Thermochromic Behaviour and Comfort Properties of Printed Woven Fabric

Abstract

Thermochromic materials have attracted interest in textile applications, particularly in printing and dyeing processes. However, their thermochromic properties and impact on fabric comfort remain underexplored. This study aimed to investigate the thermochromic properties of printed fabrics with green-to-brown transitions and evaluates their comfort attributes. In the present study, a thermochromic dye paste was applied to nylon/cotton medium-weight fabric via screen printing process. The brown pigment paste was applied first, followed by the thermochromic olive green dye. The printed fabrics were tested for thermochromism, morphology, Fourier Transform Infrared Spectroscopy (FTIR), and comfort properties. Comfort properties were assessed via air permeability, water vapour permeability, and moisture management tests. The results show reversible colour changes from green (25 °C) to brown (40 °C), with increasing lightness (L*) and shifting green–red coordinates (−a*). The scanning electron microscopy (SEM) confirmed uniform dye dispersion, and the FTIR validated the presence of thermochromic pigments. The printed fabrics showed a reduction in air permeability from 40.2 mm/s to 0 mm/s, while water vapour permeability decreased by 62.50% compared to the pristine fabric due to the coating layers. The overall moisture management properties of the printed fabric remained similar to those of the unprinted fabric, with a grade of 1. These findings highlight the potential of thermochromic textiles for adaptive camouflage, particularly in military uniforms, contributing to the advancement of intelligent textiles with enhanced thermal responsiveness.

1. Introduction

Thermochromy can be defined as colour change that occurs when a material is heated or cooled [1,2]. In principle, thermochromic transition happens when the molecular structure typically changes while heating the thermochromic colorant above room temperature [3]. The exceptional properties of the thermochromic colorant make it a promising candidate for application on fabric materials.

Several studies have explored the incorporation of thermochromic colorants into fabrics. The most commonly used application method is screen printing, which enables controlled deposition of thermochromic dye pastes onto textile surfaces. Ajeeb et al. reported that thermochromic dye paste is typically formulated by combining leuco thermochromic dye, acrylic binder, and an emulsion thickener, and applied via screen printing at specific temperatures. The printed fabric exhibited a colour transition with increasing temperature, shifting from orange at 37 °C to pink at 38 °C, and ultimately becoming colourless at 41 °C. Among the different fabric compositions, polyester exhibited the highest thermochromic response compared to cotton and wool due to its thermal properties [4]. Similarly, Vikovà and Pechovà investigated the use of thermochromic inks in adaptive camouflage fabrics for military uniforms. Their study employed screen printing with four distinct colour variations (dark brown, light brown, light green, and dark green), where the thermochromic ink exhibited colour transition temperatures ranging from 30 °C to 39 °C [5]. Borhan developed thermochromic printing paste formulas in olive green, black, and light brown specifically for Malaysia’s combat fabric [6]. Despite these advancements, the thermochromic behaviour of printed fabrics remains underexplored, leaving a significant knowledge gap. In addition, this study introduced a reversible green-to-brown colour transition, which is distinct and specifically functional for camouflage purposes. In contrast, previous studies primarily focused on colour changes from one hue to colourless or to lighter shades of the original colour.

The thermochromic mechanism involves a phase transition in both molecular structure and colour when exposed to heat. In Figure 1, the colour change is reversible and occurs in two stages: (1) during heating, the material shifts from Colour-A (Green) to Colour-B (White or colourless), and (2) during cooling, it reverts from Colour-B back to Colour-A, depending on the temperature range. Throughout a multi-phase heating process, the thermochromic colorant undergoes transformations in both its molecular and geometrical structures. When heated above room temperature, the molecular configuration of the dye changes, resulting in a visible colour shift. As the temperature decreases, the molecular structure returns to its original state, restoring the initial colour.

Furthermore, the understanding of the comfort properties of fabric printed with thermochromic dyes is insufficiently understood. Comfort in textiles refers to the ability of a garment to maintain the body’s thermal equilibrium under varying environmental conditions [7]. Key comfort properties, such as moisture management, water vapour permeability, and air permeability are assessed using standard testing methods. The moisture management test evaluates the ability of a fabric to transport moisture across its surfaces [8], while the water vapour permeability test measures the diffusion rate of water vapour through the fabric at specified temperature and humidity conditions [9]. The air permeability test determines the rate of airflow through the fabric when subjected to a specified pressure difference [10]. Demirbağ-Genç et al. reported that the application of thermochromic microcapsules exhibited a decrease in both air and water vapour permeability of the fabric, along with poor moisture management properties [11]. Ajeeb et al. also observed a similar trend, where the thermochromic pigment application affected the air permeability of printed fabrics [4]. These findings indicate that the application of thermochromic on fabric may affect the comfort-related properties of textiles. The reduction in air and water vapour permeability, as well as moisture management, is largely attributed to the presence of binders and the aggregation of pigments on the fabric surface. Therefore, an investigation into the comfort performance of thermochromic-printed fabrics is essential in the current study.

