Now You Know Digital Printing on Textiles: Evolution, Progression and Techniques
Sunday 27 May 2018
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Digital Printing on Textiles
Muhammad Saad Asif
NED University of Engineering and Technology Karachi, Pakistan
Email: saadsiddiqui2000@hotmail.com
Muhammad Saad Asif
NED University of Engineering and Technology Karachi, Pakistan
Email: saadsiddiqui2000@hotmail.com
ABSTRACT
In our project we have discussed the effect of pretreatment on color strength and dye fixation of digitally printed cotton fabric by using different thickeners. This report consists of three main parts such as introduction of report, methodology and the results. The first part of the report contains the details about the inkjet printing and the reason of the pretreatment. The second fragment includes the methodology in which we discussed the experimentation, the pretreatment chemicals, recipe which we followed and the testing which we performed. The third and the last part conclude the results.
CHAPTER 1
1. INTRODUCTION
Ink jet printing is also known as digital printing. It has become the major printing technology in the desktop/network printing markets. The advent of digital color printing has opened up many new application areas for ink jet including wide-format graphic arts and increasingly industrial applications such as textiles, which, until recently, were the exclusive domain of the traditional analogue printing technologies.
As the printing industry moves towards these new industrial ink jet markets then the media, whether it be coated paper, films or textiles, becomes an integral part of the technology and knowledge of the chemistry of the interaction of the ink, colorants and the media becomes vitally important.
Ink jet is a technology wherein there is no printing master and hence only the ink drops make contact with the substrate. It is therefore classified as a non-impact printing method.
The ink jet formulation, the specific print head and the complex interactions between them have all to be considered when we start to develop total ink jet solutions for industrial applications. Ink jet is a technology wherein there is no printing master and hence only the ink drops make contact with the substrate. It is therefore classified as a non-impact printing method. (1)
As the printing industry moves towards these new industrial ink jet markets then the media, whether it be coated paper, films or textiles, becomes an integral part of the technology and knowledge of the chemistry of the interaction of the ink, colorants and the media becomes vitally important.
Ink jet is a technology wherein there is no printing master and hence only the ink drops make contact with the substrate. It is therefore classified as a non-impact printing method.
The ink jet formulation, the specific print head and the complex interactions between them have all to be considered when we start to develop total ink jet solutions for industrial applications. Ink jet is a technology wherein there is no printing master and hence only the ink drops make contact with the substrate. It is therefore classified as a non-impact printing method. (1)
1.1 COMPONENTS OF INKJET PRINTER
Basically Ink jet has three basic components. These are the print head, the ink, and the medium all of which need to work well in order to produce an acceptable output.
1.2 INKJET TECHNOLOGIES
There are 2 types of inkjet technologies.
- CIJ (Continuous ink jet)
- DOD (Drop on demand).
In CIJ, ink is squirted through nozzles at a constant speed by applying a constant pressure. The jet of ink is unstable and breaks into droplets as it leaves the nozzle the drops are left to go to the medium or deflected to a gutter for recirculation depending on the image being printed. The deflection is usually achieved by electrically charging the drops and applying an electric field to control the trajectory. The name `continuous' originates in the fact that drops are ejected at all times.
 |
| Figure 1: Continuous inkjet (CIJ) |
In DOD ink jet, drops are ejected only when needed to form the image. The two main drop ejector mechanisms used to generate drops are piezoelectric ink jet (PIJ) and thermal ink jet (TIJ)
In PIJ, the volume of an ink chamber inside the nozzle is quickly reduced by means of a piezoelectric actuator, which squeezes the ink droplet out of the nozzle. In TIJ, an electrical heater located inside each nozzle is used to raise the temperature of the ink to the point of bubble nucleation. The explosive expansion of the vapor bubble propels the ink outside the nozzle. (2)
In PIJ, the volume of an ink chamber inside the nozzle is quickly reduced by means of a piezoelectric actuator, which squeezes the ink droplet out of the nozzle. In TIJ, an electrical heater located inside each nozzle is used to raise the temperature of the ink to the point of bubble nucleation. The explosive expansion of the vapor bubble propels the ink outside the nozzle. (2)
 |
| Figure 2: Drop On Demand (DOD) |
The idea of digital printing on textiles has been around for some time. The inkjet printing technology used in digital printing was first patented in 1968. Carpet inkjet printing machines have been used since the early 1970s. Digital ink jet printing of continuous rolls of textile fabrics was shown at ITMA in 1995. In the 1990s, inkjet printers became widely available for paper printing applications. Again at ITMA in 2003, several industrial inkjet printers were introduced to the market place which made digital printing on textiles the new industry standard. The technology has continued to develop and there are now specialized wide-format printers which can handle a variety of substrates – everything from paper to canvas to vinyl, and of course, fabric. These new generation machines had much higher outputs, higher resolution printing heads, and more sophisticated textile material handling systems allowing a wide variation of fabrics to be printed.
