Selection of Extenders for Water-Borne Coatings


Extenders – the ignored ingredient in Coating Formulations:


Most Coating formulators spend maximum time concentrating on Polymer & Pigment Discussion, leaving Extenders and Solvents as of least importance.


A simple question that arises here is
Why Extenders are being overlooked?
Is it because:


-              They do not contribute to opacity?

-              They are economical and carry the least cost impact?

-              They are available in plentiful?


Whatsoever may be the reason, but still the fact.


Why add Extenders?


To achieve the desired PVC, the addition of true pigment above the level required for Colour, Hiding, UV absorption works out to be expensive. Hence incorporation of Extenders becomes essential.

Extenders also improve


following properties Aesthetic Properties

Gloss Control




Dirt Pick-up & Cleanability

Texture and Smoothness


Wet Paint and Processing Properties


Flow & Leveling

Ease of Dispersion

Sedimentation / Settling


Mechanical & Physical Properties


Impact Resistance

Film Reinforcement


Cracking / Checking Resistance


Performance and Resistance Properties

Abrasion, Brushing and Scrub Resistance


 Corrosion Resistance

Chemical Resistance

Thermal Properties (Insulation & Conductivity)

Electrical Properties (Insulation & Conductivity)


Wet Paint and Processing Properties


Flow & Leveling

Ease of Dispersion

Sedimentation / Settling



Cost Control


Extender Selection Parameters:


On the basis of Oil Absorption, Particle Size & Particle Distribution By knowing Oil Absorption Values one can modify Viscosity. This property can be further manipulated with the help of Particle Size and Particle Distribution.


Depending on Shape Modular Extenders:

Nodular Extenders possesses low Oil Absorption Values Example Calcium Carbonate, Barytes, and Amorphous Silica – Contributes little to Mechanical or Aesthetic Values of Film.


Flat Platy Extenders:

Mica, Some Talc & Clays – highly absorbent because of more surface area:


01.          Enhances Mechanical Properties by lateral reinforcement – reduces transmission – water, oxygen & ionic solution (like Venetian blind)


02.          Flat-platy shape & parallel orientation to substrate  decrease in porosity of applied the film reduces bubbling when recoated


Acicular (Rod Shape / Needle Like):

Rod shape – Wollastonite (Cal. Silicate) – some Talcs - low the surface area as compared to Flat / Platy shape – Low Oil Absorption – No increase in Viscosity


Used for Mechanical Strength in the same way as chopped fiberglass and helps to prevent cracking




Most exclusively Asbestos –

01.          Imparts Mechanical Strength by Reinforcement

02.          By Cross linking hold Coating together – Block UV radiation penetration - used in light-sensitive (asphalt) finishes – roof coatings prevent Cracking / Alligatoring

03.          Imparts heavy thick consistency – high build – high Bridging – Nonsagging system generally having texture.


 Joint Compound, Textured, and Special Effect Paints, Roof Coatings

Depending on Hardness / Softness of Extenders Hardness:

Hardest Extenders are silicas – especially Quartz type Crystalline Silica

01.          Improves Hardness & Wear Resistance

02.          Optimises Scrub Resistance in Interior Latex Paint

03.          Abrasion Resistance in Floor Finishes & Maintenance Coatings

04.          Coarse Grade – protrude thru surface – gives “toothing” – improving Recoatability

05.          Where slippage is likely – extra coarse silica broadcasted on wet film and on drying unbounded material swept out.


Main Use - Traffic Paint / Road Marking Paint


Soft Extenders:

Talcs & China Clays (Calcined clays are slightly harder than water washed hydrated Aluminium Silicate)

 Imparts good sanding properties – used in sanding sealers with good barrier properties

Extenders  &  Properties  they  impart


Positive Effects

Negative Effects


Whiting Calcium Carbonate

Ppt Ca CO3 (Acicular Shape)


Calcite (Trigonal Crystals)


-          Consistency

-          Finer grades give good pigment distribution

-          Improve Wash Resistance

-          Minimize Cracking


-          Tends to Settle

-          Natural Grade imparts dirtiness to Shade and affects Gloss adversely

-          Specific type Cal. Carbonate should be checked for water solubility or the liberation of Cations that adversely affect material such as Casein or Emulsifying Agent.



Hydrated Aluminium Silicate


-          Imparts Flow

-          Helps Pigment Distribution

-          Improves Hiding Power above CPVC

-          Shelf Stability of Paint


-          Natural Hydrated Grade Water Sensitive

-          Imparts Dirtiness to Shade

-          Responsible for Cracking of Paint Film


Quartz Silica - Amorphous


-          Easy Brushability

-          Sag Resistance

-          Adhesion

-          Flow & Leveling

-          Low Dirt Pick-up

-          Better Stain Resistance

-          Crystalline Silica helps inter-coat adhesion


-              Toxicological Hazard





Tech. Director – Harmony Additives Private Limited

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