Any unwanted modification in the properties of a material brought about by the essential actions of organisms is called Bio-deterioration. Present article is concerned with the degeneration of textile materials produced by microorganisms like fungi and bacteria, and the solutions to avert or reduce their effects.
Not all failures of materials by microorganisms are unwanted. When we throw away any objects not required any more, we wait for “Nature” to clear away what has then become waste. Such deterioration is a vital course of action for the protection of the world in which we live, and it is a process of recycling many of the vital components held by these materials. However, it can be a critical problem to both manufacturers and users when it is an undesirable process, when textiles are influenced by rot or mildew.
Under proper environment microorganisms, which dwell in soil, water, and air can grow and flourish on textile materials. These organisms encompass species of microfungi, bacteria, actinomycetes (filamentous bacteria), and algae. Textiles manufactured from natural fibres are normally more vulnerable to biodeterioration than are the synthetic man-made fibres. Microbial expansion can also be advanced by products like starch, protein derivatives, fats, and oils used in the finishing of textiles. Micro-organisms may attack the whole substrate, i.e. the textile fibres, or they may attack only one constituent of the substrate, such as plasticizers enclosed therein, or grow on dirt that has built up on the surface of a product.
However, even minor surface tumour can make a fabric look ugly by the emergence of undesirable pigmentation. Heavy infestation, which gives rise to decaying and failure of the fibres and consequent physical variations such as a loss of firmness or flexibility, may produce the fabric that fail to serve. The material is attacked chemically by the action of extra-cellular enzymes produced by the microorganism for the objective of acquiring food. However, microbial activity can be reduced by saving the dryness of vulnerable materials because surface expansion will only take place when the relative humidity is high. Therefore, some kind of chemical shield is generally needed with textiles expected to be used in hostile conditions under which they stay wet or damp for long time.
Plant fibres like cotton, flax (linen), jute and hemp are very vulnerable to attack by cellulolytic (cellulose – digesting) fungi. Certainly, the complete degeneration of cellulose can be achieved by enzymes created by the fungi and recognized as cellulases. Diagram 1 gives details of the chemical process involved. The spores of these microfungi are there in the atmosphere and when they settle on proper substrates they can grow fast under positive conditions of temperature and humidity. The typical growth form of these “mould” fungi is recognised as mildew, a outward growth, which may discolour and spoil the fabric with stain, as many microfungi are able to produce pigments. The best safeguard against mildew is to ensure that the fabrics are dry when put in storage and that they do not turn out to be wet in storeroom. Fabrics which are to be used outdoors for awnings, beach umbrellas, military uniforms, sails, tarpaulins, tents, truck and boat covers, shoes and shoe linings, are processed with a fungicidal finish to save them from mildew damage and rotting. Algal greening may also appear on fabrics, which stay damp for long time and can create particular problems in the tropics.
In proportion to plant fibres, animal fibres are less affected by mildew growth. Pure silk, if completely degummed, is less vulnerable. Wool decomposes only slowly but chemical and mechanical harm during procedure can intensify its vulnerability to biodeterioration. When stored under very unfavourable conditions wool will finally rot by the action of the proteolytic (protein-digesting) enzymes concealed by many microfungi and bacteria.
Man-made fibres obtained from cellulose are vulnerable to microbial degeneration. Viscose (rayon) is easily struck by mildew and bacteria; acetate and triacetate are more unaffected although discoloration can take place if the fabrics are improperly stored. Fibres produced from synthetic polymers (e.g. acrylic, nylon, polyester, polyethylene, and polypropylene fibres) are very resistant to attack by microorganisms.
The hydrophobic character of these polymers is possibly a significant aspect deciding their resistance. Also, these synthetic polymers have chemical bonds, which do not take place or are rare in nature, and perhaps therefore they have not been around long enough for microorganisms to develop the proper enzymes required to start their analysis. Although the substance of a synthetic fibre by itself will not hold up microbial development, pollutants of low molecular weight (e.g. remaining marks of the caprolactam monomer of nylon 6) and mixtures such as lubricants and spinning oils used in the finishing of textiles may give satisfactory nutrient for mild surface evolution of a microorganism. In most cases this will not influence the health of the fabric but can result into staining and discolouration, which are often not easy or impossible to eliminate.
Various kinds of plastic materials have surfaced as sections of textile products, for instance, to give waterproof coatings for rainwear. Plastics, which are produced mainly or entirely from polymers such as polyethylene, are generally highly resistant to microbial expansion. However, two types of plastic used significantly as coatings for textile materials, plasticized polyvinyl chloride (PVC) and polyurethanes, are vulnerable to biodeterioration. In the case of PVC, the polymer itself does not willingly supply a means of nutrients for bacteria and fungi. The vulnerability of PVC formulations to microbial attack is associated with the amount and types of plasticizers, fillers, pigments, and stabilizers, etc., inserted during processing. Many of these additives are organic compounds of comparatively low molecular weight. For instance, plasticizers (predominantly esters of organic acids, polyesters, and chlorinated hydrocarbons), which are put in to enhance the flexibility of an otherwise fragile polymer, will in most cases nourish microbial expansion and their degree of vulnerability applies a deep impact on the propensity of the textile coating to biodeterioration; such microbial exploitation of the plasticizers may cause crack of the PVC coating during use. With polyurethanes on the other hand the actual polymer is able to prop up microbial evolution because of the resemblance of some of the chemical connections in polyurethanes to those discovered in nature. Therefore, biocides are often included in both plasticized PVC and polyurethanes as a practical measure.
