The article summarises the benefits of using Xeros bead cleaning technology when applied to textile care. Along with the results for cleaning performance, and the impact of beads on fabric lifetime, the low environmental impact of using beads versus water in conventional commercial laundry processes is also discussed.
Xeros began its technology journey with the research of Professor Stephen Burkinshaw and Dr. Jane Howroyd working at the University of Leeds in 2006. Dr. Howroyd’s Ph.D. thesis describes the cleaning performance generated using beads and their interstitial water when wetted, as applied to various textile substrates. The laboratory work included tumbling beads, water and detergent with stained fabrics in sealed containers in a domestic tumble dryer. Good cleaning performance was observed, and the process of polymer bead cleaning born. A worldwide patent application was subsequently granted based on these data. Since then, the patent portfolio associated with this technology has grown to become 25 applications strong.
The Xeros Process
The Xeros bead cleaning process is based on a step-change reduction in the amount of water required to clean a textile substrate. The essential feature allowing this to happen is the addition to, interaction with, and removal from the washload, of a multiplicity of polymer beads. These perform a number of functions in replacing the water, depending on their physical form and chemical composition.
Mechanical Action & Bead Physics
Firstly, the beads generate a controllable and uniform mechanical action on the cloth, which allows reduction of the other factors controlling wash performance, as described by Sinner’s circle.
This schematic is a widely accepted interpretation of the interdependent factors influencing cleaning performance in washing. It obviously assumes the presence of an effective transport medium for the wash process – i.e. water for conventional, and water plus beads for Xeros. If the effectiveness of any one of temperature, time, mechanical action, or chemistry is increased, then advantage can be taken in reducing the other factors. So here for example, improving the uniformity and level of the mechanical action acting on the cloth by adding beads, could be used to shorten the wash time, or allow a reduction in the amount of cleaning chemistry used, or result in a lower temperature wash process, thereby saving energy. The advantage of improved mechanical action could be spread across all three of these other factors of course, but to a lesser extent than if focussed on one individually. So Xeros beads change the way we think about Sinner’s circle. There is now effectively a new element in the wheel-beads!
In a conventional wash the mechanical action on the cloth can be highly exaggerated at creases, and become too low in areas trapped within folds. Beads however effectively help by acting as a buffer between surfaces, whilst at the same time pinning the cloth, thereby reducing folding and hence creasing. Furthermore, the degree to which this happens can be controlled by altering the overall mass of beads used, and through individual bead parameters such as size, shape, and density.
Removal of water presents other issuesfor the beads to address however, not just mechanical action. The beads must also act to remove soil from the fabric surface during the wash, by acting as an effective transport medium – one which subsequently inhibits redeposition of soil. Here the effect of the polymer beads (e.g. polyesters) can be to provide a polar surface to attract the soil, and remove it (adsorption). With certain polymers however (e.g. polyamides) there is an additional effect of absorption as the glass transition temperature (Tg) of these decreases when wet. This generates significant molecular motion within the bead even at ambient wash temperatures, and hence diffusion and absorption of soluble soil into the bead structure. Here therefore, there is effectively an additional contribution to the chemistry in Sinner’s circle, and so a further means to reduce the contributions required from the other factors to achieve an effective cleaning process.
Thus, for all polymers, there is a balance of mechanical action (which is dominated by the physical characteristics of the beads – the overall mass used, and the individual bead size, shape and density), adsorption (due to polarity) and absorption (due to Tg decrease). With both improved mechanical action and these additional effects added by the beads, it is thus easy to appreciate why Xeros has been able to reduce wash temperatures, times and detergent dosages, and yet still achieve superior cleaning to conventional washing. Note also that the overall cycle time for Xeros (i.e. wash plus rinse plus bead separation from the washload) is comparable to current commercial laundry cycle times.
The specific bead, parameters that play key roles here are the bead: cloth ratio, bead size and geometry, and bead composition. The bead: cloth ratio can be considered the mass of beads used in the process to the mass of dry wash load. Extensive experimentation has been undertaken to determine the amount of bead required for effective cleaning, with many bead candidates considered. The bead : cloth ratio has accordingly been set at 2:1 based on these cleaning experiments, although this is not a fixed proportion necessarily for our future beads.
