Structural characterization of RHT-PU
Reactive hydrophilic triblock polyurethane was synthesized from HDIt, PEG (Mn = 10000) and MEKO, according to the method shown in Scheme 1. The chemical structures of raw materials, HDIt, MEKO, PEG (Mn = 10000) and final products RHT-PU10000 were analyzed by FT-IR, and the molecular weight (MW) distributions of RHT-PU10000 (10000 represent the molecular weight of PEG in RHT-PU) and PEG (Mn = 10000) were measured by GPC. The results are shown in Fig. 1.
As shown in Fig. 1 (a), in the FTIR spectrum of HDIt, the absorption peak at 2260 cm− 1 and 1680 cm− 1 were ascribed to NCO and C = O stretching vibration (Hu et al. 2016). For the FT-IR spectrum of MEKO, the peaks around 3220cm− 1 and 1670cm− 1 were attributed to the stretching vibration of OH and C = N respectively (Hierso et al. 2000). And the peak around 1110 cm− 1 was due to the stretching vibration of C-O-C groups in PEG spectra (Naz et al. 2018). The spectrum of RHT-PU showed two new peaks at 3350 cm− 1 and 1720 cm− 1 corresponding to the N-H and C = O stretching vibrations of the carbamate structure, respectively (Li et al. 2022). The peak located at 1110cm− 1 represented C-O-C stretching vibration in polyoxyethylene ether structures. Meanwhile the NCO characteristic absorption peak at 2260 cm− 1 disappeared, indicating that isocyanate groups completely reacted with hydroxyl groups of PEG and MEKO to form carbamate groups.
The MW of PEG (Mn = 10000) and RHT-PU10000 were analyzed by GPC, as shown in Fig. 1 (b). RHT-PU10000 performed a MW of 11,364, with a relatively narrow PDI of 1.29. Compared with the MW of PEG, there was a difference of 1291, which was very close to the theoretically calculated difference value of 1308. MW distribution curves of RHT-PU10000 and PEG10000 were also very similar to each other, which indicated that most of the product was triblock polymers without branched structure and chain growth phenomenon.
Properties of RHT-PU emulsion
As amphiphilic block copolymers with hydrophilicity-hydrophobic-hydrophilicity structure, the hydrophilicity of RHT-PU depended on the copolymer structure and the hydrophilicity-hydrophobic interactions. In order to study the effect of hydrophobic chain length and polyoxyethylene ether on hydrophilicity properties and self-emulsification, different MW of PEG (MW = 1000, 3500, 6000 and 10000) were used to synthesize RHT-PU, the effects of PEG MW on the properties of the emulsion were investigated. The results are shown in Fig. 2.
As shown in Fig. 2, there was obvious water-oil separation phenomenon for RHT-PU1000. In contrast, RHT-PU3500, RHT-PU6000 and RHT-PU10000 did form stable white emulsion, and the particle sizes gradually increased with the increase of polyoxyethylene ether chain. RHT-PU was the triblock copolymer with hydrophilic-hydrophobic amphiphilic characteristics, in water, the hydrophobic HDIt blocks aggregated to form the inner core, and the hydrophilic PEG block stretched out to present petal-like orientation, forming a flower-like micelle, as shown in Fig. 2 (c). Loop-shaped hydrophilic structure formed a stable hydration layer, which made the emulsion stable for a long time (Ma et al. 2003)。RHT-PU10000 with the longest hydrophilic chain was able to form the largest petal diameter, and therefore had the largest particle size. RHT-PU1000 with the shorter hydrophilic chain, had a hydrophilic-lipophilic balance (HLB) value of 8.49 (< 10), indicating that hydrophobicity was stronger than hydrophilicity, and thus the emulsion of RHT-PU1000 showed hydrophobicity in water, which made it difficult for it to produce a self-emulsifying emulsion.
Study on the reactivity of RHT-PU to lyocell fabrics
RHT-PU could be grafted on lyocell fiber surfaces based on the reaction between isocyanate and hydroxyl. To verify the successful construction of the block copolymer grafted on lyocell fiber, the fabric was treated with 0.02mol L− 1 RHT-PU10000 and then washed five times according to the 4N procedure in ISO-6330 2012. The untreated fabric, the treated fabric and the treated after-washing fabric were monitored by fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS). The results are shown in Fig. 3.
