Introduction to Printing Inks.
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- Shivaji Nagar, Waliv, Vasai(E) - 401 208
Introduction to Printing Inks.
This part includes the nature of the product and its details.
The nature of Printing Inks :-
Inks are coloured, usually liquid, materials for writing or printing. They are used for many reasons, primarily as a tool to convey a message and give instant decoration. Inks are extremely versatile and can be applied to a wide variety of different surfaces, whatever their texture, size of shape, Paper, plastic, metal, glass and textiles will all accept ink. Printing inks are applied to such surfaces by printing presses of various design and are conveniently divided into five classifications according to the type of press Ink films, unlike paint films, are very thin varying from 2 to 30 um depending on the print process. Inks consist essentially of two components, the colourant; and insoluble solid or a dye, and the vehicle, a liquid in which the colourant is suspended or dissolved. The combined components form a fluid capable of printing from a printing press. The secondary function of the vehicle is to ‘dry’ and bind the colourant firmly to the substrate.
Although the primary functions of an ink are to convey a message or decorate, it is of no value if it will not transfer to the substrate nor adhere satisfactorily after printing. In addition to
This chapter will discuss each of these properties of an ink in turn and outline how each of the fundamental building blocks of an ink colourants, resin’s, solvents and additives, affect each property, Individual chapters in this manual will deal with each of these characteristics in more detail.
VISUAL CHARACTERISTICS OF INKS
The visual characteristics of an ink are defined in terms of its colour, its transparency or capacity and its gloss and are largely determined by the quantity and nature of colourant (pigment or dye) used.
The colour of inks
Colour is a complex concept and needs to be described in three interrelated ways. Firstly the ‘hue’ of a colour indicates what kind of colour it is, i.e. red, blue, green, etc. Secondly, the ‘strength’ of a colour is a measure of its intensity or saturation. The third property of its ‘purity’ which indicates how bright or dark it is.
There are many factors that affect each of these parameters to one degree or another. Clearly, the chemical structure of the colourant has a fundamental effect on its hue, but so do particle size, surface characteristics and the amount of colourant contained in a given ink film. In general, the higher the concentration of a particular colourant, the greater the strength of the colour, although in many cases an optimum concentration occurs beyond which there is no further increase in strength.
The types of resins, oils or solvents used in the ink vehicle can alter the hue or purity of a particular colourant.
The transparency and opacity of printing inks
To achieve the desired visual appearance, inks need to have varying degrees of opacity and transparency. One of the major influences on the transparency of an ink is the choice of colourant and the degree to which it is dispersed
Difference colourants behave differently towards light. The more opaque colourants have a greater tendency to reflect and refract light, and this is influenced by particle size and refractive index.
Thus, the high refractive index and the particle size of titanium dioxide ensure that it effectively reflects, refracts and, therefore, scatters light of all visible wavelengths, making it one of the most opaque pigments in use in printing inks. Soluble dyes, on the other hand, have a relatively low refractive index and small particle size and give highly transparent ink films.
The gloss of printing inks
The gloss of an ink is a measure of its ability to reflect incident light and depends to a large extent on whether or not the ink forms a smooth film on the surface of the substrate and masks any irregularities. When an ink penetrates a substrate it tends to lose gloss. The degree of gloss depends on the nature of the colourant, its particle size, shape and surface characteristics and the amount of resin and its ability to form a continuous film. In general, the more resin that is present in relation to the colourant the higher the gloss will be. The choice of resin can also be critical to gloss through its ability to disperse pigment – the better the pigment dispersion, the better the gloss.
THE NATURE OF PRINTING INKS AS DETERMINED BY THE PRINTING PROCESS.
The formulation of a printing ink and its physical appearance are determined greatly by the method by which the image is produced and the transfer of the inked image on to the substrate to be printed.
Flexographic and Gravure Inks
Inks which are printed by the flexographic and gravure printing process are characterized by their extremely fluid nature. They are generally termed liquid inks.
The fluidity of gravure inks allows the recessed cells of the gravure cylinder to be filled rapidly. Similar fluidity is required for the flexographic process as the inking systems depend upon an engraved cylinder metering the ink on to the raised rubber image. Both inking systems involve only a short time and distance between the bulk volume supply of the ink and the accurately metered film of ink on the printing image. Therefore, highly volatile solvents can be used and most flexographic and gravure inks are fast drying and the ink is transferred to the substrate as quickly as possible.
Lithographic and letterpress inks
When compared to the fluid nature of flexographic and gravure inks, lithographic and letterpress inks are much more viscous and paste-like. Although the mechanisms of image generation of the latter processes are very different, the thick nature of these inks is determined primarily by the method by which the image is inked and then transferred to the substrate.
