Printing inks composition


Printing inks are complicated mixtures of chemical compounds. The composition varies by printing process, by whether printing is sheetfed or web, and by substrate.

The raw materials for ink production are pigments, binders, solvents and additives.
Pigments- colour the ink and make it opaque
Resins- bind the ink together into a film and bind it to the surface
Solvents- make the ink flow so that it can be transferred to the printing surface
Additives- alter the physical properties of the ink to suit different situations
 
Pigments: Pigments are considered to be the chief constituent of ink and contribute about 50 per cent of its cost. A pigment is essentially any particulate solid - coloured, black, white or fluorescent - that alters the appearance of an object by the selective absorption and/or scattering of light. It occurs as a colloidal suspension in ink and retains a crystal or particulate structure throughout the colouring or printing process. Colour Index System number is generally used to identify the organic pigments in modern inks. It reflects the colour shade or hue, and structural and chronological details (order of synthesis) of the pigment. For example the well-known blue pigment copper phthalocyanine blue is PB 15.  As the particle size reduces, the colour intensity (strength) of a pigment increases and the opacity peaks around a particle size of 0.3µm. The molecular structures of four important pigments used in ink are shown in Fig.1.
      
Pigments colour the ink and provide gloss, abrasiveness and resistance to light, heat, solvents, etc. Special pigments such as extenders and opacifiers are also used. Extenders are transparent pigments that make the colours of other pigments appear less intense, and opacifiers are white pigments, which make the paint opaque so that the surface below the paint cannot be seen.

 
Resins: Resins are primarily binders that bind the other ingredients of ink together so that it forms a film; they also bind the ink to paper. They also contribute gloss, resistance to heat, chemicals and water. More than one resin is typically used in an ink formulation.

Solvents: These are used to keep the ink in liquid form from the period when it is applied to the printing plate or cylinder until when it has been transferred to the surface to be printed. At this point the solvent separates from the ink to allow the image to dry and bind to the surface. Some printing processes such as gravure and flexographic require a solvent that evaporates rapidly.

 
Additives:Additives are used to alter the final properties of the formulation. 

Source “Polymer Science - Coatings and Adhesives and Technology of Printing Inks”

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