Uses of UV technology in screen printing Screen 2003 July Authors: Thomas Enk, Michael Jansen, Dr. Wolfgang Schäfer 1.1. Introduction 1.2 Advantages of UV technology 2. How do UV cured inks work? 2.1 Curing mechanism 2.2 Conditions for UV curing 2.3 UV dryer 2.4 UV emitter 2.5 Reflectors 2.6 Cooling 3. Characteristics of UV inks 3.1 Print substrate 3.2 Important ink characteristics 3.3 Reactivity 3.4 Screen permeability 3.5 Viscosity/Thixotropy 3.6 Weather stability 4. Processing hints 4.1 Adhesion and scratch resistance 4.2 Radiation distribution 4.3 Verification of UV radiation capacity 4.4 Factors influencing the curing speed 4.5 Ink additives 5. Labelling and safety at work 5.1 Labelling of UV cured products 5.2 Aspects regarding packing and toys 5.3 Health and Safety at work 6. Concluding remarks 1.1. Introduction Seite 1/17 werke GmbH & Co. KG Telefon: +49-(0)7141-691-0 info@marabu-druckfarben.de www.marabu-druckfarben.de
During the past few years, there have been rapid and continual developments in printing ink technology, and these developments are not confined solely to the field of screen printing. UV curing and solvent free ink systems have been introduced in many areas, including printing inks and varnishing. They take account of increased environmental awareness and fulfil the requirements for rapid and safe production processes. In addition, there is a constant requirement for improved quality as far as the finished, printed products are concerned. The newly developed ink systems must fulfil the quality requirements of the end product in all areas of application. In addition, the product must be safe to use and permit a rational, interference-free production process with high-quality, reproducible print results. 1.2 Advantages of UV technology In contrast to traditional solvent-based inks, the new UV ink technology uses completely new raw materials and reaction mechanisms that ensure a faster and more reliable screen printing production process. On closer examination UV technology offers advantages for all concerned: Advantages for the printing industry UV inks have a drying time measured in seconds, which is also known as radical radiation curing. This offers the possibility of printing at speeds that have not been possible with solvent-based inks. For industrial applications, in particular, much higher cycle times can be achieved on modern, high-speed machines The unrestricted screen permeability of UV inks is an additional positive argument in favour of these ink systems for the screen printer. Downtime is considerably decreased during the printing process. Since the ink contains no volatile components, and the viscosity does not change (shade shift) a consistent batch print quality can be achieved. The fact that the ink does not dry in the screen permits the faultless printing of the smallest details and half-tones. This is a very positive feature for long print runs and multiple elements, in particular. In addition, the use of UV inks permits the perfect reproduction of the print output for subsequent orders. These advantages ensure that a positive increase in productivity is possible in many application areas. They ensure improved economic and competitive attributes when dealing with long print runs or high throughput (faster processing/packing). The absence of solvents is very user friendly and is another reason for using UV inks. The fact that (according to the experts) UV curing requires less energy than a hot air dryer and the small size of the dryer or inline module provides further support for this interesting and forward-looking technology. Advantages with respect to the quality characteristics of the print material Seite 2/17 werke GmbH & Co. KG Telefon: +49-(0)7141-691- 0 info@marabu-druckfarben.de www.marabu-druckfarben.de
Compared to conventional ink systems, UV cured inks and varnishes permit a much higher gloss quality. This provides further possibilities for the creation of gloss effects. The advantages of UV screen printing can be fully utilised also in combination with other print processes, such as full-surface or partial finishing and the protection of offset printing. UV inks and varnishes also ensure the highest chemical and mechanical resistance for the printed ink surface. UV printing fulfils the highest demands for abrasive and fill material resistance, especially for industrial use, for example, the printing of labels and packaging. The perfect printing of the smallest details means that many more applications are possible using screen printing. The interaction of these qualities endows the products with improved quality and attractiveness. Advantages for the environment The most important argument with respect to environmental protection is the fact that no harmful solvent emissions occur during the printing process. This ensures that the ink system is a clear favourite in the debate on environmental protection and safety in the workplace (see also Section 5). 