GB2152514A - Aqueous printing inks - Google Patents

Abstract

The transfer properties of aqueous printing inks are improved by adding polyacrylamide having intrinsic viscosity of from 2 to 4.

Description

SPECIFICATION Aqueous printing inks Aqueous printing inks, for instance for flexography or intaglio printing, comprise water, pigment and binder and various optional ingredients such as surfactant, dispersing agent, wax and thickening agent. The ink is generally made by forming a varnish of some or all of the binder and water and ball milling the pigment with this varnish and any other additives that are desired, such as wax. The various additives are selected in known manner having regard to the rheological and other requirements of the ink.

The printing onto paper or other substrate of printing ink from a printing roller, plate or other printing surface requires that the ink shall be transferred from the printing surface to the substrate as a result of contact of the substrate with the surface. It is naturally desirable that the greatest possible degree of transfer should occur.

A reasonable degree of transfer is obtained with many conventional printing inks but it is known to add material for the specific purpose of improving transfer. In particular it is proposed in US Patent 4,014,833 to add low molecular weight polyethylene oxide, and in particular to add the material sold under the trade name Polyox WSRN-80. This material has proved useful in practice. Unfortunately low molecular weight polyethylene oxides such as this are rather expensive and not readily available and so it would be desirable to achieve the same or better transfer properties by adding more readily available and cheaper materials.

We have now surprisingly found that the transfer properties of an aqueous printing ink can be improved by incorporating into the ink a polyacrylamide having an intrinsic viscosity of between 2 and 4.

An aqueous ink according to the invention generally comprises water, pigment, binder and the polyacrylamide.

The polyacrylamide preferably has an intrinsic viscosity of at least 2.5. Normally it is below 3.5 and best results are obtained with intrinsic viscosities of from 2.5 to 3.1.

Preferably a 5% solution in water of the polyacrylamide has a viscosity, measured at 25"C using a 3A suspended level viscometer, of from 370 to 600 centistokes, most preferably from 400 to 560 centistokes.

The polyacrylamide is preferably a substantially non-ionic homopolymer, having no deliberate addition of anionic or cationic groups. However the low level of anionic impurity that may often be present commercially (for instance up to around 1% anionic), is acceptable. Deliberate increase in the amount of anionic groups seems to render the product less satisfactory as a transfer additive although useful transfer effects can be obtained with anionic polyacrylamides containing, for instance, 5% or more anionic groups and indeed useful effects can be obtained with polyacrylamides containing quite high amounts of carboxylic acid groups, for instance up to 10, 30 or even 40% carboxylation of the polyacrylamide.

The polyacrylamide can be made by conventional solution or other polymerisation techniques to the desired intrinsic viscosity.

The amount of polyacryiamide included in the aqueous ink is generally from 0.1 to 1.5% by weight of the ink, most preferably 0.2 to 1%, with best results generally being achieved at about 0.5%.

The other components of the aqueous ink may be conventional and so will include water, pigment, binder and other conventional additives and these, and their amounts, will be selected in known manner such that the ink is suitable for the particular printing process involved. The invention is of particular value in flexographic printing inks and processes but it is also of value in other aqueous inks, for instance inks intended for gravure or other intaglio processes.

The pigment may be any insoluble or other colourant suitable for use in aqueous inks designed for the intended printing process and the binder may be any binder suitable for use in such inks. As is known, binders based wholly or mainly on resins that are wholly acrylic in nature are often very suitable in aqueous printing inks, especially when they have a relatively high molecular weight, and so can be used in the invention. However the invention is of particular value with lower molecular weight acrylic binders and with binders of other chemical types, for instance styrene acrylate binders, rosin maleate binders and shellac binders, as inks containing these other binders tend, before the invention, to have rather poor transfer properties.

Additives suitable for use in the inks, and suitable methods of making the inks, may all be conventional, for instance all as described in US Patent 4014833. It is desirable not to apply high shear to the ink after the polyacrylamide has been added and so preferably the polyacrylamide is added after the completion of the milling of the pigment with the other components of the ink.

However the viscosity of the ink may be adjusted by conventional dilution after the polyacrylamide addition.

We find that the defined polyacrylamides used in the invention have the advantage of giving satisfactory transfer properties whilst avoiding the expense and poor availability problems associated with the polyethylene oxide. Also it has been found that not only do the defined polyacrylamides give satisfactory transfer results, but they often give transfer results better than those obtainable with the polyethylene oxide.

The defined polyacrylamide appears to have unique advantages not possessed by other polymers that might have been postulated as possibly being useful and it appears that these advantages are not directly associated with the viscosity that the polymer imparts to the ink.

Polyox WSRN-80, as proposed in US Patent Specification No. 4,014,833 is a non-ionic polymer.

We have established that another non-ionic poly mer that might have been expected to be useful, namely polyvinyl pyrrolidone is unsatisfactory.

