Iranian Polymer Journal https://doi.org/10.1007/s13726-018-0626-5

ORIGINAL RESEARCH

Shape-stable phenolic/polyethylene glycol phase change material: kinetics study and improvements in thermal properties of nanocomposites Faranak Samani1 · Ahmad Reza Bahramian1   · Alireza Sharif1 Received: 5 January 2018 / Accepted: 23 April 2018 © Iran Polymer and Petrochemical Institute 2018

Abstract Storage, transformation, and absorption of energy play effective roles in application and performance of heat and thermal energy beneficiary. Phase change materials (PCMs) are substances with high heat of fusion which can be utilized to design thermal protective and thermal energy storage systems. However, PCM leakage in phase changing process is a well-known disadvantage of the PCM containing systems. One of the approaches to avoid PCM leakage is to prepare shape-stabilized PCM in polymeric composites. In this study, polyethylene glycol (PEG), as a PCM, was shape-stabilized with low leakage in the novolac colloidal structure with no solvent and through a sol–gel in situ polymerization process. Supercooling is a negative associate phenomenon in these systems, which may occur due to the low rate of nucleation and nucleation growth. Nanoclay was used to avoid supercooling of PEG. PEG supercooling significantly decreased when 2.5 wt% of nanoclay was incorporated. This is due to the role of nanoclay particles as the crystal nuclei. The sol–gel polymerization kinetics of novolac resin in the presence of nanoclay and molten PEG was also studied using the Kamal–Sourour model. Results showed that 85 wt% of PEG was preserved with leakage less than 3.5 wt% by shape stabilization encapsulated with colloidal structure of the phenolic resin. Nanoclay improved the thermal properties of the system and reduced the supercooling about 20%. Moreover, based on Kamal–Sourour model, polymerization kinetics could suggest a lower novolac curing rate in the presence of molten PEG and nanoclay. Keywords  Thermal insulator · Phase change material · Nanocomposites · Thermal properties · Reaction kinetics

Introduction Nowadays, latent heat for thermal energy storage is one of the most important technologies that plays an effective role in the use of energy [1, 2]. A phase change material (PCM) absorbs or releases a large amount of energy during the phase change process [3, 4]. PCMs are used for thermal applications, due to their capability for high latent heat storage per unit volume, via phase change at desired operating temperature range [5–7]. However, PCMs are not easy to use, because of their melt flow, weak thermal stability, flammability, and low thermal conductivity [7]. To * Ahmad Reza Bahramian [email protected] 1



Department of Polymer Engineering, Faculty of Chemical Engineering, Tarbiat Modares University, P.O. Box 14115‑114, Tehran, Iran

solve this problem, PCMs can be encapsulated in a proper material. Encapsulation is a process to encompass the PCM with suitable material [8]. This process was first invented by Barrett K Green in the 1940s [9]. This additional requirement not only increases cost, but also reduces the thermal performance of the system, due to increased thermal resistance of system [4] and holding the PCM isolated from the surrounding [9]. Beside, encapsulation prevents the leakage of PCM from the system [10], while its shape is stabilized during phase change. Recently, impregnating the solid–liquid PCMs into porous carriers is becoming a promising encapsulation technique in the field of shape-stabilized PCM [8, 11]. These porous carriers can be easily divided into numerous standalone energy storage units, which can improve the heat transfer efficiency of PCMs during the phase change process [8]. In addition to the leakage, PCMs have shortcomings of supercooling [12, 13]. When some molten PCMs are cooled, they solidify at a temperature below their melting points,

