- Email: [email protected]
Preparation of acrylic based microcapsules using different reaction conditions for thermo-regulating textiles production
Accepted Manuscript Preparation of acrylic based microcapsules using different reaction conditions for thermo-regulating textiles production A.S. Carreira, R.F.A. Teixeira, A. Beirão, R. Vaz Vieira, M.M. Figueiredo, M.H. Gil PII: DOI: Reference:
S0014-3057(16)31580-4 http://dx.doi.org/10.1016/j.eurpolymj.2017.05.027 EPJ 7883
To appear in:
European Polymer Journal
Received Date: Revised Date: Accepted Date:
27 November 2016 24 April 2017 16 May 2017
Please cite this article as: Carreira, A.S., Teixeira, R.F.A., Beirão, A., Vaz Vieira, R., Figueiredo, M.M., Gil, M.H., Preparation of acrylic based microcapsules using different reaction conditions for thermo-regulating textiles production, European Polymer Journal (2017), doi: http://dx.doi.org/10.1016/j.eurpolymj.2017.05.027
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Preparation of acrylic based microcapsules using different reaction conditions for thermo-regulating textiles production
A. S. Carreiraa,b, R. F. A. Teixeirac, *, A. Beirãob, R. Vaz Vieirab, M.M. Figueiredoa, M. H. Gila
a
CIEPQPF, Department of Chemical Engineering, University of Coimbra, Rua Sílvio Lima, Polo II, 3030-790, Coimbra, Portugal
b
Devan - Micropolis S.A., Tecmaia - Parque da Ciência e Tecnologia da Maia, Rua Eng. Frederico Ulrich, 2650, 4470-605, Maia, Portugal c
Devan Chemicals N.V., I.Z. Klein Frankrijk - Klein Frankrijkstraat 8, 9600 Ronse, Belgium
Corresponding author: Tel: +32 55 23 01 12, Fax: +32 55 23 01 19 e-mail address: [email protected] (R. F. A. Teixeira)
Abstract Acrylic based microcapsules containing octadecane as a phase change material (PCM) were prepared via suspension polymerization, using different types of initiator and/or polymerization temperature. Benzoyl peroxide (BPO), azobisisobutyronitrile (AIBN) and Trigonox 23 (TRIG) were used as thermal initiators in a range of temperatures from 40 ºC to 80 ºC. A comprehensive characterization of microcapsules was undertaken, regarding their chemical composition (by infrared spectroscopy), morphology (by scanning electronic microscopy (SEM)), particle size distribution (by laser diffraction) and thermal properties (by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA)). The 1
experimental results show that the studied reaction parameters influenced the monomer conversion profiles while their impact on the microcapsules properties was not quite relevant. The microcapsules exhibited mean particle sizes around 12 µm, melting enthalpies of approximately 175 J/g, PCM content about 70 % and were thermally stable up to 170 ºC, the temperature corresponding to 5% of weight measured by TGA. This study confirmed that octadecane can be successfully encapsulated via a suspension polymerization technique, exhibiting a well-defined core/shell structure. Moreover, employing TRIG as initiator enables the use of a reaction temperature as low as 40 ºC. The acrylic shell microcapsules with encapsulated octadecane show a good potential to be incorporated in textile substrates in order to produce thermo-regulating textiles.
Keywords: Thermo-regulating textiles; Phase change material; Microencapsulation; Suspension polymerization technique
1. Introduction The design and development of textiles with thermo-regulating properties has attracted much attention in recent years [1]. This new functionality can be added to the textile substrates through the incorporation of phase change materials (PCMs). When the ambient temperature increases above the melting point of the PCM, the material changes from solid to liquid and the latent heat is absorbed, interrupting the increase in temperature of the textile substrate. When the temperature decreases, the PCM solidifies and the stored heat energy is released, providing a heating effect [2,3]. In order to allow the incorporation of PCMs into textile substrates and to withstand the repeated phase changes, these compounds must be externally protected. This protection may
2
conveniently be achieved by microencapsulation processes [2,4,5]. The encapsulation of PCMs through the production of acrylic microcapsules via suspension polymerization technique has been revealed to be a very promising technique [6–10]. In this technique, the polymeric chains are formed by consecutive linking of monomers at the reactive end of a growing chain (chain reaction polymerization) [11]. In most cases, the monomers require an initiator to start the polymerization reaction. Upon a trigger, this initiator produces a free radical (reactive species) that, added to a monomer, starts the formation of one polymer chain (free radical polymerization) [11,12]. For thermal initiators, the decomposition rates are normally expressed in terms of their half-life, which is defined as the time necessary to decompose one-half of the initiator originally present, at a given temperature. The temperature at which each initiator has a half-life of 10 hours is one of the most used parameters (the 10 hour half-life temperature corresponds to the temperature at which a 50 % of the initiator is consumed in 10 hours) [11,13]. For the majority of PCM microencapsulation processes via suspension polymerization, both benzoyl peroxide (BPO) (using styrene [14– 16] or acrylic monomers [6,9,10,17]) and azobisisobutyronitrile (AIBN) (using acrylic monomers [7,8,18–22]) have been used as thermal initiators. These initiators present a 10 hour half-life temperature of 70 ºC and 65 ºC, respectively. Concerning the PCM microencapsulation based on acrylic polymers, the polymerization temperature is normally above 50 ºC [23]. In particular, the suspension polymerization processes involve temperatures over 70 ºC [6,9,10,20,24]. Qiu et al. [23] tried to reduce the reaction temperature when preparing methyl methacrylate microcapsules containing octadecane, by combining AIBN and redox initiators. However, after an initial period of 2 hours at low temperature (between 35 ºC and 55 ºC), the reaction temperature had to be increased to 85 ºC during 4 hours [23]. Moreover, and as the authors recognized, the prepared microcapsules exhibited heat capacities and thermal stabilities lower than the microcapsules
3
prepared with AIBN or BPO at 85 ºC [23]. In the present work, efforts to reduce the reaction temperature were made by using a thermal initiator with a low half-life decomposition temperature - the Trigonox 23 (TRIG), which has a 10 hour half-life temperature of 48 ºC. We have found no evidence that TRIG has been used to encapsulate PCMs, nor to produce acrylic microcapsules. The final goal in using temperatures below 50 ºC is not only to reduce energy consumption but also to develop a process suitable to encapsulate core materials other than octadecane and that may degrade at moderate temperature. In this study, acrylic based microcapsules were produced via suspension polymerization technique using three initiators (BPO, AIBN and TRIG) at different reaction temperatures and with and without octadecane. For each experiment, the monomer conversion was followed and the prepared microcapsules were fully characterized with respect to their morphology and chemical and thermal properties, having in mind their potential to be further incorporated into substrates to produce thermo-regulating textiles.
2. Experimental procedure 2.1. Materials Benzoyl peroxide (BPO, purity 75% - remainder water) supplied by ACROS, azobisisobutyronitrile (AIBN, purity 98 %) purchased from Sigma-Aldrich and Trigonox 23 (TRIG, purity 95 %) from Akzo Nobel Polymer Chemistry (kindly provided by Companhia Industrial de Resinas Sintéticas, CIRES Lda) were used as thermal initiators. The stabilizer poly(vinyl alcohol) (PVA, Mw = 31000 – 50000 g/mol and degree of hydrolysis = 87-89 %), was purchased from Sigma-Aldrich. All the acrylic compounds that constitute the monomers mixture and crosslinkers were purchased from Sigma-Aldrich. The core material, octadecane
4
(commercial grade) was supplied ex. stock from Devan Chemicals N.V. All the reagents were used as received.
2.2. Microencapsulation procedure Suspension polymerization was carried out in a triple-walled glass reactor equipped with an external circulating heating bath, a condenser, a nitrogen inlet and an overhead stirrer (with an axial impeller stirrer, four-bladed). For the synthesis of microcapsules two different solutions were prepared. Solution (1) was obtained by mixing the mixture of monomers (25 g), the crosslinkers and the thermal initiator (1.2 wt.% related to the monomers mixture). This solution was heated up to 40 ºC and the melted PCM (50 g of octadecane) was subsequently added, maintaining under magnetic stirring for 15 minutes at 40 ºC. Solution (2) was prepared by mixing 2.5 g of PVA in 250 mL of water. Lastly Solution (1) was then added to Solution (2) drop wise and the mixture emulsified using a homogenizer at 6000 rpm for 10 minutes. The obtained emulsion was transferred to the reactor, previously heated at 40 ºC, under nitrogen flux and mechanical stirring (300 rpm). After 10 minutes, the temperature was raised to 80 ºC (or other specified value, Table 1) and the reaction was carried out during 5.5 hours under nitrogen atmosphere and continuous stirring. The microcapsules so obtained were washed with water at 50 ºC, repeated three times, to remove the unreacted monomers, stabilizer and free octadecane. After each washing cycle, the sample was filtered under vacuum to separate microcapsules from the water (using a filter paper with pore size of around 2-3 µm). The washed microcapsules were collected and stored as a wet cake (50 70% of solid content). Different types of initiators as well as reaction temperatures were used, as indicated in Table 1. For comparison purposes and to study the copolymer properties, blank experiments were also carried out (B-Bp80, B-Trig48 and B-Trig40). These experiments were performed
5
following the procedure previously described, but without the addition of octadecane to the monomers mixture.
Table 1: Initiator type and reaction temperatures used. Experiment
Initiator
Reaction Temperature (ºC)
Addition of octadecane
PCM-Bp80
BPO
80
Yes
PCM-Bp70
BPO
70
Yes
PCM-Aibn65
AIBN
65
Yes
PCM-Trig48
TRIG
48
Yes
PCM-Trig40
TRIG
40
Yes
B-Bp80
BPO
80
No
B-Trig48
TRIG
48
No
TRIG 40 No B-Trig40 B: refers to “blank experiments” that were conducted without the addition of octadecane.
2.3. Monomer conversion assessment The monomer conversion as a function of time was monitored via gravimetry. At predefined periods of time, a sample of approximately 2-3 mL was removed from the reactor (using a syringe), placed in an aluminium dish with known mass and weighed immediately. The dish was positioned in a highly ventilated place to cool the sample and to remove the volatile compounds. The collected samples were dried until constant weight. The monomer conversion percentages were calculated using a simple mass balance [25,26], between the mass of formed polymer (mpolymer) and the mass of monomers initially introduced (mmonomer mixture). Equation (1) and (2) were used to calculate the percentage of the dry extract (Es) and the mass percentage of monomers conversion [27]. =
× 100
(%) = =
× × 100 ! ! ! "
# !
! ! ! "
6
× 100
(1)
(2)
Where, in equation (1), the msolution is the mass of collected solution and mdry is its dried mass. In equation (2), mtotal is the total mass of solution in reactor, mmonomer
mixture
is mass of
monomers mixture initially introduced in reactor a Es corrected is the dry extract corrected by the mass of non-volatile compounds [22]. Samples were measured in duplicate and the standard deviation was always lower than 3 wt.%.
