Pigment Settling and Stability

Settle Down: Factors that Influence Pigment Settling and Stability

Posted on by chemical dynamics

Introduction -The first steps in the pigment dispersion process are wetting and separation of the pigment. However, if the pigment dispersion is not properly stabilized, flocculation1 (fig. 1, 2) will result. Flocculation is a result of pigment particles being attracted to each other to form loose aggregates that can be redispersed under mild shear.

When pigment particles are strongly attracted to one other a cementing or agglomeration of the particles can occur. Agglomerates (chemically bound pigment aggregates that are encapsulated by resin or wetting agent) cannot be readily redispersed. Flocculation can be reversed by the application of low shear to the paint. Flocculation can have an adverse effect on color development, gloss and hiding.

Fig. 3: Relationship between Primary Pigment Particles, Flocculation and Agglomeration2 SOURCE: MDPI.com

The two main mechanisms to obtain pigment stabilization are steric and charge. In charge repulsion, particle surfaces with like charges repel each other (more applicable to waterborne systems, Fig. 4) whereas steric stabilization is a more common mechanism in solvent born paints (Fig. 5). Properly stabilized pigment dispersions prevent flocculation and agglomeration.

Fig. 4: Charge Repulsion Stabilization Mechanism3 SOURCE: Dow Coatings.com
Fig. 5: Steric Stabilization Mechanism4 SOURCE: Inkline.gr

Pigment dispersion in aqueous media uses the same principles as inorganic solvent media: that is, proper wetting, pigment dispersion and stabilization. However, the surface tension of water and high polarity makes it more problematic in wetting low polarity pigments. In many cases, water interacts aggressively with the surface of the pigment, destabilizing the dispersant on the pigment surface. Ensure that the pigment dispersion is uniform and stabilized (elimination of pigment flocculation of one pigment with the exclusion of other pigments). Thirdly, the use of suitable wetting agents/surfactants help to ameliorate differences in polarity and surface tension between pigments that contribute to pigment destabilization.

Inorganic pigments such as iron oxides, titanium dioxide, calcium carbonate, zinc oxide, and silicon dioxide, calcium carbonate and barium sulfate and many other filler pigments have a very polar surface. However, water alone normally does not adequately wet the pigment surface. Accordingly, they require a surfactant to wet and stabilize the dispersion.

Also, many pigment manufacturers supply surface-treated pigments to help pigment stabilization. Many manufacturers modify the surface of organic pigment to increase polarity by adding a layer of inorganic oxide to improve pigment wetting.

No discussion on pigment stabilization is complete without considering the effect of pigment settling with time.  These factors all influence the degree of pigment settling and resistance to hard settling:

  • Quality of the pigment dispersion
  • pigment particle size
  • oil absorption
  • shape
  • distribution
  • pigment density
  • paint viscosity

A more complete discussion of the impact of each of these parameters on pigment hard settling and stability would require several articles to adequately describe.  However, Figure 6 provides a simplified relationship of pigment and paint parameters to pigment settling.

Fig. 6: Relationship of Parameters to Settling

Finally, the use of an appropriate thixotrope helps to build sufficient viscosity and a network structure that discourages pigment hard settling. A suitable thixotrope can improve resistance to hard settling by a few different mechanisms.

  • Improves resistance to hard settling by increasing low shear viscosity
  • Forms an association with the pigment to decrease the effective density of the settled pigment layer.

However, one must be sure that there is acceptable compatibility between the thixotropic and dispersant. Thixotropes commonly used to promote soft settling include clays treated with quaternary ammonium compounds to provide higher organophilicity for solvent born coatings. Attapulgite clays are used in both waterborne and solvent born coatings, as the needle like clay particles associate to increase viscosity that easily breaks down under shear. Other polymeric thickeners can be effective by increasing viscosity and by promoting readily redispersible soft settling, such as:

  • Fine particle silicas
  • castor oil derivatives
  • basic calcium sulfonate
  • colloidal aluminum silicate

To read the rest of the article please click here to head over to UL Prospector.

__________

Ron Lewarchik, Author of article & President of Chemical Dynamics

As a contributing writer, Ron pens articles on topics relevant to formulators in the coatings industry. He also serves as a consultant for the Prospector materials search engine, advising on issues related to optimization and organization materials within the database.

