Salt Leaching As a Green Method for The Production of Polyethylene Foams for Thermal Energy Storage Applications

Materials able to store thermal energy can be a useful strategy in order to reduce energy consumption of buildings and to decrease greenhouse gases emissions. In this work, for the first time, the technique of salt leaching has been used for the production of novel polyethylene foams containing different amounts of a microencapsulated phase change material (PCM) with a melting point of 24 °C, to be potentially applied in building insulation. The microstructural, thermal and mechanical properties of the produced foams have been comprehensively investigated. The prepared foams were characterized by high values of open porosity (about 60 %) and by density values around 0.4 g/cm 3 . Differential scanning calorimetry tests revealed that the adopted production process caused a partial loss of PCM, resulting in an effective PCM content of around 33 % for the sample with the highest PCM loading (56 wt%). Infrared thermography analysis demonstrated that the time required from the samples to reach a set temperature, thanks to the presence of PCM, was up to two times higher with respect to the reference foam. Shore-A measurements evidenced that the addition of PCM generally led to a softening of the foams. Tensile mechanical tests confirmed the softening effect provided by the addition of the microcapsules, with a decrease of the stiffness and of the strength of the material. Interestingly, strain at break values were considerably increased upon PCM introduction.


Introduction
Recently, the improved living conditions of people and the increase of growth rate of population have led to an enhancement of the energy demands for buildings. Recent studies have found that the primary source of energy in the world will continue to be fossil fuels up to 2030 [1]. The development of sustainable building and renewable energy resources is thus becoming an important issue for our society. In fact, the building sector is one of the most energy consuming, with an overall energy consumption of 30 % in the world [1] (40 % in Europe [2]) and it is responsible for one-third of the worldwide emissions of greenhouse gases [3]. Considering that the annual solar energy incident on buildings is greater than the energy needs for hot water and heating, the possibility of a thermal management of renewable energy is a crucial point both in building and automotive sectors in order to minimize energy consumption and toxic gases emissions [4,5]. The ability of a material to temporarily store heat and release it in a second time is a process called thermal energy storage (TES) [4,6,7]. In order to store large amounts of thermal energy, latent heat thermal energy storage systems are particularly promising, because of their capacity to store heat at constant temperature upon a phase change in the material. Phase change materials (PCMs) are thus designed for the storage of a considerable amount of heat when they undergo a reversible solid-toliquid transition at a constant temperature [8][9][10][11][12]. Examples of PCMs are paraffin waxes [13][14][15][16], fatty acids [17], polyethylene glycol (PEG) [18], fatty alcohols [19], salt hydrates and metals [12,20]. Paraffins are organic PCMs and they are widely used for thermoregulation of buildings [10,21], sportswear [22] and smart fabrics [23][24][25], thanks to their low cost, high heat of fusion, cheapness, non-corrosive behaviour, chemically inertness, thermal stability, little volume changes on melting and low vapor pressure in the molten state [8,9,26,27]. PCMs can be encapsulated in order to avoid leakage above the melting temperature, increasing thus their strength, their durability and thermal stability, their easy-handling and reliability [25,28,29]. A second strategy to avoid PCM leakage is the shape stabilization [15,30,31]: it consists in the confinement of the PCM within a polymer matrix, such as high-density polyethylene [32], polypropylene [33], poly(methylmethacrylate) [34], polyurethane copolymers [35], acrylic resins [36], styrenebutadiene-styrene rubber and ethylene-propylene-diene-monomer (EPDM) rubber [13].
Polymeric foams are expanded materials in which the porosity is obtained using chemical or physical blowing agents. In comparison with bulk polymers they present low density, low thermal conductivity, high insulation capacity, elevated thermal stability and also good mechanical properties in terms of toughness and impact resistance [37]. In recent years, polyolefins have found wide interest as matrices for the foaming process. In fact, polyolefin foams are characterized by a wide range of properties that allow them to be employed as insulation materials in various fields such as buildings, automotive, military, aerospace, aircraft, sports etc. [38,39]. Polyethylene foams are generally used for production of gaskets and vibration pads, sound insulation, water barriers, expansion joints, pipe insulation, glazing seals. They are used in a wide range of density from 0.15 to 0.60 g/cm 3 [40]. Physical blowing agents are inert gases, which are introduced into the polymer matrix during a saturation process, usually at high pressure [37]. The most widely used physical blowing agents are nitrogen (N2), carbon dioxide (CO2) or hydrocarbons such as pentane. Chemical blowing agents are powders that can induce an exothermic or endothermic reaction inside the polymer matrix through their thermal decomposition, generating thus a gas phase. The most common one is azodicarbonamide (ADC) which decomposes at around 170-200 °C and possesses high gas yield, releasing nitrogen and heat [38,[41][42][43]. Examples of endothermic chemical blowing agents are sodium bicarbonate and zinc bicarbonate [44]. Despite the extensive use of chemical foaming agents for the production of expanded polymers, many of them are hazardous for health and their use is not allowed in the EU [43]. For this reason, safer physical foaming agents, like water or hydrocarbons, are often required [45,46]. However, physical blowing agents generally show some problems related to the difficult manufacturing process and the use of additional and expensive equipment [43].
A possible environmentally friendly alternative to traditional foaming agents is represented by the "salt leaching" technology, which involves the addition of water-soluble salt particles (such as sodium chloride, potassium chloride, etc) into the polymer matrix and their subsequent dissolution in hot water, leading to the formation of a porous cellular structure within the polymer matrix [47][48][49][50].This technology has been widely used for the production of scaffolds [48,49,51], plastic semiconductors [52], open cell nitrile-butadiene-rubber (NBR) sponges [53], EPDM foams [54,55], applied in tissue engineering and for soft sensors [56]. By tuning the polymer-to-salt ratio and by varying the leachable particle size, the porosity of the resulting scaffolds may range between 70 % and 95 %. Moreover, salt leaching maybe used in combination with other techniques (gas foaming, compression moulding) in order to further modify the structure of the foams, increasing the pore interconnectivity [57].
Despite the environmental advantages that could arise from the combination of polyethylene foams  Da, was acquired from Alfa Aesar (Kandel, Germany) and used to improve the leaching efficiency.
Sodium chloride, commercial grade (density = 2.16 g/cm 3 ), was grinded and sieved using a 230mesh sieve in a granulometry lower than 63 µm. Preliminary tests were carried out in order to determine the granulometry value that allowed to minimize the density of the foam and a salt size lower than 63 µm was thus selected. The salt particles were then dried and stored in an oven at 60 °C until use. MPCM24 microcapsules containing a wax with a melting point of 24 °C and a melting enthalpy of 145 J/g were purchased from Microtek Laboratories Inc. (Dayton, OH, USA) and used as phase change material. Polyethylene powder, PEG and microcapsules were used as received.

