Introduction

Dimethyloldihydroxyethyleneurea (DMDHEU) has been used as an anti-wrinkle crosslinking reagent for years due to its high efficiency and low cost (Qi et al. 2016a; Schramm et al. 2014). However, the DMDHEU-treated cotton fabrics and garments could release formaldehyde continuously during their commercial applications (Yang et al. 1998). Formaldehyde had been confirmed to cause cancer and skin allergy to human body (Harifi and Montazer 2012). In order to avoid the release of formaldehyde, formaldehyde-free anti-wrinkle crosslinking reagents have always gained worldwide attention, such as 1,2,3,4-butanetetracarboxylic acid (BTCA) and citric acid (CA) (Ji et al. 2019; Yang et al. 1997). BTCA has shown a prospective effect with sodium hypophosphite (SHP) as a catalyst (Ji et al. 2015). However, BTCA results in a significant strength loss to the treated fabrics, and it is very expensive for commercial use compared with DMDHEU (Ji et al. 2016a, 2018). CA is an eco-friendly crosslinking reagent, but it causes yellowing to the treated cotton fabrics (Tang et al. 2016), which is a main disadvantage for its application.

In our previous research, we investigated the low-molecular-weight copolymers [P(MA–AA)] of maleic acid (MA) and acrylic acid (AA) for anti-wrinkle finishing of cotton fabrics in detail (Liang et al. 2020). Under the similar conditions, results indicated that the wrinkle recovery angle (WRA) of the P(MA–AA)-treated cotton fabrics showed higher than that of the CA-treated ones. More importantly, the whiteness index (WI) of the P(MA–AA)-treated fabrics almost remained unchanged. The tearing strength retention (TSR) of the P(MA–AA)-treated fabrics was 16% higher than that of the BTCA-treated ones, which could be attributed to the more flexible molecular structure of P(MA–AA) (Ji et al. 2016b, c).

However, it is still a main defect for the practical application of P(MA–AA) that it caused about 50% tearing strength loss (TSL) to the treated fabrics. The TSL of crosslinked fabrics is attributed to the acid degradation of cellulose (TSLD) and crosslinking of cellulose chains (TSLC) by polycarboxylic acid (Ji et al. 2016b, c; Qi et al. 2016b), and a more flexible molecular structure may bring more uniform crosslinking inside the cellulose fiber, which will result in less TSLC to the treated fabrics.

In order to further reduce the TSL of treated fabrics, low-molecular-weight copolymers [P(IA–AA)] from itaconic acid (IA) and AA were synthesized in this study. Macromolecular copolymers of IA and AA have been used in the synthesis of resins and scale inhibitors (Wan 2014). The low-molecular-weight P(IA–AA) used as a crosslinking reagent for cotton fabrics has not been reported as far as we know. In this research, the influence of molar ratio of IA:AA and reaction time on the anti-wrinkle effect of P(IA–AA)-treated fabrics was systematically investigated. The proton nuclear magnetic resonance spectroscopy (1H NMR), mass spectroscopy (MS), and attenuated total reflection (ATR) infrared were all employed to characterize the obtained low-molecular-weight P(IA–AA). The effects of the concentration of P(IA–AA), curing temperature, curing time, and pH value of the finishing bath on the anti-wrinkle properties of the treated fabrics were studied. Under the similar conditions, the anti-wrinkle effect of P(IA–AA) was compared with that of BTCA, P(MA–AA), CA, and IA. Moreover, the TSL of treated fabrics attributed to TSLD and TSLC was separated from each other, which was correlated with the molecular structure of crosslinking reagents.

Experiment

Materials and chemicals

Fabrics consisting of 100% cotton were purchased from Warren Printing and Dyeing Co., Ltd. (Shanghai, China), which were desized, scoured, bleached, and mercerized. The fabric was 40 s × 40 s plain-woven with weighing 117 g/m2, and the warp and weft density was 133 × 72 threads/inch. Acrylic acid (AA) and itaconic acid (IA) were purchased from TCI Chemical Industry Development Co., Ltd. (Shanghai, China). Analytical reagents including citric acid (CA), 1,2,3,4-butanetetracarboxylic acid (BTCA), potassium persulfate (K2S2O8), and sodium hypophosphite monohydrate (SHP) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals were used as received.

