1 Introduction

In the last decades, increased attention is paid to comfort properties of textiles and garments. Clothing comfort, which can be defined as a state of well being and comfortable feeling, is very important, especially for next-to-skin garments. Consumers of textile and clothing products are becoming increasingly aware of the importance of comfort. With increasing demand for garment comfort, there are many studies related to the comfort properties of fabrics [1–7]. The comfort of textile materials has been the focus of many investigations since the concern for personal well-being and improving the quality of life started to become more significant. The comfort provided by clothing depends on several factors. Clothing comfort can be induced by thermal, pressure-related, and tactile properties, etc. Among these factors affecting clothing comfort, the thermal factor is the most decisive one affecting the comfort level. Thermal comfort is an important aspect of functional fabric. Thermal comfort is an element of clothing utility comfort. It is defined as a state of satisfaction with the thermal conditions of the environment. Providing thermal comfort is considered as one of the most important requirements for a clothing system. Nowadays, more and more people are interested in thermal-comfort fabric.

With the development of textile technology, the requirement for fabrics involves not only mechanical and dimensional properties but also thermal comfort properties. For this reason, it becomes necessary to benefit from the special characteristics of fibers for high levels of comfort by using fiber blends. The blending of different types of fibers is a widely practiced means of not only enhancing the performance but also the aesthetic qualities of textile fabric. Blended yarns made from natural and man-made fibers have the particular advantage of successfully combining the good properties of both fiber components, such as comfort of wear with easy-care properties. The major purpose of blending fibers in textiles is to provide better balance of mechanical and comfort properties. These advantages also permit an increased variety of products to be made, yielding a stronger marketing advantage [8].

Regenerated cellulose fibers are gaining importance in the textile industry with the increase in human population and the advances in technology. The bamboos constitute an economically important group of plants, especially in Asia. Bamboo fibers possess many excellent properties when used as textile materials such as high tenacity, excellent thermal conductivity, resistant to bacteria, and high water and perspiration adsorption. Bamboo cloth can quickly absorb and evaporate human sweat. Bamboo cloth is an indulgence with its wonderful silky softness.

Many researchers have conducted studies to evaluate and analyze thermal comfort. It is thus possible to find papers in the literature focused on the thermal comfort properties of textile fabrics.They examined the effects of the fiber type and the fabric composition on thermal comfort.

Bartkowiak and Szucht [9] studied a two-layer fabric containing both hydrophilic and hydrophobic layers to improve the comfort of clothing. Su et al. [10] pointed out that if the amount of cotton fiber in the cotton/PES blend increases, the water absorption of the structure also increases but the drying rate of the yarn decreases. Reagan and Villasi [11] have investigated that the thermal transmittance characteristics of fabric is based on the mass, thickness, density, fiber content, and construction characteristics. Mao et al. [12] found that thermal properties of spacer knitted fabrics depend on fiber and fabric thickness.

Shoshani and Shaltiel [13] noted that thermal insulation increases with decreases in the density of fabric. Milenkovic et al. [14] reviewed that fabric thickness, enclosed still air, and external air movement are the major factors that affect the heat transfer through fabric. Greyson [15] and Havenith [16] noted that heat and water-vapor resistance increases with the increment of material thickness and air entrapped in the fabric. Dhinakaran et al. [17] noted that the comfort characteristics of fabrics mainly depend on the structure, types of raw materials used, mass, moisture absorption, heat transmission, and skin perception.

Stankovic et al. [18] compared the thermal properties of plain knitted fabrics made from different natural and regenerated cellulosic fibers. Behera and Mishra [19] found that the thermal insulation property of worsted fabrics is largely dependent on the areal density.

