Since the 1970s, heat transfer printing with disperse dyes with small molecular structure has been widely used on polyester fibers in China. At that time, the cellulose fiber needed to be modified by hydrophobic polymer compound, and then heat transfer printing was carried out with the above process, but the inherent moisture absorption and permeability of cellulose fiber were lost.

The wet cold transfer printing process of reactive dyes on cotton fabrics has the advantages of less water, low heat energy, less chemical consumption and little waste water. It fully meets the requirements of clean production of water saving, energy saving, dyes saving and other chemicals, and is suitable for medium high speed and mass production.

Development of wet cold transfer printing

The wet cold transfer printing of reactive dyes on cotton fiber was first developed by Dansk Printing Company of Denmark in 1984 with the "Dansk" printing process of "Cotton 20 Art". Kustor of Germany specially designed and produced the wet cold transfer printing machine for it, with the design speed of 20 m/min, which is much faster than the speed of several yards of inkjet printing. It can be used for wet cold transfer printing of natural cellulose fibers (various cotton fibers) and regenerated fibers (various regenerated fibers including viscose fibers). After several years, Shanghai Changsheng Textile Products Co., Ltd. developed the "Cooltrans" cold transfer printing machine and conducted pilot test in Changzhou Xurong Printing and Dyeing Factory; Changzhou Hanyuan New Printing and Dyeing Factory and Shaoxing Zerui Digital Control Printing Technology Co., Ltd. independently developed the wet cold transfer printing machine and corresponding technology, and used BOPP film or PET film in the printing medium. This is because the printing paper is also a cellulose fiber, which has a certain affinity for reactive dyes, making the transfer of printing dyes dirty. However, the plastic film has no affinity for dyes, and the dyes on the transfer film only adhere. After transfer printing, more than 98% of the dyes will be transferred to the surface of the fabric, making the pattern of the printed fabric clear, accurate, and eliminate color difference. The plastic film can be recycled after cleaning.

Method of Wet Cold Transfer Printing

The so-called wet transfer printing is a method of printing the fabric in a moisture containing state. Reactive dye printing must be carried out under a certain moisture content so that the fiber can be fully expanded, and the dissolved dye can diffuse and penetrate into the fiber gap and combine with the fiber. Therefore, the transfer printing of reactive dyes, using wet transfer completely conforms to the dyeing principle of dyes. What is the best moisture content in cotton fiber?

Generally, the liquid carrying rate (rolling residual rate) of fabric after dip rolling is 60% - 70%, and the fabric contains more moisture. The moisture on the fabric is mostly balanced moisture and free moisture depending on its distribution or combination with the fiber. There is always a part of moisture in wet fabrics that cannot be removed, which is called balance moisture. This part cannot be removed during the drying process. The water that can be removed during the drying process is only the part of the fabric that exceeds the equilibrium water, which is called free water. The moisture contained in the fabric can be divided into bound moisture and non bound moisture according to the difficulty of its removal.

Unbound water is the water outside the fiber on the fiber surface or in the pores of the fabric structure. It does not interact with the fiber, has weak adhesion and is easy to flow on the fabric. The second state is the binding water that can be removed. They are distributed in the network between fibers in the fabric. They do not directly interact with fiber molecules and are easier to remove.

Balanced water refers to the bound water that cannot be removed. They refer to the water that enters the fiber or combines with fiber molecules on the fiber surface. They cannot move, but they are divided into two states. One is the water in the pore network in the amorphous area of the fiber, which cannot flow freely, and is the main component of the water inside the fiber. Although the water in this state does not act directly with the cellulose fiber, it is obviously different from the water outside the fiber, so it is difficult for the water to flow in the pore network. The other state is near the pore wall, which combines with cellulose fiber through hydrogen bond or dipole force. It is not only difficult to flow, but also has different properties from ordinary water.

The free water and the bound water in the former state are easy to remove, while the bound water in the latter state is not easy to remove. The water in the combined part of the two is called the critical moisture content of the fabric, which is usually determined by experiments. The test proved that 30% cotton fiber, 38% viscose fiber and 1% polyester fiber.

When the cotton fiber contains 30% water, the expansion degree of the fiber is the largest. At this time, the inner pore network of the fiber is saturated with water, so the structure size of the fiber reaches the maximum, and the accessibility of the internal structure of the fiber is also the largest, so that the dye liquor can enter the fiber to the maximum extent. When the moisture content of cotton fiber is lower than 30%, the water content in the fiber pore network is insufficient, and the dye dissolution and fiber swelling are insufficient; When the water content exceeds 30%, the free water increases, which is not conducive to dye uptake, but increases the possibility of hydrolysis of reactive dyes, and also causes infiltration and blurring of printed patterns, affecting printing quality.