Methods

Synthesis of P(IA-AA)

IA and SHP were mixed and transferred into a 150 mL dry three-neck round-bottom flask, and distilled water was charged into the flask as a solvent. And then a reflux condenser, a magnetic stirrer, and a dropping funnel were assembled with the flask. Thereafter, the mixture of IA and SHP was kept stirring at 70–75 °C till dissolved completely. Then AA was added dropwise to the reaction system with a dropping funnel, and within 30 min, K2S2O8 as an initiator was added in batches. The SHP and K2S2O8 were 71% and 12% (w/w) of IA respectively, the molar ratio of distilled water to IA was 18:1, and the molar ratio of IA to AA was 0.83:1, 1:1, 1.2:1, 1.5:1, and 1.8:1, respectively. The synthetic reaction was completed under the temperature range of 70–75 °C after 2–6 h by maintaining reflux throughout the process. And then the flask was charged with three-fold volume of absolute ethanol, and the macromolecular products and residual K2S2O8 were precipitated and removed by filtration. Finally, the filtrate was evaporated to remove the ethanol, and the low-molecular-weight P(IA–AA) was obtained, which was a colorless syrupy liquid. The yield of the obtained P(IA–AA) was calculated, and furthermore it was characterized with 1H NMR, MS, and ATR infrared, respectively.

Characterization of synthesized compounds

1H NMR analysis

The P(IA–AA) was measured with 1H NMR on a 400 MHz NMR spectrometer (Bruker Biospin AG, Karlsruhe, Germany). The deuterium oxide (D2O) was used as the solvent and 50.0 mg obtained P(IA–AA) samples were dissolved in 0.5 ml D2O for measurement.

Infrared spectroscopy

Attenuated total reflection (ATR) infrared spectroscopy was employed to detect P(IA–AA), IA, and AA, and the spectra were recorded on a Fourier transform infrared (FTIR) spectrometer (Thermo Fisher Scientific Co., Ltd., Shanghai, China). Before measurement, the water molecules in chemical samples were removed by drying at 50 °C for 1 h, and then 25.0 mg chemical samples were tested.

The FTIR of fabrics were performed in an absorbance mode to examine the ester bonds of the treated fabrics. Before test, the unreacted carboxyl groups in the treated samples were eliminated by ringing fabrics in a 0.1 mol/L NaOH solution for 4 min and then dried. Firstly, we took the warp yarns apart from the weft yarns, and then mixed them together evenly. Thereafter, we used a scissors to cut the yarns into fine powders carefully. Finally, 1.0 mg sample powders and 100.0 mg dried potassium bromide (KBr) were mixed well and compressed to a transparent tablet. All the spectra were recorded in the range of 4000 to 400 cm–1 with a resolution of 4 cm–1 and scan times of 32.

Pyrolysis LC–MS analysis

The P(IA–AA) was measured with a CBM-20A LC–MS (Shimadzu, Tokyo, Japan) to analyze the molecular weight. In detail, 25.0 mg P(IA–AA) were dissolved in the distilled water, the cracking temperature was 300–900 °C, and the molecular weight was determined as 406.

Fabric treatment and test

Fabric treatment

A specific polycarboxylic acid of P(IA–AA), BTCA, CA, P(MA–AA), or IA was dissolved in the distilled water, and then SHP was added as the catalyst. Cotton fabrics were firstly impregnated in a finishing solution and padded through two dips and two nips with a wet pickup about 100%. After being dried at 85 °C for 5 min, the fabrics were cured at 150–190 °C for 1.5–5 min in a curing oven (Werner Mathis, Switzerland). The unreacted polycarboxylic acids as well as the catalyst SHP on the treated fabrics were washed away with a tap water for 5 min. After washing, the treated fabrics were dried at 85 °C for 5 min. Before test, the samples were conditioned for 4 h at a 65 ± 2% relative humidity and 20 ± 1 °C condition.

Wrinkle recovery angle

The measurement of wrinkle recovery angle (WRA) of the fabrics was performed with a M003A wrinkle recovery tester (SDL-Atlas, Rock Hill, South Carolina, USA) according to the method of AATCC 66-2008 “Fabric Wrinkle Recovery: Recover Angle Method”.