Knight et al. [20] studied the thermal transmittance characteristics of fabric based on the mass, thickness, density, fiber content, and construction characteristics. The thinnest fabrics exhibited the highest thermal transmittance coefficients. Barker et al. [21] have proven that knits made with finer diameter polyester fibers had the highest water-vapor transmission rate. Karahan et al. [22] stated that natural bamboo fiber provided functional features to textile products due to its excellent moisture absorption, enabling fast evaporation, as well as its antibacterial properties. Li et al. [23] showed that the heat transfer process, which is influenced by fabric thickness and porosity, significantly impacts moisture transport processes.

This study is focused on the thermal comfort parameters of knitted fabrics made from 100 % cotton yarn, 100 % bamboo yarn, and cotton/bamboo blended yarns. For this aim, we focus on air permeability, water-vapor permeability, thermal conductivity, and thermal resistance behavior of these fabrics.

2 Materials and Method

In this research work, three different yarn linear densities of bamboo, cotton, and bamboo/cotton blended yarns \((20^{\mathrm{s}},\; 25^{\mathrm{s}},\; 30^{\mathrm{s}}\) Ne) were selected. The bamboo/cotton blended yarns were fabricated by using a single jersey knitting machine, Model MV4, gauge 24 GG, diameter 23 in (58 cm), speed 30 rpm, feeders 74, and number of needles 1728; the ambient knitting-room atmosphere had a humidity of 65 % and a temperature of \((30 \pm 2)~{^\circ }\text{ C}\). Samples were produced with three different loop-length values of 3.1 mm, 2.9 mm, and 2.7 mm.

The fabric structural and physical properties such as weight (mass per unit area), and thickness were evaluated (Table 1). The thermal comfort properties (thermal conductivity, thermal resistance, water-vapor permeability, and air permeability) of the fabrics were also evaluated. The Alambeta instrument was used to measure the thermal conductivity, fabric thickness, and thermal resistance; the water-vapor permeability was measured on a Permetest instrument working on the simulated skin principle as recommended in ISO 11092; the fabric air permeability was measured according to TS 391 EN ISO 9237 using Tester FX3300. All measurements were performed under standard atmospheric conditions, and they were repeated five times for each of the knitted fabrics. From five readings the averages were calculated.

Table 1 Structural and thermal comfort properties of the single jersey knitted fabrics
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3 Results and Discussion

3.1 Air Permeability

From the results presented in Fig. 1, it can be stated that the highest air permeability value was obtained for 100 % bamboo fabrics. In our previous study the bamboo yarns have a lower diameter than the equivalent cotton yarns [8]. The yarn linear density in combination with the large loop length results, as expected, in an open structure. The obvious decrease in fabric thickness and weight may also be clearly seen in Table 1. The results showed that the fabric thickness had a significant effect on the air permeability values of the bamboo blended fabrics, since the air permeability tended to decrease as the thickness increased, independent of the loop length. The thickness and mass per square meter of bamboo-blended fabrics are also lower as compared to those of cotton fabrics made from the same yarn count. The lower thickness and mass per square meter also facilitate the passage of air through the fabric. The results show that for a fabric of a given composition, the air permeability increases as the loop length increases. Also, the air permeability increases with the bamboo fiber content in the fabric, independent of the loop length. The air permeability of the 100 % bamboo fabric is around 200 % that of the cotton for all the loop lengths. It may be noted from Table 1 that the thickness and weight of bamboo-blended fabrics decrease with bamboo fiber content. They are also lower than those of the cotton fabrics made from yarn of the same count.

Fig. 1
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Response surface plot of air permeability of the single jersey knitted fabrics

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3.2 Thermal Conductivity and Thermal Resistance

Figure 2 shows a comparison of the thermal conductivity and thermal resistance results of the fabrics investigated.

Fig. 2
figure 2

Response surface plot of thermal conductivity and thermal resistance of the single jersey knitted fabrics

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The thermal resistance values of these fabrics were compared, and it was found that as the yarn gets finer, the thermal resistance and thermal conductivity decrease. In fact, the general expectation was to register an inverse relationship between the thermal resistance and thermal conductivity (\(R_{ct}=h/ \lambda ;\; R_{ct}\): thermal resistance, \(h\): thickness, \(\lambda \): thermal conductivity).