Figure 3 (a) shows reaction illustrations between RHT-PU and fabrics. There were four isocyanate groups blocked by butanone oxime at individual RHT-PU chain and they could be unblocked during the curing process and releasefree NCO groups. RHT-PU was tethered to the fabric interface by the reaction of four free NCO groups and hydroxyls, forming a loop-shaped brush structure. Two small-spans and one large-span bridge-structure were constructed on the fabric surface by four isocyanate groups in the RHT-PU. Two adjacent NCO groups at the end of RHT-PU chain formed a small-span cross-linking, which strengthened the radial binding force between the cellulose chains and reduced the tendency of the microfibril splitting along the fiber’s axial direction. The two small-span cross-linked structures together formed a large-span bridge-structure, giving loop-shaped hydrophilic lubrication structure higher laundering durability.
Figure 3 (b) shows the FT-IR of fabrics, in the treated fabrics FT-IR, there were two new absorption peaks located at 1680cm− 1 and 1720cm− 1, corresponding to keto carbonyl stretching vibration in HDIt and the ester carbonyl stretching vibration in the carbamate group, respectively, which indicated that RHT-PU has been successfully grafted onto the fabric. After five times washing, these two absorption peaks were also detected at 1680 cm− 1 and 1720 cm− 1, and the intensity of the absorption peaks did not change significantly, indicating that covalent bonds between RHT-PU and lyocell fiber were stable and had a certain laundering durability. Figure 3 (c) is the XPS survey spectra of fabrics. Compared with the untreated fabric, the intensity of the signal for N 1s increased in treated fabrics before and after washing, which is related to RHT-PU grafted on the fabric. In order to further determine the chemical bonding type, a high-resolution C 1s scan was performed, shown in Fig. 3 (d). For the untreated fabric, the C 1s signal was separated into three peaks located at 284.6 eV (C-C), 286.5 eV (C-O), 288.2 eV (C = O), which were characteristic peaks of cellulosic structure (Chen et al. 2020). For the treated fabric, a new signal at 289.6 eV corresponded to the N (C = O) O group appeared, which was from the reaction of RHT-PU with OH in the fabric during the curing process and the carbamate groups in RHT-PU (Canteri et al. 2003). After five washings, the peak at 289.6 eV still presented on the fabric, which further verified the good washing fastness of the grafted copolymer, which was consistent with the FI-IR results.
Study on the lubrication properties of modified lyocell fabric
The hydrophilic polyoxyethylene ether in RHT-PU could combine with water molecules through hydrogen bonding, forming a hydration lubrication layer on the surface of the fabric to reduce friction. In order to appraise the effects of RHT-PU on the hydration lubrication properties of lyocell fabric surface, four samples (RHT-PU1000, RHT-PU3500, RHT-PU6000 and RHT-PU10000) were employed to investigate the effects of PEG chain length, grafting concentration, temperature and load on lubrication properties. The results were shown in Fig. 4.
Fig. 4 (a) shows that the effect of PEG chain length on the lubrication capability of modified fabrics, the treatment concentration was 0.02 mol L-1 and testing load was 1N.
It is seen from Fig. 4 (a) that the COF decline with the increasing PEG chain length. RHT-PU10000 showed the lowest COF with data stabilized at approximately 0.32, a reduction of approximately 28.8% compared to untreated fabric. The COF of RHT-PU1000 was slightly higher than the untreated fabric, which indicated that COF was related to the length of hydrophilic chain, as polyoxyethylene ether, with the longer hydrophilic chain shows a lower COF in water. This because that large numbers of water molecules are adsorbed by the hydrophilic groups through hydrogen bonding, forming a hydration layer on the fabric surface. The thickness and uniformity of the hydration layer on the fiber surface increases with the growth of polyoxyethylene ether chain, which could better segregate the two friction interfaces and create a lower COF. While, the short length ether hydrophilic chains in RHT-PU 1000 might have been insufficient to form a lubricious water film, resulting in a higher COF (Røn et al. 2021).
Figure 4 (b) shows the comparison of COFs of modified fabrics in dry and water conditions. In dry conditions, COF of modified fabrics was lower than that of untreated fabrics, the mean COFs decreasing by 5.1%, 6.2%, 11.7% and 13.5% respectively with the polyoxyethylene ether chain growth. This indicated flexible macromolecules grafted on the fabric are well ordered and firmly attached on the fabric surfaces, forming a solid-like layer segregating the friction interface, appearing boundary lubrication regime (Ma and Luo 2016). Under water conditions, the COFs of the untreated fabric increased by 4.3%, which was related to the increased roughness caused by swelling. In contrast, the COFs of the fabrics modified by hydrophilic RHT-PU3500, RHT-PU6000 and RHT-PU10000 were significantly lower than the COFs in dry condition, decreasing by 3.6%, 5.8% and 13.7%, respectively. That was because the strong interaction of RHT-PU and water molecules formed an effective strongly bound water layer between the sliding surfaces, thus obtaining excellent lubrication performance. Comparing COFs in dry and water conditions, fully demonstrated the effective effect of the hydration layer in reducing COF.
