Printing inks are, according to their viscosity, divided into

  • liquid inks
  • paste inks.
  • Liquid inks are employed in gravure and flexo printing, while paste inks are used in letterpress and lithography. Screen inks are intermediate between paste and liquid inks.
    Basically, all printing inks are made up of

  • colourant
  • vehicle (or varnish)
  • solvent
  • additives.

  • Colourants

    Colourants are grouped into

  • pigments (tiny crystals, insoluble in the vehicle) and
  • dyestuffs (soluble).
  • In printing inks, pigments are used almost exclusively, save with flexo inks.

    Pigments, by their chemical nature, are further divided into

  • inorganic pigments
  • organic pigments.
  • Furthermore, there are metallic pigments, pearlescent pigments, fluorescent pigments, and others more.

    Pigments are usually referred to by their Colour Index name or formula number
    (e.g. P.Y. 12, CI No. 21090 = Pigment Yellow 12, formula number 21090).


    Dyes (or Dyestuffs)

    are soluble in the material they are used in.

    They are rather scarce in printing inks, but they are of some importance in flexo inks and for some special applications, e.g. heat transfer printing, invisible (i.e. fluorescent) inks, and cheque security inks.

    Some of these dyestuffs, called basic dyes, are quite popular because of their high tinctorial strength and brillant shades, but they soon fade when exposed to light. They also need some mordant, which helps to make the prints more resistant to water.

    As basic dyes usually are soluble in water, they are a bit more frequent in water-based inks then elsewhere.

    One of the most popular dyes is eosine, which is well known from the teachers' red fountain pen ink; it also shows a strong yellow-green flourescence. (Explain, why the usually visible and fluorescent inks are this way round, while the other would be impossible.)

    Transparency and opacity

    transparent ink opaque ink

    With transparent inks, the substrate is responsible for scattering; with opaque inks, the pigments scatter the light.


    Pigments

    Inorganic pigments

    account for the achromatic inks.

    The most important white pigment is titanium dioxide, which serves to make white inks; calcium carbonate, also a white pigment, is only used as an extender.

    The most important pigment at all is carbon black, as it is the only pigment used in the manufacture of the most important printing ink, the black one.

    There are several processes to form carbon black; they all rely on the thermal decomposition or the incomplete burning of hydrocarbons such as fuel oil or natural gas. "Furnace black" and "channel black" are produced most frequently.


    Particle size in some carbon black pigment

    1 Channel Black, surface about        110 m²/g
    2   Furnace Black, surface about 80 m²/g
    3 Acetylene Black,          surface about 65 m²/g
    4 Lamp Black surface about 20 m²/g
    5 Blacking surface about 15 m²/g

    As inks containing carbon black as the only pigment show a brown shade, Milori Blue is added to counter that.

    Use of inorganic pigments

     
    titanium oside
    iron osides - - -
    carbon black -
    chromate pigments - - - - -
    molybdenum
    red pigments
    - - - - - - -
    zinc pigments - - - -
    cadmium pigments - - - - - -
    chromium oside
    pigments
    - - - - - -
    ultramarine blue - - - - - -
    iron blue - - - - - -
    very important
    medium importance
    rarely used
    - not used


    Organic pigments

    account for the coloured inks.

    Inorganic coloured pigments are very rarely used nowadays, as they usually contain toxic heavy metals (chromium, cadmium, lead, etc.).

    For green and blue shades, phtalocyanine pigments are employed. These molecules contain a very stable system of aromatic rings.

    For red and yellow inks, azo pigments are most frequently used. Their formula contains the azo group, -N=N-.

    The human body is able to cleave the azo group into the compounds which it is made of, thus producing aromatic amines, some of which are carcinogenic.

    Hence, under EC legislation, azo dyestuffs are to be regarded as carcinogenic, too, if the underlying amine is. With pigments, the risk is considered very small; whether it is nil remains to be seen.

    Molecules absorb light in the visible range if there is a conjugated system of double bonds in the molecule, that is, if single and double bonds alternate.

    The most simple example to illustrate this are polyenes of the typeformula

    Obviously the wave-length of maximum absorption is the longer the more expanded the conjugated system is. If it is big enough, enters the visible region. Then the molecule shows the colour complementary to the colour absorbed.

