The Chemistry Of Ink

Introduction

Ink can be understood as a sol, gel, solid, liquid or semi-liquid, which provides colour to the surface on which it is used. The use of ink can be traced back to ancient times when people started writing for the purpose of communication. People then used substrates like paper, rock, wood, etc., on which they would write, draw, or calligraph using different instruments. These writing instruments mainly used ink from various organic extracts of plants, animals and minerals. The ingredients in ink varied according to the place where it was produced. For example, people from China and Japan made black-coloured ink from the soot of animal bone and tendon, whereas people from ancient Rome made ink from salts like ferrous sulphate and tannin extract from gallnut plants. The ink component would be a soluble dye or an insoluble pigment mixed with a carrier solvent. The Chinese were the first to develop organic ink with the help of pulverised forms of black stone and lake colours. Chemically, ink is primarily composed of three substances, namely, a colouring agent (pigment or dye), a vehicle and additives. With the advancement in technology, the characteristics of ink have developed for different purposes. Various modifiers are added to ink to increase properties like adherence to the writing surface, the ability to resist mechanical abrasions and lower the chances of premature drying of the ink.

Qualitative Analysis Of Ballpoint Pen Inks Based On TLC & Their Forensic Significance

Basic Components Of Ink

Ink is mainly composed of vehicle, colourant, and various additives, depending on the use of ink.

1. Vehicle

The vehicle is the most important component of ink, which can be organic or aqueous in nature. Earlier, water was used as the only solvent for ink; later, chemists employed other solvents for this purpose. The main function of the vehicle is to act as a solvent for the dye to dissolve or pigment to remain suspended. The vehicle acts as a carrier for ink and facilitates its transfer onto the paper surface, helping the ink remain bound to the surface. The vehicle can be plant-based or solvent-based, both of which have different drying mechanisms. Plant-based vehicles like linseed or oil from woods dry up by oxidation or penetration, but solvent-based vehicles like kerosine dry up by the process of evaporation.

Colouring agent

The substance that provides colour to the ink and imparts visibility to the writing is known as the colouring agent. The colouring agents block the light transmission to the undersurface, making only the writing visible. The colourants can be either a pigment, which is a fine particle insoluble in solvents, or a soluble dye. Sometimes, even lacquers are used as colourants. Although during ancient times, only organic colourants were used, nowadays, ink manufacturers mostly use synthetic colourants.  The earliest developed ink was only blue and black, obtained from indigo and lamp black, respectively. The origin of the modern blue-black colour is a salt of iron and tannin extract. Other colourants in ink include titanium oxide for white colour, an alloy of copper and zinc for golden, eosin for red, molybdenum for orange, etc. The colourants should have characteristics like opacity, stability and inability to fade. The dyes in ink are better than the pigments owing to their strength in producing colour density per unit mass on the paper. Pigmented inks are, however, better suited for printing purposes because they have a tendency to retain more on the paper surface, therefore leading to lesser use of ink.

3. Additives

These substances are added to ink to improve specific features. The quantity in which an additive has to be added to the ink depends on the purpose of the ink and the surface on which it has to be used. The additives provide desirable properties to ink like gloss, shine, durability, smoothness, resistance to damage and many more. Glycerides are added to ink to improve the smoothness of paper and impart easy gliding action. Triethanolamine is added to maintain the pH of the ink so that the pen does not get damaged. Substances like silicates act as fillers for ink. Stabilising polymers are employed to prevent the clotting of ink, which might occur if the pigment or dye forms a clump. Earlier resins from plants and the albumin of eggs served as stabilisers. However, nowadays, synthetic polymers are used to stabilise ink. Additives, in other words, provide those characteristics to ink that the colourant and vehicles are otherwise unable to impart. Substances like oil and waxes can act as lubricants and increase their durability. The presence of driers in ink makes the drying of ink efficient. Some additives also prevent the growth of fungal activities and defoaming and act as good surfactants.

Bi-luminescent Security Ink

Different Types of Ink And Their Composition

Printer ink

The characteristics of printer ink are visibility to the human eye owing to which vibrant colours are used, adherence to the printing surface, and ability to get dried under given conditions; they should also have resistant properties. The components of printer ink are colourants, solvents, binders and excipients. The most desired feature of printing ink is that it should form a very thin layer on the surface where it is applied. The colouring agents like peacock blue are responsible for blue ink, and varnish-containing linseed oil, along with carbon black, gives the black ink, etc. Dichloropropane is mostly used as a solvent in printer ink because it does not have impurities or odour and is colourless and cheap. The binders, particularly resins like ketones and acrylics in printer ink, provide good adherence to the surface along with properties like gloss and resistance to wear and tear. The excipients in paint can act as good wetting agents and stabilisers. Along with these additives, humectants, defoamers, and pH modifiers are also present.

