IFSCC MONOGRAPH Number 3

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An Introduction to Rheology

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ts,· (f 360/ET/ES OF CC)shicric j IFSCC MONOGRAPH Number 3

An Introduction to Rheology

Published on behalf of the International Federation of Societies of Cosmetic Chemists by

~~ MICELLE PRESS ~

Originally published 1997. This booklet first issued in a Print-on-Demand edition in 2011.

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission in writing of the International Federation of the Societies of Cosmetic Chemists.

A catalogue record for this book is available from the British Library

ISBN: 978-1-870228-16-9

Published by

Micelle Press 12 Ullswater Crescent, Weymouth, Dorset DT3 5HE, England http://www. micellepress.co.uk E-mail: [email protected] and

RO. Box 1519, Port Washington, NY 11050-7519, USA http:/Mww.scholium.com E-mail: [email protected]

on behalf of the International Federation of the Societies of Cosmetic Chemists IFSCC Secretariat Suite 6, Langham House East, Mill Street, Luton, Beds LU 1 2NA, England http://www. ifscc. org E-mail: enquiries@ifscc. org

Printed and bound in Great Britain by CPI Antony Rowe, Eastbourne IFSCC Benefactors

The current list of benefactors, to whom we offer our profound thanks for their continuing support ofthe IFSCC, are shown below:

Amore Pacific Corporation Iwase Cosfa Company Ltd. [Koreal [Japan] BASF [Germany] Kanebo Cosmetics Inc. [Japan] Bioland Ltd. [Korea] Kose Corporation [Japan] COESAM [Chile] Lipo Chemicals Inc. [USA] COGNIS Care Chemicals Matsumoto Trading Co. Ltd [Germany] [Japan] Cosmetics & Toiletries [USA] Nalco Company [USA] CRB [Switzerland] Nikko Chemical Company [USA] 1 Croda, Inc [USA] Plantapharm Chem [Austria] DROM [Germany] Pola Chemical Industries Inc. DSM [Switzerland] [Japan] Evonik Goldschmidt [Germany] Provital SA [Spain] Firmenich SA [Switzerland] Shiseido Co. Ltd [Japan] Greentech [France] Silab [Francel Ichimaru Pharcos Co. Ltd Symrise Inc. [USA] [Japan] T. Hasegawa Co. Ltd. [Japan] 'Induchem Ltd [Switzerland] Takasago Inc. Corp. [Japan] Interlees Corporation [Koreal Toyo Beauty Co. Ltd [Japan] ISP Europe [UK] Unilever [USA]

The following companies also make a substantial contribution to the IFSCC in supporting members of the Praesidium, and our grateful thanks also go to: Amore Pacific Corporation Mae Fah Luang University [Korea] [Thailand] Australian Photobiology Testing Nordbak [South Africa] Facility [Australia] Pertech Associates [UK] Beiersdorf AG [Germany] Shiseido Co. Ltd [Japan-] Chanel, Inc. [USA] Symrise [Spain] Gigot Cosmetics [Argentina] Tri-K Industries [USA] JW Solutions [The Netherlands] General Preface to the Series

There are many excellent, authoritative textbooks covering the different areas which comprise Cosmetic Science. However, certain topics cut across these various disciplines and are best studied individually. From the study of such a topic, a better appreciation can be achieved of the practical use of that topic in the cosmetic field. This series of IFSCC monographs is a collection of such intersecting themes. It is hoped that the knowledge gained from identifying activities common to a number of areas will be transferable when a chemist moves from project to project. This series of monographs will cover a wide range of themes compiled by experts in their fields, providing both the novice and the experienced individual with valuable reference books on the major topics of Cosmetic Science.

Monographs already published in this series are

IFSCC Monograph No. 1 : Principles of Product Evaluation: Objective Sensory Methods IFSCC Monograph No. 2 : The Fundamentals ofStability Testing IFSCC Monograph No. 3 : An Introduction to Rheology IFSCC Monograph No . 4 : Introduction to Cosmetic Emulsions IFSCC Monograph No . 5 : An Introduction to Cosmetics Micro-. biology IFSCC Monograph No. 6 : Antiperspirants and Deodorants: Principles of Underarm Technology IFSCC Monograph No. 7 : Microemulsions in Cosmetics

In a further series, the IFSCC has also published

Cosmetic Raw Material Analysis and Quality: Hydrocarbons, Glycerides, Waxes and Other Esters, edited by Hilda Butler

Cosmetic Raw Material Analysis and Quality: Analysis of Polymers for Cosmetics, by Janusz Jachowicz Foreword

The application of rheology in many different industries has been stead- ily increasing. Terms which were once reserved for only the nnost ab- stract articles are now creeping into our common vocabulary. The IFSCC has recognized the need for a practical review of this discipline with cosmetic rheology in mind. It is hoped that by promoting a more widespread understanding of the underlying principles of rheology, cosmetic science itself may be advanced. This monograph has been written and published with that in mind.

The IFSCC is grateful to Dennis Laba for preparing this monograph on rheology. Contents

Page

IFSCC Benefactors General Preface to the Series iv Foreword

1 Introduction to Rheology 1 Cosmetic rheology 1 Definitions 3 Types of flow 7

2 Using Cosmetic Rheology 13 , Formulating with rheology in mind 13 Rheology as a manufacturing tool 15 Rheological principles in stability testing 17 Predicting consumer acceptance 18

3 Rheological Additives 22

4 Instrumentation 27

5 Interpreting Rheological Profiles 31

6 Suggested Reading 35 Introduction to Rheology 1

1 Introduction to Rheology

Cosmetic rheology Rheology is the science or study of flow. Cosmetic rheology, therefore, involves the characterization or measurement of flow that takes place in cosmetic products. To the average consumer, proper cosmetic flow may hardly be noticeable. That same consumer, though, is likely to be acutely aware of the lack of an expected kind of flow. If a hand lotion does not easily flow from the bottle, something is wrong; if a lipstick does not transfer smoothly to the lips, something is wrong; if a shampoo doesn't have enough body momentarily to be held neatly in your hand, something is wrong. That 'something' is the product's rheology. During the course of product development, the cosmetic scientist must design key rheological characteristics into the product. Formu- lators must take into account the variable desired flow characteristics during processing, packaging, transport, storage, and consumer use. Developmental scientists have a responsibility to their profession, to their company, and to the consumer to ensure that the cosmetic can be reproducibly manufactured and will remain stable and usable through- out the life of the product. Of course, the cosmetic scientist is not alone in this challenge; many other industries take rheology very seriously. The paint and coatings industry, for example, is acutely aware of the required rheological characteristics of their products. Nail polish scientists share many of the same requirements, since they are also in the 'coatings' area. The formulators come face to face with rheological challenges while the product is waiting to be used, while it is being transferred from bottle to nail, and finally during application. Initially, the formulator must make sure that the pigments stay in suspension as long as possible to present a uniform color and appearance to the consumer. The product must then be 'picked up' appropriately by the brush, and stay neatly on the applicator while it travels through the air without dripping. Application properties are then held in delicate balance with the leveling properties. On the one hand, formulators must prepare a product that is apparently thick enough not to drip or run off the nail when applied, yet is thin enough to allow the brush marks to flow together, presenting a smooth, uniform surface before the film dries. All these interconnected properties are carefully balanced in the search for the perfect nail enamel system. 2 IFSCC Monograph No. 3

While it is true that each cosmetic product category is unique, many share common rheological concerns.

