Abstract
In this study, the heat transfer and moisture management properties of fabric combinations for office wear consisting of underwear and shirt fabrics, including three different fiber types and three kinds of weave types, were investigated. A sweating torso methodology was applied in order to characterize fabric combinations in terms of thermophysiological comfort for office occupants. This study showed that the cooling effect due to perspiration, accumulated moisture, and drying time of the combinations with cotton underwear were unaffected by both weave type and fiber type of shirt layer. However, initial cooling and drying time values of the combinations with underwear consisting of hydrophobic fibers were affected statistically significant by the fiber of the shirt layer. The percentage of moisture accumulation in the fabric layers was directly related to the fiber type of the underwear fabrics. Based on an extended data analysis, it was concluded that the combination of polyester underwear and cotton polyester twill shirt fabrics was the most recommendable of all the combinations used in this study with regard to the thermophysiological comfort of office occupants.
Keywords
Office occupants work indoors at least eight hours a day. The thermal environment, particularly one reducing body heat dissipation, is likely to affect their mental performance, health, and work productivity. In order to maintain the thermal comfort of the occupants in office buildings, environmental factors, such as air temperature, radiant temperature, relative humidity, and wind speed, as well as personal factors, such as clothing and metabolic heat production, have to be taken into account. Most of the environmental factors can be controlled by the heating, ventilation, and air conditioning systems (HVAC) of the office building. However, many occupants still work in traditional buildings, which have no or insufficient HVAC systems. This leads to less-controlled office conditions, reflecting general weather conditions and more fluctuating conditions due to solar irradiation and/or intensity of ventilation. On the other hand, there are also some advantages of naturally ventilated buildings so that a certain amount of control over ambient conditions is left to the occupants. The occupants may adjust conditions to suit themselves, and tend to adapt more to changes through their clothing.1,2 Thus, clothing becomes more prominent in terms of the maintenance of thermal comfort for office occupants working in naturally ventilated buildings, predominantly in warm to hot conditions.
Thermophysiological comfort is defined as an ability to maintain thermal equilibrium during the interactions of the human body, clothing, and the environment. 3 The body core temperature of human beings is about 37.0℃, and it should be kept within a narrow range for thermal equilibrium. 4 Therefore, excessive metabolic heat has to be dissipated from the body to the environment. If dry heat transfer is insufficient to dissipate it, sweat will be secreted by the human body to intensify heat loss through evaporation. As a consequence, water vapor and liquid transfer in clothing are very important aspects to maintain thermophysiological comfort. It is expected that clothing will support the evaporative cooling of sweating in skin proximity. 5 Furthermore, the clothing layer next to the skin should dry out quickly after sweating in order to maintain wearing comfort. 6 As a consequence, evaporative cooling efficiency during sweating is an important clothing property for thermal comfort besides the dry thermal resistance. 7 Thus, office wear should be optimized in this regard to ensure clothing is appropriate to the indoor environmental conditions of the office building.
In daily life, clothing mostly consists of multiple layers. The assessment of clothing comfort for clothing ensembles is more complicated than for single layer fabrics. The properties of the single layers as well as the interactions between these layers affect the heat and mass transfer from the skin to the environment. The evaluation of the single layer properties only does not reflect reality thoroughly, and provides only an estimation of clothing comfort. From this point of view, researchers have examined the effects of several fabric and garment properties on heat and moisture transfer of clothing ensembles. Bakkevig and Nielsen 8 have determined the effects of fiber type, weave type, and wetness of underwear on thermal comfort and people's thermoregulatory responses for two-layer clothing systems. They indicated that weave type had an impact on the rate of evaporation, and the thickness of underwear had more influence on thermal comfort than the fiber types used in the study. Kim and Spivak 9 explored the impact of fiber type on the change of inner surface temperature of fabric and vapor pressure throughout the dynamic moisture transfer by microfine hygrometry and thermometry. They observed that the ensembles with a cotton inner layer and cotton outer layer were drier and warmer than polyester inner/polyester outer layers and mixed fabric layers of cotton and polyester at the onset of sweating. Celcar et al. 10 examined the impact of environmental and sweating conditions on the thermal comfort properties of male business clothing systems. They reported that condensation and dry heat loss through the clothing systems increased with decreasing ambient temperature. An investigation of the effects of moisture content and clothing fit on clothing wet insulation of two-layer clothing assemblies has been conducted by Wang et al. 11 Clothing fit had a minimum impact on wet thermal insulation, whilst the moisture content added to underwear caused an overwhelming decrease of thermal insulation.
