WO2009092743A2

WO2009092743A2 - Improved set of printing ink compositions

Info

Publication number: WO2009092743A2
Application number: PCT/EP2009/050676
Authority: WO
Inventors: Martin THÜRING; Robert Howell; Lars Keller
Original assignee: Sun Chemical Corporation
Priority date: 2008-01-21
Filing date: 2009-01-21
Publication date: 2009-07-30
Also published as: WO2009092743A3

Abstract

A set of printing inks is described, wherein the tack of the printing ink printed as the last one has been reduced by at least 10% when compared to the printing ink printed as the first one within the set. The set of printing inks and a respective printing process provide a solution to the problem of back-trapping.

Description

Improved set of printing ink compositions

Introduction

The present invention relates to a set of printing inks, wherein the tack of the printing ink printed as the last one (the "last printed ink") has been reduced when compared to the printing ink printed as the first one (the "first printed ink") within the set.

An ink set is formed by all inks that are consecutively printed onto a single substrate during a single printing process. Preferably, the ink set according to the present invention consists of four differently coloured inks. Usually the colours black, cyan, magenta and yellow constitute an ink set. The printing sequence preferably is black-cyan-magenta-yellow. Therefore, usually the yellow ink will have a reduced tack when compared to the black ink within the set according to the present invention.

The present invention also refers to a printing process making use of the set of printing ink compositions according to the present invention.

The improved set of printing inks according to the present invention and its application in a printing process solve the problem of back-trapping.

The problem of back-trapping occurs in many printing techniques. The expression means that the reservoir for one of the printing inks is contaminated by another previously printed ink from the set, because this previously printed ink is drawn into the reservoir of the later printed ink. Consequently, back-trapping leads to an unclean printing result. Once back-trapping has occurred, it is very time-consuming to clean the printing press. Firstly, the reservoir of the contaminated printing ink has to be cleansed from the contaminated ink. Afterwards the contaminated rollers have to be cleansed in order to restore a satisfactory printing result. After having done the aforementioned steps, printing can be contin- ued for some time until back-trapping reoccurs. There are printing processes during which this happens every 15 to 30 minutes depending on the printing press.

Prior art documents do not address the tack differences between different print- ing inks forming an ink set. In particular, the prior art did not reflect so far that tack modification of the first and last printed ink within a set, such as for example the black and yellow colored ink, can improve printing quality. There is not even a hint in the art that the back-trapping problem can be solved by lowering the tack of the last printed ink to a particular extent when compared to the tack of the first printed ink.

There is a strong need to solve the back-trapping problem, i.e. to improve the printing inks such that no decrease in printing quality is visible.

As printing inks are complex mixtures of many different components, all of which serve important purposes, it is not possible to randomly alter an ink composition. Inks are usually composed of at least one pigment and at least one ink vehicle. Choice of ink has important impact on overall printing ink properties. Apparently, the solution to the above-mentioned problem should therefore also be of such a nature that it could easily be applied to many different printing inks without running the risk of concomitantly influencing other important ink properties.

The object of the invention is solved by the subject-matter of the patent claims.

It has now surprisingly been found that a set of printing inks wherein the tack of the last printed ink is reduced by at least 10 % when compared to the tack of the first printed printing ink is capable of avoiding back-trapping during printing processes. It is further preferred that the tack of the first printed printing ink is at least 15% lower and most preferred at least 20% lower than the tack of the first printed printing ink within the set of printing inks according to the present invention. However, the tack of the last printed ink should not be reduced too far as it might happen that the tack is so low that the last printed ink will not transfer from the reservoir to the rollers or from the rollers to the paper. Therefore, tack reduction should preferably not exceed 50% when compared to the first printed ink. According to preferred embodiments, tack reduction should not exceed 30% when compared to the first printed ink.

According to ISO 12634:1996(E) tack is defined as: "Restoring force between two rotating rollers of a given width caused by the splitting of a printing ink or vehicle film on the roller surfaces".

One definition mentioned in the ASTM standard for tack measurement D 4361 -97: is "Tack - a function of the force required to split a thin fluid film of a printing ink or vehicle between two rapidly separating surfaces; it is a rheological parameter indicative of internal cohesion of the fluid".

Tack of printing inks controls their high-speed transfer properties. It is also meaningful to predict paper picking in multi colour printing.

