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Coating approach for a Phase Change Material (PCM)
Accepted Manuscript Coating approach for a Phase Change Material (PCM)
B.B. Paulo, K. Andreola, O. Taranto, A.D. Ferreira, A.S. Prata PII: DOI: Reference:
S0032-5910(18)30186-4 doi:10.1016/j.powtec.2018.03.003 PTEC 13234
To appear in:
Powder Technology
Received date: Revised date: Accepted date:
8 September 2017 21 February 2018 3 March 2018
Please cite this article as: B.B. Paulo, K. Andreola, O. Taranto, A.D. Ferreira, A.S. Prata , Coating approach for a Phase Change Material (PCM). The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Ptec(2017), doi:10.1016/j.powtec.2018.03.003
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ACCEPTED MANUSCRIPT Coating approach for a Phase Change Material (PCM) B.B. PAULO1*, K. ANDREOLA2, O. TARANTO 2, A.D. FERREIRA3, A.S. PRATA1 1
Department of Food Engineering, School of Food Engineering, State University of
Campinas - UNICAMP, Albert Einstein Avenue 500, 13083-862, Campinas, Brazil, Phone: +55 (19) 3521-0069
2
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*[email protected]
Department of Process Engineering, School of Chemical Engineering, State University
of Campinas - UNICAMP, Albert Einstein Avenue 500, 13083-852, Campinas, Brazil,
3
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Phone: +55 (19) 3521-3926
Department of Mechanical Engineering, University of Coimbra, Pólo II, 3030-788,
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Coimbra, Portugal, Phone: (351) 239 790 727
ABSTRACT
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PCMs (Phase Change Materials) goes through volumetric variation during the solidliquid phase transitions. Encapsulation enables the preservation of their thermal efficiency and prevents adherence on the surface where they are applied. For PCM
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application, particles with high load are desirable, and the coating in fluidized bed represents a scalable way to protect them. However, this process requires PCMs in solid
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state and, thus, it is restricted to their phase change temperature. Carnauba wax (CW) has a high enthalpy and relatively high melting point value, which can be a promising
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organic PCM and desirable to be employed in processes of coating. This work aimed to coat CW particles in fluidized bed. Some strategies were adopted to increase the load of wax in a particle and reduce experimental assays of coating. For this purpose, CW particles were produced by cold extrusion and a preliminary selection of potential substances to be used as coating material (chitosan, Eudragit® L30-D55, gum Arabic, maltodextrin and sodium alginate) was performed based on their rheological and adhesive properties. Adhesive property was evaluated through the contact angle between the CW and the coating materials. Suspensions containing sodium alginate and Eudragit® showed the lowest contact angle (θ ≅ 40 °), low viscosity, and could restrain the volumetric variation of CW particles (coating efficiency = 55 %) under heating at
ACCEPTED MANUSCRIPT 100 °C for at least one hour. Coating solutions/suspensions with contact angle above 58 ° and high viscosity were not effective to coat the particles. This study allowed a successful coating process of CW particles in fluidized bed, with potential application to other PCMs and, consequently, enlarging the range of thermal responsive textile and construction materials, and food packages. Keywords: fluidization; fluidized bed; thermal energy storage; encapsulation
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1 INTRODUCTION
Several food and biological products are sensitive to temperature changes and must be properly transported and stored to ensure microbiological, biochemical, physiological, sensory and physical quality. Thermal responsive packages can be developed and they are interesting for food industry when used to keep the temperature
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of “ready-meals” or to avoid thermal variation of fresh and highly susceptible products to microbiological contamination products, to minimize waste [1]. Except expanded polyestyrene, standard materials traditionally used for
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packaging (plastic or cardboard) generally have limited thermal capacity. One possible way to increase it and keep the product at a desirable temperature is through storage of
packaging material [2].
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thermal energy by incorporating phase change materials (PCMs) into the structure of the
Phase Change Materials (PCMs) are substances with high fusion heat that,
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during the solid-liquid phase transition, are capable to store large amounts of thermal energy, and, isothermally, release it [3–6]. PCMs have demonstrated potential in several
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applications related from electronics industry, thermoelectric devices and photovoltaic materials [7–10] to renewable energy as solar heating systems[11–14], but also for
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transforming substrates giving them a thermal tunable function, as building and textiles materials[2,15–23]. Carnauba wax has been studied as a promising PCM [24], but, during the
transition solid-liquid, it can gradually adhere on the surface where they are applied, and this may reduce its [25,26] thermal efficiency. One possible way to enable its application is through microencapsulation. This technology can provide a resistant film around the PCM and control its volumetric variation during the phase transition, reducing the reactivity of the PCMs with the packaging material and increasing the surface area of heat transfer [25,27].
ACCEPTED MANUSCRIPT The main techniques used for the purpose of microencapsulation of PCMs are complex coacervation [24,28] and interfacial polymerization [17,29], which result in moist particles with low load, undesirable for some applications. Recently, Ushak et al. [30] studied the coating of inorganic PCMs in fluidized bed, which presents the advantage the large-scale production of dried particles. However, organic PCMs, such as waxes and lipids, present relative low melting point with great susceptibility to phase change during the coating process, which is the major drawback for operation in
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fluidized bed. Different classes of lipids have been used only serving as coating materials in fluidized bed [31,32]. To the best of our knowledge, there have been no reports of the use of the fluidized bed to coat lipid particles.
The coating of meltable lipid in fluidized bed requires a well outlined procedure for the successful performance of the operation. Accordingly, the purpose of
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the work reported here was to establish a basis for the choice of operational conditions for coating carnauba wax particles. Thus, a preliminary study for establishing ranges of thermo-fluid dynamics was based on the thermo-physical properties of carnauba wax
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particles. After, a careful selection of coating suspensions was carried out with basis on rheological analysis and adhesion properties onto particles surface to avoid excess of
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trial-and-error experiments. Finally, the validation of previous analysis was carried out, ensuring the coating process and the effectiveness of coating layer in the restraining of
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volumetric variation of carnauba wax during phase transition.
2.1 Material
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2 EXPERIMENTAL
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Commercial carnauba wax (CW) (type 3, Carnaúba do Brasil Ltda, Brazil)
was used as phase change material. Gum Arabic (GA) (Synth, Brazil), sodium alginate (SA) (Manugel DMB, FMC BioPolymer, USA), chitosan (CHI) (Polymar, Brazil), maltodextrin (MD) (Mor-Rex 1910, Corn Products, Brazil), Eudragit® (L30-D55, Evonik Röhm GmbH, Germany), Polyethylene glycol (PEG) 1500 (Synth, Brazil), magnesium stearate (Sigma-Aldrich, Brazil) and talcum powder (Colorcon do Brasil Ltda, Brazil) composed coating suspensions/solutions. Acetic acid (Synth, Brazil) was used for the preparation of aqueous CHI solution.
2.2 CW particle preparation
ACCEPTED MANUSCRIPT CW powders were extruded in a twin-screw extruder ZSK-30 (Werner and Pfleiderer, USA) with 30 mm barrel diameter at 150 rpm screw speed, 16.2kg/h feed rate and under temperature below the melting point (Tm = 78 °C) of the CW. The twinscrew extruder is divided in four controlled temperature zones, which were set at 30, 50, 60 and 60°C successively from the supply window to the die (2mm hole diameter). The extruded product was gently broken into a mill (2.8mm sieve opening) and characterized in relation to their size by sieving, bulk and apparent densities, porosity,
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and sphericity[33].
a) Particle diameter and morphology
The morphology of the CW particles was observed in steromicroscope Citoval 2 (Carl Zeiss Jena, Germany). The mean particle diameter was determined by sieving the particles in standard set of Tyler sieves. The particle size distribution was
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measured after sieve particles using a shaker Magnet 100 (Bertel Indústria Metalúrgica Ltda, Brazil) for 10 minutes, in a set of Tyler 9, 10, 12, 16 and 10 series screens, whose openings are 2.00, 1.70, 1.40, 1.00 and 0.85 mm. Then, the CW particles were separated
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in the particle size range between 1.70 and 1.40 mm, obtaining a mean diameter of 1.55
b) Sphericity
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mm, to carry out the other physical characterization tests and coating experiments.