2.4. Fourier Transform Infrared Spectroscopy Fourier transform infrared (FT-IR) spectra were acquired in the range of 4000-550 cm-1 at room temperature, using a Jasco FT/IR-4200 spectrometer, equipped with a Golden Gate Single Reflection Diamond ATR. Data were collected with 4 cm-1 spectral resolution and 128 scans. Samples were previously dried in oven at 40 ºC.
2.5. Scanning Electron Microscopy The microcapsules morphology was assessed by scanning electron microscopy (SEM). For that, specimens of microcapsules were diluted using distilled water and placed onto carbon tape in appropriate supports. The specimens were dried at room temperature and subjected to a sputtering treatment to deposit a gold nano layer. Micrographs were obtained by using an electron beam voltage of 15 kV in a Leica Cambridge S360 equipment.
2.6. Particle size distribution Particle size distribution was measured by laser diffraction using a Malvern MasterSizer 2000s. The microcapsules were dispersed in distilled water prior to measurement. The particle size distribution (volume based) and the corresponding statistical mean values were determined to an average of at least three independent measurements per aliquot and 2 to 3 aliquots.
7
2.7. Differential Scanning Calorimetry Thermal behaviour was studied by differential scanning calorimetry (DSC), in a Netzsch DSC 200 F3 instrument with a cooling unit. Samples were analysed in aluminium crucibles with closed lid. Three thermal cycles were programmed: firstly the temperature was cooled, at 5 ºC/min, from 20 ºC to -10 ºC; then it was increased, at 5 ºC/min, from -10 ºC to 50 ºC; finally, the sample was cooled, at 5 ºC/min, from 50 ºC to -10 ºC. Isothermals of 3 minutes before each cooling and heating cycle were performed. Only the second and third cycle, which corresponded to the heating and cooling processes, respectively, were considered for analysis. Samples were previously dried in an oven at 40 ºC and a range of weights between 7 and 10 mg was used. DSC analysis was carried out in triplicate and results expressed as average ± standard deviation. The experimental PCM microcapsules content was estimated according to Equation (3) [23]:
$% && (%) =
∆(!,* # +,- + ∆( ,* # +, × 100 ∆(!,+,- + ∆( ,+,-
(3)
where ∆Hm,µcapsPCM and ∆Hc,µcapsPCM are, respectively, the melting and crystallization enthalpies of microcapsules containing octadecane, and ∆Hm,PCM and ∆Hc,PCM are the melting and crystallization enthalpies of octadecane [23]. Moreover, the theoretical PCM content was calculated based on equation (4):
/ℎ&12 $% && (%) =
m+, × 100 m +,- + m ! !
(4)
where mPCM and mmonomers are, respectively, the mass of octadecane and the mass of monomers used during the encapsulation experiment [17].
2.8. Thermogravimetric analysis 8
Thermogravimetric analysis (TGA) was evaluated using a TA Instruments Q500 equipment, at a heating rate of 10 ºC/min, in the range of 25 ºC to 600 ºC, using a nitrogen purge with a flow rate of 100 mL/min. Samples were previously dried in oven at 40 ºC and a weight range from 5 to 8 mg was used.
3. Results and discussion 3.1. Monomer conversion curves Figure 1 compares the monomer conversion curves of experiments carried out with the three initiators at respective 10 hour half-life decomposition temperatures: 70 ºC for BPO (PCM-Bp70), 65 ºC for AIBN (PCM-Aibn65) and 48 ºC when using TRIG (PCM-Trig48). As can be seen, the conversion-time curves obtained for these experiments almost overlap, denoting similar conversion rates. Nonetheless, the final conversion achieved for the experiment PCM-Aibn65 is slightly lower than that of the other experiments (92 % for PCMAibn65 versus approximately 97 % for the other two). In these experiments, the values of monomer conversion only starts to increase after around 30 minutes, which corresponds to the induction period of reaction (i.e., time during which no observable change occurs in the chemical reaction) [28]. This study demonstrates that, when the encapsulation procedure is carried out with the three different initiators at their 10 hour half-life temperatures, the monomer conversion present a similar rate.
9
Figure 1: Monomer conversion as a function of time for the three types of initiators used at the respective 10 hour half-life temperature: BPO at 70 ºC, AIBN at 65 ºC and TRIG at 48 ºC. See Table 1 for details.
In order to study the effect of the reaction temperature on the monomer conversion of suspension polymerization, additional experiments using the same initiators were carried out at another temperature: 80 ºC for BPO (cf. 70 ºC), Figure 2 (a), and 40 ºC for TRIG (cf. 48 ºC), Figure 2 (b). In the latter, due to the low monomer conversion rate, the reaction time was extended up to 7.5 hours. As expected, for both initiators, the slope of the conversion-time curves increases as the reaction temperature increases, denoting a faster polymerization rate. This effect is particularly marked for the initiator TRIG. Indeed, the values of monomer reaction for experiment PCM-Trig48 started to increase after 40 minutes, reaching 95 % of conversion after approximately 2.5 hours, while for PCM-Trig40, the reaction was initiated only after 2.5 hours and a monomer conversion of 97 % was reached only after 6 hours. This behaviour was expected since for a polymerization at higher temperature, the half-life of the thermal initiator is lower and its initiation activity is higher. For instance, BPO has a half-life of 10 hours at 70 ºC and a half-life of 1 hour at 91 ºC [13,29]. Although direct comparisons can not be made with the literature, due to differences in the initiator type, reaction temperatures and encapsulated PCMs, the reaction times detected in this work are of similar magnitude as those reported by other workers (e.g. Sánchez-Silva et
10
al. [30]). These authors encapsulated a PCM (PRS® paraffin wax) also using the suspension polymerization method and BPO as thermal initiator (at 98 ºC).
Figure 2: Monomer conversion as a function of time for experiments performed with BPO and TRIG at different temperatures: (a) BPO at 70 ºC and 80 ºC in the presence of octadecane and BPO at 80 ºC in the absence of octadecane; and (b) TRIG at 48 ºC and at 40 ºC in the presence and absence of octadecane (reaction time was extended up to 7.5 hours). See Table 1 for details (B refers to “blank experiments” that were conducted without the addition of octadecane).
Regarding the effect of the presence of octadecane (Figure 2), it is clear that in the absence of octadecane the reaction rate is faster, presenting a smaller induction period, but it shows a lower final monomer conversion. The shorter induction period is easily detected when comparing experiment B-Trig40 with PCM-Trig40: the monomer conversion values increased after 50 minutes in the former case instead of 2.5 hours in the latter. As previously mentioned,
11
for suspension polymerization technique, the polymeric shell is formed by free radical polymerization. Therefore, its reaction rate depends on the microenvironment and on the local monomer concentration where the free radicals are produced and where the initiation process takes place [25,31]. In the absence of octadecane, the concentration of the monomers, crosslinkers and initiator within the dispersed phase is higher, originating a fast polymerization rate (Figure 2).
3.2. Chemical characterization The FT-IR spectra of pure octadecane and of samples prepared with BPO at 80 ºC with and without octadecane are presented in Figure 3. The octadecane is an alkane mostly constituted by methylene groups. Thus, in its spectrum, two strong peaks at 2911 cm-1 and 2847 cm-1 that are caused by asymmetric and symmetric C-H stretching vibration of CH2, respectively, can be identified. In addition, two other peaks must be emphasized: one at 1470 cm-1, attributed to the bending vibration of methylene group, and another at 716 cm-1, ascribed to the alkane long chain [32]. In spectrum of sample B-Bp80 (without octadecane), from the acrylic copolymer, more specifically from the ester group, the strong peak at 1728 cm-1 from C=O stretching vibration and the band at 1190 cm-1 for its C-O-C stretching vibration are identified. Moreover, the peak that characterizes the monomers mixture (corresponding to the stretching vibration of double bond C=C) and that normally appears around 1635 cm-1, was not detected [32]. For microcapsules PCM-Bp80 (containing octadecane), the FT-IR spectrum includes the main peaks previously identified in the spectra of both octadecane and sample B-Bp80, confirming the presence of octadecane and the production of acrylic shell.
12
Regarding the remaining samples, prepared using other initiators and reaction temperatures (with and without octadecane) identical findings were detected. The FT-IR spectra of all samples are presented as supplementary information (Figure S1).
Figure 3: FT-IR spectra of pure octadecane, sample B-Bp80 (without octadecane) and sample PCM-Bp80 (with octadecane).
3.3. Morphology and particle size distribution Figure 4 shows the SEM micrographs of microcapsules prepared using BPO as thermal initiator at 80 ºC, with and without octadecane. As this figure illustrates, experiment PCMBp80 resulted in spherical microcapsules with different sizes exhibiting a rough shell surface (Figure 4 (a)). An example of a micrograph collected by optical microscopy, in which this morphology is visible, was added in the supplementary information - Figure S2. Regardless the type of initiator and reaction temperature selected to perform the experiments with 13
octadecane (Table 1), microcapsules with identical morphology were obtained (SEM micrographs presented as supplementary information - Figure S3). In contrast, the experiments performed without octadecane produced spherical microparticles but with smooth surface, as exemplified in Figure 4 (b) for sample B-Bp80.
Figure 4: SEM micrographs of samples (a) PCM-Bp80, (b) B-Bp80 (magnification x1000) and (c) broken microcapsules PCM-Bp80 (sample previously crushed to highlight the core/shell structure) (magnification x3000).
14
In suspension polymerization, the shell formation starts at the water-oil droplet interface. As the polymerization reaction proceeds, the monomers are consumed and transferred from the oil phase to the interface, increasing the shell thickness. This mechanism occurs towards the inner side of the microcapsule [33]. As observed in Figure 4 (c), the produced microcapsules exhibit a well-defined core/shell structure, with a thickness of approximately 1 µm. In the encapsulation procedure, the formation of this core/shell morphology is favoured when the polarity of the polymer and of the core material are distinct. As mentioned in the literature [9,14,30], it is the driving force between these polarities that creates a phase separation and produces a core/shell structure. In addition, the roughness of the microcapsules shell surface (Figure 4 (a and c)) seems to be associated to the density variations that occur during their formation. When the monomers react to form the copolymer shell, an increase in density occurs (the copolymer naturally presents higher density than that of individual monomers) [33]. This increase in density and the corresponding volume reduction seems to be responsible for the shell shrinkage and for the production of the observed rough surface [33]. However, it should be pointed out that density variation is not exclusive of the polymer: the octadecane also changes its density with temperature (0.81 g/cm3 for solid state and 0.69 g/cm3 for melted state at 80 ºC [33]). Consequently, after the reaction, when the microcapsules are cooled down from reaction temperature to room temperature, the octadecane volume could also be reduced. Many authors describe the combination of these events as the generation of reserved expansion space in the PCM microencapsulation [7,10,33].
