Microlayer structures bring big boost 
in barrier and formability to food packaging

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. 

While a layer multiplier can be designed to produce dozens of microlayers, one of the most promising applications involves selective multiplication of the layer produced from a barrier resin such as ethylene vinyl alcohol (EVOH). At a process laboratory in EDI’s Technology Center in Chippewa Falls, WI, the company has carried out a series of tests involving multiplication limited to the EVOH core layer in sheet used for thermoforming single-portion retort cups, such as those for packaging fruit cocktail and diced peaches. EDI extruded the sheet and used a commercial cup mold to solid-phase-pressure-form sheet with one, four, eight, and 16 EVOH layers.

All sheet structures were 50 mils in thickness, with thick (41%-43%) skin layers of polypropylene and tie layers between the skins and the EVOH core. Barrier resins included a standard general-purpose EVOH as well as a grade suitable for retort.

Thirty days after retort, cups with a single layer of the retort grade of EVOH exhibited three to six times higher oxygen transmission rates (OTR) than cups where a layer of similar material had been multiplied. In that same period, total oxygen ingress was also three to six times greater (see table at left).

By transforming a single thick layer of the crystalline, relatively brittle EVOH polymer into four thin layers, the multiplier demonstrated another important benefit—increased formability in thermoforming and greater flexibility in skin packaging. In general, this means greater quality assurance and less material consumption. In the case of deep-draw containers, where corner-thinning is a problem with less formable materials, multiplication of the EVOH layer raises the possibility of conserving raw material.

Why more is better
EDI’s research also yielded these insights on how to maximize the advantages of layer multiplication:
• The thicker the structure, the better the barrier properties. 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. Thickness matters particularly in the case of the barrier microlayers. EDI researchers have found that barrier properties fall off as layer thickness goes below 1 µm; they recommend a target thickness per barrier layer of 4 µm.
• Results vary with the choice of barrier materials. As noted above, barrier properties improved markedly with use of a retort grade of EVOH, but EDI found that performing exactly the same tests with a general-purpose EVOH yielded markedly different results. Some time after the seventh post-retort day, portion cups with four barrier microlayers demonstrated higher OTR and oxygen ingress.
• Results vary with the location of the microlayers. Skin layers, which are in direct contact with the walls of the flow channel, are subject to greater shear forces as the structure moves through the die—forces that could deform and disrupt exceedingly thin microlayers.

In another example of the importance of location, microlayer structures with contiguous layers of EVOH and nylon actually exhibited higher OTR than conventional structures with these materials. Because the two materials are very compatible, the nylon constrains the crystalline structure of EVOH—the key to its barrier properties. EDI has found that enhanced barrier can be achieved by using what it calls a “protective boundary layer” between the EVOH and nylon layers. The material tested for this layer in EDI’s research is maleic anhydride-modified polypropylene, conventionally used as a tie or bonding layer in coextrusions.

Applications
EDI’s experience with layer multiplier technology goes back a number of years, beginning with work carried out under a U.S. Defense Dept. contract for developing long-shelf-life containers for military food supplies. This government program continues, with EDI perfecting process technology for high-barrier trays and heat-sealed lids to be used in what are called Unitized Group Ration trays (UGRs)—packages for entrees that can be heated onsite during military operations.

In the process of carrying out its work with these and other microlayer structures, EDI has developed an advanced layer multiplier system, introduced to the industry in mid-2009. The company believes this new system is easier to deploy, more compact, more accurate, and more versatile than other layer multipliers, including one previously offered by EDI. Tooling based on the new design can be supplied for applications over a large range of widths. The system can be engineered to multiply materials from three, five, seven, nine, or more melt streams.

An example of the compactness and productivity of the system is available in the case of a layer multiplier for a product with a total of 84 microlayers. The tool consists of a series of four components: a five-layer coextrusion feedblock, two multiplier modules, and another five-layer feedblock. The length of the tool is only 30 inches (762 mm), representing the total distance from the inlet of the polymer streams into the tool to the exit of the multiplied structure. The tool is being used at a typical line speed of 1500 lb (680 kg) per hour.

This compactness in a layer multiplier is important for maintaining streamlined flow, developing layer uniformity, and adapting to normal variations in material viscosity or extrusion rate. Layer multipliers with massive tools that have long flow paths can compromise layer uniformity, limit adaptability, and multiply materials from a limited number of melt streams.

The new layer-multiplier tooling can be installed in an existing extrusion line with only minor modifications. The downtime required for cleaning out the system can be as little as 4 hours.

Gary Oliver is VP of technology at Extrusion Dies Industries LLC (EDI; Chippewa Falls, WI), a leading manufacturer of flat extrusion dies and coating heads.