Designing for Blown Film: Equipment, resins provide film designers with new possibilities

When it comes to extruded film, complexity today appears to be the way to go. Forget monolayers – and many applications traditionally featuring three or five layers are migrating to additional layers as well.

“Definitely there is more demand for multilayer film driven by savings in expensive barrier, tie, and outside layers,” says Mirek Planeta, president of processing machine builder Macro Engineering & Technology (Mississauga, ON). Franz Mahler, divisional manager, film extrusion at Hosokawa Alpine (Augsburg, Germany), says 10 years ago, five layers would have been the maximum produced by many processors. But five years ago, seven layers became the standard for food packaging, pushing up the demand for barrier materials, such as EVOH and nylon. Christof Herschbach, general manager business development at Windmöller & Hölscher (W&H; Lengerich, Germany), says from his experience the demand in barrier EVOH has quadrupled in the last 10 years.

In non-barrier applications, generally 3-layer designs and the larger part of the multilayer sector, processors are looking to up structures to five layers to remain competitive and provide more flexibility in their offering.

Basell skin layers Good thermal resistance of Clyrell RC1601, a copolymer PP from LyondellBasell specifically designed for use in skin layers of metallized BOPP, prevents metal crazing during film processing. Alpine blown-film line Downsizing of multilayer films and combinations of alternative materials, which are possible on this Hosokawa Alpine blown-film line, are becoming more common. Reifenhauser blown-film line Formerly, thinning out barrier layers could compromise product shelf life. With today’s equipment advances, such as this 5-layer blown-film line with REI2cool blown-film cooling system from Reifenhäuser, processors have more assurance of layer tolerances. Brampton chart As the barrier film market turns more competitive, controlling costs becomes imperative and the barrier thickness of a structure is coming under higher scrutiny—as in this example from Brampton.

Bernd Schroeter, product manager blown-film division at Reifenhäuser (Troisdorf, Germany), see trends of 3- and 5-layer films being extruded in wider (from an average of 1m to up to 2.8m) widths to provide overall better quality control. Increased output, up to 30% more thanks to recent developments in cooling rings, is also a high demand.

“Price pressure is forcing many [processors] to go to five layers to maintain precise layer control and more efficient use of post-production recyclate,” Schroeter says. “They have the opportunity to put less expensive resins and reclaim in the middle layers to cut costs and also downsize structure.”

Although barrier film processing represents a much smaller market, Adolfo Edgar, regional sales manager for Brampton, says the growth in flexible food packaging has also translated into higher demand of barrier films in seven and nine layers. “The main reason is economics. It is cheaper to make a 5-layer structure in seven layers and a 7-layer in nine layers,” he says.

Edgar admits that in the past many barrier film structures were over-engineered since end users were not too concerned about how much barrier was enough for their product as long as the film did its job. But as resin prices in recent years jumped, more control of costs became imperative and the barrier thicknesses required came under higher scrutiny.

He gives an example of a 7-layer film (see chart) that originally required 45% nylon, 10% EVOH, and 10% tie on each of the two layers. The film design also needed an expensive polyethylene (PE) sealant layer making up 25% of the total. After evaluation it was shown that 5% EVOH would provide the needed barrier yet it was impossible to run this thin on the original equipment.

A better option was seen with a 10-layer coextrusion (structure A) that not only achieved the 5% EVOH requirement but also cut costs by splitting the sealant layer and using only 15% of the expensive tie as well as including 15% cheaper filler resin. Further evaluation (structure B) illustrated the flexibility of more layers by determining that only 35% nylon is sufficient to provide adequate barrier as long as the outer layer remains 15% nylon while structure C offers further savings by increasing the low cost PE filler resin content.

Such flexibility only comes with increased layer use, says Reifenhäuser’s Schroeter. “We’re seeing very realistic barrier layer designs coming onto the market today, less over-engineered than in the past because gauge control has improved so much in the last five years and end users see less need to build in that extra thick barrier to ensure film integrity,” he says. W&H’s Herschbach says complex film structures allow, in some cases, the possibility of eliminating altogether the extra cost of lamination that is done in a second processing step.

As good as the recent processing machinery improvements are, the benefits multilayer film processors are achieving would not be possible without the latest developments in resins. Doug Wilson, global business manager for flexible packaging at polymer producer Honeywell (Morristown, NJ) says, “Due to cost concerns, package designers are becoming more cognizant of the amount of materials required to provide specific levels of barrier. No longer is it acceptable to over-engineer a package ‘just to be safe.’” Companies are becoming more sophisticated in barrier measurement techniques and are working diligently to translate barrier levels directly into shelf life, he says.

Hans-Jürgen Bach, application development and technical service film manager, LyondellBasell (Frankfurt, Germany), says the sophisticated structures produced today are requiring single resins that provide more than one function in different layers, such as softness but stiffness, or low haze and gloss but high sealability. Designers are combining many more different resins today in layer structures to achieve materials that can be downgauged yet run on existing packaging equipment. Polypropylene (PP), traditionally run on cast equipment, is finding more application, he says, as new grades are introduced to run on existing PE blown-film equipment.

Yet Brampton’s Edgar warns that producing a 7-plus-layer structure to meet specific criteria such as moisture, gas, UV light barriers, and clarity can be highly challenging for the designer. “The chosen resins and desired layer thicknesses of each might not process well as a whole, resulting in shear rates and pressures unsuitable for the design parameters of the die,” he says. His company is offering, with each SCD coextrusion die it sells, Prophecy 2.0 simulation software that allows the processor and film designer to analyze polymer flow through the die in “what-if” scenarios to predict performance of the new structure.—[email protected]