So, if we consider a preferred bead it can be seen that as for a typical 25kg washload machine there will be 50kg of beads added, and that this will result in some 1.43 million beads being used in the wash. Another important statistic here is that this amounts to over 62m2 of active polymer surface area – we will return to what ‘active’ means later.
The Xeros process increases the mechanical action quadrant in Sinner’s circle primarily through improved uniformity, generating the potential for reductions in temperature, chemistry (detergent) and time, whilst still delivering good cleaning performance.
A key part of the process is the separation of the beads from the washload. To this end a conventional washing machine has been redesigned to allow bead recirculation during the wash.
Beads are continually flowing into the drum and then out via the drain holes therein, to a collection sump below. A bead recirculation loop allows pumping of a bead/water slurry from this sump up to a separator unit, which then filters the beads back into the drum whilst passing the water directly back to the sump. When the wash is finished the bead pumping is stopped, and there is no more recirculation. The washload is then spun to dry it slightly before continuing to tumble, thus allowing the remaining beads to fall free from the washload, and exit down to the sump. This type of separation process typically removes 99.99% of the beads added.
As can be imagined, spherical beads separate better than cylindrical beads, and larger, higher density (i.e. higher mass) beads better than smaller, less dense beads. The shape effect here is pronounced. The flat surfaces at the ends of cylindrical beads can adhere by water surface tension effects to the cloth and become problematic – unless they are particularly high mass. Also, smaller beads can more easily enter pockets, seams, and other structured garment trapping points, and be difficult to remove. So, large, dense,spherical beads are the best option for good separation.
There is less creasing and less fabric damage from using Xeros’s polymer beads. Even the possibility of less ironing.
Another key part of the process is that, once the beads have been removed from the washload at the end of the cycle, they can be cleaned (if required), and re-used in subsequent washing processes, without significant detriment to their performance. A separate bead cleaning cycle allows different chemistry to be utilised compared to the wash, and with
regular cleaning a 500 cycle lifetime is a minimum achievable by the beads (depending on polymer type). After this, the beads may be recycled into noncolour critical applications (e.g. injection moulded parts for automotive, consumer goods, or similar).
The cleaning cycle for the beads is very simple. It is run with no washloadin the machine, and uses detergency and elevated temperature (60oC). The cycle runs for 40 mins and is performed as required depending upon the level of soiling in the washloads cleaned.
Each machine went through its standard wash and rinse cycle at each wash temperature tested, and the results show there is very little difference between Xeros and the controls used. It could in fact be argued that there is a 1 dE unit improvement (the level at which changes become visible to the naked eye) with Xeros in one case, but this is not thought significant. Hence, there is certainly no greying (redeposition) problem in using the Xeros ‘flow-through’ wash process.
Dye transfer Inhibition
The test involved adding four new, unworn red T shirts (250g each) to 20kg of white cotton ballast, and washing to monitor dye transfer by colour measurement versus appropriate controls. These were the commercial Competitor three washer extractor at 20kg a washload (24kg capacity rated), 10% detergent dosage; and a domestic washing machine at 3kg washload (6kg rated), 100% detergent dosage. For the latter we also added two color catcher sheets – sold specifically to prevent dye transfer. The Xeros machine was again run at a lower wash temperature (20oC) and at lower detergent dosage (50%), where it generated equivalent cleaning performance. With much better dye transfer stopping power, Xeros polymer beads mean much less sorting of white and colour washloads.
The ability of the polymer beads to inhibit redeposition and dye transfer is due to the ‘active’ nature of the bead surface.
A brief explanation of this mechanism is given below. Certain polymers – polyamides being the example here – change their molecular structure in the solid state when they absorb water.
