Acetylated treatment and the hydrophobic property
The effect of acetyl chloride modification on the hydrophobic properties of windmill palm fiber was discussed according to the orthogonal experimental design. The static contact angle was treated as the evaluation index. The effects of acetyl chloride and pyridine concentration, treatment temperature and time on hydrophobic modification of fiber were analyzed. The specific results were shown in Table 3. All the samples under different conditions had a static contact angle of more than 90 o. The maximum reached 148 o. The perfect hydrophobic property of acetylated windmill palm fiber may gift oil absorption ability(Do et al., 2020).
The higher the R value was, the more significant the influence of the corresponding factors on the static contact angle will have. The blank row of RE bigger than RC and RD in the control group indicated that the treatment temperature and time had no significant effect on the static contact angle of palm fiber in the range of 40 ~ 55 ℃ and 3 ~ 6 h, respectively. The other two factors judge that A (concentration of acetyl chloride) had a more significant influence on the results than B (concentration of pyridine) according to the value of R. The proper process level can be chosen when the value of k was the largest. Therefore, A2B3 was the best process for the modification of acetyl chloride. The results showed that the concentration of acetyl chloride was 12.5 vt.%, and the concentration of pyridine was 7.5 vt.% was the best solution.
Table 3
Hydrophobic characteristics of acetyl chloride modified fibers treated by different conditions
Samples | | | Factors | | | CA/ ˚ |
A | B | C | D | EBlank |
1 | 1 | 1 | 1 | 1 | 1 | 128 |
2 | 1 | 2 | 2 | 2 | 2 | 130 |
3 | 1 | 3 | 3 | 3 | 3 | 136 |
4 | 1 | 4 | 4 | 4 | 4 | 120 |
5 | 2 | 1 | 2 | 3 | 4 | 125 |
6 | 2 | 2 | 1 | 4 | 3 | 127 |
7 | 2 | 3 | 4 | 1 | 2 | 148 |
8 | 2 | 4 | 3 | 2 | 1 | 116 |
9 | 3 | 1 | 3 | 4 | 2 | 124 |
10 | 3 | 2 | 4 | 3 | 1 | 117 |
11 | 3 | 3 | 1 | 2 | 4 | 122 |
12 | 3 | 4 | 2 | 1 | 3 | 120 |
13 | 4 | 1 | 4 | 2 | 3 | 97 |
14 | 4 | 2 | 3 | 1 | 4 | 98 |
15 | 4 | 3 | 2 | 4 | 1 | 112 |
16 | 4 | 4 | 1 | 3 | 2 | 109 |
K1 | 514 | 474 | 486 | 494 | 473 | |
K2 | 516 | 472 | 487 | 465 | 511 | |
K3 | 483 | 518 | 474 | 487 | 480 | |
K4 | 416 | 465 | 482 | 483 | 465 | |
R | 100 | 53 | 13 | 29 | 46 | |
The optimum design scheme of acetic anhydride modified windmill palm fiber was discussed through the orthogonal experimental design. The results were shown in Table 4. As with acetyl chloride modification, the contact angles of all samples were above 90 o. The maximum static contact angle of acetic anhydride modification was 143 o, slightly lower than the acetyl chloride modified sample.
