Investigating the Effects of PU-Based Back-Coating with Boric Acid and Titanium Dioxide Additives on Flame Retardancy Levels and Comfort Properties of 100% Cotton Denim Fabric

The technical specifications of the denim fabric utilized in this study are presented in Table 1. H₃BO₃ was purchased from Eti Maden (Turkey) commercially. RUCO-COAT PU 1130 (an anionic, water-based, aliphatic polyether polyurethane dispersion), supplied by Rudolf-Duraner (Turkey), was used as the PU binder. RUCO-COAT FX 8011 and RUCO-COAT TH 5020 were utilized as a crosslinker and thickener, respectively, obtained from Rudolf Duraner (Turkey). TiO₂ nanopowder (~20 nm primary particle size, 99.5% (w/w)) and methanol were obtained from Sigma–Aldrich (USA). The chemicals used were of analytical grade and used without any additional purification.

The procedures below were used for the preparation of back-coating formulations: (I)

Functional additives were converted into a dispersion form for use in the coating paste. To accomplish this, TiO₂ nanopowder was dispersed in methanol at a concentration of 40% (w/v), and aqueous dispersion of H₃BO₃ was prepared by mixing H₃BO₃ powder and distilled water at 60% (w/v) concentration. The resulting dispersions were sonicated for a period of 10 min using an ultrasonic probe (Bandelin Sonopuls HD 2070 (20 kHz), Bandelin Electronic GmbH & Co. KG, Berlin, Germany) that operated at 9 kHz frequency with a variable power up to 500 W.

The next part of the experiment involved the application of back-coating on denim fabrics using a laboratory type scraping-knife motorized coating machine, with a knife width of 300 mm and roller diameter of 40 mm (Figure 1). The back-coating paste was spread onto the sample fabric, which was fixed in a sample holder, using the back-coating machine at a speed of 5 m/min. The scraping knife angle was adjusted to +10°. The thickness of the back-coating paste applied to the fabric surface was assessed by modifying the distance between the fabric and the scraper using a feeler gauge (0.4 mm). Both denim fabric samples were coated on both sides. Upon coating, the samples were dried in an oven at 150 degrees for 10 minutes.

Fig. 1.

In this study, TiO₂ and H₃BO₃ were incorporated into PU as back coating materials to improve the thermophysiological comfort and flame retardancy properties of cotton denim fabric. The performance was evaluated by analysing the air permeability, water vapour permeability, thermal resistance, thermal absorptivity, and burning behaviour of the coated denim samples.

The air permeability of textile materials is commonly defined as the capacity of air-permeable fabric to allow the passage of air under specific and well-defined conditions [30]. The air permeability of the textiles was assessed using an SDL Atlas Air Permeability Tester (M021A) at a consistent pressure of 100 kPa over a test area of 20 cm², following the EN ISO 9237:1995 standard, with 10 repetitions. Air permeability, measured in millimetres per second [mm/s], represents the rate at which air passes through the fabric sample.

The water vapor transmission rates of the fabrics were assessed using an SDL Atlas Water Vapour Permeability Tester (M261) in accordance with ASTM E96-22, the water method, which outlines the standard test methods for the water vapour transmission of materials. Each denim sample underwent 3 tests following this standard procedure. A shallow cup containing approximately 50 ml of distilled water held a circular sample with a diameter of 74 mm, secured by clamping it in place. The assembly, including the cup and sample, was periodically weighed over the course of a day. The weightings determined the rate of vapour movement into the sample from the water in the controlled atmosphere. Calculation of the water vapour permeability (%) was then performed according to Equation (1), where WVP is the water vapour permeability (g/m²/day), G the weight change (g), t the time during which G occurred (h), and A is the test area (m²) [31]. The coefficient 24 in equation (1) helps to convert the rate into a daily measure, since the unit of water vapor transmission (g/m²/day) indicates how much water vapor passes through a square meter of fabric in a 24-hour period. (1) WVP=Gx24/t⋅A WVP = \left( {Gx24} \right)/\left( {t \cdot A} \right)

The thermal resistance and thermal absorptivity were assessed using an Alambeta instrument (Sensora, Liberec, Czech Republic), which adheres to ISO 8302 standards and features an upper measuring head designed to simulate human skin. The test involved assessing the heat flow through a sample of material positioned between two metal plates. The upper plate was heated to 35°C, simulating body temperature, while the lower plate maintained ambient temperature. The modified sample was inserted between the plates to allow heat flow through the fabric towards the aerogel layer. The plates were pressed against the sample with approximately 200 Pa of pressure. This test was conducted with 5 repetitions.

