For many years, the use of the advantage of thermal transfer printing with disperse dyes has been applied to the printing of natural fibers. However, none of the existing equipment can meet the requirements of the general market for hand-feeling and color fastness. This article describes a new device based on reactive dyes.
Keywords: transfer printing, reactive dyes, process
1 Existing system
In order to understand the reasons for the lack of color fastness, it is necessary to know the color theory of polyester dyeing with disperse dyes. In order to dye the polyester fiber, the fiber is heated above the glass transition temperature. Above the glass transition temperature, the dye dissolves into the polyester fiber causing the fiber to color. The dye is anchored to the fibers as they cool.
When dyeing natural fibers (such as cotton), the disperse dyes are insoluble so that the fibers cannot be colored and the dyes only adhere to the surface of the fibers. In order to ensure that natural fibers can be colored, a polymer (like a polyester) must be used to dissolve the disperse dyes.
2 Color fastness issues
In order to ensure good wet fastness, it is necessary to use a polymer with a glass transition temperature higher than that required for water washing, otherwise the dye will dissolve and bleed when washed. The polymer must also adhere well to the fibers to prevent mechanical wear.
Photofastness is a common problem when all dyes are present in thin polymer layers. If the dye is destroyed in the thin layer of polymer, the basic color of the textile will change, which is considered to be disadvantageous.
3 A new process
In fact, in the past 20 years, the above problems have not found a solution, which forced people to seek new ways.
When Dansk issued its patent in 1993, a solution was born. This patent includes the types of dyes known in the dyeing and printing industry.
The advantage of this method is its inherent fastness and no hand-feel (except dyes for printing and dyeing). When printing, 60% of the cellulose fibers are significantly too high, while about 18% of the polyester and 17% of the polyester/cotton blends can be printed on the thermal transfer printing paper, so a suitable cellulose fiber must be found. Printed dye category. From a technical point of view, it is clear that reactive dyes are the most suitable.
4 reactive dyes
Reactive dyes have the advantage of having good colorfastness. From a technical point of view, its advantage lies in the use of cold-fixing dyes, without using excessive steam and maintaining a low fixing cost. Therefore, the chemical materials used in the dyeing of cold pad dye piles, as well as in the pre-treatment and post-treatment processes, are known.
5 Process Introduction
5.1 Paper
The current process uses 30 g/m2 of transfer printing paper with reactive dyes with cold fixing. To prevent the dye from reacting with cellulose, the paper is coated with a varnish before printing. Like the use of thermal transfer printing paper, the same roller and the same gravure printing machine can be used for printing.
5.2 Textiles
As with conventional printing, textiles can be pre-treated because the transfer printing itself is similar to short-term stacking and must ensure good water absorption.
5.3 Transfer Printing
The transfer printing was performed in a wet state on a calender specially developed by Klieverik (Fig. 1). The fabric is first soaked in a dyebath and passed through a pad dyer to a specific humidity. The paper and fabric are then fed into the calender. Printing without heating can be performed with only pressure. Therefore, two adjustable printing cylinders are installed on the calender, and the linear pressure of the printing cylinder can be adjusted to 50 N/mm.
The fabric and paper are then separated and rolled up separately. In order to prevent the printed pattern from contaminating the back of the fabric, the fabric is rolled up with a film. In order to fix the dye, the rolled fabric was placed at 20 °C for 12-16 h. The fabric is then separated from the film and washed with water as if using a reactive dye. Due to the high degree of fixation, this process requires less water to flush than direct printing.
6 calender
The maximum transfer rate of the calender is 30 m/min. In fact, the current transfer printing rate is about 20-25 m/min. The reason why the higher transfer printing rate can be achieved is that the actual transfer process is increased in the pre-processing stage. In the pre-treatment process, the paper and the fabric come into contact with each other at low pressure, and the dye adheres to the printing paper. Only in the high pressure printing stage does the dye transfer to the fabric.
The penetrability of the print can be increased/decreased by raising/lowering the pressure during the printing stage. For this reason, 90% of fabrics can be printed with standard baths. The prescriptions listed in Table 1 need to be adjusted only when processing highly demanding textiles. The transfer calender is designed to be manipulated by a single person. In order to keep the fabric tension constant, the dancer roller is built into all drives.
In order to ensure that the circular knit fabric can be printed, an open roll is provided at all key points to prevent curling. Through a special unwinding machine, when the fabric is wound, the printed side can be rolled inwards or rolled outwards.
When a thin textile is printed, the calender can be equipped with a cleaning device because the interlining is contaminated.
In general production, there is no need to clean the printed lining cloth.
Source: China Washing & Dyeing Industry Information Center