Conventional instruments determine the force exerted on a measuring roller that is positioned on the printing ink film of a driving roller. Fig. 1 shows a schematic drawing of a three-roller-tackmeter. The operation principle of tack measurement instruments is as follows: A defined weight of printing ink is placed on a three roller system. The roller system consists of a middle, metallic driving roller, a printing ink distribution roller and a measuring roller for tack determination. These two outer rollers are covered with an elastomeric layer. After speed adjustment and temperature stabilization the axial force on the measurement roller is determined. This axial force is used as indication of tack. The higher the axial force, the higher the determined tack number.

Different manufacturers of tackmeters have established their own arbitrary scales. In the examples a Tack-o-scope® (Testprint BV, Netherlands) is used to measure tack. According to preferred embodiments of the present invention, the last printed ink is a yellow coloured printing ink. It is further preferred that the first printed printing ink is black.

The teaching of the present invention can be applied to all printing inks and to all printing processes in which back-trapping occurs. However, these processes will usually be lithographic printing process. Therefore, the set of printing inks according to the present invention is preferably a set of lithographic printing inks. It is further preferred that the ink set is a set of offset printing inks. Offset printing inks are preferably inks selected from the group of sheet-fed offset, cold-set and heat-set offset. It is most preferred that the set of printing inks is a set of heat-set offset printing inks, preferably heat-set web offset (HSWO) printing inks.

In case proportions of constituents of the printing inks are expressed as percentages by weight, the percentage is relative to the ready-to-use printing ink composition of the respective colour.

Offset printing inks are preferably characterized by a composition comprising about 10 to 20% by weight of pigments. The pigments may for example be added as pigment pastes or dry pigments. If the pigment is added in form of pigment paste, the printing ink compositions preferably comprise pigment pastes in amounts of between 1 % and 60% by weight, preferably between 25% and 50% by weight and most preferably between 30% and 45% by weight. If dry pigments are used, the respective amounts are 1 to 30% by weight, preferably 10 to 25% by weight and most preferably between 15 and 20% by weight. Preferred pigments are conventional organic or inorganic pigments, most preferred are Diarylide Yellow (for example pigment yellow 12, 13, 14, 174), Lithol Rubine Magenta (for example pigment magenta 57:1 ), Phthalocyanine Cyan (for example pigment cyan 15:3), Carbon Black for printing inks (for example pigment black 7). The ink compositions present in the ink sets of the present invention preferably comprise solvents in amounts of from 10 to 50% by weight. These solvents are preferably hydrophobic which means that they show a distribution coefficient D between n-octanol and water that is at least above 3 and preferably in a range of from 3.9 to 8.0. Preferably at least one solvent is a mineral oil solvent.

Further, the inks within the printing ink sets of the present invention preferably comprise resins in amounts of from 10 to 25% by weight. Preferred resins are rosin modified phenolic resins, maleic resins, rosin esters or hydrocarbon types.

Usually additives are present to adjust the ink water balance. These additives can be ionic, non-ionic or amphoteric surfactants but also polyols, oleate or amine derivates as well as oleamides.

Additionally, the last printed printing ink comprises up to 10% by weight of a tack reducing agent. Preferably, this agent is present in amounts of from 1 % by weight to 6% by weight, preferably from 1 .5 % by weight up to 6 % by weight based on the printing ink composition. Tack reducing agents preferred according to the present invention are compounds with a boiling point of between 300 and 500⁰C. Preferably, these compounds have a melting point of between 30 and 60⁰C. In a preferred embodiment, hydrocarbon compounds comprising at least one aliphatic hydrocarbon with at least 15 carbon atoms is added as the tack reducing agent. Examples for preferred tack reducing agents are polyethylene (PE) wax and petrolatum. Petrolatum is a mixture of solid and liquid hydrocarbons with chain lengths between 20 and 30 carbon atoms.

In a special embodiment the tack reducing agent is substituted by halogen, preferably fluorine. A preferred example is polytetrafluoroethylene (PTFE wax).

In a preferred embodiment petrolatum is present in the last printed ink in amounts of from 3% by weight to 5% by weight and more preferred in amounts of from 4% by weight. The printing ink composition according to the invention preferably comprises at least one varnish in an amount of from 15 to 30% by weight. Preferably the varnish comprises at least one vegetable oil, preferably stand oil.

According to the present invention all printing inks within the set of printing inks except the last printed ink are substantially free of the tack reducing agents described above, i.e. these inks have higher tacks than the last printed ink composition described herein. In terms of this description the expression "substantially free" is understood in such that the respective compound has not been purposefully added to the composition.

