Microlayer technology for thick film and sheet that promises to extend the shelf life of retort, hot fill, and flexible food packaging, while increasing formability in thermoforming, has been developed by Extrusion Dies Industries, LLC (EDI). The layer multiplier is a special tool that can be engineered to multiply some or all of the layers within a coextrusion “sandwich” provided by a feedblock. The resulting microlayer structure then passes into the manifold of an extrusion die, where it is transformed into film or sheet of target width and thickness profile. No matter how many microlayers there are in the structure, the overall thickness is no greater than that of a conventional coextrusion, and the structure contains the same amount of raw material.
At its Technology Center, EDI is carrying out the first scientifically rigorous study of the benefits and limitations of layer multiplication, according to Gary D. Oliver, vice president of technology. “One result thus far is the development of a technique for selective multiplication of the barrier-resin layer of a multilayer film or sheet structure. By transforming a single layer of a resin such as EVOH into several microlayers, it is possible to reduce oxygen transmission rate (OTR) by 60 to 80%. While EVOH is a crystalline, relatively brittle material, replacing a single thick layer with multiple microlayers increases formability for thermoforming and flexibility for vacuum skin packaging.”
The resulting benefits for food packaging include:
? Prolonged shelf life. Besides reducing OTR, layer multiplication substantially reduces total oxygen ingress over an extended period.
? Enhanced quality and lower material consumption. In the case of deep-draw containers where corner-thinning has been a problem with less formable materials, for example, multiplication of the EVOH layer may eliminate the need to offset thinning by increasing sheet thickness.
EDI extruded the sheet and used a commercial cup mold to solid-phase pressure form sheet with 1, 4, 8, 16 EVOH layers. All sheet structures were 50 mils in thickness, with thick (41 to 43%) skin layers of polypropylene and tie layers between the skins and the EVOH core. Barrier resins included a standard general-purpose EVOH and a “retort” grade of EVOH. 30 days after retort, cups with a single layer of the “retort” grade of EVOH exhibited 3-6 times higher oxygen transmission rates (OTRs) than cups where a layer of similar material had been multiplied. In that same period, total oxygen ingress was also 3-6 times greater. “Layer multiplication is especially promising for sheet and thick film packaging such as rigid retort and hot-fill containers, stand-up retort pouches, and vacuum skin packaging for meats,” said Oliver. “Thickness matters—particularly in the case of barrier microlayers. Our researchers have found that barrier properties fall off as layer thickness goes below 1 micron. We recommend a target thickness per barrier layer of 4 microns.”
Choice of barrier materials also matters, according to Oliver. “While barrier properties improved markedly with use of a retort grade of EVOH, we found that performing exactly the same tests but with a general-purpose EVOH yielded portion cups with markedly higher OTR and oxygen ingress.”