Texture control of freeze-dried resorcinol–formaldehyde gels

Kocklenberg, Régine; Mathieu, B.; Blacher, Silvia; Pirard, René; Pirard, Jean-Paul; Sobry, R.; Bossche, G. Van den

doi:10.1016/s0022-3093(98)00101-x

Cited by 64 publications

(24 citation statements)

References 7 publications

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“…[24,28] Otherwise, the expansion of the aqueous solution upon freezing not only may result in the destruction of the gel structure, but it may also result in very large pores (macropores) due to ice crystal growth. [31] Rinsing the aquagel twice with tert-butanol, for example, is sufficient for exchanging the aqueous solution. [32] Moreover, the density change of the cryogels freeze-dried with tert-butanol is much smaller than that with water, [34] and it causes the formation of more mesopores.…”

Section: Freeze-dryingmentioning

confidence: 99%

Preparation and Properties of Resorcinol–Formaldehyde Organic and Carbon Gels

Al‐Muhtaseb¹,

Ritter²

2003

Advanced Materials

986

795

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A brief overview on the preparation and properties of resorcinol–formaldehyde organic and carbon gels reveals very interesting features about their structural and performance characteristics. The resulting nanostructure was very sensitive to the various synthesis and processing conditions. This leads to a remarkable potential for designing and tailoring these materials to fit specific applications. Based on step‐by‐step comparisons of the published studies, approximate generalizations on the specific roles the synthesis and processing conditions play on the final properties are provided. Overall, resorcinol–formaldehyde organic gels undergo two main stages during synthesis. The first stage is associated with the preparation of the sol mixture, and the subsequent gelation and curing of the gel. The second stage is associated with the drying of the wet gel. The most important factors that affect the properties of the organic gel during the first stage are the catalyst concentration, the initial gel pH, and the concentration of the solids in the sol. The most important factors that affect the properties of the organic gel during the second stage are the drying procedure (e.g., super‐ or subcritical drying), and the difference between the surface tensions of the solvent before and after drying. The corresponding resorcinol–formaldehyde carbon gels are produced from the organic gels during a third stage, which is associated with carbonization or activation. Depending on the conditions, carbonization and activation both impact the structural and performance characteristics significantly.

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Section: Freeze-dryingmentioning

confidence: 99%

Preparation and Properties of Resorcinol–Formaldehyde Organic and Carbon Gels

Al‐Muhtaseb¹,

Ritter²

2003

Advanced Materials

986

795

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“…Hydrogels with various pore textures were prepared by polycondensation of resorcinol, solubilized in water, with formaldehyde, in the presence of Na 2 CO 3 . Three different pH of the precursors solution were achieved by changing the Resorcinol/Sodium carbonate molar ratio, R/C, fixed at 300, 500 and 1000.…”

Section: Preparation Of Rf Hydrogelsmentioning

confidence: 99%

“…Even though some attempts have been made to simplify the drying procedure by using freezedrying [3] or solvent exchange followed by evaporation [4], drying processes used up to now remain time consuming, expensive and discontinuous, which hampers the transition to an industrial scale production.…”

Section: Introductionmentioning

confidence: 99%

Towards the production of carbon xerogel monoliths by optimizing convective drying conditions

Job

Sabatier

Pirard

et al. 2006

Carbon

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Resorcinol-formaldehyde hydrogels prepared at various resorcinol/sodium carbonate ratios, R/C, were convectively air dried. The influence of the drying operating conditions, i.e. air temperature and velocity, on the pore texture, shrinkage and cracking of the dried gels were investigated. Shrinkage was found to be isotropic. The shrinkage behaviour and the textural properties of the gels are independent of the drying operating conditions, but are completely determined by the value of the synthesis variables. The analysis of the drying kinetics shows two main drying periods. During the first phase, shrinkage occurs and the external surface of the material remains completely wet: heat and mass transfers are limited by external resistances located in a boundary layer. When shrinkage stops, the second period begins: the evaporation front recedes inside the solid and internal transfer limitations prevail. The drying time can be reduced by increasing the air temperature and/or velocity, but the temperature increase is limited when monolithicity is required, especially when the pores are small. For example, at a temperature of 160°C and a velocity of 2 m/s, about 1 h is needed to dry a 2.8 cm in diameter and 1 cm in height cylinder containing macropores (pore width > 50 nm after drying). The same cylinder presenting small mesopores (pore width = 10 to 15 nm after 2 drying) requires 20 h at 30°C and 2 m/s to reach complete dryness without the development of cracks.

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“…The gels can be directly synthesized in acetone [9] or other solvents to bypass the solvent exchange step, but since water is produced during the polymerisation reaction, surface tensions problems are not completely avoided during supercritical drying, which sometimes induces shrinkage. Freezedrying was envisaged as an alternative to supercritical drying [10][11][12], but the pore texture obtained is often heterogeneous, especially in the case of low-density materials, and monolithicity cannot be easily maintained. A few studies were conducted using subcritical drying performed on resorcinol-formaldehyde gels after solvent exchange (water to acetone) in order to minimize the shrinkage due to surface tensions [13][14][15].…”

Section: Introductionmentioning

confidence: 99%

Synthesis optimization of organic xerogels produced from convective air-drying of resorcinol–formaldehyde gels

Job

Panariello

Marien

et al. 2006

Journal of Non-Crystalline Solids

110

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Resorcinol-formaldehyde gels were produced at 50, 70 and 90°C and with three different R/C ratios (500, 1000 and 2000). The effect of these variables combined with that of aging time was studied in order to optimize the synthesis conditions. The convective air-drying process was used, and the drying duration was studied with regard to the synthesis conditions. The aging time has no effect on the pore texture after 24 h at 90°C or 48 h at 70°C, whatever the R/C value. The synthesis-aging step can be shortened by increasing the temperature.Nevertheless, the pore size tends then to decrease, especially when R/C is high, but this can be counterbalanced by increasing R/C. Moreover, bubbles often appear in the gel at high synthesis temperature, which limits the temperature to about 70°C in the case of monolithic parts. At 70°C and with an air velocity of 2 m/s, the elimination of 90% of the solvent Manuscript 2 requires 1 h drying when the pore size reaches 400-600 nm, 2.5 h for 50 nm wide pores and 3 h when the pore size decreases to 15-20 nm. The drying duration does not exceed 8 h in all cases and could be shortened by increasing the temperature at the end of the process.

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Texture control of freeze-dried resorcinol–formaldehyde gels

Cited by 64 publications

References 7 publications

Preparation and Properties of Resorcinol–Formaldehyde Organic and Carbon Gels

Preparation and Properties of Resorcinol–Formaldehyde Organic and Carbon Gels

Towards the production of carbon xerogel monoliths by optimizing convective drying conditions

Synthesis optimization of organic xerogels produced from convective air-drying of resorcinol–formaldehyde gels

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