The critical moisture content of fabric is the key point of wet cold transfer printing. The critical moisture content of various fibers is different. Even the recycled fibers that are cellulose fibers, including ordinary viscose fibers Lyoeal Richcel (i.e. Polynosic and Model), have a critical moisture content of 38%, while polyester fibers have a critical moisture content of only 1%. In the 1980s, in the Pearl River Delta region, disperse dyes were widely used for heat transfer printing on polyester fibers. Because the local relative humidity is very high, and the critical moisture content of polyester fiber is 1%, they can use Stork technology to select 200~300 mesh powdered thermosol and add it to ordinary adhesives to make printing paste without washing or spraying. The hot melt adhesive can also be made into printing paste by adding thermoplastic elastic polymer compounds (such as PU, PA, etc.) into disperse dyes. Then the hot melt adhesive melts into liquid and sticks during hot pressing, which can be firmly bonded with polyester fiber. At the same time, the pressure during hot pressing makes the printing paste seep onto the fabric, so as to achieve the purpose of transfer printing.

Key to Wet Cold Transfer Printing

Reactive dye wet cold transfer printing is not so simple. First, control the fabric to reach the critical moisture content. Whether using a roller, evaporate some of the excess moisture (generally 60%~70%) on the fabric by drying, and retain 30% of the moisture on the fabric. On the other hand, the water supply is directly formed through single-sided water supply or spray, and the water content adjusted to the fabric is 30% of the fabric mass. The water content will also change with the change of the fabric weight. For wet cold transfer printing, the correctness of 30% of the fabric water content is a key.

The method is to use the microwave humidity sensor system on the equipment, which is online. It can measure the correct moisture content of the fabric without contacting the fabric to be measured. The change of the moisture content of the fabric can be measured by using the microwave through the change of the dielectric constant of water in the fabric, so as to adjust the moisture content and keep the moisture content of the fabric at 30% from the beginning, so as to facilitate the quality of wet cold transfer printing.

This technology is first of all HMF type of Manor Company in Germany and AF310 microwave humidity sensor system of Prehuang Company. Now the School of Electronic Information of Xi'an University of Engineering has also successfully studied microwave online measurement of moisture content on fabrics. The error is affected by fabric structure, thickness and yarn count density, and it needs to be fine tuned according to different fabrics.

Process of wet cold transfer printing

The second step of the process is to enter the wet transfer step after the fabric moisture reaches the standard. The fabric and the transfer film complete the transfer process through a group of rollers at the same time. No matter how the fabric is supplied with water, the fabric pH should be kept at 10~l1.5 to ensure the next color fixation process. At this time, it is necessary to adjust the alignment between the fabric edge and the transfer film edge. Both the fabric and the transfer film must be flat and wrinkle free. The fabric after transfer printing is separated from the transfer film, and the transfer film can be recycled after being packaged and washed.

The third step of the process is that the fabric after wet and cold transfer printing enters the baking stage, which means that the reactive dyes can be firmly combined with the fabric in the color fixation stage. This stage is relatively short, basically controlled above 80 ℃, and the color fixation requirements can be met in 30-60 seconds.

The fourth step of the process is post finishing. First, after the residual alkali agent and the covalent bond are formed, the ionic steaming water washing base and alkali combined salt need to be washed clean to complete the water washing process. The requirements for the post finishing agent to be added and the finished products such as door width and shrinkage are the same as those for conventional printing.

Application of Reactive Dyes in Wet Cold Transfer Printing

Next, we will discuss what kind of reactive dyes, what kind of pastes, and what other chemicals are added. The most important thing is to make transfer printing films.