Polymers absorb water to greater or lesser extents, with polyesters such as polyethylene terephthalate (PET) absorbing up to 0.5% w/w at saturation, and polyamides such as nylon 6,6 (N6,6) absorbing up to 10.0% w/w. The effect in both cases here is to allow the molecular chains to separate slightly in the bead, and become more mobile – on a molecular level. With N6,6 this effect is more pronounced, and the increase in chain mobility makes the bead structure effectively more open (like a sponge), with the cavities created in the molecular structure able to absorb water – and hence water soluble soil or dye. The increased mobility of the N6,6 (and PET) chains can be measured by a reduction in the glass transition temperature of the polymer (Tg), which is the temperature at which chain mobility is first detected.
Tg Changes for Nylon 6,6 and PET Measured Using ASTM D3418 The drop in Tg for PET is smaller and less significant in terms of the Xeros wash process. With N6,6 the fact that the Tg reduces to become negative, means that essentially at all wash temperatures including ambient, N6,6 is in a state ready to absorb soluble soil and dye, and hence enhance cleaning, redeposition, and dye transfer inhibition performance. Note that PET beads are still useful in the Xeros process – they just do not have this additional benefit.
These results indicate that Xeros bead cleaning not only produces improved mechanical action and stain removal, but also delivers improvements in redeposition and dye transfer compared to conventional washer/extractors.
Sustainable Textile Care
In a commercial laundry setting, it is not much use to deliver improved cleaning performance and lower utility consumption if the treatment is damaging to the fabric and accelerates degradation (meaning, for example, a reduced lifespan of towels and/or sheets in a hotel). In the series of tests described below we have demonstrated why the Xeros system can actually be considered superior to conventional systems in key fabric care areas.
The benefits of Xeros beads for fabric care are perhaps best shown by wash tests made using a WFK 19811- AISE dye swatch (14 standard dyes). Here the colour fading is recorded after 10 washes in a Xeros machine versus a competitor commercial laundry process – Xeros giving superior cleaning. The results were again measured using dE derived from CIE L, a and b colour data, but here obviously lower dE numbers represent less colour fading from the unwashed original.
This is perhaps simplest thought of as the uniform mechanical action of the beads allowing a lower wash temperature and the use of less chemistry, and so generating these fabric care benefits. The fact that cleaning is superior versus the conventional processes is, of course, the breakthrough.
Whites stay whiter for longer.
Less color fade with Xeros means colors stay brighter for longer.
The separation of the beads from the cloth then has to be controlled so that they can be relatively easily removed at the end of the wash cycle.
This removal process is achieved by switching off the pump and hence not adding any more beads to the drum, which then continues to tumble to allow the beads to separate out. A series of three rinses completes the cleaning process, with the last two rinses collected for use as bead transport water in the next cycle. At the end of each cycle the machine door can be opened and the cleaned washload removed, essentially bead-free.
The appearance of the Xeros machine is not dissimilar to that of a conventional machine. Considering the nature of the technology here, the low number of changes made to the conventional hardware and the simplicity of this process ensures the Xeros machine is as reliable as the best conventional washer extractors.
Xeros is headquartered in the UK with offices in the U.S. and China.
The Xeros Machine
The hardware enabling the Xeros wash process is also worthy of special mention. As described above, the Xeros wash process essentially consists of addition of the beads to the washload in the machine, their interaction with the washload, and then a controlled ‘flow-through’ of beads throughout the wash cycle. Essentially, the beads are stored in a sump below the wash drum and pumped up in transport water to a filtration device which passes the beads into the top of the drum, and the transport water back down to the sump. The beads then interact with the washload and tumble out of the drum through its drain holes and fall back into the sump. The pump is then controlled to pulse throughout the wash cycle such that an equilibrium level of beads is maintained in the drum (@ 2:1 beads : cloth) – hence the term ‘flow-through’.
The significant colour transfer to the backing on the competitor image is soil re-depositing onto the surface of the cloth. It is how whites turn grey over time with repeated washing.
With a conventional process, the white material is effectively taking a bath in dirty water as it is washed. With Xeros however, the material is taking a shower in clean polymer beads. Hence soil redeposition is much lower.