Fountain Solution in Offset Printing INK ACADEMY

Transcription

1 INK ACADEMY Fountain Solution in ffset Printing

2 Fountain Solution in ffset Printing Interaction between ink and water 2 The basis water 3 Demands on fountain solution concentrates 4 Monitoring parameters in the fountain solution 5 Alcohol in the fountain solution 7 Corrosion protection in offset presses 7 Microorganisms in the fountain solution 8 Metering fountain solution concentrates 8 Fountain solution waste from print shops 8 Grain: The fineness of the pores and roughness of the surface of a printing plate Emulsion: A mixture of two, technically speaking, non-miscible liquids that are finely distributed and show no signs of separating Interfacial surface tension: Forces that arise between surfaces of substances that are present in different forms solid, liquid, gaseous Interaction between ink and water The offset process is the only printing process in which two different liquids are used on the printing forme. ne of these liquids, the viscous ink, is and is supposed to be visible after completion of the press run, while the other one, the fountain solution, is only visible in the print image if something has gone wrong. When viewed this way, the fountain solution seems to assume a high degree of importance only whenever things don t work out well. Non-image areas Image areas Fig. 1: H H H Attracting the water H Water Al oxide Metal Ink Ink R R R R Repelling the oily ink Ink Ink R R R R R R R R R R R R Copying layer Al oxide Metal Attracting the oily ink Diagram of the wetting phenomena R = organic molecule group H = hydrophilic substance groups H = H 2 = water Repelling the water The water-bearing layer of a lithographic printing plate consists of anodised (electrochemically oxidised) aluminium and is porous. The printingplate manufacturer determines the thickness of this layer, the fineness of the grain and, as such, what are known as the dampening characteristics of the plate. It is these characteristics, that the fountain solution must manage perfectly in the non-image areas, in order to prevent the ink from adhering to the aluminium oxide surface of the plate. In turn, this means that no ink can be transferred from the non-image areas of the plate to the blanket. The diagram opposite illustrates this principle, although it does ignore the complex processes of emulsification and interfacial surface tension. By contrast, the fountain solution (primarily water) does not adhere (or adheres only inadequately) to the image areas of the plate. The image areas can be inked freely and the interface between image and nonimage areas is clean. 2

3 The basis water Water, as it flows out of the tap, differs greatly from region to region in terms of its make-up. It consists of the chemically defined combination of two hydrogen atoms and one oxygen atom, plus variable amounts of dissolved substances. Among these dissolved compounds, the majority are gases and various types of salt. Some of these constituents of water can have negative effects on the offset printing process, while others create conditions that need to be taken into account when adding fountain solution concentrates. It is therefore highly advisable to analyse a number of parameters of your local tap water. The values of some of these parameters provide information as to how suitable the local tap water is for use in the lithographic process. Parameter Explanation Target value Total hardness in dh Influences the water balance and, when the value is very low, has been known to cause problems with oxidative drying of inks and with printing plate surfaces dh High values are not desirable because of the risk of stripping. Carbonate hardness in mg/l Determines how acidic a fountain solution concentrate has to be, so that the ph for the production conditions required is obtained. No fixed target value Conductivity in µs/cm ph Halide ions, sulphates, nitrates Describes the quantity of dissolved ions in the water. If tap water is already very highly conductive, this points to a high salt content. In individual cases, this can lead to negative effects. The ph value is an indicator of the concentration of hydrogen ions in a liquid and provides information as to the liquid s character, i.e. acidic or alkaline. f little importance in the case of tap water. These are corrosive ions, which, in order to protect structural steel, should not be present in concentrations that exceed specified values. This is particularly important in the case of dampening systems that lead to the formation of mists owing to the character of the system (spray and centrifugal dampening systems). No fixed target value No fixed target value < 20 mg/l nitrate < 25 mg/l chloride < 50 mg/l sulphate When does water need to be treated? In some regions, the tap water values measured fluctuate so greatly throughout the course of the year that it is not possible to adapt to the prevailing water quality all the time. Some water supplies are derived from different sources (e.g. lake water and groundwater, depending on availability) and the values cited above are subject to large, unforeseeable fluctuations several times a day. In such cases, or if one of the above limit values is exceeded, water treatment or conditioning should be taken into consideration. Experience shows that softening of the water followed by reverse osmosis and controlled re-hardening produces the most reliable results. Degrees of German hardness The chart opposite shows the German scale for the total hardness of water compared with a common international unit of measurement, 1 German hardness (1 dh) corresponds to a concentration of mg of dissolved calcium oxide (Ca) per litre of water or ppm. This value reliably expresses the amount of calcium left behind when the carrier medium, water, evaporates. Water evaporates readily even at room temperature very soft soft medium hard fairly hard hard very hard ppm Ca per litre too soft tolerable optimal conditionally suitable too hard Fig. 2: German hardness scale 3

