Technical Information 50.G.001 Radiation-curing Systems General Information, Services

Transcription

1 Technical Information 50.G.001 Radiation-curing Systems General Information, Services NewV UV curing inks and varnishes for offset printing Direction for use NewV offset Direction for use 06/2017 en

2 TABLE OF CONTENT Composition of UV-curing inks and varnishes... 3 Hindrance of reaction... 4 Technical requirements... 4 Rubber blankets, roller coverings... 4 Printing plates for UV offset printing... 4 Washup solution... 4 UV lamps... 5 Using UV-curing inks... 5 Handling UV-curing inks, varnishes and wash-up solutions... 5 Disposing of inks and varnishes... 6 Recycling of UV prints... 6 Substrates... 6 Adhesion on films, foils, metals and cast-coated substrates... 6 Using UV-curing inks and varnishes on thermal papers... 7 UV varnishing of conventional ink surface... 7 UV varnishing of UV ink surface... 7 Cracking of cured UV ink and varnish films... 8 Using the press in dual-mode... 8 Dampening... 8 Set-off in the stack... 8 Suitability for gluing... 9 Hot-foil stamping... 9 Greying of the cardboard surface... 9 Water and weather resistance of UV-curing inks and varnishes... 9 Thermal transfer, laser printing on UV-cured ink and varnish films... 9 Fittings and bearings for varnish and ink pumps Auxiliaries Food and confectionery packaging Classification Shelf life, storing NewV offset Direction for use 06/2017 Page 2/10

3 Composition of UV-curing inks and varnishes The special drying method of these products requires special formulation. The UV inks and varnishes are made from pigments, vehicles, additives and photoinitiators. Pigments (in inks) Vehicles (acrylates) Pre-polymers high viscosity Vehicles (acrylates) Monomers low viscosity Photoinitiators Additives Stabilizer Lubricant Extender With a few exceptions, the applied pigments are the same types which are used in conventional offset inks. The pigment uptake of UV vehicles is limited and this has a negative effect on the consistency and intensity of the inks. By this reason the UV offset inks are short, compact and have different flow characteristics. The vehicles are manufactured from purely synthetic basis. These are acrylates with different chemical structures and varying viscosity. Together with the photoinitiators, they are highly reactive. These photoinitiators are responsible for triggering and finishing the crosslinking reaction. The used additives can be stabilisers, extenders and lubricants. UV inks are not miscible either with conventional oil based offset inks or with auxiliaries for these conventional ink systems, only with products that are recommended for UV ink systems. In addition to process and standard spot colours, chromatic colours, opaque white, metal-pigmented UV-curing inks and fluorescent versions are also available. Curing Curing is a photochemical reaction, a polymerization of the ink/varnish, the transformation from liquid to solid. There are two types of reactions: cationic and radical. The hubergroup NewV inks and varnishes are radical curing products. Cationic reaction: Acid is formed and reacts slowly with the binders. The reaction is completed after hours. Radical reaction: Formed right after UV irradiation, free radicals react with unsaturated groups of acrylates % of the crosslinking is done within a fraction of a second. UV rays Liquid vehicle with initiator Initiator forms radicals Effect of radicals on vehicle Polymerisation Hard, tack-free ink film The energy potential of electromagnetic rays is inversely proportional to their wavelength. The shorter the wavelength, the higher is the energy potential of the radiation. With other words: the short wave UV light has greater energy than the long-wave light. NewV offset Direction for use 06/2017 Page 3/10

