule size in the extruded polymer but are not large enough to be visible. As the number and size of precursor gels increases, the more likely that we will have gel formation. All polymers contain precursor gels.
Summary of strategies to deal with gel formation
Replace worn screws
Use a screw design optimized for the polymer being extruded.
Reduce the temperature profile and melt temperature where possible.
Simplify the production cycle so that polymer of similar viscosity is being extruded head to head.
Perform preventative strip and clean operations every 6 months.
Use processing aids where the cost can be recovered.
Use antioxidant masterbatch, provided these do not affect downstream operations.