Case bonding

There are many compounds which do not conduct electricity when solid or fused indicating that the bonding is neither metallic nor ionic. Lewis, in 1916. suggested that in such cases bonding resulted from a sharing of electrons. In the formation of methane CH4 for example, carbon, electronic configuration l.s 2.s 2p. uses the tour electrons in the second quantum level to form four equivalent... 

Bond stretching is most often described by a harmonic oscillator equation. It is sometimes described by a Morse potential. In rare cases, bond stretching will be described by a Leonard-Jones or quartic potential. Cubic equations have been used for describing bond stretching, but suffer from becoming completely repulsive once the bond has been stretched past a certain point. 

Propellants cast into rockets are commonly case-bonded to the motors to achieve maximum volumetric loading density. The interior of the motor is thoroughly cleaned, coated using an insulating material, and then lined with a composition to which the propellant binder adheres under the environmental stresses of the system. The insulation material is generally a mbber-type composition, filled with siUca, titanium dioxide, or potassium titanate. SiUca-filled nitrate mbber and vulcanizable ethylene—propylene mbber have been used. The liner generally consists of the same base polymer as is used in the propellant. It is usually appHed in a thin layer, and may be partially or fully cured before the propellant is poured into the rocket. 

CH COLOC Correlation via long-range CH coupling, detects CH connectivities through two or three (more in a few cases) bonds in the CH COSY format, permits localisation of carbon nuclei two or three bonds apart from an individual proton... 

Fig. 12 Plot of the shear index value, N, as determined using simplified shear cell methodology, versus Worst Case Bonding Index, BIW.

Along with a very wide synthetic application the Cope rearrangement continues to be a subject of intense debates. The key mechanistic question is whether the rearrangement of 1,5-hexadiene derivatives is concerted and passes via a six-electron aromatic transition state, or whether it involves the formation of a diradical intermediate, i.e. a cyclization-cleavage mechanism. In the former case, bond making and bond breaking occur synchronously (a survey of this question has been published210). 

The situation is greatly improved in this respect when the inhibitor is not in contact with the hot gases. This is the case with case-bonded grains where the thin chamber liner acts as restrictor (Ref 2)... 

Figure I. Inert diluent process flow diagram Adhesives for Case Bonding...

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... 

The requirements, manufacturing procedures, designs of case-bonded solid propellant rockets, and the performance characteristics of a useful... 

Compatability of ingredients is always important. Particularly important to safe handling is the autoignition temperature and friction sensitivity of the propellant. Small changes in the formulation can often effect these important properties. An example is the sensitivity of some propellant formulations to extremely small amounts of chlorate. Safety precautions must always include consideration of the chemistry. By following this practice, the propellant industry has experienced an excellent safety record while making unusual progress in the application of viscoelastic materials in case-bonded solid propellant rockets. 

Hiestand has developed many indices that are useful for tablet formulations (16 18). These include the frequently cited brittle fracture index (BFI), the best-case bonding index (Bib), the worse-case bonding index (BI ), and the viscoelastic index (VI) (15,16). 

The other way of providing protection to a motor is to apply a polymeric material or rubber to its inner surface and cast propellant directly into the insulated motor. Before casting propellant mix, a liner material is also applied to the motor insulation in order to ensure a strong bond between them. Such propellant grain in known as case-bonded grain which is shown in Figure 4.17. 

Such polymeric materials or rubbers are known as insulating materials or insulators and the process of their application is known as insulation. The motor insulation not only provides protection to motor but also functions as an inhibitor. The case-bonded motors have a little less inert mass and as a result, give slightly better performance. However, they are more stressed and more difficult and expensive to manufacture. Many tactical missiles and almost all larger rockets or missiles for defense and space applications use case-bonded motors. 

Figure 4.17 Schematic of a case-bonded propellant. Reprinted from G.P.Sutton, Rocket Propulsion Elements An Introduction to the Engineering of Rockets, 1992 1992, John Wiley and Sons, Chichester, UK.

Good ageing characteristics so that the case-bonded motor has a minimum 10-12 years of Service life. 

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