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because of the rate of nucleation or the slow rate of growth of the nuclei (or both). Supercooling will reduce the performance of PCMs, and can also completely prevent heat recovery if it is very severe. Supercooling can be overcome by adding an appropriate nucleating agent with a crystal structure similar to PCM [4, 14]. Jeong et al. [15] prepared shapestabilized PCM that contained sodium montmorillonite and exfoliated graphite nanoplatelets, to improve supercooling and thermal conductivity of PCMs, and prevent leakage of PCMs in their liquid state. In Sary [16] work, PEG600/gypsum and PEG/natural clay composites were prepared as kinds of shape-stabilized PCMs for low-temperature solar passive applications in buildings. The maximum absorption ratio of PEG600 in gypsum-based and natural clay-based composites was found to be 18 and 22 wt%, respectively [16]. In Tang et al. [12, 17, 18], Yang et al. [13], Qian et al.’s [11] studies, PEG as a PCM was used by sol–gel method to prepare shape-stabilized PCM, with a maximum ratio reaching to about 80 wt%. In Chai et al.’s work [19], a sort of novel bifunctional microencapsulated PCM was designed by encapsulating n-eicosane into a crystalline titanium dioxide ­(TiO2) shell, which have a potential in applications such as: heat recovery, intelligent textiles and medical protective clothing, preservation and sterilization of foods, and solar energy storage. Jiang et al. [20] designed the magnetic microcapsules based on an n-eicosane core and F ­ e3O4/SiO2 hybrid shell as a new type of dual-functional PCM. With such a dual-functional feature, the magnetic microcapsules developed by this study could provide a new applicable route to fulfill both thermal energy storage and magnetic effectiveness. He et al. [21] synthesized a series of n-alkanes/silica composites as shape-stable PCMs using sodium silicate precursor. It is anticipative that, owing to the easy availability and low cost of sodium silicate, the synthetic technology developed by this work has high feasibility in industrial manufacturing of shape-stable PCMs. In Li et al.’s [22] study, a novel type of multifunctional microcapsules based on an n-eicosane PCM core and a ZnO shell were designed not only for latent heat storage but also for photocatalytic and antibacterial activities. Moreover, the microcapsules achieved an excellent energy storage capability and a high working reliability [22]. The goal of this work is to propose an effective new method to prepare PCM shape-stabilized nanocomposite based on PEG as phase change material, without using solvent, and with very low amount of leakage. Novolac colloidal structure was used as supporting material through in situ polymerization. Nanoclay was incorporated to improve the thermal properties while playing a role as supporting material. This system can be used to effective thermal energy technology, as well as energy storage and heat insulators. In addition, in this study, it was tried to lower supercooling of PEG using nanoclay as a nucleating agent of crystallization.

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The curing kinetics of novolac sol–gel in the presence of molten PEG and nanoclay was also studied. Specifically, in this work, the nanocomposite shape-stable PCM was prepared with higher PCM content (about 85 wt%) and lower leakage than other related recent works (about 3.5%).

Experimental Materials Novolac-type phenolic resin (IP-502) containing 9  wt% of hexamethylene tetramine (HMTA) which acts as the cross-linking agent with density of 1.27 g/cm3 (Resitan Co., Iran) was used to prepare the initial sol in molten PEG. Cloisite 15A is a natural montmorillonite that is modified with N ­ a+(CH3)2(HT)2, [T:tallow composition as (65%C18, 30%C16, 5%C14)] with density of 1.66 g/cm3, d001 = 31.35  Å, and cation exchange capacity (CEC) of 1.25 meq/g (Southern Clay Products, Inc., USA) was used to prepare nanocomposites. Polyethylene glycol (PEG) with density of 1.21 g/cm3, d001 = 22.15, and 27.02 Å was used as phase change material with melting point of about 50–55 °C (PEG2000 Merck Co., Germany).

Preparation method of PCM nanocomposite samples Organoclay was well distributed in molten PEG by stirring for 4 h at 90 °C. The precondition for addition of novolac resin was to avoid its simultaneous curing, which starts at about 80 °C. Therefore, the temperature of organoclay/ molten PEG mixture was reduced to 65 °C. Under this condition, novolac powder was dissolved in molten PEG and prepared the initial sol. The homogeneous mixture containing of 15 wt% novolac powder and organoclay in molten PEG (sol) was heated up to its curing temperature (120 °C) and kept for 5 h to complete gelation, and then post-cured at 140 °C for 2 h [23, 24]. Compositions of the nanocomposites are given in Table 1.

Characterization FTIR spectroscopy (FTIR, PerkinElmer, Frontier, USA) was applied to characterize the chemical structures of the composites using a KBr sampling sheet. X-ray diffraction was used with a diffractometer (XRD, XPert MPD, Philips, Holland) to analyze the microstructures of clay, PEG, and composites. Novolac initial particle size was determined by dynamic light scattering (DLS, Zetasizer, Malvern Instrument, England). The morphology of the composites’ fracture surface was investigated using a scanning electron microscopy (SEM, XL-30, Philips, Japan).

