
==== Front
Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)13243-4
10.1016/j.heliyon.2024.e37212
e37212
Research Article
Thermal and mechanical performance assessment of composite (CPCM) for energy storage
Khalaf Alshammari Naif a
Trigui Abdelwaheb abdelwaheb.trigui007@gmail.com
b⁎
Hassani Rym rhassani@jazanu.edu.sa
c⁎⁎
a Mechanical Engineering Department, Engineering college, University of Ha'il, Hail, 8148, Saudi Arabia
b Laboratory of Multifunctional Materials and Applications (LaMMa), Faculty of Sciences of Sfax, Tunisia
c Chemistry department, center for environmental and nature research, Jazan University, P.O Box 114, Jazan, Saudi Arabia
⁎ Corresponding author. abdelwaheb.trigui007@gmail.com
⁎⁎ Corresponding author. rhassani@jazanu.edu.sa
31 8 2024
15 9 2024
31 8 2024
10 17 e3721214 5 2024
28 8 2024
29 8 2024
© 2024 The Authors. Published by Elsevier Ltd.
2024

https://creativecommons.org/licenses/by-nc/4.0/ This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).
Phase change materials (PCMs) face obstacles in being widely used due to issues with heat transfer and maintaining their shape. In this research, instead of using binders, the Hexadecane (H) is melted in such a way that the capillary forces of the molten wax allow it to be impregnated into the low-density polyethylene (P) molecules and bind it together as a composite. It was found that the hot melt extrusion (HME) combines the two materials at the micro-scale, forming a phase change composite (CPCM) with various geometries that possesses superior latent heat and shape stability during phase transition. The structure can incorporate a higher percentage of PCM (60 %) using this method, which also results in lower costs. According to the thermal analysis, (H60P40) provides great thermal stability and can store a lot of energy per unit of weight. It has a high capacity of storing latent heat at 129.56 J/g and can also prevent Hexadecane leakage. Based on the mechanical properties results, hexadecane acts like plasticizer thus the addition of PCM decreases Young's modulus, stress in break, and stress at yield. This trend is observed as the PCM content increases. The high values of elongation at break also indicates the strong plasticizing properties of PCM. Based on the obtained results, the CPCMs as a potential candidate for an application in buildings for thermal regulation, reducing energy consumption, and reducing indoor temperature swing.

Graphical abstract

Image 1

Highlights

• Shape-stabilized (H/E) composites were prepared by hot-melt extrusion (HME).

• The guarded hot plates method presents an optimized measuring time of CPCMs and economical built up.

• (H60P40) achieves great thermal stability and prevents leakage of PCM (H).

• CPCM is a candidate for industrially application in buildings.

Keywords

Composite phase change materials (CPCMs)
Latent heat thermal energy storage (LHTES)
Hexadecane
Thermal stability
Hot-melt extrusion (HME)
==== Body
pmc Nomenclature

e	Thickness of composite samples	Q	Energy per mass stored,kJ/kg	
t	Time, s	φ	Heat flux density, W/m2	
ρ	Density of composite,kg.m−3	λ	Thermal conductivity, W.m−1.K−1	
T	Temperature, °C	L	latent heat,kJ/kg	
Cp	Specific heat capacity,kJ/kg.°C			
Subscripts	
1,2	Bottom and top faces of the composite	sens	Sensible	
s	Solid	init	Initial thermal steady state	
l	Liquid	end	Final thermal steady state	
m	Melting			

1 Introduction

Environmental protection and energy conservation represent two main challenges facing the world in the 21st century. Energy conservation is now more important than ever in today's world. Without a sustainable energy conservation plan, strong economic growth is not possible. In addition to the long-term environmental benefits, properly managing energy consumption can also lead to improved financial performance in the short term. The principle of latent heat storage involves using phase change materials (PCMs) that have a melting/freezing temperature similar to the range of thermal comfort. PCMs can be categorized by low-temperature (<∼100 °C), mid-temperature (∼100–200 °C), and high-temperature (>∼200 °C) based on their melting temperatures [1]. For low-temperature applications, PCMs can be incorporated into building insulation materials [2], used for domestic water heating [3]. Direct heating or heat pumps are suitable for a variety of applications, including auxiliary HVAC for building, off-peak electricity storage, solar air conditioning, etc [4].

