276636 Removal of Model Fats From Cotton Fabrics with Fabric Abrasion Using a Tribometer

Monday, October 29, 2012
Hall B (Convention Center )
Ruben Mercade-Prieto, Chemical Engineering, University of Birmingham, Birmingham, United Kingdom, Serafim Bakalis, Department of Chemical Engineering, University of Birmingham, Birmingham, United Kingdom and Carlos Amador, Procter and Gamble Technical Centre, Newcastle upon Tyne , United Kingdom

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.

Figure 1. Effect of abrasion in cotton fabrics stained with lard kept at ~22°C for 1 day, using water as ‘cleaning' solution. Abrasion was performed in a tribometer at 10 mm/s with a load of 1 N for 12 min, following a previous soaking step of 12 min at the desired temperature. ‘Initial' data points represent the amount of lard in the dry stained fabric. Inset photograph shows the location of the ‘Non-abraded' and the ‘Abraded' sampling areas. Vertical error bars are the standard deviation of 3 sampling areas; horizontal error bars represent the standard deviation of the water temperature in the tribometer cell. Dotted lines are a guide to the eye.

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 (labs,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.

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