Samples preparation
Polyethylene foams were prepared by melt compounding in an internal mixer (Thermo HaakeRheomix ® 600), equipped with counter rotating rotors. The compounding process was performed at a temperature of 153 °C and a rotor speed of 50 rpm. The polymer matrix and the PCM were manually mixed and then added to the melt compounder, where they were mixed for 5 minutes. Salt and PEG were then gradually added into the mixer, reaching a total mixing duration of 20 minutes. The total amount of materials inserted in the mixing chamber was of about 40 g. The resulting compounds were then compression moulded at a temperature of 180 °C for 13 minutes.
The pressure was set at 1 bar for the first 3 minutes and at 7 bar and for the next 10 minutes. In this way, square sheets of polyethylene (dimensions of 100x100x2 mm 3 ) with different PCM amounts were obtained. The leaching method described by Scaffaro et al. [49] was taken as a reference for the present work. Pressed samples were thus immersed in boiling demineralized water for 3 hours in order to obtain a proper salt dissolution. The obtained foams where then dried overnight in an oven at 60 °C.
The list of the samples with their codes is reported in Table 1. Their designation is formed by the term PE followed by the average size of the salt particles (< 63 μm) used for the foams production and by the PCM content expressed in phr (per hundred resin).

Residual salt and water uptake measurements
The amount of residual salt (RS) not dissolved during the leaching process has been calculated according to the Equation (1): where mdry is the mass of the material after the drying process subsequent to the leaching and mtot is the theoretical mass of the material in the case of complete salt dissolution, evaluated according to Equation (2): where m0 is the mass of the sample before salt leaching, mNaCl is the initial mass of the salt in the blend (30 g) and mPEG is the initial mass of the PEG in the blend (2 g). With the aim of evaluating the continuity of the porosity and therefore the dissolution of the foaming agents (PEG, NaCl), also a foam connectivity parameter (C) has been calculated according to Equation (3) [49]: Water uptake measurements were carried out in order to identify the influence of the porosity on the water absorption tendency of the foams. These tests were carried out following the procedure reported in the ASTM D570 standard: samples were previously dried for 24 hours in oven at 50 °C and then weighted, they were then immersed in demineralized water for a certain amount of time, and then weighted again. The mass of the samples was measured using a Kern KB3600 balance (resolution of 10 mg). For each sample the mass measurements were carried out after immersion in water for 30 minutes, 1 hour, 2 hours, 8 hours and 24 hours. The water uptake (WU) has been calculated according to Equation (4): where mwet is the mass of the sample after the immersion in water and mdry is the mass of the sample after drying.

