Relationship between appearance and physical properties of raw cotton

Relationship between appearance and physical properties of raw cotton

Cheng, K P S

ABSTRACT

Raw cotton appearance, expressed as the degree of yellowness and the amount of trash, is analyzed. Physical properties of cotton fibers such as micronaire, length, strength, length uniformity, short fiber content, and elongation are determined. The correlation between the appearance and the physical properties of raw cotton is evaluated. The results indicate that the higher the degree of yellowness or the more trash in raw cotton, the more desirable its physical properties.

The market value of cotton is determined by some critical factors, one of which is the grade given to a cotton bale. This grade is based on the appearance of the cotton fibers such as their color and trash [2, 3, 5]. Exposure to various elements in the environment and contamination from harvesting and ginning techniques cause variations in the color and trash content of raw cotton [2, 4-6].

Cotton may become spotted by the action of fungi, insects, soil stains, or plant diseases, and frost or drought may cause various degrees of yellowness. Cotton can be discolored by oil or grease used in mechanical harvesting [2, 4-5]. Traditionally, the cotton industry places great emphasis on the color of raw cotton and regards it as an indication of quality. It is even generally believed that the presence of colors different from the normal white indicates there will be some problems in subsequent processing, and the color of finished goods will be adversely affected [2]. This raises a question as to whether a color other than white in raw cotton affects the color of the final goods. Attempts have been made to provide answers to this question.

For example, Aspland and Williams [1] researched the effect of raw cotton color grade on the color of fabric after normal wet processing. They collected raw cotton samples that were grown under the same environmental conditions, and classified them into different color grades. The cotton fibers were used to produce some knitted fabric samples, and they found that all the fabric samples were more or less indistinguishable in color after bleaching. The results indicated that the color of final goods is unlikely to be adversely affected by the color of raw cotton used to produce them.

Apart from the color of raw cotton, the amount of trash in it is also greatly emphasized by the cotton industry. Trash and foreign matter are of no use to cotton buyers, who ultimately wish to use clean cotton for their processing lines, but it is impossible to get completely clean, trash-free raw cotton. Some materials such as leaves, stems, whole seeds, seed coat fragments, and grass get into raw cotton naturally. The amount of foreign matter remaining in cotton after ginning largely depends on the trash content and the condition of the cotton at the time of harvest, the amount of cleaning during the ginning process, and the drying equipment used in ginning [2, 4-51. Because of high expectations of customers, everincreasing processing speeds, and more advanced cotton spinning systems, less contaminated raw cotton is more favorable.

Another relevant question is whether only white, clean raw cotton can provide desirable physical properties to meet cotton spinners’ requirements. In this paper, we attempt to provide a better understanding of the relationship between the appearance and the physical properties of the raw cotton by carrying out a preliminary test.

Experimental

We collected several kinds of raw cotton fibers for this preliminary test, and determined their appearance, expressed as the degree of yellowness and the amount of trash. We also measured the physical properties of these cotton fibers, including micronaire, length, strength, length uniformity, short fiber content, and elongation. The equipment used to measure the appearance and physical properties of the raw cotton samples is listed in Table I, and the results of the measurements are shown in Table II. All samples were conditioned at a temperature of 20 +/- 2(deg)C and a relative humidity of 65 +/- 2% for 24 hours prior to testing.

We also investigated the correlation between raw cotton appearance and the physical properties of the cotton fibers. The correlation coefficient (r) tells us the strength of the relationship between raw cotton appearance and the physical properties of the fibers.

Results and Discussions

The correlation between yellowness and trash content of the cotton samples is plotted in Figure 1. Figures 2 through 7 show the correlation between yellowness and physical properties of the cotton samples, while Figures 8 through 13 show that between trash content and physical properties. The strength of the relationship between raw cotton appearance and its physical properties is shown in Table III.

RELATIONSHIP BETWEEN YELLOWNESS AND TRASH IN COTTON FIBERS

In this investigation, we measured raw cotton appearance, including the degree of yellowness and the amount of trash. The yellowness of the cotton fibers is expressed as b* determined by the CIELab system: a positive increase in b* represents an increase in the degree of yellowness. The trash of the cotton fibers is expressed as visible foreign matter % as measured by AFIS. The higher the value of visible foreign matter, the more trash in the cotton fibers. As shown in Table III, there is a significant positive correlation between trash content and the yellowness of the cotton samples. This indicates that cotton fibers with more trash tend to have a higher degree of yellowness, as illustrated in Figure 1, and it implies that the presence of trash is one of the factors accounting for the yellow color of the fibers.

