Soap fastness refers to the degree of fading of dyed fabrics after soaping in soapy liquid under specified conditions, including fading as it is and staining with white cloth. The original fading is the fading of the dyed fabric before and after soaping. The white cloth stain is a situation in which the white cloth is sewed together with the dyed material, and after the soaping, the white cloth is stained due to the fading of the dyed object.
The soaping fastness is related to the chemical structure of the dye and the state of bonding of the dye to the fiber. In addition, the soaping fastness is also related to dye concentration, dyeing process, soaping conditions and the like.
The test conditions for soaping fastness vary with the type of fiber constituting the fabric. The usual soaping temperatures can be divided into three types of 40 ° C, 60 ° C and 95 ° C (each dyeing plant has its own specific soaping temperature). After the test sample is tested, rinsed, and dried, it is rated by the “grey fading sample card” according to the national standard. The soaping fastness is divided into five grades and nine grades, of which the first grade is the worst, the fifth grade is the best, and the staining color is also divided into five grades and nine grades. The first grade staining is the most serious, and the fifth grade is not staining.