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Organic rheology modifiers come in many shapes and sizes. They are used throughout various industries, from cosmetics to paints and beyond, to change many properties of a formulation-based product. The addition of these modifiers can bring various benefits depending on the product and the modifier. In this article, we look at the properties of some of the most common organic rheology modifiers.
What are Organic Rheology Modifiers?
Rheology modifiers are compounds which are added to formulations to increase their viscosity, and to control the properties and characteristics of fluid products. Rheology modifiers come in the form of organic and inorganic modifiers, but this article is only concerned with the organic additives. They can be used in a wide class of mediums including waterborne systems, solvent systems and solventless systems; and in specific products such as paints, coatings, creams, lotions and inks. Some of the most common organic rheology modifiers include, castor oil derivatives polymers, cellulose, alkali-acrylic emulsions, hydrophobic ethoxylated urethane resins, polyurea, polyamides and calcium sulfonates.
There are two main categories of organic rheology modifiers which are determined by the thickening mechanism. These are associative and non-associative modifiers. Non-associative modifiers thicken a formulation through hydrodynamic means, i.e. the thickening is controlled by the molecular weight of the additive as they entangle the molecules in the formulation and make it more viscous. Associative modifiers rely on non-specific interactions between the end-groups of a thickener molecule and the surrounding formulation (as well as with themselves).
Most (but not all) organic rheology modifiers contain some kind of polymer. Polymers are known to be excellent additives for many reasons. The main property being that the viscosity of a polymer solutions is linear to its molecular weight, making it easy to determine how much to add into a formulation to increase the viscosity by a given amount. Polymers can also introduce both shear thinning and thickening behaviour to a formulation.
Properties of Organic Rheology Modifiers
Products that employ organic rheology modifiers are often less prone to dripping, have an increased shelf-life, possess a different textural and sensorial profile, an enhanced electrolyte tolerance and different viscosity/flow properties compared to non-modified formulations. Without the addition of these additives, many products would be runny or would drip when applied – properties which are a hinderance to the function of many of these products.
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Here we look at the specific properties of some of the most common organic rheology modifiers:
Castor Oil Derivatives
Castor oil derivatives are often used in non-aqueous systems because the hydroxyl groups provide good interfacial characteristics for modifying the rheology of a formulation. Castor oils are often incorporated into a product at 35-70 °C because they lose their rheological properties at high temperatures. Castor oil derivatives are known to possess excellent thixotropic flow, shear thinning, levelling and sag resistance properties.
Modified polyurea changes the rheological properties of a formulation through hydrogen bonding between the urea and modified urea groups, and by interacting with the binder molecules. Polyurea’s are versatile additives because many changes to the functional group can be made, such as changing the polarity of the additive. Modified polyurea’s exhibit a resistance to settling and a good sag resistance.
There are various types of polyamide additive available and each have their own application. In general, polyamides increase the thickness through a chelating effect and through the formation of micelles from the hydrophilic and hydrophobic end groups. Hydrogen bonding also plays a small role in increasing the thickness and stability of a formulation. Polyamides require a high incorporation temperature of 150 °C, but there is no max incorporation temperature (i.e. no loss of properties at high temperatures). They also possess an excellent thixotropic flow, strong shear thinning and a high film build.
Calcium sulfonates increase the thickness of a product by forming microstructures to form a stable gel-like structure. Calcium sulfonate additives exhibit a good sag resistance, high-shear thinning and an ease of incorporation into formulations.
Cellulose is a type of non-associative additive, so it thickens the formulation through hydrodynamic volume exclusion. When cellulose is added, it occupies some of the space within the solution, which then thickens the solution because the cellulose doesn’t allow the solubilized molecules to flow as freely through the internal voids. Cellulose additives can be used for a wide range of applications and possess excellent shear thinning properties, good compatibility with colorants, good sag control and a long shelf-life.
Alkali Acrylic Emulsions
There are two types of alkali acrylic emulsion. One is associative and uses the interactions of hydrophobic groups and the other is a non-associative additive that employs a unique version of the hydrodynamic volume exclusion mechanism. When the alkali emulsion is added into a formulation, a neutralization reaction occurs. This creates a large number of like charges in the formulation. These like charges repel each other, which causes the molecules in the solution to swell and occupy larger volumes, thus, increasing the viscosity.
These emulsion additives possess a resistance to microbial degradation, strong shear thinning, low cost, anti-sag properties, anti-settling properties, good spray properties and are easy to incorporate. Additionally, non-modified alkali acrylic emulsions are insensitive to pH, whereas hydrophobically modified emulsion additives have a high film build and possess a high resistance to moisture.
Hydrophobic Ethoxylated Urethane Resins
Hydrophobic ethoxylated urethane resins (HEURs) are an associative additive, so they utilize hydrophobic interactions between the additive and the molecules in the solution to thicken the formulation. HEURs possess low shear thinning, high resistance to microbial degradation, are insensitive to pH, can provide pseudoplastic or Newtonian characteristics, a simple incorporation process into formulations, minimized spattering, high film build and excellent flow and levelling properties.
- SpecialChem: https://adhesives.specialchem.com/selection-guide/rheology-modifiers-selection-for-adhesives/organic-rheology-modifiers-for-waterborne-systems
- BASF: https://www.dispersions-pigments.basf.com/portal/load/fid793184/BASF%20Rheology%20Modifiers%20Practical%20Guide.pdf
- Munzig: https://www.munzing.com/fileadmin/_migrated/content_uploads/TAFIGEL-PUR-Rheology-Modifiers_01.pdf
- Lubrizol: https://www.lubrizol.com/Personal-Care/Products/Rheology-Modifiers
- “Polymers as Rheology Modifiers”- Glass J. E. et al, ACS Symposium Series, Vol. 462¸ 1991, DOI: 10.1021/bk-1991-0462.ch001