Pre-treatment of Calotropis gigantea fibers with functional plasticizing and toughening auxiliary agents

Abstract

Calotropis gigantea (C. gigantea) has not yet been utilized in the textile industry due to its poor spinning capability. Here, we demonstrate the use of a functional auxiliary agent to improve the spinning possibility of C. gigantea fibers. Firstly, a functional plasticizing and toughening auxiliary agent (PTAA) with concentrations of 6%, 9%, and 12% (w/w) were sprayed on C. gigantea fibers, which were then dried at various drying temperatures for different drying periods. The physical properties, secondary structure, crystallization, and weight gain rate were examined before and after the processing treatment, and compared to another two commonly used commercial auxiliary agents, namely waterborne polyurethane and wook lubricating oil. The maximum breaking strength was achieved when the optimum concentration of the PTAA was 9 wt.%, the optimum drying temperature was 80℃, and the optimum time was 1 h. The C. gigantea fiber treated by the PTAA had better spinning possibility compared to the fibers treated by the other two commercial auxiliary agents in terms of weight gain rate (p < 0.001), initial modulus (p < 0.05), and reduced crystallinity (35.6%). The results showed that PTAA treatment is an effective way to improve the spinning possibility of C. gigantea fibers. The development of our new auxiliary agent and understanding of its effects on improving the spinning possibility of C. gigantea fibers could facilitate the development of C. gigantea fibers as a lignocellulosic raw material for applications in industrial textiles.

Keywords

pre-treatment functional auxiliary agent spinning possibility

The global demand for natural fiber has been steadily growing with the depletion of natural resources. Thus, it is important to develop alternative lignocellulosic raw materials.¹ As a potential lignocellulosic raw material, Calotropis gigantea (C. gigantea) has received intense attention in recent years. C. gigantea fiber grows in the fruit of C. gigantea and has become one of the important cellulose fibers that are derived from original plants. Its leaf extracts contain palmitic acid, diterpene, triterpene, and linoleic acid as the major phytochemical groups.² C. gigantea fibers comprise up to 80–90% hollow structures, similar to kapok fibers;³ thus, they exhibit outstanding hydrophilic or oleophilic properties.⁴ Compared to cotton fibers, C. gigantea fibers have fewer natural curls in the longitudinal direction, and they are lighter in weight,⁵ resulting in a smoother and softer surface. Moreover, C. gigantea fibers demonstrate excellent biomedical performance, such as antibacterial, anti-mosquito, and anti-mildew properties, since the extracts of Calotropis procera exhibit potent anti-inflammatory activity.^6,7 For example, it has been reported that the rate of anti-Staphylococcus aureus exceeds 99%,⁸ which can be used to develop antibacterial underwear, socks, and other personal clothing and bedding textiles. In addition, it was found that a mixture of C. gigantea fibers with cotton or synthetic fibers, such as rayon or polyester, could be used to make high-quality textile fabrics.⁹ Therefore, C. gigantea fibers promise a new, eco-friendly, and ultra-light natural plant fiber that can be used as a novel raw material in the textile industry.^1,10

However, C. gigantea has a poor spinning capability,¹¹ causing it to easily break; thus, it has not yet been utilized for potential applications. Although several surface modifications have been performed by researchers on C. gigantea's surface, the properties of these treated fibers have not been thoroughly investigated and understood, and current auxiliary agents have failed to significantly improve the key properties of the fibers, such as weight, mechanical properties (e.g., tensile strength and elastic modulus), and cohesiveness during yarn spinning processes. Luo¹² and Luo and Zhao¹³ used starch, sodium carboxymethyl starch, and polyvinyl alcohol (PVA) with various degrees of polymerization to treat the surface of C. gigantea fibers. The tensile properties of treated fibers were measured, and the results showed that the tensile strength, elastic modulus, and breaking extension ratio were significantly increased due to the film formation on the surface of C. gigantea fibers. However, the characteristics of these films and their mechanisms to improve the tensile properties of C. gigantea fibers were not thoroughly investigated. In another study,¹³ C. gigantea fibers were treated with different concentrations of waterborne polyurethane (W-PU). The results showed that the breaking extension ratio was insensitive to the concentration of W-PU, but the tensile strength and elastic modulus were increased due to the membrane generated on the surface of the fibers. A high concentration of W-PU formed clustered fibers that were difficult to separate; thus, mechanical damage could occur while reducing the tensile strength.