The history involves the following series of inkjet printers;
- FESPA 1996
- FESPA and ITMA 1999
- ITMA 1999
- DPI 2001
- ITMA 2003
- Drupa 2004
- SGIA 2004
- FESPA 2005
Over the last few years we have seen major changes in the global textile printing market: more individual designs, shorter run lengths and the movement of printing to Far East markets. The use of ink jet printing technology to reduce the overall cost of sample and coupon printing costs has become well established in the textile printing industry in the developed printing markets. Also the adoption of wide format ink jet printers for small scale textile print production such as the flag / banner and sportswear industries is another area that ink jet printing is making “in-roads”. We are now seeing developments in the use of industrial ink jet piezo “drop on demand” print heads capable of increasing production rates and the introduction of more ink jet machines specifically developed for textile printing production. (3)
With ink jet printing there are a wide range of print head technologies, each with specific ink physical and chemical requirements that must be satisfied for the ink formulation to perform reliably in a specific printer platform. These physical and chemical specifications are very precise and require the development of textile ink jet ink formulations, which differ from normal printing pastes, in that they cannot contain the majority of chemicals, required to achieve color yield, definition or color fastness. In addition the textile ink jet ink formulations must be developed with the aim of achieving excellent operability and firing performance, together with chemical compatibility with the materials used in the manufacture of the print head.
Digital printing includes pre-treatment of fabric prior to printing process. Pre-treatment of textiles in preparation for ink-jet printing is carried out because inclusion of auxiliary chemicals and thickeners into the low viscosity ink has proved troublesome. Thus the methodology is akin to `two-phase' conventional printing as opposed to the `all-in' approach. In the latter case all the dyes, chemicals and thickeners required are included in the print paste, whereas in the former some of the ingredients, particularly chemicals, are applied before, or after, printing.
When printing cotton the choice has generally been between reactive dyes and pigments. The pigment printing process is simpler, as it involves three main stages (print, dry, bake/cure), whereas reactive printing has two extra processes (print, dry, steam, wash-off, dry). Pigment printing is therefore a more economical procedure but we avoid its use in inkjet printing because pigments produced much duller shades than could be achieved with dyes, and there was a tendency for nozzles to block, in other words the `run-ability' was poor.
Reactive printing by the `all-in' method is the normal approach for screen printing, but for jet printing it has certain dangers. As a result the jet printing of cotton, wool and silk has generally been carried out by the `two-phase' method, the ink containing only purified dyes, the thickener and chemicals being applied to the substrate in a pre-treatment. Although the quality of the resulting prints is excellent, the extra expense of pre-treating the fabric by a pad/dry process makes the process uneconomical for anything but short runs. (4)
With ink jet printing there are a wide range of print head technologies, each with specific ink physical and chemical requirements that must be satisfied for the ink formulation to perform reliably in a specific printer platform. These physical and chemical specifications are very precise and require the development of textile ink jet ink formulations, which differ from normal printing pastes, in that they cannot contain the majority of chemicals, required to achieve color yield, definition or color fastness. In addition the textile ink jet ink formulations must be developed with the aim of achieving excellent operability and firing performance, together with chemical compatibility with the materials used in the manufacture of the print head.
Digital printing includes pre-treatment of fabric prior to printing process. Pre-treatment of textiles in preparation for ink-jet printing is carried out because inclusion of auxiliary chemicals and thickeners into the low viscosity ink has proved troublesome. Thus the methodology is akin to `two-phase' conventional printing as opposed to the `all-in' approach. In the latter case all the dyes, chemicals and thickeners required are included in the print paste, whereas in the former some of the ingredients, particularly chemicals, are applied before, or after, printing.
When printing cotton the choice has generally been between reactive dyes and pigments. The pigment printing process is simpler, as it involves three main stages (print, dry, bake/cure), whereas reactive printing has two extra processes (print, dry, steam, wash-off, dry). Pigment printing is therefore a more economical procedure but we avoid its use in inkjet printing because pigments produced much duller shades than could be achieved with dyes, and there was a tendency for nozzles to block, in other words the `run-ability' was poor.
Reactive printing by the `all-in' method is the normal approach for screen printing, but for jet printing it has certain dangers. As a result the jet printing of cotton, wool and silk has generally been carried out by the `two-phase' method, the ink containing only purified dyes, the thickener and chemicals being applied to the substrate in a pre-treatment. Although the quality of the resulting prints is excellent, the extra expense of pre-treating the fabric by a pad/dry process makes the process uneconomical for anything but short runs. (4)
1.4 REASONS FOR PRE-TREATMENT
The main reasons for separating the dye ink from thickeners and other chemicals and applying them separately to the fabric are as follows.
- 'All-in' inks are less stable and have lower storage stability, e.g. reactive dyes are more likely to hydrolyze when alkali is present in the ink.
- Chemicals in the ink cause corrosion of jet nozzles; the deleterious effect of sodium chloride on steel surfaces is well known, for instance; inks for use in `charged drop' continuous printers should have low electrical conductivity.
- Thickeners in the ink often do not have the desired rheological properties.
- Some chemicals can be utilized in pre-treated fabric but would cause stability problems in the ink; e.g. sodium carbonate as alkali for reactive dye fixation is acceptable on the fabric but not in the ink.
