Manufacturing and Properties of Various Ceramic-Embedded Composite Fabrics for Protective Clothing in Gas and Oil Industries Part II: Thermal Wear Comfort via Thermal Manikin

Abstract

Thermal wear comfort for workwear clothing plays a vital role in maintaining comfortable water- and moisture-vapor-permeable properties while wearing clothing. In particular, thermal wear comfort measured using a thermal manikin is required in the protective workwear clothing market because their use provides objective data concerning the actual wearing performance of the clothing. This paper investigated the thermal wear comfort properties of various ceramic-embedded composite fabrics for workwear clothing worn in gas and oil industries produced from new schemes. The thermal insulation rate (Clo value) of Al₂O₃(Aluminum oxide)/graphite, ZnO(zinc oxide)/ZrC(zirconium carbide) and ZnO/ATO(antimony tin oxide)-embedded clothing was greater (25.5, 24.7 and 16.9%, respectively) than that of regular clothing (control), which was in accordance with the results (15.0, 13.8 and 11.3% higher, respectively) of the heat retention rate (I) of fabric specimens. It revealed that ZnO- and ATO-embedded yarns mixed with ZrC particles enhanced thermal wear comfort and had superior anti-static and UV-protective properties. Considering UV-protective and anti-static protective clothing worn in gas and oil industries and cold weather regions, it can be concluded that ZnO/ZrC-incorporated fabric is suitable because it showed superior thermal wear comfort with excellent UV-protective and acceptable anti-static properties. Meanwhile, assuming high functional performance for protective clothing worn in winter and factories, ZnO/ATO-incorporated fabric is pertinent to fabricating protective clothing for cold weather regions.

1. Introduction

The engineering of textiles that exhibit the desired heat release/storage properties has been explored and commercialized with success through the use of various methods, including applying the heat of absorption, the phase change material (PCM) method, and the co-spinning of ceramic particle-embedded filaments [1]. Of these methods, heat release/storage textiles embedded with various types of ceramic particles have recently attracted considerable attention. Interrelations between the heat emitted from the ceramic embedded in fabrics and far-infrared (FIR) are of particular importance, especially when considering which ceramic particles can achieve high-temperature heat-release textile products. Therefore, many studies related to their interrelations have been carried out using various ceramic-embedded yarns treated with various types of ceramic particles [2,3,4,5,6,7]. Apart from studies on the application of ceramic-incorporated fabrics for workwear clothing worn in various industries, some studies related to the basic structure of the protective clothing worn by firefighters have been conducted [8,9,10,11]. They introduced a multilayer assembly structure to firefighter clothing made of three layers: an outer shell, a moisture barrier and a thermal liner [8]. Another study divided firefighting protective clothing into four layers, including a comfort layer [9]. Even though the structure of protective clothing is critical to protect the human body from various physical and chemical hazards, this study focused on the application of ceramic-incorporated fabrics for workwear clothing. Regarding studies [4,5,6,7] on the infrared radiation emitted from various ceramics, such as Zirconium Carbide (ZrC), Aluminum Oxide (Al₂O₃) and Silicone Dioxide (SiO₂) embedded in yarns, Negish et al. [4] discovered that ZrC-incorporated fabric absorbs and reflects heat, and some of the heat emitted from human bodies is converted to FIR. Xu et al. [5] investigated the absorbing index of the infrared output of various fabrics coated with ZrC exposed to infrared radiation. Furuta et al. [6] developed a heat-release and storage fabric using ZrC-incorporated Polyethylene Terephthalate (PET). Bahng et al. [7] developed a heat-generating PET using Al₂O₃ and SiO₂ particles. Recently, Kim et al. [12,13] have intensively explored the heat generation and wear comfort characteristics of ceramic-incorporated fabrics with a newly designed experimental scheme. Kim et al. [12] explored the wear comfort of ZrC-incorporated PET fabrics produced using bi-component yarns in comparison with regular PET fabrics and reported that ZrC-embedded PET fabrics exhibited superior drying and thermal insulation properties to regular PET fabrics. They examined various heat and moisture permeable properties of ZrC and Al₂O₃-incorporated PET fabrics, and water absorption, drying characteristics, breathability and thermal wear comfort, through the use of a thermal manikin experiment, were assessed and examined in the context of regular PET fabrics [12]. Investigating prior studies, various types of ceramic materials, such as ZrC, Al₂O₃, Zinc Oxide (ZnO), SiO₂, and Titanium Dioxide (TiO₂), were used to produce ceramic incorporated fabrics, and heat generation properties and wear comfort characteristics of the ceramic-incorporated fabrics were explored in terms of their FIR radiation properties.

