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Rheology is a branch of physics that describes the deformation and flow of matter, in particular the non-Newtonian flow of liquids and the plastic flows of solids. The study of rheological properties is important as it can affect every stage of a material’s usage across numerous industries, from formulation development and stability, to processing and product performance.
The rheologic properties of a material are determined by the composition and concentration of the ingredients it consists of, such as binders, solvents, pigments, fillers and additives. Most commonly-used materials have complicated rheological properties, the viscosity and viscoelasticity of which changes depending on the external conditions – stress, strain, temperature or timescale - being applied to it. Measurements taken with a rheometer might comprise of viscosity profiling, viscoelastic fingerprinting, assessing dispersion stability, determining thixotropy of paints and coatings, and measuring the ability of food and personal care products to spread or pump, amongst many others.
Rheology modifiers – also known as thickeners or flow control agents - are a key ingredient added to materials to control their viscosity and meet the desired rheological profile, either Newtonian, dilatant, pseudoplastic or thixotropic. The paints and coating industry account for around a third of the modifiers market, with cosmetics and personal care products, and adhesives and sealants following. The demand for rheology modifiers is ever increasing, especially in the paints and coatings industry, and in personal care products.
Without rheology modifiers paints, coatings and inks, for example, would be very runny, spatter and have a short shelf life – rendering them practically useless for their purpose. Adding a modifier improves the medium’s viscosity and application characteristics, ensuring the end product reaches the desired balance of consistency, durability and good application properties. In personal care products like creams and nail polish, rheology modifiers increase the viscosity while also giving the product the feeling of improved quality. The product might be Newtonian, thixotropic or pseudoplastic and the structure of the modifiers can affect its sensory properties as well as performance characteristics, in other words how well it is poured from the packaging, how effortlessly it rubs into the skin, or how easy it is to remove after use for example.
Modifiers can be organic – based on cellulosics, polyacrylates and polyurethanes – or inorganic, based on clays and silicas. The latter, which includes organoclays like hectorite and attapulgite, minerals such as treated and untreated silicas and metal organic gellants like zirconates are often supplied as a powder and if dispersed properly, they usually act as a suspending or gelling agent. Inorganic rheology modifiers tend to have high yield values and are considered as thixotropic. They often have a lesser usage as an extender for pigments and are sometimes added to aqueous formulations as secondary thickeners to improve anti-sag, anti-settling and anti-spattering properties, particularly when it comes to paints.
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Organic rheology modifiers are much more diverse. Modifiers might be based on natural raw materials such as cellulose, polysaccharides like carageenan and xanthan, or those deriving from castor oil or cellulosic fibres. Other are founded on synthetic organic chemistry such as polyacrylates and polyurethanes, or fibres such as aramid and polyester. Organic modifiers can be further divided into associative and non-associative modifiers.
Non-associative modifiers work on the basis of entanglement. Soluble, high molecular weight polymer chains become tangled and thicken the product via hydrodynamic thickening. The effectiveness of the modifier is controlled by the molecular weight of the polymer, and products often have limited flowability.
Associative modifiers work to thicken the product by non-specific interactions between hydrophobic (water-repelling) end groups on the modifier molecule themselves, and with other molecules in the product. The result is known as a physical network, where parts of the different molecules interact and thicken the product, thus increasing its viscosity.
There are so many rheology modifiers available and combination of various modifiers can give each product its desired rheological properties, altering everything from the way the paint adheres to the wall to how nail varnish coats a fingernail to the way a cream feels when spread on the body.
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