Glass Products with Controlled Porosity

Besides etching of masked glass substrates, see Sect. 6.1, the leaching of phase-separated glasses, see for instance Figs. 1.14-1.16, is completely an other process and also allows for the fabrication of microstructured glass bodies. A schematic of the process to fabricate a porous glass membrane, which was developed from the classical VYCOR process, is shown in Fig. 6.4. 

Before the removal of the more soluble minor phase, the glass is shaped by mechanical cutting into thin plates. This is important to define the membrane thickness and reduce the sample thickness to enhance the effectiveness of the leaching process. 

Porous glasses can also be produced by inducing partial crystallisation in the glass rather than a phase separation and utilising the difference in solubility between the glass and crystalline phase. This process is similar to the photostructuring process (see Chap. 9). 

The filler process typically results in open porous glass or glassceramic parts with an open pore volume of around 60vol % and only 8vol % of deadend pores and 2 vol % of closed pores. 

The filler process has been used for the preparation of open porous glass carrier balls, such as Siran [452], and also to produce complex open porous glass and glassceramic carrier bodies [466]. They are used in biotechnology for storage, conduction and distribution of liquids and in filtration and separation processes and also as catalyst support material. 

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Aluminates are refractory materials and their synthesis often simply involves solid-state growth of mixtures of purified oxides. Alternative synthesis routes are also used in specialist applications, for example in production of materials with controlled porosity and these invariably involve sol-gel methods. For glasses, one notable, commercially important method of production is container-less synthesis, which is necessary because of the non-Arrhenius (fragile) viscosity of aluminate liquids. 

Most acrylic production is for staple fiber. Huge spinning plates with up to 60 000 holes are used, and tow drawing and crimping are included in the process. Drawing takes place in a hot-water bath, which is possible because the wet glass transition temperature is about 75°C. The above-mentioned porosity of acrylics is used to control luster. Dried yams are very lustrous but can be made dull again by a hot wet treatment. 

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