Many studies [14,15,16,17,18,19,20,21,22,23,24,25,26,27] to improve ultraviolet (UV)-protection and anti-static properties have been carried out using various ceramic-particle-embedded and coated fabrics treated with various ceramic powders. According to previous studies [14,15,16,17,18,19,20], ZnO and TiO₂ are known to be more efficient for protecting against UV than other ceramics. Some studies [16,17] conducted using TiO₂ nanoparticles (NPs) examined the primary UV-protective effects of TiO₂. Other studies [18,19,20] focused on UV protection reported that ZnO-coated fabric was much more efficient in protecting against UV than conventional fabric.

In contrast, ZnO and antimony tin oxide (ATO, Sb₂SnO₅) impart excellent anti-static properties with electrically conductive properties [21,22] that effectively dissipate the static charge accumulated on the fabric surface, and ATO has heat-insulating and shielding effects [23]. In previous studies, however, the UV-protective and anti-static properties of ceramic-treated fabrics obtained via the coating method limited the durability of their functions when wearing clothing. Hence, ZnO/ATO particle-embedded fabrics produced from new methods of preparing specimens have been developed.

On the other hand, the hazards posed by static electricity and UV radiation are considerable in cold and dry conditions with sunlight, in particular when wearing workwear in oil and gas industries [28,29]. Therefore, workwear clothing in winter requires high functional characteristics (anti-static and UV) with superior thermal wear comfort. Hence, comprehension of how the high-performance properties of different ceramic-incorporated fabrics are influenced according to the various ceramic-incorporated fabrics is critical for workwear clothing worn in the oil and gas industries.

Therefore, we examined the high functional characteristics (anti-static and UV) of Al₂O₃/ZnO/ZrC/ATO-embedded composite fabrics that maintain the thermal wear comfort critical for workwear protective clothing, which was performed in two parts. For these purposes, bi-component yarns were spun with newly designed experimental schemes, non-coated method, and various ceramic particles, such as Al₂O₃, ZnO, ZrC and ATO, were prepared and mixed to enable superior multi-functional characteristics required for workwear protective clothing.

A prior study [30] was the first to examine the anti-static and UV-protective properties in terms of thermal radiation of an Al₂O₃/ZnO/ZrC/ATO-embedded composite fabric required for workwear clothing worn in oil and gas industries.

However, the thermal radiation properties evaluated in a prior study [30] were examined by measuring the surface temperature of the ceramic-incorporated fabrics, i.e., which is not an actual clothing performance assessment as the thermal radiation was measured in relation to fabrics. Moreover, the market for protective clothing requires thermal wear comfort characteristics obtained using thermal manikin equipment.

Accordingly, based on a prior study [30], this article is the 2nd paper to explore the thermal insulation properties (Clo value) of workwear protective clothing using thermal manikin equipment with various types of ceramic particle-incorporated fabrics, such as Al₂O₃, ZnO, ZrC and ATO. Furthermore, a thermal manikin test was performed under light on and light off conditions to simulate exposure to sunlight, and the Clo values were calculated according to the positions of the thermal manikin (stomach, chest and total), which was analyzed using the Clo values measured with and without light.

Finally, the thermal wear comfort of clothing assessed using a thermal manikin was compared with the thermal insulation value (TIV) of the fabrics measured using a Kawabata Evaluation System (KES)-F7 apparatus [12].

2. Methodology

2.1. Master Batch (M/B) for Conjugated Yarns Specimens

Different ceramic particle-embedded conjugated filaments were produced using a master batch chip, a process based on a prior study [30]. Each M/B chip was produced using the compounding machine. A detailed manufacturing method for M/B is quoted in a prior study [30], as follows.

“The concentration (wt. %) of the Al₂O₃ M/B chip was 10 wt. %, which was produced using a polymer combined with 10 wt. % Al₂O₃ and 90 wt. % PET chip (intrinsic viscosity (IV): 0.635 ± 0.005, melting temperature: 250 ± 4 °C). In the same manner, 20 wt. % ZrC, 5 wt. % graphite, and 20 wt. % ZnO and ATO chips were prepared. A mesh ranged of 600–800 nm and 200–300 nm was used to filter the ceramic particles. Average size in

Table 1. Specification of ceramic embedded M/B [30].

Ceramic Embedded PET Yarn Specimen		PET Chip Mass	M/B Chip Mass		Mixed Polymer Mass	Mixing Ratio (wt. %)
		(kilogram)	(kilogram)		(kilogram)	Al2O3	Graphite	ZnO	ATO	ZrC	Total
1	Al2O3/ Graphite	45.5	Al2O3	Graphite	50	0.7	0.1				0.8
			3.5	1.0
2	ZnO/ZrC	48.0	ZnO	ZrC	50			0.5		0.3	0.8
			1.25	0.75
3	ZnO/ATO	48.0	ZnO	ATO	50			0.5	0.3		0.8
			1.25	0.75

means diameter obtained in particle size distribution curve assessed using Hydro 2000S [12]. The detailed measuring methods for particle size and intrinsic viscosity (IV) are referred to prior studies [12]”.

