Abstract :
Paper with its properties and the way of producing in the process of offset printing changes properties of fountain solution. This is especially in respect with newspaper paper, where for the reasons of alkaline paper production process and its composition and struc-ture, pH value in the printing process significantly changes. Except pH value, electrical conductivity and surface ten-sion changes, as well other physical-chemical parameters.
Investigations in this paper covered physical-chemical measurements of fountain solution in dependence on the quantity of newspaper printing paper, as well interacting time of the paper to the fountain solution. The results show that the addition of paper to the solu-tion raises all physical chemical para-meters. The wetting process is carried out in a mild acid media, in a pH range 4.4 to 4.9. Increasing pH value above 6 could cause a permanent damage of boemite Al2O3 structure that forms nonprinting elements, and that could cause changes in nonprinting elements structure and change their hydrophi-lity. The increase of surface tension and contact angle of the fountain solu-tion could cause significant changes in physical-chemical properties. These changes are evident in disturbing the emulgation ratio and the ink-water ratio.
The investigations we made are of si-gnificance in the field of graphic reproduction. The possibility of dama-ging the printing plate surfaces which do not accept ink in the printing process could cause disturbance of ink-water balance and prints of poor quality. With defining the influence time of the paper on the fountain solution, as well as with other charac-terristics of the fountain solution the limits of optimal quality reproduction were determined.
Key words: contact angle, electrical conductivity, fountain solution, paper, pH value, surface tension
1 Introduction
The offset printing technique is based on different physical-chemical proper-ties of printing and nonprinting areas. The printing areas must have explicitly hydrophobic properties, while the non-printing areas must be explicitly hydro-philic whereas selective adsorption of molecules of different formulation and opposite properties appears on the sur-face of the printing plate. According to its properties and the way of produc-tion, paper changes the properties of the fountain solution on the offset printing process. The change of charac-teristics of the fountain solution is particularly possible during newspaper production because this paper has an alkaline character of the surface, which can influence the change of pH value, surface tensions and electric conducti-vity of the solution during the printing process. Except that the other physical-chemical parameters change and this can have negative influence on the final product. The investigations in this area are rather insufficient, particularly because the physical-chemical charac-terristics of the fountain solutions as well as the surface properties of paper are thoroughly studied and the results of their interaction and eventually bad consequences on the reproduction have been insufficiently observed, and they are the basis and the problem of each printing system. The investigations in this work have been directed towards the observation of paper influence on physical-chemical properties of the fountain solution by determining the pH value of the solution, surface tension and the electric conductivity in dependence on the quantity and time of the interaction with paper.
The damping process happens in mild acid medium, in pH area from 4.4 to 4.9. The increase of pH value of the fountain solution above 6 can cause permanent damage of the boehmite structure Al2O3 which is the constituent of the nonprinting areas and which can change their hydrophilic ability (Pour-baix, 1966). The increase of the surface tension value as well as of the contact angle of the fountain solutions can cause significant change of their physic-cal-chemical properties. This change is visible in the disruption of the emulsi-fication relationship and the balance in the relationship water-ink which can also have influence on the final repro-duction (Lovrecek et.al., 1999).
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2 Background
Today the offset printing is the most often used printing technique. Because of the increased printing speed, the size it is printed on, prepress, modernizing the control of the whole process as well as because of the printing substrates, which can be printed on today, the offset printing, is in constant develop-ment. Today it is printed on all kinds of paper and board in offset technique, and the machines can be quickly adap-ted to the transition from one kind of the printing substrate to another. In regard to the quick development the offset printing has the top-grade possi-bilities of reproduction today. It is essential that the whole process is performed in controlled and defined conditions and that the care has been taken about the influence of particular factors and their mutual interaction.
The principle of printing in the offset process is based on selective adsorption caused by the opposite physical-chemical affinities where the nonprin-ting areas are damped with water whose molecules are of polar character, and the printing areas are covered by ink which contains non-polar molecu-les of the higher fat acids. Fountain solution has an important role in reproduction. The characteristics of the fountain solution for damping influence considerably the printing process and the quality of reproduction. By salt adsorption for hydrophilization from the fountain solution the necessary hydrophilic ability of the nonprinting areas on the printing plate and rubber blanket has been maintained. This decreases the print deformity. Except that the fountain solution decreases the possibility of corrosion of the machine parts, it keeps the constant pH value, it cools the printing units and the printing plate and it give the necessary tackiness and viscosity to the printing ink. Which quantity of the liquid will be absorbed depends on the technologically caused quantity of the fountain solution, which is applied with the damping unit and on the specific surface to which it is proportional. Generally speaking, it depends on the surface characteristics. There is the tendency to decrease the damping quantity, which takes part in printing to the minimal possible level, which prevents toning, because the absorbing of the fountain solution into the mass of the printing substrate (if it concerns paper or board) appears there. The absorption of the fountain solution into the printing substrate causes the swelling of cellulose fibers or ground-wood pulp, and the result of it is the increase of sheet size particularly in the cross grain
direction, which is mostly expressed in multicolor printing. There are also factors which can influence the fountain solution itself. Firstly it is paper which, because of its properties, can influence the change of physical-chemical characteristics of the fountain solution.