The sphericity of the particles was determined by image analysis obtained in
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the stereomicroscope (Carl Zeiss Jena, Germany), using ImageJ software. Equation 1
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describes the principle used by the software to determine the sphericity. ∅=
Dci
(1)
Dcc
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where Dcc and Dci are the diameters of the circumscribed and inscribed circle, respectively, determined from the projected area of the particle.
c) Bed porosity Particle bed porosity was determined by the ratio of apparent density to bulk
density. ε=1−
d) Bulk density
ρbulk ρap
(2)
ACCEPTED MANUSCRIPT A mass of 5 g particles was put in a 10 mL calibrated beaker, without forcing them to settle, and the volume occupied by the particles was measured. This test was performed at least 10 times. Bulk density was calculated according to Equation (3). ρbulk =
mp
(3)
V
where mp is mass of particles (g), V is volume occupied (cm³).
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e) Apparent density A 50 mL pycnometer was filled with water, and the corresponding mass was determined (mliquid). Then, an amount of particles (mparticles) was added to the empty pycnometer, the volume was completed with water, and the new mass of the liquidparticle system (mLP) was quantified. The apparent density of the particle was calculated by Equation 4.
[mparticles − (mLP − mliquid )]
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2.3 Coating material preparation
ρliquid × mparticles
(4)
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ρap =
Increasing concentrations of solutions of each polymer (2-4 wt% CHI, 3045 wt% GA, 45-60 wt% MD, 1-3 wt% SA) were prepared by homogenization at room
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temperature with a magnetic stirrer until complete dissolution in distilled water, except for chitosan, which is soluble only in acidic conditions and, therefore, 1% (v/v) acetic
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acid solution was used. Three coating polymeric suspensions (Table 1) were also
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prepared using a mechanical stirrer for 10 min.
2.3.1 Rheology
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Rheological measurements of solutions/suspensions were performed in triplicate after 24h of preparation using a controlled stress rheometer AR 1500ex (TA Instruments, England), with stainless steel cone-plate geometry (60mm, 1.59°). Flow curves were obtained using an up-down-up steps program with shear rate ranging from 0 to 300s-1 at 25°C. The third flow curve data was fitted to the Herschel-Bulkley’s model (HB).
2.3.2 Contact angle Twenty contact angle (θ) measurements were performed with each coating solution/suspension using a goniometer (Contact Angle Meter, Tantec, USA). A drop of
ACCEPTED MANUSCRIPT the coating solution was deposited on a flat and smooth surface of CW, whose image was projected onto a graduated bulkhead and then visually measured the contact angle.
2.4 Coating of CW particles in fluidized bed Experiments of coating of CW particles were performed using a fluidized bed (Figure 1[34]) with a transparent acrylic Plexiglas® conical-cylindric shape (conical base: inlet air diameter=0.075m, height=0.15m; cylindrical column:
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diameter=0.15m, height=0.60m) and a perforated stainless-steel screen (hole diameters= 43μm).
The bed pressure measurements were obtained using a water millimeter differential manometer. The temperature and relativity humidity of the fluidizing air were monitored by Pt-100 thermoresistances (Novus, Ø 3.0mm x 100mm) and thermo-
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hygrometers (Novus, RHT-XS) during the experiments. These data were recorded by a data acquisition system (NI cDAQ-9172, National Instruments) and processed through the LabVIEW 8.6 software building a virtual instrument (VI).
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A calibrated peristaltic pump (Cole Parmer, model 7518-10, Masterflex L/S type) pumped the coating material through a bi-fluid nozzle (SU12A, Spraying
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Systems), with atomization pressure, supplied by a 2 cv compressor (Schulz, CSA 7.8/25).
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2.4.1 Evaluation of fluid dynamic behavior of particles The geometric and aerodynamic properties of particulate affect the onset
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of fluidization, and the main parameters of processes in fluidized beds. Theoretical equations are normally used to obtain some relevant parameters for the fluidization and
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predict the fluid dynamics behavior of particles [35,38]. The minimum fluidizing air velocity (umf) was determined by measuring
the bed pressure drop (ΔP) as a function of fluidizing air flow for the particles without binder atomization. The theoretical umf and terminal velocity (ut) of the particles were determined using the Equations5 and 6[35], 7 and 8[36], respectively. 𝐑𝐞𝐦𝐟 =
𝛒𝐠 𝐮𝐦𝐟 𝐝𝐩
𝐑𝐞𝐦𝐟 = [(𝟐𝟖. 𝟕)𝟐 + 𝟎. 𝟎𝟒𝟗𝟒 ( 𝐮𝐭∗
𝟏𝟖
= [𝐝∗ + 𝐩
(5)
𝛍 𝐝𝐩 ³ 𝛒𝐠 (𝛒𝐩 − 𝛒𝐠 )𝐠
)]
𝛍𝟐
𝟐.𝟑𝟑𝟓−𝟏.𝟕𝟒𝟒∅𝐬 (𝐝∗𝐩 )
𝟎.𝟓
𝟎.𝟓
− 𝟐𝟖. 𝟕
(6)
−𝟏
]
(7)
ACCEPTED MANUSCRIPT 𝐝∗𝐩 = 𝐝𝐩 [
𝛒𝐠 (𝛒𝐩 − 𝛒𝐠 )𝐠
]
𝛍𝟐
𝟏/𝟑
(8)
In which: ρp and ρg: particle and air densities (kg/m³), respectively; dp: particle diameter (m); g: acceleration of gravity (9.8 m/s²), µ: air viscosity (Pa s), ∅s: sphericity of particle.
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2.4.2 Process of particle coating
For all the runs, these conditions were fixed: 0.20 kg of the CW, fluidizing air velocity u=1.27 m/s (u>umf, regarding that the size and the mass of the particles would increase along the coating process), the atomizing air pressure of 1.0 bar, the nozzle height at 200 mm from the lower base of the cone, and the air temperature was 60°C. The CW particles were preheated for 10 min under the fluidization conditions in
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order to ensure the equilibrium of the system at 60 °C during the coating process. 250 mL of polymeric solution/suspension was atomized onto the particles at room
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temperature and fixed flow rate, which depended on the composition.
2.4.3 Characterization of coating process and coated particles
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The coating efficiency (CE) is defined as the ratio between the mass of solids adhered to the particles (real) and the solids mass of the suspension added to the
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fluidized bed (theoretical) (Equation 9)[37]. The real coating mass was determined by
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dissolving the coating of the waxy particles. CE (%) =
Mf − Mi Qat ρcs Cs t
x 100
(9)
In which: Mf and Mi: mass of particles with and without coating (g),
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respectively, Qat: mass flow rate of atomization of the coating suspension (mL/min), ρcs: density of the coating suspension (g/mL), Cs: solids concentration (fraction) of the suspension, t: process time (min). The relative growth (δ) of the particle represents the amount of coating (Mf- Mi) over the particle (Mi). The agglomeration index (Agl) is defined as the ratio between the mass of agglomerated particles and the total mass of particles loaded to the bed at the beginning of the test. Particles that had a diameter greater than 3 times the average size of the extruded particles without coating were considered agglomerates.
ACCEPTED MANUSCRIPT The morphology of all particles was evaluated in a Citoval 2 stereomicroscope (Carl Zeiss Jena, Germany). Melting and crystallization curves of the free and coated CW particles were determined by DSC technique (DSC 2920 Modulated, TA Instruments, USA). Heating and cooling cycles at 2°C/min rate were performed for each sample using 4 mg of sample weighed in airtight aluminum capsules. Samples were heated at 100°C and maintained for 15 min at 100°C, cooled to 0°C and maintained at 0°C for 15 min. The
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thermograms obtained were normalized based on the mass of the samples. The melting and crystallization temperatures (Tm;Tc) and the phase transition enthalpies (ΔHfusion/crystallization) were determined by the TA Universal Analysis software.
3 RESULTS AND DISCUSSION
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CW serving as PCM can be used for the manufacture of oven or microwaves packages as standard materials traditionally used for packaging (plastic or cardboard) have limited thermal capacity.
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Fig. 2 shows the thermal behavior of CW particles, which confirms its use as organic PCM due the high melting/crystallization enthalpy[12,19,38]. Tm was 78°C and it was determined from the last peak of the thermogram, whereas a softening point
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was considered at 60°C, which is extracted from the 2nd peak of the thermogram and can represent the initial melting of minor fatty acids of the composition of the CW , .
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This point allows to set the operational temperature during the extrusion and coating processes.
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The PCM encapsulation is important for avoiding the leakage of the liquid PCMs from the substrate. Coating in fluidized bed can provide dried particles with high
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load, but the coating process is highly complex, because it includes the fluid dynamics study of the particles through their packaging properties, the understanding of the adhesion properties of the coating material and the determination of the coating operating conditions. An approach for each topic above was taken to achieve the coating process and reduce the number of trial-and-error experiments.
