Methanol is used commercially as a stabilizer in solutions of formaldehyde to prevent its precipitation. However, the methanol content of commercially available formaldehyde solutions differs from one supplier to another. The pH, dilution and R/F ratio have been demonstrated to be interdependent variables that can be manipulated to tailor the porous properties of RF carbon xerogels. This work considers the methanol contained in formaldehyde solutions as a new variable to be studied in conjunction with those just mentioned. For the purpose of this study, the influence of methanol on the final porous properties of RF carbon xerogels has been evaluated. It was found that carbon xerogels synthesized using formaldehyde solutions with lower concentrations of methanol showed a higher total pore volume and pore size, and in turn, a lower density and a greater porosity. The porosity of RF carbon xerogels could therefore be radically modified depending on the commercial formaldehyde solution used for their synthesis.
a b s t r a c tResorcinol-Formaldehyde xerogels are organic polymers that can be easily tailored to have specific properties. These materials are composed of carbon, hydrogen and oxygen, and have a surface that is very rich in oxygen functionalities, and is therefore very hydrophilic. Their most interesting feature is that they may have the same chemical composition but a different porous texture. Consequently, the influence of porous characteristics, such as pore volume, surface area or pore size can be easily assessed. In this work, a commonly used desiccant, silica gel, is compared with organic xerogels to determine their rate and capacity of water adsorption, and to evaluate the role of surface chemistry versus porous texture. It was found that organic xerogels showed a higher rate of moisture adsorption than silica gel. Pore structure also seems to play an important role in water adsorption capacity. The OX-10 sample, whose porosity was mainly composed of micro-mesoporosity displayed a water adsorption capacity two times greater than that of the silica gel, and three times higher than that of the totally macroporous xerogel OX-2100. The presence of feeder pores (mesopores) that facilitate the access to the hydrophilic surface was observed to be the key factor for a good desiccant behaviour. Neither the total pore volume nor the high surface area (i.e. high microporosity) of the desiccant sample, is as important as the mesopore structure.
The polymerization reaction that takes place between resorcinol and formaldehyde is spontaneous but slow. For this reason, compounds are often used to increase the reaction rate and reduce the synthesis time. These compounds can be basic or acidic and their nature and concentration can be used to modify the mechanisms of the reaction and the final properties of the materials. In this work the differences in the final properties of the organic xerogels obtained with basic or acid boosters have been studied. It was found that, irrespective of the nature of the booster, none of the end-product materials showed any differences in their chemical properties. Moreover, the concentrations of the components of the precursor solution (i.e. monomers, water and methanol) were observed to have the same effect on the porous properties of the materials regardless of whether an acidic or a basic booster was used. However, differences in the porous properties were observed. It was found that the methanol content was crucial to tailor the porosity over the entire nanoscale when an acidic booster is used. These results are of great importance as acidic boosters allows to decrease synthesis time and, hence, to produce more competitive materials.
A new macroporous, waterproof and breathable material has been synthetized. The synthesis of these materials consists in a modified version of the classical resorcinol-formaldehyde (RF) xerogels synthesis. This new process includes a further treatment, after the gelation-curing-drying process, with methanol at 240 ºC in order to passivate the hydrophilic phenolic groups of the surface of the RF xerogel. It was found that the treatment time strongly depends on the pore size of the xerogels. After reaching this threshold time, the hydrophilic materials become superhydrophobic. It is postulated that the exohedral inner surface of the porous xerogels, different from most porous materials, is responsible for this behavior. Although the materials exhibit a superhydrophobic behavior against liquid water and maintain its waterproofness for a long period of time, they are able to adsorb/desorb water vapor, which makes them also breathable.
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