High-thermal storage used for used for power plant [5], metallurgical applications [6], or to shift peak heating loads [7], etc. However, an obstacle preventing the widespread use of phase change materials in practice is their poor performance in heat transfer and shape stability. Additionally, there is a lack of understanding regarding the relationship between manufacturing processes and PCM performance. Several studies have been conducted on a latent heat storage system that utilizes an encapsulated phase change material [[8], [9], [10]]. However, this system has presented various challenges that needs to be addressed, including the high cost of encapsulating the material, the risk of leakage if the capsule shell is damaged, and heat resistance caused by the capsule shell. In most cases, phase change materials must be contained within a module to prevent any loss due to leakage when the material is in its liquid state. This containment module is located adjacent to the heat source. The module adds more thermal resistance to the system, along with extra weight and cost. As a result, there is a desire to get rid of the containment module. A new type of phase change material called shape-stabilized phase change material has been created. This material can maintain its shape in solid form even when exposed to temperatures above its melting point. As a result, issues related to encapsulated phase change materials are resolved by utilizing these composite phase change materials (CPCMs). CPCMs are able to preserve their shape and prevent any leakage of the PCM when it melts. This is achieved by combining the PCM for heat storage with a supporting material and/or additive that enhances thermal conductivity. The supporting materials must be compatible with the PCM chemically and structurally under working conditions. This second method includes combining a liquid phase PCM with a melted polymer to form a supportive polymer matrix that resembles a foam matrix within the PCM [[11], [12], [13]]. By this situation, the polymer structure's resistance is working alongside the PCM, rather than being in a series connection between the PCM and the heat source. This arrangement is expected to enhance the thermal response. The polymer matrix keeps the shape of the PCM compact and when the PCM melts, the liquid phase is completely contained within the matrix because of capillary forces. This eliminates the need for a separate containment module. Experimental work has identified promising materials for support, including polyethene, polyurethane, acrylic resin, and polyolefin. High-density and low-density polyethene (HDPE/LDPE) are popular choices due to their cost-effectiveness, chemical stability, and durability. Polymer materials are often mixed with organic PCMs like paraffin and fatty acids that have low to medium operating temperatures due to the polymers having a maximum working temperature of usually less than 300 °C. Chen [14] created various paraffin-polymer combinations using HDPE, LDPE, and LLDPE. The miscibility between paraffin and polymers affects the phase change enthalpy of the paraffin as discussed broadly in this work. The miscibility is observed to be lower between paraffin and HDPE compared to LDPE or LLDPE, but the heat enthalpy of paraffin is highest with HDPE. Hexadecane has gained attention in TES systems due to its favorable characteristics, such as chemical stability, non-corrosiveness, and thermal stability. Among various organic PCMs, Hexadecane is widely used as the notable materials due to its suitable phase change temperature, high latent heat storage capacity, good chemical and thermal reliability and is commercially available at competitive prices [15]. The advantage of high latent heat, congruent melting behaviour, and non-corrosiveness make hexadecane a competitive candidate for modern PCMs [15]. However, there are several disadvantages to using hexadecane, including instability when melted, low ability to conduct heat, problems with leaking, and the difficulty in binding with encapsulation materials [16]. In the area of technical engineering research, there is a growing focus on composite phase change materials (CPCMs). In their study, Chriaa et al. [17] prepared the shape-stabilized PCMs (Hexadecane) using SEBS elastomeric copolymers and low-density polyethylene as excellent thermostable matrix.

They also included expanded graphite (EG) to enhance thermal conductivity. The composite, which contains 10 % EG and 75 % hexadecane by weight, effectively prevents PCM leakage, thanks to the high hexadecane content. Rahmalina et al. [18] stabilized paraffin wax with high density polyethylene (HDPE) at 15 % HDPE by weight. The thermal analysis performance of the paraffin/HDPE composite were reported, but no structural properties and no mechanical properties were provided.

The incorporation of LDPE, HDPE, and LLDPE in the paraffin mixture has proven to be beneficial, enabling the composite to be suitable for large-scale production. This topic has been widely explored in various studies, particularly those focusing on simulating the extrusion process for PCM and paraffin composites [19]. M. Q. Wu et al. [12] tackle the trade-off between constructing shape stable organic PCMs with high enthalpy values and low leakage rates. The maximum amount of HDPE that can be added is 20 % by weight, which still results in a significant 29.7 % reduction in the latent heat of fusion [20].

Therefore, in this study an attempt has been made to increase the mass percentage of PCM inside supporting matrix as a material shape stabilizer while the thermophysical properties or the transient response to thermal cycles remains guaranteed also be considered. The idea of blending hexadecane (H) and low-density polyethylene (P) is investigated as a composite form-stable phase change material (CPCM) with hot melt extrusion (HME) method which would be beneficial in technical engineering applications. This will provide steady of thermophysical properties of the paraffin and LDPE composite during the latent heat storage system operation. In this study, different formulations of a LDPE/hexadecane blend were extruded and investigated for thermal conductivity, thermal storage capacity, and mechanical characteristics of the composites.

2 Experimental section

2.1 Materials

The current study utilizes organic paraffin as the phase change material. Hexadecane (H) a saturated hydrocarbon of the alkane's family with a phase change temperature of 18–20 °C, was used as the PCM thanks to its high latent heat (over 223.66 kJ/kg). This melting point has been considered suitable for using interior building envelopes. Low density polyethylene (P) in powder form, as the supporting materials for their good compatibility with Hexadecane, has a melting temperature of 110 °C, a density of 0.9 g/cm3, a specific enthalpy of melting of 109.8 J/g, a particle size <50 μm and an MFI = 7.0 g/10 min).

2.2 Preparation of CPCMs

The powdered low-density polyethylene (P) and liquid paraffin were combined in specific percentage by weight before being extruded. The composite materials (H/P) were then processed in the Plasti-Corder PLE machine from Brabender in Germany, operating at 140 °C and 35 rpm for a duration of 10 min. While mixing, it is clear from observation that the two materials blend easily and harmoniously, indicating that they are compatible. Mixing takes 30–60 min, depending on the sample [[21], [22], [23]]. Pour the mixture into a mold 200×200×2mm3 and let it harden. The prepared PCM mixture was first compressed at 19.5 MPa for 1 min to remove the compressed air. The final step was to melt the samples at 140 °C at 19.5 MPa for 10 min using an AMS 10 ton melting press and then gradually lower the temperature until it reaches room temperature, all while keeping the pressure constant. (Fig. 1). The various mass ratios of the constituent elements in each sample are displayed in Table 1. PCM content exceeding 60 wt% resulted in substantial phase separation of the blends and compromised their mechanical strength.Fig. 1 Preparation steps of the CPCM (H/P).