Table 1 Rheological concerns in cosmetic products

Cosmetic product Rheological concerns Lipstick Suspension of pigments in molten state Thermal stability during storage Even flow or payout onto the lips Mascaras Suspension of pigments Spreadability Proper viscosity for use Emulsion stability Shampoo Proper viscosity Proper flow characteristics Actives suspension in therapeutic formulas Nail enamel Suspension of pigments Proper flow and leveling Proper viscosity Creams/Lotions Proper 'body' or viscosity Emulsion stability Spreadability Extrudability Antiperspirants Suspension of the actives Flow control of the roll-on formula Thickening Spreadability Payout of the stick formula

Though pigments in lipstick are certainly different from the active ingredients used in antiperspirants, both are solids that need to be held in uniform suspension. The cosmetic scientist must look beyond the obvious differences and deal with the underlying rheological need. For- tunately, scientists now have many tools available to help them form- ulate products with the needed rheological characteristics. An arsenal of additives has been developed to provide specific properties for either Introduction to Rheology 3 water or oil systems. Some of the additives are as seemingly simple as clays, while others are complex reaction products of multiple raw materials. To keep watch over the effects of these products, specialized instrumentation has been developed, and is now used regularly in quality control and research and development (R&D) laboratories to characterize accurately the types of flow encountered. The formal science of rheology, though not a new field, has experienced a re-awakening recently in the cosmetic industry. Great strides have been made within the last twenty years, and, without the proper background or training, some formulators may feel a little out of touch with this important aspect of cosmetic science. This monograph is designed to help bridge that gap. Though its purpose is not to be an all-inclusive, aut)oritative guide to cosmetic rheology, it does explain many of the basic principles involved as they pertain to this field. Sections of this work will help cosmetic scientists to understand the types of flow they are encountering, the kinds of products and instrumentation that are available to help them, and most importantly, how to apply their knowledge of rheology to their own situations.

Definitions One of the most important steps in understanding any technical field is to learn the 'language' that the field has developed. There are a number of terms used in the field of rheology that need definition, starting with the name of the science itself. The word rheology comes from the Greek word rheo, which rneans 'flow'. A textbook definition of rheology is 'how materials deform and flow under the influence of external forces', which, simplified, means: how materials flow. The external force talked about above is usually referred to as the shear stress, which is defined as a force applied over an area. The unit of shear stress is called the pascal (Pa), which is measured in newtons/ meteri and 1 Pa = 10 dynes cm-2. To define clearly the next term, we must refer to figure 1, which shows a force (F) being applied to a layered material in which the top layer is movable, and the bottom layer is stationary. When this shear stress (or force) moves the top layer in the direction of the arrow, the second layer is dragged along with it, 6ut at a slightly slower velocity. 4 IFSCC Monograph No. 3

FORCE 7~~~ AREA ,//1\ ~~/' T~KNESS

FORCE SHEAR STRESS = AREA

VELOCITY SHEAR RATE = THICKNESS

SHEAR STRESS VISCOSITY = SHEAR RATE

Figure 1. Rheological definitions

Each successive layer is also dragged along by the one directly above it, but at an ever-decreasing velocity until the bottom stationary layer is reached. This velocity gradient, or ratio of the velocity of the material to its distance from a stationary object, is termed shear rate. The units of shear rate are (meters/second)/meter, or reciprocal seconds (sec -1). Pouring a lotion from a container typically represents a much lower shear rate value than rubbing that same lotion into the hands because, in the first case, the lotion is significantly thicker and is being 'sheared' at a slower speed. The next term is the one most closely associated with the science of rheology : viscosity. The viscosity of a material is defined as its ' resistance to flow', and is arrived at through the ratio of shear stress (force applied) to shear rate (movement). Water has a lower viscosity than honey since there is more movement with water for a given amount of force. When recording the viscosity of a product, a scientist must report the shear rate range at which the results were achieved. Without this information, reproducibility by another scientist may be difficult. In the Systtme international d'unit6s (International System of Units, abbreviated SI), the unit of viscosity is the pascal second (Pa-s) and in the centimeter-gram-second (cgs) system, the unit is the poise (P). For Introduction to Rheology 5 conversion purposes, 1 pascal second = 10 poise = 10 dyne sec cm-2. Often viscosity is expressed in centipoises (eps or cP) or millipascal seconds (mPA·s). In this case, 1 eps = 1 mPa·s, and 1000 eps = 1 Pa·s. A viscosity profile is a graph of how a material 's viscosity responds to changes in shear rate. Not all materials react to shear in the same way. The viscosity of some liquids is unaffected by shear, some de- crease in viscosity with increasing shear, and some actually increase in viscosity as the shear increases. Many cosmetic products are exposed to a wide range of shear rates, so it is important to understand how the product will react under varying conditions. Viscosity profiles are normally graphed with shear rates reported on the x-axis and viscosity reported on the y-axis. The opposite of a viscosity profile is a 'single point measurement', recorded at only one shear rate. This kind of measurement can be useful for previously defined systems but does not provide any information about shear responses. To understand the difference between a viscosity profile and a single point viscosity, examine two different materials with viscosity profiles depicted in figure 2.

~~- B

(PaS)

VISCOSITY

0 SHEAR RATE (1/sec) .~

Figure 2. Viscosity profiles 6 IFSCC Monograph No. 3

In the middle of the shear rate range, material A exhibits the same viscosity as material B, although at low shear rates, A is much more viscous. To make matters more confusing, when measured at high shear rates, material A is now lower in viscosity than material B. Without knowing the shear rate range discussed, one cannot say for sure how the viscosity of material A relates to material B. If comparisons are based on single point viscosities (measurements at only one shear rate) it may be said that material A is either higher, lower or equal in viscosity to material B. The conclusion depends on what shear rate was used for the measurement. Yield value is defined as the minimum amount of force necessary to induce flow. To visualize the effect yield value plays on our perception of viscosity, refer to table 2. While mayonnaise appears to be more viscous than honey, it actually has a much lower viscosity. The property that makes it appear thick is its high yield value. Materials with high yield values do not flow under gravitational force and do not appear to be fluid, even though they may be of a low viscosity.

Table 2 The effect of yield value on the perception of viscosity

Viscosity, mPaS Yield value, Pa Water 1 0 Honey 11 000 0 Mayonnaise 600 85

A viscometer, sometimes called a viscosimeter, is an instrument that measures viscosity. There have been many different types used over the years, ranging from simple bubble tubes to the more complex Brook- field viscometers which allow measurements at various shear rates by adjusting spindles and speeds. The Brookfield viscometer is widely usdd in the cosmetic field, and will be discussed further in the section dealing with instrumentation. The drawback of using a viscometer for all applications is that it typically gives a viscosity at a single shear rate. As discussed above, it does not automatically provide the formulator with a viscosity profile or an indication of what will happen to the system if it is exposed to very low or very high shear rates. To obtain that kind of information, a rheometer must be used. Introduction to Rheology 7

A rheometer is an instrument that can automatically adjust itself to measure viscosity over a specified shear rate range. It is designed to provide a 'shear profile', which could include information about the effects at the extremes of shear rates as they relate to viscosity or to shear stress. The versatility of the rheometer exceeds that of the viscometer since it can quickly take hundreds of readings per sample at ever- changing stresses. Most rheometers are computer-controlled, which makes it very easy to print out a viscosity profile or modify the graph to highlight the shear range in which one is most interested. All this versatility, though, does not come without a price. A rheometer is usually many times more expensive than a viscometer.