Table 1 lists the M/B chip mass of each ceramic particle, PET polymer chip mass and the compounding ratios of the M/B used to produce the yarn specimens, which is quoted in a prior study [30], as follows. “The Al₂O₃/graphite conjugated yarn was spun using a polymer (50 kg) combined with 0.7 wt. % Al₂O₃ (3.5 kg) and 0.1 wt. % graphite (1.0 kg) M/B chips, as shown in Table 1. The ZnO/ZrC sheath/core yarn with 0.5 wt. %, ZnO (1.25 kg) and 0.3 wt. % ZrC (0.75 kg) and ZnO/ATO sheath/core yarn with 0.5 wt. % ZnO (1.25 kg) and 0.3 wt. % ATO (0.75 kg) M/B chips were spun”.

2.2. Spinning of the Bi-Component Yarn Specimens

Figure 1 presents a bi-component spinning machine [30]. The detailed spinning method was quoted in a prior study [30], as follows. “Each sheath/core yarn was made with the PET polymer of 50 wt. % in the sheath part and the ceramic-embedded PET polymer of 50 wt. % in the core part as shown in Figure 1a. Spinning temperatures at the spinbeam and manifold ranged 280 to 295 °C (Figure 1b). Extruder heating temperature in sheath ranged 310 to 320 °C, and in core ranged 287 to 315 °C. As shown in Figure 1b, 1st and 2nd godet roller (GR) speeds ranged about 1300 and 4050 m/min, respectively. 1st GR speeds for Al₂O₃/graphite, ZnO/ZrC, and ZnO/ATO bi-component yarns were 3160, 1360 and 1300 m/min, respectively, and 2nd GR speeds were 3100, 4010 and 4050 m/min, respectively. 1st and 2nd GR temperatures ranged 80 to 120 °C. Feed roller (F/R) speed for each yarn was 3100 and 4000 m/min, respectively and yarn linear density was 75d/24f, finally sheath/core ratio was 50/50. Cross-sections of the yarn specimens were assessed using SEM”.

2.3. Fabrication of Composite Fabric Specimens

Composite fabric specimens were fabricated on a weaving machine using four ceramic-embedded yarns as a weft with a warp yarn of polyamide (PA, nylon): Al₂O₃/graphite, ZnO/ZrC, ZnO/ATO, and one regular (a control yarn). The pattern of the composite fabric was plain.

Composite fabric specimens were dyed and finished, and their detailed processes and processing conditions are referred to in a prior study [30].

Table 2. Details of the composite fabric specimens [30].

Specimen	Yarn Type		Fabric Sett (Picks, Ends/cm)		Thickness(mm)	Mass (g/m2)	Weave Pattern
	Wp	Wf	Wp	Wf
1	nylon	Al2O3–graphite PET	51.6	35.4	0.128	84.2	Plain
2	nylon	ZnO–ZrC PET	51.6	35.4	0.130	84.6	Plain
3	nylon	ZnO–ATO PET	51.6	35.4	0.131	84.8	Plain
4	nylon	Reg PET	51.6	35.4	0.122	83.1	Plain

shows the specifications of the composite fabric specimens, which are quoted in a prior study [30]. The fabric surface and its cross-sections were assessed to find out their structural characteristics using SEM.

2.4. FIR Emissivity Measurement and Ingredient Analysis of Yarn Specimens

FIR rays emit heat in the electromagnetic wave through radiation [11] as ceramic embedded in the yarn is heated or exposed to sunlight. FIR emission of the yarn specimens was examined using a Fourier transform infrared (FT-IR) spectrometer. A square arrayed yarn specimen 38 × 38 mm in size was prepared and inserted on the specimen holder and then pre-heated for two hours. The emissive powers of the black body and specimen were obtained through this experiment, respectively, and the emissivity (γ) was calculated using Equation (1) [31].

$$\mathrm{Emissivity} \left(\mathsf{\gamma }\right)=\frac{the sum of emissive power by electomagnetic wave of the speicmen}{the sum of emissive power by electromagnetic wave of the black body}$$

(1)

Figure 2 presents diagrams of the emissive power and emissivity obtained in this experiment [31]. The mean for five readings was obtained via deviation. Elemental analysis of yarn specimens was performed using energy-dispersive X-ray spectroscopy.

2.5. Measurement of Thermal Property of the Composite Fabrics

The thermal property measurement method used in relation to the fabric specimens was quoted in a prior study [31], as follows. “The heat retention rate (I) of the ceramic embedded composite fabrics was assessed using KES-F7, and compared with the Clo value (a measure of clothing thermal insulation) of clothing by the thermal manikin experiment. Figure 3 presents a schematic diagram of the KES-F7 [31], which was quoted in a prior study [31]”.

“As shown in Figure 3, the composite fabric specimen was inserted between plate (cupper) and water bath. The B.T. Box temperature was adjusted to 30 °C, and water was circulated in a water bath with a constant temperature 20 °C. Heat streams from electrical system (B.T. box) to a water bath through a cupper plate and composite fabric. Five readings were performed for each specimen. The heat retention rate (I) was calculated using Equation (2)”.

$$\mathrm{I} \left(\%\right)=\left(1-\frac{\mathrm{b}}{\mathrm{a}}\right)\times 100$$

(2)

where a and b are referred to in the previous study [31].