Iranian Polymer Journal Table 1  Composition of samples Row Sample

Solid content/ PEG

Novolac resin (wt%)

Clay 15A (wt%)

1 2 3 4 5 6 7 8 9 10

15/85 15/85 15/85 15/85 15/85 15/85 15/85 100/0 0/100 100/0

15.0 14.5 13.5 12.5 12.0 11.5 10.5 100.0 0.0 0.0

0.0 0.5 1.5 2.5 3.0 3.5 4.5 0.0 0.0 100.0

N15C0 N14.5C0.5 N13.5C1.5 N12.5C2.5 N12C3 N11.5C3.5 N10.5C4.5 N100C0 (novolac) PEG Clay 15A

Burning test was used to analyze the homogeneity of samples in an oven (Nabertherm, Germany), according to the standard test method of ASTM C 831-18. The samples were heated to 650 °C and preserved for 1 h in air atmosphere. Char yield and residual solid of the composites after burning test were calculated using their residual/initial mass ratios. The homogeneous sample has the same residual solid in all its pieces. Dynamic differential scanning calorimetry (DSC, Netzch Co., Germany) was performed to analyze the curing and thermal properties of the samples. All measurements were performed at a heating and cooling rate of 10 °C/min in air atmosphere. To investigate phase change characteristics of the cured composites and their curing behavior analysis, the heating scan was run sweep from − 10 to 100 °C and for uncured samples from 0 to 200 °C, respectively. Shape stabilization and leakage experiments were studied using an electrical oven (Fater Co., Iran). All samples in cylindrical shapes with their each respective diameter of 1.9 and 0.4–0.5 cm height were kept at 40, 50, 60, 70, 80, 90, and 100 °C for 1 h. For leakage evaluation, mass changes of the composites were measured before and after heating.

Results and discussion Chemical structure analysis Figure 1 shows the FTIR spectra of initial components and composite samples. Most absorption peaks of PEG main functional groups (about 1100 and 2900 cm−1) appear in the spectra of the sample, with a very slight shift. In PEG/ novolac sample, frequency shift of the functional group indicates that there are some chemical interactions between PEG and novolac [24]. These interactions are responsible for leakage prevention performance [25, 26]. In addition to

Fig. 1  FTIR spectra of the initial materials and samples

these chemical forces, there are physical interactive forces (hydrogen bonds, interface interaction,…) between the PEG melt and the novolac structure for shape stabilization of molten PEG in composite systems. Hence, a little leakage was observed when the temperature was raised above the melting point of PEG [27–29]. With clay content increases, there was no change observed in the characteristic peaks. It meant that there were no chemical reactions between PEG, novolac, and clay during sol–gel polymerization of novolac. The 463 cm−1 peak which is an indicator peak for the clay is slightly shifted to lower wavenumbers in the nanocomposites. Empty orbital of Al in border of clay sheet acts as Lewis acid and has interaction with polar section of PEG (because of high electronegativity difference between carbon and oxygen). Therefore, it causes a reduction of Al–O peak which is due to the clay/ polymer interactions. This peak is presented in Fig. 2. This interaction illustrates the act of nanoclay to preserve PCM leakage.

Microstructure and morphology analysis Figure 3 presents XRD patterns of clay and composite samples; XRD information data are also shown in Table 2. The clay characteristic peak (defined by d001) in composites is omitted or shifted to lower angle which suggests that clay is exfoliated or intercalated in the samples, and a nanocomposite morphology is obtained. Moreover, the small peak appearing at 8.23 (defined by d002) may be due to the crystallographic planes of the clay layers [30]. The intensity of XRD is generally taken as a measure for classifying microstructure and the intensity is increased with increasing filler concentration [31]. Therefore, d002 confirms the presence of clay all over the nanocomposite, as well as FTIR in Fig. 2.

