Study on the Ramie Fabric Treated with Copper Ammonia to Slenderize Fiber for Eliminating Prickle

ABSTRACT Ramie fabrics are suitable for summer clothing due to their excellent moisture absorption and breathability. Still, their application in the garment industry is limited by the presence of stiff, coarse hairiness that tends to prickle when it comes into contact with the skin. Herein, a low concentration copper ammonia solution was used to slenderize the fiber of the ramie fabric for a short time at room temperature to eliminate prickle and improve the softness of the fabric. Scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and thermal gravity analysis (TG) were used to investigate the morphology and structure of treated ramie fabric. And its softness, smoothness, fiber diameter, breaking strength, air permeability, and moisture absorption were also studied. The results showed that the fibers were slenderized, the stiff, coarse hairiness was reduced, and the softness of the fabric was improved, while its other properties remained good after the ramie fabric was treated under appropriate conditions. The use of copper ammonia solution at room temperature to eliminate the prickle of ramie fabrics was simple and inexpensive, suitable for use in practical production.


Introduction
With the benefits of wide available, environmental friendliness, biodegradability, and low price, natural cellulosic fibers are widely used in textiles, paper industry, biocomposite production, and renewable energy (Kicinska-Jakubowska, Bogacz, and Zimniewska 2012, Sun et al. 2022). As one of the natural cellulosic fibers, ramie has the advantages of long, strong, moisture absorbing, breathable, and light compared to cotton. Five hundred thousand tonnes of ramie fiber are produced annually in China, accounting for approximately 96% of global production (Kipriotis et al. 2015). Large quantities of ramie yarn and fabric are exported to Japan and Europe each year (Rehman et al. 2019). Ramie agriculture, industry, and trade support the livelihoods of approximately 5 million people ). However, due to the high crystallinity and orientation of ramie fibers (Tang et al. 2022), there is stiff, coarse hairiness on the surface of fabrics, which causes a strong prickle when in contact with the skin, limiting the application and development of ramie in the textile and garment industry (Peng et al. 2016).
The prickle is a physiological reaction that occurs when the fabric is subjected to a certain external force, the stiff, coarse fibers and hairiness in the fabric come into contact with the skin and stimulate the nerve endings in the lower layer of the human epidermis (Wang et al. 2021). Researchers have conducted studies to reduce or eliminate the prickle of ramie fabrics for their better application and promotion in the garment industry. These studies had two main thoughts: reducing the number of stiff, coarse hairiness or retaining them and softening. In early industry, hairiness was reduced from the fabric by singeing and shearing or using adhesives to attach it to the fabric (Liu 1995, Zhou 1985. However, after singeing and shearing, the hairiness became stiffer, and the prickle increased; the use of adhesives to attach hairiness was difficult to achieve permanence. These treatments cause damage to the body of the fabric and destroy the feel of the fabric itself. Cellulase could be used to soften the fibers to reduce the prickle (Ni et al. 2015). But cellulase demanded high culture and storage conditions, and the high cost of its use made it difficult to promote in the industry.
Zhang et al. developed a solvent of cellulose, NaOH/urea solution pre-cooled to −10°C, in which the dissolution of cellulose was achieved within 2 min (Cai and Zhang 2005) and demonstrated that it could also dissolve cellulose from ramie (Li et al. 2015). NaOH/urea solutions were subsequently used to soften ramie fabrics to reduce their prickle (Qiuran et al. 2015). By dissolving the fibers and hairiness on the surface of the fabric and then coagulating them, together with rolling and pressing, the stiff, coarse hairiness was removed to reduce the prickle. At the same time, the coagulated cellulose fixed the surface fibers and prevented them from hairiness again (Hu et al. 2017). But the NaOH/urea solutions were not suitable for largescale use in production due to the pre-cooling required before use. Copper ammonia solution was found to be a suitable solvent for cellulose in 1857, and copper ammonia can be used to dissolve cellulose at room temperature and for the production of regenerated cellulose (Dias et al. 2020).
In our previous study, the treatment of ramie fabrics with a high concentration of copper ammonia solution at a low temperature (−10°C) achieved the same effect as NaOH/urea solutions treatment in eliminating the prickle (Yang et al. 2021). However, owing to excessive dissolution on the surface of the fabric to form a cellulose film, the softness and strength of the fabric inevitably deteriorated, affecting the wearing properties of the ramie fabric. Moreover, the high concentration and lowtemperature treatment made it more expensive and unsuitable for practical use. In this work, a low concentration copper ammonia solution was used to slenderize the fiber of the ramie fabric at room temperature while removing the hairiness of the fabric to eliminate prickle and improve the softness of the fabric. The ramie fabric's breaking strength, air permeability, and moisture absorption was well retained. This strategy could eliminate the prickle of the ramie fabric without damaging its good wearing properties, and with the advantage of simple operation and low cost, suitable for use in practical production.

