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Rheology Modifiers Selection for Paints & Coatings

Rheology modifiers are vital additives used in almost every coating. But, choosing the right rheology modifier for your formulation can be complex keeping in view the existing VOC regulations.

Are you also facing challenges while selecting right paint rheology modifier? We are here to help you!

Check out here the major types of rheology modifiers available for waterborne and solvent borne coatings along with their key benefits, limitations and applications.

We would like to acknowledge Johan Bieleman for providing technical information needed to develop this guide.

What Are Rheology Modifiers?


TAGS:  Rheology Modifiers    

Rheology Modifiers for PaintAs you know, rheology is crucial for the success of your paint at every step: manufacturing, storage and final application. In both solvent-based and waterborne formulations, rheology modifiers (also called thickeners for paints) help to achieve the desired rheological behavior.

Rheology modifiers help in controlling paint shelf stability, ease of application, open time / wet edge and sagging. They influence leveling, settling and film forming. Depending on rheology modifiers features and required adjustments, they are added to the mill-base or during let-down.


There is a broad choice of additives for paints and coatings that you can use for this purpose. The question is therefore to find the right one!

 » Explore all Commercially Available Rheology Modifiers for Paints!


Relation b/w Rheology and Viscosity of Pigments & Coatings


Rheology modifiers are added during the production process mostly before the dispersing stage - type and amount adjusted to the dispersing equipment - to obtain optimal flow behavior to the mill base. If the pigments are dispersed in a medium which is too thin a turbulent flow behavior occurs which leads to a great loss of the energy supplied, so it is not used in an optimal way for the dispersing process.

During storage, the paint should have a viscosity sufficiently high so that heavy pigment particles do not sedimentate. In a similar way by the viscosity adjustment a lot of application characteristic properties are determined, for example:

  • The flow and leveling
  • Sag resistance and brush ability, as well as
  • Film thickness and opacity

The rheology behavior of a system under various conditions is described by a viscosity profile, showing the viscosity as a function of the applied shear conditions.

Newtonian flow defines a system where viscosity is at indicated pressure and temperature constant regardless of applied shear rate and time. Therefore a single viscosity measurement will give a true value for the viscosity; the rate of shear is directly proportional to the shearing force. Examples for Newton liquids are water and pure solvents.

Pseudo-plasticity is a form of structural viscosity, illustrated by a decrease in viscosity as shear rate is increased. Most paints and varnishes show some degree of pseudo plastic (shear thinning) flow behavior!

The figure below shows the rheology profile for various systems:

Rheology Profile
Rheology Profile of Various Systems


Thixotropy is a time-dependent flow behavior; the viscosity is reduced at a constant shear rate (or constant shear stress) with ongoing time; after ending the stress the viscosity increases again.

Rheology and Viscosity Measurement


The selection of most appropriate measuring method for paint rheology is much linked to the purpose:
A: An indicative method of the in-can appearance and on-spot application viscosity
B: QC purposes
C: A sophisticated method for precise rheology-measurement

Regarding A, it is customary to use the running test from a spatula. The performance is expressed as "thin/ thick", "short/ long" and the subjective judgment is very much based on experience. Application viscosity of diluted, sprayed paints is determined using flow or efflux cups, such as the DIN, Ford or ASTM cup. Flow cups measure the time to flow through a fixed orifice and can be used only for near-Newtonian liquids.

Rotational viscometers, such as the Brookfield, are widely used for QC purposes because of being easy to handle and allowing quick measurement. This is a relative measuring system, so not allowing absolute rheology measurements. Various disk- and pin-like spindles can be used, depending on the expected viscosity range. The cone and plate viscometer offers additionally the advantage of requiring only low amount of test sample material and being very easy to clean.

For absolute rheology measurement systems rotational and oscillatory rheometers are used, such as the cone and plate rheometer. These instruments can work in a controlled shear stress or shear rate mode and allow the determination of viscoelastic behavior, creep and relaxation tests.


