(179q) Removal of Model Fats From Cotton Fabrics with Fabric Abrasion Using a Tribometer

The detergency of
fabrics with oily stains using surfactants has been extensively researched in
the past. In recent years, significant efforts have been made to reduce the large
consumption of energy during laundry by lowering the washing temperature. With time,
the washing temperature in washing machines will approach that commonly used
during hand washing: that of the tap water. These energy saving measures bring
an old challenge: at ?cold' temperatures (eg. <30 °C) many fats solidify, limiting
the detergency of surfactants. This and other problems, persistent in the
old-fashioned hand washing in ?cold' waters used everywhere around the world, are
being tackled by the industry using different strategies, for example by
developing new enzymes efficient at ?cold' temperatures.

The present study
considers the non-chemical action involved during laundry cleaning, the
mechanical effects that occurs with the hydrodynamic flow, during flexing and
when abrading the fabrics. Mechanical action, abrasion in particular, has been
found to be efficient in the removal of water insoluble stains, eg. carbon
black with mineral oil (Lee et al. Fibers and Polymers (2008) 9:101). The
understanding of this common sense observation is very scarce, particularly due
to the lack of systematic and controlled experiments.

Here, it is reported
the first methodology to systematically study the effect of abrasion between
two fabrics on the removal of fats. The fats selected are those relevant for
?cold' washing: have melting points around room temperature. Well controlled
abrasion conditions were possible by using a tribometer (mini Traction Machine,
PCS Instruments, UK), using a rotating silicone disk covered with a soiled
fabric and a static silicone ball covered with non-soiled fabric (inset Figure
1). The silicone disk was rotated at 10 mm/s for 12 min (following the washing
time suggested in the ASTM D4265-98), resulting in about 57 rotations, while
the load of the static ball was typically kept at 1 N. The contact diameter
between the ball and the disc is about 2 mm, resulting in an estimated contact
pressure of ~0.3 MPa. Soiled samples were prepared using commercial white
cotton fabrics fully immersed in the given fat with an optic lipophilic dye
(Sudan Black B) in the liquid form (ie. at a temperature higher than the
melting temperature) for 5 min. Excess fat on the fabric was absorbed with
tissue paper under pressure, and the samples where left overnight in a fridge
or at room temperature. The amount of fats in the samples was quantified by
solubilising the fats in propanol followed by UV quantification of the
lipophilic dye. About 40-60% of the weight of the stained fabrics is fat.

Tribology experiments
performed at different temperatures (10-45°C) while using dionized water as the
?cleaning' solution shows the remarkable influence of the melting temperature
in pure fats stains. In fats that easily crystallize, such as octadecane (mp
~28°C), little fat is removed using water and abrasion does not improve cleaning.
Above the melting temperature, the quantity of octadecane in the abraded and
non-abraded parts of the fabric is reduced significantly (about to half the
initial fat amount), but in the abraded part more fats is removed. Similar
behaviour was obtained using hexadecane (mp ~16°C) and heptadecane (mp ~21°C),
although abrasion was found more effective in hexadecane bellow the melting
temperature provably due to incomplete crystallization on the fabric. Hence,
fabric abrasion below the melting point in these pure fats has a limited
effect, regardless of the actual temperature. Remarkably, although abrasion
significantly reduces the fat left in the abraded parts of the fabric above the
melting point, this improvement is found constant also with temperature. Hence,
temperatures higher than the melting temperature of the fats do not improve the
removal of the abraded and non-abraded fabrics in water.

The use of a chemically
different model fat, undecanoic acid (mp ~26°C), shows a similar temperature
dependence than the three previous alkanes, however abrasion is much less
effective. Only at temperatures around the melting point does abrasion
significantly improve the removal of the stains.

The abrasion effect
observed in pure fats differs significantly with that found in a complex fat
mixture that is lard, representing a more real stain. In lard, abrasion was
found to have a significant effect at all temperatures, except at highest
temperature tested (~45°C) when not much lard is left in the fabric (Figure 1),
higher than the melting point.

Tribology experiments
on lard stained cotton fabrics using 0.5 g/L linear alkylbenzene sulfonate
(LAS) solutions at different temperatures shows that this simple surfactant
solution had a negglegible effect on removal of lard compared to water. In
order to simulate a commercial laundry detergent, the standard detergent IEC
60456 A* was used at a concentration of 4 g/L. The detergent solution provides
marginal improvements at temperatures <26°C, but lard removal is greatly
enhanced at higher temperatures. Hence, using abrasion similar low remaining
lard concentrations can be achieved in water or in detergent solutions but at
much lower temperatures. Or in other words, if the water temperature is reduced
for energy conservation as discussed previously, fabric abrasion can offset the
lower washing efficiency.

In pure fats the
efficiency of the detergent solution was more salient at temperatures above the
melting points, where the amount of fat remaining in the fabric in abrading and
non-abrading conditions was reduced compared to water only (except for
hexadecane where the difference was not significant). Abrasion still significantly
reduces the alkanes left at temperatures higher than the melting point, but it
had a negligible effect for undecanoic acid.

The use of a tribometer
allows studying parameters relevant in abrasion in a controlled manner. Experiments
using lard stains show that the load had no effect of the removal between the
0.3-1 N tested. Kinetic experiments in water show two distinct behaviours. On
one hand in pure fats (below or above the melting point) as well as lard above
the melting point (44°C) the effect of abrasion changes little between 1 min
and 1 h. On the other hand, for lard below 40°C the amount of lard in the
abraded areas diminishes with time, while in the non-abraded remains constant.

The effect of abrasion
in lard is suggested to be related to the spreadability of lard, which depends
on the solid content fraction, as for instance lard is observed to be
accumulated outside the abraded area particularly at low temperatures. In
liquid or in crystalline fat stains, whatever abrasion can do is done very
quickly (5 revolutions or less).

The mechanistic effects
of abrasion on the removal of fats were further studied using fluorescency
microscopy (Leica MacroFluo). Cotton fabrics were stained as before but using
Pyrromethene 546 (l_abs,max
~495 nm, Exciton, USA) as a fluorescent lipophilic dye. These samples were then
tested under the tribometer as above. Quantification of the fat removal in the
abraded areas using fluorescence microscopy is comparable to the solubilisation
method. The abraded areas show damage in the fabric, as observed from the
larger number of superficial loose fibers. Interfabric abrasion works best to
remove the superficial fat stains, whereas is less effective when the stains
are far from the fabric surface, for example in the contact region between
yarns. Abrasion also improves fat removal even when the water temperature is
above the melting point of the stain (eg. >40°C for Lard, Fig. 1). Microscopy
shows that the stain concentration in the abraded yarns is lower, see Fig. 2,
provably some liquid fats have been squeezed out. Moreover, in the non-abraded
fabric free liquid fat droplets are found trapped between yarns and loose fibres
(Fig. 2). Abrasion facilitates the detachment of these droplets improving fat
removal.

Figure 2.
Fluorescence microscopy of a cotton sample soiled with lard and a lipophilic
fluorescent dye after a tribology experiment in water at 44°C. Dotted line
shows the approximate locus of the abraded (left) and non-abaraded (right)
regions. Note the higher fluorescence in the non-abraded yarns, as well as the
entrapped liquid lard drops.