By Bill Crawford, Waytek Corporation

(Editor’s Note: Bill will be speaking about more material variables at his ICMA Workshop Presentation in Chicago, IL June 7-8, 2006).

Lamination; controlled process or black art? For years man has attempted to comprehend the many variables in printed card manufacturing. It is generally accepted that there are three variables to the lamination cycle: heat, pressure, and time. This article attempts to simplify the process even further and investigate only one of these variables. That being temperature.

To a materials supplier in the plastic card industry, the hydraulic lamination cycle represents a “B-stage” adhesion application. That is, at room temperature the materials must exhibit the easy handling necessary to print and collate. This is the materials’ “A” stage. Later in the process of making cards the materials must perform their second stage of activity – bonding at an elevated temperature. To be successful, the material must be formulated in such a way as to meet both of these requirements.

As an adhesives and coatings supplier to the card industry, Waytek looks at the lamination cycle as the opportunity to create bonds that will last a lifetime. A materials supplier must consider the lamination equipment’s “recipe” to achieve the required results for their customer. In this article, I would like to explore the recipe’s temperature variable in an attempt to simplify the process.

We can start with a few assumptions. The sheet materials are loosely collated and clamped between steel plates forming a book. The book starts at room temperature. As time passes in the lamination cycle, the temperature increases. A chart provided by Bürkle on sheet temperature in a standard cycle demonstrates the time delay of heat getting to the sheets in a book. See Figure 1. Let’s assume for a minute that the variety of activities that must happen for all card layers to bond together will take place at different times in the cycle. We should be able to start and stop the process and see what happens along the way.

A set of experiments was performed to test what happens at specific temperatures in the standard PVC card cycle.

Equipment

The test equipment included a one opening 6x6 inch Carver lab press Model 3912. See Figure 2. The equipment was capable of holding a constant pressure and achieving accurate temperature set points in a stable lab environment.

Materials

A 30-mil card construction was chosen to create standard split core copolymer cards. The construction included full bleed UV inks and a magstripe back. The raw materials chosen were those commonly used in the US market for conventional PVC card body manufacture.

Method

A book consisted of nine materials. The cross sectional drawing of the book is Figure 3. One book per cycle was planished in the Carver press. The cycle time was 19 minutes. Twelve identical books were hand collated to be laminated at twelve different temperatures.

Pressure was held the same for each test. The cycle’s maximum temperature set point was the only variable. The temperatures were varied from 100 to 320 degrees F (38 to 160°C) in 20-degree F increments.

No changes were in place to test variations in press pads, plates, electric vs hot oil heat, or single stack vs transfer presses. This study did not consider book thickness temperature gradients or pressure changes during the cycle. We set out to discover what simply happens at each temperature in the lamination cycle.

Results were recorded by identifying the changes in appearance and adhesion of the book. Particular attention was paid to the surface appearance of the nine material layers.

Observations

The test sheets from each temperature were visually inspected after lamination. Observations of the components were categorized as follows:

· PVC split cores start to seal and ultimately bond permanently.

· PVC clear overlay loses its embossed surface and clarifies to a gloss appearance.

· Trapped air between the overlay and corestock is removed and the materials are planished.

· Coated overlay develops a bond to the ink and has a measurable peel value.

· PVC corestock starts to flow excessively and distorts the image.

· Inks discolor or crack.

· The raised magnetic stripe becomes mirror like and planishes level with the surface of the back overlay.

Results

Observations were found to fit well into a matrix within 20 degree F temperature gradients. A color code was used to represent the results at each temperature: Red meaning no action had taken place during the lamination cycle; Yellow meaning some slight changes were taking place; and Green meaning the physical change required had completed its course. The matrix of results is Figure 4. It was apparent that the seven categories of laminating activity took place at different temperatures in the cycle.

Additional Studies

Further analysis was performed on overlay peels. Three commercially available coated overlays were applied to three UV ink colors. ISO/IEC 10373-1 peel tests were performed. The coated overlays all exhibited similar properties in the lamination cycle. See Figure 5. With these results we can expect that full adhesion of the overlay to ink is one of the last steps to reach completion, just before split core sealing.

Discussion

Are things happening in the right order during planishing?

The matrix seems to indicate materials react throughout the cycle in a sequence. This seems to be the basis of many concerns in card lamination. For example, the complete overlay bonding to corestock should not occur until after all air has been removed between the overlay and corestock. In the matrix (Figure 4) we observed the development of overlay gloss occurring before the overlay adhesives obtain a significant bond. Whether by design or by luck, the sequence of events in card lamination must be carefully controlled and monitored. The more we understand about these processes the greater control over the end result will be.

What is vicat and does it tell us anything about my recipe?

Vicat softening temperature tests give us a close approximation to a material’s softening point under pressure. It says nothing about flow, tackifying, or bonding. Vicat softening temperature tests like ASTM D1525 and ISO 306 are primarily used for comparison studies between materials.

Here’s how it works. The material to be studied is planished into a 3 mm thick plate. A small specimen from this plate is placed in a temperature regulated oil bath. A 1 mm diameter needle weighted with 1 Kg sits on the surface ready to penetrate the sample. See Figure 6. As the temperature in the bath increases at the rate of 50°C per hour, the linear movement of the needle is recorded. The temperature in the bath when the needle has penetrated 1mm is the ‘vicat’ temperature. It doesn’t sound like a planishing operation, but it’s the best measurement we have. There are two loads and two heating rates used in the tests, so be sure to check the test method before comparing values from different sources. See Figure 7. Intra-laboratory correlation has also been shown to be poor between labs with this test. Two values within +/- 5°C between labs is considered consistent in our industry.

Can adhesives be used to fix a lamination problem?

Yes, it is possible to alter the polymeric structure of adhesives to give a variety of performances in a high temperature environment. The peel/temperature curve in Figure 5 can look very different between adhesive coatings of different manufacturers. In addition, adhesives can be tailored to fit the adhesion parameters required in a given recipe. We have all experienced the effect a change in overlay adhesive has in lamination; good and bad. Overlay suppliers and card manufacturers alike are learning that the one-adhesive-fits-all is just not possible.

Why does the addition of chips, wires, metallic foils require recipe changes?

Materials used in plastic card constructions can be separated into two categories. ‘Thermoplastic’ materials are those that soften and change form when heated within our lamination range of 20 to 150°C. ‘Thermoset’ materials are those that do not soften with heat. Thermoplastic materials include PVC, APET, adhesive coating, varnish, screen ink, hot melt adhesive, and the magstripe heat seal coating. Thermoset materials include OPET, holographic foils, magnetic stripe, wire, antennae, module, UV cured ink, and some toner based ink. These materials will not react the same in lamination and require recipe changes to compensate.

Summary

Be smart. The plastic card lamination process appears to be fraught with overwhelming material variables and perilous equipment. I submit to you a strong understanding of the science of the materials can help distill the “magic” and “art” of the process into an understandable set of controllable parameters that can be mastered.

My advice when experiencing laminating problems? Review past history. Perform experimental tests whereby you change only one variable at a time and measure the results. And don’t forget to involve the materials suppliers—they seem to be fairly helpful.

ABOUT THE AUTHOR: Bill Crawford is the V.P. of Sales and Marketing and an owner of Waytek Corporation in Franklin Ohio USA. Waytek has been providing film coating services for the card industry since their inception in 1986. Bill received a B.S. Chemistry degree from the University of Cincinnati in 1981 and currently directs all technical sales of Waytek products. Contact Bill by e-mail wjc@waytekcorp.com or by phone 1-937-743-8080.