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Home > MacroLetter > 4.1 - Winter 1998
Contents:
World's First Single-Pass PVdC Extrusion Coating Process
The story of Macro's successful development of the new PVdC extrusion coating system was revealed by Michael C. Gabriele, Senior Editor of Modern Plastics magazine. The following is a reprint of the editorial that appeared in the November 1997 issue.
Extrusion Coating: Single-Pass Process Permits 35-Plus-Micron PVdC Layers
By Michael C. Gabriele, Senior Editor- Modern Plastics & Modern Plastics International
Macro Engineering & Technology Inc., Mississauga, ON, Canada, has launched a novel extrusion coating system for polyvinylidene chloride, a resin known for its outstanding moisture and oxygen barrier properties. An initial commercial line has recently been installed for a processor in China and a second line is to be shipped early next year to an undisclosed U.S. firm.
According to Macro officials, a tangle of process technology challenges has to be confronted in developing an extrusion coating system that could fully exploit the barrier properties of PVdC. “This means that PVdC barrier coating of any thickness, and not just up to the current practical limit of 1 mil (25 microns), is possible,” comments Ed Green, technical director for Dow Plastics PVdC (Saran) Group, in Midland, MI. Conventionally, PVdC is coated using water dispersion, which imposes limits on the thickness of the barrier and raising its cost. Dow has assisted Macro in its recent effort. To the benefit of PVdC, the resin, in contrast to EVOH, the major competing barrier resin, offers an oxygen barrier that is much less adversely affected by high moisture and high relative humidity environments. At 100% relative humidity, PVdC has 0.08 to 0.2 oxygen permeability (measured as cc/mil/100 sq.in).
Macro’s process breakthrough stems from its parlaying of established expertise in the coextrusion of PVdC in barrier films.
Mastering Temperature Control and Residence Time
Jim Stobie, Macro’s marketing manager, says temperature control is an essential element in the success of PVdC, because of its auto-catalytic nature, has a narrow temperature processing window (160-170 Celsius or 320-340 Fahrenheit). In contrast, the typical melt temperature range for extrusion coating of polyolefin, the materials most commonly used for the process, is broader.
Stobie adds that residence time also is critical: the coating system must maintain a sufficient flow velocity to insure a hopper-to-die residence time of less than two minutes. Macro has devised a cooling control package to maintain a tight (+- 2 Celsius or 3.6 Fahrenheit) temperature profile for the die and extrusion zones. The “package” includes dedicated control software and an oil-based heating/cooling system. Die design is another key aspect of the development. Macros fabricated die components with high-nickel-alloy steel to protect against corrosion. PVdC requires minimal shear and cannot tolerate hang-ups or dead spots in the die, since it degrades rapidly and produces hydrochloric acid emissions. For this reason screen changers also cannot be used.
Paul Asturi, Macro’s assistant marketing manager, says the coating substrates with PVdC for improved barrier property is done today by a gravure coating process using latex or solvent base coating. He says this process involves multiple passes, due to limits on the thickness of coating achievable in single pass. He also points out that gravure coatings tend to be brittle, often resulting in micro cracking that undermines the resin’s barrier properties.
In the Macro system (priced at about US $2 million), an adhesive primer is first applied to the given substrate. Then a single-pass extrusion coating delivers a uniform thickness of 35 microns (1.4 mil) or more, a level suitable for most medical, pharmaceutical, and food barrier packaging applications.
Asturi says his firm benefited from cooperative research efforts with Dow, which developed an experimental PVdC grade that offers enhanced heat stability, providing improved processing characteristics.
Macro claims this is the first commercial extrusion coating system able to employ PVdC. Macro previously developed cast film lines that employ PVdC as a core barrier layer, as well as biaxially oriented PVdC monolayer and multilayer blown film systems.
Other machinery builders confirm that they cannot recall specifying PVdC for extrusion coating. Dave Garret, sales team leader at Black Clawson Converting Machinery Corp., Fulton, NY, says his firm has designed flexible packaging systems that do PVdC solution roll coating film “but we don’t use PVdC as an extruded resin,” he says.
Frank Orsini, extrusion coating product manager for Egan Davis-Standard Corp., Somerville, NJ, says he, too, is familiar with PVdC only in non extrusion liquid applications for coating film.
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