Preparation of copper ammonia
1 g Cu(OH) 2 was dissolved in 100 mL ammonia solution to prepare a copper ammonia solution with a Cu 2+ concentration of 100 mmol L −1 . The prepared solutions were diluted 2, 5, 10, 20 and 30 times with distilled water to obtain copper ammonia solutions with Cu 2+ concentrations of 50 mmol L −1 , 20 mmol L −1 , 10 mmol L −1 , 5 mmol L −1 and 3 mmol L −1 , respectively.

Eliminating the prickle of ramie fabrics
The purchased ramie fabrics were desized in the solution containing 10 g L −1 of NaOH at 100°C for 60 min, then washed and dried under ambient conditions. The desized ramie fabrics were treated with different concentrations of copper ammonia solution for 5 min and repeatedly rinsed in tap water until the fabric did not feel sticky to the touch. Then the treated ramie fabrics were immersed in a 1 wt% aqueous hydrochloric acid solution to remove the residual copper ammonia present in the fabrics. The obtained fabrics were washed repeatedly in tap water and dried at 60 °C.
In the same way, ramie fabrics were treated for 2 min, 5 min, 10 min, 20 min and 30 min at the optimum concentration of copper ammonia solution determined in the above experiments. The concentration of copper ammonia solution and treatment time for optimum process conditions were determined by testing.

Characterization methods
The morphology of the samples was observed by a scanning electron microscope (SEM, Phenom Pro X, NED). The X-ray diffraction (XRD) patterns were characterized by an X-ray diffractometer (TD-3500, CHN) and collected by reflection in the range of 5°--50° (2θ) in steps of 0.02° (35 kV, 25 mA) with Cu Ka radiation (k = 0.154 nm). The chemical composition of the sample was analyzed by Fourier transform infrared spectroscopy (FTIR, Nicolet iS10, USA) within the wavenumber range of 4000-400 cm −1 . The thermal behavior of the sample was studied by using thermal gravity analysis (TG, NETZSCH TG 209F3, GER). The bending rigidity of the fabric was tested using an automatic fabric stiffness tester (YG(B)022D, CHN) and set at a bending angle of 45°. The breaking strengths of ramie fabrics were measured with an electronic tensile tester (HD026N, CHN) according to ASTM D 5035-2006ASTM D 5035- (2008. The air permeability was tested by a digital fabric air permeability instrument (YG(B) 461D-II, CHN).
The samples were placed in a chamber with a temperature of 25°C and 65% humidity for 24 h, then dried to a constant weight.
The formula for calculating the moisture adsorption value (MAV) was Eq. (1): where, W 1 (kg) is the weight of the fabric at 25°C and 65% humidity for 24 h, and W 0 (kg) is the constant weight of the fabric after drying.
The apparatus for testing the static frictional coefficient of the samples was shown in Figure 1a. The static frictional coefficient of the fabric was measured using a ruler and stacked coins to construct an inclined surface. The fabric was taped to the ruler, and a 10 g weight was placed on the sample. One end of the ruler was placed on the table, and another end was lifted to a suitable height with stacked coins until the weight could start to slide (critical state of sliding); record the height (h). At this point, the weight was in the critical state of sliding, and its force analysis diagram is shown in Figure 1b, where the static frictional coefficient (μ) of the fabric could be calculated.
The formulas for calculating the static frictional coefficient (μ) were Eq.
(2) and Eq (3): where, μ is the static frictional coefficient, F (N) is the maximum static friction force, m (kg) was the mass of the weight, which is 0.01 kg, g (m s −2 ) was the gravitational acceleration, which is 9.80 m s −2 , α (rad) is the inclination angle of the inclined plane (ruler)when the weight was in the critical state of sliding. Yarns were randomly taken from the ramie fabrics before and after treatment, and the yarns were untwisted to obtain ramie fibers to make preparation. The diameter of the fibers was then measured under the optical microscope using a microscope-micrometer. Thirty fibers were selected from each sample, and the diameter of each fiber was averaged over three sites.

