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The rapid developments in the field of genetic engineering have given a new impetus to the biotechnology. This introduces the possibility of 'tailoring' organisms in order to optimize the production of established or novel metabolites of commercial importance and of transferring genetic material (genes) from one organism to another. Biotechnology also offers the potential for new industrial processes that require less energy and are based on renewable raw materials. It is important to note that biotechnology is not just concerned with biology, but it is a truly interdisciplinary subject involving the integration of natural and engineering sciences. Defining the scope of biotechnology is not easy because it overlaps with so many industries such as the chemical industry or food industry being the majors, but biotechnology has found many applications in textile industry also, especially textile processing and effluent management. Environmental protection is becoming a serious concern for textile wet processors. Dyes discharged from textile dyeing and finishing processes are a priority pollutant because of their visibility at low concentrations. Most dyes have low toxicity but their components and breakdown products can be more toxic. Physical and chemical treatment techniques are effective for colour removal but use more energy and chemicals than the biological processes. They also concentrate the pollution into solid or liquid streams requiring additional treatment or disposal. Biotechnological methods can completely mineralize pollutants and are usually cheaper.
Biotechnology in Textiles: Biotechnology has its roots in the dawn of the history. It is not new to textile industry, it has been used in textile processing for the enzymatic removal of starch sizes from woven fabrics for most of this century and the fermentation vat is probably the oldest known dyeing process. The scope of enzymes in textile industry is very wide. This can be understood by looking at the wide spectrum of applications in textiles. The major areas of application of biotechnology in textile industry are Improvement of plant varieties used in the production of textile fibres and in fibre properties, Improvement of fibres derived from animals and health care of the animals, Novel fibres from biopolymers and genetically modified micro-organisms, Replacement of harsh and energy demanding chemical treatments by enzymes in textile processing, Environment friendly routes to textile auxiliaries such as dyestuffs, Waste management. Textile processing industry is characterized by high consumption of energy and resources and time consuming processes. Lot of pollutants are generated by textile industry, especially textile processing sector produces most of the pollutants, mostly water pollutants. Waste water from textile industry, especially process houses, is characterized by high COD and BOD, suspended solids and intense color due to the extensive use of dyes. This type of water must be treated before discharging it into the environment. The water must be decolorized; harmful chemicals must be converted into harmless chemicals. . Biological treatments have been used to reduce the COD of textile effluents. Instead of using the chemical treatments various biological methods can be used to treat the waste water from the textile industry. These methods include, Biosorption, use of Enzymes, Aerobic and anaerobic treatments etc. Only biotechnological solutions can offer complete destruction of the dyestuff, with a co-reduction in biological oxygen demand (BOD) and chemical oxygen demand (COD). In addition, the biotechnological approach makes efficient use of the limited development space available in many traditional dyehouse sites Many azo dyes and harmful chemicals, like p-chlorophenol have been banned from use. The area in which biotechnology is required exactingly is the treatments on waste water generated during textile processing. The presence of chemicals, like formaldehyde, salts, phenols, dyes, alkalis and acids, heavy metals, polyvinyl alcohol in waste water is very harmful to the environment. So these chemicals must be either removed or converted into harmless chemicals by treating them chemically or biologically.
Though the industrial Biotechnology is in the early stages of development but its innovative applications are increasing and spreading rapidly into all areas of manufacturing. It is already providing useful tools that allow for cleaner, more sustainable production methods and will continue to do so in the future. Adoption of biotechnology ensures the cleaner environment; also it cuts the cost of the processes. Textile industry, which is responsible for generation of lot of pollutants in all forms, must adopt the biotechnology, especially in the processing sector, to reduce the consumption of energy as well as other resources.
Navanath Pingle, Laxmikant Jawale, Prashant Vispute. Department of Fibres and Textile Processing Technology University Institute of Chemical Technology University Of Mumbai, India
Abstract
Whilst, the India has abolished the textile quota regime of textiles, one has to face tremendous amount of competition from producers of all over the world. One has to be very productive, economical, quality conscious, environment friendly with assured reproducibility. Modern technologies are going to play very major role in this. Ink jet printing is one such promising technique, which will be a critical tool in the hands of production units. In this article attempt is made to familiarize ink jet printing.
Introduction Drastic changes are taking place in the textile printing; quick response, globalization and ecology impose substantial demands on the different components of the printing process. They force the textile printers to focus on: shortening and flexibility of the pre-print process; printing right first and the next time, reducing down time of the printing machine, short run production, and stock risks. Consumers are demanding a greater variety of colour and design. They want fabrics that express their individuality in their environments and in the clothes that they wear. Today's ecological stringent demands ask for eco processes minimizing waste of raw materials and pollution of the environment. In short these demands have common denominations: flexibility and versatility. Conventional printing lacks the above demands. The answer lies in digital printing. The jet-printing technology has yet to make the leap into mainstream. Although digital printing is already claimed as the most significant technological advancement to the textile printing industry in over 30 years, the adaptation of these new systems has been much slower than many had anticipated or expected, due to specific technological limitations (hardware, print head or nozzle, ink classes). These hurdles must be overcome for Commercial exploitation of these new technologies.
