ADHESIVES FOR SECURITY
Advanced self adhesives are key to many of today’s tamper-evident, security and anti-counterfeiting devices.
The successful use of many security papers, foils and films depends on the technology of chemical fastening systems – especially self adhesives. These are adhesives activated not by heat or by the evaporation of water or some other solvent, but simply by the act of application – by pressure. These adhesives provide the mean whereby laminations, substrates and seals are made effective. In addition to their physical properties they are often required to posses optical properties to allow the security materials to be visibly active and indeed the adhesive system may itself contribute as a carrier for a variety of security materials.
Recent advances in adhesives chemistry have made it possible to achieve virtually all the required physical performance characteristic combined with a choice of optical properties ranging from total opacity to invisibility and including controlled translucency and tinting.
The implications for security printing and packaging are important. Opacity is easy to achieve, for example by loading the adhesive with aluminium powder, by the selection of totally opaque materials like metallized film or by various printing processes, But achieving transparency is a different matter, and transparency is essential for applications involving the protection of documents, photographs etc with a clear film over-laminate. Obvious examples would be for passports, visas and other personal identification. Some security devices may themselves require protection, for example holograms or embossings.
Transparency in the test laboratory is not enough. The Australian driving licence is stuck to the windscreen, so the transparency of the adhesive must be sustained over long periods without deterioration due to prolonged UV exposure, climatic conditions or aging.
The commercial label market has helped to push the technology forward. There is a strong demand for the ‘no-label look’ for packaging of clear plastic and glass containers where the contents can be easily seen without interference and where wording or symbols can be read through the container. You see this increasingly with pharmaceuticals, cosmetics and toiletries, even bottled beer.
Achieving transparency is one thing but this property must be combined with all the physical properties required of the self adhesive. First there is the question of permanence, re-positionability and removability and the degree to which these features are required. Secondly many complications arise from the range of materials to which the adhesive must be anchored and the range to which it will be applied and must bond. Obviously these surfaces vary from those with the highest surface energy (polycarbonate for example) to those apolar surfaces engineered for minimum attraction (PTFE – Teflon for example)
In addition to achieving the required bonds the integral strength of the adhesive must match. So for effectiveness we have already got five critical points – the integral or cohesive strength of the two bonded surfaces, the chemical bond between the adhesive film and each surface, and the ‘z direction’ resistance of the adhesive film itself.
All the other performance demands are familiar to the chemist – peel strength, shear strength, resistance to again, to exposure, to humidity and water, to chemical contamination (including grease, dust and just plain dirt), resistance to temperature extremes. All the properties must be maintained at every stage from original manufacture through conversion, distribution, storage, application and use. Finally the adhesive itself may be a carrier for security features, chemical indicators, microscopic identification particles, other visible additives. The Italian Government self-adhesive postage stamp is constructed with a watermarked face paper and a pressure sensitive adhesive containing a responsive dye. (And the adhesive had to allow for perforation with absolutely no edge bleed, ever.)
Once the chemist has done his best and published all the appropriate guidelines and disclaimers there are still the practical problems of the market place. If the end-user carefully reads the instructions on the packet and follows them conscientiously then generally adhesives to perform according to specification. But that is not the real world and the manufacturer cannot reliably predict the conditions in which his products will be used. And there is no way that all possible conditions can be accommodated nor all the conflicting requirements be met – especially with the added consideration of cost constraints.
I was once asked to provide a coloured vinyl for aircraft marking. The specification for the adhesive was that it would be repositionable at the time of application and then develop a bond that would survive wind and rain in a several hundred mph air stream, -50°C at 30,000 feet and +120°C when parked on a runway in Saudi Arabia. Then, please, they would like to be able to remove and re-use the decal when the aircraft was leased to another airline and the livery had to be changed. Well, we coped with all the requirements except the final removability bit. But that is an example of how the layman tends to take adhesive performance for granted.
A big range of transparent films is readily available and in widespread use. In terms of clarity the best performer is polyester (PET) followed by polypropylene with vinyl (case and calendered), polystyrene and polyethylene probably bringing up the rear. It is vital that any imperfections in the clarity of the film should not be exaggerated by any lack of transparency in the adhesive. So transparent adhesives have been created too, but it did require some considerable investment in R&D. After all we are talking about an adhesive film typically 10 microns thick (sometimes much more) coated on to a surface at something like 20 grammes per square metre. An important factor in achieving transparency for a pressure sensitive adhesive is the choice of material for the release liner which protects the adhesive until its use. Traditionally this backing has been glassine or clay coated kraft paper but for ultra-clear adhesive it is necessary to have the extra smoothness provided by a filmic backing. So the very best clear film laminates have a polyester face with a polyester or polypropylene release liner construction – ‘clear on clear’ as the market knows it.