2. The functioning of UV curing systems 2.1 Curing mechanism Under the influence of UV radiation, liquid UV cured inks and varnishes transform within seconds into a cured ink film. The hardening is initiated by defined UV radiation causing a chain reaction, which produces a firm, very tough ink film following the interaction of the individual components. This curing process is based on a chemical process in which short chains of molecules are strengthened by changing into three-dimensional molecular structures. This reaction mechanism is known as radical polymerisation. The reaction speed of the ink in the curing process is dependent on the formula property values and components of the UV ink. The most important components of a UV screen printing ink are as follows: Prepolymer/oligomer = viscous synthetic resin: They form the basis of a UV ink and function as the binding material in the ink system. They determine important qualities of the ink such as adhesion, durability and flexibility. Monomer = reactive thinners: They influence the curing speed and impart the required viscosity to the ink for printing. Their functionality determines and controls the hardening thickness of UV ink. Use of pre-polymer and monomer allows selection of a flexible or hard ink system. Photo initiators = Reaction starter: They absorb incident UV light and separate out into socalled free radicals. Radicals are highly reactive molecules that can form unsaturated double Seite 3/17 werke GmbH & Co. KG Telefon: +49-(0)7141-691- 0 info@marabu-druckfarben.de www.marabu-druckfarben.de
bonds, which leads to polymerisation of the acrylate. The matching of the photo initiators to the colour system is largely responsible for control of the curing process. UV-Härtung gedruckter Farbfilm UV-Bindemittel UV-Monomere Photoinitiator TC 2002 UV-Härtung Kettenreaktion UV-Licht Einwirken von UV-Licht löst Kettenreaktion aus Photoinitiatoren bilden freie Radikale UV-Harz UV-Monomere Photoinitiator TC 2002 UV-Härtung Polymerisation Seite 4/17 UV-Harz UV-Monomere TC 2002 chemische Reaktion ist beendet Dauer: Farbfilm ist sekundenschnell vernetzt After polymerisation is complete, the cured ink film displays excellent chemical and mechanical resistance combined with a high gloss. werke GmbH & Co. KG Telefon: +49-(0)7141-691- 0 info@marabu-druckfarben.de www.marabu-druckfarben.de
UV-Härtung mechanische Haftung kein Anlösen der Oberfläche Haftung durch Adhäsion UV-Harz UV-Monomere TC 2002 As the ink contains no solvents, adhesion to the substrate is purely mechanical and is created by strong adhesive forces. UV-Härtung UV-Systeme härten zu 100 % UV-Licht 10 my ca. 10 my TC 2002 The wet film and dry film strengths of the printed ink layer are identical, as it does not contain any volatile components. 2.2 Conditions for UV curing The above information makes it clear that UV screen printing inks undergo a complex chemical process when hardening. The hardening depends a great deal on the UV energy applied, which is generated by a UV dryer with appropriate output data. It depends initially on applying just enough radiation dosing to the UV reactive material to bring about curing and good adhesion to the substrate. The aim is to achieve complete hardening of the various components without using too much energy, which could have a detrimental effect. In addition to other factors, the applied energy is determined by the following values: Energy output of the UV emitter/dosing Seite 5/17 werke GmbH & Co. KG Telefon: +49-(0)7141-691- 0 info@marabu-druckfarben.de www.marabu-druckfarben.de
Reflector quality Lamp distance focal point Belt speed in m/min The conditions for fault-free UV curing depend on the correct interaction of these values. 2.3 UV dryer Special drying units are required for UV curing. These consist of the following elements, which work in conjunction to ensure reliable curing of the UV systems. UV emitter Reflector Cooling Conveyor belt The functioning of the special UV drying components is described next. 2.4 UV emitter UV emitters consist of quartz tubes, which typically incorporate mercury in an internal vacuum atmosphere. The emitter itself is manufactured from quartz glass, which ensures a high permeability for UV radiation. In addition to the mercury a rare gas, which is important for the generation of the arc, is also contained within the tube. On application of an ignition voltage, this filling permits starting of the UV tube. The gas filling is ionised and forms an arc, which causes the temperature to rise and the mercury to vaporize. This phase produces the light spectrum that is important for UV curing, which operates at a wavelength range of 200 to 400 nm. This range of UV radiation corresponds to the sensitivity of the integrated