What You Need to Know to Make World-Class Stretch Film
The dynamics of the stretch film market are continuously evolving. Worldwide, the big trends and drivers in cast stretch are:
• Downgauging: The average gauge of hand wrap has gone from 25 microns to 10 microns (µ) and the average gauge for machine wrap and power pre-stretch has gone from 35µ to 15µ. Cast-film lines are being engineered to allow production of thinner films at higher-than-ever winding speeds (up to 2300 ft/min or 700 m/min). More flexible inline winding has been developed to reduce labor costs and scrap rates.
• Higher stretch percent: The upper end of the stretch range has risen from 200% to 300%.
• Increased use of machine wrap by end users: The trend is moving from hand wrap to machine wrap—a result of increasing labor costs.
• Higher machine wrapping speed among end users: Average wrapping speed has moved from 25 rpm to 60 rpm (the latter applies only to orbital/rotary arm-wrapping machines).
• More layers: The movement towards multilayer films continues to grow and diversify. Film configurations range from three layers all the way up to dozens of nanolayers. Nanolayer structures give the film a “plywood” effect that enhances mechanical properties such as puncture and tear resistance and allows thinner pure-metallocene PE layers.
In Europe, nanolayer technology is growing quickly. Progressive processors include the likes of Apeldoorn Flexible Packaging B.V. (AFP), a Dutch producer of a wide range of blown and cast films for packaging, which has been utilizing nanolayer technology since January 2009. AFP’s original nanolayer launch was for 27-layer films, but Eddy Hilbrink, who heads up strategic R&D projects, told Plastics Technology of plans to push the envelope further with the installation of a third nanolayer line, this one for more than 50 layers (see May Close-Up).
Among North American processors, no one seems willing to even discuss numbers of layers. A 2012 patent lawsuit probably explains why. In February of that year, a technology licensing firm called Multilayer Stretch Film Holdings filed separate lawsuits against nine leading North American stretch-film processors, claiming they had violated a patent covering stretch cling films with seven or more layers. Industry sources report that most of the processors named have since settled out of court. Last November, however, the Federal District Court in Memphis ruled in favor of stretch-film processor Berry Plastics, Evansville, Ind. Multilayer Stretch Film Holdings has appealed that ruling. Berry would not comment on the matter.
Nonetheless, signs suggest that more processors in North America are moving beyond the five-layer structures that have been generally considered “state of the art” in the NAFTA region since Peter Cloeren launched Chaparral Films in Orange, Tex., in 1994. In August, Sigma Stretch Film, Lyndhurst, N.J., the largest producer of stretch film in North America, announced that it would be installing a nine-layer cast-stretch line from SML of Austria (U.S. office in Gloucester, Mass.) with a Cloeren die/feedblock package. The line, which is expected to be delivered by the first half of next year, will be used to run 20-in.-wide rolls nine up. Another major processor, Inteplast Group’s AmTopp Stretch Film Div., also announced a major expansion recently (see sidebar at the end of this article). Market consultant Mastio & Co., St. Joseph, Mo., projects stretch film will grow at 4.5%/yr through 2017, when it will consume more than 2.2 billion lb of PE.
How does a processor serving this market kick it up a notch from a technology standpoint? While stretch lines are large and complex, three keys to developing world-class stretch film are materials, feedblocks/dies; and winding. In this article, industry leaders in each of these areas—ExxonMobil, Cloeren, and Windmoeller & Hoelscher, respectively—share their expertise.
IT STARTS WITH MATERIALS
In trials with customers and leading machinery suppliers, ExxonMobil has found that certain of its resins offer desirable properties for stretch film:
• Enable metallocene-based PE (mPE) resin provides high holding force at low film thickness.
• Exceed mPE resin provides high holding force and puncture resistance at high stretch ratios.
• Vistamaxx performance polymers are propylene-based elastomers that provide high tear-propagation resistance at high stretch ratios. These resins are also commonly used to provide reliable, cost-effective cling in these multilayer films.
