Dispersion and its role in ink quality
Anuj Johri, AGM - R&D (Ink) Uflex Limited (Chemicals Division) C-5-8, Sector 57, Noida UP 201 301, India.
PERFORMANCE of a packaging ink is characterized by its high quality of printing, trouble free runs and no rejections on press at printer's end. This high quality performance is further characterized by high quality of ink and high quality of ink is mainly dependent on flawless manufacturing thru perfect pigment dispersion, optimal pigment particle size and long term stabilization of dispersed particles in the ink formulation. All these 3 parameters are the results of an excellent dispersion process. Role of dispersion becomes critical and important for an ink. Let us understand the process and its role in ink quality.
The dispersion of a pigment in the ink to produce stable suspension can be understood through wetting and grinding process.
In pigment wetting, all of the air and moisture is displaced from the surface and between the particles of the pigment agglomerates and is replaced by the binder solution. The solid/gaseous interface is transformed into a solid/liquid interface. The complete wetting out of the primary sized pigments particle helps to enhance the technical performance of an ink that depends very much on interaction between the pigment particles and the binder system. Dispersing additives, which adsorb on the pigment surface, also facilitate liquid/solid interfacial interactions and help to replace the air/solid interface by a liquid medium/solid interface. The efficiency of the wetting depends primarily on the comparative surface tension properties of the pigment and the binder solution, as well as the viscosity of the resultant mix. The wetting step of dispersing processes can be intensified by the use of wetting agents and/or binders with lower viscosity and surface tension. On the other hand, a resting of pigment/binder premixes prior to their dissolving or grinding helps to accomplish the wetting stage and always eases and accelerates dispersing processes.
In grinding, the pigment agglomerates are broken up and dispersed through mechanical energy (impact and shear forces) in the mill. At this stage, the cohesive forces inside the agglomerates are overcome. Energy is added to the system and therefore smaller particles (with a larger interface to the resin solution) are formed. This result in loosened inter-particle contact durability which eases the destruction of pigment clusters under the action of shear stresses, applied in dissolvers and mills. As the pigment powder is broken down to individual particles by mechanical shear, higher surface areas become exposed to the binder and larger amounts of binder is required to wet out newly formed surfaces. Once dispersed, the primary particles have a tendency to re-agglomerate. This process is called flocculation. Now the flocculates are very similar to the agglomerates having filled interstitial spaces between the pigments with resin solution rather than air. The grinding process can be regarded as a de-flocculation process.
The aim of stabilization is to keep the pigment particles separated and to control the degree of pigment particle size. Therefore, immediately after grinding pigment suspensions must be stabilized by the addition of let down mixtures as per formulation. Stabilization is achieved through absorption of stabilizing molecules on the pigment surface, so that repulsive forces prevent other particles from approaching close enough to cause agglomeration.
The main objective of the dispersion process is to produce a stable colloidal system in which the pigment particle size distribution ranges between 50 and 500 nm. Colour strength, gloss, and opacity are achieved maximum in this range. The particle size and particle size distribution (PSD) fundamentally affect the ink performance.
A balanced formulation and good dispersion process can give a stable and superb performance of ink at press and a poor dispersion can lead to huge number of quality issues followed by losses to all stakeholders. If dispersion of an ink is not upto the mark, it can be responsible for number of quality issues like less strength, poor printability, low ink transfer, poor in-can stability, pigment floating, ink separation & settling, less gloss, less transparency, plate or cylinder wear, unstable flows, poor hiding power, shade variation during printing...etc.
As ink manufacturer, one should focus on dispersion process, to give consistent & best quality of his / her products.