Fig. 1

Table 1 Properties of CPCMs (H/P).

Table 1Properties of blends	P100	CPCMs (H/P)	
H10/P90	H20/P80	H30/P70	H40/P60	H50/P50	H60/P40	
Mass (g)	86.3	64.7	62.6	61.9	61.1	58.7	57.1	
Densities (kg/m3)	970.62	823.71	873.34	863.63	838.67	657.09	616.16	
Mass of Hex (g)	_	6.4	12.5	18.8	24.4	29.4	34.3	

Ideally, it would be best to use a high percentage of paraffin PCM. However, previous research [24] has shown that the upper limit is around 75–85 %. A comprehensive overview of the thermophysical properties of CPCMs are tabulated in Table 1.

2.3 Characterizations

The comprehensive phase change transition of the PCM (Hexadecane) were assessed using a Perkin–Elmer Diamond DSC differential scanning calorimeter with an intra-cooler. The DSC analysis calibrated with an indium standard in the range from −20 °C to 120 °C. The signal is analyzed without subtraction. The sample (6 mg) was sealed in an aluminum pan and heating rate of 10 °C/min. The temperature accuracy was ±0.01 °C.

The guarded-plate heat flow method, as shown in Fig. 2 (a), offers another way to study the thermo-physical properties of PCMs. Typical thermal analysis methods, like DSC, are made for testing small, pure, and uniform samples. However, DSC cannot measure thermal conductivity. The guarded hot plates method, which is suitable for use as a concrete sample, establishes a steady state one-dimensional heat flux through the test sample between the two plates [25,26]. The guarded hot plates system as depicted in Fig. 2 (b). The purpose of this method was to.- Measure how the melting and freezing temperature of the CPCM changed;

- Analyze how the ratio of LDPE affected the amount of latent heat released during the fusion of Hexadecane;

- Investigate how the ratio of LDPE impacted on the thermal properties of the CPCM.

Fig. 2 The guarded plate heat flow method (GPH): (a) Schematic plan, (b) Diagram of experimental setup.

Fig. 2

The instrument provides an isothermal and temperature-controlled boundary condition using upper and lower temperature-controlled two aluminum plates and a well-insulated side wall. The experimental CPCM sample is placed in between the top and bottom heat exchangers. To insulate the sample, a 20 mm thick polyethylene expanded foam (PEF) is wrapped around it. This configuration permits heat flow through the samples in an adiabatic manner. Accordingly, the heat transfer through the CPCM sample can be calculated assuming one-dimensional thermal condition. Two heat flux transducers (Captec, FR) are less than 1 mm in thickness to measure the enthalpies through sample during testing processes. Six K-type thermocouples (TC Ltd., UK) attached at the surface on both sides of the CPCM to measure the temperature variations from 15 °C to 50 °C. Measuring thermal storage properties of the samples was recorded using an external hybrid multimeter/DC power supply (U3606, Keysight, MY). With the guarded hot plates system, the thermal properties of CPCMs are taken as an important factor in the latent heat storage system. They describe the operated characteristics, specifically the thermal conductivity, the specific heat capacity and the latent heat which affect the operation of the latent heat storage unit. Additionally, it is feasible to study the thermal efficiency of CPCMs by looking into factors like their thermal stability and energy consumption in order to maintain a constant indoor temperature. The amount of heat is given by equation (1) [27,28]:(1) Q=1ρ.e∫tinittendΔφ.dt=Cp.(Tend−Tinit)[kJ/kg]

where Δφ represents the cumulative heat rate entering the sample and Cp is the apparent specific heat capacity of the composite (kJ/kg. °C). This measured experimental density heat can also be calculated from equation (2):(2) Q=Qsens+Lm=(Cps.ΔTs+Cpl.ΔTl)+Lm[kJ/kg]

where Cps and Cpl are the average of the measuring specific heats of the PCM solid and liquid, respectively; ΔTs and ΔTl are the temperature variations for the material in the solid and liquid phases, respectively; and Lm is the latent heat of melting.

DMA was conducted to assess how the glass transition temperature of blend components changed with varying hexadecane content, as well as to examine the mechanical properties of the composite materials at room temperature. Tensile testing was carried out following the ASTM D638 standard, with a testing rate of 50 mm/min. The analysis included obtaining average values and standard deviations from at least five measurements. Unfortunately, pure hexadecane could not be analyzed by DMA because it was too fragile to withstand the dynamic forces used in the tests.

