The stable triazine ring of melamine contains three reactive amino groups in 2, 4 and 6 position. Therefore, melamine is capable of undergoing various reactions (Bann and Miller 1958). The most important reaction is the methylolation of the amino groups by means of formaldehyde resulting in the formation of a melamine resin, a durable thermosetting plastic (Fink 2005). The amine group of melamine also can be acylated at elevated temperatures by means of anhydrides or amides (Bann and Miller 1958).
Thus melamine was reacted with hydrolyzed TESP-SA at various molar TESP-SA/MEL ratios (3/1, 2/1, 1/1). Subsequently, the cellulosic material was impregnated with the reaction mixtures and subjected to a thermal treatment at 160 °C and 220 °C. The physicomechanical data of the as-prepared cotton samples were evaluated. Various investigations have shown that the reaction of an amino functionality with vicinal carboxylic groups at elevated temperatures results in the formation of a cyclic imide group, which can be very well detected applying FT-IR spectroscopy (Ahmad et al. 2009; Clemson et al. 1997; Schramm and Rinderer 2015; Schramm et al. 2014).
The cotton samples are denoted as follow: RM: raw material; The first number indicates the molar ratio of TESP-SA/MEL (1 = 1/1, 2 = 2/1, 3 = 3/1), the second number refers to the amount of the impregnation steps, whereas the third number indicates the curing temperature (160 °C, 220 °C).
The physicomechanical data
The cotton samples were impregnated once, twice or three times with the various treating solutions. Then the samples were dried and cured at elevated temperatures. The value of the add-on, which indicates the amount of the chemical agent being applied to the cotton sample, are listed in Table 1. A most significant increase can be observed when the molar ratio of TESP-SA/MEL is 2/1 and 3/1. The data make evident that the second impregnation step gives rise to an increase in the add-on values. The cotton samples which were treated at 220 °C exhibit a moderate increase in the add-on values compared to the samples having been cured at 160 °C.
Dry crease recovery angle
N-methylol compounds are the most commonly used cross-linking agents, which are very effective in conveying crease-resistant properties to cellulose-based material. Examples of these compounds include dimethylol urea (DMU), dimethylol ethylene urea (DMEU), trimethylol triazine (TMT), dimethylol methyl carbamate (DMMC), uron, triazone and the most effective dimethylol dihydroxy ethylene urea (DMDHEU). However, they suffer from various disadvantages, such as the tendency to release the irritant formaldehyde (Hewson 1994). Therefore, intensive efforts have been undertaken to replace DMDHEU.
Table 1 shows the DCRA values of the TESP-SA/MEL-treated cotton samples. Compared to the untreated fabric an improvement of the DCRA values can be observed. However, they do not reach the level of the commercial available durable press values.
The bending properties of woven fabrics are of high interest for industrial, household and apparel applications (Chattopadhyay 2008; Ghosh et al. 1990). Therefore, the flexural rigidity values of the TESP-SA/MEL-treated cotton samples were measured. They are shown in Table 1 and make evident that an increase in the molar ratio of TESP-SA/MEL results in an enhancement of the flexural rigidity. The same tendency can be observed with respect to the number of impregnation steps. The highest value of flexural rigidity is obtained for 3-2-220.
The air permeability of a fabric is an indication of how well it enables the passage of air through it. The passage of air through a woven fabric is of importance for a number of fabric end uses such as industrial filters, tents, parachutes, raincoat materials, shirtings, and airbags (Morrissey and Rossi 2014; Ogulata 2006; Xiao et al. 2012).
The data of the measurement of the air permeability are shown in Table 1. The results make evident that no significant change can be observed, when the molar ratio was changed. A second impregnation step leads to a reduction of the air permeability indicating that the interstices in the cotton fabrics were reduced.
When chemically modified cellulosic material is subjected to a thermal treatment various degradation processes of the cellulose and chemical agent may occur resulting in a reduction of the whiteness index. The WI values are presented in Table 1 and reveal that the curing process at 220 °C results in a significant reduction of the WI. This discoloration can mainly be ascribed to the reactions of carbonyl groups of the cellulosic material with the amino groups of melamine forming Schiff bases (De la Orden et al. 2004; De la Orden and Urreaga 2006).
Due to the presence of hydroxyl groups in cellulosic materials, cotton is capable of taking up water. One portion of the water is bound to the surface and the other part remains between the fibers because of capillary actions. Therefore, the chemical modification of the cotton surface exerts an influence on the water retention value. The WRV are shown in Table 1 and make evident that an increase of the portion of melamine results in a moderate decrease of the water retention ability.
Water vapor permeability
The values of the water vapor permeability (WVP) which is governed by several structural and physicomechanical properties of the fabric are shown in Table 2 (Huang 2016). The data make evident that a significant increase of the WVP can be observed for those samples having been treated with finishing baths containing TESP-SA and MEL in the molar ratio 1:3.
Morphology of the treated cotton fabrics
The SEM images of the cotton samples which have been impregnated once with TESP-SA/MEL solutions containing the components at different molar ratio (1/1, 1/2 and 1/3) are shown at two magnifications (500×, 3000×) in Fig. 2a–f (2a, b: 1-1-220, 2c, d: 2-1-220, 2d, f: 3-1-220). The SEM observations confirm that an obvious change in the surface morphology of the cotton fabrics can be seen after sol–gel coating. The typical longitudinal fibril structure of the native cotton disappears in all treated samples and the surface of fiber appears as a rough coating.