Types of flow There are a number of different types of flow responses that are encountered in the cosmetic field. If the viscosity of a system does not change in response to a change in shear, the material is said to be a Newtonian fluid. If it does exhibit a viscosity response to an increase in shear, the material is classified as a non-Newtonian fluid, and further categorized depending on its response. Definitions of the various types of flow are given below, and are depicted in figures 3 and 4.

A. Newtonian flow is the simplest type of flow, best characterized by water and simple oils. No matter what shear is applied to New- tonian fluids, their viscosity remains unchanged. If a bottle con- taining a Newtonian fluid is tipped, the liquid will flow out. The speed of flow will be determined by the viscosity of the fluid, but even highly viscous products will eventually flow.

B. Non-Newtonian flow encompasses many different types of fluids that exhibit reactions to changes in shear.

Plastic flow is defined as the flow of materials that exhibit a yield value and show a drop in viscosity with increasing shear rate when the yield point is exceeded. It is important to note that until enough force is applied to exceed that yield point, the material does not flow. Mayonnaise exhibits plastic flow since it seems to be a solid when at rest (due to its high yield value), yet is easily stirred or spread when sheared. When the shear is taken 8 IFSCC Monograph No. 3

=- PLASTIC

0 0 NEWTONIAN 5

DILATANT PSEUDOPLASTIC 0 SHEAR RATE (1/sec) m.~

m~-

(PaS)

THIXOTROPY

VISCOSITY

0 SHEAR RATE (1/sec) =~I.

Figure 3. Viscosity vs. shear rate

Introduction to Rheology 9

(Pa)

STRESS

SHEAR

0 SHEAR RATE (1/sec) »~

(Pa)

THIXOTROPY

STRESS

SHEAR

0 SHEAR RATE (1/sec) =4Ii

Figure 4. Shear stress vs. shear rate 10 IFSCC Monograph No. 3

away, plastic materials immediately revert to their original, solid- like state. Pseudoplastic flow is similar to plastic flow, except that it does not include a yield point. Pseudoplastic materials show a drop in viscosity in response to an increase in shear, starting to 'shear- thin' as soon as a shearing force is applied. As above, the material immediately reverts to its original viscosity as soon as the shear is taken away. Many colloidal systems are pseudo-plastic. Dilatant flow is best characterized by quicksand. As a force is applied to a dilatant system, the material's response is an immediate increase in viscosity. In quicksand, this viscosity response makes it harder to escape if you try to move through it quickly. Usually, the only time cosmetic chemists encounter a dilatant material is when working with heavily pigmented con- centrates. When there is just enough liquid to wet out the solids, any movement through the fluid will cause an increase in particle interactions since the particles cannot move out of each other's way fast enough, and show a dramatic increase in viscosity or resistance. When the shear is taken away, the material flows easily once again. The process engineering department must be made aware of dilatant materials since high-speed mechanical pumping of these materials could be disastrous. Thixotropic flow is different from other types of flow because the recovery of the original viscosity is time-dependent. Thixotropic materials start out pseudoplastic, exhibiting a shear-thinning effect. When the shear is removed or reduced, thixotropes re- cover their original viscosity slowly, over a period of time. On a rheological profile of a thixotrope there are two curves, one describing the viscosity effect of increasing the shear rate, and one describing the recovery of the viscosity as the shear rate is brought back to zero. The area between the two curves, called the hysteresis loop, can be used as a measure of the degree of thixotropy. In general, the larger the hysteresis loop, the more thixotropic behavior a material exhibits. Ketchup is an example of a familiar thixotropic material. When the container has been sitting undisturbed, the ketchup is hard to pour since its viscosity is high; but if shaken or otherwise sheared, it will flow out more easily. Letting it rest undisturbed Introduction to Rheology 11

for a period of time allows it to build back up to its original high viscosity. Thixotropic effects allow a cosmetic formulator to take advantage of higher viscosities when a material is not being used, and still have the reduced viscosity or easy flow once it is sheared during use. Clay additives tend to make water systems thixotropic and, when used in emulsions, can cause pseudoplastic materials to take on more of a thixotropic character.

Many materials do not respond in a simple manner to changes in shear rates and cannot be fully defined by a single type of flow. To account for this non-ideal nature of most materials, rheologists study how the elastic and viscous components of systems interact, and call the resultant material viscoelastic. When a material is ideally viscous, all external forces acting on it are lost after the deformation takes place. Conversely, when materials are ideally elastic, any deformation forces acting on them are fully stored and fully released once the outside force is removed. The concept of a spring can be used to visualize an elastic system. When an outside force is applied, the energy is stored within, , until the external force is removed and all the stored energy is released. Dynamic, or kinetic, measurements are necessary to characterize - more fully the viscoelasticity of a material. This is most commonly : done through oscillation studies. These studies can take two forms. holding the oscillation frequency constant and increasing the strain amplitude (called a strain sweep curve) or holding the strain constant and increasing the oscillation frequency (called an oscillation sweep curve). Some rheometers will display a strain sweep curve with the elastic component as G' (called the storage modulus) and the viscous component as G" (called the loss modulus). In a completely elastic system, the stress curve during an oscillation study is fully synchronous with the strain curve, while in a completely viscous system the strain and stress curves are 90° out of phase. There- fore, the closer the phase angle of a material is to 0°, the more elastic- like it behaves, and the closer the phase angle is to 90° (where the stress/strain curves are completely out of phase) the more viscous-like it behaves. 12 IFSCC Monograph No. 3

