Rheology Modifiers Selection for Paints & Coatings

Rheology in Coatings and Role of Rheology Modifiers

Rheology modifiers are vital additives used in almost every coating and ink to adjust the rheological characteristics. Apart from getting desired viscosity, these additives also help in controlling paint shelf stability, ease of application, open time / wet edge and sagging.

Before we dig into the use of rheology modifiers, it may be useful for you to understand what exactly rheology means, why does it matter so much in coatings & inks as well as get clarification on some of the technical terms, such as viscosity, shear rate, yield stress, etc.

Get started with rheology with our exclusive guide on rheology basics.

Rheology in Paints and Coatings - Essential Concepts

In paints and coatings, rheology examines the behavior of flow and deformation properties during manufacturing, storage, applications as well as film formation. It influences key characteristics such as vertical flow, leveling, gloss, adhesion, film thickness, covering power, spattering tendency, brush and roll resistance, sedimentation tendency and pigment stabilization of a formulation. During the coating production, rheology is useful to obtain optimal flow behavior to the mill base.

Rheology is so crucial for your formulation that you should be in control and at ease to play with it.

The starting selection criteria are basic as we are adjusting the mill base at this stage:

Average shear rate
Coatings system: waterborne, or solvent-based

Of course, you may have extra criteria to meet all your requirements. For example, you may try to limit VOCs. Additives such as VOC free hydrophobically ethoxylated urethanes (HEUR) and Hydrophobically modified polyether (HMPE) provide a high level of efficiency in the mid-shear range.

Rheology Modifiers Chemistries: Which one is right for your system?

Today, we are aware of several thickening technologies that influence the rheology profile of paints and coatings. From a broader perspective, rheology modifiers can be divided into inorganic and organic chemistries.

Rheology Modifier Chemistry	Suitable Example
Organic	Based on Natural raw materials (cellulose, xanthum gum)
Organic	Based in synthetic organic chemistry (polyacrylates, PU) Associative Non-associative Other solvent-based
Inorganic	Clays Silicas Specialty (modified) Clays

Organic and Inorganic Rheology Modifiers
Further, rheological additive technologies can also be divided into thickeners suitable for waterborne and solvent-based systems.

Coating System	Rheology Modifier Chemistry
Waterborne Systems	Cellulosics Non-associative (ASE/HASE) Associative (HEUR, HMPE) Specialty clays
Solventborne Systems	Organoclays Hydrogenated castor oils Fumed silicas Polyamides Overbased Sulphonates

Rheology Modifiers for Waterborne and Solvent-based Systems

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 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 based on thickening functionality:

Rheology modifiers that just thicken the aqueous phase, and
Products that thicken by interaction with other paint ingredients - Associative thickeners represent this group.

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. Alike waterborne systems, variations regarding the chemical composition of these rheology modifiers are extraordinarily versatile.

» Explore a variety of rheology modifiers grades that offer good sag resistance

Inorganic Rheology Modifiers

Organoclays & organically modified laminar silicates are among the 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-based rheology modifiers are the fumed silica, colloidal silica, precipitated silica, etc.

Undoubtedly finding the right rheology modifier & achieving the optimum balance of properties is quite complex. Let move on with an overview of these chemistries & their performance properties.

Rheology Modifiers for Waterborne Paints

For water-borne paints some chemistries will be better suited depending on your end application, as summarized in the table below.

Type of Coating	Cellulose	Acrylics	Associative Thickeners	Clays
Architectural	*	*	*
Industrial, Automotive		*	*	*
Protective		*	*	*

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 (Non-Associative)

Should you work with waterborne architectural paints, cellulosic modifiers, either alone or in combination, are among the most widely used rheological additives. They are the oldest class of rheology additives used in waterborne coatings. Cellulosic modifiers are naturally sourced and are majorly available in a powder form.

Strengths	Limitations
Offer a wide range of application Shear thinning for easy application Compatible with a number of colorants Open time control & sag control Biodegradability, lack of toxicity	Can cause problems with flow and leveling Roller spattering Negative effects on water and scrub resistance

Cellulosic modifiers can be altered chemically too. 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

Some cellulosic modifiers used with waterborne coating systems are listed below - you can click on links to explore commercially available grades per chemistry.

Methyl cellulose
Hydroxy ethyl cellulose (HEC)
Carboxy methyl cellulose (CMC)
Methyl hydroxyethyl celluloses (MHEC)
Hydroxy propyl cellulose (HPC), and
Hydrophobically modified hydroxyethyl cellulose (HMEC) – discussed under associative modifiers

The viscosifying ability these additives is primarily controlled by:

Structural parameters, such as molecular weight,
The quantity of cellulosic thickener,
Degree of substitution (DS), the nature of the substituent group and amount of substitution groups.

Cellulosic non-associative modifiers thicken a formulation through hydrodynamic means. This means the thickening is controlled by the molecular weight of the additive as they entangle the molecules in the formulation and make it more viscous.

Therefore, while selecting cellulosic non-associative thickeners, 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.

In general, the molecular weight is between 10⁵ and 10⁶ g/mol.

Polyacrylates /Acrylates

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
Hydrophobic Alkali Swellable Emulsions (HASE)

ASE: Alkali Swellable Emulsions (ASE) are dispersions of acid functional acrylic polymers in water. They are non-associative thickeners supplied at low pH and the acid groups on the polymer chains need to be neutralized to allow the polymer to swell and thicken. The concentration of acid groups, the molecular weight and degree of crosslinking of the polymer are important factors influencing the rheology profile and thickening efficiency of acrylic thickeners.

HASE: Hydrophobically Modified Alkali Swellable Emulsion (HASE) associative thickeners are also commonly found in latex paints. HASE thickeners differ from ASE products in that they also contain long-chain hydrophobic groups in addition to acid groups distributed throughout the polymer chain.

