Difference between revisions of "Dyeing & Printing"

Latest revision as of 10:04, 27 April 2015

Colour is a critically important part of a fabric or garment, and one of the single most important factors in the appeal and marketability of an apparel product, particularly during the "interest" phase of the consumer purchasing decision. An inappropriate or unattractive colour may make a garment unmarketable no matter what the quality of the fibre, the yarn, the weave or knit, or the finish. Conversely, a poor-quality fabric may achieve big seller status purely because of its colour. Aside from its attractiveness, colour permanence (i.e., fastness) is important; most problems that consumers have with textile and apparel products are associated with colour fading, bleeding or colour staining (crocking). At one time, colourants were limited to natural dyes and pigments obtained from plants, insects and minerals. The first synthetic dye was developed in 1856 and by the early 20th century, a wide range of synthetic dyes were readily available. Today, the textile and apparel industry uses well over 1 million tonnes of colourant annually, almost all of which is manufactured synthetically. Adding colour to a textile substrate is a complex process. Slight differences in fabric caused by minor irregularities in fibre, yarn, fabric, or finishing can result in obvious colour variations in finished products. Fibre chemistry also plays an important role. A match between the chemistry of the colourant and that of the fibre is necessary in order for the colour to be permanent. In addition, during use, any textile may be exposed to a wide variety of potential colour degradants such as detergent, perspiration, dry-cleaning solvents, sunlight, makeup, etc. To achieve a durable colour, the colourant must be attached to or trapped within the fibre by using a combination of heat, pressure and chemicals. Fibre crystallinity, chemical finishes, and fabric and yarn structure are all factors that influence the success of dyeing and printing.

Overview of environmental concerns in dyeing & printing

• Formaldehyde common ingredient of dispersing agents (for vat, sulfur, and disperse dyes), printing pastes (ingredient of resins added to promote cross-linking between binders and fibres), and colourant fixatives
• Heavy metals found in dye-stuffs, dyeing auxiliary chemicals and print pastes (as PVC stabilizers); often an unintended contaminant found in numerous chemicals
• PVC and phthalates used in plastisol printing pastes
• Residual colour in wastewater due to poor exhaustion and/or fixation of colourants
• Salt used to promote exhaustion of reactive and direct dyes onto cotton substrates
• Volatile organic compounds (VOCs) in print pastes (particularly solvent-based)
• High biological oxygen demand (BOD) or chemical oxygen demand (COD) caused by substances in the wastewater after dyeing and finishing. BOD and COD create environments that are hostile to aquatic plants and animals.

Synthetic colourants

Synthetic colourants are man-made and cost less than natural colourants, are offered in a diverse range of colours, are more colour-fast and easy to apply.
Chromophores are an essential part of the colourant's chemical structure, and are partly responsible for a chemical's ability to project colour. Chromophores are limited in terms of the fibres upon which they can be used (i.e., limited to certain dye classes), the number of hues possible, the intensity of colour required, and/or cost.
More than 50% of all commercial colourants contain one or more functional chromophores known to the azo group. Many dye classes make use of azo groups (e.g., direct, azoic, reactive, acid, basic), as do some pigments, so their presence is not limited to any particular textile fibre or substrate. Under certain conditions, they can break down to form aromatic amines, which can then be released from the fabric or garment and may be carcinogenic. The use of certain colourants that contain azo groups— mostly those that can release higher concentrations of amines— is forbidden in many parts of the world. This includes around a dozen acid dyes (normally used with nylon and wool) and numerous direct dyes (used with cellulosic fibres).
Anthraquinone is the second most common chromophore. Anthraquinone chromophores are found in vat, reactive, disperse, acid dyes, and in some pigments, so they can be used on cellulosic fibres such as cotton or rayon and on synthetic fibres such as polyester and nylon. Anthraquinone colourants are often brighter than their azo counterparts, but are limited in terms of shade depth.

Potential impacts

Chemicals used in textile and garment dyeing and printing are developed to be resistant to environmental influences. This durability sometimes limits the biodegradability of colourants and makes them difficult to remove from wastewater generated by dyeing or printing processes. Conventional treatments tend to transfer waste from one place to another. For example, solids extracted from wastewater are sometimes hazardous and are disposed into special landfills, where they can cause groundwater contamination, gas formation and noxious odours.
Certain types of dyes are suspected carcinogens and mutagens, while other disperse dyes are known to have a sensitizing effect on skin and should be avoided. Turquoise blue and greens contain metals, such as copper and nickel, as part of the dye molecule. Metals can cause toxicity in aquatic environments. Metal-containing colourants can be replaced with colourants that do not contain metals or contain lower metal content, though this is sometimes at the expense of colourfastness. These dyes are carcinogenic or mutagenic colourants and should be completely avoided: CI Basic Red 9, CI Disperse Blue 1, CI Acid Red 26, CI Basic Violet 14, CI Disperse Orange 11, CI Direct Black 38, CI Direct Blue 6, CI Direct Red 28, and CI Disperse Yellow 3.

