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| Inkjet technology is a relatively recent development. The ink / printhead pair are the limiting factors. If we could make inks that did not clog, or printheads that were easily fixed, then inkjet would become the universal way of printing. Ink may be an old idea dating back as much as 5,000 years, but it only became easy to use in the second half of the 20th Century with "Quink" inks and then ball-point pens. Ink still presents some problems. In an inkjet printer anything that gives strong, lasting colour on the page has a nasty tendency to dry and clog in the printhead. | On This Page.
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Ink Importance.
Delivering material through microscopic channels is an idea that is inspiring a lot of research. The idea of using a ceramic or silicon chip to form small dots of material extends well beyond ordinary printing - it has all sorts of possibilities and applications in medicine and manufacturing as well.
The relatively direct use of ink in an inkjet looks like a huge advantage for all sorts of printing tasks. In principle a thin bar of printhead fed with ink could produce anything from a school essay to wallpaper with each individual dot placed as it has been told by a computer. There is a lot of potential, for instance:
Books could be printed on demand - something happening on a limited scale now, but with low cost inkjet machines this could be done on bookshop or print-works scale.
Newspapers and magazines could be written for the reader - addressing their specific interest areas - and obviously with much better targeted advertising.
Personalisation can be applied to any surface - not just wallpaper but clothing and car bodies. Personalised number plates would seem a bit naf if the whole paint job could be unique.
Commercial printing is moving in this direction. Scitex have high speed colour printers. Heidelberg can add an inkjet personalisation unit onto their presses. In 2006 the majority of printing is still done with conventional presses but the market is switching around to digital methods. Big commercial printers can be based on inkjet or laser methods. Inkjets have the advantage that they use straight forward direct methods and "digital inks" are a thinner version of conventional printers ink. Laser printer processes need several stages.
Low cost home computer printers are opening up all sorts of new possibilities. Home printers are commonly aimed at photography; business printers at letterheaded correspondence, personalised mailings from databases and catalogue production.
Perhaps one of the problems with inkjets is that people expect one kind of cartridge to do too many things. A cartridge good for colour photography is not ideal for printing out low cost text. Neither will be very good for labelling CDs or pickle jars. There is a growing market for specialised cartridges but perhaps printer makers aren't helping the market as much as they might. If you need a special cartridge where do you keep it when not in use? The foil pack it came in has been opened - perhaps it would be better if manufacturers shipped them in tins?. A part used cartridge should probably be kept cool - would putting it in the fridge help? Inkjets would be less troublesome if people knew what was going on.
A limit at present is that highly coloured fluids that might go through an inkjet head all have a tendency to clog. At the moment human ingenuity stumbles just trying to get an inkjet fax machine cartridge to last a week without streaking.
Ink History.  |  |
Even without all the future possibilities printing is a large industry - estimated world-wide business is something like $400 billion. At the moment the computer share is might be about 10% - HP has half the market and its printer division did about $24Bn in 2004. They have a stated ambition to double business in the next ten years.
That any aspect of printing could grow so rapidly may seem surprising. Ink production is a well developed practical art. The basis of printing since the invention of the process by the Chinese and its importation into Europe by Gutenberg and Caxton has been the application of ink to paper.
People have been trying to colour things for a long time - since the age of cave art. Nature uses colour as a signal and so do people. In fact by taking blood, berries and flowers people can make all sorts of colours. The disadvantage is that these things usually turn brown and flake off in a day or so which is rather disappointing to the artist.
Nature rather favours browns and greens. People look at the sky and and are intrigued by blue. Getting a strong blue for years meant grinding up lapis lazuli from Persia to get azure (pigment) or growing woad, rotting it and mixing the result with urine (dye). There have been thousands of years of pigment, dye, paint and ink development. In China an ink made from soot, vegetable oil and gelatine was used to colour carvings as long as 5,000 years ago.
