Development and Evaluation of an Integrated Phase Change Material Oriented Strand Board for Thermal Energy Storage in Building Walls

Abstract

In this study, a phase change material (PCM) in the form of technical-grade octadecane and oriented strand boards (OSBs), which are boards made from wood strands, are used to develop a latent heat storage board with the aim of utilizing this material in building construction while lowering energy consumption. The incorporation of PCM into buildings is difficult for several reasons, including the organic phase material’s flammability and leakage during phase change. These obstacles have been overcome to a significant extent in the engineered OSB material. To avoid PCM from leaking throughout the phase change regime, PCM was hosted in the oriented strand board (OSB) using high-density polyethylene to develop a shape-stabilized phase change wood-based board (SSPCM-OSB). To improve the binding between PCM and OSBs and reduce the flammability, additional additives were added. Extensive testing was conducted to determine the physical and thermal properties and heat transfer characteristics of the developed SSPCM-OSB. The newly developed oriented strand board with SSPCM integration has a lower heat flux than a conventional OSB and comparable flammability characteristics.

1. Introduction

As global temperatures continue to rise, the need for thermal comfort becomes increasingly important, thereby increasing the energy demands on air conditioning systems [1,2]. Each day that is warmer than 90 °F (32.2 °C), compared to days that are between 65 and 70 °F (18.3–21.1 °C), increases monthly electrical usage by over 3% [2]. During summer days, the highest electricity demand is during the peak hours (typically from 12 p.m. to 6 p.m.). To meet power demand during peak hours, power companies resort to load shifting to off-peak hours. Load shifting has been examined by integrating PCM into a building wall by Halford and Boehm [3]. If some of the power load is moved to off-peak hours, there could be a substantial financial gain because of the price difference between the high electric demand period versus low electric demand period [4]. There are numerous types of PCMs available for utilization within a thermal storage system, including inorganic and organic [5]. The integration of PCMs enhances energy storage in building envelopes, reduces room temperature fluctuations to increase comfort levels and assists in maintaining temperatures inside a building near the ideal range, as well as helping shift maximum air temperature to a period outside of the peak electricity demand [4]. Energy storage improves energy utilization and conservation. Sensible heat storage is the common form by which thermal energy is stored in buildings. However, latent heat storage methods based on the use of PCMs provide some benefits over sensible heat storage due to their higher energy storage density compared to sensible storage materials [4,6,7].

When PCM melts, energy is stored and released when it solidifies. A key aspect to be addressed is the leakage of PCM during phase change; hence, containment is required. Different types of containments have been used to address the issue of PCM loss when melted [6]. Shape-stabilized phase change material (SSPCM), a compound material, has received a lot of interest. It consists of PCM and other materials as supporting or host material, such as high-density polyethylene (HDPE), which is utilized as a supporting material in this study. The SSPCM maintains its shape when the PCM melts in the matrix. This enables its application for thermal storage in structures without the need for PCM capsules. The SSPCM work well because they have a skeleton that is like the organic PCM [7,8,9]. The SSPCM can be used directly without using capsules and is cheaper compared to encapsulation. Additionally, it can be made as plates with different dimensions [9]. Simulations of a building with an SSPCM layer in the envelope walls by Zhu et al. [10] have shown an 11% electricity cost reduction and over 20% peak load reduction. Thus, there is supporting evidence on the utility of shape-stabilized phase change materials applications in buildings to save energy and peak load shifting.

In addition, there have been a few studies that incorporated PCMs into wood-based materials to study the effects on energy savings and indoor air environment. However, there are some limitations that need to be overcome, such as low thermal conductivity, potential negative effect on the mechanical properties and possible PCM leakage over time [10].

Li et al. [11] developed a latent heat system consisting of paraffin capsules supported by high-density polyethylene and wood using graphite to enhance thermal conductivity. They found that this system has the potential to store thermal energy without leakage during melting. Zhang et al. [12] mixed paraffin–HDPE–SBS with concrete to store thermal energy. They reported that for PCM floors or wallboards, the appropriate melting temperature of PCM is 2 °C higher than the temperature of the room without PCM. Lin et al. [13] investigated the potential of using SSPCM as an underfloor heating system and released the heat that had been stored throughout the day by using cheaper electricity at night rather than more expensive daytime electricity. Barreneche et al. studied the impregnation of RT 21 and RT 27 paraffins into wood and coating with polystyrene. They found that it helps with increasing the thermal mass and regulates the temperature fluctuations. Moreover, it can be considered as a potential solution for decreasing energy consumption in buildings [14]. Guo et al. [15] fabricated paraffin, wood flour, high-density polyethylene and expanded graphite composite to be used as a thermal storage material. Their results revealed that the composite has good impact strength; however, the flexural strength decreased. Their study concluded that the prepared composite material can be used as a building material if the mechanical properties are not very important. Can et al. [16] integrated three different types of paraffin PCMs into sapwood using vacuum impregnation. Their results demonstrated that this material reduced the temperature fluctuation in building applications and has a positive effect on fungal resistance. It has the potential to be used as a wall and roof insulator. Liu et al. [17] prepared thermal energy storage material containing polyethylene glycol and PVA/wood PEG gel with enhanced tensile strength and stiffness to be considered as a sustainable thermal management material. Can et al. [18] used vacuum impregnation to prepare solid wood and myristic acid shape-stabilized phase change material. The resulting samples weight were increased in a range of 9–22%, which shows the effectiveness of the impregnation method. Zhang et al. [19] studied the properties of polyethylene glycol and biochar derived from Cu-based preservative-treated wood as a highly conductive thermal energy storage material. The results revealed that the prepared material has significant potential in thermal management field. Deng et al. [20] prepared flame-retardant wood-based PCM composite using expanded graphite as a base coating material. Moreover, the coating can also be used to prevent PCM leakage. They found that the prepared material has the potential to be used in solar energy systems and thermal management.