photo initiators in the UV ink and ensures initialisation of the rapid polymerisation. In addition to the 15-25% of actual UV radiation, there is also a visible light component and a considerable amount of infrared radiation. The high proportion of infrared radiation causes a large amount of heat to be generated, which is beneficial for UV curing, but can be a problem for many print substrates and so must be reduced by suitable measures. Current UV emitters, also known as medium or high-pressure mercury vaporisation emitters, are available in the 80-240 Watt/cm range. By dosing the lamps with special metal halogen additives (e.g. gallium or iron) in addition to the mercury, the radiation spectrum can be displaced and made to match the reaction conditions of the radiation sensitive UV inks. Another way of varying the UV output of curing equipment is to use an infinitely variable conveyor belt or a variable cycle speed. Commercially available UV emitters generally function for more than 1000 hours, although a continual reduction in output is experienced over time. Towards the end of its life, a UV emitter may appear to be operating correctly, but will not in fact provide the correct UV output. Seite 6/17 werke GmbH & Co. KG Telefon: +49-(0)7141-691- 0 info@marabu-druckfarben.de www.marabu-druckfarben.de
It is therefore important that the UV emitter is regularly and carefully checked and changed if necessary. A clear reduction in output below normal UV emissions invariably means curing problems during the manufacturing process at a constant production speed. UV-Strahler Strahlertyp Quarzstrahler Quecksilberdotierung Mittel- oder Hochdruckquecksilberdampfstrahler TC 2002 2.5 Reflectors The output gain of a UV emitter without a reflector would be only 40% of the possible energy. By using reflectors the remaining 60% of energy can be employed for UV curing. This means that the reflectors amplify the energy, but also take over the task of bundling/reflecting the beam onto the focal point by means of the appropriate reflector geometry. The following factors distinguish a good aluminium reflector: polished aluminium for optimum reflection quartz coating for protection against corrosion focussed reflection for precise focussing. Aluminium is a very good reflective material and is very suitable for the task of bundling the UV radiation. Polished metal is used especially for this task, in order to maintain the highest possible gain from the secondary radiation. The UV radiation is focussed to a specific bandwidth depending on the respective application and according to the geometry of the reflector. Elliptically arranged reflectors provide the best possible focussing, i.e. the highest power density of UV radiation, but also develop the most heat on the substrate. The temperature occurring on the material surface can be reduced by setting the reflector geometry to semi-focussing. This will, however, reduce the power density to some extent. Dichroic reflectors are a special method of achieving lasting heat reduction. Several layers of vaporised quartz glass reflect the UV rays that are important for curing and transmit the IR rays responsible for heat generation to an external cooled absorber. This ensures optimum control of temperature with highest possible utilisation of energy. Seite 7/17 werke GmbH & Co. KG Telefon: +49-(0)7141-691- 0 info@marabu-druckfarben.de www.marabu-druckfarben.de
2.6 Cooling In addition to the various reflector arrangements for the reduction of temperature to an acceptable level, there are additional constructive cooling measures associated with UV dryers. It is very important to limit the high temperatures that occur in order to avoid damaging the reflector by overheating; efficient cooling is also essential in the processing of heat sensitive print substrates to ensure dimensional stability. The quality of the cooling system is important and the appropriate compromise must be found in order to avoid losses in the curing output. The following cooling systems can be found: Air cooling outside the reflector chamber, using fans and circulating air to cool the outside of the reflector. This system is known as a hot system as the heated air is not removed at source. Air cooling within the reflector chamber, with cold air being fed into the reflector chamber and heated air being led away to the outside. This method involves loss of some of the UV output, as the oxygen from the supplied air reacts to form ozone. Indirect water-cooling on the outside of the reflector by a water circulation system without affecting the UV output. Direct water cooling by a water-filled tube surrounding the UV emitter. In this way a considerable amount of IR radiation is absorbed, but a proportion of the UV radiation is also filtered out. There are also output penalties caused by absorption losses in the radiation media transfer. These losses can only be compensated for by a higher emitter output. Temperature management is an important variable for successful operation of UV equipment. For the polymerisation process of UV systems, heat is desirable, even essential but it must be applied in a monitored and controlled fashion. The interplay between the emitter output, the reflector characteristics and the heat reduction measures during UV curing is an important part of a successful UV process. 