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Fig. 2  a Characteristic peak of clay and b characteristic peak of clay in nanocomposite in obvious scale

Fig. 3  XRD results of samples

Table 2  XRD results of samples Row

Sample

Position (2θ, °)

d-spacing (Å)

d-spacing difference (Å)

1 2 3 4 5 6 7 8 9

N15C0 N14.5C0.5 N13.5C1.5 N12.5C2.5 N12C3 N11.5C3.5 N10.5C4.5 Clay 15A PEG

… 1.06 1.57 … 1.94 1.37 ... 3.27 22.16 27.02

… 96.573 65.326 … 52.870 74.702 … 31.356 4.645 3.831

… 65.216 33.970 … 21.514 43.346 …

Intensity of d002 decreases by lowering clay component and it confirms its homogeneity in nanocomposites. According to Table 2, samples of N12.5C2.5 and N10.5C4.5 are exfoliated and other samples are intercalated nanocomposites. DLS result shows that the particle size of novolac is about 2 nm, so it can penetrate between the clay sheets. According

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to the studies; the polymerization is believed to be the indirect driving force of exfoliation. Clay, due to its high surface energy, attracts polar monomer molecules in its galleries until equilibrium is reached. The polymerization reaction occurring between the layers reduces the polarity of intercalated molecules and disrupts the equilibrium. This allows new polar species to penetrate into the layers and progressively exfoliates the clay. Therefore, the nature of the curing agent as well as the curing conditions is expected to play a role in the exfoliation process [30]. Based on XRD test results, basal space of clay layer is 31 Å, so it can be assumed that, in addition to the compatibility [32] between PEG/novolac composite and nanoclay, in situ polymerization can occur between the clay sheets in increasing their intensity of intercalation and exfoliation. A typical result of SEM image of sample is presented in Fig. 4. Figure 4a, b shows the images of cross section of colloidal structure of novolac filled and unfilled with PEG, respectively. The microstructure of composites indicated that novolac continuous network is not formed, due to low concentration of novolac in molten PEG. Therefore, under this condition, the colloidal morphology of novolac in Fig. 4b was obtained in PEG, as highlighted spots in Fig. 4a.

Homogeneity of the composites To investigate the homogeneity of samples, burning test was done according to the standard test method for residual and apparent residual carbon of polymer and composite (ASTM C 831-18). Results were preserved in Fig. 5 in red (a) columns, and showed that char yield for different parts of samples were approximately equal, which meant that clay dispersion in specimens was homogeneous. Beside, the same procedure was carried out for the initial components. Therefore, the residual mass of PEG, clay, and N100C0 at burning condition were 0.00, 57.30 ± 0.01, and 0.09 ± 0.01%, respectively. These results are used to calculate the char yield based on the initial component percentages which are rendered in Fig. 5 in green (b) columns.

Iranian Polymer Journal

Fig. 5  Results of burning test

In the first step, the pure novolac was fitted to three types of the following modeling formulas of nth order (Eq. 1), autocatalyzed (Eq. 2), and Kamal–Sourour (Eq. 3):

Fig. 4  SEM images of fracture surface of typical samples: a filled with PEG and b without PEG

The results of columns a and b were different and it became more obvious by increasing the clay content. In other words, the results indicated to lower char yield than the calculated amount based on row material. It seems that, in nanoclay filled samples, the organic parts of organoclay have been burnt out. In the initial clay, the small basal space between the sheets can protect the modifier from burning. Moreover, it can confirm the intercalation and exfoliation of clay sheet.

Kinetic study of sol–gel polymerization The kinetics of novolac curing reaction in molten PEG was investigated in different conditions. There are many techniques to monitor the curing kinetics of thermosetting resins. DSC is among the techniques employed to study the kinetics of curing reactions.

d𝛼 = K(1 − 𝛼)n , dt

(1)

d𝛼 = K𝛼 m (1 − 𝛼)n , dt

(2)

d𝛼 = (K1 + K2 𝛼 m )(1 − 𝛼)n , dt

(3)

where α, dα/dt, K, n, and m are fractional conversion, reaction rate (1/s), rate constant (1/s), and reaction orders, respectively [33]. The basic assumption in DSC kinetic measurements is that the change in heat flow is proportional to the change in extent of reaction [34]. Fractional conversion is defined by formula of Eq. 4, which HT and dH/dt are total heat generated from reaction and rate of heat generation, respectively:

𝛼=

dH∕dt . HT

(4)

Materials in nth order kinetics category will have the maximum rate of heat evolution in zero reaction time, while an autocatalyzed material will have its maximum heat evolution at 30–40% of the reaction. Kamal–Sourour model is a combination of nth order and autocatalyzed models. K1 is the rate constant associated with the noncatalytic reaction, while K2 is the rate constant associated with the autocatalytic reactions [35]. In other words, K1 is the beginning of the reaction rate constant. It is obtained