Tearing strength retention

Tearing strength of the fabric was evaluated with tearing strength tester (Wenzhou Darong Textile Instrument Co., Ltd., Wenzhou, China) on the basis of the ASTM Testing Method D-1424-1996, “Tear Strength of Fabrics by Impact Pendulum Method”. The calculation of tearing strength retention (TSR) referred to formula (1).

$$TSR = \frac{TS1}{TS2} \times 100\%$$
(1)

where TS1 indicates the tearing strength of the fabrics after treating, and TS2 indicates the tearing strength of the fabrics without treating. TSR of the cotton fabrics without treating is 100%.

Whiteness index

Whiteness index (WI) of the fabric was measured according to the method of AATCC 110-2005 “Whiteness Determination of Textiles”. And a Datacolor D650 instrument (Datacolor Inc., Lawrenceville, New Jersey, USA) equipped with a color matching system was used to examine the WI of the cotton fabrics.

Washing durability

The washing durability was tested with a soaping machine according to the test method of “AATCC 61-2009 Washing Color Fastness: 2A of Fast Method”.

Tearing strength loss

A 0.5 mol/L NaOH solution was used to hydrolyze the ester crosslinking in the cotton fabrics treated with P(IA–AA) or P(MA–AA) (Yang et al. 2003). After 50 h hydrolysis at room temperature, WRA of the treated fabrics decreased to 140°, which was close to that of the control, indicating the ester crosslinking on the fabrics were completely removed. The total tearing strength loss (TSL) of treated fabrics was calculated by the formula (2).

$$TSL = 100\% - TSR$$
(2)

where TSR was calculated by formula (1). The tearing strength loss due to crosslinking (TSLC) was calculated by the formula (3).

$$TSLC = \left[ {\frac{TS3 - TS4}{TS2}} \right] \times 100\%$$
(3)

where TS3 is the tearing strength of the treated sample after alkaline hydrolysis, TS4 is the tearing strength of the treated sample before alkaline hydrolysis, and TS2 is the tearing strength of the control. The tearing strength loss due to cellulose degradation (TSLD) was calculated by the formula (4).

$$TSLD = TSL - TSLC$$
(4)

Calculation of Connolly solvent excluded volume (CSEV) and number of rotatable bonds (NRB)

The CSEV and NRB of the chemicals were calculated with a ChemBio3D Ultra software as follow: Calculation → MM2 → Minimize Energy → Compute Properties → Chem ProStd → Connolly Solvent Excluded Volume (CSEV), or → Compute Properties → Molecular Topology → Number of Rotatable Bonds (NRB).

Results and discussion

Effects of synthetic factors

In our previous study (Liang et al. 2020), the synthetic conditions, including the molar ratio of IA:AA and the synthetic time, of the copolymers were found to show an impact on the anti-wrinkle properties of treated fabrics. The influence of molar ratio of IA:AA (0.83:1, 1:1, 1.2:1, 1.5:1, and 1.8:1, respectively) on the anti-wrinkle performance of treated cotton fabrics were investigated. After the reaction was finished at 3 h, the low-molecular-weight P(IA–AA) was obtained and further applied as a new crosslinking reagent to treat cotton fabrics.

Figure 1a demonstrates that the WRA of the treated fabrics ascended with the molar ratio of IA:AA increasing and then diminished. The maximum WRA of the treated fabrics was around 230° at the molar ratio of IA:AA = 1:1, almost 90° higher than that of the control. And the fabric WI was not affected much after the P(IA–AA) treating (Fig. 1b). As shown in Scheme 1, the P(IA–AA) presents a longer molecular chain, which results in a flexible structure of P(IA–AA). The flexible structure of the crosslinking reagent can bring uniform crosslinking inside the cellulose fibers, which reduces the TSLC. The high TSR (Fig. 1c) proved that the flexible structure of P(IA–AA) reduced the TSL of the treated fabrics. The existence of unsaturated polycarboxylic acids may result in yellowing of the treated fabrics (Tang et al. 2016). However, the high WI of the treated fabrics (Fig. 1b) indicated the inexistence of C=C bonds in the obtained P(IA–AA) and the occurrence of the reaction between IA and AA (Scheme 1). Both IA and AA are unsaturated dicarboxylic acids. The strong electron-withdrawing effect of the carboxylic group in the molecule endues a high reactivity to the unsaturated C=C bond (Peng et al. 2012; Yang and Gu 2001; Bajaj et al. 1993), and therefore it is easy for IA and AA to radically polymerize. The results in Fig. 1a–c showed that the molar ratio of IA:AA = 1:1 benefit the improvement of anti-wrinkle properties of the treated fabrics.

