Coating composition for offset paper

Abstract

 Coating for an offset paper comprising a catalyst for fixing polymerizable or crosslinkable constituents of the offset ink. The chemical drying time can be substantially reduced if such a catalyst system is added to the coating, wherein preferentially such a catalyst is a transition metal complex, like Mn (bipy).

Description

TECHNICAL FIELD

[0001] The present invention relates to a coating composition in particular for sheet-fed lithographic offset printing paper, as well as to a paper coated with such a coating, and to methods for applying such a coating to a substrate.

BACKGROUND OF THE INVENTION

[0002] One important application of wood-free coated fine paper is in the field of sheet-fed lithographic offset printing processes. There is a clear trend in this market towards shorter times to re-print and for converting in order to reduce the time of the production process and to facilitate handling.

[0003] Printers will have a clear advantage when paper can be almost instantly re-printed and converted (i.e. within 0.5 hours) as this is leading to a much higher efficiency of the process. Workflow in the printing industry today has been fully digitized, enabling same-day processing of a complete print-job (like e.g. CD-inserts), provided that the print-process in itself would enable to do so. The only component in the complete workflow that prevents speeding up of the full process is the interaction ink-paper, i.e. sufficient drying before converting. One can therefore say that ink drying time is the bottleneck or the rate determining step in the full sheet-fed lithographic offset printing process.

[0004] There is a strong belief that the shorter time to converting requires both an adequate physical drying component as well as a sufficient (but not necessarily 100% completed) chemical drying component of printed ink.
As supported by results of several studies the physical drying component can e.g. be increased by adjustment of the porosity and/or of the surface energy of coating layers in a way that:

• initial ink setting during residence time of the paper on the press is not too fast
• ink setting is as fast as possible for ink setting times directly after printing.

[0005] An induction period with respect to initial ink setting is necessary to avoid runnability problems on the press and to avoid loss in quality of the printed surface. Adjustment of surface energy appears also to be necessary to obtain superior print quality.

[0006] In the sheet-fed offset lithography printing process the quickset inks involved in general are mainly composed of ink colour pigment, resin, drying oil (which is a vegetable or biological oil) and a high boiling hydrocarbon (mineral) solvent (e.g. for adjusting the total flow characteristics). When printed on coated paper an initial physical absorption process starts, with rapid rather selective penetration of the mineral oil phase into the paper coating and the raw paper-base. The residual resin- and drying oil rich phase precipitates due to the concomitant change of ink composition. It ends up with a relatively high viscosity and as a result it is (with ink colour pigment incorporated) more or less consolidated (often called "set") on and somewhat on top of the coating surface, providing best conditions for optimal printing gloss properties.

[0007] In general such 'set' ink film is sufficiently rigid to withstand limited mechanical forces and enables the sheet to be re-printed on the second side of the sheet very soon after completing the first side. However its rigidity (especially wet rub resistance, abrasion resistance) normally has not well developed enough for 'safe' instantaneous further handling or converting (e.g. folding, cutting) of the printed paper without damaging printed images. In fact several hours up to a day or more might be needed before these next converting steps can be performed. In order to keep printing process economics viable, it is essential for printers to have this interval time minimised.

[0008] A well-known present method is to start up an additional chemical drying step of the printed ink layer, a so-called oxidative polymerisation or cross-linking reaction. Both the vegetable drying oil part, e.g. linseed oil and the resin part are partly based upon (preferentially conjugated) unsaturated fatty acids. Oxygen in the air (or between the sheets in stack) adds to the double bonds of these fatty acids and resins to initially form hydroperoxides. After consecutive degradation of these hydroperoxides the resulting free radicals are very reactive. These radicals attack other fatty acid molecules and attach, forming new (larger) free radicals. This causes polymerisation to finally form a cross-linked ink network. The rate-determining step, formation and degradation of hydroperoxides can appreciably be speeded up by the presence of special catalytic species (so-called primary/secondary/auxiliary driers or siccatives) in the ink. Possible is the addition of fatty acid salts (e.g. naphtenates or octoates) of transition type metals like cobalt to the ink prior to the printing. These catalysts are being added in small amounts to the printing inks, appreciably speeding up drying time from 100-200 h (non-catalysed situation) towards 1 - 10 h (catalysed situation). The complex mechanism of this ink cross-linking reaction path is visualised schematically in Fig. 1.

[0009] Former catalytic drier systems in ink systems are similarly applied in e.g. commercial alkyd resin, solvent-based paints, also to speed up their chemical drying behaviour and to provide them with consumer-friendly behaviour. Latest developments on the paint market are water-based paint systems. In order to also speed up their chemical drying behaviour after application, specially adapted water-dispersible catalytic drier systems have been developed. In fact known primary/secondary/auxiliary drier systems have been modified with dedicated emulsifier combinations to make them sufficiently water-dispersible.

[0010] The present regular working method of printers therefore is to add so-called drier systems to the ink prior to the printing to speed up chemical drying. This however has several drawbacks. For instance a practical point is the appreciable reduction of the so-called 'open time' of the ink system, requesting a printer to clean-up the printing machine at the end of every regular 8h working day cycle, or toxic anti-skinning agents like e.g. oximes have to be added to the ink. Another drawback is that a printer is forced to deal not only with standard ink but also to use several types of (more expensive) printing inks with an added drier system, depending on the absorptive and other printing properties of respective paper qualities.