4 Hindrance of reaction The short wave UV radiation energy absorption of pigments and fillers is high, but the penetration depth of the UV rays into ink films is low. Generally pigments can take up to 30% of the overall ink composition, the pigments in the opaque whites may take even bigger part. One of the characteristics of the pigments is, that they absorb those short wave rays which are necessary for the reaction. Therefore they extract activation energy from the reactive components of the ink - from the photoinitiators and the vehicle. The absorption property of dark pigments and opaque white inks is high and it grows with the pigment concentration and with the ink film thickness. The high energy, short wavelength UV radiation is consumed at the surface of ink film. This means that in case of a thick ink layer the crosslinking of the vehicle is hindered down to the lower layers of the print. Therefore in case of wet-on-wet printing the highest UV absorption ink has to be printed first. Technical requirements First, to understand why many of the accessories of the conventional oil-based offset printing are not suitable for the UV inks, it needs to be fixed, that oil-based inks and varnishes are non-polar chemicals, while by the reason of the acrylates, the UV inks and varnishes are polar. They react the opposite way with materials. Polar solvents react with polar molecules, and non-polar will react with the non-polar chemicals. The same polarity creates a penetration/migration problem, the chemicals may dissolve the surface of the coverings and this may end-up in the well-known swelling problem. This specific chemistry of UV-curing inks and varnishes needs special UV compatible materials. This means switching from a conventional oil-based ink to a UV ink does not only mean to change the ink. It also requires checking the suitability of the plates, blankets, roller coverings, press chemicals, etc. Rubber blankets, roller coverings By the reason of the above mentioned difference in the chemistry, the polar nitrile or nitrile/pvc based blankets and coverings work very well with conventional offset inks, but make trouble with UV inks. These nitrile compositions can swell and soften when using them with polar chemistry. A non-polar EPDM material (ethylene-propylene-diene-rubber) provides a very good resistance against UV products. This is recommended when using exclusively UV-curing products. It is not possible to run conventional oil-based inks on EPDM rollers. Choosing the right blanket is essential for the required print quality. EPDM coverings are sometimes harder than the conventional ones, however printers prefer the softer rollers for a better performance. But softer coverings require more plasticisers, and high amount of plasticizer in combination with UV inks, can cause the shrinking and hardening of the EPDM covering. Nitrile-butadiene-rubber based (NBR) blankets and roller coverings are suitable for dual purpose use. They are recommended for printers who switch back and forth between UV and conventional jobs. However it is still a compromise. This is why the ratio of the oil-based jobs has to be higher. Neither these coverings are suitable for continuous exposure to UV products. Printing plates for UV offset printing For information about the most suitable plates, please contact your plate supplier. Important is, to use a plate that can handle the UV products. It might be also necessary to use special plates, as the UV press chemicals may dissolve the coating of the plate and shorten the planned run length. Baking the plate helps to increase its lifetime. Washup solution Because of the specific chemistry, use only wash-up solutions that were developed for UV inks and varnishes. The wash-up solutions of oil-based offset inks are not suitable for UV products. For details about our wash-up solutions please find the related Technical Information sheet of the NewV sup auxiliaries on our webpage. NewV offset Direction for use 06/2017 Page 4/10

