Polycarbonate, a very useful material for ID-card manufacturing, is relatively sensitive to influences such as chemical stress and ageing. Similar to other applications, a wider range of properties and a better peak performance can be achieved by material combinations that provide synergies to each other. This article gives an insight into the latest developments in ID-card materials and illustrates the superior performance that can be achieved by using intelligently balanced mixtures of materials. Based on results from the Agfa Gevaert R&D laboratories, Rainer Rettig looks into the performance of ID-cards made from composite materials from all relevant angles, including the most essential one, the cost of such a product.
Polycarbonate, a very useful material for ID-card manufacturing, is relatively sensitive to influences such as chemical stress and ageing. Similar to other applications, a wider range of properties and a better peak performance can be achieved by material combinations that provide synergies to each other. This article gives an insight into the latest developments in ID-card materials and illustrates the superior performance that can be achieved by using intelligently balanced mixtures of materials. Based on results from the Agfa Gevaert R&D laboratories, Rainer Rettig looks into the performance of ID-cards made from composite materials from all relevant angles, including the most essential one, the cost of such a product. ID-documents with a validity of 10 years or more demand an enhanced robustness of materials used. Although the current applications are pushed to their limits, the evolution of materials is artificially slowed down. More than 85% of today’s tenders for ID-1 sized documents face a substantial error in their basic material design. Instead of defining properties or requesting proof of performance and guarantees, the Requests For Quote define a polymer that has become the industries’ standard answer to cards that require a long life span, namely polycarbonate or PC. By bending the rules of a tendering process bidders are prevented to offer new solutions and superior materials to enter the scene of long life ID documents.
It is obvious that polycarbonate is a well-suited material for ID-card manufacturing. However, although it possesses a lot of notable properties, there is no doubt that it is considerably sensitive to ageing and chemical stress. Furthermore, when electronic structures such as microprocessors are incorporated in a polycarbonate document, tension can cause cracks in the areas surrounding them. In order to minimize these effects through sophisticated processing, substantial expertise is required. To avoid such problems and to optimize properties, composite materials may provide the answer.
Examples of the use of composite materials
In various areas composite materials are already used on a large scale and have shown to offer a number of advantages: In nature, the chemical compound lignin binds and shelters wood fibres, allowing trees to reach high and nevertheless survive storms. The fastest race cars do not rely on steel for the car body and the break discs, but on carbon fibre composite materials. High performance yachts combine fibre materials and foam or web-structured materials to achieve a supreme rigidity-to-weight ratio. Wherever there is a request for the highest performance, carefully selected and well-constructed composite materials beat monomaterials almost every time.
In polymer technology, polymer strings can be used to take on the role of reinforcing fibres, especially if they can be orientated during production. In film production, one way of achieving this is by biaxial stretching (figure 1). This process is used on an industrial scale for the production of olefin films (used in polymer banknotes) and in the manufacturing of crystalline polyester film for ID-card films. When it comes to ID-cards, performance is measured in security, expected life span and costs. Although security is paramount, it will not be discussed in this article. Instead, the focus will be on the latter two parameters which will be compared to the current solutions.
Expected life span
The life span of a product is determined by the amount of stress applied and the structural robustness of the materials and the construction itself. The stress that is applied to an ID document during its life span can vary tremendously. A test scenario which wants to reflect that has to imitate a worst case scenario, during which an attempt is made to purposefully destroy the document. A large number of specialists have worked and are still working on identifying test scenarios that would be widely accepted as reflecting the lifetime stress of an average document. Selected tests try to simulate the main stress scenarios, i.e. mechanical stress, thermal stress and chemical stress and combinations of these scenarios (table 1). The cards are usually stressed in cycles until failure, and the number of cycles survived is related to the expected life span.
The focus will be on the physical breakdown and the related optical and electronic failure in reading, and the manner in which the various materials react in a mechanical, thermal and chemical way.
Mechanical endurance of various materials
The performance of both monomaterials and composite materials in a mechanical bending test according to ISO 7816 is charted in figure 2. Monomaterials are represented by samples of PVC and polycarbonate or PC samples, while PETix (crystalline polyester overlays combined with PETG core material) and Teslin+PVC (PVC overlay combined with Teslin core) are examples of composite structures.
In this test the outer layers, the first to show failure when subjected to bending, represent areas of maximum stress. In the case of Teslin+PVC the soft core accepts deformation and reduces stress of the outer layers of the PVC overlay. The overlay lasts longer than if it was coated over a comparatively rigid PVC core. The composite material PETix with the PETG core material shows an enormous stability of the crystalline polyester which easily endures the alternating tension without breaking. To a large extent the mechanical performance of a thermoplastic polymer depends on the presence and ratio of modifiers and additives as well as on the length, variation and distribution of the length of the molecules within the structure. These parameters can cause a product made of thermoplastic polymers to break or withstand mechanical stress.