Enable mPEs are branched metallocene resins. This metallocene resin family has higher shear thinning, which allows for low-melt-index grades to be used in cast-film extrusion to obtain improved physical properties. The strain-hardening curve illustrates that these resins have a distinct second yield point, which provides a step change in tensile strength. Using these resins in cast stretch films yields high tenacity and high holding force across a wide stretch range, which delivers additional value in cast hand-wrap and machine-wrap applications.
Exceed mPE resins have become an industry standard for high-stretch, high-puncture-resistance stretch films. A broad portfolio of resins—including Enable mPE and Exceed mPE resins, and Vistamaxx polymer—is utilized to tailor solutions for specific equipment and application requirements.
Vistamaxx performance polymers offer improved puncture and tear-propagation resistance at high stretch and tension. In addition, they provide a more efficient cling solution.
Achieving cling in stretch film has been an evolutionary process that began with the use of polyisobutylene (PIB). PIB is a sticky liquid that’s very difficult to meter into the product. Next came metallocene elastomers, which provided a cleaner, more precise alternative to achieve cling by blending as much as 30% into the cling layers. Today, adding about 10% or less Vistamaxx often provides comparable cling performance at a better value than either of the previous options.
A variety of multilayer configurations are used in the stretch-film industry. Solutions using Exceed mPE resin and Vistamaxx polymers have been developed that provide significant unit-cost savings versus the alternative by providing these benefits:
• Improved toughness at thinner gauge.
• Outstanding pre-stretch and high-speed wrapping performance.
• Less film, by weight, to wrap a pallet at equal load stability.
Figures 1 and 2 review two multi-layer configurations that provide examples of value-added films. The seven-layer example is representative of a multi-layer approach and can be adapted to other layer configurations, whether higher or lower in layer count. In both instances, thinner film yields equivalent physical properties.
There are many options for customizing stretch-film performance. Options that ExxonMobil Chemical has proven to be successful, for a variety of scenarios, include the following:
• Enable mPE resin can be used to adjust load force/working range.
• Vistamaxx 3980FL has provided improved stretch/puncture resistance in a nanolayer structure.
• Exceed mPE resins have been used to improve many stretch-film properties. Exceed 3812CB has provided cling /toughness improvement. Exceed 7518CB has provided process continuity, cling, cling retention, and stretch performance. Using it as a cling layer with Vistamaxx performance polymers provides improved cling and blocking resistance. It also provides softer, higher-stretch film used in skin and/or core layers.
Moreover, it improves extrusion processing, edge-flow stability, and web stability, which allows stable operation at higher rates, as well as improving film continuity and consistency.
FEEDBLOCKS, DIES & NANOLAYERS
With the commercial introduction of metallocene resins in the mid-1990s, the 15-year standard of three-layer stretch films was challenged, and the world’s first commercial five-layer film was introduced to the marketplace in 1994 by Chaparral Films. Was this development “smoke and mirrors” or “hocus-pocus”? Something new had upset the industry status quo, and the industry icons of the time, who had yet to understand the technology and its promise, did not respond kindly.
Fast-forward to 2015, and films are being processed comprising 50+ layers, and the 1995 song of “smoke and mirrors” again rings in some corners of the industry. As baseball legend Yogi Berra reportedly once said, “It’s like deja vu all over again.”
The industry did not jump from five-layer films to 55 layers in one fell swoop. Instead, it took two decades to arrive there. The industry saw commercialization of seven-layer films at the turn of the 21st century and the earnest use of polypropylene as a tear-propagation-resistance layer. Between 2000 and 2005, some firms even ventured into nine-layer films. The first nanolayer film, comprising 21 layers, was introduced to the market by Pinnacle Films (now part of AmTopp). Next, industry expert and visionary Hilbrink of AFP introduced a 27-layer film and now is adding a third nanolayer line with more than 50 layers.
Today, it is rare to see new five-layer and even seven-layer capacity being acquired on large-scale commercial lines. It’s fair to say that nine- and 11-layer capacity is the current norm, and nanolayer capacity not so unusual. Figure 3 shows the current distribution of purchased layer capacity during 2010-2015, by geographical region.
So why more and more layers? One reason is that the stable of polymers available today—including mPEs, propylene-based elastomers, olefin block copolymers, LLDPEs, ULDPEs, and VLDPEs—to construct higher-performance films is significantly larger than a decade ago. When you combine the available selection of polymers and the so-called “plywood effect,” more layers are inevitable.
