Progress in the improvement of printers’ materials is nowadays so rapid that it is highly important for the printer to keep in touch with modern developments in the basic materials of the trade. The main advances in printing ink technology have been the progressive improvement of colours, varnishes, grinding methods, etc.; the introduction of outstanding properties in commonly used inks; and the discovery of entirely new types of inks. These notes are intended to give a few modern lines.
The aim of the ink maker is to produce inks which can be used straight from the tin without having to be doped by the addition of boiled oil, driers or similar materials. A first-class black should, without incorporation of any other material, possess the following properties:
- High density and brilliance of colour.
- Sufficient softness and freedom from tack to prevent plucking and picking of the paper.
- Rapid drying on the paper but slow drying on the machine, so that the ink remains wet for 24 hours, or preferably 50 hours, on the rollers.
- Such consistency and texture that it feeds well without hanging back in the duct, and does not fill up half-tones.
- Speed of penetration so that the print does not set-off, even under the pressure of a considerable pile of superimposed sheets.
- Quick hard setting, enabling backing up to be done soon after printing.
- Freedom from spray, even on high-speed presses.
- Applicability to a wide variety of different papers.
With regard to the last point, we all know that different classes of paper behave differently with the same ink. It is easy to formulate an ink which does not set off on one paper yet sets off markedly on another. Realizing that the printer’s choice of paper is often limited by practical considerations, the ink maker so formulates his standard lines that they are usable on as many different papers as possible.
The modern introduction of high-speed presses has led to the making of special inks for these machines. An ink which has given excellent results on a comparatively slow machine may spray badly when it is tried out on a fast-running one.
During the past year or two, much attention has been paid to improving the setting of blacks and preventing set-off. There are now obtainable some new blacks, the physical properties of which are such that there is a very rapid and firm set on the paper, allowing of quick backing up, while set-off* is eliminated.
Non-skinning Bronze Blues
It is rarely indeed that the ink maker puts into his bronze blue inks any driers such as cobalt, manganese or lead. This is because the bronze blue pigment is itself a powerful drier, and being, of course, present in very high proportion, causes rapid drying and skinning.
Expressed differently, bronze blue, like cobalt, manganese and lead driers, is a catalyst for the reaction between linseed oil varnish and oxygen — that is, bronze blue is a substance which increases the rate at which the linseed oil varnish, which is present in the ink, combines with the oxygen in the air to produce a dry film. Catalysts are used for quickening up many chemical processes.
When the printer requires a bronze blue ink which will not skin on the rollers in, say, two days, the ink maker is faced with a problem of a different type from the usual problem of driers. It is, of course, impossible to reduce effectively the bronze blue which is at once the pigment and the drier. Tiie problem is best solved by using a substance, known as a “negative catalyst,” which behaves in the opposite way to an ordinary catalyst; the negative catalyst reduces the rate at which the ink skins. By using the right negative catalyst in suitable proportion (only a very little is required), it is possible to produce a bronze blue of brilliant lustre which will not skin or dry on the rollers even in two days and yet will dry satisfactorily on the paper.
Coloured News Inks Although coloured news inks are by no means new, it is only in recent times that they have been used to any large extent for advertising purposes. Good quality coloured news inks, which work well and do not fill up even on long runs, are now obtainable at a price suited to newspaper work, and it is probable that the near future will see a considerable increase in the amount of coloured advertisements.
Aniline Inks and Syrups
Aniline inks are being widely employed nowadays with good results. These inks consist essentially of dyes dissolved in methylated spirit, while other ingredients are added to impart fastness to water, brightness, etc., and to prevent excessive drying on the rollers.
Aniline inks are much used in paper bag and similar manufacture. The drying of the inks is mainly dependent on the rate of absorption and evaporation of the volatile spirit, and since this is very fast, the printed sheet can pass directly from the rubber stereo to be processed into the completed bag.
Generally, a good aniline ink will be found suitable for most papers, but there are a few exceptions. For printing on kraft paper, for instance, special anilines are generally required in order to obtain the maximum brightness of colour. Special surfaces like tinfoil and cellophane also require special inks for best results.
