Clear Barrier at Atmospheric Pressure – the Second Phase

The barrier properties of transparent layers deposited on flexible plastic substrates are of interest to many in the packaging industry. Numerous methods have been used to manufacture transparent barrier coatings with varying degrees of success to address evolving environmental laws requiring the reduction or elimination of volatile organic compounds (VOCs), which are the byproduct of curing liquid topcoats. There is also a consumer preference to visually inspect packaged products through packaging prior to purchase. This paper will present new evidence since the 2006 SPE FlexPack Conference of the potential for clear barrier at atmospheric pressure through the use of plasma processing as integral steps in a composite, or several stage, process for deposition and polymerization of functional barrier coatings. XPS analysis of polymerized film showed presence of silicon, carbon and oxygen in ratios different from the monomer, and in fact approaching a Si:O atomic ratio of 1:2 confirming cross linking effects, and the plasma polymerized organo-silicon films displayed good functional barrier properties without the environmental concerns of VOCs. Introduction The continuing and growing interest in clear barrier for flexible packaging of food, electronics and other consumer products has inevitably led to additional development effort in this area as people become aware of the opportunities. Clear barrier means ceramic; typically inorganic oxides or nitrides of metals. While deposition of inorganic oxides at reduced pressure is fairly routine such processes, and the equipment to implement them, are expensive. The prospect of being able to add value by depositing an inorganic oxide barrier layer using an inline, inexpensive, atmospheric pressure process is extremely attractive. Organo-silicon compounds, in particular, are a logical choice. These materials are easily selectable as liquids at room temperature, with low boiling points, and are considered good candidates for thermal evaporation into the plasma gas stream. It is not surprising that much of the current development effort is directed toward plasma polymerization and oxidation of organo-silicon compounds to yield a thin functional layer of SiOx on a substrate such as a plastic film or sheet. Results and Discussion Initial development experiments using substituted siloxanes were immediately followed by X-Ray Photoelectron Spectroscopy (XPS) analyses of the treated film surfaces which confirmed the addition of silicon to the film surfaces. Figure 1 presents a typical XPS response for untreated PET film while Figure 2 shows the response for the same film following Plasma Enhanced Chemical Vapor Deposition (PECVD) with a silicon compound. The additional peaks at 100 eV and 155 eV in Figure 2 confirm the presence of silicon atoms on the surface of the treated film. A useful feature of the “cold” atmospheric PECVD environment is the potential for control of the polymerization process outcome. Energy management and control of reactant flow rates can be used to provide a broad spectrum of polymerization reaction outcomes ranging from essentially atomic to somewhat molecular. Atomic polymerization comprises cleavage of all bonds in the CVD material followed by addition of individual atoms to the building polymer layer on the surface of the deposition substrate. Molecular polymerization comprises cleavage of only the most labile bonds in the CVD precursor molecule followed by addition of the nearly intact precursor molecule to the building polymer on the surface of the deposition substrate. Figure 2. XPS of CVD with Si-Cmpd 2 Recently we have been working with a large producer of plastic sheet to develop equipment and process methodology for deposition of a thin functional coating of SiOx on plastic sheet that will display anti-fog functionality. The equipment shown in Figure 3 is currently in place at one of the customer’s pilot facilities and is successfully depositing a very thin SiOx anti-fog coating on plastic sheet. It is not thick enough to be a barrier coating but it is tantalizingly close and is considered a significant milestone on the path to clear barrier. Preliminary single stage samples have just been prepared in our labs as described below and, while the atomic composition, coating thickness, and modest reduction in moisture transmission rate all say we are moving in the right direction, we know that to get good barrier we will need the procedural capability to perform multi-stage plasma polymerization. We are currently making equipment modifications in our labs to make multi-stage plasma polymerization and composite deposition operations possible on a routine basis. First Phase Methodology / Experimental Figure 4 shows the schematic diagram of the atmospheric PECVD reactor used in this experiment for the deposition of SiOx at a low temperature. As the plasma source, a RF power supply operated at 1kW 100kHz was used for the high dissociation of gas molecules. The substrate was maintained at temperature of 72 degrees F. at 50% relative humidity. Gases were supplied to the treatment assembly using mass flow gas controls located within an Enercon power supply/PLC prior to a turbulence chamber for mixing and subsequent dispensing to the assembly. The gas mixture, inclusive of tetramethyldisiloxane (TMDSO) and compressed air, was used to deposit SiOx. In this gas mixture, TMDSO was delivered to the treatment assembly by maintaining the TMDSO liquid source at 72F using a an evaporating chamber strapped by heating bands and by sweeping it from the evaporation chamber with Helium through PE tubing which did not required heating. The flow rates of TMDSO (15 lpm), Helium (15 lpm) and compressed air (15 lpm) were varied for the optimization of the SiOx film. SiOx was deposited on 1 mil. PET film at 50 fpm and immediately exposed to a “curing” helium plasma at the same power and frequency settings as the deposition plasma. Following deposition and curing at several coating thicknesses, the PET film surface was studied to determine the characteristics of the deposited film and to measure the water permeation properties. The chemical compositions and binding states of the deposited SiOx film were investigated using an X-ray photoelectron spectrometer (XPS). The coating thicknesses were measured using a Filmetrics F-20 Reflectometer. The water vapor transmission rate (WVTR) was measured using a WVTR measurement system (MOCON Inc., PERMATRAN-W-700). Table 1 shares the results of First Phase tests: Table 1. First Phase MVTR Using Atmospheric Pressure Plasma SAMPLE FILM THKNS COATING THKNS MVTR