Results and discussion
The prickle was caused by the mechanical stimuli exerted on the skin by the protruding fiber ends on the fabric, so removing the stiff, coarse hairiness of the fabric could reduce the prickle to some extent (Garnsworthy et al. 1988). In our previous studies on the elimination of prickle in ramie fabric, the fibers and hairiness on the fabric's surface were dissolved with a cellulose dissolving agent to reduce the prickle. Then the dissolved cellulose coagulated to fix the surface fibers without using additional adhesives and prevent them form hairiness again (Hu et al. 2017, Yang et al. 2021). However, the dissolved cellulose coagulated on the surface of the fabric formed a cellulose film, making the fibers difficult to slide with each other and compromising the softness of the fabric; at the same time, causing stress concentration to affect the strength of the fabric, all of which had a negative impact on the wearing properties of ramie fabric. Moreover, using a highly concentrated solution to treat ramie fabrics at low temperatures increased the cost and was not conducive to mass production. In this work, the ramie fabric was treated with a low concentration of copper ammonia solution for the appropriate amount of time to slenderize fiber of the ramie fabric at room temperature while removing the hairiness of the fabric to eliminate prickle and improve the softness of the fabric. The treatment with a low concentration of copper ammonia solution for the appropriate amount of time was intended to dissolve the ramie fabric at a low degree to remove the hairiness while avoiding the intense dissolution to form a dissolved cellulose layer on the surface. Since there was no dissolved cellulose layer, the copper ammonia solution could enter the fabric and slenderize the fiber.
The process of eliminating the prickle in ramie fabric with copper ammonia solution is shown in Figure 2. The main component of ramie fiber was cellulose, whose intramolecular and intermolecular hydroxyl groups formed hydrogen bonds linking the cellulose molecular chains firmly together. The copper ammonia complex could interact with the two hydroxyl groups on the cellulose to form a molecular compound, which broke the hydrogen bonds and dissolved the cellulose (Burchard et al. 1994). When ramie fabrics were immersed in the copper ammonia solution, the hairiness floating on the fabric's surface was more easily dissolved than the tightly bound fibers. Firstly, the stiff and coarse hairiness of the ramie fabric was dissolved to reduce the prickle. At the same time, as the concentration of the copper ammonia solution and the treatment time were controlled to limit the dissolution of the ramie fibers, the outer cellulose layer of the fibers in the main part of the ramie fabric was dissolved while the inner layer remained intact. Then the dissolved cellulose was removed by washing to obtain smaller diameter fibers. Prickle was not only related to the number of stiff, coarse hairiness but also the diameter of the hairiness; the finer the hairiness, the less prickle (Naylor 1992). The finer the fibers in the main part of the ramie fabric, the less prickle it would feel if the new hairiness were produced again during subsequent use. And the removal of dissolved cellulose prevented the fabric from becoming stiff, improving its softness.
When using copper ammonia solution to treat ramie fabric, if the degree of dissolution were too small, it would not be sufficient to slenderize the fiber or remove the stiff, coarse hairiness on the surface; if too large a degree of dissolution would make it difficult to remove too much dissolved cellulose, and the fabric would become stiff and affect wearing comfort. The stiffer the fabric was, the less likely it was to bend and deform when in contact with the skin. The less room there was for cushioning the hairiness on the fabric's surface, the stronger the stimulation to the skin and the increased prickle. The bending rigidity of ramie fabrics after different process treatments was tested to determine the optimum process conditions for eliminating the prickle with copper ammonia solution.
As shown in Figure 3a, the bending rigidity of the ramie fabric treated with the lower concentration of copper ammonia solution increased after being treated for 5 min compared to the raw ramie fabric. It was possible that the lower concentration of copper ammonia solution only caused swelling of the ramie fibers without more significant dissolution, resulting in increased fiber diameter and additional bending rigidity of the fabric. As the concentration of copper ammonia solution increased, the diameter of the fibers decreased, the bending rigidity of the fabric decreased, and the bending rigidity of the ramie fabric treated at 10 mmol L −1 , 20 mmol L −1 , and 50 mmol L −1 was reduced by about 30 mN cm −1 compared to the raw ramie fabric. The concentration of 10 mmol L −1 copper ammonia solution was chosen for the subsequent investigation experiments to save cost.
The degree of cellulose dissolution was not only affected by the concentration of the copper ammonia solution but also by the treatment time. Before maximum solubility was reached, the longer the treatment time, the more cellulose was dissolved. The ramie fibers were dissolved from the outermost ring and gradually dissolved inwards as the treatment time increased. At the suitable treatment time, the outer layer of cellulose was dissolved and could be removed by washing to obtain smaller diameter ramie fibers. Smaller fiber diameters resulted in a fabric with less bending rigidity. The bending rigidity of the ramie fabric was reduced from 252 mN cm −1 to 187 mN cm −1 when treated with 10 mmol L −1 copper ammonia solution for 2 min (Figure 3b). As the treatment time increased, the bending rigidity of the ramie fabric increased, which may be because more cellulose was dissolved. Amounts of dissolved cellulose were difficult to remove from the fabric by washing, and coagulated to form a cellulose adhesive layer between the fibers that impeded fiber slippage, resulting in higher bending rigidity. After consideration, a Cu 2+ concentration of 10 mmol L −1 in a copper ammonia solution for 2 min was selected as the best condition to eliminate the prickle of ramie fabric.
