SPI Magazine Gems

WEB_02swatchesEntering the digital era has reduced costs, time of delivery, machinery needed, fabric storage, space, energy and water usage. Still, some advantages of screen printing relating to the quality expected by customers were not available in direct digital printing on fabric for many years, one being the ability to print on fabric blends using pigments. This, however, has changed significantly and is now possible.

Pigments vs. dyestuffs

Dyestuffs are colour molecules specially designed for a specific kind of textile fibre. The most common dyes used by the textile industry are reactive dyes for natural cellulose fibres like cotton and flax, plant-derived fibres like viscose and rayon, and also for silk. Acid dyes are for natural protein fibres, like wool and all types of animal hair, but also silk and nylon. These two types of inks equire special chemical processes to create a permanent bond with the fibre, without changing its properties. They are also used for dyeing fibres and fabrics, and for high quality screen printing and digital printing on fabric. Dyestuffs are transparent; only white or very light fabrics can be dyed or printed with them. They are not used to print on finished garments, with screens or digitally, because a complex process of pre-treatment, steaming and post-washing is necessary to fix the colour and eliminate all chemicals added, which would spoil the garment. What is more, they require large amounts of water and can only be printed on 80% to 100% blends of a fibre to obtain good colours and fastness.

Other very popular dyes in the textile industry are disperse dyes for polyester. These dyes are known for their capability to sublimate to vapour and enter synthetic fibres on exposure to heat and pressure, this being a very simple method of adding colour to a material. The bond is more physical than chemical, and it can last forever, as long as the fibre itself. The substrate must have a high percentage of the fibre to obtain good colour, but because polyester is not a suitable fabric for use directly on the skin, this process is only recommended for external garments such as jackets, home decorating items or even advertisement.

WEB_03swatchesPigments are colour molecules unrelated to a specific fibre. They do not bond with the fibres and instead have to be “glued” to them using specially designed binders. Binders are monomers dissolved in printing pastes that polymerize on exposure to heat and transform into in a kind of network which, in this case, holds the pigment molecules to the fabric. Pigments are incorporated in printing pastes for screen printing. They are mixed with various thickeners and binders to create special effects on fabric.

Unlike dyestuffs, pigments can be used on dark fabrics and, depending on the type of paste, they can be transparent or opaque, thin or thick, water- or plastic-based. White pigment is used for highlights and as a background for coloured images on dark fabrics. Metal effects are also considered to be a type of pigment. Although chemical methods have been developed for using pigments to dye fabrics and in digital printing, pigments are more commonly used in screen printing on garments because they are simple to apply, the printed fabrics can be fibre blends, and they reduce water consumption. For these reasons, pigments have been on the minds of developers for decades for digital printing on fabric, but for many years the results were dull colours that would lose their richness after only a few washes. But these issues have been resolved today.

WEB_05swatchesDigital printing with pigments

Pigments are not a single type of colour molecule with fixed characteristics. Almost every colour available for any application can have a different formula and will react in a different way with the pastes and substrates. What we are experiencing now is the result of extensive work to create a set of inks capable of binding to fibres without the need for highly complex processes. I always define printing as a method of “staining” fabrics in controlled fashion: We decide which areas will be coloured and which will not, and this produces the image. Keeping a colour in place for a long
period of time without staining other areas of a substrate is another way to see it.

The pigments developed for digital printing “stain” the fibres without requiring complex processes. Pigmented inks therefore can be used with or without pre-treatment, which in this case is a secret formula that is added to the fabric prior to printing using a foulard to obtain an evenly distributed coating on a dry and wrinkle-free roll of fabric. After printing, the fabric has to be fixed by applying heat and pressure. Depending on the machine used to fix the colours, the pressure level will influence the results. With a calander, the pressure applied will make the colours last longer, but may compromise their saturation because they are essentially “pushed” into the fibres.

Using a foulard avoids applying pressure on both sides of the material at the same time, allowing the colours to be evenly fixed and saturated. If no pressure is the colour brightness will be maintained but the washing fastness will be reduced, as it is the case of a heat tunnel, only recommended to cure the fabric before fixing. After fixation, the fabric is ready to be sewn. If it has a coating, it will feel slightly hard, but no extra washing is needed because the formula uses the minimum amount of coating required. Without pre-treatment, the fabric will feel softer to the touch.

The two swatches at the top were printed with pre-treatment, the ones below without. Shown on the right are the same swatches after washing. It is difficult to see the difference in a photo.
The two swatches at the top were printed with pre-treatment, the ones below without. Shown on the right are the same swatches after washing. It is difficult to see the difference in a photo.

Printing with or without pre-treatment?

These newly developed inks are capable of staining a fabric in a semi-permanent way, to obtain a printed material that should last at least as long as the garment itself. Pre-treatment helps more pigment to penetrate and hold to the fibre, but it is not fundamental for obtaining an attractive printed fabric. To prove this, experiments were conducted on the same fabric, using the same printed design, machine and fixing settings, created using d-gen inks. The sample without pre-treatment is “lighter” than the one with pre-treatment. After washing half of each swatch, it was evident that the colours did not fade on either one.

The coating therefore is needed to enhance colour but not washing fastness.

The best results are obtained on 100% cotton and cottonpolyester blends with less than 35% polyester. This may indicate that the pigments are more suitable for cotton than for synthetic fibres.

Machinery

Pre-treatment
When using pre-treatment, a foulard is required to add the binder and fix the colours. If you decide to print without pretreatment, a calander will suffice. The investment differs greatly, because a calander is a much simpler and inexpensive system than a foulard. It may be possible for a nearby textile company to provide pre-treatment, or for a digital print supplier to offer the service to those who buy a printer.

WEB_07printerPrinter
We used the Arachne Hexa printer from d-gen for samples and testing. This model prints on woven and low-elastic fabrics with a maximum width of 1.8 meters. It has 6 colours and reaches a speed of 52 m2/hour at 900 dpi, 40 m2/hour at 1200 dpi and 28 m2/hour at 1800 dpi. It can print reactive, acid and disperse dyes, but we recommend textile pigment ink for a waterless printing process that may be the way to transform digital printing into a truly sustainable solution.

The information for this article was obtained from Salcolor, a specialist in distributing printing solutions in greater Bogota. Salcolor offers its clients a wide range of printing machinery and services.