Abstract
Due to the small thermal conductivity of wooden furniture, there is a better cooling effect in the initial contact with the human body during the summer. However, after long-term contact, heat cannot be exported, resulting in heat accumulation, which affects the thermal comfort. Phase change microcapsules (PCM) can be added to water-based coatings for wood furniture to achieve a lower contact temperature by using the heat absorption properties of its phase change process. Thus, it improves the thermal comfort of the human body for long-term use. In this study, the PCM were added to a water-based paint for wooden furniture. By testing the microscopic properties and thermal properties, it was found that the PCM powder could undergo phase change and absorb heat after dispersing in water-based paint. Secondly, fiberboard coated with different solid content of microencapsulated coatings was tested by homemade equipment at 27 °C room temperature. The results showed that the addition of PCM can significantly change the contact temperature. And as the amount of microcapsules added increased, the cooling effect was more obvious. At the same time, the addition of microcapsules can cause matting of the paint film and have an effect on the hardness of the paint film. Therefore, the cooling effect can be achieved by stacking the paint film or controlling the amount of microcapsules.
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Preparation of Wood Furniture Cooling Coatings Based on Phase Change Microcapsules and its Performance Study
Jue Bai,a Yu Li,b Shuangshuang Jiang,c and Huiyuan Guan d,*
Due to the small thermal conductivity of wooden furniture, there is a better cooling effect in the initial contact with the human body during the summer. However, after long-term contact, heat cannot be exported, resulting in heat accumulation, which affects the thermal comfort. Phase change microcapsules (PCM) can be added to water-based coatings for wood furniture to achieve a lower contact temperature by using the heat absorption properties of its phase change process. Thus, it improves the thermal comfort of the human body for long-term use. In this study, the PCM were added to a water-based paint for wooden furniture. By testing the microscopic properties and thermal properties, it was found that the PCM powder could undergo phase change and absorb heat after dispersing in water-based paint. Secondly, fiberboard coated with different solid content of microencapsulated coatings was tested by homemade equipment at 27 °C room temperature. The results showed that the addition of PCM can significantly change the contact temperature. And as the amount of microcapsules added increased, the cooling effect was more obvious. At the same time, the addition of microcapsules can cause matting of the paint film and have an effect on the hardness of the paint film. Therefore, the cooling effect can be achieved by stacking the paint film or controlling the amount of microcapsules.
DOI: 10.15376/biores.17.1.1319-1337
Keywords: Phase change microcapsules; Water-based coatings; Temperature-reducing properties; Paint film properties; Wooden furniture
Contact information: a: College of Furnishings and Industrial Design, Nanjing Forestry University, Nanjing, Jiangsu Province, China; b: College of Chemical Engineering, Nanjing Forestry University, Nanjing, Jiangsu Province, China; c: College of Chemical Engineering, Nanjing Forestry University, Nanjing, Jiangsu Province, China; d: College of Furnishings and Industrial Design, Nanjing Forestry University, Nanjing, Jiangsu Province, China; *Corresponding author: nlydh2018@163.com
INTRODUCTION
With the rapid development of modern economy, people’s requirements for the comfort of the indoor environment are getting higher and higher. An indoor environment that is too hot or too cold is not conducive to staff productivity and productive performance. In a moderate thermal comfort environment, office workers can easily perform their work tasks (Maula et al. 2016). From a human health perspective, indoor heat affects all aspects of human health, with the strongest evidence being the effect of high temperatures on respiratory health, diabetes, and the core symptoms of schizophrenia and dementia (Tham et al. 2020). Therefore, it is important to reduce indoor temperature and enhance human thermal comfort through furniture design and other means. Wooden furniture has also become one of the most common furniture choices due to its better temperature perception properties. However, in higher temperature indoor environments, wood furniture often shows better thermal comfort performance in the early stages of use. In long-term exposure, the temperature absorbed by the wood material cannot be dissipated in time, leading to heat buildup, producing a stuffy situation. And compared to other materials with faster thermal conductivity, such as metal, wood material is unable to overcome the stifling feeling in long-term contact in summer. However, these highly conductive materials are too cold at the beginning of contact and do not solve the problem of thermal comfort very well. Therefore, it is necessary to improve the thermal conductivity of the wood by modifying its surface so that it can exhibit better thermal comfort properties.