3 Results and discussion

3.1 Leakage-proof properties

The evaluation of leakage-proof properties is important for assessing the form stability, efficiency, and durability of CPCMs. To determine shape stability, samples are heated above the PCM's melting point and compared. The process involves placing the initial sample mass (M0) on a filter paper in a thermostat set at 50 °C. The sample is then allowed to cool naturally, with its weight measured hourly using an analytical balance. This cycle is repeated with the filter paper replaced after each hour. The mass of the sample after each heat treatment cycle is noted as (Mn). equation (3) provided in Refs. [29,30] can be used to calculate the amount of leakage in this context, allowing for a quantitative assessment of the leakage-proof properties of CPCMs.(3) L(%)=M0−MnM0.100

Fig. 3 shows photographs of the CPCMs samples after being heated at 50 °C for various durations from 1 h to 20 h. Fig. 4 shows the evolution of mass loss as a function of the number of thermal cycles in a comparative study containing different mass fractions. Fig. 4 shows that the leakage rate after 1 h for the H10P90 to H60P40 at 50 °C are respectively 3.64 %, 0.29 %, 0.42 %, 0.39 %, 0.10 %, and 0.17 %. As the quantity of PCM was augmented, the rate of leakage increased. The incorporation of polyethylene can maintain the stability of PCMs and resolve the issue of leakage. This is due to various interactions occurring between the matrix and the PCMs, including surface tension, hydrogen bonding, and capillary forces. At 50 °C, the leakage curves for all CPCM are significantly stable after 14 h as evident in Fig. 4. Moreover, the utilization of (P) in the composite hindered the mobility of paraffin chains within the matrix. As a result, the combined effects of the crosslinked network of (H) and (P) synergistically contribute to shape stability. The observed excellent stability can be attributed to the strong compatibility of the polyethylene and the hydrocarbon chain of Hexadecane. This specific blend has the capacity to absorb and retain a substantial volume of liquid Hexadecane, bringing about a thermally stable PCM.Fig. 3 Shape-stable photographs of all samples after consecutive hours of heating for 50 °C.

Fig. 3

Fig. 4 Leakage tests of CPCMs samples according to time at 50 °C.

Fig. 4

3.2 Thermal stability

Thermogravimetry was performed to assess the thermal stability of samples. The TGA curves of P100, H100 and CPCMs are illustrated in Fig. 5. The test temperatures ranged from 25 °C to 600 °C at a heating rate of 20 °C/min. As seen from Fig. 5, the weight curve of P100 and H100 present a mainly degraded in one step. The degradation of these polymers begins at around 200 °C and finishes at around 500 °C. For the CPCMs, the thermal degradation was a two-step process, corresponding to the degradation of H100 and P100, which is typical for immiscible blends different of degradation temperatures. The first step ranged from 250 °C to 320 °C corresponding to the degradation of Hexadecane and the percentage of mass loss during this step corresponds to the quantity of Hexadecane mixed in the composites. The second step at above 400 °C represents the degradation of the polymer matrix. The results suggest that the decomposition of (H/P) composites is negligible below 200 °C. It was found that the hot melt extrusion (HME) combines the two materials, forming a phase change composite (CPCM) that possesses superior shape stability during phase transition. This improves the reliability and durability of the PCM based system.Fig. 5 TGA curves of H100, P100 and CPCMs.

Fig. 5

3.3 Thermal analysis of hexadecane "differential scanning calorimetry"

DSC is used to examine latent heat enthalpy of Hexadecane by measuring area under the curve corresponding to phase change of sample in the DSC thermograms (Fig. 6). The latent heat enthalpy can be used to find out heat capacity and the thermal stability of a material. Fig. 6 shows distinct endothermic and exothermic peaks, which indicated consistent phase change behaviour. The Hexadecane melts at 21.15 °C and solidifies at 16.96 °C, according to the DSC results, which is beneficial to the practical application of CPCM in solar based thermal energy storage [26,31]. The degree of supercooling is the difference between the melting and freezing temperature, (Tm−Tf). The melting peak temperature is 25.57 °C, whereas the freezing peak temperature is 14.04 °C. This suggests that Hexadecane has a relatively high supercooling effect of approximately 11.53 °C which is in a good agreement with the result obtained by Ref. [32]. The results illustrated hexadecane's endothermic enthalpy (ΔHm) of 223.65 J/g and an exothermic enthalpy (ΔHs) of 220.86 J/g, is greater when comparing to other semi-crystalline polymers such as polyesters [33]. This indicates a strong thermal heat storage capacity for hexadecane, particularly in low temperature, making it suitable for seasonal passive building envelope components [34]. In the molten phase, self-reinforcement is present in both hexadecane and PE, preserving their structure above their respective melting points. However, hexadecane has a higher phase change enthalpy comparing LDPE, making composites with higher percentages of hexadecane have great energy storage capacity. The hexadecane contents and dimensional stability are considered to be key factors affecting the thermal energy storage capability of CPCM.Fig. 6 DSC curves of endothermic (stored heat) and exothermic (release heat) processes of Hexadecane.

Fig. 6

3.4 Apparent thermal conductivity measurements of the CPCMs

The CPCM is placed between two plates. Typically, the lower plate is set to a colder temperature, known as the bottom plate, while the top plate is set to a higher temperature. To maintain a constant flow of heat in one direction, a specific thermal gradient is applied to the sample. By analyzing the steady temperatures, the sample thickness, and the heat input to the hot plate, the thermal conductivity can be calculated by equation (4) [35]:(4) λs,l=e.Σφs,l2.ΔTs,l

where e is the distance between two sides of the sample; Σφs,l is the quantity of heat passing through a unit area of the sample in unit time; ΔT is the temperature difference across the specimen (K).

