Mistakes in science often lead to inventions. Polymer science is no exception. Around 1961, a laboratory scientist processing fluoroelastomers and polyolefins on the same extrusion line, discovered that for some reason he was able to process the most difficult polyolefins obtaining good appearance at higher throughput rates. An important patent was granted as a result of this work; the patent claimed the addition of small amounts of fluoropolymers (0.1 – 2.0 wt %) to polyolefins gave amazing processing benefits . This is how fluoropolymers and fluoroelastomers gained their fame as polymer processing aids (PPA).
The beginning – Goodbye to melt fracture
After the patent rights expired around 19841, a flood of activity centered around this modification to polyolefin processing. The early focus was on Linear Low Density Polyethylene (LLDPE) blown films and high-speed wire & cable extrusion. This is a result of the well-known challenges to process LLDPE. At that time, several polymer manufacturers, high-end compounders and film processors began contacting suppliers of fluoroelastomers made from vinylidene fluoride (VF2) and hexafluoropropylene (HFP). They began creating high quality low density polyethylene products at up to triple previous production rates on extrusion machines that had previously been only used to process LLDPE using narrow die gaps.
Figure 1: Split screen photo of hexene-based LLDPE plus masterbatch containing 1% of Kynar Flex® 2821 PPA: left side before adding the PPA and right side almost immediately after adding the PPA
A split screen photograph taken in 1987 (Figure 1), shows the immediate effects of adding a fluorinated PPA to a LLDPE blown film, which has been running for 40 minutes full of melt fracture. It cleared up in less than 4 minutes after the addition of a 1% masterbatch of LLDPE + Kynar Flex® 2821-00. With such dramatic improvements, it is understandable why polyethylene extrusion and polyolefin resin manufacturers and compounders got excited.
These benefits resulted in a flurry of research on fluoropolymer PPAs that produced rapid results. Fluorinated resin manufacturers were exploring new monomers, varying VF2 and HFP ratios, varying molecular weights, changing levels of PPA addition, studying the effects of particle size, investigating the importance of particle distribution, and analyzing the effects of processing temperature while using fluorinated PPAs.
The next important patented breakthrough resulted from the study of interactions of common polyolefin additives in blends with PPA. Researchers found that polyethylene glycol had a synergistic effect on the performance of the PPA. It helped to reduce the levels of PPA required to minimize sharkskin defects, torque, gauge control, processing temperatures and pressure, while increasing processing throughput, and film transparency2.
During the same year, the Kynar® PVDF PPA team at Pennwalt (now Arkema) commissioned a blown film extruder. The technical staff of the supplier stated that based on existing knowledge it would be impossible to run LLDPE on a 2.5”, 30/1 LD extruder with a 10” diameter die and a 0.025” die gap. Figure 2 shows the actual reduction trend of pressure versus temperature in the virgin hexene-based LLDPE having less than 1 MI (Melt Index) with the same resin with and without 1% Kynar Flex® 2821 masterbatch. The masterbatch compound contained a let down ratio of 22:1 into the host resin resulting in 450 ppm concentration of Fluorinated PPA. The results were outstanding but typical of the results that film processors were obtaining in the 1980s. It is interesting to observe that the pressure reduction shift is almost a parallel line with 800 – 1200 PSI pressure reduction regardless of the melt temperature of the extrusion.
Figure 2: Extruder Pressure Comparison of Virgin LLDPE and LLDPE with 450 PPM Fluorinated PPA at a range of Melt Temperature
In the same study, the output of clear film without sharkskin (surface melt fracture) had more than tripled (i.e. from 66 pounds per hour to 204 pounds per hour) at the same melt temperature. Output was limited by the equipment’s throughput since screw speed could only be increased from 34 RPM to 102 RPM (see Table 1).
Table 1: Processing Parameters Comparison of LLDPE Control vs. LLDPE with 450 ppm of Fluorinated PPA
Figure 2 and Table 1 show how fluorinated PPAs bring large benefits to LLDPE processors through benefits that include equipment longevity, production capacity improvement, higher strength, improved appearance, and reduced scrap. In addition, these additives enabled the use of existing equipment to be used to manufacture products made from more than one version of a polymer. Figure 3 gives an example of a microscopic view of a PE film with a visible melt fracture that was processed without a fluorinated PPA and a comparison with the same PE resin extruded with just 300 ppm of fluorinated PPA.
As the industry soared along with this technology, manufacturers with less obvious issues began considering that fluorinated PPAs could have incremental benefits that would differentiate them from their competitors. Compounders started to supply PPA compounds for extruding LDPE, HDPE, UHMW-PE and Cross-linked polyethylene (PEX) that could improve the appearance of marginally smooth products. Injection and blow molders could fill molds by up to 8% more easily, without going to a higher melt flow rate resin with lower performance properties. In all types of processing, other processing aids like stearates that were affected by chemicals and sunlight could be replaced by fluorinated PPAs, thus reducing longer-term exposure problems of the polymer. The use of fluorinated PPAs has been extended to applications with Polypropylene, Polyurethane, and PVC in addition to the various polyethylene resins.