In order to measure the particle size distribution, several samples were analysed by laser diffraction. The size distribution curves, on volume basis, for sample PCM-Bp80 and B-Bp80 are illustrated in Figure 5. Besides a broad size distribution (from submicrometer range to a 1
15
-20 µm), both curves exhibit a bimodal distribution (with one peak, around 10 - 11 µm, much more intense than the other at 1 - 2 µm), being this result is consistent with the SEM micrographs (Figure 4). This bimodal trend found for all samples regardless the type of initiator (see supplementary information - Figure S4). The presence of small particles could be attributed to the process of secondary nucleation [30]. This process occurs when a portion of monomers diffuses to the aqueous phase, forming small polymeric particles that do not contain any core material. This phenomenon is favoured by the hydrophilicity of the monomers as well as by the absence of inhibitors in the aqueous solution [30]. Figure 5 also shows that the size distribution curve of the sample without PCM is displaced to the left relatively to the microcapsules with octadecane.
Figure 5: Particle size distribution curves of microcapsules prepared with BPO at 80 ºC in the presence and absence of octadecane.
In order to compare the microcapsules sizes, the values of the mean volume diameter (D4,3) of all samples were plotted in Figure 6. These results confirm that the initiator type does not affect significantly the mean size of the microcapsules that is around 12 µm. Additionally, they show that the presence of octadecane leads to a considerable increase in particle size. This trend could be induced by the difference in viscosity of the dispersed phase of the 16
experiments carried out with octadecane relatively to those without octadecane. In fact, at 40 ºC (temperature of the emulsification process) the viscosity of octadecane is much higher than that of the monomers mixture (3.1 mPa.s vs 0.6 mPa.s) and thus the formation of larger droplets and, consequently, larger microcapsules is expected [9].
∑ 5 657
Figure 6: Comparison of the mean particle size (D4,3 = ∑
5 658
) of microcapsules obtained with different reaction
conditions. See Table 1 for details (B refers to “blank experiments” that were conducted without the addition of octadecane).
3.4. Thermal properties As the microcapsules containing PCM are to be incorporated in textiles to impart thermoregulating properties, their thermal characterization is of utmost importance. Phase change enthalpies and PCM content (estimated from Equation (3)) were determined by DSC whereas microcapsules thermal stability was assessed by TGA. As illustrated by the DSC curves of Figure 7, both octadecane and the microcapsules exhibit two peaks, one endothermic peak (downwards) for the heating process and the other exothermic peak (upwards) for the cooling process (supplementary information for the remaining sample is given in Figure S5). As shown in Table 2, the melting and crystallization
17
onset temperatures (Tom and Toc) of octadecane are, respectively, 26.7 ºC and 25.0 ºC. For the encapsulated octadecane, Tom values are similar to that of the octadecane and independent of the initiator type and temperature, while the values of Toc are considerably lower (between 15 ºC and 19 ºC) and slightly dependent on the reaction conditions. This decrease in Toc of microencapsulated octadecane compared to the pure core material is typical of the microencapsulation of n-alkanes and is known as the supercooling effect. It is considered an obstacle for the microcapsules application as thermal energy storage system and many alternatives have been sought to avoid this phenomenon [34–36]. However, in the present study, this effect was not explored.
Figure 7: DSC curves of pure octadecane and microcapsules prepared with BPO at 80 ºC (sample PCM-Bp80).
18
Table 2: Influence of initiator type and polymerization temperature on thermal performance (average ± standard deviation): melting and crystallization enthalpies, respectively ∆Hm and ∆Hc; melting and crystallization onset temperatures, respectively Tom and Toc; and PCM content, calculated using equation (3). Sample
T om (ºC)
∆Hm (J/g)
Toc (ºC)
∆Hc (J/g)
PCM content (wt.%)
Octadecane
26.7±0.1
252.3±2.8
25.0±0.1
252.1±3.0
--
PCM - Bp80
26.4±0.1
174.4±0.9
19.0±0.2
175.5±1.8
69.3
PCM - Bp70
26.5±0.0
178.1±8.8
18.4±0.1
180.6±9.3
71.1
PCM - Aibn65
26.4±0.1
207.8±7.6
18.7±0.3
209.0±8.4
82.6
PCM - Trig48
26.3±0.0
181.1±5.3
15.2±0.1
183.2±4.7
72.2
PCM - Trig40
26.3±0.1
183.5±4.6
15.8±0.1
185.4±5.5
73.1
In general, for microcapsules containing octadecane, the measured phase change enthalpies of melting and crystallization (∆Hm and ∆Hc) were around 175 – 185 J/g, corresponding to calculated PCM contents around 69 – 73 wt.%. The only exception was sample PCM-Aibn65 that exhibits a ∆Hm above 200 J/g (PCM content of 82.6 %). These values show that the produced microcapsules present a thermal performance quite satisfactory for energy storage applications. The values of PCM content depend on the relative quantity of octadecane with respect to the total quantity of the sample (octadecane and copolymer) (see equation (3)). According to this definition, a value lower than the theoretical value indicates that not all the octadecane was encapsulated, being removed during the microcapsules washing. On the other hand, a higher value could reveal that not all the monomers reacted and an amount of copolymer smaller than the theoretical was formed. The theoretical PCM content for the prepared microcapsules is 67 % (calculated using equation (4)) [17]. Therefore, for all the experiments, the PCM content is higher than the theoretical value, suggesting that, despite of any the possible presence of free and/or residual
19
octadecane, not all the monomers used in the formulation were able to react and form the polymeric shell. The results summarized in Table 2 suggest that the influence of the type the initiator and temperature on microcapsules thermal performance are not very relevant. Nevertheless, the use of AIBN as thermal initiator seems to produce microcapsules with high PCM content. As previously described, this sample also presents the lowest final monomer conversion (near 92%), while the other samples containing octadecane presented values around 97 % (Figure 1 and Figure 2). This lower monomer conversion could be responsible for a reduction of the quantity of produced copolymer and for an increase in the percentage of octadecane of this sample. However, this may not be the only cause for the smaller production of copolymer. The initiator properties can also be associated to this behaviour. Indeed, from the three initiators used, AIBN is the only azo-initiator that, compared to BPO, has much higher water solubility (350 mg/L at 25 ºC for AIBN versus 9.1 mg/L for BPO). Thus, for AIBN, more free radicals can be produced in the aqueous phase which probably react with the monomers that diffuse from the dispersed phase instead of reacting in the droplet interface to form the shell of the microcapsules.
Thermal stability was evaluated for the microcapsules containing octadecane as well as for the copolymer microparticles (without PCM) and for the pure octadecane. The curves of weight loss as a function of temperature are exemplified in Figure 8, showing the results obtained for octadecane and samples prepared with BPO at 80 ºC (information for the remaining samples is provided in Figure S6). The most relevant data extracted for all samples are summarized in Table 3. The curve of pure octadecane shows that the weight loss occurs in one stage, with an extrapolated onset temperature for the first degradation stage (Ton1) of 145 ºC (Table 3). This
20
profile was originated by octadecane evaporation before its flash point (166 ºC) and boiling point (308 ºC) [8,10,29]. For sample B-Bp80 (without PCM), one main weight loss is also observed but at a higher temperature (around 352 ºC), corresponding to the copolymer degradation. In this sample, a small weight loss (about 2 wt.%) at Ton1 of 127 ºC was detected, which could be caused by adsorbed water and/or the evaporation of unreacted monomers.
Figure 8: TG curves of pure octadecane and of samples prepared with BPO at 80 ºC with (PCM-Bp80) and without octadecane (B-Bp80).
Regarding the microcapsules PCM-Bp80, the thermogram reveals two distinct stages of degradation: the first one is originated by the weight loss of octadecane, that diffuses out through the shell membrane and evaporates; and the second one is created by the degradation of the polymeric shell [16]. For the other samples of microencapsulated octadecane, the same curve profile was obtained. As listed in Table 3, all samples of microcapsules containing octadecane present higher values of Ton1 compared with that of pure octadecane (185 ºC – 200 ºC versus 145 ºC), being the same tendency observed for the values of temperature corresponding to 5% of weight loss (T5%). This difference can be caused by the resistance and protection that the polymeric shell
21
promotes for the octadecane evaporation. Among these samples, small differences between the onset temperature values could be detected, revealing slight differences between their shell properties. In this respect, sample PCM-Bp80 is the one with the highest thermal stability, reaching a T5% of about 180 ºC. Regarding the values of weight loss, and as for DSC analysis, only microcapsules PCM-Aibn65 differ from the other ones (the weight loss in the first degradation stage is 80.2 % against around 70 % for the remaining samples). It is worth noting the similarity between the percentage of weight loss in the first degradation stage determined by TGA and the PCM content calculated from DSC data. For instance, for PCM-Bp80, the first weight loss determined by TGA is 70.2 % (Table 3) and the corresponding value of PCM content determined by DSC is 69.3 % (Table 2). This excellent agreement confirms that the weight loss in the first degradation stage corresponds in fact to the octadecane degradation/evaporation. The thermogravimetric results confirm that octadecane was successfully encapsulated using the suspension polymerization technique. They also demonstrate that the microcapsules are stable below T5% of 170 ºC, thus allowing their incorporation in textile substrates without undergoing thermal degradation.
22
Table 3: Influence of initiator type and polymerization temperature on thermal stability: T5%, temperature corresponding to 5% of weight loss; Ton1 and Ton2, extrapolated onset temperatures of the first and second degradation stage; and weight loss (%) in each degradation stage. 1st stage Sample
T 5% (ºC)
Octadecane
2nd stage
T on1 (ºC)
Weight Loss (%)
T on2 (ºC)
Weight Loss (%)
117.8
145.0
99.8
--
--
PCM - Bp80
181.2
199.7
70.2
351.6
29.3
PCM - Bp70
172.4
190.1
70.8
347.7
29.0
PCM - Aibn65
175.5
195.2
80.2
361.8
19.6
PCM - Trig48
172.2
188.4
69.7
354.4
30.0
PCM - Trig40
168.5
184.4
71.4
361.3
28.4
B - Bp80
278.3
126.7
1.9
345.5
96.8
B - Trig48
280.2
103.9
2.3
344.0
96.6
B - Trig40
284.0
99.4
2.7
344.2
96.4
B: refers to “blank experiments” that were conducted without the addition of octadecane.
4. Conclusions The use of different types of initiators (BPO, AIBN and TRIG) and temperatures revealed that the polymerization reaction at the respective 10 hour half-life temperatures present identical monomer conversion rates. Additionally, and as expected, with the increase of the reaction temperature the polymerization rate also increases. It was also demonstrated that the presence of octadecane decreases the reaction rate. Moreover, all experiments exhibited high rates of monomer conversion (except PCM-Trig40), reaching final conversions above 90 %. This work also confirmed that it is possible to prepare microcapsules using reaction temperatures of 40 ºC, making it possible to adapt the encapsulation procedure to entrap core materials that degrade at relatively low temperatures. Despite the variations in the monomer conversion profiles, the microcapsules exhibited only slight differences between them with respect to morphology and thermal properties.
