What is Antimicrobial Finishing of Textiles?

22.7.17

Antimicrobial Finishing of Textiles

It has long been recognised that micro-organisms, particularly bacteria, can thrive on textile substrates. Natural fibres such as cotton are more susceptible than synthetics because their porous hydrophilic structure retains water, oxygen and nutrients, providing a perfect environment for bacterial growth.

Most textile materials currently used in hospitals and hotels are conductive to cross infection or transmission of diseases caused by microorganisms. The spread of HIV and hepatitis viruses by contact of contaminated materials has created increased pressure for protection of personnel with functional clothing . Also, all articles of apparel and home textiles are susceptible to problems of hygiene in normal daily use- for example, socks, sportswear and working clothes as well as mattresses, floor coverings and shoe linings. Textiles for outdoor use are constantly exposed to the influence of microbes.

This microbial infestation has several unpleasant consequences : not only foul odours but also mould and mildew stains are produced. An unpleasant odour develops when, among other things, bacteria convert human perspiration into some foul smelling substances such as carboxylic acid, aldehydes and amines. As microbes often attack the additives applied to textiles, discoloration and the loss of the textile’s functional properties such as elasticity or tensile strength can also occur. Microbes may severely disrupt manufacturing processes, textile dyeing, printing and Antimicrobial Finishing operations through reduction of viscosity, fermentation and mould formation.

Another aspect which is now becoming important is the occurrence of allergies provoked by mould fungi or excretions of house dust mites. Mould fungi of the aspergillus type have been known to produce lung diseases. Some other common kinds of microbes and their effects are reported

Antimicrobial finishes

It has been realised that the microbial infestation cannot be removed even by the most frequent washing, with the exception of washing at boiling temperature, which is out of question with many textiles. A variety of antimicrobial finishes have now been developed for application to textiles. In addition to the effective control of bacteria, molds and fungi, such finishes must also fulfil many other requirements . They should exhibit :

Durability of activity to laundering, dry cleaning or leaching.
Selective activity towards undesirable micro organisms
Acceptable moisture transport properties
Compatibility with other finishing agents
Absence of toxic effects for both the manufacturer and consumer
Easy to apply
Applicable with no adverse effect on the fabric

Protection against biological attack may be subdivided into three categories:

protection of the wearer or user of a textile material against microorganisms for aesthetic (suppressing or killing odor-causing bacteria), hygienic (preventing skin and related infections usually caused by dermatophytic fungi) or medical purposes (suppressing or killing pathogenic and /or parasitic microorganisms problematic in hospitals and public institutions)
protection of the textile itself from biodeterioration caused by mold, mildew, and rot-producing fungi ; and
protection of textiles from insects and other pests for preservation of the fiber and or protection of persons wearing clothing from insects and pests.

Mechanisms of antimicrobial action

General strategies for protecting textiles and their users against biohazards are given in
Physical barrier or blocking action is achieved by either using inert films or coatings for physically blocking bacteria, or by films & coatings having direct surface contact activity against bacterial growth. Coatings can be universally applied to all fiber types and other surfaces to produce activity against a broad range of micro-organisms and be durable to normal washings. Hydrolysis product of trialkoxysilyl quaternary ammonium salt for example is applied through surface bonding to give such protection

Micro encapsulation, although not a chemical Antimicrobial Finishing process, is a physicochemical technique where a substrate reservoir of antimicrobial compound is held between two layers of protective plastic. As the active compound is used up, it is replaced by additional amounts from the reservoir by a controlled release mechanism. Substrates like polyester, cellulosics, vinyl acetate and polyethylene can be so treated. Mattress covers for example are protected against mites and other microbes for over 6 years this way.

However, the majority of antibacterial finishes function by the controlled- release mechanism. It is based on the principle of applying a chemical finish that would produce an active germicidal species continually regenerated by, say, addition of a bleaching agent during laundering, or, exposure to UV light which would break some strategic covalent bond in the chemically modified fibre during regeneration. Thus the model has theoretically an unlimited reservoir of antibacterial agent. The micro-encapsulation technique comes closest to this model, though its reservoir of antibacterial compound is not unlimited.

The other chemical methods involve insolubalization of chemical reagents in or on the fiber. Insolubalization is achieved by incorporation of agents into spinning baths for synthetic or regenerated fibres, or by padding natural or synthetic fabrics with solutions that when evaporated by curing or other methods, deposit a water-insoluble for slightly water-soluble agent onto the fiber.

Broad spectrum anti-microbial activity has been imparted to acrylics, nylon, poly vinyl chloride, cellulose acetate, polypropylene, and polyethylene fibers by chemically modifying the fibers using insolubalisation of 0.5-2% of various nitro compounds into wet or dry spinning baths. Compounds like 5-nitrofurfural, 5-nitro 2-furfurylidene 3-amino 2-oxazolidone etc are used. Cellulosics are modified using a different strategy viz. by introducing carboxylic and sulphonic acid groups and immersing them in cationic germicides.