Table 4
The hydrophobic characteristics of acetic anhydride modified treated by different conditions
Samples | | | Factors | | | CA / ˚ |
A | B | C | D | E Blank |
1 | 1 | 1 | 1 | 1 | 1 | 143 |
2 | 1 | 2 | 2 | 2 | 2 | 123 |
3 | 1 | 3 | 3 | 3 | 3 | 134 |
4 | 1 | 4 | 4 | 4 | 4 | 107 |
5 | 2 | 1 | 2 | 3 | 4 | 123 |
6 | 2 | 2 | 1 | 4 | 3 | 127 |
7 | 2 | 3 | 4 | 1 | 2 | 108 |
8 | 2 | 4 | 3 | 2 | 1 | 91 |
9 | 3 | 1 | 3 | 4 | 2 | 126 |
10 | 3 | 2 | 4 | 3 | 1 | 137 |
11 | 3 | 3 | 1 | 2 | 4 | 114 |
12 | 3 | 4 | 2 | 1 | 3 | 121 |
13 | 4 | 1 | 4 | 2 | 3 | 130 |
14 | 4 | 2 | 3 | 1 | 4 | 136 |
15 | 4 | 3 | 2 | 4 | 1 | 114 |
16 | 4 | 4 | 1 | 3 | 2 | 132 |
K1 | 507 | 522 | 516 | 508 | 485 | |
K2 | 449 | 523 | 481 | 458 | 489 | |
K3 | 498 | 470 | 487 | 526 | 512 | |
K4 | 512 | 451 | 482 | 474 | 480 | |
R | 63 | 72 | 35 | 68 | 32 | |
The extreme value of each factor was more significant than the comparison column RE, which indicated that all factors significantly influenced the results. The influence of various factors on the hydrophobic property of fiber was B (concentration of aluminium perchlorate) > D (treatment time) > A (concentration of acetic anhydride) > C (treatment temperature), according to the value of R. The optimum technological level of each factor in the modification process was A4B2C1D3, due to the highest k value. When the concentration of acetic anhydride was 68 vt.% (at this time, the concentration of acetic acid is 32 vt.%) and the concentration of aluminium perchlorate was 0.12 wt.%, the best hydrophobic property of palm fiber velvet can be obtained after being treated at 70 ℃ for 4 h.
The best process then treated windmill palm fibers of acetyl chloride modification and acetic anhydride modification. The water static contact angles were 148 and 145 o, respectively, showing excellent hydrophobic properties.
Microstructural Characteristics
Figure 1 described the surface morphology of windmill palm short fiber before and after acetylation modification under different enlargement times. Windmill palm short fibers extracted by alkali treatment had smoother and cleaner surfaces due to the damage to the fiber cell wall. At the same time, the surfaces of acetylated modification windmill palm fibers contained many small holes and partially broken cell wall. Acetylate treatment destroyed hydrogen bonding and damages fiber cell wall(Chen et al., 2018). The windmill palm short fibers modified by acetyl chloride were twisted along the axial direction showed in Fig. 1e. The spiral-liked fiber had a shorter length after modification. The fiber surface was not smooth, with irregular granular adhesions. After acetic anhydride modification, the fiber surface appeared serious fibrillation phenomenon(Gudayu et al., 2020), and the pores on the cell wall increased significantly compared with acetyl chloride treatment. SEM imaging of Fig. 1i revealed that the pores on fiber cell wall are open and discontinuous. Acetylation modification greatly influenced the surface morphology of fibers, which made the dense structure loose, leading to the decrease of mechanical properties.
Microscopic Morphology Of Palm Fiber
Windmill palm fibers will become paper-like thin sheets with hydrophilic properties through natural air dried or oven-dried. These short fibers were connected by hydrogen bonding, which is difficult to disperse. The methylene blue-dyed liquid can be instantly absorbed by freeze-dried windmill palm fiber (Fig. 2a), showing solid hydrophilic properties. The colour of acetylated fiber was light yellow with a hydrophobic surface. The blue water drops were spherical on the surface of fiber pile (Fig. 2b). Acetic anhydride-modified fiber was white and similar to alkali-treated fiber. At the same time, the hydrophilic and hydrophobic properties were quite different. Water droplets can maintain a spherical structure on acetic anhydride modified fiber (Fig. 2c). It had a good water repellency.
Sound Absorption Property
Windmill palm fiber velvet treated with acetyl chloride and acetic anhydride were used as raw materials to prepare the nonwoven mats to study the sound absorption property. The effect of treated methods and surface density on the acoustic property was shown in Fig. 3. The sound absorption coefficient exceeds 0.2 in most frequency bands, indicating the samples all had good sound absorption performance. In contrast, the acetylated windmill palm fiber had the highest sound absorption coefficient. The alkali treated windmill palm fiber nonwoven mat had the highest acoustic property with the sound absorption coefficient of 0.43 when the surface density reached 0.187g/cm2. The lumen in the middle of the fiber and the pits in the cell wall becomes continuous pores that made sound waves pass through multiple interstices(Ferreira et al., 2021).