The plywood effect is the mutual reinforcement of plies (layers) to obtain unique properties. Plywood relies on orientation of grain structure in varying or opposing directions to obtain its unique strength and pliability. While the coextrusion film-forming process does not allow for different directional orientation of each ply, discrete layers of differing polymers do, in fact, result in different crystalline structures per layer. These differing crystalline layer structures, when produced at the appropriate thicknesses of apposite polymers, provide mutual reinforcement. Hence, more, and thinner, layers.
What constitutes a nanolayer? A nanometer is 1/1000 of a micron. There are 25,400 nanometers in one mil. In a cast film, layers can be a thin as 100 nanometers. Then, when this film is stretched above 300%, you arrive at layer thicknesses of less than 25 nanometers (0.0009 mil).
With many films produced today at or less than 10 microns, for example, even five-layer films could comprise sub-micron, or nanometer-scale, layer thicknesses. However, as a practical matter, it appears that the industry is coming to the resolution that a “nanolayer film” is comprised of a majority of layers of less than one micron each. Perhaps some nine- and 11-layer films, and certainly all 20-50 layer films, technically fit the developing consensus definition of nanolayer film.
The principles of laminar flow apply to all thermoplastic extrusions and coextrusions. The number of layers is indifferent to these principles. However, the required attention to design detail does increase significantly, if not exponentially. We at Cloeren assumed we knew a lot of things well, but it turned out that we didn’t know as much as we thought. Increasing the number of layers while simultaneously reducing the layer thicknesses has a way of humbling someone really fast.
To meet customer expectations, new software had to be developed, new tooling had to be implemented, and new manufacturing techniques had to be applied. Feedblocks that house nanolayer technology are substantially larger than conventional (three- to 11-layer) feedblocks. Usually, as things become larger, allowable manufacturing tolerances also become larger and more forgiving. That is not the case with nanolayer feedblocks: The tolerances actually had to be tightened up to provide the required precision of mass distribution, which is proportional to the number of layers involved.
At the same time, it was clear that flexibility could not be sacrificed. This required a modular design concept to be able to change layer position, polymer selection, and the like. The modularity and precision levers typically don’t move in the same direction, but in this case they had to if customer expectations were to be met.
Then came the die. To distribute nano-thickness layers uniformly across a 3.5-5.5 m (150-220 in.) die required a second look at die design. Flow-channel shapes had to be reexamined, and precision of tolerances had to be tightened up, in order to meet the process demands associated with such thin layers.
So what are the benefits of nanolayer stretch films? When looking at standard laboratory testing, or controlled testing standards, the results for nanolayer films are not overwhelmingly apparent when compared with conventional seven- to 11-layer films. However, when looking deeper, or further downstream in the process, other benefits present themselves.
Where nanolayer films appear to excel in practice is in the wrapping process itself. Typical orbital wrapping speeds are in the range of 20-25 rpm. When nanolayer films are applied to the same wrapping process, all else being equal, we see attainable and reliable wrapping speeds at least double those of conventional films. To big bottlers and packagers, this equates to big money—twice as many pallets per wrapping machine per hour.
What these field results tell us is that nanolayer films yield significantly higher allowable acceleration forces. One can then further postulate that if the allowable acceleration forces are measurably higher, then so too are the allowable deceleration forces. Allowable deceleration forces are particularly important in Europe where pallets are side loaded, and nesting of pallets is not inherently attainable, as is the case with end-loaded trucks in the U.S.
WINDING IT UP
High winding quality is essential for reliable and safe usage of stretch film in tertiary packaging. The majority of challenges during winding are tightly connected to the extrusion process, so it’s essential that extrusion and winding be closely integrated. Film profile tolerances in the sub-micron scale are inevitable and add up layer by layer in the roll of film, so that the film could be deformed and damaged inside the wound roll. In some applications, stretch winders have to be designed for frequent roll changes so that the extrusion line speed can be kept high.