The number of comprehensive studies reported on office wear comfort is low. Most of these studies did not include various fabrics allowing for a systematic investigation of fabric properties. However, such a systematic approach is a prerequisite for a better understanding of the effects of fabric properties on wearing comfort. Therefore, in this study, we aimed to investigate heat transfer and moisture management properties of combinations of a systematic set of woven shirt fabrics and knitted underwear fabrics made out of different fiber types (cotton, bamboo, and polyester) and weave types (plain, twill, matt). These combinations include the most preferred shirt and underwear fabrics for office wear. The results of this study provide new insights into the effect of fabric properties on the thermophysiological comfort properties of office wear combinations. This knowledge will help to select optimal underwear–shirt combinations in order to minimize discomfort in office occupants.
Materials and methods
Materials
The properties of yarns used in the study
S3: number of hairs with a length equal to or exceeding 3 mm
Three different common weave types were selected for shirt fabrics (plain, 3/1 twill, and matt derivative, Figure 1). Greige woven fabrics were singed, washed at 70℃, and dried at 95℃. After the processes, the fabrics were treated with micro silicone softener (30 g/L) and were dried at 140℃. For cotton/polyester fabrics, thermal fixation was done at 185℃ for 48 seconds. Sanforization and calendaring were applied for cotton and bamboo viscose fabrics, whilst cotton/polyester fabrics were only treated sanforization.
Weave types of the shirting fabrics (a) plain, (b) 3/1 twill, and (c) matt derivative.
Underwear knitted fabrics were manufactured out of cotton, polyester, and cotton/polyester blend as single jersey, which is one of the preferred knitted structures for underwear. All the knitted fabrics were produced on a Mayer & Cie circular knitting machine with 28 gauges on 34 diameters using constant setting values. Bleaching was applied for cotton (110℃) and cotton/polyester (80℃) fabrics for 20 minutes. Preliminary scouring was done for polyester fabrics at 80℃ for 15 minutes.
Structural and physical properties of shirt and underwear fabrics. Data is shown as mean (standard deviation)
PV: volume porosity.
R3D: average pore radius (three dimensional).
Methods
The physical and structural properties of the woven and the knitted fabrics were measured according to dedicated standards. Warp and weft densities of the woven fabrics were determined according to ISO 7211-2:1984. The loop length and the cover factor of the knitted fabrics were determined based on the formulas used in the studies of Peirce 12 and Wardiningsih and Troynikov, 13 respectively. In addition, volume porosity and average pore radius of the woven fabrics were calculated theoretically. Mass per unit area was measured according to ISO 3801:1977. Fabric thickness tests were conducted using ISO 5084:1996. Air permeability of the fabrics was measured by using the Textest FX 3300 Air Permeability Tester (Textest AG, Schwerzenbach, Switzerland) according to ISO 9237:1995. Thermal resistance was determined using a sweating torso in accordance with ISO 15831.
The combined heat and moisture transfer for the different combinations of underwear and shirt fabrics was investigated by using the sweating torso (Figure 2).
14
This heated sweating cylinder enables investigation of insulating and cooling properties of fabrics applied on a cylindrical shape in an upright condition, as is the case for clothing worn on the human body. The experimental set-up does not include air gaps in order to evaluate functional properties of the fabrics in a very controlled manner and how they are affected by weave type and raw material. Therefore, the results provide an important theoretical input concerning the improvement of comfort properties, but do not perfectly reflect the realistic conditions of wearing comfort.
The sweating torso and calculated parameters.
15
IC: initial cooling, SC: sustained cooling, PC: post cooling intensity.
The sweating torso was placed inside a climatic chamber in order to control ambient conditions. Environmental temperature was set at 20℃ (±1℃) with 50% relative humidity (±5%), and applying 1 m.s−1 (±0.1 m.s−1) air speed. The experimental protocol included three phases: acclimatization phase, sudatory exposure phase, and recovery phase. The duration of each phase was set to 60 minutes. For the first phase, the torso surface temperature was kept constant at 35℃ and the required heating power to maintain the surface temperature was recorded. During the sudatory exposure, a constant heating power of 125 W was chosen (in order to simulate body heat accumulation due to elevated office room temperature and radiation (sun, windows, and walls)) and a sweat rate of 230.7 g/h/m2 were set to investigate and compare the functional properties of fabrics in order to support cooling of the body surface. In the recovery phase, heating power was reduced to and maintained at 25 W (57.7 W/m2) without sweating. Surface temperature, heating power, and weight of the torso device were recorded throughout the three phases.