Some security applications require the overlaminating film to be strong, resistant to tear, burst or delamination and those films I have described so far do have those properties. But many applications require that the film is highly destructible – so that any attempt to remove, reposition or alter the film will damage it. Cellulose acetate does this very effectively and there are also destructible cast vinyls, delaminating polypropylenes and so on to answer this need.
In the commercial market the choice of film may often be influenced mainly by recycling considerations, use of the same material for the label as is used for the container means that the whole thing can be recovered for re-use with minimum contamination of the resin.
Pressure-sensitive laminates are produced from three different adhesive coating systems – solvent based, aqueous based and hot-melt. While there has been real and convincing progress by the adhesives manufacturers in producing formulations for each of the systems it is generally acknowledged that the best results are achieved with the (older) solvent borne acrylic adhesives. These seem to combine the best optical clarity with the anti-aging properties demanded by security applications. However solvent systems are everywhere subject to criticisms on environmental grounds and the manufacturers of both hot-melt and emulsion acrylics have demonstrated an effective response.
Solution acrylics are fully synthetic pressure sensitive adhesives that can be tailored to give a wide range of performance features. They are chemically relative simple products with the performance characteristics built into the polymers rather than created by post formulation or by blending with other ingredients (as is typically the case with hot-melt or water borne technologies). The absence of components sensitive to water and UV light makes solution acrylics particularly suitable for outdoor uses and applications demanding longevity and non-yellowing performance.
Adhesives are most effective when engineered for known conditions of use. There are many surfaces to which it is inevitably difficult to achieve satisfactory adhesion – low energy surfaces, silicone treated, waxed, greasy or wet; rough surfaces where contact is intermittent, wood, open-weave fabrics, unglazed pottery; cold surfaces especially with condensation or frost; dusty or friable surfaces like asbestos or rusty metal. All these pose their own problems to the user and hence to the supplier. And then there are surfaces that are damaged by the application and especially by the removal of adhesive, weak bonded surfaces, paint, flock or natural fibre; and tarnishable surfaces like silver.
So far I have spoken only about pressure sensitive laminates where the adhesive film is protected until use by a release treated backing. But there are also available over-laminating films which are self-wound, that is with no release liner. Now more or less the same films as those using the traditional technology are available and there are well developed adhesive formulations but problems do arise with manufacture and usage. First, with a typical liner-backed construction the adhesive is coated on to the liner which has been release treated (not coated on to the face material). When the liner and the face materials are laminated together the adhesive transfers from the liner and anchors to the underside of the face. This process means that the face material is not exposed to the adhesive curing process. But with self-wound products the adhesive must be coated directly on to the underside of the face material. Then in the case of laminates when the time comes for use, the face material is easily and evenly peeled away from the siliconised backing. But with self-wound products the adhesive must release from the top side of the face. A clever combination of surface treatments (to stimulate release on the one side and/or to improve anchorage on the other) and proprietory adhesive formulations make this possible but the release is typically not an easy one – hence the familiar noise and disturbance of the adhesive film. This may create imperfections in the clarity of the adhesive which may or may not be corrected in time as the adhesive is allowed to ‘flow’ after application.
But self-wound producers are solving these problems and of course there is a big cost saving potential from the elimination of the throw-away release liner.
This is an absolutely key factor in security requirements. With tamper-evident seals the contribution of the adhesive is to ensure that the seal cannot be broken without it showing and the seal cannot be broken and repositioned, replaced or re-used. There are various ways of achieving this. The simplest way, in use for many years in price making, is to die-cut the sticker so that it fractures with any attempt to remove or alter it. All you need is an adhesive bond that is stronger than the integral strength of the paper label. Not to much of a problem in the environment of the supermarket.
But to protect against the determined efforts of the expert criminal when dealing for example with valuable documents is a totally different matter. The principles are straightforward – the adhesive must resist effort to de-activate it and the bonded surfaces must be visibly damaged if disturbed. As with most criminal activity there is a running battle between the chemists on both sides but the good guys have more or less kept ahead. Most successful forgery or mis-usage involving pressure sensitive adhesives is the user to implement agreed procedures and inspections.
However –here are the basic ways to remove ‘permanent’ adhesives: peel slowly – using a knife if the adhesive ‘strings’; soak in hot water using a wetting agent (household detergent); soften the adhesive with a household solvent (lighter fuel, nail varnish remover); use stronger solvents (acetone, MEK); scrape gently; use hot-air gun (hairdryer); refrigerate (professionals use freon).
Fortunately most of these methods create the problems they are trying to solve – whatever weakens the effectiveness of the adhesive bond does obvious damage to the material surfaces. But the experienced criminal using a combination of these methods can often achieve sufficient success for his purposes. So the security seeker must construct his own combinations with a choice of materials that will react predictably and visually to the criminal’s efforts to break down the adhesive and the bonds.
As is so very often the case in security matters it is the informed combination of techniques that achieves real cost effectiveness.