Kevin Callahan is chief operating officer of both Charlottesville, Va.-based Barrier Plastics Inc., and BP Polymers LLC, having founded both companies in 2008. He has led the development of Baritainer® containers made by Barrier Plastics. On Page 12, he and Christopher Lind examine the performance of Kortrax, a polyamide resin, in barrier layers. He can be reached at firstname.lastname@example.org. Download
The topic of safe, effective barrier properties for plastic containers was thrust back into the spotlight in mid-January when the U.S. Environmental Protection Agency published new findings related to per- and polyfluoroalkyl substances (PFAS) contamination. The EPA stated that “Through a coordinated effort with both the Commonwealth of Massachusetts and a pesticide manufacturer, the agency has determined that fluorinated high-density polyethylene (HDPE) containers that are used to store and transport a mosquito control pesticide product contain PFAS compounds that are leaching into the pesticide product.”
While acknowledging its investigation is still in the early stages, the agency urged companies using fluorinated containers to “engage in good product stewardship and examine their distribution chains to identify potential sources of contamination.” Therefore, we are revisiting an alternative barrier technology here.
Barrier properties add functionality to plastic containers. They allow the use of plastic packaging to hold permeating solvents and permeation-sensitive chemicals that historically had been contained in lined or unlined steel or tin-plated containers. This article reviews the history of one such barrier, an engineered polyamide (PA) additive for polyolefins, Kortrax Barrier Resin (BR).
We review recent developments that improve and resolve issues with impact resistance, compatibility, and delamination. We also assess how the sustainability and recyclability of plastics modified with this PA are a considerable improvement over alternatives while maintaining the integrity of the packaging and its contents.
The Need for Barrier Properties
Barriers in plastic packaging are designed to impede or prevent the migration of chemicals from the contents to the outside, and/or impede or prevent the ingress of oxygen, carbon dioxide, or water vapor. Inhibiting permeation out of the container improves the shelf life of the material by protecting the formulation. Solvents and carriers that escape through the wall degrade the formulation of the contents. This also may necessitate adding more active ingredients or carriers than is ideal to protect against permeation. The permeants can cause container failure and the ingress of gas and water vapor can damage delicate formulations.
Kortrax BR is currently used by makers of agricultural chemicals, food additives, consumer health and beauty aids, automotive fluids, and fuel additives. The pharmaceutical industry also takes advantage of Kortrax BR for its ability to protect against water vapor transmission (WVTR) and oxygen transmission (OTR).
History of Kortrax Engineered PAs
Plastic packaging has long been plagued by permeability issues that lead to container failure and the collapse of stacked drums in warehouses. Steel or metal containers resolve this issue but are heavier and are subject to corrosion and wide fluctuations in raw material costs.
PA gained favor as a barrier additive that required special processing parameters but could be used with existing machinery. The problem with older PA technology was that it was highly sensitive in terms of getting the heat right — PAs used for this application generally have higher melting points than HDPE — which could slow throughput. Further, it often resulted in delamination of the PA-HDPE layer and yellowing of the bottle or jerrycan.
Ethylene-vinyl alcohol (EVOH) was developed as an effective barrier for solvents and WVTR and OTR in film and injection molded and blow-molded parts. Products made with EVOH needed to have five or more layers to sandwich the EVOH and its top and bottom tie layers in the PE matrix. Instead of one or two extruders, five or more extrusion systems were needed, which greatly increased the cost.
When BP Polymers was incorporated in 2008, it took the extant PA chemistry and started to adapt a PA6 formulation for a barrier resin additive that would offer the lightweight, corrosion resistance, and BPA-free formulation of HDPE while offering the same or better solvent and gas resistance as an EVOH system.
At the time, fluorination was also in favor but, as seen with recent developments in environment and health, there is concern of fluorine providing precursors for “forever chemicals,” such as perfluorooctanoic acids (PFOS) and other recalcitrant fluorine compounds. This further impelled the development of a fluorine-free alternative. For traceability, BP added an FDA-grade UV tracer to Kortrax BR that can easily be spotted with a UV flashlight. In addition, Kortrax is formulated for enhanced molecular bonding between the PA6 and the base PE resin. This enhancement improved cold impact resistance and prevented delamination during the deflashing process.
Permeation of materials through PE to the atmosphere is sorption of the material into the product contact layers of the container, diffusion migration through the PE, and desorption and diffusion into the atmosphere. Barriers act to inhibit the initial sorption of the material, preventing the diffusion and migration through the PE matrices; provide a contiguous layer of less-permeable polymer within the matrix to block or at least slow the diffusion through the PE; or provide multiple discontinuous layers of impermeable or less permeable material to block and slow the diffusion of permeating chemistry.
The Kortrax BR-modified PA technology uses the latter approach — creating a tortuous path that inhibits diffusion and migration.
Effects of Permeation on Plastic Packaging
PE packaging exposed to permeating chemicals may fail within the designated shelf life of the contents. The PE may be softened to the point where it panels or “elephant foots” under load. Severe paneling can lead to the collapse of stacked packages, or in combination with stress cracking, cause a containment failure.
Migration of solvents out of the container can cause label-adhesion failures. This migration can also change the composition of the formulation of the contents enough to affect performance.
To counter loss of active ingredients or to counter the adhesion or sorption of the active ingredient to the inside container wall — known as “scalping” — barrier layers can be added.
Oxygen or water vapor ingress can also have a negative impact on the color of a product. Active ingredients that are oxygen- or water-sensitive (water reactive) have historically been packaged in steel or other metal containers to prevent OTR and WVTR.
Barriers –– Pro and Con
Fluorination is a traditional barrier. To apply it, the plastic container is placed in a chamber that is heated and filled with fluorine gas. The fluorine produces a layer on the inside and outside that is more or less impervious for at least the shelf life of the container and its contents.