Phase change microcapsules (PCM) are a collection of materials made based on phase change principles. Microcapsules are added to coatings as fillers to achieve the effect of heat absorption and temperature reduction. When the external ambient temperature reaches the phase change temperature, the phase change material particles wrapped in the microcapsule shell material change their phase state, thus absorbing the external heat (Khadiran et al. 2016). Paraffin hydrocarbons are the most studied core materials for microcapsules, with phase change temperatures close to room temperature and large enthalpy of phase change, as well as being non-toxic, non-hazardous, and inexpensive (Huang and Wang 2012). The shell material of PCM can be divided into inorganic polymer material and organic polymer material (Peng et al. 2020). Polymethyl methacrylate (PMMA), as one of the more common shell materials for PCM, has the advantages of no formaldehyde residue, non-toxicity, and non-polluting nature. In addition, the PMMA reaction monomer is safe and widely available, and the polymerization reaction is easy to control (Sari et al. 2014). Therefore, PMMA can be used as the shell material to wrap the paraffin wax core material if it is necessary to make the system clean and environmentally friendly. Thus, PMMA can be used in making economical, phase change microcapsules. In the continuous exploration of phase change microcapsules by many scholars, the phase change microcapsules with PMMA as the shell material and paraffin as the core material have been further developed in terms of the synthesis method, the ratio of core material to shell material, and the characterization method (Alay et al. 2011; Alkan et al. 2011; Han et al. 2015; Xu and Yang 2019).
Microencapsulated phase change materials are used in a wide range of fields. In the field of architectural coatings, they are often used in cement mortar, concrete, walls, floors, and other building materials to achieve heat storage, cooling, energy saving, and other functions (Feldman et al. 1991; Pasupathy et al. 2008; Hunger et al. 2009; Jeong et al. 2012). Yu and Pei (2020) prepared coatings with thermoregulation function by using methyl methacrylate and ethyl methacrylate as wall raw materials and paraffin wax encapsulation by suspension polymerization. In furniture coatings, research related to microencapsulation technology has gradually increased. Yan et al. (2019a,b, 2021a,b) have conducted considerable research on the effect of microcapsules added to coatings on the properties of paint films, which makes the study on the characterization of the properties of microcapsules added to wood furniture coatings more and more mature. At present, there are more studies that have been carried out on the mechanism of phase change microcapsules and coating preparation, but there has been a lack of published work on the applications of phase change microcapsule coatings to wooden furniture surfaces, and the use of phase change properties of phase change microcapsules to achieve the cooling effect.
In this study, after mixing PCM powder with water-based varnish for wood furniture in certain proportions, the effect of adding different proportions of PCM on the thermal properties of the coating was analyzed. Next, after the prepared coatings were applied to the MDF surface, the effect of PCM addition on the hardness, adhesion, and gloss of the water-based paint film was tested. Finally, through use of a homemade device to simulate the constant temperature of the human body exothermic process, further exploration of the heat absorption and cooling function of the paint is needed to understand whether it can be sent to the requirements of the regulation of temperature awareness.
EXPERIMENTAL
Materials
The wood furniture material used in this experiment was medium density fiberboard (MDF) with a density of 778 kg/m3 and dimensions of 500 × 500 × 20 mm. The core material of the phase change microcapsules was paraffin, and the shell material was polymethyl methacrylate. The water-based varnish for surface finishing was a modified polyvinyl acetate-ethylene copolymer water-based varnish. The main experimental apparatus is shown in Table 1.
