469396 Modelling the Deposition of Actives on Cotton Fabrics during the Washing Process
Thus, during the washing process of textiles in a commercial washing machine, whiteness maintenance depends on many different transformations, namely (a) soil removal from fabrics’ structure, (b) suspension and anti-re-deposition of removed soil, (c) bleaching of soils remaining in fabrics, (d) deposition of shading dye actives, which are used to shift the yellowness of fabrics to a more preferred blue hue and (e) deposition of Fluorescent Whitening Agents (FWAs), also known as optical brighteners.
Soil Release Polymers (SRPs) play a key role in detergency due to its use for soil release from fabrics and prevention of re-adsorption of soils. FWAs are widely used in the textile and paper industry for improvement and maintenance of whiteness. These molecules absorb light in the UV region (350 nm) and emit on the blue region of the spectrum compensating yellowing of materials [5, 6].
Understanding and modelling the chemical and physical processes driving deposition and retention of the actives driving each of the vectors mentioned above is key for optimising whiteness performance by an optimisation of the detergent formula.
In the present work the adsorption behaviour of two stilbene derivative FWAs typically used in detergent formulas onto flat cotton fabrics has been investigated and modelled as a function of temperature, water hardness, concentration of FWAs in solution, fabric: water ratio and detergent concentration. Furthermore, the impact of SRPs on soil release and soil anti- redeposition on cotton fabrics is investigated.
The adsorption of actives on textiles is tracked by a spectrophotometry based real – time methodology which allows obtaining high time resolution data of the active remaining in solution.
The resulting whiteness of textile garments is assessed by measuring their reflectance under different light conditions by spectrophotometry. Calibration curves that correlate the adsorbed mass of active per area of flat cotton fabrics with the resulting whiteness of textiles have been developed which allow predicting the final whiteness for any adsorbed concentration of actives. This correlation is typically non – linear where the whiteness benefit (delta b* in L*a*b* colorspace) plateaus as we reach high levels of adsorbed active.
Based on experimental data, a mechanistic model for molecular deposition has been developed which considers the following transformations in the system: (I) Dissolution of active in the washing liquor, (II) convective mass transfer flow into the fabrics, (III) Fickian diffusion of molecules to the fabric surface across boundary layer and stagnant layer of water in yarns and (IV) adsorption/desorption on fabrics’ structure (adsorption/desorption isotherms).
The time to reach 95% of equilibrium as well as the equilibrium concentration, increase with increasing water to fabric ratio for a fixed active concentration since there is more mass of active per surface area to be adsorbed. Therefore, depending on the type of washing machine (Top Loader vs Front Loader Washing Machine) where the water to fabric ratio varies significantly (typically 0.02 m3:kg vs 0.004 m3:kg), washing time plays a key role on the final adsorbed mass of active on cotton fabrics and therefore on the resulting whiteness of textiles requiring almost twice the time for actives in the higher water:fabric ratio machine to reach equilibrium. The importance of these models is that experiments can be run under laboratory conditions with less variable high water to fabric ratios and results scale up to real wash conditions.
A lack of fit is observed for actives depositing on cotton fabrics when Fickian diffusion across a single boundary layer is considered. The hypothesis presented to explain this disagreement between experimental and predicted data is the presence of a complex dual porosity fabric structure . Fabrics have two main porosities, inter-yarn porosity due to the spaces between the yarns of the textiles and intra – yarn porosity due to spaces between the fibres forming the yarns. As a consequence, molecules diffuse across different diffusing distances before they reach the fabric surface. Initially a rapid decay of the bulk concentration is experimentally observed which is thought to correspond to the adsorption of molecules on the outer surface of the fabrics, which progressively decreases until saturation is reached leading to diffusion to deeper areas of fabrics which would promote a slower decrease of the bulk concentration due to larger diffusion distances. The approach followed for modelling the entire process is based on the consideration of two different surfaces with different surface area and diffusion distance where molecules can diffuse and get adsorbed simultaneously in parallel.
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