GENERAL CHARACTERISTICS OF INKS

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Printing ink is essentially an intimate mixture of pigments, oils, varnishes, driers, and, frequently, waxy or greasy compounds. The ink must possess suitable physical characteristics such as viscosity, flow and tack to adapt it to the particular type of printing press and stock on which it is to be used. Each type of press and each kind of grade of stock requires an ink of different physical characteristics to insure that it will distribute properly and transfer correctly from the type to the paper. It must print sharply and give legible prints of the desired color. The ink must dry sufficiently fast to enable the printed sheets to be handled within a reasonable time without offsetting or smudging. Besides these general properties, it is usually important that the ink print a sufficient number of impressions. Many other properties such as resistance to acids, alkalies, paraffin, sunlight and solvents, may be required and are obtained through formulation and selection of ingredients.

Printing inks are divided into three main classes, typographic, planographic and intaglio; corresponding to the three major divisions of printing. Each of these three different classes of inks must be essentially different in character and properties from the other two. Typographic inks are used to print from raised, or relief, surfaces such as ordinary type, line and halftone cuts. These inks consist of soft pigments ground in a varnish or vehicle composed essentially of heat-bodied linseed oil, or, more recently, of synthetic resins dispersed in drying oils. Since the paper, or other drying surface on which they are printed is generally more or less non-absorbent, they must dry mainly by oxidation of the vehicle. They possess considerable " tack, " i.e., stickiness and pull, so that they will distribute evenly on the press and transfer properly from type to paper.

Planographic inks are used to print from plane surfaces, as in the lithographic and offset processes. These inks are generally considerably heavier than typographic inks, that is, they possess considerable tack. In other words, they are more viscous and have higher yield values than typographic inks. The pigments and vehicles used in these inks must not be affected by water, since this process depends upon the fact that grease (in this case planographic ink) and water are mutually repellent. In the old, basic lithographic process, a design was drawn with a greasy crayon on a smooth limestone slab, and the whole surface of the slab was then impregnated with a gum arabic solution. After subsequent drying, it was found that lithographic ink would adhere to the design, but not to the remainder of the slab, providing it was kept moist. In the most modern version of the planographic process, grained metal plates take the place of the cumbersome limestone slabs, and the greasy images, or designs, are formed on the plates by photographic means.

Intaglio inks, originally called steel or copper plate inks, are used to print from engraved, or depressed, surfaces. The character of the ink depends upon the kind of plate, the type of press being used and the speed of operation; thus hand printing of fine engravings requires an ink which will dry by oxidation, and which possesses buttery consistency. At the same time, it must be quite greaseless so that the excess ink may be wiped off the face of the plate without disturbing the ink in the lines, and so the ink will lift readily from the lines of the engravings when the paper is pressed against it. Since the paper used in this class of printing is generally dampened before use, the inks also must exhibit no tendency to bleed, or dissolve, in water. At the other extreme are intaglio inks, which are used to print from engraved copper rolls at high speed. Here the drying is mainly by absorption and evaporation and the inks must be very thin and consist of low gravity pigments suspended thoroughly in a highly fluid vehicle which contains little, or no, drying oil.

Упражнение 11. Ответьте на вопросы к тексту 5А.

1. What is the formulation of printing ink?

2. What physical characteristics must ink possess?

3. Why must ink dry fast?

4. What other properties of ink may be required?

5. What classes are printing inks divided into?

6. What do typographic inks consist of?

7. Are water and grease mutually repellent?

8. Can you describe the old lithographic process?

9. For what purpose are intaglio inks used?

10. What does the character of intaglio ink depend upon?

Упражнение 12. Переведите предложения, содержащие парные союзы.

1. Either relief or gravure image carriers can be used on this rotary press.

2. You may use either hand composition or photographic methods for this unusual work.

3. The engineer technologists said that neither ink nor paper were suitable for this method of printing.

4. The inks for platen press must be neither too sticky nor too fluid.

5. You can see both sheet and web feeders in this pressroom.

6. Both rotary and offset presses can be equipped with either sheet or " web" feeders.

7. Lithographic inks have neither raised nor recessed image areas.

8. Both water and different solutions are used for damping.

Упражнение 13. Переведите следующие словосочетания:

brass alloy; opaque liquid retouching medium; register bar; halftone areas; insert wrong-reading halftone positives; bi-layer masking film; inset positive halftone foil; background positive halftone; colour separation technique; four-colour halftone plate; colour correction; initial colour-corrected screen separations; punch register system; high fidelity duplicate transparencies; photopolymer plate materials; photo electronic control techniques; colour reproduction unit; accurate end product prediction; optimum reproduction equipment; graphic communications industries; graphic arts show.