In this research, thermochromic dye pastes were applied to blended nylon/cotton medium-weight fabrics to develop a functional thermochromic printed fabric. The findings of the study offer valuable contributions, particularly in military applications, by enhancing the fabric camouflage effects.

2. Materials and Methods

2.1. Materials

Materials used in this study were medium-weight blended nylon/cotton woven fabrics, pigment, print paste from BRITEX™ (Lumico Colors Creation, Kuala Lumpur, Malaysia), thermochromic dye (Kolortek®, Kolortek Co., Ltd., Huaian, China), and acrylic binder (Liquitex®, Colart International Ltd., London, UK). The properties of the medium-weight woven fabrics are tabulated in

Table 1. The properties of nylon/cotton medium-weight woven fabrics supplied by STRIDE.

	80% Cotton/20% Nylon
Weight of the fabric	222.78 g/m2
Fabric structure	Twill 2/2
Fabric density	140 warp/inch × 79 weft/inch
Fabric thickness	0.45 mm

.

2.2. Thermochromic Printing of Fabrics

The optimal mixture to produce brown pigment paste and thermochromic olive green dye paste has been described in our previous studies [12,13]. To produce the brown printing paste, 3% red, 34% yellow, and 63% black pigments were added into the BRITEX™ (Lumico Colors Creation, Kuala Lumpur, Malaysia) acrylic-based binder. Meanwhile, the thermochromic olive green paste was prepared by mixing 47% green, 45% yellow, and 8% black dyes into the Liquitex® (Colart International Ltd., London, UK) acrylic binder. In the current study, an A3-sized screen-printing frame with a mesh size of 77 threads per inch (TPI) was used. Using the screen-printing frame, the brown pigment paste was applied first on the medium-weight woven fabric’s surface, allowed to dry in the oven for 5 min at 100 °C, and cured at 150 °C for 3 min without washing off. Then, the following step was printing the thermochromic olive green dye paste on the same fabric. The printed fabrics were air-dried for 24 h at room temperature, as suggested in the binder instructions. The resultant printed fabric is shown in Figure 2. The pristine nylon/cotton and printed nylon/cotton woven fabric samples were then tested for thermochromism, morphological, FTIR analysis, and comfort, such as air permeability test, water vapour permeability test, and moisture management test.

2.3. Thermochromism Assessing of Printed Woven Fabric

The thermochromism of the fabric samples was assessed using the International Commission on Illumination (CIE) L* a* b* colour space. CIELAB space is a standard colour difference formula widely used to present colours, as shown in Figure 3. The L* indicates the lightness and darkness of the material, scaled from 0 (perfect black) to 100 (perfect white). A higher L* value describes a lighter hue, and a low L* value describes a darker hue of the dyed or printed material. On the other hand, a* is the red or green coordinate. A dyed or printed material with a positive a* value indicates red and a negative a* value indicates green. Other than that, b* is a yellow or blue coordinate. The value of b* positive indicates yellow, while b* negative indicates blue [14,15]. Figure 3 displays the three-dimensional of CIELAB colour space.

In the present study, the thermochromic printed woven fabric was heated within temperatures range from 25 °C to 40 °C, reflecting the typical temperatures of equatorial regions, such as Malaysia. A hot plate with a tile on it was used. The fabric was placed on top of the tile. When the hot plate was heated, the tile also heated up. Hence, the heat was transferred to the fabric above it. CIE L*a*b* values were taken for each temperature increase from 25 °C until reaching 40 °C using a portable colourimeter (CS-10 digital handheld colourimeter, Shenzhen Caidawei Technology Co., Ltd., Shenzhen, China). This method was carried out to see the colour change at each temperature and to test the reversible characteristics of the green–brown thermochromic camouflage fabric. The fabric colour changes were observed using a polarised optical microscope with hot stage (SOPTOP-CX40M, Sunny Group, Ningbo, China).