Fig. 7 (a) displays the results of the solid phase case study. The experiment involved a composite material that initially contained solid paraffin in a temperature of 15 °C, which was below the melting point (Tm). This temperature was maintained until equilibrium was reached, resulting in the composite phase change material (CPCM) being entirely solid. At time t > 0, the composite was heated by increasing the temperature on one side only to 20 °C, still below the melting point. A similar experiment (Fig. 7 (b)) was conducted for the liquid phase, where the material was exposed to a temperature gradient, with both sides at temperatures higher than the melting point (40 °C and 50 °C). The apparent thermal conductivities of the tested samples were found to be satisfactory, listed in Fig. 8. Adding hexadecane reduced the thermal conductivity of LDPE based PCM blends. This continued to lower with an increase in PCM mass fraction in the blends. The behaviour was attributed to low thermal conductivity of paraffin. When the hexadecane loading increases, they act as insulators, which cause a decrease of conductivity. This significant decrease the composite's heat transport due to a higher ratio of PCM in the LDPE matrix. Additionally, the polar OH groups of cellulose are involved in inter and intramolecular hydrogen bonds, making them less available for conduction compared to the LDPE matrix. This decrease is expected because the hexadecane thermal conductivity (0.19 W m−1. K−1) is lower than the matrix (0.218 W m−1. K−1). Analyzing the results indicates that with a constant concentration (e.g. H10P90) as temperature rises, thermal conductivity decreases slightly from 0.257 to 0.241 W/m/K. The temperature during processing does impact the adhesion between hexadecane and the matrix, which is crucial for the composite's overall properties.Fig. 7 Measurements of apparent thermal conductivities of composite (H20P80).

Fig. 7

Fig. 8 Apparent thermal conductivities and associated uncertainties of the composites with different Hexadecane contents.

Fig. 8

3.5 Phase transition temperature and enthalpy measurement

• Apparent thermal heat capacity

The study investigated the heat capacity of the material in both its solid and liquid states as it transitions between the two phases. This was done by measuring the total sensible heat accumulated by the material as it was subjected to different temperatures. The process involved initially setting the surface temperature of the sample at 15 °C until it reached a thermal steady-state. The heat flux was then reduced to zero at the start time of t = 0s. It was also noted that there were minimal thermal losses at the sides of the sample. At a specific time (tinitial), a change in temperature was introduced in the controlled temperature bath, causing the material to undergo a thermal transition (storage) until it reached a new equilibrium state at a temperature of 20 °C.

The amount of heat energy Q, which represents the change in the system's internal energy, is influenced by the heat transfer properties of the solid or liquid region of the PCM. To calculate the specific heat capacity of the PCM in its solid state, the initial temperature (Tinit) and final temperature (Tend) must be below the melting point. For determining the specific heat capacity in the liquid state, the experiment is conducted similarly but at temperatures above the melting point (Tinit = 40 °C, Tend = 45 °C). At the start of the test, there is a rapid increase in current which eventually drops to zero, indicating that system has reached a new equilibrium state. This specific heat capacity of the solid or liquid can then be calculated by integrating the heat flows from the initial state (tinit) to the final state (tend). A summary of the investigated CPCMs, phases, and experimental parameters for axial flow methods applied to CPCMs are given in Table 2. According to the data collected, adding hexadecane to LDPE results in an increase in apparent thermal heat capacity in both phases. The specific heat capacity also rises as the amount of paraffin in the composite increases, because hexadecane has a high specific heat capacity.• Latent heat of fusion/release determination

Table 2 Amount of heat stored and apparent latent heat for various composites (H/P).

Table 2Sample	Qsens(kJ/kg)	Cp (kJ/kg. °C)	Q(kJ/kg)	Lm(kJ/kg)	
solid (15–20 °C)	Liquid (40–50 °C)	Solid (15–20 °C)	liquid (40–50 °C)	evolution (15–50 °C)	
H10P90	16.84	30.38	3.41	4.27	126.88	79.66	
H20P80	9.27	37.94	2.23	3.78	129.20	81.98	
H30P70	17.7	41.11	3.54	4.14	144.57	85.76	
H40P60	21.32	37.45	4.51	3.78	154.94	95.16	
H50P50	22	37.7	4.39	3.78	186.29	126.63	
H60P40	225.13	39.16	5	3.9	193.98	129.56	

The latent heat capacity of melting and freezing is a key factor in a latent heat storage system, as it determines the storage capacity. Compared to sensible heat systems, the latent heat storage of PCM can provide larger energy storage capacity and constant supply temperature during the phase transition. In this study, the latent heat of fusion was determined by calculating the total amount of heat (Q) stored between two stable thermal states (15 °C and 50 °C). The transition from 15 °C to 50 °C allowed us to observe the complete melting process of the composite material, where a significant amount of energy was stored as sensible heat (Qsensible) and latent heat (Lm). The experimental latent heat of Phase Change Materials (PCM) made from CPCMs (H/P) with varying hexadecane mass fractions at different temperatures is of great interest due to the ability of composites to provide high-energy storage density in a quasi-isothermal process, which is important in Thermal Energy Storage (TES) applications. Fig. 9 illustrates the changes in flux density at the bottom side for each sample. It also highlights the significance of the heat stored through latent heat in comparison to that from sensible heat (P100). The graph displays the temperature evolution and heat flux on both sides of the sample within a temperature range of 15–50 °C. The temperatures selected are significantly beyond the melting point, allowing us to categorize the material definitively as either being in a solid or liquid state. The phase change, occurring between temperatures of 15 °C and 50 °C, leads to the establishment of a new thermal equilibrium in slightly more than 60 min. The fusion process exhibits distinctive qualities as the sample smoothly transitions from a solid state to a liquid state with clearly defined phases. Each test measures the material's transient heat response by integrating heat flux over time. Multiple tests were conducted to verify the repeatability of the experiments. Table 3 displays the flow and temperature changes for various temperature differences in order to illustrate the significance of thermal storage through latent heat.Fig. 9 Evolution of the heating process (15 °C to 50 °C) for all CPCMs.