- The presence of large amounts of salts in aqueous inks reduces the solubility of the dyes; concentrated inks are required in jet printing due to the small droplet size.
- The advantage of applying thickeners and chemicals separately from the dyes is that it allows the wettability and penetration properties of the fabric to be adjusted. (5)
| Comparable Factors | Screen Printing | Digital Printing |
| Tools | The process involves making a stencil using a drawn/digitized image or a photograph, attaching to a screen, placing it over the desired canvas and spreading the ink over the image. | All need is a computer and a printer with ink cartridges of every color. |
| Efforts | Takes a lot of time consuming effort, because the screens need to be made and the process takes long | As it is so easy to operate and gives results at the touch of a key. It is relatively quicker. |
| Quality | Offers better quality imaging as the ink gets deeply absorbed and lasts longer. Screen printing also gives clearer edges to the image printing, because of the precision that carefully created stencils offer. | The ink does not spread because the image is directly printed on the fabric, but tends to fade quicker that the screen printed images. However, person has a colorful image to imprint, then this is an option, all the colors are present in the single image and person does not need separate screen for the same. |
| Cost | Costs escalate with the numbers of screens. If person want a more complex image with many colors, then individual slides for every color are created. It also required trained labor which adds to the cost. It is most apt if person want a large quantity. | The computer and printers are one time investments and digital printing is cheaper compared to screen printing as the charge is offer for per imprinted image. |
Table 1: Comparison between conventional and digital printing
1.6 TIME TO INTRODUCE A NEW PRODUCT
| Component | Conventional | Digital |
| Color separation/Design editing | 2 weeks | 2 weeks |
| Digital fabric samples | - | 2-4 days |
| Screen engraving | 1 week | - |
| Strike off | 1 week | 1 week |
| Production yardage | 3-4 weeks | 1 week (low yardage) |
| Total | 7-8 weeks | 2-3 weeks |
Table 2: Comparison for time to introduce a new product in conventional and digital printing.
1.7 ADVANTAGES OF INKJET PRINTING
Digital printing has revolutionized the way businesses create their printed materials. It is fast, effective, and provides an alternative to the more traditional method of textile printing.
- Quality: When it comes to quality, nothing surpasses digital printing. Images are essentially flawless, alignment and registration issues are non-existent, and color is vibrant. Digital printers can also use the entire length of a printable item.
- Speed: Digital printing’s ability to switch over to a new label almost instantly is another perk of using digital printing. Because there’s no lost time setting up plates and printing machinery, your order is likely to reach its intended destination days, if not weeks earlier.
- Short run printing advantage: Digital textile printing efficiently produces designs at run lengths as low as one yard of fabric without the need for screen changes.
- Lower water and power consumption: Digital textile printing eliminates the substantial amount of water and electrical energy one requires for rotary screen preparation, printing and cleanup. Even greater water and power savings can be achieved with disperse/sublimation and pigment digital textile inks, which only require a heat-fixation step for post treatment.
- Less chemical waste: Digital textile printing results in significantly less ink usage and waste relative to screen-printing. Taking into account the additional chemistry and chemical waste from screen production, printing digitally offers a greener advantage for printing.
- Large repeat sizes: Digital textile printers can print large designs (e.g. cartoon characters on sheets and blankets) on roll fabric without the usual rotary screen-printing limitation in pattern repeat size.
- Reduced production space requirements: By not having to prepare and store customer screens for future use, the production footprint for digital printing is a fraction of the size one requires for a rotary screen print facility.
- Less printed inventory needed: Digital textile printing permits the option to print a design at will. This means that manufacturers with an integrated digital printing system in their production chain can keep a stock of unprinted textiles on hand to print as required. This reduces the need for pre-printed inventory of fabric that may or may not be used.
- Sampling and production done on same printer: By being able to print samples (strike-offs) on the same printer one uses for production, digital textile print shops can present their customers with proof samples of designs that will exactly match the final printed material.
- Print flexibility: Printing houses utilizing both digital and screen technologies can choose to print a small quantity of designs with different color combinations (color ways) first with their digital textile printing solutions for test the market. They can later opt to print higher volumes of the most desired color designs using rotary screen technology.
- Variety of creative design choices for printing: Digital textile printing provides the option to print photographic/continuous tone images, spot color pattern designs or a combination of both. This expands the creative printing alternatives for fashion and interior designers.
- Low capital investment: The relatively low capital investment to setup a digital textile print shop, especially compared to rotary screen-printing production, makes it possible to start small and expand as business grows.
- Limitation of particle size: Metallic colors cannot be printed by these machines due to large particle size.
- Large Volumes are expensive: Without getting too technical, digital printing presses run at a maximum of about 50 feet per minute. While this speed is sufficient for low volume (10,000 – 15,000 item) projects, larger volume work will benefit from using traditional presses that can run at speeds between 300 and 500 feet per minute. Although traditional presses are more expensive to configure and operate, they will save you money if your jobs are very large.
