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Rheology modifiers are vital additives used in almost every coating. Choosing the right rheology modifier for your formulation can be complex keeping in view the existing VOC regulations. This selection guide will help you select the right rheology modifier for your coating formulation. It will first detail the chemistries available for waterborne and solvent borne coatings. It will also uncover benefits and limitations as well as suitable applications.

 

Rheology Modifiers: Additive Overview



Rheology ModifiersAs 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) 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 that you can use for this purpose. The question is therefore to find the right one!


 

Chemistries available to control rheology

Rheology_Modifier


The primary role of rheology modifiers 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).

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

  • Required rheological properties
    1. Sag resistance
    2. Flow- and levelling
    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


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

 

Rheology Modifiers for Waterborne Coatings


Most of the waterborne liquid paints do not show an ideal rheological 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:
 

 

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 coatings

 

*

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 rheology modifier family:


We will detail 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.


 

Cellulosic Modifiers


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

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
Methyl cellulose, Hydroxy ethyl cellulose (HEC), Carboxy methyl cellulose (CMC), Hydroxy propyl cellulose (HPC), and Hydrophobically modified HEC.

 

Cellulosic Modifiers Strengths and Limitations

 

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


 

Polyacrylates/Acrylates

If you are looking for rheology 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.


 

Polyacrylates/Acrylates Strengths and Limitations

 

Strengths

Limitations

Strong shear thinning

pH sensitivity

Anti-settling and anti-sag

Reduce water and scrub resistance

Low costs

Easy incorporation

Good spray properties


 

Associative thickeners

 

Associative thickeners (AT) are also known as HEUR/ PUR 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 rheology 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.

 

 


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.


 

 

Fig. Associative thickeners Structure


 

Associative Thickeners Strengths & Limitations

 

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


 

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.
 

Fig. Hectorite Clay mode of action


For full development of their rheological properties in a formulate 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.

 

Hectorite Clay Strengths and Limitations

 

Strengths

Limitations

Sag- and settling resistance

Incorporation

Heat-resistance. Controlling syneresis

Gloss, flow and leveling issues.
Open time control is less


 

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 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.
 

 

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 modifiers will be better suited, as summarized here:
 

 

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:
 



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.

 


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.


 

Organoclay Strengths & Limitations

 

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



 

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




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.


 

Hydrogenated castor oil Strengths & Limitations

 

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



 

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.

 

Polyamides Strengths & Limitations

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



 

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.

 

Fumed Silica Strengths & Limitations

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



 

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.


 

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.
 

 

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

 

 

Reference

By Johan Bieleman

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