ASE and HASE additives thicken at a pH above 7 through repulsion of carboxylate anions along the polymer backbone. However, HASE polymers present enhanced viscosity because the hydrophobic groups aggregate together in the water phase in a manner similar to the way in which surfactants form micelles.

While ASE modifiers are used in low-cost paints and inorganic pigment slurries, HASE are used in automotive basecoats.

Strengths	Limitations
Strong shear thinning Anti-settling and anti-sag Low cost Easy incorporation Good spray properties	pH sensitivity Ca++ ion sensitivity Reduce water and scrub resistance Water whitening

Associative Thickeners

Associative modifiers are often aqueous solutions of low molecular weight polymers containing both hydrophilic and hydrophobic regions.

Associative Thickeners Structure
The structure is represented schematically as containing a hydrophilic polymer backbone, which is end-capped by hydrophobic heads.

The thickening mechanism is based on formation of physical interactions with other components in the paint formulation. They work by coupling themselves with other paint components, notably the polymer binder surface. The hydrophobic regions are then able to associate with the hydrophobic moieties while the hydrophilic regions can associate with the hydrophilic moieties. Thus, a continuous network is formed, which results in increased viscosity and unique rheological properties.

Associative modifiers are mostly employed in mid and low PVC-grades dispersion paints and industrial coatings. They show better performance in sag and leveling, moisture resistance, spatter resistance and coverage vs other cellulosics and ASE non-associative thickeners.

However, one of the key challenges associated with these thickeners is viscosity instability with the addition of colorants. The issue is known in the industry as “viscosity loss” and results in poor coating properties (thin coatings, poor coverage, increased sagging and poor color rub-up) and increased formulation complexity.

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

Associative thickeners represent a class of ground-breaking materials based primarily on hydrophobically modified polymers. Some of the key chemistries are listed below.

Hydrophobically Modified Alkali-soluble Emulsions (HASE)
Hydrophobically Modified Celluloses (HMEC)
Hydrophobically Modified Ethoxylate Urethanes (HEUR)
Hydrophobically Modified Polyethers (HMPE)
Hydrophobically Modified Polyacetal-Polyether (HM-PAPE)

Borchi® Gel - HEUR Thickeners for Water-based Coatings

Clay / Hectorite Clay

While formulating protective coatings for excellent water resistance, clays are the perfect choice as modifiers. Hectorite clay 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

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
Sag- and settling resistance Heat-resistance. Controlling syneresis	Incorporation Gloss, flow and leveling issues Open time control is less

Common Rheology Modifier Combinations in Waterborne Systems

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.

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.

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
Required water sensibility paint film	Extremely low	Clay, AT

Rheology Modifiers & Waterborne Systems Performance

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, Leveling	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 in the table below.

Type of Coating	Organoclays	Hydrogenated castor oil	Polyamides	Silicas
Architectural	*	*
Industrial, Automotive	*
Protective	*	*	*	*

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

Organoclays

Organoclays are hydrophillic and must be reacted with specific organic quaternary ammonium compounds to make the surface hydrophobic. Organoclays show excellent performances in controlling sag- end settling resistance in coatings, while maintaining flow and levelling, notably in industrial, protective and automotive, coatings.

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 versatile while hectorite is more specifically preferred for use in polar systems.

How organoclays work?

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.

Cation exchange sites on platelet surface

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

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

Hydrogenated Castor Oil (HCO)

Hydrogenated Castor Oil is a preferred group of thickeners for achieving shear-thinning viscosity build, sag control and excellent flow and leveling. They are offered in powder or paste form.

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

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 swelling characteristics of Hydrogenated castor oil thickeners in liquid medium highly influence the performance.

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

Swelling Mechanism

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 Contributes to improved shelf stability Sag resistance	May cause inter-coat adhesion failure Workability, ease of incorporation 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.

Strengths	Limitations
Thixotropic flow Excellent anti-settling Heat resistant Effective also in low viscosity liquids	Ease of incorporation, gel time related to system composition System dependency as per type of silica Risk of floatation

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	Organoclays, Hydrogenated Castor oil
	Mid	Organoclays, Hydrogenated Castor oil
	High	Organoclays, Polyamide

Yield value	Low	Hydrogenated Castor oil, Polyamide
	High	Organoclays, Fumed silica
	High	Organoclays

Recovery	Low	Hydrogenated Castor oil, Polyamide
	Mid	Fumed silica
	High	Organoclays

Rheology Modifiers & Solvent-based Systems Performance

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	Leveling	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

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

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, flow adjustments, the compatibility with other additives, their fit with your regulatory constraints (VOC for example).

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

Formulation Stage	Required rheological properties	Liquid paint performances	Paint film performance properties
Manufacturing Stability: sedimentation, syneresis Application performances: spraying, brushing (brush-drag) Ease of incorporation Total cost of the formulation	Sag resistance Flow- and leveling	Appearance (transparency, color, stability) Bio-stability	Gloss Transparency/ opacity Water resistance Durability Regulation Compliance

Now when you have the suitable rheology modifier options shortlisted according to your system, do not forget that there can be unwanted interactions with other paints ingredients (colorants, surfactants…). It is time to optimize thickeners efficiency - the right loading at the right time, no unwanted interaction, etc.

Take a tour with rheology expert David Davilla C. into the practical aspects of rheology and predict structural behavior, storage behavior, film formation kinetics… with step-by-step demo of rheological data interpretation. With this exclusive course on applied rheology, launch your products faster in the market by reaching the desired performance level with much needed practical rheological guidance to drive your formulation work.

Applied Rheology for Improved Performance in Paints and Coatings

Rheology Modifiers/ Thickeners for Paints, Coatings and Inks

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