2.4. Morphological Analysis Using SEM

The morphological structures of the pristine nylon/cotton and printed nylon/cotton woven fabric samples were analyzed using scanning electron microscopy (SEM) (Hitachi TM3000, Hitachi High-Technologies Corporation, Tokyo, Japan). This SEM analysis would show the dispersion of the brown pigment paste and the thermochromic dye paste on the fabrics. The printed fabrics and pristine fabric samples were cut with a size of 1 cm × 1 cm square. Prior to the SEM analysis, a gold layer was sputter-deposited onto each sample. The study used the magnification of 1000× with a constant acceleration voltage of 15-kV.

2.5. FTIR Analysis

The Fourier Transform Infrared Spectroscopy (FTIR) analysis was conducted using the ATR-FTIR (Attenuated Total Reflectance Fourier Transform Infrared) (Perkin Elmer, Hopkinton, MA, USA) to identify the chemical functional group present in the pristine nylon/cotton and thermochromic printed nylon/cotton woven fabric. The fabrics were cut with a square dimension of 1 cm × 1 cm and placed onto a diamond crystal. An appropriate pressure was given so the fabric sample had enough contact with the diamond crystal. The analysis was conducted within a wavenumber range of 4000–630 cm⁻¹, with each sample spectrum obtained as an average of 20 scans.

2.6. Air Permeability, Water Vapour Permeability, and Moisture Management Testing Analysis

Three types of comfort tests were carried out in the study: air permeability, water vapour permeability, and moisture management. The air permeability properties of the pristine nylon/cotton and thermochromic printed nylon/cotton woven fabrics were tested using MESDAN Air-Tronic, MESDAN S.p.A, Brescia, Italy, in compliance with the standard test method ASTM D737-04/2008 [16]. An air pressure of 100 Pascal (Pa) with an air volume of 10 litres was applied to the fabric surface with an area of 38 cm². The air permeability value was expressed as millimetres/second (mm/s). The average air permeability was calculated based on five tested samples for each fabric.

The fabric’s water vapour permeability (WVP) properties were tested using the SDL Atlas International Water Vapour Permeability tester (M261), SDL Atlas Ltd., Rock Hill, SC, USA, in accordance with the ISO8096 standard method [17]. The WVP tester has eight cups with water reservoirs, a standard permeable fabric cover, a sample holder ring, and a precision drive system. The fabric samples were placed on top of each cup filled with 60 mL of water, and subsequently, the samples were run for 5 h. The water weight loss in the cup through the fabric sample was measured, and the fabric’s water vapour permeability was calculated using Equation (1).

$${\mathrm{WVP} (\mathrm{g}/\mathrm{m}}^2/\mathrm{h})={\displaystyle \frac{24\times \mathrm{M}}{\mathrm{A}\times \mathrm{T}}},$$

(1)

where M is the mass loss (g), T is the period in hours, and A is the cup internal area (m²) [18].

The moisture management properties of both pristine nylon/cotton and thermochromic printed nylon/cotton woven fabrics were assessed using a Moisture Management Tester (MMT) (SDL Atlas, USA), following the American Association of Textile Chemists and Colorists (AATCC) 195-2009 standard test method [19]. A fabric sample measuring 80 mm × 80 mm was placed between the upper and lower sensors. Then, a saline solution was pumped onto the centre upper surface of the fabric to represent liquid perspiration. The fabric’s performance against moisture penetration can be concluded with the obtained overall moisture management capacity (OMMC) value, interpreted from 0 to 1. The grading scale is shown in

Table 2. Grading of the overall moisture management capacity (OMMC).

Index	Grade
OMMC	1	2	3	4	5
	0–0.2 (Poor)	0.2–0.4 (Fair)	0.4–0.6 (Good)	0.6–0.8 (Very Good)	>0.8 (Excellent)

, where 0–0.2 indicates “Poor”, 0.2–0.4 indicates “Fair”, 0.4–0.6 indicates “Good”, 0.6–0.8 indicates “Very Good”, and 0.8 and above shows “Excellent” moisture penetration.

3. Results

3.1. Thermochromism of the Printed Nylon/Cotton Medium-Weight Woven Fabrics

In the current study, the brown pigment and olive green thermochromic dye were printed on medium-weight woven fabrics made of nylon/cotton. The printed fabrics were then subjected to heating and cooling to observe the reversible green–brown colour change. Figure 4a–c demonstrates the thermochromism of the printed fabric. At room temperature of 25 °C, the thermochromic printed fabric appears green, as shown in Figure 4a. Upon reaching a temperature of 40 °C, the thermochromic printed fabric undergoes a colour change and exposes the brown colour of the base layer [Figure 4b]. This suggests that the thermochromic dye molecules in the printed fabric undergo phase transitions during heating and cooling. When the thermochromic colorant was heated above room temperature, the dye molecules underwent a change. Similar observations were also reported in previous studies [3,20,21]. Furthermore, when it was cooled to a specific temperature of 25 °C, it reverted to its original green colour [Figure 4c]. Therefore, it is proven that the printed fabric can reverse its colour from green to brown and vice versa in response to fluctuations in temperature.