INKJET TECHNOLOGY
Ink-jet printing involves squinting droplets of ink, which make contact with substrate image. Ink-Jet can be divided into two major technology types: Continuous and Drop-on-Demand. Each of these subdivided further as shown in figure:
Fig 1. TYPES OF INK JET PRINTING
l.Continuous Ink-Jet Printing (CIJ): As the name implies, in Continuous Ink-Jet systems a continuous stream of ink droplets is ejected from nozzle on to the substrate. Two designs are possible in the designing of this method. In the first design, charged ink droplets are deflected on to the paper to form the image and the uncharged droplets are collected in gutter. This is called as raster scan continuous Ink-Jet method. In the second design, the uncharged ink droplets form the image and the charged ink droplets are deflected into the gutter. This is the Binary Continuous Ink-Jet system. While in Hertz technology, fine mist of the uncharged droplets form the images. Hertz Technology is more suited to color printing than the previous two methods.1 Continuous Ink Jet printing is further divided into two main types viz; Piezoelectric and Thermal Excitation.
A) Piezo-eleetric Technology Piezo-electricity is a phenomenon of producing electricity by application of pressure on a crystal, which is capable of conducting electricity through it.4 This method has been suitably exploited for ink jet printing technology. Piezo ink jet printing relies on different principles for the expulsion of ink from the cartridge nozzles. In this technology, an electrical charge is applied to the cartridge nozzles which excites a small piezo crystal that is inside. As the piezoelectric crystals are stimulated, the crystals change shape and squeeze the ink chamber. This action is similar to the action of squeezing an oilcan, and forcefully expels the ink from the nozzle.
After leaving the nozzle, the drops are electically charged by an amount that depends on the image to be printed. The drops then pass through an electric field to cause then to deflect. There are two ways of deflecting the drops in piezoelectric- driven Continuous Ink Jet. In the binary deflection method droplets are directed to a single pixel location in the medium or to the recirculation gutter. In the multiple deflection method the deflection is variable so the drops can address several pixels. These two concepts are illustrated in the Fig.3 (a) and (b).
In the Hertz method, the amount of ink deposited per pixel is variable. This is achieved by generating very small drops at the speed of 40m/s with excitation frequencies of over 1 MHz (see Fig 3 (c)). The drops not intended to reach the medium are charged ad deflected to the gutter. The printing drops are given the smaller charge to prevent them in merging in flight.
Since the piezoelectric process does not utilize heat, the cartridge life of these printers is greatly expanded, cartridges should last a minimum of one year under heavy usage. Piezoelectric print heads can use a wider range of inks than thermal inkjet printers because the heat is removed from the process. This means that solvent-based ink systems and pigmented-ink formulations will be more readily available, which increases the development capabilities for better inks in the future. Although piezo is currently the lesser-utilized technology, many experts predict that the long-term development of ink jet print devices will use the piezo technology because of the greater through-put speeds offered and the wider latitude with the types of inks that can be developed 6.
3. DROP ON DEMAND (DOD) Drop-on-Demand, or impulse, inkjet systems differ in two major respects from continuous Ink-Jet systems. First, all the ink droplets are used to form the image none are wasted. Ink droplets are ejected only where a dot is required on the substrate, i.e. they are produced "on demand ". Secondly, the droplets are not charged. Hence there is no deflection involved. Drop on Demand Ink Jet systems may be subdivided into two broad types, namely piezo and thermal (or bubble jet). In DOD printing a significant proportion of the solid ink resides on the surface of the paper giving the print an embossed feel.(2,3,4)
Principle of Drop On Demand printing (5)
BUBBLE JET PRINTING Bubble-Jet or thermal technology is a well-known technology. The technology relies on a thermal pulse to generate the ink drop. This technology was the first of the drop on demand. The technique boils the water content of the ink and the resulting steam pressure forces a droplet of ink out of the nozzle. In these engines, the computer signal heats a resistor to a high temperature(>360 C) which creates a vapor bubble in a volatile component in the ink, the vapor bubble expands and exits the nozzle followed by a contraction of the bubble causing a drop of ink to be ejected on the textile substrate. The vapqur bubble must then cool and collapse allowing the ink chamber to refill from a reservoir. Cycle time is limited to approximately 10,000 drops per second and the volume per drop of ink is typically 150 to 200 Pico liters.
