Resorcinol–formaldehyde Solution Research Articles

Evaluation of porous structure of resorcinol-formaldehyde hydrogels by thermoporometry

The porous structures of resorcinol-formaldehyde (RF) hydrogels were evaluated by thermoporometry (TPM) based on the thermodynamic relationship between the size of the pores and the solidification temperature of the freezable pore water in the pores. Adding the thickness of the nonfreezable pore water layer to the ice crystal size distribution of the freezable pore water calculated from the thermogram (a recording of the heat flux during the solidification of the freezable pore water) allowed for the determination of the porous properties of the RF hydrogels. It was confirmed that the RF hydrogels possess a fairly uniform mesoporous structure formed between the interconnected primary particles, and that the size of the mesopores clearly increased with the increase in the molar ratio of resorcinol to catalyst ( R/ C) used in the preparation of the initial RF solution. The dependence of both the mesopore volume and the surface area of a given RF hydrogel on the R/ C ratio were also established.

Improvement of mesoporosity of carbon cryogels by ultrasonic irradiation

Mesoporous carbon gels are usually obtained by pyrolyzing resorcinol–formaldehyde (RF) gels, which are synthesized via the sol–gel polycondensation of resorcinol with formaldehyde in a slightly basic aqueous solution followed by drying. However, mesoporous carbon gels cannot be prepared under the conditions of high catalyst concentration or high pH of RF solution even by using supercritical drying or freeze drying. In this work, mesoporosity of carbon cryogels is improved by ultrasonic irradiation to RF solution. It is found that the gelation time of RF solution becomes greatly short by ultrasonic irradiation and that ultrasonic can improve mesoporosity of carbon cryogels prepared at high catalyst concentration (C/W). Although the carbon cryogels prepared from C/W = 80 mol/m 3 have no mesopores, the carbon sonogels prepared by ultrasonic irradiation under the same catalyst condition have sharp mesopore size distribution. The utilization of ultrasonic in the preparation of RF gel is an interesting way in improving mesoporosity of carbon gels prepared at high C/W or pH.

Capacitance control of carbon aerogel electrodes

Carbon aerogels are promising materials as electrodes for capacitive deionization (CDI) units and electrical double layer capacitors (EDLCs). An optimum process is presented for synthesis of nanoporous carbon aerogels via pyrolyzing resorcinol–formaldehyde (RF) organic aerogels, which could be cost-effectively manufactured from RF wet gels by a modified ambient drying technique using acetone exchange/controlled evaporation instead of conventional supercritical drying. The linear shrinkage of RF aerogel during ambient-drying of wet gels was controlled to be less than 4%, almost same as that of supercritical drying. Carbon aerogels obtained by pyrolyzing of RF aerogels (specific surface area 300–400m2/g) retains high specific surface area (400–700m2/g), low density (0.40–1.16g/cm3), and ultrafine pore size (<50nm) depending on the pH3.0–6.5 range of the starting RF solution. The specific capacitance of carbon aerogel electrodes activated at 450°C in air environment for 1h has been improved to 220F/g.

Controllability of pore characteristics of resorcinol–formaldehyde carbon aerogel

Resorcinol–formaldehyde (RF) aerogels were synthesized by sol–gel polycondensation of resorcinol with formaldehyde in a slightly basic aqueous solution, followed by drying with supercritical carbon dioxide. The RF carbon aerogels were prepared by carbonizing the RF aerogels at a high temperature under a nitrogen atmosphere. On changing the catalyst species and the catalyst ratios in synthesizing the RF hydrogels, we investigated their influence on the pore characteristics of the RF carbon aerogels prepared. We found that the order in the magnitude of the peak radii in the size distributions agrees with that in the magnitude of the gyratory radius measured from the small-angle X-ray scattering curves of the RF solutions during their gelation process. The gyratory radii and the form of the primary particles constituting the RF hydrogel were found to determine the pore characteristics of the RF carbon aerogels, relating to the polymerization conditions such as the catalyst species and the catalyst ratios in synthesizing the RF hydrogel.

Interpretation of structure formation during the sol-gel transition of a resorcinol-formaldehyde solution by population balance

Growth of colloidal particles formed during the sol-gel transition of a resorcinol-formaldehyde (RF) solution was simulated based on the population balance equation by using the discrete-sectional model (DSM). During the early stage of the sol-gel transition, the transient change of sizes of colloidal particles estimated by this method agreed well with the previous experimental observation by dynamic light scattering (DLS), which confirmed the influence of the catalyst concentration of a starting RF solution on the growth rate of the particles. From the size distribution of colloidal particles predicted at the gelation time, the surface area of a RF hydrogel after the completion of the sol-gel transition was estimated, which coincided with the BET surface area of a RF aerogel because the porous structure of a hydrogel was maintained and few micropores were formed during supercritical drying.

Dynamic and Static Light Scattering Study on the Sol-Gel Transition of Resorcinol-Formaldehyde Aqueous Solution

Structure formation during the sol-gel transition of resorcinol-formaldehyde (RF) solutions was traced by dynamic light scattering (DLS) and static light scattering (SLS) techniques. The decay time spectra obtained by DLS revealed that both the growth rates of colloidal particles formed during the early stage of the sol-gel transition and the time required for the colloidal particles to form a firm network structure could be related to the ratio of catalyst to water (C/W) of the starting RF solution. SLS results indicated that the molecular weight of colloidal particles increased with the progress of the sol-gel transition, the rate of which was also affected by the value of C/W. The mesoporosity of RF aerogels, which were prepared by drying RF hydrogels with supercritical carbon dioxide, was confirmed to depend on the size of colloidal particles estimated from the decay time spectrum collected at the last stage of the sol-gel transition.