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Optimal Selection and Use of Pigment Dispersing Agents

SpecialChem / Johan Bieleman – Dec 14, 2011

TAGS:  Dispersing Agents      Wetting Agents      Pigments    

Optimizing Pigment DispersionDispersing agents are additives used in coating formulations to facilitate the dispersion of the solid constituents in the liquid phase, during manufacture, storage and application of the coating material.

Complete dispersion of pigment and extender in the liquid medium is essential for optimization properties, such as:

  • Processability
  • Shelf-life stability
  • Color development
  • Opacity, and
  • Gloss

Moreover, the quality of the dry paint film or ink is very much related to the uniformity of the distribution of the pigment. Inferior mechanical film properties and low film transparency are often related to insufficient pigment dispersion
.

In order to optimize the stage of dispersion, well controlled conditions for the dispersion process and dispersion stabilization need to be maintained. Dispersing agents play a key role in accomplishing this condition.


The Pigment Dispersion Process


Dispersion is described as a three stage process:

  1. Wetting: particle surfaces must be wetted and adsorbed air replaced with liquid medium
  2. Disruption: breaking up clumps of flocculated, agglomerated and aggregated particles
  3. Stabilization: avoiding reflocculation.

Wetting is defined as being "the contact between a liquid and a solid surface, when the two are brought together". Wetting and dispersing agents are required to enable complete wetting of the pigment particles: to replace the adsorbed air and water onto pigment particle by the liquid medium in the mill-base. As a consequence of complete wetting a more Newtonian flow behavior at lower viscosity of the mill-base is obtained (reduction of the "apparent pigment volume fraction"), meaning optimization of the conditions for the efficiency of the milling process.

Optimal pigment wetting is essential for proper dispersion conditions
Optimal Pigment Wetting is Essential for Proper Dispersion Conditions



Role of Wetting/Dispersing Agent for Effective Wetting


During the mechanical disruption process, fresh pigment/ liquid interfaces are being formed; spontaneous wetting of the newly formed surfaces is essential for maintaining optimal rheological conditions. The use of effective pigment wetting and dispersion agents enables to increase the speed at which the liquid phase wets the pigment surface. This is valid to aqueous as well as non-aqueous mill-bases.

A main contribution of dispersing agents is avoiding re-flocculation of the freshly dispersed particles. In the absence of repulsive forces, the disrupted and suspended pigment particles will flocculate in response to attractive forces (London-van der Waals forces) between approaching particles. Explained by Brownian motion and contributing to a lesser extend gravitational forces, particles may collide and form flocculates: the smaller the particle size, the stronger the Brownian movement of the particle and the higher the probability to encounter a second particle. Flocculation due to Brownian movement is most pronounced for small particles sized organic pigments. However flocculation is prevented by introducing repulsive forces onto the pigment particles. A main tool for accomplishing this condition is through adsorbed dispersing agent molecules onto the pigment surface.

Pigment dispersions are stabilized only if the repulsive forces are stronger than the attractive forces. Repulsive forces between two dispersed particles may arise from following mechanisms:

  1. Electrostatic repulsion
  2. Steric repulsion

Electrostatic repulsion involves the presence of a repulsive potential created by interaction of diffuse electrical charge double layers at the particle and liquid interface. Coulombic repulsion of equally charged particles keeps the particles apart. This stabilization mechanism is most effective in systems of high polarity (notably aqueous systems), enabling full dissociation of ionic compounds, brought onto the pigment surface.

Steric stabilization is explained by adsorbtion of monomeric or -more efficiently- polymeric species onto the pigment surface. The adsorbed layer provides a steric barrier preventing approaching particles from attraction and collision. Obviously, the effectiveness of steric stabilization is related to the structure and thickness of the adsorbed layer.

Stabilization mechanisms for pigment dispersions
Stabilization mechanisms for pigment dispersions


Overlap of the extended parts of the adsorbed molecules between two approaching pigment particles result in the formation of a repulsive force. Solvent molecules penetrate into the zone of the interpenetrating polymer, as a result of osmotic pressure, thus re-separating the two particles.