3.1 Study of fluidization conditions and maximum drying temperature In fluidization process, particles move randomly in a fluid-like state through an upward flow of another fluid which maintains the suspended bed of particles[36]. Thus, the limit of operation ranging from the beginning of the fluidization
ACCEPTED MANUSCRIPT regime (umf) to maximum air velocity without dragging the particles (ut) can be estimated by theoretical equations (equation 5-8) based on density and diameter of particles as well as properties of the fluid used. The physical characteristics of the CW particles are shown in Table 2. Based on these equations, the theoretical range of fluidization is from 2.2 to 10.5 m/s for CW particles. Starting from this reference, the fluid dynamic of the particle bed was previously analyzed to establish the real conditions for the coating
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experiments. Fig.3 shows the pressure drop curve as a function of the air fluidization velocity. This curve did not represent the behavior normally observed for fluidization regime. In this regime, the velocity increases with the pressure drop across the bed until a maximum value, falls slightly, reaching a constant value, at which the pressure drop through the bed is equal to the bed weight per unit cross-sectional area.
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In contrast, experimental data show that, with the increase of air velocity, the pressure drop does not reach a constant value, tending to reduce. This behavior is similar to spouted beds[39], in which the movement of particles is very vigorous, due to
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their large size, which makes it difficult to establish a point of minimum fluidization. The experimental value umf=0.7m/s, lower than the theoretical value (2.2 m/s),
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confirmed a deviation of classic fluidization regime. In spouted regimes, at low air velocities, the pressure drop increases with the air velocity , because the fluid only percolates between the particles and the system behaves as a fixed bed. However, with
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the further increase of the air velocity, the maximum pressure drop is reached due to the rupture of the particle bed by the fluid, similarly to the fluidization regime. In this study
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this behavior was observed, but continuing with the increase of the air velocity, the pressure drop decreased, due to the instability in the porosity of particle bed, result of
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the frequent formation of bubbles and the oscillation in bed height. The pneumatic transport will occur only above ut of particles (10.5m/s). Tests were performed to establish the thermal profile inside the drying
chamber, with and without particles, by coupling temperature sensors throughout the bed. For a set point of 60°C, the temperature within the bed is below or equal to 60°C, indicating that the coating process could be carried out carefully, since this temperature is the softening point of the CW particles (Fig. 2).
3.2 Study of coating process
ACCEPTED MANUSCRIPT The PCM coating must protect the CW particles against the volumetric changes that occur during the liquid-solid phase transition, reducing the reactivity of this component with the external environment[5]. Thus, the protection film formed must have mechanical strength to withstand collisions during the coating process, and the stress caused by the melting/crystallization behavior of the CW. However, the coating of the CW particles has some experimental limitations. These particles initiate their phase change around 60°C (Fig. 2) and cannot
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be reused after coating. In addition, the combination of processing and film formation are related to some properties of the coating material, which determine the wettability of the solid. Its molecular weight is responsible for rheological and elastic behaviors and, consequently, for the maintenance of the droplets over the particle surface. Thus, to minimize experimental tests of coating, some factors should be considered: a) T<60°C,
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which reduces the driven force for evaporation. Thus, a maximum flowable concentration of solids of coating material was chosen to reduce the drying time and favor faster formation of the coating film. Both aspects minimize the effects of
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temperature on the shape of the wax particle; b) The film formed must adhere efficiently to the particle surface during drying, without unsticking; c) The film formed must
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restrain the CW volumetric variation during the phase change.
3.2.1 Rheology of coating solutions
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Since rheological behavior of the coating materials is an indication of the flow during the atomization, it was considered an initial criterion for the selection of
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coating material, aiming to operate with fluids at the maximum solids concentration. The potential coating solutions were produced in three concentrations, in which the
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minimum value was based on information from the literature[40–43]. Fig. 4 shows the rheological properties obtained for all polymer solutions
used, whose coefficient of determination (R²) obtained was 0.999 for the adjustment of all systems to the HB’s model. GA and MD, even in high amounts of solids, showed a behavior closer to the Newtonian (n=1). In contrast, SA and CHI obtained pseudoplastic behavior (n<1.0), which accentuates according to the increase of solid concentration. However, these compounds were very viscous even at low concentrations, such as solutions of 1 wt% SA and 3 wt% CHI, which have viscosities equivalent to solutions of 30 wt% GA and 45 wt% MD. This change in viscosity can be a consequence of the highest molecular
ACCEPTED MANUSCRIPT weights of the polymers SA and CHI, and the interactions between their chains when dispersed. In general, all coating material solutions at the highest solid concentration presented very high viscosity and bubbles, which could cause discontinuities of the coating on the particle. These high viscosities hampered the complete homogenization of the solutions and could impair the flow during the atomization. In this perspective, solutions of coating material with ηap<1 Pa s were chosen for contact angle analysis,
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which are solutions with 1 and 2 % (w/w) SA, 30 and 35 % (w/w) GA, 45 and 50 % (w/w) MD, 2 and 3 % (w/w) CHI.
3.2.2 Expectation of adhesion of the coating material to the particle
The contact angle is commonly used to determine the wettability and,
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therefore, as another selection criterion for coating material, since it is possible to predict the best material that will adhere onto the particle. Complete spreadability or wettability can be found with lower adhesion
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work, and therefore, low contact angles favor the coating, since it makes the work of liquid-solid adhesion equal to or greater than the cohesion of the liquid. Another
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important property is surface tension of the polymer solutions and it can present a trend to reduce with the increase of solid concentration [44]. Donida et al.[45,46] studied the coating efficiency of inert particles with
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different polymer suspensions in spouted bed. The limit value of θ=75° indicated good wettability on the particle surface resulting in higher adhesion values, and,
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consequently, higher CE.
In our study, the contact angle of the polymeric solutions ranged from
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89.2° to 66.5° (Table 3), in which the increase of the solid content resulted in the reduction of the contact angle. For synthetic/inert substrates, the contact angle is more reliable to
adhesion behavior because the substrate surface used for measuring the contact angle is similar to the real particle. However, CW presented irregular surface and, besides the contact angle and surface tension, other parameters may also be involved in the adhesion behavior, such as hydrophobicity, roughness of substrate and chemical interactions[47].
3.2.3 Coating validation test
ACCEPTED MANUSCRIPT From the previous analysis of viscosity and contact angle, the criteria were established: viscosities lower than 1 Pa s and the lowest contact angles. Thus, two coating solutions, GA (35% w/w, θ=78.2°) and SA (2% w/w, θ=66.5°) solutions, were chosen to represent a Newtonian and non-Newtonian fluid, respectively. Regardless of the rheological characteristic of the solutions, the coating tests (Tair=60°C; Qat=2.5mL/min) with both GA and SA solutions were ineffective, as the solutions dried prematurely and the coating powder was elutriated. These results
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may be associated to the low adhesion of the coating material onto the particle surface. The low contact anglesmay not have been low enough to form a film on the particle. Therefore, new formulations (Table 1) based on the preliminary tests and the studies developed by Donida[48] with some modifications were prepared to reduce the contact angle.
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The components present in the formulations have specific characteristics that improve the coating process. Among them, PEG1500, a plasticizing agent that has the function of giving greater flexibility to the coating film to withstand the mechanical
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stress, and adjusting the viscosity and consistency of suspensions; talcum powder and magnesium stearate are smoothing and separating agents to prevent tackiness of the
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film, reducing its agglutination when dried; magnesium stearate is responsible for reducing the surface tension of the suspension, facilitating its spread on the particle surface and on the dry film layer on the particle[49].
