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Paper (and Ink) Optical Brighteners
Optical Brighteners including stilbenes and flourescent dyes such as umbelliferone absorb energy in the UV portion of the spectrum and re-emit in the blue. As a result a white surface emits more blue than shines on it, making it appear brighter. The blue hides yellow and brown tones. Optical brighteners are common in detergents and papers.Banknotes don't use optical brighteners so a common test for forged notes is to look for flourescence in UV light.
Inks
Many possible formulations for ink exist.Dyes in solution will not settle out, giving it a long shelf life. Unfortunately it is difficult to achieve a truly intense colour with a water-based solution. The low viscosity of water means dye runs into the paper and gives feathered edges to the print. Water based inks also tend not to be UV -fast and resistant to ozone.
PolyVinyl Pyrrolidone (PVP) has two useful properties - having a polarity itself it binds to polar molecules very well (including those in water and paper) and it gives viscosity. The polarity and viscosity make it eady to form films so it is widely used in coatings. When dry it is a white hygroscopic powder used in pharmaceutical tablets because it is innert to the human metabolism.
Pigments can be organic or metal salts.
Most of the metal salts are mildly toxic and a few markedly so
Salts of
Cadmium (Red Yellow Orange Green)
Chrome (Green Yellow)
Cobalt (blue)
Lead (White Yellow Red)
Copper (Green)
Iron (Prussian Blue, Oxide Red, Venetian Red)
Sulphur (Sodium silicate - Ultramarime Blue, Mercury - Vermillion Orange)
Titanium (White)
Cadmium pigments are often used to produce brilliant colours, particularly as sulfides and sulphoselenides at the red / orange / yellow end of the spectrum where they are popular artists colours. The same materials are used in plastics although they are sometimes banned in toys owing to toxicity.
Prussian Blue obtained by adding iron(III) chloride solution to potassium ferrocyanide, the solution thickens and the colour changes to the characteristic prussian blue. Dissolving in water and addition of heated oxalic acid gives a useful blue ink.
Pigment based inks generally deliver solid colour. Unfortunately suspensions may tend to settle or floculate and will then either lose colour or clog the print-head.
Solvent
Alcohols appear in several forms - Methanol, Ethanol, Isopropanol and tert-Butanol for instance (in order of complexity). Alcohol is commonly used as a solvent for organic material. Alcohols are a hydroxyl group bonded to a hybridized carbon.Some alcohols will mix with water - others will not. The hydroxyl group gives an alcohol molecule polarity and tends to make it soluble in water whilst the carbon chain resists it. Methanol, ethanol and propanol are miscible in water because the hydroxyl group wins. Butanol is only moderately miscible. Pentanol is insoluble because the hydrocarbons dominate.
Alcohols with two hydroxyl groups are glycols. Oxidation creates ketones or carboxylic acids
The simplest dye is a solution or suspension of very small particles in liquid that will settle into the fibrous kaolin filled mat provided by paper. No chemical reaction is expected between the dye and the paper. The vector used for this sort of process tends to be mainly water. Most actual inks are much more complicated – separating ink components out using blotting-paper chromatography can be quite amusing.
Specialised Inks
Complex processes can be devised. The printing can be done onto a sheet which accepts ink, which is then placed face down on cloth. Ironing the sheet causes the ink loaded polymer on the backing material to melt, transferring the pattern onto the fabric.Specialist inkjet cartridges intended to print directly on plastics and fabric are now becoming available – in principle an inkjet cartridge can be made to print directly onto anything which can go through the printer mechanism. Specialist inks are likely to become a significant market.
The basic message of this section is that paper and ink are not simple materials. Fast machine printing requires detailed attention to chemistry and engineering.
Outdoor Inks
User expectations are always a problem. For instance, should ink perform well when paper is damp?. The alcohol-based inks used on offset litho machines often perform very well in soaking conditions as posters and placards demonstrate. Biro and dot-matrix inks are also waxy in their consistency and do work surprisingly well on damp paper. Laser printer output is vinyl powder hearted into the paper and a laser or photocopy sheet will work excellently as a poster. People have become used to this performance. The appearance of an inkjet-printed envelope can be spoiled slightly if the postman delivers it in the rain. Because inkjets can produce solid printed posters people can be surprised when inkjet printing runs and smudges in the rain.Used Tyres to Printer Ink.
New Scientist (26th July 2005) reports on a patent to recover carbon black from vehicle tyres. of sufficient quality for use in inkjet ink Tyres are baked at 800 centigrade which breaks them down into a molten mass. Shaking the material through a magnetic seive separates off the steel and wire components. Semi-pure powder is produced by baking the smaller particles.
Flushing hydrochloric acid through the material should remove all metals and sulphur. A second flush with alkali should remove silica.
The three inventors from Derbyshire reckon the process not only recovers valuable materials but also releases far less carbon dioxide.
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© Graham Huskinson 2010
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