Almost any coloured liquid can be used as ink, but most single components aren't very good at the job. Either there isn't enough colour, or it isn't stable and fades in light, or it reacts with the material used as paper and fades. A common problem is that colours tend to wash out and smear- even just handling a document can damage it. Ink needs to have the right surface tension and electrostatic properties so that it spreads exactly as required, but not too far, then binds to the fibres.
An ink recipe used for many centuries uses ferrous sulphate made by usting iron in sulphuric acid together with tannin from oak gall, wine and a thickener such as gelatine. This ink is blue-black when fresh, fading to brown with time. Pens and paper were as much a limitation as ink - writing with a split goose-feather on sheepskin parchment may work but it isn't exactly effective.
Application methods are very important in developing an ink. European artists adopted brushes rather than pens. The brush can hold a much thicker material that holds its colour better - paint is pigment in varnish. Brushes were good for Chinese calligraphy as well - relatively complex characters holding a lot of meaning. Latin alphabets work better with small characters and suited pen and ink.
For printing with presses new inks were needed. Soot, turpentine and walnut oil made a greasy ink paste that worked quite well.
Inkjets really only became practical in the 1970s. Drop on demand inkjet printing relies on computer intelligence continually generating the pattern to be used. Inkjet is a new application method, so it may take a while to develop the best possible inks.
Old formulations for inks, dyes and paints were largely serendipitous discoveries by alchemists. German chemists in the later half of the 19th century began to be successful with long-lasting colour dyes.
Ink and dye are clearly related - things which will dye a cloth might be used as an ink. However the process to dye clothing can use strong alkali, acid or high temperature. The cloth can be treated with alum mordant. Any treatment is acceptable as long as the colour persists but the cloth is left wearable. Ink usually has to applied without extreme force, heat or chemical reaction - although laser and thermal printers do use a certain amount of heat, pressure and chemistry .
Parker "Quink" - quick drying ink - is often considered a breakthrough. To get the quick drying properties Parker used a solution including isopropyl alcohol and water. The rapid evaporation of the isopropyl part reduced or eliminated the need for blotting paper. Quink did, however, need a new design of pen because the isopropyl damaged the plastics of most other pens. Parker made more money from the ink than the pen - a relationship inkjet manufacturers have carried on.
Modern Inks.   |  |
Inks for modern digital printers come in several forms. Just as inks, dyes and paints are inter-related so are printer materials.
| Laser printers use solid powders. The carrier is a plastic such as styrene, acrylate or polyester resin mixed with colourant and materials to give the right electrostatic properties. |
| Thermal printers use waxes or resins mixed with the colourant. The printhead locally melts material to transfer it to paper. |
| Impact printers use a greasy ink held in a ribbon. Impact printer ink is often most like traditional printers formulations. |
Liquid ink is a dye or pigment mixed in some kind of liquid carrier. The liquid inks used in most inkjet printers owe more to modern industrial chemistry than to alchemy - but there is still a bit of both. The primary aim is to colour the paper, doing this neatly, reliably and repeatedly using a printhead with hundreds of tiny chambers firing thousands of times per second is difficult.
An ink can have several components matching it to the mechanism that will be applying it. They include:
 | Colourant - is what the rest of the ink is there to deliver. It can be a dye or a pigment. |
 | Carrier - or "base" is the liquid that transports the colourant to its point of use. Usually the ink is mainly carrier and the carrier is mainly water. |
 | Other Things - Surfactants, Buffer, Biocide, Chelating Agent, Defoamer, Solublizer. |
Modern inks are quite intriguing but unfortunately the actual formulations are often the basis of some industrial fortune - so they aren't made public. Hewlett Packard's formulation is just as secret as that for Coca Cola - and possibly even more widely imitated.