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In general, the polymeric suspensions presented viscosities and contact angle remarkably lower than those presented by solutions. Viscosity corresponded to
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2.6, 0.6 and 4.7 mPa s at 100 s-1 for F1(SA), F2(Eudragit®) and F3(GA), respectively, being all suspensions showed near-Newtonian behavior (n≈1), and contact angle was
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43.1, 40.4, and 57.2° for F1(SA), F2(Eudragit), and F3(GA), respectively. Both suspension F1 and solution with SA at 1 % (w/w) presented different values of viscosities and contact angle, confirming that the addition of the coadjuvant materials contributed to reduce the viscosity and the contact angle. The coating was performed, and Table 4 summarizes the coating process results and parameters. For application of PCM particles, the formation of agglomerates is undesirable, and the number of agglomerates was extremely low for all formulations tested. The formation of agglomerates is related to two main factors. The first one is the intrinsic characteristic of the coating material, as was observed by Albanez, Nitz and
ACCEPTED MANUSCRIPT Taranto[50], when they used different polymers submitted to the same coating process conditions. Although such polymeric materials had adhesion to the particle surface, the fractions of agglomerates obtained were different. The other factor is related to the inadequate operating conditions used during the process. Temperature and relative humidity are driven forces for coating by evaporation in fluidized beds, and operational parameters influencing on the size distribution of droplets of atomized suspension should be controlled to avoid premature drying (very small droplets) or overwetting
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(very large droplets). In this perspective, many parameters influencing the heat and mass transfer have a significant role such as temperature and atomization rate[50–53]. As for the coating of PCMs, the air temperature is restricted to that of phase change of the material, the atomization flow was the main variable and it was adjusted based on a mass and energy balance applied between the outlet (60°C, RHo=25%), and the
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saturation conditions of the fluidized bed. According to this analysis, the maximum water flow to be evaporated would be 3.9 mL/min. Therefore, the atomization rate used was 2.5 mL/min for F2(Eudragit®) and F3(GA), and 2.0 mL/min for F1(SA), ensuring
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that this condition was enough to avoid agglomeration in these systems. Growth value is a parameter to predict the efficiency of coating onto
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particles, whose the range found in the literature is normally between 1 to 5%[37]. Higher value was found for F2, which had the lower contact angle. Hemati et al.[52] also verified the increase of the particle growth with the decrease of the contact angle.
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Fig. 5 shows the relation between the CE of the suspensions and their respective obtained contact angles. SA (θ=43.1°) and Eudragit® (θ=40.4°) suspensions
CE
were efficient for coating CW particles, presenting CE=55%. In contrast, GA suspension dried and did not adhere onto CW particles surface, reaching a very
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inefficient level of coating (CE=15%), and, consequently, data related to the particles such as growth or agglomeration were not obtained. This result was confirmed by checking the morphology of the free and
coated CW particles as function of a time and temperature. CW particles coated by the F1(SA) and F2(Eudragit®) could restrain the volumetric variation for at least 1h at 100°C (Fig. 6). In contrast, the particles resulting from the process using the F3(GA) showed a very similar behavior to that of the free CW particles. This poor adhesion of GA onto CW particles was indicated by highest contact angle, which can be associated to chemical interactions, as, for example, to the hydrophobicity of the materials. Eudragit®, as derived from ester of acrylic acids is
ACCEPTED MANUSCRIPT more hydrophobic than GA and SAs. Conversely, all polymers, SA, GA and Eudragit® L30-D55 are negatively charged in their natural pH, but with different magnitudes. Therefore, there are additional intrinsic parameters of each material, for example, molecular weight, surface tension, molecular conformations and arrangements, and other physicochemical properties of the coating materials that result in different interactions with the substrate. In general, CE was lower than that obtained by Donida et al.[37,46,54] and
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Paulo Filho, Rocha and Lisboa[55], indicating other influencing factors besides the selection of appropriate coating material. Properties such as bulk density, porosity and shape have a great influence on the fluid dynamics of the particle bed and on the drying of the suspension. The spherical shape of the particle minimizes the friction between the particles during the coating, aiding the formation of a homogeneous film. High porosity
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facilitates the adsorption of the suspension and prevents the presence of free moisture on the particles and liquid bridges that bind them, maintaining the intense circulation of these in the bed, even with the addition of moisture[37,47].
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Fig. 7 shows the thermal behavior of free and coated particles obtained by differential scanning calorimetry (DSC). All particles presented similar values of Tm, Tc,
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ΔHm and ΔHc. Therefore, besides the coating was effective in restraining the volumetric variation, it also did not modify the thermal properties and behavior of the particles
4 CONCLUSIONS
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during the solid-liquid phase transition, ensuring their application as PCMs.
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CW particles produced by cold extrusion at 16.2 kg/h showed a good bubbling fluidization behavior, when using umf=0.7m/s and T=60°C, the softening point
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of CW particles, which required a very well stablished protocol for coating. Rheological and adhesive characteristics were effective criteria for
selection of appropriate coating materials. Coating materials with θ>58° could not coat CW particle. SA and Eudragit® suspensions containing adjuvant materials (θ=40°, CE=55%) could restrain the volumetric variation during the phase transition of the PCM, indicating that Eudragit, a high cost synthetic polymer, can be replaced by a natural one. This study allowed a successful coating process of meltable particles in fluidized bed, with potential application to other PCMs and, consequently, enlarging the range of thermal responsive materials.
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ACKNOWLEDGEMENTS This work was supported by São Paulo Research Foundation – FAPESP [grant numbers 2011/04230-5 and 2015/11629-2].
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ACCEPTED MANUSCRIPT List of the figure captions Figure 1. Scheme of the fluidized bed [34] Figure 2. Thermal behavior for free carnauba wax particles (ΔHm; ΔHc: melting and crystallization enthalpy, respectively; Tm; Tc: melting and crystallization temperatures, respectively) Figure 3. Pressure drop for carnauba wax particles
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Figure 4. Rheology of the coating solutions Figure 5. Relation between coating efficiency and contact angle
Figure 6. Checking the coating efficiency to restrain volumetric variation at 100 °C: A) Free; B) F1 (SA); C) F2 (Eudragit®); D) F3 (GA)
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Figure 7. Thermal behavior of free and coated CW particles (ΔHm; ΔHc: melting and crystallization enthalpy, respectively; Tm; Tc: melting and crystallization temperatures, respectively): A) crystallization; B) melting
ACCEPTED MANUSCRIPT Table 1. Chemical composition of the coating suspensions Formulation (% w/w) Component F2
F3
1.00
-
-
Eudragit® L30-D55
-
16.70
-
GA
-
-
16.70
PEG1500
0.80
0.80
0.80
Magnesium stearate
1.10
1.10
1.10
Talcum powder
3.75
Pigment
0.20
Water
93.15
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SA
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F1
3.75
3.75
0.20
0.20
77.45
77.45
ACCEPTED MANUSCRIPT Table 2. Physical characteristics of CW particles CW particles
Dp (mm)
1.5 ± 0.1
ρbulk (g/cm³)
0.54 ± 0.01
ρap (g/cm³)
0.80 ± 0.06
ε
0.326
Roundness
0.7 ± 0.2
Sphericity
0.8 ± 0.1
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Properties
Morphology
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Dp: particle diameter; ρbulk, ρap: bulk and apparent densities, respectively; ε: bed porosity
ACCEPTED MANUSCRIPT Table 3. Contact angle (θ) between CW and coating solutions at different concentrations (% w/w) θ (°)
CHI - 2 %
75.6c ± 5.3
CHI - 3 %
75.6c ± 5.2
GA - 30 %
82.0b ± 4.0
GA - 35 %
78.2bc ± 4.1
MD - 45 %
89.2ª ± 4.6
MD - 50 %
77.5bc ± 3.7
SA - 1 %
76.6bc ± 5.8
SA - 2 %
66.5d ± 4.5
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Coating solutions
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Different letters indicate a significant difference (p<0.05) by Tukey’s test
ACCEPTED MANUSCRIPT Table 4. Results of experiments of coating Suspension
Qat (ml/min) Ti (°C) Tan (°C) To (°C) RHi (%) RHo (%) Hp (%) η (%) Agl (%) δ (%)
θ (°)
F1_SA
2.0
58.7
58.7
46.6
34.7
21.8
0.3
55.21
0.05
2.4
40.4b ± 4.5
F2_Eudragit®
2.5
60.0
54.8
44.3
52.4
28.7
0.3
55.23
0.40
4.0
43.1b ± 4.1
F3_Gum Arabic
2.5
59.1
57.2
45.4
35.7
23.1
0.2
15.33
-
-
57.2a ± 4.2
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Qat: flow rate of atomization of the coating suspension; Ti, To, RHi, RHo: temperature and relative humidity of air in the inlet and outlet, respectively, Tan: temperature in the conic region of the bed, Hp: humidity of particles at the end of the process, η: coating efficiency, Agl: proportion of agglomerates, δ: particle growth, θ: contact angle between the CW particle and the coating suspension. Different letters indicate a significant difference (p<0.05) by Tukey’s test.
ACCEPTED MANUSCRIPT HIGHLIGHTS
Extruded carnauba wax particles were evaluated in relation to their fluidization behavior
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The coating suspensions with the lowest contact angle were able to coat the particles
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Coated carnauba wax particles resisted at least for 1 hour at 100 °C, without volumetric variation
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The thermal behavior of coated and free PCMs particles was similar.