Упражнение 14. Переведите следующие производные слова:

impression, viscosity, tendency, solution, emulsification, adhesion, successfully, consistency, carrier, undesirable, sticky, importance, portion, naturally, various, durability, expensive, practically, immediately, greasy, receptive, condition, planographic, equipment, extremely, production, suitable, additive, penetration, resistance, hydrophilic, arrangement, repellent, rapidly, fatty, replace, requirement, thickness, oxidation, absorption, polymerize, evaporate, contribution, absorbent, conventional, initiator, pollution, elimination, embrittlement, considerable, uncoated, minimize, insoluble, desensitized, dissolve, emulsify, drier, inability, roller, protective.

Text 5B

Задание 1. Прочтите текст, стараясь понять основное содержание.

(Этот текст надо прочесть за 6 минут).

Printing Ink

Ink was first developed in China about 2, 500 to 3, 000 B.C. The early inks are thought to have been made by mixing lampblack with oil and gum. Different kinds of paper, different printing surfaces and printing presses require different kinds of ink. Printing inks are mainly composed by colour pigments and varnish ground together in proper relation to one another to suit the various works for which they are intended. The pigments furnish the colour and the varnish the binder which holds the colour to the paper. Pigments for various colours are obtained from various sources. The pigment in black ink is lampblack produced by burning fats or oils in such a manner аs to sесure a heavy deposit of soot or carbon.

There are water-colour and oil-base inks. The water or oil, in which the pigment is carried, is called vehicle. The most common vehicles are linseed oil and rosin oil. The former is used for better grades of ink. It has the property of absorbing oxygen, and, when spread out in a thin film, it forms a very smooth, hard coating which after drying a few hours, will not rub off. The rosin varnishes, which do not dry so rapidly, are used in cheaper grades of ink. They are intended for printing on softer papers which will easily absorb the ink. There are many kinds, or classes of printing inks differing from each other in composition depending on the work to be printed. For example, a cheap grade news ink is used for printing on rough, uncoated paper such as newsprint. It has no drying properties and dries only by absorption into the paper and therefore is not suitable for better classes of printing.

Printing inks are available in many colours. If you need a lighter tint than you have, you can add some white into the ink. A thick piece of glass with a white piece of paper under it makes a good mixing surface for ink. To check the mixed colour one should spread a thin layer of the mixed ink on a piece of printed paper to be used.

Задание 2. Выберите утверждение, соответствующее содержанию

текста.

1. Linseed oil is used:

a) for bad grades of ink;

b) for better grades of ink;

c) for all kinds of ink.

2. The rosin varnishes are intended

a) for printing on softer papers;

b) for printing on coated papers;

c) for expensive grades of ink.

3. The pigment in black ink is:

a) a special plant;

b) varnish;

c) burnt fats or oils.

a) Newsprint is an expensive grade of paper;

b) newsprint is a coated paper;

c) newsprint is a rough, uncoated paper.

Задание 3. Ответьте на следующие вопросы.

1. Where and when was ink first developed?

2. Why are different types of ink required?

3. What are printing inks mainly composed of?

4. How is lampblack produced?

5. What are the most common vehicles?

6. Where are rosin varnishes used?

7. How does news ink dry?

8. What do you do when you need a lighter tint than you have?

9. What makes a good mixing surface for ink?

10. How do you check a mixed colour?

Text 5C

Задание 1. Прочтите текст.

On a printing press, ink must be transformed from a massive volume to a thin film which can then be transferred to a surface being printed. There are two ways of providing this distribution: one is by means of a doctor knife, whereby ink is spread at one go, and the other is by means of a train of distribution rollers, whereby ink is split in the nip between successive rollers in the train. For successful doctor blade spreading, the viscosity of the ink must be low. Photogravure and flexographic inks are of low viscosity, so doctor blade spreading can be adopted. An example in the handling of letterpress and offset inks is the movement of ink out of the ink duct. As the duct roller rotates, a film of ink is spread on the roll by the duct blade.

