Textile knowledge: analysis of similarities and differences between Chinese and American Cotton grading Standards

The United States is a big country in cotton production and consumption in the world. Half of the cotton produced each year is used for domestic consumption and half for export. Therefore, the cotton standard of the United States has an important influence in the world and is used by 24 countries and regions, which is also known as the universal cotton standard. The following is a brief introduction to the similarities and differences between Chinese and American cotton grading standards:

Cotton grade is a reflection of the comprehensive performance of cotton quality. there is a cotton grade test in the cotton standards of most cotton-producing countries in the world. In the American cotton standard, the grade standard was originally determined comprehensively by color sign, rolling and leaf shavings. Because all cotton processing adopts automatic processing technology, 99% of the processing quality meets the requirements, and the rolling index is basically not taken into account; while the leaf shavings are separated from 1998 and the leaf chip grade is set up separately. Therefore, now the grade of American cotton is actually just a color grade. In our country, the grade conditions are maturity, color characteristics and rolling quality, and the grade is determined comprehensively by these three conditions.

1. Color level

The color grade of American cotton is determined by reflectivity (Rd) and yellowness (+ b), and there is a corresponding color map. According to the yellowness, American cotton can be divided into five types: White cotton (code 1), light stain cotton (code 2), spot stain cotton (code 3), yellowish dye cotton (code 4) and yellow dye cotton (code 5). Each type is divided into several grades according to its grayscale (that is, reflectivity, indicating the degree of light and shade of cotton). At present, American upland cotton has 25 formal color grades and 5 external grades, of which 15 have physical standards, and other color grades have no physical standards, only descriptive standards. The physical standards of American cotton are all midline standards (China's is the bottom line standard).

The color level is represented by a two-digit code, the first bit represents the level, the second bit represents the type, such as code 11 represents white cotton GM level, code 42 represents light dirty cotton SLM grade, and so on. American cotton is divided according to its grayscale, while in China, the grade is divided by yellowness, so the change of cotton grade in China is equivalent to that in the United States.

The color level can be tested by an instrument, and the color level of the corresponding instrument will be obtained where the test results fall on the color map of American cotton. It can also be graded by manual method, and the color grade can be determined by comparing with the physical standard. When there is an objection between the two, the sensory test results of the grader shall prevail, because the instrument classification can only test the surface condition of the sample and can not fully reflect the characteristics of the cotton sample.

2. Length

The significant difference in the length index between Chinese and American cotton standards lies in the difference in grade distance, which stems from the different usage habits of the measurement unit system between China and the United States. The length of cotton in the United States is divided into 19 grades with a distance of 1 0.794mm (32 inches). Our country is divided into 7 length classes according to the distance of 1mm. Another difference is that the expression of cotton length is different. American cotton uses the average length of the upper half, which is widely used in the world, while our country still uses the main length of cotton. The experimental results show that the results of the two representation methods have a very high correlation (the correlation coefficient is 0.99) and are basically the same in numerical value. The following is a comparison table of the conversion of 1x32-inch units of measurement to metric length.

3. Micronaire value

Micronaire value is a comprehensive index reflecting the fineness and maturity of cotton. In the American cotton standard, micronaire value is the quality assessment index of upland cotton in the United States, and is directly related to the price of cotton. The Macroclone value is a normal micronaire value in the range of 3.5-4.9, and the price is not deducted; the price is increased in the range of 3.7-4.2; outside the range of 3.5-4.9, it is deducted from the price, and the greater the deviation from the normal micronaire value, the greater the deduction.

The micronaire value is also used as the assessment index of cotton quality in the new cotton standard in China, and the division method is consistent with that of the American cotton standard.

4. Fracture specific strength

As an important physical property index of cotton fiber, the breaking specific strength has an important influence on the yarn quality. In the United States, because cotton production is concentrated in the hands of a small number of farmers, the production scale is large, the cotton variety is relatively single, the degree of mechanization of harvesting is high, and the technology level of cotton ginning is advanced. Therefore, in the late 1980s, the United States began to use HVI900 to test the breaking specific strength of almost all American cotton bales one by one as the basis of settlement price.

There are many methods to test the fracture specific strength, different instruments, different spacing length or using different calibration cotton samples, the fracture specific strength is different, and the measurement unit of fracture specific strength is also different. The following evaluation index of the breaking specific strength of American cotton uses the HVI instrument, 1/8in (3.2mm) spacing, and the unit of measurement is g / Tex.

In the cotton standard of our country, only the specific strength of fracture is taken as the reference index, which is the basis for making the physical standard of cotton grade. The reference value of the specific strength of all levels of China's cotton standard listed below, the distance is 3.2mm ICC check level, the unit of measurement is Li Niu / Tex.