5 UV lamps Most commonly standard mercury UV lamps are used for curing the UV inks. For specific applications amongst others, nitrogen-neutralised, gallium-doped, iron-doped mercury lamps are also preferred by the market. The iron-doped mercury lamps compared to the standard mercury lamps have increased radiation intensity in the UV-A and UV-B range. By the reason of the narrower radiation range, they require special high reactive, high sensitive inks and varnishes. For less environmental impact and electrical efficiency LED lamps are also popular. Light emitting diodes are based on semiconductor compounds, and emit light in a very narrow band of wavelength. In comparison to the previously mentioned UV lamps, LEDs have much narrower spectral output, therefore they require special, high reactive inks. Opposite to conventional UV curing methods UV LED lamps require no start-up, warm-up time, they are ready for use immediately. By the emission in a narrow wavelength no infrared radiation and no ozone is generated. LEDs require the less space and due to the low heat formation it is highly recommended for temperature-sensitive substrates. The shorter is the wavelength radiation output of the LED, the lower is the efficiency. By this reason, LED UV lamps in printing machines are placed much closer to the printing substrate then the conventional UV lamps. This determines also the thickness of the possible substrate in offset printing. Important: To ensure the most efficient curing, the UV inks and varnishes must be adapted to the specific UV spectra of the different UV lamps! To provide the right drying of the inks and varnishes, the condition of the lamps (eg. operation time, damage, cleanness) and the reflectors (dirty, distorted) must be checked regularly. For more information about the proper curing results of lamps please contact your UV lamp manufacturer. Please consider that not only the lamp, but many further parameters of the printing process have effect on the curing: room temperature, humidity, printing speed, substrate, dampening, etc. Cooling Since a big part of the energy used in the lamp is not transformed into UV radiation, but into visible light and infrared radiation (eg. heat), cooling of the system is essential. Extraction The interaction of short-wavelength UV light with oxygen generates ozone. Extraction is necessary for removing this and also for removing the heat. It also helps cooling the lamps. Shutter Shutters are recommended for safety reasons to prevent the operator exposure to UV and visible light. In case of an immediate stop, automatic shutters are closing and the lamp power is reduced, to avoid the risk of the ignition of the substrate. Ink agitators (recommended) UV inks have a tendency to sleep in the duct. Therefore we recommend using ink agitators. Using UV-curing inks Handling UV-curing inks, varnishes and wash-up solutions UV-curing inks and vanishes irritate and wash-up solutions degrease the skin. The effect on the skin depends on the intensity and the duration time of the contact, as well as the sensitivity of the user s skin. To avoid this, special handling and safety instructions must be observed. For further information please read the EuPIA "Guideline for Printers on the Safe Use of Energy Curing Printing Inks and Varnishes", ask the International Paint and Printing Ink Council ( or your ink and varnish supplier. NewV offset Direction for use 06/2017 Page 5/10

6 Please also keep the following instructions: Avoid skin contact. Wear suitable protective gloves. In case of skin contact, wash the contaminations down immediately with water and ph neutral soap. Never use solvent or detergent. If there is a risk of ink, varnish, additive or wash-up solution splashing into your eyes, wear protective goggles. In case it goes into the eyes, rinse them thoroughly with water and consult a doctor. Use skin care product. Protective creams can be applied on clear skin, but never use cream on contaminated skin. Contaminated clothes have to be changed and washed immediately. Eating, drinking and smoking are strictly prohibited in the area where UV curing inks and varnishes are handled and stored. If you spill ink, varnish or wash-up solution, clean it up immediately. Store used cleaning materials in separate containers. Further details can be found in the safety data sheet of the products. Disposing of inks and varnishes The waste of UV products must be handled as hazardous waste (including cleaning material, substrate with non-cured UV inks and varnishes, wash-up solution residue and also non-returnable containers). Please find further information about the handling of wastes in the safety data sheets. Recycling of UV prints While the printing materials represent only a small proportion of the waste substrate, they do not affect the recyclability of the end product. Prints with dried UV inks and varnishes can be recycled together with the conventional offset print scrap. Substrates UV offset inks are suitable for printing on the following substrates 1 : Coated and uncoated papers and card stocks Top-coated grades of board Metal coated papers Thermal papers Pretreated (corona or gas flame) or pre-primed, non-absorbent substrates such as PE, PET, PVC, PS, PP, BOPP, etc Other substrates, but only after technical evaluation. Generally, previous test are always recommended to see the result in real life conditions. Adhesion on films, foils, metals and cast-coated substrates Treated metals, films, foils and cast-coated substrates can have a negative effect on the adhesion of inks and varnishes. Their lubricant and plasticizer components have a tendency to migrate to the surface of the substrate and step into chemical reaction with the substances of the ink/varnish layer. UV inks and varnishes needs to have a good tape (Scotch) resistance in order to ensure trouble-free further processing. But the good result tape test does not consequently mean good scratch resistance (nail test) as well. Overcoating the ink layer with UV varnish that contains lubricants can improve scratch resistance. However, the adhesion to a substrate cannot be improved this way, it can be the result. 1 Non-absorbent substrates must have a surface tension of at least 38 mn/m in order to ensure optimum ink adhesion. In view of the large number of substrate suppliers and different substrates and substrate grades available, we recommend to carry out an adhesion test before beginning the commercial print run. NewV offset Direction for use 06/2017 Page 6/10