Major Types of Rheology Modifiers


These additives can be grouped into:

  1. Organic rheology modifiers
    • For waterborne paints
    • For solvent-borne paints
  2. Inorganic Rheology modifiers

Organic Rheology Modifiers for Waterborne Paints: Most organic rheology modifiers are surface active; furthermore they may be part of the polymeric film matrix during film-formation. This explains, for instance, the excellent coating layer properties like, for example, improved appearance, gloss and flow. The variations regarding the chemical composition of these rheology modifiers are extraordinary versatile.

For waterborne paints different types of organic rheology modifiers are distinguished on the basis of thickening functionality:
  • Rheology modifiers that just thicken the aqueous phase, and
  • Products that thicken by interaction with other paint ingredients

Associative thickeners represent last group.

Organic Rheology Modifiers for Solvent-borne Coatings: Organic rheology modifiers are used with success to optimize the rheological characteristics in Solvent-borne coating materials, such as imparting resistance against sagging and sedimentation. Various products are available for the coating formulator. The variations regarding the chemical composition of these rheology modifiers are extraordinarily versatile.

Inorganic Rheology Modifiers: Organoclays, organically modified laminar silicates, are the most widely used inorganic Rheology modifiers and are being applied for many purposes in the paint and coating industry. Main representatives of laminar- or phyllo-silicates are hectorite and bentonite. Other important silica rheology modifiers are the synthetic amorphous silicates.


Chemistries Available to Control Rheology


Rheology_Modifiers for Waterborne and solventborne Coatings

The primary role of rheology additives is obviously to play on the rheology of your coatings (open time / wet edge, sag resistance, leveling, settling, film forming…). Yet, for an optimal selection, you will also have to consider their cost, the compatibility with other additives, their fit with your regulatory constraints (like VOC for example).

Rheology Modifiers for Coatings


Here is a list summing-up the main things to consider when selecting your rheology modifiers thickeners for paint:

  • Required rheological properties
    1. Sag resistance
    2. Flow- and leveling
    3. Application performances: spraying, brushing. (brush- drag)
    4. Stability: sedimentation, syneresis
    5. Manufacturing
  • Liquid paint performances
    1. Appearance (transparency, color, stability)
    2. Bio-stability
  • Paint film performance properties
    1. Gloss
    2. Transparency, opacity
    3. Water resistance
    4. Durability
    5. Regulation Compliance

 » Improve Surface of Your Coating Film Using Appropriate Surface Modifier! 

No doubt it’s complex to find the right balance and many will spend hours in trial & error. Instead, read on and get guided in your selection process:

  • Rheology Modifiers for Waterborne Paints & Coatings
  • Rheology Modifiers for Solvent-based Paints & Coatings


Continue reading or click to go on specific section of the page:

Rheology modifiers in waterborne coatings:

 » Types of rheology modifiers
 » Common rheology modifier combinations
 » Correction paint flow
 » Selection

Rheology modifiers in Solventborne coatings:

 » Types of rheology modifiers
 » Common rheology modifier combinations
 » Correction paint flow
 » Selection



Rheology Modifiers for Waterborne Paints


Most of the waterborne liquid paints do not show an ideal rheological modifiers profile at the required solids content and pigment / binder ratio without adding rheology modifiers. Depending on your end application, some chemistries will be better suited, as summarized here:

Type of Coating

Cellulose

Acrylics

Associative Thickener

Clays

Architectural

*

optimal flow characteristics, stability and water retention

*

costs and ease of incorporation

*

best flow and leveling in semi-gloss paints; additionally to control high shear viscosity, ease of addition

Industrial, automotive

*

*

*

Protective

*

for ease of addition, anti-settling, anti-sagging

*

levelling, hydrophobicity, enabling low VOC

*

for sag resistance, anti-settling


Now, let’s have a closer look at each family of paints and coatings additives for rheology - their pros & cons, as well as their main use:


As world is never black or white, you will also find common combination that you can try to tweak specific rheological properties.

Cellulose Rheology Modifiers


Should you work with waterborne architectural paints, cellulosic modifiers are the rheology modifiers thickeners of choice. They are the oldest class of rheology additives used in waterborne coatings. These modifiers are naturally sourced and can be altered chemically too. You will find cellulosic modifiers in the powder form.

