3.1 Preparation of organosilicon polymer containing disulfide bonds
According to the reaction equation in Fig. 1, weigh a certain amount of AFD and epoxy-terminated silicone oil in a molar ratio of 1:0.8, and place it in a water bath at 80°C for 0.5 h to synthesize the organosilicon prepolymer. Then it was transferred into a polytetrafluoroethylene template for further polymerization at a high temperature to obtain the organosilicon polymer film. The infrared spectra of the reaction product organosilicon polymer film and the reaction raw materials are shown in Fig. 3.
As shown in Fig. 3, in the FTIR spectrum of the epoxy-terminated silicone oil, the stretching vibration peak of the epoxy group (C-O-C) is at 912 cm− 1. In the FTIR spectrum of AFD, at 3300 ~ 3500 cm− 1, there are typical double absorption peaks of stretching vibration of the amino group (-NH2). They all disappeared in the FTIR of the reaction product.
In the FTIR spectrum of the reaction product, the characteristic absorption peak of hydroxyl (-OH) formed by the ring-opening reaction of the epoxy group appeared at 3361 cm-1. At 1593 cm− 1 and 520 cm− 1, the skeleton vibration absorption peak of the benzene ring and the absorption vibration peak of the disulfide bond (S-S) appeared similar to those in AFD, respectively. At 1042 cm− 1, there is the antisymmetric stretching vibration peak of the silicon-oxygen bond (Si-O-Si) similar to that in the organosilicon polymer chain. The results of FTIR spectral analysis show that the chemical reaction between epoxy-terminated silicone oil and AFD occurs as shown in Fig. 1, which indicates organosilicon polymer containing disulfide bonds was synthesized.
3.2 Effect of raw material ratio on the resilience of organosilicon polymer film
According to the reaction equation shown in Fig. 1, the organosilicon polymer film was prepared with the raw materials molar ratios of epoxy-terminated silicone oil and AFD as 1:1, 1:0.9, and 1:0.8, respectively. The elastic properties of the organosilicon polymer film were tested with the dynamic thermomechanical analyzer DMA Q800. The organosilicon polymer film was stretched to 120% deformation at a deformation rate of 30%/min. After the external force was removed, the organosilicon polymer film rebounded quickly and then underwent a slow recovery process until the deformation no longer changed. The recovery of 15 S after removing the external force was defined as the rapid elasticity recovery stage, the recovery of 15 S ~ 15 min was defined as the delayed elasticity recovery stage, and the remaining unrecovered deformation was defined as the permanent deformation. The recovery of the organosilicon polymer film after being stretched by the external force is shown in Fig. 4.
It can be seen from Fig. 4 that the deformation ratio of rapid elasticity recovery of organosilicon polymer film prepared by different raw materials is very high, indicating that the synthesized organosilicon resin has high elasticity. As the proportion of AFD in the raw material decreases, the deformation ratio of delayed elasticity recovery of the organosilicon polymer film gradually increases, and the residual permanent deformation of the polymer film decreases. This is because in the reaction system with the raw material molar ratio of 1:1.0, more linear macromolecules (Fig. 1a) are synthesized by the reaction between the epoxy-terminated silicone oil and AFD. The intermolecular force of this linear macromolecule is weak. When subjected to external force, the straight chains of macromolecules easily slide relative to each other, and the center of gravity of macromolecules moves, so the deformation is difficult to recover.
When the ratio of AFD decreases, the reaction synthesizes more network macromolecules (Fig. 1b). In the synthetic organosilicon polymer, the molecular weight of the polymer increases, and the crosslink density between the macromolecular chains also increases. When subjected to external force, the polymer macromolecular segments slip with each other. However, due to the large molecular weight of macromolecules and the cross-linking of disulfide bonds, the center of gravity of macromolecules is not easy to move. When the external force is removed, the macromolecular segments in the organosilicon polymer film are affected by the interaction force between the molecular chains of the network structure, and gradually return to the original state of the segments. Therefore, the more complex the macromolecular chain network cross-linked structure of the organosilicon polymer film is, the better the delayed elasticity recovery performance of the polymer film and the less the permanent deformation will be. When the raw material molar ratio is 1:0.8, the proportion of permanent deformation is only 0.41%.
3.3 Effect of the amount of emulsifier on the performance of organosilicon resin emulsion
According to the preparation process in Fig. 2, the organosilicon resin emulsion with a solid content of 30% was prepared with the raw material molar ratio of 1:0.8. The ratio of emulsifier to raw materials respectively was 6.67%, 13.33%, 20%, 26.67%, 33.33%, and the emulsifying time was 30 min. The performance of the prepared organosilicon resin emulsion is shown in Fig. 5.
