Promat high temperature insulation - Production and classification

Amorphous fibres are melted in an electric arc furnace at approximately 1900 °C and then either blown with pressed air or cast by counter-rotating discs. During blowing, the melt stream is split into tiny drops whichsubsequently extract the fibres. These fibres have unsorted lengths up to 50 mm and a diameter of approximately 2-3 µm.
Fibre formation is carried out via centrifugal force applying the centrifugal method. The length of these fibres can reach 250 mm and the diameter is approximately 3- 5 µm. In both processes as the head of the fibrous body, a small massive ball remains. This breaks off during cooling and is added to the raw fibres as a particle that has not been fiberized. The share of these particles (shots) is between approximately 40 and 60 %. However, it is reduced to approximately 30 % of the total amount by mechanical separation during the process. Those components which have not been fiberized can be almost completely removed by elutriation
or air separation.
The production of fibres from the melt is possible up to an Al203 content of approximately 60 %. Higher alumina contents will effect increased surface stress on the melt which will result in short fibres or even no fibre formation at all. Hollow spheres (balls) will form.
The production of polycrystalline fibres is technically more difficult and this has an effect on the price of this high-quality product. Aluminium salts are the base materials. Organic polymers are added to these salts in order to obtain fibres in a spinning or centrifugal process. Silicic acid is added as a crystal stabiliser for further thermal treatment. A fibre which has a semi-amorphous and semi-crystalline structure forms at approximately 1100 °C. Finally, the fibre is converted to a pure polycrystalline form by firing and the porosity disappears completely.
Finally the fibre consists of mullite, corundum or their mixture depending on the silicic acid content.
The fibres used for thermal insulation have ungraded lengths ranging from 5 to 100 mm with a median diameter of approximately 3 µm. The process enables regulation of the fibre diameter between approximately 1 and 30 µm. The amount of particles which have not been fiberized is < 1 % and is of no further significance.
The classification temperature of ceramic fibres is determined according to DIN EN 1094 and is based on irreversible linear change (shrinkage). For fibre mats the shrinkage may not exceed 4 % after a period of 24 hours. Only 2 % shrinkage is allowed for shaped parts.

The amorphous fibre crystallises above approximately 900 °C. Mullite (3Al203 2Si02) forms in addition to cristobalite. Cristobalite forms from the excess silicic acid which cannot enter a bond with the alumina in rystalline form. Shrinkage of the fibres is due to recrystallisation, softening of the glassy share and crystal growth. The fibres become brittle during recrystallisation.
The shrinkage process continues beyond a 24 hour period (graph 5). Shrinkage and embrittlement, which also depend on the chemical composition of the fibre, determine the service limit for fibre products. Less shrinkage is noticed here at classification temperature due to the crystalline structure of the alumina fibre.

Once in service, it is possible to minimise or compensate shrinkage of fibre productsby specific design measures. Preliminary stress is applied by various processes into the mat linings, joints are relocated, joints which occur are „fixed“ with plastic fibre masses, or coatings are applied.