1 Cyde > 1- 05 \-/'~ 900 < 3 , Elastic Behavior Z

TIME

Viscous Behavior Z

TIME

Figure 5. Dynamic elastic and viscous behavior Introduction to Rheology 13

2 Using Cosmetic Rheology

Formulating with rheology in mind Cosmetic scientists must keep in mind that they are working on rheologically active systems. Since all materials flow, all products have a rheological component. This flow may be obvious to a creams and lotions chemist, but even a lipstick goes through a rheological state when the mass is molten, and suspension of the pigments is a concern. To address these kinds of concerns most effectively, the formulating scientist must be aware of the various types of rheological additives available on the market today. As will be discussed later, there are many different types of additives, some for water systems and some for oil systems, but without an understanding of what they can do, they can be of no help at all. In addition to what we normally think of as 'rheological additives', many standard cosmetic additives can have signifi- cant rheological effects. Salt itself is used as the thickener of choice for many shampoo formulations due to its special properties when used in combination with certain surfactants and its cost effectiveness in these systems. Even the suspension of solid particles can be improved by something as easy as decreasing the particle size. Not all rheological effects must come from specific rheological additives. Though some cosmetic formulating seems more like an art than a science, there are many places where science is obviously taking over. One such place is in emulsion technology. There has been a large number of articles written about this fascinating part of cosmetics, and many of them contain information about the flow and stability characteristics encountered. Emulsions are intimate mixtures of two (or more) im- miscible liquids. These two liquids want to be as separate as they can, which leads to separation of the two phases. The formulator knows this, and tries to keep these phases from separating by making sure the surfactant blend and level are correct for that particular system. Proper stability can be achieved through chemistry in most cases. There are some cases, though, that seem to need more than just the right chemistry. This is when the formulator must know what is happening rheologically to his system and how to control it. During phase separation there is movement (or flow) occurring, allowing the internal or dispersed phase to pass through the external or continuous phase. One rheologically based answer to phase separation would be to increase the viscosity of the external phase, which might still allow movement 14 IFSCC Monograph No. 3 through this phase, but at a slower rate. Another choice would be to include a rheological additive with a high yield point in the formulation, making the movement possible only if enough energy were present to overcome the resistance to flow. This kind of movement is usually much more difficult for the internal phase, leading to a more stable system. It is important to know the kind of response wanted from your pro- duet during all phases of manufacturing, storage and use. In designing a shampoo, for example, it will need to have some degree of viscosity, but not too much. It is up to the formulator to decide how much is too much. It may be sufficient for formulating purposes to judge visually how a shampoo flows when poured, and how easily it remains in the hand, but those visual characteristics are not easily relayed to the quality control (QC) technicians who have the responsibility of approving or rejecting a batch. For that part of the product's life cycle, it would be better to rely on an instrumental check to determine the acceptance of the viscosity. The QC technicians will be looking to you, as the formulator, to set an acceptable viscosity range. Once it has been set, you should be aware of what effects each of your ingredients will have on the in-process and final viscosity. Before it goes into production, ask yourself, What will happen if manufacturing accidentally uses too much or too little of any ingredient? Are there ways of correct- ing these mistakes? Thinking about these kinds of questions beforehand can help highlight potential trouble spots in manufacturing, and can save valuable time when the product is being produced. There is an advantage in knowing how a product holds up under various shear conditions. Simple mixing is a low shear process and is very gentle on the system; even fragile encapsulated products can usually stand up to propeller-type mixers. Other mixers, with other con- figurations and speeds, may be an entirely different story. A shear- sensitive product, exposed to a high shear process like a colloid mill, homogenizer, or triple-roller mill, may irreversibly break down. For example, a product relying on a long-chain polymer for its viscosity could very well thin down when it is sheared, yet another product using a clay-based additive for viscosity build may, in fact, gain additional viscosity upon shearing. The basis for rheological additive choice should not be made on one property alone: each additive can affect many of the final product's characteristics and should be chosen carefully. Introduction to Rheology 15

In addition to understanding the shear rates of processing conditions, one must keep in mind the differences in shear rates used in delivery or application of the final product. Pouring a lotion from a bottle is a very gentle action in comparison to spraying an aerosol or applying a lipstick. The shear rates associated with the last two actions can be several orders of magnitude higher than the first. When a thixotropic material is aerosolized, it responds by thinning down, and taking some amount of time before regaining its original viscosity. To compare an aerosolized product's application characteristics to its unaerosolized ' version would not give an accurate comparison.

Rheology as a manufacturing tool

The manufacturing and process development departments must not only understand the rheological phases the product passes through, but also be able to control the rheological character of the formulation and use it to their own advantage. Processing departments must overcome the challenges associated with scale-up and manufacture of large quantities. This often means that if a batch becomes too thick, specialized equipment has to be used to mix it adequately. In another case, if the batch exhibits a high yield value in the mixing kettle, it may be necessary to use side scrapers in the kettle to empty it adequately during the filling operation to avoid the excessive build-up on the walls of the vessel. In ' order to be ready to handle these potential problems, the company may have to study the ingredients to understand how they interact, or find out exactly when they interact. It is at this point that the formulator's knowledge of the raw materials can be of great assistance. The formulator may be able to suggest ways to postpone building viscosity until ·later in the manufacturing process, making it easier to handle. - In the manufacture of emulsions, it may be possible to keep the temperature of the emulsion high until all the ingredients have been added. This may help to avoid inadequate mixing at lower temperatures where some temperature-sensitive bodying agents may start to build their viscosity. If the product contains a polymer that needs neutral- ' ization with a base before it thickens, the formulator could suggest adding the neutralizing agent as late as possible, to keep the batch fluid for easy mixing. Mixing in a water-soluble polymer may, in fact, be easier if it can first be dispersed in a non-aqueous ingredient, then added to the water phase. 16 IFSCC Monograph No. 3

If a non-aqueous product needs rheological control, there are other choices of rheological additives available. Waxes, of course, can be used, but they do not give, rheologically, more than a viscosity increase. Two materials that have found use as rheological additives are fumed silicas and organoclays. The mechanism of thickening behind fumed silicas is long-chain entanglement. Though the process is very handy in many cases, formulators should be aware that this particular thickening mechanism makes this kind of product unstable against any prolonged shear, which may break down the long chains. If the system must be sheared, the shearing must happen before the addition of the fumed silica. In the case of the organoclays, there are also a few rheological tricks that process formulators should be aware of. Since this is the type of product that needs shear and polar activation before a viscosity increase is seen, it is possible to hold off these steps until one is ready to handle a thickened product. In many cases, it is possible to apply the high shear just before the filling stage. This provides more time at low viscosity so that the mixing is simplified and more effective. It also means that any shear-sensitive products will have to be kept back until the viscosity is developed. Often, a formulator will be surprised at how many in-process viscosity checks a processing department will run. While preparing the batch in the laboratory, it does not seem necessary to be overly con- cerned since it seems to be an easily made product. In the world of process development, though, it is known that what appears to flow easily in a one-liter batch may not show acceptable flow at 1000 liters. Here, process developers must anticipate whether the product can be gravity-fed or if it needs mechanical assistance. Will the product cling excessively to the sides of the vessels or will it flow out nicely with a minimum amount of loss? As it is being pumped from one place to another, will it significantly increase in viscosity owing to the shearing action of the pump and possibly cause a blockage in the equipment? What will happen to the product if it is oversheared or overheated? What will happen if the batch must be held in the vessel overnight? Is it better to keep mixing it, or will that cause a viscosity change? These are all questions that should be answered before the situations arise. If they are not, a considerable amount of product and time could be lost if those situations arose in the middle of a production run. Once the product has been made, the work is far from over. The product can still undergo changes and must be tested for stability by the quality assurance department. Introduction to Rheology 17

