Differences in structure and thermal property among kapok, brown cotton, white cotton and green cotton which included light green cotton and dark green cotton were studied. Five kinds of fiber were tested by infrared spectrometry, X£­ray diffraction approach and thermogravimetric analysis.

The results indicate that the waxiness content of green cotton was higher and the brown cotton was lower, which mostly consisted of aliphatic alcohol; lignin and hemicelluloses content of kapok were higher; comparing with dark green cotton, suberin content of light green cotton was higher; crystallinity of colored cotton and kapok were less than that of white cotton; among colored cotton, crystallinity and grain orientation of brown cotton were higher than that of green cotton; crystallinity and grain size of kapok was the lowest in five kinds of fibers; thermal stability of dark green cotton was the best and thermal degradation temperature of kapok was the lowest.

 

With environmental protection and health being taken into account, development and utilization of natural fiber resources were attached more importance day by day, especially for all kinds of colored cotton and kapok. The performance and structure of fiber were closely related, for example, moisture regain, mechanical property and thermal stability of brown cotton, green cotton and kapok were very different and the spinnability as well, which was finally caused by the different structure for classes of these fibers. Thus, the research on the structure difference of these fibers in detail was much important to the using and development of them.

 

In this paper, a well comprehensive study on differences in hydrogen bond structure, crystallinity, grain size, orientation degree and thermal property among brown cotton, kapok and green cotton, included light green cotton and dark green cotton, which were compared with white cotton in these aspects, was made.

 

1 Experiment

1.1 Materials

White cotton, brown cotton and two types of green cotton (dark green and light green) from Sinkiang, kapok from Guangdong.

 

1.2 Infrared spectrum

With the resolution 2  cm  -1, each sample was scanned for 100 times to get the fourier-transform infrared spectrum £¨FTIR£©tested by Nicolet NEXUS670 laser Roman spectrometer. The mixture tabletting consisted of 5.0 mg dry fiber dust and 200 mg KBr was made for infrared spectrum experiment.

 

1.3 Wide angle X-ray diffraction spectrum (WAXS) test

The x-ray diffraction spectrum of fiber sample was scanned and analyzed by D/max-2550PC 18 kW rotating target X-Ray Diffractometer. Putting sample on the glassy sample stand and analyzing it under the stable condition. Measure condition was shown as follows: Ni filtering, Cu target Ka-ray, pipe pressure 40 kV, pipe flow 40 Ma, scanning speed 2¡ã/min, from 5¡ã to 50¡ã.

 

1.4 Thermogravimetric analysis

Thermogravimetric analysis was made by Perkin Elmer TGA7 Thermogravimetric analyzer. Initial mass of fiber samples were controlled in the range of 5£þ6 mg. All the tests were performed under the same N2 flow rate, with heat rate 10¡æ/min and thermal degradation temperature 25¡æ-500¡æ.

 

2 Results and Discussions

2.1 Infrared spectrum analysis

The C£þH stretching vibration was located in the range of 2,852 cm-1 £þ 2,920 cm-1 (Fig.1(a)). In this area, a single peak for white cotton appeared at 2,902 cm-1 and a familiar single peak for kapok appeared at 2,919 cm-1. while a double peak for each of two types of green cotton appeared at 2,919 cm-1 and 2,846 cm-1 respectively, and one triplet for brown cotton appeared at 2,919 cm-1, 2,902 cm-1 and 2,846 cm-1 respectively. The absorption peak at 2,919 cm-1 and 2,846 cm-1 was attributed to C£þH asymmetric and symmetric stretching vibration of non-cellulosic polysaccharides by Andreeva etc.. However, Elesini found out one double peak of C£þH asymmetric and symmetric stretching vibration of waxiness (fatty acid, fatty alcohol and fatty acid ester) also appeared at 2,919 cm-1 and 2,846 cm-1 respectively. In the area of C-H stretching vibration, the single peak for kapok was mainly caused by semicellulose (xylan), however, the double peak for two green cottons was waxiness, which was proved by the stretching vibration area of acetyl group in the range of 1,700 cm-1 - 1,800 cm-1, because, in the infrared spectrum of two green cottons, there was another absorption peak at 1,716 cm-1 was due to the acetyl group in fatty acid (Fig.1(b)), except for the absorption peak at 1,737 cm-1 was attributed to semicellulose or ester group in fatty acid ester. The waxiness content of brown cotton was less than green cotton, because C£þH stretching vibration peak of cellulose could be distinguished by infrared spectrum  which was disappeared in two green cottons. Moreover, the waxiness in brown cotton, which was not the same to green cotton but the white cotton, mainly consisted of fatty alcohol, because no absorption peak at 1,716 cm-1 and 1,737 cm-1 for fatty acid and fatty acid ester in the infrared spectrum of green cotton was found.