The friction between the fabric and the human skin surface created resistance, stimulating the skin and leading to prickle. The frictional coefficient of fabric could reflect the degree of prickle generated by the fabric in use. The change in the frictional coefficient of fabric is probably related to the change in hairiness on its surface. The stiff and coarse hairiness on the surface of the ramie fabric led to a rough surface and high frictional coefficient. After treatment of ramie fabric with copper ammonia solution, the frictional coefficient of ramie fabric was first reduced compared to the raw ramie fabric (Figure 3c,  d). As shown in Figure 4a, there was vertical and irregular length hairiness on the surface of raw ramie fabric. And the ramie fabric treated with copper ammonia did not show apparent coarse hairiness (Figure 4b-e), indicating that the stiff and coarse hairiness on the surface of the ramie fabric could be largely removed after treatment. The frictional coefficient of treated ramie fabric decreased with the disappearance of hairiness.
As the concentration of copper ammonia solution increased (Figure 4b,c), the dissolution of the fibers intensified, resulting in fiber breakage and increased hairiness of the fabric after treatment (Figure 4c). The frictional coefficient of ramie fabric increased with the addition of more new hairiness. As a result, the frictional coefficient of the treated ramie fabric increased as the concentration of the copper ammonia solution increased (Figure 3c). . Photographs of surface hairiness of raw ramie fabric (a), the ramie fabric after treatment with10 mmol L-1 (b) and 50 mmol L-1 (c) copper ammonia solution for 5 minutes and 10 mmol L-1 for 2 minutes (d) and 30 (e) minutes. SEM images of raw ramie fabric(f) and the ramie fabric after treatment with 10 mmol L-1 (g) and 50 mmol L-1 (h) copper ammonia solution for 5 minutes.
As mentioned earlier, the degree of cellulose dissolution was affected by the concentration of the copper ammonia solution and the treatment time. Before maximum solubility was reached, the longer the treatment time, the more cellulose was dissolved. As the treatment time increased (Figure 4d,e), more of the cellulose was dissolved, resulting in fiber breakage and increased hairiness of the fabric after treatment (Figure 4e). Similarly, the frictional coefficient of ramie fabric increased with the addition of more new hairiness. So, the frictional coefficient of the treated ramie fabric increased as the treatment time increased (Figure 3d).
SEM investigated the surface morphology of the raw ramie fabric and treated ramie fabric. The SEM image showed that some of the unbonded single fibers extending from the yarn of the ramie fabric disappeared after treatment with 10 mmol L −1 copper ammonia solution for 5 min (Figure 4g) compared to the raw ramie fabric (Figure 4f). As the concentration of copper ammonia solution increased, treatment with 50 mmol L −1 copper ammonia solution for 5 minutes caused the ramie fabric to dissolve too much, and the fibers broke. More fibers extending out of the fabric were observed in the SEM image, with the ends splitting into more fibril (Figure 4h).
The frictional coefficient, the surface morphology of the fabric, and the bending stiffness, indicated that over dissolution treatment of ramie fabrics would lead to new problems such as producing new hairiness and higher bending stiffness. Treating ramie fabrics under the appropriate conditions could be the best performance. Ramie fabrics treated with a Cu 2+ concentration of 10 mmol L −1 in a copper ammonia solution for 2 min (Treated Ramie Fabric) were characterized and compared with untreated ramie fabrics (Raw Ramie Fabric).
Naylor proposed the following formula for calculating the deformation force (F) of fiber (Naylor 1992): where F (N) is the deformation force of fiber, E (Pa) is Young's modulus, D (m) is the fiber diameter, and L (m) is the fiber length. From Eq. (4), F and D 4 were proportional to each other. Ideally, only considering the change in the fiber diameter, and as the fiber diameter decreased, the deformation force of fiber decreased. When the fabric and the skin friction, the fibers were deformed, and the smaller diameter fibers acted with less force on the human skin, reducing the prickle. Fibers were randomly selected from raw ramie fabric (RRF) and treated ramie fabric (TRF) to test the diameter. The mean diameter of the treated ramie fabric was 24.6 μm, a reduction of 11.2 μm from the raw ramie fabric of 35.8 μm. The diameter of the fibers was significantly smaller after the treatment, and the diameter distribution was still uniform, as observed in the number distribution graph (Figure 5a). Images under the optical microscope likewise observed a thinning of the fibers after the treatment (Figure 5b). The above results indicated that the fibers of the treated ramie fabric were finer, so even if new hairiness was generated during subsequent use, its prickle was lower than that of the raw ramie fabric.