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Fig. 7  Kinetic parameter of Kamal formula for curing novolac in the presence of PEG Fig. 6  Fitting DSC results of curing novolac by nth order, autocatalyzed, and Kamal–Sourour formula Table 3  Compositions of samples used in DSC analysis Row

Sample

Solid content/ Novolac resin PEG (wt%)

Clay (wt%)

1 2 3 4 5 6 7

N45C0 N40C0 N35C0 N30C0 N39C1 N37C3 N35C5

45/55 40/60 35/65 30/70 40/60 40/60 40/60

0 0 0 0 1 3 5

45 40 35 30 39 37 35

by primary changes of conversion at the beginning of the reaction. In this work, kinetic parameters of reaction (m, n, and K2) were derived by the MATLAB software. As shown in Fig. 6, Kamal–Sourour model can fit well on experimental data; therefore, it is a suitable model for curing of novolac resin. According to Fig. 6, the kinetic parameters of Kamal–Sourour model were calculated for K1 and K2 as 0.0007 and 0.05 (1/s), and m and n were 0.7 and 1.1, respectively. Small value of K1 parameter was shown that the initial stage of curing reaction of novolac resin in molten PEG was very slow. DSC results for samples listed in Table 1 show a small amount of heat generation which the analyzer device cannot sense any peak. Hence, kinetic investigation was performed on samples listed in Table 3, with large amount of novolac to show the heat of reaction. Kinetic parameters for curing of novolac resin in the presence of molten PEG listed in Table 3 are investigated. The results show that the changes in K1 are negligible, so this parameter can be assumed constant. K 1 value is in

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Fig. 8  Polymerization conversion (α) of composite samples without nanoclay

order of 1­ 0 −5/s, which is one order of magnitude lower than pure novolac. This can be due to the presence of PEG which has depressed the encountering probability of novolac molecules. Other parameters (m, n, and K 2) are introduced in Fig. 7. Catalytic reaction rate constant (K2) is decreased by increasing PEG, which suggests that PEG heat of consumption is responsible for this phenomenon. In this respect, the reaction orders (m, n) are decreased by decreasing novolac in showing that PEG medium slows down the reaction rate as it encounters the rate increasing effect of the novolac content. Figure  8 shows curing conversion (α-time) diagram of composites and pure novolac. It shows that in  situ

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polymerization of novolac in the presence of PEG significantly increases the curing time. Nanoclay has changed the viscosity of reaction medium and curing kinetics due to matrix/nanoclay interaction. Consequently, this influences the rate of cure, reaction time, enthalpy, vitrification, activation energy, viscoelastic properties, and cross-linking density of the final composite [32]. Other factors such as type and amount of clay fillers and organic modifiers and their interaction with the intended host polymer molecules and their curing agents have all shown to influence properties of polymeric composites during processing [32]. The kinetic parameter of curing of novolac in the presence of PEG and clay was investigated. As shown in Table 3, solid containing samples were adjusted to 40 wt% (row of 5, 6, and 7). The same result obtained for K1 was in the order of ­10−4/s. Therefore, clay intercalation and exfoliation have not played a role in the beginning of reaction. Other parameters (m, n, and K2) are introduced in Fig. 9. While samples’ solid content was the same, the kinetic parameters (m, n, and K2) decreased by increasing nanoclay, which could contribute to the viscosity increment of the medium caused by clay intercalation and exfoliation. In addition, the clay sheets which were much bigger in size than PEG molecules prevented novolac molecules to encounter; hence, the kinetic parameters became lower than the sample without clay (N40C0). Figure 10 shows curing conversion (α-time) diagram of nanocomposites samples and N40C0. It shows that in situ polymerization of novolac in the presence of PEG and nanoclay slightly increases the curing time. In samples containing nanoclay, at conversions around of 0.2–0.3, the reaction stops for a short period of time. Perhaps, in this conversion, novolac molecules have the same size as clay sheet and the latter becomes a barrier for novolac molecules to

Fig. 9  Kinetic parameter of Kamal formula for curing novolac in the presence of PEG and clay

Fig. 10  Polymerization conversion (α) of nanocomposites

meet. However, by increasing temperature, thermal motion of novolac increases and reaction continues again.

