- February 01, 2007, David J. Bentley, Jr., Editor
Stretch Film Production — Control Of Extrusion And Casting Variables
by Mark A. Jones, Battenfeld Gloucester Europe
APPLICATION: Using this qualitative and practical guide to the effect of line settings on film properties with observations on common problems encountered when these settings are adjusted too far in any direction provides better stretch film production.
Several studies have shown the relationship between extrusion line settings and cast stretch film properties. Because of variations in line design, replicating the line settings published in such studies will rarely reproduce the film properties that were quoted. When faced with the need to tune a commercial line in the field, it is more useful to have an understanding of the effect of various process conditions on film properties than a list of “recommended” settings.
Among the variables mentioned in this paper, grade selection of polymer is critical to achieving the correct mix of price and performance in a film. Much data is available to assist in this choice. The effect of polymer choice on line operation also requires consideration. High performance resins often allow performance enhancement, down-gauging, or both. Such changes can have consequences for line operation.
Film properties depend critically on line settings. One should experiment with line settings to optimize film properties. Careful adjustment can allow down-gauging or selection of a lower cost resin with no loss of film performance. Always ensure that line settings are thoroughly documented and followed. Small and seemingly insignificant changes to settings and conditions can have an unexpected and major effect on film properties. Achieving film properties depends on line design and maintenance. Line design is usually set by the manufacturer. Specific upgrades may be worth considering. Effective maintenance of the line is necessary. Modern lines especially use a wide range of sensors to provide feedback to the operator and a computer-based process control system. Without this feedback, the fine control required to give a consistent product is more difficult to achieve.
Understanding Why Adhesion in Extrusion Coating Decreases With Diminishing Coating Thickness, Part II: Non-porous Substrates
by Barry A. Morris, DuPont Packaging and Industrial Polymers
APPLICATION: In the extrusion coating process, adhesion to aluminum foil and other non-porous substrates decreases with decreasing coating thickness. Modeling and experimental results show that cooling in the nip and stress have the greatest impact.
Extrusion coating involves extruding a molten polymer through a flat die onto a fast moving substrate and quenching with a cold roll. The performance of the resulting structure depends on a number of processing and polymer related properties. Of particular importance is the adhesion between the coating and substrate. As the coating thickness decreases, the adhesive strength generally decreases. Part I of this study looked at adhesion to porous substrates and found that adhesion relates to penetration of a polymer into pores of a substrate.
This work covers adhesion to non-porous substrates. Although the focus is adhesion to aluminum foil, the conclusions should be valid for any non-porous substrate. Adhesion to nonporous substrates requires good wetting of polymer to the substrate. In some cases, chemical interactions occur at the interface. The motivation for this study began with observations from an earlier study involving blending an additive into LDPE to enhance its adhesion to foil. That study involved a statistically designed experiment that varied the coating temperature at the die exit, coating thickness, time in the air gap, and the percentage of the adhesion-enhancing additive in the blend. It measured peel strength to foil.
Several mechanisms are possible for the observed drop in adhesion with thickness. These include less time in the air gap (reduces oxidation and increases stress), greater cooling in the air gap (increases viscosity and reduces wetting), faster cooling in the nip (reduces time for wetting and chemical interaction), and greater stress during drawing. The study reported in this paper proposes a hypothesis for each of these potential mechanisms and tests the hypothesis using a combination of modeling and experiments.
Polyethylene Gels: A Primer
by Norman Aubee, Rolf Saetre, and Tony Tikuisis, NOVA Chemicals Corporation
APPLICATION: Film imperfections such as gels commonly occur in the extrusion process. The paper reviews the different types of gels typically found in polyethylene, a suggested troubleshooting process, and methods to reduce specific gel types.
In the context of this paper, a gel is a visible dome-shaped imperfection in the film matrix due to the embedding of an incompatible material. The size of a gel can vary from 50 microns in diameter to several millimeters. Common types of gels include polyethylene (cross-linked, high density, high molecular weight), oxidized polyethylene, other polymers (EVA, ionomers, cyclic olefins, etc.), fibers (cellulose), additives, and miscellaneous materials such as metal shards, dust particles, etc. The presence of a gel causes a bulge on one or both sides of the film. The bulge represents the observed size of the gel whereas the size of the embedded material is significantly smaller.
