Since the last part of the century an increasing improvement in
qualitative standards of human lifestyles has brought to a greater sense
of comfort, and cleanliness. People are more and more looking for fresh
public living surroundings and a higher level of hygiene in home areas.
A wide class of micro-organisms coexists in a natural equilibrium with
human body and living environments, but a rapid and uncontrolled
multiplication of even non pathogenic microbes can seriously
compromise the hygienic and healthy personal standards. Because of
their capillary spread in the human living spaces, the textiles have been
involved in this research of a growing quality of hygienic living
conditions.
Actually, several combinations of temperature, humidity and other
climate factors added to the presence of dust, soil and fat-stains on the
textile surfaces can transform the textiles themselves in an optimal
enrichment culture for a rapid multiplication of micro-organisms. In
such a case two contemporary effects occur: the first is an uncontrolled
proliferation from textile surfaces into the surrounding environment
with a consequent increase of bio-burden level and potential health risks
or, at least, of discomfort for the unpleasant odours produced by high
concentrations of micro-organisms; the second one is the onset of
degradation phenomena as colouring and discoloration of the textile
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fibres. Many efforts have been performed by textile industry with the
aim to score two goals: the protection of the living environments and of
the textile fibres from an uncontrolled proliferation of microorganisms
like bacteria.
2) – Micro-organisms action on textile surfaces.
In the broad spectrum of existing bacteria there are pathogenic and
non-pathogenic organisms. Both of them can multiply abnormally on
the textile surfaces with an accumulation that compromises the hygienic
cleanliness.
Tab.1 shows some examples of pathogenic and non pathogenic microorganism.
Tab. 1
MICRO
ORGANISM
PATHOGENICIT
Y
EFFECTS
Bacillus subtilis Generally non
pathogenic
Food spoiling
occasionally conjunctivitis
Escherichia coli Low pathogenic Food spoiling
occasionally urinary bladder
infection
Klebsiella
pnuemoniae
Pathogenic Pneumonia,
urinary bladder infection
Pseudomonas
aeuroginosa
Pathogenic Multi-infections
Proteus vulgaris Low pathogenic Infiammations
Staphylococcus
epidermidis
Low pathogenic Surgical wound infections
Staphylococcus
aureus
Pathogenic Toxic shock, purulence,
abscess, fibrin coagulation,
endocarditis
The proliferation of pathogenic micro-organisms has to be fought for
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the physiologic impact to the human health, while non pathogenic
micro-organisms have to be controlled for the visual, olfactory and
tactile effects produced by their metabolism.
The textile materials, on which source of nutrients are present (food
contamination, oil, fat, protein, sugar, skin secretions like sweat and
sebum etc.) become a medium for a rapid multiplication of microorganisms.
Many bacterial colonies produce, in their metabolism,
coloured pigments that protect them against light and UV radiation.
In fig. 1 some colouring pigments, synthesized by different bacteria,
are reported:
Fig. 1
These substances cause colouring of textiles through adhesion to the
surface. The pigments attached to the fibre cannot be adequately
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removed by normal washing and, as time passes, colour stains firmly
bond to the textile with no possibility of removal, even after repetitive
washings. In some cases, according to the type of fibre material and
attached pigment, only bleaching agents can be helpful, but it is quite
difficult remove the pigment even by the oxidation and reduction
reactions of the bleaching process.
In their growth on a fibre, micro-organisms can produce volatile
compounds of unpleasant odour as decomposition by-products of their
feeding. Spilled foods and drinks, dirt, dust, organic stains, secretions
from the human body like sweat and sebum are decomposed from
bacteria with a production of bad smelling components like : fatty
acids (acetic, propionic, butyric, valerianic, caproic), n-methylamines,
ammonia, aldehydes, sulfides, mercaptans, aromatics and lactones.
Other micro-organisms transform the human steroid hormones in foul
ketones and steroids with the same odour of urine.
3) – Bio-active fibres.
A general term that is adopted to indicate the textile fibres with activity
against micro-organisms growth is “bio-active fibres”. A distinction can
be made according to the possible end-uses: hospital uses, home textiles,
carpets, furnishing, mattress and pillows fillings, air-liquid filters, nonwovens,
protective clothing, sportswear etc. Each of these application
fields will demand a different bioactivity performance from the fibre.
Man-made antibacterial fibres are manufactured by two basic methods:
the first is kneading antibacterial additive during the spinning stage and
the second is an after-treatment method in which an antibacterial agent
solution is used.