- Ink limitations: While digital printing certainly handles color and ink well, digital inks have a tendency to fade more quickly than offset inks when exposed to direct sunlight. Also, the opacity of digital ink isn’t quite up to par with offset ink, because digital ink is naturally thinner (though the difference between the two is only noticeable when dealing with clear or metallic media). There are types of laminations available to help prevent this problem from occurring.
Fabric pre-treatment is essential for textile printing with reactive dyes to ensure efficient inkjet print performance, for example to achieve acceptable color strength and fastness properties, and to control droplet penetration and spread for optimum image quality, because the auxiliary chemicals required, such as urea, alkali and migration inhibitor, cannot normally be incorporated into the inks. Therefore, the aim of our project is:
- To study the effect of the fabric pre-treatment on color strength and dye fixation of a digital printed cotton fabric.
- To optimize a pre-treatment recipe and to analyze the effect of the fabric pre-treatment on color strength and dye fixation of a digital printed cotton fabric.
CHAPTER 2
2. LITERATURE REVIEW
Ahmad Wassim Kaimouz et al. studied the significance of pre-treatment chemicals and their relationship with color strength, absorbed dye fixation and ink penetration. In this paper, the inkjet printing performance on Tencel fibers (standard Tencel and Tencel A100) using a reactive dye based ink is reported, and some comparisons made with cotton. The fabric was first pretreated by padding and then printed using reactive inks and the relationships between the concentrations of pre-treatment chemicals and steaming time, color strength, dye fixation and ink penetration, have been established. The pre-treatment chemicals Thermacol MP (migration inhibitor), Alcoprint AIR (penetration agent) and Lyoprint RG (reduction inhibitor) supplied by Huntsman, Urea and sodium bicarbonate were used. The liquors were applied to the fabrics cut to A4 size, by padding with 75–80% pick up. The fabrics were dried for 5 min at 120°C and conditioned for 24 h at 20°C and 65% relative humidity. Printed samples were steamed at 102°C. Then the fabric was washed by de-ionized water. The mean color strength of the printed Tencel fabric was greater than cotton and Tencel A100, while the mean absorbed dye fixation values of Tencel A100 was greater than Tencel and cotton. Tencel A100 has the lowest color strength value as its cross linked structure limits the development of deep shades at the fiber surface. (6)
Soleimani Gorgani and M. Jalili studied ink-jet printing of cotton with cationic reactive dye based inks. In this study, cotton fabric was printed with two types of reactive dyes in different conditions. Cotton fabric was first pre-treated by padding using the pre-treatment chemicals sodium alginate or chitosan, sodium bicarbonate and urea and then it is printed with the commercial anionic reactive inks. Secondly, the untreated fabric was printed with the novel cationic reactive inks. Color yield and absorbed dye fixations of both printed cotton fabric were compared. The results indicated that printed untreated cotton fabric with cationic reactive dye based ink at optimum pH exhibited higher level of reactive dye fixation than commercial anionic reactive dye based inks on alkali pre-treated cotton fabrics. All reactive dye based inks are demonstrating excellent washing and dry/wet crocking color fastness. The light fastness of each reactive dye based ink fixed to cotton fabrics was moderate. (7)
Atasheh Soleimani-Gorgani and Najva Shakibanalyzed the effect of the structure of reactive dye on cotton ink-jet printing. Cotton fabric was printed upon with three commercial cellulosic reactive dyes which are based on the similar chromophore and possess different numbers of reactive and anionic groups and then the printed cotton fabrics were air dried and then put into a steamer to fix the reactive dye on to the cotton fabric. Color yield and absorbed dye fixation of the printed cotton were evaluated at various pHs. The results indicated that the absorbed dye fixation levels increased by decreasing the numbers of anionic groups and appeared to be dependent of reactive group’s numbers. All reactive dye based inks are demonstrating excellent to washing and dry/wet crocking color fastness. (8)
C.W.M.Yuen et al. evaluated the effect of various compositions of pretreatment paste constituents with different steaming time on final color yield of inkjet printed cotton fabric. The parameters that were changed were amounts of alginate, alkali, urea and the steaming time. After making different compositions of pre-print paste, it was then applied on the fabric and subsequently dried in oven. Inkjet printing was performed then followed by steaming for different time period, thereafter fabric was conditioned and color yield was measured. Result deduced from these observations was that, as the amount of Sodium alginate was increased the color yield was improved as well as the sharpness of the printed patterns and wash fastness properties of the fabric. But when the amount of sodium alginate was too high it reduced the fixation of the dye because thickener acted as a diffusion barrier so less dye could fix on the fibers. (9)
Atasheh Soleimani Gorgani et al. analyzed the dye fixation and fastness properties of printed pretreated and non-pretreated fabric. In this study, attempts have been made to develop a reactive ink-jet print in a single-phase process by adding an organic salt to the ink formulation and hence removing the need to pretreat fabrics. The behaviour of a novel reactive ink formulation for ink-jet printing on to cotton fabric was evaluated at different pH vlaues. The results at optimum pH indicated that printed non-pretreated fabrics with ink containing organic salts exhibited a higher level of reactive dye fixation than printed pretreated fabric containing no organic salt ink. The yielded prints demonstrate excellent color fastness to washing and dry/wet crocking properties.
