The two key factors influencing a product’s functional and sensory performance are formulation and processing. Formulation ingredients, the underlying microstructure, interactions between ingredients, and impact of these on physicochemical properties such as rheology, tribology and surface activity, can have a critical impact on the product’s final performance and consumer acceptability of the product. These factors can influence ease of processing and manufacturing. High stresses in processing can impact the underlying microstructure, too. This link between structure-property-processing-performance must be better understood and optimized for cosmetic and consumer products. This article will highlight some of the key processing challenges that may be encountered in the manufacturing of these complex fluids/soft matter-based consumer products.
Mixing is a key step in the formulation process. Formulating with various products that must be mixed is often a bigger challenge than it seems. Issues such as non-Newtonian behavior, viscoelasticity, yield stress and thixotropy, can all provide a very significant roadblock in the formulation process.1 A non-Newtonian fluid is a fluid whose viscosity is dependent on the amount of stress applied to the system.2 Once a fluid is classified as a non-Newtonian fluid, it can then be categorized into either a shear thinning fluid or a shear thickening fluid. A majority of cosmetic products are shear thinning fluids, meaning their viscosity decreases with increasing shear rate.2,3
A great way to picture this is to think of a skin moisturizer. Straight from the product’s container, the moisturizer is usually pretty thick. As the consumer adds stress onto the moisturizer by rubbing it into their skin, the moisturizer starts to decrease in viscosity in order to fully absorb into the skin. Non-Newtonian fluids are very complex and studying them using rheometry techniques can allow optimizing product stability. When formulating a non-Newtonian fluid, it is important to engineer rheological properties such as viscosity and yield stress to enhance product stability.3 Mixing non-Newtonian fluids can be very difficult due to the changes in viscosity. The more viscous the fluid, the harder it will be to fully incorporate into the product. This can leave unmixed portions of product in the mixing tank, resulting in a product that is not up to ideal standards. The easiest fix for this is to reassess the mixing impellers used in the mixing tank. In most cases, a bigger impeller can be used to mix more viscous materials, or even multiple smaller impellers at a higher power.
Thixotropy is defined as a property of some products which change their viscosity with time when subjected to constant shear force; e.g., in the mixing stage.4 As a thixotropic fluid is subjected to a constant force over a period of time (constant mixing), it will decrease in viscosity, making the mixing stage a tricky one. An example of a thixotropic fluid is nail polish. The viscosity of nail polish decreases as the bottle is shaken allowing easy spreading on the nail and then recovers the structure/viscosity back once its applied on the nail. The viscosity of thixotropic fluids ultimately returns back to normal after a period of time. Thixotropic fluids can cause issues in mixing because they can damage the impellers that are used to move the fluids around. The impellers need to be moving at a constant speed over a period of time in order to reach the stage at which the fluid goes from high viscosity to low viscosity. After it reaches this stage, the fluid needs to be constantly mixed at a certain power to keep the viscosity as low as possible, so it is fully incorporated. If the machinery used to mix the fluid uses too little power, the fluid will be too viscous and can ultimately damage the impellers. It is very important to understand how the fluid will act when subjected to force because it may cause a much larger problem in the end. It is therefore critical to optimize the operating conditions of the impeller based on an understanding of the thixotropic response of the fluid.
Viscoelastic fluids are a type of non-Newtonian fluid formed by a viscous component and an elastic component.2,5 A fully elastic material has a very solid-like behavior whereas a fully viscous material has a more liquid-like behavior. Viscoelastic materials are a mixture of both, exhibiting both solid and liquid behaviors. Most cosmetic and consumer products are viscoelastic fluids. When it comes to mixing, fluids with high elasticity can cause significant issues. If an impeller or blade is used that cannot withstand highly elastic characteristics, they can potentially snap and break while trying to mix the material. As noted previously, highly viscous materials can cause issues when mixing as well. Lastly, yield stress is another issue that can cause problems when mixing materials.2
Many cosmetic products such as hair conditioners or skin creams possess a yield stress; i.e., a critical stress that needs to be surpassed in order for the product to start flowing. When mixing a yield stress fluid, certain regions close to the impeller where the yield stress has been surpassed will yield and flow and fully incorporate. The region of fluid distant from the impeller will not mix and therefore form a stagnant layer of fluid in the mixing tank surrounding a small area of fluid that is mixing. The diagram below shows what a typical setup for mixing a yield stress fluid looks like.
The issue of non-uniform mixing of yield stress fluids can be overcome through using larger impellers or using multiple impellers organized in vertical direction.