Initial cooling and sustained cooling during sudatory exposure, and post cooling intensity during the recovery phase, were measured for the single layers and the fabric combinations without an air gap between the layers. Initial cooling is the rate of temperature reduction observed after the onset of cooling induced by sweating. The constant change in surface temperature of the sweating torso device followed after initial cooling during sudatory exposure was designated as sustained cooling. Post cooling intensity is the temperature minimum observed during the recovery phase due to the evaporation of moisture retained in the fabrics (post cooling effect). The dry weight of fabrics was recorded previous to the test and the wet weight of underwear and shirt fabrics were recorded for the fabric combinations at the end of the sudatory exposure in order to determine the percentage of moisture accumulation in the separate layers. Drying time is the time elapsed during the recovery phase until the weight of the single layers and the fabric combinations stabilized after becoming dry again. The amount of retained moisture within the single layers at the end of the sudatory exposure was measured as non-evaporated moisture accumulation. 15 The measurements were repeated three times. In addition, the moisture accumulation in the fabric combinations was determined once to get more information.
Statistical analyses were performed with SPSS version 22.0 (IBM, Armonk, USA). The assumptions of equal variance and normality of the data were examined using analysis of variance (ANOVA). For the homogeneous data, the univariate analysis of variance was used to analyze the effects of fiber and weave type on measured properties at the 95% confidence level. Otherwise, the Brown–Forsythe robust ANOVA was used, and for the data not normally distributed, the Kruskal–Wallis H test was used.
Results and discussion
In this study, we firstly examined the heat and moisture transfer properties (initial cooling, sustained cooling, moisture accumulation, post cooling intensity, and drying time) of single layer fabrics and then those of fabric combinations.
Single layer fabrics
Cooling effect due to perspiration and moisture accumulation in single layer fabrics. Data is shown as mean (standard deviation)
First letter, fabric type (U: Underwear; S: Shirt); Second letter, fiber type (P: Polyester; C: Cotton; CP: Cotton/polyester; B: Bamboo); Third letter, weave type of shirt (P: Plain; T: Twill; M: Matt).
The effect of weaving type on heat transfer and moisture management became obvious for cotton and cotton/polyester shirt fabrics. These yarns have a smoother surface than bamboo yarns (Table 1). Taking into account the initial cooling of cotton and cotton/polyester shirts, the plain fabrics had a higher temperature reduction than the matt and twill fabrics. Raja et al. 6 pointed out that sweat transfer rate was high mainly for fabrics consisting of more floats (i.e. more air space) and a lower thickness. In this study, even though the plain shirt fabrics had fewer floats, the temperature reduction of the plain fabrics was higher as compared to others at the onset of sweating because of low fabric thickness. Therefore, it is possible to say that fabric thickness has an influence on initial temperature reduction of the shirt fabrics used in this study. In addition, matt shirt fabrics had the highest post cooling intensity and drying time. The reason may be the high fabric thickness of the fabric. Crow and Osczevkski 17 reported that the amount of water held by a fabric was associated with fabric thickness, and the drying process of thick fabric took longer owing to the amount of water in the fabric. As can be seen in Table 3, the bamboo shirt fabrics had the highest moisture accumulation and the highest drying time values compared to other fabrics. This may be due to the superior water absorption ability of bamboo fibers. 18 Thus, the bamboo shirt fabric will not be advantageous for office conditions. The cotton/polyester shirt layer fabric with plain weave type can be recommended for office wear if the shirt is worn as a single layer, that is, without underwear.
Combinations of underwear and shirt fabrics
Evaporative cooling highly affects surface temperature and is an indicator of how effective sweat can be evaporated temporally and spatially.
7
It is influenced by various clothing properties (e.g. fiber type, fabric thickness, multi-layer system), as well as environmental conditions.19,20 Figure 3 shows initial cooling values of the combinations consisting of the shirt and underwear fabrics. In office conditions, immediate cooling is preferable in the case of too warm ambient conditions. High initial cooling values indicate an immediate cooling effect. The combinations of polyester underwear–cotton/polyester plain shirt and polyester underwear–cotton plain shirt had the highest initial cooling of all the samples (Figure 3). The reason may be because of low fabric thickness of plain shirt fabrics. A decrease in fabric thickness leads to an increase in cooling efficiency. The finding was supported by Wang et al.