• Pros: Traditional. Does not impact processing while recycling the plastic. Can have many levels of protection. Improves label adhesion. Very good for solvents.
• Cons: Performed by third-party contractors that control costs and technology rights. The potential health effects of fluorinated compounds (PFOS, PFAS, etc.) may preclude their use in human-contact applications. Does not prevent the ingress of gases. Legal/ liability exposure, given the statutory and regulatory environment.
EVOH barriers use a layer of EVOH and two tie layers for products with five or more layers.
• Pros: Provides decent solvent permeation and excellent WVTR and OTR reduction.
• Cons: Cost. Difficult to recycle/reprocess. May involve additional processing hassles and expertise.
Nanosilicate additives are blends of PA and nano silicate clay. They are added to the process through the blow molding machine’s existing additive feeder.
• Pros: Ease of feeding. Older, familiar technology.
• Cons: High letdown ratios (LDRs) of up to 40 percent to provide the same barrier as EVOH or fluorination. May reduce UN/DOT performance for the transport of hazardous materials. Nanosilicate can wear screw flights and extruder barrels. Expensive. Nanosilicate products are not FDA-approved. Silicate clay may inhibit recycling and/or reprocessing, resulting in disposal instead of reuse.
Kortrax BR is an engineered PA specifically formulated for compatibility with polyolefins and HDPE. It is a single component designed to be fed into monoor multilayer systems.
• Pros: Easy one-component additive. Straightforward processing adjustments. Very good solvent and permeation control. Good or very good WVTR and OTR reduction. LDRs vary with aggressiveness of desired barrier, but 5 percent to 12 percent is most common. Does not impair recycling or recyclability of plastic.
• Cons: No current formulations for injection, film, rotomolding, or vacuum/thermoforming. High LDR may reduce cold impact resistance (not in every container design).
Blow molding the Kortrax BR product requires a short learning curve to adjust heat and throughput for optimal melting, mixing, and distribution to provide the discontinuous layers affecting the barrier. The processing manual provides details, but here are a few items to allow the reader to determine their situation.
The mixing portion of the extruder screw should provide enough shear to effectively distribute the Kortrax BR into the HDPE. “Pineapple” types of high-shear mixing ends are not recommended due to the amount of shear, which disperses the barrier resin without enough time for it to redistribute and layer before extrusion.
The heat needs to be gradually increased along the barrel in stages or zones from 173 degrees Celsius maximum at the feed zone to not more than 223 degrees Celsius at the head — all depending, of course, on the heat generated in the barrel by the flights and barrel configuration. Experience has proven these numbers can be reduced in more-efficient extrusion systems.
Parison wall distribution systems (PWDS) are helpful in maintaining a proper profile since the higher heat lowers the viscosity of the parison. Mold closing speeds may be slowed significantly, if necessary.
HDPE resins currently being used with Kortrax BR have the following properties: high-load melt index (190 degrees Celsius at 21.6 kilograms) 2.5 to 12 grams/10 minutes; density 0.940-0.960 grams/cubic centimeter; monomodal and bimodal; melt index for smaller parts 3 grams to 10 grams/10 min (190 degrees Celsius at 2.16 kg). Whether the HDPE is made via the slurry loop, gas-phase or pressure-catalysis processes has no effect on its compatibility with Kortrax BR.
Improving the functionality of plastic packaging to allow for more reuses and keeping the contents in the containers rather than allowing permeation into the environment is a good step. Barriers provide this to a degree depending on the choice of barrier.
Reusability or recyclability of the container is another factor. Can the containers be reused, and the plastic be reused in another form? In this area, fluorination and Kortrax BR have the advantage of not impacting reprocessing.
Fluorination and fluorinated plastics can contain perfluorinated complex organic chemicals with severe human health implications. Technology exists to remove some of those chemicals and their precursors from the recycling stream, but how to sort and identify them is a concern. Kortrax BR has no such chemicals, precursors or end products and does not have fluorine compounds added.
A major producer of food and flavoring ingredients had used steel drums. Repeated lining failure and seven-figure product returns prompted a look at plastic, which, because of paneling issues, had never been previously considered. A study on the sustainability of a steel drum versus a plastic drum made with Kortrax BR showed that the plastic drum had a 58 percent reduction in carbon footprint; a 38 percent reduction in fossil fuel depletion; and a 60 percent to 90 percent reduction in other properties, like the creation of inhalable particulate matter (PM10), acidification (from sulfur dioxide or its equivalents) and photochemical oxidation.
The quest for optimal barrier properties and performance continues as BP Polymers continues its R&D efforts. In progress are the evaluation of new technology for newer formulations, even though the current iteration has produced excellent results; new PA chemistry and copolymers of PA6, PA66, PA12 and their modifications; improvement of OTR and WVTR to extend shelf life of package contents; new cold-impact modifiers for higher UN ratings; and formulations under test with LLDPE for rotomolding. On the horizon are injection and thermoforming additives.
Barrier properties can be achieved in plastic packaging through multilayer and multicomponent technology such as EVOH; chemical enhancement of surfaces like fluorination; or the addition of advanced formulations of PAs that produce layers within the PE matrix.
Fluorination is not an answer for gas permeation and has issues with PFOS and cost. EVOH is a reliable performer but has issues in cost and sustainability and is not quite as good as a strong solvent barrier.
Kortrax BR — which already is being used to mold jerrycans as well as various shapes of bottles for detergents, automotive fuel additives, and pesticides — is the result of 12 years of product development and product applications. It provides a more sustainable and cost-effective alternative to metal containers, fluorination, and EVOH-type technologies.