2.6. Thermal Insulation Rate (Clo) Measurement by a Thermal Manikin Apparatus

The thermal insulation characteristics of the clothing were measured under light-on and-off states. The jacket and trousers worn by a thermal manikin were made using the fabric specimens (Table 2). Figure 4 presents the model of the jacket and trousers used in this study. Total thermal resistance (R_t) and sub-thermal resistance (R_ti) of the clothing at each position on the thermal manikin were assessed according to the KSK ISO 15831 L 2004 standards [32] in a climatic chamber (Tabai Espec, Osaka, Japan). The sub-thermal resistance assessment was conducted at thirteen positions of the thermal manikin (chest, forearm, up thigh, calf, upper arm, hand, foot, head, stomach, face, shoulder, back, and low thigh), as quoted in a prior study [13].

Figure 5 shows a model of the human body of the thermal manikin with sensor positions. Temperature sensors were attached to twenty positions on the thermal manikin [13].

A thermal manikin was exposed to a light bulb (Iwasaki Electric, Osaka, Japan) with illumination of 9000 Lux 100 cm from the thermal manikin to detect the thermal wear comfort characteristics in relation to the FIR radiation from the composite fabrics. Figure 6 presents the thermal manikin wearing the clothing with and without light. As shown in Figure 6a, the light was directed to the stomach and chest positions of the thermal manikin. A detailed measurement method is quoted in a prior study [13], as follows.

“Skin temperature (T_s) of the thermal manikin was set to 35 °C for each body part. Ambient temperature (T_a) was 20 ± 0.5 °C with 48 ± 2% RH and an air velocity of 0.1 m/s. Average skin temperature (T_si) on the nineteen points and heat loss (H_i) were also measured for 30 min. Total thermal resistance (R_t) was calculated using Equations (3) and (4), which were quoted in a prior study [13]. This experiment was measured using a serial method [32].

$$R_t={{\displaystyle \sum }}_{i=1}^{20}{f}_{i }\times \left[\frac{\left(T_{si}-T_a\right)\times a_i}{H_i}\right]$$

(3)

$$f_i=\frac{a_i}{A}$$

(4)

where R_t is total thermal resistance of the clothing and air layer; f_i is area coefficient; H_i is heat loss on point i of the manikin, and T_si is the skin temperature on point i of the thermal manikin. T_a is mean ambient temperature; a_i is surface area of point i, and A is surface area of the thermal manikin. Total Clo value (Clo_t), which indicates the thermal insulation rate of clothing, was calculated using Equation (5). The measurement and calculation were performed under the light on and off states, respectively. Five independent replications of this test were conducted.

Clo_t = R_t × 6.45

(5)

3. Results and Discussion

3.1. FIR Emissivity (γ) of the Yarn Specimens

The main concern of this study focused on which ceramic-embedded yarn exhibits the best heat release among various ceramic-embedded composite fabrics to acquire superior thermal wear comfort while wearing clothing in the gas and oil industries. Therefore, the FIR emissivity characteristics affecting the heat release of the three yarn specimens were assessed and compared with that of regular yarn. The emissivity and F-test results of the yarn specimens are quoted in a prior study [30], as follows. “Statistical analysis (F-test) among average value of the yarn specimens for emissivity was conducted to verify the statistical significance with 95% confidence limit. Average value between each specimen was significant, respectively, as F₀ (V/Ve) > F (3, 16, 0.95) and p < 0.05”.

In addition, the emissivity of the ceramic-incorporated yarns was greater than that of regular yarn. In particular, Al₂O₃/graphite and ZnO/ZrC-embedded yarns exhibited greater emissivity than ZnO/ATO-embedded yarn. These phenomena were attributed to greater heat generation from Al₂O₃/graphite and ZnO/ZrC ceramic particles than ZnO/ATO ceramic ones incorporated in the yarns. FIR characteristics with more heat released from Al₂O₃ and ZrC ceramic particles were explained previously, which is quoted in a prior study conducted by Kuo et al. [3], as follows. “When the Al₂O₃ and ZrC particles in the yarns are heated or irradiated, the energy generates electronic excitation, and then the electrons become steady, transferring from orbit to other one. In this period, the FIR transmits the heat energy in electromagnetic wave by radiation. On the other hand, the human body emits energy, which is absorbed by the fabric and released in a far-infrared ray back to the human body. This procedure repeats, thus keeping the human body warm”.