In a troubleshooting process, collecting appropriate information to develop an understanding of the scope and severity of the gel issue is of key importance in enabling a processor to locate the cause in a timely manner. Determining the source of the gels is paramount to developing procedures and practices to prevent reoccurrence. The paper lists several questions that will help in developing an understanding of the problem.
The next step in the investigation process that can be performed while collecting information on the scope of the problem is gel characterization. Once the gel type is known, one can focus on steps to investigate and eliminate that specific type of gel. If no gel characterization capabilities are available on site, a sample of film containing the gels should be forwarded to the raw material supplier for characterization. Analysis of five to ten gels will provide an indication of the types of gel present. If an improved level of confidence is necessary, the total number of gels characterized should be increased to twenty.
A processor does not have the luxury of waiting for the gel characterization results if a third party performs the analysis. Some type of immediate corrective action to improve the gel problem must therefore occur so that commercially saleable material results. A systematic approach to troubleshooting is strongly recommended. Simultaneously changing multiple parameters can lead to incorrect conclusions. We recommend using an information-gathering step — a formal problem solving process — before making changes to a process.
Gel issues can result in significant downtime and cost processors money. Using relatively simple techniques to characterize gels with a thorough and well-designed troubleshooting process will allow a processor to understand and resolve such issues quickly. This will save time and money. Implementing best practices and procedures based upon the findings from previous troubleshooting investigations can prevent or reduce the reoccurrence of certain types of gel problems.
Fresh Cut Produce Packaging: New Developments In PLA Films
by Rich Eichfeld, Plastic Suppliers Inc.
APPLICATION: Advancements in PLA films are in high demand from suppliers, converters, retailers, and end users who are seeing the clear benefit of using such environmentally-friendly bio-products.
Film requirements in the produce industry will vary depending upon product and shelf life requirements. The market in the United States is different than other parts of the world and is more difficult because a longer shelf life is necessary. The ideal sustainable package would be made entirely of PLA film and perform exactly as the produce type requires. Compounding this difficult situation is that each type of produce has different respiration, transpiration, and ripening requirements that maintain its survival. Various factors influence these processes. They include temperature, physical damage, and concentrations of water vapor, oxygen, carbon dioxide, and ethylene. The likelihood of development of a single PLA film that could provide a flawless package for all forms of produce while providing a perfect barrier for respiration, transpiration, and ripening processes is small. Different grades of PLA films can nevertheless be developed to satisfy specific needs.
Advancements are occurring on a daily basis to perfect PLA films for many industries including produce. Produce companies understand the benefit of using such a sustainable packaging film and find that this packaging provides marketing leverage where petroleum-based films do not. Consumers will notice that the brand owner is respectful of the environment and conscious of the need to contribute to its preservation. This environmental goodwill can be sufficient to clench the sale.
The sale and use of PLA films in the produce industry have been slow to come because they have fallen short of barrier and machining requirements in the past. Strong demands from brand owners and retailers have prompted a unified effort in making these films work for produce. Victories in produce packaging are still occurring as PLA films are tailored to specific fruits and vegetables. PLA films are as good as and oftentimes better than petrochemical-based films in such areas as machining, printing, product finishing, overall appearance, and environmental friendliness. Yesterday's disadvantages in PLA film function are today's successes as manufacturers, scientists, and technicians collaborate to improve the properties of the film Developments in PLA films have made them suitable for produce packaging because of their optimal performance values accompanied by stellar energy and environmental benefits.
Receive technical information delivered to your computer every other Monday from TAPPI's PLACE Division. Subscribe on-line at www.tappi.org by selecting “Newsletters” under the Publications/Bookstore heading.
To obtain the complete version of these papers, go to www.tappi.org, and select “the PLACE” under the Publications/Bookstore heading.