Editorial Feature

What are Rheology Modifiers?

Under the influence of gravity, fluid materials like paint exhibit sagging, forming “curtains” and “tears.” Controlling their flow properties for proper application and long-term stability on vertical structures is important.1 This can be achieved using rheology modifiers or thickeners, improving the liquids’ in-can stability and application properties.2

What are Rheology Modifiers?

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Rheology modifiers, initially developed for coatings and paints, are today used to modify various products such as cement pastes, food items, pharmaceutical preparations, cosmetics, and automotive lubricants.1 This article explores the basic functioning and applications of rheology modifiers.

Defining Rheology Modifiers

Rheology modifiers manipulate the flow of liquids by altering their viscosity, improving the functionality and long-term durability of the final products. They can be associative and non-associative, each functioning differently to enhance product performance and consistency.

Non-associative modifiers can influence the pseudoplastic and elastic behavior of liquids but often result in storage instability and require high concentrations to achieve acceptable flow properties.2

Associative modifiers are polymers with very high molecular weights, comprising hydrophobic and hydrophilic parts.1 Self-association and hydrophobic interaction with other liquid components, such as surfactants or binders, govern the functioning of associative modifiers. They are further categorized as hydrophobically modified alkali swellable emulsion (HASE) and hydrophobically modified ethoxylated urethane (HEUR).2

HASE works by swelling and increasing the hydrodynamic volume of its particles at a pH above 7. Alternatively, HEUR rheology modifiers are non-anionic and act by associating with the hydrophilic groups and adsorbing hydrophobic groups on the surface of the binder particles.2

Depending on their source, rheology modifiers offer different functionalities. For example, cellulose derivatives such as carboxymethyl, ethyl, hydroxyethyl, and hydroxypropyl exhibit aqueous thickening and stabilizing properties. Animal fat derivatives offer wax emulsion and corrosion inhibition properties, while castor oil derivatives exhibit strong sag resistance and thixotropic effects in solvent-based coatings.1

Among inorganic rheology modifiers, metal oxides improve flow behavior using their high specific surface area, silica fumes promote adhesion through nano-sized primary particles, and attapulgite is an exceptionally effective gellant. Polymeric modifiers like acrylate polymers and polyamide wax exhibit stable viscosity over a wide pH range and are applicable in most solvent-based synthetic resin systems.1

Gum-based rheological modifiers can reduce manufacturing time by up to 80 % compared to conventional gel systems, which degrade at extreme pH and temperature. Alternatively, gelatin protein, being biocompatible, is widely employed to control the rheology of food products, cosmetics, drug capsules, and medicines.1

Applications in Various Industries

Rheology modifiers have found applications in several industries. In electronics, polydimethylsiloxanes are employed as adhesives and sealants. They are modified with carbon allotropes, such as graphite and carbon nanotubes, to build a stable and interconnected network. Additionally, adhesives for electronic packaging containing SiO2 nanoparticles are altered using organosilane to lower viscosity and the force required for mixing.1

In agriculture, rheological products enhance crop protection and seed coatings. Improving the rheological properties of pesticides with organosilicon and polyelectrolyte-based modifiers enhances their deposition and spreading effect on crops.1

The thermophysical properties and lubricity of lubricating or engine oils for automobiles are modified using nano-clay. The large surface of the nanoparticles improves their dynamic viscosity of oil dispersions. In the coatings and paint industry, rheology modifiers improve the durability, applicability, and resistance to microorganism growth.1

In the construction sector, modifiers like xanthan gum enhance the viscosity of Portland cement pastes. Adding superabsorbent polymers and nano-silica improves the casting performance of cement pastes.1

Starch-based hydrogels are applied in medical, pharmaceutical, and cosmeceutical industries because of their safety, biocompatibility, hydrophilicity, and biodegradability. In cosmetics, hyper-thixotropic modifiers are used in foundations, mascaras, hair care, lip formulations, and other personal care products. This ensures low product viscosity during the application and a rapid rise in viscosity to inhibit migration of the applied formulation.1