3. Characteristics of UV inks Compared to solvent-based inks the curing mechanism of UV inks caused by the radical polymerisation reaction imparts quite original characteristics to the printed ink film. 3.1 Print substrates In recent years, the number of print substrates available for UV inks has increased due to new raw materials on the market. In addition to traditional materials such as PVC, polystyrol, polycarbonate, PMMA, PE and PP as well as paper, cardboard and card, new print substrates such as pre-treated polyester, varnished foundations, metal and glass are now available. This means that for most screen printing applications there is now a choice between UV cured or solvent-based screen printing inks. Further details to assist in making a decision can be obtained from technical data sheets. Seite 8/17 werke GmbH & Co. KG Telefon: +49-(0)7141-691- 0 info@marabu-druckfarben.de www.marabu-druckfarben.de
3.2 Important ink characteristics Gloss grade In general, all UV cured inks and varnishes have a very high gloss grade. This can be measured by gloss grade measurement equipment according to DIN 67 530, with values in excess of 85 gloss units being achieved without problem. For matt surfaces there is a range of special UV inks. If a specific gloss grade is required (e.g. silk gloss), this fine-tuning cannot be achieved by matt-finish powder or other additives on site, but must be incorporated into the formula when the ink is manufactured. Opacity The simplest and best curing of UV inks is still obtained with non-pigmented varnishes, as in this case the crucial deep curing of the UV radiation through the printed varnish film to the print substrate base is not obstructed by disruptive ink materials such as pigments. This means that the method of pigment mixing and the quantity of additive in the UV cured binding medium is limited and cannot be increased as required, for reasons related to ink technology (reaction speed). Consequently, many colour tones are currently mono-pigmented, very brilliant and less opaque compared to solvent-based colour tones. The thickness of the printed ink layer, which is determined by selection of the correct fabric, is decisive for optimum hardening of the ink. Flexibility A great deal of progress has been made in the past few years in the field of UV. Whereas inks were very hard and brittle with well-known features such as raised edges on printed adhesive films, there are now flexible ink systems that permit extreme film stretching or even deep-draw processes. Durability This is an area where a number of great advantages of UV inks come to the fore. For example, gloss UV ink has a very high mechanical resistance to dry rubbing. Chemical resistance to various fill materials, cleaning materials or test liquids (in the acidic and alkali ranges) is also very high, with more flexible UV inks being generally less resistant than harder multifunctional cured systems. One difference in comparison with solvent-based inks is resistance to water. As UV inks adhere only to the surface of the material, the ink layer will be infiltrated if submerged in water for a long period and adhesion to the print substrate will be severely reduced as a consequence. This danger can be limited by the addition of adhesion enhancers. 3.3 Reactivity As far as curing speeds are concerned, there have also been great advances in highly reactive inks in the past few years. Nowadays speeds of up to 6000 copies/h, e.g. for CD or body printing, or 80 to Seite 9/17 werke GmbH & Co. KG Telefon: +49-(0)7141-691- 0 info@marabu-druckfarben.de www.marabu-druckfarben.de
100m/min for roll label printing can be achieved, and with a much thicker ink application (8 to 12µ) compared to other printing processes. This clear increase in curing speed opens up further opportunities for screen printing inks on high-speed printing machines in combination with other printing processes. In addition to the ink formula, decisive factors for a high curing speed are the controlled ink application (layer strength), the output of the UV dryer unit, the colour of the print substrate and the applied shade. A slower curing speed must be expected when processing flexible UV ink systems, as the hardening grade and reactivity are reduced. These special influencing factors are described in more detail in Section 4.4. 