Natural colourants

Natural colourants are produced or extracted from plants, arthropods and marine invertebrates (e.g., sea urchins and starfish), algae, bacteria, fungi, and minerals. Sources for natural colourants include cochineal, mollusks, roots, berries, bark, lichen, carrots, artichokes and other natural matter.
In order to achieve acceptable colourfastness, mordants are almost always necessary to properly fix natural colourants to textile and apparel substrates. Mordants increase colourfastness by combining with both the colourant molecule and the fibre molecule. The most commonly used mordants for natural dyes are chromium, aluminum, iron, copper, tin and other heavy metal salts.
A major challenge with natural colourants is producing natural colourants in large quantities, at a reasonable cost, and to achieve comparable colourfastness.
Obtaining a full palette of colours using natural colourants remains a challenge, as does repeatability, light fastness and durability to wear. Furthermore, in production it’s extremely difficult to produce the same natural dye shade twice, even when using exactly the same dyeing technique and procedure. For these reasons, natural dyes remain niche in the commercial fashion world, and are well suited to the “slow fashion” movement, which emphasizes locality, difference and diversity, though new technologies may at some point bring natural dyes to wider economic viability.

Potential impacts

It requires approximately 1 pound of fresh plant to produce the colouring power of one gram of synthetic colourant. In a world of increasing human population and declining natural resources, land use for food vs. colouring material will be hotly contested.

Printing: application of colour

Printing is the patterned application of colour. Since the colourants and auxiliary chemicals used in printing are similar to those used in dyeing, many of the environmental concerns are shared between these processes. However, to obtain the sharply defined, precise, reproducible patterns typical in printed textiles it's necessary to use special liquids, known as pastes or inks that have a high degree of viscosity (i.e., they're in a "gel" state). The printing ink, which typically includes colourants, binders, softeners, thickeners and other auxiliary chemicals, can be directly applied to the substrate using mesh screens, engraved rollers, ink-jet printers and can be indirectly applied to the substrate using pre-printed transfer paper.
The two main types of printing inks are pigmented emulsions and plastisol inks.

Emulsion inks

Emulsion inks are used mainly for direct printing of fabrics and are typically based on aqueous dispersions (i.e., water-based) of a binder and cross-linking agent. Emulsion inks can be solvent-based, although their use is rare in textile and garment printing.

Potential impacts

Solvent-based inks have high volatile organic content (aliphatic, aromatic and oxygenated organic solvents), and can cause problems with air pollution and waste disposal.

Plastisol inks

Plastisol inks are primarily used for direct and indirect (transfer) printing of garments and are typically vinyl resin (PVC) dispersed in plasticizer. In garment printing, PVC serves as a binder that melts into, or fuses with, the garment while bearing the solid pigment. Plastisol inks contain plasticizers, which soften the naturally rigid PVC to give it the flexibility to keep from cracking. When used as a transfer, the plastisol ink is screen-printed onto a release paper and cured to a dry film, which is then stored until being transferred onto a garment using a heat transfer process.

Potential impacts

Plastisols do not biodegrade. One byproduct of their production (and of disposal via incineration) is dioxin, an acutely toxic substance. If landfilled, heavy metals sometimes used as PVC stabilizers, such as lead or cadmium, can contaminate groundwater. Plasticizers are often phthalate esters, which may leach out of the print or may evaporate and be released during drying, either during the production process or in the home. Exposure to phthalates is known to cause adverse health effects, and several phthalates are banned by the California’s Prop 65 law in the United States.

Discharge (removal of colour)

Whereas printing is the patterned application of colour, discharge printing is the patterned removal of colour. In other words, the fabric or garment is dyed prior to printing (commonly known as the "ground" colour or shade), and then printed with a paste or ink containing a chemical discharge agent. The discharge agent is capable of destroying the chromophoric system of the original colourant(s) under appropriate conditions, thereby severely degrading/fading the colour or removing it altogether. Many reducing agents, oxidizing agents, acids, salts and alkalis can function as discharge agents. All colourants react differently to these agents; some are dramatically affected, while others are largely or completely resistant. If more-resistant and less-resistant colourants are combined into one dyed ground shade and then discharge printed, the area to which the agent is applied creates a shift in hue. Colourants that are resistant to the discharge agent can also be included in the print paste itself, so that they effectively "replace" the discharged colourant(s). This replacement colour is sometimes referred to as the "effect" colour.
The most common reducing agents used in discharge printing are metal salts of formaldehyde-sulphoxylic acid, such as zinc, sodium or calcium formaldehyde-sulphoxylate. An alternative, but less frequently used agent, is thiourea dioxide.