7 Using UV-curing inks and varnishes on thermal papers UV-curing inks and varnishes can be printed on thermographic paper. But a number of thermal papers react to UV vehicles. Depending on the paper quality, the paper may be stained (discoloured) and/or the cured UV ink and varnish film can have poor adhesion. To avoid such problems in and after the real print run, we recommend conducting a prior test. UV offset inks are often not or only conditionally suitable for printing on economy thermal papers* due to the fact that they darken the thermal layer. This is not the case for some new, special paper types. (*Economy thermal paper: Economy quality, white thermal paper without protective surface coat layer.) UV varnishing of conventional ink surface Wet-on-wet coating of conventional printing inks with UV-curing overprint varnishes does not always provide the desired result. The two systems are not compatible. Trapping problems may occur, and the result can be not even surface, pin-holing, the well-known orange peel effect, or the poor adhesion to the ink layer. This problem is caused by the decomposition products which may form in smaller or larger quantities during the oxidative drying of the conventional inks. The drying process of the conventional oxidationdrying inks and the formation of decomposition products is influenced by the climatic conditions at the workplace, the type of the substrate and its surface structure, the applied ink amount, the type of pigments, the composition of the fount solution as well as the amount of emulsified fount solution. Another significant factor that affects the adhesion of the UV varnish is the degree to which the conventional inks had dried. Poorly or insufficiently dried conventional inks have a negative influence on the adhesion. Cast-coated stocks and substrates of low absorbency, or no absorbency, often create conditions that do not assist to the adhesion of UV varnishes even on dry conventional inks. This risk can be smaller in case of using special absorption-drying offset inks, and not using siccative in the formulation. The inks also have to be fully dried, before they get the UV varnish on the top. Despite all of this, long term storing and stacking of such prints can lead to blocking problems. The products may simply stick to each other. For the safe and repeatable high quality result, we recommend to use water-based primer varnish to cover the conventional ink layer. We recommend our water based ACRYLAC primer for these applications. Please find more information in the technical information sheet 50C032 for coating unit primers. UV varnishing of UV ink surface Wet-on-wet coating of UV-curing inks with water-based and UV-curing varnishes generally does not present any problems. In case of thick ink and varnish layer, or poor efficiency of the curing unit, the curing of the ink film may be affected as the varnish film absorbs the UV radiation. The result is insufficient adhesion of the different layers (cured ink and varnish films) and also reduced gloss of the print. For the sufficient drying and for achieving the full gloss of the varnish, a curing unit before and after the coating unit is recommended. Without an intermediate dryer, the risk that the varnish layer damages, especially in case of thermal or mechanical strain (eg. hot-calendaring or foiling) is higher. Inks with a high absorption rate of UV radiation, such as black or dark blue are particularly prone to cause the problem described here. Also opaque whites can cause trapping problem with higher possibility by the reason of their high pigment ratio and their natural behaviour, that they reflect the light very well. Inks containing poor fastness property pigments as well as mixtures from these colours may change shade after UV varnishing! Therefore we recommend making print test before the commercial production. Further information about the fastness properties of inks and varnishes, please find in our product related Technical information sheets. NewV offset Direction for use 06/2017 Page 7/10