The high percentages of fixation attained with organic salt, which has higher ionic strength, would support that cations from the salts can counter the negative charge of the fabric, thereby facilitating absorption of dye anions on to the fabric. In all cases, excellent wash and dry/wet crocking fastness properties were achieved, and light fastness was improved by adding organic salt to the ink formulation. (10)
P S R Choi et al. pretreated the cotton fabric using chitosan as a thickener and evaluated the color fastness properties of digitally printed fabric. In this study, chitosan was applied separately on cotton fabric for ink-jet printing. A two-bath method was developed to separate the chitosan paste from sodium bicarbonate and urea before being padded onto the fabric surface so as to minimize the neutralization effect. A two-bath method helps to achieve a better color yield. Experiments have been done to evaluate the possibility of using chitosan as a thickener in the pretreatment print paste for ink-jet printing. The final color yield obtained from chitosan containing cotton fabrics depended greatly on the stage of chitosan application. Nevertheless, the color fastness properties and the outline sharpness of the prints of cotton fabric were moderately improved by the chitosan treatment. In addition, it was found that chitosan could also impart higher anti-bacterial property onto the cotton fabric. (11)
Soleimani Gorgani and M. Jalili studied ink-jet printing of cotton with cationic reactive dye based inks. In this study, cotton fabric was printed with two types of reactive dyes in different conditions. Cotton fabric was first pre-treated by padding using the pre-treatment chemicals sodium alginate or chitosan, sodium bicarbonate and urea and then it is printed with the commercial anionic reactive inks. Secondly, the untreated fabric was printed with the novel cationic reactive inks. Color yield and absorbed dye fixations of both printed cotton fabric were compared. The results indicated that printed untreated cotton fabric with cationic reactive dye based ink at optimum pH exhibited higher level of reactive dye fixation than commercial anionic reactive dye based inks on alkali pre-treated cotton fabrics. All reactive dye based inks are demonstrating excellent washing and dry/wet crocking color fastness. The light fastness of each reactive dye based ink fixed to cotton fabrics was moderate. (7)
Atasheh Soleimani-Gorgani and Najva Shakibanalyzed the effect of the structure of reactive dye on cotton ink-jet printing. Cotton fabric was printed upon with three commercial cellulosic reactive dyes which are based on the similar chromophore and possess different numbers of reactive and anionic groups and then the printed cotton fabrics were air dried and then put into a steamer to fix the reactive dye on to the cotton fabric. Color yield and absorbed dye fixation of the printed cotton were evaluated at various pHs. The results indicated that the absorbed dye fixation levels increased by decreasing the numbers of anionic groups and appeared to be dependent of reactive group’s numbers. All reactive dye based inks are demonstrating excellent to washing and dry/wet crocking color fastness. (8)
C.W.M.Yuen et al. evaluated the effect of various compositions of pretreatment paste constituents with different steaming time on final color yield of inkjet printed cotton fabric. The parameters that were changed were amounts of alginate, alkali, urea and the steaming time. After making different compositions of pre-print paste, it was then applied on the fabric and subsequently dried in oven. Inkjet printing was performed then followed by steaming for different time period, thereafter fabric was conditioned and color yield was measured. Result deduced from these observations was that, as the amount of Sodium alginate was increased the color yield was improved as well as the sharpness of the printed patterns and wash fastness properties of the fabric. But when the amount of sodium alginate was too high it reduced the fixation of the dye because thickener acted as a diffusion barrier so less dye could fix on the fibers. (9)
Atasheh Soleimani Gorgani et al. analyzed the dye fixation and fastness properties of printed pretreated and non-pretreated fabric. In this study, attempts have been made to develop a reactive ink-jet print in a single-phase process by adding an organic salt to the ink formulation and hence removing the need to pretreat fabrics. The behaviour of a novel reactive ink formulation for ink-jet printing on to cotton fabric was evaluated at different pH vlaues. The results at optimum pH indicated that printed non-pretreated fabrics with ink containing organic salts exhibited a higher level of reactive dye fixation than printed pretreated fabric containing no organic salt ink. The yielded prints demonstrate excellent color fastness to washing and dry/wet crocking properties.