It was reported that the spinning capability of other fibers was also improved by modifying fibers through auxiliary agents¹⁴ or blending with other cellulose fibers, such as cotton.¹⁵ For example, to improve the cohesion of fibers, Bahi et al.¹⁴ prepared milkweed blended yarns after the surface was modified by sodium hydroxide and binding agents. However, to the best of our knowledge, improving the cohesiveness of C. gigantea fibers has not been addressed. Furthermore, Sakthivel et al.¹⁵ found that it was difficult to spin 100% Mudar yarn (Calotropis procera) without blending with other fibers. Meanwhile, a 75/25 Mudar/cotton blend was successfully spun. Hence, the lack of effective methods to increase the spinning capability of C. gigantea fibers motivated us to investigate their properties in detail.

In this work, a new functional plasticizing and toughening auxiliary agent (PTAA) was prepared to improve the spinning possibility of C. gigantea fibers during yarn spinning processes. The surface morphology, mechanical properties, secondary structure, crystallization, and weight gain rate (WGR) were examined to investigate the effects of the processing treatment of the PTAA. Two commercial auxiliary agents, W-PU and wook lubricating oil, were compared with the PTAA regarding their performance.

Materials and methods

Materials

The following materials were prepared: C. gigantea fiber (Yixing zhongchang yarn-dyed Co., Ltd, China), sodium alginate (chemically pure, Changzhou Runli Co., Ltd, China), fatty alcohol-polyoxyethylene ether (AEO, Kunshan Taiyuan Co., Ltd, China), sodium laurylsulfonate (C₁₂H₂₅NaO₃S, chemically pure, Sinopharm Chemical Reagent Co., Ltd), calcium chloride anhydrous (CaCl₂, chemically pure, Sinopharm Chemical Reagent Co., Ltd), peregal O (Kunshan Taiyuan Co., Ltd, China), W-PU (molecular weight 20,000–50,000, Fushan H.J.Unkel Ltd, China), and wook lubricating oil (the main ingredients are mineral oil, spindle oil, turkey red oil, oleic acid, emulsifiers, softeners, and surfactants, TongXiang YongJin Textile Auxiliary Co., Ltd, China). Other reagents and solvents were analytical grades.

Processing treatment

The PTAA is a mixture of high molecular weight sodium alginate (thickeners, stabilizers, and emulsifiers), softeners (AEO) (emulsification, level dyeing, and osmosis), emulsifiers (sodium laurylsulfonate) (emulsification, flotation, and soaking), non-organic metal salts (CaCl₂) (effect on weight-increasing), and non-ionic surfactants (peregal O) (emulsification, wetting, solubilization, dispersion, foaming). The PTAA was prepared by mixing these together by emulsification. According to the preliminary experiments, dyeing temperature, dyeing period, and additive concentration are the primary factors for treatment. Firstly, the PTAA with concentrations of 6%, 9%, and 12% (w/w) were prepared by adding deionized water. Then, they were sprayed on C. gigantea fiber (10 g PTAA solution per 1 g fiber). After standing for 10 min, the samples were placed in a vacuum oven for drying. The drying temperatures were 60℃, 80℃, and 100℃, the vacuum degree was 0.08–0.09 MPa, and the drying periods were 0.5, 1, and 2 h. To maximize the coverage of tests with having many permutations and combinations of the three independent variables (drying temperature, drying period, and additive concentration), we designed the tests using the orthogonal array testing technique. As a result, nine orthogonal testing groups were produced, which are given in Table 1.

Table 1.