Morphological properties
Cryofractured surfaces of the foams were observed through a Zeiss Supra 40 field emission scanning electron microscope (FESEM), operating at an accelerating voltage of 2.5 kV inside a chamber under a vacuum of 10 -6 Torr. Before the observations, the samples were metallized through the deposition of a thin electrically conductive Pt/Pd coating.
Helium pycnometry density (ρpicn) measurements were conducted using a gas displacement Residual salt was taken into account for the density measurements; therefore, mixture rule (Equation 5) has been used.
where ρF is the effective density of the foam considering the residual salt content, ρc is the geometric density of the foam (containing residual salt), WRS and WF are the weight fraction of residual salt and expanded material (without residual salt), respectively, and ρRS is the density of salt (2.16 g/cm 3 ) [58]. According to ASTM D6226 standard, it was possible to calculate the total porosity (Ptot) and the fraction of open porosity (OP) and closed porosity (CP) according to Equations (6-8): where is the density of the non-expanded polymer matrix (equal to 0.914 g/cm 3 ) obtained from pycnometric measurements.

Thermal properties
Investigation of the thermal degradation behaviour of the prepared materials was carried out through thermogravimetric analysis (TGA) using a Mettler TG50 thermobalance under a nitrogen flow of 10 ml/min in a temperature range between 30 and 700 °C, at a heating rate of 10 °C/min.
The temperature associated to a mass loss of 5% (T5%), the temperatures associated to the maximum rate of degradation of the PCM and of the PE matrix (respectively denoted as Tpeak1, Tpeak2) and the residual mass at 400 °C (m400) were determined.
Differential scanning calorimetry (DSC) measurements were performed on the prepared samples using a Mettler DSC30 calorimeter under a nitrogen flow of 100 ml/min. A first heating scan from -30 to 70 °C was followed by a cooling stage from 70 to -30 °C and by a second heating scan from -30 to 70 °C. All the thermal ramps were carried out at 10 °C/min. In this way, the melting temperature of the PCM during the first and the second heating scan (Tm1,Tm2), its crystallization temperature (Tc,) and its specific melting and crystallization enthalpy values (ΔHm1, ΔHc, ΔHm2) were obtained. Moreover, the effective PCM content in the foams was determined during the first heating scan (PCM 1 ), the cooling scan (PCM ) and the second heating scan (PCM 2 ) as the ratio between the specific enthalpy of the samples and the corresponding specific enthalpy values of the neat PCM, as shown in Equations (9-11): where ΔHm1PCM, ΔHcPCM, ΔHm2PCM are the specific phase change enthalpy values of the neat PCM collected during the first heating scan, the cooling and the second heating scan, respectively.
The thermal energy storage performance of the materials was investigated monitoring their surface temperature evolution using an infrared thermal camera FLIR E60 (emissivity = 0.86). The specimens were heated in an oven at 40 °C overnight and then inserted in a climatic chamber at a temperature of 5 °C; their surface temperature was then monitored until the thermal equilibrium between the samples and the environment was reached. In the same way, the specimens were cooled in a refrigerator at 0 °C overnight and then inserted in an oven at a temperature of 40 °C. In the heating tests the time required to reach a temperature of 40 °C (t40) has been evaluated, in the cooling tests the time required to reach a temperature of 5 °C (t5) has been determined.

Mechanical properties
Shore A hardness test was performed using a Hilderbrand Prufstander OS2 durometer following the ASTM D2240 standard. Square samples 20 mm wide and 2 mm thick were tested, and Shore A hardness value was determined after a loading time of 5 seconds. At least five measurements were performed for each composition.
Tensile properties of the prepared foams under quasi-static conditions were measured on ISO 527 type 1BA dumbbell specimens (gage length equal to 30 mm) using an Instron 5969 tensile testing machine equipped with a load cell of 100 N and operating at a cross-head speed of 1 mm/min. The elastic modulus (E) has been calculated as a secant modulus between strain values of 0.0005 mm/mm and 0.0025 mm/mm. The stress at break (σR) and the strain at break (εR) have been also evaluated. At least five specimens were tested for each composition. Both the elastic modulus and the stress at break values were normalized for the geometrical density of the foams. probably because of an inhomogeneous salt dispersion during the production stage. However, the connectivity decreases as the content of PCM inside the foam increases, passing from 80% for the neat PE foam up to 47% for the PE_<63s_130_p sample. This could be explained by the fact that the PCM filled some of the pores, thus limiting the interconnections among them.

