Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Patent
1983-08-18
1985-01-15
Welsh, Maurice J.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
524218, 524721, 524724, 528 52, 528 53, C08L 7504
Patent
active
044939167
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to production processes where a prepolymer is cured and particularly to a method of avoiding in such processes a too rapid curing reaction, which gives an extension of the so-called pot life.
Solid propellants, so-called composite propellants, for use for instance in rocket and missile propellant charges are pyrotechnical compositions mixed with for instance a thermosetting resin. At the production thereof one starts from a liquid prepolymer, e.g. hydroxy terminated polybutadiene (HTPB) or carboxy terminated polybutadiene (CTPB), which comprises chemically reactive groups and which, after various additions, is crosslinked and cured by means of a curing agent. As part of solid propellants there are often metal comprising additives, e.g. combustion catalysts such as iron oxide and ferrocene compounds.
However, it has been found that transition metals, e.g. iron and copper, even in very small amounts catalyze the curing reaction, and because of this they shorten in a grave way the so-called pot life, i.e. the time that you have at your disposal to mix and cast the composition before it turns too viscous.
Thus, the metal comprising additives that are added for instance to catalyze the combustion of the produced propellant have because of this as a side-effect that they might heavily shorten the pot life, sometimes so heavily that a production in larger scale turns out to be impossible.
The present invention relates to a method that will eliminate said difficulty. The characteristics of the invention are evident from the subsequent patent claims.
According to the invention there is added in the mixing process a very small amount of a compound, the structural formula thereof comprising the metal binding part ##STR2## wherein each of R.sub.1 and R.sub.2 is hydrogen or a hydrocarbon group, for instance a tetracycline compound, to form complexes with the transition metals that are present in the mixture. As a result thereof the pot life will become considerably extended in the production of the propellant without the combustion of the produced propellant being influenced in a negative way.
Very small amounts of the metal complex forming component are required. Suitably there is added between 0 and 3 weight percent based on the produced propellant. Preferably there are used so small amounts as 0.05-1 weight percent.
Certain types of solid propellants comprise metals, e.g. magnesium or aluminium, to increase the energy content. However, this will also cause a heavy intensification of the activity of the combustion catalysts as curing catalysts, causing difficulties in the production. Even in this case the method according to the invention has a favourable effect. It removes the negative influence of the metal on the curing course of reaction, and long pot lifes will again be obtained.
The invention is generally applicable in production processes, where a prepolymer is being cured and where there is a wish to remove the undesirable catalytic action of the transition metals present on the curing reaction, for instance in the production of plastic-bound explosives or other types of encapsulation in plastic.
The method of the invention is also applicable in all polyurethane productions where a hydroxy comprising prepolymer is mixed with a multivalent isocyanate. An undesirable metal catalysis of the curing reaction might occur here because metals will contaminate the source chemicals during the production and storage thereof in metal vessels. Metals will also enter at the actual polyurethane production from the apparatus which is usually made from metal. It is difficult to get control over the metal contamination which often varies with the varying production batches. The metal contamination disturbs both the production process and the following utilization; the shelf life is influenced among other things.
Such undesirable metal catalysis of the curing reaction is prevented according to the invention by adding before the curing a small amount of a compound, whose structural formula comprises
REFERENCES:
patent: 4180643 (1979-12-01), Moss et al.
Forsvarets Forskningsanstalt
Welsh Maurice J.
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