The orthogonal array testing for the experimental parameters of plasticizing and toughening auxiliary agent treatment

Sample	A	B	C	Drying temperature	Drying period	Additive concentration
Sample	A	B	C	(℃)	(h)	(wt.%)
1	1	1	1	80	0.5	6
2	2	3	1	60	2	6
3	3	2	1	100	1	6
4	1	2	2	80	1	9
5	2	1	2	60	0.5	9
6	3	3	2	100	2	9
7	1	3	3	80	2	12
8	2	2	3	60	1	12
9	3	1	3	100	0.5	12

By comparison, the fibers were treated with W-PU and wook lubricating oil. These two agents are the most commonly used auxiliary agents in practice; thus, they were chosen as references. A total of 25 wt.% W-PU was prepared and sprayed on C. gigantea fiber (10 g W-PU was sprayed on 1 g fiber). After standing for 10 min, the samples were placed in a container with a rough surface and dried in a vacuum oven until a constant weight was reached. Meanwhile, wook lubricating oil and water with a ratio of 1:9 was sprayed on C. gigantea fiber (10 g oil–water was sprayed on 1 g fiber). All samples were placed at room temperature for 24 h to allow uniform penetration, and then used for further performance evaluation.

Test of weight gain rate and moisture regain

The sample was placed in a vacuum oven and dried at 80℃ until it reached a constant weight. The WGR was calculated according to Equation (1)

WGR (%) = 100 \times (W_{0} - W) / W_{0}

(1)

where W₀ represents the dry weight before fiber treatment and W represents the dry weight after fiber treatment.

In accordance with Chinese standard GB/T 6102.1-2006, the moisture regain value was measured by the method of oven drying.¹⁶ The moisture regain value was calculated according to Equation (2)

W (%) = (G - G_{0}) / G_{0} \times 100 %

(2)

where W represents the moisture regain value, G represents the wet weight of fibers, and G₀ represents the dry weight of fibers. The higher the calculated moisture regain value, the better the hydroscopicity of the fiber.

Microscopic observations

The surface morphology of the treated fibers in each group was examined by a scanning electron microscope (SEM) (Hitachi TM3030, Tokyo, Japan). All samples were selected randomly, and their longitudinal and transverse structures were imaged at 3 kV and 10 μA. Liquid nitrogen was used to chop and obtain the cross-sectional images of a single fiber at 1000 × and 5000 × magnifications. The surface roughness of untreated and modified C. gigantea fibers were characterized using an atomic force microscope (AFM) (Dimension Loon, Bruker Company, USA). The tapping mode was selected, the scanning range was 10 µm, and figures were saved in three-dimensional (3D) format.

Mechanical property characterization

In accordance with British standard BS EN13895:2003, the mechanical properties of treated fibers were measured.¹⁷ As shown in Figure 1, a paper was cut into a 3 cm by 4 cm rectangle, from which a 1 cm by 2 cm rectangle was cut from the center. C. gigantea fiber was put upright on a piece of paper and fixed from the upper and lower ends of the paper with scotch tape to ensure that the distance between the two ends of C. gigantea fiber was 10 mm.¹⁸ The tensile properties, including breaking force and elongation at break of treated single C. gigantea fibers, were recorded by a universal material testing machine (INSTRON-3365, Instron, USA). Each treatment was repeated 20 times under the same testing conditions to obtain reliable representative data. Pre-tension was used to remove the stress to maintain the standard testing condition for all samples. The pre-tension of fiber was given as 0.5 ± 0.05 cN/tex, the gauge length was 10 ± 0.2 mm, and the stretching speed was 5 ± 0.1 mm/min (standard testing environment: 20℃ ± 2℃, relative humidity 65% ± 3%).^19,20 In order to spray agent solution on the fibers uniformly, a novel fiber spraying equipment (China patents, No. 201810917088.1 and 201810917087.7) was installed, and this can spray the agents on the front and back sides of the fiber layer and spray as evenly as possible, and then the samples were placed in a vacuum oven for drying before the spinning process. Also, we can see from the stress–strain curves that the mechanical properties of the majority of fibers are increased significantly, which indicated that most of the fibers were sprayed evenly.

Figure 1.

(a) Illustration of the sample preparation and images of the mechanical measurement process in the laboratory. (b), (c) Tensile stress–strain diagram of treated and untreated fibers. (d) Laboratory setup to study single Calotropis gigantea (C. gigantea) fibers. (e) The paper was cut into a rectangle. (f) Measurement of the mechanical properties of a single fiber.

