By Jan H. Schut
New technologies appearing at the 2013 SPE International Polyolefins Conference February 24-27 in Houston (www.spe-stx.org/conference.php), include a developmental 77-nanolayer polyethylene shrink film with an extremely high stretch ratio and the first full conference session devoted to the emerging science of haptics in product design.
The unusual shrink film, developed by BBS Corp. in Spartanburg, S.C. (firstname.lastname@example.org), achieves high stretch ratios of seven times in both machine and transverse directions (7 x 7), where the normal stretch ratio for shrink film is 5 x 5. “Seven-by-seven is something to crow about,” says Henry Schirmer, principal of BBS, who presents the new film in “Nano-Layer Structural Advances in Shrink Films,” co-authored by Jean-Francois Glez, R&D and product manager for the packaging division at Bollore S.A. (www.bollorefilms.com) in Quimper, France. Bollore, which makes biaxially oriented shrink films and thin capacitor films, worked with BBS for a year on the nanolayer shrink film.
Schirmer has given papers at Polyolefins conferences for over ten years on previous test work with his micro layer blown film die, called the Modular Disk Die, and on a Layer Sequence Repeater device that fits inside the die to form nanolayers. So that part of his development will be familiar. But this is his first report of stacking Layer Sequence Repeaters together to get to higher numbers of layers.
HOW 77 NANOLAYERS STACK UP
To reach 77 layers, Schirmer combined three Layer Sequence Repeaters inside a 6-inch die. The middle repeater forms 25 nanolayers; the other two form 26 layers, including a surface layer. The whole stack consists of 80 metal plates or disks, between which polymer flows to form successive layers onto the die mandrel. “Each and every nanolayer surface is formed separately between two metal surfaces with a mini-gap of 0.017 inches between them,” he explains.
This intensive contact of polymer with metal creates “parallel flow,” aligning polymer molecules without stretching them like parallel coiled springs, Schirmer postulates. When the film is biaxially oriented, the molecular springs uncoil, reaching the very high stretch ratios. Flow is slow through the die, and pressure is low–800 psi on the lab extruders, though at production rates, die pressure would be much higher (5000 psi). The relative thickness of layers is controlled by extrusion rate.
BBS’s nanolayering approach is quite different from creating nanolayers by splitting polymer flow and folding or stacking the multilayer flow onto itself to make more layers. That flat die technology was variously developed by Nordson EDI in Chippewa Falls, Wis. (www.extrusiondies.com); Dow Chemical Co., Midland, Mich. (www.dow.com); and Case Western Reserve University in Cleveland, Ohio (www.case.edu), and is used commercially to make cast films with anywhere from 25 to 800 nanolayers (see also this blog July 6, 2012).
BBS’s 77-nanolayer blown film alternates layers of two different polyethylenes, a departure for Schirmer, whose nanolayer R&D has mostly been with barrier resins like nylon, EVOH and COC (cyclic olefin copolymer). Bollore, however, had recently introduced a thin cross-linked shrink film (40-gauge) called BTTX110, which alternated metallocene PE and a different PE in a 7-layer structure, and Bollore wanted to test more layers to make even thinner, stronger shrink film.
Schirmer made test film with different structures of up to 77 nanolayers and shipped the rolls to Bollore in France, where the film was slit, partially cross-linked, and biaxially oriented on an R&D tenter stretching line with sequential MD/TD stretching. A 77-nanolayer film with two alternating PEs could be biaxially oriented 7 x 7 times to a thickness of 0.026 mil (26 gauge) or 6.5 micrometers with tensile strength of 23,057 psi, elongation at break of 79%, flex modulus of 300 MPa, and tear strength of 10.2 grams–all in TD. Good tear strength is the main advantage of the nanolayers, Bollore’s Glez notes. The film was also simultaneously stretched with the same test results, Glez adds.
BBS’s 77-layer film isn’t the first nanolayer blown shrink film, but it has the most layers and the highest stretch ratio. In 2009 the Cryovac division of Sealed Air in Duncan, S.C. (www.cryovac.com) commercialized 29-layer shrink film with 25 microlayers—which is still believed to be the most layers made commercially in blown film (see this blog June 17, 2011). Cryovac also describes achieving very high stretch ratios of 6 x 6 times.