Fig. 1
figure 1

Effect of synthetic conditions a–c molar ratio of itaconic acid: acrylic acid (IA: AA), and d–f reaction time on the anti-wrinkle performance. Notes The concentration of P(IA–AA) was 180 g/L, the curing temperature and time was 180 °C × 2 min. WRA and WI of the control sample are around 140° and 74.00, respectively

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Scheme 1
scheme 1

Proposed synthetic mechanism of P(IA–AA)

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By controlling the molar ratio of IA:AA = 1:1, the synthetic reaction time of IA and AA was selected as 2 h, 3 h, 4 h, 5 h, and 6 h, respectively. The fabrics treated with P(IA–AA) obtained at different reaction times are shown in Fig. 1d–f. It was seen in Fig. 1d that the WRA of treated fabrics presented the highest value of 230° at the reaction time of 3 h, and then it showed a downtrend with reaction time further going. Moreover, the WI of treated fabrics always remained constant, and the TSR is constant at a high value of 65% (Fig. 1e, f). It can be explained as that when the reaction time was 3 h, the appropriate molecular size of the obtained P(IA–AA) benefited its diffusion into the fiber and crosslinking with cellulose. If the reaction time was prolonged, macromolecular products could be generated in the synthetic system (Chen et al. 2005). As reported, macromolecular products did not easily diffuse into the amorphous region of cellulose fiber to crosslink with cellulose (Qi 2017), which inhibited the improvement of anti-wrinkle properties of the treated fabrics.

The suitable synthetic conditions were demonstrated as molar ratio of IA:AA = 1:1, temperature of 75 °C, and reaction time of 3 h, the yield of obtained P(IA–AA) was calculated around 60%. The obtained P(IA–AA) was characterized with MS, 1H NMR, and ATR respectively, and results are presented in Fig. 2.

Fig. 2
figure 2

a MS, b 1H NMR spectrum of P(IA-AA), and c ATR infrared spectrum of different chemicals

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As shown in the MS spectrum of P(IA–AA) (Fig. 2a), the molecular weight of obtained product is deduced as 407 − 1 = 406.

The 1H NMR spectrum of P(IA–AA) was demonstrated in Fig. 2b, the analysis of 1H NMR (400 MHz, D2O) are as follow, (δ): 1.6–1.9 (6H, CH3); 2.0–2.4 (4H, CH2); 2.5–2.9 (6H, CH, CH2), H of –COOH does not signal in D2O.

Figure 2c presented the disappearance of transmittance peaks at 1645–1620 cm−1 and 860–820 cm−1, which corresponded to the vibration of C=C bonds of IA and AA, in the P(IA–AA) spectrum, which indicated the successful polymerization of IA and AA. The strong transmittance peak observed around 1700 cm−1 is the typical of carboxylic carbonyl groups, and the transmittance peak located near 3300 cm−1 was attributed to hydroxyl groups (Luo et al. 2018; Vadakkekara et al. 2020). According to the results of 1H NMR, MS, and ATR infrared spectra, it can be concluded that the maximum molecular weight of the synthesized P(IA–AA) is 406, which is the product of the polymerization of two IA and two AA molecules.

Effects of different treating factors

As known, the anti-wrinkle properties of treated fabrics are strongly influenced by the treating conditions. Firstly, cotton fabrics were treated with five different concentrations (100 g/L, 120 g/L, 140 g/L, 160 g/L, and 180 g/L, respectively) of P(IA–AA) aqueous solution, and the anti-wrinkle properties of the P(IA–AA)-treated fabrics are shown in Fig. 3a–c.