Figure 3: Microscopic comparison of extruded metallocene PE film before (left) and after addition of 300 ppm of Kynar® 5300 PPA (right)
Fluoropolymer producers have made great efforts to focus on the form of the additives to allow good mixing with any synergists to obtain the best mixing and proper dispersion of the small amounts of PPA to gain the desired effects. The dispersion of PPA in the polymer matrix is critically important and should be considered carefully. Much has been written on combining the concentrates and the fluorinated PPA. There are many patents in this area; one important patent is US 8053502 B23 which reports that PPA reduces or eliminates extrusion defects without causing deterioration in the yellowing index of the extruded resin. If the polymer or the master batch containing the PPA does not match up well with the melt flow rate of the host polyolefin, the PPA can lose its ability to coat the die and aid in the reduction of melt fracture. In other words, the PPA can add to the problem. Studies have shown that surface melt fracture is a die exit phenomenon4. It has been reported5 that visualizing polymer/die interfaces in high resolution images of PPA/PE blends can show that PPA coating occurred in 3 stages.
The Future - Can PPA help in reducing die build-up?
Die build up is the newest phenomenon that fluorinated PPAs are helping to avoid. Companies extruding pipes and profiles with relatively slow extrusion speeds normally would not see any production speed benefit or increased product gloss by using a fluorinated PPA, yet they are now turning to these additives to reduce unwanted buildup on the die that can compromise the integrity of the final product. Die buildup can do more than create unsightly residue on the extrusion. It can attach to the product and generate a weak point. It can accumulate and degrade at the die creating safety issues. Die build up can be an acute problem in products containing high levels of fillers and additives.
Research is continuing on fluorinated PPAs to address the concern that die build up adds to the complexity of the process. Processors still want good film, wire, or pipe with lower stresses on their extruder, but also, they want to avoid the negative effects of die build up. PPAs that work best for processing effects may not be optimal for reduction of die build up, and some PPAs that may reduce die build up well may not be very effective in reducing melt fracture. As a result, this has become a major area of study in 2018, leading to the introduction of newer grades of fluorinated PPA materials. These are designed with specific melting points, molecular weights, and ratios of co-monomer, and VF2-based polymers rather than elastomers. Figures 4 & 5 show examples of die build up from a capillary and tube extrusion without PPA and Figure 6 shows a comparative reduction over time of die build up with the use of a specific fluorinated PPA.
Figure 4: Example of die build up in highly filled Polyethylene Figure 5: Common problem of die build up in tubing extrusion
Figure 6: Die build up in a capillary of highly calcium carbonate filled PE without PPA (left) and 30 minutes after the introduction of 1260 ppm of Kynar® 705 PPA (right)
Die build up is caused by many variables such as concentration and weight of additives, temperature of extrusion, rate of extrusion, molecular weight of the host resin, and design of the dies. The selection of the loading of the fluorinated PPA becomes even more critical to the elimination of melt fracture. Too much PPA may add to die build up, and not enough PPA may mean the effect is severely restricted by the other components in the compound. Additionally, if the choice of molecular weight of the fluorinated PPA is not made very carefully, too much PPA can pass through the process without getting to the die wall in time to compete with the other additives in the stream. It is common to add as much as 1200 ppm of fluorinated PPA to reduce the die build up. In contrast, some processors have eliminated melt fracture and improved pressure reduction in the melt with levels as low as 150-400 ppm.
Since the idea of fluorinated PPAs used in polyolefins was accidentally discovered in 1961 and was used by one company for about the next 23 years, the market for these products is now in the thousands of tons globally. Today, the breadth of its use is commonly presented at conferences on every continent. What started out as a dramatic processing improvement additive for one polymer (LLDPE), has emerged as a standard processing additive for countless others. The current trend is for continued incremental improvements in either the processing, appearance or performance properties of the final polyolefin product in injection molding, blow molding, extrusion, and casting.
1. Blatz, P.S., US Patent 3,125,547 (March 17, 1964)
2. Duchesne, D., Johnson, B. V., US Patent 4,855,360 (August 8, 1989)
3. Bonnet, A., Laffargue, J., Triballier, K., Beaume, F., US Patent 8053502 B2 (November 8, 2011)
4. Cogswell, F.N. ; J. Non-Newtonian Fluid Mech. 2, pp. 37-47, 1977
5. Meillon, M.G., Morgan, D., Bigio, D., Migler, K., ANTEC Proceedings, PP. 96-100, 2005
David A. Seiler received his B.S. in Chemical Engineering from Penn State University in 1983. He is employed for Arkema Inc as the Americas Business Manager Industrial / Global Manager Polymer Processing Aids, and has worked in the area of Fluoropolymers for 34 years.
Jason Pomante graduated from Lafayette College in 2002 with a B.S. in Chemical Engineering and MBA from Saint Joseph’s University in 2010. Jason has worked at Arkema for 15 years and is currently the North American Market Manager in the Technical Polymers Division.
Robert Lowrie graduated in May of 2017 with a B.S. in Chemical Engineering from Villanova University. He is currently a Technical Marketing Specialist for Arkema Inc. in the Technical Polymers Division.
|David A. Seiler||Jason Pomante||Robert Lowrie|