3.4 Screen permeability This is an enormous advantage of UV inks, as the ink cannot react without UV radiation and therefore productivity with unrestricted screen permeability is always higher than for other ink systems. 3.5 Viscosity/Thixotropy UV inks are normally press-ready and need only be thoroughly stirred before printing. Various ink viscosities are available according to the application area and printing speed. By adding reactive thinners, the various viscosities can be further reduced. The thixotropy (flow capability) of UV ink is adapted ex-works to the printing conditions of the respective application. 3.6 Weather resistance Nowadays, high-quality UV ink systems are available that are every bit as good as solvent-based inks as far as long-term weathering is concerned. If the print material is fully covered with an equally weather-resistant UV protective coating, then in a moderate central European climate, a resistance duration of up to five years is currently possible, and in special cases, can be even longer under the appropriate conditions. The only proviso in this respect concerns shades containing a high white or varnish component (more than 30%). 4. Processing hints Seite 10/17 werke GmbH & Co. KG Telefon: +49-(0)7141-691- 0 info@marabu-druckfarben.de www.marabu-druckfarben.de
Success with UV inks depends in the first instance on correct ink selection and on a fraction of a second during the curing process in the UV dryer. In order to ensure optimum operation in this respect a number of processing hints and general requirements must be observed: Uniform radiation across the full path width Adaptation of the UV dose at variable speeds Prompt exchange of the UV emitter depending on the operating hours (max. 1200-1500 hrs) Optimum serviced and cleaned UV dryer (especially reflectors and UV emitter) No change in the gap between the emitter and the material surface and therefore the focussing Belt drive, cycling or unrolling beneath the UV emitter must be uniform 4.1 Adhesion and scratch resistance Directly after UV curing and a short period for cooling to room temperature, the printed ink film must pass the adhesion and scratch resistance test in accordance with DIN 53 151. This is tested using a screen section and adhesive tape test, as well as a fingernail test. For this, the only acceptable values are GT 0 or 1, which certify good ink adhesion to the print substrate. If the test methods cause large ink particles to break free, then the printed ink film is not fully cured or the selected ink type is not suitable for this application. What is important in this respect is deep curing, i.e. the UV radiation through the printed ink film to the boundary surface of the print substrate/ink. This decides the quality of the ink adhesion. For optimum deep curing and associated ink adhesion, the longer wave UV radiation of the UVB and UVA ranges is especially important. In addition to this, the quality of the surface curing is also very important. The aim is a durable and adhesive-free surface that does not separate or block in a stack or on a roller. Energy-rich short wave UV radiation in the UVC range is very important in this respect. In general, almost every UV ink displays post-curing in the first 24 hours, which improves the adhesion and scratch resistance as well as the chemical resistance. This is good, but during the adhesion test one cannot speculate about the efficacy of this improvement directly after UV curing. 4.2 Radiation distribution For a uniform UV illumination across the full path width, it is necessary to know that the UV output of the emitter diminishes considerably the nearer it gets to the edge. To avoid differences in curing in this respect, the emitter length must always be approx. 10 cm larger that the maximum format to be printed 4.3 Verification of UV radiation capacity Seite 11/17 werke GmbH & Co. KG Telefon: +49-(0)7141-691- 0 info@marabu-druckfarben.de www.marabu-druckfarben.de
To maintain a consistent UV curing over time, the capacity of the UV dryer must be regularly (once per week preferably) measured and checked. For this, there are measuring instruments such the UV integrator, or professional devices (e.g. UV Power MAP) with optical indication of the UV capacity via a specific spectral path with temperature development. With these instruments, you can measure the radiation dose (intensity x time) precisely in millijoules pro cm² and always determine the amount and quality of the UV radiation. Information about the quality of the UV dryer is available at all times, together with details of the operating hours counter and the optical cleanliness of the internal components, such as reflector and emitter. It is only in this way that the radiation dose can be held constant and consistent curing results can be achieved. 