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Troubleshooting and Mitigating Gels in Polyolefin Film Products

Modified on Monday, 11 May 2015 12:16 PM by mpieler Categorized as Paper of the Month

Troubleshooting and Mitigating Gels in Polyolefin Film Products

Mark A. Spalding, The Dow Chemical Company, Midland, MI
Eddy Garcia-Meitin and Stephen L. Kodjie, The Dow Chemical Company, Freeport, TX
Gregory A. Campbell, Clarkson University/Castle Associates, Jonesport, ME


The term “gel” is commonly used to refer to any small defect that distorts a film product. Eliminating gel defects from extruded polyolefin film products can be difficult, time consuming, and expensive due to the complexity of the problem and the high levels of off-specification product produced. This paper discusses the identification of gel types, the common root causes for gels, and the technical solutions for mitigating gels in film products produced using single-screw extruders.


Troubleshooting extrusion processes where gels are appearing in polyethylene (PE) film products can be difficult due to the number of different gel types that are possible. For these processes, the troubleshooter must be able to diagnose the problem quickly and provide an economically viable technical solution [1]. Because gels can originate from numerous sources, the troubleshooter must be able to identify the characteristics of the gel and recognize the likely possibilities of the source. Process changes must then be performed to mitigate the gel defects.

There are many types of gels [2] and the most common include: 1) highly oxidized polymeric material that appears as brittle black specks, 2) polymeric materials that are crosslinked via an oxidative process, 3) highlyentangled polymeric material (such as high molecular weight species) that are undispersed but not crosslinked, 4) unmelted resin, 5) filler agglomerates from masterbatches, and 6) a different type of resin or contaminant such as metal, wood, cloth fibers, or dirt. A crosslinked resin gel is typically formed during an oxidation process, resulting in the crosslinking of the resin chains and the generation of discolored gels. Highly-entangled gels are typically high molecular weight polymer chains that are entangled and thus difficult to disperse during the extrusion process. When analyzed using a hot stage microscope, this gel type will melt as the stage temperature is increased. When the stage temperature is then decreased, the gel will crystallize, creating the appearance of a gel as a solid polymer fragment. Since these gels are not oxidized they are not associated with color. They are commonly referred to as undispersed or unmixed gels. Unmelted resin exiting with the discharge can sometimes occur, especially at high extrusion rates. These gels will melt during heating with a hot-stage microscope, and typically they will not reform during the cooling phase. Numerous sophisticated methods are available for analyzing gels including epi-fluorescence microscopy, polarized light microscopy, and electron microscopy with x-ray analysis. These methods are discussed in the next sections.

Gels can be generated from many different sources and include: 1) the resin manufacturer, 2) the converting process, 3) pellet blending of resins with significantly different shear viscosities, 4) pellet blending of different resin types, and 5) direct contamination. Modern resin manufacturing processes exclude oxygen from the system and are very streamline such that process areas with long residence times do not exist. As such, crosslinked and oxidative gels are likely not generated by the manufacturer. Improperly designed extrusion equipment and processes, however, are common, leading to the oxidative degradation of resins and crosslinked gels. Several case studies in the next sections show how poorly designed processing equipment can lead to crosslinked and unmixed gel contamination of film products.

The goal of this paper is to describe the different type of gels that are likely to occur in polyolefin film products, techniques for identifying the gel type, and technical solutions to mitigate them from single-screw extrusion processes.

Protocols for Gel Analysis

Established protocols for gel analysis in polymer films are well documented in the literature [2-4]. For example, gels can be identified using the schematic process [4] shown in Figure 1. Typically a film with defects is visually inspected using a low power dissecting microscope. The gels can be classified based on size, color, and shape, and isolated using a razor blade or scissors. Cross sections of the gels ranging from 5 µm to 10 µm thick are collected at temperatures below the glass transition temperature (Tg) of the film using a cryogenic microtome; i.e., -80°C to - 120°C. For optical examination, a thin section containing the gels is placed on a glass microscope slide with a drop of silicon oil and covered with a glass cover slip. Additional sections are collected for examination via hot stage microscopy and for compositional identification if needed.

