Seite/Page: 1 Sunlight sensors A range of pigments has been developed which fade at a controllable rapid rate in response to light exposure. They can be used to produce light exposure or fading indicators which can be easily read with the naked eye. Potential applications range from ensuring that fluorescent safety materials have retained their high visibility to the production of temporary self-erasing markings. Pigments that fade rapidly act as visual indicators of light exposure Rami Ismael * Quick-fading pigments lose their colour rapidly, and may fade completely within a short period of time in response to light exposure (see Figure 1). This is in contrast to conventional pigments, where in general good lightfastness is required. Novel types have been developed in order to allow the approximate light dosage to which any product has been exposed to be monitored with the naked eye. The main interest for these materials lies in indicators or products with a high level of shade change or fade ratio. These products can, for example, be attached to visible areas of safety clothing in order to monitor the stability of their brilliance and ensure their optimal functionality. These novel smart "Quick-Fading" pigments fade to a very large extent when the safety garments no longer conform to the EN471 norm for colour. This would alert the wearer to the problem that they are no longer functioning as expected. The same technology is also of interest for monitoring indicators that can be attached to traffic signs, fluorescent coatings on fire-brigade vehicles and other items coloured with high visibility paints. Light stability issues can be complex The light stability of pigment-based safety clothes and other high visibility coatings depends on the following main factors: Pigment type and pigment particle size; Pigment loading the higher the loading, the better the lightfastness; Thickness of the end-product the higher the thickness, the better the lightfastness; Binder type, additives incorporated (UV absorbers, etc.); Design of the coating (topcoat, special effects such as reflectors); Intensity and angle of the incident sunlight. Most commercially available fluorescent coatings tend to move out of the range of the EN471 norm for colour at light exposure dosages between 75 and 500 MJ. Most safety waistcoats sold in the EU satisfy the norm at the time of purchase (Figure 2). For some at least, this is soon no longer the case, as the waistcoats fade. Wearing a faded waistcoat might increase risk levels as the suggested safety brilliance is no longer fully available. It is important, however, to keep in mind that the colour fading of safety clothes could be due not only to light exposure, but might be caused by other factors. The EN471 norm principally takes into account (based on DIN 58124 and DIN 16954) the following factors: Colour measurement: new state and after exposure to (xenon) light; Colour fastness: rubbing, perspiration, laundering, dry cleaning, (NaOCl bleached and hot pressed); Mechanical properties: tensile strength, water penetration, water vapour resistance, flexibility. How visual fading monitors are designed The objective in creating fading sensors is to colour a sticker or a small piece of textile so that it can be adhered to the safety clothes as an indicator of light exposure. This piece would ideally be coloured on one half with the fading pigments and the other half with a light stable (mostly inorganic) pale yellow colour. Under exposure to light, the fluorescent coating colour will at a certain moment move outside the EN471 norm specifications. The fading piece will fade to a shade between pale yellow and colourless. The pale yellow part (coloured with inorganic pigments) of the piece will remain almost unchanged. The system must offer scope to adjust the colour of the fading part in order to achieve the same shade as the adjacent pale yellow indicator, as soon as the fluorescent coating moves out of the EN471 norm specifications. This can easily be arranged if two different fugitive pigments are used to colour the test piece. One of the pigments used fades completely upon intensive exposure to light (500 MJ) while the other fades under weaker light exposure (75 MJ). Pigments covering the higher exposure field have been available in the company s product range for many years and are already used in (among other purposes) temporary road marking. New products have been developed to cover the low exposure doses.figure 1, already referred to, shows an application made at 12 µm wet film thickness of a side-by-side comparison of two paints. After drying, both paints contain 57 % pigment and 43 % acrylic resin. The application shows visually the impact of xenon light ("Suntest XLS+" instrument from Atlas) on "Radglo" "Quick- Fading" (QF) pigments. Both pigments, "QF-23" (left) and "QF-3-0741" (right) are vivid orange fluorescent colours with high chroma values. Mixing two pigments of the same shade such as these with two different fading rates (see DE after exposure to 10 MJ of Xenon light) would allow the fading rate to be adjusted to suit a product marked directly or via indicators; for example, to monitor the effectiveness of a UV protection cream or the functionality of safety clothes. In these examples, the monitoring system would indicate different renewal times for different sun protection creams or safety coatings or clothes. This methodology could be used across all applications where the visual monitoring of exposure time to light provides added value. Test results confirm flexibility of system