The high percentages of fixation attained with organic salt, which has higher ionic strength, would support that cations from the salts can counter the negative charge of the fabric, thereby facilitating absorption of dye anions on to the fabric. In all cases, excellent wash and dry/wet crocking fastness properties were achieved, and light fastness was improved by adding organic salt to the ink formulation. (10)
P S R Choi et al. pretreated the cotton fabric using chitosan as a thickener and evaluated the color fastness properties of digitally printed fabric. In this study, chitosan was applied separately on cotton fabric for ink-jet printing. A two-bath method was developed to separate the chitosan paste from sodium bicarbonate and urea before being padded onto the fabric surface so as to minimize the neutralization effect. A two-bath method helps to achieve a better color yield. Experiments have been done to evaluate the possibility of using chitosan as a thickener in the pretreatment print paste for ink-jet printing. The final color yield obtained from chitosan containing cotton fabrics depended greatly on the stage of chitosan application. Nevertheless, the color fastness properties and the outline sharpness of the prints of cotton fabric were moderately improved by the chitosan treatment. In addition, it was found that chitosan could also impart higher anti-bacterial property onto the cotton fabric. (11)
CHAPTER 3
3. SCOPE OF PROJECT
Our project is a step towards establishing Optimum condition concerning the content of pretreatment print paste and steaming time for ink-jet printing to achieve improved color strength and dye fixation. The scope of our project is immensely broad. Market may see new players in the future which will be motivated by our work which in turn may generate new concepts to improve color yield as well as drive prices down. Not only will our project help to improve the quality of inkjet printing but also will help reduce the amount of effluents discharged into sewage which will reduce the pollution of marine environment. Obviously the cost per unit of printing will decrease because of lower wastage of dyes as a result many customers will be able to buy because they will be able to afford the cost. These developments in the technology will probably shape the future of this technology as well as provide incentive for new entrants to put up their efforts in various areas of technology.
Digital inkjet printing of textiles opens doors to new opportunities and creates new markets. Creative designs can be digitally printed that cannot be screen-printed. The largest screen printers have no more than 12 screens, which equates to a limitation of 12 spot colors. With process color there can be an almost unlimited number of colors in a design, allowing much more than 12 colors in a specific design. Design cycle times are reduced and sample production can be done immediately. The ability to do economical short runs allows reductions in the size of inventories. Restocking of a `hot' apparel item is made easy by digital printing and the store doesn't have to discount its prices. Today's markets are changing faster and customers are becoming more demanding than ever. Digital textile printing allows the production of goods and services to match individual customers' needs.
Digital inkjet printing of textiles opens doors to new opportunities and creates new markets. Creative designs can be digitally printed that cannot be screen-printed. The largest screen printers have no more than 12 screens, which equates to a limitation of 12 spot colors. With process color there can be an almost unlimited number of colors in a design, allowing much more than 12 colors in a specific design. Design cycle times are reduced and sample production can be done immediately. The ability to do economical short runs allows reductions in the size of inventories. Restocking of a `hot' apparel item is made easy by digital printing and the store doesn't have to discount its prices. Today's markets are changing faster and customers are becoming more demanding than ever. Digital textile printing allows the production of goods and services to match individual customers' needs.
CHAPTER 4
4. METHODOLOGY
4.1 MATERIAL
The fabric used was 100% Cotton, ready to print, plain weave (125 g/m2) which was supplied by Yunus Textile Mill(YTM).
4.2 CHEMICALS
The chemicals which are used in the pre -treatment of cotton fabric are;
- Thickeners
II. Thermacol min (Synthetic thickener)
III. Prepajet uni ( Synthetic thickener)
- Urea
- Alkali (Sodium bicarbonate)
- Anti-reduction agent (Lyoprint RG-GB)
- Penetrating agent (Lyoprint air)
- Reactive inks
Thickeners are employed in printing to preserve the sharpness of edges and outlines by countering the natural wicking effect of the substrate. In addition they hold moisture to enable dyes and chemicals to dissolve and enter the fibers during the steaming stage after printing and drying. They also modify the flow properties (rheology) of the ink or print paste. The thickening agent should not react with either the dye or other chemicals present because, if they do, an insoluble product usually results. This does not wash off and the fabric becomes stiff.
1. SODIUM ALGINATE
The chemical compound sodium alginate is the sodium saltof alginic acid. Sodium alginate is a gum, extracted from the cell walls of brown algae. A major application for sodium alginate is in reactive dye printing, as thickener for reactive dyestuffs (such as the procion cotton-reactive dyes. Alginates do not react with these dyes and wash out easily, unlike starch-based thickeners. It is poly anionic in nature. It is this property that prevents the anionic reactive dyes from reacting with the thickener, since both have negative charges and so repel each other.
The uses of alginates are based on three main properties.
- The first is their ability, when dissolved in water, to thicken the resulting solution (more technically described as their ability to increase the viscosity of aqueous solutions).
- The second is their ability to form gels; gels form when a calcium salt is added to a solution of sodium alginate in water.
- The third property of alginates is the ability to form films of sodium or calcium alginate and fibers of calcium alginates. (12)
- Chemical constitution: aqueous solution of an acrylic polymer.
- Ionic character: anionic
- Physical form: colorless liquid.
- Storage stability: store at 20°C more than 1 year.
- Compatibility: compatible with anionic and nonionic auxiliaries.
Prepajet UNI is high concentration synthetic thickener for reactive printing with high electrolyte stability ensuring excellent color yield & sharp definition.
4.2.2 ALKALI
Reactive dyes react with cellulose under alkaline conditions to form covalent bonds between fibre and dye. There are various classes of reactive dyes, monochlorotriazine (MCT), vinyl sulphone etc., and these require different strengths of alkali for optimum fixation. Sodium bicarbonate is generally recommended for `all-in' pastes and inks, as it causes least hydrolysis of the dye on storage.