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Graphics Abstract
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
B.B. Paulo, K. Andreola, O. Taranto, A.D. Ferreira, A.S. Prata PII: DOI: Reference:
S0032-5910(18)30186-4 doi:10.1016/j.powtec.2018.03.003 PTEC 13234
To appear in:
Powder Technology
Received date: Revised date: Accepted date:
8 September 2017 21 February 2018 3 March 2018
Please cite this article as: B.B. Paulo, K. Andreola, O. Taranto, A.D. Ferreira, A.S. Prata , Coating approach for a Phase Change Material (PCM). The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Ptec(2017), doi:10.1016/j.powtec.2018.03.003
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ACCEPTED MANUSCRIPT Coating approach for a Phase Change Material (PCM) B.B. PAULO1*, K. ANDREOLA2, O. TARANTO 2, A.D. FERREIRA3, A.S. PRATA1 1
Department of Food Engineering, School of Food Engineering, State University of
Campinas - UNICAMP, Albert Einstein Avenue 500, 13083-862, Campinas, Brazil, Phone: +55 (19) 3521-0069
2
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*[email protected]
Department of Process Engineering, School of Chemical Engineering, State University
of Campinas - UNICAMP, Albert Einstein Avenue 500, 13083-852, Campinas, Brazil,
3
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Phone: +55 (19) 3521-3926
Department of Mechanical Engineering, University of Coimbra, Pólo II, 3030-788,
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Coimbra, Portugal, Phone: (351) 239 790 727
ABSTRACT
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PCMs (Phase Change Materials) goes through volumetric variation during the solidliquid phase transitions. Encapsulation enables the preservation of their thermal efficiency and prevents adherence on the surface where they are applied. For PCM
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application, particles with high load are desirable, and the coating in fluidized bed represents a scalable way to protect them. However, this process requires PCMs in solid
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state and, thus, it is restricted to their phase change temperature. Carnauba wax (CW) has a high enthalpy and relatively high melting point value, which can be a promising
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organic PCM and desirable to be employed in processes of coating. This work aimed to coat CW particles in fluidized bed. Some strategies were adopted to increase the load of wax in a particle and reduce experimental assays of coating. For this purpose, CW particles were produced by cold extrusion and a preliminary selection of potential substances to be used as coating material (chitosan, Eudragit® L30-D55, gum Arabic, maltodextrin and sodium alginate) was performed based on their rheological and adhesive properties. Adhesive property was evaluated through the contact angle between the CW and the coating materials. Suspensions containing sodium alginate and Eudragit® showed the lowest contact angle (θ ≅ 40 °), low viscosity, and could restrain the volumetric variation of CW particles (coating efficiency = 55 %) under heating at
ACCEPTED MANUSCRIPT 100 °C for at least one hour. Coating solutions/suspensions with contact angle above 58 ° and high viscosity were not effective to coat the particles. This study allowed a successful coating process of CW particles in fluidized bed, with potential application to other PCMs and, consequently, enlarging the range of thermal responsive textile and construction materials, and food packages. Keywords: fluidization; fluidized bed; thermal energy storage; encapsulation
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1 INTRODUCTION
Several food and biological products are sensitive to temperature changes and must be properly transported and stored to ensure microbiological, biochemical, physiological, sensory and physical quality. Thermal responsive packages can be developed and they are interesting for food industry when used to keep the temperature
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of “ready-meals” or to avoid thermal variation of fresh and highly susceptible products to microbiological contamination products, to minimize waste [1]. Except expanded polyestyrene, standard materials traditionally used for
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packaging (plastic or cardboard) generally have limited thermal capacity. One possible way to increase it and keep the product at a desirable temperature is through storage of
packaging material [2].
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thermal energy by incorporating phase change materials (PCMs) into the structure of the
Phase Change Materials (PCMs) are substances with high fusion heat that,
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during the solid-liquid phase transition, are capable to store large amounts of thermal energy, and, isothermally, release it [3–6]. PCMs have demonstrated potential in several
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applications related from electronics industry, thermoelectric devices and photovoltaic materials [7–10] to renewable energy as solar heating systems[11–14], but also for
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transforming substrates giving them a thermal tunable function, as building and textiles materials[2,15–23]. Carnauba wax has been studied as a promising PCM [24], but, during the
transition solid-liquid, it can gradually adhere on the surface where they are applied, and this may reduce its [25,26] thermal efficiency. One possible way to enable its application is through microencapsulation. This technology can provide a resistant film around the PCM and control its volumetric variation during the phase transition, reducing the reactivity of the PCMs with the packaging material and increasing the surface area of heat transfer [25,27].
ACCEPTED MANUSCRIPT The main techniques used for the purpose of microencapsulation of PCMs are complex coacervation [24,28] and interfacial polymerization [17,29], which result in moist particles with low load, undesirable for some applications. Recently, Ushak et al. [30] studied the coating of inorganic PCMs in fluidized bed, which presents the advantage the large-scale production of dried particles. However, organic PCMs, such as waxes and lipids, present relative low melting point with great susceptibility to phase change during the coating process, which is the major drawback for operation in
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fluidized bed. Different classes of lipids have been used only serving as coating materials in fluidized bed [31,32]. To the best of our knowledge, there have been no reports of the use of the fluidized bed to coat lipid particles.
The coating of meltable lipid in fluidized bed requires a well outlined procedure for the successful performance of the operation. Accordingly, the purpose of
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the work reported here was to establish a basis for the choice of operational conditions for coating carnauba wax particles. Thus, a preliminary study for establishing ranges of thermo-fluid dynamics was based on the thermo-physical properties of carnauba wax
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particles. After, a careful selection of coating suspensions was carried out with basis on rheological analysis and adhesion properties onto particles surface to avoid excess of
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trial-and-error experiments. Finally, the validation of previous analysis was carried out, ensuring the coating process and the effectiveness of coating layer in the restraining of
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volumetric variation of carnauba wax during phase transition.
2.1 Material
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2 EXPERIMENTAL
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Commercial carnauba wax (CW) (type 3, Carnaúba do Brasil Ltda, Brazil)
was used as phase change material. Gum Arabic (GA) (Synth, Brazil), sodium alginate (SA) (Manugel DMB, FMC BioPolymer, USA), chitosan (CHI) (Polymar, Brazil), maltodextrin (MD) (Mor-Rex 1910, Corn Products, Brazil), Eudragit® (L30-D55, Evonik Röhm GmbH, Germany), Polyethylene glycol (PEG) 1500 (Synth, Brazil), magnesium stearate (Sigma-Aldrich, Brazil) and talcum powder (Colorcon do Brasil Ltda, Brazil) composed coating suspensions/solutions. Acetic acid (Synth, Brazil) was used for the preparation of aqueous CHI solution.
2.2 CW particle preparation
ACCEPTED MANUSCRIPT CW powders were extruded in a twin-screw extruder ZSK-30 (Werner and Pfleiderer, USA) with 30 mm barrel diameter at 150 rpm screw speed, 16.2kg/h feed rate and under temperature below the melting point (Tm = 78 °C) of the CW. The twinscrew extruder is divided in four controlled temperature zones, which were set at 30, 50, 60 and 60°C successively from the supply window to the die (2mm hole diameter). The extruded product was gently broken into a mill (2.8mm sieve opening) and characterized in relation to their size by sieving, bulk and apparent densities, porosity,
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and sphericity[33].
a) Particle diameter and morphology
The morphology of the CW particles was observed in steromicroscope Citoval 2 (Carl Zeiss Jena, Germany). The mean particle diameter was determined by sieving the particles in standard set of Tyler sieves. The particle size distribution was
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measured after sieve particles using a shaker Magnet 100 (Bertel Indústria Metalúrgica Ltda, Brazil) for 10 minutes, in a set of Tyler 9, 10, 12, 16 and 10 series screens, whose openings are 2.00, 1.70, 1.40, 1.00 and 0.85 mm. Then, the CW particles were separated
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in the particle size range between 1.70 and 1.40 mm, obtaining a mean diameter of 1.55
b) Sphericity
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mm, to carry out the other physical characterization tests and coating experiments.