3 The principle of the offset printing
The printing process in the offset printing, as the indirect printing tech-nique, happens in the following proce-sses: application of damping solution, application of the printing ink and production of reproduction. The prin-ting plate which consists of texts and illustrations is covered first by the damping solution by means of the damping roller system. The damping solution is absorbed into the nonprin-ting areas preventing the later possible adsorption of the printing ink into the same area (Technical Library, 2004). By turning the plate cylinder, the printing plate comes on the inking system and transfers the ink on the printing elements on which the dam-ping solution did not adhere. From the printing plate (plate cylinder) the paste ink and the transparent solution are transferred on the offset cylinder cove-red by the rubber blanket which is placed between the plate and impre-ssion cylinder, and the printing sub-strate (most often paper) passes between the offset and impression cylinder. The offset cylinder has the role of the ink transfer between the printing plate and the printing substrate and we speak about the indirect printing technique. Because of that the printing plate is laterally reversed. Great advantage of such printing technique is the adaptation of the rubber blanket to different substrates. In this way the determined printing quality level even on a bad paper can be maintained. The role of the impre-ssion cylinder is to realize the corres-pondding pressure and to transport the printing substrate. To enable this process, the printing and nonprinting areas (on the offset printing plate) must differ in their physical-chemical cha-racteristics. Water adheres to the non-printing areas and ink to the printing areas. The technique is based on selec-tive damping of the nonprinting areas by the fountain solution. The printing elements and the nonprinting areas are on the same level, i.e. they have negligible geometrical difference. It is several micrometers. The nonprinting areas are hydrophilic, they attract water, i.e. they are oleophobic because they reject ink while the printing areas are oleophilic. i.e. hydrophobic because they absorb the ink, based on oil. Their
hydrophobic ability is not so much expressed and this is the reason why it is necessary to apply the fountain solution first so that it can cover the nonprinting areas and prevent the toning. Consequently, in production of the printing plate for the offset printing it is essential to obtain the necessary physical-chemical characteristics of the printing and nonprinting areas.
3.1 Characteristics of the fountains solution
The fountain solution in the offset printing, as one of the most important factors, has manifold role and the great majority of the fountain solutions is prepared in similar way. But it is known that the solution has to satisfy several conditions. It has to prevent the application of ink on the nonprinting areas, maintain hydrophilic character of the nonprinting areas, enable quick cleaning of the printing ink from the nonprinting areas, enable quick sprea-ding of water over the plate surface, enable the uniform water flow over the damping rollers, to lubricate the plates and rollers, control the emulsification of water and ink and to control the cooling. All the additives influence the size of the tolerant area, the physical-chemical stability as well as the quan-tity of damping solution which is nece-ssary to prevent the smearing (Mahovic et al., 2003). If the process is performed correctly, water and ink are in a balance, nonprinting and printing areas are cove-red with the thin film of primarily adsorbed substances, i.e. with the foun-tain solution and the ink. But besides very important differences between these areas the water and the ink can adhere to both of them. The mixing of water into ink is acceptable up to certain limits while the reverse process must not happen. In the case of bad damping, the washing out of the printing processes can happen because of too much foun-tain solution and toning, and the ink adheres to the nonprinting areas.