Although steric stabilization being the dominant mechanism for stabilization in apolar systems, it applies also to aqueous systems, where it is typically being applied in conjunction with electrostatic stabilization.

In order to be effective the stabilizing species (pigment dispersant) must provide strong anchoring groups for interaction to the pigment surface. Furthermore, the non-adsorbed moiety of the molecule is preferably solvated and highly compatible with the liquid phase, as otherwise the stabilizing layer collapses, which means, low layer thickness and poor stabilization. This is an important criterion for the selection of dispersing agents that should provide steric stabilization! The solvent or resin molecules need to be able to migrate easily into the penetrating layers. Therefore, the effectiveness of the dispersant is much related to the compatibility of the dispersant with the solvent and binder molecules.


Pigment Dispersing Agents - Main Types and Functions


Main contribution of pigment dispersing agents is pigment wetting and stabilization against flocculation. However, the necessity to contribute to wetting depends on the pigment liquid phase characteristics. Easy wetted pigments, like TiO2 in water, do not require additional wetting support, so emphasize on the contribution of the dispersant is very much on stabilization effect. The main group of dispersants as used in white waterborne dispersion paints is sodium-polycarboxylate. Indeed, this dispersant provides excellent electrostatic stabilization, but provides only limited wetting activity. Dispersants as offered for organic pigments in water demonstrate strong wetting support, as well as stabilization activity. A wide range of products is offered, however, having in common of offering surfactant (reduction interfacial tension pigment and liquid phase) as well as strong stabilization properties.

Main dispersing agents are polymeric. Common polymers are polyacrylates, polyester, polyether or polyurethane-based.

Functional groups, such as amine, amides, sulphonate, phosphate, for firm anchoring onto pigment surfaces are present in the polymer structure3. Amine-functional anchoring groups are efficient in apolar systems and interact with the pigment surface through hydrogen bonding, dipole or London-van der Waals interactions. Acid or amid functionalities are typically preferred as anchoring moiety for various aqueous applications.

Schematic Presentation Interaction Polymeric Dispersing Agent and onto Pigment
Schematic presentation interaction polymeric dispersing agent and onto pigment


High molecular mass dispersants (Mw approx 5000-30 000) are most widely used in industrial paints. And as compared to low molecular mass dispersants; they typically provide:

  • Superior performances
  • Workability
  • Best coloristic properties
  • Gloss
  • Film transparency
  • Film integrity
  • Low risk of being extracted from the dried film, and
  • Minimal side effects


High Versus Low Molecualr Mass-based Dispersants


High molecular mass based additives tend to be more system specific as compared to low molecular mass dispersants and a careful selection and evaluation procedure is required.

Oligomers of medium high molecular mass (Mw around 1000-2000) typically show widest range of compatibility and superior (fast) pigment wetting properties in comparison to high molecular mass products, whereas still outperforming low molecular molecular mass products in dispersion stability and film consistency properties.

With few exceptions, monomolecular surfactant based additives are less commonly used as dispersing agent. This is because, this group typically is inferior in contributing to dispersion stability and has high risk on effecting film properties such as:

  • Water sensibility
  • Hardness, and
  • Substrate adhesion

However, occasionally low Mw-based surfactants are being applied in conjunction with polymeric dispersants to optimize pigment wetting.


Ionic and Non-ionic Dispersants


For use in waterborne coatings, anionic charged and nonionic dispersing agents are to be considered. Excellent wetting and dispersion performance in mill-bases for dispersion paints has been collected using a combination of sodium- or ammonium-polycarboxylate and polymeric nonionic surfactant additive. A main nonionic additive is alkyl phenol ethoxylate (APE) and more precisely nonyl phenol ethoxylate, NP 10 ( ethylene glycol chain of 10 units). Due to toxicity concerns, NP 10 is being replaced now with an APE free nonionic, possessing same HLB-value. HLB stands for hydrophilic lipophilic balance and is used as indicative value for comparing nonionic surfactants from different hydrophobic nature. The HLB value can be calculated from the percentage hydrophilic components in a molecule, divided by 5.