Rheological principles in stability testing Even before the formulating scientist passes the product to a process development department, there should be a fairly complete picture of the product's stability profile. The profile should contain tests that will predict how the finished product might change when it encounters certain physical stresses such as aging, thermal stress, freeze-thaw cycling or vibrations. Predictive stress-testing is a very important part of a product's portfolio. To be truly predictive, the tests should mimic the real-life situations as closely as possible. The best indicator of what a product will be like two years from now is an actual sample of the product that is two years old. Since a two-year wait would drastically slow down the formulator's timetable, methods for quick stability tests are used. The most commonly used way to speed up that process is to subject the U product to high temperatures. The Arrhenius equation tells us that in . general for each 10°C rise in temperature, a chemical reaction rate doubles. If a chemical reaction is affecting rheological characteristics, elevated temperature testing may prove valuable. If a product can with- stand 45°C for 3 months, it should be able to hold up at room temperature for 2 years. A viscosity measurement will be taken initially, and the product is then split into a number of containers: a 5°C control, a room temperature (25°C) sample, a 37°C sample and a 50°C sample. Peri- odically, the samples are taken off stability testing and checked after equilibrating to room temperature. Changes in viscosity or phase separation can be noted and, when plotted, will show a trend which will be interpreted by the formulator. Temperature can play a critical role in the apparent viscosity of a product. In order to be reproducible, viscosity testing should be carried out under identical conditions each time. If the sample to be tested was held at a certain temperature for stability purposes, it should be allowed to equilibrate before the viscosity determination. The internal temperature of the product should be checked and recorded with the test results. Knowing the shear profile of a product can aid in understanding some of the variables which it will be necessary to control in your testing procedure. If the product shows non-Newtonian flow, then the shear history of the product may affect the viscosity. In other words, if you start with two identical samples, but shear one product more than another, you will often see different viscosity values for the products. If a sample must be shaken or stirred before testing, it must be done in the 18 IFSCC Monograph No. 3 same way each time or false viscosity readings could be recorded. Even the time interval between the shearing of the product and the testing should be kept uniform if reproducible results are expected. Vibrational testing can be carried out to simulate the actual shipping of materials by truck or rail. When products are shipped, they encounter vibrations which could lead to creaming in emulsions, flocculation in pigmented products, and a viscosity breakdown or settling of solids in products like antiperspirants. In the case of solids, the vibration could cause a hard cake to form on the bottom of the container, which may be much harder to resuspend than a routine settling test may predict. Placing samples in a car's trunk is a simple and inexpensive 'vibration' test that could indicate how rheologically sound a product would be when shipped by truck. Often, shipping is a combination of both thermal and vibration stress, and the combination may be more detri- mental than when each stress is tested individually. Centrifuge testing is often proposed to test the stability of products. When it is used, the formulator must be careful in interpreting the results. There are many products that contain rheological additives which impart a yield value to the system. This yield value may help to hold the system together by stabilizing the emulsion or suspending the solids. If the force exerted on the system by the centrifuge exceeds the yield point, separation or sedimentation could occur. Since the majority of cosmetic products will never encounter a gravitational situation higher than 1 G, this test could lead to false negative results; you cannot state that a product which failed the centrifuge test will necessarily fail under normal conditions. On the other hand, some people look at products that pass centrifuge testing as products less likely to cream or separate than failed products.

Predicting consumer acceptance Many of the terms used to describe a product's aesthetics are actually terms that characterize the flow or rheological properties of the system. Words such as 'tacky' or 'sticky' are normally associated with products that exhibit high viscosity under high shear conditions, whereas 'non- tackiness' could be associated with low viscosity under low shear conditions. 'Smooth', 'silky', and 'creamy' are also terms that are related to the flow characteristics of the product. In figure 6, some of the actions that cosmetics undergo are placed on a graph in the approximate area that depicts their shear rate and viscosity factors. Here, it is easy to Introduction to Rheology 19

see the importance that rheology plays in product design. Judicious use and selection of additives with known rheological effects can help a formulator to achieve the desired properties. In the not too distant past, this selection was based only on the formulator's own 'instinct' and training, instead of any shear profiles which may have been available. Today, much more information is available about the rheological character of the raw materials used in cosmetics, taking some of the guess- work out of formulating.

SUSPENSION

EXTRUSION ...4 SAGGING

LEVEUNG

POURING VISCOSITY

SPREADABILITY

SPRAYING

2 10 -4 10 4 1 10 10 4

SHEAR RATE (1/sec)

Figure 6. Viscosity vs. shear rate of common cosmetic actions

' By obtaining samples of commercially successful products and hav- ing rheological profiles run on them, a formulator can get a better understanding of what kinds of properties lead to consumer acceptability. During the examination of the curves, formulators might ask them- selves the following: 20 IFSCC Monograph No. 3

How does the material react under stress? Is it Newtonian or non-Newtonian? Might there be a tackiness associated with the product? Does it retain some of its viscosity even under the high shear conditions of rub-in, or does it thin down considerably? If the viscosity breaks down with shear, does it eventually build back up? How long will that take? If the product is used in a very hot climate, will it appear thinner to the consumer there than to one in a colder climate? How will this product flow out of the bottle? If the consumer shakes the bottle first, will the product run out quickly or will it retain enough body still to flow out as expected? This kind of questioning can help to give an overview to the formulator and may save time during the formulation process. A number of articles have been written about correlating rheological profiles with consumer acceptance. Most of them have been

BOHLIN RHEOMETEA SYSTEM Vlscometry test

CREAM

kPas

0.1-

Viscoalty LOTION ~NX

0.01

0.65 0.1 0.'2 0.5 'i '2 Shear rate t/s

Figure 7. Rheological profiles of a typical cream and lotion Introduction to Rheology 21

written in retrospect; first the evaluation of the lotion or cream, then the explanation of how the curves fit the results. One thing to remember is that a study of the curves can indicate in which direction to go in the 'fine tuning' of the formula. Often, formulations are prepared and, i f the initial evaluation is favourable, are not modified significantly. By understanding the rheological profile and how the ingredients affect it, one may be able to show that there is an optimum amount of a certain emol- lient, or a better ratio of oil phase to water phase for desired effects. With reference to figure 7, the rheogram of a lotion should show a high viscosity at low shear rate ranges where appearance and pouring characteristics take place, and low viscosity at high shear rate ranges, where spreadability properties register. If the reverse were true, the , consumer would be faced with a low viscosity product when pouring it out of the bottle, which could easily lead to splashing or running uncon- trollably. Then, when the product was rubbed in, a high viscosity value at high shear rate ranges would make the product feel extremely thick and hard to rub in. Consumers have come to expect a certain kind of rheology within their products, and, if it is different, it is immediately noticed. Different is not necessarily bad: there are many 'unusual' products on the market that have become great successes. A cream product can also be characterized by its rheogram. Very high viscosity values at low shear conditions indicate a 'richer-looking', more bodied product than a lotion, and higher 'high shear' values indicate that a thicker film of product will be left behind during application. In facial masks, a shear-thinning rheology is needed for ease of application, yet a fairly fast recovery must happen if sagging of the formulation is to be avoided. In this case, the cosmetic chemist should look for either a thixotropic system or a pseudoplastic one. The study of rheology is not strictly the domain of the formulators; analytical chemists and engineers must also be knowledgeable of it. Rheological profiles have been used as analytical tools to determine the inner workings of emulsions. In one instance, temperature-controlled rheometers and photomicrographs were used to help characterize the temperature-dependence of the emulsion-stabilizing effect of cetyl palmitate in oil-in-water creams. (See under Suggested Reading, Pena, et al.) As more cosmetic scientists start embracing rheology, they will begin to use it in ways not yet dreamed of. The versatility of the science itself will open doors to a better understanding of the overall field. 22 IFSCC Monograph No. 3