This paper presents the development of a method for fabricating a latent heat-storing wooden board utilizing an integrated shape-stabilized PCM. The board utilizes technical-grade octadecane as the PCM and HDPE as the host for the PCM to produce shape-stabilized material. The oriented strand board (OSB) wood-based panels were made by combining the SSPCM with wood chips. The latent heat-storing wooden boards have been developed to replace the conventional oriented strand boards, as shown in Figure 1, that are widely used in the construction of residential and some commercial buildings. The heat transmission properties and viability of lowering the thermal load in building envelops were then investigated using the various thermal energy-storing OSB panels. To achieve the specific capabilities further described in Section 2, several additives were utilized.

In North America alone, over 20 billion square feet of OSB panels are used primarily in the construction of buildings. Thus, this material is an important candidate for product engineering to store thermal energy and investigating its energy savings potential. The end-use energy consumption of residential and commercial buildings in the United States is about 21 quadrillion BTU. Of this, only residential energy consumption for heating/cooling is approximately 6 quadrillion BTU per year. If we can save a modest half percent or 0.03 × 10¹⁵ BTU on residential energy use, it would result in energy cost savings of USD 90 million a year (assuming USD 3 per 1 million BTU from natural gas). This level of natural gas savings could result in the reduction of carbon dioxide emissions by nearly 55 million pounds or 27,000 metric tons per year.

We have used wood as our primary research material, as it is a sustainable material, and we have investigated how to engineer wood to provide greater energy efficiency in a building. Furthermore, HDPE is a fossil-based polymer, which is widely recycled and frequently reused in construction-related applications [21]. Recycling helps to lessen its overall environmental burden and conserve natural resources. However, if HDPE and other additives end up as waste or in the environment, they could have a negative impact. Regarding the cost of the OSB-SSPCM, it may be noted that all the synthetic constituents are commercially available at relatively low costs. In addition, the hot-processing time of the new material is the same as for conventional OSBs, so there is no additional cost or a need for changing the machinery used for conventional OSBs. Thus, it is anticipated that the increase in cost would be marginal, which would be more than offset by energy cost savings.

2. Experimental Methods

2.1. Materials Used in OSB-PCM Panels

Technical-grade octadecane, C-18 phase change material, with an 83 °F (28.3 °C) melting point and a heat of fusion of 104.76 BTU/lb (243.67 kJ/kg) was provided by (Roper Thermals, Clinton, CT, USA). Medium-molecular-weight, high-density polyethylene (Alathon H5112) in pellet form with a softening temperature of about 262 °F (127.7 °C) and with a melt index of 12 was obtained from (LyondellBasell, Houston, TX, USA) to use as the supporting material for the PCM. To strengthen the binding between HDPE and the OSB, two kinds of coupling agents were applied, maleic anhydride grafted polyethylene of 65,000 molecular weight (Epolene C-26) and linear low-density, medium-viscosity polyethylene of 7200 molecular weight (Epolene C-70), which were obtained from (Westlake Chemical Corporation, Houston, TX, USA). Wood strands (hardwood with some pine wood) were supplied by (Weyerhaeuser Co, Seattle, WA, USA). For enhancing thermal conductivity, purified expanded graphite (EG) with a purity of 99.95% were obtained from (Superior Graphite, Chicago, IL, USA) and graphite nanoplatelets (xGnP-05) were supplied by (XG Sciences, Lansing, MI, USA). A flame-retardant, nano magnesium hydroxide (purity 99.8%, particle size 50 nm) was purchased from (SkySpring Nanomaterials, Inc. Houston, TX, USA) Phenol Formaldehyde Primax™ W9-137 (54.5% solids) was obtained from (Hexion, Columbus, OH, USA) and used as a binder in the OSB manufacturing process.

2.2. Preparation of Shape-Stabilized Phase Change Material (SSPCM)

Prior studies have determined that the most advantageous PCM melting point for construction usage is approximately 1–3 °C above ambient temperature. Additionally, Zhou et al. [1] reported that the preferred melting point for PCM in buildings is about 18–30 °C. The results of Liu and Zhang [22] demonstrated that the temperature of the room affects the PCM melting temperature used for thermal comfort. In this study, however, it is envisaged that the PCM-OSB wall panels will be in the external envelope of the building, namely the side walls and the roof; thus, technical-grade octadecane was chosen as the phase change material, with a fusion temperature of 83 °F (27 °C). Paraffin is used widely as a PCM in building applications because it offers a number of favorable properties, including high heat of fusion, not suffering from phase segregation and low cost; however, it is flammable [23]. HDPE was utilized to host the PCM.

The process followed to make shape-stabilized phase change material (SSPCM) is as follows:

Technical-grade octadecane (or wax) was chosen as the phase change material, and the supporting or host material was HDPE. The processing steps are listed below.