SUMMARY OF THE INVENTION

[0011] The objective problem underlying the present invention is therefore to provide improvements for the printing process, in particular improvements allowing to reduce the time which has to be waited until the printed sheet-fed paper can be further treated or converted.

[0012] The present invention solves the above problem by providing a coating for an offset paper comprising a catalyst system for fixing polymerizable or crosslinkable constituents of the offset ink.

[0013] As catalyst system shall be understood a system comprising one or several (as a mixture) catalyts or catalytically active components including additives, ligands, salts etc. supporting the total activity of the catalyst system.

[0014] The object of the present invention is therefore a coating according to claim 1, a paper according to claim 26, a method according to claim 28 as well as a use according to claim 30.

[0015] The key feature of the invention is therefore the fact that surprisingly it is possible to incorporate in particular water-dispersible or water-soluble catalytic primary drier systems, preferentially combined with a well-balanced ligand/chelate system, into the top coat layer (or also or in the alternative into a middle coating, e.g. in order to hide sometimes possibly slightly coloured catalyst systems behind the top coat) of for example a wood free coated grade for sheet-fed offset. In this way it is possible to provide a new type of graphical coated paper with intrinsic chemical drying potential incorporated directly in the paper coat itself, in addition or even to replace the existing chemical drying potential of an ink system itself. Chemical ink drying time can be reduced appreciably by this approach among others for the following reasons :

• The chemical drying process is now started at both sides of the printed ink layer.
• Due to an adequate transportation process via physical absorption, unsaturated ink components to be chemically reacted (the catalysts are fixing polymerizable or crosslinkable constituents of the offset ink) are in closest vicinity with catalytic dryer species at the inside of the paper.
• Due to the presence of enough oxygen in the porous coating system close to the catalytic species, oxygen diffusional limitations are minimised with best consequences for minimal induction time of chemical reaction.

[0016] In case of coatings according to the state-of-the-art, the chemical drying component (=cross-linking of biological oil vehicle and unsaturated resin part of the ink) seems, if at all, not to be significantly influenced by the paper surface. It can surprisingly be shown, that, as a matter of fact, incorporation of an appropriate catalyst system into the top coat of a paper reduces the chemical drying time sometimes even more than does the corresponding incorporation of a catalyst system into the ink. In addition to that, surprisingly, such drier systems which are added to the coating are stable even if the paper is stored for a long time. In particular the synergistic combination of driers added to the in the ink and driers present in the coating can lead to drying times which are far below values that can be achieved using systems according to the state-of-the-art. The catalyst catalyzes the oxidative polymerisation or the crosslinking of unsaturated constituents of offset ink printed onto the coating. In particular the catalyst fixes fatty acid and resin parts of the offset ink.

[0017] The drying time of offset printing ink applied to such a coating can be reduced below 2h, even below 1h, and in some cases to values equal or below 0.5h.

[0018] In principle, primary drier systems which are known as additives for offset printing inks can be used as catalysts for the coating according to the invention. In a first preferred embodiment of the present invention, the catalyst is a transition metal complex, wherein preferentially the metal ion of the transition metal complex is selected from the group of Ti, V, Cr, Mn, Fe, Co, Ce, Cu, or a mixture thereof.

[0019] Particularly preferred are primary drier catalysts which are based on a transition metal selected from the group of Mn, V, Fe or a mixture thereof, wherein in particular Mn alone proved to be particularly powerful and efficient.

[0020] The effect of primary drier catalyst systems as given above can be supported by so-called secondary driers which are additionally present. Such secondary driers can be based on Pb, Bi, Ba, Al, Sr, Zr e.g. in ionic form as salts, complexes or the like.

[0021] Even further or alternative support of the catalytic effect can be generated if auxiliary driers are additionally added like e.g. Ca, Zn, Li, K, again in ionic form as salts, complexes or the like.

[0022] According to another preferred embodiment the transition metal complex which is used as the catalyst is a carboxylate and/or a naphthenate complex. In case of a carboxylate complex, carboxylates with an alkyl chain of 2-18 carbon atoms, preferably of 6-12 carbon atoms, which may be unsubstituted or substituted can be shown to be efficient. A particularly suitable system is a 2-ethylhexanoate-complex, in particular a Mn 2-ethylhexanoate-complex. In case of a naphthenate complex, the naphthenic acid anion has an alkyl chain of 1-12 carbon atoms, preferably of 4-8 carbon atoms, and the alkyl chain as well as the cyclopentane unit may be unsubstituted or substituted.

[0023] According to another preferred embodiment of the coating according to the invention, the transition metal complex used as the catalyst comprises or is supplemented by at least one bidentate ligand. Such a bidentate ligand can advantageously be used in combination with the above-mentioned carboxylate or naphthenate ligand system. Particularly useful are bidentate systems which lead to chelate-rings with e.g. 5 atoms. The atoms which are used for providing the link to the metal atom may be selected from the group N, O, S, and/or P or combinations thereof. Therefore, useful bidentate systems include organic molecules with appropriate sp3 or sp2 hybridised N-atoms and/or O-atoms which are available for forming a bond to the metal atom.

