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 brighteners for cotton were used to whiten the cotton cloth. The main structure of the five brighteners is shown in Figure 1, and the color-forming group is shown in Table 1.
Analysis of Factors Affecting Whiteness of Thermal Transfer Printing Products of Cotton Fabrics
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The whitened cotton cloth was hot pressed with a heat transfer machine. The whiteness values ​​before and after pressing (GANZ brightness 1 and GANZ brightness 2) were measured, and their whitening effect and heat resistance were analyzed.
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.
Analysis of Factors Affecting Whiteness of Thermal Transfer Printing Products of Cotton Fabrics
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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.
CP-3D, CP-3L, NC, SC, these four kinds of whitening agents for cotton have good whitening effect, and the whiteness value of the cotton fabric after pressing is also relatively close. It can be considered that the additive in the whitening agent product The effect on the heat resistance of brighteners is not significant, that is, it has little effect on the whiteness of cotton thermal transfer products.
2.3 The effect of bridging agent on the whiteness of cotton thermal transfer products
In order to test the effect of bridging agent on the whiteness of cotton thermal transfer products, the cotton cloth treated with different brighteners was treated with bridging agent to measure the whiteness of the fabric's GANZ 3, and then it was pressed and heated with a pressing machine. GANZ brightness 4.
The whiteness of different whitening agents for whitening cotton fabrics has increased in whiteness after treatment with bridging agents. It can be speculated that this bridging agent may have a synergistic effect with the brightener molecules and play a role in the fluorescence. Enhanced effect.
The level of fluorescence quantum yield mainly depends on the molecular structure of the optical brightener itself and the substituents introduced. In addition, the influence of the chemical environment of the brightener on the fluorescence intensity must not be ignored. The fluorescent whitening agents used in this experiment have the same conjugated chromogenic system. The system has a π*-π transition that emits strong fluorescence, and is an important transition type for fluorescence generation. When a bridging agent is added to the fluorescent whitening agent solution, the bridging agent is rich in hydroxyl groups, and an intermolecular interaction between the two forms a hydrogen bond. These hydrogen bonds can cause the electron cloud distribution of the fluorescent whitening agent molecules to shift. Increase the degree of conjugated π electrons. The larger the degree of conjugation, the larger the nonlocality of the electron, the easier it is to be excited, the easier to produce the π*-π transition, and the increase in fluorescence quantum yield.
Under high temperature hot pressing conditions, the whiteness of the cotton cloth treated by bridging and the cloth not treated by the bridging agent also increased. This may be because the high-temperature protective agent contained in the bridging agent protects the whitening agent so that it is not easily oxidized, thereby increasing the whiteness of the cotton cloth. UV absorption curve for fluorescent whitening agent VBU. It can be found that the maximum absorption wavelength of VBU is at 350 nm, and its absorption value is significantly increased after the bridging agent treatment, indicating that the synergistic effect of the bridging agent makes the absorption of the VBU in the ultraviolet region enhanced. According to the visible light reflectance curve of the optical brightener VBU on the cotton cloth, the maximum reflection wavelength of the VBU is in the blue-violet light region of 440 nm. After adding the bridging agent, the fluorescence reflection in this region is also significantly enhanced.
It can be seen that the bridging agent can enhance the absorption of the fluorescent whitening agent in the ultraviolet region, reflecting a stronger blue-violet light in the blue-violet visible region, and the high-temperature protective agent in the bridging agent can also enhance the whitening agent. Heat resistance, helps increase the whiteness of thermal transfer cotton cloth.
3 conclusions
There are two ways to improve the whiteness of cotton thermal transfer products:
1) Improve heat resistance by screening brighteners or modifying the brightener structure;
2) Increase initial whiteness and heat resistance by applying builders such as bridging agents.
Reprinted from: China Textile Economic Information Network