In order to acquire further insight into effects of the RHT-PU grafting density on lubrication properties, different concentrations of RHT-PU10000 were grafted on the fabric, testing the average COFs within 180s under 1N load and 600/s shear rate, the results are shown in Fig. 4(c). As grafting density of RHT-PU10000 on fabric surfaces increased, COF showed a significant downward trend. The fabric with a high grafting density of 0.02 mol L− 1 show a decreased COF of 22.3% compared to a low grafting density of 0.001 mol L− 1. This was due to the fact that at lower grafting density, polyoxyethylene ether chains were far away from each other and steric repulsion between monomers was weaker, so that the conformation of RHT-PU10000 tended to flatten rather than stretch upward at the fabric interface. The flattened conformation resulted in a thin hydration layer in which, when subjected to mechanical force, the load-bearing capacity of the hydration layer was poor and easily destroyed. As grafting density increased, polyoxyethylene ether chains were closer to each other, so that steric repulsion between the monomers forced the chains to stretch perpendicular from the surface (Divandari et al. 2019). High-density grafting increased the thickness and density of the hydration layer on the fabric interface and improved the stability and load-bearing capacity, resulting in a highly lubricated surface.
Figure 4 (d) shows the relationship between temperature and lubrication performance. The COFs of modified fabrics and untreated fabrics were tested under a 1 N load and a 600/s shear rate. CoFs were seen to increase with increased ambient temperature. For untreated fabric, friction properties were mainly influenced by the roughness of the material itself. Under high temperature condition, wet swelling of fiber was aggravated and roughness increased, resulting in increased COFs. COFs of untreated showed a significant increasing with time at 40°C and 60°C, this was because fibrous fuzz gradually formed hairballs on the fabric surface during the friction process, further increasing COFs. That suggested higher temperature accelerated the wear of untreated fabrics. For modified fabric, the hydrogen bonding between the polyether chain and the water molecule is weakened with the increase of temperature, which resulted in a thinner hydration layer at the fabric interface and an increase in COFs. However the results showed there was less COFs change with temperature, with only 4.95% increase at 60 ℃, and the change over time was not too much. We inferred that the hydrogen bonding between the polyether chain and water molecule may be less affected by temperature in the experimental temperature range. In addition, the COFs of modified fabrics increased slightly with time at 40℃ and 60℃. On one hand, because of the presence of a hydration layer, friction actually occurred between the upper plate of the rotational rheometer and hydration layer, as the increase in roughness on the fabrics produced little effect. On the other hand, due to the lower COF, fibrous fuzz and hairballs were hardly produced on fabric surfaces during rubbing and thus there was no further increase in roughness. It is indicated that hydration layers effectively separated direct contact of the two solid surfaces, weakening the influence of fabric swelling and roughness increasing on COFs.
In general, COFs were related to the roughness of the material surface and is independent of applied load between the friction surfaces. However, for the polymers and their composites, when the load was higher, the load affected COF by changing the surface contact state. In order to investigate the relationship between normal load and COF on the fabric surface, the COFs of untreated and modified fabrics were tested under different loads. As shown in Fig. 4 (e), the COFs of the untreated fabric decreased with increased load, at a rate of 20.6%. Also COFs of the modified fabrics firstly decreased with an increase of load and then tended to be stable, decreasing by 13.6%. This was mainly because the effect of load on COFs was related to the surface deformation state of the material. The micro-bumps with certain elasticity existed on the fabric surface. When the load was lower, the contact between the fabric surface and upper plate of rotational rheometer was in an elastic or viscoelastic state, where the fabric surface had elastic resistance, so the friction coefficient was larger. With the increase of load, the micro-bumps on the fabric surface tended to flatten and the contact between fabric surface and plate changed from elastic or viscoelastic state to plastic or viscoplastic state. Thus, the elastic resistance decreased, resulting in a reduction of the COFs (Tian et al. 2023). At different loads, COFs of modified fabrics were significantly lower than the untreated fabric, and stayed stable after 7N, which indicated that the hydration layer formed on the fabric surface could bear huge loads. Thus low COFs surfaces were still obtained at higher load. In addition, micro-bumps on the modified fabric surface were covered by hydration layers, reducing the roughness. Applying a certain load could make the micro-bumps on the fabric surface tended to flatten, from elastic state to plastic state, COFs stayed stable. However, higher pressure was required for untreated fabric surfaces to change from elastic state to plastic state. At a pressure of 9N, the COF of untreated fabric was still not stabilized.