Phase change behavior and thermal analysis Phase change behavior is studied by dynamic DSC analysis with a heating rate of 10 °C/min in air atmosphere. The results are shown in Fig. 11 and the phase change parameters obtained by DSC are represented in Table 4. Melting temperature (Tm), solidification temperature (Ts), and these temperatures differences (Tm − Ts) are listed in Table 4. The values of Tm − Ts show the supercooling effect of the samples. It was concluded that clay changes the range of melting and solidifying temperatures. In a pure molten PCM, nucleation of crystallization, due to cooling, is homogeneous. In this condition, melting and solidification have not been done

Fig. 11  DSC results of cured samples

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Table 4  DSC result analyses of samples, Part 1

Sample

∆HS (J/g)

∆Hm (J/g)

Ts (°C)

Tm (°C)

Tm −Ts (°C)

Supercooling improvement (%)

PEG N15C0 N14.5C0.5 N13.5C1.5 N12.5C2.5 N12C3

206.40 202.62 154.52 178.49 170.34 177.30

206.29 169.03 159.45 184.80 188.82 185.77

23.56 19.85 17.16 20.53 24.86 18.53

52.54 52.30 51.14 50.93 51.99 51.03

28.98 33.98 32.45 30.40 27.13 32.50

– Reference 4.5 10.5 20.2 4.3

at the same time. In the presence of nanoclay, the heterogeneous nucleation occurred in solidification process. Therefore, nanoclay has reduced the solidification time. In sample without clay (N15C0), supercooling is increased as a result of small volume of PCM in the nodules, but, in organoclay-filled samples, supercooling is decreased especially in N12.5C2.5. Data are interpreted by nucleating effect of the clays. PEG starts to crystallize on clay layers which acts as heterogenic nucleation agent and decreases the supercooling effect. In N12.5C2.5, nanoclay overcomes the supercooling effect in a way that it solidifies at higher temperature even more than pure PEG. A 2.5 wt% of dispersed nanoclay in PEG can increase the crystalline phase of PEG, as well as heat of fusion, due to heterogeneous nucleation in solidification. Three parameters can characterize the phase change performance that could be calculated according to the DSC results [34–36]. Encapsulation ratio, R, indicates the effective performance of a PCM inside the capsule for latent heat storage. The encapsulation ratio is defined by the following equation:

R=

ΔHm,encapPCM ΔHm.PCM

(5)

× 100.

The encapsulation efficiency, E, describes the effective encapsulation of PCM within a microcapsule. E is given by the following equation:

E=

(ΔH)m,encapPCM + (ΔH)s,cncapPCM (ΔH)m,PCM + (ΔH)s,PCM

Table 5  DSC result analyses of samples, Part 2

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(6)

× 100.

Another parameter that is calculated by DSC results is the thermal storage capability (φ), indicating the effective amount of PCM to save energy, φ is given by the following equation: ΔHm,encapPCM +ΔHs,encapPCM

𝜙=

R

ΔHm,PCM + ΔHs,PCM

(7)

,

which (ΔH)m,encapPCM  , (ΔH)s,cncapPCM  , (ΔH)m,PCM  , and (ΔH)s,PCM are latent heat of melting of encapsulated PCM, latent heat of solidification of encapsulated PCM, latent heat of melting of PCM, and latent heat of solidification of PCM, respectively [1, 9, 36]. Results show that PCMs can be well encapsulated and have high efficiency at the same time. In addition, thermal storage capability shows that almost the whole PCM stores the energy through the phase changing. Although the parameters are not equal for all samples, results show that the addition of clay increases the encapsulation ratios which suggest that nanoclay can make PCM to act effectively inside the capsule. However, encapsulation efficiency of the clay containing samples is slightly lower compared with the sample N15C0; because the clay sheets act as crystallizing agents and reduce ∆Hs of the samples. In Tables 4 and 5, PCM melting and solidification parameters are listed. Results show that encapsulation increases melting duration for all the samples. It can be justified that novolac colloidal structure with high specific surface area and small size [37] prevents polyethylene glycol chains to

Sample

R (%)

E (%)

φ (%)

Time of solidifica- Time of melttion (min) ing (min)

Crystallinity ± 1 (%)