Fig. 3
figure 3

Effect of treating conditions: a–c the concentration of P(IA-AA), and d–f the curing temperature on the anti-wrinkle finishing performance of the treated fabrics. Notes a–c The curing temperature and time was 180 °C × 2 min; d–f P(IA–AA) concentration was 160 g/L, and curing temperature and time were 150–170 °C × 3 min, or 180 °C, 190 °C × 2 min. WRA and WI of the control sample are 140° and 74.00, respectively

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It was observed that the WRA of the treated fabrics increased as the concentration of P(IA–AA) increased to160 g/L, and then it almost stagnated with the concentration further rising (Fig. 3a), while the cotton fabrics showed a high WI after treatment regardless of the concentration of P(IA–AA) (Fig. 3b). The TSR decreased greatly but always kept higher than 60% as the concentration of P(IA–AA) increased to 160 g/L (Fig. 3c). This could be explained as that when the concentration of P(IA–AA) increased, the crosslinking reaction between cellulose chains and P(IA–AA) were promoted, which resulted in a significant increase of WRA. However, after the concentration of P(IA–AA) reached to 160 g/L, the crosslinking reaction between P(IA–AA) and cellulose reached saturation with no significant WRA increase as the P(IA–AA) concentration continued to increase. At the same time, the acidic degradation of the treated fabrics from the excessive P(IA–AA) led to an abrupt decrease of the tearing strength. Therefore, the suitable concentration of P(IA–AA) was 160 g/L.

The crosslinking reaction between P(IA–AA) and cellulose occurs in the curing stage. At a high temperature, the adjacent carboxylic groups of P(IA–AA) form cyclic anhydride first, and then esterification of the cyclic anhydride and cellulose occurs (Yang 1991; Yang and Wang 1996). With the suitable concentration of P(IA–AA) being 160 g/L, the curing temperatures were changed to 150 °C, 160 °C, 170 °C, 180 °C, and 190 °C, respectively. The performance of the P(IA–AA)-treated fabrics are described in Fig. 3d–f. The WRA of the treated fabrics increased as the curing temperature increased, and at 180 °C, the maximum WRA of the treated fabrics almost reached to 243° (Fig. 3d). In the case of TSR (Fig. 3e), it always decreased with rising the curing temperature from 150 °C to 190 °C, while the WI of the treated fabrics remained constant (Fig. 3f). This could be explained as that when the curing temperature increased, the intermolecular collision between the P(IA–AA) molecules and cellulose chains was improved, thus enhancing the reactivity of P(IA–AA) as well as the WRA of the treated fabrics. However, at a higher curing temperature, the acidic degradation of cellulose chains was also accelerated, which resulted in a sharp decrease of TSR. The suitable curing temperature was selected as 180 °C to balance the WRA and the TSR.

In order to investigate the effect of curing time on the anti-wrinkle performance of the P(IA–AA)-treated fabrics, the curing temperatures were set at 170 °C and 180 °C, respectively. As shown in Table 1, at 170 °C with a longer curing time, the P(IA–AA)-treated fabrics indicated a higher WRA. At the curing conditions of 170 °C × 4 min or 170 °C × 5 min, the WRA of the treated fabrics almost showed the same. However, the TSR of the treated fabrics decreased greatly with a longer curing time. With the curing time increasing from 1.5 to 2.5 min, the WRA of the treated fabrics cured at 180 °C was higher than that cured at 170 °C. At the curing conditions of 180 °C × 2 min or 180 °C × 2.5 min, the WRA values of treated fabrics were comparable, but TSR decreased with the curing time being prolonged. The WI of treated fabrics almost showed the same as that of the control at 170 °C and 180 °C. The data in Table 1 indicated that 180 °C × 2 min is the optimal curing condition.

Table 1 Effects of curing times at 170 °C and 180 °C on the anti-wrinkle effect of treated fabrics
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Besides the concentration of P(IA–AA), curing temperature, and curing time, the pH value of finishing bath also shows an influence on the anti-wrinkle properties of treated fabrics. To investigate the influence of pH value of finishing bath on the anti-wrinkle performance of the treated fabrics, 0.1 mol/L NaOH solution was applied to adjust the pH value of the finishing bath to 2.1, 2.5, 3.0, and 3.5 respectively before padding the fabrics.