In the mixed spinning technology, the antibacterial agent is supplied
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into the polymer stream before the spinneret or blended into the
spinning polymer feeding. The additive characteristics have to be
compatible with spinning conditions (e.g. particle diameter, heat and
chemical stability, no degradation interactions with polymer, lack of
adverse effects on fibre quality). A reserve of antibacterial additive is
englobed into the fibre and, after migration to the surface, it can
practise its bioactivity against micro-organisms.
In the post-process finishing technology, the most common techniques
to apply antibacterial agents are: spraying, immersion, padding and
coating. The textile surfaces are often treated in the final dyeing and
finishing stages of their manufacturing process. Antibacterial agent is
linked to the surface through physical bonds or anchored by a crosslinking
on the fibre. The most used additives are based on organic
compounds like halogenated salicylic acid, anilides, organotin
compounds, quaternary ammonium compounds, organosilicon
quaternary ammonium salts, and quaternary ammonium sulphonamide
derivatives . Since most of them are highly water soluble and weakly
anchored to the fibre surface, they have to be constantly reapplied.
According to the manufacture technology and the antimicrobial agent
nature, the antibacterial fibres can exhibit two kinds of bioactivity
mechanism: an elution mechanism and a non-elution mechanism. In the
first the additive gradually migrates out from the fibre to the solvent
external medium, while in the second mechanism it does not dissolve
out. Although, sometimes, the two kinds of mechanism coexist in the
antimicrobial activity of a bioactive fibre, generally, one of them is the
predominant.
4) – Antibacterial activity tests .
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Antibacterial activity of bioactive fibres is not immediately evident, but
it can be evaluated by opportune test methods. Since the early
appearance of bioactive fibres, several standard methods have been
defined and, at the moment, there is not a unique test protocol that is
suitable for all the sorts of the antibacterial fibres. Each of the existing
methods has its own approach and application field, so that, if two of
them are adopted to characterize the same antimicrobial textile, they
often show opposite results.
A first overall classification is carried out on the basis of the kind of
the evaluation of the micro-organism population reduction: quantitative
and qualitative. In the quantitative methods the number of bacteria, still
living after an opportune contact time, is counted. Besides, the
quantitative evaluation can be differentiated further in other two classes
according to the main test conditions. For example, a small amount of
liquid culture medium is used to cover a specimen in the static method
ATCC 100, while the fibre specimen is immersed in a larger amount of
liquid culture when the dynamic Shake Flask Test Method is carried
out.
In the qualitative methods the test specimen and an untreated control
are pressed into intimate contact with an agar culture medium inoculated
with the test bacteria solution. If antibacterial activity is present, it will
be possible observe a clear zone around the treated sample comparing to
the zone of bacterial growth around and over the untreated control
sample after the same contact time.
These qualitative methods provide a formula to measure the inhibition
zone width, but this is a qualitative evaluation and it can not be
considered as a quantitative indication of the antibacterial activity.
The most important antibacterial activity test methods, with their main
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features, are listed in Tab. 2
Tab. 2
Method Name Origin Evaluation Suitability
AATCC 100/1988-98
USA Quantitative
Textiles treated with
antibacterial finishes.
Hydrophilic fibres
AATCC 147/1988-93 USA Qualitative Textiles treated with
fast migrating/leaching rate agents
SN 195924/1983 SWITZ. Quantitative Hydrophilic fibres
SN 195920/1983 SWITZ. Qualitative Textiles treated with
fast migrating/leaching rate agents
AFNOR XP G 39010: 99 FRANC
E
Quantitative Textiles treated with migrating agents
SHAKE FLASK TEST
JAPAN
USA
Quantitative Textiles with antibacterial
properties inherent to the
structure
Hydrophilic/Hydrophobic fibres
JIS L 1902-8 (1998) JAPAN Quantitative
and
Qualitative
Textiles treated with
fast migrating/leaching rate agents
The results that these methods can give depend strongly on the
antibacterial additive mechanism of activity and on the hydrophobic or
hydrophilic nature of the bioactive fibre. In each analysis, the
measurement of the activity of a reference sample of nature similar to
the antibacterial fibre but without additive must be carried out.
After the time contact, three cases of bioactivity can present as result
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of testing a textile:
1) a significant increase of the initial bacteria population
2) an inhibition of the bacteria growth comparing the antimicrobial
product with the control sample for which there is a multiplication of
test bacteria population inoculated at the beginning of the time
contact.
3) a quantitative reduction of the number of test bacteria inoculated at
the beginning of the time contact.
The second and the third cases indicate an antibacterial activity from
the bioactive fibre and the terms used to differentiate the two
performances are biostatic and biocide.