PEG N15C0 N14.5C0.5 N13.5C1.5 N12.5C2.5 N12C3

100.00 81.94 77.29 89.58 91.53 90.05

100.00 90.05 76.08 88.03 87.03 87.97

100.00 109.90 98.42 98.26 95.08 97.69

2.52 4.03 2.78 1.86 2.29 2.57

57.00 46.70 44.06 51.06 52.17 51.33

2.43 4.77 5.19 5.63 4.40 4.61

Iranian Polymer Journal

crystallize normally due to its molecular limited movement. Therefore, the phase change behaviors of PCMs, confined into tiny pores, are different from the normal situation [8]. The solidification time in N12.5C2.5 sample increases and clay sheet can act as a nucleation agent to lower the crystallization time compared with N15C0. Therefore, encapsulated PCM with nanoclay takes more time to store the energy but releases it relatively faster than pure PEG. Consequently, it can be a good insulator in at certain temperature. To compare the performance of PCM systems, a couple of affected parameters are investigated in Table 6. In this table, all PCMs in some recent papers are polyethylene glycol. To compare the productivity of samples, encapsulation ratio would be a suitable parameter. All the selected papers in Table 6 were shape-stable PCM, prepared by sol–gel method, and PEG was used as a PCM, however, with different ­Mw.

Shape stabilization and leakage behavior Samples were heated up from 40 to 100  °C, and phase change occurred around 50 °C. Samples preserved their shapes, while PEG melted, which meant that the shape-stabilized PCM was achieved, but, in N11.5C3.5 and N10.5C4.5, the shapes changed at 80 and 90 °C, respectively. Leakage behavior of samples is demonstrated in Fig. 12. It can be concluded that the obtained composite offers good leakage properties, but all the samples do not show the same behavior. When novolac solid content is decreased, supporting that structure provides larger pore size, which cannot hold the PEG chains fully; so that it leaks out of structure in the molten state. Results indicate that increasing the novolac solid content contributes to less leakage. On the other hand, in sample N12C3 with lowest amount of novolac, lowest leakage is observed. It seems that, in this case, clay sheets, hand in hand with novolac network, prevent the leaking of molten PEG. However, in samples with higher percentage of clay (N11.5C3.5 and N10.5C4.5), composite cannot keep molten PEG, because novolac is not able to make continuous structure. In other Table 6  Comparison of the present achievements with the past researchers’ work

Row Sample

Fig. 12  Leakage tests on the samples

words, increase of nanoclay leads to higher viscosity which prevents the leakage; at the same time, it increased viscosity and depressed novalac content prevent novolac networking. These factors are in competition. In cases with clay content lower than 3 wt%, leakage increases by increasing the clay. N12C3 has lowest amount of leakage. N11.5C3.5 and N10.5C4.5 lose their shapes completely at 90 and 100 °C, respectively.

Conclusion Shape-stabilized PCMs were prepared by in situ sol–gel polymerization of novolac in molten PEG and nanoclay. The kinetics of curing reaction of novolac in the presence of PEG and nanoclay was investigated. The obtained nanocomposite can stabilize 85 wt% of PEG with just below 3.5 wt% of leakage. In addition, nanoclay can reduce the encapsulation supercooling effect, even lower than pure PEG. Moreover, the results showed that nanocomposites have good thermal performance and nanoclay can improve the performance of PCM.

Method Maximum R (%) Melting process

Solidifying process

References

Hm (J/g) Tm (°C) Hs (J/g) Ts (°C) 1 2 3 4 5 6 7

PEG/SiO2 PEG/SiO2 PEG/SiO2 PEG/SiO2/Cu PEG/SiO2/Al2O3 PEG/SiO2 PEG/novolac

Sol–gel Sol–gel Sol–gel Sol–gel Sol–gel Sol–gel Sol–gel

80.0 80.0 80.0 76.0 83.0 79.3 85.0

171.0 122.0 128.4 100.4 123.8 151.8 169.03

59.0 36.0 57.4 58.2 57.1 58.09 52.30

105.1 118.3 – 102.8 126.4 141.0 202.62

44.0 23.3 – 45.8 42.0 42.34 19.85

[38] [39] [40] [41] [42] [43] This work

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Iranian Polymer Journal

Acknowledgements  The authors would like to thank Tarbiat Modares University and Iran Nanotechnology Initiative Council (INIC) for the support of this work.

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