Calculation of the initial modulus

The initial modulus of the fiber was calculated as 100 times the stress at 1% elongation when stretched, which was examined by the Instron 3365 machine. Four groups of fibers were tested: each group of fibers was tested for 20 roots, the average value was taken for each group, and precision was expressed as the standard error with the standard deviation divided by the square root of the sample number (=σ/n^1/2).

Calculation of breaking tenacity

Considering the uneven thickness of each fiber, we calculated breaking tenacity according to Equation (3)

P_{0} = P / D

(3)

where P₀ is the breaking tenacity (cN/tex), P is the breaking force (cN), and D is the fiber linear density (dtex).

Structure characterization

The secondary structure of the C. gigantea fibers was examined using Fourier transform infrared spectroscopy (FT-IR) spectroscopy (Nicolet 5700, USA). FT-IR wave numbers were set from 4000 to 400 cm⁻¹ during 32 scans with a resolution of 2 cm⁻¹. The crystal structures of untreated and surface-modified C. gigantea fibers were analyzed by an X-ray diffractometer (X'Pert-Pro MPD, PANalytical, Almelo, Holland) with the Cu Kα anode at a current of 30 mA, a tube voltage of 40 kV, and a scan speed of 6°/min. The degree of crystallization was calculated according to Equation (4)

X_{c, a} (%) = I_{c} / (I_{c} + I_{a}) \times 100 %

(4)

where I_c is the integrated area of the crystalline reflections and I_a is the integrated area of the amorphous underneath the crystalline reflections.

Spinning trail

A pilot spinning experiment using cotton fibers and C. gigantea fiber (70/30) after PTAA treatment was conducted and C. gigantea fiber/cotton blended yarns were spun out by ring spinning. Breaking strength, breaking tenacity, elongation at break, and moisture regain of C. gigantea fiber/cotton blended yarns were measured and compared with previous studies. The surface morphology of the C. gigantea fiber/cotton blended yarns was also examined by the SEM.

Results and discussion

Breaking strength and rate of moisture regain of C. gigantea fibers

Figures 1(b) and (c) and Table 2 show the measured breaking strength and rate of moisture regain of untreated samples and treated samples. Compared to untreated samples, the PTAA-treated samples generally had increased breaking strength by 135% and rate of moisture regain by 35%, except Samples 1, 5, and 8. The peak values of breaking strength and moisture regain reached 6.97 ± 2.41 cN and 17.18 ± 1.84%, respectively, in sample groups 4 and 6. The overall increase of breaking strength may be because C. gigantea fibers sprayed by the PTAA were covered with some amounts of agents on the surface and a small amount of PTAA was penetrated into the hollow of the fibers. The decrease of breaking strength of Samples 5 and 8 may be caused by the insufficient binding between the wet fiber and auxiliary agents under the relatively low drying temperature at 60℃ within short drying periods (0.5 and 1 h).

Table 2.

The breaking strength and moisture regain rate after plasticizing and toughening auxiliary agent treatment

Sample group ID	Breaking strength	Rate of moisture regain
Sample group ID	(cN)	(%)
0 (untreated)	2.96 ± 0.65	12.70 ± 0.84
1	4.14 ± 1.21	11.39 ± 1.22
2	3.92 ± 1.13	14.35 ± 0.95
3	3.04 ± 1.36	15.72 ± 1.35
4	6.97 ± 2.41	15.12 ± 0.56
5	1.78 ± 0.52	15.20 ± 1.17
6	3.34 ± 1.36	17.18 ± 1.84
7	3.34 ± 1.02	16.18 ± 1.45
8	2.73 ± 0.53	15.44 ± 1.21
9	3.62 ± 1.04	16.82 ± 1.46

A Duncan multiple comparison test was then carried out on the breaking strength data using SPSS software, and the results are provided in Table 3. The effect of drying temperature was slightly more prominent compared to the effect of drying time; meanwhile, increasing the drying time was more effective to generate higher breaking strength than increasing additive concentration. The best test combination was A₁B₂C₃, where the drying temperature was 80℃, the drying time was 1 h, and the additive concentration was 9 wt.%. Thus, the parameters processed using the PTAA were as described above in the next experiment.