The fountain solutions contain the fo-llowing main groups of components which equal the basic functions of the fountain solutions: water soluble gum arabic, buffer system for regulation of pH values, hydrophillization salts, buffers, surfactants, solvents, additives for lubrication, agents for emulsify-cation control, additives for viscosity increase and some others such as bio-cides, agents for defoaming and colo-rants (Kohler, 2004). All these addiives influence the physical-chemical stability of the printing and nonprinting areas. They influence, more or less, the size of the tolerant area as well as the
quantity of the fountain solution, which is necessary to prevent smearing. The decrease of the necessary quantity of the fountain solution is possible only when the damping is increased in the given system: printing plate-fountain solution, respectively, when the contact angle is decreased. It is possible with the decrease of the surface tension of the solution or the inter phase tension between the fountain solution and the offset plate (TS Fuji Hunt Photographic Chemicals, Inc., 2003). The additives for the correction of pH value do not influence the surface tension of the solution but with different pH values the important processes on the surface of the printing plate happen. Offset plate is more or less hydrophilic, respectively hydrophobic. The buffer system consists of weak acid and the corresponding salt or weak alkali and the corresponding salt. Buffers keep the pH value in a narrow limit suitable for the contact with the ink and they enable in this way a quick drying and they prevent oxidation etc. Most often the buffers are the solutions of gum arabic and salt of the phosphoric acid. The most suitable pH value for keeping the maximal hydrophilic ability of the nonprinting areas during the exploita-tion of the printing plate is from 4,5 to 5,5 (on monometallic plates), while the acceptable tolerance is from 4,8 – 6,0. The oxidation process of drying above pH 5 will become more difficult up to the unacceptable level. At the same time the damage of the anodic layer formed on aluminum can appear. Under the value of pH 4,5 the hydro-philic anodic layer begins to dissolve and with further decrease of pH value the risk of metal corrosion is increased. The surface tension of the fountain solution can be decreased by adding the alcohol which influences neither the pH of the solution nor the existing physical-chemical properties (Traber, 1996). It is most often 2-propanol. The decrease of the surface tension depends on the kind and concentration of the used alcohols. They are added in the amount of 5% to 15%. In order to pre-vent the coagulation, alcohol must be put first and after that the additive for pH correction. The fountain solutions prepared in such a way influence suita-bly the hydrophillization of the non-printing areas, the cleaning of the sur-faces from ink, better damping of the nonprinting areas, the decrease of the printing plate wear by decreasing the friction and emulsification control. Cleaning agents such as acids and solvents have a secondary role. They clean the surface with the tendency of removing the fat accumulations. If the sedimentation of these
agents happens, such accumulations will attract ink and it will result in toning. By using the phosphoric acid glossy surface appears, but because of its aggressiveness etching appears in which a thin layer of metal, only several molecules thick, dissolves from the plate, exposing the new surface in this way. Some salts react aggressively with the metal aluminum forming the hydrophilic compounds. All such salts contain strong polar bonds which attract the hydrophilic molecules of water. They are most often silicates (SiO4) and phosphates (PO4). Both groups contain oxygen and the polar oxygen atoms act with the water molecules on the surface.
3.2 Properties of paper
Printing can be defined as the hetero-geneous process which begins with the production of the printing plate and finishes with the production of prints. In all the processes, regardless the prin-ting technique, four elements are neces-sary for production of prints: printing plate, printing material (paper and board), printing ink and printing machi-ne. Each of the mentioned elements has its own specific properties which can be static (they stay unchanged during the printing process) or dynamic (they change during the printing process). In an ideal example paper should be the element with static properties. But its properties change more or less. For preparation of the pulp the fiber mate-rial is used consisting of ground-wood pulp, cellulose and waste paper. All the working processes necessary for the preparation of all the needed raw materials and their mixing into the mass ready to go to the paper machine are meant under the terms preparation of pulp. The main working processes are dissolving and grinding of raw materials, mixing of different fiber substances and adding of fillers, colorants and glue. The usage of parti-cular fiber raw materials as well as other auxiliary raw materials depends on the properties which are demanded from the products – paper, board and cardboard. The additives which are added to the mass as the auxiliary raw material are glues, fillers and colorants. Papers cannot be considered homoge-neous products. The network structure can be noticed which interspaces are partly filled with fillers, parts of fibers, resins and colorants. Fibers and fillers differ very much (fillers are mainly inorganic pigments) and their mutual relation, i.e. their interacting is weak, which is visible in
rather weak yield of fillers. Considerable part of the volume is filled with air in paper the share of which can be over 80%. Fillers are primarily used today for improving the paper characteristics, especially for improving the possibilities of printing. Except for bringing many advantages to papers the fillers have some disad-vantages: the strength regularly decree-ases with the increase of fillers quantity as well as with the degree of sizing. Except that they are partly responsible for picking and dusting of paper. Paper is a fibrous substance consisting of great number of single fibers. Each fiber is a tube able to accept liquid and to swell. Acceptance of liquid is sup-ported by the capillary activity of the pores. In this way paper can influence more or less the fountain solution in changing its chemical formulation. The conditions for that are in the properties of the used paper, formulation of the fountain solution, contact surface of paper and solution, as well as the time, solution flow through the machine and the thickness of the film. An important parameter which directly influences the damping level in the machine is the nature of the printing substrate. For example, porous paper will “use” more fountain solution. But the quantity of the absorbed water can negatively influence the paper causing the accu-mulation appearance, picking etc. Inks for sheet fed and web offset printing together with the participation of water solutions for damping are the keys for printing without problems and for the real quality of the printed product. Today it is possible to justify and choose the inks and fountain solutions for material to be printed.
