|LEAD Action News Volume
13 Number 1, November 2012, ISSN 1324-6011
Incorporating Lead Aware Times ( ISSN 1440-4966) and Lead Advisory Service News (ISSN 1440-0561)
The Journal of The LEAD (Lead Education and Abatement Design) Group Inc.
Editor-in-Chief: Zac Gethin-Damon
An extract from Printing Ink Manual, 1961
By Editor-in-chief, Reginald F. Bowles; Associate Editors: F.A. Askew; R.F.G.HOLNESS; H.A.IDLE; R.H.LEACH, commissioned by the technical training board of the Society of British Printing Ink Manufacturers
Origin: By coupling of tetrazo-benzidine with aceto-acetanilide derivatives.
Hue: Yellows varying from lemon to golden yellow and in opacity from semi-opaque to transparent.-
Bulk value: 85. Concentrated paste: 40 per cent.
Application: Widely used in many types of inks. Particularly useful for soap- and heat-resistant inks. Used to replace chrome yellows where lead pigments are objectionable.
Lead Chrome Yellows
Origin: By co-precipitation of lead chromate and lead sulphate together with basic salts.
Hue: Yellows ranging from pale greenish yellow to deep orange, all opaque.
The individual pigments are known as: primrose chrome, lemon chrome, golden chrome (medium chrome), orange chrome.
Bulk value: 16-20. Concentrated paste: 80 per cent.
Application: Their high specific gravity necessitates attention to formulation to obtain the best working qualities. Being compounds of lead, the pigments are not suitable for use in foodstuff inks. They catalyse the oxidation of drying oils and formulae must be adjusted accordingly.
Tartrazine Yellow Lakes
Origin: A dye-alumina complex prepared by the precipitation of tartrazine with barium chloride in the presence of alumina.
Hue: A range of shades varying from lemon yellow to orange.
Bulk value: 40 + according to extender. Concentrated paste: 40 per cent.
Cadmium Yellows and Reds
Origin: Mixtures of cadmium sulphide, cadmium selenide and lithopone.
Hue: A range of colours varying from pale greenish yellow to orange, scarlet and deep bluish red. All opaque.
Bulk value: 23. Concentrated paste: 65 per cent.
Application: Used in inks where resistance to heat and light and alkalis are of great importance. Not acid-resisting. Cadmium pigments are resistant to organic solvents and oil. Must not be mixed with lead pigments, being sulphides.
Iron Oxides, Ochres, Umbers, Siennas, etc.
Origin: Iron oxides and silicates.
The qualities used in printing inks nowadays are mostly synthetic forms, prepared by heating ferrous sulphate, or precipitation of the hydroxide followed by ignition.
Hue: Colours ranging from dirty lemon yellow to deep reddish brown.
Bulk value: 25. Concentrated paste: 75 per cent.
Application: Coarse pigments, difficult to grind. Their use is limited. Also used in silk-screen inks.
Origin: Azo colour based on acid dyestuff (barium salt). Made by coupling diazosulphanilic acid with B naphthol.
Origin: Lead chromate, lead molybdate and lead sulphate in the form of mixed crystals.
Bulk value: 18. Concentrated paste: 80 per cent.
Application: A very opaque orange pigment of wide general use. Limitations exactly as chrome yellows.
Metallic driers are used in two forms, as oil-soluble soaps, which comprise the liquid driers, and as dispersions in oils of inorganic salts, which comprise the paste driers.
The soluble driers are prepared by the conversion of suitable organic acids to their heavy metal salts or soaps.
The liquid driers are prepared with metal concentrations of the order of 3-8 per cent, with the exception of lead, 25 per cent. At such metal concentrations, the addition of between 0.5-4 per cent of driers is the amount normally necessary to achieve adequate drying in printing ink films.
Inorganic metal salts when dispersed in drying oils are converted to a limited extent to the soluble soap. The dissolved soap is in heterogeneous equilibrium with the insoluble metallic salt. Such a paste dries slowly but has the advantage of acting as a source of metal to replace adsorbed soluble drier.
The metals most widely used in the preparation of liquid driers are the following:- Cobalt, manganese, lead, zinc, calcium, cerium, zirconium and lithium.
Cobalt forms a powerful drying catalyst, violet in colour. It is the most widely used of the metallic driers. Its chief limitations are its tendency to discolour tints and whites, and its ready solubility in inorganic acids.
Manganese is generally less powerful in its action than cobalt. It is brown in colour, affects pale colours less than cobalt, and is less readily leached out of lithographic inks. Manganese usually requires lead to be present in considerable quantity to act as an activator.
Lead is a slow-acting drier, of pale colour, now very limited in use in view of the many lead-free specifications for printing inks. It is unsuitable for use in alkyds, being precipitated as lead phthalate. It is frequently used in conjunction with manganese.
Cerium, zirconium and lithium are driers of medium efficiency and pale colours. They have replaced lead in alkyd vehicles and are usually used in conjunction with cobalt or manganese.
Calcium and zinc are both poor catalysts and it is doubtful whether they are of much value in printing ink films as driers.
Before the introduction of the present synthetic type of vehicle, the paint industry drew a distinction between through drying and surface drying or skinning. Lead and zinc in particular were said to promote through drying, and cobalt and manganese surface drying.
