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The anti-graffiti issue is at the centre of mass transit preoccupations. A solution based on two new technologies has lead to a gelcoat meeting both G1 class anti-graffiti requirements (NF F 31-112) and the M1F2 or M2F1 fire and smoke classification (NF F 16-101).
(Published on July 2006 – JEC Magazine #26)
CHRISTIAN AMORIN, MARKETING MANAGER, INTERNATIONAL RAILWAY BUSINESS MÄDER GROUP HERVÉ FARGE, OPERATIONAL MANAGER GTI PROCESS
Mass transit vehicles, as they are intended for collective transportation, have to meet two requirements linked to their service conditions: resistance to vandalism - especially graffiti - and safety, mainly in terms of fire and smoke behaviour. They also have to comply with an economic reality that imposes easy maintainability, as railway materials are designed for a minimum service life of 30 years.
Fire and smoke behaviour refers to standards such as the German DIN 5510 or, more generally, the French NF F 16-101. Parts to be fitted inside coaches have to meet specific performance requirements depending on their location (side panels, ceilings, seats parts, etc.). While metallic parts are not an issue, composite parts are made of fire-resistant resins and thus the decorative elements should not affect their classification.
The behaviour of materials subject to graffiti is more difficult to understand and each country - sometimes even each railway network - has its own standards for evaluating performance. For example, the Italian network Trenitalia uses the ST 373947 technical specification: the surface is tagged with several kinds of paints then dried and cleaned 10 times successively, before the result is shown to the customer for approval. French railway companies (SNCF & RATP) use the NF F 31-112 standard: the surface is tagged with special inks defined by the network, then dried and cleaned three times successively, before the colorimetric changes are measured (∅E) in an authorised range. The substrate is then classified from G0 to G4 depending on the type of removal product used: G0 for water, G1 for a product without safety labelling, G2 for a Xi product, etc.
The French standard, which is probably the most stringent, is increasingly used as a reference. Indeed, graffiti removal accounts for a significant part of railway network budgets (for example, the Stockholm sub-area network is spending 2.5 million euros a year for only 163 trains in service), (Figures 1 and 2).
Conventional anti-graffiti solutions
Up to now, several solutions have been used for graffiti protection, but they all have their own disadvantages.
The first solution is painting, which generally consists in applying 3 or 4 coats of an epoxy-polyurethane system: a 50-μm layer of primer, an 80-μm layer of surfacer and a finishing system composed of a 50-μm layer of high-gloss enamel or a 50-μm layer of clearcoat over a 15-μm layer of coloured basecoat. While this solution is very flexible in terms of colour design and is applicable to all parts of the train, it exhibits medium to poor anti-graffiti properties (from G2 to G4) depending on the products and suppliers. Moreover, in the case of composite parts, painting may affect the fire and smoke behaviour of the substrate.
The second solution consists in using adhesive films, often in the form of a sandwich: an acrylic adhesive layer providing adhesion to the substrate, a pigmented PVC film giving the colours and, finally, a clear polyester film providing graffiti protection. This is actually the best protection against graffiti as it is classified G1 and is quite easy to replace in case of damage. It also makes it possible to combine advertising with the protective function.
However, this is a rather expensive solution because the film has to be applied on a homogeneous, smooth surface, which means that the part needs to be painted before filming. Moreover, films are often inapplicable on 3D parts, leading to unprotected areas in the train. Last of all, two other problems are sometimes encountered: over-vandalism with destruction of the film (and sometimes of the substrate) with sharp tools and prohibitive removal costs when the film has been left too long on the train (due to the ageing of the acrylic bonding system).
Composites are mostly employed to match 3D parts and, in this case, the film solution is not possible. Coloured gelcoats are used with this kind of parts but also with more conventional parts. In this case, cost is optimised, the fire and smoke behaviour is perfectly managed and there is no geometric limitation, but the anti-graffiti performance is not sufficient (G2 to G4 depending on the gelcoat). There is also a design limitation as metallic colours are difficult to match with gelcoats.
The composite anti-graffiti solution
Facing this imperfect situation (See Table 1), the Mäder Group has developed an anti-graffiti gelcoat which meets G1 requirements while offering significant advantages compared to existing solutions.
Table 1: Summary of conventional anti-graffiti solutions
To develop this product, the phenomena involved in the tagging process and in the graffiti removal process were studied very carefully. First, as regards the graffiti aggression process, it has been proved that the products used are composed of a binder, pigments and very aggressive solvents. When tagging, the solvents soften the polymer in the coating layer, allowing the binder and pigments to penetrate inside both the polymer network and the small voids between the fillers and the polymer. The solvent then evaporates while the binder and pigments remain inside the coating.
To remove graffiti, a strong solvent is applied, which also softens the polymer in the coating and dissolves the binder of the pigments in the graffiti. After total dilution in the solvent, the binder and pigments disappear. To ensure the safety of cleaning staff, the products used for graffiti removal are becoming softer and softer each year: current regulations require GI class products (unlabelled) with the objective of eventually using water-based or dry cleaning (G0 class) products. This is in contradiction with the need for cleaning products with a high dissolution power.
Based on this analysis, research has focused on two simple ideas: (1) a low-porosity surface is necessary to avoid the graffiti’s binder and pigments from penetrating; and (2) the surface should be as chemically-resistant as possible to withstand the long-term action of the graffiti’s solvents or cleaning products.
The first difficulty has been to obtain a low-porosity surface while retaining excellent fire and smoke behaviour (M1F2 or M2F1).
Very good fire resistance is obtained by adding particular fillers in the gelcoat - the more fillers, the better it resists. But the addition of fillers always increases the micro-porosity of the surface, caused by interstices between the fillers and the polymer. This problem has been solved by using a patented process called Giralyse®, which allows grafting fillers on the polymer. The Giralyse® process consists in replacing the OH radical on the surface of the filler by an organic radical with a reactive double bond. During the polymerisation stage, this double bond reacts with one of the reactive monomers (styrene or acrylic monomers), so that the fillers are grafted on the polymer network. The result obtained is shown in Figure 3.
A very high chemical resistance could be obtained with highly crosslinked acrylic gelcoats. The second problem is related to the fact that acrylic gelcoats cannot fully polymerise in small thicknesses and in the presence of air, nor are they fully compatible with the polyester resins used for lamination. At this stage, another patented technology, the B-Cure process, has been used. This process allows full polymerisation of the acrylic monomers, even in open moulds. Finally, perfect compatibility with polyester resins has been obtained by using a special primer applied directly in the mould.
It is now possible to produce polyester parts that are fully compliant with both the G1 anti-graffiti performance and M1F2 & M2F1 fire and smoke behaviour requirements. This solution, based on the Giracoat Antigraff® AG 5200 gelcoat, allows the manufacture of graffiti-resistant parts (meeting all European standards) at optimised cost and without geometric limitations. This solution will undoubtedly help solve another major problem of the railway sector: weight reduction.