4.2.3 UREA
Urea is a very common constituent of print pastes as it acts as both dye solvent and hygroscopic agent (or humectant). The main functions of urea are:
- To increase the solubility of dyestuffs with low water solubility. This hygroscopic effect does not influence fixation rates significantly.
- To increase the condensate formation necessary for the migration of dyestuffs from paste to fibers.
- To form condensate with increased boiling point, thus the requirement on steam quality can be reduced.
Anti-reduction agents (LYOPRINT® RG-GB) protect dyes against shade changes in printing, hence achieving excellent dyeing with good reproducibility.
4.2.5 PENETRATING AGENT
Penetrating agent or De-aerating agents (LYOPRINT® AIR) are used to remove air from print pastes and improve ink penetration in printing systems. (13)
4.3 EXPERIMENTATION
4.3.1 RECIPE FOR PRETREATMENT
Recipe#1
| Ingredients | Amount (g/l) in 500ml solution |
| Thickener (Sodium alginate) | 100 |
| Anti-reduction agent (lyoprint RG-GB) | 10 |
| Penetrating agent (lyoprint air) | - |
| Urea | 100 |
| Alkali | 30 |
| Deionized water | 94ml |
| Steaming time | 5 min at 102°C |
Table 3: Recipe #1
Procedure
- Add 94 ml water in beaker.
- Then add the given amount of ingredients in water in following sequence; urea, alkali, reduction inhibitor, thickener, penetrating agent.
- Stir with Stuart SS20 overhead stirrer until the preprint paste consistency becomes thick.
- Pre-treat the cotton fabric by using padding mangle at conditions of 1.4 bar pressure and 1.5 rpm speed to achieve 75-80% pick up.
- Dry the fabric at 120°C for5 min.
- Digitally print the fabric by using reactive inks.
- Steam the printed fabric at 102°Cfor 5 min.
- Fabric is then washed in the following sequence (tap water, hot water and cold water).
| Ingredients | Amount (g/l) |
| Thickener (Prepajet UNI) | 80 |
| Anti-reduction agent (Lyoprint RG-GB) | 20 |
| Penetrating agent (Lyoprint air) | 6 |
| Urea | 200 |
| Alkali | 40 |
| Deionized water | X |
| Steaming time | 7-10 min at 102C |
Table 4: Recipe #2
Procedure:
- Add x ml of water in a beaker.
- Then add the given amounts of ingredients in water in the following sequence urea ,alkali, reduction inhibitor, thickener ,penetrating agent.
- Stir with Stuart SS20 overhead stirrer until the preprint paste consistency becomes thick.
- Pre-treat the cotton fabric by using padding mangle at conditions of 1 bar pressure and 1 rpm speed to achieve 70-80% pick up.
- Dry the fabric at 120°C for 5 min.
- Digitally print the fabric by using reactive inks.
- Steam the printed fabric at 102°C for 7-10 min.
- Fabric is then washed in the following sequence (tap water, hot water and cold water).
| Ingredients | Amount (g/l) |
| Thickener (Thermacol MIN) | 100 |
| Anti-reduction agent (Lyoprint RG-GB) | 20 |
| Penetrating agent (Lyoprint air) | 10 |
| Urea | 150 |
| Alkali | 40 |
| Deionized water | X |
| Steaming time | 7-10 min at 102C |
Table 5: Recipe #3
Procedure:
- Add x ml of water in a beaker.
- Then add the given amounts of ingredients in water in the following sequence urea, alkali, reduction inhibitor, thickener ,penetrating agent
- Stir with Stuart SS20 overhead stirrer until the preprint paste consistency becomes thick.
- Pre-treat the cotton fabric by using padding mangle at conditions of 1 bar pressure and 1 rpm speed to achieve 70-80% pick up.
- Dry the fabric at 120°C for 5 min.
- Digitally print the fabric by using reactive inks.
- Steam the printed fabric at 102°C for 7-10 min.
- Fabric is then washed in the following sequence (tap water, hot water and cold water).
When the pre-treated fabric has been dried and then jet printed there is usually little need to provide a drying station to dry the print, as the printing process is so slow. By the time the fabric is batched on a roll it has dried by exposure to the warm atmosphere in the room. However, in most instances fixation and washing will be necessary. This not only ensures that the full fastness properties of the dyes are realized, but also brightens and alters the colors significantly.
4.4.1 FIXATION
Steaming is the process normally used to fix printed textiles. During the process steam condenses on the fabric and is absorbed by the thickener and hygroscopic agents in the printed areas. Dyes and chemicals dissolve and form extremely concentrated dye baths within the thickener film. The digitally printed fabrics were steamed at 102°C (saturated steam) in the range of 5-10 min.
4.4.2 WASHING-OFF
After printing and steaming, washing of ink-jet printed fabric is carried out. The reason for this is that thickener, auxiliaries and loose dye should be removed under conditions where the dye is unlikely to stain white or unprinted ground shade areas.
The washing was done in three steps:
- First the fabric was cold rinsed.