Residual salt and water uptake measurements
In Figure 1 the water uptake values, calculated according to Equation (4), are reported as function of the testing time. In this plot, also the water uptake of the bulk polyethylene sample (PE) is reported. The parabolic shape of the curves indicates that the water absorption in these foams is mainly governed by a diffusion mechanism [59] and that the absorption rate decreases with time, reaching in some cases a plateau after 24h. As it could be expected, the bulk PE sample does not absorb water, in fact the water uptake value after 24 hours is zero. It is interesting to notice that increasing the PCM amount inside the foams, the water uptake decreases. This could suggest that the addition of more PCM leads to a decrease in the porosity and thus, in the volume available for water absorption. This feature can be also correlated to the decrease of the connectivity observed in Table 2. In fact, the increase of the PCM content in the foams leads to the formation of a less interconnected structure and thus to a lower water absorption capability.

Morphological properties
The morphological features of the foamed samples were studied through scanning electron microscopy (SEM) observations. In Figure 2 Table 2) and an open pores structure (see Figure 2). In these conditions, the density values obtained through pycnometric measurements are close to bulk density of PE (taking also into account the PCM content and the residual salt concentration).

Thermal properties
Thermogravimetric curves of the prepared foams, along with the corresponding derivative plots are presented in Figure 4(a,b) while the most significative results are listed in Table 3 and Table 4.
Comparing PE and PE<63s samples, it is evident that the foaming stage produces a slight decrease of the thermal stability of the sample, with a shift of thermogravimetric curves towards lower temperatures. This behaviour can be explained considering the increased surface area exposed to degradation in the case of foams, the presence of preferential pathways for release of by-products  Table 3 it is evident that both Tpeak1 and Tpeak2 are not substantially affected by the PCM content in the foams. As it could be expected, the thermal degradation stability of the foams is negatively influenced by the PCM addition. As it can be seen in DSC curves of the prepared samples and of the neat PCM are presented in Figure 5(a-c) and the most significant results are summarized in Table 4. In Figure 5a and 5c the first and second heating scans are reported and in both figures two endothermic peaks can be observed. The first peak, approximately at 20-25 °C, is associated to the melting of paraffin and its intensity increases with the PCM content in the foams. The second peak, observed at around 50 °C, is associated to the melting of PEG that remained in the foam and has not been fully dissolved.
In Figure 5b the thermograms referred to the cooling scan are represented. In this case a double exothermic peak in the temperature interval from -15 °C to 20 °C, corresponding to the crystallization of the PCM, can be noticed, and its intensity increases with the PCM amount. As it could be expected, the melting/crystallization enthalpy values increase with the PCM amount, and the PE_<63s_130_p foam shows a ΔHm1 value of 50 J/g. However, taking into account the PCM weight concentration within the foams, reported in Table 1, it is possible to see that ΔHm1, ΔHc, ΔHm2 values are systematically lower than the expected ones. Consequently, the effective PCM contents (PCM 1 , PCM , PCM 2 ) are considerably lower than to the nominal values reported in Table 1. For instance, the sample PE_<63s_130_p has a PCM 1 value of 33 % (instead of 56 %) and the sample PE_<63s_25_p a value of 1.6 % (instead of 20 %). These discrepancies are partly due to the presence of residual salt that reduces the effective PCM content and also due to the PCM loss during the production process of samples. Considering that TGA tests highlighted that the differences between the real and the theoretical PCM contents within the samples are realtively small, the discrepancy detected in DSC tests can be also attributed to the breakage of some capsules during the preparation stage. In other words, the paraffin that flew out of the capsules is not able to melt/crystallize within the PE matrix in an efficient way.
The melting temperature of PCM (Tm1 and Tm2) slightly increases with the PCM amount. On the other hand, the crystallization temperature (Tc) decreases as the PCM content increases. This trend is not due to an intrinsic property of the material, but rather than to the lower thermal conductivity of the foams with respect to the neat PCM. In these conditions, all the thermal transition are shifted towards higher temperature in the heating scan and to lower temperature in the cooling stage. A similar behaviour was also observed for PCM microcapsules dispersed in an acrylic matrix reinforced with carbon-fibres, that showed lower crystallization temperature with respect to neat PCM microcapsules [61]. The melting enthalpy values in the first and second heating ramps (∆Hm1 and ∆Hm2) are quite similar, meaning that the capability of samples to store heat is retained even when the thermal history of the samples is deleted. Further tests will be made in the future to evaluate the retention of the TES properties upon the application of repeated thermal cycles.   Table 5 highlight that PE_<63s_130_p foam is characterized by values of t5 and t40 which are double in comparison to those of the foam without PCM. The results indicate that, even if part of the PCM was lost during the production process (or it is not able to melt/crystallize once inserted in the PE matrix), the prepared foams are able to store/release an interesting amount of thermal energy in the considered temperature interval.