According to the fact of an obvious stretching peak for CH2 flexural vibration of semicellulose existing at 1,456 cm-1 in infrared spectrum, while the same peak was not appeared in white cotton and colored cotton, the semicellulose (xylan) content of kapok was proved highest (Fig.1(b)). Lignin was almost not found in white cotton. The absorption peak at 1,506 cm-1 was ascribed to aromatic skeleton vibration of lignin in the infrared spectrum of two green cottons and kapok, the stretching absorption of kapok was especially strong which indicated the highest content of lignin in it. The lignin content in brown cotton was lower for no clear absorption peak appeared (Fig.1 (b)). C£þH characteristic absorption peak for lignin at 1,465 cm-1 was clearly observed in the infrared spectrum of dark green cotton, however only one weak shoulder appeared in the infrared spectrum of light green cotton, which stood for the lignin content of dark green cotton was higher than that of light green cotton. In addition, it should be paid attention to that the absorption and vibration of acetyl group, at 1,737 cm-1 in infrared spectrum of kapok, was far stronger than we expected, because the chemical analysis showed that the content of waxiness and benzene/ethanol extractives in kapok was pretty low. So, the acetyl group degree of substitution, of which, was only in the range of 2%-4% for those unconventional xylan of angiosperms and glucan of gymnosperms. Abnormally high substitution degree of xylan in kapok was showed. That was in accordance with the study of Liyama etc., who believed that the acetyl group substitution degree of xylan in kapok was high to 1.43-1.38.

 

The resolution of stretching vibration band of infrared spectrum had been improved by second derivative technique (Fig.2). The difference in second derivative FTIR between the two green cottons was not clearly, but the wave at 3,298 cm-1 was caused by O6-H¡­O hydrogen bond stretching vibration of  crystal plane. Wave intensity of dark green cotton was higher than the light green one. On the contrary to the wave vibration at 3,298 cm^-1, wave at 1,201 cm^-1, although was related to CH₂OH group in cellulose, was due to the OH in-plane bending vibration.

Absorption peak of vibration of light green cotton was a little higher than that of dark green cotton. It was obviously that the absorption peak of vibration of white cotton was far lower than colored cotton.

Some related research indicated that there was suberin existing in cotton fiber. In the process of fiber being made, cellulose structure was distorted by suberin, which led to the absence of O6-H¡­O bridge bond. For the distortion of structure, the peak at 1,201 cm-1, due to OH in-plane bending, was appeared in FTIR. Therefore, the suberin content was fewer, which have a serious effect on cellulose structure, though the waxiness content of dark green cotton was much higher than light green cotton.

 

2.2 X-ray analysis

X-ray curve fitting of colored cotton and kapok was shown in Fig.3.  The peak of crystallization, near by 44.4¡ã, for metallic element of two green cottons and kapok, especially for the dark green cotton was found. However the peak was absence in brown cotton and white cotton, which indicates the metallic element content of dark green cotton was the highest and then followed by light green cotton and kapok, and last it was much lower for white cotton and brown cotton.

The test results on crystallinity, grain size and orientation degree of several fibers were shown in Table 1.

The crystallinity of white cotton was the highest. It was obviously the amorphous element, including lignin, semicellulose and waxiness etc, result in the crystallinity reduction of colored cotton and kapok. It was kapok; in particular, having the lowest ton and kapok. It was kapok, in particular, having the lowest crystallinity, since the semicellulose and lignin of it was far higher than that of other three fibers. Among natural colored cotton, the crystallinity of brown cotton was higher than two green cottons. With the same case as crystallinity, the grain size of kapok was the smallest in all these fibers. About the two green cottons, the crystallinity and grain size of light green cotton was all lower than dark green cotton. Especially for 101, 021 and 002 grain size of crystal plane, dark green cotton was obviously bigger than light green cotton. As shown in the analysis of infrared spectrum, there was more higher suberin content in light green cotton, which was especial enough to cause the absence of O6-H¡­O bridge bond in cellulose, therefore, it seemed to be having a remarkable effect on the crystal grain formation of cellulose. Results on the orientation calculation show that the axis orientation degree of kapok crystal grain was the best, in spite of the lowest crystallinity and grain size in five fibers. The orientation degree of brown cotton crystal grain was the same as white cotton, while that of two green cottons was lower than white cotton, especially for light green cotton.