The colour change in the printed fabrics at each temperature increment was analyzed by measuring the CIE L* a* b* values. Figure 5 displays the variations in lightness, hue, and intensity of the colour for the printed nylon/cotton medium-weight fabric. From the analysis, it shows that the L* value increases with each rise in temperature, as shown in Figure 5. This implies that the fabric becomes lighter in colour as the temperature rises. The L* value was initially expected to decrease as the temperature increased. This is because the lighter olive green thermochromic dye on the top layer would reveal the darker brown base layer. However, the opposite occurred due to the material utilised to produce olive green thermochromic dye paste. The olive green thermochromic dye was incorporated with a white Liquitex® binder to produce olive green thermochromic dye paste. Upon heating, only the olive green thermochromic dye underwent a colour change, transitioning to a colourless state, while the Liquitex® binder retained its white appearance. The binder on top of the dark brown layer had caused a noticeable alteration in the lightness and saturation of the dark brown colour on the bottom layer. Hence, the L* value increased from 34.19 to 39.07, as the temperature rose from 25 °C to 40 °C.

Figure 5 also displays the plotted data for the −a* value of the printed nylon/cotton fabric, represented by the green colour. As the temperature rose from 25 °C, the a* value gradually increased from −4.75 to −2.86 at 40 °C. This indicates that the a* value of the fabric remained in the green region, while gradually shifting toward the red region, revealing a lighter brown shade. In addition, the presence of the binder on the printed fabric had caused a slight modification in the hues and brightness of the brown colour.

As the temperature increased from 25 °C, the b* value of 4.04 remained consistent. At 32 °C, the b+ value gradually fluctuated from 3.82 to 2.26 as the temperature increased to 40 °C. Thus, the analysis reveals that the +b* value of the printed nylon/cotton fabric indicates the presence of yellow hue in the fabric samples.

3.2. Morphological Structures of Thermochromic Camouflage Fabrics

This study evaluated the morphological structure of nylon/cotton and printed nylon/cotton medium-weight fabrics using scanning electron microscopy at a magnification of 1000×, as illustrated in Figure 6a,b.

In Figure 6a, the morphological structures of the nylon/cotton medium-weight fabric consist of fibres arranged in closely packed, parallel layers. It comprises elongated cylindrical fibres and fibres with a twisted ribbon-like structure, approximately 12.60 micrometres in size. This structure demonstrates a blended nylon/cotton fabric. After a printing process, the fabric surface morphology had significant changes, making the fibres no longer visible, as illustrated in Figure 6b. The surface morphology appears identical to that presented in the study by [6]. A round-shaped particles of the thermochromic dye and several holes have been observed on the fabric surface. It shows that the brown pigment paste and olive green thermochromic dye paste deposition are evident on the surface of uncoated medium-weight fabrics.

3.3. FTIR Analysis

In Figure 7, the FTIR spectra of the pristine fabric (a) and the thermochromic printed fabric (b) are compared. The pristine nylon-cotton blended fabric exhibits a sharp absorption peak at 3294.10 cm⁻¹, corresponding to the O–H stretching vibration of the cotton component. A broad band around 3300 cm⁻¹, along with peaks at 1633.10 cm⁻¹ and 1461.36 cm⁻¹, are attributed to the N–H stretching, C=O stretching, and N–H bending vibrations, respectively, confirming the presence of amide and carbonyl groups characteristic of the nylon.

The printed sample (b), which was coated with two functional layers, a bottom layer of brown pigment with binder and a top layer of thermochromic dye with binder, exhibited additional absorption peaks. The bands observed in the 1200–1300 cm⁻¹ region are attributed to C–O and C–N stretching vibrations of the thermochromic dye in its leuco-form, which is phenols-based, aligning with several findings [22,23]. Furthermore, the broadening and slight shift in the N–H stretching peak (~3300 cm⁻¹) in the nylon component may result from hydrogen bonding interactions between the amide N–H groups in nylon and the acidic O–H groups of the phenols-based thermochromic dye. Phenolic compounds are known to form strong hydrogen bonds due to their highly acidic hydroxyl groups. In addition, the incorporation of the binder and brown pigment introduces new absorption bands at 1150 cm⁻¹ (C–O stretching) and 1724.72 cm⁻¹ (C=O stretching), corresponding to the acrylic-based binder system.