Effect of the dispersing agent on color development PBk in a stoving enamel
Effect of the Dispersing Agent on Color Development PBk in a Stoving Enamel.
High molecular Mass Dispersant (L), Right Low Molecular Mass Dispersant (R)


Related to the high degree of ionic dissociation in water, applying combinations of anionic and cationic dispersants in aqueous systems should be avoided; reaction between the anionic and cationic products may result in insolubility and changed surface activity. Cationic, amine functional dispersants are successfully used in solvent-borne systems, for instance to support the wetting and dispersing process. Due to the low degree of dissociation the effect of the electronic charge is less evident in a-polar systems.


Selection of Pigment Dispersing Agents for Coating Formulations


The variety in product characteristics and chemistry of dispersing agents as offered to the industry is huge. A careful selection and evaluation in the specific formulation is required. For the selection process of dispersing agent for a particular coating formulation, following characteristics are to be considered for a first dispersant selection approach:

  1. Solvent composition: polarity of the liquid phase
  2. Pigment characteristics: ease of wetting
  3. Binder system; compatibility


#1. Solvent


Solvent characteristics largely contribute to ease of pigment wetting as well as required stabilization mechanism.

Polar solvents, such as water, show high surface tension and therefore poor wetting properties for low surface tension substrates, as typical for organic pigments. Thus, additional wetting support is required, through the use of specific wetting agents or pigment dispersing agents, showing strong wetting activity. Most inorganic pigments show much higher surface tension and are easily wetted by water; therefore having no need for additional wetting agent. The selected pigment dispersing agent best provides electrostatic stabilization (refer to paragraph 2) for pigment dispersions in polar media is.

Although apolar solvents, such as mineral spirits and xylene, show low surface tension characteristics, the interfacial tension with pigment surfaces (air/ water surface) is high and requires further wetting adjustment. The preferred solution is selecting dispersing agents, contributing to pigment wetting and dispersion stability, by steric stabilization.

#2. Pigment


Typically untreated inorganic pigments show high surface tension and are easily wetted by water; therefore having no need for additional wetting agent. The selected pigment dispersing agent best provides electrostatic stabilization2 for pigment dispersions in polar media is. However, for use in apolar systems, dispersing agents with wetting and stabilizing properties are required.

Organic pigments typically demonstrate hydrophobic character, low surface tension and require dispersants providing optimal wetting and stability properties. This is true for aqueous as well as apolar solvent based systems.


#3. Binder


The non-adsorbing moieties of the dispersing agent should demonstrate excellent compatibility with the binder system. This is essential for optimal stability in the liquid pait (see paragraph 2) as well as best film performance. Any incompatibility may result in pigment flocculation in the wet paint, even during film formation.

Typically medium Mw dispersants are preferred for use in architectural coatings: best cost/ performance, wide range of compatibility.

High Mw dispersants are preferred for high quality industrial paints and OEM: best optical performances, high degree of being system specific.

In summary, following guideline is suggested for the selection process of dispersing agent:

  • Use polycarboxylate for best stability of hydrophilic pigments and extenders in aqueous medium
  • Use surface active polymeric dispersant for best wetting and stability of hydrophobic pigments in aqueous medium
  • Use surface active polymeric dispersant (medium Mw) for best wetting and stability of all pigments in apolar, architectural coatings
  • Use application system specific- surface active polymeric dispersant (high Mw) for outperforming wetting and stability properties of pigments in apolar, industrial and OEM coatings
Rheology Modifiers Solutions



Commercially Available Dispersing Agents for Pigments






References

  1. Bieleman, JH (ed.), Additives for Coatings. Wiley-VCH, Weinheim (2000). ISBN 3 527 29785 5
  2. Pirrung, F, Chimia 2002, 56, no 5, p.170
  3. Company brochure Patcham: www.patchamltd.com

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