THE DRYING CHARACTERISTICS
An ink in its supply form is a liquid but after application it is required to change to a solid. There are many mechanisms by which the liquid ink is transformed into a solid permanent mark. This change of state is referred to as ink drying and can be physical, chemical or a combination of both processes.
Absorption during
An ink dries by absorption when it penetrates by capillary action between the fibres of a substrate and also when it is absorbed into the substrate itself. The ink remains liquid but because of the degree of penetration, is effectively ‘dry’.
Oxidation drying
An ink dries by oxidation when the oxygen in the atmosphere chemically combines with the resin system converting it from a liquid to a solid.
The oxidation process can be affected by a number of factors, including pigments and additives, as well as the printing process and the substrates. The ink maker’s task is to produce carefully balanced formulations which ensure that high quality and high production speeds can be maintained whilst the drying process proceeds at a satisfactory rate.
Evaporation drying
A wide range of inks is formulated to dry by the physical removal of volatile solvents from the ink formulation, leaving the resin material behind to bind the pigment to the substrate. The rate of drying depends upon the evaporation rate of the solvents selected and also the affinity of the resin system for the solvents. Generally, the greater the affinity of the resin for the solvents the slower the rate of solvent release. This can greatly affect the properties of the ink especially with respect to printability, drying speed and retention of solvents in the ink film. Each type of ink requires a very careful balance of properties if the characteristics of the ink are to meet the requirements of the printing process and the end use of the printed article.
Chemical drying
Oxidation drying has already been explained, but there are other chemical reactions used by the ink maker to convert a liquid into a solid. Some systems comprise a polymerisable compound which requires the presence of a catalyst and sometimes heat to set off a chemical cross linking reaction. Other rely on the reaction of at least two different chemical types which when mixed together combine to produce a solid that binds the colourant to the substrate. Each of the various systems used has its own specific conditions under which the chemical reactions occur and give the dry ink films required.
THE ADHESIVE NATURE OF PRINTING INKS
One of the fundamental functions of an ink is that it adheres and binds the colourant to the substrate on which it is printed, and keeps it there during the life span of the printed product.
Colourants have little or no effect on the adhesive nature of the inks, although if they are not adequately bound by the vehicle may be because of poor dispersion, the ink will not exhibit good adhesion.
THE RESISTANCE PROPERTIES OF PRINTING INKS
All inks have to resist certain forms of chemical and physical attack during their ‘life span’. Firstly, the raw materials must withstand the manufacturing process used to make the ink in the first place. Having manufactured the ink, it must be able to withstand the rigorous of the printing process.
Light fastness.
The degree of resistance to light will depend on the exposure, application and life expectancy of the ink. The action of light can cause a colour to become weaker, dirtier and/or change its shade.
The light resistance of an ink is primarily determined by the light fastness of the colourant used. Colourants can be placed in broad classes of light fastness and before attempting to use a colourant in any application, its light fastness against a shaded scale, i.e. blue wool scale, should be checked. A few pigments, including carbon black, ultramarine and some iron oxides, can be considered permanent, but the remainders are all to some extent fugitives.
Heat resistance
Printed packages sometimes have to resist excessive heat in their processing or application and the choice of resin and pigment are both critical to the heat resistance properties of the ink.
Pigments vary in their ability to resist heat. Heat can dirty a colour, cause strength loss and alter shade. Pigments must be evaluated and chosen so that they will withstand the temperatures encountered both during manufacture and end use of the printed article.
Abrasion resistance
Most, if not all, printed articles are subject to different forms of rub and abrasion during their life cycle. All types of printing inks are formulated with this in mind.
The degree to which a dried ink film can resist abrasive forces depends greatly on the degree to which the pigment is bound. The more resin that is present the greater is the ink’s resistance to abrasion, and the hardness and flexibility of the ink film will affect its ability to withstand abrasion and mechanical wear. Additives such as waxes, which promote surface slip, are used extensively to increase the scuff resistance of inks. By their very nature, most printing inks contain some wax of some kind.
Product resistance
Ink in its many application in the packaging field has to be able to withstand contact with the substances packaged. Colourants vary widely in their resistance to acids, alkalis, oils, fats, detergents and other substances and must, therefore, selected with the end use of the ink in mind. Resins, also, have differing resistance properties and must be evaluated in conjunction with the colourants to determine their suitability for a particular application. Specific additives can be used to impart specific product resistance properties. For food packaging work, ink components must be selected to avoid odor and taint problems.
THE GRAVURE PROCESS
In the GRAVURE process the printing image is engraved into a cylinder in the form of cells which become filled with ink. Printing is achieved by passing the substrate between the GRAVURE cylinder and an impression roller under pressure.