21
Besides, examining the combinations with polyester underwear in Figure 3, the weave type and the fiber type of the outer layer had significant effects on initial cooling values (p<0.05).
Initial cooling of the underwear–shirt fabric combinations. Data is shown as mean values with error bars indicating 1 standard error.
In contrary to the single layer polyester underwear fabrics, the combinations with polyester underwear generally had a low cooling effect due to the second layer. Sweat can be transferred from a hydrophobic inner layer to a hydrophilic outer layer through wicking. 22 However, in order to wick water from one layer to the other layers, a sufficient amount of water must be absorbed in the capillaries of the inner layer, which are formed between fibers and yarns.23,24 The sweating rate applied during the sudatory phase might not provide sufficient moisture to induce moisture transport from the capillaries of the polyester underwear to the outer layer at the onset of sweating. Vapor diffusion, which is a mechanism for moisture transfer between the layers at low moisture levels, may only take place between polyester underwear and outer layers. 23 As can be seen in Figure 3, the combinations with polyester underwear and bamboo shirt fabrics had the lowest initial cooling values. This could be related to the higher hairiness of bamboo yarns (Table 1). This might cause low porosity and low air permeability, which prevent the occurrence of liquid transport between the layers.25–27 The initial cooling of the combinations of cotton/polyester underwear was generally higher than the cotton underwear combinations. During sudatory exposure, water absorbed by cotton/polyester underwear may be transported faster to the shirt, thanks to the wicking ability of hydrophobic polyester fibers in its structure, and finally evaporated. Regarding the initial cooling of combinations with cotton/polyester underwear, no statistically significant changes were found according to weave type of shirt layer, whilst the fiber of shirt layer had a significant effect on initial cooling. In addition, there was no statistically significant difference among initial cooling values of the cotton underwear combinations with regard to weave type and fiber type of the shirt layer (p < 0.05).
The results of sustained cooling are shown in Figure 4. As there is no continuous intensive sweating during office work, the amount of accumulated moisture which leads to cooling is low, and it should be evaporated immediately. Thus, a low sustained cooling value is advantageous for office wear comfort, contrary to initial cooling. The combinations of polyester underwear had the lowest sustained cooling of all the samples. Although the combinations of cotton underwear generally had higher sustained cooling as compared to those of the cotton/polyester underwear in Figure 4, no statistically significant difference was observed between the sustained cooling values of these combinations, similar to the results for single layers (p < 0.05). The sustained cooling of cotton underwear fabrics might be higher owing to moisture accumulation. If the equilibrium between microclimate and absorbed water by fibers occurs, diffused water vapor can transfer to the environment without the formation of moisture accumulation. Nevertheless, the increase in vapor concentration within the fibrous system and moisture saturation may cause moisture accumulation. The increase in moisture in the fabric layers facilitates conductive heat transfer through the fabric and reduces its thermal resistance. This finally leads to a surface temperature reduction.
28
The fiber type and weave type of the shirt layer had no statistically significant effect on the sustained cooling of the combinations with polyester, cotton/polyester, and cotton underwear fabrics, separately (p < 0.05). During sudatory exposure, the combinations of polyester underwear–cotton/polyester plain shirt and polyester underwear–cotton plain shirt were the combinations with the highest cooling performance, since they had high initial cooling and low sustained cooling values.
Sustained cooling of the underwear–shirt fabric combinations. Data is shown as mean values with error bars indicating 1 standard error.
The percentage of moisture accumulation on each of the layers of the fabric combinations (the values were calculated according to the assumption of sample weight of 100 g)
The maximal temperature reduction for the fabric combinations during the recovery period is illustrated in Figure 5. A low reduction in temperature during recovery is preferable in order to maintain office wear comfort. It was observed that the combinations of polyester underwear with bamboo twill and bamboo matt shirt fabrics had the highest post cooling intensity of all the samples. This could be because of the hairiness of the bamboo yarns and the high fabric thickness of the twill and matt fabrics. Possibly, water moved slowly from polyester underwear to bamboo shirt through wicking and vapor diffusion due to hairiness of the bamboo yarns during sweating. Even if some water evaporates through the fabric combinations which had low initial cooling and low sustained cooling properties, the remaining moisture may be accumulated in the bamboo shirt fabrics, which have good water absorbency capability. This accumulated water can continue to evaporate in the recovery phase, which leads to undesired cooling after the sudatory exposure. Thus, the combinations of polyester underwear with bamboo twill and bamboo matt shirt fabrics do not seem to be suitable for office wear application.