• SSPCM mixture materials were weighted according to their percentage in the mixture.
• High-density polyethylene was melted in a pan.
• After all the HDPE melted, wax was added gradually and mixed manually in the melted HDPE using a motorized mixer. Manual mixing was continued until the mixture appeared homogeneous.
• The mixture was cast into a thin sheet using two flat plates lined with parchment paper by pouring the mixture and letting it cool till it solidified. The solidified sheet was crushed into small flakes, which can be seen in Figure 2.
• The addition of Epolene (C-26, or C70) was carried out by melting Epolene with HDPE before the addition of the paraffin. For the SSPCM containing expanded graphite, exfoliated graphite nanoplatelets (xGnP) or nano magnesium hydroxide, the addition of these materials was performed gradually to the molten HDPE after adding about half the amount of wax because the viscosity of the mixture at this stage is not too high and the mixing process is easier. These materials and wax were added alternatively until all the weighted materials had been added.

Oriented strand board fabrication was conducted using the following process:

• First, 6 wt.% phenolic resin was applied on a solid basis compared to dry wood weight. The strands were sprayed with the binder and then mixed in a container by shaking for a few minutes to ensure even distribution of the binder.
• A wooden box-type mold with dimensions of 5 in × 5 in × 4 in was used to form mats. Randomly oriented mats with 80% by weight of wood strands and 20 wt.% SSPCM were laid up as shown in Figure 2.
• The layered mat of about 4-inch height was pressed for 4 min at a temperature of 415 °F and a load of 15,000 pounds using a hydraulic benchtop press, as shown in Figure 2, to produce the final specimen.

Table 1. Identification and proportions of OSB-SSPCM in samples (by weight%).

Sample Designation	Wood %	SSPCM %	SSPCM Composition	% Composition of SSPCM
Group C (control)	100	0	-	-
Group 1	80	20	PCM-HPDE	80-20 75-25 70-30
Group 2	80	20	PCM-HPDE-Epolene C70	72-24-4 73.5-24.5-2
			PCM-HPDE-Epolene C26	72-24-4 73.5-24.5-2
Group 3	80	20	PCM-HPDE-xGnP	74-25-1 74.5-25-0.5 80-19.5-0.5
Group 4	80	20	PCM-HDPE-EG	74.75-25-0.25 74.5-25-0.5
Group 5	80	20	PCM-HDPE-Epolene C70-EG	74-21-4-1 74-20-4-2 74-19.5-4-2.5 80-15-4-1 80-18-1-1
Group 6	80	20	PCM-HDPE-nano magnesium hydroxide	60-20-20

lists the several OSB-SSPCM compositions used in this study.

Oriented strand boards (OSBs) were fabricated in the lab to dimensions of 5 × 5 × 0.4 in (127 × 127 × 10.6 mm) using liquid phenolic resin as a binder for the wood strands. The phenolic resin was applied on a solid’s basis compared to dry wood weight. The strands were laid up in a container, and the binder was applied using a paint brush. To ensure even distribution of the binder, the strands were shaken manually in a container for over 2 min. A wooden box-type mold with dimensions of 5 × 5 × 4 in (127 × 127 × 106 mm) was used to form mats. Randomly oriented mats with 80% by weight of wood strands and 20 wt.% SSPCM were laid up as shown in Figure 2. For the samples containing SSPCM, the SSPCM strands were randomly distributed in the interior layers. Then, the mat was pressed for 4 min at a temperature of 415 °F (213 °C) at a load of 15,000 lb (66,723 N) using a Carver hydraulic benchtop press [24]. The high-temperature compression loading in the hot press, as shown in Figure 2, decreased the thickness of the pre-formed mat in the mold to 0.4 in (10.6 mm) in the finished sample. More details of the fabrication are provided in reference [25]. Epolene coupling agents enhance the compatibility and interfacial adhesion between the wood fibers and the polymer resin matrix, which leads to increased flexural strength and stiffness. For example, in Epolene C-26 the maleic anhydride grafted polyethylene coupling agent allows for effective bonding with the hydroxyl group in wood fibers [26]. Nano magnesium hydroxide enhances the fire resistance because of the formation of MgO during the combustion of nano magnesium hydroxide, which builds a char layer that isolates the underlying material and restricts the escape of volatile materials [27]. When compared to regular magnesium hydroxide, nano magnesium hydroxide exhibits better flame retardance by reducing the rate of degradation of inner matrix and increasing the char residue at high temperatures during a fire [28].

2.3. Experimental Testing Methods

Mettler Toledo’s differential scanning calorimeter (DSC) was employed for assessing the thermal storage capabilities of SSPCM. The ASTM E793-06 standard was followed when conducting the tests [29]. A specimen capsule was filled with 35 mg of the SSPCM. The specimen capsule was covered with a lid and sealed at room temperature. Dry nitrogen was used to purge the chamber. First, the necessary calibrations were carried out. Then, after loading the specimen into the DSC chamber, the specimen was heated from 0 to 60 °C (32–140 °F) and then cooled from 60 to 0 °C (140–32 °F) at a rate of 1 °C/min for the heating and cooling process.

For determining the heat flow through the thickness of the boards, we used a test cell of 60 in × 60 in × 70 in (1.5 m × 1.5 m ×1.78 m) size, which holds two OSB samples of sizes up to 12 in × 12 in (0.3 m × 0.3 m). The test cell was used to test the heat transfer characteristics of various SSPCM-integrated OSB specimens. The exterior walls were made of 0.75 in (19 mm) hard wood and insulated with 2 in (51 mm) extruded polystyrene foam. The interior walls were made of 0.5 in (12.7 mm) thick OSBs. A high-wattage light bulb was used as the heat source, which was connected to a thermostat. To ensure uniform temperature distribution in the test cell, a velocity fan was used. The temperature of the room where the test cell was located was set between 67 and 69 °F (19.4–20.5 °C). In this configuration, the inside of the simulator simulates the exterior of an actual building on a hot summer day, while the exterior of the simulator reflects the inside conditioned space in a building.