It can be seen from Fig. 5, that when the dosage of the emulsifier is 6.67%, the particle size and PDI value of organosilicon resin emulsion are large, and the prepared emulsion is unstable and prone to delamination. With the increase of the amount of emulsifier, the particle size of the emulsion showed a decreasing trend. When the amount of emulsifier was 26.67%, the particle size and PDI value of the emulsion reached the lowest value, and the emulsion showed high stability. When the amount of emulsifier continues to increase, the particle size and PDI value of emulsion tend to increase. It shows that in this emulsion system, increasing the amount of emulsifier can increase the adsorption of surfactant on the interface of organosilicon resin particles, reduce the interfacial tension of emulsion particles, and reduce the interfacial energy of the emulsion system. Thus, the particle size of the emulsion particles is reduced and the stability of the emulsion is improved. However, excessive use of emulsifiers may be unfavorable for the close arrangement of surfactants on the surface of emulsion particles. The particle size distribution of the emulsion particles is uneven, and the PDI value increases, thereby affecting the stability of the emulsion.
By detecting the Zeta potential of the emulsion particles, it is found that the emulsion particles have a positive charge due to the use of cationic surfactant. This increases the charge repulsion between the emulsion particles, improves the stability of the emulsion, and is also beneficial to the adsorption of the emulsion particles on the cotton fiber surface. With the increase of the amount of emulsifier, the Zeta potential of the emulsion particles increases, and the stability of the emulsion is also improved. However, when the amount of emulsifier is 33.3%, the Zeta potential decreases, which needs to be studied in the future.
3.4 Effect of emulsifying time on the performance of organosilicon resin emulsion
According to the preparation process in Fig. 2, the organosilicon resin emulsion with a solid content of 30% was prepared with the raw material molar ratio of 1:0.8. The proportion of the emulsifier is 26.67%, and the high-speed homogeneous emulsifying time is 10, 20, 30, 40, and 50 min respectively. The performance of the prepared organosilicon resin emulsion is shown in Fig. 6.
As shown in Fig. 6, with the extension of homogeneous emulsifying time, the particle size and PDI value of the emulsion decreased greatly. When homogeneously emulsified for 20 min, the particle size and PDI value of the emulsion tended to be stable. It can be seen that prolonging the emulsifying time can increase the mechanical energy provided to the emulsion system, making the emulsion particles smaller and more uniform in size. After 20 min of emulsification, the emulsion particles can reach 200 ~ 300 nm. And the particle size of the emulsion no longer decreases when the emulsification time continues to be extended. Therefore, the excessive extension of the emulsification time has no visible significance in improving the performance of the emulsion.
The Zeta potential of the emulsion gradually increased to a stable value with the extension of emulsifying time. When the emulsion was emulsified for 20 min, the Zeta potential of the emulsion reached + 46.6 mV. Continuing to prolong the time, the Zeta potential of the emulsion no longer increased significantly. Therefore, a good emulsification effect can be achieved by 20 min of emulsification.
3.5 Effect of organosilicon finishing agent concentration on the elastic properties of cotton fabric
The application process is as described in the " Application of organosilicon resin emulsion" section. The cotton fabric was immersed in different concentrations of organosilicon resin emulsion finishing agents. After the cotton fabric was padded and pre-baked, it was baked at 160°C for 6 min. Then according to ISO 2313‑1:2021, measure the rapid and delayed elasticity recovery angle of the finished cotton fabric. The results are shown in Table 1.