In particular, in studies [31,33,34,35,36] reported most recently, Kim [31,33] examined that Al₂O₃/ATO incorporated yarns exhibited higher FIR emissivity than TiO₂ incorporated regular yarns. The following is quoted in a prior study [31] “Kim reported that highly ATO-embedded yarn showed lower FIR emissivity than low ATO-embedded yarn, which was due to heat-shielding effect of the ATO particles embedded in the yarns. That is, the heat shielding of the ATO particles decelerates the temperature rise due to lower FIR emissivity emitted from the highly embedded ATO particles in the yarns”. In addition, heat shielding and lower FIR emissivity of highly embedded ATO particles in the yarns might be explained by three shielding modes, which is quoted in a previous study [35], as follows. “There are three shielding modes of electromagnetic waves, where the energy of electromagnetic waves is dissipated through absorption, reflection, and multiple reflection. Electromagnetic waves absorbed by the shielding of ATO particles in the yarns are converted into heat energy and attenuated. Reflected electromagnetic waves are dissipated as a result of the discontinuous inductive reactance between ATO particles in the yarns, and they are repeatedly reflected inside of ATO particles”. Accordingly, energy loss due to shielding is caused by absorption, reflection and multiple reflection, which enables highly ATO-embedded yarns to lower the emissivity of FIR, resulting in lower heat release of highly ATO-embedded yarns.

Summarizing the current findings in comparison to a prior study [30], ZnO/ZrC-incorporated yarns combined with ZnO to enhance UV-protective and anti-static characteristics exhibited high FIR emissivity, which indicated that ZrC particles are the dominant factor affecting the high FIR emissivity of ZnO/ZrC-embedded yarns compared to ZnO particles. On the other hand, the FIR radiation of the mixed ZnO and ATO-embedded yarns (specimen 3) was inferior to Al₂O₃-incorporated yarn 1 and ZrC-incorporated yarn 2 mixed with other ceramic particles. The reason why is quoted in a prior study [37], as follows. “This was due to the lower heat release of ZnO particles and the heat shielding effect of ATO particles. In other word, the ATO particles in the yarns enable them to shield the FIR emitted from the light, resulting in lower emissivity of the ZnO/ATO embedded yarns”.

The FIR emissivity of various ceramic-incorporated yarns is dependent on the heat release emitted from FIR and the irradiation of the ceramic powders distributed in the yarns. Hence, the distribution of the ceramic particles embedded in the sheath/core yarns was examined using SEM images of the fabric cross-sections in the next section.

3.2. Ingredient Analysis with SEM Images of Yarn and Fabric Specimens

Figure 7 presents the ingredient analysis with SEM images of the yarn cross-section, which is quoted in a prior study [30], as follows. “Peaks for Al and Ti in Figure 7a are shown, for Zn, Zr and Ti in Figure 7b, and for Zn, Sb, Sn and Ti in Figure 7c. Figure 7d presents the ingredients in the regular PET yarn, in which Ti, C and O peaks appear”. The regular PET yarn includes TiO₂ particles (0.36 wt. %). Comparing the SEM images in Figure 7e–h with the ingredient peaks in Figure 7a–d, white spots in Figure 7e were verified as being Al₂O₃ and TiO₂ particles through elemental analysis, and the white spots in Figure 7f appeared to be ZnO, ZrC and TiO₂ particles through examination of the ingredient peaks. The white spots shown in Figure 7g were observed as being ATO particles composed of Sn and Sb and ZnO and TiO₂ through elemental analysis, as shown in Figure 7c.

On the other hand, Figure 8 presents the SEM images of the surface and cross-sections of the composite fabric specimens, which is quoted in a prior study [30] as follows. “Some white spots on the composite fabric surface in Figure 8a–d appeared, which was assumed to be TiO₂ particles embedded in the sheath of the yarns in the composite fabrics. Large white spots in the circular yarns of the composite fabric cross-section (Figure 8e) were shown as Al₂O₃ particles, and white spots in Figure 8f appeared as ZrC and ZnO particles, whereas, white spots in Figure 8g were ZnO and ATO particles”. As shown in Figure 8e–g, the particles (white spots) size of Al₂O₃ (Figure 8e) were greater than the ZrC and ZnO particles (white spots) (Figure 8f) and also greater than white spots of the ZnO and ATO particles (Figure 8g). The sizes of white spots in Figure 8e–g might be compared with the particle sizes and their distribution shown in a prior study [37]. Followings are quoted in a prior study [37], “the particle size of Al₂O₃ was distributed between 0.1 and 10 µm with average size 869 nm, and ZrC was distributed between 0.05 and 1.1 µm with average size 548 nm. The particle size of the ZnO and ATO were distributed between 200 and 300 nm”.

3.3. Thermal Insulation Characteristics (Clo Value) of Clothing

The thermal radiation assessment of the fabric conducted in a prior study [30] could not provide objective data because it measured the thermal properties of the fabric and not the actual performance of the clothing. Accordingly, the thermal insulation properties of the clothing were examined using thermal manikin equipment.

Table 3. Thermal resistance and Clo value of the composite fabric specimens.

