What are the similarities and differences between Chinese and American cotton grading standards?

The United States is a major cotton-producing and cotton-consuming country, with half of its annual cotton production used for domestic consumption and half for export. As a result, American cotton standards have an important influence worldwide and are used by 24 countries and regions; they are also known as the universal cotton standards. Below is a brief introduction to the similarities and differences between Chinese and American cotton grading standards:

Cotton grade is a reflection of the overall quality performance of cotton, and the cotton standards of most cotton-producing countries in the world include a cotton grade inspection item. In the U.S. cotton standard, the grade standard was originally determined comprehensively by color, ginning preparation and leaf trash. Because cotton processing is entirely automated, and 99% of processing quality meets requirements, the ginning preparation index is basically not considered; starting in 1998, leaf trash was separated out and a separate leaf grade was established. Therefore, the current U.S. cotton grade is in fact only the color grade. In our country, however, the grade conditions are maturity, color characteristics and ginning quality, and the grade level is determined comprehensively by these three conditions.

1. Color grade

The color grade of American cotton is determined by reflectance (Rd) and yellowness (+b), and there is a corresponding color chart. American cotton is divided by yellowness into 5 types: white cotton (code 1), light spotted cotton (code 2), spotted cotton (code 3), light tinged cotton (code 4) and tinged cotton (code 5). Each type is further divided into several levels according to its grayness (that is, reflectance, indicating the degree of lightness or darkness of the cotton). At present, American upland cotton has 25 official color grades, plus 5 below-grade levels; among them, 15 color grades have physical standards, while the other color grades have no physical standards, only descriptive standards. The physical standards for American cotton are all midline standards (ours are bottom-line standards).

The color grade is expressed by a two-digit code, with the first digit representing the level and the second digit representing the type. For example, code 11 indicates white cotton GM grade, and code 42 indicates light spotted cotton SLM grade, and so on. American cotton is graded by its grayness, while our country uses yellowness to determine grade level. Therefore, changes in the grade level of our country's cotton correspond to changes in type in the United States.

The color grade can be tested with instruments. Whichever region of the American cotton color chart the test result falls into gives the corresponding instrument-graded color grade. Manual grading can also be used, determining the color grade by comparison with physical standards. When the two disagree, the grader's sensory inspection result prevails, because instrument grading can only test the surface condition of the sample and cannot fully reflect the characteristics of the cotton sample.

2. Length

The notable difference in the length index between Chinese and American cotton standards lies in the difference in class intervals, which stems from the different customary measuring unit systems used in China and the United States. American cotton length uses 1/32 inch (0.794mm) as the class interval and is divided into 19 classes. Our country uses 1mm as the class interval and has 7 length classes. Another difference is that the methods of expressing cotton length are not the same: American cotton uses the upper half mean length, which is commonly used worldwide, while our country still expresses it by the staple length of the cotton. Test results show that the results of the two expression methods have an extremely high correlation (correlation coefficient 0.99) and are basically equivalent in numerical value. Below is a conversion table from the 1/32 inch unit of measure to metric length.

3. Micronaire value

The micronaire value is a comprehensive index reflecting cotton fineness and maturity. In the U.S. cotton standard, the micronaire value is a quality assessment index for American upland cotton and is directly related to cotton price. A micronaire value in the range of 3.5-4.9 is normal micronaire, with no premium or discount; within the range of 3.7-4.2, a premium is added to the price; outside the range of 3.5-4.9, a discount is applied to the price, and the greater the deviation from the normal micronaire value, the larger the discount.

In our country's new cotton standard, the micronaire value is also used as a quality assessment index for cotton, and its classification method is consistent with that of the U.S. cotton standard.

4. Breaking specific strength

Breaking specific strength, as an important physical performance index of cotton fiber, has an important influence on yarn quality. In the United States, because cotton production is concentrated in the hands of a small number of farm owners, the production scale is large, the cotton varieties are relatively uniform, the degree of harvesting mechanization is high, and the ginning technology level is advanced. Therefore, in the late 1980s, the United States began using HVI900 to test the breaking specific strength of almost all American cotton bales bale by bale, and used it as a basis for pricing.

There are many methods for testing breaking specific strength. Different instruments, different gauge lengths, or the use of different calibration cotton samples yield different breaking specific strength values, and the units of measurement for breaking specific strength are also different. The evaluation index for American cotton breaking specific strength given below uses the HVI instrument, 1/8in (3.2mm) gauge, and the unit of measurement is grams/tex

In our country's cotton standard, breaking specific strength is only used as a reference index and is the basis for making physical standards for cotton grade. The reference values of specific strength at each level in our country's cotton standard listed below use a 3.2mm gauge, ICC calibration level, and the unit of measurement is centinewtons/tex.