4 Experimental part
The phenomena on the boundary phase’s liquid, solid and gas are called surface phenomena. The principle of the printing plate for offset printing is based on physical-chemical properties of the surface. The phenomena usually appear on the surfaces which are in contact and they are caused by different position of the molecules of the same type. These phenomena are absorption, damping, and decanting, capillary pe-netration e.t.c. In this work the physical-chemical properties of the fountain solution which appear on the contact with the coated paper for magazines were investigated by measuring the pH va-lues, surface tension, electric conduc-tivity and contact angle. The influence of the newsprint paper quality on the changes of the mentioned
electroche-mical parameters of the fountain solution was investigated. The fountain solution was prepared by diluting the concentrate with the demineralized water in the volume concentration of 2.5%. The defined quantity of the newsprint paper (45dm-2) in the quantity of 0.02, 0.06 and 0.10 gcm-3 is immer-sed into the damping solution. The samples of the damping solution are prepared by mixing the concentrate with the demineralized water in the concen-tration of 2.5 vol%. The conductivity of the prepared solutions is measured by means of the conductometer LF 330/SET of the firm WTW GmbH. pH value of the solutions was measured by pH meter pH 330/SET of the firms WTW GmbH with the standard electro-de and calibration and the surface ten-sions were measured by the method of stalagmometer. A measurement of the specific solution density was performed by picnometer. As the reference solution the distilled water was used whose volume was constant at the determined temperature and pressure (72.75x10-10 mNm-1 at 21oC). Measure-ments of the contact angle were per-formed by goniometer NPL
5 Results
Measurements of the mentioned phy-sical-chemical parameters of the foun-tain solution show important changes in values at the solution which were influ-enced by paper in relation to their va-lues without the influence of paper. These value increments of physical-chemical parameters of the fountain solution can have negative influence on the printing process itself. Increasing the pH values of the fountain solution above 6.0 can cause continuous damage of the boehmite structure of the printing plate surface with Al2O3 which the non-printing areas are composed of (fig. 1).
Fig. 1. Dependence of pH value of the solution on time .
As it is visible in figure, by the time increase of paper influence on the foun-tain solution, pH increases up to the value of 6.5 if the volume share of paper is greater and the surface tension of the solution increases up to 52x10-2 mNcm-1 (fig. 2). From the curve for the first sam-ple it is visible that the value of the surface tension grows quickly at the be-ginning and after 30 minutes the value increase of the surface tension becomes slowly. From the diagram is also visi-ble that the samples of solutions ob-tained with 0,02 gcm-3 paper show somewhat lesser value increase of the surface tension than the samples ob-tainned with 0,06 gcm-3, and the sam-ples obtained with 0,06 gcm-3 paper show somewhat lesser increase than those obtained with 0,10 gcm-3 paper. All the curves have similar flow and show logarithmic dependence.
Fig. 2. Dependence of surface tension of the solution on time .
Fig. 3. Dependence of electric conductivity on time of paper influence .
In dependence on time of paper influence on the fountain solution the electric conductivity grows by conductive parti-cles dissolved from the news-print paper as well (fig. 3). It is visible from the dia-gram that the dependence of
electric con-ductivity on time of exposing the dam-ping solution to the influence of the newsprint paper shows great logarithmic dependence. In dependence on the paper quantity the increase of electric conduc-tivity is dependent. It is visible from the diagram that by the increase of exposing time of fountain solution to the influence of paper the electric conductivity grows quickly at the beginning and after 30 minutes this growth becomes more mo-derate. It is also visible that the quantity of paper influences the increase of elec-tric conductivity of the fountain solution. From the diagram in figure 4 it is visible for the first sample that the cosine of the contact angle on nonprinting surfaces grows quickly (angle is decreasing) up to the time of 30 minutes. After that the cosine of the contact angle shows milder growth. The sample with 0,10 gcm-3 paper shows the greatest change. In dependence on the paper quantity the contact angle is changed. All the curves show the logarithmic dependence.
Fig. 4. Dependence of the contact angle on time of paper influence .