Paint Film Defects

Overcoming Paint Film Defects: Causes and Remedies

Posted on by chemical dynamics

Paint film defects can appear during or immediately after application or become more apparent after the coating is cured. While there is no standard convention for the nomenclature of film defects, this article will separate film defects into the two categories mentioned above.

Crawling, crafters, crazing - a variety of paint defects can occur after application or curing. Learn the causes and solutions here.
Example of crazing.
Copyright: paylessimages / 123RF Stock Photo

Paint film defect causes

The largest number of paint defects is from dirt particles1embedded in the paint. Most other paint defects are the results of:

  • lack of cleanliness
  • surface preparation
  • application error
  • attention to detail

Surface tension

Many coating defects are related to surface tension issues. Surface tension is the elastic tendency of liquids that make them acquire the least surface area possible. This occurs when the forces at the interface of a liquid differ from those within the liquid, attributed to uneven force distribution of molecules at the surface. A common unit of surface tension is dynes/cm2 (force/unit area).

For example, applying a coating with a higher surface tension than the substrate may cause dewetting, crawling, pinholing, holidays and telegraphing.

Likewise, the difference in surface tension at the paint surface can result in cratering or fisheyes.

Table 1: Surface tension of paint Solvents

Solvent Surface Tension Dynes/cm
Water 72.8
Toluene 28.4
Isopropanol 23.0
n-Butanol 24.8
Acetone 25.2
Methyl propyl ketone 26.6
Methyl amyl ketone 26.1
PM acetate 28.5

 

Table 2: Liquid surface tension of Polymers used to reduce surface defects

Polymer mj/m2
Poly(dimethylsiloxane) 22.6
Poly nButyl Acrylate 33.7
Poly nButyl Methacrylate 31.2

 

Highly polar molecules (e.g. water) have a higher surface tension than less polar materials (see Tables 1 and 2). Surface defects can often be reduced or eliminated by using small amounts of additives with low surface tension such as polydimethyl siloxanes (DMS), poly butyl acrylate or poly 2-ethyl hexyl acrylate. These additives tend to migrate to the surface to help flow and leveling.

Table 3: Defects that can occur during or soon after application

           Defect Appearance                Causes          Remedy
Crawling Uneven film thickness, dewetting High surface tension paints applied to a substrate with lower surface tension. For example, paint on steel with oil on the surface
  • Proper surface cleaning of metallic or plastic surfaces
Craters/fish eyes Small round depressions in the surface of the coating Small particles of a low surface tension contaminant (e.g. oil, grease, silicone oil, wax) on the substrate or that embeds in the coating
  • Proper spray booth air filtration and the contaminant elimination.
  • The addition of surface wetting agents such as DMS and/or polyacrylates with a low glass transition (Tg).
Crazing, cracking Small cracks formed in the coating. This can occur on recoat or if coating is applied to solvent sensitive plastics Application of coatings on plastics where the paint contains strong solvent that solvates the underlying coating layer or plastic substrate
  • Use solvent that will not crack or craze the plastic.
  • Test spot resistance of substrate with suitable solvent.
Dirt, contamination Small raised imperfections in the surface of the coating
  • Surface not carefully cleaned.
  • Dirty spray booth and/or booth filters.
  • Pressure in the spray booth too low.
  • Unsuitable work clothes.
  • Inadequate paint filtration
  • Ensure cleanliness of the environment where the coatings are applied
Loss of gloss, blush Areas of low gloss or a white haze Humidity condenses on the wet paint due to the cooling effect of solvent evaporation when the substrate temperature is below the dew point. Causes:

  • Unsuitable reducers
  • Poor air circulation in drying oven
  • Film thickness too high or low
  • Proper humidity control