    Absorption spectra of two triphenylmethane dyes

    Absorption spectra fo two triphenylmethane dyes            chemical formula
    chemical formula



    Pigments for coloured printing inks

    chemical formula of yellow (azo pigment)
    yellow (azo pigment)
    chemical formula of magenta (lithol pigment)
    magenta (lithol pigment)
                     chemical formula of cyanblue (copperphthalocyanin) beta-form
    cyan (copper phthalocyanine)
     - form



    Crystals of Cu phthalocyanine pigments
    Crystallstructures of Cu-Phthalocyanin-Pigments
     [Full screen 82 KB]
    (Source: Fonds der Chemischen Industrie, Germany; imageseries "Farbstoffe und Pigmente")


    Special pigments

    Pigments for metallic inks consist of small metal flakes, which act as tiny mirrors. They are, however, larger than usual pigments (several µm as opposed to fractions of a µm). "Silver" pigments are made of aluminum, while "gold" pigments are made of brass or dyed aluminium.

    Pearlescent pigments are coated with very thin layers of titanium dioxide, some silicate material, etc. The thickness of these layers is only fractions of the wave-length of visible light, so parts of the reflected light are extinguished by interference.

    Interference colours produced on TiO2-covered mica
    Interferencecolours visible by TiO2-covered glossyplates
    [Full screen 56 KB]
    (Source: Fonds der Chemischen Industrie, Germany; imageseries "Farbstoffe und Pigmente")

    Fluorescent pigments give extremely brillant inks. They absorb light of the visible or UV-range and emit it again at some other, longer wave-length. Hence, most fluorescent pigments are red or yellow. They are most effectful when illuminated in the dark with ultraviolet light. They may also be employed for security purposes (stamps, banknotes, etc.).

    Interference effects in nature
    Interference effects in nature 1
    Interference effects in nature 4
    Interference effects in nature 3

    (Source: Fonds der Chemischen Industrie, Germany; imageseries "Farbstoffe und Pigmente")


    Lightfastness

    describes, how fast a colourant fades when exposed to light.

    In general, the following rules apply:

    Lightfastness is specified in eight steps, ranging from 8 (excellent) to 1 (very poor). Note: "Good" is only level 5 !

    This scale is a logarithmic one, that is, one step higher means, the ink can tolerate the double amount of sunlight until it starts to fade (level 1: 20 h, level 8: > 3000 h).

    Lightfastness is tested by exposing a print to sunlight itself or to a xenon arc with a UV-filter, which has an emission spectrum similar to sunlight, but is much stronger and so saves time.

    The fading of an ink is due to the action of light, that is, by the energy it carries

    (E=h*v)

    This energy is absorbed to some extent (dependent on the wave-length) and, if strong enough, is able to decompose the colourant molecule.

    So it is easily understandable that red and yellow organic pigments are more susceptible to fading than green and blue ones, as they do not only possess more stable chemical bonds, but also absorb light of higher energy.

    Of course, lightfastness is not only a property of the colourant, but also depends on the light source.

    Hence, a certain print will remain unchanged for a longer time in Lapland in winter but in Sicily in summer. It will also fade faster at higher altitudes, as there the light is more intense, especially in the UV-region of the spectrum, which is richest in energy.

    Hence, the sample print should always be exposed to light together with a series of standards.



    Vehicles (or varnishes)

    serve to bring the pigment in a printable form and fix it onto the printing stock.

    The use of the terms "vehicle","varnish" and "binder" is somewhat confusing. Some say, a varnish is made up of a binder and a vehicle; others say, vehicle and varnish are the same.

    The components of the vehicle mainly depend on the drying process and hence on the printing process envisaged.


    Binders

    Binders, i.e. that part of the vehicle which remains on the printing stock, may be

    In cases 2 and 3, the formation of the binder may, in part, be performed in the manufacture of the varnish.

    Resin

    Resin is the comprehensive expression for a broad selection of naturally occuring, semi-synthetic or synthetic materials which are employed as (e.g) binders for printing inks.

    Chemically, they are polymers. They are solids or rather viscous liquids.Most of them are of a non-crystalline structure.

    Natural resins include

    Semi-synthetic resins include

    Synthetic resins are virtually innumerable.

    Important examples include


    Solvents

    are used to dissolve the binders of printing inks. They are also used, by the manufacturer and by the printer, to adjust the viscosity of the ink to the printer's requirements.

    The solvents used in printing inks include mineral oil, other aliphatic and aromatic hydrocarbons, ketones, esters, and alcohols. These substances do not take part in any chemical reaction.

    Important examples include

    Next to their chemical nature and, hence, their solubilizing properties, the boiling point is the most crucial property when choosing suitable solvents.