Post cooling intensity of the underwear-shirt fabric combinations. Data is shown as mean values with error bars indicating 1 standard error.
As illustrated in Figure 5, the maximal temperature reduction of the combinations with cotton underwear was generally high during recovery due to on-going moisture transport. Statistical analysis revealed that weave type and fiber of the shirt fabrics had no significant effect on post cooling intensity of the combinations of cotton underwear and of cotton/polyester underwear. Regarding the combinations with polyester underwear, plain shirt fabrics had lower temperature reduction than matt and twill fabrics because of the fact that they removed the absorbed water during sudatory exposure due to low fabric thickness. There were statistically significant differences among the combinations with polyester underwear according to weave type of and fiber type of the shirt fabrics (p < 0.05).
The transportation time of sweat from skin to environment is a quantitative indicator of drying behavior of clothing, and the amount of absorbed sweat of clothing, pore structures, and fabric thickness can dictate drying time.
31
The drying time of the fabric combinations is presented in Figure 6. It was observed that the combinations of polyester underwear and cotton/polyester shirt had the lowest drying time values among all the combinations. This may be due to hydrophobic polyester fibers within the fabrics. As hydrophobic fibers have low water absorbency, they can dry faster compared to hydrophilic fibers.
32
The drying time of the combinations with bamboo shirt fabrics was generally higher than that of the combinations of cotton shirt fabrics and cotton/polyester shirt fabrics, due to the outstanding water absorbency of bamboo fibers. The fiber type of the shirt layer was statistically significant for drying time of the combinations with polyester underwear and with cotton/polyester underwear, whereas there was no significant effect of weave type of the shirt layer (p < 0.05). The combinations of bamboo shirt and polyester underwear are not suitable for office wear during recovery. On the other hand, the combination of polyester underwear and cotton/polyester shirt, which had low drying time and low post cooling intensity, provides an advantage for office wear.
Drying time of the underwear–shirt fabric combinations. Data is shown as mean values with error bars indicating 1 standard error.
General overview of the findings
First letter, fabric type (U: Underwear; S: Shirt); second letter, fiber type (P: Polyester; C: Cotton; CP: Cotton/polyester; B: Bamboo); third letter, weave type of shirt (P: Plain; T: Twill; M: Matt).
Conclusions
The objective of this study was to investigate the effects of fiber and weave type on the heat and moisture transport properties of underwear–shirt fabric combinations to determine the combinations with the highest cooling performance with regard to the thermophysiological comfort of the office occupants.
The weave type of shirt fabrics did not have significant effects on the measured properties for combinations with cotton and with cotton/polyester underwear fabrics. Cooling properties of the combinations with polyester underwear showed a statistically significant difference depending upon the weave type of the outer layer and fabric thickness. The percentage of moisture accumulation in the shirt fabrics of office wear combinations was directly related to the fiber type of underwear fabrics. The fiber type of the outer layer affected significantly initial cooling and drying time of the combinations with underwear consisting of hydrophobic fibers (i.e. polyester and cotton/polyester fabrics). However, when cotton underwear fabric was used as the inner layer, the effect of the properties of the outer layer fabric was not statistically significant on heat and mass transfer. Therefore, office occupants may choose suitable shirt materials if they wear underwear including hydrophobic fibers in terms of providing clothing comfort inside office building without air conditioning systems. Having all these factors in mind, the fabric combination of polyester underwear–cotton polyester twill shirt was the most advantageous combination of all the samples used in the study because of its high cooling effect during the simulated activity and its fast drying time during recovery.
The findings of this study contribute to the basic understanding of the effects of fabric properties on the thermophysiological comfort properties of office fabric combinations. As an additional aspect, various knitted structures should be applied for underwear fabrics in order to investigate the effect of the water vapor permeability of underwear on thermophysiological comfort. Furthermore, future studies are required to evaluate the coupled heat and moisture transport of the combinations considering air gaps between the skin and the fabric and between the fabrics, as well as the application of realistic environmental office conditions, to be able to make suggestions for real use conditions.
Footnotes
Declaration of conflicting interests
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by The Scientific and Technical Research Council of Turkey (TÜBİTAK) – 2214-A Research Fellowship Program.