BBS’s 77-nanolayer film also isn’t the most layers reported for blown film, at least not yet. In 2011, Dow introduced a nanolayer feed block technology, which creates a nanolayer melt and feeds it into a modified crosshead blown film die to make 114-layer blown film with 108 nanolayers (see this blog May 17, 2011). Schirmer, however, already plans more layers. He only stacked three Layer Sequence Repeaters together in the 6-inch die because that was the most that could physically fit. The repeater reduces the diameter of the 6-inch die to only 2 inches. Schirmer plans to scale up to a larger diameter die. “When that happens, it will double the number of potential layers,” he says.
PUTTING SCIENCE IN TOUCH
Scientific interest in haptics is growing. Many big companies today have research staffs studying perceptions of touch, which govern many purchasing decisions from car interiors to diapers. Yet there are still no test instruments or procedures to measure touch objectively. Color, sound, gloss, even odors can be accurately measured, but the sensation of touch is harder to understand and apply to product design, explains session organizer Cris Schwartz, an associate professor at Iowa State University in Ames (www.iastate.edu).
Schwartz has presented papers on haptics at Polylefins conferences before, but this time has assembled seven speakers, all doing active research in the new field. The seven speakers come from a wide range of perspectives, including that of a household products company, a business school, university research, and a resin company.
Session moderator Schwartz presents “Investigating the Haptics of Textured Polypropylene Using Friction Coefficient.” He uses mechanical wands with neoprene and silicone balls on the tips to measure the coefficient of friction off plain and textured surfaces. Human skin is 40 Shore A hardness, neoprene is 55, and silicone is 70, so they are similar in hardness. He compares COF data from smooth polypropylene panels and from panels with raised parallel ridges with different spacing by swiping human, neoprene and silicone “finger tips” in both parallel and perpendicular directions to the direction of the texture. Results show that smooth surfaces produce the highest COF, as expected. A surprising result is that some textures produce the same COF when slid both with and across the direction of the texture.
Marc Masen, a mechanical engineer at the University of Twente in Enschede, the Netherlands (www.utwente.nl), reports on the contact mechanics of micro textures against human skin in “Friction Effects in Tactile Contacts.” His R&D began serendipitously when researchers at the university used lasers to cut micro textures into mold inserts and molded samples with micro textures. They discovered that some micro textures that were too small to see could make plastic feel soft to the touch.
Vicky Polashock, a technical leader at Kimberly-Clark Corp. in Roswell, Ga. (www.kimberly-clark.com), shows how to apply the latest haptic research to product design in “Fundamentals of Haptic Processing of Everyday Textures.” Her research focuses on how neural receptors in finger tips are triggered by touching different surface textures, and how to use that information to predict sensory perception of objects. The skin in your finger tips and feet has four different types of neural receptors, which respond to sharp, rounded, smooth and rough surfaces and are well known. By increasing the frequency vibration of a stimulus to one type of receptor, she can dull sensitivity to that sensation, allowing her to study perception of different surfaces by the other neural receptors. She is trying to isolate how people interpret sensory information from each of the four types of receptors.
Dianne Pawluk, associate professor of Biomedical Engineering at Virginia Commonwealth University in Richmond (www.vcu.edu) describes “Haptic Perceptual Organization: Basic and Applied Research.” Her own research into haptics focuses on how to give better information through tactile markers to assist the blind and sight impaired. Her work aims at improving public policy on access requirements to help the sight impaired.
Joann Peck, associate professor at the School of Business of the University of Wisconsin in Madison (www.wisc.edu), presents “Does Touch Matter? Insights from Haptic Research in Marketing,” analyzing what motivates shoppers to touch. Her research finds that people have very different psychological needs to touch objects before they buy. Roughly half of us have an acute need to touch things and are frustrated, for example, by online buying when they can’t touch, she explains. The other half doesn’t need to touch. She further finds that giving an enlarged image of a surface texture online for a product to show how it feels to the touch only satisfies people with a low need to touch in the first place, but still frustrates people with a high need to touch. In case you wondered why touching products is important, she explains that “if people can touch something, they’ll pay more for it.”
Roger Barker, director of the Textile Protection and Comfort Center in the College of Textiles at North Carolina State University in Raleigh (www.tx.ncsu.edu/tpacc), gives the science behind a variety of attempts at quantifying haptic responses in his “Overview of Modern Methods for Evaluating Tactile Response to Textile Materials.” Tanya Fry, Sensory Science Leader at Dow, presents “Making Sense of Polyolefin Product through Sensory Science.”