Fig. 9

Table 3 The thermal properties and performance of PCM (H100) and CPCMs (H/P) with varying contents of Hexadecane.

Table 3
Sample	Melting enthalpy (J/g)	Solidification enthalpy (J/g)	Heat storage efficiency	Impregnation ratio	Impregnation efficiency	Thermal storage
capability	
ΔHm	ΔHS	γ(%)	R (%)	E (%)	φ(%)	
PCM	H100	223.66	220.87	1.25	–	–	–	

CPCMs	H10P90	126.88	123.66	2.53	56.72	56.36	99.36	
H20P80	129.20	125.6	2.78	57.76	57.32	99.23	
H30P70	144.57	140.63	2.72	64.63	64.16	99.27	
H40P60	154.94	151.14	2.45	69.27	68.85	99.4	
H50P50	186.29	181.04	2.81	83.29	82.63	99.21	
H60P40	193.98	190.18	1.95	86.73	86.41	99.64	

When the temperature decreases from 50 °C to 15 °C inside the cool case, the material becomes solid. Symmetrical changes in heat fluxes are seen in both faces of our parallelepipedic samples during this process, as shown in Fig. 10. At first, there is a normal decrease in heat flux as the liquid phase cools down. However, at (t = 33 min), the heat flux changes direction when the surface temperature nears 28 °C. From this critical point, the sample continues to cool and solidify gradually until it reaches the desired temperature of 15 °C. It takes at least 1 h and 30 min for the sample to reach equilibrium, indicating that heat restitution is a lengthy process. During the initial stages of solidification, a layer of solid PCM forms on the surface in contact with the cooling plate, creating a barrier between the liquid phase and the cooling source. The slow solidification process is attributed to the low thermal conductivities of the solid SS-PCMs. The surface temperature of the sample only reaches 15 °C by the end of the test, showing the prolonged nature of the solidification process.Fig. 10 Flux density and temperatures evolutions (50 °C–15 °C) “heat releasing processes of PCM”.

Fig. 10

Fig. 10 displays a comparison for the changes in flux density on the underside of various samples. It illustrates the unpredictable nature of crystallization and emphasizes the significance of the heat released during crystallization compared to the heat obtained from the surroundings. This outcome can be attributed to the unpredictability of hexadecane's behaviour, which changes with each solidification process as a result of random crystal rearrangements.

Successful utilization of the latent heat energy storage system depends considerably on the thermal reliability and stability of the phase change materials (PCMs) used. Thermal stability of phase change material can be established by measuring the thermo-physical properties of the PCM after a number of repeated thermal cycles. A comprehensive knowledge of thermal stability of the PCMs as functions of number of repeated thermal cycles is essential to ensure the long-term performance and economic feasibility of the latent heat storage systems [30,36,37]. Fig. 11 presents the stability of the composite (H60P40) after 120 h of heating-cooling cycles.Fig. 11 Flux density and temperatures evolutions (10 °C–30 °C) “thermal storage of PCM”.

Fig. 11

Fig. 12, Fig. 13 demonstrate that latent heat energy storage is more effective than sensible energy storage when considering operating temperature and storage density. Furthermore, adding Hexadecane correlates with an increment in latent heat of fusion for the CPCM. Interestingly, adding LDPE at different ratios reduces the melting/solidification temperatures of CPCM insignificantly. The reason for this can be explained by applying the second law of thermodynamics to the phase change process identified by equation (5).(5) T=ΔHΔS

where T is the transition temperature of CPCMs, ΔH the phase changes heat per unit mass, and ΔS is the entropy changes during phase change transition.Fig. 12 Evolution of stored latent heats of (H/P) composites with PCM mass fraction.

Fig. 12

Fig. 13 Evolution of heat stored/released for all CPCMs.

Fig. 13

With a higher charging rate of PCM, the enthalpy change (ΔH) improves, while entropy change (ΔS) decreases. Consequently, the temperature at which phase change occurs decreases. The overall amount of heat absorbed by a CPCM that has 60 % hexadecane when the temperature changes from 15 °C to 50 °C is 1.76 times greater than the heat absorbed by plaster, a typical construction material. Plaster can store around 129.56 kJ/kg of latent heat.

By examining the line diagram depicting the relationship between the mass fraction of hexadecane and the enthalpy of the CPCM (Fig. 14), the calculated latent heat of melting of CPCM is lower than the measured latent heat of melting of PCM, due to the presence of the supporting matrix that reduces the effective PCM content in the CPCM. The heat loss percentage of all the prepared composites (γ) remains largely unchanged after thermal cycle testing. To further assess the phase change performance of the prepared composite PCM, a commonly used measures of heat storage efficiency was utilized, and equation (6) is presented below:(6) γ=(1−ΔHsΔHm)*100%

where ΔHm and ΔHs are the melting and solidification phase change enthalpies, respectively.Fig. 14 The relation between the PCM mass fraction, enthalpy and the corresponding heat loss percentage for all samples.