RELATIONSHIP BETWEEN YELLOWNESS AND PHYSICAL PROPERTIES OF RAW COTTON

Correlation coefficients indicating the strength of the relationship between yellowness and the physical properties of cotton fibers are given in Table III. Here, we see that yellowness is strongly correlated with length, strength, and micronaire, with correlation coefficients of 0.89, 0.89, and -0.87, respectively. The data indicate that the cotton samples with higher yellowness tend to have lower micronaire, but higher length and strength. These phenomena are shown in Figures 2, 3, and 4. As shown in Table III, short fiber content and length uniformity are highly correlated with cotton yellowness. This shows that yellower cotton fibers exhibit a higher tendency for lower short fiber content, but higher length uniformity. Figures 5 and 6 illustrate these tendencies. From Table Ill, we see that there is a small, positive correlation between yellowness and elongation of the cotton samples. This indicates that the yellower cotton fibers tend slightly to exhibit higher elongation, as shown in Figure 7.

In this investigation, we see that the yellower cotton fibers have more desirable physical properties, such as higher length and strength and lower micronaire. (Generally, lower micronaire fibers have smaller perimeters and are thus finer.) It is commonly known that there is a close relationship between the micronaire/fineness, length, and strength of cotton fibers. Longer fibers tend to be finer and stronger. This inherent interrelationship of physical properties can also be found in the yellower fibers.

The results of our investigation show that other desirable physical properties can also be found in the yellower cotton, including lower short fiber content and higher length uniformity. These physical properties are associated closely with the strength and length of the fibers. Stronger fibers tend to have lower short fiber content because strength is an important factor in resisting fiber damage. The stronger cotton fibers are, the fewer fibers are broken and hence the lower the short fiber content. Fiber length is associated with length uniformity, and longer cotton fibers seem to have higher length uniformity.

RELATIONSHIP BETWEEN TRASH AMOUNT AND PHYSICAL PROPERTIES OF RAW COTTON

It is also interesting to know how trash content correlates with the physical properties of raw cotton. As Table III shows, trash is strongly correlated with the short fiber content and length uniformity of raw cotton, with correlation coefficients of -0.77 and 0.70, respectively. This reveals that cotton samples with more trash strongly tend to have lower short fiber content but higher length uniformity, as shown in Figure 8 and 9.

As shown in Table III, the strength and length of cotton samples are highly correlated with the trash amount of the cotton samples. Cotton fibers with more trash tend to have higher strength and length, and these tendencies are illustrated in Figures 10 and 11.

From Table III, we see that there is a small, negative correlation between trash content and micronaire. This indicates that cotton fibers with more trash tend slightly to exhibit smaller micronaire, as shown in Figure 12. Table III and Figure 13 reveal that there is no correlation between trash content and elongation of the cotton samples.

According to this investigation, the cleaner cotton samples seem to have more unfavorable physical properties. They tend to have higher short fiber content, shorter length, lower length uniformity, and strength. One of the reasons for such findings may be the consequence of excessive cleaning during ginning. The prove cotton’s market value is usually accompanied by fiber damage and increased short fiber content. Hence, some physical properties of cleaner cotton fibers are undesirable.

Conclusions

There is a significant relationship between yellowness and trash content of our cotton samples. They also exhibit somewhat similar relationships for short fiber content, length, length uniformity, and strength. As the cotton samples become yellower and have more trash, they tend to have lower short fiber content and higher length, length uniformity, and strength. Consequently, the physical properties contributing to yarn and final product quality should not be overlooked and should be emphasized as strongly as the appearance of cotton fibers when determining the market value of cotton. Such market values can reflect the actual value of cotton, and all the valuable properties of cotton fibers can be explored.

This preliminary investigation reveals that it is possible to obtain more desirable physical properties from cotton samples with higher degrees of yellowness and more trash. For a more thorough understanding of the relationship between appearance and physical properties of raw cotton fibers, and to consolidate this preliminary investigation, further experimental work will be required.

ACKNOWLEDGMENT

We gratefully thank Sun Hing Industries Holding Limited for funding and providing samples for this study.

Literature Cited

1. Aspland, J. R., and Williams, S. A., The Effect of Cotton Grade on the Color Yield of Dyed Goods, Textile Chem. Color. 23(2), 23-25 (1991).

2. Barton, S. E., The Effects of Cotton Fiber Color Grade and Growth Area on Dyed Color Variation in 100% Cotton Knitted Fabrics, Masters thesis, Institute of Textile Technology, Charlottesville, VA, 1998.

3. El Mogahzy, Y. E., Broughton, R. M., and Lynch, W. K., A Statistical Approach for Determining the Technological Value of Cotton Using HVI Fiber Properties, Textile Res. J. 60(9), 495-500 (1990).

4. Hamby, D. S., “The American Cotton Handbook: A Ref

erence and Text for the Entire Cotton Textile Industry,” 3rd ed., Interscience, NY, 1965-1966.

5. Kothari, V. K., “Testing and Quality Management,” Ist ed., L4-FL Publications, New Delhi, 1999.

6. Steadman, R. G., and Smirfitt, J. A., Cotton Testing, in “Textile Progress,” v.27, no. 1, Textile Institute, England, 1997.

Manuscript received March 19, 2002; accepted August 9, 2002.

K.P.S. CHENG AND Y.S.J. CHENG

Institute of Textiles and Clothing, The Hong Kong Polyethnic University, Hung Hom, Kowloon, Hong Kong

Copyright Textile Research Institute Mar 2003

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