Plastisol propellants

Chloride Plastisol Propellants in Propellants Manufacture, Hazards, and Testing , ACS Advances in Chemistry Series 88, Washington (1969) 8) CondChemDict (1971), 714... 

Table I gives a range of typical ingredient types and percentages for potentially useful nitrocellulose plastisol propellants.

Table I. Typical Ingredients for Nitrocellulose Plastisol Propellants...

The advanced applications for nitrocellulose plastisol propellants require that they be integrally bonded to the motor case. Successful case bonding for the multiyear storage life of a rocket calls for special adhesives and liners which are completely compatible with these highly plasticized propellants. Best results have been obtained with a combination of an impervious rubber liner and a crosslinked adhesive system with a limited affinity for the plasticizers used in the propellants. Examples of effective liners are silica-filled butyl rubber and chlorinated synthetic rubber. Epoxy polyamides, isocyanate-crosslinked cellulose esters, and combinations of crosslinked phenol-formaldehyde and polyvinyl formal varnishes have proved to be effective adhesives between propellant and impervious liners. Pressure curing of the propellants helps... 

It is in the technique of solidifying the mass that plastisol propellants differ so markedly from composite propellants. In composite propellants, the nonvolatile liquid is comprised of monomers or low molecular weight prepolymers. Solidification is accomplished by completion of the polymerization reactions. Much attention must be given to the degree of completion of these reactions during manufacture so as to minimize changes in physical properties as a consequence of continued slow polymerization, or so-called post-cure, following manufacture. 

In plastisol propellants, however, all polymerization reactions are complete before propellant manufacture begins. Solidification is accomplished through solvation (or solution) of the solid resin (or polymer) particles in the nonvolatile liquid, which has been selected to be a plasticizer for the resin. Solvation or curing is accomplished by heating to a temperature at which the resin particles dissolve rapidly (within a matter of a few minutes) in the plasticizer to form a gel which on returning to room temperature has the characteristics of a rubbery solid. 

The resin used to manufacture plastisol propellants must be dispersion grade. The resin particles should be spherical (18) (or nearly so), preferably a maximum diameter of about 30p or less (14, 17), free from porosity (14), and have a clean surface (15). This will permit the formation of a smooth, creamy plastisol when mixed with approximately an equal weight of the usual plasticizers for the polymer in question. Further, the plastisol of the resin and plasticizer must be capable of being heavily loaded with oxidizer and other fine solids to permit the formulation of a useful propellant composition. 

In the plastisol propellant process, it is essential that the resin particles not solvate too rapidly at processing temperature since a rapid increase in viscosity of the propellant mix interferes with the mixing and casting operation. There must be adequate pot life of the mixed propellant. The resin-plasticizer system itself is the dominating influence on pot life, and for this reason certain combinations cannot be used in the plastisol process. 

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... 

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