6 Discussion
From the diagram in figure 1 it is visible that the pH value of the fountain solution grows quickly at the beginning and after 30 minutes the pH value growth becomes slowly. From the diagram it is also visible that the samples of the solutions obtained with 0,06 gcm-3 and with 0,10 gcm-3 pa-per show lesser pH value increase than the samples obtained with 0,02gcm-3 pa-per. The curves show the logarithmic de-pendence. By determining the dependen-ce of the surface tension on time (fig. 2) one can conclude that the value of the surface tension grows quickly at the be-ginning of the paper influence and after 30 minutes the increase of the surface tension value becomes slowly.
Comparing the dependence of pH values and the contact angle on the paper mass immersed into the fountain solution, one can notice that the
additives from paper, while dissolving, increase the pH value with the simultaneously increase of the contact angle on the nonprinting areas (fig. 5). This is visible in the first 15 to 20 minutes of the paper impact on the fountain solution. By the increase of pa-per impact time on the fountain solu-tion the concentration of the dissolved substances increases as well. But there is considerable difference between the paper quantity and the dissolved additi-ves. Namely, increasing the paper quan-tity after 0,06 gcm-3 there is neither con-siderable increase of pH value (fig. 1) nor electric conductivity (fig. 3). The cause for that can be the generating of the saturated solution with no further salt dissociation present in paper. Based on these obtained investigation values it can be seen that by the increase of the volume share of paper and the duration time of paper influence on the solution the pH value and the surface tension of the solu-tion increase (fig. 6).
Fig. 5. Dependence of pH value and the contact angle of the solution on the paper mass .
Fig. 6. Dependence of pH value and the surface tension of the solution on time .
Fig. 7. Dependence of the surface tension and the contact angle of the solution on the
paper mass.
Except that, the increase of the volume share of paper and the duration time of the paper impact on the fountain solu-tion cause the increase of the surface tension and with it the increase of the contact angle which causes weaker damping and weaker toning of prints as the negative effect in the offset printing (fig. 7).
Surfactants added into the fountain solution cause the decrease of the sur-face tension (Taber, 1966).
At the same time the electric conduc-tivity is dependent on the number of ion kinds which transmit the electric charge. Therefore the increase of elec-tric conductivity by the conductive par-ticles dissolved from the newsprint pa-per causes the increase of the surface tension and with it the increase of the contact angle which results in weaker damping of the nonprinting surfaces of the plate (fig. 8).
Fig. 8. Dependence of the electric conductivity and the contact angle of the solution on the paper mass.
During offset plate damping the layer of molecules which increases the ability of bonding molecules of water to it by salts present in the damping solution is rest-ored.
Except these salts for hydrophi-llization the surfactants (SAS) which cause the surface tension of water such as alcohol, carboxy-methyl-cellulose, gly-col, diethyl glycol, propylene glycol, glycerol etc. are added to it. In depen-dence on the quantity and the kind of added surfactants (SAS) in the fountain solution their physical-chemical proper-ties change considerably. But the dissol-ved substances from the paper can con-siderably change the impact of SAS as well as the added salts from hydro-phillization.
It is estimated by measuring that there appears the penetration of the boundary of puffer impact which negatively influences the function of the nonprinting areas on the offset printing plate.
The increase of paper quantity in the prepared damping solution causes the increase of all the considerable phy-sical–chemical parameters (except the contact angle on the printing elements) which is the consequence of fillers dissolving present in paper as well as the remaining additives from the paper production process whether the new paper or the recycled one.
7 Conclusion
The obtained results show that all the physical-chemical parameters of the fountain solution increase by adding the paper into the fountain solution. By in-creasing the time of paper impact on the fountain solution the concentration of the dissolved substances increases which is characteristic for the first 15–20 minutes. By increasing the paper quantity after 0,06 gcm-3 there is not considerable increase of other physical-chemical para-meters. The cause for that can be the generating of the saturated solution where there is no further dissociation of salts present in paper. The damping process happens in a mild acid medium, in the pH value area of 4.4 to 4.9. The in-crease of pH value of the fountain solution above 6 can cause permanent damage of the boehmitic structure of Al2O3 which is the constituent of the nonprinting areas which can cause the change in structure of the nonprinting areas and the change of their hydro-philic ability. The value increase of the surface tension as well as of the contact angle of the fountain solution can cause considerable change in their physical-chemical properties. This change is visi-ble in disturbing the emulsification re-lation and the balance ink-water which is one of the most important
parameters which must be observed and maintained constant during the reproduction.
New observations with the aim of achie-ving more simple and accurate ways of controlling the parameters of influence on the print quality are yet to come. These observations are to be made in the de-pendence of the type of paper used, various fountain solutions and the quality requirements. Another important goal of the control of the mentioned parameters is to predict the possible changes in the observed reproduction flow.
8 References
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