8 Cracking of cured UV ink and varnish films Cracking is generally caused by the tension difference among the applied layers and the substrate, the change in the temperature (thermal strain) and/or by mechanical stress, especially if there is insufficient bond between the different layers. In extreme cases, layers can flake off. Despite of this, standard UV varnishes after curing are brittle. If a UV varnish is applied on absorbent stocks, it penetrates into the top layer of the substrate and this effect may be even stronger. To decrease the possibility of this problem, avoid applying heavy varnish films and check the absorbent stocks for suitability, or use flexible UV varnish. Using the press in dual-mode By the reason of the chemistry of conventional and UV products, described at the Technical requirements, switching back and forth from UV inks to conventional oil-based inks, needs a careful work process and also attention and time. For keeping the efficiency of the print production, it is advisable to make longer blocks for UV and non- UV jobs. The longer blocks will result the better efficiency from time and also from cost point of view. By the reason of the problems that may occur after a not-sufficient preparation of the change, it is always recommended to consider the different needs of UV and conventional inks described earlier. But it is not only the preparation before printing. UV inks generally require a slightly lower running temperature in comparison to oil-based inks. These inks do not have the same pigmentation as the conventional inks, therefore to achieve the required PSO results, more adjusting is needed to get the right colour, which means not only time loss, but also more substrate, more ink, more energy consumption, more waste generally. In case of a switch from oil-based inks to UV a very thorough wash should be the first step, in order to remove the conventional ink and its residues from the press. Residues of conventional vehicle systems may cause ink transport problems in the inking unit as well as drying problems. If necessary, repeat the cleaning 2-3 times. After cleaning and before inking up, make sure that the inking unit is free from all the wash-up solution residues. Even the slightest residue of wash-up solution causes splitting, curing and scumming problems. Dampening Print with the possible minimum amount of fount solution. UV-curing inks are sensitive to the ink / water balance. They absorb more water than the conventional inks. In case of not well adjusted dampening, curing and adhesion problems may occur. When printing and coating non-absorbent substrates on both sides with UV-curing inks and varnishes and in case an unsuitable fount solution additive is used, a greasy, usually water-soluble film with a strong odour may form in the stack after prolonged storage. This problem is caused by the non-volatile products, contained in many fount solution additives. Also in case of UV inks, choosing the right fount additive is essential: use suitable additive if you print on non-absorbent substrates, and use a different one in case of paper and cardboard. Amongst others, the lubricant content of the dampening additive can cause poor adhesion if used for printing on nonabsorbent substrates. Note: Wash-up solution residue must not enter the dampening unit, otherwise dampening problems occur risk of scumming. If this happens replace the fount solution with freshly prepared one. Set-off in the stack UV-cured inks and varnishes are thermoplastic materials. Particularly on non-absorbent materials (films, foils, tinplate) they may soften as a result of increased pressure and temperature. The problem it can cause can be noticed easily: the printed products stick to each other. To avoid this, proper curing is essential, also using the right dampening additive and settings, and a further action can be to air the printed sheets after they have passed the UV drier and stack them in as cool condition as possible (< 30 C). NewV offset Direction for use 06/2017 Page 8/10