Table 1. Main Test Instruments
The phase change microencapsulated coating cooling performance test apparatus was a homemade apparatus, simulating the human forearm exothermic device, and its external dimensions were 200 × 30 × 20 mm. It consisted of five parts, as shown in Fig. 1: (1) digital display thermostatic water bath (HH-2); (2) adjustable speed peristaltic pump; (3) latex tube; (4) simulated human upper limb quartz square bottle; and (5) simulated human skin of 2 mm elastic silicone rubber plate. The device worked by circulating the temperature of water to simulate the temperature of the upper limb to achieve the effect of constant temperature and exotherm.
Methods
Human temperature sensory test contact experiment
Ten adult males and females, aged between 20 and 30 years old, were selected as test subjects who felt no significant abnormalities in temperature and humidity. The experiments were conducted using the palm of the upper limb in contact with the experimental material (Fig. 2), and the temperature acquisition test points were selected from the tip of the middle finger (T1), the greater interphalangeal muscle of the palm (T2), and the lateral side of the forearm (T3), respectively (Fig. 3) (Wang et al. 2000). This worked by connecting the temperature sensor on the multiplex temperature tester at the test point and by testing the contact temperature at each of the three points. The laboratory temperature was set at 27 °C and the humidity was 50% ± 10. After the test started, the subject’s upper limbs were in contact with the test material in a natural fit, and the temperature change at three points was recorded for 30 min. The average taken of the three test points temperature was the test temperature T.
Fig. 1. Parts of cooling performance testing devices
Fig. 2. Hand position on the furniture surface
Fig. 3. Temperature test points on the hand of the subjects
The subjective method was used to test the thermo-perceptual properties of the wooden tabletop while testing the temperature. The subjects rated the predicted mean vote (PMV) of the material on a scale of “cold (-3 points), cool (-2 points), slightly cool (1 point), comfortable (0 points), slightly hot (1 point), warm (2 points), and hot (3 points)” at one-minute intervals (Lan et al. 2019).
Preparation of phase change microencapsulated coatings and lacquer films
In this test, PCM was mixed with water-based varnish in the proportions of 5%, 10%, 15%, and 20% by mass, and the appropriate amount of dispersant was added and mixed well with high-speed stirring. After stirring well, a brush was used to apply the appropriate amount of varnish and spread it evenly on the surface of MDF board with a finishing amount of 150 g/m2. According to the wood furniture finishing requirements, after finishing three layers (one primer and two topcoats), the wood furniture was placed at room temperature environment to cure and dry for more than 7 days. After the paint was completely dry, its performance was tested.
Phase change microcapsules and paint film characterization for coatings
The particle size and morphological characteristics of the microcapsules were analyzed by scanning electron microscopy (SEM), followed by the analysis of the surface characteristics of the paint film after film formation of the waterborne paint.
The thermal properties can be determined by recording the heat absorbed or released by the microcapsules and varnish film as a function of time using a differential scanning thermal analyzer (DSC) to determine the phase change capability of the phase change microcapsules. The temperature range was -30 to 180 °C with nitrogen as the protective gas and the flow rate of the protective gas was 40 mL/min and the flow rate of the purge gas was 60 mL/min.
Phase change microencapsulated waterborne coating film performance test
To observe the effect of the addition of PCM powder on the paint film adhesion, the method of the paint film adhesion test was carried out according to ISO 2409 (2020). A hand-held multi-blade cutting tool was used to characterize the degree of peeling of the paint film after cross-cutting.
The gloss of the paint film was tested using a three-angle gloss meter with reference to ISO 2813 (2014). The gloss reflectance of the paint film was characterized by the gloss reflectance at different angles. Referring to ISO 15184 (2012), to observe the effect of the PCM powder addition on the hardness of paint films, the hardness was characterized by pencil type.
