The New Standard Of Silk Quilt: How To Measure The Weight Gain Of Silk Floss?

In May 1, 2020, the GB/T 24252 - 2019 "silk quilt" standard was formally implemented. Compared with the 2009 version, the standard revised the old indicators which were no longer suitable for the new problems appearing in recent years, and added new assessment contents to further improve the quality of silk quilts. But after the implementation of the new standard, many testing organizations are also faced with some problems: how to operate the new inspection items? Is there a uniform operation standard? What problems should the inspectors pay attention to?

Therefore, we invited Sun Chenxiao, an expert of Suzhou Fiber Inspection Institute, one of the drafting units of GB/T 24252, to interpret the test of silk weight gain.

Sun graduated from China Medicine University in 2007 and graduated from Soochow University in 2014. He has been engaged in textile chemistry testing for more than 10 years. Since 2014, he has been the head of chemistry project of Suzhou Fiber Inspection Institute, and has participated in a number of national and provincial scientific research projects. GB/T 24252 - 2019, one of the drafters of the new standard of silk quilt.

1 the new standard requires that silk can not be treated with weight gain and has added a qualitative test method. What is the basis of this test?

In recent years, the results of risk monitoring test showed that silk was seriously weighted by silk floss. In 2018, the market surveillance and Supervision Bureau of Suzhou carried out the risk monitoring of silk. The detection rate of the weight gain of silk products was 15% (40 batches of 6 batches were detected). In 2019, the detection rate of weight gain was 6.25% in the silk inspection conducted by the national product quality safety risk monitoring center of the State Administration of market supervision (5 batches of 80 batches were detected). Moreover, according to the test results, the weight gain ratio of the treated silk is 20%~100%, and the average increase is around 50%, which is equivalent to that of the consumer buying a three kg silk quilt. The phenomenon of weight gain has seriously disturbed the healthy development of silk market and infringed the legitimate rights and interests of consumers.

However, there is no obvious difference between the weight increasing treated silk and the conventional silk in terms of micro morphology and dissolution state. Conventional component testing methods such as microscopy, dissolution and combustion can not detect weight gain components. Therefore, in the new silk quilt standard, the test of increasing weight of silk is added. In order to standardize the behavior of enterprises and protect consumers' legitimate rights and interests under the new standards,

2 what kinds of silk weight is added to silk? What are the main methods in the new standard?

According to the literature, the weight gain methods of silk fiber include glycerol weight gain, metal salt weight gain, sericin immobilization technology weight gain, silk fibroin immobilization technology weight gain, polyacrylamide grafting technology weight gain and so on. The most common factor is the weight gain of glycerol and the weight gain of polyacrylamide grafting technology. In early sample studies, glycerol weight gain accounted for a large proportion. The weight gain of glycerin is mainly due to the high absorbency of glycerol. Therefore, abnormal weight data usually appear in moisture regain test with glycerin weight increasing silk floss.

But in recent years, through a lot of research and testing practice, it has been found that most of the weight gain of silk fibroin is increased by polyacrylamide grafting technology. It is difficult to detect the weight of silk silk with polyacrylamide graft. The qualitative and quantitative determination of fiber content, moisture regain, oil content and residual gum content, including many other technical indexes, are not significantly different from those of silk without added weight.

In the new standard, there are 4 kinds of tests on silk weight gain agents. They are GB/T 32016 - 2015, determination of silk amino acids and appendix C, D and E in GB/T 24252 - 2019 silk quilt. Appendix C and D methods can only test the grafting technology of polyacrylamide, appendix E and standard GB/T. The 32016 method can test most types of weight increasing silk floss.

3 how to operate the silk weight increasing agent? What is the requirement for the test result?

In the new edition of silk quilt, the appendix C method is to add concentrated hydrochloric acid (12moL/L, 36%~38%) into silk silk floss, then form a light yellow solution after dissolving in room temperature. After a certain period of time, the weight of silk can be determined by changing the color of the solution. Under normal circumstances, the weight increase of silk floss will become purple, but the weight increase will not change. As shown in Figure 1.

Fig. 1 appendix C results of weight gain test of silk

Appendix D method is to add a certain amount of hydrochloric acid (6moL/L) to silk silk floss, and put it into a constant temperature drying box, and put it in a high temperature (110 C) for a period of time to hydrolyze it. Generally speaking, no weight gain of silk floss generally does not precipitate, and the weight increasing silk floss will precipitate. As shown in Figure 2.

Fig. 2 appendix D results of weight gain test of silk

The pretreatment method of appendix E and GB/T 32016 standard is similar to appendix D method. After pretreatment, the acidic hydrolysate is removed and then dissolves with the corresponding reagent. Appendix E method is pre column derivatization, the sample solution is derivatized and determined by HPLC, and the content of amino acids is determined.

GB/T 32016 is the simple solution of sample solution detected by amino acid analyzer, because the equipment is equipped with post column derivatization system, so it can be derivatized in the instrument and determine the content of amino acid. The standard stipulates that the total amount of all amino acids in silkworm protein should be over 95%. If the content of 4 amino acids (serine, glycine, alanine and tyrosine) is determined, the sum of 4 amino acids should reach more than 77%.

4 what is the difference between the weight increasing methods of silk floss in the standard?

Appendix C and D methods are relatively simple, require less equipment and test time, but can not quantify the weight gain, and have limitations of weight increasing agents. These two methods, which are mainly oriented to enterprise factories, can be used as an effective means for internal quality control of enterprises.

Appendix E pretreatment is similar to appendix D. After acid hydrolysis, the content of amino acids is obtained by pre column derivatization, and then the amino acid derivatives are determined by liquid chromatography. The amino acid content is used to verify whether the weight of silk quilts is increased. This method has relatively long testing time and has certain requirements for instruments and equipment. This method is suitable for most of the testing institutions in China. Daily detection of silk is an effective means of weight gain.

GB/T 32016 standard can effectively measure all amino acids in silk protein. The result of amino acid content test is high accuracy and high accuracy, but the equipment investment is large and the detection cycle is long. It is suitable for the institutions with amino acid analyzer to carry out weight gain test or silk protein composition research.

5 what are the difficulties in the test and what are the requirements for the testing personnel and equipment?

There are two main difficulties in silk fibroin weight gain.

The first is the limitation of weight increasing agents. Appendix C and D can only test the weight increasing silk of polyacrylamide grafting technology, and can not effectively monitor other kinds of weight gain. Appendix E and GB/T 32016 may not be able to accurately test the weight gain of protein substances. However, most of the weight gains of silk silk floss are all grafted by polyacrylamide, so these 4 methods are feasible.

Two, appendix C and D are not very suitable for testing tussah silk. Appendix E method and standard GB/T 32016 method can interfere with tussah silk test. When testing problems, we can use multi method joint testing.

For the testers, the appendix C and D methods in the new version of the standard are relatively simple to operate, as long as they are qualified by some simple safety trainers. Appendix E method and standard GB/T 32016 method. Instruments and equipment require the laboratory to be equipped with high performance liquid chromatography or amino acid analyzer. The experimenter needs a certain instrument and equipment operation basis and has certain analytical chemistry knowledge, and has higher requirements for personnel and equipment.

6 from the test results, what is the impact of weight gain on silk products?

First of all, economic interests are affected. The study found that the most important influence of weight gain on silk is that the silk protein of unit quality is replaced by the weight increasing agent, and the quality and comfort of silk are affected.

Second, it has potential health hazards. The weight gain agent itself may be toxic, for example, the weight gain of metal salt will have the risk of heavy metal residues.

The effect of increasing weight silk floss on the physical properties of silk is not clear and needs further study.