However, the test results revealed that as the thermal resistance decreases, the thermal conductivity decreases as well. This contradiction might be explained by the fabric thickness. When a finer yarn is used in the fabric, the yarn diameter and therefore the fabric thickness decrease. If the amount of the decrease in thickness is more than the amount of the decrease in thermal conductivity, the thermal resistance also decreases. It may be observed that the thermal conductivity and thermal resistance values of 100 % bamboo fabrics are lower than those of 100 % cotton fabrics for all the constituent yarn linear densities, with the cotton/bamboo blended fabrics showing intermediate values. Although bamboo fiber is well known for its comfort properties, the properties are not as good as those for cotton, which would therefore still be regarded as the ‘cooler’ fiber. Moreover, the known morphological differences between the two fibers and the fact that the bamboo yarn is finer than a cotton yarn of comparable linear density tend to mask the inverse relationship between the thermal conductivity and thermal resistance. It is borne out from Fig. 2 and the data in Table 1 that the finer the constituent yarn in a fabric, the lower is the fabric thermal conductivity. The greater amount of entrapped air in fabrics composed of finer yarn acts as a barrier to thermal transmittance. It is also evident that for any given type of fabric, the thermal resistance of fabric composed of finer yarn is generally higher. It is observed from Fig. 2 that as the linear density and loop length of bamboo fiber increases, the thermal conductivity of the knitted fabrics is reduced. For the same loop length, finer yarns show lower thermal conductivity.

3.3 Water-Vapor Permeability

Figure 3 illustrates the values of the water-vapor permeability with respect to the blend ratio, loop length, and linear density. As the yarn count increases, the relative water-vapor permeability value also increases significantly for bamboo and its blended fabrics. The fabrics produced from coarser yarns have a more porous structure. With the increase of porosity, the water-vapor permeability also increases as mentioned in previous papers. It may be seen that the water-vapor permeability increases with bamboo fiber content in the fabric. The water-vapor transmission due to diffusion may also be higher for the bamboo fabrics as the moisture regain of bamboo fiber is higher than that of cotton. The higher water-vapor permeability of bamboo blended fabrics can be attributed to the lower values of fabric mass per square meter and thickness, which facilitate the easy passage of the water vapor through the fabrics. As the yarn count is coarser, the fabric thickness and mass per unit area increase, resulting is less flow of water vapor across the fabric surface, and vice versa, if the yarn count is finer, the fabric thickness and mass per unit area decrease, resulting in a higher flow of water vapor.

Fig. 3
figure 3

Response surface plot of water-vapor permeability of the single jersey knitted fabrics

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4 Conclusions

The effect of blend proportion, linear density, and loop length on air permeability, thermal conductivity, thermal resistance, and water-vapor permeability was studied

  • It is observed that the thermal conductivity depends on the proportion of bamboo component in the yarn blend. The thermal conductivity of knitted fabrics is reduced as the proportion of bamboo fiber increases in the yarn. The lowest thermal conductivity is observed at 100 % bamboo with a finer linear density. It is observed that the parameters of air permeability, thermal resistance, water-vapor permeability, and thermal conductivity are significantly affected by the fiber blend ratios. An increase in the bamboo fiber content in the fabric affects the thermal properties.

  • The knitted fabrics made from bamboo-blended yarns have a lower thickness and a lower mass per square meter than the cotton fabrics. The water-vapor permeability and air permeability shows a concomitant increase as the proportion of bamboo fiber increases.

  • As the loop length increases, the thermal conductivity also increases independent of the fabric packing density. As far as the water-vapor permeability is concerned, the increase in loop length decreases the flow rate of water vapor because of the hindrance of air layers.

  • As the linear density of the yarn increases, the thermal conductivity decreases as more air is caught by fibers, and vice versa. In the case of a higher water-vapor permeability, the linear density of yarn and the thickness of the fabric increase and the flow rate will be less.