Vapor transfer printing was developed in the 1960s for printing fabrics made of synthetic fibers. To date, there are no commercial methods for transfer printing fabrics made from natural fibers.
The printing of polyester fabrics with low molecular weight disperse and solvent dyes, which readily sublime at around 200 °C, is the most important vapor transfer printing method. . The dyes used have only a limited number of polar substituents and are relatively free of dispersing agents and diluents.
Transfer Printing Definition
Transfer printing in the textile industry usually means the sublimation of thermally stable dyes from a colored design on paper at high temperature followed by absorption of the dye vapors by synthetic fibers in the fabric. The paper presses against the fabric and dye transfer occurs without any distortion of the pattern.
Dye Selection for Transfer Printing
Dye selection is critical. The dyes must sublime on heating but must also have adequate fastness to washing and hot pressing. This is possible for printing polyester but is not usually the case with other types of synthetic fibers.
When printing with dye mixtures, compatible dyes with comparable transfer properties are essential to ensure color constancy. Incompatible dyes may give width and length variations in color because of differences in their rates of vaporization and of absorption.
- Dyes should sublime at 195-210°C
- Nonionic volatile dye
- Belong to disperse dye class
Trade names of some dyes
Which Fabrics Can Be Printed With This Technique?
- The fabric usually has a high proportion of hydrophobic fibers such as polyester since the vaporized dyes are not strongly absorbed by natural fibers.
- Cotton/ polyester fabrics with up to 50% cotton can be transfer printed provided a resin finish has been applied. The vaporized dyes absorb into the polyester fibers and into the resin finish in the cotton.
- With melamine-formaldehyde pre-condensates, curing of the resin and vapor transfer printing can be combined into one operation.
- The fabric must be dimensionally stable up to a temperature of 220 °C during the transfer period to ensure good pattern definition.
- Heat setting or relaxation by scouring prior to printing is therefore essential. The process also eliminates spinning and knitting oils.
A key step is production of the appropriate paper with the colored design. The inks containing the volatile dyes are printed onto one surface of the paper.
- The paper has low permeability towards the printing ink solvent to ensure good pattern definition.
- Strong paper
- Paper should be dimensionally stable
- Paper weight 35-115 GSM
- Good release of dye vapor from ink layer
- Should have smooth surface
- Should be metal free
- Resistant to heating to 220 °C, is necessary. At this temperature, the release of the dye vapor from the ink layer must be quite rapid.
- The paper is printed using a variety of techniques, the most popular being rotary screen printing. This technique allows more color on the paper surface, and printing of wider papers than with other continuous printing methods.
- Resins in the ink bind the dyes to the paper and prevent marking-off onto the back of adjacent paper layers when the paper is rolled up.
- The resin, and any other additives such as thickeners, should not hinder dye vaporization nor transfer onto the fabric themselves.
Paper Printing Method
The industrial methods for paper printing are:
- Off-set lithography
- Digital printing
- Screen Printing
How Transfer Printing Actually Works?
- Even though the paper is in contact with the fabric during printing, there is a small air gap between them because of the uneven surface of the fabric. The dye vaporizes when the back of the paper heats up and the vapor passes across this air gap.
- For vapor phase dyeing, the partition coefficients are much higher than for aqueous systems and the dye rapidly adsorbs into the polyester fibers and builds up.
- There is an initial temperature gradient across the air gap but the fiber surface soon heats up and the dye can then diffuse into the fibers. In most respects, the printing mechanism is analogous to Thermosol dyeing in which disperse dyes are vaporized from cotton and absorbed by polyester fibers.
Image Source: Basic Principle of Textile Coloration
Transfer Printing Process
- In garment printing with flat-bed printers, production rates are quite low. The paper and fabric are uniformly pressed face to face. The back of the paper contacts a uniformly heated plate at around 200 °C. The transfer takes from 20 to 60 s.
- Continuous fabric printers use heated calendars and give printing speeds around 20 m min–1. A boiling liquid inside the calendar condenses all over the inner metal surface and ensures uniform heating of the outer shell.
- The paper winds around the heated calendar, its printed face in contact with the fabric. The two layers are held in place under slight pressure by an endless Nomex blanket.
- Nomex is a heat resistant aramid (aromatic polyamide) fiber manufactured by Du Pont. A backing paper inserted between the fabric and blanket prevents marking-off onto the latter.
- The pressure and high temperature required for vapor transfer printing can cause some compression and surface glazing of the thermoplastic fabrics.
- Both the fabric winds around a partially evacuated perforated cylinder with the paper on the outside. No blanket is required as the air pressure holds the two layers in place. Infrared heaters placed around the cylinder increase the temperature to vaporize the dyes.
- Alternatively, a flexible metallic mesh holds the paper and fabric against a heated calendar surface. A vacuum chamber with suitable seals covers the mesh around about three-quarters of the calendar surface and pulls air through the paper and fabric layers.
- Vapor transfer printing with vacuum is possible using shorter heating times and allows effective dye transfer at temperatures around 160 °C.
Image Source: Basic Principle of Textile Coloration
Limitations of Transfer Printing
- The major restrictions of vapor transfer printing include the rather limited range of volatile but fast dyes.
- The difficulties of printing wide materials.
- The inability to print fabrics containing high proportions of natural fibers.
- Each length of material gives an equal length of used print paper. This is not usually suitable for recycling, nor would this be practicable.
- In some cases, the dye penetration into the yarns tends to be rather poor and fabrics often expose underlying white fibers when stretched.
- Once printed cannot be corrected
- Long storage of paper is a little bit risky
Advantages of Transfer Printing
- One major advantage of transfer printing is that no washing-off is needed.
- No need of much chemicals.
- Very little surface dye to remove from the fabric.
- A skilled printer is required
- A clear and sharp definition object is required.
- No adverse effect on luster and feel of fabric
- Eco friendly
Sum Up on Transfer Printing
Nowadays, transfer printing is on a hit list while the printers want to print on t-shirts. Though the installation process is a little bit costly but the result and satisfactory production of transfer printing process has earned much popularity.