According to the experimental data and analysis presented above, the lubrication mechanism of fabric modified by RHT-PU was described as Fig. 4 (f). In dry conditions, RHT-PU was anchored to the fiber and polyoxyethylene ether chains curled on the fabric surface, forming a solid layer. When the upper plate of the rotational rheometer contacted with fabrics, the solid layer prevented the fabric from directly contacting with the upper plate, thus reducing friction. In water, large numbers of water molecules were adsorbed by the hydrophilic polyoxyethylene ether groups through hydrogen bonding, forming a hydration layer on fabric surfaces. As the upper plate of the rotational rheometer rotated, friction actually occurred between the hydration layers formed on the fabric. Meanwhile, as the increase of modified concentration and polyoxyethylene ether molecular weight, the number of bound water molecules increased, which formed more dense and stable hydration layers at friction interfaces, resulting in a lower friction interface. Because of strong interaction between polyoxyethylene ether and water molecules, water molecular were adsorbed strongly to provide a high load bearing capacity.
The effects of lubrication properties on anti-fibrillation performance of modified lyocell fabrics
When lyocell fibers are subject to wet abrasion, microfibrils can split along their axis, producing fibrillation tendency, imparting a ‘frosty-white’ appearance, especially for dark hues fabrics. Reducing abrasion during wet friction is an effective way to improve fibrillation. In order to investigate the dependence between lubrication properties and fibrillation, two methods, mechanical rubbing and household washing, were performed to generate fibrillation. The effects of RHT-PU with different molecular weight on anti-fibrillation has been analyzed by abrasion and color fading. Figure 5 shows the SEM image of the fabric surface after mechanical friction test.
As shown in Fig. 5, for the untreated fabric, fracture of multiple fibers were visible on surface after wet rubbing. In the 500x magnified image, it is clearly seen that the rupture caused by multiple splitting of lyocell fibers, which suggested that the surface of the untreated fabric was severely abraded after 80 times rubbing. In Fig. 5 (a), fabrics modified with RHT-PU1000 presented less abrasion than the untreated fabric. Although the COF of the fabric modified by RHT-PU1000 was a little higher than the untreated fabric, fibrillation was improved to a certain extent. This indicated that the cross-linking between the NCO in the RHT-PU and the OH in the fiber improve the lateral cohesion of cellulose chains, thus, reduce the splitting of fibers along the axial direction. Comparing Fig. 5 (a) to Fig. 5 (d), the fibrillation was reduced with increased hydrophilic chains length in RHT-PU and the modified concentration. For fabrics modified with RHT-PU6000, rupture of fibers was significantly improved and the damage mainly microfiber splitting. When modified with RHT-PU10000 at 0.01mol L− 1 and 0.02mol L− 1, there was no obvious fibrillation phenomenon on fabrics surface and the appearance of fabrics were clean, without the "frost-white" phenomenon. This was consistent with lubricating properties of the fabrics, indicating that the construction of water lubricated interface effectively reduced fibrillation. In addition, from SEM images, splitting of microfibers on the modified fabrics adhered to the fiber surface rather than protruding on the fiber surface. This might have been because, in water, polyoxyethylene ether chains stretched and, after drying, the chains curled and were pulled by the anchored isocyanate groups at both ends to form a solid film. The solid film adhered microfibers on the fabric surface, reducing light scattering caused by protruding microfibers on the surface, in turn reducing the "frost-white" appearance.
