1. Field of the Invention
The present invention relates to the processing of materials wherein calibrated sensors comprising microcapsules which contain detection materials ar incorporated with the materials to be processed, the microcapsules being adapted to rupture at predetermined processing conditions with release of detectable amounts of the detection material, this providing an indication that the predetermined conditions had been equaled or exceeded.
Although the invention has wide application in the mixing of materials, for example in the food industry, in extrusion, in extruder design and calibration, and the like, the invention is especially useful in the continuous mix processing of solid rocket fuel and plastic bonded explosives. In this preferred practice of the invention, a microencapsulated, readily detectable material such as a dye is incorporated with the conventional solid fuel components during mixing of the components in, for example, a twin screw extruder. The microcapsules are formulated to rupture when shear rates exerted on the fuel components exceed predetermined levels with the release of the detectable material. Through the use of appropriate monitoring, appearance of the detectable material can be determined evidencing the fact of the microcapsules rupture, and the mixing process can be slowed or halted before shear rates become so high as to be hazardous. Similarly, in other mixing and/or extrusion applications, the calibrated sensors are formulated to rupture at certain conditions, thus enabling materials processing to be controlled.
2. Description of the Prior Art
The use of large solid fuel rocket motors is essential to space and military programs of the United States and other countries. These rocket motors may contain tens of thousands of pounds of fuel, the components of which must be carefully and accurately mixed and loaded into the motor.
Generally speaking, laborious and expensive batch techniques have in the past been employed. Solid fuel components have been batch mixed in vats in quantities of up to 27,000 pounds and the resulting mixtures loaded into rocket motors. Such procedures have been hazardous, and, where uniformity or quality was not satisfactory, entire batches were wasted.
Continuous mixing techniques, for example employing extruders, have inherent advantages over batch techniques. Much smaller quantities are present at any one time in the mixer, thus reducing hazards. Monitoring and sampling are facilitated, and in this way quality control can be greatly improved.
There are, however, problems with the continuous mixing of solid rocket fuel components since the fuels are combustible and potentially explosive. In extruder-type continuous mixers, there always is the danger that the fuel components will be subjected to excessive shear rates causing localized overheating and potential safety problems.
The present invention addresses these problems and provides a method for determining shear rate during the continuous mixing of the solid rocket fuel components whereby operation can be controlled to achieve high throughput and to avoid unsafe conditions.
In the food industry, for example, excessive shear rates during mixing can impart unacceptable taste to the mixed product resulting in substantial amounts of product being unsuitable for sale.
In polymer extrusion processes, it is frequently important to avoid excessive shear rates in order to prevent overheating and/or discoloration of extruded material.
The concept of microencapsulating detectable materials is not novel. For example, U.S. Pat. Nos. 3,016,308 and 3,179,600 describe the use of such materials in "carbonless paper". U.S. Pat. No. 3,469,439 describes the use of microencapsulated color components to measure and record forces over a surface. The microcapsules size and wall characteristics are controlled to provide groups of microcapsules which break at different pressures. Explosives have been tagged by means of vapor fumeable microcapsules containing volatile fluorinated materials; see U.S. Pat. No. 4,399,226. Explosives have also been tagged by addition of luminescent material according to U.S. Pat. No. 3,835,782, by addition of magnetic material according to U.S. Pat. Nos. 4,363,678, 4,198,307, and 4,152,271. Other patents having to do with tagging explosives include U.S. Pat. Nos. 4,018,635, 4,131,064 and 3,772,200. However, this prior art does not relate to the use of microencapsulated sensors in detecting changes in shear conditions and pressures involved with mixing or extruding various materials.