Fig. 14

Three important factors, including the impregnation ratio (R), impregnation efficiency (E) and thermal storage capability (φ) were utilized to assess the phase change capabilities of the samples that were prepared. These parameters were determined using three equations (7), (8), (9) respectively [38,39]:(7) R=(ΔHm,BCPCMΔHm,PCM)*100%

(8) E=(ΔHm,BCPCM+ΔHs,BCPCMΔHm,PCM+ΔHs,PCM)*100%

(9) φ=(ΔHm,BCPCM+ΔHs,BCPCMRΔHm,PCM+ΔHs,PCM)*100%

The determined results of (R), (E) and (φ) of all CPCMs are shown in Fig. 15.Fig. 15 The calculated results of (R), (E) and (φ) for phase change performance of the prepared samples.

Fig. 15

R denotes the impregnation of hexadecane within the LDPE structure, while E describes the PCM's useful performance in the composite for storing latent heat. But inside the tiny PCM pores, the phase change hardly happens because of the confinement effect on molecular motion. Consequently, some PCM cannot be used as phase change materials (PCMs). Interestingly, the prepared composites demonstrated a thermal storage capability of 100 % (±0.15 %), proves that nearly all hexadecane molecule chains can efficiently store or release heat via a phase transition.

Recently, the benefits of extrusion method have attracted a lot of researchers applying this approach to produce high-performance PCM, especially CPCMs. A state of art of CPCMs fabricated by hot-melt extrusion methods (HME) is displayed in Table 4.Table 4 State of art of the CPCM prepared by extrusion.

Table 4CPCM	Screw speed (rpm)	Method description	Melting point (°C)	PCM ratio (%)	References	
HDPE/paraffin	175	1. Extruded to composite pellet; 2. Compressed to sheets (70 bar for 5 min).	18 -23; 56 -58	75	[40]	
HDPE/PureTemp 42	35	Mixed and extruded.	42	21.4	[41]	
HDPE/paraffin	80, 100, 150	melted and mixed, then extruded	30	75	[42]	
HDPE, LDPE, LLDPE/octadecane paraffin	80	1. mixed with molten paraffin; 2. Extruded.	24.8–26.9	90	[14,43]	
HDPE/wood flour (WF)/paraffin/EG	80	1. impregnated EG with paraffin; 2. blended with HDPE/WF by twin extruder; 3. compressed at 4 MPa and 160 °C.	20	27	[44]	
Hexadecane/LDPE	35	1. mixed with molten paraffin; 2. Extruded; 3. compressed at 19.5 MPa and 140 °C.	25.57	60	This study	

3.6 Evaluation of mechanical properties by dynamic mechanical analysis (DMA)

Playing a supportive role in Phase Change Materials (PCM), LDPE is crucial to provide high structural strength with a minimal amount of LDPE. According to the data in Table 5, blending hexadecane with LDPE has a substantial impact on the mechanical properties of the materials. Across all tested LDPE/hexadecane blends, the stress in break decreased as the charging rate of PCM increased. This decline is attributed to the high presence of low molecular weight hexadecane, which weakens the tensile strength of the blend. Hexadecane itself lacks strong tensile properties, and its crystals in the amorphous phase of the polymers serve as weak points for stress cracking initiation and propagation. The tensile strength of LDPE was more greatly affected by higher levels of hexadecane, likely due to the increased amorphous content in the polymer. Lower values of stress at break were also observed by Mtshali et al. [45], after blending LDPE with hexadecane. A rise in hexadecane levels causes a drop in the elongation at break of the blends being studied. This is likely due to hexadecane crystals which create weak spots that make it easier for stress cracking to start and spread. Additionally, the immiscibility of the components can also affect the elongation at break. As phase separation occurs in all blends, the materials experience a loss of drawability and a significant decrease in elongation at break. A decrease Young's modulus with an increase in hexadecane content is observed for the LDPE blends, indicating that the modulus of the hexadecane is lower than the polymer. This is probably associated with its lower degree of crystallinity. This is a logical behaviour since the mechanical properties of low molecular, soft hexadecane are significantly lower than the mechanical properties of LDPE [46]. Paraffin hexadecane acts like plasticizer which reduces Young's modulus, stress at break as well as the stress at yield. All these parameters decrease with an increase of the hexadecane content. The strong plasticizing character of hexadecane is clear also from the relatively very high values of elongation at break. The materials blended with hexadecane up to 40 wt % show cold flows. Neat LDPE and its blends with 10 wt % of hexadecanes show and orientation hardening. The orientation hardening is suppressed for the blends having hexadecane content over 10 wt %.Table 5 Mechanical properties of LDPE and CPCMs (H/P): tensile mode, T = 25°C.

