1. Field of the Invention
This invention relates to a new process for strengthening geological formations in underground mining or other underground operations, in particular in coal mining by forming polyurethanes from mixtures of organic polyisocyanates and polyhydroxyl compounds containing special dihydric alcohols as viscosity reducing additives in order to improve the mixing process, accelerate hardening and regulate the foaming process.
2. Description of the Prior Art
The strengthening of geological formations below ground, particularly in coal mining, by means of polyurethanes which foam and harden within the formation is already known (see e.g. the Journals, Gl/u/ckauf (1968), pages 666 to 670, Gl/u/ckauf (1977) pages 707 to 711 and Bergbau (1977), pages 124 to 129, DE-PS 1,129,894, DE-PS 1,758,185, DE-PS 1,784,458, DE-PS 2,436,029, DE-PS 2,623,646 and DE-PS 3,139,395). Such mixtures which react to form polyurethanes are also used to seal the structures against water and/or gas. This known process is generally carried out as follows.
The two reaction components, the polyisocyanate and polyhydric alcohol (polyhydroxyl component), are conveyed separately to a position in front of a borehole where they are continuously combined in a static mixer and pressed through the borehole into the rock formation where they foam up and harden.
The polyhydroxyl component used in this state of the art process is preferably based on mixtures of polyether polyols and castor oil. Fat derivatives containing hydroxyl groups, in particular castor oil, increase the binding property and strength of adherence as well as the flexibility of the polyurethane mixtures. At the same time, these polyols act as defoaming agents, thereby suppressing excessive foaming of the polyurethane resin which would impair the mechanical properties. The castor oil is generally used in quantities of 1 to 50% by weight, preferably 5 to 20%, based on the polyol mixture. It therefore constitutes a significant cost factor, especially since the price of this natural product is subject to wide fluctuations and is on average considerably higher in price than other polyols. Moreover, being a natural product, castor oil is also subject to wide variations in quality.
Another very serious technical disadvantage of these polyols mixtures containing castor oil is that they have little compatibility with polyisocyanates. Although a homogeneous emulsion may be obtained by prolonged and vigorous mixing of the two components, such an emulsion at least partially separates into its components within a short time such that the components do not always react quantatively in the gaps of the formations. The known strengthening process therefore becomes unreliable. Another disadvantage is the long curing time (2 to 2.5 hours) of the polyurethane resin obtained from the polyol mixtures containing castor oil and the above-mentioned polyisocyanates. Since the quality of the reinforcement depends upon the strength of the hardened polyurethane resin, it will be obvious that a long hardening time results in considerable delays before work may be resumed.
Practice has shown that the high viscosity of polyol mixtures containing castor oil may also have an adverse effect on the degree of strengthening obtained.
Therefore, there was a need for another solution which, while retaining the advantageous effect mentioned above, would not have the disadvantages described.
It has now surprisingly been found that by using certain dihydric alcohols not only can the disadvantages described above be obviated, but the properties of the polyurethane can be improved.