Amateur sleuths can take a drop of ink and some damp filter paper and attempt to figure out the formulation using liquid chromatography - dye based inks develop all sorts of coloured rings. The major inkjet makers maintain labs to reverse engineer one another's products - although they get annoyed when people crack their own secrets.
| Colourant. |  |
Light is electromagnetic radiation visible to the human eye. Colours are wavelengths or frequencies of electromagnetic energy often characterised in particle form as "photons". Photons hitting the atomic structure of an object can be:
| absorbed -raising its temperature so the energy is re-emitted at a much lower frequency. |
| reflected - bouncing off (which involves creating a localised electromagnetic field). |
| absorbed and immediately re-emitted at a different frequency - as with fluorescence. |
These effects happen at the level of the "wavelength" - about 380 nanometres for violet and 740 nm for red light. The nature of the material influences what happens, which is what makes sight such a useful sense. Sight is a way of remote-sensing the nature of things.
Film, TV, painting and printing are ways of misleading the eye about the nature of things.
| Film and TV add light sources together - usually red, green and blue. |
| Painting and printing subtract and absorb light from what will be reflected. For instance, magenta ink (which looks and is red-blue) absorbs the green and reflects the other two colours. |
Colourants are often classed as dyes or pigments. The distinction between dye and pigment is rarely clear to the end user, but often made in discussion about materials.
| Dye - a coloured liquid. The colouration comes from molecules of some substance(s) dissolved in the liquid. Because the dye is a pure liquid it shouldn't clog even the smallest channels of a printhead - which is a very helpful property. A simple test for a dye is that it passes through filter paper unchanged. If there is a deposit left on the paper that isn't just staining with the dye and won't wash through then there are other components - gels perhaps or pigments. |  |
| Dye carrier diffuses into paper, dries and leaves the dissolved material behind. | |  |  |  |  |
A dye will soak into the paper which should help give a zone of colour that is not easily removed. Because the dye sinks in a lot of dye may be used in printing. If the absorbency of the material is not what was expected the image may feather or bleed at the edges.
Dye on paper that sinks into the fibre is lost to view and makes less contribution to the returned light. The liquid component will first be absorbed, then it will evaporate leaving small dry crystals of material behind. The cover and distribution of the crystals gives the strength of colour seen. Because the material is deposited so finely it should look even.
The colour of the dye is generated at a molecular scale. Exposure to ultraviolet light tends to break the molecules and hence the colour down. If the dye reacts with an ingredient in the paper it may also change colour. Ozone, O3 which is a small but natural component of air will also react with dye components - probably turning them to grey oxides.
| Paper designed to take up dye well needs a certain amount of filler to trap the dye material close to the surface where it will be seen. Dye that sinks below the fibres is lost to sight. The small crystals and infilling in cracks in fibres should mean that a dye adheres to the paper fairly well, but it may not colour it all that intensely and may fade quite quickly. |  |
The latest generation of inkjet dye based inks intended for photography claim very long stable lives - HP says 108 years.
The liquid nature of a dye is obviously a good thing in an inkjet since it shouldn't block the nozzles.
| Pigments - are particles with the required colour. In an ink its a suspension of particles in a liquid. |
The particles are not dissolved. Small particles will stay in suspension easily due to Brownian motion but very small and regular particles may not be easy to make.
Pigment particles are often a fine powder - the product of grinding a mineral for instance.
Emulsification can also be used - chemical compositions precipitates out of solution.
Carbon black is the most common pigment - used in inkjet ink, laser toner and many other black things like vehicles tyres. The production process is basically smoky combustion of oil.
To prevent particles being inclined to separate out they need to be educed to a scale where they are just several microns across. This suits inkjet heads as well, the nozzles in a head may be 10 microns across. A possibility with fine powders is that they may be uneven and block a nozzle or may flocculate.
Most of a pigment settles on top of the paper, which means comparatively little need be used to give colour. A further advantage is that the pigment material is usually chemically inert and less vulnerable to ultraviolet. Something in the liquid may be needed to bind the pigment to the paper.
| Pigment carrier diffuses into paper. Particles of material are left on the surface. |  |  |  |  |
Pigments are usually divided into two groups - inorganic and organic
Inorganic pigments are minerals. Iron gives red and green colours. Cobalt and titanium salts are used for other colours. Chrome salts are popular - they give a wide colour range. Lead became unpopular because white lead in paint and tetraethyl lead in petrol created political storms. Some lead salts are insoluble and biologically inert so they could be used but are banned.