The Hukesflux HFP01 heat flux sensor, Hukseflux Thermal Sensors, Center Moriches, NY, USA was used to measure through the thickness heat flux by positioning it in the middle of the 5 in × 5 in (127 mm × 127 mm) OSB sample with a thin film of arctic silver high-density thermal compound on the attached side to prevent any air film over the contact area. During testing, data logging was performed with a Campbell Scientific CR1000 data logger, Logan, UT, USA.

To determine the heat flux, the following expression was used:

$$Heat Flux,q\left({\displaystyle \frac{Watts}{sq.m}}\right)={\displaystyle \frac{differential voltage}{Sensitivity of the sensor}}\ast 1000$$

where sensitivities of the heat flux sensors were provided by the company in a calibration sheet. Temperature measurements were made using K-type thermocouples, while a heat source connected to a thermostat was used to control the temperature inside the test cell. Two HOBO U12-013 data loggers, Onset Computer Corporation, Bourne, MA, USA were used to record the temperatures inside and outside the test cell. They had an in-built temperature sensor, which senses and logs at the same time. The measurement interval of HOBO and CR1000 were set to 1 min. In Figure 3, the test arrangement is displayed. The tests were conducted over a 24 h duration, where the heat source was “on” for 12 h and turned “off” during the remaining 12 h of the test. Two OSB specimens were tested at the same time in the hot box apparatus: the first one was a commercial OSB without PCM as a control sample, and the second was the fabricated OSB with SSPCM.

The flammability tests were conducted on various 4 × 4 × 0.4 in (101.6 × 101.6 × 10.16 mm) SSPCM-integrated OSB panels in a cone calorimeter. The tests were conducted by an external entity, Govmark Labs, Farmingdale, NY, USA (presently known as SGS Co.). The total samples tested were eight, with two samples of each type, as illustrated in

Table 2. Sample designation of OSB-SSPCM used in the flammability testing.

Sample Designation	SSPCM Composition	% Composition of SSPCM
A0	None (commercial OSB)	None
A1	PCM-HPDE-Epolene C26	72-24-4
A2	PCM-HDPE-Nano magnesium hydroxide-Epolene C26	60-24-10-6
A3	PCM-HDPE-nano magnesium Hydroxide	60-20-20

. The test method used was according to ASTM E1354 [30] using a 50 kw/m² heat source.

The test specimen is mounted into the specimen holder, which sits on a load cell. The opening of a cone-shaped radiant heat source faces the test specimen. Specimens are burned in ambient air conditions. Sparks were used to ignite the off-gases. During that time, the exhaust gas flow rate and oxygen concentrations were recorded. Using the oxygen concentrations present during combustion, pressure flow rate and thermocouple temperature, the mass consumed at any given time can be calculated. Heat release values are then determined based on the premise that 13.1 MJ of heat is released per kg of oxygen consumed during combustion [30]. Simultaneously, the optical photometric, or smoke obscuration, measuring system is gauging smoke release while the weigh cell is tracking specimen mass loss. The smoke value is reported as the specific extinction area (SEA).

3. Results and Discussion

All the OSB-SSPCM boards were manufactured using 80 wt.% wood strands, while the SSPCM concentration was maintained at a total of 20 wt.% that included all the different material compositions, as shown in Table 1. From here onwards, the SSPCM compositions are identified as follows: the first number is wt.% PCM, the second number is wt.% HDPE, and third and beyond are the additives, with each sample totaling 100% in the SSPCM part of the specimen. During each experiment, a control OSB without SSPCM and the experimental OSB with SSPCM were installed in the test cell.

3.1. Differential Scanning Calorimeter Tests

To evaluate the heat storage properties of SSPCM, differential scanning calorimeter (DSC) was used. This method is used to determine the enthalpy of fusion and crystallization [29]. The heating and freezing curves from DSC are shown in Figure 4. The major peak of octadecane indicates where the phase change in octadecane takes place and was determined using a differential scanning calorimeter (DSC).

When compared to PCM, which had a phase change temperature of 23.4 °C (74.1 °F), the SSPCM’s temperature ranged from 26.4 °C to 28.4 °C (79.6 °F to 83.1 °F). For the samples containing 0.25% and 1% expanded graphite, the solidification temperatures were 22.7 °C and 21 °C (72.8 and 69.8 °F), respectively. The solidification temperature was 23 °C (73.4 °F) for the samples containing PCM-HDPE, and PCM-HDPE-nano magnesium hydroxide, which means that there was no supercooling.

There were no chemical interactions between octadecane and HDPE throughout the making of the SSPCM, as evidenced by the paraffin phase change peak still being present in the sample [14,26,31].

Theoretically, the latent heat of SSPCM can be calculated using the following equation [32]:

$$∆H=m{∆H}_{pcm}$$

(1)

where

ΔH = SSPCM’s latent heat;

m = mass fraction of the PCM;

ΔH_pcm = PCM’s latent heat as measured by DSC.