If an ink is of low viscosity and has little or no yield, elaborate distribution systems are not needed. With photogravure and other machines using liquid inks, the duct is simply a trough from which the ink is withdrawn by contact.

If an ink has a yield value this is indicative of flocculation, so it is necessary to use a more elaborate distribution system which is capable of performing sufficient work on the ink to break up the flocculates. This is the purpose of the roller trains and ancillary devices used on offset and letterpress machines.

Once the ink has been broken down into a thin film, it must be transferred to the substrate being printed. This final printing action can be regarded as a special case of ink distribution and transfer in which one surface initially has a zero ink film thickness, and is commonly absorbent to ink.

Notes to the text:

1. doctor blade – ракельный нож

2. nip – зазор

3. yield – текучесть

4. flocculation – выпадение хлопьями, флоккуляция

Задание 2. Ответьте на вопросы.

1. How many ways of ink distribution do you know?

2. What inks have low viscosity?

3. When are elaborate distribution systems needed?

4. What is indicative of flocculation?

5. For what purpose are the roller trains and ancillary devices used on offset and letterpress machines?

Задание 3. Озаглавьте текст.

Text 5D

Задание 1. Прочтите текст.

Drying Principle

Offset printing inks form a solid printing ink film on the substrate by means of drying or curing processes.

These processes can be based on a physical or a chemical mechanism or on the effect of both of these two processes.

The physical drying process can be divided into absorption and evaporation phases and is accelerated by heat.

When using absorbent substrates such as paper or cardboard, it is mainly the low-viscosity components of the paste inks which penetrate into the capillaries of the substrate, thereby enriching the binder on the surface and quickly forming a tack-free ink layer. This method of drying is called absorption. Absorption is an important drying mechanism for offset and letterpress printing inks. Web offset newspaper printing (coldset) and rotary letterpress newspaper printing are solely based on this drying mechanism. Therefore, the absorbency of newsprint is very significant for drying.

In heatset web offset printing, the paper web is run through a drying oven where the thinner contained in the inks evaporates at temperatures far in excess of 120 °C. The temperature of the circulating air in the dryer is between 160 °C and 220 °C, and generally heats up the paper webs to temperatures between 90 °C and 130 °C. In the subsequent purification process the evaporated components in the exhaust air are extracted, usually by thermal or regenerative afterburning.

Oxidative drying is a chemical process. So-called 'drying' oils and resins are cross-linked by a reaction with the oxygen in the air. To accelerate the reaction, the printing ink contains siccatives which act as catalysts. In general, these are metal soaps consisting of cobalt, manganese, calcium, iron and similar metal compounds with fatty acids such as linoleic acid or other organic acids such as octanoic acid and naphthenic acid. The entire drying process takes several hours. Heat and careful airing of the stack accelerate this chemical reaction.

Ultraviolet (UV) and electron-beam (EB) radiation are used to harden specially designed printing inks and varnishes. The radiation energy generates highly reactive particles in the ink layer which trigger an abrupt chain reaction, resulting in a solid, very resistant and high-gloss ink layer being formed. This process is used for printing inks which do not contain any thinners, meaning that an excellent result can be achieved even when printing on non-absorbent substrates such as foils, aluminized paper etc.

The two principles of drying mentioned, physical absorption (or evaporation) and chemical oxidation, are used in different proportions with offset printing inks. There are also examples of each of the pure reaction processes. With the majority of inks, the two processes are applied in combination. On the basis of the varying proportions in the process, the entire range of offset printing inks can be divided into categories of more or less oxidative drying or more or less physical drying.

The coldset inks used in newspaper printing, which employ a single drying process, can be cited as an extreme example in the category of physical drying. The solidification of the printing ink film takes place solely by physical separation of the low-viscosity ink components from the solids content by means of absorbency of the substrate - namely the newsprint. The capillary action of the paper surface is the sole driving force in this separation, and it is clear that a change in the paper surface has a major influence on the solid/liquid separation effect, and therefore also visibly affects the so-called drying of the printing ink. This is the case, for example, with high-finish newsprint or high-glazed paper and also grades of soft-calender treated paper. Changing these paper properties can even lead to a situation where it is necessary to alter the composition of the printing ink to be able to guarantee certain printability properties when switching to this kind of paper.