The present invention also refers to the use of the set of printing inks as defined herein in a printing process. This process preferably is a lithographic printing process. It is further preferred that the process is an offset printing process. Offset printing processes are preferably selected from the group of sheet-fed offset, cold-set and heat-set offset processes. It is most preferred that the set of printing inks according to the present invention is used in a heat-set offset printing process.

The printing process according to the present invention comprises the step of applying to a substrate a set of printing inks wherein the last printed ink has reduced tack when compared to the first printed ink, i.e. the set of printing inks de- scribed above. The invention is most preferably directed to a heat-set offset printing process comprising the step of applying to a substrate a set of printing inks wherein the last printed ink has reduced tack when compared to the first printed ink.

In preferred embodiments, the printing process is a process of consecutively print- ing four printing inks on the substrate. The sequence of inks is preferably black- cyan-magenta-yellow.

Preferably, the set of printing inks is printed on paper. According to the present invention, the set of printing inks preferably consists of four printing inks with the above described properties. The second and third printed printing ink may have higher or lower tacks than the first printing inks. In a special embodiment, the second printing ink has a higher tack than the first print- ing ink. In preferred embodiments of the present invention, the second printed printing ink has a tack that is at least 5% higher than the tack of the first printed ink.

Examples

Example 1 :

This ink set is based on "1/3 varnish" and varnish "7131 ", which are described in WO 2008/132192.

The printing ink composition comprises pigment paste (pigment level 30%). Mineral oil B solvents with low boiling range (210-230⁰C) were chosen for improved drying. Mineral oil C solvents with medium boiling range (240-270⁰C) were chosen. A high pigment level allows thin printing ink films on printing press which speeds up drying. In order to solve the back-trapping problem, the tack of the yellow printing ink was reduced by using petrolatum.

Composition of printing ink Series according to the present invention:

Component Yellow Magenta Cyan Black

Yellow pigment paste 38

Magenta pigment paste 45

Cyan pigment paste 39 Black pigment paste 45

Extender paste 5 5 5 5

Petrolatum 4 . . .

Varnish 7131 29 17 20 25.5

1/3 varnish 10 14 18 15.5

PTFE wax paste 2 2 2 2

Antioxidant solution 1 1 1 1

Mineral oil B 5.9 1 1 1 1 6

Mineral oil C 4.5 5 4 -

Total (wt. %) 100.0 100.0 100.0 100.0

The components of the printing ink were mixed in a dissolver at temperatures up to 6O⁰C.

Example 2:

The following example shows that the set of printing inks according to the present invention is effective in solving the back-trapping problem that would usually have occurred in the described process. This example is divided into several "printing steps". Each printing step is a test printing wherein four inks with different colors are printed on paper. From step to step the tacks of defined inks are modified. Each printing step contributed to show that surprisingly the tack difference between first and last printed ink is the decisive factor in solving the back-trapping problem. Printing was done on an 8 page Heatset Web Offset press equipped with a four zone hot air dryer (web width: 52 cm, speed: 50.000 cp/h = 6.2 m/s, runtime: 15.000 copies). After every printing step the blankets and ink duct rollers of yellow were cleaned. All printing operations were conducted using a four colour ink set in the sequence black-cyan-magenta-yellow.

Procedure:

The first printing step was done with a conventional ink set (series 12100 as described in WO 2008/132181 ). The paper was a conventional 65 gsm paper. This step was conducted in order to have obtain a printed paper showing back- trapping as it appears with printing inks known in the art. As expected significant back-trapping occurred. The yellow roller and container were blatantly contaminated by the black ink.

In the second printing step the amount of fountain solution was increased at the yellow unit. As a result, there was little less back trapping, because of the lower tack of the more emulsified yellow ink. But further increase of fountain solution leads to printing problems like over-emulsification resulting in low print quality.

In the third printing step, the black printing ink was changed to another ink (series 12000 as described in WO 2008/132181 ) which had higher tack than the black ink used in the first step. There was less back trapping when compared to the first step.

In the fourth printing step, also the yellow coloured printing ink was replaced by a modified yellow ink with lower tack than the one used in the steps before. Modifying the yellow printing ink to an ink with lower tack decreased back- trapping significantly. In a fifth printing step, the set of inks was designed such that the tack of all inks forming the set was reduced (see ink 16000 in WO 2008/132190) within which all the tacks of the printing inks are reduced. There was back trapping.

In a sixth step, the black coloured ink in the set was modified in order to reduce its tack. There was even more back trapping.

In a seventh step, petrolatum was added to the yellow coloured ink. The black ink was not modified. No back-trapping could be observed.