Using Equation (1) and the latent heat of octadecane (89.6 BTU/lb) with its mass fractions, the result is the hypothetical latent heat of the SSPCM comprising 80, 75, 74.75 and 60 wt.% octadecane, which is about 71.7, 67.2, 66.97 and 53.76 BTU/lb, respectively. In some samples, the latent heat of melting measured by DSC is less than the theoretical values, as shown in

Table 3. The characteristic temperatures and latent heat of PCM and SSPCM by DSC measurement.

Sample Composition	Tm* (°F)	ΔHm* (BTU/lb)	Ts* (°F)	ΔHs* (BTU/lb)
Octadecane (PCM)	80.1	89.6	74.1	90.4
75%PCM-25% HDPE	79.6	67.5	73.4	66.6
74.75 PCM-25%HDPE-0.25% expanded graphite	83.1	54.9	72.8	56.7
80% PCM-15% HDPE-1% expanded graphite-4% Epolene C70	81.32	66.9	69.8	66.7
60% PCM-20% HDPE-20% nano magnesium hydroxide	81.32	53.3	73.4	51.3

* Subscript: m: melting; S: solidifying.

. The reduction in latent heat is brought on by the three-dimensional net structure made by HDPE and the other additives, which act as an obstacle to molecular movements of octadecane throughout the phase change [26,32]. This indicates that the features of thermal storage are negatively impacted by the addition of HDPE and other additives.

3.2. Heat Flow Properties

The heat flux through the control board and experimental boards were tested in the hot box. Regarding the experimental setup shown in Figure 3, the tests were conducted using a lab-scale hot box rather than a full-scale wall assembly. A hot box is widely used for material-level thermal performance evaluation in a controlled environment, and in this study, it was utilized to simultaneously study the difference in thermal performance of the control specimen (without SSPCM) with the experimental specimen (with SSPCM).

The compositions of SSPCMs of group 1 OSB-SSPCM boards were 80-20%, 75-25% and 70-30% PCM-HDPE. Figure 5 shows the results of three tests, which are the hourly average of heat flux through the thickness of OSB–SSPCM experimental boards and the control boards (OSB without SSPCM) of group 1. During the 12 h heating period, the heat flux through the SSPCM board was lower than the heat flux through the control board for the three group 1 specimens. However, after the shutdown of the heat source (light bulb) heat flux in the SSPCM board gradually reached the heat flux of the control board, and sometimes, it exceeds the heat flux of the control board by a small amount. This is because the SSPCM began to solidify and released the absorbed heat during the heating cycle. The average heat fluxes for the first 12 h (heating cycle) through the PCM/HDPE ratio of 80/20, 75/25 and 70/30 experimental boards were −225.1, −218.3 and −210.1 Btu/h*ft² (−71.1, −69 and −66.4 W/m²), respectively, which is a corresponding reduction of 11.7%, 14% and 18.9% in the heat flux. Figure 6 displays the temperature both within and outside the test cell over the duration of group 1 tests reaching a maximum of 103.8 to 105 °F. (39.9–40.5 °C) in the cell while the room temperature was between 67 and 69 °F (19.4–20.6 °C) for the three tests. The temperature in the hot box and the space outside the box are not reported for the rest of the tests, as they were in this range.

Group 2 panels contain PCM-HDPE and Epolene C26 or Epolene C70. There was a small favorable change in the reduction percentages as compared to 75–25 OSB-SSPCM specimens. The reduction in heat flux in the 4 wt.% Epolene C70 and Epolene C26 specimens was 14.8% and 15%, respectively. On the other hand, the average heat fluxes of 2% Epolene C70 and Epolene C26 were −241.8 and −211.7 Btu/h*ft² (−76.4 and −66.9 W/m²), respectively, which is a corresponding reduction of 10.8% and 14.9%, as shown in Figure 7. The highest temperature reached in the cell was 106.7–107 °F (41.5–41.7 °C), and 67–69 °F (19.4–20.6 °C) was the room temperature during group 2 tests.

OSB panels in group 3 contain SSPCM with exfoliated graphite nanoplatelets (xGnP) as an additive. The heat flux through the OSB containing SSPCM with 74% PCM-25% HDPE-1% xGnP was higher than the control after five hours, as shown in Figure 8. The heat flux reduction percentage was only 1%, which is very low. By using a lower graphite nanoplatelet percentage, the heat flux reduction through thickness became 14.3% for the 74.5% PCM-25% HDPE-0.5% xGnP sample. For 80-20-0.5% PCM-HDPE-xGnP, the heat flux reduction was 11.9%. During group 3 tests, the test cell’s maximum temperature ranged from 105.4 to 106.6 °F (40.8 to 41.4 °C), whereas the ambient temperature ranged from 67 to 69 °F (19.4 to 20.6 °C). Thus, the use of graphene for greater fire resistance is not viable, as the heat flux increases significantly.

The OSB-SSPCM boards of group 4 contain SSPCM with different percentages of PCM, HDPE and expanded graphite (EG). The boards of this group contain EG as an additive to potentially reduce PCM leakage, even with a relatively higher concentration of PCM, while providing increased fire resistance. As can be noticed in Figure 9, the heat flux through the thickness of SSPCM boards was less than the heat flux through the control boards. For the OSB-SSPCM board containing 0.25% expanded graphite, the reduction of heat flux was 13.6%. The average heat fluxes were −223.1 Btu/h*ft² (−70.5 W/m²) for the SSPCM board and −258.2 Btu/h*ft² (−81.6 W/m²) for its control during the first 12 h. For the OSB containing SSPCM with 0.5% EG, the reduction in heat flux was 10.9%. The average heat fluxes during the first 12 h were −226.3 and −253.96 Btu/h*ft² (−71.5 and −80.5 W/m²) for the SSPCM board and the control sample, respectively. The maximum temperature during the tests was 105.6–107 °F (40.9–41.7 °C), and the room temperature was between 67 and 69 °F (19.4–20.6 °C). The use of expanded graphite has a deleterious effect by increasing the heat flux through the boards.

