Part One of a Two Part Series

By Mario Ognibeni, INEOS Films

Overlay films are used as external protective layers in the card structure body; the main objective of the overlay film is to protect the artwork from scratches. Very often the final quality level of a card depends on the correct process performance of the adopted overlay type. For this reason it is extremely important to understand the different kind of services that are required from these clear material layers. They can contain the magnetic stripes and the holograms. They can provide the right laser personalization performance, they can be printed, they can give stress whitening effects and they can give fluorescence emission. In many situations they also solve critical card production process problems. Overlay films are very versatile and they are able to cover a wide range of completely different technical criteria.

This article, focused mainly on PVC and ABS polymers, will cover the technical aspects regarding the specific and the unusual application fields of these important transparent film layers.

Chemical and physical properties
VicatA50 (1 Kg pressure in oil)
The majority of overlay films fall within the VicatA50 temperature range of 69°C - 100°C. PVC film, most commonly used, are located inside the 69°C - 78°C temperature range, with the PVC copolymer overlays in the lowest range. When the PVC homopolymer increases, inside the overlay formulations, it moves the VicatA50 value up until it achieves 78°C with a 100 percent PVC homopolymer overlay grade. A further VicatA50 increase can only be obtained by adding special “modifiers” to the basic PVC recipe or using different kind of polymers. In INEOS Films case, VicatA50 values of 100°C can be achieved with ABS overlays. The higher the VicatA50 point, the better the overlay temperature resistance. This means, in terms of card production process, there is less material distortion but more severe lamination process conditions in terms of temperature and pressure.

Roughness level
In the card market sector, roughness level is very important and must be correctly optimized in order to assure the best trapped air expulsion rate, during the lamination process, while minimizing the impact on the ink surface quality. The trapped air between overlay and core material is primarily responsible for the “frosting” effect on the laminated sheet surface. Frosting is the presence of opaque spots or disturbed areas affecting the sheets surface brightness homogeneity. It is important to understand that the higher the surface roughness level, the better the volumetric pumping effect and therefore air expulsion performance. When metallic inks are used, the overlay roughness can affect the original printing ink surface, under the lamination pressure effect, creating uneven metallic particles orientation changing the way the inked surface quality appearance is perceived by sight. A matte surface result is generally better than an embossed one, maximizing the air expulsion effect and minimizing the impact on the ink surface. Orientation of the metallic ink particle could become a serious quality problem if flow lines or other physical defects are present on the overlay surface, highlighting the poor quality of the overlay. There are however some exceptions, due to technical requirements to cover non-standard lamination processes. An example of this is the Melzer process, where it is a technical constraint to operate with a very high overlay roughness level in order to assure the best final lamination result.

Overlay color
The normal expectation for the overlay is to maintain, unchanged, the card artwork colors, assuring a quite neutral effect. This is not easy to achieve due to some technical constraints that force the use of special pigments. During the overlay production process the polymers tend to become yellowish due to the thermal degradation effect. In order to contrast it, a thermal stabilization system is used in combination with a certain quantity of blue pigment that is able to mask undesired yellowing. This is the reason why most of the overlays have a bluish appearance. Generally, when an overlay must guarantee a laserable performance, special pigments must be dispersed inside the material matrix and, due to their presence, the final overlay color assumes a grayish or brownish tone.

Stress whitening and fluorescence
It is well known that in order to enhance the overly films mechanical properties, a certain quantity of modifier must be used inside the formulation. The kind of modifier has a visual impact when the overlay is folded or punched and may have a whitening effect. The market application for the card drives the customer choice. A light overlay fluorescence is normally considered a material defect, disturbing the security ink emission, visible under an UV light source. This undesired effect is due, for example for the PVC overlays, to the formation of some conjugated double bonds that can occur as a result of overlay thermal degradation. This is why the overlay production process requires a strict control level of its parameters in order to assure the best film final quality. Sometime the overlay fluorescence is a precise market requirement, and in this case, it is possible to obtain the required effect with an optimized dosing of a suitable optical brightener agent inside the overlay recipe.

UV stabilization
It is a normal production procedure to protect the final overlay film against the aging yellowing effect generated by the sun exposure. The most dangerous for the material is the high power radiation associated with UV wavelength exposure. In order to assure to the films long performance without undesired color changes, special UV absorber agents are used inside the process. Depending on the final overlay application, this kind of protection can be modulated in order to meet the more stringent technical requirements.

Thickness value
The overlay films are mainly in the thickness range of 50 - 110 microns with few exceptions. These thicknesses are comprised of the film roughness values of the two sides of film, therefore they are the result of peak measurements. Generally, 60 microns, 80 microns and 100 microns seem to be ideal. This kind of oriented thickness demand can be justified by the markets technical card requirements. The more frequent use of collator machines in the cards production processes is forcing overlays films to have better “free flowing” properties in order to avoid frequent machine stops. The sticky effect is generally the result of static charge. Typically this effect tends to be is less significant with overlay thickness greater than or equal to 80 micron. This thickness effect can be explained by the fact that higher the overlay thickness, the higher its specific weight, allowing the overlay sheets to overcome the sticking and static adhesion forces during their collator machine handling. In opposition to the advantages of thicker films, there is the necessity to face the thickness requirements coming from the contactless cards market sector. The high tech inlays present normally a thickness value around 480 micron and, in order to complete the contactless card structure, two printed cores and two overlays have to be added to the inlay. The most suitable thickness value for this kind of application is 50 microns and 60 microns. The main technical reason for this is the better antenna masking effect with a higher printed core thickness.

Overlay shrinking
The overlay shrinking is strictly connected with the overlay thickness. During the calendering process, the overlay film final thickness is obtained by a combination of two different and synergetic actions. The calender cylinders are closed to the minimum possible distance and a pulling force is mechanically applied to the film in order to reduce its thickness to the final planned value. The combined operations give origin to an internal tensioning inside the film matrix that remains “blocked” as mechanical memory. When the overlay film is heated the shrinking effect takes place generating a material tensioning stress. This stress entity is directly proportional to the overlay shrinking percentage. Taking into consideration overlay thickness values inside the range 50 - 110 microns, the associated shrinking effect can vary respectively between -35 percent and -15 percent. This sensitive variation level is able to explain why a non-symmetric card structure could suffer bending problems due to the mechanical tension on the two card sides.

To be continued….
Look for part two in the July/August Card Manufacturing.