    For printing inks, the following boiling point ranges are common:

    flexo and gravure 80140 °C
    screen printing 130210 °C
    heat-set web-offset 240280 °C
    cold-set web-offset, letterpress 280320 °C

    In a sense, the "drying oils" in chemically drying inks and the reactive fluids in UV-curing systems also are solvents; they serve every purpose of a non-reactive solvent, although in the end they are not removed, but become part of the binder.

    Evaporating solvents are a major source of environmental pollution by printing plants. They should be recovered or, at least, destroyed.

    The following combinations of binders and solvents are common for printing inks:

    Process Binder Solvent
    newsprint hydrocarbon resins,
    e.g. asphalt
    mineral oil
    offset drying oils
    alkyd resins
    modified rosin
    hydrocarbon resins
    mineral oil
    metal decorating alkyd resins
    melamine resins
    mineral oil
    publication gravure modified rosin
    hydrocarbon resins
    toluene
    flexo,
    packaging gravure
    nitrated cellulose
    polyvinyl acetate
    polyamide resins
    alcohols
    esters
    ketones
    mineral spirits
    water based flexo,
    packaging gravure
    maleic resins
    acrylic resins
    shellac
    water
    alcohols


    Additives

    make up only a few percent of the total ink, but may have tremendous effects on the performance of the ink. They are the best part of the manufacturers' know how.

    Additives include


    Optical brighteners

    are used to make inks more brillant.

    They are fluorescent chemicals similar to fluorescent pigments; they absorb ultraviolet light and usually transform the energy contained into blue or bluish-green light.

    As the eye (or the brain) regards bluish white as particularly white, such inks are perceived as extremely brillant.


    Driers

    usually are cobalt or manganese compounds. These heavy metals act as catalysts to accelerate oxdative drying.


    Anti-skinning agents

    are employed to prevent the ink from drying on the printing machine.

    For this purpose, among others, phenols are empolyed. These chemicals are more readily oxidized than the drying oils contained in the ink and consume the oxygen. So the drying of the ink does not start until the anti-skinning agent is exhausted - even if the printer would like them to do so now.


    Thixotropy promoters

    diagram of agitation

    Finely dispersed silicic acid hydrated castor oil
    silicic acid
    CH2-O-COR
    |
    CH2-O-COR
    |
    CH2-O-COR
    
    R=CH3-(CH2)5-CHOH-(CH2)10-
    



    Substance Example
    finely dispersed silicid acid Aerosil
    modified sheet mica Bentone
    hydrated castor oil Thicatrol ST



    Aerosil 200: medium particle diameter: 12 nm
      specific surface: 200 m²/g
      preparation: burning of volatile silicon compounds


    Adhesion promoters

    Titanium chelates, among others, from chemical bonds between binder and printing stock. Both must contain hydroxyl (-OH) groups.

    Example:

    P-OH + Ti(acac)2 (OR)2 ---> P-Ti(acac)2 OR + ROH

    P-OTi(acac)2 OR + P-OH ---> P-O-Ti(acac)2 O-P + ROH

    Example: P-OH is a pretreated polyethylene foil
      R-OH is a modified cellulose binder


    Waxes

    may be natural or synthetic in nature. They are employed to improve mar-resistance, slip and water repellancy of the print.

    The term "wax" is used for any material suitable for these purposes, whatever its chemical nature (polyethylene waxes, other hydrocarbon waxes, teflon waxes, beeswax, carnauba wax,etc.)
    Waxes are usually employed as pastes.


    Plasticizers

    are used to make the dried film of printing ink more flexible.

    Their mode of action has been explained in the general polymer section. The plasticizers used in printing inks usually are esters of medium sized alcohols with phthalic acid (dioctyl phthalate, DOP, being the most important of all), citric acid, stearic acid, etc.


    Surfactants

    are employed to reduce surface tension and thus to minimze wetting problems, especially with difficult printing stocks.


    Defoaming agents

    may become necessary if surfactants are contained in the ink. Usually silicones are employed.


    Biocides

    prevent microbial degradation of printing inks.

    Conditions in a printing ink are not so hostile to microbial life as one might expect. This is particularly true for water-based inks.


    Deodorants

    do not prevent, but mask unpleasant odours.

    Some manufactureres even add peppermint, lavender, or vanillin aroma to their inks.


    Micro-encapsulated perfumes

    may be liberated by rubbing the print with one's nail. They are usually contained in coatings. So a print of roses smells of roses, etc.
    These inks, however, lose their fragrance fairly quick.




               Copyright © 1997     gomail Feedback     Updated on: 20. January 1998