PVC plastisol propellant

The first successful static firing of plastisol propellant took place late in 1950 as part of a broad program conducted by Atlantic Research Corp. to investigate and evaluate plastisol propellants and methods for their manufacture (16). Major attention was directed to poly (vinyl chloride), cellulose acetate, and nitrocellulose, although other polymers were tested for their suitability (17). Patent applications were filed for plastisol propellant compositions and manufacturing processes, based on poly(vinyl chloride) (PVC) (19) and on nitrocellulose (18). The commercial availability of dispersion grade PVC enabled work with this resin to advance rapidly. The balance of this paper is devoted to a discussion of PVC plastisol propellants and their manufacture. 

Basically, PVC plastisol propellants are systems of binder,.oxidizer, and metallic fuel (if used). Minor ingredients, normally comprising less than 2% of the total, consist of wetting agent, stabilizer, opacifier, and burning rate modifier. The composition of three typical PVC plastisol propellants is shown in Table I. 

The upper limit depends quantitatively on the viscosity that can be processed through the casting fixtures in reasonable time. Too high a viscosity may also lead to problems under certain flow conditions as, for example, when propellant folds over on itself to form a void space which may remain as a defect in the cured grain. If the propellant grain is to be formed and cured by screw extrusion, however, somewhat higher viscosities can be handled. A viscosity of 1600 poise has been reported (9) for a PVC plastisol propellant processed this way. 

Figure 1. Flow of uncured PVC plastisol propellant in pipes at 70 2°F. (17)...

Since no chemical reactions are involved in curing PVC plastisol propellant, the heat required to raise the propellant to curing temperature... 

Figure 4. Effect of curing temperature on tensile characteristics of a typical PVC plastisol propellant (17)...

Figure 5. Time to heat slabs of PVC plastisol propellant to various curing temperatures by conduction...

Figure 6. Unaccomplished temperature change of midplane or midpoint of various shapes in curing PVC plastisol propellant by conduction...

In the simple two-component system of PVC binder and oxidizer, the important propellant properties of specific impulse, density, adiabatic flame temperature, and burning rate increase with an increase in solids loading. This is shown in Figure 8, where theoretical calculated values of specific impulse, adiabatic flame temperature, and density are given for a range of oxidizer content for PVC plastisol propellants comprised of only binder and oxidizer. [Calculated values of specific impulse reported throughout this paper are for adiabatic combustion at a rocket chamber pressure of 1000 p.s.i.a. followed by isentropic expansion to 1 atm. pressure with the assumptions that during the expansion process chemical compo-... 

Figure 10. Burning rate of PVC plastisol propellant oxidized with potassium perchlorate (equal parts PVC and dibutyl sebacate) (10)...

There is an unmistakable tendency in PVC plastisol propellants oxidized with ammonium perchlorate for the burning rate to become more or less insensitive to pressure in the pressure interval 200-700 p.s.i.a. when... 

Table II. Burning Rates Along 5-mil Wires of Various Metals Embedded in PVC Plastisol Propellant...

Table III. Combution Temperature and Products of Typical PVC Plastisol Propellants...

Motor applications of PVC plastisol propellants have been notably free of unstable burning problems. 

PVC plastisol propellants are quite safe to work with. Binder ingredients are nonexplosive and nontoxic. Ammonium perchlorate, the principal... 

PVC plastisol propellant having the chemical composition of standard Arcite (composition given above) ignites at 540° to 550°F. when its temperature is increased about 30°F./min. (16, 17). Thus, there is an ample margin of safety between the autoignition temperature and the curing temperature, which is about 200 °F. lower (19). 

In the manufacture of PVC plastisol propellants, the usual care is taken to assure that all ingredients meet specifications for chemical composition, dryness, and particle size. Since PVC plastisol propellants do not involve chemical reaction in their curing process, however, the presence of certain impurities in the ingredients is far less critical than with those propellant systems that are solidified by polymerization reaction. Ammonium perchlorate, the most common oxidizer, can normally be procured from the supplier with the required low moisture content. If not, it is dried immediately before use. Tray dryers or other drying means may be used, depending upon the quantity of material. Normally, some or all of the oxidizer must be ground to achieve the desired mean particle size and particle-size distribuion. High speed hammer mills have proved... 

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