The main advantage of the thermal ink jet technology is the low cost of nozzle fabrication. It is made using the mass production technique based on the integrated circuits. The system restricts the use of binder containing pigment inks. The major problem with the thermal ink jet is the high nozzle and resistor failure rate resulting from rapid thermal cycling. As the heater to boil the water has to work in a semi-explosive way, the temperature can rise up to 360 C, which can cause the nozzle to burn out. The high temperatures cause often decomposition of ink components, which leads to poor heat transfer and / or nozzle clogging. Therefore only thermal stable inks can be used. These defects are unpredictable.
The two major classes of colorants, namely dyes and pigments. The choice of dye depends upon the ink used, whether it is aqueous, solvent or hot-melt and on the type of printer (thermal or non thermal). However irrespective of the solvent system, all ink-jet dyes have to satisfy a number of stringent criteria.
1. Color: Though Black is the predominant color, for full color printing the three subtractive primary colors of yellow, magenta and cyan are also required. The key parameters are the absorption maxima (peak wavelength), and the shape of the absorption curve, in particular its broadness and the presence (or absence) of unwanted secondary absorption. Ideally, bright dyes (which have narrow absorption curves) are required in order to produce a comprehensive balanced color gamut because bright dyes can be made duller (e.g. by adding a shading color such as a black), but dull dyes cannot be made bright.
2. Color Strength: The color strength of the dye should be as high as possible for several reasons. One reason is to enable prints having high optical densities to be produced, Other one is the increased flexibility possible in the ink formulation. The ink staprecipitation and also kogation on thermal ink-jet systems.
3. Solubility: A dye for ink-jet should have as high solubility as possible to minimize any tendency for the dye to crystallize and cause problems such as nozzle blockage. Generally greater the number of sulfonic acid groups per molecule, the greater the water solubility. Carboxylic acid groups, especially in the form of salts, also confer water solubility on dyes.
4. Electrolytes / Metals: Anion such as sulfate and especially chloride are undesirable in ink-jet inks due to the corrosion problem they cause to the metal print heads. Certain metal cations particularly divalent cations such as calcium, need to be removed, since these can precipitate with certain anions such as sulfate and with dye itself so the permissible amount is 100 ppm. Which is generally accomplished by dialysis or ukrafihration8
5. Light Fastness: The final printed document must have reasonable light fastness (resistance to fading by light) if it is to serve any useful purpose. However it is well-known fact that in general dyes have much poorer light fastness than pigments.
6. Water Fastness: The final print should be resistance to water so that if the paper gets wet or is rubbed by moist fingers, smudging does not occur. On the one hand high solubility is required for the ink, but on the other hand, water insolubility is required once the dye is on the paper. Hence a compromise position has to be reached to adequate solubility and adequate warer fastness. One of the promising approaches to achieving high water fastness is by using the concept of differential solubility.
7. Smear Fastness: This is effectively resistance to smear when using a highlighter pen. Such pens usually contain aqueous inks and can cause a dye to smear by facilitating the formation of water soluble species. Again dyes from solvent or wax / resin inks are less prone to smear than those from aqueous inks.9 :
8. Shade The shade or hue appears the same irrespective of the substrate on which it is printed. :The shade or hue appears the same irrespective of the substrate on which it is printed.This can be achieving science the substrate varies enormously in physical properties. For instance, paper varies in texture, adsorption, additives, and pH, and ideally the dye should be insensitive to this differences.10
9. Toxicology: Because of the increased awareness of Environmental health and safely, in recent years. It has been made imperative that screening test for mutagenicity (in bacteria) is the Amens test, and it is normally required that dyes should be Amens negative, i.e. not cause mutations in bacteria. Therefore, ink-jet dyes should ideally be Amens negative 10. Thermal stability If the dye is required for thermal ink-jet system such as canon's bubble jet or Hewlett Packard's Think Jet, then the extra parameter of thermal stability or kogation fastness is required. Since the temperatures involved are as high as 300°C, then dyes stable to these conditions are required. Good kogation is when these deposits are absent or very low, whereas poor koagtion is when the deposits substantial. Poor kogation produce by two factors The first is the presence of inorganic impurities such as iron, copper and silicon. The second is the degradation of the dye itself Solving the second cause of kogation, namely dye degradation, has proved more difficult
Aqueous Ink-Jet Dyes: Colorants comprising the first generation of inkjet dyes were selected for their vividness (high chroma), good aqueous solubility, and the stable inks derived therefrom which gave a reliable printing performance InkJet printers have become considerably more sophisticated over the years, and demand for dyes and inks capable of producing very high print quality has grown. Ink formulation research has complimented dye development and has been aimed towards achieving the following print properties: · Good optical densities · No feathering · Minimal black to color bleed · Uniform and controlled drop spreading · Good water fastness · Rapid dry time · Smear resistance · Media compatibility12 Solvent Inks:
Solvent based inks find application in continuous printers in the industrial segment where they are used to print bar codes, batch numbers, sell-by dates and other such information on packaging materials. The used of solvent based inks rather than aqueous inks affords a faster dry time and makes the inks more suitable for printing hydrophobic or non-porous substrates. Traditionally, ketonic solvents like methyl ethyl ketone are used but these are being replaced by less flammable alcohol. Solvent dyes are usually used as colorants and the predominant color is black. A typical example of a black dye used in this area is the trisazo dye. Another dye used in solvent inks is CI Solvent Black 35. This dye exhibits high light fastness.