The WRA of the treated fabrics gradually decreased with the pH value increasing, but the TSR showed an opposite trend as the pH value increased from 2.1 to 3.5 (Table 2). The WI of the fabrics treated at different pH conditions were not decreased compared with that of the control. The TSL of the treated fabrics was attributed to the acid degradation of cellulose (TSLD) and the crosslinking (TSLC) of the fabrics. With the pH value of the finishing baths increasing, TSLD was declined. At the same time, less crosslinking occurred on the treated fabrics and consequently increase the TSR of the treated fabrics. Results showed that when the pH value of the finishing baths was about 2.1, WRA, TSR, and WI were appropriate for the treated fabrics.

Table 2 Effects of the finishing bath pH values on the anti-wrinkle performances of the P(IA–AA)-treated fabrics
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For the treated fabrics, the durability to washing is very important for its practical applications. With the suitable concentration of P(IA–AA) being 160 g/L, curing temperature at 180 °C, curing time for 2 min, and pH value at 2.1, the fabrics were treated and then the durability of treated fabrics was performed according to the test methods. The change of WRA was presented in Fig. 4. It indicated that after washing with 20 times, the WRA of the treated fabrics was decreased by 15.3°. It is well known that the crosslinking between cellulose chains and P(IA–AA) molecules is ester bond (Yang 1991; Yang and Wang 1996). The pH of the washing solution was measured as 9.3, the decrease of WRA might be attributed to the hydrolysis of ester bonds between cellulose chains and P(IA–AA).

Fig. 4
figure 4

Change in wrinkle recovery angle (WRA) after different washing times

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Comparison of different polycarboxylic acids

To investigate the difference among P(IA–AA), P(MA–AA), BTCA, CA, and IA in anti-wrinkle treatment of cotton fabrics and the profound reason of the high TSR of the P(IA–AA)-treated fabrics, a comparison research with equimolar concentration was designed, and results are shown in Table 3.

Table 3 Comparison of anti-wrinkle performance of the fabrics treated with equimolar IA, CA, BTCA, P(MA–AA), and P(IA–AA), respectively
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Under the similar conditions, Table 3 showed that the ester bond absorbance of treated fabrics is positively correlated with the WRA. The polycarboxylic acids crosslinks with the cellulose chains by esterification (Yang 1991; Yang and Wang 1996), and therefore the rising of crosslinking extent, accompanied by an increase in the ester bond absorbance, results in the increasing WRA of the treated fabrics. Moreover, the P(IA–AA)-treated fabrics showed a comparable WRA value about 245° to the CA-treated ones, but higher than the IA-treated ones. The fabrics treated with P(IA–AA) showed the highest WI of 77.30, about 26 higher than that treated with CA (51.66). Among the mentioned polycarboxylic acids in this study, the P(IA–AA)-treated fabrics showed the highest TSR of 59.30%, which was about 24% higher than the BTCA-treated ones. As presented in Table 3, the tearing strength loss of cotton fabrics treated with P(MA–AA) was almost 8% lower than that treated with P(IA–AA). In a word, the synthesized P(IA–AA) can be used for the anti-wrinkle finishing of cotton fabrics instead of P(MA–AA), IA, CA, or BTCA.

As reported (Qi 2017), the size of crosslinking reagents had an impact on the diffusion of crosslinking reagents into the fibers. Moreover, the flexibility of crosslinking reagents showed an influence on the crosslinking efficiency, as well as the TSR (Qi 2017). To further clarify the effects of the size and flexibility of different crosslinking reagents on the anti-wrinkle properties of treated fabrics, the connolly solvent excluded volume (CSEV) and the number of rotatable bonds (NRB) of crosslinking reagents were calculated according to the test methods. As the NRB value of crosslinking reagents increased, the molecular structure of crosslinking reagents presented more flexible. As presented in Fig. 5, the crosslinking reagents with flexible structure will enhance the strength (Qi 2017).