[0024] Particularly preferred are ligands in the form of a diamine or alkanolamines, like for example selected from the group 2,2'-bipyridine, 2-aminomethylpyridine, 2-hydroxymethylpyridine, or 1,10 phenanthroline, which may be substituted or unsubstituted. The ligands are preferably substituted by side groups, which increase the stability and/or increase the solubility or dispersibility of the catalyst system in water, which is important since coatings are deposited onto a substrate on a water basis.

[0025] A particularly suitable system is given by a catalyst consisting of or comprising a Mn 2,2'-bipyridine system, in which e.g. Mn is present as a salt complex and additionally bpy is present, typically in slight molar excess. As mentioned above, such a system can be provided as a combined system with a Mn carboxylate or a Mn naphthenate, suitable is for example a combination of Mn 2-ethylhexanoate with (Mn) 2,2'-bipyridine.

[0026] According to another preferred embodiment of the coating according to the invention, the catalyst, i.e. the metal part of the primary drier complex, is present in the coating in 0.01 - 0.5 weight-% of the total dry weight of the top paper coating, preferably in 0.05-0.2 weight-% of the total dry weight of the coating.

[0027] In addition it can be shown that superior catalyzing efficiency can be achieved if one of the ligands is present in well balanced and controlled excess compared to the transition metal ion of the primary drier system. Therefore, according to another preferred embodiment, the transition metal complex preferentially comprises at least one bidentate ligand and the ratio of metal to ligand is in the range of 1:1 - 1:8.

[0028] In order to provide a catalyst system which is adapted to the process of coating in the paper machine, it is advantageous to add additives for increasing the solubility/dispersibility of the transition metal complex and the ligands present. Additives for enhancing dispersibility can e.g. be chosen from alcohols or glycol-ethers like e.g. 1-methoxy-2-propanol or propylene-glycol-monomethyl-ether. Those additives can either be added to the coating formulation or they can be added to the solution/dispersion of the transition metal complex prior to its introduction into the coating formulation.

[0029] In principle, the present concept can be applied to any (aqueous) coating formulation. It however proves to be advantageous if the coating has a high degree of porosity and an appropriate morphology of the porosity and/or appropriate surface energy of pore walls. According to another preferred embodiment, the coating comprises 100 parts in dry weight of pigment substantially supplemented by 5-20 parts in dry weight binder and additives like lubricants, thickener etc., wherein the pigment part comprises ultra-fine CaCO₃ and/or kaolin or clay wherein up to 10-20 parts may be substituted by synthetic solid or vacuolated polymeric pigments which may e.g. be made of poly(methyl methacrylate), poly(2-chloroethyl methacrylate), poly(isopropyl methacrylate), poly(phenyl methacrylate), polyacrylonitrile, polymethacrylonitrile, polycarbonates, polyetheretherketones, polyimides, acetals, polyphenylene sulfides, phenolic resins, melamine resins, urea resins, epoxy resins, polystyrene latexes, polyacrylamides, and alloys, blends, mixtures and derivatives thereof. Possible are also Styrene maleic acid copolymeric latexes (SMA) or styrene malimide copolymeric latexes (SMI), mixtures of these with the above mentioned structures and derivatives thereof.

[0030] In principle the catalyst is only added to the top coating, it may however also be added to layers which are beneath the top coat. The top coat comprising the catalyst system typically has a thickness in the range of 10-30g/m², preferably in the range of 10-15 g/m².

[0031] The present invention additionally pertains to a paper coated with a coating as given above, preferentially as a top coat. Beneath such a top coat there is preferably an additional coating, which in particular supports the physical absorption process of the ink in the layers structure. Possible is a formulation of the additional middle coating as follows: 100 parts in dry weight fine CaCO₃; 5-10 parts styrene butadiene synthetic binder; 1 part lubricant; 1 part modified starch; 1 part PVA; 1 part CMC.

[0032] Furthermore, the present invention relates to a method for the production of a coating as given above, wherein the transition metal complex is added, preferentially as an aqueous solution or dispersion, to a stirred coating formulation, and the final coating formulation is coated onto a paper substrate. The coating process can be carried out using regular techniques like a blade coater, a roll coater, a spray coater, a curtain coater or other coater systems, and the paper may be calendered after the coating process.

[0033] According to a preferred embodiment of the above method, concomitantly with the addition of the transition metal complex a chelating agent, preferably in excess to the transition metal content (on a molar basis), is added to the coating formulation, wherein the chelating agent is added as an aqueous solution or dispersion and may contain one or several additives (cosolvents using the sulubilization principle) to increase the solubility/dispersibility or to increase the stability of the catalyst system.

[0034] Further, the present invention relates to the use of a catalyst for fixing polymerizable or crosslinkable constituents of the offset ink as an additive for a coating. Such a catalyst is preferentially a water soluble or water dispersible transition metal complex, and has the characteristics as outlined above.

[0035] Further embodiments of the present invention are outlined in the dependent claims.