Chemical modification is necessary in order to provide the cellulose water solubility and to control the thickening properties.

Modification is usually achieved through esterification as shown in figure below. The kind of modification largely affects the relative thickening properties and product features.

Hydroxy Ethyl Cellulose
Hydroxy Ethyl Cellulose


In your selection process, you need to pay attention to the molecular weight: Low molecular weight cellulosic modifiers offer good spatter resistance and open time. On the other hand, the ones with high molecular weight bring good thickening efficiency. So, lower quantities are needed to get ideal thickening effect. Some cellulosic modifiers used with waterborne coating systems include:
Strengths
Limitations
Offer a wide range of application
Can cause problems with flow and leveling
Shear thinning for easy application
Roller spattering
Compatible with a number of Colorants
 Negative effects on water and scrub resistance
Open time control
Sag control

Cellulosic Modifiers Strengths and Limitations

Polyacrylates /Acrylates


If you are looking for rheological modifiers that are widely compatible and easy to handle, then polyacrylates are the ideal ones for your formulation. These rheology modifiers exhibit strong pseudoplastic flow. Being synthetic in their origin, they are less prone to bacterial and fungal attack. This category comprises of Alkali Swellable Emulsions (ASE) and Hydophobic Alkali Swellable Emulsions (HASE). While ASE modifiers are used in low cost paints and inorganic pigment slurries, HASE are used in automotive basecoats.

Strengths
Limitations
Strong shear thinning
pH sensitivity
Anti-settling and anti-sag
Reduce water and scrub resistance
Low costs
Easy incorporation
Good spray properties

Polyacrylates /Acrylates Strengths and Limitations

Associative Thickeners


Associative thickeners (AT) are also known as HEUR/ PUR rheology modifiers thickeners. These consist of aqueous solutions of low molecular mass polymers; the thickening mechanism is based on formation of physical interactions with other components in the paint formulation. These rheological modifiers offer easy handling, best flow and leveling, low risk of roll spatter and film hydrophobicity. They work by coupling themselves with other paint components. Main representatives of this class are nonionic copolymers based on polyurethanes. Associative modifiers are mostly employed in mid and low PVC-grades dispersion paints and industrial coatings. The structure is represented schematically as containing a hydrophilic polymer backbone, which is end-capped by hydrophobic heads.

Associative Thickeners Structure

Associative Thickeners Structure

Associative thickeners Structure


The term “associative thickeners” is related to the thickening mechanism, which is based on the formation of associates, links, between the hydrophobic groups o the thickener with paint ingredients, notably the polymer binder surface. Thus a continuous network is formed, which results in increased viscosity.

Associative Thickeners Mode of action 


Strengths
Limitations
Excellent flow/ leveling, high gloss
Limited colorant compatibility
Low shear thinning; high film build
Weak sag control
Liquid; ease of handling
Roller spattering
Water and scrub resistance

Associative Thickeners Strengths & Limitations


Clay / Hectorite Clay


While formulating protective coatings for excellent water resistance, clays are the perfect choice as modifiers. Hectorite clay (HEC) is basically sodium magnesium lithium silicate powder. It offers excellent suspension properties while maintaining the ease of application. You will generally find it in the form of elongated platelets. Hectorite clays are widely used in industrial and automotive coatings.

Hectorite clay mode of action

Hectorite Clay mode of action

For full development of their rheological properties in a formulated product, such as paint; the hectorite clay must be subjected to wetting and shear to break up agglomerates of platelets. Specific surface charge effects between the platelets result in the formation of a flocculated gel network, which greatly influences the rheological properties.

Strengths
Limitations
Incorporation 
Heat-resistance. Controllingsyneresis
Gloss, flow and leveling issues.
Open time control is less

Hectorite Clay Strengths and Limitations


Common Rheology Modifier Combinations in Waterborne Systems


  • HEC in the millbase and post-add AT for high shear adjustment.
  • AT in conjunction with silicate for anti-settling.
  • Metal effect lacquers: HEC for viscosity and Hectorite for metal suspension.
  • ASE for standard viscosity, low cost and hectorite or attapulgite clay for additional anti-settling and separation.

