THE EFFECT OF HUMIDITY ON THE OPTICAL PROPERTIES OF HRI (ZNS) COATING

THE EFFECT OF HUMIDITY ON THE OPTICAL PROPERTIES OF HRI (ZNS) COATING

Professor Nadir A G Ahmed

Idvac Ltd.

Manchester, UK

Over the past few years holograms have grown into a complex business to prevent counterfeiting of security cards, banknote and security documents such as passports and identity cards. The rapid advances in holographic technology have led to the development of see-through, semi-transparent diffractive optically variable image device, or DOVID, holograms. The surface relief pattern of the DOVID is usually embossed into a clear base coat, which is applied onto flexible films such as Polyester (PET) substrate. The pattern is then vacuum metallised with high refractive index material (HRI) such as Zinc Sulphide (ZnS) to enhance reflectance and iridescence.

Zinc Sulphide material, which has a refractive index >2 increases the reflectance of the embossed substrate from about 10% to over 30% due to the diffraction of light. For semi-transparent DOVID applications, the ZnS coating has to fulfil certain requirements such as high transparency, easy to detect colour shift, durability, stability (particularly on a passport or other document which is valid for ten years), as well as consistency (in terms of colour uniformity etc), and low cost.

At present, there are so many companies around the world offering high and low qualities HRI (ZnS) coated PET films. The selection of the best performing ZnS coated films requires scientific analytical equipment and vast experience in the field.

There are different methods to metallise ZnS. Among these methods, the thermal evaporation technique is the most used technique inside R2R vacuum web metallisers. The advantages of this technique are high stability process, high reproducibility, high deposition rate, large area deposition, and low cost.

However, the final properties of ZnS coated films are highly dependent on various deposition parameters including moisture retention level in film, types of base coat and top coat, evaporation process, coating thickness, residual gas composition, pressure, condensation rate and line speed inside the R2R vacuum metalliser. For example, the presence of water or solvent within the PET film matrix prior to coating can affect the optical and mechanical properties of ZnS. Consequently, this would influence the final durability of coating when applied onto holographic embossed film.

The effect of moisture on optical coatings has been studied in the past, but not a great deal has been published. It is well known that in many optical coatings voids in the microstructure are large enough to allow water which condenses from the atmosphere or from within the film structure to enter and spread laterally through the coated layer. This process may take place over a long period of time and is an important cause of temporal instability of the optical coating properties. It has been reported before that the most prominent optical effect is a shift of the spectral profile of the coating toward longer wavelengths due to the increase in effective index of the wet material.

Further studies have shown that water absorbed in the polymeric films such as PET generally has a deleterious effect on the adhesion properties of the coating-film interface. Factors affecting the content of moisture in the film are: the season during which the film was manufactured by the vendor, the length of time in storage, and conditions during shipping. This will increase the variability of the actual moisture content of the film at the time of introduction into the vacuum metallising process and consequently will affect the final properties of the metallised product.

In our investigation at Idvac ltd, we found that one of the main problems of ZnS metallised PET film is the durability of its optical properties such as reflectance and transparency when exposed to humid environment from outside or within the ZnS coating matrix. This effect becomes very important to elucidate if ZnS is used as a coating onto embossed security holograms used in passport or travel documents. Such documents are usually kept inside the pocket in clothing which is considered as a high corrosive environment material for optical coatings.

Pockets in clothing are known to have a certain microclimate with high humidity, salty sweat, high temperature, etc which could affect the durability of any optical coating used in documents kept inside jackets or exposed to external warm humid environment.

Idvac ltd has conducted a preliminary development project to test the effect of humidity on the optical reflectance and transmission of ZnS metallised PET films supplied by different suppliers. Various thicknesses of ZnS coating have been selected to investigate their durability against moisture at fixed humidity and temperature levels. The samples have been tested in a controlled humid and temperature over certain period of time.

The main aim of this test was to simulate the effect of humidity on the optical reflectance?and opacity of ZnS as used in passports or identity cards where the whole laminated document is exposed to humidity and body temperature.

The samples have been tested in a humidity chamber in which the temperature was kept constant at 25oC and humidity level was 80% RH. Samples with the same ZnS coating thickness were selected from diferent rolls to minimize statistical error. The samples were then kept in the humidity chamber for a total period of 32 days. The test was only interupted for measurement purpose were samples have been removed for a reflectance and optical observation measurement then placed back in the shortest possible time. It should be noted that in all tests the ZnS coating, which is applied onto a base coat, have not been protected by a top layer, i.e the ZS coating was exposed directly to the humid environment.

One of the test results is shown in the top figure. It is clear that ?the optical reflectance of the un-protected ZnS metallised PET film drops by nearly 12% after 32 days from exposure to humid environment. This could be due to the porous nature of thin ZnS coating structure where moisture can penetrate easily to react with Zinc to form Zinc hyrdoxide which has a milky/white colour. Eventually, this would affect ZnS refractive index and increase coating abosorption leading to a continuous drop in reflectance. At this stage the coating or the laminate may look milky bloom. In fact it was observed that some of?the ZnS coated films with certain coating thicknesses have changed from semi-transparent into a milky colour at the end of the test.?It is possible to speculate that a change in moisture content within the ZnS film matrix increases the absorption coefficient of ZnS coated films leading to a milky bloom for certain ZnS thicknesses. This may also affect lamination and result in a de-lamination when a hologram with a layer of ZnS is laminated for certain document applications

During the development of present project, Idvac ltd has found ways to minimize the impact of humidity on the optical properties of ZnS metallised PET films.

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Idvac Ltd, Manchester, UK, is a leading company in the development of advance vacuum metallising processes for holographic, packaging and other niche markets.

Since its establishment in 2004, Idvac has been successful in developing and introducing new vacuum metallising processes/products and technologies including retrofits.

The company was the pioneer in introducing ZnS process technology, knowhow and retrofits to many leading companies worldwide.

Idvac ltd also offer other process knowhow and retrofits for R2R vacuum metallising including Copper, Copper Alloy, Germanium, Gold metallised finish (no inks or dyes), SiOx, AlOx, Colour Shift liquid Crystals and other niche products.

For further details please contact:

Professor Nadir Ahmed

Idvac ltd,

Manchester, UK

Email: [email protected]

Website: www.idvac.co,uk???

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