After collecting the sections, the polished block-face containing the remainder of the gel is retained. In many instances, gels arise from inorganic contaminants such as the metallurgy from pellet handling equipment, extruders, or components from masterbatches. Examination of these inorganic components are best performed with the blockface sample using a scanning electron microscope (SEM) equipped with an energy dispersive x-ray detector (EDX) [5,6]. In some cases, additives or inorganic residues are present in low concentrations within the gels. A method to enrich the concentration of these materials is to expose the block-face containing the gel to oxygen plasma. Etching will preferentially remove the polymer at a much faster rate than the inorganic materials, enriching the inorganic components for elemental analyses. It must be noted that prior to SEM and EDX analyses, a thin conductive coating like carbon is typically evaporated onto the sample to render it conductive under the electron beam.

Figure 1. Methodology for characterizing defects in polymer films [4].

The next sections will demonstrate various methods of analysis used for common gel types.

Oxidized Gels
The most common type of gel is caused by oxidative processes that crosslink the PE chains. The best way to identify this gel type is by observing them with polarized light and ultraviolet (UV) light sources. Transmitted polarized light microscopy represents an effective technique [7] that can be used to investigate structures in crystalline films. For example, black speck gels were contaminating a multilayer film product. The gels were relatively brittle when cut for analysis. The source was unknown. The detail of a gel is clearly visible using transmitted polarized light, as shown in Figure 2a. Close examination of this gel using epi-fluorescence with an ultraviolet light source caused an intense fluorescence emission, as shown in Figure 2b. This type of emission suggests thermal oxidation and crosslinking of the polymer. Micro-infrared analysis of the gel indicated that it contained oxidized PE and maleic anhydride, as shown by the spectrum in Figure 3 (for clarity, this figure can be found at the end of this paper). This material likely formed on the metal surfaces of the extruder and then flaked off during a minor process instability. The material then flowed downstream and contaminated the film as a gel.

Figure 2. Transmitted polarized light images of a thermally oxidized and crosslinked gel in a multilayer film: a) photograph in polarized light, and b) the gel fluorescing under UV light.

Crosslinked Gels
Crosslinked gels are oxidized gels, but the level of oxidation may not be enough to cause them to fluoresce under UV light. These gels may have a level of crystallinity and thus be birefringent under polarized light. For example, the slightly birefringent gel shown in Figure 4a was studied using a temperature programmable hot stage, polarizing light microscope [7]. The optical melting temperature (Tm) of the gel was measured at 128°C and consistent with the PE used to make the product, as shown in Figure 4b. To determine if the gel was unmixed (highly entangled but not crosslinked), the gel was held above the melting temperature (135°C) and then stressed. A dental tool was used to stress the top of the glass cover slip. Crosslinked gels will appear birefringent, (Figure 4c) in response to the anisotropy of stress distribution in the gel to polarized light. The gel dimensions and shape remained after cooling verifying crosslinking, as shown in Figure 4d. If the gel was highly entangled and not crosslinked, the gel would have disappeared after the stress and cooling were applied.

Gels from Foreign Contamination
The origin of defects causing discoloration in polyolefin pellets can be identified using light and electron microscopy. For example, PE pellets from an in-plant recycle re-pelletizing process contained pellets that were off color and had black specks, as shown in Figure 5a. One of these defects was isolated using the cross sectioning technique, as shown in Figure 5b. The cross section revealed an intense reddish particle that caused the discoloration of the pellet.

Figure 4. Hot stage microscopy of a crosslinked gel in a crystalline monolayer film: a) below the melting temperature, b) optical melting point at 128°C, c) appearance of birefringence after stressing at 135°C, and d) intact crosslinked gel after cooling to 30°C.

Figure 5. Photographs of foreign contamination in pellets of a re-pelletized reclaim stream: a) photomicrograph of discolored polyolefin pellets containing dark defects, and b) transmitted polarized light micrograph of a pellet cross section containing a defect.

SEM and EDX microanalysis were used to determine that the defects contained primarily iron and oxygen, and it likely was iron oxide. Figure 6 shows a backscatter electron image (BEI) of the pellet block-face sample showing the defect causing the discoloration and the elemental spectrum. Metallic based defects can originate from process equipment, railcars used for shipment, pellet transfer lines, and poor housekeeping. The origin of the iron oxide was likely from a storage bin.