In an eighth step, polyethylene wax was added to the yellow coloured ink. The black ink was not modified. There was no back-trapping and simultaneously the slip of the resulting prints was improved.

In a nineth step, the same yellow ink as used in the eighth step above remained in the press and additionally the tack of the black ink was reduced with petrolatum, too. As a result, back-trapping re-occurred.

In a final step, the whole printing ink set was again changed to series 12000 and the tack of the yellow ink was reduced with petrolatum. No back-trapping could be observed.

Discussion:

It can be seen from the steps above that the concept of the present invention is able to solve the back-trapping problem for different printing ink sets. Obviously, the modification of the tack of the black coloured ink does not solve the back- trapping problem. Rather, the tack of the yellow ink relative to the black ink must be lowered to achieve the advantageous result of no visible back-trapping in accordance with the invention.

The type of tack reducing agent does not seem to play an important role as the effect can be achieved, for example, with petrolatum or PE wax. The outcome of this test is actually surprising as the person of ordinary skill in the art would have thought that the tack of the black, i.e. the first printed, ink had to be reduced in order to solve back-trapping. This is due to two factors: 1. It is the black ink, which sticks to the yellow roller and is thus carried to the yellow ink container, therefore it would be obvious to reduce the tack (aka stickiness) of the black ink. 2. Reducing the tack of the black ink slightly improves back-trapping (see third and sixth step), and this slight improvement was interpreted as a hint that the tack of the black ink would be the key to solving the back-trapping problem. In other words, the knowledge of the skilled person teaches away from such the present invention.

During the printing trial comprising the above described printing steps it was clearly shown that the last printed ink has to have significant lower tack than the first printed ink.

Ink sets wherein the tacks of first and last printed inks were equal or tack of the first printed ink was higher showed significant back-trapping.

Example 3:

In the following four tables are presented showing the effect of the extent of tack reduction on the occurrence of back-trapping. The values are results of tack measurements and printing trials with the respective printing ink sets specified in the headlines. The printing process was a heat set offset printing process.

Tack measurement was conducted with a Tack-o-scope® instrument using a speed of 300 m/min. Table 1 :

Y - K defines the tack difference between the yellow and the black colored ink within the ink set.

It can be seen from the tables above that tack reduction of the last printed ink (yellow in this case) has to amount to at least 10% in comparison to the first printed ink (black in this case) in order to solve the back-trapping problem. If, in contrast, the last printed ink had a higher tack than the first printed, severe back-trapping occurred.

Example 4:

The table below shows results of a printing trial on a sheet-fed printing machine. Tack measurements were conducted with a Tack Thwing Albert Instrument at a speed of 800 rpm.

It can be seen that back-trapping was only perceivable where the difference in tack between first and last printed ink (black and yellow in this case) was below 10%. This example also shows that the concept according to the present invention works with different types of printing processes.

Claims

1. A set of printing inks wherein the tack of the last printed ink in the set is at least 10% lower than that of the first printed ink.

2. The set of printing ink compositions according to claim 1 , wherein the last printed ink comprises a tack reducing agent in amounts of up to 10% by weight.

3. The set of printing ink compositions according to claim 1 or 2, wherein the last printed ink comprises a tack reducing agent selected from hydrocarbon compounds with a boiling point of 300 to 500⁰C or a melting point of 30 to 60⁰C.

4. The set of printing ink compositions according to one or more of the preceding claims, wherein the tack reducing agent is selected from petrolatum, PE and PTFE.

5. The set of printing ink compositions according to one or more of the preced- ing claims, wherein the last printed ink is yellow.

6. The set of printing ink compositions according to one or more of the preceding claims, wherein the first printed ink is black.

7. The set of printing inks according to one or more of the preceding claims, wherein the set comprises four differently colored inks.

8. The set of printing inks according to claim 7, wherein the four colors in the ink set are black, cyan, magenta and yellow.

9. The set of printing ink compositions according to one or more of the preceding claims, wherein the printing inks are lithographic printing inks.

10. A printing process comprising the step of applying a set of printing inks to a substrate, wherein the tack of the last printed ink in the set is at least 10% lower than that of the first printed ink.

11. The printing process according to claim 10, wherein the substrate is paper.

12. The printing process according to claim 10 or 11 , wherein the printing process is a lithographic printing process.

13. The printing process according to claims 10 to 12, wherein the process is selected from heat-set offset, sheet-fed offset or cold-set offset printing processes.