Flow instabilities are another well-known issue in the cosmetics industry, which can occur during pipe flow of cosmetic and consumer products or during mixing. There are many different types of flow instabilities including wall slip, shear banding and elastic turbulence.
Wall slip is a common trait with polymers, gels or microgels, colloidal gels, pastes and foams. Wall slips cause issues in process design, pipe flow and rheological measurements. Wall slip begins with attachment points between the wall and the bulk material within the pipe. Once the material starts flowing, there are three realities: weak slip, strong slip or no slip. Strong slip occurs when a film of polymer that is covering the surface of the pipe completely disconnects from the bulk causing a change in the velocity profile. The severity of the change in the velocity profile depends on the thickness of this polymer layer on the pipe’s surface. The opposite of this is no slip where there is nothing on the surface of the pipe. Lastly, weak slip occurs when some connectivity remains as the bulk material flows through the pipe. This is determined by the chemistry between the polymer and the pipe wall, including factors such as temperature, pressure, molecular weight, molecular architecture, interfacial conditions, dependence on the surface coating, and flow of immiscible blends. A surface coating on the wall could be an easy fix to avoid this wall slip from occurring and is a more direct solution than changing the polymer. Wall slip can be quantitatively assessed via the Weissenberg Number, which is the product of the relaxation time and shear rate. A higher Weissenberg Number concludes a greater interfacial disentanglement, meaning the polymer wall interface is weaker than the entangled bulk material, giving rise to separation.
Wall slip, flow properties and yield stress are all intertwined: below yield stress flow occurs due to slip, yield stresses between the yield stress and 1.5 times that value have flow due to slip and deformation. Finally, at higher yield stress values the slip decreases and shear stress increases. Process engineers must view these as competing effects and decide the best next steps for individual products.
Another type of flow instability that occurs in some cosmetic products such as shampoos is shear banding. Here, due to microstructural heterogeneity in the product, the fluid separates into two different flow bands. These two different layers of flow then have two different flow rates, as seen in Figure 1. This shear banding behavior can be detected by plotting a flow curve showing viscosity versus shear rate or shear stress versus shear rate on a rheometer. On this flow curve, a plateau region will develop if there is shear banding in the system. This is not an infallible test but definitely a strong indication of shear banding.
Majority of cosmetic products are either oil-in-water emulsions or water-in-oil emulsions. When formulating emulsions, there are many issues that can arise. Emulsion stability is of highest importance here; a product that will phase separate on the shelf after a few weeks or months is not at all desirable. A high stability emulsion yields a longer shelf life and an overall better product. But ensuring that the emulsion itself has high stability is challenging. Processing conditions and emulsifier choice play a critical role.
During emulsion processing it is essential to choose the proper processing technique so that the desired product is obtained with good long-term stability. The decision on a high-energy or low-energy process technique should be sensitive to the droplet size, droplet breakup, volume ratio, surfactant absorption and absorption rate. During emulsion processing, droplet formation can be done via low energy or high energy processing techniques.
Low-energy processing conditions produce a smoother consistency with individual droplets but also a lower output. Higher-energy processing techniques require high shear stress, high pressure costly process. The combination of the high-energy mechanical processes and increased temperature can influence a change in the emulsification; therefore, the formulator and process engineer must choose the emulsification properties. If high-energy processing is determined to be most efficient for the chosen emulsion system, the process should be carefully monitored, as high shear can break up certain emulsifier classes such as high molecular weight polymers.
During emulsion processing, the formulator should be knowledgeable of the influence that temperature and shear have on the desired emulsifier or stabilizer. Both can degrade the polymer chain size or protein, which in hand may disrupt emulsion stability. If the flow condition is classified as turbulent due to high shear stress, polydisperse emulsions will develop. If the flow condition is classified as cavitational, there is an accelerated pressure decrease that will cause vapor bubbles and polydisperse emulsions too. Since cavitation forces have abrupt temperature and pressure conditions within the vanities, droplet breakup occurs. If the flow pattern is classified as laminar, the emulsion will have low polydispersity since it flows in a defined pattern. This laminar flow condition is found in lower-energy homogenization devices where the liquid is injected into the membrane pores or microchannels, leading to droplet formation and detachment.
Powders are essential in cosmetics such as eyeshadow, blush, bronzer and foundation. Powder are also critical in pharmaceuticals, food and beverages, agriculture, petrochemical and specialty chemicals. Powders require proper handling; otherwise processing problems can arise. Some of these difficulties may include rat holes, erratic flow, dead zones, channeling and flooding during powder transfer. Dead zones may occur during mass flow where the powder becomes compacted in the bin and acts as a solid, instead of flowing through the opening. In other instances, dead zones can occur on the perimeter of the bins while only the powder in the center falls through which results in poor mixing. This in the end disturbs the integrity of the cosmetic product.