Zinc formaldehyde-sulphoxylate (zfs)

Zinc formaldehyde-sulphoxylate (commonly known as ZFS) is particularly popular, because it helps to cure the acrylic binders commonly included in the discharge print paste. When combined with appropriate inks and a humectant, ZFS can be used in dry as well as wet or moist heat conditions.

Potential impacts

Releases formaldehyde during the discharge reaction process. Formaldehyde is retained in the fabric. Formaldehyde is a toxic air pollutant, a volatile organic compound, is allergenic and/or carcinogenic in certain conditions, and is heavily regulated.

Thiourea dioxide

Unlike formaldehyde-sulphoxylates, thiourea dioxide neither contains nor releases formaldehyde. However, its effectiveness in discharging colour is limited to a narrower range of colourants and its effect is noticeably weaker than formaldehyde-sulphoxylates. Thiourea dioxide also requires steaming and thorough washing after the discharge print paste is applied.

Potential impacts

Requires steaming and thorough washing after the discharge print paste is applied.

Dyeing: Reactive and directive dyes

When reactive or direct dyes are used to dye cotton, the use of salt—usually sodium chloride or sodium sulfate (also known as Glauber's salt)—is necessary to promote exhaustion (uptake and fastness) of the dye onto the cotton substrate. Sodium chloride is less expensive and contains more sodium per unit mass than Glauber’s salt, so less salt is needed in the dye solution. However, Glauber's salt is less corrosive (particularly to dyeing machines) and produces brighter shades when used with some classes of dyes. In general, reactive dyes require 5-10 times more salt than direct dyes, and dark shades require 5-10 times more salt than light shades.
In addition to dye class and shade depth considerations, the amount of salt required in dyeing cloth is dependent upon the volume of the dye bath solution in relation to the mass of the material being dyed. This “liquor ratio” can vary widely depending on the dyeing equipment used. Some garment dyeing machines require as much as 400 gallons of dyebath for every pound of material dyed, whereas the commonly used jet dyeing systems require 80 gallons or less. In other words, the former would require five times more salt in the dying process than the latter.

Potential impacts

Salt use in textile dyeing is a serious environmental issue, since it is used in such large quantities and is a major source of aquatic toxicity in wastewater. Moreover, the removal of salt from wastewater is extremely difficult and expensive using current treatment methods.
Low- or no-salt developments are therefore of great interest in the chemical dye industry. Direct dyes use less salt than reactive dyes, but their attraction to cotton is relatively weak, and they are often treated with special fixing agents to improve colourfastness. Cationic fixing agents (usually quaternary ammonium compounds) are commonly used to improve wash fastness, and copper sulfate is sometimes used to improve light fastness. Both are major concerns in terms of aquatic toxicity.

Reduce environmenal impacts of dyeing & printing

Naturally coloured cotton

Naturally coloured cotton has existed for more than 5000 years. Naturally coloured cotton has pigmentation in the center, or lumen, of the fibre and the colour depth and shade varies with growing conditions, location and climatic factors. Generally speaking, natural colours range from shades of cream and tan to tones of brown, red and green, although purple, mauve, grey and black cottons are theoretically also possible.
Over the past twenty years, considerable work has been done to cross breed coloured cotton fibres to both expand the range of available colours and to improve the fibre length and quality. As a rule, naturally coloured cotton suffers from lower agricultural yields, making it economically challenging to produce, and therefore expensive to the mills and manufacturers. Brown fibre is typically 2-3 times the cost of white cotton fibres, and green fibre approximately 4 times the cost of white fibres. Coloured cotton fibre also lacks important quality characteristics such as fineness, length and strength, and requires special handling through ginning and spinning, since the colour can catch on equipment and contaminate the batches of white cotton fibre most commonly processed through the facility.
For these reasons, naturally coloured cotton fibres are usually blended with white cotton to improve quality, facilitate processing and reduce costs. Though blending reduces the colour intensity of the end product, washing the yarn or laundering the product in alkaline solution can enrich hues. The strength of 100% coloured cotton can also be improved by plying several ends of yarns together (2 or 3 ply), though this increases cost. Coloured cotton yarns can also be plied with white cotton yarns to bring cost down. Coloured cotton fibre properties can differ profoundly by colour and are largely dictated by the cultivation practices by the type of seed used. In terms of colourfastness, brown and beige cottons generally outperform greens.

“right-first-time" dyeing

Without question, the most effective pollution prevention practice in colouring cloth is "right-first-time" dyeing. Corrective measures such as reworks, re-dyes, stripping, shade adjustments, top-ups or "adds" are all chemically intensive and contribute significantly to pollution, since each corrective action increases colourant and/or chemical and water use.

Spun dyeing

Synthetic fibres can be coloured by introducing pigments or dyes into the polymer melt prior to extrusion. Spun dyeing reduces water and pollution associated traditional dyeing, and can also produce favorable chartacertisitcs, such as such as uniform colouration, level shade, a high degree of light fastness, relatively low cost and cleanliness.