Fig. 5
figure 5

Anti-wrinkle mechanism by crosslinking

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As presented in Table 4, among the selected crosslinking reagents, CA shows the smallest CSEV value of 137.12, indicating the best diffusibility into fibers to esterify with cellulose. BTCA, with the CSEV value of 174.44, also shows a suitable molecular size for diffusion into fibers to crosslink with cellulose. However, as seen from Table 3, the anti-wrinkle performance of the CA-treated fabrics cannot compete with the BTCA-treated ones. It is possible that the higher crosslinking efficiency is related to the higher NRB of crosslinking reagents (Qi 2017; Hu et al. 2018), consequently BTCA showed a better anti-wrinkle finishing effect than CA. P(MA–AA) and P(IA–AA) showed a larger molecular size than both CA and BTCA, with a value of CSEV being 301–304 and 335–339, respectively. It would be more difficult to diffuse into fibers to cause crosslinking reaction. Owing to the larger molecular size of P(IA–AA), the P(MA–AA) showed a promising anti-wrinkle effect (Table 3), although the P(IA–AA) showed higher NRB value. The results indicated that only the suitable CSEV and NRB can improve the anti-wrinkle performance effectively.

Table 4 Comparison of CSEV and NRB of different crosslinking reagents
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In order to further study the strength loss of cotton fabrics treated with P(IA–AA) or P(MA–AA), the treated fabrics were alkaline treated according to the test methods section. Consequently, TSLD and TSLC of the treated fabrics can be separated from each other. Results are summarized in Table 5. Table 5 showed that the TSL of the cotton fabrics treated with P(MA–AA) was 48.80%, almost 8% higher than that treated with P(IA–AA). The TSLD of the fabrics treated with P(IA–AA) was about 10% lower than that treated with P(MA–AA). And the TSLC of the fabrics treated with P(IA–AA) was almost equivalent to that treated with P(MA–AA). This suggested that the size and the flexibility of the crosslinking reagent affected the strength loss of the treated fabrics (Ji et al. 2016b).

Table 5 Comparison of tearing strength loss of the fabrics treated with P(IA–AA) and P(MA–AA), respectively
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P(MA–AA) (Scheme 2), which indicated a smaller molecular size than P(IA–AA) (Table 4), had an advantage to diffuse into the cotton fabrics more rapidly and deeply, thus it established more uniform crosslinking in the fabrics. Consequently, the P(IA–AA)-treated fabrics presented a little higher TSLC than the P(MA–AA)-treated ones. However, it was much easier for the small-size P(MA–AA) to diffuse into fibers to cause the irreversible acidic degradation of cellulose. At the same time, pH values of the P(MA–AA) and the P(IA–AA) aqueous solutions were about 1.8 and 2.1, respectively, therefore P(MA–AA) solution could cause more acid degradation of cellulose than P(IA–AA) solution. As a result, the TSL of the P(IA–AA)-treated fabrics was lower than that of the P(MA–AA)-treated ones.

Scheme 2
scheme 2

The chemical structure of different crosslinking reagents

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Conclusions

Low-molecular-weight copolymers of P(IA–AA) were successfully synthesized with the optimal conditions that the molar ratio of IA:AA was selected as 1:1, and reaction temperature and time were 75 °C and 3 h, respectively. The characterization results showed that the maximum molecular weight of the synthesized P(IA–AA) was 406, which was composed of two IA and two AA molecules. For the anti-wrinkle finishing of fabrics, the recommended P(IA–AA) concentration was 160 g/L, and the recommended curing temperature and time were 180 °C and 2 min, respectively. By comparing the anti-wrinkle effect of P(IA–AA) with that of BTCA, P(MA–AA), CA, and IA under the equimolar concentrations, the WRA of the P(IA–AA)-treated fabrics was about 245°, which was equivalent to that of the CA-treated ones. TSR (59.3%) and WI (77.30) of the P(IA–AA)-treated fabrics showed the highest values among the selected polycarboxylic acids, and TSR showed about 24% higher than that of the BTCA-treated fabrics. Furthermore, by comparing TSL values, the TSLD of the P(MA–AA)-treated fabrics showed about 10% higher than that of the P(IA–AA)-treated ones, because the pH value of the P(MA–AA) solution was 1.8, lower than that of the P(IA–AA) solution at 2.1. The TSLC of the P(IA–AA)-treated fabrics showed 1.4% higher than that of the P(MA–AA)-treated ones, which may be attributed to the higher molecular size of P(IA–AA). Overall, the obtained P(IA–AA) presented a potential application in anti-wrinkle finishing of cotton fabrics to retain a high tearing strength of the treated fabrics.