 

2.3 Thermogravimetric analysis

The TG and DTG curves of samples were shown as Fig.4.

It was clearly the initial wave of DTG curve from 40¡æ to 100¡æ was cossresponding with the volatilization of water and then followed by the main degradation peak (Fig.4 (b)), in which a great difference among these fibers was revealed. There was a stronger shoulder at 280¡æ for kapok and a weak shoulder at 300¡æ for dark green cotton, apparently, of which the former was attributed to the degradation of rich semicellulose of kapok and the latter was likely to be caused by waxiness but not semicellulose, as the waxiness content of dark green cotton was a little higher and the semicellulose content of dark green cotton was lower than light green cotton. With the same case as white cotton, the main degradation peak of light green cotton appeared in a single peak and two peaks in front were all disappeared in FTIR of light green cotton. According to TG and DTG curves, it could be inferred the heat degradation performance between brown cotton and light green cotton was similar.

 

Data about heat degradation of several fibers were showed in Table 2.

Usually, the initial thermal degradation temperature Ti was the most important one to characterize the fiber heat stability. Using the initial temperature as an index to judge the heat stability of five fibers, it could be found out that heat stability of dark green cotton was the best and the initial temperature was 30¡æ higher than that of ordinary white cotton, and moreover, that of light green cotton, brown cotton and white cotton was similar. About these five fibers, thermal degradation temperature was the lowest, which was mainly because of the excessively high content of semicellulose and low crystallinity of kapok. Much the same to Ti, other characteristic index, such as maximum rate of degradation temperature Tp and the half degradation temperature T1/2, show the identical regularity. By means of analyzing (Tp - Ti) and (Te- Ti), a conclusion was drawn that, for fibers, the order for temperature difference was accordant, namely, kapok£¾white cotton£¾light green cotton£¾brown cotton£¾dark green cotton. For whatever the temperature span from initial to the maximum rate of degradation or the whole range of degradation temperature, kapok was all the highest and the dark green cotton was the lowest, which illustrate that, for dark green cotton, both reaching the maximum degradation rate and finished degradation was made in a short time after reaction, though having the highest degradation temperature, and it was opposite to kapok.

 

3 Conclusions

(1) The waxiness content of green cotton was a little higher, which was mostly contributed to fatty acid. As for brown cotton, the waxiness content was a little lower, which consisted chiefly of fatty alcohol. There were much higher content of lignin and semicellulose in kapok, while having an abnormally high substitution degree of xylan. Among natural colored cottons, lignin content of two types of green cottons was much higher.

 

(2) Among green cottons, the waxiness content of dark green cotton was much higher than that of light green cotton, while suberin of light green cotton was more higher and the high suberin content should result in the absence of O6-H¡­O bridge bond so as to affect the integrity of hydrogen bond structure.

 

(3) The crystallinity of colored cotton and kapok was lower than that of white cotton. Among natural colored cottons, crystallinity of brown cotton was higher than that of two types of green cotton, and crystallinity and grain size of light green cotton was all lower than that of dark green cotton. For kapok, the crystallinity and grain size was the lowest in five fibers; however, the axis orientation degree of crystal grain was the best. The orientation degree of two types of green cotton was lower than white cotton, especially for light green cotton.

 

(4) The thermal stability of dark green cotton was the best, and moreover, which of light green cotton, brown cotton and white cotton was more approximate. Heat degradation temperature of kapok was the lowest.

 

References

[1] Sun Jingxia, Wang Fumei, Liu Wei. Test and Analyses of Java Cotton & Cotton Blending Yarn Property [J]. Cotton Textile Technology, 2005, 33(6): 34-36.

[2] Yang Li, Jin Hongli. Analyses of Kapok Yarn Property [J]. Cotton Textile Technology, 2008, 36(5): 16-18.

[3] Chen Li, Huang Gu. Analyses of Influence on Green Colored Cotton Property of Pectase Treatment [J]. Cotton Textile Technology, 2006, 34(4): 26-29.

Cotton Textile Technology English Version - 2010 / 3

(2010-07-28)