Fluids used in deep-water oil and gas drilling thicken at low temperatures, resulting in lost circulation, high flow resistance, and leakage. Non-ionic and anionic surfactants are added to water-based drilling fluids to improve their rheological and filtration characteristics and high-temperature resistance.1

Leading Manufacturers and Innovations

Leading manufacturers of rheology modifiers include Dow Coating Materials, Münzing, BASF, and Elementis. Dow and Münzing are primary manufacturers of HEUR and HASE rheology modifiers for the water-based coatings industry, covering the complete rheological profile from highly pseudoplastic to Newtonian.3

BASF is a leading supplier of rheology modifiers for water-based, solvent-based, and solvent-free systems. Its additives for aqueous systems include non-ionic and anionic associative and non-associative anionic methods. Additionally, Elementis produces associative, acrylic, organic thixotropic, and clay-based rheology modifiers.3

Emerging trends in the rheology modifier industry include thickeners that act only in specific regions of the rheology curve for targeted viscosity regulation. Another focus of the field is to create rheology modifiers with uniform performance across different applications.3

There is also an increased focus on bio-based, sustainable, and environmentally friendly additives. Manufacturers are developing pollutant-free modifiers without parabens, volatile organic compounds (VOCs), or harmful preservatives. BASF, for instance, offers modifiers that provide environmental advantages such as being free from VOCs, odors, and heavy metals and added functionalities like wetting properties.3

Future Outlook

Despite the well-established advantages of rheology modifiers, the industry faces several challenges. It is difficult to simultaneously optimize a modifier's performance while ensuring its compatibility with a medium's ingredients.

Additionally, establishing the appropriate rheology balance with novel latexes is difficult as their particle sizes are being reduced, influencing the performance of rheology modifiers and the storage stability of the liquid.3

Formulation experts are developing novel rheology modifiers to overcome the existing functionality limitations. For instance, a recent study in Advances in Colloid and Interface Science explored stimuli-responsive viscosity modifiers, which enable smart material fabrication using various stimuli for switching, including pH, temperature, light, and salinity.

These modifiers can respond to multiple stimuli, exhibiting high functionality and adaptability through multi-level and precise actuation. These stimuli-responsive modifiers are promising as hydrogels and polymeric vessels for tissue engineering and site-specific drug delivery.4

Overall, the market of rheology modifiers was estimated to be US$6.56 billion in 2018 and is anticipated to reach US$8.2 billion by 2026.1 This growth can be sustainably accelerated through enhanced investment in research and development by the leading manufacturers.

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References and Further Reading

  1. Wypych, G. (2022). Handbook of Rheological Additives. ChemTec Publishing, Toronto, Canada. ISBN: 978-1-927885-98-7
  2. Cibangwa., MK. (2021). Rheological characterisation of water based paint using associative rheology modifiers. [Online] Cape Peninsula University of Technology. Available at: https://etd.cput.ac.za/bitstream/20.500.11838/3538/1/Manasse_Cibangwa_211087300.pdf
  3. American Coatings Association. (2022). Rheology Modifiers: Roundtable Q&A. [Online] American Coatings Association. Available at: https://www.paint.org/coatingstech-magazine/articles/rheology-modifiers-roundtable-qa/
  4. Bhat, B., Pahari, S., Kwon, JS., Akbulut, MES. (2023). Stimuli-responsive viscosity modifiers. Advances in Colloid and Interface Science. doi.org/10.1016/j.cis.2023.103025

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Nidhi Dhull

Written by

Nidhi Dhull

Nidhi Dhull is a freelance scientific writer, editor, and reviewer with a PhD in Physics. Nidhi has an extensive research experience in material sciences. Her research has been mainly focused on biosensing applications of thin films. During her Ph.D., she developed a noninvasive immunosensor for cortisol hormone and a paper-based biosensor for E. coli bacteria. Her works have been published in reputed journals of publishers like Elsevier and Taylor & Francis. She has also made a significant contribution to some pending patents.  

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