Table 1
Effect of finishing agent concentration on elastic properties of cotton fabric
Finishing agent concentration(g/L)
|
Recovery Angle
(°)
|
Recover angle increase ratio
(%)
|
Rapid elasticity
|
Delayed elasticity
|
Rapid elasticity
|
Delayed elasticity
|
Comparison sample
|
92
|
127
|
—
|
—
|
10
|
115
|
162
|
25.0
|
27.6
|
20
|
141
|
185
|
53.3
|
45.7
|
30
|
148
|
188
|
60.9
|
48.0
|
40
|
139
|
180
|
51.1
|
41.7
|
50
|
138
|
175
|
50.0
|
37.8
|
As can be seen from Table 1, compared with the comparison sample, the crease recovery angle of cotton fabric after being finished with organosilicon resin emulsion showed a trend of first increasing and then slightly decreasing. When the concentration of the finishing agent is 30 g/L, the increase ratio of the rapid and delayed elasticity recovery angle of finished cotton fabric reaches the maximum. Continue to increase the concentration of the organosilicon resin finishing agent, and the wrinkle recovery angle of cotton fabric tends to decrease. We believe that the organosilicon resin emulsion finishing agent forms a layer of elastic polymer film on the surface of cotton fiber under the condition of high-temperature baking, which improves the elasticity of cotton fibers. Thus, the elasticity of the cotton fabric is improved, and the recovery angle of rapid and delayed elasticity is also improved. However, when the concentration of the finishing agent is too high, a thicker polymer film is deposited on the fiber surface. At the same time, more "fiber cross-linking points" bonded by the polymer film will be formed in the tissue structure of finished cotton fabrics. In this case, the increase in fiber diameter changes the elasticity of the cotton fiber. Moreover, the "fiber cross-linking points" formed by the deposition of excessive finishing agents weaken the removability of the fiber under external force. This reduces the elastic recovery of the cotton fabric after the external force is removed. Therefore, the crease response angle of cotton fabric is reduced and the elasticity is decreased. To sum up, when the concentration of the finishing agent is 30 g/L, the cotton fabrics can obtain better rapid and delayed elasticity performance.
3.6 Effect of baking temperature on the elastic properties of cotton fabric
The cotton fabric was immersed in the organosilicon resin emulsion finishing agent with a concentration of 30 g/L. After the cotton fabric was padded and pre-baked, it was baked at 130, 140, 150, 160, 170, and 180°C for 6 min, respectively. Then the rapid and delayed elasticity recovery angles of the finished cotton fabric were measured. The results are shown in Table 2.
Table 2
Effect of baking temperature on the elastic properties of cotton fabric
Baking temperature(℃)
|
Recovery Angle
(°)
|
Recover angle increase ratio
(%)
|
Rapid elasticity
|
Delayed elasticity
|
Rapid elasticity
|
Delayed elasticity
|
Comparison sample
|
92
|
127
|
—
|
—
|
130
|
105
|
143
|
14.1
|
12.6
|
140
|
132
|
175
|
43.5
|
37.8
|
150
|
145
|
185
|
57.6
|
45.7
|
160
|
143
|
182
|
55.4
|
43.3
|
170
|
140
|
180
|
52.2
|
41.7
|
180
|
141
|
179
|
53.3
|
40.9
|
It can be seen from Table 2, that with the increase of the baking temperature (from 130℃ to 150℃), the rapid and delayed elasticity recovery angles of cotton fabric increase. When the baking temperature reaches 150°C, the wrinkle recovery angle of cotton fabric reaches the maximum value, and the increase ratio of the recovery angle reaches the highest. Continue to increase the baking temperature, the fabric recovery angle shows a decreasing trend. We believe that there are still underreacted epoxy and secondary amine groups in the organosilicon resin emulsion finishing agent. The high temperature during baking provides the conditions for its full reaction, which increases the molecular weight of the organosilicon polymer. A larger network polymer structure is formed, and a polymer film with greater elasticity is obtained, so the elasticity of the cotton fabric is improved. However, when the baking temperature is too high (from 160℃ to 180℃), the disulfide bonds in the polymer structure may be oxidized, resulting in a decrease in the number of cross-linking points constructed by the disulfide bonds. The cross-linked network structure of the resin is destroyed, and the molecular weight of the silicone resin finishing agent is reduced. As a result, the elasticity and strength of the organosilicon polymer film are reduced, and the resilience performance of the cotton fabric is reduced accordingly. In summary, the elastic finishing of cotton fabric with the organosilicon resin emulsion finishing agent containing disulfide bonds can obtain good resilience performance at a baking temperature of 150°C.
3.7 Effect of baking time on the elastic properties of cotton fabric
The cotton fabric was immersed in the organosilicon resin emulsion finishing agents containing disulfide bonds with a concentration of 30 g/L. After the cotton fabric was padded and pre-baked, it was baked at 150°C for 2, 4, 6, 8, and 10 min respectively. Then the rapid and delayed elasticity recovery angles of the finished cotton fabric were measured. The results are shown in Table 3.