23
SEM results revealed that in all experiments carried out in the presence of octadecane, spherical and individual microcapsules with a well-defined core/shell structure were produced. The existence of an acrylic based shell and of octadecane in the produced microcapsules was confirmed by FT-IR analysis. The size distribution resulted in a bimodal curve with a mean diameter (volume based) of around 12 µm. In addition, these microcapsules showed a ∆Hm around 175 J/g, a PCM content of 70 % and T5% around 170 ºC. In conclusion, the overall results show that acrylic shell microcapsules can be prepared to encapsulate octadecane by suspension polymerization using a variety of thermal initiators and reaction temperatures. As currently most of the microcapsules containing PCMs and industrially used are based on melamine-formaldehyde shell, the production of acrylic microcapsules is a good alternative, presenting the added value of being a formaldehyde-free product and thus more environmentally friendly. Although these microcapsules exhibit proper thermal performance and stability to produce textiles with thermo-regulating properties, they can also be used for many other thermal energy storage purposes, such as solar heating systems, heat exchangers, thermal comfort in buildings and air-conditioning systems [30,37].
Acknowledgement: A. S. Carreira thanks Fundação para a Ciência e Tecnologia (FCT) and Devan Micropolis, S.A. for her Ph.D. grant (SFRH/BDE/51601/2011). The authors thank Dr. John Ellis (Devan Chemicals NV) for proof-reading the manuscript.
References [1]
N. Sarier, E. Onder, The manufacture of microencapsulated phase change materials
24
suitable for the design of thermally enhanced fabrics, Thermochim. Acta. 452 (2007) [2]
S. Mondal, Phase change materials for smart textiles - An overview, Appl. Therm. Eng. 28 (2007) 1536–1550.
[3]
L. Sánchez-Silva, J.F. Rodríguez, P. Sánchez, Influence of different suspension stabilizers on the preparation of Rubitherm RT31 microcapsules, Colloids Surfaces A Physicochem. Eng. Asp. 390 (2008) 62–66.
[4]
S.K. Ghosh, Functional Coatings: by Polymer Microencapsulation, WILEY-VCM, 2006.
[5]
L. Liu, G. Alva, X. Huang, G. Fang, Preparation, heat transfer and flow properties of microencapsulated phase change materials for thermal energy storage, Renew. Sustain. Energy Rev. 66 (2016) 399–414.
[6]
R. Al-Shannaq, M. Farid, S. Al-Muhtaseb, J. Kurdi, Emulsion stability and crosslinking of PMMA microcapsules containing phase change materials, Sol. Energy Mater. Sol. Cells. 132 (2015) 311–318.
[7]
X. Qiu, L. Lu, J. Wang, G. Tang, G. Song, Preparation and characterization of microencapsulated n-octadecane as phase change material with different n-butyl methacrylate-based copolymer shells, Sol. Energy Mater. Sol. Cells. 128 (2014) 102– 111.
[8]
X. Qiu, G. Song, X. Chu, X. Li, G. Tang, Preparation, thermal properties and thermal reliabilities of microencapsulated n-octadecane with acrylic-based polymer shells for thermal energy storage, Thermochim. Acta. 551 (2013) 136–144.
[9]
L. Sánchez-Silva, J. Tsavalas, D. Sandberg, P. Sánchez, J.F. Rodriguez, Synthesis and
25
Characterization of Paraffin Wax Microcapsules with Acrylic-Based Polymer Shells, Ind. Eng. Chem. Res. 49 (2010) 12204–12211. [10] X.L. Shan, J.P. Wang, X.X. Zhang, X.C. Wang, Formaldehyde-free and thermal resistant microcapsules containing n-octadecane, Thermochim. Acta. 494 (2009) 104– 109. [11] M.P. Stevens, Polymer chemistry: an introduction, Oxford University Press, New York, 1999. [12] G. Alva, Y. Lin, L. Liu, G. Fang, Synthesis, characterization and applications of microencapsulated phase change materials in thermal energy storage: A review, Energy Build. 144 (2017) 276–294. [13] C. Barner-Kowollik, P. Vana, T.P. Davis, Chapter 4. The Kinetics of Free-Radical Polymerization, in: K. Matyjaszewski, T.P. Davis (Eds.), Handb. Radic. Polym., John Wiley & Sons, Inc., Hoboken, NJ, USA, 2002: pp. 187–259. [14] L. Sánchez, P. Sánchez, A. de Lucas, M. Carmona, J.F. Rodriguez, Microencapsulation of PCMs with a polystyrene shell, Colloid Polym. Sci. 285 (2007) 1377–1385. [15] P. Sánchez, M.V. Sánchez-Fernandez, A. Romero, J.F. Rodríguez, L. Sánchez-Silva, Development of thermo-regulating textiles using paraffin wax microcapsules, Thermochim. Acta. 498 (2010) 16–21. [16] L. Sánchez-Silva, J.F. Rodriguez, M. Carmona, A. Romero, P. Sanchez, Thermal and Morphological Stability of Polystyrene Microcapsules Containing Phase-Change Materials, J. Appl. Polym. Sci. 120 (2011) 291–297. [17] S. Lashgari, H. Arabi, A.R. Mahdavian, V. Ambrogi, Thermal and morphological
26
studies on novel PCM microcapsules containing n-hexadecane as the core in a flexible shell, Appl. Energy. 190 (2017) 612–622. [18] P. Han, L. Lu, X. Qiu, Y. Tang, J. Wang, Preparation and characterization of macrocapsules containing microencapsulated PCMs (phase change materials) for thermal energy storage, Energy. 91 (2015) 531–539. doi:10.1016/j.energy.2015.08.001. [19] T. Feczkó, L. Trif, B. Németh, D. Horák, Silica-coated poly(glycidyl methacrylateethylene dimethacrylate)
beads containing organic
phase change
materials,
Thermochim. Acta. 641 (2016) 24–28. doi:10.1016/j.tca.2016.08.016. [20] X. Qiu, W. Li, G. Song, X. Chu, G. Tang, Microencapsulated n-octadecane with different methylmethacrylate-based copolymer shells as phase change materials for thermal energy storage, Energy. 46 (2012) 188–199. [21] X. Qiu, G. Song, X. Chu, X. Li, G. Tang, Microencapsulated n-alkane with p(n-butyl methacrylate-co-methacrylic acid) shell as phase change materials for thermal energy storage, Sol. Energy. 91 (2013) 212–220. [22] Y. Ma, S. Sun, J. Li, G. Tang, Preparation and thermal reliabilities of microencapsulated phase change materials with binary cores and acrylate-based polymer shells, Thermochim. Acta. 588 (2014) 38–46. [23] X. Qiu, W. Li, G. Song, X. Chu, G. Tang, Fabrication and characterization of microencapsulated n-octadecane with different crosslinked methylmethacrylate-based polymer shells, Sol. Energy Mater. Sol. Cells. 98 (2012) 283–293. [24] M. Zhang, X.-M. Tong, H. Zhang, J.-H. Qiu, Preparation and Characterization of Poly (MMA- co -AA)/Paraffin Microencapsulated Phase Change Material for Thermal Energy Storage, Energy Sources, Part A Recover. Util. Environ. Eff. 34 (2012) 396– 27
403. [25] F. Jahanzad, S. Sajjadi, B.W. Brooks, Characteristic intervals in suspension polymerisation reactors: An experimental and modelling study, Chem. Eng. Sci. 60 (2005) 5574–5589. [26] R.F.A. Teixeira, H.S. McKenzie, A.A. Boyd, S.A.F. Bon, Pickering Emulsion Polymerization Using Laponite Clay as Stabilizer To Prepare Armored “Soft” Polymer Latexes, Macromolecules. 44 (2011) 7415–7422. doi:10.1021/ma201691u. [27] G. Henini, F. Souahi, Y. Laidani, Tracking Offline Conversion Solution Polymerization of Methyl Methacrylate / Vinyl Acetate in Toluene in a Reactor Calorimeter, Procedia Eng. 33 (2012) 225–233. [28] M.S.M. Alger, Polymer Science Dictionary, Springer Science & Business Media, 1997. [29] W. Li, G. Song, G. Tang, X. Chu, S. Ma, C. Liu, Morphology, structure and thermal stability of microencapsulated phase change material with copolymer shell, Energy. 36 (2011) 785–791. [30] L. Sánchez-Silva, J.F. Rodríguez, A. Romero, A.M. Borreguero, M. Carmona, P. Sánchez, Microencapsulation of PCMs with a styrene-methyl methacrylate copolymer shell by suspension-like polymerisation, Chem. Eng. J. 157 (2010) 216–222. [31] I. Capek, Radical polymerization of polar unsaturated monomers in direct microemulsion systems, Adv. Colloid Interface Sci. 80 (1999) 85–149. [32] D. Pavia, G. Lampman, G. Kriz, Infrared spectroscopy, in: Introd. to Spectrosc., 3rd ed., Cengage Learning, Washington, 2001: pp. 13–101. [33] M. You, X.C. Wang, X.X. Zhang, L. Zhang, J.P. Wang, Microencapsulated n-
28
Octadecane with styrene-divinybenzene co-polymer shells, J. Polym. Res. 18 (2011) 49–58. [34] N. Sarier, E. Onder, Organic phase change materials and their textile applications: An overview, Thermochim. Acta. 540 (2012) 7–60. [35] X. Zhang, Y. Fan, X. Tao, K. Yick, Crystallization and prevention of supercooling of microencapsulated n-alkanes., J. Colloid Interface Sci. 281 (2005) 299–306. [36] X. Tang, W. Li, X. Zhang, H. Shi, Fabrication and characterization of microencapsulated phase change material with low supercooling for thermal energy storage, Energy. 68 (2014) 160–166. [37] G. Fang, F. Tang, L. Cao, Preparation, thermal properties and applications of shapestabilized thermal energy storage materials, Renew. Sustain. Energy Rev. 40 (2014) 237–259.
29
Preparation of acrylic based microcapsules using different reaction conditions for thermo-regulating textiles production A. S. Carreiraa,b, R. F. A. Teixeirac, *, A. Beirãob, R. Vaz Vieirab, M.M. Figueiredoa, M. H. Gila
Highlights: Production of thermo-regulating textiles has attracted much attention in recent years. Suspension polymerization technique was used to encapsulate a phase change material. Reaction temperature and type of initiator influenced the monomer conversion profile. Acrylic based microcapsules with a well-defined core/shell structure were prepared. Microcapsules exhibit suitable thermal performance/stability for textile application.