The sphericity of the particles was determined by image analysis obtained in
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the stereomicroscope (Carl Zeiss Jena, Germany), using ImageJ software. Equation 1
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describes the principle used by the software to determine the sphericity. ∅=
Dci
(1)
Dcc
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where Dcc and Dci are the diameters of the circumscribed and inscribed circle, respectively, determined from the projected area of the particle.
c) Bed porosity Particle bed porosity was determined by the ratio of apparent density to bulk
density. ε=1−
d) Bulk density
ρbulk ρap
(2)
ACCEPTED MANUSCRIPT A mass of 5 g particles was put in a 10 mL calibrated beaker, without forcing them to settle, and the volume occupied by the particles was measured. This test was performed at least 10 times. Bulk density was calculated according to Equation (3). ρbulk =
mp
(3)
V
where mp is mass of particles (g), V is volume occupied (cm³).
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e) Apparent density A 50 mL pycnometer was filled with water, and the corresponding mass was determined (mliquid). Then, an amount of particles (mparticles) was added to the empty pycnometer, the volume was completed with water, and the new mass of the liquidparticle system (mLP) was quantified. The apparent density of the particle was calculated by Equation 4.
[mparticles − (mLP − mliquid )]
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2.3 Coating material preparation
ρliquid × mparticles
(4)
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ρap =
Increasing concentrations of solutions of each polymer (2-4 wt% CHI, 3045 wt% GA, 45-60 wt% MD, 1-3 wt% SA) were prepared by homogenization at room
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temperature with a magnetic stirrer until complete dissolution in distilled water, except for chitosan, which is soluble only in acidic conditions and, therefore, 1% (v/v) acetic
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acid solution was used. Three coating polymeric suspensions (Table 1) were also
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prepared using a mechanical stirrer for 10 min.
2.3.1 Rheology
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Rheological measurements of solutions/suspensions were performed in triplicate after 24h of preparation using a controlled stress rheometer AR 1500ex (TA Instruments, England), with stainless steel cone-plate geometry (60mm, 1.59°). Flow curves were obtained using an up-down-up steps program with shear rate ranging from 0 to 300s-1 at 25°C. The third flow curve data was fitted to the Herschel-Bulkley’s model (HB).
2.3.2 Contact angle Twenty contact angle (θ) measurements were performed with each coating solution/suspension using a goniometer (Contact Angle Meter, Tantec, USA). A drop of
ACCEPTED MANUSCRIPT the coating solution was deposited on a flat and smooth surface of CW, whose image was projected onto a graduated bulkhead and then visually measured the contact angle.
2.4 Coating of CW particles in fluidized bed Experiments of coating of CW particles were performed using a fluidized bed (Figure 1[34]) with a transparent acrylic Plexiglas® conical-cylindric shape (conical base: inlet air diameter=0.075m, height=0.15m; cylindrical column:
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diameter=0.15m, height=0.60m) and a perforated stainless-steel screen (hole diameters= 43μm).
The bed pressure measurements were obtained using a water millimeter differential manometer. The temperature and relativity humidity of the fluidizing air were monitored by Pt-100 thermoresistances (Novus, Ø 3.0mm x 100mm) and thermo-
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hygrometers (Novus, RHT-XS) during the experiments. These data were recorded by a data acquisition system (NI cDAQ-9172, National Instruments) and processed through the LabVIEW 8.6 software building a virtual instrument (VI).
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A calibrated peristaltic pump (Cole Parmer, model 7518-10, Masterflex L/S type) pumped the coating material through a bi-fluid nozzle (SU12A, Spraying
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Systems), with atomization pressure, supplied by a 2 cv compressor (Schulz, CSA 7.8/25).
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2.4.1 Evaluation of fluid dynamic behavior of particles The geometric and aerodynamic properties of particulate affect the onset
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of fluidization, and the main parameters of processes in fluidized beds. Theoretical equations are normally used to obtain some relevant parameters for the fluidization and
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predict the fluid dynamics behavior of particles [35,38]. The minimum fluidizing air velocity (umf) was determined by measuring
the bed pressure drop (ΔP) as a function of fluidizing air flow for the particles without binder atomization. The theoretical umf and terminal velocity (ut) of the particles were determined using the Equations5 and 6[35], 7 and 8[36], respectively. 𝐑𝐞𝐦𝐟 =
𝛒𝐠 𝐮𝐦𝐟 𝐝𝐩
𝐑𝐞𝐦𝐟 = [(𝟐𝟖. 𝟕)𝟐 + 𝟎. 𝟎𝟒𝟗𝟒 ( 𝐮𝐭∗
𝟏𝟖
= [𝐝∗ + 𝐩
(5)
𝛍 𝐝𝐩 ³ 𝛒𝐠 (𝛒𝐩 − 𝛒𝐠 )𝐠
)]
𝛍𝟐
𝟐.𝟑𝟑𝟓−𝟏.𝟕𝟒𝟒∅𝐬 (𝐝∗𝐩 )
𝟎.𝟓
𝟎.𝟓
− 𝟐𝟖. 𝟕
(6)
−𝟏
]
(7)
ACCEPTED MANUSCRIPT 𝐝∗𝐩 = 𝐝𝐩 [
𝛒𝐠 (𝛒𝐩 − 𝛒𝐠 )𝐠
]
𝛍𝟐
𝟏/𝟑
(8)
In which: ρp and ρg: particle and air densities (kg/m³), respectively; dp: particle diameter (m); g: acceleration of gravity (9.8 m/s²), µ: air viscosity (Pa s), ∅s: sphericity of particle.
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2.4.2 Process of particle coating
For all the runs, these conditions were fixed: 0.20 kg of the CW, fluidizing air velocity u=1.27 m/s (u>umf, regarding that the size and the mass of the particles would increase along the coating process), the atomizing air pressure of 1.0 bar, the nozzle height at 200 mm from the lower base of the cone, and the air temperature was 60°C. The CW particles were preheated for 10 min under the fluidization conditions in
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order to ensure the equilibrium of the system at 60 °C during the coating process. 250 mL of polymeric solution/suspension was atomized onto the particles at room
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temperature and fixed flow rate, which depended on the composition.
2.4.3 Characterization of coating process and coated particles
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The coating efficiency (CE) is defined as the ratio between the mass of solids adhered to the particles (real) and the solids mass of the suspension added to the
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fluidized bed (theoretical) (Equation 9)[37]. The real coating mass was determined by
CE
dissolving the coating of the waxy particles. CE (%) =
Mf − Mi Qat ρcs Cs t
x 100
(9)
In which: Mf and Mi: mass of particles with and without coating (g),
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respectively, Qat: mass flow rate of atomization of the coating suspension (mL/min), ρcs: density of the coating suspension (g/mL), Cs: solids concentration (fraction) of the suspension, t: process time (min). The relative growth (δ) of the particle represents the amount of coating (Mf- Mi) over the particle (Mi). The agglomeration index (Agl) is defined as the ratio between the mass of agglomerated particles and the total mass of particles loaded to the bed at the beginning of the test. Particles that had a diameter greater than 3 times the average size of the extruded particles without coating were considered agglomerates.
ACCEPTED MANUSCRIPT The morphology of all particles was evaluated in a Citoval 2 stereomicroscope (Carl Zeiss Jena, Germany). Melting and crystallization curves of the free and coated CW particles were determined by DSC technique (DSC 2920 Modulated, TA Instruments, USA). Heating and cooling cycles at 2°C/min rate were performed for each sample using 4 mg of sample weighed in airtight aluminum capsules. Samples were heated at 100°C and maintained for 15 min at 100°C, cooled to 0°C and maintained at 0°C for 15 min. The
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thermograms obtained were normalized based on the mass of the samples. The melting and crystallization temperatures (Tm;Tc) and the phase transition enthalpies (ΔHfusion/crystallization) were determined by the TA Universal Analysis software.
3 RESULTS AND DISCUSSION
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CW serving as PCM can be used for the manufacture of oven or microwaves packages as standard materials traditionally used for packaging (plastic or cardboard) have limited thermal capacity.
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Fig. 2 shows the thermal behavior of CW particles, which confirms its use as organic PCM due the high melting/crystallization enthalpy[12,19,38]. Tm was 78°C and it was determined from the last peak of the thermogram, whereas a softening point
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was considered at 60°C, which is extracted from the 2nd peak of the thermogram and can represent the initial melting of minor fatty acids of the composition of the CW , .
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This point allows to set the operational temperature during the extrusion and coating processes.
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The PCM encapsulation is important for avoiding the leakage of the liquid PCMs from the substrate. Coating in fluidized bed can provide dried particles with high
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load, but the coating process is highly complex, because it includes the fluid dynamics study of the particles through their packaging properties, the understanding of the adhesion properties of the coating material and the determination of the coating operating conditions. An approach for each topic above was taken to achieve the coating process and reduce the number of trial-and-error experiments.
