Extrusion Technical Services

Gels

Extrusion Gels and other Contaminates

Gels and contaminants (referred to as gels in this text) in extrusion can cause disruption in the extrusion process and quality problems. Here we hope to discuss and address the problem by listing sources and cures. Of course Extrusion Technical Services would be happy to help you improve your extrusion process.

Unmelted resin:

Since the early days of extrusion unmelted resin has been the number one cause of extrusion and quality problems. Unmelted resin occurs when the extruder pumps the resin faster than it can melt it. This is a common situation. Unmelted resin can be difficult to test for and to resolve. If your gel problem seems to have the same color as your base resin, then this is most likely the source of your problem. The quick fix is to slow down your extruder until the problem goes away. Sometimes additional or denser screen packs can be added to increase backpressure and improve melting. A better solution is to have a screw designed to melt your material at the desired output rate.

Degraded residue:

This is a common source of gels. Resin that stagnates on process surfaces can polymerizes, cross link, degrade and/or oxidize. The resultant materials can then release from the surface and contaminate the flow. These contaminates can have a huge range of color. They can also have a large range of physical strength from soft to hard and non-flowing. Solutions to this problem involve screen packs, formulation stabilization, equipment cleaning, screw design, adaptor design, die design, and checking for wear and corrosion. Some extruders run a highly stabilized blend through a clean extruder first to coat the surfaces. You can analyze these contaminates for size and strength to see if it came from before the screen pack, or after.

Degraded resin and additives:

It is very possible for resin and additives to degrade in the extrusion process and for the degradation products to form carbon and other compounds. If the gel problem your having involves a lot of brown or black specs which are not sourced from other contaminants, then there are two courses of action; 1) Evaluate the extrusion process to lower the melt temperature and residence time, 2) Evaluate all raw materials for temperature stability. One method of determining temperature stability is to compression mold a plaque of the resin (with compounded in additives) and hold it at high temperature for an extended period (example HDPE at 550F for 15 minutes). If there is a stability problem many dark spots, or charring will occur. Of course be watchful for smoke or fire during this test. Some resins can degrade at high processing temperatures and can cause gassing and foaming. These gasses are usually highly dangerous so please be careful. Vary the time and temperature to see what the system can tolerate. Some systems, like flame retardant ones, can tolerate very little time at temperature. If the formulation shows signs of instability, work with your resin supplier to obtain a more stable formula.

True gels:

A true gel comes from the resin producer. Some resins are more likely to produce gels in the reactor. A true gel is defined as a very high molecular weight version of the same resin, and may also be somewhat cross-linked. The viscosity of a true gel is so high that while the gel will flow with the melt stream, it will not disperse into the melt stream. A true gel will normally have similar optical properties as the base resin, but it will clog screen packs and distort extrusions. True gels also have the unique property of being able to flow through tight mesh screen packs and end up with a particle larger than the screen mesh hole on the other side. Solutions to this problem should always be the responsibility of the resin producer. Find out whether the resin you are using is subject to large gel concentrations when produced. One way to check for gels is UV light as many gels will fluoresce under UV. Other tests involve thin films, various light sources, and counting techniques. A standard test should be developed with the supplier and material should be certified before shipment. Most resins do not have this problem so it is best to look elsewhere for the source of your “Gel” problem. If you are a resin producer or a user who has to deal with a true gel problem yourself, solutions come in the form of “don’t create-em”, “break-em” or “trap-em”. Not creating gels has to do with reactor operation, design, and maintenance. Breaking gels is a proven method of controlling the problem by using an extrusion process that can stress the gels and reduce their size so they are no longer a problem. Recent work with extensional mixing concepts has proven useful to in gel reduction. Trapping works fairly well with gels, and should be used in conjunction with breaking. Trapping involves using filters with depth, like stacking five 325 mesh screens together. While gels can flow through a single screen, multiple screens seems to stop them. If you have even a mild gel problem, the stacked screens can be quickly clogged. When you take the screen out however they will seem completely clean because of the gel’s optics being similar to the base resin.

Foreign contamination:

Wood from pallets, paper, plastic bags, dust, higher temperature resin, pellets stuck to shoes, transfer pipe cross contamination, can all cause problems in extrusion. These contaminants can enter the feed stream at any point. Screen packs can stop a lot of these material from ruining the product or process. When foreign contaminates are suspected examine the extrusion and screen packs microscopically to help in determination of the type and sort. One additional suggestion is to check your screen packs for the possibility of material flowing around the edges.

Un-dispersed additives:

Additives are added to resins in powder or masterbatch pellet form for inclusion in the melt. The additives need to be melt mixed; dispersed and distributed. In production of concentrate master batches, or when using powdered additives directly, the powdered additives often experience high pressure and form agglomerates. These agglomerates are very difficult to break down after they are formed. The results will be white or colored specs in the extrusion. Agglomerates that are visible to the eye are normally not supplied by the additive powder producer, but a screen mesh test of the powder can be performed to rule the additive supplier out as the possible source. If you are purchasing pre-compounded pellet masterbatch it is the responsibility of the masterbatch producer to make sure your masterbatch is agglomerate free (down to a certain size). You, as the extruder, normally can not disperse an agglomerate once it has been produced, otherwise the masterbatch producer would have been able to disperse it. If you are using powdered additives, a dispersion aid (usually a wax, surfactant, or fluoropolymer) finely blended onto the powder will help prevent agglomerates from forming. Also a screw design which keeps pressures low while the resin is solid will help. Additionally new screw mixer designs can help break up agglomerates which have formed.

Un-dispersed modifiers:

Typically materials added to the resin above 5% or so are now considered modifiers and no longer considered additives. Modifiers can range from minerals, to organic, to other polymers and rubbers. It is a mistake to attempt to modify the resin and produce an extruded product at the same time unless you have a specially designed process. Typically modifiers are added in the reactor or in an extrusion compounding step. It is the responsibility of the resin producer or compounder to make sure the resin is free if gels. If you have a modified resin, and seem to have gel problems, contact your resin supplier to see if the resin is subject to improperly dispersed modifiers.

Moisture:

Moisture is a very common source of extrudate defects. Your resin supplier should be contacted to determine the maximum allowable moisture content for your resin. Some resins can be processed with higher moisture as long as the extruder is vented, or vacuum vented. Different resins react differently to the presence of moisture, and to different level of moisture. Some resins will degrade when moisture is present, which generally will result in a low viscosity melt, and will not produce gel-like defects. Other resins will form bubbles that may be elongated or not, and may look like gels.

Air bubble or Void?

Often an extrusion is made where large bubbles are present. If these bubbles are randomly dispersed, then air entrapment in the screw may be the cause. Increasing the back-pressure, slowing the RPMs, or a change in screw design may help. If the air bubbles seem to be generally centered in large cross sections, and semi-continuous then product shrinkage could be at fault. In this case slowing down the product cooling rate should help.