In another example, a multilayer film product was experiencing occasional gels. The gels were isolated and the cross sections were collected as shown in Figure 7a. These gels contained highly birefringent particles that resided in the core layer. The outer film layers appeared amorphous and the core layer was slightly birefringent. The optical melting temperature of the core layer was determined to be 123°C while the birefringent gels melted at 265°C. The melting temperature of 123°C was consistent with the PE resin used to produce the core layer. The higher melting temperature material and micro-infrared analyses of the defects indicate that they were foreign contaminants, and they were identified as a polyester resin. The polyester resin was used in another process in the converting plant, and it inadvertently contaminated the PE feedstock.

Figure 6. EDX microanalysis of an inclusion in a polyolefin pellet cross section (Figure 5b). The analysis indicated that the particle was likely iron oxide.

Figure 7. Photographs of gels in the core layer of a three layer film: a) transmitted polarized light, and b) hot stage microscopy was used to determine the melting temperatures of the core resin and defects.

Another common contaminant that produces gels is fibers, as shown in Figure 8. In many cases, these contaminants are cotton fibers from clothing and gloves or cellulosic fibers from packaging materials. Fourier transform infrared (FTIR) spectroscopy is one of the best techniques for determining the chemical functionality of organic based defects in PE films. The infrared absorbance characteristics of the defect were determined using FTIR spectroscopy, as shown in Figure 9 (for clarity, this figure can be found at the end of this paper). The broad absorption bands near 3600 cm-1 to 3100 cm-1 are due to hydroxyl (-OH) stretching vibrations, the C-H vibration stretch is near 2916 cm-1 to 2851 cm-1, and the ester carbonyl group absorption is near 1734 cm-1. Based on the infrared absorption characteristics, the defect in the PE film is a cellulosic fiber with degraded PE resin.

Once the contaminant is identified, the troubleshooter must determine how the material entered the feedstock stream. Process controls must be identified and implemented to mitigate the contaminant source.

Figure 8. Transmitted bright-field image of PE film containing a fibrous gel.

Case Studies

Oxidized gels, carbon specks, and unmixed gels can be created inside the extrusion processing line. Crosslinked gels and black specks occur due to regions in the process that are stagnant and have very long residence times in the extruder. Unmixed gels and solid polymer fragments occur because the resin was not subjected to a high stress level during processing. This section provides several case studies where these types of gels occurred. The technical solutions to mitigate the gels are then presented.

Gel Showers in a Cast Film Process
Crosslinked gels can form in stagnant regions of screw channels, transfer lines, and dies. The time required for these gels to form range from about 30 minutes for linear low density polyethylene (LLDPE) resin up to 12 days for low density polyethylene (LDPE) resin. Stagnant regions can occur at entries and exits of mixers [1] and barrier sections, and they can occur when the metering channel of smooth-bore extruders is not controlling the rate. In these cases, a section upstream of the metering section is rate limiting, causing portions of the metering section to operate partially filled [8,9]. When these channels operate partially filled the main flow is on the pushing side of the channel while the trailing side operates void at first. After a period of time, clean resin gets into the void regions and rotates with the screw for long durations. Eventually the resin will degrade, forming crosslinked gels. Slight process upsets can dislodge this material, allowing the material to flow downstream creating a gel shower in the film.

A film plant was extruding a LDPE resin into a specialty product using a cast film process [8,9]. Due to high demand, a new 88.9 mm diameter, 33 L/D extruder was installed in the plant. Soon after startup the product was acceptable and high quality. After 12 days, the line began to experience intermittent discharges of crosslinked material (gel showers) and carbon specks. Photographs of these gels are shown in Figure 10. In some cases, the gel showers were observed 2 to 3 times per day and would last from 1 to 5 minutes. The gels were clearly crosslinked and were brown in color. The extrudate temperature was higher than expected for the process. The intermittent gels resulted in production downtime due to purging and in numerous customer complaints. A high and costly level of quality control was required to remove the gel contaminated product from the prime product. Due to the high amount of downtime and the high levels of quality control needed, the operation of the new line was considerably more expensive than planned.

Figure 10. Photographs of crosslinked gels in a LDPE film.