The vertical flame test, conducted using a flame control module based on the EN ISO 15025:2016 standard method, was applied to 3 samples in the warp direction and 3 samples in the weft direction for each fabric. The flame was directed towards the lower portion of the vertically placed specimen. Following a 10-second flame exposure, measurements were taken for the after-flame time and char length. To document the process, photographs of the samples were captured using a digital camera (IPhone 13, Apple). The LOI (Limiting Oxygen Index) was assessed by igniting 5 samples measuring 5×10 cm, following the guidelines outlined in ISO 4589-2. The flame was directed towards the upper edge of the samples within a glass tube filled with an O₂/N₂ mixture.

The breathability of clothing depends on body geometry, air velocity, fabric breathability, and air gaps between garment layers [30]. Air permeability is closely linked to the fabric’s structure, including weight, thickness, and pore structure, which are influenced by fiber type, yarn structure, weft density, and the weaving method. There is a strong correlation between air permeability and porosity, as larger pores increase air flow [32,33]. The decrease in the air permeability of cellulosic fabric obtained by PU coating application is an expected phenomenon, as the breathability of textile materials primarily depends on fabric porosity, including the cover factor and the structure of the fabric [21,22,34]. According to the air permeability results obtained, B/PU and BT/PU exhibit air permeability values that are, respectively, 54% and 61% lower than those of the untreated denim fabric (figure 2). In this investigation, it is observed that the application of a PU-based coating onto denim fabric tends to close the fabric’s structural pores. This results in the coated fabric being less porous and, therefore, less breathable. Moreover, the higher surface mass and thicker structure of coated denim fabrics also result in a decrease in air permeability values. Acceptable air permeability limits for denim fabrics can vary depending on factors such as intended use and the target market. For instance, denim fabrics with higher air resistance are deemed suitable for outerwear, as they offer enhanced thermal insulation, particularly in colder climates.

Fig. 2.

Water vapour resistance refers to a fabric’s capability to permit the transmission of moisture vapour through it. The water vapour permeability of fabrics is one of the main factors that determine comfort of the wearer [1]. As shown in Figure 3, the untreated denim fabric has a water vapour permeability (WVP) value of 1891 g/m²/day (Exposure conditions: 65% RH; 20°C. Air layer thickness: 10 mm). PU-based coating applications on the same fabric resulted in lower WVP values [34,35]. These outcomes are understandable given that thicker fabrics generally exhibit increased resistance to water/moisture vapour transmission [36]. There exists a significant difference in the WVP rates of B/PU and BT/PU fabrics (t-test P-value: 0.0073), which can be attributed to the greater thickness of BT/PU. Although B/PU and BT/PU exhibit decreases in water vapour permeability by 38% and 46%, respectively, they still maintain a level of permeability that can satisfy the wearer’s comfort where the minimum limit for the water vapor permeability value of existing commercial fabrics is ~450 g/m²/day [37].

Fig. 3.

The thickness of textile materials is the key factor in determining their thermal resistance, and there is a linear relationship between these two parameters [38]. The thicker the material, the greater its ability to protect against heat loss. The application of coatings resulted in elevated fabric thickness values, generating additional area and air within the fabric structure, consequently enhancing insulation properties. The literature includes studies examining fabrics that exhibit increased thickness and, consequently, enhanced thermal resistance as a result of coating applications [39,40,41]. Applying a thin layer of functional coating to fabric surfaces can modify thermal conductivity without significantly altering the fibrous structure, porosity, dimensions, or moisture evaporation properties. There are studies focusing on enhancing the thermal insulation capacity of fabrics by reducing their thermal conductivity through the application of coatings [42]. Results of thermal conductivity and thermal resistance obtained for samples are presented in figure 4. The results indicate that the content of H₃BO₃ in the coating layer enhanced the thermal resistance by reducing the thermal conductivity value of the sample fabric. The presence of TiO₂ nanoparticles does not exert a significant influence on the thermophysical properties; namely the thermal conductivity and thermal resistance; of the cotton denim fabric. (The t-test P value for the thermal conductivity results of B/PU and BT/PU is 0.1773).