The printing unit
The printing unit of a gravure press consists of an ink duct in which the etched cylinder rotates in a fluid solvent-based ink. A metal doctor blade, which reciprocates from side to side scrapes excess ink from the cylinder surface. The substrate is fed from reels into a nip between the etched cylinder and a rubber covered impression roller which supplies the cylinder and a rubber-covered impression roller which supplies the pressure needed to transfer ink from the cells to the substrate Fig.
The printed web runs upwards through a heated drying system where the solvents are evaporated and extracted and the ink is thus dried. In gravure printing each colour must be nominally dry before the succeeding colour is printed over it, therefore each printing unit has its own integral drying equipment. The ink which is usually
Stored underneath each unit, is pumped up to be ink trough and continuously circulated, and usually viscosity control is incorporated in this system. Because each printing unit has an integral drying system and impression roller, most presses consist of units arranged in line, where the web travels between units in a horizontal plane. As the impression cylinder is not gear driven, but obtains its drive through contact with the gravure cylinder, cylinders of different size can be used to provide variable print repeat dimensions within certain limits.
The inking system
The oldest and simplest method which is still widely used is to place the cylinder in the ink trough. This has its deficiencies, however, since the ink is not well agitated, lots of fumes are given off and it is unsuitable for high speeds since the cylinder tends to carve a trough in the ink.
An alternative uses transfer rollers. The roller does not actually touch the cylinder – there is a gap of about 0.006 inches (0.15 mm). This method was used on some gravure machines printing from plates rather than cylinders.
Doctor blades
The function of the doctor blade is to remove surplus ink from the surface of the cylinder leaving the ink in the cells. There are many possible configurations for the doctor blade and they have an effect on the printed result. The thickness of the blade is generally 0.006 – 0.01 inches (0.15 – 0.25 mm) and it is made from high-carbon flexible steel. Doctor blades are usually supported by a blacking blade to give extra support (say 0.03 inches or 0.76 mm thick)
Drying system
Slow-speed sheet-fed machines may not have any drying system other than an extended delivery transport but some form of cold air blower is more normal. High-speed web machines demand more sophisticated driers between each unit. A web tends to carry a solvent layer with it and consequently high-velocity hot air is used to break up this layer and remove the solvent.
COMPOSITION / INFORMATION ON INGREDIENTS
Chemical Characterization:
Weight% CAS
Physical Appearance: Specific colored Liquid with a characteristic odor.
Potential Health Effects
Eyes : May cause eyes burning or irritation.
Skins : May cause irritation in the form of redness, burning or dermatitis.
Ingestion : Solvents present in this mixture may be fatal if swallowed. Also small quantity may affect nervous system and cause abdominal discomfort. Contact local poison control centre or physician immediately. Do not induce vomiting unless instructed so by a physician. If vomiting occurs, keep head lower than hips to help prevent aspiration. Get medical attention immediately.
Vapour can flow along surfaces to distant ignition sources and flash back.
Personal Safety
Extinguishing Media : Carbon Dioxide, Dry Chemical Foam and Water Spray. Hazardous Combustion Products : Oxides of carbon and noxious fumes.
Fire Fighting : Wear full protective equipment and a self contained Equipments breathing apparatus with the full face covering operated in the pressure demand mode. Water spray may be used to keep fire-exposed containers cool until they can be evacuated.
Sensitive to Static Discharge : Yes.
Hazardous Decomposition Products: Hazardous polymerization will not occur.
ACCIDENTAL RELEASE MEASURES.
Comments: Ventilate area of leak or spill. Remove all ignition sources. Wear appropriate personal protective equipment. Contain and recover liquid when possible. Use non-sparking tools and equipment. Contain liquid in an appropriate container or absorb with an inert material i.e. sawdust, dry sand, universal binder, acid binder or cloth etc., and place in a chemical waste container. DO NOT FLUSH TO SEWER. If leak or spill has ignited use water spray to disperse any vapor and flush spill away from personal or other ignitable materials. When picked up, treat material as prescribed under heading “DISPOSAL”.
HANDLING AND STORAGE
General Procedures: Protect container against physical damage. Store in a cool, dry well ventilated location away from any area where a fire hazard mat be acute. Containers should be bonded and grounded for transfer to avoid static sparks. Do not permit smoking in storage areas. Use only non-sparking tools and equipments around the liquid. Empty containers may be hazardous from residual liquid and vapor. Observe all warnings and precautions listed.
Use: This product is used as a printing ink in packaging & paper industry.
WE BELIVE ALL THE DATA AND INFORMATION GIVEN IS ACCURATE AS OF DATE OF PREPARATION AND IS OFFERED IN GOOD FAITH BUT WITHOUT WARRANTY OR REPRESENTATION. SINCE CONDITIONS OF USE ARE BEYOND OUR CONTROL, WE DISCLAM ALL LIABILITY FOR RELIANCE THEREON. THIS IS OFFERED SOLELY FOR CONSIDERATION, INVESTIGATION AND VERIFICATION