Hot-Melt These inks are solid at room temperature but become fluid when heated at 60-125°C. They are fired using a piezo printer. The ink vehicle includes Cig-24 fatty carboxylic acids and alcohol. Dyes that are soluble in the vehicle tend to be hydrophobic, an example of which is the modified xanthenes magenta.13 Ink Jet Printing Technology for Textile Definition of type: There are two types of ink-jet printer. The coarse resolution type & Stork true cote within these two types are further sub groups. It is in this area of fine resolution that there has been the most recent research activity2.
Coarse Ink Jet Printer These are normally based on valve technology and have essentially found use only in the carpet industry. There are two main commercial available systems. The Millitron system use an array of jets with continuous streams of dye liquid which can be deflected by a controlled air jet. The chromo jet uses computer-activated on/off valve systems to control the flow of the dye liquid. The use of electrochemical valves which are computer controlled to open and close rapidly so that liquid is fired in a succession of short pulses (13,14,15) The contribution of BTTG in Manchester on the type of technology but with increased resolution.17
Strock Trucolor Jet Printer
The Strock Trucolor pronter is a development based on the continuous stream technology (using the binary method). Essentially a dye formulation is pumped at constant pressure through nozzle, 14.4in diameter. The continuous stream is brolen up into droplets by modulation 625kMz, meaning that 625000 drooplets of colorant are formed each second 18 Because of the number of droplets per colour in any one pixel area can vary between zero and 15,16 color level possible. On each pixel of pattern which allows the production of smooth continuous tones. This contrast with drop on demand printing, which has to relay on dither patterns to naked eye, instead of the snooth shade afforded by continuous stream method19.
Pre and post Treatments Printing with reactive and acid dye inks generally involves pre and post treatment in order for the dyestuff to fix onto the fabric. This is a multi- step process with a substantial degree of complexity.
Reasons for Pre Treatments The main reasons for separating the dyes 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 hydrolyse when alkali is present in the ink. · Chemical in the ink cause corrosion of jet nozzle; the detrimental effect of the 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 de 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 droplets size. When pre treated fabric has been dried and then jet printed there is usually little need to provide a drying station to dry the print.
Fixation Steaming is the process normally used to fix printed textile. Reactive and acid dyes are steamed under atmospheric pressure at just over 100ºC. During the process steam condensed on the fabric and is absorbed by the thickners and hygroscopic agents in the pronted areas. Dyes and chemicals dissolves and form extremely concentrated dyenath within the thickener film. As the result of extremely low liquor ratio fixation is much more rapid than in exhaustion dyeing. High temperature steam is necessary for the fixation of disperse dyes on polyester. The Tg of polyester in steam is lower than it is in dry air, and fixation is more efficient. Usually steam is heated to 170-180ºC at atmospheric pressure, but sometimes pressure steaming at 130-150ºC is used. Pigment prints are cured hot air in a stenter or a roller baker.
The Advantage of the difital ink jhet printing · When consider as a whole, it can be said that digital ink- jet printing above all saves time. It speeds up the process between design and industrial production in an almost dramatic way in an emergency, Sampling for shade Prototypes Design variation ate possible overnight
· There is also considerable cost reduction in these areas the cost savings are between 50% and 90%. Ink jet printing enables the production of luxury goods (designer fashion etc), custom made textile and the economical production of short runs and small batches. And yet, I the course of technical progress, the profitability ceiling of short production runs is fast moving upwards and could very soon reach a level where the average production quantities could be a interesting proposition. · A further advantage of high speed printing is that for the first time ‘Just In Time” production and delivery are possible. This opens up entirely new business opportunities and at all level reduces the stock level and fashion related risks. · In addition the digital production is more environmentally friendly as far as effluents; energy consumption and waste are concerned. Because of software aided optimization processes, there is hardly any textile waste and it is possible to make an exact calculation of the amount of the colorant to be applied.(20,21)
Conclusion In the end we can conclude that, ink jet printing is one of the state of art technique available in terms of quality, productivity and assured reproducibility in spite of initial high investment. Further research has to be focused for the betterment of technology.
References
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