2010-Eco-friendly Solutions for Inkjet Printing of Cellulosic Fibres

Eco-friendly Solutions for Inkjet Printing of Cellulosic Fibres

A. W. Kaimouz, Heriot-Watt University, United Kingdom


Introduction

Inkjet printing has become the new frontier in textile printing, offering advantages in process efficiency, ease of use, cost effectiveness and environmental impact. Effective use of inkjet technology with dyes requires fabric pretreatment with auxiliaries such as thickener, urea and alkali, since they cannot be used in inkjet inks as they may adversely affect the printhead and the ink properties.
Inkjet inks for textiles use dyes selected from those already commercially available.
These inks are formulated from conventional dyes with further purification to ensure good
compatibility with the printhead and to increase the ink shelf lifetime. Inkjet formulation also involves simultaneous adjustment of the ink rheological behaviour. The ideal inkjet ink should exhibit Newtonian behaviour with a constant viscosity under increasing shear rate.
The main ingredients which the ink may contain are humectant, surfactant, buffer, co-solvents, viscosity modifier, dye solubiliser, UV-blocker, and anti-oxidant. Chemical selection depends to an extent on the substrate and printhead type. The main inkjet printhead type currently used in textile printers is piezo drop-on-demand (DOD). Viscosity, surface tension, pH, conductivity, evaporation rate, dissolved oxygen content and purity requirements all vary according to the printhead type. For a DOD printhead, the general guidelines are viscosity <12 cp and surface tension >32 N m-1. The pH is
selected according to the colorant. For example, strongly alkaline conditions are avoided with reactive dyes to minimise dye hydrolysis, while acidic conditions can cause printhead corrosion.
Typically, the pH of inks containing reactive dyes should be in the range 6.5~8. Ink evaporation is a crucial characteristic of water-based inkjet inks since it has a major effect on the generation of the first drop. After a long idle period, the printhead may encounter difficulties in ejecting ink droplets, giving rise to misdirection, velocity change or no droplet at all. The evaporation rate can be modified with a humectant. A dissolved oxygen content < 4 mg/g is desirable for a DOD printhead to ensure optimum ink compressibility and is achieved commonly by degassing or ultrasonication.
In inkjet inks, monofunctional reactive dyes are favoured over bifunctional dyes because of ease of wash off of unfixed and hydrolysed dye. It has been reported in an evaluation of commercial inks that monofunctional dyes do not yield fixation greater than 70%. Such low fixation does not fulfil requirements of process efficiency and environmental protection. This paper describes a comprehensive investigation into the formulation of inkjet inks containing two bifunctional reactive dyes (a hetero-bifunctional red and homo-bifunctional mixed black), comparing their behaviour in terms of fixation, colour strength, image quality and fastness properties when applied to optimally pretreated Lyocell (standard Tencel and Tencel A100) and cotton.