3.1 Study of fluidization conditions and maximum drying temperature In fluidization process, particles move randomly in a fluid-like state through an upward flow of another fluid which maintains the suspended bed of particles[36]. Thus, the limit of operation ranging from the beginning of the fluidization
ACCEPTED MANUSCRIPT regime (umf) to maximum air velocity without dragging the particles (ut) can be estimated by theoretical equations (equation 5-8) based on density and diameter of particles as well as properties of the fluid used. The physical characteristics of the CW particles are shown in Table 2. Based on these equations, the theoretical range of fluidization is from 2.2 to 10.5 m/s for CW particles. Starting from this reference, the fluid dynamic of the particle bed was previously analyzed to establish the real conditions for the coating
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experiments. Fig.3 shows the pressure drop curve as a function of the air fluidization velocity. This curve did not represent the behavior normally observed for fluidization regime. In this regime, the velocity increases with the pressure drop across the bed until a maximum value, falls slightly, reaching a constant value, at which the pressure drop through the bed is equal to the bed weight per unit cross-sectional area.
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In contrast, experimental data show that, with the increase of air velocity, the pressure drop does not reach a constant value, tending to reduce. This behavior is similar to spouted beds[39], in which the movement of particles is very vigorous, due to
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their large size, which makes it difficult to establish a point of minimum fluidization. The experimental value umf=0.7m/s, lower than the theoretical value (2.2 m/s),
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confirmed a deviation of classic fluidization regime. In spouted regimes, at low air velocities, the pressure drop increases with the air velocity , because the fluid only percolates between the particles and the system behaves as a fixed bed. However, with
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the further increase of the air velocity, the maximum pressure drop is reached due to the rupture of the particle bed by the fluid, similarly to the fluidization regime. In this study
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this behavior was observed, but continuing with the increase of the air velocity, the pressure drop decreased, due to the instability in the porosity of particle bed, result of
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the frequent formation of bubbles and the oscillation in bed height. The pneumatic transport will occur only above ut of particles (10.5m/s). Tests were performed to establish the thermal profile inside the drying
chamber, with and without particles, by coupling temperature sensors throughout the bed. For a set point of 60°C, the temperature within the bed is below or equal to 60°C, indicating that the coating process could be carried out carefully, since this temperature is the softening point of the CW particles (Fig. 2).
3.2 Study of coating process
ACCEPTED MANUSCRIPT The PCM coating must protect the CW particles against the volumetric changes that occur during the liquid-solid phase transition, reducing the reactivity of this component with the external environment[5]. Thus, the protection film formed must have mechanical strength to withstand collisions during the coating process, and the stress caused by the melting/crystallization behavior of the CW. However, the coating of the CW particles has some experimental limitations. These particles initiate their phase change around 60°C (Fig. 2) and cannot
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be reused after coating. In addition, the combination of processing and film formation are related to some properties of the coating material, which determine the wettability of the solid. Its molecular weight is responsible for rheological and elastic behaviors and, consequently, for the maintenance of the droplets over the particle surface. Thus, to minimize experimental tests of coating, some factors should be considered: a) T<60°C,
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which reduces the driven force for evaporation. Thus, a maximum flowable concentration of solids of coating material was chosen to reduce the drying time and favor faster formation of the coating film. Both aspects minimize the effects of
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temperature on the shape of the wax particle; b) The film formed must adhere efficiently to the particle surface during drying, without unsticking; c) The film formed must
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restrain the CW volumetric variation during the phase change.
3.2.1 Rheology of coating solutions
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Since rheological behavior of the coating materials is an indication of the flow during the atomization, it was considered an initial criterion for the selection of
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coating material, aiming to operate with fluids at the maximum solids concentration. The potential coating solutions were produced in three concentrations, in which the
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minimum value was based on information from the literature[40–43]. Fig. 4 shows the rheological properties obtained for all polymer solutions
used, whose coefficient of determination (R²) obtained was 0.999 for the adjustment of all systems to the HB’s model. GA and MD, even in high amounts of solids, showed a behavior closer to the Newtonian (n=1). In contrast, SA and CHI obtained pseudoplastic behavior (n<1.0), which accentuates according to the increase of solid concentration. However, these compounds were very viscous even at low concentrations, such as solutions of 1 wt% SA and 3 wt% CHI, which have viscosities equivalent to solutions of 30 wt% GA and 45 wt% MD. This change in viscosity can be a consequence of the highest molecular
ACCEPTED MANUSCRIPT weights of the polymers SA and CHI, and the interactions between their chains when dispersed. In general, all coating material solutions at the highest solid concentration presented very high viscosity and bubbles, which could cause discontinuities of the coating on the particle. These high viscosities hampered the complete homogenization of the solutions and could impair the flow during the atomization. In this perspective, solutions of coating material with ηap<1 Pa s were chosen for contact angle analysis,
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which are solutions with 1 and 2 % (w/w) SA, 30 and 35 % (w/w) GA, 45 and 50 % (w/w) MD, 2 and 3 % (w/w) CHI.
3.2.2 Expectation of adhesion of the coating material to the particle
The contact angle is commonly used to determine the wettability and,
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therefore, as another selection criterion for coating material, since it is possible to predict the best material that will adhere onto the particle. Complete spreadability or wettability can be found with lower adhesion
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work, and therefore, low contact angles favor the coating, since it makes the work of liquid-solid adhesion equal to or greater than the cohesion of the liquid. Another
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important property is surface tension of the polymer solutions and it can present a trend to reduce with the increase of solid concentration [44]. Donida et al.[45,46] studied the coating efficiency of inert particles with
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different polymer suspensions in spouted bed. The limit value of θ=75° indicated good wettability on the particle surface resulting in higher adhesion values, and,
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consequently, higher CE.
In our study, the contact angle of the polymeric solutions ranged from
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89.2° to 66.5° (Table 3), in which the increase of the solid content resulted in the reduction of the contact angle. For synthetic/inert substrates, the contact angle is more reliable to
adhesion behavior because the substrate surface used for measuring the contact angle is similar to the real particle. However, CW presented irregular surface and, besides the contact angle and surface tension, other parameters may also be involved in the adhesion behavior, such as hydrophobicity, roughness of substrate and chemical interactions[47].
3.2.3 Coating validation test
ACCEPTED MANUSCRIPT From the previous analysis of viscosity and contact angle, the criteria were established: viscosities lower than 1 Pa s and the lowest contact angles. Thus, two coating solutions, GA (35% w/w, θ=78.2°) and SA (2% w/w, θ=66.5°) solutions, were chosen to represent a Newtonian and non-Newtonian fluid, respectively. Regardless of the rheological characteristic of the solutions, the coating tests (Tair=60°C; Qat=2.5mL/min) with both GA and SA solutions were ineffective, as the solutions dried prematurely and the coating powder was elutriated. These results
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may be associated to the low adhesion of the coating material onto the particle surface. The low contact anglesmay not have been low enough to form a film on the particle. Therefore, new formulations (Table 1) based on the preliminary tests and the studies developed by Donida[48] with some modifications were prepared to reduce the contact angle.
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The components present in the formulations have specific characteristics that improve the coating process. Among them, PEG1500, a plasticizing agent that has the function of giving greater flexibility to the coating film to withstand the mechanical
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stress, and adjusting the viscosity and consistency of suspensions; talcum powder and magnesium stearate are smoothing and separating agents to prevent tackiness of the
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film, reducing its agglutination when dried; magnesium stearate is responsible for reducing the surface tension of the suspension, facilitating its spread on the particle surface and on the dry film layer on the particle[49].