4 Softened water with various salts but no hardness minerals (calcium, magnesium) cation exchange resin Fig. 3: Principle of softening Purified (treated) water (50-75%) with approx. 3% of all salts smosis membrane Service water with various salts and hardness minerals (calcium, magnesium) Concentrate (25-50%) with approx. 97% of all salts Service water with various salts and no hardness minerals (calcium, magnesium) parts per million. nly water that falls within the green zone guarantees trouble-free printing. If the hardness value of your tap water puts it in the white or red zone, we urgently recommend water treatment. Benefits of water treatment In practice, effects on the quality of a printed image can be observed, if fluctuations in water hardness or other quality of the fountain solution occur. For instance, dot gain is one factor that can experience marked effects from changes in the fountain solution. Since, in the quest to standardise the offset process, dot gain is one of the key parameters that have to remain within certain tolerances, it is important to standardise the fountain solution, too. This requires not only suitable measurement and control equipment for metering the correct quantities of fountain solution concentrate, but also a constant quality of the employed water, i.e. tap water. The decision to install a water treatment plant becomes unavoidable if the aim is, to achieve standardisation in accordance with the rules laid down in IS to 5 and the quality of your tap water is only conditionally suitable or fluctuates. Hardening Hardening refers to the adding of calcium or magnesium ions to previously treated water. In regions with hard water, softened water can be rehardened to an appropriate level by cutting (blending) it with untreated water. Before it is used in the offset process, the virtually pure water derived from reverse osmosis is usually brought to the desired residual hardness of 8 to 12 dh by adding a hardening agent. The technologies used to treat water vary. The method, type and design of the water treatment plant must be chosen in line with the specific purpose, requirements and quantities of service water required. We strongly advise you to consult an expert. Fig. 4: Principle of reverse osmosis Demands on fountain solution concentrates Impact on the water Impact in the press Impact on the printing plates Impact on offset inks Impact on the substrate Setting and long-term stabilisation of the ph Targeted setting of the surface tension Partial bonding of the hardness materials Suitable for various dampening system models Protection of steel parts, rollers and plastics Ensuring optimum productivity Good wetting of non-image areas Fast plate runoff Provides good corrosion protection for the printing plates Formation of a stable water-in-ink emulsion Fast adjustment of the ink/water balance No deterioration of the ink drying characteristics No partial dissolving of the paper coating Leaves no separating layer that could have a negative influence on ink adhesion 4

5 All the requirements on this list are met by deploying a mix of functional, chemical substances. The functions of the various substances can be grouped together into four main categories: Buffer components ensure a stable ph of the fountain solution Plate protecting components remain on the non-image area after the printing plate has dried and facilitate rapid re-dampening Wetting agents set the surface tension of the fountain solution and guarantee uniform wetting with a minimal amount of water Solubility promoters ensure that the wetting agents are miscible with the water and that the fountain solution concentrate has an adequate shelf life Monitoring parameters in the fountain solution ph The ph scale is used to measure the acidity or basicity (alkalinity) of a solution and goes from 0 to 14. The range from 0 to 7 is defined as acidic and 7 to 14 as alkaline (or basic), while ph 7 is neutral. Each drop in ph of one unit means a tenfold increase in acidity. The ph influences a number of variables of relevance to the printing process. As the offset printing process has been further developed, the range between ph 5.0 and 5.3 has proven to be favourable. In the USA, however, printers frequently work with a ph of 3.5 and still produce good prints. That said, the domestic paper and ink industries there have ph Neutral adapted to these low values, because the ph has a significant influence on inorganic metal compounds in the paper coating and in the ink. ther branches of the industry in the USA, such as newsprinting, work with neutral or slightly alkaline fountain solution concentrates for various reasons related to the paper. All this reveals that there is a variety of options and philosophies concerning this matter. In Europe, however, the ph range from 5.0 to 5.3 has become established among the manufacturers of fountain solution concentrates. With the quantity normally expressed as a concentration (%) of fountain solution to be added specified and constant, this solution must set the ph to the desired range and maintain it over a long period. For this reason, fountain solutions are buffered. range favourable for printing Concentration of hydrogen ions (mol/l) increasingly acidic increasingly alkaline ph (pondus hydrogenii) Negative common (decadic) logarithm of the concentration of hydrogen ions Fig. 5: ph and acid concentration Inorganic metal compounds such as calcium-containing constituents, can be released from the paper coating and cause piling or disrupt the sensitive ink/water balance. What is more, driers in the ink are also affected. They can lose their effectiveness and consequently retard drying of the ink. The term buffered is used to describe aqueous solutions whose ph does not noticeably change when they are subjected to external influences. 5

