Printing is defined as the reproduction of images to a substrate. To create an image there must be differentiation between the image area (ink on) and non-image area (ink off). Main printing processes accomplish this either by raising the image (flexo and letterpress), differentiating chemically (lithography), recessing the image (gravure), stencil, (Screen print) electronically, (toners).
Printing inks are broadly classified into two main categories; paste and liquid. It can be formulated using chemistries based on oil solvent, water or energy curable.
Raw materials used in inks can be classified into three classifications
1 Vehicles- carry pigment to substrate during the printing process. Vehicles are sometimes referred to as varnishes. Usually fluid in nature, they consist of resins binders plasticizer and solvents. Can be free flow gelled or viscous
2 Colorants- pigments or dyes that provide the color – Pigments are insoluble and need to be dispersed whereas dyes are dissolved in vehicle. Pigments are more common
3 Additives –materials or compounds that control or modify specific properties. Often added in small amounts they cover a wide range of chemistries-properties controlled or enhanced by additives include, rubrestance, drying, film formation, viscosity etc. Wax is an additive.
While it is theoretically possible to make ink without additives most modern inks contain multiple additives. Some additives are specific to certain ink types others such as waxes are use in a wide variety of inks. Additives may be added in the vehicle during processing while others are added at ink mixing stage
Role of Waxes
Almost all inks with exception of inks that will be coated or laminated contain waxes fo some type. Waxes provide physical attributes e.g. rub/scuff resistance, water/ solvent or grease resistance etc. Wax can influence coefficient of friction introducing slip.
In the ink industry waxes can vary with natural and synesthetic waxes being used. Waxes are supplied as micronized stir in powders, votated oil/varnish compounds, oil dispersion for sheet fed and heat set inks, water dispersion water based flexo/gravure. Typically waxes are used less than 5% w/w however certain emulsions /dispersion additives can contain much higher percentage of wax.
For stir in particle size is an important property, 4-6 microns is typical for sheet fed and high speed heat set. Flexo/gravure can tolerate much higher particle size (6-20 microns).
Polyethylene. MW 500-6000 high or low density. (Acculin is high density MW 500-3000) used in all major ink types supplied as dry (micronized), oil
Paraffin wax: good moisture barrier but need to be used cautiously for overprinted or coating after drying by forming a low energy surface.
Microcrystalline. Used extensively in heat set inks. Often combined with PTFE for good slip and rub
FT Wax Flexo and gravure inks sometimes limited because lower fractions can become soluble if the ink becomes too hot in manufacture.