Table 5Sample	εy ± Sεy/%	σy ± Sσy/MPa	εb ± Sεb/%	σb ± Sσb/MPa	E ± SE/MPa	
LDPE	11.4 (0.3)	10.6 (0.2)	488 (108)	11 (2)	148 (6)	
H10P90	11.2 (0.6)	8.4 (0.2)	495 (67)	8 (1)	124 (4)	
H20P80	10.7 (0.2)	4.6 (0.3)	65 (15)	4.9 (0.2)	101 (4)	
H30P70	10.3 (0.3)	4.2 (0.2)	75 (10)	4.0 (0.1)	78 (4)	
H40P60	3.5 (0.2)	3.8 (0.3)	54 (5)	3.6 (0.1)	71 (2)	
H50P50	br	Br	30 (20)	2.1 (0.1)	44 (4)	
H60P40	br	br	33 (18)	0.7 (0.2)	24 (4)	
εy, σy, εb, σb, E − elongation at yield, yield stress, elongation at break, stress at break, and Young's modulus of elasticity; Sεy, Sσy, Sεb, Sσb, SE are the standard deviations.

br refers brittle rupture.

The impact of reduced mechanical properties due to plasticization on the performance of Composite Phase Change Materials (CPCMs) in building applications is a significant concern, yet it remains underexplored in current literature. Plasticizers are added to polymers to enhance flexibility and processability, but they often lead to decreased mechanical strength, which can adversely affect the structural integrity and longevity of materials used in construction. However, this comes at the cost of mechanical properties such as tensile strength and Young's modulus. Specifically, plasticized polymers exhibit lower tensile strength and stiffness, meaning they can deform more easily under stress, which is critical in load-bearing applications [47,48]. For instance, studies have shown that the tensile modulus can decrease significantly with increasing plasticizer content, leading to materials that are less resilient and more prone to failure under mechanical loads [48]. CPCMs are used in building applications for their ability to absorb and release thermal energy, contributing to energy efficiency. However, if the mechanical properties of these materials are compromised due to plasticization, their effectiveness in structural applications may be diminished. Reduced tensile strength and stiffness can lead to:

Decreased Load-Bearing Capacity: Structures may not withstand the required loads if the CPCMs lose significant mechanical strength, potentially leading to structural failures.

Increased Deformation Under Load: Enhanced flexibility might result in excessive deformation, affecting the overall stability and performance of building elements.

Durability Issues: The long-term performance of building materials is critical, and plasticization can lead to accelerated wear and degradation, especially under varying environmental conditions [49].

While plasticization can enhance certain properties of polymers, the trade-off in mechanical performance poses challenges for the use of CPCMs in construction. It is essential for future research to focus on balancing the benefits of plasticizers with the mechanical integrity required for building applications, ensuring that materials can perform effectively in real-world conditions. This balance will be crucial for advancing sustainable building practices that rely on the thermal management capabilities of CPCMs.

4 Conclusion

In this study, we investigate experimentally the thermal potential of a novel composite phase change materials (CPCMs) composed of hexadecane (H) and low-density polyethylene (P) prepared by extrusion method. The CPCMs have been acknowledged as cost-effective materials that possess adequate thermal capabilities for construction purposes. These findings show that the composite made has a great thermal energy storage capability. The phase change material, when sealed inside the matrix, can effectively absorb, store, and release latent heat in a nearly uniform temperature transition. This good stability is actually a result of the polyethylene's high compatibility with the hydrocarbon chain of hexadecane. The maximum weight percentage for PCM dispersed in the CPCMs without any leakage of the melted PCM was found as high as 60 %. Based on the above discussion, following conclusions can be drawn:• The LDPE/Hexadecane is cheap, easy to prepare, and possess a suitable latent heat.

• H60P40 had a large enthalpy of 129.56 J/g and suitable melting temperature (T = 25.57 °C), which presents great potential in mid-high temperature TES applications.

• Five important parameters, including the impregnation ratio (R), impregnation efficiency (E), thermal storage capability (φ) , heat storage efficiency γ (%) and apparent thermal conductivity (λ), which might represent the phase change performance of the prepared samples.

• Based on the mechanical properties results (which is obtained from the tensile strength test), it is seen that the all investigated mechanical parameters decrease with an increase in the hexadecane content. This is a logical behaviour since the mechanical properties of low molecular, soft hexadecane are significantly lower than the mechanical properties of LDPE.

HME involves the continuous melting and mixing of CPCMs under controlled temperature and shear conditions. The choice of processing conditions, such as temperature, screw configuration, and feed rate, directly influences the final properties of the extrudate, including its morphology, thermal stability, and mechanical properties. Thermo-physical and mechanical properties obtained are very important for studying and simulating their behaviour. This phase change characteristic makes it suitable to use in buildings located in hot and dry climates, where daytime temperatures are typically very high (above 40 °C) and drop significantly at night (around 1–10 °C). These weather conditions allow the CPCM to switch back and forth between the two phases, helping to minimize temperature fluctuations inside the building.

Data availability statement

No data was used for the research described in the article.

CRediT authorship contribution statement

Naif Khalaf Alshammari: Supervision, Project administration, Conceptualization. Abdelwaheb Trigui: Writing – original draft, Validation, Methodology, Investigation, Formal analysis. Rym Hassani: Writing – review & editing, Validation, Investigation, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The author (Rym HASSANI) extends their appreciation to the Deputyship for Research & Innovation, 10.13039/100009950 Ministry of Education in Saudi Arabia for funding this research work through project number ISP23-106.
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