30
Preparation of acrylic based microcapsules using different reaction conditions for thermo-regulating textiles production A. S. Carreira, R. F. A. Teixeira, A. Beirão, R. Vaz Vieira, M. M. Figueiredo, M. H. Gil
31
S0014-3057(16)31580-4 http://dx.doi.org/10.1016/j.eurpolymj.2017.05.027 EPJ 7883
To appear in:
European Polymer Journal
Received Date: Revised Date: Accepted Date:
27 November 2016 24 April 2017 16 May 2017
Please cite this article as: Carreira, A.S., Teixeira, R.F.A., Beirão, A., Vaz Vieira, R., Figueiredo, M.M., Gil, M.H., Preparation of acrylic based microcapsules using different reaction conditions for thermo-regulating textiles production, European Polymer Journal (2017), doi: http://dx.doi.org/10.1016/j.eurpolymj.2017.05.027
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Preparation of acrylic based microcapsules using different reaction conditions for thermo-regulating textiles production
A. S. Carreiraa,b, R. F. A. Teixeirac, *, A. Beirãob, R. Vaz Vieirab, M.M. Figueiredoa, M. H. Gila
a
CIEPQPF, Department of Chemical Engineering, University of Coimbra, Rua Sílvio Lima, Polo II, 3030-790, Coimbra, Portugal
b
Devan - Micropolis S.A., Tecmaia - Parque da Ciência e Tecnologia da Maia, Rua Eng. Frederico Ulrich, 2650, 4470-605, Maia, Portugal c
Devan Chemicals N.V., I.Z. Klein Frankrijk - Klein Frankrijkstraat 8, 9600 Ronse, Belgium
Corresponding author: Tel: +32 55 23 01 12, Fax: +32 55 23 01 19 e-mail address: [email protected] (R. F. A. Teixeira)
Abstract Acrylic based microcapsules containing octadecane as a phase change material (PCM) were prepared via suspension polymerization, using different types of initiator and/or polymerization temperature. Benzoyl peroxide (BPO), azobisisobutyronitrile (AIBN) and Trigonox 23 (TRIG) were used as thermal initiators in a range of temperatures from 40 ºC to 80 ºC. A comprehensive characterization of microcapsules was undertaken, regarding their chemical composition (by infrared spectroscopy), morphology (by scanning electronic microscopy (SEM)), particle size distribution (by laser diffraction) and thermal properties (by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA)). The 1
experimental results show that the studied reaction parameters influenced the monomer conversion profiles while their impact on the microcapsules properties was not quite relevant. The microcapsules exhibited mean particle sizes around 12 µm, melting enthalpies of approximately 175 J/g, PCM content about 70 % and were thermally stable up to 170 ºC, the temperature corresponding to 5% of weight measured by TGA. This study confirmed that octadecane can be successfully encapsulated via a suspension polymerization technique, exhibiting a well-defined core/shell structure. Moreover, employing TRIG as initiator enables the use of a reaction temperature as low as 40 ºC. The acrylic shell microcapsules with encapsulated octadecane show a good potential to be incorporated in textile substrates in order to produce thermo-regulating textiles.
Keywords: Thermo-regulating textiles; Phase change material; Microencapsulation; Suspension polymerization technique
1. Introduction The design and development of textiles with thermo-regulating properties has attracted much attention in recent years [1]. This new functionality can be added to the textile substrates through the incorporation of phase change materials (PCMs). When the ambient temperature increases above the melting point of the PCM, the material changes from solid to liquid and the latent heat is absorbed, interrupting the increase in temperature of the textile substrate. When the temperature decreases, the PCM solidifies and the stored heat energy is released, providing a heating effect [2,3]. In order to allow the incorporation of PCMs into textile substrates and to withstand the repeated phase changes, these compounds must be externally protected. This protection may
2
conveniently be achieved by microencapsulation processes [2,4,5]. The encapsulation of PCMs through the production of acrylic microcapsules via suspension polymerization technique has been revealed to be a very promising technique [6–10]. In this technique, the polymeric chains are formed by consecutive linking of monomers at the reactive end of a growing chain (chain reaction polymerization) [11]. In most cases, the monomers require an initiator to start the polymerization reaction. Upon a trigger, this initiator produces a free radical (reactive species) that, added to a monomer, starts the formation of one polymer chain (free radical polymerization) [11,12]. For thermal initiators, the decomposition rates are normally expressed in terms of their half-life, which is defined as the time necessary to decompose one-half of the initiator originally present, at a given temperature. The temperature at which each initiator has a half-life of 10 hours is one of the most used parameters (the 10 hour half-life temperature corresponds to the temperature at which a 50 % of the initiator is consumed in 10 hours) [11,13]. For the majority of PCM microencapsulation processes via suspension polymerization, both benzoyl peroxide (BPO) (using styrene [14– 16] or acrylic monomers [6,9,10,17]) and azobisisobutyronitrile (AIBN) (using acrylic monomers [7,8,18–22]) have been used as thermal initiators. These initiators present a 10 hour half-life temperature of 70 ºC and 65 ºC, respectively. Concerning the PCM microencapsulation based on acrylic polymers, the polymerization temperature is normally above 50 ºC [23]. In particular, the suspension polymerization processes involve temperatures over 70 ºC [6,9,10,20,24]. Qiu et al. [23] tried to reduce the reaction temperature when preparing methyl methacrylate microcapsules containing octadecane, by combining AIBN and redox initiators. However, after an initial period of 2 hours at low temperature (between 35 ºC and 55 ºC), the reaction temperature had to be increased to 85 ºC during 4 hours [23]. Moreover, and as the authors recognized, the prepared microcapsules exhibited heat capacities and thermal stabilities lower than the microcapsules
3
prepared with AIBN or BPO at 85 ºC [23]. In the present work, efforts to reduce the reaction temperature were made by using a thermal initiator with a low half-life decomposition temperature - the Trigonox 23 (TRIG), which has a 10 hour half-life temperature of 48 ºC. We have found no evidence that TRIG has been used to encapsulate PCMs, nor to produce acrylic microcapsules. The final goal in using temperatures below 50 ºC is not only to reduce energy consumption but also to develop a process suitable to encapsulate core materials other than octadecane and that may degrade at moderate temperature. In this study, acrylic based microcapsules were produced via suspension polymerization technique using three initiators (BPO, AIBN and TRIG) at different reaction temperatures and with and without octadecane. For each experiment, the monomer conversion was followed and the prepared microcapsules were fully characterized with respect to their morphology and chemical and thermal properties, having in mind their potential to be further incorporated into substrates to produce thermo-regulating textiles.
2. Experimental procedure 2.1. Materials Benzoyl peroxide (BPO, purity 75% - remainder water) supplied by ACROS, azobisisobutyronitrile (AIBN, purity 98 %) purchased from Sigma-Aldrich and Trigonox 23 (TRIG, purity 95 %) from Akzo Nobel Polymer Chemistry (kindly provided by Companhia Industrial de Resinas Sintéticas, CIRES Lda) were used as thermal initiators. The stabilizer poly(vinyl alcohol) (PVA, Mw = 31000 – 50000 g/mol and degree of hydrolysis = 87-89 %), was purchased from Sigma-Aldrich. All the acrylic compounds that constitute the monomers mixture and crosslinkers were purchased from Sigma-Aldrich. The core material, octadecane
4
(commercial grade) was supplied ex. stock from Devan Chemicals N.V. All the reagents were used as received.
2.2. Microencapsulation procedure Suspension polymerization was carried out in a triple-walled glass reactor equipped with an external circulating heating bath, a condenser, a nitrogen inlet and an overhead stirrer (with an axial impeller stirrer, four-bladed). For the synthesis of microcapsules two different solutions were prepared. Solution (1) was obtained by mixing the mixture of monomers (25 g), the crosslinkers and the thermal initiator (1.2 wt.% related to the monomers mixture). This solution was heated up to 40 ºC and the melted PCM (50 g of octadecane) was subsequently added, maintaining under magnetic stirring for 15 minutes at 40 ºC. Solution (2) was prepared by mixing 2.5 g of PVA in 250 mL of water. Lastly Solution (1) was then added to Solution (2) drop wise and the mixture emulsified using a homogenizer at 6000 rpm for 10 minutes. The obtained emulsion was transferred to the reactor, previously heated at 40 ºC, under nitrogen flux and mechanical stirring (300 rpm). After 10 minutes, the temperature was raised to 80 ºC (or other specified value, Table 1) and the reaction was carried out during 5.5 hours under nitrogen atmosphere and continuous stirring. The microcapsules so obtained were washed with water at 50 ºC, repeated three times, to remove the unreacted monomers, stabilizer and free octadecane. After each washing cycle, the sample was filtered under vacuum to separate microcapsules from the water (using a filter paper with pore size of around 2-3 µm). The washed microcapsules were collected and stored as a wet cake (50 70% of solid content). Different types of initiators as well as reaction temperatures were used, as indicated in Table 1. For comparison purposes and to study the copolymer properties, blank experiments were also carried out (B-Bp80, B-Trig48 and B-Trig40). These experiments were performed
5
following the procedure previously described, but without the addition of octadecane to the monomers mixture.
Table 1: Initiator type and reaction temperatures used. Experiment
Initiator
Reaction Temperature (ºC)
Addition of octadecane
PCM-Bp80
BPO
80
Yes
PCM-Bp70
BPO
70
Yes
PCM-Aibn65
AIBN
65
Yes
PCM-Trig48
TRIG
48
Yes
PCM-Trig40
TRIG
40
Yes
B-Bp80
BPO
80
No
B-Trig48
TRIG
48
No
TRIG 40 No B-Trig40 B: refers to “blank experiments” that were conducted without the addition of octadecane.
2.3. Monomer conversion assessment The monomer conversion as a function of time was monitored via gravimetry. At predefined periods of time, a sample of approximately 2-3 mL was removed from the reactor (using a syringe), placed in an aluminium dish with known mass and weighed immediately. The dish was positioned in a highly ventilated place to cool the sample and to remove the volatile compounds. The collected samples were dried until constant weight. The monomer conversion percentages were calculated using a simple mass balance [25,26], between the mass of formed polymer (mpolymer) and the mass of monomers initially introduced (mmonomer mixture). Equation (1) and (2) were used to calculate the percentage of the dry extract (Es) and the mass percentage of monomers conversion [27]. =
× 100
(%) = =
× × 100 ! ! ! "
# !
! ! ! "
6
× 100
(1)
(2)
Where, in equation (1), the msolution is the mass of collected solution and mdry is its dried mass. In equation (2), mtotal is the total mass of solution in reactor, mmonomer
mixture
is mass of
monomers mixture initially introduced in reactor a Es corrected is the dry extract corrected by the mass of non-volatile compounds [22]. Samples were measured in duplicate and the standard deviation was always lower than 3 wt.%.
