Rheology in Coatings and Role of Rheology Modifiers
Rheology in Coatings and Role of Rheology ModifiersRheology 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.
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?
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)
| Based in synthetic organic chemistry |
Further, rheological additive technologies can also be divided into thickeners suitable for waterborne and solvent-based systems.
|Coating System||Rheology Modifier Chemistry|
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
Rheology Modifiers for Waterborne PaintsFor 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|
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.
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.
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.
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.
- 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)
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.
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|
|Cost sensitiveness paint||Low||AT|
|Mid||Clay, HEC, HASE|
|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,|
|Required water sensibility paint film||Standard||ASE, HEC, HASE|
|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|
Rheology Modifiers for Solvent-based Coatings
Rheology Modifiers for Solvent-based CoatingsIn 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|
- 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 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.
The formed organoclays enable application of the clay in a wide range of organic solvent-based coatings.
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.
The optimal incorporation temperature depends on the coating formulation and is generally between approx. 35 and 70°C.
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.
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.
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|
|Yield value||Low||Hydrogenated Castor oil, Polyamide|
|High||Organoclays, Fumed silica|
|Recovery||Low||Hydrogenated Castor oil, Polyamide|
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|
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|
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Rheology Modifiers/ Thickeners for Paints, Coatings and Inks
View a wide range of rheology modifiers available today, analyze technical data of each product, get technical assistance or request samples.