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In general, the polymeric suspensions presented viscosities and contact angle remarkably lower than those presented by solutions. Viscosity corresponded to
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2.6, 0.6 and 4.7 mPa s at 100 s-1 for F1(SA), F2(Eudragit®) and F3(GA), respectively, being all suspensions showed near-Newtonian behavior (n≈1), and contact angle was
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43.1, 40.4, and 57.2° for F1(SA), F2(Eudragit), and F3(GA), respectively. Both suspension F1 and solution with SA at 1 % (w/w) presented different values of viscosities and contact angle, confirming that the addition of the coadjuvant materials contributed to reduce the viscosity and the contact angle. The coating was performed, and Table 4 summarizes the coating process results and parameters. For application of PCM particles, the formation of agglomerates is undesirable, and the number of agglomerates was extremely low for all formulations tested. The formation of agglomerates is related to two main factors. The first one is the intrinsic characteristic of the coating material, as was observed by Albanez, Nitz and
ACCEPTED MANUSCRIPT Taranto[50], when they used different polymers submitted to the same coating process conditions. Although such polymeric materials had adhesion to the particle surface, the fractions of agglomerates obtained were different. The other factor is related to the inadequate operating conditions used during the process. Temperature and relative humidity are driven forces for coating by evaporation in fluidized beds, and operational parameters influencing on the size distribution of droplets of atomized suspension should be controlled to avoid premature drying (very small droplets) or overwetting
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(very large droplets). In this perspective, many parameters influencing the heat and mass transfer have a significant role such as temperature and atomization rate[50–53]. As for the coating of PCMs, the air temperature is restricted to that of phase change of the material, the atomization flow was the main variable and it was adjusted based on a mass and energy balance applied between the outlet (60°C, RHo=25%), and the
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saturation conditions of the fluidized bed. According to this analysis, the maximum water flow to be evaporated would be 3.9 mL/min. Therefore, the atomization rate used was 2.5 mL/min for F2(Eudragit®) and F3(GA), and 2.0 mL/min for F1(SA), ensuring
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that this condition was enough to avoid agglomeration in these systems. Growth value is a parameter to predict the efficiency of coating onto
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particles, whose the range found in the literature is normally between 1 to 5%[37]. Higher value was found for F2, which had the lower contact angle. Hemati et al.[52] also verified the increase of the particle growth with the decrease of the contact angle.
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Fig. 5 shows the relation between the CE of the suspensions and their respective obtained contact angles. SA (θ=43.1°) and Eudragit® (θ=40.4°) suspensions
CE
were efficient for coating CW particles, presenting CE=55%. In contrast, GA suspension dried and did not adhere onto CW particles surface, reaching a very
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inefficient level of coating (CE=15%), and, consequently, data related to the particles such as growth or agglomeration were not obtained. This result was confirmed by checking the morphology of the free and
coated CW particles as function of a time and temperature. CW particles coated by the F1(SA) and F2(Eudragit®) could restrain the volumetric variation for at least 1h at 100°C (Fig. 6). In contrast, the particles resulting from the process using the F3(GA) showed a very similar behavior to that of the free CW particles. This poor adhesion of GA onto CW particles was indicated by highest contact angle, which can be associated to chemical interactions, as, for example, to the hydrophobicity of the materials. Eudragit®, as derived from ester of acrylic acids is
ACCEPTED MANUSCRIPT more hydrophobic than GA and SAs. Conversely, all polymers, SA, GA and Eudragit® L30-D55 are negatively charged in their natural pH, but with different magnitudes. Therefore, there are additional intrinsic parameters of each material, for example, molecular weight, surface tension, molecular conformations and arrangements, and other physicochemical properties of the coating materials that result in different interactions with the substrate. In general, CE was lower than that obtained by Donida et al.[37,46,54] and
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Paulo Filho, Rocha and Lisboa[55], indicating other influencing factors besides the selection of appropriate coating material. Properties such as bulk density, porosity and shape have a great influence on the fluid dynamics of the particle bed and on the drying of the suspension. The spherical shape of the particle minimizes the friction between the particles during the coating, aiding the formation of a homogeneous film. High porosity
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facilitates the adsorption of the suspension and prevents the presence of free moisture on the particles and liquid bridges that bind them, maintaining the intense circulation of these in the bed, even with the addition of moisture[37,47].
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Fig. 7 shows the thermal behavior of free and coated particles obtained by differential scanning calorimetry (DSC). All particles presented similar values of Tm, Tc,
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ΔHm and ΔHc. Therefore, besides the coating was effective in restraining the volumetric variation, it also did not modify the thermal properties and behavior of the particles
4 CONCLUSIONS
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during the solid-liquid phase transition, ensuring their application as PCMs.
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CW particles produced by cold extrusion at 16.2 kg/h showed a good bubbling fluidization behavior, when using umf=0.7m/s and T=60°C, the softening point
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of CW particles, which required a very well stablished protocol for coating. Rheological and adhesive characteristics were effective criteria for
selection of appropriate coating materials. Coating materials with θ>58° could not coat CW particle. SA and Eudragit® suspensions containing adjuvant materials (θ=40°, CE=55%) could restrain the volumetric variation during the phase transition of the PCM, indicating that Eudragit, a high cost synthetic polymer, can be replaced by a natural one. This study allowed a successful coating process of meltable particles in fluidized bed, with potential application to other PCMs and, consequently, enlarging the range of thermal responsive materials.
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ACKNOWLEDGEMENTS This work was supported by São Paulo Research Foundation – FAPESP [grant numbers 2011/04230-5 and 2015/11629-2].
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ACCEPTED MANUSCRIPT List of the figure captions Figure 1. Scheme of the fluidized bed [34] Figure 2. Thermal behavior for free carnauba wax particles (ΔHm; ΔHc: melting and crystallization enthalpy, respectively; Tm; Tc: melting and crystallization temperatures, respectively) Figure 3. Pressure drop for carnauba wax particles
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Figure 4. Rheology of the coating solutions Figure 5. Relation between coating efficiency and contact angle
Figure 6. Checking the coating efficiency to restrain volumetric variation at 100 °C: A) Free; B) F1 (SA); C) F2 (Eudragit®); D) F3 (GA)
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Figure 7. Thermal behavior of free and coated CW particles (ΔHm; ΔHc: melting and crystallization enthalpy, respectively; Tm; Tc: melting and crystallization temperatures, respectively): A) crystallization; B) melting
ACCEPTED MANUSCRIPT Table 1. Chemical composition of the coating suspensions Formulation (% w/w) Component F2
F3
1.00
-
-
Eudragit® L30-D55
-
16.70
-
GA
-
-
16.70
PEG1500
0.80
0.80
0.80
Magnesium stearate
1.10
1.10
1.10
Talcum powder
3.75
Pigment
0.20
Water
93.15
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SA
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F1
3.75
3.75
0.20
0.20
77.45
77.45
ACCEPTED MANUSCRIPT Table 2. Physical characteristics of CW particles CW particles
Dp (mm)
1.5 ± 0.1
ρbulk (g/cm³)
0.54 ± 0.01
ρap (g/cm³)
0.80 ± 0.06
ε
0.326
Roundness
0.7 ± 0.2
Sphericity
0.8 ± 0.1
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Properties
Morphology
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Dp: particle diameter; ρbulk, ρap: bulk and apparent densities, respectively; ε: bed porosity
ACCEPTED MANUSCRIPT Table 3. Contact angle (θ) between CW and coating solutions at different concentrations (% w/w) θ (°)
CHI - 2 %
75.6c ± 5.3
CHI - 3 %
75.6c ± 5.2
GA - 30 %
82.0b ± 4.0
GA - 35 %
78.2bc ± 4.1
MD - 45 %
89.2ª ± 4.6
MD - 50 %
77.5bc ± 3.7
SA - 1 %
76.6bc ± 5.8
SA - 2 %
66.5d ± 4.5
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Coating solutions
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Different letters indicate a significant difference (p<0.05) by Tukey’s test
ACCEPTED MANUSCRIPT Table 4. Results of experiments of coating Suspension
Qat (ml/min) Ti (°C) Tan (°C) To (°C) RHi (%) RHo (%) Hp (%) η (%) Agl (%) δ (%)
θ (°)
F1_SA
2.0
58.7
58.7
46.6
34.7
21.8
0.3
55.21
0.05
2.4
40.4b ± 4.5
F2_Eudragit®
2.5
60.0
54.8
44.3
52.4
28.7
0.3
55.23
0.40
4.0
43.1b ± 4.1
F3_Gum Arabic
2.5
59.1
57.2
45.4
35.7
23.1
0.2
15.33
-
-
57.2a ± 4.2
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Qat: flow rate of atomization of the coating suspension; Ti, To, RHi, RHo: temperature and relative humidity of air in the inlet and outlet, respectively, Tan: temperature in the conic region of the bed, Hp: humidity of particles at the end of the process, η: coating efficiency, Agl: proportion of agglomerates, δ: particle growth, θ: contact angle between the CW particle and the coating suspension. Different letters indicate a significant difference (p<0.05) by Tukey’s test.
ACCEPTED MANUSCRIPT HIGHLIGHTS
Extruded carnauba wax particles were evaluated in relation to their fluidization behavior
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The coating suspensions with the lowest contact angle were able to coat the particles
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Coated carnauba wax particles resisted at least for 1 hour at 100 °C, without volumetric variation
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The thermal behavior of coated and free PCMs particles was similar.
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Graphics Abstract
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7