Functions of dispersing additives in ink

Functions of Dispersing Additives in Ink

By :Adesh KatariyaManager- R&D, Tirupati Inks Ltdplast.adesh@gmail.com

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Pigment

A pigment is a coloring particle which is insoluble in the application media.

Particle size and crystal structure of pigments determine the application properties like gloss, tinting strength etc

Primary particles are single crystallites or sub crystallites which are strongly connected by their surface areas. They can not be destroyed during normal grinding processes.

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Pigment Properties

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Pigments in Comparison

Property Organic Pigment

Inorganic Pigment

Particle size (µm) Small Large

Surface area (m2/g) Large Small

Polarity Non-polar Polar

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Objective of Pigment Dispersion

To separate the pigment agglomerates which are formed (by hydrophilic aggregation) during the drying processes in pigment manufacture.

As particle size is reduced, the surface area so created increases, leading to an improvement in optical properties, such as tinctorial strength, gloss, brightness, opacity or transparency.

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Pigment Dispersion

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Improved Dispersion and Dispersing Additive

The improved dispersion means a smaller average particle size with a narrower particle size distribution.

Small particles are generally more prone to re-agglomeration or flocculation.

With dispersing additives reducing inter-particle attraction, dispersions are significantly more stable to flocculation and agglomeration than those produced by conventional means.

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De-flocculation -Flocculation

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Wetting and Dispersing Process

High brilliance and color strength are characterized by a perfect pigment dispersion, optimal pigment particle size, and long-term

stabilization of the dispersed particle in the formulation.

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Pigment wetting:

All of the air and moisture is displaced from the surface and between the particles of the pigment aggregates and agglomerates (clusters) and is replaced by the resin solution.

The solid/gaseous interface ( pigment/air) is transformed into a solid/liquid interface (pigment/resin solution).

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Proper Wetting

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Wetting and Surface tension

The efficiency of the wetting depends primarily on the comparative surface tension properties of the pigment and the vehicle, as well as the viscosity of the resultant mix. The adsorption mechanism depends on the chemical nature of the pigment and the types of dispersing agents used.

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Grinding stage:

Through mechanical energy (impact and shear forces), the pigment agglomerates are broken up and disrupted into smaller units and dispersed (uniformly distributed).Ideally, a fully deflocculated state will arise, in which all pigment particle agglomerates have been broken up into their primary particles.

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Stabilization of pigment suspension :

The pigment dispersion is stabilized by the adsorption of binder species or molecules at the pigment surface

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Pigment Stabilization

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Pigment Stabilization Mechanism

System Stabilization Mechanism

Solvent-based Steric stabilization

Water Based Electrostatic / Steric stabilization

Emulsion Electrostatic stabilization

Polymerics (UV) Steric stabilization

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WHY Dispersant Required ?

Dispersing Additives have two fundamental roles in surface coatings;o they produce improved pigment dispersion o they reduce inter-particulate attraction within that

dispersion.

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Dispersing Agent and Rheology

The greater steric repulsion generated by the addition of polymeric dispersants  moves the minimum in the Potential Energy Curve, and thus

reduces the overall viscosity.

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Dispersants familiesIn term of chemical structure one can divide dispersing agents into the two following classes:

1. Polymeric dispersants -Sterical stabilization 2. Surfactants-Electrostatic stabilization

The main differences of those two types of dispersants being the molecular weight, the stabilization mechanism and the resulting let down stability.

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Electrostatic stabilization

A charge is generated on the pigment surface, and a more diffuse cloud of oppositely charged ions develops around it.

As two particles approach each other the charge effectively provides a barrier to closer particle interactions.

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Sterical stabilization

Steric Stabilization occurs by the adsorption of a layer of resin or polymer chains on the surface of the pigment.

Effective in media of low dielectric constant.

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Mechanism of Steric Stabilization

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Mechanism of Steric Stabilization

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Mechanism of Steric Stabilization

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Mechanism of Steric Stabilization

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Mechanism of Steric Stabilization

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Polymeric dispersants

Polymeric dispersants stabilize ink systems via a steric stabilization mechanism previously described. They have a two-component structure which combines the following two very different requirements:o It must be capable of being strongly adsorbed into the

particle surface and thereby possess specific anchoring groups .

o The molecule must contain polymeric chains that give steric stabilization in the required solvent or resin solution system.

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Structure of Wetting and Dispersing Additives

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Pigment affinic groups

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Dispersing Additives :Advantages

Higher gloss Lower haze Higher color strength Improved hiding power Better transparency Low viscosity / Newtonian Flow No flooding & floating

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Co-polymer/functional polymer configurations

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Dispersing Additives : How to stabilize the pigments

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Dispersing Additives : How to stabilize the pigments

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Adsorption/Anchoring of Dispersing agent on Pigment Surface

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Anchoring Mechanisms

As the nature of the surface of pigments differ, according to their chemical type, many different chemical groups can be found as anchor groups for polymeric dispersants. This wide range of anchoring possibility enables polymeric dispersants to disperse inorganic pigments as well as pigments with polar surfaces. The actual anchoring can then take place through a variety of mechanisms;. 1. Through Ionic or Acidic/Basic Groups.2. Through Hydrogen-Bonding Groups3. Through Polarizing Groups4. Through Solvent-Insoluble Polymer Blocks

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Anchoring Through Ionic or Acidic/Basic Groups

When a pigment particle has a relatively reactive surface (eg: inorganic pigments) it is possible to form an ion-pair bond between a charged site on the particle surface and an oppositely charged atom or functional group on the dispersant.