3 Rheological Additives

There is a large choice of additives today that are capable of changing significantly the rheological properties of cosmetic systems. One of the hardest jobs for a formulator, faced with so many choices, is keeping an open mind about the use of the best additive for any particular system. It is very tempting to become familiar with only a few of the additive choices, then force them to try to work in every situation, but many times that can lead to settling for a result that falls short of what was really needed. To stay current, one must be willing continually to read the trade journals and the literature from new suppliers, as well as to attend conferences and meetings at which new ingredients are discussed. As in many fields, you don't have to know everything, but it helps a great deal if you know who to talk to, or how to find the needed information. To discuss more easily the vast array of rheological additives available, they will be split into two groups: additives for water-based systems, and additives for oil-based systems. However, flexibility must be maintained even in this simple set-up, since many cosmetic products are based on emulsion technology, which uses both oil and water phases. In the case of emulsions, it is possible to use both kinds of additives in a single system to achieve a unique effect. Table 3 shows a general breakdown of the major types of products that are commonly used to change the flow characteristics of water and oil systems. There are many different products in each category, but it goes beyond the scope of this monograph to give more than a brief characterization of the general categories. Other reference books go into great detail to describe the individual ingredients and their properties, and the reader is particularly referred to Chapter 4 of Rheological Pro- perties of Cosmetics and Toiletries, listed in the Suggested Reading section (p. 35). Items like waxes and salt, though used for their thickening abilities in some formulations, are not usually characterized as rheological additives. Each wax certainly has its own unique properties and, when carefully blended, can give a vast array of effects. Salt is also very versatile and is used as a thickening agent in some shampoo formulas, thanks to its special interaction with the surfactant system. Introduction to Rheology 23

Table 3 Major groups of rheological additives

Rheological additives for: Water-based systems Oil-based systems Gums Organoclays Clays Polyethylenes Cellulosics Trihydroxystearin Al/Mg hydroxide stearate Polyethylene glycols Polymers Silicas

The first category of additives for water-based systems contains many different kinds of gums. These products are natural derivatives from plants or microbial fermentation. This group contains products such as guar gum, xanthan gum, carrageenan and karaya. Within the gum group, one can find thixotropy, elasticity, viscoelasticity or pseudoplasticity, depending on which gum is used and how it is used. Carrageenan, for example, is derived from seaweed and can exist in three separate forms, kappa, iota or lambda carrageenans. The iota forrn has a yield value in water, while the kappa form does not. Formulators should be aware of the 'long' rheology or stringiness associated with some gums, and that there are often synergistic effects between gums and other raw materials. For example, the addition of magnesium aluminum silicate to a xanthan gum enhances suspension ability, while the addition of guar boosts the viscosity profile. Natural gums are not used as much today as they were in the past because of the inherent varia- bility found with these naturally derived products, and the development of new rheological additives. Clay products are also used to modify the flow properties of water. Clays are very small particles of hydrous aluminum or magnesium silicates. As with the gums, these natural products are also available in a number of varieties: bentonites (aluminum-based), hectorites (magnesium-based), and magnesium aluminum silicates. Some definite differences exist between the clay types, but not in the overall rheology achieved. All three groups swell in water and provide a thixotropic gel, but the viscosity profiles will be different. Hectorites tend to build high- 24 IFSCC Monograph No. 3 er viscosities than the other two since the smaller particle size allows for more bonding structure to build up between the platelets or particles. Of course, there are other characteristics of each clay that must be taken into account by the formulator before deciding which one will do the best job in a particular system. Another product group for water-based systems is the cellulosics. These materials are all initially derived from cellulose, a water-insol- uble polysaccharide. Cellulose derivatives have played a large part in the development of cosmetics. Some of the familiar product names include carboxymethylcellulose, hydroxyethylcellulose, methylcellulose and hydroxypropylcellulose. All these derivatives tend to provide pseudoplastic rheology, but will differ in their abilities to gel alcohols or other organic solvents, as well as in their electrolyte tolerance. The first group of additives for oil-based systems is called 'organoclays', since such compounds are composed of an organic quaternary compound attached to an inorganic clay. The quaternary compound con- fers oil compatibility on a previously hydrophilic clay base. This allows the rheologically active hydrophilic clay to affect the rheology of oil systems by building a hydrogen-bonding structure between platelets of clay. Typically, the rheology is thixotropic and provides a yield value to help the suspension of pigments or active ingredients. Formulators should be aware that there are different kinds of organoclays, some based on hectorite and some on bentonite, as well as the different quaternary compounds used for compatibility with solvents and oils of different polarities. If an inappropriate organoclay is used, the rheological effects will not be optimally achieved. The polarity of the quaternary can be used as an indicator for oil or solvent compatibility. Highly polar quaternaries tend to make organoclays which work better in highly polar oils, and organoclays made with low polarity quaternaries tend to work better in low polarity oils. Polyethylenes are used to increase the melting point of non-aqueous formulations and can improve water resistance. Mineral oils and other aliphatic solvents can also be gelled by these polymers and copolymers of ethylene. Some of the materials need high incorporation temperatures, and formulators must keep in mind that the cooling procedures can affect the appearance of the final gel. If handled according to the suppliers' recommendations, polyethylenes can provide a stabilizing influence in formulations, as well as being a viscosity-increasing agent. Trihydroxystearin can be used to thicken and suspend solids in non- aqueous systems. This product is derived from castor oil, and needs Introduction to Rheology 25

both shear and heat for full activation. When it is used in lipsticks, it acts as a thixotropic bodying agent and imparts water repellency. In water-in-oil emulsions, it can be used in the outer phase to increase the stability of the formulation by imparting a yield value to the system. Aluminum magnesium hydroxide stearates can be used to gel a number of cosmetic oils and provide suspending properties. As with organoclays and polyethylenes, elevated temperature stability is also imparted to the oil systems. Polyethylene glycols come in a variety of molecular weights. The higher molecular weight versions are used as water-soluble thickening agents in aqueous systems and add lubricity to the formula. They are also used in non-aqueous stick formulations to impart both structure and some water solubility. Synthetic polymers are now available in many types, and have given formulators a chance to work with man-made materials that are very effective gel formers at low concentrations. The earliest products were + alkali-swellable homopolymers of acrylic acid which delivered highly pseudoplastic rheology. Today, there are many modifications of that · early theme, involving copolymers of other materials, and cross-poly- , mers which utilize cross-linking agents to hold together two different - kinds of monomers. Some of the modifications are even allowing formulators to produce stable emulsions by minimizing or, in some cases, - I eliminating traditional emulsifiers. Through the use of appropriate neutralizing agents, some synthetic polymers form gels in low molecular weight alcohols and solvents. In general, the rheology achieved by this product group is pseudoplastic. Formulators should bear in mind that the rheology from these products can be affected by electrolytes, pH and shear. In most cases, formulations that use these polymers tend to stay between pH 5 and 8. Silicas, or synthetic silicon dioxides, can be used to increase viscosity, suspend solids, or build thixotropic character into either water- based or oil-based formulations. They build structure in a system 1 through a network of long chains and hydrogen bonds. The pH of aqueous systems determines the effectiveness of the viscosity-building properties. In water, the effectiveness decreases dramatically above a pH of 7·5, which is the isoelectric point. Though silica gels are thixotropic, re- building their viscosity after shear is applied, they can also be destroyed through the use of prolonged shear. This problem can be avoided through proper planning of the processing steps and avoidance of prolonged shear after addition of the silica. 26 IFSCC Monograph No. 3

To use properly the available rheological additives, there are a number of important variables that must be thought about ahead of time. Items like pH, processing temperature, ideal viscosity, shear levels, clarity, odor, lubricity, incompatibilities and synergies, all have to be decided upon in advance if the choice of additives is to be successful. Introduction to Rheology 27

