Since the primary function of the top ply in a coupon construction is consistent and controlled removeability, the adhesive used to hold the ply in place must possess special properties. A combination of application ease and post-dry, tack-free properties across a broad temperature range dictates the use of a water base adhesive. Water base adhesives are easy to apply flexographically and can exhibit excellent wetting and flow characteristics over relatively low dyne-level base substrates. When suitably formulated, they can be designed to act as nonpermanent wet laminating adhesives. When applied to a suitable release base and overlaminated with the absorptive, uncoated side of a paper substrate, they can act as the binder in a coupon construction. A dry, tack-free surface remains following delamination of the coupon from the release base.
Various materials can act as release bases for coupon constructions. Untreated (low dyne level) films such as polypropylene, polystyrene, and polyester can be used, as well as release coated paper stocks. UV curable topcoats, when suitably formulated, applied and cured, can also act as coupon release bases on print treated films or clay coated paper stocks.
Drying and substrates
A water base laminating adhesive can be designed to laminate the top coupon ply to a release base while still wet. Drying prior to laminating is unnecessary, and typically undesirable. The tack of the wet adhesive is used to hold the two plies together during processing. Since the adhesive is applied to the base and laminated wet, it is imperative that the top ply be an absorptive material.
The non clay-coated side of a C1S paper stock is the coupon material of choice ("C1S" is assumed to mean "clay coated, one side", not simply "coated one side".). Clay-coated two side (C2S), synthetic, and poly top-ply coupon stocks are not as absorptive and are highly undesirable and not recommended for coupon constructions.
Adhesive performance issues
The apparent strength and consistency of the adhesive's bond or holding power is controlled by three primary factors. These factors include applied coat weight, coupon basis weight, and release base properties.
Higher coat weights of adhesive will bond more tightly after drying, but might lead to post-aged coupon lock-up, substrate and ink picking, or fiber tear. Conversely, lower coat weights of adhesive might provide insufficient holding power which could lead to premature coupon dispensing and/or flagging during processing and product labeling. Additionally, consistency in the applied adhesive's film thickness is paramount in obtaining precise coupon release properties. Therefore, precise metering of the adhesive with an anilox doctor blade is a necessity.
The apparent adhesive bond is also affected by the basis weight of the construction's top ply. Thicker, heavier weight papers will require larger amounts of adhesive, at the risk of coupon lock-up, to properly hold them in place. On the other hand, lighter weight stocks might require significantly less adhesive to anchor them properly, although the potential for premature coupon delamination becomes a concern.
The third factor affecting the adhesive's bond deals with the properties of the release base. A material with a higher level of release will require a higher coat weight of adhesive while the converse is also true. Concerns with respect to adhesive coat weight and coupon performance still hold true as previously discussed. Since the properties of the release base are integral to adhesive performance and subject to variation, it is important to understand them on their own.
Release performance issues
Release properties for both untreated synthetic and UV topcoated base films are primarily controlled by the solid surface tension (dyne level), and the inherent moisture/chemical resistance of the films. Dyne levels under 34 dynes are desirable for both types of release bases.
Suppliers of synthetic films create the solid surface tension of their products during manufacturing. Dyne levels of cast, blown, or extruded poly films vary by chemical composition, manufacturing process, and age. Dyne levels of UV release topcoats, on the other hand are primarily controlled by the cure, or "dose" supplied to the coating by the converter.
When UV release base coatings are cured they undergo shrinkage, which effectively "squeezes" silicone-type release agents to the surface. The stronger the cure, the higher the applied shrink/squeeze and resulting release level, due to the dyne level drop caused by silicone migration. Curing the UV release base less will reduce the silicone migration and provide a relatively high dyne level at the coating's surface, thereby tightening the release of the coupon adhesive.
Changes to the cure applied to the UV release base (dose) can be implemented by adjusting one or two variables. The first involves adjusting the line speed up or down, to effect a change in exposure time under the UV lamp, while the second would be adjusting the input voltage and resulting output of the lamp. Both changes will have an impact on the energy applied to the UV topcoat and subsequent changes in dyne (release) level.
Base release films in coupon construction must provide a chemical and moisture barrier against attack of the underlying print and substrate by the residual moisture and tackifiers contained in the laminating adhesive. Failure to do so will most definitely result in ink picking or smearing and/or coupon lock-up. Synthetic manufactured films are inherently resistant to adhesive attack because of their uniformity in thickness and molecular structure. Converter-applied UV release coatings, on the other hand, are much more susceptible to attack.
Inconsistent application methods — use of dirty or worn aniloxes, applicator rolls, or plates — will result in low coat weights, voids, or pinholes in the UV coating. This can provide openings for the laminating adhesive to penetrate and anchor into the coating and underlying substrate. Just as doctor blades are necessary when applying the adhesive, they are a must for consistency in the application of the UV coating. Additionally, a marginally cured UV release base can possess a relatively low molecular cross-link density; this can result in a weak polymer matrix which the adhesive can more readily penetrate and potentially degrade. Insufficiently dried inks can also generate a surface that creates difficulty for the wetting out and proper flow of the UV release.
Once assembled, coupon constructions might display performance changes and be susceptible to variations in environmental temperature and humidity. While adhesive bond strengths will typically increase over a 24-48 hour period, sudden changes in ambient temperature or relative humidity can have a dramatic impact on the release properties of the coupon and adhesive from the release base. It is for this reason that comprehensive long term and accelerated age testing, including low to high temperature and humidity conditions, must be conducted to assure stability of the finished construction.
Furthermore, process control verification should be conducted prior to every production run and should include but not necessarily be limited to coating adhesion, applied UV cure/dose, release and adhesive coat weights, and applied wet film uniformity.
About the author
Kevin Rea is the UV/EB Product Manager at Northwest Coatings LLC in Oak Creek, WI. He received his BS in Chemistry from Hope College, in Holland, MI, and attended graduate school at the University of Minnesota. Rea has been working in the coatings industry since 1988, and has spent the past nine years with Northwest Coatings.