Pretreatments
English (en) svenska (sv)
'Pretreatment includes production steps such as singeing, desizing, washing and boiling.
Contents
Singeing
Woven garments often undergo singeing before wet pretreatments. This means that the fabric is passed over a gas burner in order to burn any surface impurities, which otherwise could cause problems for further processing and later when used. It is know that air emissions arise from the burning but this is believed to cause little environmental impact.
Desizing
Desizing is a textile process that is used as a pre-treatment in the manufacture of woven fabrics. Desizing involves removal of the warp glue (size) that has to be impregnated on the warp threads in order to protect them from mechanical abrasion during the weaving process. The adhesives currently used are starch products such as polyvinyl alcohol (PVA), polyacrylic acid (PAC), carboxymethylcellulose (CMC). Furthermore galaktomannan and polyester are used. Adhesive agents must be removed before dyeing and processing. The requirements are therefore that they should be:
- Easy to remove.
- Resistant to alkali.
- Easy to demonstrate in order to be able to control that the glue has been removed.
Starch adhesive are usually removed by adding enzymes that simply breaks down starch adhesives and make them water soluble. PVA, PAC and CMC are water soluble. PVA is somewhat sensitive to alkali and peroxide; these substances can change them and make them difficult to wash out. Certain adhesive PACs are thermally unstable and can become insoluble if heated. Imported raw fabrics already contain warp glue (size), while in Sweden you can make the decision yourself. This normally means that the choice of adhesive is not controlled from within Sweden. PVA and PAC-sensitive adhesive is often used in the synthetic material while the starch adhesive is mostly used on cellulosic fibres. In general there are often required higher amounts of starch and cellulose derivatives other than the synthetic glue (size) calculated as weight per cent of fibre materials.
Environmental Facts - desizing
Starch products are easily biodegradable. CMC, PVA, and PAC are partially biodegradable. Some observations suggest that some modified starch adhesives are persistent. Adhesive agents and their enzymatically degraded variants show no fish toxicity. The persistent adhesives end up in sewage treatment plants with other sewage sludge. In processing woven fabrics more than 50% of the sewers organics consists of adhesive agents. Most of the adhesive quantities are usually composed of starch adhesives that are easily biodegradable. In addition, it may also be degraded by desizing. Other solutions data has been found in the Tox Info Manual and also data from the adhesive manufacturers Examples of adhesive that are commonly used are shown in the following points:
- Polyvinyl alcohol (PVA) is prepared from polyvinyl acetate to form acetic acid, which is easily biodegraded. According to earlier data, PVA is persistent, but more recent data from the manufacturers indicates that the substance is biodegradable. Various molecular weights may be the cause of these conflicting data and the adaptation of sludge organisms.
- Polyacrylic acid (PAC) is easily degradable in the aquatic environment but toxic to algae. Acrylic polymer is partially persistence, probably because of the degree of polymerization (molecular size) and the adaptation of sludge organisms.
- Carboxymethyl cellulose (CMC) is probably somewhat more difficult to degrade than cellulose, which is due to its degree of carboxylation.
Washing
Washing of adhesives has to be made in order for the following treatments can be implemented. For water-soluble adhesives washing can be done directly, but the most common starch adhesives must firstly be decomposed. This can be done either by oxidatively sodium bromite and alkali, or enzymatically, a starch-decomposing enzyme, e.g. amylase. This “washing-out” technique is usually performed in special wash machines with several wash boxes connected in sequences, where wash water flows towards the weft direction, in other words, counter-current washing to conserve water. Shorter woven lengths also run discontinuously in an apparatus with several changes of water.
Boiling / Bykning (Swedish)
Boiling is used to remove various impurities such as natural seed hulls, waxes, and pectin, and also previously applied avivage and spinning oils. Boiling is performed with the use of surfactants and alkali agents. Seed coats and waxes are especially difficult to wash out and they require high alkalinity and high temperatures in order to be removed. This is also called boiling or bykning (Swedish). When boiling methods are used on cotton it contains starch pastes. Boiling is not suitable for synthetic fibers - especially if mixtures of synthetic adhesives have been used. Cotton that is amassed by machine usually contains a lot of metals that increases water hardness (calcium, magnesium and iron). To remove these metals in the washing process before bleaching and not risking the fibers to be damaged complexing agents and mineral acids are used.
Environmental Facts - washing, boiling / bykning
The waste water from pre-treatment often contains more than 50% of the contaminants from the textile processing. Depending on what type of textiles used in the different variants of pre-treatment, eg is the boiling finished with washing? This means that the wash water will contain many variations of spinning lubricants, knitting oils, weave oils, glue types, dirt, seed coat, waxes, pectin, and wool or other animal fur particles. Furthermore, the water will contain chemical pesticides, heavy metals and the like from the production. The wash water will contain the boiling and laundry detergents. Depending on the fiber, use of cultivation and chemicals used in manufacturing, spinning and weaving techniques, the wash water will contain mixture of these substances.