4 Instrumentation

Over the years, scientists have used many types of instruments to help them express the rheological character of their products. The earliest type of viscometer must have been the combination of the human hand and eye. People will still feel a product, or pour it out of a bottle to make a judgment as to its viscosity or flow characteristics. To judge a viscosity more objectively, timed flow was introduced. If the product to be measured was clear, a comparison to a bubble viscometer could be made. This early instrument consisted of a set of various oils of different viscosities in tubes. When turned upside down, the air bubbles left in each one would take a different amount of time to reach the top. By placing a sample in a similar tube and running it alongside the set, one could get an objective, relative idea of how viscous the product was. Other early viscometers still in use measure the time it takes for a certain volume of material to run out of a cup with a specific-sized hole in the bottom. These Ford or Zahn flow cups are handy for opaque liquids which are of fairly low viscosity. Another type of measurement still made is done by timing the fall of a standard ball or rod through a cylinder filled with the material in question. If necessary, the weight or size of the falling ball or rod could be changed to accommodate very different viscosity fluids. One of the major disadvantages of the falling ball or rod viscometers is that the material tested must be clear enough for the analyst to follow the falling object. A capillary viscometer is used for very thin or dilute systems. In this apparatus, the fluid flows through a thin capillary and the time it takes is compared to the time it takes a certain volume of a standard fluid (like water) to flow through a similar capillary. Another instrument still used to give an indication of viscosity is commonly called a penetrometer. This is a handy device to characterize the thickness or body of very viscous materials. It consists of a free- falling cone suspended over the test material. Once activated, the cone falls into the material to a certain depth, which is measured in milli- meters. The cones are available in various sizes, shapes and weights, depending on the material being tested. Penetrometers are used to measure the consistency of materials from light creams to crayons. Rotational viscometers rotate a paddle or spindle through materials and measure the resulting torque or resistance. There are principally three types of rotational viscometers: Stormer, ICI, and the Brookfield. To use a Stormer viscometer, the analyst adds weights to a pulley 28 IFSCC Monograph No. 3 system which powers a rotating paddle. The viscosity is related to the amount of weight it takes to cause the paddle to rotate at a certain speed within the material. The Stormer viscometer uses this principle to measure low shear rates while the ICI viscometer, which is more meehan- ized and automated, is used to measure high rates of shear. The Brookfield viscometer is the most widely used instrument of its kind in the cosmetic field. It is a rotational viscometer that has both adjustable speeds and spindles, giving it more flexibility to handle widely varying materials. The instrument itself comes in several different models for high, medium or low viscosity measurements. When using the Brookfield to measure a viscosity, the analyst must be careful to record the model, spindle and speed that was used for the measurement, since the scale reading will have to be multiplied by a 'factor' which is dependent on all three variables. Once the multiplication is done, the final answer is given in 'centipoises'. If thick creams or ointments are tested on a rotational viscometer, a cavity can be formed inside the test material as the spindle rotates. To avoid this phenomenon, an attachment can be added to the Brookfield, called a helipath. This device very slowly raises or lowers the spindle. For highly viscous materials, special spindles have also been designed, which resemble an inverted letter 'T'. When the helipath is activated, and the special 'T-bar' spindles are used, the viscometer slowly lowers the spindle into the material while the spindle is slowly spinning around. This set-up cuts a 'helix' into the product, and allows the T-bar constantly to measure the resistance of new, unsheared material. In viscous materials, multiple readings are usually taken as the spindle descends, then they are averaged to minimize any effects caused by air entrapment. Because of its reliability and ease of use, the Brookfield viscometer has become the standard tool of viscosity measurements in many R&D and QC laboratories. As important as knowing what this viscometer can do, though, is knowing its limitations. Though the Brookfield company makes more sophisticated rheometers, the majority of cosmetic chemists still use the basic RVT and RVF models, which measure the viscosity only at one shear rate at a time. The problem with this is that not enough information is supplied to tell all that should be known if one is really looking to study a material's rheological profile. To be able fully i to understand and characterize a material, multiple tests must be run. This is usually more important for R&D than it is for a QC laboratory, since the testing in QC is usually done on well-defined systems, and Introduction to Rheology 29

single point measurements may have been found to be adequate for routine testing. To assist further in the characterization of materials, more sensitive and more powerful rheometers are available. One type is the concentric cylinder rheometer, which has a cylindrical spindle within an outer cylindrical cup. The test material is put in the small space between the faces of the two cylinders, and one of the cylinders is rotated. In the Searle system, the inner cylinder rotates and is connected to a sensor device, while the outer one remains stationary. The opposite arrange- ment exists in the Couette system: the inner cylinder is stationary while the outer cylindrical cup rotates and has the sensoring device. These kinds of rheometers are usually computer-controlled and automatically run through a long string of measurements, giving a rheological profile from very low to very high shear rates. Another sensitive type of rheometer system is based on the cone and plate configuration. A cone is situated point down on a smooth surface, with a very small angle between the cone and plate. The test material lies between the two surfaces and, as in the concentric cylinder set-up, either the cone rotates or the plate rotates. The advantages of this system is that it can handle very small sample sizes (0·2 to 5 ml) and can usually measure very viscous materials. The disadvantages include fast evaporation and the potential for some of the sample to be thrown out of the small gap when high rotational speeds are reached. For some measurements, such as viscoelasticity, the rotation that takes place in some rheometers could disrupt the internal structure of the test material before enough information is extracted. To avoid this occurrence, one can use an oscillatory rheometer which imparts an al- most imperceptible oscillating movement. The test material then under- goes a strain without an immediate breakage of its structure. The oscillation frequency or strain can be gradually increased until the measurement is done, or the material exceeds its viscoelastic limit. Many of the more technically advanced rheometer systems include an oscillatory mode as well as a rotational mode. Measuring rheological properties at extremely low shear rates takes a special kind of rheometer. Suspension, settling, and yield values are all phenomena that take place under very low shear conditions. Control- led-stress rheometers are specifically designed to apply a very small amount of stress, and slowly increase it while measuring the resultant strain. By gradually stepping up the stress level, the point at which the structure breaks down can be found. This point would be an indication 30 IFSCC Monograph No. 3 of the material's yield value. On other machines not as sensitive in the low shear rate range as the controlled-stress rheometer, the yield values must be extrapolated from the curves and are prone to inaccuracy. Testing materials on the wrong type of instrument will obviously give misleading answers. If formulators want to know something about the suspension strength of a liquid, they should pick an instrument with accuracy in as low a shear rate range as possible. If they want to know how a nail polish will hold up under the shear applied during applica- tien, then the product should be tested under high shear conditions. If a quick viscosity test is needed to ensure that every batch of a manufactured lotion will pour out of the bottle in the same way, then a Brookfield viscometer will do the trick in the mid shear rate range. A formulator should be comfortable enough with rheological concepts to know that the range of shear rates encountered in cosmetics can vary dramatically (from 10-6 to 106 sec-1). Though there is sometimes a considerable amount of overlap, in general, oscillatory and controlled-stress rheometers handle the low end, cone and plate rheometers and concentric cylinder rheometers handle the high end, while rotational viscometers handle the middle. Introduction to Rheology 31

5 Interpreting Rheological Profiles

Initially, the interpretation of rheological profiles can be an intimidating experience. Not only do different instruments present their data in different ways, but the appearance of the final graph can be greatly man- ipulated by the whims of the creator, thanks to the power of user- friendly software packages. It is important not to lose sight of what it is that the profile is trying to depict and exactly what it is saying. To do this, one must critically examine each part of the profile before it can be truly compared to other profiles and understood. To help in this examination, we will look at each important section of a typical profile individually. In the following description, the letters refer to the regions designated in figure 8.