It was hypothesized that the extruder was operating partially full due to the low specific rate during operation. To determine if partially filled channels were the root cause of the reduced rate, high discharge temperature, and degraded material, screw rotation was stopped and the screw was removed while hot from the extruder. Examination of the polymer on the screw indicated that in the meter section about half of the channel width on the trailing sides of the flights for all but the last diameter were filled with a dark colored, partially carbonized LDPE resin, indicating that these regions were stagnant. The reduced flow rate caused these regions to be partially filled, creating void regions on the trailing side of the channel. Some of the resin adhered to the trailing side of the screw in the void regions and stayed there for extended time periods, as shown in Figure 11. The resin adhering in the void regions eventually degraded into the dark-colored, crosslinked material. Small process variations dislodged some of this material and caused the intermittent gel showers that contaminated the product. Moreover, compacted solids were found wedged in the channel at the entrance to the barrier section. The wedged material was caused by the relatively large width of the entering solid bed being forced into the continually decreasing width of the solids channel of the barrier section.

Figure 11. Photograph of a removed screw showing the resin flow and degraded resin due to stagnant regions [9].

The technical solution to eliminate this problem was a simple modification to the entry of the barrier melting section. For this modification [8], some of the metal in the melt conveying channel was removed along with a portion of the barrier flight, allowing some solid material to enter the melt channel and reducing the restriction at the entry. By reducing the restriction, the rate limiting step of the process changed from the entry region of the barrier section to the metering section. After the modification was made, the gels were eliminated from the process.

Unmixed Gels
As stated previously, unmixed gels are highly entangled species that are molten when they are discharged from the die, but solidify first upon cooling to produce a gel that appears as a solid polymer fragment. These types of gels are easily removed from an extrusion process by subjecting all molten resin to a one-time high level of stress near the discharge of the extrusion screw. This stress is easily applied using a Maddock-style mixer with a relatively tight clearance on the mixing flight. A film process was producing a monolayer film that had a low level of gels. The gels were tested using hot stage microscopy and identified as highly entangled species (unmixed gels). These gels melted and then disappeared when heated and stressed via pressure smearing using a dental tool, as shown in Figure 12. The unmixed gels were removed by increasing the stress level in the Maddock mixer. The stress level was increased by decreasing the clearance on the mixing flight. The stress level required to disperse unmixed gels depends on the resin and the level of chain entanglement. In past experiences, the stress level required to disperse PE unmixed gels is about 100 to 200 kPa.

Figure 12. Photographs of an un-mixed gel at select temperatures using a hot-stage microscope. The un-mixed gel melted at about 135oC. When the gel was smeared by moving the glass cover slip, the stress was enough to disentangle the polymer chains such that the gel would not reappear upon cooling.

A similar problem with solid polymer fragments occurred for a thermoplastic polyurethane (TPU) resin [10]. For this case, a combination of a lower compression ratio, a longer barrier section with a very small barrier flight clearance, a Maddock mixer with a small mixing flight clearance, and deeper metering channels allowed the TPU resins to extrude at twice the rate and provide high quality extrudates that were free of solid polymer fragments.

The shear stress that the material experiences for flow across the mixing flight of the Maddock mixer can be estimated using Equations 1 and 2. The shear stress level is responsible for breaking up the entangled species. This calculation is based on screw rotation physics [1].

where YM is the average shear rate for flow over the mixing flight in 1/s, N is the screw rotation rate in revolutions/s, ή is the shear viscosity at the temperature of the mixing process and at shear rate YM, Db is the barrel diameter, u is the undercut distance on the mixing flight, λ is the main flight clearance, and tM is the shear stress that the material will experience for flow over the mixing flight.

Carbon Specks in a Film Product
Carbon specks can be generated in the extruder channels and in downstream transfer lines and dies if stagnant regions are present. In general, these regions are not very large like those in Figure 11. Instead, they are thin coverings that occur at the flight radii or at entry and exits of mixing devices [1]. In general, the region will first create small crosslinked type materials that adhere to metal surfaces. With additional residence time, the crosslinked material will form a thin carbon layer of highly oxidized material. When the layer breaks away from the metal, it is discharged as black specks in the PE film. These specks will fluoresce under UV light.

A LLDPE blown film line was experiencing black specks in the product. In order to locate the source, a Maddock solidification experiment [11] was performed where a small amount of a red color concentrate was added to the feedstock resin, after the red color appeared in the extrudate screw rotation was stopped, and the resin was solidified in the channels. A photograph of the experimental sample [12] is shown in Figure 13. Here a thin layer of carbonaceous material was formed at the pushing flight due to the formation of Moffat eddies [13]. Moffat eddies are recirculation or vortices that occur at sharp corners as shown in Figure 14. When fluid is put in motion with top driven cavity flow the main circulation is shown in Figure 14. A secondary circulation is set up in the stationary corners of the channel, creating a low velocity helical eddy that is outside the high velocity flows of the main part of the channel.