Seite/Page: 2 The novel pigments were produced by utilising proprietary resins, coloured with different proprietary fluorescent dyes and ground to fine powders (D50 = 3-5 µm) after the colouring process while monitoring the process with tight quality control. The resins and the dyes are modified so as to accelerate the fading process. From multiple fading studies, the following types were chosen for testing: Colourant 1 = "Radglo QF-5-0987" red = fluorescent red dyed polyester-3. Colourant 2 = "Radglo QF-5-0988" red = fluorescent red dyed polyester-3, with lower colour strength than Colourant 1. Colourant 3 = Dyed (glycidoxyphenol)propane/ bisamino methylnorbornane copolymer at 99.5 %, proprietary fluorescent red dye at 0.5 %. Colourant 4 = Dyed (glycidoxyphenol)propane/ bisamino methylnorbornane copolymer at 99.9 %, proprietary fluorescent red dye at 0.1 %. Colourant 5 = "Radglo QF-5-0989" red = Fluorescent red dyed melamine formaldehyde resin. Each of these prepared pigments was mixed with "Paraloid F-10" acrylic resin (at a 2:3 ratio) to create paints, which were then applied on a "Penopac 19BR" drawdown to give a wet thickness of 12 µm. The dry films were exposed to 10, 20, 30 and 40 MJ in a xenon light tester ("Suntest XLS +" from Atlas). The measured DE of the exposed drawdowns is shown in Figure 3. Once a DE of higher than 120 had been measured, no further exposure was carried out. This because such a high DE represents complete colour fading to the naked eye. Visual sensors have many other applications Another major area of interest apart from protective clothing are sun protecting creams, lotions and milks for children (or adults), where using a visual monitor patch would give a warning before the protective effect of the sun cream is gone and it must be applied again. Mixing two pigments of the same shade with two different fading rates would again allow the fading rate to be adjusted to the effectiveness of the sun cream, or to indicate different renewal times in different sun protection creams. The tuned mix of pigments might also be incorporated onto/ into a plastic bracelet, that will fade shortly before the UV protection factor of the protection milk or cream is passed. The tuned mix of pigments might also be incorporated onto/ in an easily removable sticker, label or film, which has to be attached to the body or clothes indicating in an adjustable time frame the need to apply the sun protecting milk or cream again. Further, depending on the protection factor of the UV protection products, different types of fading stickers might be developed for every product. Personal care or cosmetic products can be given an indicator to determine the time when they need to be removed or renewed. Such indication might be added - for example - to lip blam, disinfection products and detergents. In order to promote the use of the monitoring stickers or bracelets among children, a light stable colorant might be mixed with the monitoring one to create a design, which would disappear or appear as a comic figure (such as Spiderman or Barbie) after fading occurs, or where a toxic or warning sign or the word itself would remain after the fading of the monitoring patch. "Quick-Fading" pigments open the way towards monitoring the expected functionality of many other safety products such as traffic signs, fluorescent coatings on fire-brigade vehicles, safety clothes such as fluorescent vests and buoys in the sea. While the colour of the safety paint or clothes is in the process of shifting out of the colour specifications (such as the EN471 norm), the colour of the indicator will fade completely, indicating the shift to outside the intended visibility area. The novel technology enables fugitive pigments of different fluorescent shades (Figure 4) to be created to satisfy different market needs. Resin-bound pigments are simple to use The fugitive pigments in this range enjoy all the advantages known for other "Radglo" resinated pigments. They are available as a "stir in" quality. They simply need to be mixed into the formulation of the intended application without the need for any extra grinding step or any high shear forces. However, applying such a production step would not harm them. The advantages of resinated pigments can be summarised as follows: Good colour properties; Special effects such as fluorescent pigments available; Unique effects such as "Quick-Fading" pigments available; Transparency (translucent); Excellent compatibility with multiple resins and formulations; "Stir in" colorants, just mix. No grinding, dispersion or shear forces, no dispersing additives; No adverse effects from grinding which may be required for other ingredients; No particle-size related problems. Technology adaptable to many colorants, additives and applications The two disadvantages with this technology are, in general, the moderate light stability and moderate colour strength. After reviewing these factors, it was considered necessary to optimise only one of the disadvantages of these pigments, their moderate light stability; but in this case the optimisation was carried out in the opposite sense to the usual i.e., reducing the lightfastness of the pigments. Practical considerations for different applications Warning clothes design makes use of several different kinds of polymers. The most important ones are PU-, PVCtextile-, PP fibres-coatings and polyester dyeing. If high visibility paints for buoys and olefins applications are added to this, a diversity of different fading formulations must be designed to suit the intended uses. As these pigments are based on dyed resins, the resins tailored for PE applications and those tailored for high visibility paints with high solvent resistance might not be interchangeable. The embedding technology which is used to make "Quick- Fading" pigments for olefins is based on the company s "RPC1" technology [1], resulting in melting pigments with no plate-out and no shrinkage problems. For the most part, PVC and PU coatings would have a white base layer before application of the top layer, where the fading test materials are incorporated. An extra topcoat or a top layer including UV absorbers is generally recommended for durable safety clothes. This should not be applied to coatings which are intended to function by fading rapidly.
Seite/Page: 3 Applications exist for use as main colourants Temporary markings (Figure 5) are applied mainly with spray cans, in colours which include fluorescent pigments. Using mixtures of standard fluorescent pigments and fastfading types or the latter alone would pave the way towards the creation of quasi-traceless temporary markings. Temporary event marks in streets (or forests), which disappear (leaving only a slight white trace) after fading would be possible. The same concept can be applied to clay pigeons (Figure 6), where colouring with rapidly fading fluorescent pigments alone is recommended to reduce the obtrusiveness of leftover fragments. Indicators based on these pigments might also be of interest for detergents, medical and agricultural products. Moreover, the pigments might be incorporated in DVDs, CDs and other optical data storage devices in order to limit their lifetime. This would be of added value, when bringing back a rented DVD to the renting store is due to transportation of higher negative environmental impact, than destroying the DVD. Pigments fpr such applications need to be designed very carefully, as they need to survive the writing process, which utilizes a very intense laser light. Luckily the dwell-time of the writing process (at the same spot) is very short. Further, a quick fading coating might be applied after finalizing the writing process. REFERENCES [1] Ismael R., New fluorescent pigments for plastics, Kunststoffe International 2007, 4, pp 76-77. Results at a glance» While it is normally desirable to maximise the lightfastness of pigments, a number of applications have been identified for fluorescent colours which fade very rapidly.» By mixing two similar colours with different fade rates in differing proportions, it is possible to produce easily read visual indicators of light exposure with tailored fading rates.» A key application for these products is in monitoring the fading of fluorescent safety clothing, to ensure it retains its desired high visibility. Other possible applications include monitoring of fluorescent colours on emergency services vehicles, checking exposure when sunbathing and the production of temporary markings which are self-erasing.» The pigments used for these applications are based on dyed resin technology, which allows easy incorporation in many coatings formulations. * Corresponding Author: Dr. Rami Ismael RADIANT COLOR N.V. T +32 11 520748 rami.ismael@radiantcolor.be
Seite/Page: 4 Figure 1: Strong impact of light on "Quick-Fading" pigments
Seite/Page: 5 Figure 2: Safety clothes satisfy the EU EN471 norm, but upon exposure to light they sooner or later fade to a less visible shade outside the specifications
Seite/Page: 6 Figure 3: Shade changes of fugitive pigments shown as the DE value upon exposure to 10, 20, 30 and 40 MJ doses of light
Seite/Page: 7 Figure 4: Possible shades of "Quick- Fading" pigments
Seite/Page: 8 Figure 5: Temporary marking products utilising fluorescent pigments
Seite/Page: 9 Figure 6: Clay pigeons are to a large extent coloured with fluorescent pigments