SHORT DESCRIPTION OF THE FIGURES

[0036] In the accompanying drawings preferred embodiments of the invention are shown in which:

Figure 1

shows a schematic illustration of the chemical processes of catalytic ink cross-linking;

Figure 2

shows the chemical ink drying performance as determined by thumb test of printed Black tempo max ink on laboratory made 'regular' MagnoStar with incorporated Mn-(2-ethylhexanoate, bpy) dryer complex in topcoat, given as a function of the molar ratio of ligand to metal for different contents in Mn (reference without Mn and at 0.1 and 0.2 wt.-% Mn9);

Figure 3

shows a set-off test of 'regular' MagnoStar 250 gsm end paper with incorporated Mn-(2-ethylhexanoate, bpy) catalytic dryer complex, at 0.1 and 0.2 wt.-% Mn9 as a function of the molar ratio of ligand to metal i.e. with varying excess bpy, printed with Black tempo Max ink; and

Figure 4

shows the chemical ink drying performance as determined by thumb test of printed Bio 2 ink on laboratory made 'regular' MagnoStar with incorporated Mn-(2-ethylhexanoate, bpy) dryer complex in topcoat, given as a function of the molar ratio of ligand to metal for different contents in Mn (reference without Mn and at 0.1 and 0.2 wt.-% Mn9).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] It was surprisingly established that under laboratory printing conditions (Pruefbau, no Fount solution) a significant reduction in chemical drying time of printed ink layer (commercial Black Tempo Max, BTM, SICPA, CH) on regular paper available under the trade name MagnoStar from the applicant is possible. For that purpose a water-dispersible manganese based catalytic dryer complex Mn- (2-ethylhexanoate) was homogeneously incorporated in the topcoat, preferentially completed with a second ligand 2,2'-bipyridyl (further mentioned as bpy). With only complex Mn- (2-ethylhexanoate) at 0.2 wt.-% Mn, chemical drying time (evaluated via Thumb test) of printed BTM ink was reduced from 4h (blank) to 2h (= 50%). In the presence of a faster setting middle-coat layer, drying time was lowered about 1 hour extra: from 3h (blank) to 1-2 h. At additional presence of bpy, results were even further improved: at 0.1 wt.-% Mn and a molar ratio bpy/Mn = 6, a chemical ink drying time 0.5 - 1 h (= 12.5 - 25%) was achieved. At 0.2 wt.-% Mn and bpy/Mn = 3 a chemical ink drying time as low as 0.5 h (= 12.5%) could even be reached.

Experimental details

Materials

Chemical drier complex

[0038] 

• Nuodex Web Mn9: 9,0 ± 0.2 wt.-% of manganese as Mn- (2-ethylhexanoate) complex, and containing special surfactant mix to make it suitable for water-borne systems, commercially available from Elementis Servo, Delden, NL.
• Drymax (commercially available from Elementis Servo, Delden, NL): a chelating agent used for additional manganese drier acceleration. It contains about 30 wt.-% 2,2'-bipyridyl (bpy, superactive ligand for manganese, next to 2-ethylhexanoate) and about up to 60 wt.-% N-methyl-2-pyrrolidon (non-active material, co-solvent to increase aqueous solubility of 2,2'-bipyridyl).

Paper substrate

[0039] 

A: Regular MagnoStar papers without topcoat layer, meant for 250gsm end-paper quality. The surface coating layer of this substrate without topcoat layer was containing 100 parts in dry weight rather coarse CaCO₃; 10 parts synthetic latex binder; 1 part modified starch; 1 part PVA; 1 part CMC; 1 part lubricant.

B: Experimental mill produced paper with specific porous middle layer without topcoat, meant for 250 gsm end paper. The surface coating layer (after application of the top coat acting as porous middle layer) of this substrate without topcoat layer was containing 100 part fine (not ultrafine as in topcoat and not rather coarse as it is regular) CaCO₃; 10 parts styrene butadiene synthetic binder; 1 part lubricant; 1 part modified starch; 1 part PVA; 1 part CMC.

Paper top coat

[0040] 100 parts in dry weight ultra-fine CaCO₃; synthetic latex binder 10 parts; modified starch 1 part; PVA 2 parts; lubricant Ca-stearate 1 part; synthetic thickener as further needed to set viscosity behaviour, e.g. 0.05 parts.

Printing ink

[0041] 

• Tempo Max (SICPA, CH), black. Like most commercial inks, no specification available of composition and unsaturation value. Selected as oxidatively rather 'quick drying' model ink. Probably contains limited amount of some drier complex.
• Bio 2 (BASF/K&E, DE), cyan: specially prepared 100% biological model ink for paper-ink . Composition: 17 parts ink pigment + 60 parts bio binder + 9 parts alkyd resin + 9.5 parts bio oil + 2 parts special additives + 2.5 parts siccative + 0 parts mineral oil. No details available of composition and unsaturation value of biological part.

Incorporation drier complex in top coat / top coat application

[0042] Small amounts of Nuodex Web Mn9 and (if required) Drymax are simultaneously added slowly (via two feeding devices) into well-stirred topcoat formulation (marine type stirrer) in a small open vessel for about 10 minutes at room temperature. Metal and ligand were added as requested, the specific figures can be found in the tables.

[0043] In case the active drier complex is assumed to be an octahedral surrounded mono-metal/ligand complex, 1 to (maximum) 8 moles bpy versus 1 mole manganese metal should be possible. It is however presumed that the complex is a polynuclear complex with several metal atoms within one moiety.

[0044] In case of certain thickening behaviour of coating after complex addition, it suffices to add some additional dispersant, e.g. Polysalz type.