Figure 13. Photograph of degradation at the pushing flight for a screw running LLDPE resin [12].

The Moffat eddies that created the degraded resin occurred because the flight radii were too small for the depth of the channel. If the flight radii would have been larger, the Moffat eddies would not have occurred and thus carbon deposits would not have formed.

The Society of the Plastics Industry, Inc. (SPI) guidelines state [14] “unless otherwise specified the root radius will not be less than 1/2 of the flight depth up to 25 mm radius.” Many screws are often designed, however, with flight radii that are very small and approach values that are between 10 and 20% of the channel depth. Previous research [12] has indicated that the SPI guideline as a minimum is appropriate for many resins. But for thermally sensitive resins, radii up to 2.5 times the depth are optimal. Flight radii sizes are shown in Figure 15. When a new screw with radii equal to the depth of the channel was built and installed into the blown film line, the black specks were essentially eliminated.

Figure 14. Two dimensional flows in a screw channel with an H/W = 1 (channel depth / channel width). The arrows show the recirculation flows. The shaded area in the lower right corner is expanded to show the Moffat eddy [1].

Figure 15. Schematic of small (R1) and large (R2) flight radii.

Filler Agglomerates
Some specialty films are produced using masterbatches with high levels of mineral fillers. The filler materials must be compounded with a properly designed process such that fillers are not agglomerated prior to dispersion into the base resin. If agglomerates are produced and contained in the masterbatch, then they are essentially impossible to disperse in the filming process, leading to optical defects in the film. For example, a compounding operation for making a specialty resin from a high impact polystyrene (HIPS) resin and specialty filler chemical was not designed properly. Here the filler chemical was partially agglomerated prior to the melting process in a twin-screw extruder. As shown in Figure 16, the resin was colored black and the filler chemical was white. These white agglomerates could not be eliminated in the final plasticating process (injection molding in this case) and created defects in the product. The goal for this type of application is to produce masterbatches that are free of filler agglomerates since the final film making extrusion process is incapable of dispersing them.

Figure 16. Photographs of specialty HIPS resin pellets made using a poorly designed process. The white specks are filler agglomerates: a) 1x magnification, and b) 4x magnification.


Gel defects are common in PE film products, and they can originate from many different sources, causing a reduction in the product quality and sometimes stopping production. Gel types, identification protocols, and mitigation strategies were presented in this paper. Mitigating or eliminating gels quickly via the best technical solution will reduce costs to the plant and maximize profits.

The equipment and techniques required to diagnose properly many of the gel types can be expensive and require highly trained people. Many small converters will not be able to afford the development of these types of capabilities. Most resin suppliers, however, have the capabilities and are willing to aid customers on the identification and mitigation of the gels.


This paper describes the different type of gels that are likely to occur in polyolefin film products, techniques for identifying the gel type, and technical solutions to mitigate them from single-screw extrusion processes.


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2. T.I. Butler, “Gel Troubleshooting,” in “Film Extrusion Manual,” Chapter 19, Edited by T.I. Butler TAPPI Press, Atlanta, GA, 2005.
3. J. Scheirs, “Compositional and Failure Analysis of Polymers,” Wiley, New York, 2000.
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9. M.A. Spalding, SPE-ANTEC Tech. Papers, 50, 329 (2004).
10. K.S. Hyun and M.A. Spalding, “Troubleshooting TPU Resin Extrusion Rate Limitations Due to Solids in the Discharge,” SPE-ANTEC Tech. Papers, 58 (2012).
11. B.H. Maddock, SPE J., 15, 383 (1959).
12. M.A. Spalding, J. Dooley, and K.S. Hyun, SPEANTEC Tech. Papers, 45, 190 (1999).
13. H.K. Moffat, J. Fluid Mech. 18, 1 (1964).
14. “Recommended Dimensional Guideline for Single Screws,” The Society of the Plastics Industry, Inc.

Figure 3. The micro-infrared spectrum of gel shown in Figure 2. The spectrum suggests it is an oxidized polyethylene gel containing maleic anhydride.

Figure 9. FTIR spectrum of defect in a polyolefin film. The spectrum indicates that the gel is cellulosic fiber and degraded PE resin.

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