- The main requirement after the first cold rinse when washing off reactive dye prints is to ensure that the temperature of the hot wash reaches a minimum of 90°C, otherwise hydrolyzed dye may not be removed. Then the fabric was washed by hot water (at the temperature of 80 - 90°C) containing 2-3 drops of surfactant (soaping SN).
- After that the fabric was again cold rinsed. (14)
We have performed two types of test namely rubbing fastness and wash fastness test, to check the effect of pretreatment on color strength and dye fixation of digitally printed cotton fabric.
4.5.1 DRY RUBBING FASTNESS
- The test was carried out following the standard (AATCC 08).
- The test specimen was placed on the base of the Crock meter.
- Place specimen holder over specimen as an added means to prevent slippage.
- After that we mount a white cloth square shape sample at the top nip of crock meter for testing of dry crocking fastness of printed and washed cotton fabric.
- Lower the covered finger onto the test specimen. Beginning with the finger positioned at the front end, crank the meter handle 10 complete turns at the rate of one turn per second to slide the covered finger back and forth 20 times. Set and run the motorized tester for 10 complete turns.
- Remove the white test cloth square, condition and evaluate the staining.
- In this test we first wet the white square shaped cloth sample and then mount it at the top nip of crock meter.
- After that we repeat the above procedure and evaluate the result by using gray scale.
The washing fastness test is carried out by following the standard BS EN ISO 105-CO6-E2S using the following ingredients;
| Ingredients | Amounts |
| Detergent | 4g/l |
| Sodium per borate | 1g/l |
| Sodium carbonate | 1g/l |
| Water | - |
Table 6: Wash fastness test recipe.
Process parameters:
- Temperature = 60°C
- Time = 30 min
- Steel balls = 25
- Make a solution of 1000 ml.
- Add detergent, sodium carbonate, sodium per borate in water in the amounts as mentioned above in the table.
- Leave the solution for 30 min so that its constituents mix homogenously before using it.
- Cut a sample of fabric of 10*4 cm and staple a cotton piece to the sample of the same dimensions.
- Put the sample and 25 steel balls in the beaker.
- Add 50 ml solution in the beaker.
- Seal the beaker with a cap and place it in high temp IR dyeing machine.
- Leave the beaker for 30 minutes at 60°C.
- Wash the samples.
- Leave the samples in open air to dry the samples.
CHAPTER 5
5. CONCLUSIONS
The two types of tests performed for the fastness properties of digitally printed cotton fabric conclude the following results;
| | RUBBING FASTNESS (wet) | RUBBINGFASTNESS (dry) | WASH FASTNESS |
| SODIUM ALGINATE | 4-5 | 5 | 3 |
| PREPAJET UNI | 4-5 | 5 | 4 |
| THERMACOL MIN | 3- 4 | 5 | 3 |
As we have used the three different types of thickeners (sodium alginate, thermacol min, prepajet UNI) for the pre-treatment of our printed fabric. We have observed the effect of pre-treatment with all these thickeners performing different fastness tests. The results of the test showed that sodium alginate gives poor fastness properties as compare to the others two because it does not fix the dye properly and the dye bleeds more while washing. While the Prepajet UNI gives the better fastness properties on digitally printed fabric in all of the three thickeners. Thus, at this stage we have omitted the sodium alginate.
CHAPTER 6
6. FUTURE WORK
In future, different types of test (i.e. % fixation, K/S) will be performed to analyze the effect of pretreatment on color strength and dye fixation of digitally printed fabric.
Further working will be done with thermacol min and prepajet UNI by doing little changes in recipe and process parameters and it is also possible that more thickeners will be used other than thermacol min and prepajet UNI for better results.
REFERENCES
- http://provost-inkjet.com/resources/SDC++Ink+JetPretreatment+4th+Dec+03.pdf
- L. W. C. Miles in “Textile Printing”, 2nd ed. (L. W. C. Miles Ed.), pp.240–274, Society of Dyers and Colourists, Bradford, 1994.
- Susan Meller and Joost Elffers, Textile Designs: Two Hundred Years of European and American Patterns for Printed Fabrics Organized by Motif, Style, Colour, Layout, and Period, Abrams, 2002.
- http://www.silk-road.com/artl/printing.shtml.
- Kulube H M and Hawkyard C J, `Fabric pretreatments and inks for textile ink-jet printing', Int. Text. Bull., Dyeing, Printing, Finishing, 1996.
- http://www.sciencedirect.com/science/article/pii/S0143720809001387?np=y.
- http://www.sid.ir/EN/VEWSSID/J_pdf/1033120130101.pdf.
- http://pccc.icrc.ac.ir/?xid=0113010121017900004&id=705&T=A .
- http://link.springer.com/article/10.1007/BF02902924.
- https://www.researchgate.net/publication/235002804_Single-phase_ink-jet_printing_onto_cottonfabric.
- http://onlinelibrary.wiley.com/doi/10.1111/j.1478-4408.2007.00094.x/full
- Dawson T L and Glover B (eds), Textile Ink Jet Printing, Bradford, UK, SDC, 2004.
- Miles L W C, Textile Printing, 2nd edn, Bradford, UK, SDC, 1994, Chapter 7, 250-252.
- www.storktextile.com.