6 Indicator liquids change colour when an acid or alkali is added. There are a number of different indicators that change from one colour to another at specific ph values. This means, that a variety of different indicator types must be used in order to measure the ph of solutions over a wide range. There are two methods available for measuring the ph of a fountain solution: Indicator strips are available in many different versions and gradations. They are simple paper strips that have been impregnated with indicator liquids that change colour and indicate the ph value on being dipped into the fountain solution. Measurement errors of 0.5 ph units or more are frequently observed when measuring buffered or coloured fountain systems. This method is therefore suitable only as a rough guide and not for taking precise measurements. Electrical measurement with the aid of a ph meter and suitable ph electrode is extremely accurate even if the solution being measured is buffered or coloured. It is, however, essential that the measurement equipment is stored, maintained and calibrated in accordance with specifications. Electric conductivity A fountain solution concentrate can only adequately satisfy the many demands made on it if the prescribed quantity for addition is complied with. Measuring the conductivity of a fountain solution indicates how well electrical charges can be carried by the solution. As the quantity of fountain solution concentrate in the fountain solution is increased, the conductivity of the solution rises proportionally. At first sight this correlation appears to make electric conductivity a perfect monitoring and control parameter; however, use of this measurement method does have its limitations in practice, which users should be aware of and must take into account: Conductivity in µs/cm Fig. 6: Differences in conductivity with differing fountain solution chemistries While, according to the metering instructions, fountain solution concentrate A produces a conductivity of 10 µs/cm when added in a 4% concentration, concentrate B is correctly metered (650 µs/cm) with a 3% concentration. Consequently, the conductivity value alone cannot be taken as the basis for a product-independent quality statement. Concentrate A Concentrate B % added Thanks to its specific composition, each particular fountain solution concentrate possesses its own range of conductivity. Changing the fountain solution concentrate, will usually also lead to the conductivity of the solution changing. wing to the fact that water also contains various salts, the electric conductivity of the fountain solution is likewise dependent on the water quality used. Fluctuations in the salinity of the water lead to fluctuations in the conductivity of the fountain solution. As you know, isopropanol is miscible with water in any proportion. That said, the alcohol does not break down into ions like the salts from the buffer system do. Isopropanol (IPA) is not capable of transporting an electric charge and consequently, the conductivity of a fountain solution drops when alcohol is added. During the printing process, contaminants that get into the fountain solution can increase (dissolved constituents from pigments or paper coating) or lower (washup solutions, vehicles from inks) the conductivity of the solution. Changes in the temperature of the fountain solution alter its conductivity, even if its composition has not been changed. The higher the temperature, the more mobile the ions and the more conductive the solution. 6