Speci Men	No.		Total Rt	Clo Value	Sub-Thermal Resistance, Rti
			(m2·°C/W)		Face	Head	Upperarm		Forearm		Hand		Chest	Shoulders	Stomach	Back	UpThigh		LowThigh		Calf		Foot		AmBient	HumiDity	Setting
					R	L	R	L	R	L	R	L					R	L	R	L	R	L	R	L	Temp. (°C)	(%)	Temp. (°C)
1	Al2O3 /graphite PET fabric	On	0.308	1.987	0.113	0.12	0.275	0.251	0.241	0.249	0.094	0.09	0.432	0.305	0.738	0.272	0.466	0.48	0.214	0.228	0.145	0.138	0.191	0.089	20	48	35
		Off	0.243	1.567	0.084	0.115	0.228	0.217	0.198	0.211	0.088	0.083	0.234	0.301	0.395	0.264	0.403	0.416	0.194	0.209	0.136	0.13	0.085	0.085
2	ZnO/ZrC PET fabric	On	0.306	1.974	0.115	0.12	0.281	0.227	0.228	0.253	0.094	0.093	0.457	0.304	0.741	0.272	0.458	0.48	0.223	0.211	0.144	0.145	0.091	0.09
		Off	0.237	1.529	0.084	0.115	0.229	0.204	0.19	0.212	0.088	0.085	0.235	0.3	0.376	0.265	0.386	0.389	0.2	0.193	0.134	0.138	0.084	0.083
3	ZnO/ATO PET fabric	On	0.287	1.851	0.115	0.127	0.243	0.231	0.251	0.254	0.099	0.089	0.418	0.306	0.558	0.267	0.429	0.478	0.212	0.226	0.147	0.135	0.089	0.09
		Off	0.232	1.496	0.085	0.119	0.206	0.204	0.201	0.215	0.092	0.084	0.23	0.301	0.33	0.259	0.369	0.407	0.191	0.207	0.14	0.126	0.085	0.083
4	Regular PET fabric	On	0.245	1.583	0.085	0.113	0.201	0.209	0.201	0.209	0.092	0.082	0.245	0.281	0.354	0.272	0.31	0.354	0.191	0.204	0.127	0.131	0.082	0.081
		Off	0.214	1.382	0.078	0.009	0.198	0.19	0.178	0.201	0.081	0.078	0.21	0.273	0.321	0.251	0.371	0.368	0.181	0.183	0.121	0.114	0.072	0.074

lists the total Clo value of the clothing and thermal resistance (R_ti) of twenty areas of the thermal manikin under with and without light conditions. The total thermal resistance was calculated using Equations (3) and (4) in terms of the measured skin temperature (T_si) and heat loss (H_i). The Clo value was calculated using Equation (5).

Table 4. Total Clo and sub-Clo values at the stomach and chest.

Experimental Condition	Fabric Specimens	Total Clo		Stomach Clo		Chest Clo
		Mean	Dev.	Mean	Dev.	Mean	Dev.
Light On	1. Al2O3/graphite PET fabric	1.987	0.257	4.760	0.610	2.786	0.212
	2. ZnO/ZrC PET fabric	1.974	0.214	4.779	0.800	2.948	0.250
	3. ZnO/ATO PET fabric	1.851	0.257	3.599	0.650	2.696	0.238
	4. Regular PET fabric	1.583	0.199	2.283	0.500	1.580	0.187
Light Off	1. Al2O3/graphite PET fabric	1.567	0.209	2.548	0.216	1.509	0.216
	2. ZnO/ZrC PET fabric	1.529	0.210	2.425	0.291	1.516	0.262
	3. ZnO/ATO PET fabric	1.496	0.234	2.129	0.293	1.484	0.223
	4. Regular PET fabric	1.382	0.227	2.070	0.347	1.355	0.256

lists the total Clo of each specimen and sub-Clo at the stomach and chest positions under light-on and -off conditions. Statistical analysis was conducted to verify the statistical significance. An F-test between each specimen (1 to 4) was conducted for each mean Clo with a 95% confidence limit.

Table 5. ANOVA analysis of each Clo value of the composite fabric specimens.

	Each Clo	F-Value (F0)	F (3, 16, 0.95)	p-Value
Light-on	Total Clo	15.91	3.24	4.64 × 10−5
	Stomach Clo	106.13	3.24	8.98 × 10−11
	Chest Clo	372.43	3.24	5.24 × 10−15
Light-off	Total Clo	10.47	3.24	4.70 × 10−40
	Stomach Clo	21.51	3.24	7.36 × 10−6
	Chest Clo	3.35	3.24	0.045

lists the F-test data of t each Clo value of the composite fabric. The mean of the composite fabric for each Clo value was statistically significant as F₀ (V/Ve) > F (3, 16, 0.95) and p < 0.05.