2.4. Fourier Transform Infrared Spectroscopy Fourier transform infrared (FT-IR) spectra were acquired in the range of 4000-550 cm-1 at room temperature, using a Jasco FT/IR-4200 spectrometer, equipped with a Golden Gate Single Reflection Diamond ATR. Data were collected with 4 cm-1 spectral resolution and 128 scans. Samples were previously dried in oven at 40 ºC.
2.5. Scanning Electron Microscopy The microcapsules morphology was assessed by scanning electron microscopy (SEM). For that, specimens of microcapsules were diluted using distilled water and placed onto carbon tape in appropriate supports. The specimens were dried at room temperature and subjected to a sputtering treatment to deposit a gold nano layer. Micrographs were obtained by using an electron beam voltage of 15 kV in a Leica Cambridge S360 equipment.
2.6. Particle size distribution Particle size distribution was measured by laser diffraction using a Malvern MasterSizer 2000s. The microcapsules were dispersed in distilled water prior to measurement. The particle size distribution (volume based) and the corresponding statistical mean values were determined to an average of at least three independent measurements per aliquot and 2 to 3 aliquots.
7
2.7. Differential Scanning Calorimetry Thermal behaviour was studied by differential scanning calorimetry (DSC), in a Netzsch DSC 200 F3 instrument with a cooling unit. Samples were analysed in aluminium crucibles with closed lid. Three thermal cycles were programmed: firstly the temperature was cooled, at 5 ºC/min, from 20 ºC to -10 ºC; then it was increased, at 5 ºC/min, from -10 ºC to 50 ºC; finally, the sample was cooled, at 5 ºC/min, from 50 ºC to -10 ºC. Isothermals of 3 minutes before each cooling and heating cycle were performed. Only the second and third cycle, which corresponded to the heating and cooling processes, respectively, were considered for analysis. Samples were previously dried in an oven at 40 ºC and a range of weights between 7 and 10 mg was used. DSC analysis was carried out in triplicate and results expressed as average ± standard deviation. The experimental PCM microcapsules content was estimated according to Equation (3) [23]:
$% && (%) =
∆(!,* # +,- + ∆( ,* # +, × 100 ∆(!,+,- + ∆( ,+,-
(3)
where ∆Hm,µcapsPCM and ∆Hc,µcapsPCM are, respectively, the melting and crystallization enthalpies of microcapsules containing octadecane, and ∆Hm,PCM and ∆Hc,PCM are the melting and crystallization enthalpies of octadecane [23]. Moreover, the theoretical PCM content was calculated based on equation (4):
/ℎ&12 $% && (%) =
m+, × 100 m +,- + m ! !
(4)
where mPCM and mmonomers are, respectively, the mass of octadecane and the mass of monomers used during the encapsulation experiment [17].
2.8. Thermogravimetric analysis 8
Thermogravimetric analysis (TGA) was evaluated using a TA Instruments Q500 equipment, at a heating rate of 10 ºC/min, in the range of 25 ºC to 600 ºC, using a nitrogen purge with a flow rate of 100 mL/min. Samples were previously dried in oven at 40 ºC and a weight range from 5 to 8 mg was used.
3. Results and discussion 3.1. Monomer conversion curves Figure 1 compares the monomer conversion curves of experiments carried out with the three initiators at respective 10 hour half-life decomposition temperatures: 70 ºC for BPO (PCM-Bp70), 65 ºC for AIBN (PCM-Aibn65) and 48 ºC when using TRIG (PCM-Trig48). As can be seen, the conversion-time curves obtained for these experiments almost overlap, denoting similar conversion rates. Nonetheless, the final conversion achieved for the experiment PCM-Aibn65 is slightly lower than that of the other experiments (92 % for PCMAibn65 versus approximately 97 % for the other two). In these experiments, the values of monomer conversion only starts to increase after around 30 minutes, which corresponds to the induction period of reaction (i.e., time during which no observable change occurs in the chemical reaction) [28]. This study demonstrates that, when the encapsulation procedure is carried out with the three different initiators at their 10 hour half-life temperatures, the monomer conversion present a similar rate.
9
Figure 1: Monomer conversion as a function of time for the three types of initiators used at the respective 10 hour half-life temperature: BPO at 70 ºC, AIBN at 65 ºC and TRIG at 48 ºC. See Table 1 for details.
In order to study the effect of the reaction temperature on the monomer conversion of suspension polymerization, additional experiments using the same initiators were carried out at another temperature: 80 ºC for BPO (cf. 70 ºC), Figure 2 (a), and 40 ºC for TRIG (cf. 48 ºC), Figure 2 (b). In the latter, due to the low monomer conversion rate, the reaction time was extended up to 7.5 hours. As expected, for both initiators, the slope of the conversion-time curves increases as the reaction temperature increases, denoting a faster polymerization rate. This effect is particularly marked for the initiator TRIG. Indeed, the values of monomer reaction for experiment PCM-Trig48 started to increase after 40 minutes, reaching 95 % of conversion after approximately 2.5 hours, while for PCM-Trig40, the reaction was initiated only after 2.5 hours and a monomer conversion of 97 % was reached only after 6 hours. This behaviour was expected since for a polymerization at higher temperature, the half-life of the thermal initiator is lower and its initiation activity is higher. For instance, BPO has a half-life of 10 hours at 70 ºC and a half-life of 1 hour at 91 ºC [13,29]. Although direct comparisons can not be made with the literature, due to differences in the initiator type, reaction temperatures and encapsulated PCMs, the reaction times detected in this work are of similar magnitude as those reported by other workers (e.g. Sánchez-Silva et
10
al. [30]). These authors encapsulated a PCM (PRS® paraffin wax) also using the suspension polymerization method and BPO as thermal initiator (at 98 ºC).
Figure 2: Monomer conversion as a function of time for experiments performed with BPO and TRIG at different temperatures: (a) BPO at 70 ºC and 80 ºC in the presence of octadecane and BPO at 80 ºC in the absence of octadecane; and (b) TRIG at 48 ºC and at 40 ºC in the presence and absence of octadecane (reaction time was extended up to 7.5 hours). See Table 1 for details (B refers to “blank experiments” that were conducted without the addition of octadecane).
Regarding the effect of the presence of octadecane (Figure 2), it is clear that in the absence of octadecane the reaction rate is faster, presenting a smaller induction period, but it shows a lower final monomer conversion. The shorter induction period is easily detected when comparing experiment B-Trig40 with PCM-Trig40: the monomer conversion values increased after 50 minutes in the former case instead of 2.5 hours in the latter. As previously mentioned,
11
for suspension polymerization technique, the polymeric shell is formed by free radical polymerization. Therefore, its reaction rate depends on the microenvironment and on the local monomer concentration where the free radicals are produced and where the initiation process takes place [25,31]. In the absence of octadecane, the concentration of the monomers, crosslinkers and initiator within the dispersed phase is higher, originating a fast polymerization rate (Figure 2).
3.2. Chemical characterization The FT-IR spectra of pure octadecane and of samples prepared with BPO at 80 ºC with and without octadecane are presented in Figure 3. The octadecane is an alkane mostly constituted by methylene groups. Thus, in its spectrum, two strong peaks at 2911 cm-1 and 2847 cm-1 that are caused by asymmetric and symmetric C-H stretching vibration of CH2, respectively, can be identified. In addition, two other peaks must be emphasized: one at 1470 cm-1, attributed to the bending vibration of methylene group, and another at 716 cm-1, ascribed to the alkane long chain [32]. In spectrum of sample B-Bp80 (without octadecane), from the acrylic copolymer, more specifically from the ester group, the strong peak at 1728 cm-1 from C=O stretching vibration and the band at 1190 cm-1 for its C-O-C stretching vibration are identified. Moreover, the peak that characterizes the monomers mixture (corresponding to the stretching vibration of double bond C=C) and that normally appears around 1635 cm-1, was not detected [32]. For microcapsules PCM-Bp80 (containing octadecane), the FT-IR spectrum includes the main peaks previously identified in the spectra of both octadecane and sample B-Bp80, confirming the presence of octadecane and the production of acrylic shell.
12
Regarding the remaining samples, prepared using other initiators and reaction temperatures (with and without octadecane) identical findings were detected. The FT-IR spectra of all samples are presented as supplementary information (Figure S1).
Figure 3: FT-IR spectra of pure octadecane, sample B-Bp80 (without octadecane) and sample PCM-Bp80 (with octadecane).
3.3. Morphology and particle size distribution Figure 4 shows the SEM micrographs of microcapsules prepared using BPO as thermal initiator at 80 ºC, with and without octadecane. As this figure illustrates, experiment PCMBp80 resulted in spherical microcapsules with different sizes exhibiting a rough shell surface (Figure 4 (a)). An example of a micrograph collected by optical microscopy, in which this morphology is visible, was added in the supplementary information - Figure S2. Regardless the type of initiator and reaction temperature selected to perform the experiments with 13
octadecane (Table 1), microcapsules with identical morphology were obtained (SEM micrographs presented as supplementary information - Figure S3). In contrast, the experiments performed without octadecane produced spherical microparticles but with smooth surface, as exemplified in Figure 4 (b) for sample B-Bp80.
Figure 4: SEM micrographs of samples (a) PCM-Bp80, (b) B-Bp80 (magnification x1000) and (c) broken microcapsules PCM-Bp80 (sample previously crushed to highlight the core/shell structure) (magnification x3000).
14
In suspension polymerization, the shell formation starts at the water-oil droplet interface. As the polymerization reaction proceeds, the monomers are consumed and transferred from the oil phase to the interface, increasing the shell thickness. This mechanism occurs towards the inner side of the microcapsule [33]. As observed in Figure 4 (c), the produced microcapsules exhibit a well-defined core/shell structure, with a thickness of approximately 1 µm. In the encapsulation procedure, the formation of this core/shell morphology is favoured when the polarity of the polymer and of the core material are distinct. As mentioned in the literature [9,14,30], it is the driving force between these polarities that creates a phase separation and produces a core/shell structure. In addition, the roughness of the microcapsules shell surface (Figure 4 (a and c)) seems to be associated to the density variations that occur during their formation. When the monomers react to form the copolymer shell, an increase in density occurs (the copolymer naturally presents higher density than that of individual monomers) [33]. This increase in density and the corresponding volume reduction seems to be responsible for the shell shrinkage and for the production of the observed rough surface [33]. However, it should be pointed out that density variation is not exclusive of the polymer: the octadecane also changes its density with temperature (0.81 g/cm3 for solid state and 0.69 g/cm3 for melted state at 80 ºC [33]). Consequently, after the reaction, when the microcapsules are cooled down from reaction temperature to room temperature, the octadecane volume could also be reduced. Many authors describe the combination of these events as the generation of reserved expansion space in the PCM microencapsulation [7,10,33].