Edible ink

Eating ink refers to the ink used to decorate food materials and consumables to attract customers. This type of ink is mostly made up of natural substances and, therefore, is safe for consumption. The colourant used can be natural (red beat and indigo) or synthetic (azo dye and acid dye). Many times, colour enhancers or mordants like titanium dioxide and alum are used, which makes the colour brighter. Binders or resins like gelatin, gum Arabic, shellac, or even chocolate can be used to act as emulsifiers or stabilizing agents. The solvents used in edible ink can be edible oils or water-based. The ink developed for this purpose is safe, tested, and verified by the food safety organisation before being applied to various products. Some examples of food that use such types of ink are confectionary products, such as sugar-coated chocolates and bakery items. The purpose of using decorative articles for food includes decoration, manufacturing details, food wrapping sheets, etc.

Tattoo ink

It is primarily composed of different types of pigments and additives like preservatives and dispersants. The components of tattoo inks can be of two types: organic and inorganic. The organic compounds include azo-coloured compounds for colourful inks and polycyclic aromatic hydrocarbons (PAH) for black-coloured inks. Inorganic compounds like iron and zinc can be used in the pigments. To prepare black tattoo ink, it has been found that iron oxides, logwood and carbon are mixed with water or alcohol-based vehicles. However, it has been found that tattoo ink manufacturers often adulterate the inks as they do not have to pass through any quality testing. These adulterations have proven harmful to human beings, causing various allergies and skin diseases.

Pen ink

The ballpoint pen has glycol solvents and synthetic dyes. Additives like resins, anticorrosive agents and surfactants are added accordingly to improve the characteristics of ink. Fatty acids are used at the ballpoint of this pen, which acts as a lubricant and resists friction. The blue-black colour fountain pen consists of synthetic blue dye in an aqueous solution, which turns black colour due to oxidation once it reaches the paper surface. These inks are very stable in terms of abrasions and solubility in water. The ink of the gel pen contains pigments that are insoluble instead of organic dyes. These inks are gel in nature and are insoluble in both aqueous and organic solvents. The marker pens use ink, which is comprised of vehicle, resin, glyceride, pyrrolidone, and dyes. The permanent marker ink contains substances like toluene and xylene.

Fluorescent ink

These inks become visible when they are excited by a certain wavelength of light. Nowadays, fluorescent inks are used to gain visibility under ultraviolet (UV) light. The colourant used in these might be a fluorescent dye, which does not show fluorescence when in undissolved conditions. Once these are dissolved in a thermoplastic carrier resin, fluorescence can be seen. One of the most important fluorescent dyes is Rhodamine B. The fluorescent ink mainly consists of a fluorescent pigment, an organic solvent that has propylene glycol monomethyl ether and a ketone resin. Such type of ink can be used for secret message transmission, road markers, etc.

Erasable ink

this type of ink is sensitive to temperature. When the ink is removed mechanically with the help of an erasure, an increase in temperature takes place due to the erasing action. This, in turn, causes the bond breakage between the colourant and the colour developer on the surface of the writing, resulting in the formation of a clear surface. Erasable ink contains a colourant, a resin that is film-forming like polyethene oxide and a release agent like polyethene glycol. Surfactants may or may not be present on these inks.

Importance of Ink in Forensic Science

The different chemical compositions of ink are particularly useful in the forensic investigation of questioned documents. The chemical composition of ink can be used to compare the original and obliterated documents. Different ink manufacturing companies use their own recipe for ink production. Therefore, by identifying the composition of ink and finding out the components present in it, the origin of ink can be traced. Ageing of ink is a useful parameter with the help of which many crimes have been solved to date. This process involves the use of dating techniques to find out the age of ink. Often, criminals alter certain parts of the writing within the same document. Many times, it has been observed that criminals only check the colour before making any alteration in a document and don’t pay heed to the composition of ink. Different ink of the same colour might have different constituents. Thus, based on the composition, the different types of ink of the same colour get detected and are useful for the detection of forgery. Ink examination is a very useful technique in solving cases of forgery.

Conclusion

Ink is a mixture of different chemical components, each of which has its own function. The ink was first prepared a long time back when people felt the need to communicate in the form of writing. Since then, the components and procedures used for manufacturing ink have developed significantly. Ink has evolved greatly in terms of composition based on its use and demand in society. This article summarises the basic components of all ink and specifically discusses certain types of ink along with their chemistry. It is found that ink chemistry is particularly relevant in solving criminal cases. In addition to forensic science, ink chemistry also finds its application in anti-counterfeiting measures, food technology, and printing technologies.  

References

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