As each group of rheology modifiers thickeners contributes typically to characteristic rheological effects, combinations may be used in order to meet specific performance requirements. For instance, applying cellulose thickeners in the mill-base an using the liquid associative thickener in the let-down, taking advantage of the ease of addition of this class of thickeners and also enabling optimization of brushing and film-build. Hectorite is used in conjunction with HEC to optimize in-can stability and sag-resistance, while maintaining dry film water resistance.

Correction Paint Flow in a Waterborne Paint


Characteristic
Feature
Prime Thickener Considerations
Desired shear correction
Low shear
Clay, HEC, ASE
 Mid shear  HASE, low shear AT
 High shear  AT

Cost sensitiveness paint
Low
AT
 Mid  Clay, HEC, HASE
 High  ASE
 
Wet edge during drying
Slow release water
HEC
 Medium  High shear AT, HASE, ASE
 Fast release  Clay, low shear AT
 
Desired gloss retention
Low
HEC, OC,  hectorite OC,
 Mid  ASE, HASE
 High  AT
 
Required water sensibility paint film
Standard
ASE, HEC, HASE
 Extremely low  Clay, AT


Rheology Modifiers Selection for Waterborne Systems


The following table informs generic and relative effects of each of the thickener classes on paint performance properties. However, it should be kept in mind, for each class as being a wide range of commercial products being offered.

Rheology Modifier

KU Viscosity Increase

Paint in-can Stability

Sag Control

Wet-Edge

Flow, Levelling

Water Sensibility Paint Film

Desired Gloss Retention

Effect on Costs Formulation

Cellulosics

4

3

2

4

3

-3

0

-1

Acrylates

3

3

3

1

2

-3

0

-1

Associative thickeners

2

2

1

2

5

2

2

-2

Clays

1

4

4

0

2

0

-1

-3


-5: significant negative effect | 0: no effect | +5: significant positive effect


Rheology Modifiers for Solvent-based Coatings


In solvent-based coatings, the main reason to use rheology modifiers is to adjust sedimentation & sag resistance. Unlike with waterborne paints, , thickeners are usually not required for paint manufacturing purposes. Depending on your end application, some of these rheology additives will be better suited, as summarized here:

Type of Coating
Organoclays
Hydrogenated castor oil
Polyamides
Fumed Silicates
Architectural
*
anti-settling control
*
control sagging
   
Industrial, automotive
*
Airless 2-pack topcoat. Stoving, sealants, epoxy primers
   
Protective coatings
*
Shop primers, anti-settling
*
Anti-selling
 *
Sag control
 *
Anti-settling


While working for the above mentioned applications you should consider the following criteria to select rheology modifiers for solvent-based coatings:

  • Rheological requirements of the liquid formulation during storage and application
  • Ease of incorporation
  • Physical properties of the dry surface coating
  • Total cost of the formulation

The following rheology modifiers can be used with solvent-based coating systems:


Organoclays


Organoclays are hydrophillic and must be reacted with specific organic quaternary ammonium compounds to make the surface hydrophobic. Here is an overview of the variety of organoclays you can choose upon:

Clay rheology modifiers


Smectite, notably bentonite and hectorite clays, represent the most widely used inorganic rheology modifiers for solvent-based paints.

Bentonite organoclay is most versitile while hectorite is more specifically preferred for use in polar systems. Organoclays show excellent performances in controlling sag- end settling resistance in coatings, while maintaining flow and levelling, notably in industrial, protective and automotive, coatings.

Cation exchange sites on platelet surface


The sodium cation site, as present on the natural clay platelets, is replaced with quaternary amine. After further purification organophilic clay (OC) is formed and this is applicable in solvent-based systems.

The formed organoclays enable application of the clay in a wide range of organic solvent-based coatings.

