Using a Chemical Primer for High Bonds & Low Oxygen Transmission

PEER-REVIEWED TECHNICAL PAPER
Following is a expanded summary of a complete paper that is available on the TAPPI web site at www.TAPPI.org. On the page, click “the PLACE” in the section designated Journals.


Email the author(s) at: gcushing@mica-corp.com


Packaging engineers seek creative and environmentally conscious ways to build oxygen barrier into multi-layer flexible packaging structures. By affording protection from oxygen, a package can inhibit spoilage and extend shelf life of its contents. Our laboratory developed an aqueous oxygen barrier dispersion based on vinyl alcohol chemistry and formulated to provide strong adhesion to film substrates. Table I shows characteristics of the barrier primer.

Table I — Properties of Barrier Primer

Solids, % by weight

10.5

pH

10

Tendency to foam

Low

Adhesion to film substrates

Excellent

Printability

Excellent

Clean up

Soap/water

Freeze/thaw stability

Yes

Chlorine content

None

Tendency to discolor

None

FDA Compliance

Yes

COATING APPLICATION Producing an optically clear film with an effective barrier to oxygen requires delivery of a smooth, continuous coating to a treated film web. Pinholes are unacceptable since they act as a window to oxygen and will defeat barrier performance. Direct gravure coaters produce coatings with an uneven appearance. In addition to poor optics, the uneven coating has high and low spots. Low spots limit oxygen barrier performance.

The preferable coating technique for this product uses a roll coating technique that employs a “wiping” action to level hills and valleys and produce an even coating. Successful techniques include direct gravure with smoothing bar, reverse gravure, smooth roll with flooded nip and variable speed pick-up roll, and indirect (offset) gravure applicator with variable speed pick-up roll.

The coating requires thorough drying. Residual moisture will compromise the adhesion to the substrate and oxygen barrier performance. Variables including line speed, oven temperature, web temperature, oven relative humidity, air velocity, air volume, etc., influence drying. A thin, dry coating will result in better adhesion and oxygen barrier performance than a thick, wet coating.

EXPERIMENTAL WORK
The laboratory work used the above mentioned barrier primer, a metallized polyester film (MOPET), and a metallized nylon film (MOPA). The metallized side of the polyester and nylon films was treated and primed on a laboratory coating line. The resulting coating was smooth and optically clear after drying. Figure 1 shows a diagram of the coating line used for this study, and Fig. 2 is a photograph of the actual unit.

The coated rolls were tested one week after manufacture for oxygen barrier performance and coating surface characteristics. Measurement of oxygen transmission rate involved placing the samples in test cells so that the primed, metallized side faced 100% nitrogen, 760 mm Hg, and 0% RH. The uncoated side faced 100% oxygen, 760 mm Hg, and 50% RH. Fig. 3 shows the data. The barrier coating was also tested on nonmetallized film substrates on the same line.

DISCUSSION
The primer contributed significant reduction in oxygen transmission rate in all coated samples tested. The polymeric barrier coating bonds strongly to treated films and metallized surfaces. It is flexible and not prone to cracking or flaking when flexed. When properly applied, it dries to a tack-free surface and is not likely to block in roll form. In one test, a primed sheet of OPET was aged for 14 months and tested for adhesion and oxygen barrier. The performance was comparable to the freshly primed roll stock. The primer is therefore probably suitable for a multiple pass process involving application, drying, and rewinding before printing, adhesive lamination, or extrusion coating.

Data from previous work helps to summarize some performance characteristics of the barrier primer. For the coating formulation tested in this study, the variables that most influence oxygen barrier performance are coating application weight — higher application weight will lead to lower oxygen transmission rate values, the continuity of the coating, and drying at the time of application. Residual moisture will cause poor adhesion and barrier performance. Excessive application weight can contribute to incomplete drying of thick coatings.

The relative humidity that reaches the coating is also important. The oxygen transmission rate is proportional to relative humidity. Oxygen transmission increases with increased humidity. We believe the coating is most suitable for dry package contents in multi-layer structures that provide moisture barrier.

CONCLUSION
The data from this work indicates that application of an aqueous barrier coating in a smooth, continuous layer to a film substrate using the differential offset gravure coating technique is possible. The film may undergo coating and rewinding with storage for later use. The new primer contributes high bond strength and low oxygen transmission rates on plain and metallized film substrates.

The coated surface should be subsequently buried within a multi-layer structure. It may have use in processes such as adhesive lamination or extrusion coating. The technology is suitable for packaging low moisture content items.



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