The OSB-SSPCM of group 5 contained PCM, HDPE, EG and Epolene C70 with different percentages. Epolene is conventionally used to improve the mechanical properties of the OSB. It may be noted that the primary goal of this study was to determine the effect of integrating SSPCM in the OSB wood board on the thermal performance in terms of heat flux through the thickness direction. Thus, the mechanical properties were not investigated. There could be potential for some reduction in the mechanical properties with the use of PCM; however, that would form part of a separate study. For the sample containing 74%PCM-4% Epolene C70, adding more expanded graphite increased the heat flux through OSB-SSPCM boards. The reduction percentages were 9.7%, 7% and 5% for 1, 2 and 2.5% EG. Higher percentages of paraffin were used in two of the group 5 samples. The sample that contains 80 PCM-15 HDPE-4 Epolene C70-1 EG provides the highest heat flux reduction of 14% in group 5. On the other hand, 5.1% was the reduction percentage of the 80-18-1-1 sample, as shown in Figure 10.

In group 6, nano magnesium hydroxide was used as a potential flame retardant. In Figure 11, it is observed that during the heating cycle and the starting hours of the cooling cycle the SSPCM board’s heat flux was less than the control sample. The temperature inside the test cell was about 105 °F (40.6 °C). The reduction in heat flux was higher in the SSPCM board by 18.4% compared to the control board. The average heat flux through the control board was −242.2 Btu/h*ft² (−76.5 W/m²) and −197.7 Btu/h*ft² (−62.5 W/m²) in the OSB-SSPCM board. Thus, the addition of 10% less PCM compared to the 70 (PCM)-30 (HDPE) board that presents the greatest reduction in heat flux did not compromise the potential for reduction in heat flux while providing required fire protection.

Figure 12 summarizes the reduction of heat flux percentages during the first 12 h of the test (the heating cycle) after integrating SSPCM in the OSB as compared to the control OSB without SSPCM.

The maximum reduction of heat flux was achieved in the 70-30 (%PCM-%HDPE) OSB, potentially as the PCM leakage was less than the 80-20 blend. The 70-30 PCM-HDPE sample had negligible leakage of PCM. The maximum paraffin percentage that can be trapped in HDPE depends on the SSPCM preparation method. Previous studies stated the range to be between 60 to 80 percent paraffin. According to certain studies, the HDPE structure may hold up to 70% paraffin, preventing molten paraffin from leaking out of the solid HDPE matrix [33,34,35]. The percentage of total paraffin leakage increases as HDPE decreases.

To strengthen HDPE and wood bonding, coupling agents such as Epolene C26 and C70 were used. The use of Epolene produces a chemical interaction regardless of the types of compatibilizers used to develop high interfacial adhesion that improves the tensile and impact strength of the wood and HDPE composite [26]. The addition of Epolene C26 or C70 enhanced the reduction in heat flux as compared to 75-25 OSB-SSPCM with no Epolene by a relatively small amount. They enhanced the reduction percentage to about 15% as compared to the reduction of 14% for 75-25 SSPCM only.

The purpose of adding graphite nanoplatelets (xGnP) was to further prevent liquid paraffin leakage in SSPCM, enhance thermal conductivity, and act as a flame retardant. Since the heat-emitted rate of PCM considerably decreases when carbon base material is added, graphite nanoplatelets (xGnP) can be employed as a flame retardant, as xGnP has better dispersion in the paraffin than expanded graphite because of the nanometer size of xGnP [36]. There was little improvement in the heat flux reduction percentages for the 0.5% xGnP samples for both the 74.5-25 PCM-HDPE and the 80-19.5 PCM-HDPE. Adding more of xGnP or EG gives lower heat flux reduction percentages. PCM has a lower heat transfer rate than PCM-(EG or xGnP) composite, and the heat transfer rate increases as the percentage of EG or xGnP increases. This is because of the increase in thermal conductivity [36,37,38]. The addition of EG decreases the melting time of the PCM [37,38]. High-conductivity material additives also reduce the melting time of the SSPCM during the hot press, which causes paraffin leakage.

There was little improvement in the heat flux reduction percentages for the 0.5% xGnP samples for both the 74.5-25 PCM-HDPE and the 80-19.5 PCM-HDPE. Nano magnesium hydroxide was therefore employed as an alternative flame retardant. An increase in fire resistance is caused by the creation of MgO during the burning of nano magnesium hydroxide, which raises the char residue at high temperatures, builds up char layers on the surface, shields the underlying material and inhibits the release of the volatile chemicals produced during decomposition [23,38]. Also, it can provide better compatibility in the wood–plastic composite (WPC) [28]. Additionally, this compatibility might enhance WPC’s tensile and elongation properties [28]. An OSB containing nano magnesium hydroxide contains about 10% less PCM than the 70 PCM-30 HDPE board but has nearly the same heat flux reduction percentage. It is also expected to provide greater fire resistance, which is further studied in the next section.