As shown in Fig. 3b, the peak value of the acetyl chloride-treated windmill palm fibre's sound absorption coefficient moves to low frequency with surface density increase. Then the line turned back to high frequency when the surface density reached 0.140g/cm2. The average sound absorption coefficients of the four materials have the same trend, which was 0.23,0.35,0.47 and 0.42, respectively. When the surface density was 0.047g/cm2, the sound absorption coefficient of the material increased uniformly with the increase of frequency. In contrast, the sound absorption coefficient line of the other materials t can be divided into three stages. In the first stage, the rising rate was the fastest and reached the peak value. Then the average sound absorption coefficient decreased slightly and remained stable at last. The maximum sound absorption of the samples reached 0.92 when the surface density was in the range of 0.093 to 0.14g/cm2, higher than the kapok fiber (Zheng et al., 2021). The d1 and d2 were defined as the frequency widths of the slowest or fastest material reaching and always maintaining above 0.5, respectively. There was no d1or d2 for alkali treated windmill palm fiber nonwoven mat for this material with a surface density of 0.047g/cm2 did not reach 0.5. d1 was about 1.4 times of d2 for acetyl chloride treated palm fiber nonwoven mat.
The peak value of sound absorption coefficient of materials treated by acetic anhydride gradually moved to low frequency with increased surface density. The average sound absorption coefficients of the four materials are increasing continuously, which are 0.15,0.26,0.33 and 0.42, respectively. The d1 was about 4.1 times of d2 for acetic anhydride treated palm fiber. It indicated that the surface density had a great influence on the sound absorption properties. All the samples exhibited absorption coefficients greater than 0.5 for frequencies of up to 2000 Hz, when the surface density was above 0.047g/cm2. It had the same acoustic property as coir fiber/polymer composites(Cheewawuttipong and Memon, 2021). Thus, the windmill palm fiber nonwoven mats were suitable for use as sound absorption materials.
Warm Performance
Cellulose fibers were in everyday use as thermal insulation for many years(Brzyski et al., 2019). The properties of windmill palm fiber with different treatments should be still investigated. The thermal conductivity of alkalized windmill palm fiber, acetyl chloride treated palm fiber, and acetic anhydride treated palm fiber were similar, which were 0.047 ± 0.015, 0.048 ± 0.016 and 0.050 ± 0.013 W/(m℃), respectively. The thermal conductivity of cotton and wool were 0.066 ± 0.015 W/(m℃) and 0.057 ± 0.011 W/(m℃). It much higher than windmill palm fiber samples under the same bulk density. High hollowness can catch a large amount of still air, which made windmill palm fiber have excellent thermal insulation performance and can be used as filler in the textile field.
Thermal Property
The thermal properties of untreated and acetylated palm fiber were shown in Fig. 6 − 5. The degradation of all samples was divided into three stages. In the first stage, the temperature was about 30–100 ℃, mainly the evaporate moisture content from the fibers cell wall(Asim et al., 2021). In the second stage, lignin, hemicellulose and cellulose began to degrade by breaking chemical bonds. The third stage was simultaneous carbonization and oxidation(Chen et al., 2016). The initial decomposition temperatures of windmill palm fiber treated by alkali, acetyl chloride, and acetic anhydride were 333 ℃, 336 ℃ and 356 ℃, respectively. Acetylation improved the thermal stability of the material slightly. The fastest change of TG curve was the central decomposition temperature of the material, which was 330 ℃, 337 ℃ and 356 ℃, respectively. When the temperature was 600 ℃, the decomposition of the material tended to end.Furthermore, the residual carbon content was stable. The residual carbon rate was the highest, with 22.4% of the windmill palm fiber treated by alkali. While the residual carbon content of windmill palm fiber treated by acetic anhydride was the lowest of 11.4%.