Fig. 4. Schematic diagram of the simulated temperature test of the device
Fig. 5. Quartz square vial for microcapsules cooling performance test
Phase change microcapsules cooling performance test
The cooling performance of PCM coatings was carried out using homemade equipment. The temperature of the constant temperature water bath was set to 39.5 °C with temperature fluctuations ≤ 0.5 °C. After the temperature of the bath reached 39.5 °C, the peristaltic pump with adjustable speed was turned on, and the volume flow rate was set to 2.52 m3/h. Water flowed in through the bottom inlet of the quartz square vial and out through the top outlet above, and the temperature of the silicone rubber surface increased and finally stabilized (Fig. 4). The physical quartz square vial is shown in Fig. 5. The temperature receiver of the multiplex temperature tester was connected to the silicone rubber surface in contact with the painted MDF and the temperature change was recorded over a period of 25 min. The ambient temperature of the laboratory room was 27 ± 0.5 °C, and the evaluation rate of the multiplex temperature tester test was once every 6 s.
RESULTS AND DISCUSSION
The Best Human Comfort Temperature Range
The test temperature T obtained in the human temperature sensory test experiment and the temperature sensory evaluation PMV can be a function of time. In other words, each minute of contact temperature T corresponds to a value of PMV, and there is a certain relationship between T and PMV. A linear regression was performed in SPSS (General Linear Model, SPSS Inc., Chicago, IL, USA) with PMV as the dependent variable and temperature T as the independent variable. The coefficient of determination after regression, R2 = 0.797, was initially judged to be a good linear fit. The significance of ANOVA, P = 0 < 0.01 < 0.05, indicated that the linear relationship regression model established by the independent variable T and the dependent variable PMV was highly statistically significant, i.e., the linear relationship was significant. Then, the linear regression equation was established as Eq. 1.
PMV = 1.015T – 33.99 (1)
where PMV is the predicted mean vote and T is the human contact temperature (°C). The optimal threshold value of PMV was between -0.5 and 0.5, and it started to feel uncomfortable when it exceeded ±1. By Eq. 1, the authors found the contact temperature T corresponding to ±1. When the human contact temperature reached about 32.5 °C, the PMV reached -1. When the human contact temperature reached about 34.5 °C, PMV reached 1. When the human contact temperature reached about 33.5 °C, the PMV reached 0. Therefore, the cooling paint should try to make the human contact temperature close to 33.5 °C and not more than 34.5 °C.
Analysis of the Structure and Thermal Properties of Phase Change Microcapsules
To further understand the surface morphology of the phase change microcapsule powder, the PCM samples were analyzed by scanning electron microscopy. Figures 6 and 7 show the SEM test results at 250ｘ and 3000ｘ field of view, respectively. As can be seen from the figure, the powder microcapsules had a clustered morphological appearance with smooth surface and mostly irregular spherical structure, and the average particle size was 25 µm. Since at 32.5 °C, the thermal comfort evaluation of the human body reaches the minimum comfort threshold, which is the lower limit of comfortable temperature, microcapsules with phase change at around 32 °C were selected as the additive material. The heat flow-temperature curve was obtained by DSC test, as shown in Fig. 8. The microcapsules reached the phase transition peak at 32.22 °C, indicating that the solid-liquid phase transition occurred during heat absorption. The enthalpy of phase change is the integral of the area enclosed by the curve and the auxiliary line divided by the heating rate, which was calculated to be 209.0 J/g. The PCM powder had a large enthalpy of phase change and could absorb more heat.
Fig. 6. 250ｘ phase change microcapsule electron microscopy
Fig. 7. 3000ｘ phase change microcapsule electron microscopy
Fig. 8. Diagram of differential scanning calorimetry analysis of phase change microcapsules
Analysis of Paint Film Structure and Thermal Properties
Scanning electron microscopy analysis of the paint film surface of PCM samples with different solid contents is shown in Fig. 9. From the figure, it can be seen that the surface of the varnish film without adding PCM powder was relatively flat except for a small amount of impurities. As the amount of powder added increased, the surface of the paint film became rough and some of the powder appeared agglomerated. In general, PCM powder was more uniformly dispersed in the varnish, with relatively little agglomeration and a more stable film structure.
