Hyperbranched polyethoxysiloxanes for the encapsulation of silicon oils : preparation and application

• Hochverzweigte Polyethoxysiloxane für die Verkapselung von Silikonölen : Herstellung und Anwendung

Weiß, Sebastian; Möller, Martin (Thesis advisor); Pich, Andrij (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2022

Abstract

Modern coating compositions typically require the use of silicones in order to carefully adjust the surface properties of the coating during preparation and after drying and curing. However, the incorporated surface properties are typically not permanent and tend to decrease during use. In this sense, solutions are needed to induce long-term effects to make coatings more durable and therefore more sustainable as well. One way to achieve such long-term effect is the use of encapsulated materials, which release its content over time. In this work, the preparation of silica-based capsules is shown, which contain silicone oils and other silicone-based materials as core materials. For the encapsulation, a process was utilized which relies on the extraordinary properties of hyperbranched polyethoxysiloxane (PEOS), a polymer which exhibits hydrolysis-induced amphiphilicity upon addition to water and thus concurrently acts as a surfactant and silica source for the shell material. The extent of amphiphilicity as well as the compatibility of these polymers can be manipulated by modification, e.g. with polyethylene glycol or alkyl groups. Initially, unmodified PEOS was used for the enclosure of silicone oils of different viscosities from 1.5 to 10.000 cSt by emulsification in water. The process is based on the dispersion of a mixture of the initially hydrophobic PEOS with the silicon oil in water. Because of the hydrolysis of the PEOS occures preferentially at the oil/water interface, it was found that a rapid formation of an initial silica skin is taking place, followed by progressive ripening of the capsule shell. The miscibility of the silicone oil with the PEOS has a decisive influence on the mechanical integrity of the capsule shell. Diameter and polydispersity depend to a large extent on the viscosity of the dispersed phase and the dispersing technique. High viscosities of the silicon oil and the associated reduced compatibility with the PEOS impeded formation of small capsules but were subsequently circumvented by diluting the mixture of PEOS and silicon oil with tetraethoxy orthosilicate (TEOS). TEOS is another silica precursor and miscible with most organic substances and silicone oils of arbitrary molecular weight. In combination with PEOS the capsule sizes obtained have been significantly reduced. However, sufficient condensation was in this case only achieved if the post-dispersion ripening process of the capsules was catalyzed with ammonia. Capsule sizes of a few micrometers could be realized with simple rotor-stator emulsification, and the capsule size could be reduced further to 200 nm by a special emulsification process using the combination of rotor-stator with a Microfluidizer®. The use of polyethylene glycol modified PEOS decreased the interfacial tension between water and silicon oil to values below 1 mN/m and allowed for further reduction of the capsule size. In combination with dilution by low molecular weight, siloxane like TEOS capsule sizes in the range of 200 nm were prepared even with high-viscosity silicone oils by means of a simple rotor-stator dispersion. Furthermore, it has been shown thatultrasonic emulsification can be used to achieve capsule sizes in the range just over 100 nm within the same systems. The prepared capsules were incorporated into clear coats as aqueous dispersions. In contrast to the addition of plain silicon oil, the encapsulated silicone oil did not interfere with film formation. While larger capsules caused some milkiness of the clear coats due to the scattering, with the smallest capsules in the tested concentrations we obtained completely transparent coatings. As the capsules are not tight and can release the silicon oil slowly, we carried out release studies. No release of the silicon oil was observed in aqueous medium. For capsules dispersed in an organic solvent it was shown that the rate of release depends on the pH during synthesis, the solvent as well as the viscosity of the silicone oil. These release studies reveal the decisive factors governing the final porosity of the capsule walls and hence the rates of diffusion from the capsules. To obtain a realistic picture of the behavior of the capsules in coatings, silicone oil was end-functionalized with vinylanthracene and encapsulated in silica. This enabled the monitoring of the diffusion processes within the coating in a time-resolved manner using confocal laser scanning microscopy. We were able to determine a homogeneous distribution of the capsules within the coating. The encapsulated silicone oils were released from the capsules into the coating matrix over time. The speed of release is largely influenced by the temperature as well as the nature of the binder of the coating. It was observed that the silicone oils accumulated in proximity of substrate as well as the surface of the coating. Water contact angle measurements were used to obtain information about the polarity and hence the presence of silicon oil of the surface. The introduction of silicone oil filled capsules into the coating caused an increase of the contact angle, permitting to conclude on a release and stratification of the oils at the coating surface. When measuring the coefficient of friction of the coating, a comparable value was measured when free and encapsulated silicone oil was present, suggesting the formation of a thin film of silicone oil on the surface of the coating. The last part of the thesis deals with polyether modified PEOS (PEOS-PEG) compounds for the synthesis of low density silica solid materials. Depending on the degree of modification, PEOS-PEG behaves differently in aqueous environment. Low levels of polyethylene glycol modification lead to the formation of highly porous silica particles with diameters of several micrometers and a sponge-like structure. While conventional low-density silica structures, known as aerogels, require supercritical conditions for the removal of the liquid phase, the silica particles prepared here withstand an ambient drying protocol without rupture of the silica skeleton. If a drying protocol was carried out in the form of a concentrated particle dispersion within a mold, stable three-dimensional silica aerogel bodies could be produced by drying-induced co-condensation of the particles. The aerogels prepared exhibited good mechanical stability and low densities of approximately 0.1 g/cm3. Additionally, the feasibility of the preparation of aerogel-like coatings entirely based on silica hollow capsules was evaluated. Therefore, PEOS-PEG with elevated modification degree were used to prepare capsules. These capsules were filled with an organic solvent like decane to avoid collapse of the structures upon evaporation of the aqueous phase in the drying stage. Casting of capsule concentrates lead to nicely stacked, intact hollow capsule coatings. Here further development is required in order to reduce the density as well as the brittleness of such materials, e.g. by introduction of a suitable binder.