Fig. 4.

Thermal absorptivity measures how fabrics feel warm or cool and is linked to surface properties. It is a material’s ability to insulate and is affected by conductivity, density, and specific heat capacity [43]. Higher thermal absorptivity ratings indicate that the fabric will feel cool to touch for very short periods of time [44]. Figure 5 shows that untreated denim fabric exhibited the highest thermal absorptivity levels, offering the coolest sensation upon initial contact with the skin. B/PU and BT/PU fabrics, on the other hand, yielded notably lower values, indicating a warmer sensation. There is no statistically significant difference between the thermal absorptivity and thickness values of B/PU and BT/PU (t-test P values are 0.305 and 0.939, respectively). The reduced thermal absorption performance of B/PU and BT/PU can be attributed to the lower thermal conductivity of coated fabrics.

Fig. 5.

It is widely recognized within the industry that each specific flammability test presents unique challenges in achieving a “pass” result. In the vertical flame test based on the EN ISO 15025:2016 standard method, a flame is brought into contact with the edge of the fabric instead of its face. The edge test is particularly challenging to pass because both sides of the fabric are burned simultaneously, making it very difficult to achieve the charring effect. Only highly effective flame retardant treatments can pass this test, which is more commonly applied to work-wear fabrics (over 200 g/m²) [45]. Therefore, a double-sided coating application was performed to enhance the flame resistance of both sides of the resulting fabric.

The flame retardant properties of both untreated cotton and back-coated cotton fabrics were assessed through the vertical flame test, the results of which are presented in Table 3. Both coating applications led to a decrease in after-flame time. Similarly, as observed in Figure 6, in terms of ash content, both coating applications resulted in lower char lengths. Specifically, the after-flame time of BT/PU was realized 30 seconds shorter than that of the untreated fabric, accompanied by an ash length of 45 mm. This phenomenon suggests that the incorporation of TiO₂ additive into the coating formulation (BT/PU) enhances the fabric’s resistance to ignition to a greater degree compared to the H₃BO₃ containing coating formula (B/PU).

Fig. 6.

A fabric with an LOI value below 20.95% is classified as flammable, while one falling within the range of 21–28% is categorized as slow-burning, and if it falls within the range of 28–100%, it is classified as self-extinguishing [46]. According to Table 3, the LOI value of UDF is 21.3%, indicating the fabric’s flammable nature. The LOI value of the B/PU increased by 18% compared to the untreated fabric, reaching 25.1%. The LOI value of the BT/PU fabric increased by 26.9% compared to the untreated fabric, demonstrating the best fire resistance performance and reaching a value of 27%, approaching the self-extinguishing threshold (28%).

Borates exhibit excellent flame retardant properties due to the formation of impermeable glass coatings when they undergo thermal degradation. These glass coatings develop on the material’s surface and contribute to the intumescent effect by creating a barrier that excludes oxygen and hinders the spread of combustion. Additionally, through endothermic decomposition, the release of water of hydration not only dilutes the flame but also absorbs the thermal energy, thus providing a cooling effect [14,47]. TiO₂, a metal oxide, has a high melting temperature during thermal decomposition. The oxides coat the surface of cotton fibers, creating an efficient physical barrier that facilitates the transformation of cotton fibers into carbon through dehydration. This process isolates the cotton fibers from external factors and restricts the spread of flames [8]. TiO₂ can also function as a heat absorber during the flame retardant reaction, aiming to interrupt the combustion cycle of cellulosic fibers [48]. In a comparable study, Bentis et al. [18] investigated the modification of cotton fabric surfaces using the solgel method with titanium-based sol and boric acid as an additive, finding that the coated fabrics exhibited good flame retardancy properties, and thermal studies by thermogravimetric analysis and differential scanning calorimetry demonstrated increased thermal stability due to the synergistic effect of Ti and B. The vertical flame resistance and LOI results of the BT/PU fabric indicate the synergistic effect of H₃BO₃ and TiO₂; these findings are consistent with those reported by the referenced study. Overall, it can be stated that TiO₂ and H₃BO₃ demonstrate an additional impact on the thermal stability and flame retardancy levels of the cotton denim fabric (BT/PU) obtained in this study.