A. Often, the type of rheometer system used for the profile will be shown on the profile itself. If it is, this may give you an idea of the reliable shear rate range when you examine the x-axis. B. Always check the label to be sure what test the rheometer system was running. It may be misleading to assume that tests are at- ways for viscosity: many systems can run both viscosity and oscillation profiles. C. The designed flexibility in today's instruments means that there is a variety of exchangeable parts. Hopefully, the originator of the curve has already used the appropriate hardware for this profile. The interpreter of the graph may be interested in knowing what hardware was used in order to compare one graph directly with another. To do this reliably, the sensing systems should be the same or at least similar. Just because two graphs were made on the same machine does not mean that, by superimposing one on the other, you can determine the true differences. There will be times when a rheologist will use multiple-sensing systems on the same product to obtain a more reliable read-out at very low or very high shear rate ranges. When this is done properly, the two curves can be computer-blended to present one, more rheologically correct profile than either system could have given alone. ' D. At what temperature was the sample run? Some materials will give different profiles depending on the temperature of the sample. Thanks to the use of special fluids, circulating baths can be 32 IFSCC Monograph No. 3

A BOHLIN RHEOMETER SYSTEM Viscometry test B

L 100

Gma. M C Bio . K U 90.00 0= m Mon >

F COT: los M.I. 5 Ho. of M. 1 1

D rter 26.4 C R 1£- E84.91 SH J O.'1-1 ' '~10 100 Stear rate . 1/s

Figure 8. Example of a rheological profile

attached to rheometers in order to test materials under elevated or very low temperatures. E. The torsion element used to record the resistance of the fluid to the movement of the spindles has a range of flexibility that leads to reliable measurements. Some machines will record the extent of the range that was used during the test. The range may be given as a percentage of the total range. In our example, 1·52 - 84·91% of the range of our chosen torsion element was used. This shows that the measurement was reliable, since the torsion element was Introduction to Rheology 33

not twisted to the extreme ends where the results may have been distorted. F. What y-axis title has been displayed on the graph? The most commonly found property is 'viscosity', but there are others, with the next most common being 'shear stress'. When shear stress is used as the y-axis, the curves take on different appearances (see figs. 3 and 4). The typical pseudoplastic curve, for example, could easily be mistaken for dilatancy if one misreads the axis title. G. Numbers can be misleading if you are not aware of the units used, i Since software packages now allow for automatic adjustment of the scales, one has to pay particular attention to the units displayed on each axis. The internationally accepted unit of viscosity is the pascal second. Through automatic adjustments, you may be seeing millipascal seconds (mPa·s) or kilopascal seconds (kPa·s) depending on whether the material tested is very 'thin' or very ' thick'. Another unit that will show up is the poise. Though not as widely used as the pascal second, the poise (P) and centi- poise (cP, but sometimes designated as cps) are still used in the United States. H. The axis scales can be displayed in either a linear or a logarithmic form. The preferred method is logarithmic, since it smooths out many insignificant perturbations, and is able to display an otherwise very long axis in a small graph. I. Check the x-axis title. It should read 'shear rate' with a unit of one reciprocal-second (abbreviated 1/sec, 1/s or sec-1). J. The scale used on the x-axis should extend into both the low and high shear rate ranges. Typically, several decades of numbers should be displayed, reaching from below 1 reciprocal-second to well over 100. Since there are some rheological additives that affect either the very high or the very low shear rate ranges, the tests should extend into these areas to be sure of the results. Extra- polations from just the mid-range section of the curves do not always portray what happens in real life. K. Cosmetic formulators should be able to look at an overall curve and be able to tell whether the material tested was Newtonian, pseudoplastic, thixotropic or dilatant. If multiple curves are 34 IFSCC Monograph No. 3

displayed on the same graph, the interpreter should be able to tell that the pseudoplastic curve with the sharpest drop displays more pseudoplasticity (shear-thinning effect) or that the thixotropic curve with the largest hysteresis loop (area between the up and down curves) is the most thixotropic.

L. If the first area displayed on the curve seems unusual in any way, it may be due to disturbances when the machine first started up. Even when looking specifically for very low shear rate profiles, it is usually a good idea to discount the first few points, and base your graph on the rest. Another way of checking the reliability of these points is to examine the numbers given as the 'range' the torsion bar was going through for these points. If they were taken under the 1% level of the bar's total range, it is quite likely that they are unreliable. Often, the generator of the curve will ignore those points altogether and pick a more reliable low shear rate point with which to start the final graph. M. If there is a single line on the graph that you are evaluating, you should know in what direction it is running. Most curves will run in the low shear to high shear direction, but many instruments can be set to sweep from low to high, then back to low. In this latter case, you want to be sure which curve is which. If only one curve is displayed, be careful not to jump to the conclusion that the material is pseudoplastic, when in reality it could be thixotropic with only one part of the curve displayed. If the two curves are very similar, as in pseudoplasticity, only one becomes necessary and will often stand alone.

As you can see, interpretation of rheological profiles can become involved, but, with a little practice, is achievable. As cosmetic technology continues to advance, it will become more and more important for cosmetic formulators to understand and utilize the science of rheology. Those who do will have a powerful tool at their disposal, which will help them to keep their competitive edge in the exciting years to come. Introduction to Rheology 35

6 Suggested Reading

Alexander, Philip. 'Rheology principles, measurement and control', Manufacturing Chemist (April 1986). Bell, D. 'The rheological behavior of creams and lotions '. Soap, Perfumery & Cosmetics (Sept. 1982). Boonfriend, Robert. 'Effects of processing on the rheological behavior of emulsions'. Cosmetics & Tbiletries, 93 (July 1978).

Holland, David. 'Measuring and interpreting the rheological properties of cosmetic products'. Rheology, 1, Issue 2, pp. 108- 12 ( 1991 ). Idson, Bernard. 'Rheology: fundamental concepts'. Cosmetics & Toiletries, 93 (July 1978). Laba, Dennis, ed. Rheological Properties of Cosmetics and Toiletries, New York: Marcel Dekker (1993). Moran~ais, J.L., and G. Vanlerberghe. 'Rheological behavior of cosmetic creams'. Preprints of the XIV IFSCC Congress, Barcelona, pp. 653-67 (1986). Pena, Lorraine E., Barbara L. Lee, and James R Stearns. 'Secondary structural rheology of a model cream' . Journal ofthe Society of Cosmetic Chemists, 45, No. 2, pp. 77-84 (March/April 1994). Sutterby, Lloyd J. 'Viscosity at home'. Chemtech, pp. 416-19 (July 1985). . Ward, John B. 'Application of rheological studies to product formulation, stability and processing problems'. Journal ofthe Society ofCosmetic Chemists. 25, (8) 437-54 (August 1974).