Figure 9 presents the Clo value diagram of the composite fabrics under the light-on and -off conditions. As shown in Figure 9a, the total Clo of the ceramic-particle-embedded composite fabrics (specimens 1, 2, and 3) was greater (25.5, 24.7 and 16.9%, respectively) than that of the regular specimen 4, which was much greater under light-on conditions. This means that the thermal insulation properties of the ceramic-particle-embedded clothing is superior to regular clothing. This was attributed to the greater FIR emissivity of the ceramic-incorporated composite fabrics than regular fabrics, as explained previously. This indicates that the greater FIR emissivity emitted from ceramic particles (Al₂O₃, ZrC, ZnO, and ATO) allows more heat to be released from the composite fabric, reducing the heat flow from the human body throughout the composite fabric, which results in the ceramic-particle-embedded composite fabrics having a greater thermal insulation value (Clo) than the TiO₂-embedded regular fabric. This result might be partly explained by the characteristics of TiO₂ particles, which is quoted in a prior study [38], as follows. “TiO₂ is a semiconductor with a wide bandgap (3.2 eV) between the low energy valance band and the high energy conduction band. Accordingly, when the TiO₂ is activated with light waves of energy greater than its bandgap, the electrons will absorb UV light due to its wide bandgap^”. This is why TiO₂-embedded fabric is less effective in terms of FIR emissivity and exhibits lower FIR emissivity than Al₂O₃ and ZrC-embedded composite fabrics, resulting in lower Clo values than Al₂O₃ and ZrC-embedded composite fabrics.

On the other hand, the total Clo under light conditions of Al₂O₃/graphite and ZnO/ZrC-embedded clothing was greater than that of ZnO/ATO-embedded clothing, i.e., the thermal insulation rate of the Al₂O₃/graphite and ZnO/ZrC-incorporated clothing is superior to that of ZnO/ATO-embedded clothing.

This finding suggests that embedding ZnO with ATO particles lowers thermal radiation in terms of FIR due to the irradiation characteristics of ZnO particles. This result could be explained in terms of the free radical characteristics of the irradiation of ZnO particles, which is quoted in prior studies [39,40], as follows. “ZnO particles have a very narrow size distribution and minimal aggregation, which results in higher levels of ultra-violet radiation blocking than that of the infrared radiation [39]. When ZnO particles is irradiated by the sun, photons with certain energy are injected into the ZnO particles. The electrons are excited from the VB to the CB, thereby leaving a hole. The excited CB electrons are combined with the hole to eliminate heat and energy. The hole changes the ambient hydroxy electrons into free radicals as a strong oxidizer during excitation [40]”. These free radical characteristics of ZnO particles protect the human body from UV damage and microbial attacks [3], and they exhibit lower FIR emissivity and thermal radiation than Al₂O₃ and ZrC particles, resulting in lower thermal insulation rates of ZnO/ATO-embedded clothing than Al₂O₃/graphite and ZnO/ZrC-embedded ones.

3.4. Thermal Insulation Characteristics (Sub-Clo Values) at Stomach and Chest Positions

The sub-Clo values at the stomach and chest positions were compared with total Clo under light-on and -off conditions. The same trend was found with sub-Clo under light-on conditions in the stomach and chest positions of the thermal manikin, as shown in Figure 9b,c. It was noted that ZnO/ZrC-embedded clothing exhibited the highest thermal insulation rate (the largest Clo value) at the stomach and chest positions under light-on conditions (Figure 9b,c). This indicates that even though ZnO powders in the ZnO/ZrC-incorporated fabric had relatively lower heat release compared to ZrC, ZrC particles mixed with ZnO are more effective in heat generation when they directly receive light, resulting in higher Clo value due to the greater emissivity of ZnO/ZrC yarns. Hence, assuming the context of workwear clothing worn in winter, ZnO/ZrC-incorporated fabric is suitable for this clothing because of its excellent thermal wear comfort presented in this study.

In particular, the difference in Clo under light-on states between the ceramic-particle-embedded composite fabrics (specimens 1, 2 and 3) and the regular fabric (specimen 4) was much greater at the stomach (Figure 9b) and chest (Figure 9c) positions than the total Clo (Figure 9a). This might be explained by the fact that more heat is released at the stomach and chest positions where the clothing is exposed directly to the light during the thermal manikin experiment, resulting in a larger difference in the sub-Clo values at the stomach and chest positions than the difference in the total Clo between the ceramic particles incorporated composite fabrics (specimens 1, 2 and 3) and regular PET one (specimen 4).