Each of the powder processing complications that may arise are influenced by the physical properties of the desired powder whether that is the particle size, shape and interactions. If the physical property of the powder is not causing the processing disruption, then it can be due to the choice of equipment or stress state of the powder. When trying to understand the physical properties of the selected powder, the Hausner ratio can be adopted to determine its flowability. The Hausner ratio is a quick and simple test to understand the initial and final states of the powder, but it can also have errors or discrepancies depending on the operator completing the evaluation. A Hausner ratio between 1 and 1.34 is recommended, but the lower the more desirable. Another method to measure the powder’s resistance to flow is with a powder rheometer which will read its rotational and vertical resistances. This would be collected as torque and force which, when combined, represent total flow energy. Total flow energy is the energy required to move the blade through the sample from the top to the bottom of the powder column.6 The powder flow properties not only influence the powder processing but can also impact final performance such as pay off and cake strength.7
The processing challenges that come about in the cosmetic industry consist of complications with powders, flow instabilities, emulsion processing and mixing challenges. The complications with powders include rat holes, channeling, flooding, erratic flow and dead zones; each is influenced by particle size, shape and interaction. The significant flow instabilities are wall slip, shear banding, and elastic turbulence while the highlighted mixing challenges are non-Newtonian behavior, viscoelasticity, yield stress and thixotropy. Lastly, the difficulties that arise with emulsion processing is destabilization and selecting a high or low energy processing technique. All present unique challenges. They require the formulator or engineer to be well-versed in proper processing; otherwise, they lead to injury, damaged equipment or failed end product.
Tackling difficult mixing problems – AICHE. (n.d.). Retrieved December 2, 2021, from https://www.aiche.org/sites/default/files/cep/20150835.pdf.
Luigi Gentile and Samiul Amin. Chapter 11: Rheology primer for nanoparticle scientists. Colloidal Foundations of Nanoscience (Second Edition). Elsevier, 2021.
A. Davis, S.Amin. Rheology of Cosmetic Products: Surfactant Mesophases, Foams and Emulsions. July 24, 2020.
Batlle, E. O. +. (2021, August 24). What is thixotropy and how does it influence the manufacturing process? Oliver + Batlle. Retrieved December 2, 2021, from https://oliverbatlle.com/en/what-is-thixotropy/.
Pérez-Reyes, I., Vargas-Aguilar, R. O., Pérez-Vega, S. B., & Ortiz-Pérez, A. S. (2018, April). Applications of viscoelastic fluids involving hydrodynamic stability and heat transfer. IntechOpen. Retrieved December 2, 2021, from https://www.intechopen.com/chapters/60426.
Webnetism.Dynamic methodology. FT4 Powder Rheometer. Retrieved January 14, 2022, from https://www.freemantech.co.uk/powder-testing/ft4-powder-rheometer-powder-flow-tester/dynamic-methodology
X.Liu, C.Drakontis, S.Amin. Designing high performance colour cosmetics through optimization of powder flow characteristics. February 6, 2020.
Meghan Hartson is a senior at Manhattan College majoring in chemical engineering with a cosmetic engineering concentration as well as a minor in chemistry. This past year, Hartson worked in the cosmetic research lab at Manhattan College where she studied smart materials and learned how to formulate hair gels using them. She has worked in the industry for two years, first interning last year at Teawolf, a food and beverage manufacturing plant owned by Döehler, and most recently interning for Nalco Water, an Ecolab Company, as a technical sales representative intern.
Ciara Coyle is a student at Manhattan College pursuing her bachelors in chemical engineering with a concentration in cosmetics. She is on track to graduate in May 2022. Coyle is also an undergraduate research student in the cosmetic lab at Manhattan College as well as the founder of the Cosmetic Engineering and Chemist Society. Her most recent industry role was as a hair care research and development intern for Aveda at the Estée Lauder Companies, and will return full time after graduation.
Samiul Amin is currently associate professor of chemical engineering at Manhattan College. At Manhattan College Chemical Engineering Department, he leads the cosmetic and consumer product engineering concentration area. Prior to joining academia in 2018, Amin worked in industry for 20 years working across Engineering, R&D and innovation management in global multinationals such as ExxonMobil, Unilever, L’Oréal and Malvern Instruments in Asia, Europe and the US. His area of expertise is formulation design, smart formulation design through automation, sustainable cosmetic formulations and advanced characterization.