The surface morphology of the RRF and TRF is shown in Figure 5c. The SEM images showed that the surface of the raw ramie fibers was smooth. After treatment, the surface was rough, and cracks and holes were observed. The dissolution of the ramie fiber was controlled to the surface, and the main part was not damaged after treatment.
The FTIR spectra of the RRF and TRF (Figure 6a) were not obviously different and both showed the dominant characteristic peaks of ramie fabrics at wavenumbers of 3436 cm −1 (−OH stretching vibration), 2891 cm −1 (C-H stretching), 1640 cm −1 (C=C stretching vibration), 1319 cm −1 (O-H in-plane distortion vibration), 1032 cm −1 (C-O stretching vibration) and 896 cm −1 (C-O-C out-plane stretching vibration) (Peng et al. 2022, Tang et al. 2022 (Figure 6a). This indicated that the ramie fabric's chemical structure remained unchanged after the treatment with copper ammonia solution.
The thermogravimetric behavior of the RRF and TRF samples is shown in Figure 6b. The maximum thermal decomposition temperature for the raw and treated ramie fabrics were 371.6 and 358.5°C, respectively. In the range of 300-390°C, the TG diagram of TRF shifted slightly forward, attributed to the degradation of the cellulose macromolecules on the fiber surface and the increase of the amorphous zone in the treated fabric.
The XRD diffractograms of the samples before and after treatment were similar, as shown in Figure 6c,d. The diffraction peaks at 2θ = 15.0°, 16.5°, and 22.7° corresponded to the typical (1-10), (110), and (200) peaks of natural cellulose fibers, respectively (French 2020), indicating that the composition of the ramie fabric was not significantly altered after treatment. The crystallinity of ramie decreased from 80.08% to 76.54% by the software, Jade® calculation analysis, which did not decrease significantly, indicating that the crystal structure of ramie was not seriously damaged during the treatment and that the dissolved cellulose was well removed from the fabric.
In order to investigate the effect of the copper ammonia treatment on the ramie fabric's strength, the breaking strength of the fabrics was tested. As shown in Figure 6e, the ramie fabric's warp and weft breaking strengths were reduced by 46 and 78 N, respectively, after the treatment. The reduction in breaking strength was related to the ramie fabric's damaged surface zone and declining crystallinity degree. However, the strength of the TRR was not significantly damaged, indicating that under the appropriate treatment conditions, the dissolution of the ramie fiber was controlled on the surface, and the main part was not significantly damaged.
The samples were tested for air permeability and moisture absorption, with the results (Figure 6f) showing that the air permeability of the fabric remained largely unchanged, and the moisture absorption increased after the treatment. It can be explained by the fact that the fiber surface was only slightly dissolved during the treatment, and most of the dissolved cellulose was removed after washing, leaving the main fabric structure unchanged. The cracks and pores on the surface of the treated ramie fibers helped absorb water and improved the moisture absorption of the fabric.

Conclusions
In summary, a low concentration of copper ammonia solution was used to slenderize the fibers of ramie fabrics at room temperature to eliminate the prickle and improve the wearing properties.
• Elimination of prickle in ramie fabrics using cellulose dissolving agent required control of the degree of dissolution, as excessive dissolution affected the fabric's bending rigidity and frictional coefficient. By reducing the concentration of the copper ammonia solution and adjusting the treatment time, optimum treatment conditions were obtained with a Cu 2+ concentration of 10 mmol L −1 in a copper ammonia solution for 2 minutes. • The ramie fabric treated under the optimum conditions had a 25.6% reduction in bending rigidity and a 9.1% reduction in frictional coefficient, which improved the softness and smoothness while removing the prickle. • The mean diameter of the fibers was reduced from 35.8 μm to 24.6 μm, a reduction of 31.3%, reducing the degree of prickle that may arise during subsequent use. • The crystallinity of the ramie fabric was slightly reduced after treatment, and the breaking strength, air permeability, and moisture absorption were well maintained. • The copper ammonia solution at room temperature to eliminate the prickle of ramie fabrics was simple and inexpensive, suitable for use in practical production.

Disclosure statement
No potential conflict of interest was reported by the author(s).