Firstly, reactive dyes with high fixation rate, high lifting rate and high fastness should be selected. Generally, reactive dyes with double reactive groups of monochlorotriazine and vinyl sulfone, such as Sumifix Supra series, i.e. domestic ME type reactive dyes, have a fixation rate of 75%~82%; If Novacron C type, FN type and dark S type with higher fixation rate are selected, they are all fluorinated reactive dyes, and some troublesome ecological and environmental problems will be encountered. Generally, the radical of monochlorotriazine is chlorine atom, which is fixed in alkaline medium to generate sodium chloride. LD50 is 3760 mg/kg, low toxicity, water solubility 34.79% (25 ℃), and post-treatment is easy to wash clean, which is conducive to the improvement of dye solubility. However, the radical of monofluorotriazine is fluorine atom, and the fixation pair generates sodium fluoride in alkaline medium. The situation is different. The national sodium fluoride is moderately toxic, with LD50 of 180 mg/kg, water solubility of only 4.3% (25 ℃), and specific weight of 2.79. It is very difficult to wash the post-treatment water. In addition, the national mandatory GB 4287-2012 included AOX in the MCCs. As early as the 1980s, European countries such as Germany and Britain limited the amount of AOX in wastewater according to the relevant methods. The original silver method used in IS0 11480:1997 could not detect fluoride ions, but now GB 4287-2012 stipulates that fluoride ions can be detected by ion chromatography. Therefore, the fluorine-containing reactive dyes are characterized as AOX substances. What's more, Remazol RR is a low temperature type, which is more suitable for wet cold transfer printing. Domestic Annuoqi has also developed a low temperature type. Unfortunately, it has not formed a series, and its molecular structure is unknown, so it cannot be casually recommended.

Urea is usually added to printing paste, but its function is to absorb moisture. In wet cold transfer printing, urea will not form a low melt solution with the dye as the fixation medium, on the contrary, urea will react with the reactive dye to reduce the fixation rate. Therefore, urea does not need to be used as a moisture absorbent in the cold transfer printing of reactive dyes, so the printing process is much simpler and the pressure of wastewater treatment is reduced accordingly, because GB 4287-2012 stipulates that total nitrogen should be measured.

The paste (size) is necessary for making printing paste. The reactive dye printing paste is commonly used with sodium alginate, which is a linear polymer composed of ß - 1,4-D-mannose residue (M) and ß - 1,4-L-gulose residue (G). The molecule contains carboxyl group, which is a hydrophilic organic molecular electrolyte. It is the most stable when the pH of solution is 5.8~11.0. The other is a variety of synthetic pastes.

Preparation of transfer printing film

The intaglio printing uses a large amount of ink, and the images and texts have a convex sense, and the layers are rich. The thickness of the lines and the thickness of the ink can be controlled at will during the engraving. The printed patterns are bright and lifelike.

The process flow of intaglio printing is: preparation before printing → printing → adjustment → formal printing → post printing treatment.

Gravure printing processes the image, separates the color and sets, and then makes the gravure. The color yield during printing is controlled by the depth of intaglio. Therefore, when making intaglio, the intaglio engraving must be designed according to the depth of the pattern, so as to achieve the correctness of the dye printed on the transfer film. In gravure printing, the ink here is specially used for reactive dye ink after being treated, so that the patterns on the gravure can be smoothly printed on the film to become transfer film. Gravure printing is suitable for mass production. The non alkali agent in the dye can ensure the effectiveness of the printing film for a long time.

Reactive dye wet transfer printing ink itself is water-soluble, different from the oil soluble ink used in the past. The transfer film is plastic film, which has no affinity with reactive dyes. The dyes on the transfer film only adhere to the fabric. After transfer printing, more than 98% of the dyes are transferred to the fabric, making the pattern color of the transfer printed fabric accurate and eliminating color difference.

For the products produced by the wet cold transfer printing process, because the transfer film is made by intaglio printing, its flower shape effect almost reaches the level of the simulation photo, with rich colors, clear layers, and fine and beautiful patterns. The genuine rate of wet transfer printing is higher than that of ordinary printing machine printing. If the defects in transfer printing are eliminated, the defect rate of transfer printing fabric caused by transfer film is almost zero. The cost of mass production can be reduced to the cost of ordinary rotary screen printing or flat screen printing. Even small batch orders will not bring significant cost increase.

The whole transfer printing process requires less water, low heat energy and little wastewater to be treated, which fully meets the requirements of water saving, energy saving and resource saving environmental printing and cleaner production. After PET film is used, it can be used for many times with simple treatment, reducing printing cost and "white pollution" and resource waste.

Transfer printing of wool and silk fabrics

Except for woolen sweater and woolen woolen fabric, most light and thin fabrics (such as velutin, serge and women's thin fabric) can be printed by digital inkjet printing, and only the critical moisture content and pH of wool need to be adjusted. Other process procedures are the same as those of cotton fabrics. Only acid dye inks for wool are not produced by many manufacturers and have poor quality. It is mainly because the acid dye ink produced by some large foreign companies can meet the requirements of color depth, shade and color fastness. Silk fabrics can also be used to produce printed fabrics by transfer printing process.