7 As long as these factors are taken into account, it does make sense to measure the electric conductivity of the (freshly prepared) solution in order to determine how much has been added. However, we do point out that unlike the ph value, the conductivity of the fountain solution is not a parameter that is directly relevant to the printing process and result. Alcohol in the fountain solution A very high percentage of the isopropanol used evaporates during the course of the printing process and finds its way into the atmosphere, with negative consequences for the environment. EU Directive 1999/13/EC on limiting the emissions of volatile organic compounds (VCs) was aimed at reducing the solvent emissions of the member states by 50% compared with 1990 levels by the end of ctober 2007 and resulted in all branches of the chemicals industry taking a far more critical look at how they handle technical alcohol. There are already similar and more stringent rules in other countries around the world and in many countries, the use of isopropanol or ethanol in fountain solution has long been banned. The fountain solution concentrates designed for alcohol-free printing contain special active ingredients for setting the surface tension and for controlling emulsification and transport of the solution. Alcohol substitutes are low-volatility, water-miscible substances that even in very low concentrations improve plate runoff and limit the amount of fountain solution inks take up. They do not slow down the drying process of the inks, nor are they harmful to health. What is more, they are suitable for use with all types of printing plate, but can slightly shorten the service life of the plate. The products available have proven how effective they are, but are often still viewed with great scepticism. A rethink in this regard is well overdue in some quarters! If we restrict ourselves to reducing the concentration of isopropanol (<5%), the degree of accuracy offered by conventional alcohol metering units equipped with a float system is inadequate. In these cases, modern, advanced systems based on other measurement methods (e.g. infrared measurement) must be used. If we stop using isopropanol altogether, there is no need for expensive measurement and metering systems and add to this the savings from no longer using alcohol. Volatile organic compounds (VCs) are carbon- and hydrogen-containing substances that evaporate at low temperatures (such as room temperature) or are present in gaseous form. There is no globally standardised definition of VCs. The various definition variants include details of the state of aggregation at the reference temperature, the boiling range, vapour pressure, environmental impact and/or classification of the chemical compound. The service life of a printing plate is measured as the number of print images the plate can produce before image content is lost or the quality of the images produced drops. This manufacturer specification is often not fulfilled if chemical or mechanical influences destroy the surface of the plate (as a rule, the image area) prematurely. Corrosion protection in offset presses A working committee set up specifically to deal with this issue drew up as long ago as 1985 not only recommendations for anti-corrosion measures on offset presses but also testing methods and limit values applicable to fountain solution concentrates. These corrosion protection guidelines for offset presses initially compiled for the newsprinting sector were broadened in 2001 to cover sheet-fed and web offset heatset, too. Products that conform to the requirements of this guideline are listed as certified on the Fogra website ( In addition to a suitable water quality and the use of certified fountain solution concentrates, it is also important to use corrosion-inhibited cleaning agents (washup solutions) and maintenance products. A fountain solution concentrate is certifiable in accordance with the Anti-corrosion Guideline only if its ph is between 5.0 and 9.0 (tolerance ±0.2). In addition, the conductivity of the service water must increase by no more than 1700 µs/cm for sheet-fed offset and no more than 1500 µs/cm for web offset when the concentrate (diluted for application) is added. Both are make-or-break criteria! 7

8 Microorganisms in the fountain solution Growth constant psychrophilic bacteria mesophilic bacteria C Fig. 7: Correlation between bacteria growth and temperature Fountain solution concentrates are incapable of killing germs when diluted for application. If this were not the case, they would have to contain large quantities of preservatives, necessitating special labelling and complex disposal procedures. Since this is far from desirable, fountain solution concentrates are set to perform the function for which they are intended. Under favourable conditions (or unfavourable, depending on how you see it), microorganisms can multiply between four and six times over every hour! Cooling the fountain solution helps stabilise it in this respect, but the degree of cooling should be chosen with a view to balancing the amount of energy (and its cost) invested and how effective the measure is at slowing down microorganism growth. We recommend that the entire system be cleaned thoroughly every 3 to 6 months (to protect the heat exchanger, too). If no special filtration is employed, alcohol-free fountain solution should be used for no longer than 200 operating hours. It is essential that metering of the fountain solution concentrate is conducted in full accordance with the manufacturers instructions, in order to ensure perfect function during production. Different bacteria have different temperature preferences. The blue curve illustrates the growth of bacteria that love the cold, while the red curve shows those bacteria that thrive at 37 C. Low temperatures have a positive influence on the cell division rate of microorganisms. It should be noted that it may be more important to ensure that there is no break in cooling than it is to cool to a very low level. Temperatures between 12 C and 18 C are sufficient under normal circumstances. Metering fountain solution concentrates If the offset printing process is to be standardised, the amount of fountain solution concentrate to be added should also be standardised. The recommended quantity of concentrate to be added to a product guarantees that all active ingredients are matched perfectly to this application concentration in the necessary dosage. Adding too much or too little concentrate is not advisable and both can lead to problems in the long term. Corrosion inhibitors and alcohol substitutes in particular require a minimum concentration for them to develop their full effect. We recommend the use of volumetric systems for metering purposes. The accuracy of metering systems should be tested at regular intervals, either by means of the conductivity of the fountain solution when freshly prepared or by means of volumetric checks. Your supplier s technical staff will be pleased to offer you any assistance you may require. Fountain solution waste from print shops MHM Holding GmbH Feldkirchener Str Kirchheim Germany wing to the fact that the fountain solution used in the offset process is completely used up during the process, fountain solution and/or rinsing solution is/are only left over as waste when the fountain solution system is cleaned. From the supplier s point of view, used fountain solution or rinsing solution currently need to be disposed of as special waste unless the print shop comes to some other arrangement with the local authority. 1D12/2E2