Polyox WSRN-80 has an intrinsic viscosity of about 0.002 and the viscosity of a 5% aqueous solution of it at 25"C measured using a 3A suspended level viscometer is about 100 centistokes. We have established that it is essential in the invention for the polyacrylamide to have an intrinsic viscosity between 2 and 4 and that polyacrylamides having lower intrinsic viscosities are unsatisfactory and that polyacrylamide giving a similar 5- solution viscosity of about 100 centistokes is also unsatisfactory. The inclusion of very high molecular weight polyacrylamides (for instance having intrinsic viscosities above 5), as thickening agents in printing pastes is well known but they are unsatisfactory as transfer additives in printing inks since they change the rheology of the inks by increasing the viscosity as a result of which the ink has to be diluted.

The addition of the defined polyacrylamide does thicken the ink slightly but we find that the same degree of thickening achieved by the addition of different polymers, for instance acrylic acid-acrylic ester copolymers does not give the improvement in transfer properties attained when the polyacrylamide is used.

It might have been thought that clean release of the ink from the printing surface would be promoted by the addition of a suitable surfactant to the ink but we find that the addition of the defined polyacrylamide gives much better release properties than the addition of various surfactants.

The following are examples of the invention.

Example 1 A conventional aqueous flexographic ink was prepared by ball milling 15 parts by weight phthalocyanine blue with 2 parts by weight of a wax (Polymist A12) and 83 parts by weight of a varnish in conventional manner. The varnish had previously been formulated in conventional manner from 40 parts by weight of a carboxylated acrylic emulsion (as binder), 60 parts by weight water and about 1.4 parts by weight monoethanolamine to adjust the pH to between 8 and 8.5.

The ink was divided into samples and to each sample was added 5% by weight of a 10% aqueous solution of a transfer additive (0.5sub by weight solids based on the weight of the ink). The additive was stirred into the ink and the viscosity of each ink was adjusted to 25 seconds, measured at 25"C through a Zahn 2 flow cup. The transfer properties were assessed by preparing draw downs in a conventional manner using a hand operated anilox roller and the results from each sample were compared.

The starting ink, to which no transfer additive was added, gave reasonably satisfactory release properties. The addition of ethoxylated lauryl alcohol, polyvinyl pyrrolidone, ethoxylated ceto-stearyl alcohol, sodium di-octyl sulphosuccinate, or ammonium polyacrylate as transfer additive gave no improvement and in some instances gave worse results. The use of Polyol WSRN-80 gave an improvement. The use of polyacrylamide sometimes gave an improvement and sometimes gave no improvement, depending upon the particular polyacrylamide that was used.

Best results were achieved using non-ionic polyacrylamide having an intrinsic viscosity of 3.06 and a 5% solution viscosity (measured as described above) and 550 centistokes. Next best results were obtained using polyacrylamide homopolymer having intrinsic viscosity 2.71 and solution viscosity of 426 centistokes. Next best results were obtained using Polyox WSRN-80, having intrinsic viscosity 0.002 and 5% solution viscosity of 100 centistokes.

The use of polyacrylamide having intrinsic viscosity 1.51 and 5% solution viscosity of 100 centistokes and the use of polyacrylamide having intrinsic viscosity 1.91 and 5% solution viscosity of 343 both gave no significant improvement compared to the ink free of transfer additives.

The transfer properties deteriorated when the polyacrylamide homopolymer having intrinsic viscosity of 2.71 was replaced with a similar polyacrylamide having a substantial degree of carboxylation (up to about 30%) but were better then in the absence of polyacrylamide.

Example 2 Inks were formed broadly as defined in Example 1 and adjusted to a viscosity of 15 seconds measured on a No. 4 Ford Flow Cup. Three identical inks were then thickened to a viscosity of 20 seconds (a typical printing viscosity) by the addition of sufficient of an additive to give this desired viscosity. One additive was a polyacrylamide having IV about 3. Another was an ethylacrylate methacrylic acid linear copolymer whilst the third was an ethylacrylate methacrylic acid lightly cross-linked polymer. The ink containing the polyacrylamide gave the best transfer properties.

When the process of this example was repeated using a larger amount of a lower molecular weight acrylic emulsion as the binder, and when it was repeated using a styrene acrylic binder instead of the carboxylated acrylic emulsion, the same trend was always observed, namely the polyacrylamide always gave the best transfer properties.

Claims (9)

1. An aqueous printing ink containing a transfer improving amount of a polyacrylamide having an intrinsic viscosity of from 2 to 4.

2. An ink according to claim 1 in which the intrinsic viscosity is from 2.6 to 3.1.

3. An ink according to claim 1 or claim 2 in which the polyacrylamide has a 5% solution viscosity, measured at 25"C using a 3A suspended level viscometer, of from 370 to 600 centistokes.

4. An ink according to any preceding claim containing from 0.1 to 1.5- by weight of the polyacrylamide.

5. An ink according to any preceding claim containing water, pigment and binder.

6. An ink according to claim 5 in which the binder is selected from acrylic binders, shellac, rosin maleate binders and styrene acrylate binders.

7. An ink according to any preceding claim selected from intaglio and flexographic inks.

8. An ink according to any preceding claim in which the polyacrylamide is substantially nonionic.

9. A method for improving the transfer characteristics of an aqueous printing ink which comprises adding to the ink a polyacrylamide having an intrinsic viscosity of from 2 to 4.