Multifunctional property fabrics are often produced by grafting polymers, homopolymers, and/ or copolymerization on to the fiber or by chemical modification of the fibre by formation of covalent bonds. Graft, homo-, and/or copolymers are usually affixed to fabrics to create a positively or negatively charged functional group on the fiber, which is then immersed in counterions. Graft polymerisation of cellulosic textiles with poly( 2-methyl –5-vinylpyridine) or poly vinylpyrrolidone followed by treatment with potassium iodide solution imparts antibacterial and antifungal activity.

Specific applications of antibacterial and antimycotic agents for textiles

Practically every class of chemical compound has been utilized to impart antibacterial activity to textiles. Earlier efforts were primarily based on insolubilization of inorganic compounds, like copper and other organometallic salts. Later organic compounds like poly-halogenated phenols, their esters and bisphenols were used. Cellulosics are best treated with various wrinkle recovery, nitrogeneous resins which also impart rot and mildew resistance. More recent approaches include synthesis of reactive dyes containing antimicrobial substituents on cotton fabrics to simultaneously dye and rotproof fabric.

Many commercial formulations are available today. Table 2 lists some of them. Dow Corning used its expertise in silicone chemical technology to incorporate a standard antimicrobial substance (a quaternary amine) into a silane. With its silane backbone, the cored antimicrobial polymer becomes an integral part of any surface to which it is applied. That means it can not volatilise into the air or leach into water or other liquids; and it is not washed away by normal cleaning.

Actigard from Santized AG, Switzerland, is designed for items that are seldom or never washed such as carpets furnishing fabrics and also fillings such as polyurethane foam. Applied to textile fibres it inhibits fungal growth and reduces mould spores. UV stable it is fast to dry cleaning but not machine washing.

Thompson Research Associates (TRA) Canada’s Ultra Fresh brand is a family of products, using a variety of chemcals, that offer
fabric protection, hygiene ( since the treated material cannot sustain and transmit bacterial infections), odour protection and anti-staining. Applied by common processes, Ultra Fresh can treat both textile and filling materials such as feathers and down. It has been used in mattress tickings, upholstery, sportswear, and medical uniforms. Recently, TRA has introduced dust mite control, which has particular relevance to alleviating the symptoms of asthma. Having studied the ecology of the dust mite the company has found that a common mould and bacteria are essential in the mite’s food chain, so by attacking these it can create an environment in which the dust mite cannot survive.

A product in the HyFresh range from Japanese manufacturer Daiwa, and supplied by LJ Specialties takes a different approach to dust mite control. This product acts as a repellent, driving the mites into areas where they can be more readily controlled by cleaning and vacuuming. The product retains its effectiveness even after 50 washes. Another member of the HyFresh range is designed for odour control. The active ingredient in the finish inhibits both bacteria and fungi. Applied by either pad or exhaust methods it can be used on synthetic or synthetic/natural blends. Still active after 40 washes it has been used in household textile and clothing. Tests have also shown that is capable of alleviating the symptoms of athletes foot when applied to socks.

Biosil from Toyobo is a well known finishing agent based on organic silicones with tertiary ammonium where the cations block the cell division. Commercial products for PET with tertiary ammonium salts are Sanitan from Kuray and Peach Fresh from Nisshinbo.

Use of chitosan

Chitosan is a very specialised product manufactured from a natural waste product (crustacean shells such as crab etc.) and its benefit is that , when applied to cellulose by cross linking, it gives both antimicrobial and moisture control properties. The technology for chitosan Antimicrobial Finishing has been successfully tested in Japan by UK based Speciality Textile Products and is undergoing trials in Europe.

Testing for efficacy of antimicrobial activity

The tests done to evaluate antibacterial textiles can be divided into two types- agar based zone of inhibition tests and bacteria counting tests. In the agar tests, a swatch of textiles placed onto a dish of nutrient agar, and suspension of bacteria inoculated on the textile. The dish is then incubated at 370C for 1-2 days. An effective finish will prevent growth of bacteria on the textile surface. Some finishes may also migrate from the textile and diffuse into the surrounding agar. This gives rise to a zone of inhibition around the textile. Large zones of inhibition suggest that the finish will not be durable. A durable finish will prevent growth on the fabric, but give no , or very little zone of inhibition.

Bacterial counting tests such as AATCC test method 100 (1993) are technically more difficult, and time consuming. However, they give a quantitative assessment of the efficacy of an antibacterial treatment. In this test a swatch of damp textile is inoculated with a bacterial suspension in aqueous nutrient solution. After incubating for 24 hours, textile is treated with a neutraliser to stop the bacterial action. The surviving bacteria are then counted.

Conclusion

A variety of antimicrobial finishes have been developed for application to textiles. All the active chemicals are designed to kill microbes and pests but the issue of their safety or otherwise to the humans continues to be an area of concern. The most recent trend therefore appears to be a search for and development of technologies for use of unconventional natural materials such as chitosan for imparting antimicrobial finishing to home and medical textiles.

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