Table 3
Effect of baking time on the elastic properties of cotton fabric
Baking time(min)
|
Recovery Angle
(°)
|
Recover angle increase ratio
(%)
|
Rapid elasticity
|
Delayed elasticity
|
Rapid elasticity
|
Delayed elasticity
|
Comparison sample
|
92
|
127
|
—
|
—
|
2
|
139
|
178
|
51.1
|
40.2
|
4
|
147
|
185
|
59.8
|
45.7
|
6
|
148
|
185
|
60.9
|
45.7
|
8
|
148
|
186
|
60.9
|
46.5
|
10
|
148
|
186
|
60.9
|
46.5
|
As shown in Table 3, under the conditions of finishing agent concentration of 30 g/L and baking temperature of 150℃, prolonging the baking time, the wrinkle recovery angle of cotton fabric can only increase slightly. When the baking time was 4 min, the rapid and delayed elasticity recovery angle of cotton fabric reached the highest value. Continue to extend the baking time, the crease recovery angle of the fabric remains unchanged. It can be considered that after baking at 150°C for 4 min, the residual reactive groups in the organosilicon resin emulsion finishing agent on the fabric surface have undergone a sufficient ring-opening reaction. A larger network macromolecular structure is formed, the organosilicon polymer film with good elasticity is obtained, and the wrinkle recovery angle of the fabric reaches the maximum value. Continue to prolong the baking time, the macromolecular structure of the organosilicon polymer film on the fiber surface does not change significantly. Thus, the baking time of 4 min was chosen as a suitable finishing time for the experiment.
3.8 Temperature-responsive shape memory properties of finished cotton fabric
According to the finishing process conditions obtained above, the padding process is used to finish the cotton fabric with silicone resin emulsion. Then clamp both ends of the cotton sample with a clip, place it in an oven at 85°C for 10 min, and then cool it to room temperature. Comparing the opening length of the comparison sample and the finished cotton sample. The testing process and the mechanism of dynamic reorganization of disulfide bonds are shown in Fig. 7.
As shown in Fig. 7, the comparison sample has a larger opening length L0 after heat treatment at 85°C, and the opening length L of the finished cloth sample is significantly smaller than the opening length L0 of the comparison sample(Fig. 7a). It shows that the comparison sample has good elasticity, and has a good property of returning to its original shape even if it is treated at 85℃ for 10 min. A layer of the elastic organosilicon resin film is deposited on the surface of the cloth sample finished with organosilicon resin emulsion, in which the macromolecular network structure contains a certain number of disulfide bonds(Fig. 7b).
At room temperature, when the finished cotton fabric is deformed by an external force, the organosilicon resin film on the fiber surface will generate a certain internal stress due to the deformation. The organosilicon macromolecular chain will produce a certain slip to reduce the internal stress in the film. However, due to the constraints of the network structure, the slippage of the macromolecular chain is relatively limited, and the internal stress in the film is difficult to be fully eliminated. Therefore, after the external force is removed, the fabric will have greater deformation recovery under the influence of the internal stress of the film.
However, when the deformed fabric is heated at 85°C, the disulfide bonds in the film will be broken to generate sulfhydryl groups(Fig. 7c,7d). The cross-linking density of the macromolecular network structure is reduced, and the chain segment slips and adjusts to a new position due to the influence of internal stress. During the 10 min heat treatment, the structure of the organosilicon macromolecular chains is constantly adjusted under the internal stress. The internal stress of the organosilicon polymer film generated by the fabric deformation is continuously reduced, and the breaking of the disulfide bonds also accelerates the adjustment process of the macromolecular segments until the internal stress of the polymer film tends to zero. When the fabric is cooled to room temperature, the new shape of the fabric is fixed in a new position through the reorganization of the disulfide bonds in the organosilicon polymer film. Therefore, the opening length(L) of the fabric sample finished with organosilicon resin emulsion is lower than that(L0) of the comparison sample. At the same time, the elasticity of the finished fabric sample did not change at room temperature. Since the organosilicon resin emulsion contains the disulfide bond structure, the finished cotton fabric has the shape memory property caused by the temperature change, which is called "temperature-responsive shape memory" functional finishing in this paper.
3.9 Surface morphology of finished cotton fabric
Scanning electron microscopy was used to analyze the cotton sample finished with the organosilicon resin emulsion. The surface morphology of the cotton sample is shown in Fig. 8.
It can be seen from Fig. 8, The fiber surface of comparison sample is smooth and clean, with small cracks caused by processing, however, the fibers of the finished cotton samples with organosilicon resin emulsion are coated with a layer of the polymer film. Due to the non-uniformity of the evaporation rate of the finishing agent during the drying process, the polymer film forms certain wrinkles. The presence of such polymer film increases the elasticity of the fibers and yarns, thereby improving the elastic properties of cotton fabrics.