Table 3.

SPSS analysis of breaking strength after plasticizing and toughening auxiliary agent treatment

Drying temperature/ ℃	Subset	Drying time/h	Subset	Additive concentration/ %	Subset
60	2.8100	0.5	3.1800	12	3.2300
100	3.3333	2	3.5333	6	3.7000
80	4.8167	1	4.2467	9	4.0300
Sig.	0.302	Sig.	0.540	Sig.	0.638

Figure 2(a) shows that the rate of moisture regain generally increased significantly with concentration, and when the drying temperature was 100℃, moisture regains of the samples with three different concentrations reached the maximum, that is, Sample 6 reached a peak of 17.18 ± 1.84% moisture regain. The reason for the improvement in the moisture regain value of the fibers is probably that the sodium alginate in the PTAA has hygroscopicity.

Figure 2.

Moisture regain (a), where 0–9 represent the groups of specimens in Table 2. Weight gain rate (b) and mechanical properties of treated Calotropis gigantea fibers. Breaking tenacity and elongation at break (c) and initial modulus (d), plasticizing and toughening auxiliary agent (PTAA), where (4) represents groups of four specimens (the drying temperature was 80 ℃, the drying time was 1 h, and the additive concentration was 9 wt.%). W-PU: waterborne polyurethane.

Comparison of WGR

As shown in Figure 2(b), the WGR of C. gigantea fibers increased after treatment with auxiliary agents. The WGR of PTAA and wook lubricating oil-treated samples reached 72.3% and 61.7%, respectively, while the WGR of W-PU-treated sample was increased by 11.2%. The increase of WGR occurs because the inorganic salt in the treatment liquid, such as CaCl₂, attaches on the fiber surface or its hollow space. One advantage of adding inorganic salt into auxiliary agents is that the dose of CaCl₂ can be regulated to modify the WGR. The improved weight can prevent the fiber from flying easily during spinning, because the fiber is extremely light. The fibers treated by the PTAA and wook lubricating oil both possessed a higher WGR (p < 0.001) compared to W-PU-treated fibers (Figure 2(b)).

Mechanical properties of individual C. gigantea fibers

Figures 2(c) and (d) show the mechanical properties of treated C. gigantea fibers, which were significantly improved. With the treatment of W-PU, the breaking tenacity and initial modulus reached the highest among the three treatments, increasing considerably from 2.96 to 7.88 cN/dtex, and from 146.6 to 420.0 cN/dtex, respectively. However, C. gigantea fibers that were sprayed with the PTAA had the best overall performance: the breaking tenacity, elongation at break, and initial modulus of C. gigantea fibers were increased from 2.96 to 6.44 cN/dtex, 1.92% to 3.24%, and from 146.6 to 365.3 cN/dtex, respectively. A slight increase was measured for wook lubricating oil-treated fiber (4.94 cN/dtex, 3.11%, and 152.9 cN/dtex, respectively). Moreover, the fiber after processing treatment with the PTAA was more prominent than wook lubricating oil in terms of initial modulus (p < 0.05) (Figure 2(d)).

It can be seen from Figure 3 that some fillers or coatings with the auxiliary agents were added to the inside or outside of the C. gigantea fibers. The components of the auxiliary agents had some high molecular weight polymers, such as sodium alginate, and these polymers increased the relative molecular mass and the total bonding force between the molecular chains, making them resistant to slip; thus, the breaking tenacity, elongation at break, and initial modulus of C. gigantea fiber were all improved.

Figure 3.

Cross-sectional scanning electron microscope (SEM) images (a)–(d), longitudinal SEM images (a1)–(d1), and atomic force microscope images (a2)–(d2) of untreated, plasticizing and toughening auxiliary agent (4)-treated, waterborne polyurethane-treated, and wook lubricating oil-treated C. gigantea fibers, respectively.