4.4 Factors influencing the curing speed Each UV ink type is formulated specifically for the individual requirements of the respective application (e.g. various print substrates, printing speed) and for the required characteristics of the printed UV ink (such as flexibility or chemical resistance) and therefore has unique hardening characteristics. This is made clear by the examples of flexible, hard or highly resistant UV inks. In order to create a flexible UV ink film, the internal hardening grade of the ink film is reduced (no three dimensional hardening) which at the same time means a reduction in the reactivity. This means that compared to hard and reactive inks, flexible UV inks do not permit such high belt speeds or cycle times for the same radiation dose. Therefore, before the start of printing, each ink type must be checked for suitability for the specific task. There are, however, other effects that need to be taken into account: Printed ink layer thickness The thicker the printed ink layer, the bigger the radiation dose (mj/cm²) required for hardening, or the production speed must be reduced. In this respect, a central role is played by the type of fabric for text, and the design of the print stencil for halftone screen printing. Normally, finer fabrics, which counteract the high ink application by means of a 100% proportion of solid particles, are used in UV printing. Different shades As described above, pigmentation generally makes UV curing more difficult. Parts of the UV radiation are deflected, reflected or even absorbed. Consequently, compared to un-pigmented varnishes, the radiation dose for pigmented ink systems often has to be increased. Different print substrate colours This also has an effect on the curing speed. In the optimum case, the substrate is white and reflects the incidental UV radiation up through the ink layer. If the substrate is transparent, coloured or even black, this effect no longer applies and a higher radiation dose is therefore required. 4.5 Ink additives Seite 12/17 werke GmbH & Co. KG Telefon: +49-(0)7141-691- 0 info@marabu-druckfarben.de www.marabu-druckfarben.de
In addition to changing the radiation dose or the printed ink layer thickness, the ink can be tuned using additives to provide optimum setting of the UV ink for the production process. By adding UV print varnish, the pigment content can be reduced and the reactivity increased. This, however, causes a reduction in opacity and external weather resistance. Another possibility is to reduce the addition of photo initiator to the UV colour, which is provided as an additive (accelerator). The advantage is that opacity is practically unaffected. In addition, there are various adhesion enhancers available for difficult substrates or for improving chemical resistance. According to the ink type, however, the mixed ink may have a limited cantime. 5. Labelling and safety at work UV curing uses UV radiation for the actual curing process and is currently used for the drying of screen printing inks, but also for offset and flexo printing inks, as well as varnishes and adhesives. Although these systems can be used without volatile solvents (VOC), which can be a risk to health in high concentrations and may be environmentally unfriendly, when handling these types of product, the labelling should be observed and particular attention paid to work safety. 5.1. Labelling of UV cured products As mentioned in Section 2.1 UV cured products normally consist of pre-polymers (oligomers) and acrylate-based monomers, which are in the widest sense derivatives of acrylic acid, and other diverse raw materials. These derivatives can be divided into two classes: Stenomeric acrylate, these are precisely defined products with a low molecular weight and a restricted molecular weight distribution. Eurymeric acrylate, which are products with a high molecular weight and a wide molecular weight distribution. Further details are contained in the UV protocol that was signed in 2000 by the German Professional Trade Association, the HSE in the United Kingdom, the CNAMFS in France and other members of the EU and which can be obtained from the German Print and Paper Processing Professional Trade Association (Berufsgenossenschaft Druck und Papierverarbeitung). The suppliers of the individual components will provide the manufacturers of UV cured screen printing inks with the appropriate Material Data Safety sheet for each raw material. The printing ink manufacturer must then prepare Material Data Safety sheets for the UV curing products offered for sale in accordance with the Dangerous Materials Act. The following directives must be considered in detail: Material directive 67/548/EEC Preparation directive - 88/379/EE Material Safety Data sheet directives 91/155/EEC and 93/112/EEC According to the composition of the UV curing products these can be classified as: Seite 13/17 werke GmbH & Co. KG Telefon: +49-(0)7141-691- 0 info@marabu-druckfarben.de www.marabu-druckfarben.de