[0045] Topcoat plus incorporated drier have been applied with available Bird applicator or with a lab-scale pilot coater onto one side of dual coated A or B substrate. Applied topcoat amount was tested as about 15 g/m²/side with layer thickness about 11-12 µm. This fits well in with mill practice for these topcoats.

Laboratory printing method

[0046] After conditioning the coated paper samples (with and without incorporated drier) in accordance with GTM 1002, printing ink (black) Tempo max or (blue) Bio 2 is applied onto paper samples at Prüfbau printing device according to directions of ESTM 2302, Multicolour ink setting, revision 0 of 11-2-2004. It means 0.24 g ink, printing pressure 1000N, printing speed 0.5 m/s, with aluminium printing reels and with standard long sample carrier.

[0047] Remark: Printed ink layer thickness was measured as about 1 - 2 µm.

Analytical drying test methods

[0048] All analytical measurements have been performed on conditioned papers (GTM 1002) in conditioned laboratory. Following analytical drying test methods have been selected and applied:

Physical 'setting' time of printed ink:

Set-off test (ESTM 2301): a paper sample is printed (100%) with a standard ink (Huber 520068) at the Prüfbau printing device. After several relatively short time intervals (15, 30, 60, 120 s), a part of the printed sample is countered (top versus bottom) against the same blank paper. The density of the transferred ink of each area on the counter paper is measured and plotted against time. This method is reported to describe the measurement of the (physical) set-off (pile simulation) of papers used for sheet-fed offset printing.

Chemical drying time of printed ink:

Thumb test, (non-standard): in line with general practice of commercial printing (and also in paint testing area) at several time intervals (15, 30, 60, 90 ....minutes) a thumb, covered with (special) house-hold tissue paper (to avoid influence of skin grease), is firmly (but always at about same force) pressed and simultaneously turned over 90° in the printed ink layer. In case of fully wet stage all ink is wiped off, leaving a clear white spot on paper substrate. In case of fully chemically dried ink no injury can be seen. It is preferred that one and the same operator is performing all series.

Results

[0049] The investigations in this report are to be subdivided into three main parts:

Part I: Pre-assessment of intrinsic catalytic ink drying activity of used manganese dryer product Nuodex Web Mn9 added to the ink.

[0050] For this purpose the physical and chemical drying performance of Black Tempo Max printing ink without ('as such') and with additionally mixed in Nuodex Web Mn9 product [0.1 and 0.5 wt.-% Mn in form of Mn- (2-ethylhexanoate) complex ] as printed upon commercial MagnoStar 250 gsm paper was determined. Furthermore 250 gsm commercial paper substrates Paper 1, Paper 2 and Paper 3 were evaluated correspondingly as printing substrates for the sake of comparison, in order to rank drying behaviour (and in a certain way its converting ability) of present MagnoStar quality relative to that of well drying commercial papers in the market.

[0051] Thumb test analysis of several commercial 250 gsm WFC (woodfree coated) papers, printed with Black tempo Max ink 'as such' was carried out. It was seen that there is dramatic differences between the behaviour of different paper substrates, so the Paper 1 shows quick drying behaviour, while Magnostar shows relatively slow drying behaviour. The results are summarised in table 1.

[0052] Thumb test analysis of several commercial 250 gsm WFC papers, printed with Black tempo Max ink + 0.1 wt.-% Mn9 and 0.5 wt.-% Mn9, respectively, was carried out. An improvement in the drying behaviour in particular of MagnoStar was clearly observed. The results are also summarised in table 1.

Part II: Incorporation of Mn- (2-ethylhexanoate) catalytic dryer complex in paper topcoat to enhance chemical ink drying performance

[0053] The required amounts manganese complex (0, 0.05, 0.1 and 0.5 wt.-% Mn9 given as weight % of metal of the primary drier compared to dry weight coating formulation) as Nuodex Web Mn9 were mixed into topcoat composition (see above). Treated topcoat was Bird applied to middle-coated paper substrate for 250 gsm end paper: A and B. End paper was laboratory printed and tested for drying behaviour.

Results were as follows:

[0054] Thumb test analysis of MagnoStar 250 gsm end paper with incorporated Mn- (2-ethylhexanoate) catalytic dryer complex in varying amounts, printed with Black tempo Max ink was carried out. One could easily recognize the increasing speed of chemical drying for increasing catalyst concentration. The results are summarised in table 2.

[0055] Thumb test analysis of 'fast setting' MagnoStar 250 gsm end paper with incorporated Mn- (2-ethylhexanoate) catalytic dryer complex, printed with Black tempo Max ink was carried out. The results are summarised in table 2.

Part III: Incorporation of Mn- (2-ethylhexanoate, bpy) catalytic dryer complex in paper topcoat to enhance chemical ink drying performance

[0056] Complementary to part II, in this part next to Mn- (2-ethylhexanoate) complex also several excess amounts ligand bpy (as Drymax product) have been (separately) mixed into topcoat, in order to intentionally react 'in situ' to Mn- (2-ethylhexanoate, bpy) dryer complex. Next to printing ink Black Tempo Max also Bio 2 type ink was applied at laboratory printing.

Results were as follows:

[0057] Thumb test analysis of 'regular' MagnoStar 250 gsm end paper with incorporated Mn-(2-ethylhexanoate) catalytic dryer complex without additional bpy, printed with Black tempo Max ink was carried out. The results are summarised in table 3.