FT-IR/ATR is an excellent tool to identify the functional group being present on the surface of cotton materials (Warren et al. 2016; Xue et al. 2016). Therefore, the spectra of the cotton samples which had been impregnated twice and which were treated at 160 °C and 220 °C have been recorded and are shown in Fig. 3. Figure 3a shows the spectrum of the untreated raw material. Figure 3b–d show the spectra of 1-2-160, 2-2-160 and 3-2-160. A peak can be observed at 1714 cm−1 which can be attributed to the stretching vibration of the carboxyl carbonyl bond present in TESP-SA. The amide I band (1631 cm−1) and the amide II band (1533 cm−1) prove the formation of a an amide functionality. The amide I band overlaps with the O–H bending mode of adsorbed water. The absorptions appearing in the spectral region of 1200 cm−1 to 800 cm−1 are assigned to the asymmetric vibration modes of the siloxane group and the asymmetric bridge C–O–C bonds of the cellulose. An inspection of the spectra of the cotton samples having been treated at 220 °C (Fig. 3e: 1-2-220, Fig. 3f: 2-2-220, Fig. 3g: 3-2-220) clearly make evident that an additional peak can be observed at 1784 cm−1. This absorption band indicates the formation of an imide group (imide I: asymmetric stretching mode of the carbonyl group). The corresponding symmetric vibrational mode can be seen at 1714 cm−1. The band at 1392 cm−1 is ascribed to the symmetric stretching vibration of the C–N group (imide II) (Schramm et al. 2014).
Cellulose comprises 1-4 linked β-d-glucose units and is known to exist in at least four polymorphic forms: cellulose I (native cellulose), cellulose II (regenerated cellulose and mercerized cellulose), cellulose III1, III11 (liquid ammonia treated cellulose), cellulose IV1, IV11 (thermally treated cellulose III) (O’Sullivan 1997). It is well known that cellulose is composed of crystalline regions (ordered) and amorphous regions (less ordered). Various methods have been used to evaluate the crystallinity of cellulose, such as FT-IR spectroscopy (Åkerholm et al. 2004; Warren et al. 2016) or X-ray powder diffraction (XRPD) (Ling et al. 2019; Park et al. 2010). In the present study XRPD was used to determine the CrI. The XRPD pattern of the treated cotton fabrics are shown in Fig. 4a–g. 4a: RM, 4b: 1-2-160, 4c: 1-2-160, 4d: 2-2-160, 4e: 2-2-220, 4f: 3-2-160, 4 g: 3-2-220). The XRD patterns with major peaks at 14.8, 16.4, 22.8 and 34.2, respectively which are assigned to the (1−10), (110), (200) and (004) diffraction planes indicate that cellulose I is present (Kim et al. 2018; Ling et al. 2019). The values of the CrI are shown in Table 1. The results reflect that the thermal treatment at 220 °C causes a reduction of the CrI values.
Mechanism of the crosslinking reaction
TESP-SA is hydrolyzed under acidic conditions. Consequently, the ethoxy groups which are attached to the silicon atom are converted into silanol groups and the anhydride moiety reacts to two carboxylic groups as shown in Fig. 5a. A nanosol solution is formed, which is added to a solution of melamine at elevated temperature. The carboxyl groups of the hydrolyzed TESP-SA react with the amino groups of melamine resulting in the formation of an amide functionality. This reaction mixture is applied to the cellulosic material. The thermal treatment at 220 °C results in the formation of the imide moiety (Fig. 5b).
The findings of the physicomechanical measurements make evident that an improvement of the DCRA values can be observed indicating that a crosslinking reaction was achieved. A tentative crosslinking scheme of the organic–inorganic hybrid with cellulose chains is shown in Fig. 6.
Bleaching of the cotton materials cured at 220 °C
As shown above, the curing process of TESP-SA/MEL-treated cotton samples at 220 °C results in a significant decrease in the WI value. Therefore, the cotton samples 3-2-160 and 3-2-220 were subjected to a bleaching process to improve the whiteness index. Two bleaching recipes were applied.
The findings which are presented in Table 2 make evident that the bleaching process gives rise to an increase in the WI. In the case of 3-2-160 the application of the bleaching bath containing 8% hydrogen peroxide causes a further improvement of the WI almost reaching the level of the raw material. The treatments of 3-2-220 with the bleaching baths give rise to an increase in the WI values. However, the level of the WI of the RM cannot be reached. Due to the alkaline conditions the values of the DCRA are significantly decreased.
Figure 7a–c show the FT-IR spectra of the raw material (7a), 3-2-220 (7b) and 3-2-220 having been treated with bleaching bath 2 (7c). The spectrum of 3-2-220 (Fig. 7b) shows the imide I bands (1782 c372 m−1 and 1708 cm−1) whose intensities are significantly reduced after the bleaching treatment (Fig. 7c). Figure 7c confirms that two new bands appear at 1556 cm−1 and at 1411 cm−1 which are ascribed to the asymmetric carbonyl stretching mode and the symmetric stretching mode of the carboxylate ion (COO−) having been formed during the alkaline bleaching process (Chung et al. 2004; Oomens and Steill 2008).