In order to measure the particle size distribution, several samples were analysed by laser diffraction. The size distribution curves, on volume basis, for sample PCM-Bp80 and B-Bp80 are illustrated in Figure 5. Besides a broad size distribution (from submicrometer range to a 1
15
-20 µm), both curves exhibit a bimodal distribution (with one peak, around 10 - 11 µm, much more intense than the other at 1 - 2 µm), being this result is consistent with the SEM micrographs (Figure 4). This bimodal trend found for all samples regardless the type of initiator (see supplementary information - Figure S4). The presence of small particles could be attributed to the process of secondary nucleation [30]. This process occurs when a portion of monomers diffuses to the aqueous phase, forming small polymeric particles that do not contain any core material. This phenomenon is favoured by the hydrophilicity of the monomers as well as by the absence of inhibitors in the aqueous solution [30]. Figure 5 also shows that the size distribution curve of the sample without PCM is displaced to the left relatively to the microcapsules with octadecane.
Figure 5: Particle size distribution curves of microcapsules prepared with BPO at 80 ºC in the presence and absence of octadecane.
In order to compare the microcapsules sizes, the values of the mean volume diameter (D4,3) of all samples were plotted in Figure 6. These results confirm that the initiator type does not affect significantly the mean size of the microcapsules that is around 12 µm. Additionally, they show that the presence of octadecane leads to a considerable increase in particle size. This trend could be induced by the difference in viscosity of the dispersed phase of the 16
experiments carried out with octadecane relatively to those without octadecane. In fact, at 40 ºC (temperature of the emulsification process) the viscosity of octadecane is much higher than that of the monomers mixture (3.1 mPa.s vs 0.6 mPa.s) and thus the formation of larger droplets and, consequently, larger microcapsules is expected [9].
∑ 5 657
Figure 6: Comparison of the mean particle size (D4,3 = ∑
5 658
) of microcapsules obtained with different reaction
conditions. See Table 1 for details (B refers to “blank experiments” that were conducted without the addition of octadecane).
3.4. Thermal properties As the microcapsules containing PCM are to be incorporated in textiles to impart thermoregulating properties, their thermal characterization is of utmost importance. Phase change enthalpies and PCM content (estimated from Equation (3)) were determined by DSC whereas microcapsules thermal stability was assessed by TGA. As illustrated by the DSC curves of Figure 7, both octadecane and the microcapsules exhibit two peaks, one endothermic peak (downwards) for the heating process and the other exothermic peak (upwards) for the cooling process (supplementary information for the remaining sample is given in Figure S5). As shown in Table 2, the melting and crystallization
17
onset temperatures (Tom and Toc) of octadecane are, respectively, 26.7 ºC and 25.0 ºC. For the encapsulated octadecane, Tom values are similar to that of the octadecane and independent of the initiator type and temperature, while the values of Toc are considerably lower (between 15 ºC and 19 ºC) and slightly dependent on the reaction conditions. This decrease in Toc of microencapsulated octadecane compared to the pure core material is typical of the microencapsulation of n-alkanes and is known as the supercooling effect. It is considered an obstacle for the microcapsules application as thermal energy storage system and many alternatives have been sought to avoid this phenomenon [34–36]. However, in the present study, this effect was not explored.
Figure 7: DSC curves of pure octadecane and microcapsules prepared with BPO at 80 ºC (sample PCM-Bp80).
18
Table 2: Influence of initiator type and polymerization temperature on thermal performance (average ± standard deviation): melting and crystallization enthalpies, respectively ∆Hm and ∆Hc; melting and crystallization onset temperatures, respectively Tom and Toc; and PCM content, calculated using equation (3). Sample
T om (ºC)
∆Hm (J/g)
Toc (ºC)
∆Hc (J/g)
PCM content (wt.%)
Octadecane
26.7±0.1
252.3±2.8
25.0±0.1
252.1±3.0
--
PCM - Bp80
26.4±0.1
174.4±0.9
19.0±0.2
175.5±1.8
69.3
PCM - Bp70
26.5±0.0
178.1±8.8
18.4±0.1
180.6±9.3
71.1
PCM - Aibn65
26.4±0.1
207.8±7.6
18.7±0.3
209.0±8.4
82.6
PCM - Trig48
26.3±0.0
181.1±5.3
15.2±0.1
183.2±4.7
72.2
PCM - Trig40
26.3±0.1
183.5±4.6
15.8±0.1
185.4±5.5
73.1
In general, for microcapsules containing octadecane, the measured phase change enthalpies of melting and crystallization (∆Hm and ∆Hc) were around 175 – 185 J/g, corresponding to calculated PCM contents around 69 – 73 wt.%. The only exception was sample PCM-Aibn65 that exhibits a ∆Hm above 200 J/g (PCM content of 82.6 %). These values show that the produced microcapsules present a thermal performance quite satisfactory for energy storage applications. The values of PCM content depend on the relative quantity of octadecane with respect to the total quantity of the sample (octadecane and copolymer) (see equation (3)). According to this definition, a value lower than the theoretical value indicates that not all the octadecane was encapsulated, being removed during the microcapsules washing. On the other hand, a higher value could reveal that not all the monomers reacted and an amount of copolymer smaller than the theoretical was formed. The theoretical PCM content for the prepared microcapsules is 67 % (calculated using equation (4)) [17]. Therefore, for all the experiments, the PCM content is higher than the theoretical value, suggesting that, despite of any the possible presence of free and/or residual
19
octadecane, not all the monomers used in the formulation were able to react and form the polymeric shell. The results summarized in Table 2 suggest that the influence of the type the initiator and temperature on microcapsules thermal performance are not very relevant. Nevertheless, the use of AIBN as thermal initiator seems to produce microcapsules with high PCM content. As previously described, this sample also presents the lowest final monomer conversion (near 92%), while the other samples containing octadecane presented values around 97 % (Figure 1 and Figure 2). This lower monomer conversion could be responsible for a reduction of the quantity of produced copolymer and for an increase in the percentage of octadecane of this sample. However, this may not be the only cause for the smaller production of copolymer. The initiator properties can also be associated to this behaviour. Indeed, from the three initiators used, AIBN is the only azo-initiator that, compared to BPO, has much higher water solubility (350 mg/L at 25 ºC for AIBN versus 9.1 mg/L for BPO). Thus, for AIBN, more free radicals can be produced in the aqueous phase which probably react with the monomers that diffuse from the dispersed phase instead of reacting in the droplet interface to form the shell of the microcapsules.
Thermal stability was evaluated for the microcapsules containing octadecane as well as for the copolymer microparticles (without PCM) and for the pure octadecane. The curves of weight loss as a function of temperature are exemplified in Figure 8, showing the results obtained for octadecane and samples prepared with BPO at 80 ºC (information for the remaining samples is provided in Figure S6). The most relevant data extracted for all samples are summarized in Table 3. The curve of pure octadecane shows that the weight loss occurs in one stage, with an extrapolated onset temperature for the first degradation stage (Ton1) of 145 ºC (Table 3). This
20
profile was originated by octadecane evaporation before its flash point (166 ºC) and boiling point (308 ºC) [8,10,29]. For sample B-Bp80 (without PCM), one main weight loss is also observed but at a higher temperature (around 352 ºC), corresponding to the copolymer degradation. In this sample, a small weight loss (about 2 wt.%) at Ton1 of 127 ºC was detected, which could be caused by adsorbed water and/or the evaporation of unreacted monomers.
Figure 8: TG curves of pure octadecane and of samples prepared with BPO at 80 ºC with (PCM-Bp80) and without octadecane (B-Bp80).
Regarding the microcapsules PCM-Bp80, the thermogram reveals two distinct stages of degradation: the first one is originated by the weight loss of octadecane, that diffuses out through the shell membrane and evaporates; and the second one is created by the degradation of the polymeric shell [16]. For the other samples of microencapsulated octadecane, the same curve profile was obtained. As listed in Table 3, all samples of microcapsules containing octadecane present higher values of Ton1 compared with that of pure octadecane (185 ºC – 200 ºC versus 145 ºC), being the same tendency observed for the values of temperature corresponding to 5% of weight loss (T5%). This difference can be caused by the resistance and protection that the polymeric shell
21
promotes for the octadecane evaporation. Among these samples, small differences between the onset temperature values could be detected, revealing slight differences between their shell properties. In this respect, sample PCM-Bp80 is the one with the highest thermal stability, reaching a T5% of about 180 ºC. Regarding the values of weight loss, and as for DSC analysis, only microcapsules PCM-Aibn65 differ from the other ones (the weight loss in the first degradation stage is 80.2 % against around 70 % for the remaining samples). It is worth noting the similarity between the percentage of weight loss in the first degradation stage determined by TGA and the PCM content calculated from DSC data. For instance, for PCM-Bp80, the first weight loss determined by TGA is 70.2 % (Table 3) and the corresponding value of PCM content determined by DSC is 69.3 % (Table 2). This excellent agreement confirms that the weight loss in the first degradation stage corresponds in fact to the octadecane degradation/evaporation. The thermogravimetric results confirm that octadecane was successfully encapsulated using the suspension polymerization technique. They also demonstrate that the microcapsules are stable below T5% of 170 ºC, thus allowing their incorporation in textile substrates without undergoing thermal degradation.
22
Table 3: Influence of initiator type and polymerization temperature on thermal stability: T5%, temperature corresponding to 5% of weight loss; Ton1 and Ton2, extrapolated onset temperatures of the first and second degradation stage; and weight loss (%) in each degradation stage. 1st stage Sample
T 5% (ºC)
Octadecane
2nd stage
T on1 (ºC)
Weight Loss (%)
T on2 (ºC)
Weight Loss (%)
117.8
145.0
99.8
--
--
PCM - Bp80
181.2
199.7
70.2
351.6
29.3
PCM - Bp70
172.4
190.1
70.8
347.7
29.0
PCM - Aibn65
175.5
195.2
80.2
361.8
19.6
PCM - Trig48
172.2
188.4
69.7
354.4
30.0
PCM - Trig40
168.5
184.4
71.4
361.3
28.4
B - Bp80
278.3
126.7
1.9
345.5
96.8
B - Trig48
280.2
103.9
2.3
344.0
96.6
B - Trig40
284.0
99.4
2.7
344.2
96.4
B: refers to “blank experiments” that were conducted without the addition of octadecane.
4. Conclusions The use of different types of initiators (BPO, AIBN and TRIG) and temperatures revealed that the polymerization reaction at the respective 10 hour half-life temperatures present identical monomer conversion rates. Additionally, and as expected, with the increase of the reaction temperature the polymerization rate also increases. It was also demonstrated that the presence of octadecane decreases the reaction rate. Moreover, all experiments exhibited high rates of monomer conversion (except PCM-Trig40), reaching final conversions above 90 %. This work also confirmed that it is possible to prepare microcapsules using reaction temperatures of 40 ºC, making it possible to adapt the encapsulation procedure to entrap core materials that degrade at relatively low temperatures. Despite the variations in the monomer conversion profiles, the microcapsules exhibited only slight differences between them with respect to morphology and thermal properties.