Environmental data on these substances are only partially known. Environmental facts about oils and adhesive types have been mentioned earlier. On the different variants and mixtures of substances one must require environmental data directly from the manufacturers. Manufacturers should also be able to describe the content of pesticides and heavy metals, although their presence can be determined by analysis of the wastewater. An analysis, of course, does not identify the sources of where the contaminants in the material originate from. The data must be provided by the manufacturer.
Regarding detergents the most important thing is to receive data on their toxicity, biodegradability, and ability to accumulate in living organisms. The laundry chemicals always contain surfactants and complexing agents. Solvents have been used in Sweden in order to speed up the washing process. It is important to get as much information as possible by detergent manufacturers. For general information about the environmental effects of various surfactants and complexing agents please see the Tox Info-guide (Tox-handboken). Often, non-ionic surfactants are used to obtain a sufficiently rapid and efficient washout of the chemicals used in spinning. Previously, the dangerous alkylphenolethoxylates (APEO) where used but they were replaced by alcohol. The alcohol should ideally have a linear hydrocarbon chain with a maximum of 18 carbon atoms. The alcohol is the primary and the number of ethylene oxide units should be no more than 20-30 in order to be biodegradable. Previously, polyphosphates were used as complexing agent during washing, but after the requirement for lower phosphate concentrations got in effect the most used substances are EDTA, NTA and phosphonates since they are very effective even at low concentrations. It is not considered suitable for the textile industry to use zeolites and polycarboxylic acids, which are used in household detergents. The criticism of phosphate substitutes may lead to a reassessment of polyphosphates regarding the water use.
EDTA, NTA and phosphonates have been questioned primarily because of their ability to loosen heavy metals. Today,all bleaching in Sweden is done with peroxides. It has increased the need for effective complexing agents that otherwise would jeopardize the bleaching of cotton grown on iron-rich soils. If he iron metals cannot be removed properly in the pretreatment, there is a limitation for potential reuse of evaporated alkaline liquors in subsequent processes. After washing there is usually a pH control (deacidification), typically in acetic acid. If the product proceeds to bleaching, the alkali residues do not need be removed. In the event of a total ban on the use of phosphates in detergents, the need for substitutes will increase for EDTA, NTA and zeolites. This may mean that the compensation solutions may need to be revalued from an environmental and health perspective because they may not be appropriate to use in large quantities. [1]
BAT (Best Available Technique) - pretreatment
Use of BAT - Best Available Techniques in the pretreatments are environmentally important. Nearly half of the amount of pollution derives from pretreatments, and consists of pollutants that enter the process with fiber materials. These contaminants consist of fiber contamination from cultivation or production facilities, and placed where – spinning preparations / avivage, warp size takes place. BAT concludes that the use of these elements and addatives should be minimized to the best possible extent. Moreover that you either use the readily biodegradable adjuvants and / or recycles and reuses them in the processes. There will undoubtedly be a follow-up on BAT recommendations that will have a major impact on the size of emissions and environmental hazards, but these measures have their limitations. If the tools are readily biodegradable, it can lead to mold and bacterial growth during transport and storage, which increases the need for preservatives. Reusing spinning preparations is hardly practicable, while the reuse of adhesive agents is already being done in several parts of the West. Normally up to about 70% will be reused, and somewhere around this number we find the practical limit due to the enrichment of contaminants from the fiber material, which deteriorates the recycled adhesive properties. If the recovered adhesive is readily biodegradable it needs either preservatives or high temperatures for thermal disinfection of glues composition. BAT also includes the limited use of environmentally harmful processing aids in pretreatments.
Summary - pretreatments
To summarize, there are the following types of chemicals used in pretreatment (desizing, washing, boiling, bleaching, mercerization). Of the mentioned substances the most common ones are the substances that are washed out. Of these, some types of adhesives can be recycled or re-used (if weaving and desizing is done in the same factory). Neither the avivagers nor natural fibers impurities have any type of area for reuse. Previously, both the oil from cotton seeds and the short seed straws were used. These are called "linters" in cotton production. Of these "linters" one have previously made viscose fibers. The next largest group of pre-treatment chemicals are the basic chemicals. Of these, a portion of alkali is reused (by mercerizing). The most used oxidizing agent is H2O2 that almost produce no pollution. In some cases NaOCl is still necessary to use for bleaching and deinking. The greatest threat to the wastewater is coming from adhesive, avivage and natural fibers impurities and contaminants from the environment in which they were produced.
Table:
- Utilities
- Basic chemicals
- Substances washed out
