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Tiny bubbles power sheet advance

Article-Tiny bubbles power sheet advance

Unwind a roll of plastic sheet through MicroGREEN Polymers’ (MGP) solid-state microcellular technology, and it will rewind wider and thicker with around 150% more material at roughly two times the thickness.

Unwind a roll of plastic sheet through MicroGREEN Polymers’ (MGP) solid-state microcellular technology, and it will rewind wider and thicker with around 150% more material at roughly two times the thickness.

The secret for this technology, which MGP (Arlington, WA) says can change the relative density of sheet from 95% down to 10%, are tiny, actually microscopic, bubbles formed in a very controlled manner within a plastic sheet, leaving a surface unmarred while reducing material at the core and adding insulative and reflective properties.

MicroGREEN Polymers Inc.’s technology allows the controlled creation of bubbles within plastic sheet, including their location and size, to impart unique properties while reducing the amount of resin used.

Thomas Malone, MGP CEO, says his company is now in a commercialization phase, which included time at November’s Pack Expo event in Chicago, pitching the unique technology to the plastics packaging industry. Speaking with MPW a few weeks following Pack Expo, Malone said there were already “some deals cooking very, very quickly,” including interest with “large partners to address high-volume packaging applications” in North America.

MuCell’s cousin

MGP’s technology traces its roots to the Massachusetts Institute of Technology (MIT) and the development there of microcellular foaming technology, which was eventually commercialized under Trexel (Woburn, MA) with MuCell. MuCell releases supercritical gases within an injection molding melt stream to improve surface finish of parts and cut cycle times, among other benefits.

Vipin Kumar was introduced to microcellular foaming at MIT and continued to pursue the technology when he left Cambridge to become a professor at the University of Washington (UW). Graduate engineering students Greg Branch and Krishna Nadella learned about microcellular plastics at UW while doing graduate work in mechanical engineering, eventually developing their own patent-pending innovations to improve the process and enable deep-draw thermoforming. From there, MGP was formed in 2002.

Under pressure

The process starts by placing a roll of plastic into a vessel and then pressurizing it with atmospheric carbon dioxide (CO2). Depending on the application, the sheet is held at a set pressure, or profile of pressures, with those parameters affecting the final product. The sheet is then removed with the CO2 trapped within the polymer matrix. Next, that roll is unwound/rewound through a heat source, like an air-flotation dryer, which lifts the plastics temperature to just below its glass transition stage.

With the plastic no longer able to constrain the CO2, little bubbles begins to form very rapidly. Depending on what the pressure was within the chamber and how long it was in the vessel, the bubbles’ size and location within the plastic matrix can be controlled. Malone says that at that point, generally speaking, the whole web has expanded by about 50% in width and 50% in machine direction, so there is essentially 150% more material being rewound.

According to Malone, if the process is run at high speeds, the bubbles would eventually come through to the surface, while at more normal line speeds like 300 ft/min, the core expands first, with the ability to create individual strata of different thicknesses with different bubble sizes.

Market opportunities

Beyond source reduction in raw materials, the microcellular bubbles impart some other unique properties that open up a variety of applications to MGP-treated sheet, with Malone saying the company’s “addressable market” constitutes some $47 billion in potential business. The company’s first licensee actually uses the technology to expand polyethylene terephthalate (PET) for a component used to reflect light in liquid crystal display (LCD) televisions.

Malone says the bubbles can be engineered so they’re shiny and lens like, allowing them to reflect light. Since there are billions of them, light is reflected in an extremely uniform manner across an entire sheet. Using the example of a mirror and a light bulb, Malone explains, “You could turn the mirror so you’re actually looking straight at a reflection of the lightbulb; it would be blindingly bright in one place but nowhere else. This is the opposite of that – no matter how you move that material, you can’t get it to have a particularly bright spot – the whole thing is bright.” Within the back-light unit of an LCD, this means the television can use the least amount of energy to make the brightest image possible. The market is already buying in, with Malone estimating that 20% of LDC TVs utilize MGP sheet.

The same principle applies to signage, where typically polycarbonate (PC) sheet utilizes opacifiers and many small lights to create a uniform sign. With the MGP technology, a PC outer material features the microcellular structure, which allows light to pass through, be reflected, and remain bright without adding opacifiers that can absorb light. Behind the light source, the material is used in the exact same manner as the LCD TV so that light reflecting 180° off the viewer is reflected back, making the image brighter. On the basis of that ability to reflect images, MGP is also pursuing printing, helping ink stand out with a highly reflective surface for printed, converted outer packaging.

Finally, the outstanding insulating properties of air, or trapped air in the form of bubbles, also presents opportunities. Malone says MGP can optimize the size of the bubble to engineer specific insulation, specifically creating a PET cup that has all the insulation characteristics of a poly-coated paper cup outfitted with a sleeve. Malone points out the combination of poly-coated cups and sleeves is a nearly ubiquitous and non-recycled application, which a single-system PET could replace with a “greener” solution.

Drop-in technology

Malone says thermoformers using the sheet can run approximately the same line speeds and heat they would with a solid material, with some advantages seen with MGP products, including slightly faster forming and lower temperatures. In terms of mechanical properties, Malone says MGP sheet exhibits differences, not necessarily positive or negative, which can be tailored to suit an application.

The process has been used with a wide variety of polymers, including commodity grades like polypropylene and polyethylene, high-end engineering systems like polyetherimides and polysulfones, and 100% recycled streams. Worth watching in the future is work with polylactic acid (PLA), where MGP was able to improve stiffness and temperature resistance of some experimental grades from PLA producer NatureWorks. [email protected]
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