[0058] Thumb test analysis of 'regular' MagnoStar 250 gsm end paper with incorporated Mn-(2-ethylhexanoate, bpy) catalytic dryer complex, at 0.1 wt.-% Mn9 and with varying excess bpy printed with Black tempo Max ink was carried out. Here the dramatic increase in chemical drying speed in the increasing presence/excess of bpy becomes obvious, and the results are summarised in table 3.

[0059] Thumb test analysis of 'regular' MagnoStar 250 gsm end paper with incorporated Mn-(2-ethylhexanoate, bpy) catalytic dryer complex, at 0.2 wt.-% Mn9 and with varying excess bpy printed with Black tempo Max ink was carried out. The effect is even more pronounced here than in table 3 and with 0.2% Mn9 with 1.68% bpy the drying time goes down to less than 0.5h, as can be seen from table 4.

Very similar results are obtained if printed with Bio 2 ink:

[0060] Thumb test analysis of 'regular' MagnoStar 250 gsm end paper with incorporated Mn-(2-ethylhexanoate, bpy) catalytic dryer complex, at 0.1 wt.-% Mn9 and with varying excess bpy printed with Bio 2 ink was carried out. The results are summarised in table 3.

[0061] Thumb test analysis of 'regular' MagnoStar 250 gsm end paper with incorporated Mn-(2-ethylhexanoate, bpy) catalytic dryer complex, at 0.1 wt.-% Mn9 and with varying excess bpy printed with Bio 2 ink was carried out. The results are summarised in table 3.

[0062] Thumb test analysis of 'regular' MagnoStar 250 gsm end paper with incorporated Mn-(2-ethylhexanoate, bpy) catalytic dryer complex, at 0.2 wt.-% Mn9 and with varying excess bpy printed with Bio 2 ink was carried out. The results are summarised in table 4.

Discussion

Conclusions Part I, Pre-assessment of intrinsic catalytic ink drying activity of used manganese dryer product Nuodex Web Mn9

[0063] Results of Thumb tests of printed Black Tempo Max ink (as such or with added commercial manganese dryer) on some 'best in class' commercial 250 gsm WFC papers in the market with respect to drying/converting performance are presented in Table 1. Corresponding MagnoStar 250 gsm (commercial end product) drying test results were also included.

Table1:

Chemical ink drying time(h) via Thumb test	BTM ink as such	BTM ink + 0.1 wt.-% Mn9	BTM ink + 0.5 wt.-% Mn9
Comparative Paper 1	2	1	1
Comparative Paper 2	3	3	3
Comparative Paper 3	4-5	3	2-3
MagnoStar 250	≥5	3	3

[0064] The following conclusions can be drawn:

• Commercial dryer product Nuodex Web Mn9 with active component Mn- (2-ethylhexanoate) complex, as being specially developed for chemical drying of waterborne paint types, is also active for chemical drying if added to the ink.
• Chemical drying time of MagnoStar, printed with Black Tempo Max ink 'as such' is significantly longer than of Paper 1 (factor > 2.0) and of Paper 2 (factor > 1.5) and somewhat longer than of Paper 3 (factor > 1.0).
• It appears that for all papers involved, except Paper 2, chemical ink drying time can be shortened significantly (up to 50% for Paper 1) by adding additional manganese dryer complex in the printing ink, as is general practice by printers in printing.
• With respect to MagnoStar, its chemical ink drying time even with additional manganese dryer complex (0.1 wt.-% Mn9) in printing ink can still not compete (factor 1.5 slower) with chemical ink drying time of Paper 1, printed with ink 'as such'. Similarly Magnostar chemical ink drying time under these conditions is equivalent to that of Paper 2 or even better than Paper 3, printed with ink as such.
• Assuming that fast chemical ink drying behaviour is essential for good converting ability, it seems clear that special measures are to be involved to enhance converting ability of MagnoStar to 'best in class' competitor paper Paper 1. Remark: Paper 1 while showing excellent drying is very sensitive for picking (only 2x free versus 4x for MagnoStar) and for low print gloss.

Conclusions Part II, Incorporation of Mn- (2-ethylhexanoate) catalytic dryer complex in paper topcoat to enhance chemical ink drying performance

[0065] Results of Thumb tests of printed Black Tempo Max ink on laboratory made uncalendered 'regular' 250 gsm MagnoStar with varying concentrations incorporated Mn- (2-ethylhexanoate) catalytic dryer complex in topcoat are summarized in Table 2:

Table2

Chemical ink drying time (h) via Thumb test	0 wt.-% Mn9 in topcoat	0.05wt.-% Mn9 in topcoat	0.1 wt.-% Mn9 in topcoat	0.2 wt.-% Mn9 in topcoat
BTM ink printed on A substr. + topcoat	4	3	2-3	2
BTM ink printed on B substr. + topcoat	3	2-3	2	1-2

[0066] The following conclusions can be drawn:

• Chemical ink drying performance of laboratory made (uncalendered) 'regular' MagnoStar without any added manganese dryer complex is slightly faster than of commercial 250 gsm (calendered) MagnoStar (see Table 1).
• In regular concentration range 0 - 0.2 wt.-% Mn9 chemical ink drying performance of laboratory made 'regular' MagnoStar was best improved from 4h (blank) to 2h (= 50% residual drying time).
• In case of laboratory made 'fast setting' Magnostar, applying a 'faster' middle coat layer of type B, in the same concentration range of 0 - 0.2 wt.-% Mn9 chemical ink drying performance was improved from 3h (blank) to 1-2 h, in fact also an improvement in drying time of about 50%. Obviously applying a 'faster' middle coat layer B in absolute sense leads to 1h improvement of chemical ink drying performance over the whole manganese concentration range concerned.