Strengths
Limitations
Sag resistance, anti-sedimentation
Incorporation, shear required
Wide range of application
Reduced gloss, poorer leveling
Shear thinning for easy application: brush and roll (alkyds). Thixotropic flow Heat resistance. Syneresis control Less thixotropy than organics
Not generally suitable for clear coats

Organoclay Strengths & Limitations


Hydrogenated Castor Oil: HCO


This class of rheology modifiers is offered in powder or paste form. The swelling characteristics of Hydrogenated castor oil thickeners in liquid medium highly influence the performance. HCO is a preferred group of thickeners for achieving shear-thinning viscosity build, sag control and excellent flow and leveling.

Swelling mechanism
Hydrogenated Castor Oil Swelling Mechanism


For activation, the powder or wax-form hydrogenated castor oil must be subjected to solvent wetting, de-agglomeration and high shear dispersion forces at specific temperatures. This process leads to partly de-agglomeration, followed by swelling of the particles in the solvent medium.

The optimal incorporation temperature depends on the coating formulation and is generally between approx. 35 and 70°C.

Strengths
Limitations
 Excellent thixotropic flow
 Temperature/ seeding control
 Strong shear thinning
 Workability: require adequate shear and dwell time
 Leveling
 Cool down before packing (false- body)
 Sag resistance
 Solvent dependency
 Recoatability

Hydrogenated castor oil Strengths & Limitations

Polyamides


A class of synthetic rheology modifiers, available in a wide variety of chemical compositions. The reached strong pseudoplastic thickening effect is partly explained by formation of intra-molecular hydrogen bonding, effective particularly in low polarity systems. Furthermore to entanglement and swelling of the polyamide, a contribution, which is strongly related to the typical solvation characteristics of the polyamide in the particular system.

Strengths

Limitations

 Steep shear thinning flow

 May cause inter-coat adhesion failure 

 Contributes to improved shelf stability
 Workability, ease of incorporation
 Sag resistance  Solvent dependency

Polyamides Strengths & Limitations

Fumed silica


Fumed silica has proven to be excellent rheology modifier for many low-viscosity systems and is consequently finding widespread application in converting Newtonian systems into pseudoplastic and thixotropic systems. However, careful selection of the right silica as per application system is essential; applicability of the silica is strongly related to the required degree of hydrophilicity respectively hydrophobicity of the fumed silica.

Strengths

Limitations

 Thixotropic flow

 Ease of incorporation, gel time related to system composition

 Excellent anti-settling  System dependency as per type of silica
 Heat resistant  Risk of floatation
 Effective also in low viscosity liquids

Fumed Silica Strengths & Limitations


Common Rheology Modifier Combinations in Solvent-based Coatings


  • Bentonite for anti-settling and hydrogenated castor oil for additional anti-sag and maintaining gloss.
  • Silicate for anti-settling and liquid castor-oil rheology modifier for post thickening and anti-sag adjustment.
  • Clay for anti-settling, polyamide for flow.

 » Know More Additives from Paint Rheology Modifiers Market for Solventborne Systems! 


Correction Paint Flow in a Solvent-based Paint


Characteristic
Required level
Prime thickener considerations
Coating solids
Low
OC, Hydrogenated Castor oil,
 Mid
OC, Hydrogenated Castor oil
 High
Polyamide, OC
Yield value
Low
Hydrogenated Castor oil, Polyamide
 Mid OC, fumed silica
 High OC
Recovery
Low
Hydrogenated Castor oil, Polyamide
 Mid Fumed silica
 High OC


Rheology Modifiers Selection for Solvent-based Coatings


The following table shows the positive and negative effects of each of the thickener classes on paint performance properties. However, you should keep in mind that these figures give a general order of magnitude. In each class you will find a wide range of commercial products nuancing one or more of these characteristics.

Type of Coating

Ease of incorporation

Viscosity build

Shear thinning

Settling

Sag control

Levelling

Cost of formulation

Organoclay

-1

2

3

3

3

-1

-2

HCO

-5

3

4

4

4

1

-1

Polyamide

-3

2

3

2

1

0

-3

Fumed silica

0

2

2

3

1

-1

-3



Enhance your Knowledge in Rheology Modifiers!


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