Thiokol physical properties

Early in the development of solid propellant, the asphalt composites were found to have poor physical properties, such as cracking under normal temperature cycling, poor tensile characteristics, etc. They were replaced with the elastomeric polymers which have become the present-day binders. The first of these was Thiokol rubber, a polysulfide rubber, whichgives the propellant with good physical properties. The presence of the sulfur atom in the Thiokol rubber decreases the performance compared to a CHO polymer thus the most frequently used binders are polyurethane, polybutadiene acrylic acid (PBAA), epoxy resin, etc. The choice of the latter binders is made with regard to physical properties rather than performance. 

Morton Thiokol, Inc. markets a series of liquid polysulfides that can be oxidized to rubbers. These rubbers have excellent resistance to most solvents, good water and ozone resistance, very low specific permeability to many highly volatile solvents and gases, objectionable odor, poor physical properties, and a service temperature range of -55 to 150°C. 

Zinc oxide is the major vulcanization agent required for Thiokol FA however, the particular grade is important as certain impurities may affect the cure rate and final properties. A French process zinc oxide, such as Midwest AZO 66L or equivalent, is recommended. Other oxides such as lead oxide or dioxide, cadmium oxide, and zinc hydroxide may also be used with varying rates of cure and physical properties. The amount of zinc oxide over or under 10 phr is not critical and will not affect the rate of vulcanization. Note that the MBTS and DPG are peptizing agents, not accelerators. 

Polysulhdes often are blended with other elastomers such as nitrile rubber, NBR, or neoprene, CR, for improved physical properties and factory processing. Traditionally, Thiokol FA is blended with neoprene for improved strength and processing for rollers, at some sacrihce in solvent resistance. Table 11.9 has information on Thiokol FA blends. If minimal loss in chemical resistance is indicated, then a blend with a high ACN nitrile is employed. A cure system that is compatible with both rubbers needs to be used in all cases. The zinc peroxide-cure system is NBR specihc, hence it is important that the recommended one be used or others be tested since many NBRs do not cure with zinc peroxide. Best results are obtained with blends if separate masterbatches are made with the individual polymers, which are then blended in the... 

Press curing of Thiokol compounds should be done in a press with cooling capabilities for best results. A minimum press temperature of 150°C is recommended for best physical properties for a time dictated by the iheometer curve at the curing temperature. At the end of the cure cycle the press should be cooled under pressure to approximately 40°C before opening the press to avoid pock marks and other surface defects. Typical cure times at 154°C for Thiokol FA is 40 min and for Thiokol ST is 30 min. 

Physical and Ballistic Properties of Minuteman Solid Propellant , Rept AS60-Y002-02622, Thiokol Chem Corp, Elkton (1960) also in Trans of 5th Syuip on Ballistic Missile Space Technol, Vol II (1960) (AD-323311) 87) G.S. 

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