Thermal resistance of various materials
The results of a thermal test defined in ISO 7816 are shown in table 2. A card is fixed vertically in a clamp. The environment is heated up and kept at a certain temperature for ten minutes, after which the temperature is increased. The point of failure is reached when a card bends for more than 10 mm during the exposure. The grey areas indicate the increase of performance through a combination of materials, as illustrated in figure 3.
Thermal failure in this test is represented by non elastic deformation. A card that failed in this test will not return to its original shape, as deformation has taken place at a temperature range above the glass transition temperature of the polymer. Nevertheless, a combination with a high temperature polymer such as crystalline polyester and a low temperature polymer such as PVC can easily resist temperatures above the possible temperature range of usage for mono PVC. The yellow areas in the table show the performance of the same PVC. The results in the first yellow line are based on a card of pure PVC; the second yellow line was tested with a combination of the same PVC core as the mono card, but with PETix outer layers. The full advantage becomes clear when this performance is combined with the cost related information in the relative cost chart of table 3. Correlating data is marked in the same colour.
Chemical resistance of various materials
Chemicals are ubiquitous in our environment, ranging from cosmetics to leather treatment for wallets, from petrol residues on our hands after filling up the car at the petrol station to air pollution in urban areas. Exposure to chemicals or UV radiation will affect the robustness of a material. UV light causes the molecules to break and will change the variation and distribution of length. It can have the same effect on additive molecules, reducing the effect of additives such as impact modifiers in PVC. Chemical stress influences the properties in a similar way: molecules can be broken and structures can be weakened through the influence of additives. The easier it is for a chemical to penetrate a material, the more harm it can do. Amorphous polymers such as PVC or polycarbonate are much more exposed to this threat than crystalline materials such as oriented polyester.
To simulate a chemical stress scenario, scientists usually confront the material with a very aggressive substance over a short period of time, which will show its behaviour with less aggressive substances over time. The result of a one hour exposure of cards to methyl ethyl keton (MEK), a solvent widely used in technical applications, is shown in figure 4.
The amorphous materials PVC and polycarbonate face significant deterioration. In contrast, the crystalline PETix material protects the PVC core and the personal data from the negative influence of the solvent attack.
One of the other risks a document project often faces is the failure to implement a product due to high costs. For a document to be implemented on a certain budget, life span extension and cost reduction are vital, and the right choice of material plays a significant role in it as well.
To be able to compare costs a variety of aspects have to be taken into account: material costs, processing costs and replacement costs. Tough materials tend to be more challenging in processing: their conversion will be causing more abrasion and has to be done using higher pressure or higher temperatures than when using less tough materials. This results in an increase in energy consumption, material costs and costs for more durable and powerful tools and equipment. Composite card constructions, when well designed, balance the burden more easily than a monomaterial.
As PVC turned out to be the cheapest and most widely used material for ID-cards in our comparison, it was used as a reference. Since the majority of ID-card projects today aim for a contactless microcontroller the cost of a prelaminate is included. Table 3 charts the relative costs of various materials.
• PVC serves as a reference and is set as value 1.
• PETG is processed at conditions similar to PVC and provides comparable durability and thermal behaviour. Prelaminates are available at a similar cost. Overall, cards from this material perform as PVC, but the material costs are 20% higher due to the actual market price level.
• PC is processed at a significantly higher temperature in prelaminate and card production and in turn the energy consumption is increased. Also the heating and cooling processes take longer, so the efficiency of the process is reduced. Since the printing inks have to withstand higher temperatures, costly inks have to be used. The market price for the material is significantly higher than for PVC or PETG. The mechanical performance of the material requires special tooling and causes increased wear and tear on the tools.
• PETix/PVC (composite material): the product in this comparison is based on a regular PVC structure in the core and crystalline polyester in the outer layer. Prelaminate manufacturing, printing and lamination can be executed at PVC temperatures and cost. No significantly increased wear and tear of tools will happen. The increase of material costs overall is moderate as the PC like price level only applies to 15-25% of the overall card structure.
Composite materials provide significant advantages over traditional material combinations and are able to solve both technical and commercial problems. They are the next step forward in technology, as already experienced in nature and in a variety of other industries. A lot of projects have a relatively low budget, especially those in large volume deployment scenarios. These projects can be realized more quickly and easier when using the advantages of composite materials, additionally reducing production costs and increasing life span. The authorities preparing tenders for ID-documents hold the key to escape the polycarbonate trap by starting to take advantage of the material evolution and by simply reviving best practice through defining requirements, not materials in future tenders.