Organic pigments are derived from chemical reactions, normally using an oil feedstock.
A potential disadvantage with pigments is that some change the texture of the paper - the collection of crystals on the surface can be visible giving the paper an odd matt finish.
| Paper designed for a pigment might ideally have a bit more texture to act as traps and filters for the particles. | |
At the moment most inkjet inks use a pigment black (carbon black) but dye based colours. Epson is using pigment based colours in the Ultrachrome ink for the Stylus C66 and Stylus Photo R800 and claiming a stable life of 80 years for them.
| Colours. |  |
People have colour vision so they ideally want something more flexible than a black colourant. The human eye has three types of colour sensor and can be "fooled" into seeing colours by changing the mix of wavelength that each receives. There are two ways to do this:
 | Additive colour builds the colour wanted using Red, Green and Blue lights on a dark background. This gives the RGB method used by screens. |
 | Subtractive colour removes the unwanted colours from white light reflected from a page. Subtractive colour uses Cyan, Magenta and Yellow colourants so this is called the CMY process. As it happens people do use blacK for a lot of purposes, so most printers also have a straight, simple black ink. Most colour printers use a CMYK process. |
Nature does provide a reasonable source of black pigment - carbon black which is essentially a specially produced soot - so ink still has some of its traditional ingredients.
Cyan, Magenta and Yellow are more difficult. The chemicals and processes used often appear to be proprietary secrets.
| Liquid Carrier. |
Inks use a liquid carrier that evaporates or is absorbed and a colour that remains behind. The flow characteristics of the liquid - its viscosity, surface tension and capillary action are important in transporting the colour. Once the colour has arrived the liquid should disappear.
Inkjet inks seem to use quite a large volume of carrier, which acts as their propellant.
Water is the obvious liquid. Water will dissolve the widest range of materials, is odourless, is absorbed quite readily by most materials and evaporates quite slowly. Disadvantages are that water reacts with metallic components in any inking mechanism. Water evaporates so slowly it can break up the fibrous mat of the paper if a lot of ink is used. Most inks are 60 to 90% water.
Alcohols will dissolve waxes and other organic materials and will carry pigments. The advantage of alcohols is that there are several types and most evaporate quite quickly. Isopropyl alcohol was a component in "Quink" and remains popular in inks. Isopropyl does react with some plastics.
Oils will also dissolve waxes and organic materials and carry pigments. Oils generally don't react with metallic components - but do with some plastics, like organic pigments.
Water and alcohol in solution have some of the properties of each in proportion to the mixture. Water and alcohol also have different surface tensions so mixture can vary the energy input needed to eject ink. Most inkjet inks are mainly water based with some isopropyl alcohol, ethylene glycol and other volatile organic solvents such as ketones.
The carrier is an essential part of all inks - they need the right characteristics of viscosity and surface tension. Thermal inkjets additionally need the ink to vaporise and condense in the right manner.
| Other Ink Components. |
Water, alcohol and a pigment colourant may not mix, or if they do so the pigment may not easily flow, may flocculate and turn into a gel, or settle out. Ink has various minor components to make it workable.
Surfactants are usually the main additional component
Buffer, Biocide, Chelating Agent, Defoamer, Solublizer.
New Scientist 20th Feb 1999 has an article outlining the problem of manufacturing reliable polymers from the dozen or so groups of monomers and a new "Vladimir Initiator" which improves the process. The discovery has immediate application in photolithography where the erratic length of ordinary photoresist polymers has tended to spoil sharp features. Another application is in block copolymers used in inkjet printers. Inks contain polymers which are soluble at one end, insoluble at the other. The insoluble end of the polymer attaches to a carbon black, the soluble portion forms a shell around the pigment so it can be suspended in water. The new Vladimir initiator would produce better polymers more cheaply.