23
SEM results revealed that in all experiments carried out in the presence of octadecane, spherical and individual microcapsules with a well-defined core/shell structure were produced. The existence of an acrylic based shell and of octadecane in the produced microcapsules was confirmed by FT-IR analysis. The size distribution resulted in a bimodal curve with a mean diameter (volume based) of around 12 µm. In addition, these microcapsules showed a ∆Hm around 175 J/g, a PCM content of 70 % and T5% around 170 ºC. In conclusion, the overall results show that acrylic shell microcapsules can be prepared to encapsulate octadecane by suspension polymerization using a variety of thermal initiators and reaction temperatures. As currently most of the microcapsules containing PCMs and industrially used are based on melamine-formaldehyde shell, the production of acrylic microcapsules is a good alternative, presenting the added value of being a formaldehyde-free product and thus more environmentally friendly. Although these microcapsules exhibit proper thermal performance and stability to produce textiles with thermo-regulating properties, they can also be used for many other thermal energy storage purposes, such as solar heating systems, heat exchangers, thermal comfort in buildings and air-conditioning systems [30,37].
Acknowledgement: A. S. Carreira thanks Fundação para a Ciência e Tecnologia (FCT) and Devan Micropolis, S.A. for her Ph.D. grant (SFRH/BDE/51601/2011). The authors thank Dr. John Ellis (Devan Chemicals NV) for proof-reading the manuscript.
References [1]
N. Sarier, E. Onder, The manufacture of microencapsulated phase change materials
24
suitable for the design of thermally enhanced fabrics, Thermochim. Acta. 452 (2007) [2]
S. Mondal, Phase change materials for smart textiles - An overview, Appl. Therm. Eng. 28 (2007) 1536–1550.
[3]
L. Sánchez-Silva, J.F. Rodríguez, P. Sánchez, Influence of different suspension stabilizers on the preparation of Rubitherm RT31 microcapsules, Colloids Surfaces A Physicochem. Eng. Asp. 390 (2008) 62–66.
[4]
S.K. Ghosh, Functional Coatings: by Polymer Microencapsulation, WILEY-VCM, 2006.
[5]
L. Liu, G. Alva, X. Huang, G. Fang, Preparation, heat transfer and flow properties of microencapsulated phase change materials for thermal energy storage, Renew. Sustain. Energy Rev. 66 (2016) 399–414.
[6]
R. Al-Shannaq, M. Farid, S. Al-Muhtaseb, J. Kurdi, Emulsion stability and crosslinking of PMMA microcapsules containing phase change materials, Sol. Energy Mater. Sol. Cells. 132 (2015) 311–318.
[7]
X. Qiu, L. Lu, J. Wang, G. Tang, G. Song, Preparation and characterization of microencapsulated n-octadecane as phase change material with different n-butyl methacrylate-based copolymer shells, Sol. Energy Mater. Sol. Cells. 128 (2014) 102– 111.
[8]
X. Qiu, G. Song, X. Chu, X. Li, G. Tang, Preparation, thermal properties and thermal reliabilities of microencapsulated n-octadecane with acrylic-based polymer shells for thermal energy storage, Thermochim. Acta. 551 (2013) 136–144.
[9]
L. Sánchez-Silva, J. Tsavalas, D. Sandberg, P. Sánchez, J.F. Rodriguez, Synthesis and
25
Characterization of Paraffin Wax Microcapsules with Acrylic-Based Polymer Shells, Ind. Eng. Chem. Res. 49 (2010) 12204–12211. [10] X.L. Shan, J.P. Wang, X.X. Zhang, X.C. Wang, Formaldehyde-free and thermal resistant microcapsules containing n-octadecane, Thermochim. Acta. 494 (2009) 104– 109. [11] M.P. Stevens, Polymer chemistry: an introduction, Oxford University Press, New York, 1999. [12] G. Alva, Y. Lin, L. Liu, G. Fang, Synthesis, characterization and applications of microencapsulated phase change materials in thermal energy storage: A review, Energy Build. 144 (2017) 276–294. [13] C. Barner-Kowollik, P. Vana, T.P. Davis, Chapter 4. The Kinetics of Free-Radical Polymerization, in: K. Matyjaszewski, T.P. Davis (Eds.), Handb. Radic. Polym., John Wiley & Sons, Inc., Hoboken, NJ, USA, 2002: pp. 187–259. [14] L. Sánchez, P. Sánchez, A. de Lucas, M. Carmona, J.F. Rodriguez, Microencapsulation of PCMs with a polystyrene shell, Colloid Polym. Sci. 285 (2007) 1377–1385. [15] P. Sánchez, M.V. Sánchez-Fernandez, A. Romero, J.F. Rodríguez, L. Sánchez-Silva, Development of thermo-regulating textiles using paraffin wax microcapsules, Thermochim. Acta. 498 (2010) 16–21. [16] L. Sánchez-Silva, J.F. Rodriguez, M. Carmona, A. Romero, P. Sanchez, Thermal and Morphological Stability of Polystyrene Microcapsules Containing Phase-Change Materials, J. Appl. Polym. Sci. 120 (2011) 291–297. [17] S. Lashgari, H. Arabi, A.R. Mahdavian, V. Ambrogi, Thermal and morphological
26
studies on novel PCM microcapsules containing n-hexadecane as the core in a flexible shell, Appl. Energy. 190 (2017) 612–622. [18] P. Han, L. Lu, X. Qiu, Y. Tang, J. Wang, Preparation and characterization of macrocapsules containing microencapsulated PCMs (phase change materials) for thermal energy storage, Energy. 91 (2015) 531–539. doi:10.1016/j.energy.2015.08.001. [19] T. Feczkó, L. Trif, B. Németh, D. Horák, Silica-coated poly(glycidyl methacrylateethylene dimethacrylate)
beads containing organic
phase change
materials,
Thermochim. Acta. 641 (2016) 24–28. doi:10.1016/j.tca.2016.08.016. [20] X. Qiu, W. Li, G. Song, X. Chu, G. Tang, Microencapsulated n-octadecane with different methylmethacrylate-based copolymer shells as phase change materials for thermal energy storage, Energy. 46 (2012) 188–199. [21] X. Qiu, G. Song, X. Chu, X. Li, G. Tang, Microencapsulated n-alkane with p(n-butyl methacrylate-co-methacrylic acid) shell as phase change materials for thermal energy storage, Sol. Energy. 91 (2013) 212–220. [22] Y. Ma, S. Sun, J. Li, G. Tang, Preparation and thermal reliabilities of microencapsulated phase change materials with binary cores and acrylate-based polymer shells, Thermochim. Acta. 588 (2014) 38–46. [23] X. Qiu, W. Li, G. Song, X. Chu, G. Tang, Fabrication and characterization of microencapsulated n-octadecane with different crosslinked methylmethacrylate-based polymer shells, Sol. Energy Mater. Sol. Cells. 98 (2012) 283–293. [24] M. Zhang, X.-M. Tong, H. Zhang, J.-H. Qiu, Preparation and Characterization of Poly (MMA- co -AA)/Paraffin Microencapsulated Phase Change Material for Thermal Energy Storage, Energy Sources, Part A Recover. Util. Environ. Eff. 34 (2012) 396– 27
403. [25] F. Jahanzad, S. Sajjadi, B.W. Brooks, Characteristic intervals in suspension polymerisation reactors: An experimental and modelling study, Chem. Eng. Sci. 60 (2005) 5574–5589. [26] R.F.A. Teixeira, H.S. McKenzie, A.A. Boyd, S.A.F. Bon, Pickering Emulsion Polymerization Using Laponite Clay as Stabilizer To Prepare Armored “Soft” Polymer Latexes, Macromolecules. 44 (2011) 7415–7422. doi:10.1021/ma201691u. [27] G. Henini, F. Souahi, Y. Laidani, Tracking Offline Conversion Solution Polymerization of Methyl Methacrylate / Vinyl Acetate in Toluene in a Reactor Calorimeter, Procedia Eng. 33 (2012) 225–233. [28] M.S.M. Alger, Polymer Science Dictionary, Springer Science & Business Media, 1997. [29] W. Li, G. Song, G. Tang, X. Chu, S. Ma, C. Liu, Morphology, structure and thermal stability of microencapsulated phase change material with copolymer shell, Energy. 36 (2011) 785–791. [30] L. Sánchez-Silva, J.F. Rodríguez, A. Romero, A.M. Borreguero, M. Carmona, P. Sánchez, Microencapsulation of PCMs with a styrene-methyl methacrylate copolymer shell by suspension-like polymerisation, Chem. Eng. J. 157 (2010) 216–222. [31] I. Capek, Radical polymerization of polar unsaturated monomers in direct microemulsion systems, Adv. Colloid Interface Sci. 80 (1999) 85–149. [32] D. Pavia, G. Lampman, G. Kriz, Infrared spectroscopy, in: Introd. to Spectrosc., 3rd ed., Cengage Learning, Washington, 2001: pp. 13–101. [33] M. You, X.C. Wang, X.X. Zhang, L. Zhang, J.P. Wang, Microencapsulated n-
28
Octadecane with styrene-divinybenzene co-polymer shells, J. Polym. Res. 18 (2011) 49–58. [34] N. Sarier, E. Onder, Organic phase change materials and their textile applications: An overview, Thermochim. Acta. 540 (2012) 7–60. [35] X. Zhang, Y. Fan, X. Tao, K. Yick, Crystallization and prevention of supercooling of microencapsulated n-alkanes., J. Colloid Interface Sci. 281 (2005) 299–306. [36] X. Tang, W. Li, X. Zhang, H. Shi, Fabrication and characterization of microencapsulated phase change material with low supercooling for thermal energy storage, Energy. 68 (2014) 160–166. [37] G. Fang, F. Tang, L. Cao, Preparation, thermal properties and applications of shapestabilized thermal energy storage materials, Renew. Sustain. Energy Rev. 40 (2014) 237–259.
29
Preparation of acrylic based microcapsules using different reaction conditions for thermo-regulating textiles production A. S. Carreiraa,b, R. F. A. Teixeirac, *, A. Beirãob, R. Vaz Vieirab, M.M. Figueiredoa, M. H. Gila
Highlights: Production of thermo-regulating textiles has attracted much attention in recent years. Suspension polymerization technique was used to encapsulate a phase change material. Reaction temperature and type of initiator influenced the monomer conversion profile. Acrylic based microcapsules with a well-defined core/shell structure were prepared. Microcapsules exhibit suitable thermal performance/stability for textile application.
30
Preparation of acrylic based microcapsules using different reaction conditions for thermo-regulating textiles production A. S. Carreira, R. F. A. Teixeira, A. Beirão, R. Vaz Vieira, M. M. Figueiredo, M. H. Gil
31