Mechanical properties
From the stress-strain curves presented in Figure 8 and from the results reported in Table 6 In a similar way, the specific strength decreases from 4.8 MPa·cm 3 /g up to 1.9 MPa·cm 3 /g for the PE<63s_130p sample. The decreased strength of the foams due to PCM addition can be probably explained by the limited interfacial adhesion between the PE matrix and the PCM microcapsules, as detected in SEM micrographs (see Figure 2f). Interestingly, the strain at break values of the foams are considerably increased with the PCM addition, passing from 0.08 mm/mm in the case of PE_<63s sample up to 0.16 mm/mm for the PE_<63s_130_p foam. It can be hypothesized that the observed increase in the strain at break values upon PCM addition can be attributed to the toughening mechanism produced by the interfacial debonding between the PE matrix and the microcapsules at elevated deformation levels. In these conditions, a considerable amount of energy must be spent to create debonding surfaces wihin the samples, with a consequent fracture resistance increase. However, further tests will be required to have a better comprehension of this effect. It is therefore possible to conclude that the addition of paraffin microcapsules within these materials plays a softening effect, with a consistent reduction of the dimensional stability and of the failure resistance, accompanied by an interesting improvement of the ultimate strain levels.

Conclusions
This work demonstrated that the salt leaching technique could be a very interesting method for the sustainable production of polyethylene foams applied in thermal energy storage applications.
Evaluation of water uptake, SEM micrographs and density measurements allowed to investigate the morphology of the foamed samples, showing that the produced foams were characterized by an open porosity with an interconnected structure. The presence of PCM in the foams filled part of the pores, increasing thus the foam density and reducing water absorption values. Thermogravimetric analysis evidenced that the thermal degradation stability decreased with the PCM amount, but the degradation temperature remained well above the maximum working temperature of these foams in building applications. DSC analysis showed that these materials were able to store/release an interesting amount of thermal energy, even if the measured melting/crystallization enthalpy value were below the theoretical ones. Infrared thermography analysis highlighted that the time required by the PE_<63s_130_p sample to reach the set temperatures was doubled with respect to the neat PE foam. The stiffness, the hardness and the strength of the foams were considerably reduced upon the PCM addition, probably due to the limited intrinsic mechanical properties of the microcapsules and the low of interfacial adhesion between the PCM and the PE matrix. Interestingly, the strain at break values were noticeably improved upon PCM addition. It could be therefore concluded that the prepared foams possessed interesting thermo-mechanical features, and they could be successfully applied for the thermal insulation and the thermal management of buildings. An optimization of the production process should be made in the future to increase the PCM concentration within the foams and to improve the interfacial adhesion between the capsules and the PE matrix. Figures   Figure 1. Results of the water uptake measurements on the prepared foams.       Tables   Table 1. List of the prepared samples. Table 2. Residual salt and connectivity values obtained for the prepared foams. Table 3. Results of the TGA tests on the prepared foams. Table 4. Results of DSC tests on the prepared foams and on the neat PCM. Table 5. Results of the thermal imaging tests on the prepared foams. Table 6. Results of Shore A hardness and quasi-static tensile tests on the prepared foams. Figure 1 Results of the water uptake measurements on the prepared foams.  Residual mass (a) and derivative of mass loss (b) from TGA tests on the prepared samples.   Temperature pro les obtained from thermal imaging tests on the prepared foams during (a) heating and (b) cooling stages.

Figure 8
Representative stress-strain curves from quasi-static tensile tests on the prepared foams.