3.4. Air and Water Vapour Permeability Test of the Nylon/Cotton and Printed Nylon/Cotton Medium-Weight Woven Fabrics

The air permeability of nylon/cotton and printed nylon/cotton medium-weight fabrics had been evaluated. The corresponding results are presented in Figure 8. Based on Figure 8, the printed nylon/cotton medium-weight fabrics have an air permeability value of 0 mm/s. These data demonstrate that the thermochromic fabric materials produced in this study are impermeable to air. This is because the medium-weight fabrics have been printed with a layer of brown pigment paste and a layer of olive green thermochromic dye paste. The authors of [24] found that increasing the number of layers in textile printing reduces the textile’s air permeability. The thick layering in this study contributed to the impermeability of the thermochromic camouflage fabrics towards air. Furthermore, the air permeability of nylon/cotton fabrics is higher than that of the printed fabrics, measuring at 40.2 mm/s. These results align with the research conducted by several studies [25,26], which found that the air permeability of unprinted fabrics is greater than that of printed fabrics. This is because the print material covers the pores of the fabrics, hindering the free flow of air through the textile materials.

Figure 9 displays the water vapour permeability (WVP) of nylon/cotton and printed nylon/cotton medium-weight fabrics. The nylon/cotton fabrics have higher levels of water vapour permeability, with WVP values of 1419.33 g/m²/h, respectively. When the uncoated medium-weight fabrics were printed with brown pigment and olive green thermochromic dye paste, there was a significant drop in water vapour permeability (WVP), as much as 62.50%. The authors of [26] explained that the structure of the unprinted fabric was more porous than that of the printed fabric. In the current study, the brown pigment and the olive green thermochromic dye paste had covered the pores of the fabrics, causing the fabric to be less permeable to water vapour.

3.5. Moisture Management Test of the Nylon/Cotton and Printed Nylon/Cotton Medium-Weight Fabrics

Figure 10 depicts the results of the moisture management test (MMT) for nylon/cotton and printed nylon/cotton medium-weight fabrics. The overall moisture management capability (OMMC) values can be used to determine the performance of the fabric against moisture penetration. Based on the results from the experimental works, it was observed that all the tested fabrics provided an OMMC grade of 1. This suggests that the application of brown pigment and thermochromic olive green dye paste through the printing process resulted in similar overall moisture management properties to that of the unprinted fabrics. This also indicates that the presence of these colorants does not significantly alter the moisture-handling characteristics of the fabric. Based on [27], OMMC grade 1 indicates that the fabrics exhibit poor moisture penetration and possess waterproof properties.

4. Conclusions

This study demonstrates the thermochromic behavior of printed nylon/cotton medium-weight woven fabrics where a reversible green-to-brown colour change was observed upon heating and cooling. The colour transition was analyzed through CIE Lab* values, confirming that the thermochromic dye molecules underwent phase transitions affecting the fabric’s hue and brightness. The morphological analysis further revealed significant surface modifications after printing, with the presence of thermochromic dye and pigment layers altering the fabric structure. The FTIR analysis confirmed the successful coating of the fabric with thermochromic materials, as indicated by characteristic absorption peaks matching those of thermochromic microcapsules. Moreover, the current study evaluated the functional properties of the thermochromic camouflage fabric, including air permeability, water vapour permeability, and moisture management. The results show that the printed fabrics had significantly reduced air and water vapour permeability due to the deposition of pigment and dye layers, which blocked fabric pores and restricted airflow. Similarly, the moisture management testing indicated poor moisture penetration, classifying the fabric as waterproof. These thermochromic fabrics have potential applications in military camouflage and adaptive textiles. Compared to conventional non-responsive camouflage materials, the thermochromic offers enhanced environmental adaptability. However, the use of thermochromic dyes may affect the overall production costs due to their high material cost and the need for specialised processing methods. While the thermochromic camouflage fabric successfully demonstrated its colour-changing properties, future studies should focus on improving air permeability, water vapour permeability, and moisture management to enhance overall fabric performance. A potential solution may involve incorporating microencapsulated thermochromic pigments into the fabric to minimize pore blockage and enhance the breathability of the printed material.