9 Suitability for gluing Only suitable water-based glues can be used for UV-coated prints. And only UV varnishes are recommended for this application which are stampable and do not include lubricant. Please contact glue suppliers for product recommendation. For the good result we recommend to leave blank or roughen the area which needs to be glued. Based on the wide range of products and technologies on the market, preliminary tests are necessary. Hot-foil stamping Only stampable UV varnishes that were developed for this reason are recommended to this application. They do not contain lubricants which presence leads to adhesion problems. The stamping pressure and the temperature need to be adjusted to achieve the required result. For the suitable varnishes please see our Technical information sheets about NewV lac varnishes. Greying of the cardboard surface The UV varnish penetrates into the upper layer of the board, makes it transparent, therefore the colour of the supporting layers shows through. It also can be caused by the light reflection in the varnish layer. Contact the cardboard supplier to choose the whitest possible cardboard. Water and weather resistance of UV-curing inks and varnishes UV-cured ink and varnish layers do not act as a barrier against moisture or water. They show changeable resistance to the influences of moisture depending on their adhesion on non-absorbent substrates (eg. plastic film and metal surfaces). The lower is the adhesion strength, the less favourable conditions are obtained under the influence of moisture. The water gets under the cured UV layer and lifts that up. In case of mechanical force this film layer separates from the surface. Once the ink and varnish have dried completely, their adhesion properties can considerably improve again. Pre-treatments, such as corona treatment or a base layer with screen-printing ink or covering the ink layer with NewV lac products can improve the resistance against moisture. In case the UV print needs to be weather resistant, suitability tests should be conducted under the customer required conditions. Thermal transfer, laser printing on UV-cured ink and varnish films The producers of laser printers demand that inks and varnishes that are used in their equipment neither soften after drying, nor contain any volatile components. Softening of the inks within the fixing station leads to blocking and build-up on fixing and press rollers and may lead finally to their destruction. Volatile components such as mineral oil evaporate within the fixing station and lead to unwanted smoke formation In order to avoid such problems, please choose stampable varnishes from our NewV lac portfolio. Please find the detailed information about the properties of inks and varnishes in the product related Technical information sheets. The good adhesion of the printed image depends on the ink ribbon, the surface smoothness of the substrate and also on the thermal transfer printing process, not only on the varnish. We recommend you to conduct real life tests before the commercial production run. In case the fusing temperature of the toner is too high, there is a possibility of deposits building up in the hot-laser printer on the fusing roller or on the fusing plate. The thicker the ink/varnish layer, the larger amount of deposits you can find, due to thermoplastic properties of the cured ink/varnish films. Any fumes that are developed during the hot-laser marking are not caused by UV inks and varnishes. They contain no solvents. Pigments with poor heat resistance properties can sublimate during the fusing process. In this case these products must be substituted by similar colour shade inks that are formulated from heat-resistant pigments. NewV offset Direction for use 06/2017 Page 9/10

10 Fittings and bearings for varnish and ink pumps Avoid using fittings and pipe systems which contain copper. Contact between them and the UV-curing inks and varnishes may start the hardening process ( dark reaction ). UV-curing inks and varnishes do not have any self-lubricating properties. This fact needs to be considered when selecting materials for bearings and ink pumps! Teflon is suitable for bearings. Auxiliaries For information please read our technical information 50.A.001 NewV sup Auxiliaries for radiationcuring offset and 50.A.002 NewV sup Auxiliaries for radiation-curing varnishes. UV curing inks has to be mixed with UV curing products and their auxiliaries. But never mix radical inks and varnishes with cationic curing products. Note for application: to avoid problems, always mix the auxiliaries well into the inks and varnishes. Food and confectionery packaging Our NewV MGA product family is recommended for printing primary food packaging or secondary packaging where the primary layer is not a barrier against migration of substances from the printed layer to the packed product. These products are recommended also for further applications where the migration of substances needs to be avoided: eg. cosmetics and pharmaceutical products More information on the subject of packaging for food, cosmetics, pharmaceutical products, tobacco can be found in the information sheet 50.G.002 NewV for food packaging. Please also find information on the webpage of the European Printing Ink Association: In case you are interested in UV inks and varnishes for the applications mentioned above, please contact us for recommendations. Classification Safety data sheet is available on request. Shelf life, storing Keep the ink/varnish containers closed. The cans need to be closed back immediately after usage. Store them in cool, dark place. The minimum shelf life of these products is 12 months from the production date if the container is not opened. But dependent on the storing and handling conditions, they can be usable much longer. Store between 5-25 C. Higher storage temperature may reduce shelf life. Lower temperatures increase the shelf life of UV inks and varnishes. Protect from frost and sunlight. Note: UV inks always begin to harden at the bottom of the container ( dark reaction ). Their consistency changes from rubber-like till hard. Skin does not form on UV inks. Do not use hardened UV inks and varnishes. Please find the shelf of life information in the product related Technical Information sheets. Contact addresses for advice and further information can be found under This Technical information sheet reflects the current state of our knowledge. It is designed to inform and advise. We assume no liability for correctness. Modifications may be made in the interest of technical improvement. NewV offset Direction for use 06/2017 Page 10/10