How to remove gels from a polymer

What are gels in a polymer?

Gels are small amorphous resin particles in a polymer. They differ from their surroundings by being cross-linked and/or having a higher molecular weight. Gels can be broadly classified as either ‘hard’ or ‘soft’. Beyond that classification, gels come in all shapes and sizes. Depending largely on their appearance, they can be described as point gels, fisheye gels, gel flurries, arrow-shaped gels, fiber gels, or irregular-shaped gels. Their appearance can be better identified under an optical microscope.

What causes gel formation?

There are many potential sources of gel formation. These include the following:

Gels could be naturally present in the resin or masterbatch.
Resins or resin blends could have only a low or moderate level of thermal stabilization.
Recycled resin could have low thermal stabilization and/or contaminants.
Contaminated resin hoppers, feeders or conveying lines can lead to gel formation.

Gels could arise from unmelted particles due to insufficient melting or mixing (e.g. poorly dispersed additives/pigments).
A large melt viscosity difference between resins could form gels.
Die fouling or die build-up can instigate gels.
Excessive melt temperatures could lead to crosslinked or degraded resin, leading to gel formation.
Carbonized resin could be present, from a dirty extruder screw/die and/or contaminated screen packs.

Why do gels in polymers matter?

Generally speaking, a gel results in lower product quality as it makes the polymer mechanically less stable. Breaking points can occur where a gel is present. Gels can prevent flow, create weak spots and cause optical defects. Specific problems include the following:

Large gels can cause film breaks or reduce the drawdown ability, especially at high line speeds.
Gels can cause weak spots and reduce mechanical strength and sealing performance.
Gels can reduce the barrier properties of laminated films, especially those with aluminum foil or metallized films.
In high-clarity films, gels can reduce optical performance and detract from the aesthetic appeal of the packaging.
In surface protection films, gels are unsightly and can cause tearing during peeling, resulting in scratches in the surface being protected.
In lamination films, excessive gel content can cause optical imperfections, printing and sealing problems.
Hard crosslinked gels can damage expensive printing plates

In fibers and poly yarns, gels can cause spin breaks. These can reduce the mechanical strength of the final material, or even bring a halt to textile production processes such as weaving. For bottle to bottle recycling where extra Gels are created Bekipor is used to reduce Gels for clarity and Bekipor is also used to capture any chemical who could act as nucleating agent and this especially in Japan.

How can gels be removed?

The best way to remove gels in a polymer is to use a filtration system. Hard gels can be removed by the filter media in the same way that any hard contaminant such as metal or dust is removed. However, soft gels can change shape and pass through the filter media, causing an increase in differential pressure. So it’s important to choose the right filter media.

What’s the best filter media to use to remove gels?

Sintered metal fiber media is the preferred filtration media to use to remove gels from a polymer, as it captures and holds both hard and soft type gels during the filtration process. In combination with the right design, the number of dead zones is considerably lower than with any other type of depth filtration media on the market today. Compared to other filter technologies, powder displays a porosity of only 20-30%. Wire mesh is not effective at all for gel removal.

Is there a difference between different types of sintered metal fibers?

Yes. Bekaert’s Bekipor® metal fiber media has a higher porosity (consistently 80%) than metal fiber media from other suppliers, which typically only reach 70%. A higher porosity is better for gel removal. In addition, Bekipor® has a labyrinthine structure of at least 70 layers, which also contribute to its high efficiency at trapping gels. Bekipor® is also proven to significantly reduce spin breaks. In tests on yarn production, Bekipor® reduced spin breaks from an average of one spin break per ton of yarn produced, to a maximum of one spin break per 10 tons of yarn.

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