Powder Technology 313 (2017) 389–401 Contents lists available at ScienceDirect Powder Technology journal homepage: www.elsevier.com/locate/powtec Review A review of bulk powder caking U. Zafar a, V. Vivacqua a,G.Calverta,1,M.Ghadiria,⁎, J.A.S. Cleaver b a Institute of Particle Science and Engineering, School of Chemical and Process Engineering, University of Leeds, Leeds LS2 9JT, UK b Department of Chemical and Process Engineering, University of Surrey, Guildford GU2 7XH, England, UK article info abstract Article history: The handling and storage of bulk powders is common in many industries such as detergent, pharmaceutical, bulk Received 31 October 2016 chemical and food. A lot of materials are susceptible to changes with time that may lead to bulk powder caking, Received in revised form 13 February 2017 resulting in an unmanageable powder and process downtime, which impacts upon profitability. In this review Accepted 14 February 2017 the current state of the art related to powder caking is explored. The relevant interparticle interactions are Available online 21 February 2017 discussed with respect to elastic and plastic deformations and the development of liquid and solid bridges due Keywords: to capillary condensation, sintering and solvent evaporation. The environmental, i.e. temperature and humidity, fl Powder caking and the mechanical conditions can heavily in uence the transformation of a bulk powder and a number of Caking mechanism studies are available that attempt to relate these conditions to caking. A significant amount of work related to Assessing powder caking the caking behaviour of amorphous powders is available in the literature. Amorphous materials are susceptible to caking due to environmental conditions influencing the glass transition temperature. Once the temperature of amorphous powders exceeds the glass transition, viscous flow occurs and cake strength increases. Crystalline solids may undergo transformations leading to caking. It can take a considerable time and cyclic environmental conditions for caking to occur. However, little research focuses on cyclic conditions and there is limited predictive capability. Finally the options available for attempting to reduce caking propensity are briefly covered and a section that discusses the available caking test methods is given. © 2017 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Contents 1. Introduction.............................................................. 390 2. Particleinteractions........................................................... 390 2.1. vanderWaals.......................................................... 390 2.2. Electrostaticforces........................................................ 390 2.3. Liquidbridgeforces....................................................... 390 2.4. Contactmechanics........................................................ 391 2.4.1. Elasticdeformation................................................... 391 2.4.2. Plasticdeformation................................................... 391 2.4.3. Surfaceroughness.................................................... 391 2.5. Solidbridgeformation...................................................... 391 2.5.1. Sintering........................................................ 392 2.5.2. Solventevaporation................................................... 392 3. Amorphouspowders.......................................................... 393 3.1. Glasstransitiontemperature................................................... 393 3.2. Viscous flowkinetics....................................................... 393 3.3. Theeffectofcomposition..................................................... 394 4. Cakingdynamics............................................................ 394 4.1. Heatandmasstransfermodels.................................................. 394 5. Cakingreduction............................................................ 395 5.1. Cakingreductionadditives.................................................... 396 ⁎ Corresponding author. E-mail address: [email protected] (M. Ghadiri). 1 Current address: Procter and Gamble, Newcastle Innovation Centre, Newcastle upon Tyne NE12 9BZ, UK. http://dx.doi.org/10.1016/j.powtec.2017.02.024 0032-5910/© 2017 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). 390 U. Zafar et al. / Powder Technology 313 (2017) 389–401 6. Cakingtestmethods........................................................... 396 6.1. Mechanicaltestmethods..................................................... 396 6.1.1. Shearcelltesting..................................................... 396 6.1.2. Uniaxialcompressiontesting............................................... 396 6.1.3. Tensiletesting...................................................... 397 6.1.4. TheICIcakingtest.................................................... 397 6.1.5. Creeptesting....................................................... 397 6.1.6. Penetrationtesting.................................................... 398 6.1.7. Powderrheometer.................................................... 398 6.2. Alternativetestmethods..................................................... 398 7. Conclusions............................................................... 399 Acknowledgements............................................................. 399 References.................................................................. 399 1. Introduction 2.1. van der Waals The undesirable transformation of bulk powders, such as caking and There are different classes of van der Waals force and more detail is deliquescence is a common problem in a number of industries including available in the literature [7,12]. The dispersion force is due to local detergent, food, fertiliser, pharmaceutical and chemical. By definition, polarisation within molecules caused by random fluctuations in powder caking may be viewed as the detrimental aggregation of parti- electrons and is consequently transient and induced; these are regarded cles, which transforms a readily free-flowing powder into a coherent as non-specific interactions. Specific van der Waals forces are due to solid. The amount of caked material can vary from large lumps, which interaction between molecules with permanent dipoles. willingly break up when stressed, to the irreversible fusion of particles that are no longer representative of the original material. The environ- 2.2. Electrostatic forces mental and mechanical conditions a material experiences can consider- ably influence bulk powder transformation. In certain conditions, a The science of electrostatics is in itself highly challenging as there are material may be so hygroscopic that it deliquesces. Obviously caking many mechanisms that influence particulate behaviour [14].Electrostatic and deliquescence can be a significant problem that may lead to a loss forces may be responsible for particle deposition of airborne particles on of material functionality and a lower quality product [1]. This could processing walls, which may promote powder make-up; however, there have a large impact on profitability. is little evidence suggesting that electrostatic forces play a significant Bulk powder caking is a very challenging topic because there are long term role on powder caking. A possible explanation for this is that many reported mechanisms that may lead to powder transformation the potential difference between particles dissipates much more rapidly and few studies have taking a holistic view of the subject [2]. Griffith compared to the time required for caking [2]. However, in the presence [3] provided a very useful and practical overview of caking and broadly of electric fields, particles adhere strongly to each other due to the classified the subject into four groups: (a) mechanical; (b) chemical; electro-clamping phenomenon [15]. (c) plastic-flow; and (d) electrical. There are many features that need On the other hand, electrical caking has been implicated as a possible to be considered when understanding caking propensity. Intrinsic mechanism that may lead to caking; although the cake strength is typical- factors such as cohesion, elasticity, yield stress, amorphous content, ly relatively weak [3]. For example, the application of a mechanical stress hygroscopicity, particle size and many others will inevitably influence can induce a charge polarisation in crystals, known as the piezoelectric caking behaviour; however, extrinsic factors such as temperature, effect, which can promote caking; pyroelectric caking can occur as a result humidity, stress, strain rate, vibration may also heavily influence caking. of charge polarisation due to heating or cooling crystalline materials; and This makes it very difficult to reliably predict how a powder will behave. finally, ferroelectric caking may happen when crystals are placed in an This is especially the case when the timescale for a powder to cake is so electric field. long that extrapolation of laboratory data is common. In this review, the current state of the art and the experimental ap- 2.3. Liquid bridge forces proaches available for investigating bulk powder caking are addressed. A summary of relevant particle interactions, contact mechanics, caking It is recognised that liquid plays a pivotal role in bulk powder caking mechanisms and dynamics is given. Finally, a section