Conclusions Part III, Incorporation of Mn-(2-ethylhexanoate, bpy) catalytic dryer complex in paper topcoat to enhance chemical ink drying performance

[0067] Results of Thumb tests of printed Black Tempo Max ink or Bio 2 ink on laboratory made 'regular' 250 gsm MagnoStar with varying concentrations incorporated Mn-(2-ethylhexanoate, bpy) catalytic dryer complex in topcoat are summarized in Table 3 {at 0.1 wt.-% Mn9 and varying ratio bpy/Mn} and in Table 4 {at 0.2 wt.-% Mn9 and varying ratio bpy/Mn}.

The following conclusions can be drawn:

[0068] 

• Chemical drying performance of printed Black Tempo Max at 0.1 wt.-% Mn9 was still further improved on co-addition of second ligand bpy from again 4h (blank) to only 0.5 - 1 h (= 12.5 - 25% residual drying time), at molar ratio bpy/Mn = 5.9. Similarly at higher concentration 0.2 wt.-% Mn9 and co-addition of second ligand bpy chemical ink drying performance of printed Black Tempo Max was further improved from 3-4 h (blank) to only 0.5 h (= 12.5 - 16.7% residual drying time), at molar ratio bpy/Mn = 3.0.

[0069] In a graphical presentation these results are summarized in Fig. 2, which gives the chemical ink drying performance of printed Black tempo max ink on laboratory made 'regular' MagnoStar with incorporated Mn- (2-ethylhexanoate, bpy) dryer complex in topcoat as a function of the bpy content.

[0070] In Fig. 3 it was verified for all tested 'regular' MagnoStar papers with incorporated Mn-(2-ethylhexanoate, bpy) catalytic dryer complex in topcoat that regular set-off test results (= initial physical ink setting) are not significantly influenced by the presence of said dryer complexes. This is important for optimum printing performance under practical conditions, e.g. with respect to minimum fouling of printing press.

[0071] Chemical drying performance of printed Bio 2 ink at 0.1 wt.-% Mn9 was still further improved on co-addition of second ligand bpy from 6h (blank) to only 0.5 - 1 h (= 8.3 - 16.7% residual drying time), at molar ratio bpy/Mn = 20.1. Similarly at higher concentration 0.2 wt.-% Mn9 and co-addition of second ligand bpy chemical ink drying performance of printed Bio 2 ink was further improved from 6h (blank) to only 0.5 - 1h (= 8.3 - 16.7% residual drying time), at molar ratio bpy/Mn = 3.0.

[0072] In a graphical representation these results are summarized in Fig. 4, which gives the chemical ink drying performance of printed Bio 2 ink on laboratory made 'regular' MagnoStar with incorporated Mn- (2-ethylhexanoate,bpy) dryer complex in topcoat.

Conclusions

[0073] 

• The Mn- (2-ethylhexanoate) catalytic dryer complex for waterborne paint systems is also active for chemical drying of printed ink layer on WFC sheet-fed paper if incorporated in the coating.
• Chemical ink drying time of MagnoStar even with additional manganese dryer complex in printing ink (a regular 'trick' in printing practice) can still not compete (factor 1.5 slower) with chemical ink drying time of other papers, printed with ink 'as such'.
• In regular concentration range 0 - 0.2 wt.-% Mn9 incorporated in top coat chemical ink drying performance of laboratory made 'regular' MagnoStar was best improved from 4h (blank) to 2h (= 50% residual drying time).
• Chemical drying performance of printed Black Tempo Max at 0.1 wt.-% Mn9 incorporated in top coat was still further improved on co-addition of second ligand bpy from again 4h (blank) to only 0.5 - 1 h (= 12.5 - 25% residual drying time), at molar ratio bpy/Mn = 5.9. Similarly at higher concentration 0.2 wt.-% Mn9 and co-addition of second ligand bpy chemical ink drying performance of printed Black Tempo Max was further improved from 3-4 h (blank) to only 0.5 h (= 12.5 - 16.7% residual drying time), at molar ratio bpy/Mn = 3.0.
• Chemical drying performance of printed Bio 2 ink at 0.1 wt.-% Mn9 incorporated in top coat was still further improved on co-addition of second ligand bpy from 6h (blank) to only 0.5 - 1 h (= 8.3 - 16.7% residual drying time), at molar ratio bpy/Mn = 20.1. Similarly at higher concentration 0.2 wt.-% Mn9 and co-addition of second ligand bpy chemical ink drying performance of printed Bio 2 ink was further improved from 6h (blank) to only 0.5 - 1h (= 8.3 - 16.7% residual drying time), at molar ratio bpy/Mn = 3.0.