This sole drying mechanism also shows the latent weakness of all coldset products, namely the tendency toward offsetting or smearing. An additional solidification of the printing ink film due, for example, to a chemical reaction (oxidation) also encompasses the possibility of interaction with the fibres of the paper. This would result in significantly stronger anchoring and therefore better rub resistance but would also certainly make it more difficult to redissolve components of the printing ink. This simple separation is, however, an indispensable requirement for paper recycling using a de-inking process.

We divide coldset inks into categories according to their viscosity, which itself is determined by the various inking unit constructions. We therefore differentiate between 'long' low-viscosity newspaper printing inks for inking units with overshot ink (duct) knives, 'short' high-viscosity inks for those inking units with undershot ink (duct) knives and also a special kind of ink for keyless anilox inking units.

With web offset heatset inks the ink drying also takes place to a large extent purely physically by separation of the low-viscosity thinner components of the ink. A small amount is absorbed into the substrate. However, by far the greatest proportion evaporates when heat is applied. Here, air that has been heated in the drying ovens is blown as turbulant blast onto the layer of printing ink. This blast of air causes the thinner to evaporate and removes it. To enable this to happen the evaporation properties of the thinner must be such that it can evaporate as quickly as possible at oven temperatures of just 160 to 220°C.

The blast of air must, however, be efficient enough to guarantee removal of the thinner which has already evaporated out of the ink film. If the correct force is not achieved, thinner can be seen condensing on the chill rollers behind the drying oven. This is predominantly on the first and third chill roller, and can lead to disturbing ink softening again in the print.

With heatset inks a certain proportion of oxidative drying takes place. This can be estimated to be around 5 to 10 %. The use of various types of paper with different surface properties (sensitivity to picking/ sensitivity to picking when wet) makes adjustment of the printing inks necessary to match these properties.

Here we make a distinction between ink formulations for coated paper, calendered or super-calendered paper and inks for newsprint.

A relatively high proportion of oxidative drying compared to these ink formulations is found with highly rub resistant inks which attain additional rub resistance due to a particular oxidative film formation of the printing ink layer. An even stronger oxidative film formation can be found with inks intended for use in the production of print products which are then personalized using a laser printer. Burning in laser printer toner takes place at a temperature of around 210°C, and the printing ink film must be able to tolerate this.

The formulation of the heatset ink series is oriented towards the demands of different substrates and does not take into account the various press configurations.

Depending on the configuration of the inking or dampening unit used, ink film splitting problems can occur at high production speeds. These effects can lead to insufficient ink transfer onto the paper, which in turn leads to insufficient saturation compared to the original. With higher printing speeds, the rotational speed of the rollers in the inking unit usually also increases and sometimes leads to considerable ink misting. The printing ink is severely decompressed at the outlet of the inking rollers and the resulting formation of strands of printing ink leads to droplets forming as the strands break in several places. This is actual ink misting. However, by selecting vehicle systems of a suitable structure it is possible to modify the rheological properties of the printing inks in such a way that they can also satisfy these additional requirements.

The next area is that of classical sheet-fed offset printing inks (printing inks with a higher proportion of oxidative drying components) where infrared drying should be seen as a physical aid to drying. Special series of inks have been formulated to meet the demands of the various processing conditions.

The quantity of physical and chemical drying components in these ink series is approximately equal.

Inks for use on non-absorbent substrates, such as those for foils and to a certain extent aluminized paper, represent the opposite extreme of purely oxidatively drying ink. The special inks used here have a composition which enables the film of printing ink to dry purely by chemical oxidation without any physical adsorption.

This overview enables a systematic classification of all inks used in offset printing.

For a complete description, it is necessary to mention that radiation-curing, so-called UV or EB inks are an inking system which contains no thinners. These inks are fundamentally different from conventional offset inks in their vehicle composition. The vehicles used are capable of reacting with themselves under the influence of UV or electron radiation (polymerization).

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