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Abstract

A fire-retardant plastic mixture contains 55%-75% by weight of boehmite which has an orthorhombic crystal structure and has the general formula AlO.sub.x (OH).sub.3-2x with x-values ranging between 0.8 and 0.99, and has a mean grain diameter d.sub.50 ranging between 0.4 and 0.7 .mu.m, a d.sub.10 value ranging between 0.7 and 1.2 .mu.m and a d.sub.90 value ranging between 0.2 and 0.4 .mu.m. The filler material is used in the form of a finely-crystallized hydrargillite which is obtained during a precipitation process according to Bayer with a grain diameter of 0.5 to 3 .mu.m, which is suspended and which, without any mechanical working, is transformed directly in a hydro-thermal process at temperatures ranging between 220.degree. C. and 240.degree. C. under turbulent conditions into a boehmite of the general formula AlO.sub.x (OH).sub.3-2x with x-values ranging between 0.8 and 0.99. The phase transformation pressure amounts to at least 24 bar after a holding time of 1-15 minutes at the maximum temperature and then the pressure is reduced to ambient pressure.

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