Where transport charges become considerable, as is the case with exported inks, aniline syrups may be used instead of aniline inks. These syrups are so made that when one part of the syrup is mixed with two (sometimes three) parts of methylated spirit, the resultis an aniline ink. The stronger syrups (those to be mixed with three parts of spirit per one part syrup) are so concentrated that, in the case of some (not all) colours, the syrup may be rather too viscous and may not immediately mix with the spirit. It is, therefore, recommended that those syrups, which are convertible by adding two parts of spirit to one of syrup, should be used, as these never give trouble on mixing. These latter syrups are of course cheaper.
Special syrups are obtainable for use on kraft papers.
Letterpress Inks for Non-absorbent Surfaces
A little may perhaps be said about letterpress printing of black and coloured inks on surfaces such as cellophane, celluloid and glassine. Inks for these materials dry almost entirely by oxidation — there is practically no drying by absorption or evaporation. Consequently special quick-drying varnishes are used.
The printer should carefully avoid the introduction of any non-drying ingredients into such inks. For instance, in washing up previous to a run, particular .care should be taken to remove traces of wash-up liquid from the rollers and the forme. If the printing were done on ordinary papers, the presence of a little non-drying liquid would not matter so much— although here, too, there is an element of danger. On non-absorbent papers, however, the danger is considerably increased, because the non-drying liquid remains with the ink on the surface and may seriously hamper the hardening of the print.
Gold and Silver Letterpress and Photogravure Inks
When the first experiments were made towards producing metallic letterpress inks, the main difficulty was to combine the desirable properties of resistance to rubbing, extremely high lustre, and non-drying on the machine during printing. A very large amount of work was in fact done on this problem, with the result that exceptionally brilliant inks which do not wipe off, or otherwise misbehave, are on the market.
More recently, metallic photogravures have been introduced, and the efforts of ink makers’ laboratories have resulted in the availability of very bright, firmly-adhering metallic gravures which work well on the machine.
As is well known, there are two main types of overprinting varnishes, one containing a proportion of volatile liquid such as methylated spirit or turpentine, and the other composed entirely of non-volatile ingredients. Although the former type is excellent as far as it goes, it is probable that it will be increasingly displaced by the non-volatile varnishes. These latter are easy of application, being printable in much the same way as ordinary letterpress inks; they can also be safely used to overprint many colours which would bleed in a methylated spirit varnish.
Research has been in the direction of increasing the gloss and improving the working qualities of the overprinting varnish. Some modern preparations are quite soft and easily workable, and yield a hard, smooth film of extremely high gloss, not only when applied to a dry non-absorbent ink layer, but also when printed directly on to the paper. Coloured overprinting varnishes are also available.
Odourless Inks and Varnishes
For some purposes, particularly for printing on food wrappers, it is sometimes advisable to use an ink having as little odour as possible. Varnishes made in the ordinary way possess a slight smell which is imparted to inks made from them.
There are two methods of producing an ink which does not possess the smell of ordinary varnish. Either there can be incorporated with the ink a small quantity of a perfume which satisfactorily masks the other smells, or odourless varnishes (made by special processes) can be used. Both methods can be combined.
Offset and Litho Inks
Improvements in offset and litho inks have included the increasing of the water resistance and the strength of colour. For — good quality offset work, particularly where in competition with photogravure processes, great colour intensity of the ink is, of course, highly desirable, since the film of ink in the offset print is so thin.
One type of modern offset ink contains special water-insoluble dyestuffs in addition to the usual pigments, a device which results in great colour strength and quite satisfactory resistance to water.
Washing Up — Ink Removers
A variety of good ink removers have been in use for a long time and are well known, but a point of which many are unaware is that there are special removers for dry ink. A roller or type face which contains patches of dry ink will require a considerable amount of laborious scrubbing with an ordinary ink remover before it is clean, but with special solvents or solvent mixtures, it is only necessary to moisten the surface. After a few minutes the powerful action of the solvent uproots the dry film. Sometimes it may be preferable to remove the worst of the dry film with the special solvent and then to finish off with an ordinary ink remover. In this way, the cleaning of dry ink can be done in a tenth of the time usually employed. Of course, the solvent does not injure the roller or type face in any way.
With regard to the cleaning of ordinary aniline inks from the machine, methylated spirit is the standard medium for this purpose. The best anilines do not dry on the rollers too quickly, but if the rollers or other parts have been left inky overnight, it may be desirable to wet the surface with a special aniline ink remover before finishing off the cleaning with methylated spirit.
This article from the British Printer, 1934 and written by J. D. Cohen, BSc, AIC