The linoleates prepared from linseed oil fatty acids are perhaps the most widely used of the soluble driers. The metal content is usually 3 to 6 per cent for all metals except lead which is commonly 16 to 24 per cent. Linoleates are mild in odour and of viscosity of the order of 10-60 poises.
Naphthenates, prepared from naphthenic acids, are supplied as solutions in hydrocarbon solvents, and are much lower in viscosity than the linoleates, usually less than 10 poises. The metal content is much higher and can go up to 12 or 16 per cent for cobalt and manganese. Their rather strong odour limits their use, but their high metal content offers economic advantages.
Paste driers are prepared by grinding mineral salts of lead and manganese in linseed oil varnishes. Lead acetate and borate and manganese borate are commonly used. The percentages vary with individual formulators, but are of the order of 40 per cent lead acetate, 8 per cent manganese borate. Small amounts of soluble manganese driers may also be added. Cobalt is not normally used in paste driers.
Cobalt driers are very inclined to yield dried prints which are not receptive to overprinting.
The whole problem of producing rub-resistant inks is very complicated owing to the large number of variable factors involved. Work carried out by the author shows that individual pigments vary widely in respect of rub resistance and that this variation is not related simply to particle size. Lead chromes, for example, which have a comparatively large particle size, are capable of yielding inks of very good rub resistance.
Inks for Food Wrappers - Toxicity
The number of toxic ingredients of present-day printing inks is small, being virtually limited to compounds of lead and to a lesser extent, zinc. Lead may be present in the form of lead-chrome pigments or in the form of soluble lead soaps as driers. Zinc may be present as a pigment as in zinc oxide or sulphide (in lithopone) or as a soap in a resin (eg. Zinc resinate).
Inks for immediate wrappings must be applied to the outside of the wrapper. Inks for this purpose should be formulated with materials other than those known to be toxic, and in particular, pigments, driers and plasticisers should be selected accordingly; for example, materials containing significant quantities of arsenic or lead should not be used.
It can be calculated that an ink with a lead content of 1 per cent printed over an area of 4 sq. in. would add less than one half-part per million of lead to a quarter pound packet of butter. If, therefore, the whole of the printing ink were consumed with the butter, the contribution of lead from the printing ink would fall below the recommended limit. With the removal of the wrapper in the normal way before eating, it is difficult to visualise any possibility of lead contamination of the butter from the ink, sufficient to influence materially the acceptability of the butter in accordance with the ‘Revised Recommendations for Limits of Lead Content of Foods.’
“The recommended limit for lead in butter is recorded in ‘Food Standards Committee Report on Lead, 1954 – Revised Recommendations for limits of Lead Content.’ 14.-(11). Edible Oils and Fats. 0.5 parts per million by weight.
The ‘Food Standards Committee Report on Lead, 1954,’ makes the following to ice cream and iced lollies:
15.-(2) ‘We feel that the lead limit for ice cream and iced lollies should be put as low as possible having regard to the large consumption by children.’
It is as well to remember that there is no evidence of any person in this country having suffered any ill effects which may be attributed to the use of toxic ingredients in inks. Nevertheless, there has been a considerable volume of correspondence on the matter and certain food manufacturers do specify that inks for packaging their products should be “lead-free.” In practice, it is virtually impossible to guarantee complete freedom from lead and certain maxima expressed in terms of lead per million are used.
INKS FOR SPECIAL PURPOSES
The machines used are similar in principle to a lithographic flat-bed press and the output is low and may only run into a few hundred copies a day. Both the damping and inking have to done with exceptional care and control of pressure, the inks have to be stiff, concentrated, and non-abrasive.
Highly concentrated offset inks, bodied still further to stiffer consistency by grinding in a proportion of a fine extender, such as alumina hydrate, are indicated.
Blue Collotype Ink
Iron blue pigment: 56 per cent
Alumina hydrate: 6 per cent
Lithographic varnish (40 poise): 32 per cent
Refined linseed oil: 6 per cent
100 per cent
With pigments which do not accelerate drying, a small amount of paste drier (Pb/Mn) should be added (eg. 2 ˝ per cent).
INKS FOR SPECIAL PURPOSES
INORGANIC PHOSPHORESCENT INKS
Phosphorescent inks are used for special purposes, the pigments being usually zinc sulphides containing small amounts of impurities which activate the molecule when irradiated, so that visible light is emitted. By choice of the metal sulphide, the activating impurity, and the conditions of preparing the pigment by calcining, various colours of emitted light can be produced and the pigments so formed are used for display or identification purposes.
INK AND FOOD
Many foodstuffs manufacturers and confectionery-makers specify the limits of such elements as lead and arsenic for printed matter used by them, and sometimes attempt to apply to inks the standards which should properly relate to package contents rather than the wrappings. Certain steps can be taken by the ink maker, such as avoiding the use of lead or cadmium pigments, but it is extremely doubtful if there is any risk attracting to the use of these in normal printed wrappings.
A plain surface, such as metal or paper, to which a coating containing selenium or zinc oxide has been applied, has a surface electrical charge applied to it by passing under a high voltage discharge.
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Updated 29 November 2012
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