Morphology and roughness of individual C. gigantea fibers after different treatments

SEM and AFM images of individual fibers with PTAA, W-PU, and wook lubricating oil treatments are shown in Figure 3 along with the untreated samples. The surface of C. gigantea fiber was extremely smooth and glossy, and the degree of hollowness from the cross-sectional view is extremely high, reaching 80–90% of its volume (Figures 3(a)–(a2)). However, after the treatment, the surface morphology of the treated fibers was roughened. This is because C. gigantea fibers sprayed by the PTAA were covered with some amount of agent on the surface by a rough film and filled with a small amount of the PTAA in the hollow. On the other hand, a thick layer of film was covered on the fiber surface after spraying with W-PU, and we observed that the fibers were clustered, which may lead to difficulties in opening and was not conducive to weaving, although its mechanical properties were the highest of all. Moreover, the surface of C. gigantea fibers was pleated when sprayed with wook lubricating oil, which still maintained the advantages of large hollowness. All of these treatments confirmed that the use of the PTAA, W-PU, and wook lubricating oil increased the surface friction of C. gigantea fibers significantly. Further examination showed that improvements in spinning possibility provided by the use of the PTAA and wook lubricating oil were more suitable compared to those obtained by W-PU treatment for potential applications.

The secondary structure of the fibers

The FT-IR spectra of the treated single C. gigantea fibers are presented in Figure 4(a). A broad and strong absorption peak appeared near 3360 cm⁻¹, which belongs to the stretching vibration of O-H, the absorption peak near 2900 cm⁻¹ corresponds to the stretching vibration of C-H, and the stretching vibration of the six-membered ring ether bond appeared near 1040 cm⁻¹, all of which show the characteristic absorption peak of cellulose fiber. However, the treatment of W-PU and wook lubricating oil resulted in the disappearance of the absorption peak at 3400–3200 cm⁻¹, indicating that these two methods both formed a thick film on the surface of fibers that was too thick to check the absorption peak of cellulose, and this is consistent with good mechanical results. By contrast, C. gigantea fibers sprayed by the PTAA were covered with some amount of agent on the surface and a small amount of the PTAA penetrated into the hollow of the fibers. On the other hand, there was some amount of non-cover fiber surface during this treatment, so that the cellulose peaks remained. The absorption peak at 1720 cm⁻¹ is attributed to the stretching vibration of the carbonyl group, the absorption peak at 1511 cm⁻¹ belongs to the vibration peak of the aromatic framework group, and the absorption peak at 1250 cm⁻¹ corresponds to the shear vibration of the phenolic hydroxyl group, all of which are characterized as the absorption peaks of lignin. The difference in the secondary structures of the treated fibers was not significant and can be used to characterize the characteristic peaks of C. gigantea fiber. It should be noted that in the range of 3400–3200 cm⁻¹, the peak intensity reduced after the treatment of W-PU and wook lubricating oil; however, there was no substantial change due to PTAA treatment, which indicates that the C. gigantea fibers sprayed by the PTAA was at the proper concentration covered few agents on the surface.

Figure 4.

Fourier transform infrared spectroscopy (a) and X-ray diffraction (b) patterns of untreated and treated individual C. gigantea fibers, plasticizing and toughening auxiliary agent (PTAA) (4), waterborne polyurethane (W-PU), and wook lubricating oil-treated C. gigantea fibers, respectively.

The crystal structure of the fibers

X-ray diffraction (XRD) patterns of the untreated and modified C. gigantea fibers are shown in Figure 4(b). The change in crystallinity of C. gigantea fibers was not obvious and the reflection peaks were all at the angle of 22.1°, where the crystallinity of untreated C. gigantea fibers was 40.3%, and the crystallization of the C. gigantea fibers that were sprayed with the PTAA dropped to 35.6%. On the other hand, the crystallization of fibers as a result of the other two treatments went up by 44.7% and 43.9%, respectively, indicating the presence of some other crystalline substances in auxiliaries of W-PU and wook lubricating oil, such as W-PU in the W-PU and some kind of oil in the wook lubricating oil. The decreased crystallinity of PTAA-treated fibers may be due to some high molecular polymers in PTAA, such as sodium alginate. It is well known that low crystallinity helps to improve dye uptake and dyeing rate, and it is also known that the lower the crystallinity, the stronger the moisture absorption ability; thus, the PTAA-treated fibers with the lowest crystallinity are likely to possess high dye uptake, dyeing rate, and moisture regain value.