Claims

1. Coating for an offset paper comprising a catalyst system for fixing polymerizable or crosslinkable constituents of the offset ink.

2. Coating according to claim 1, wherein the catalyst system catalyzes the oxidative polymerisation or the crosslinking of unsaturated constituents of offset ink printed onto the coating.

3. Coating according to any of the preceding claims, wherein the catalyst system fixes fatty acid and resin parts of the offset ink.

4. Coating according to any of the preceding claims, wherein the catalyst system comprises a transition metal complex.

5. Coating according to claim 4, wherein the catalyst system is water soluble or water dispersible.

6. Coating according to one of the claims 4 or 5, wherein the transition metal in the transition metal complex is selected from the group of Ti, V, Cr, Mn, Fe, Co, Ce, Cu, or a mixture thereof.

7. Coating according to one of the claims 4 or 5, wherein the transition metal in the transition metal complex is selected from the group of V, Mn, Fe or a mixture thereof, wherein the transition metal complex is preferably Mn.

8. Coating according to any of claims 4 - 7, wherein the transition metal complex based primary drier catalyst system is supplemented by a secondary drier system comprising Pb, Bi, Ba, Al, Sr, Zr or a mixture thereof in ionic form as salts, complexes or the like.

9. Coating according to any of claims 4 - 8, wherein the transition metal complex based primary drier catalyst system is supplemented by an auxiliary drier system comprising Ca, Zn, Li, K or a mixture thereof in ionic form as salts, complexes or the like.

10. Coating according to one of the claims 4-9, wherein the transition metal complex is a carboxylate and/or a naphthenate complex.

11. Coating according to claim 10, wherein the transition metal complex is a carboxylate with an alkyl chain of 2-18 carbon atoms, preferably of 6-12 carbon atoms, which may be unsubstituted or substituted.

12. Coating according to claim 11, wherein the transition metal complex is a 2-ethylhexanoate-complex.

13. Coating according to one of claims 4-12, wherein the transition metal complex comprises at least one bidentate ligand.

14. Coating according to claim 13, wherein the ligand is a diamine.

15. Coating according to one of claims 13 or 14, wherein at least one ligand is selected from the group 2,2'-bipyridine, 2-aminomethylpyridine, 2-hydroxymethylpyridine, or 1,10 phenanthroline, which may be substituted or unsubstituted.

16. Coating according to claim 15, wherein the ligands are substituted in order to increase solubility and/or dispersibility and/or stability.

17. Coating according to any of the preceding claims, wherein the catalyst system consists of or comprises Mn 2,2'-bipyridine.

18. Coating according to any of the preceding claims, wherein the catalyst system comprises a metal part and wherein the metal part of the primary catalyst system is present in the coating in 0.01 - 0.5 weight-% of the total dry weight of the coating, preferably in 0.05-0.20 weight-% of the total dry weight of the coating.

19. Coating according to any of the preceding claims, wherein the catalyst system comprises a transition metal as well as at least one bidentate ligand and wherein the ratio of metal to ligand is in the range of 1:1 - 1:8.

20. Coating according to any of the preceding claims, wherein additives for increasing the solubility/dispersibility of the transition metal complex and/or the ligand comprised in the catalyst system are present.

21. Coating according to any of the preceding claims, wherein it has a gloss on the surface of more than 75 % according to TAPPI 75deg.

22. Coating according to any of the preceding claims, wherein it has a gloss of more than 45, preferably more than 50 according to DIN 75deg.

23. Coating according to any of the preceding claims, wherein it comprises 100 parts in dry weight of pigment supplemented by 5-20 parts binder and additives, wherein the pigment part comprises CaCO₃, in particular ultra fine or clay CaCO₃ or a mixture thereof, and wherein the pigment part may be substituted up to 10-20 parts by a solid or vacuolated synthetic polymer pigment.

24. Coating according to any of the preceding claims, wherein it is applied as the top coat with a thickness in the range of 10-30g/m², preferably in the range of 15 g/m².

25. Coating according to any of the preceding claims, wherein the drying time of offset printing ink applied to the coating is below 2h, preferentially below 1h, and particularly preferentially equal or below 0.5h.

26. Paper coated with a coating according to any of the preceding claims as top coat.

27. Paper according to claim 27, wherein beneath such top coat there is an additional coating, preferentially also or in the alternative provided with the catalyst system.

28. Method for the production of a coating according to any of claims 1-25, wherein the transition metal complex comprised in the catalyst system as a water solution or dispersion is added to a stirred coating formulation, and the final coating formulation is coated onto a paper substrate.

29. Method according to claim 28, wherein concomitantly with the addition of the transition metal complex a chelating agent, preferably in excess to the transition metal content, is added to the coating formulation, wherein the chelating agent is added as a water solution or dispersion and may contain agent to increase the solubility/dispersibility of the chelating agent.

30. Use of a catalyst system for fixing polymerizable or crosslinkable constituents of the offset ink as an additive for a coating, in particular or paper coating of a high gloss offset paper.

31. Use according to claim 30, wherein the catalyst system comprises a water soluble or water dispersible transition metal complex preferably as a carboxylate and in combination with a bidentate ligand.

32. Use of a paper according to any of claims 26 or 27 in an offset printing process, preferably using an offset ink which also comprises a catalyst system for improving chemical drying of the ink.

Drawing