Gel cracking

During vacuum drying, the gel will feel cold relative to the surrounding platen if it is not completely dry. When the gel temperature is the same as the platen, the gel is dry and the vacuum and dryer can be turned off. Gel cracking can be caused by releasing the vacuum before the gel is dry. 

Figure 2. Appearance of a gel crack during electro-osmosis tests.

Krieger, R.I., L.A. Rosenheck and L.L. Schuester (1997). Adult and infant abamectin exposures following Avert 310 and pressurized gel crack and crevice treatment. Bull Environ. Contam. Toxicol, 58, 681-687. 

Silica gel cracking catalysts have also been studied. These materials are amorphous and yield no powder diagrams but they do give a very maiked small angle scatter. If the particle distributions obtained from small angle scatter are used to calculate surface areas and these areas are related to the activity, fairly reasonable correlations result. Certain complications concerning these relationships are introduced by various types of treatments applied to the catalysts and by the iron content of commercially deactivated samples. 

There are many divergent opinions about the causes of gel cracking and many models have been suggested based on experiments and observed results. The macroscopic model and the miaoscopic model are among the more representative models reported. 

If the solvent in gel pores is a mixture of water and methanol (or ethanol), appreciable amount of water may be left when drying is finished because methyl alcohol is easier to volatilize than water. But if some surfactants of low surface tension and volatility (for example, A,A -dimethylformamide [DMF]) are added to the wet gel before drying, the possibility of gel cracking is reduced. 

Certain drying conditions produce gel particles of particular shapes. As a thin layer of gel dries, the shrinkage causes it to crack usually into scales or ribbons. A fibrous form of gel 5-25 microns wide and several inches long is obtained by drying a film of concentrated silica sol on an inert surface. The gel cracks into parallel ribbons especially when drying progresses along the surface in one direction (269). "Microballoons" or discrete hollow spherical particles are formed when water solutions of SiO, are spray-dried with a small amount of gas-former such as ammonium carbonate to inflate the droplets (270). 

The microscopic model [6] assumes that the main cause of gel cracking is asymmetry of pores. According to this model, behind the critical point evaporation of liquid occurs in the larger pores initially, thus the stress on the larger pores can be relaxed. However, stress is still present in the smaller pores, which... 

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