Selecting the right ink and process for digital textile printing - ink types
Digital inkjet technology currently accounts for roughly 1% of every metre printed in the textile industry. However, this figure is expected to rise as an increasing number of end users begin to run digital and analogue technology side by side or switch over to digital completely. Digital technology has already become an attractive choice for major textile applications, such as printing flags, banners, apparel fabrics, garments, technical textiles and point of purchase displays. While screen printing remains dominant in the textile industry, digital has particular advantages such as reduced setup costs, dramatically improved lead times and environmental benefits. Variable data, short runs and fast turnaround times are being demanded more frequently which plays to the strongest attributes of digital. As prepared for print fabrics and digital printing systems and inks continue to improve it is reasonable to assume demand for the technology will increase. To get the best results from a digital textile printer, considerations for the production process and its distinct but related stages are essential. This article covers the entire process from start to finish, including ink selection, fabric selection, pre-treatment and fixation, with ink selection covered first.
Textile inks are split into four main categories, each with chemical and physical properties specifically designed to work with the target fabric.
1) Reactive dye-based inks (or reactive inks) are ideal for printing cotton, linen, silk and plant derived fibres such as hemp and jute. Reactive inks have good light fastness as well as typically bright colours. Wash fastness is also good due to the chemical bond created between the dye and the fabric. Readily available and inexpensive pre-treated fabrics are often used when printing with reactive inks, although major manufacturers often do their own pre-treatment. After printing, these inks need to be post-treated by steaming and washing.
2) Acid dye-based based inks (acid inks) are designed for printing nylon, silk, wool and even leather. They are used to manufacture sportswear, swimwear, lingerie, flags, banners and accessories including ties and scarves. Like reactive inks, acid based inks are also bright but typically have better light fastness. Wash fastness is also good due to the strong bond created between the substrate and dye stuff. Pre-treated fabrics and post-process steaming and washing are also a necessity.
3) Disperse dye-based inks (disperse inks) are used for printing polyester fabrics. They are generally not as bright as acid and reactive inks. When heated to high temperatures the dye becomes gaseous and is absorbed into the polyester fibres. As the dye cools and condenses it gets trapped in the fibres, making fabrics printed with disperse inks highly resistant to laundering. Disperse inks are typically divided into low and high energy inks. The former (also known as dye sublimation inks) have good light fastness and can be printed onto paper and then transferred to polyester or printed directly on the polyester itself and heated in an oven or transfer press. Due to their excellent light fastness, high energy direct disperse inks are often chosen for the most taxing outdoor applications such as lawn furniture. All high energy disperse inks must be printed direct to fabric and then undergo heat fixation.
4) Pigmented inks use a pigment (insoluble in the ink carrier) rather than a dye (soluble in the ink carrier) to provide colouration. They contain resin binders which help the pigment particles to adhere to the fabric, meaning that the inks can be used across a very wide range of fabric types. However, the inclusion of resin in the composition can limit the amount of pigment possible, as both components raise the ink viscosity. Typically inks with high amounts of colour have low amounts of resin and thus low wash fastness. Conversely, pigment inks with good wash fastness often have higher resin and lower colour intensity. The binder properties and concentration also need to be chosen carefully to avoid affecting the 'feel' of the fabric. Pigment inks typically have very good light fastness and the ability to be used on a wide range of fabrics is a significant advantage. UV curing or heating is sufficient for fixation, and so the inks are simple to use.
In the next article we will look at fabric selection, pre-treatment and post-treatment/fixation.