Thermal transfer printing is a dry printing process without liquid medium. It has the advantages of vivid patterns, fine patterns, and clear layers. At present, thermal transfer printing has been widely used in synthetic fiber products, and industrial fiber production has not yet been realized on natural fibers. Our group has developed a bridging agent. After treated with this bridging agent, the cotton fabric can be thermally transferred and printed with disperse dyes. The washing resistance, friction resistance, and perspiration resistance of the product can reach national standards. However, during the experiment, it was found that the whiteness of the cotton fabrics with different sources and the same organization (referred to as the yarn count) was greatly different after the heat transfer, that is, some yellowish, some were dark, and some had good whiteness.

The heat resistance of the cotton cloth itself is relatively good, and the difference in whiteness is mainly determined by the heat resistance of different additives retained on the fabric. After the conventional pretreatment and bridging treatment of the cotton cloth before thermal transfer, the auxiliaries remaining on the cloth are mainly fluorescent whitening agents and bridging agents. It is speculated that the above phenomenon may be caused by the joint action of bridging agent and fluorescent whitening agent at a high temperature of 200°C. As there are only 1 types of bridging agents developed by this group, and there are many kinds of fluorescent whitening agents, the varieties used by different printing and dyeing plants are not the same. It is clear that the phenomenon of different whiteness is mainly caused by fluorescent whitening agents.

In order to explore the structure of bridging agents and fluorescent whitening agents and the effects of additives in commercial products of fluorescent whitening agents on the whitening effect and heat resistance, nine commonly used fluorescent brighteners were selected for testing in this paper. Industrialization of transfer printing technology provides a certain reference.

1 experiment (slightly)

1.1 Drugs and Fabrics

Fluorescent whitening agent VBL (powder), fluorescent whitening agent CXT (powder), fluorescent whitening agent VBU (liquid), Hangzhou Huayang Chemical Co., Ltd.; fluorescent brightener NC (liquid), fluorescent brightener SC (liquid ), fluorescent whitening agent CP-3D (body), fluorescent whitening agent CP-3L (liquid), Hangzhou Green Code Chemical Co., Ltd.; fluorescent brightener BA (powder), fluorescent brightener NT-3 (powder) , Nantong Lisi Chemical Co., Ltd.; this group of self-made bridging agent; fabric for the scouring, bleached cotton plain cloth.

1.2 Process flow

1) Cotton → Absorption method fluorescent whitening (dosage, temperature, time, pH, etc. in accordance with the best conditions for each brightener operation) → drying;

2) Whitening cotton cloth → bridging agent treatment (two dip two rolling, with liquid rate of about 80%) → drying;

3) Whitening cotton cloth (or bridging agent-treated cotton cloth) → Pressing at 220°C for 30 seconds with a press machine.

2 Results and Discussion

2.1 The Effect of Molecular Structure of Fluorescent Whitening Agent on the Whiteness of Thermal Transfer Products of Cotton Fabrics

Five kinds of known triazine aminostilbene fluorescent whitening agents for cotton were used to whiten the cotton cloth.

Brighteners have different whitening effects, with CXT being the best whitening effect. From the difference in whiteness, the heat resistance of CXT, NT-3, and BA was poor. The whiteness of high-pressure presses decreased by 21.92, 22.13, and 22.32, respectively, while the whiteness of VBU and VBL only decreased by 7.74 after pressing. 10.17. This may be because NT-3 and BA molecules contain multiple -OH, -OH is easily oxidized in the presence of high temperature and oxygen, resulting in a decrease in whiteness; there are two -OH in CXT molecules, the formation of hydroxyl Ethylamino is also easily oxidized under high temperature and aerobic conditions; while there is no -OH in VBU molecule, and its heat and oxidation resistance is relatively good. Therefore, it can be said that the fewer oxidizable groups contained in the whitening agent molecule, the better the heat resistance, and the whiteness of the whitened cotton cloth after thermal transfer is the better.

2.2 The effect of additives in whitening agents on the whiteness of cotton thermal transfer products

In order to investigate the effect of additives in fluorescent whitening agents on whiteness, four types of products containing the same whitening agent structure and different additives were selected for the detection of ultraviolet spectrum. The absorption of the four brighteners in the UV region is exactly the same. It can be confirmed that the structure is the same. Adding different additives is to enable them to be used in silk, acrylic, polyester, etc. in addition to the whitening of cotton. Fabrics; different additives also give them their own advantages, some good light fastness, while others have a wider range of pH applications. Then, do these additives have an effect on its whitening performance? For this purpose, whitening the cotton cloth with the above four brighteners, and then pressing the whitened cotton cloth with a thermal transfer machine, respectively, before and after pressing The whiteness values ​​(GANZ Whiteness 1 and GANZ Whiteness 2) were analyzed for their whitening effect and heat resistance.