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Anchoring Through Hydrogen-Bonding Groups

A strong interaction may be developed between the pigment particle and a polymeric dispersant containing many hydrogen-bond donors and acceptors in its anchor chain

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Anchoring Through Polarizing Groups

An interaction can also take place between polarized or polarizable groups on an organic pigment particle surface, and similarly polarized or polarizable groups on the anchoring function of the polymeric dispersant. Again, these interactions will often be relatively weak, but strong interaction may be developed with a polymeric dispersant possessing an anchor chain composed of several of these groups.

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Anchoring Through Solvent-Insoluble Polymer Blocks

It is possible to anchor a polymeric dispersant onto a pigment particle surface simply via van der Waals interactions and without recourse to ionic, hydrogen-bonding, or polarizing effects.

The polymeric block within the dispersant must simply be insoluble in the medium

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TiO2: Surface Treatments

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TiO2: Surface Treatments

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Groups on Carbon black Surfaces

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Interaction W/D Additives with other Pigments

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Wetting and Dispersing Additives: Class

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Low molecular weight dispersants

Found in anionic, cationic, electro-neutral or non-ionic Stage

The molecular weight of these products is low, usually between 300 and 2,000 g/mol.

Example : mono functional oleo-alkylene oxide block copolymers.

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Oligomeric dispersants Based upon fatty acid

chemistry, having polar heads based on tertiary amines.

Typically, these molecules are oligo-functional, meaning that more than two amino anchoring groups are present.

The molecular weight ranges from 1,000 to 3,000 g/mol.

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High molecular wt dispersants High molecular weight dispersing agents can be linear or branched

molecules with molecular weights between 5,000 and 20,000 g/mol.

Excellent stabilization, due to a high number of anchoring groups along the polymer backbone which bind to numerous sites on the pigment surface.

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Polyurethanes

Best suited dispersants for viscosity depression ,higher pigment loads, more economical mill base formulations and lower VOCs.PU dispersants usually have a branched backbone with a three dimensional network structure. Different anchoring groups are introduced at various points on this network structure.

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Star-shaped dispersing polymers

Having core-shell morphology. During the dispersing step the

polymer segments in the core adsorb on the pigment surface.

The very high density of pigment affinic groups results in a very strong adhesion on the pigment surface.

The polymer chains in the shell orient into the solvent and stabilize the pigment very effectively.

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Block copolymers Dispersant Based on Controlled free Radical Polymerization (CFRP)

technology , enables the precise design of polymer structures.

With the CFRP technology, well-defined block copolymers can be prepared that are designed to optimally fit pigment and resin chemistry.

Typically, a longer stabilizer block is synthesized first, which has to be compatible with the relevant ink systems. The anchoring block contains functional groups which interact strongly with the pigment surface to allow for efficient and stable adsorption. For demanding applications like organic pigments, the anchoring block typically contains aminic groups, which can optionally be modified further.

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CFRP Technology based Dispersant's structure

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Polyacrylates

Polyacrylic dispersing agents have linear structures with a C-C backbone that bears various functional side groups and short side chains. The main difference to polyurethane-based dispersants is their higher molecular weight.

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Co-polymer dispersants :

Anionic dispersing agents based on poly-carboxylic co-polymers .

Their narrow molecular weight distributions provide

optimum dispersion efficiency, translating into maximum performance at

the lowest possible formulation cost.

Anionic dispersants are especially effective in stabilizing inorganic pigments and fillers

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Comparison in Poly Acrylic and Polycorboxylic co-polymer base D.A.

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Surfactants

Surfactant molecules are able to modify the properties and, in particular, they lower the interfacial tension between the pigment and the resin solution.This surface activity arises because the surfactants' structure consists of two groups of contrasting solubility or polarity.In aqueous systems, the polar group is known as a hydrophilic group and the non-polar group as hydrophobic or lipophilic. In non-aqueous systems, the polar group is known as the oleophobic group and the non-polar group as oleophilic.

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SurfactantsSurfactants are classified according to their chemical structure and, more specifically, their polar group: anionic, cationic, electroneutral and non-ionic .

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Surfactants : Points

Surfactant’s effectiveness is determined by:o The absorption of the polar group onto the pigment

surface. The anchoring groups can be amino, carboxylic, sulfonic, phosphoric acids or their salts.

o The behavior of the nonpolar chain in the medium surrounding the particle. This part of the molecule (aliphatic or aliphatic-aromatic segments) must be highly compatible with the binder system.

The stabilization mechanism is electrostatic. Due to the Brownian movement the pigment particles

frequently encounter each other in the liquid medium thus having a strong tendency to re-flocculate on the let down stage

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Types of Surfactants

o Fatty acid derivativeso Phosphate esterso Sodium polyacrylates / polyacrylic acid o Acetylene diolso Soya lecithin

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Fatty Acid Derivatives

Nonionic fatty acid derivatives such as the alkyl phenol ethoxylates (APEs) and fatty alcohol ethoxylates (FAEs)

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Phosphate Esters

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Polyacrylic acid/ Sodium polyacrylate

Polyacrylic acid (PAC) and salts of polyacrylates are anionic surfactants.

Polyacrylic Acid structure and conversion to sodium polyacrylate

Acetylene Diols

To reduce the side effects of standard surfactant types of dispersing agent such as foaming, oligomeric acetylenic ethoxylate glycols have been developed with multi-functional properties and especially defoaming property .

Ethoxylated acetylene diols

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