The article doesn't seem directly accessible, but use this link to New Scientist and search for "vladimir initiator" 
| Inkjet Inks. |
When Frank Cloutier and his team at HP were developing their first inkjet printer it seems that they weren't initially all that interested in the ink itself. The initial focus was on the novel delivery mechanism. They wanted the ink in a cartridge so users could keep their hands clean, but initially thought ordinary fountain-pen ink would do.
In fact the main benefit of the inkjet printer is the cheap basic mechanism and the way it can handle colour easily.
Problems are outlined in the following list:
| Thermal inkjets need to vapourise the ink violently. The temperature reached seems to be of the order 300 centigrade. This creates sufficient pressure to overcome viscosity and surface tension. Ink carrier is acting as a propellant. |
| Piezoelectic inkjets pump ink mechanically so ink composition isn't a major issue - the two office printer types are the Epson series which uses aqueous ink and the Xerox Phaser which uses molten wax. Air locks are a problem. |
| Viscosity and surface tension have to be right to flow through the manifolds, chambers and - under pressure - the nozzles. Ink should not flow through the pads and pressure relief holes |
| Ordinary inks in a thermal head produce "kogation" - a build-up of scale on the heater-resistors in thermal cartridges. |
| The chemical composition of the ink attacks adhesives such as the silicone sealant initially used to glue the cartridges together. |
| Ink attacks metal and plastic components. It also proved possible for inks to dry out through the plastic body. |
| Ink particle size becomes very important as nozzle size is reduced. At 1200 dpi the dots on the page are 20 microns across and the nozzles of the printer are probably less than 10 microns. A fragment of dust in the ink would completely block a nozzle. |
| Air bubbles will break the capillary action that maintains ink flow in the head. Bubble might migrate into the channels if a cartridge is inverted. Dissolved gasses or air in the ink is highly likely to separate out with the pressure waves near the printhead nozzles. |
| Colour stability on the page is important and until recently inkjet inks have not delivered it. |
So whilst "Quink" might work in an inkjet it usually isn't ideal.
Inkjets are in some ways a very favourable environment for ink handling. The material is normally delivered in a carefully designed tank or cartridge that remains sealed until it is about to be used. The tank isn't usually very large, partly because it has to be mounted on the carriage and would make it heavy, but also because printheads don't last terribly long. Manufacturers may also have an eye to profits. In principle if the ink needs ultrasound or heat to activate it that can be supplied - though so far these potentialities haven't been much used.
The ink will be delivered in droplets measured in picolitres from tubes less than 100th of a millimetre across (at 2500 dpi). The ink mustn't settle out or dry in the tubes. Most importantly, if it is used in a thermal printhead, the liquid component needs to vapourise easily when a heater turns on then condense rapidly when it turns off.
Besides a strong colour there are two premier properties required in inkjet formulations: mobility in the head and immobility on the paper. Whilst the ink is in the printhead it should be very mobile and not adhere to or react with any of the working parts. In a thermal printhead the ink must also contain a component that will evaporate suddenly when the element heater is tuned on so the ink ejects. When the ink hits the paper it should ideally turn to stone, any flow within the paper beyond that needed to make it adhere is undesirable. Xerox "Phaser" solid inks match this need ather well.
Printer manufacturers generally rely on absorption into the paper and evaporation to dry the ink. Absorption is the main initial route. If the paper is not absorbent, the carrier evaporates slowly or the dye is a solution when the printer design called for a pigment then the ink will smudge in the paper. Material in suspension will settle out more quickly and not disperse through the paper. If ink diffusing into paper is a real problem then heating the paper will help – the HP1200C uses a halogen lamp to drive wetness out of the paper just after printing.
Target properties include:
- Precisely reproducible colour and colour density
- Correct spreading qualities (viscosity, surface tension and electrostatic)
- Mixable with other colours
- Chemical inertness especially to other inks and to common paper ingredients
- Insolubility once used,
- Colour stability in ultraviolet,
Older generations of inkjet ink weren't stable, particularly if they were used unframed as pictures in a sunlit place. Recent inks like Epson's UltraChrome and HPs 343/344 ink are extremely stable when used with the ight paper.
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© Graham Huskinson 2010
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