Spray-based polymeric core-shell microcapsules production for fabric care applications
- Autores: Albert Gasull Morales
- Directores de la Tesis: Ricard Garcia Valls (dir. tes.), Johan Smets (codir. tes.)
- Lectura: En la Universitat Rovira i Virgili ( España ) en 2013
- Idioma: español
- Tribunal Calificador de la Tesis: Marta Giamberini (presid.), Alberto Vaca Puga (secret.), Moira Ambrosi (voc.)
- Materias:
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- Resumen
Since microencapsulation as it is known nowadays was mostly born in the 1950s in the carbonless paper industry, its interest has constantly grown and its applications multiplied over the last decades. Analogously, spray drying was already commercially available in the early 1930s even though its applications were mainly focused in the dairy industry and its use in microencapsulation has been limited to the production of matrix configuration microcapsules, i.e. the core and the shell are heterogeneously mixed.
The main objective of microencapsulation is to protect or isolate a content (core) with a container (shell). After this objective a large number of applications may arise. In the case of the carbonless paper, for instance, microencapsulation had the objective of containing or protecting ink until its release was needed, i.e. printing. Fish oil is microencapsulated in order to prevent it to degrade in contact with the environment but also to reduce its strong scent when manipulated. In the pharmacy industry drugs are microencapsulated within an excipient in order to facilitate its dose, as normally doses are in the scale of micrograms, and to protect the active ingredients from the aggressive conditions are found in the case of oral dosing, etc. Finally, in the case of the laundry care industry, proper core-shell microencapsulation is now being vastly applied to fabric conditioners in order to provide a scent release during the different steps that take part in the washing cycle. Obviously, in this last particular case, even though perfume microcapsules are commercially available and increasingly implemented many limitations in terms of processing and materials incompatibility are still encountered. Thus, this work is born with the aim of researching one alternative and promising route for the production of microcapsules for its use in the fabric care industry.
Microcapsules have been satisfactory produced by means of the particular microfluidic Flow Focusing® (FF) technology that can continuously produce monodisperse droplets in combination with spray drying. In more detail, a polymeric solution and an active agent are pumped through the outer and the inner capillaries, respectively, of a concentric Flow Focusing® nozzle. Thereafter, the concentric drops exiting the nozzle are dried in a spray dryer. The evaporation of the solvent from the outer drop during the drying step brings to the formation of a spherical film around the agent active chosen, i.e the microcapsule. Several polymers have been identified and tested in order to assess their potential as film-forming materials for its use in microencapsulation. Several polymers have been tested but Shellac, which is a resin secreted by an Asian origin bug, Resine K10, a dimeric rosin acid, and Polyvinyl alcohol (PVA) have been mostly used due to their convenient properties.
Three different active agents with different characteristics and purposes have been encapsulated with the technology aforementioned. Highly reactive solid bleach (¿-phthalimido-peroxy-hexanoic acid, PAP) was first tried. The encapsulation of solid particles by means of the methods used present several challenges. Firstly, the particle size distribution of the bleach agent needs to be below certain range due to the micron scale of the nozzle geometry. Secondly, the combination of flowing solids through a microfluidic device requires the addition of a suspending agent in order to homogeneously distribute solid particles and avoid them to sediment or agglomerate through the pumping process. Several suspending agents and concentrations were prepared by different methods and characterized under the microscope. PAP could not be finally used due to its high particle size distribution. Tetraacetylethylenediamine (TAED), which is a booster of the bleaching process, was then proposed as it was available in solid form and had right particle size diameter. Also in this case several concentrations and suspending agents were tried and the addition of plasticizer tested for its influence in the film-fomring properties. Only shellac was used to encapsulate this first active agent in order to reduce variables and for its suitable characteristics for the process. After these initial steps, an optimum solution for spraying was selected. Finally, monodisperse core-shell microcapsules were obtained by means of the proposed method. The microcapsules obtained were characterized with different techniques, mainly microscopy. Scanning electron microscopy (SEM) was used in order to observe the microcapsules morphology. SEM micrographs were also used for image analysis with the ImageJ software in order to calculate the particle size distribution. Shell thickness was also measured in the same way but with an additional step that required the use of a cryotome. The production of core-shell TAED microcapsules was confirmed although the microcapsules produced finally had to have a low payload due to the extremely difficulty presented during the pumping through the FF nozzle and the spraying steps.
Next, a second active agent of interest for fabric care was proposed. In this case solid particles were avoided and Liquitint® VioletDD (LVDD), a liquid polymeric colorant that is used as a brightener for fabrics, was used. LVDD was miscible with the polymeric solutions tried and so, potential miscibility could occur. LVDD was encapsulated with three different shell materials: shellac, resine K 10 and PVA. Again, the microcapsules produced where characterized with SEM and the particle size distribution obtained was narrower compared to the first experiment. Core-shell encapsulation was demonstrated via optical microscopy, where the colorant was preserved in the whole capsules and leaked out after cutting them with the cryotome method. The microcapsules obtained where compared to another encapsulation process. Although the colorant dye was observed to be perfectly enclosed within the shell with the optical microscope, the addition of a fluorophore and posterior analysis with confocal microscopy did not confirm the initial results. Potential mixing in the flow focusing step was proposed according to literature.
In a third case, after the encapsulation of solid particles and the use of miscible fluids, the encapsulation of immiscible silicone oil (Decamethylcyclopentasiloxane; D5), which is used as suds suppressor in laundry care, was tested. Again, three different shell materials were used: shellac, resine K 10 and PVA. After a first screening PVA was clearly superior due to its superior film-forming properties that allowed to not only produce higly monodisperse and smooth-surface microcapsules but also increase the payload. After the process conditions were optimized the system reliability was tested and satisfactory confirmed. The microcapsules obtained were characterized by several techniques in order to confirm a perfect core-shell and monodisperse microencapsulation. As in the other cases, SEM was used to observe the surface morphology and the particle size distribution. Confocal microscopy confirmed a perfectly defined core-shell configuration. Raman microscopy, SEM/EDS (Energy-dispersive X-ray spectroscopy) and X-ray tomography results indicated the silicone oil was confined within the capsules. Results described a payload up to almost a theoretical value of 70% of silicone oil withon the capsules and narrow particle size distribution. Once the optimal conditions were found and reliability checked different steps were followed in order to better understand the process and increase the payload/encapsulation efficiency.
The suds suppressor microcapsules proposed were thought to degrade or dissolve in water or, at least, after certain time in contact with water. The PVA initially used did not satisfy that condition, as they dissolved immediately. Other PVA commercially available grades and types were then tested. In order to compare the different PVA a method that qualitatively compared dissolution time was proposed and implemented. The method proposed resulted dependent on the PVA grade and its acetate content so its end was finally limited. Additionally, the dissolution time of specially-made polymeric microcapsules was quantitatively calculated by means of laser diffraction, with little accuracy.
In order to better understand the microcapsules formation with Flow Focusing®, computational fluid dynamics (CFD) were used in order to reproduce the drop formation at microfluidics scale within the FF nozze of the suds suppressor experiments. The CFD simulation was constructed from the experimental data of the suds suppressor microencapsulation. The jet diameter obtained in the simulation was used to predict, with the expressions found in the literature, the microcapsules size. The microcapsules predicted were in agreement with the experimental results.
Additionally, a mathematical model was constructed from the experimental data in order to predict the whole process, i.e. Flow Focusing® formation and spray drying, and optimized with the general algebraic modeling system (GAMS). From the results obtained in the optimization a cost analysis from a calculation basis was performed, proposing the microencapsulation cost.