 
Mottling Uneven appearance of metallic paints
  • Dirty spray gun nozzle
  • Incorrect air pressure
  • Incorrect reducer
  • Faulty spray technique
  • Incorrect spray viscosity
  • Use proper viscosity cup to obtain spray viscosity.
  • Clean and maintain spray guns on a regular basis.
  • During application maintain spray gun parallel to the substrate and maintain correct distance from gun to substrate.
  • Follow Technical Data Sheets instructions.
Poor hiding · Uneven paint coverage
  • Nonuniform substrate surface
  • Uneven or inadequate paint coverage to mask the substrate color
  • Uniform and sufficient paint application to obtain proper hiding.
Runs and sags Drips and sags
  • Paint applied too thick or too wet to a vertical surface and the force of gravity overcomes the forces resisting the downward flow of paint (viscosity).
  • Temperature too low to enable proper solvent evaporation (solvent born paint), or humidity too high (waterborne paint).
  • Adjust low shear viscosity of paint with appropriate thickener.
  • Use proper reducer and viscosity adjustment for environmental conditions.
  • Adjust spray gun and apply thinner wet coats. If a waterborne paint, apply paint in a lower humidity environment.
Skips/holidays Incomplete paint coverage
  • Paint applied too thin
  • Minute areas on the substrate surface of low surface tension, causing inadequate film flow and coverage.
  • Proper paint application and ensure surface cleanliness.
Striping, banding Stripes of uneven paint appearance (e.g. differing color) Uneven paint application
  • Use proper viscosity cup to obtain spray viscosity.
  • Clean and maintain spray guns on a regular basis.
  • During application, maintain spray gun parallel at the correct distance to the substrate and maintain
Telegraphing Highlighting of the surface of the coated substrate through the coating. Such defects as fingerprints, sand scratches and water spots on the substrate become visible on the coating surface Coating with high surface tension applied to a substrate with lower surface tension. e.g. Fingerprints or silicone oil on a substrate surface.
  • Ensure that the substrate is thoroughly clean and absent of low surface tension oils and fingerprints.
Wrinkling, lifting, aligatoring Upon applying an overcoat, the existing paint film shrivels, wrinkles or swells; may also occur during drying. Solvents in the new paint swell the underlying paint finish.
  • Allow sufficient cure times of underlying paint
  • Ensure that the new paint is compatible with the undercoat
  • Proper application of the new paint (not too wet).

 

Table 4: Defects that are more apparent after cure

Defect Appearance Causes Remedy
Air entrapment Similar to solvent popping or bubbles Paint pump sucking air when paint level is low. In two component urethanes, moisture present reacts with isocyanate to cause CO2 generation.
  • Proper attention to paint line conditions.
  • Ensure use of urethane grade solvents and proper spray gun air filtration through desicant.
  • Addition of moisture scavenger in paint.
Barnard Cells Hexagonal pattern in the surface of a cured paint film. Convection pattern from pigment segregation as a result of surface tension differentials Adjust formulation to overcome flooding and differential surface tension at surface
Blisters Bubbles near the surface of a film during oven cure that do not break through the surface. Viscosity of the surface of the film increases to a high level, trapping the volatile solvent at a lower level.
  • Proper oven staging to enable slow release of solvent.
  • In an acid catalyzed system, use an acid salt to slow the cure and enable solvent release.
  • Increase flash time before bake.
  • Use slower evaporating solvent.
  • For spray application, apply additional thinner coats to build film rather than fewer thick coats.
  • For waterborne coatings, use a dehydration bake lower than the boiling point of water, followed by a second bake to cure.
Orange peel Rough surface that resembles the surface profile of an orange Paint applied at high viscosity or under conditions deleterious to proper flow and leveling.
  • Adjust paint to proper viscosity with correct reducer per technical data sheets.
  • Apply at proper fluid delivery rate and atomizing air pressure.
Solvent pop Broken bubbles at the surface of a film that do not flow out during oven cure Viscosity of the surface of the film increases to a high level, trapping the volatile solvent at a lower level. The bubbles break the surface when the solvent volatilizes.
  • Proper oven staging to enable slow release of solvent.
  • In an acid catalyzed system, use an acid salt to slow the cure and enable solvent release.
  • Increase flash time before bake.
  • Use slower evaporating solvent.
  • For spray application, apply additional thinner coats to build film rather than fewer thick coats.
  • For waterborne coatings, use a dehydration bake lower than the boiling point of water followed by a second higher bake to cure.
  • Lastly, the use of lower Tg resins along with lower dry film thickness decrease popping.

 

Search Prospector for formulating remedies to overcome paint film defects:

Defect Remedy material
Crawling and substrate wetting
Craters and fish eyes
  • PDMS
  • polyalkyl acrylates
Runs and sags
Telegraphing
Air entrapment
Solvent pop, blisters For melamine cure systems:

 

To read more, please click here to head over to UL Prospector.

__________

Ron Lewarchik, Author of article & President of Chemical Dynamics

As a contributing writer, Ron pens articles on topics relevant to formulators in the coatings industry. He also serves as a consultant for the Prospector materials search engine, advising on issues related to optimization and organization materials within the database.