Use of biocides
The perfect technique of preventing microbial degeneration is to use synthetic materials, which are naturally resistant to attack. Another method is to apply antimicrobial chemicals known as “biocides” which are generally included into the finished textile product. So far no additive agent has been unearthed, which provides neither complete safety nor is without some drawback. Perfect biocides include following requirements:
. Efficient against a large range of microorganisms, especially bacteria and microfungi.
. Operative during the life of the product.
. Of low mammalian toxicity and non-toxic to humans at the concentrations used.
. Lacking colour and odour.
. Influential at low concentrations.
. Not expensive and easy to use.
. Resistant to sunlight and percolating from the fabric.
. Fabric handle and health are unaffected.
. Adaptable with water-repelling and flame-reducing agents, dyes, and other textile accessories.
. Does not intensify the fabric to destruction by light or other effects.
It would be an endless journey if somebody sets off to find the ideal biocide and the compromise choice of a proper product is not always easy. Some chemicals, for instance organo-mercury compounds, have been discarded because of their lasting and increasing toxic effects in the environment. Textile materials, which are to be used outdoors, need a constant fungicide that has anti-rain wash properties and capability to suffer breakdown by light. If the environment is tremendously damp, monitoring of algae and bacteria becomes more significant. However, many compounds, which are efficient against microfungi, are not essentially good bacteriocides and vice versa.
Regularly used biocides in the textile industry are organo-copper compounds, organo-tin compounds, and chlorinated phenols. These function by intervening in the energy-producing procedures of microbial cells. Copper naphthenate and copper-8-hydroxyquinolinate are greatly multipurpose biocides, very efficient against fungi, bacteria, and algae. They are specially used to look after textiles prone to be bare to soil and to harsh weathering conditions, e.g. cotton and flax canvases, awnings, tarpaulins, cordage, ropes, sacks, tents, military uniforms and military gears. The main drawback is that they give a yellow-green colour to processed materials. Pentachlorophenol esters, conventionally pentachlorophenyl laurate (LPCP), are resistant to percolating by rainwater and so are applied as fungicides for the rot-proofing of a large array of textiles together with cotton, flax, and jute fabrics used as covers, tarpaulins, shop blinds, tents, etc.; also carpet backings, coated fabrics, hospital materials, mattress covers, pressed felts and woollen textiles. Some biocides can provide more than one objective; thus organo-tin mixtures can work as stabilizers for plastic formulations as well as fungicides.
Use of biocides in textile fabrics for rotting and mildew-proofing is generally performed as a final finishing treatment. The fabric is soaked in either a solvent (usually white spirit) solution or, more commonly, an emulsion of the biocide; it is then pressed and dried out using a cylinder dryer, a stenter, or other appropriate tools. The fabric may be polished first but more usually and especially with heavyweight materials, the biocides are applied to loomstate material without polishing. Very often they are co-applied with water-repelling, fire-retardants, and pigments. In vinyl polymers like PVC, the biocide is generally diffused in the plasticizer, which is by and large the most biodegradable constituent. As the surface film is eliminated, new plasticizer will shift to the surface, taking with it a continuous source of biocide. However, these products finally lose their protecting merits through seasoning even though 70%-80% of the biocide continues to be chemically unaffected in the formulation. One possible cause is that under the influences of heat and ultraviolet radiation, depolymerisation of the vinyl resin and consequent cross-linkage may condense the biocide, checking its transfer to the surface where biodeterioration occurs.
Biocides are also applied to give hygienic finishes for fabrics that will be used in health-care goods. These finishes are categorised as either renewable or long-lasting, although long-lasting finishes are detached gradually during laundering. Renewable finishes can be substituted during laundering, for instance quaternary
ammonium compounds used to resist napkin rash. Some safeguard against the microfungi responsible for athlete’s foot is also asserted for hygienic finishes that are applied to socks and linings for footwear.
Measuring the scope of biodeterioration of textile materials is not easy but it must be estimated in terms of millions of pounds sterling annually in the UK alone. Breakdown of materials can often be a complicated event resulting from a mixture of chemical, physical, and microbiological reasons. The problem is most critical with fabrics used outdoors but other products such as floor coverings, rug backings, shower curtains, vinyl baby pants, and mattresses may also need an antimicrobial finish to check fungal and bacterial surface expansion. If possible, materials naturally resistant to microbial attack should be chosen rather than materials, which need protection. Thus, synthetics, which resist mildew, contend positively with cotton in manufacturing sails for boats or shower curtains. Advance research is needed into techniques of producing both natural and synthetic materials more resistant to biodeterioration by chemical alteration of their make-up, especially because uncertainties have now been given rise about the toxicity and environmental constancy of some of the so far well recognized biocides. It is also envisioned that enhanced biocides will surface on the market to meet new set of laws introduced by governments worried about the environmental impact of current compounds.