unlabelled (occurs only in rare cases) skin irritant irritant for the eyes sensitising For most UV products, this means attachment of a label with a diagonal cross and the text irritant, and in a few cases, the text dangerous to health, as well as appropriate indications about specific dangers (R clauses) and safety advice (S clauses). 5.2. Aspects regarding packaging and toys For packaging, both absorbent and non-absorbent substrates are used. Absorbent print substrates are unsuitable for UV cured screen printing inks, as monomers and photo-initiators can be absorbed into the substrate and be removed from the curing reaction. In addition, one must distinguish printed substrates in which the screen printing ink comes into direct or indirect contact with the content of the packaging. For applications in which there is direct contact with the packaging content, the use of UV cured screen printing inks cannot be recommended. In the case of direct contact with the content of the packaging, the problem is as follows: for UV cured screen printing inks/products, the level of curing (conversion of the acrylate double binding) in production cannot be determined by simple methods and can only be established by special analytical investigations. In this way, it can be proved that even under good conditions, radically cured UV screen printing inks still contain small amounts of unhardened monomers, photoinitiators or fragments of them in the ink film, which migrate under certain conditions and consequently represent a potential danger. In order to minimize the potential danger, it is extremely important to monitor the curing process precisely. As described above, the reflectors must be continually checked and the lamp output measured at regular intervals. Reliable implementation of these conditions means that UV cured screen-printing inks/products can be used for the external printing of packaging containing food, skin care products, cleaners or medical products. Additional safety can be achieved in the form of a certificate of exemption awarded as a result of investigations by independent institutes who analyse, under production conditions, printed examples for the migration of unhardened components. The printing of toys with UV cured screen printing inks/products represents a special case. In general, we can only advise against this, as on contact with saliva, any unhardened monomers, photo-initiators, or their fragments may separate out, which represent a considerable danger to children. 5.3. Health and Safety at work Seite 14/17 werke GmbH & Co. KG Telefon: +49-(0)7141-691- 0 info@marabu-druckfarben.de www.marabu-druckfarben.de
A number of points must be considered with respect to health and safety at work, as well as the effect on the environment due to the different composition of the UV cured products and the specific processing method. Avoidance of VOC In UV cured screen printing inks/products there are normally no flammable organic solvents (VOC). Therefore, employees and the environment are not affected by products found in conventional inks such as these. In addition, no flammable solvent vapour-air mixture can form, so that there is no specific danger of fire or explosion. Naturally, UV cured screen printing inks can be ignited by a naked flame or polymerised by heating. Pigmentation Normally, UV cured screen printing inks contain no heavy metals (DIN EN 71, Part 3), but do contain organic pigments. Skin irritation or sensitisation When working with UV inks, avoid direct contact with the skin, as this can lead to skin irritation or sensitisation. For sensitised people, renewed contact with UV cured screen printing products - even in small doses - can lead to an extremely strong reaction. Therefore, gloves must be worn at all times when direct contact with the skin cannot be avoided. Cleanliness UV cured products do not dry in the air. Therefore, when working with these products extreme cleanliness must be maintained at all times. Areas of skin that come into contact with the products must be cleaned immediately with soap and water. The hands must not be cleaned with solvents as the skin will be degreased and components can be more easily absorbed by the skin. Dirty clothing must be changed immediately and contaminated cleaning cloths must be immediately disposed of in sealed containers as instructed. Cleaning process When cleaning, take care that the skin does not come into contact with the cleaning solution and that contaminated cleaning fluid does not splash into the eyes (wear protective goggles!). UV radiation The effect of short-wave UV-C radiation on the skin can cause sunburn, or in the worst cases irreversible damage. UV-C radiation has serious consequences if it strikes the cornea and conjunctiva of the eyes. Therefore, the UV light from the UV dryers must be fully shielded by appropriate measures. Ozone Seite 15/17 werke GmbH & Co. KG Telefon: +49-(0)7141-691- 0 info@marabu-druckfarben.de www.marabu-druckfarben.de