Properties of C. gigantea/cotton blending yarn

C. gigantea fiber/cotton blended yarns were spun out by ring spinning successfully; the testing results of such blended yarn are shown in Figure 5 and the data is given in Table 4. It can be seen that the C. gigantea fibers with hollow cavities are wrapped within cotton fibers, and a small number of C. gigantea fibers are distributed in the outer layer of blended yarns (Figures 5(B) and (C)). There is an increase in breaking tenacity and elongation at break from 12.13 to 13.35 cN/tex and from 6.86% to 8.24%, respectively, which is better than the type of C. gigantea fiber/cotton (30/70) blended yarns developed by Luo et al.²¹ In addition, it is well known that the moisture regain of cotton yarn is between 6% and 8%. By contrast, the moisture regain of C. gigantea fiber/cotton (30/70) blended yarns reached 9.2%, which increased by nearly 2%. All of these data indicated that the use of the PTAA can increase the possibility of spinning.

Figure 5.

Images of Calotropis gigantea (C. gigantea) and cotton blended yarns: (A) the blending yarn cone; (B), (C) longitudinal and cross-sectional scanning electron microscope images of C. gigantea and cotton blended yarns.

Table 4.

Physical properties of Calotropis gigantea (C. gigantea)/cotton blending yarn

Parameter	Value
Components	C. gigantea/cotton (30/70)
Yarn count/Ne	32
Twist	380
Breaking strength/cN	220.52 ± 28.47
Breaking tenacity/cN/tex	13.35 ± 1.56
Elongation at break/%	8.24 ± 0.64
Moisture regain/%	9.2 ± 1.4

Conclusions

In this work, a functional auxiliary agent was prepared to solve the shortcomings of C. gigantea fibers during yarn spinning processes, and was compared to two other commercial auxiliary agents, wook lubricating oil and W-PU. During W-PU treatment, the C. gigantea fibers were more likely to cluster; thus, W-PU could not be sprayed uniformly, and the mechanical defects seemed to be caused when the fibers were separated, which might lead to the difficulty of weaving fibers. In addition, the treatment of W-PU and wook lubricating oil resulted in the disappearance of the absorption peak at 3400–3200 cm⁻¹, indicating that these two methods both formed a thick film on the surface of fibers that was too thick to check the absorption peak of cellulose, and is consistent with good mechanical results. On the other hand, C. gigantea fibers sprayed by the PTAA were covered with some amount of agent on the surface by a rough film and filled with a small amount of the PTAA in the hollow. We conclude that the effects of PTAA treatment are superior in that it offers not only the highest WGR but also improved mechanical properties, including breaking tenacity, elongation at break, and initial modulus. The orthogonal test results showed that the maximum value of breaking strength (6.97 ± 2.41 cN) could be achieved when the concentration of the PTAA was 9 wt.%, the drying temperature was 80℃, and the drying period was 1 h. The crystallinity of the C. gigantea fibers that were treated only by the PTAA was reduced, offering a better dye uptake, dyeing rate capability, and moisture regain value. Therefore, a better spinning possibility of C. gigantea fibers after being treated by the PTAA could be achieved compared to fibers treated by the other two commercial auxiliary agents in terms of WGR (p < 0.001), initial modulus (p < 0.05), and reduced crystallinity (35.6%). Finally, C. gigantea fiber/cotton blended yarns were spun out by ring spinning successfully, which indicated that the use of the PTAA does increase the possibility of spinning. Our findings can be useful for the development of C. gigantea fibers as a lignocellulosic raw material for industrial applications.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Natural Science Foundation of China (51603140), the Natural Science Foundation of Jiangsu Province (BK20150372), the University Science Research Project of Jiangsu Province (16KJB540003), the China Postdoctoral Science Foundation (2017M620223), and the Science and Technology Project of Suzhou (SYG201638 and SYG201849; projects 2018914 and 2018054).

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