Learning Point │ Analysis and Solution of the Causes of Yellowing of Pure Cotton Products 【Production Application】

In order to meet the needs of exporting special white products, knitted apparel companies have made many tests on bleaching aids (especially fluorescent whitening agents) and bleaching processes, which have increased the whiteness of bleached products and met foreign requirements. However, in recent years, the phenomenon of yellowing of special white products has attracted the attention of foreign investors. Some manufacturers' products have a good whiteness during production. However, they have undergone significant yellowing during storage and sales. This article has made a analysis.

1 Photodegradation of fluorescent brighteners on textiles

Special white products are generally treated with fluorescent whitening agents. Some manufacturers have made many tests for the selection of fluorescent whitening agents in order to increase whiteness, but they tend to focus only on the improvement of whiteness and neglect the use of whitening agents. Degree indicator test. It is understood that some brands of T-shirt garments on the Japanese market are sold under unpacking. Under long-term lighting conditions, fluorescent brighteners may degrade photostribution, causing yellowing of special white products. In this case, fluorescence The lightfastness and heat resistance of brighteners are very important.

Fluorescent whitening agents under the effect of ultraviolet light, its molecules are gradually destroyed , so the effect of whitening fabrics gradually decreases with the increase of exposure time. Various kinds of fluorescent whitening agents have different light fastnesses, and the light fastness mainly depends on the molecular structure of the fluorescent whitening agent, the nature and position of the substituents, and the improvement of light fastness is the development focus of fluorescent whitening agents. . Increasing the purity of fluorescent whitening agents can also prevent yellowing to a certain extent, which is also a development direction of brighteners. In order to reduce the yellowing of special white products, attention should be paid to the use of fluorescent brighteners with good light fastness and high purity.

2 factors of the production process

2.1 Insufficient pre-processing

Cotton fiber contains 90% cellulose, and also contains about 10% of natural impurities (called cellulose symbiont): including pectin, nitrogenous substances, waxy substances, ash (inorganic salts), pigments, and cottonseed hulls . Due to defects in the scouring and bleaching process, cotton impurities are not removed or the bottom white is too bad, which will lead to yellowing.

Therefore, for the special white products, we must pay attention to strengthening the pre-treatment, bleaching as much as possible to enhance the fabric white. Our company adopts the bleaching process of sub-oxygen bleaching. It is favored by foreign investors because of its good white background, high whiteness, long-term whiteness, and a clear, translucent feel.

2.2 Selection of softener

Cationic softeners are generally tertiary amine salts or quaternary ammonium salts derived from fatty acids or fatty amides, have excellent softening effects, and have a high adsorption capacity to the fibers, and can be sufficiently adsorbed even at a low concentration, which is economically advantageous. Therefore, many manufacturers choose cationic softeners.

However, cationic softeners have poor compatibility with fluorescent whitening agents for cotton. During the long-term storage and marketing of the product, the amine groups of the softener may also be oxidized, resulting in a certain degree of panning of the special white products. yellow. Therefore, it is best not to use cationic softeners for special white fabrics. At present, some Japanese companies and testing organizations have requested that special white products not use cationic softeners and require the use of anionic softeners.

Anionic softeners are generally composed of alkyl sulfates, sulfated fatty acid esters, sulfate esters of polyoxyethylene alkyl ethers, alkyl phosphates and the like. Anionic softeners have a smooth feel, rewetting properties, good water absorbency on cotton fibers, and poor wash fastness. Anionic softeners have good compatibility with fluorescent whitening agents for cotton and generally do not reduce brightener effects or discoloration. 24 tex (24S) chin cloth was softened with softener PA and softener GA, and then subjected to light resistance test. The test results are shown in Table 1.

3 Flue gas, especially nitrogen oxides related to antioxidants

In the presence of antioxidants and nitrogen oxides, the effects of the special white products are described below.

3.1 The existence and impact of antioxidants

Antioxidants may be present in the fibers themselves, such as cotton fibers that use insecticides in their growth. It may also originate from the textile processing process. The auxiliaries used in spinning, weaving and dyeing and finishing processes may contain antioxidant components. Antioxidants are more predominantly present in packaging materials. Grey cloths and garments are generally packaged in plastic materials. In order to prevent aging, plastic materials are generally added with antioxidants. Commonly used phenolic antioxidants: 2,6-di-tert Butyl-4-methylphenol (BHT for short). BHT will change in the presence of nitrogen oxides (NOX) (see Figure 1).

It can be seen from the reaction that BHT is nitrated in the presence of NOX, and the nitrating derivative is yellow under neutral to alkaline conditions and is colorless under acidic conditions.

3.2 Solutions to Yellowing Problems Caused by Antioxidants

3.2.1 Avoid the presence of nitrogen oxides in production and storage locations, especially if there are no direct gas heating devices or large engines in the vicinity. Monitor the nitrogen oxides in production and storage locations. Some testing organizations in Japan use a special blue fabric to hang it on site and measure it according to its color change.

3.2.2 Avoid the use of chemicals containing antioxidants in the textile processing process. In particular, try to avoid the use of packaging materials containing phenolic antioxidants.

3.2.3 Control the cloth surface pH value 5-6. As mentioned above, BHT nitration derivatives are colorless under acidic conditions. But at the same time taking into account the general cotton fluorescent brightener acid resistance is not high. Therefore, it is considered that the pH of the cloth surface of the control cloth is preferably 5-6. It can be treated with proper amount of acid during the post-treatment of the grey cloth, preferably non-volatile acids such as citric acid and tartaric acid. At the same time, a process test should be performed to control the pH value of the cloth surface of the grey cloth (refer to the pH value of the extract).

Aqueous solution pH test method: Take (5.0 ± 0.1) g of the fabric to be tested, cut into pieces of 1cm × 1 cm, in a 200 mL flask, inject 50 mL of distilled water, cover, boil, keep micro-boiling for 2 min. Then keep the flask away from the heat source, put the pieces of cloth, cover the flask lid, and place it for 30 minutes. During this time, loosen the lid and shake the flask. After 30 minutes, the pH was measured with a pH meter.

Through the above measures, the problem of yellowing due to antioxidants can be alleviated.

4 Conclusion

The yellowing of pure white cotton products during storage and sale is the result of the intertwining effects of various factors, mainly the photodegradation of fluorescent whitening agents, softeners that may change color, pH value of the fabric, and smoke, especially The effect of nitrogen oxides associated with antioxidants. For this reason, it is necessary to pay attention to the degree of pretreatment in production, the selection of fluorescent whitening agents and softeners, and the control of the pH value of the fabric should be adapted to the requirements for maintaining the stability of whiteness. The nitrogen oxides in the place to be placed should be monitored. Gray cloths and garments should be stored in locations with high nitrogen oxide concentrations. Avoid direct sunlight and keep the storage place cool and dry. In this way, the stability and durability of the whiteness of the special white product can be maintained to meet the customer's requirements.