Auxiliary chemicals

Auxiliary chemicals can be selected to minimize or reduce the environmental impact of dyeing and printing processes. For example, acetic acid, which has a relatively high biological oxygen demand (BOD), is used in a variety of textile and garment processes for pH adjustment. Formic acid, which has a much lower BOD, or dilute mineral acids, which have no BOD, can sometimes be substituted for acetic acid.
In addition to chemical substitution, harmful auxiliary chemicals can sometimes be completely avoided by changing operating conditions. For example, "carriers" are organic chemicals that are often used as dyeing assistants when dyeing hydrophobic synthetic fibres such as polyester. In essence, carriers "open" the synthetic fibre, thereby increasing the rate of dyeing. The most common carriers are chlorinated benzenes and biphenyl. As a rule, carriers are extremely volatile and are also toxic, and they contribute significantly to toxic air pollution. But if dyeing takes place at a high enough temperature, carriers are unnecessary. In order to reach the required temperature (at least 129° C), a pressurized dyeing vessel is necessary. Many dyeing facilities have pressurized dyeing machines, but not all.

Low-liquor-ratio dyeing

A typical dye bath comprises salt, acids and alkalis, lubricants, and dispersing agents, all of which can contribute to pollution. These chemicals are measured in proportion to the volume of water used, and so lower volume dye baths greatly reduce chemical use and disposal in wastewater. Lower volume dye baths also require less energy for heating. Standard liquor ratios range from 10:1 to 15:1 for many exhaust dyeing operations. Low- liquor-ratio machines are capable of dyeing at liquor ratios closer to 5:1, with some as low as 3:1. Some dye systems can operate effectively at room temperature, eliminating the need for heating altogether. It's important to note that low-liquor-ratio dyeing often limits the choice of dye class used (i.e., to more water-soluble dyes), and that existing equipment cannot normally be "retrofitted" to make it operate at lower liquor ratios; investment in new equipment is usually necessary.

Dyebath reuse

Dyebath reuse is the process by which exhausted hot dye-baths are analyzed for residual colourant concentrations, replenished and, rather than being dispelled as wastewater, are reused to dye further batches of fabric. Dye-bath reuse requires that colourants undergo minimal change during the dyeing process. Direct, disperse, acid or basic dyes are therefore best for reuse applications. Dye-bath reuse carries a greater risk of shade variation, because impurities can build up and decrease the reliability of the process over the longer term. Capital is also required to purchase and install the appropriate infrastructure (e.g., holding tanks, pumps) to effectively reuse dye baths. When properly controlled, some dyebaths can be reused for 5-25 cycles.

Pad-batch dyeing

Pad-batch dyeing is a cold dyeing method mainly used for dyeing cellulosics (100% cotton and polyester/cotton blends) and can result in significant reductions in pollution and water and energy consumption (50-80% water and energy savings are common). No salt or chemical auxiliaries are necessary and the colourant exhaustion is much higher (which means less colour released into the wastewater). Moreover, quality is often more consistent compared to other exhaust dyeing techniques. Capital outlay is also low. However, pad-batch dyeing requires significant floor space to store dyed batches for long periods of time to allow the colour to permeate the cloth. Many dye houses lack the needed space, and brands don’t always have the time to accommodate the longer production processing time.

Ink-jet printing (also known as digital printing)

Ink-jet printing is arguably the cleanest printing technology. Ink-jet printing is a noncontact printing method that works much like an office printer—droplets of colourant are propelled toward a substrate and directed to a desired spot. Colourant types that work best with ink-jet printing include reactive, vat, sulfur, and napthol dyes, although acid, basic, and disperse dyes, or even pigments, can also be used in some cases. Ink-jet printing eliminates the need for many printing auxiliary chemicals (e.g., thickener), eliminates the need for screen, squeegee, and machine cleaning (which also dramatically reduces water consumption), and reduces waste generated from strikeoffs. Though ink-jet printing machines represent a capital investment, and production speeds are still relatively slow compared to analogue printing, ink-jet can offer significant savings for short production runs.

Transfer printing

With transfer printing, dye is printed on paper and then the paper carries the dye to secondary process where the colour is moved onto the fabric without the use of water and associated pollution associated with more typical jet dyeing systems. Transfer printing has poor fibre penetration, however.
AirDye is a propriety technology heat transfer printing technology that achieves greater penetration than traditional heat transfer printing by using a proprietary set of dyes. The AirDye process creates rich hues and achieves higher colour fastness. AirDye is suitable only for synthetics, and is readily used on polyester and nylon.

Optimize sustainability benefits

Sources

http://www.fibre2fashion.com AirDye Environmental Profile: Life Cycle Assessment (undated).