In summary, the OSB-SSPCM (with 70% PCM-30% HDPE) board was determined to provide the greatest reduction in the heat flux through the thickness of the board, followed closely by the 60% PCM-20% HDPE -20% nano magnesium hydroxide specimen. The heat flux is reduced by 18.9% as compared to the control board. It should be noted that we do not have comparative studies using SSPCM integrated into a wood board. Most of the studies utilize PCM in a contained layer or other methods of incorporation, which could be subject to leakage. With this limitation, the results from some studies show that adding PCM as a layer provided higher reduction in heat flux percentage than integrating PCM as a composite. Adding PCM as a thin layer in building walls can provide a reduction in heat flux of about 29.7–51.3%, as reported by Lee et al. [39]. According to Li et al. [40] a different heat flux reduction was obtained by varying the type of PCM embedded in a multilayer wall. It ranges from 15.6 to 47.6%, and 2 to 7.8% for different types of PCM. Jin et al. [41] reported a heat flux reduction of 11% when using PCM as a thermal shield in a gypsum board building wall.

3.3. Flammability Properties

The flammability properties of four different types of OSB-SSPCMs were tested using a cone calorimeter, which is an instrument for fire testing of materials that could experience a fire, such as in building materials. Cone calorimeter readings have been shown in prior research to be highly correlated with large-scale fire testing, and their data may be valuable in forecasting how flammable materials will perform in actual fires [22,25]. The most important parameter is the peak heat release rate (PHRR) when assessing materials flammability. The oxygen consumption rate is used to make the determination; a lower PHRR means the material is more fire resistant [42,43,44,45]. The test results are summarized in

Table 4. Cone calorimeter data of OSB-SSPCM boards.

Property	SSPCM Composition
	Sample A0	Sample A1	Sample A2	Sample A3
	OSB with no SSPCM	72% PCM-24% HPDE-4% Epolene C26	60% PCM-24% HDPE-10% Nano Magnesium Hydroxide-6% Epolene C26	60% PCM-20% HDPE-20% Nano Magnesium Hydroxide
Time to ignition (s)	35	23	23	24
Test end (s)	1055	1088	1132	1196
Peak heat release rate (KW/m2)	220	251	231.7	223.4
Average heat release rate at KW/m2
Sec 60	85.9	117.8	106.4	103.8
Sec 180	96.2	149	158.7	137.9
Sec 300	97.4	150	169.8	147.1
Average mass loss Rate (g/m2 s)	11.4	11.8	12	10.8
Total heat release (KW/m2)	100.3	134.2	158	162.7
Visible smoke development of material (SEA) at m2/kg
Sec 180	40	90	124	98
Test end	78	114	150	149
Effective heat of combustion (MJ/kg)	12.8	16.4	17	18.1

. Each result is the average of two samples.

After the SSPCM is added, the peak rate of heat release rises from 220 to 251 KW/m². As demonstrated in Figure 13, adding nano magnesium hydroxide to SSPCM as a flame retardant reduces the PHRR of the OSB-SSPCM from 251 to 231.7 and 223.4 KW/m² for the samples containing 10% and 20%, respectively, of nano magnesium hydroxide. The sample designations, A0, A1, etc., are listed in Table 2. The peak heat release rate of the OSB increases by 14% when SSPCM is added without nano magnesium hydroxide but only by 1.5% when 20% nano magnesium hydroxide is added to SSPCM. Using SSPCM without nano magnesium hydroxide raises the OSB’s peak heat release rate by 14%, whereas doing so with 20% nano magnesium hydroxide reduces the difference to just 1.5%.

Figure 14 displays the heat release rate curves for the four specimen types tested, with two identical samples tested for each specimen type. The OSB-SSPCM’s heat release rate was reduced as the fraction of nano magnesium hydroxide increased. Since nano magnesium hydroxide’s endothermic decomposition releases water, which reduces heat and cools and dilutes vapors produced by fire, nano magnesium hydroxide acts as a filler, reducing flammability. Furthermore, the leftover magnesium oxide crust that remains after combustion can shield the lower layers from the outside heat. The sample cools without being locally overheated because nano magnesium hydroxide disperses very well and undergoes uniform decomposition [46].

The burning begins, and a sharp peak is created. The HRR was then reduced as a char layer was created. The slow burning of the samples is seen in the center of the figure. The second peak was immediately generated by an increase in the amount of volatile compounds forming in the small, not-burned area of the sample. The last segment of the curve represents burning without flames. After the volatiles were consumed, it occurred. The second peak intensity decreases with an increasing nano magnesium hydroxide percentage. The total HRRs of A0, A1, A2 and A3 versus time are shown in Figure 15.

Average HRR values at 60, 180 and 300 s were determined, as shown in Figure 16. The average HRR of the A3 specimen was lower than those of A1 and A2, though it is still higher than that of A0. The difference in the HRR between A0 and A3 was lower at the beginning of burning than at longer periods after ignition. The importance of the lower HRRs at the onset of burning is substantially greater than that of the latter [46]. As a result, A3 is more flame resistant than A2 in terms of mean HRRs.

The time to ignition is the shortest amount of time available before the specimen ignites and continues to burn in a flaming state. To cause ignition, the cone calorimeter’s spark igniter is employed [47]. The OSB-SSPCM boards (A1, A2 and A3) ignited in about 23–24 s, while the OSB without SSPCM (A0) ignited at 35 s, as shown in Figure 17. To get a uniform and consistent thickness, the OSB-SSPCM boards were machined before the test. This machining process causes the layers that contain SSPCM to appear in the external surface and be subjected to direct spark. Without machining, the SSPCM would be in the internal layers only, and the time to ignition is anticipated to increase.