Ozone is formed by the effect of short-wave UV-C radiation (< 250 nm) on oxygen. It produces a poisonous gas that has a characteristic and very intense odour (mass spectrometer value 01 ppm, odour limit: 0.01 ppm). Therefore, all UV curing units must have a good extraction system. The generation of ozone can be avoided by using special UV lamps, but this means that a considerable proportion of the short-wave radiation that is important for the curing of UV inks is lost. By using activated carbon filters, for example, it is also possible to filter out the ozone from the UV dryer expelled air. Disposal There are no special requirements for the disposal of printed and cured material. These can be disposed of or incinerated as normal, or they can be recycled in accordance with the procedures for the specific printed material. Cleaned tins are either incinerated or recycled. In contrast, uncured ink residue must be treated as special waste (waste code 55903) and must be handled in accordance with the specific regulations. This also applies to printed materials in which the screen printing or varnish is not fully cured. Therefore, when using UV cured products all waste paper and spoiled prints must be fully cured. The same applies to UV cured screen printing inks as for conventional screen printing inks: A danger for humans and the environment only exists if instructions are not followed correctly. To ensure that the specific conditions of UV cured screen printing inks/products in daily use are observed, it is important that all employees involved with the use of UV cured inks are fullytrained and warned of the appropriate dangers. These themes play an important role in the UV seminars held by the manufacturer of the printing ink systems. In addition, the employees concerned should have access to the appropriate Material Data Safety sheets for all UV cured screen printing products used. If these self-evident safety measures are adhered to during the processing of UV cured screen printing inks and products, there is currently no danger to human health or harmful environmental effects. 6. Concluding remarks The past few years have seen a continual increase in the proportion of UV cured systems in the total volume of screen printing inks and varnishes. The advantages already outlined have been so positive in some areas of application that UV technology is used almost exclusively in the manufacture of many products. Positive examples of this are the CD industry or the printing of packaging, both direct printing on synthetic packaging and rotating screen printing for the manufacture of high-quality labels. In this respect, the enormous increase in productivity on multicolour machines would simply not have been possible without UV technology. UV inks and varnishes have also become very popular for many graphical screen printing applications, for example on multi-colour machines for the printing of posters, citylight posters or displays. Currently, new machines in all these market sectors are being fitted solely with UV drying. Seite 16/17 werke GmbH & Co. KG Telefon: +49-(0)7141-691- 0 info@marabu-druckfarben.de www.marabu-druckfarben.de
This positive development will continue in the next few years. For many applications, UV inks could lead to further increases in productivity and in the quality of the print output. Many users still feel a measure of uncertainty and reluctance to give up tried and tested working methods and switch to a new method of working. This is understandable and therefore, the manufacturers of screen printing inks, as well as fabric, stencil material and machines place great emphasis on objectively informing and training interested users. For there are still application areas in which solvent-based ink systems have many advantages. Therefore, if a user wishes to change, good advice is still of prime importance, so that any problems when converting to UV technology can be sorted out in advance. If a company does not have any in-house UV curing equipment, it can consult its ink manufacturer with a view to discussing and investigating its specific requirements. It can then make a sound decision about investing in a suitable UV drying unit. By means of this type of individual consultation, training, symposia and other publicity work, a great deal can be done to ensure that screen printing, in all its various applications, can use the advantages of UV technology to the optimum effect. In this way, the company will be in a position to improve its performance in the interests of all concerned, not least the environment. Seite 17/17 werke GmbH & Co. KG Telefon: +49-(0)7141-691- 0 info@marabu-druckfarben.de www.marabu-druckfarben.de