Figure 6 shows abrasion SEM images of fabrics after household washing, which is closer to the fabric abrasion in the practical application. Comparing the two methods, the wear degree of the fabric surfaces after washing were lighter than the mechanical friction. This might have been related to the normal load and contact area. During mechanical friction test, the fabric was subjected to higher normal load and the two friction surfaces were closer to each other. There were clearly fiber fractures observed on the untreated lyocell fabric, while the surface wear of the other modified fabrics were mainly fiber splitting, except for fabrics modified with RHT-PU1000. This indicated that reducing the wet COF of fabrics effectively improved anti-fibrillation, which was consistent with mechanical friction results. However, from Fig. 6 (b) to (d), it is found that the fabrics modified with 0.02 mol L− 1 RHT-PU3500 and 0.01 mol L− 1 RHT-PU6000 shows the best anti-fibrillation performance, while fabrics modified with RHT-PU10000 presented the worse. This indicated that higher molecular weight and larger grafting density had a negative impact on the anti-fibrillation performance during household washing. This result was contrary to the lubrication performance of the fabrics studied above. In contrast to the flat state of fabrics in mechanical friction test, the fabrics would be creased and bent, during tumbling in the household washing process. The creases formed were more susceptible to abrasion, which in turn generated fibrillation. We found that when the higher molecular weight and higher grafting density would cause fabrics to stiffen, which increased slipping resistance between yarn. If the forces were sufficient magnitude, the yarns will be forced out of the fabric plane and a crease developed, resulting more serious abrasion. This meant the lubrication properties of fabrics could improve fibrillation, such that it is equally important to maintain a softness handle.
Fibrillation of lyocell fibers caused microfibers protruding from the fiber surface, which increased light scattering, making the fabric colors lighter and appearing "frost-white" appearance. Thus, from a macro perspective, the performances of anti-fibrillation could be evaluated by K/S. Color fading grades are shown in Table 1, along with the KS values and its change rate of fabrics before and after anti-fibrillation tests.
Table 1
Color fading grade, KS value and its change rate of fabrics before and after anti-fibrillation tests.
| | Before | Mechanical friction | Household washing |
| | KS value | KS value | Change rate of KS | Color fading grade | KS value | Change rate of KS | Color fading grade |
| Untreated | 26.17 | 20.23 | 22.70% | 3 | 19.51 | 25.45% | 3 |
| RHT-PU1000-0.005 mol L− 1 | 27.37 | 21.02 | 23.20% | 3 | 21.05 | 23.09% | 3 |
| RHT-PU1000-0.01 mol L− 1 | 27.68 | 21.59 | 22.00% | 3 | 21.08 | 23.84% | 3 |
| RHT-PU1000-0.02 mol L− 1 | 27.88 | 21.99 | 21.13% | 3 | 21.48 | 22.96% | 3 |
| RHT-PU3500-0.005 mol L− 1 | 28.24 | 22.03 | 21.99% | 3 | 22.84 | 19.12% | 3–4 |
| RHT-PU3500-0.01 mol L− 1 | 28.52 | 22.8 | 20.06% | 3 | 25.01 | 12.31% | 4 |
| RHT-PU3500-0.02mol L− 1 | 28.58 | 23.56 | 17.56% | 3–4 | 27.32 | 4.41% | 4–5 |
| RHT-PU6000-0.005mol L− 1 | 28.39 | 24.09 | 15.15% | 3 | 24.59 | 13.38% | 4 |
| RHT-PU6000-0.01mol L− 1 | 28.62 | 25.21 | 11.91% | 4 | 26.92 | 5.930% | 4–5 |
| RHT-PU6000-0.02mol L− 1 | 29.19 | 28.27 | 3.15% | 4–5 | 26.01 | 10.89% | 4 |
| RHT-PU10000-0.005mol L− 1 | 28.65 | 25.42 | 11.27% | 4 | 26.56 | 7.29% | 4 |
| RHT-PU10000-0.01mol L− 1 | 29.08 | 27.98 | 3.78% | 4–5 | 25.94 | 10.80% | 4 |
| RHT-PU10000-0.02mol L− 1 | 29.41 | 29.26 | 0.51% | 5 | 24.24 | 17.58% | 3–4 |
As shown in Table 1, the KS values of modified fabrics slightly increased, indicating a darkening effect of RHT-PU. The K/S change tendency was consistent with abrasion. In mechanical friction, the KS value change rate of RHT-PU10000-0.02mol L− 1 was the lowest, only 0.51%, with a color fading grade was 5 almost no color difference. And, in household washing test, the K/S changes of fabrics modified with RHT-PU3500-0.02mol L− 1, RHT-PU6000-0.01mol L− 1 and RHT-PU10000-0.005mol L− 1 showed smaller changes of 4.41%, 5.93%, 7.29% respectively, with a color fading grade can be increased by 1 to 1.5 levels.