On the other hand, a comparison of light-on and-off conditions at the stomach and chest positions (Figure 9b,c) revealed larger differences in Clo values between light-on and-off conditions in terms of the ceramic-particle-incorporated composite fabric specimens 1, 2 and 3 than those of the regular fabric specimen 4. This may be explained by heat generation due to the FIR radiation released from the ceramic particle incorporated composite fabrics when they receive light. In other words, more heat from the ceramic-particle-incorporated composite fabrics is released under light-on than light-off conditions at the stomach and chest positions of the thermal manikin, as the stomach and chest are directly exposed to the light, i.e., more lights are directed at the stomach and chest than other places in the manikin, resulting in a larger difference in the Clo values between with and without light conditions at the stomach and chest positions. This indicates that the FIR ray released from the ceramic particles (Al₂O₃, ZnO, ZrC and ATO) enhances heat generation from the fabric when exposed to sunlight. This suggests that the Al₂O₃/graphite, ZnO/ZrC, and ZnO/ATO-embedded yarns and composite fabrics have superior thermal insulation properties than regular PET fabric, and that imparts excellent thermal wear comfort during wearing workwear clothing in gas and oil industries exposed to sunlight in cold weather regions.

Furthermore, Al₂O₃/graphite and ZnO/ZrC particles are more effective in terms of heat release and thermal insulation properties than ZnO/ATO particles, indicating that ZnO and ATO are needed to be combined with ZrC and Al₂O₃ to enhance the thermal wear comfort of workwear clothing worn in winter. However, while based on multi-functional properties for high-performance workwear clothing, ZnO/ATO-incorporated fabric is useful for fabricating workwear clothing for gas and oil industries due to its relatively good thermal wear comfort (Clo value) obtained in this study with superior UV-protective and anti-static properties.

3.5. Comparison between Clo Value of Clothing and Heat Retention Rate (I) of Fabric

Thermal insulation (Clo value) of the ceramic incorporated clothing measured using a thermal manikin system was compared with the heat retention rate (I) of the composite fabrics.

Table 6. Heat retention rate (I) and F-test of the composite fabrics [37].

Specimens Physical Properties	1			2			3			4
	Al2O3/Graphite			ZnO/ZrC			ZnO/ATO			Regular
	M	D		M	D		M	D		M	D
Heat retention rate (I, %)	40.7	+0.9	−0.7	40.3	+1.3	−0.9	39.4	+1.4	−0.8	35.4	+1.1	−0.8
F-test result	F-value (F0)				F (3, 16, 0.95)				p-value
	44.77				3.239				5.24 × 10−0.8

Note: M: mean. D: deviation.

lists the I values, which are quoted in a prior study [37]. The D (deviation) in Table 6 denotes the difference (+ and −) between the maximum (minimum) and mean values. ANOVA was conducted to certify the statistical significance, which was carried out between each specimen (1 to 4) of the mean value of the heat retention rate with a 5% significance level. The mean of the four specimens for the heat retention rate was statistically significant, as F₀ (V/Ve) > F (3, 16, 0.95) and p < 0.05 (Table 6). Detailed analysis for I according to the ceramic-embedded fabrics was quoted in a prior study [37], as follows. “The heat retention rate (I) of the ceramic particle-embedded composite fabrics (specimen 1 to 3) was higher (15, 13.8 and 11.3%, respectively) than that of the regular fabric (specimen 4), which was attributed to the more heat release from the absorption (or accumulation) of the higher FIR radiation released from the ceramic particles in the yarns, which was previously certified by the greater FIR emissivity of the ceramic incorporated yarns than the regular one. On the other hand, the heat retention rate (I) of the Al₂O₃/graphite and ZnO/ZrC-embedded composite fabrics was greater than that of the ZnO/ATO-embedded composite one”.

These results were in accordance with the higher Clo values of the ceramic-incorporated clothing than regular clothing (Table 4), which was also consistent with the greater Clo of the Al₂O₃/graphite and ZnO/ZrC-incorporated clothing than ZnO/ATO-embedded clothing (Table 4).

4. Conclusions

This study examined the thermal wear comfort (Clo) of workwear clothing incorporated with various ceramic particles worn in winter and factories. The Clo value of the clothing measured using a thermal manikin was compared with the heat retention rate (I) of the composite fabrics measured using a KES-F7 system. The results were verified via the FIR emissivity measured from the ceramic-incorporated yarns. The Clo values of the Al₂O₃/graphite, ZnO/ZrC and ZnO/ATO-embedded clothing were greater (25.5, 24.7 and 16.9%, respectively) than regular PET clothing (control), respectively, which was consistent with the results (15.0, 13.8 and 11.3% higher, respectively) of the heat retention rate (I) of fabric specimens. It revealed that ZnO and ATO-embedded yarns mixed with ZrC particles enhanced thermal wear comfort with superior anti-static and UV-protective properties.

In the context of workwear clothing worn in the winter and industries, ZnO/ZrC-incorporated fabric is suitable for this clothing because of its excellent thermal wear comfort (i.e., superior thermal insulation rate) that appeared in this study. However, considering the high-performance properties required for protective clothing worn in cold weather and factories, ZnO/ATO-incorporated fabric is pertinent for the fabrication pf workwear clothing for use in winter because of its relatively good thermal wear comfort that appeared in this study with superior UV-protective and anti-static properties obtained in a prior study [30]. Finally, this study revealed the possible applications of ZnO and ATO particles for workwear protective clothing wearing in gas and oil industries.