The “TNT” principles of winding are the basic parameters that control the build-up of pressure inside the roll: tension of the film, nip force between contact roll and film roll, and torque on the winding shaft at the center of the roll. The TNT settings determine the general tightness of the film layers within the roll. At the same time, the outer layers of film in the roll act as compression tapes on the inside layers and particularly on the paper core, where the pressure is the highest. Depending upon film resin formulation, the film may “age” by thermal shrinkage and post-crystallization after production. Stretch films with a functional layer of PP, for example, exhibit a significant change in their mechanical properties and subsequently tend to develop higher core pressures during storage.
Three different types of stretch films are commonly produced: hand wrap, machine wrap and super-power stretch wrap. These films have application-optimized properties concerning ultimate stretch, holding force, puncture, tear-propagation resistance, and cling. All of these parameters affect slitting and winding processability.
For example, the winding tension for thin super-power stretch wrap should be as low as possible to prevent core crushing. Furthermore, the application affects the roll dimensions. For compatibility with the wrapping machine, rolls for machine wrap have a standardized roll diameter of about 10 in. and fixed film widths of 20, 30, or even 40 in. Hand-wrap rolls, in contrast, might have any width in the range between 10 and 20 in. The roll diameter for the final hand application is less than 10 in. Sometimes there are only a few thousand feet of film on the core.
Hand–wrap rolls can be produced inline with extrusion or offline by slitting larger “jumbo” rolls with typical diameters of up to 16 in. High production speeds of up to 2300 ft/min, combined with small roll diameters, result in roll-change cycle times from 30 sec to several minutes. Depending on roll and die widths, the extruded web is split into a maximum of 12 webs in the slitting station of the winder.
Usually, a “bleed” trim is cut between the webs, so that the paper cores in the center of the rolls extend over the edges of the rolls. These trims are typically tacky and highly extensible; they must be cut reliably at high speed and re-fed into the extrusion process for better material and energy efficiency. All in all, one stretch-film product can differ a lot from another on the winder. As a conclusion, the winder needs to provide high flexibility, easy handling of complexity, high reliability, and—last but not least—good roll quality.
High-speed production of stretch films requires a dedicated winder that incorporates all three TNT principles. In the case of W&H´s Filmatic PS winder, two winding units are positioned on top of each other in a laterally offset arrangement, each winding one half of the web. Identical web paths through both winding units guarantee consistent roll quality across the complete web. All rolls in the winder are optimized for enhanced traction at low web tensions for thinner films.
In addition, the tension zones for slitting and winding are mechanically isolated from one another to widen the process window. A unique, stiff bearing assembly for the winding shafts ensures minimal deflection during winding, ensuring perfect roll edges and consistent roll hardness at high speeds. Furthermore, the winder offers an innovative feature to optimize and maintain the winding hardness by a fine adjustment of the air entrained in the winding gap.
Traditionally, elimination of air in the rolls was paramount. Now, with the advent of higher-performing, thinner stretch films, running at higher line speeds, there is the need to control air distribution throughout the roll.
Rolls with controlled softness offer these benefits:
• Low unwinding forces cause fewer film breakages in the stretch wrapper head.
• Low unwinding noise is a prerequisite for fully automated packaging lines in warehouses.
• Higher possible cling levels are critical for load stability and higher load-holding forces.
• Elimination of film wrinkles gives few film breakages during stretching on the wrapper.
• The ability to run thinner machine film cores provides huge potential savings to the film converter.
An automation system is mandatory at high web speeds to achieve operator safety and process stability. Each winding turret features three winding shafts so that paper cores can be loaded onto a shaft and rolls can be pulled from another shaft in parallel to the winding process.
At the same time, film-width format flexibility has to be maintained. Therefore each winding turret has core bins for two different core lengths. Cores from either bin can be combined with a manually fed core size in a programmable core pattern. All core bins for both turrets are easily accessible to the operator in a single location.
A new trim-suction system in the slitting station offers width-format flexibility and easy operation. Position of the flow-optimized suction pipes can be adjusted seamlessly according to the roll widths for best slitting performance and stability. Finally, specialized high-speed shafts are available that can cope with any core pattern that might be necessary.
Winding a good roll of stretch film can be the most demanding part of the production process. The features enumerated above simplify the art of winding with additional adjustability and enhanced flexibility.