Figure 18 shows the time required to burn the samples completely through the thickness. The OSB sample (without PCM, A0) needs 1055 s to burn, while OSB-SSPCM without nano magnesium hydroxide (A1) needs about 1088 s. The addition of nano magnesium hydroxide in samples A2 and A3 increases the burn time to 1132 s and 1196 s, respectively. However, this variation in the burning time may be due to the difference in composition of the SSPCM and the A3 sample having the highest concentration of nano magnesium hydroxide.

The mass loss rate is another essential flammability parameter for materials [48]. The lowest average mass loss rate was in the OSB-SSPCM samples (A3) containing 20% nano magnesium hydroxide, as shown in Figure 19, which means a slow decomposition of the sample. The mass loss rates versus time are shown in Figure 20, and the masses of the samples versus time are shown in Figure 21.

The state of flame burning accounts for almost all the effective heat of combustion measured in a cone calorimeter and, consequently, to the burning of vapors from a substance. It is determined by dividing the values of total heat evolved by mass loss within a discrete time [49], demonstrated in Figure 22.

Effective heat of combustion is a measure of the amount of oxygen required for combustion. If EHC is high, it means more oxygen was used in the combustion process and ignites more easily and releases more heat per unit mass of wood (MJ/kg). Higher EHC generally implies lower fire resistance. The use of fire retardants can also affect the heat of combustion and result in higher EHC but promote char formation and reduce volatile release and lower the heat release rate. However, EHC alone should not be considered as a measure of fire resistance, as it is dependent on the system-level fire behavior that includes other parameters, such as time to ignition, peak heat release rate, mass loss rate, smoke release rate, etc. It should be noted that peak heat release rate is considered as the most important parameter for measuring fire resistance. In this study, specimen A3 produces the lowest heat release rate, which is in favor of higher fire resistance compared to other specimen types. In addition, the mass loss rate is lower and burn end time is higher for A3 samples containing nano magnesium hydroxide fire retardant.

The specific extinction area (SEA) is a measure of the smoke’s ability to disperse during a fire test in a cone calorimeter. It is calculated as the ratio of smoke production to mass loss, and it could be affected by the addition of fire retardants. The SEA is an indicator of smoke behavior during an actual fire. The smoke formation from OSB burning (A0) has two distinct peaks. The addition of SSPCM makes the second peak higher in A1 specimens and also A2 and A3 specimens containing nano magnesium hydroxide, which is favorable, Figure 23. However, the total SEA released at the end of the test was higher in samples that have nano magnesium hydroxide [50].

The culminating results of the flammability tests are shown in Figure 24. The fire-related behavior of the samples demonstrated that adding paraffin increases the flammability, which is expected. When 20% nano magnesium hydroxide was added, the material becomes less flammable, as seen by a decrease in the rate of heat release and mass loss, which indicates a slower rate of decomposition. Nano magnesium hydroxide functions as a filler to lessen flammability because its endothermic decomposition releases water, which lowers heat and cools and dilutes fire-produced fumes. Moreover, the lower layers can be isolated from the external heat by the magnesium oxide crust that is left over after combustion. Because nano magnesium hydroxide disperses and decomposes uniformly, the sample cools without being locally overheated [46]. The reduced heat release rate and mass loss rate improve the fire performance of the OSB-SSPCM building wall, which is related to reduced fire growth potential and enhanced evacuation safety.

4. Conclusions

This study has developed a wood-based oriented strand board (OSB) integrated with shape-stabilized phase change material (SSPCM) that incorporated technical octadecane into a high-density polyethylene host polymer. To the best of our knowledge, there is limited literature or such a comprehensive study on the application of SSPCM in wood for use as building materials. OSB-SSPCM panels were fabricated, which enables the storage of latent heat along with sensible heat to potentially reduce indoor temperature fluctuations, reduce operating costs by moving electrical consumption to off-peak periods of power supply and reduce the cooling–heating load. The OSB-SSPCM (with 70% PCM-30% HDPE) board was determined to provide the greatest reduction in the heat flux through the thickness of the board. The heat flux is reduced by 18.9% as compared to the control board. The use of the nano magnesium hydroxide fire retardant in OSB-SSPCM panels produces the same reduction in heat flux as compared to OSB-(70%PCM-30%HDPE), despite the lower PCM concentration and potentially greater fire protection. As a consequence, the sample containing 60% PCM-20% HDPE-20% nano magnesium hydroxide performed the best, since it achieved both a reduction in heat flux and flame retardancy characteristics. This study provides the framework and feasibility for the application of OSB-SSPCM in buildings to advance energy efficiency and sustainability goals.

5. Future Research Directions

Future research on the OSB-SSPCM boards may involve expanding the experimental study to full-scale OSB-SSPCM specimens of 4 ft × 4 ft or larger; conducting tests using a full-scale wall assembly in place of a hot box apparatus; studying the effect of various additive and SSPCM concentrations on the mechanical properties of the wood board; studying the effects of environmental factors, such as thermal cycling and humidity, on the durability of the wood boards; and developing a multiscale model to simulate the effects of PCMs on the overall thermal performance of the OSB-SSPCMs.