Fuel binder for composite propellant
United States Patent 1 Fukuma et al.  
 [ Apr. 22, 1975 [22] Filed: Dec. 6, 1973 [21] Appl. No.: 422,181  
 [30] Foreign Application Priority Data Dec. 25, 1972 Japan 48-729 [52] US. Cl 260/23.7 R; 149/199; 260/458 N {51] Int. Cl. C08c 11/72 [58] Field of Search 260/23.7 R; 260/458 N;  
 [56] References Cited UNITED STATES PATENTS 3.507.837 4/1970 Hidinger 260/87.3  
 OTHER PUBLICATIONS Skeist. Handbook of Adhesives, 1962, pp. 255-259 relied on.  
 Primary E.\&#39;aminerDonald 1E. Czaja Assistant Examiner-William E. Parker Attorney, Agent, or Firm-Frank J. Jordan [57] ABSTRACT In carboxyl-terminated polybutadiene fuel binders, tris I-(Z-methyl) aziridinyl phosphine oxide (MAPO) is converted into a bifunctional derivative with a fatty acid so that the catalytic effect of MAP0 on the burning rate reduction is retained and the adverse effect on the elasticity of the cured polymer is eliminated.  
 3 Claims, No Drawings FUEL BINDER FOR COMPOSITE PROPELLANT The present invention relates to composite solid propellants and particularly to polybutadiene fuel binders for composite propellants of relatively low burning rates.  
  In the field of composite solid propellants for rockets. gas generators etc., it is still now a problem of great interest to provide a propellant of a relatively low burning rate without making sacrifice of its physical properties. Although some substances such as lithium fluoride, strontium carbonate and oxamide are known as catalysts for reducing the burning rate of composite propellants, the adverse effects of these catalysts on physical properties of the resulting propellants are almost inevitable and too significant to be ignored.  
  In carboxyl-terminated polybutadiene (CTPB) propellants, which have been holding an important part in the field due to their excellent physical properties over a wide temperature range, it is well known that a typical curing agent of CTPB, namely tris l-(2-methyl) aziridinyl phosphine oxide (MAPO) has a remarkable effect of reducing the burning rate. However, the use of a large amount of MAP to achieve a considerable reduction of the burning rate also causes the resulting propellants to have some undesirable physical properties such as a decreased elongation and increased Young&#39;s modulus. In short, a rigid propellant is always obtained when the burning rate is reduced by MAPO. 1  
  it is therefore an object of the present invention to provide a CTPB-based fuel binder for composite propellants of a relatively low&#39; burning rate and adequate elasticity.  
  Briefly stated, a fuel binder in accordance with the present invention comprises a carboxy-terminated polybutadiene, MAP0 and a fatty acid so that at least a portion of MAP0 is converted into a bifunctional derivative combined with the fatty acid. The bifunctional MAPO derivative, while retaining the catalytic nature of MAPO. has little influence on the elasticity of the propellant since it extends the polymer chains without cross-linking them.  
  MAPO serves as a curing agent of CTPB by crosslinking the polymer chains because it is a trifunctional imine. When, however, MAPO is used in order to reduce the burning rate in an amount larger than sufficient for an adequate degree of cross-linking. the cured binder has naturally an excessively cross-linked structure, losing a featuring property of CTPB binder, an adequate elasticity.  
  Despite this drawback, MAPO is considered too dominant to be abandoned both as a curing agent and as a burning rate reduction catalyst for CTPB. We have therefore contemplated to turn MAPO into a bifunctional derivative retaining the catalytic nature. The denaturation can be accomplished by opening one imine ring of MAP0 with a monocarboxylic acid. When the acid is mixed with MAPO equimolarly, the reaction may be expressed as follows:  
 Preferably monocarboxylic acids are fatty acids, for example acetic acid, valeric acid, caproic acid, caprylic acid and oleic acid. Some aromatic acids, for example benzoic acid, may also be used. The obtained bifunctional MAPO derivative no longer works as a crosslinking agent for CTPB which has functional groups at MAP0 unreacted with the acid so that CTPB will be cured to a cross-linked structure corresponding to the required properties such as elongation and Young&#39;s modulus.  
  The denaturation of MAP0 in accordance with the invention may be carried out either by mixing the three components of the fuel binder, namely a CTPB, MAP0 and a monocarboxylic acid, all at once, or by treating MAPO alone with the acid prior to the mixing with a CTPB. If, in the former case, Y mole of MAP0 is desired to be combined with X mole of a CTPB from a burning rate viewpoint (where 0.7X 5 Y X as mentioned before), the amount of a monocarboxylic acid is determined to be. equal or less than (3Y 2X) mole. In this instance, an oxidizer for composite propellants may be simultaneously mixed with the above three components of the fuel binder of the invention like a common propellant mixing process. When MAPO alone is treated with the acid in advance. calculated amounts of the denatured and untreated MAPO are mixed with a CTPB. If Y mole of total MAPO is required to X mole of a CTPB, at least 2(X Y) mole of untreated MAPO is used in addition to at most (3Y 2X) mole of the denatured MAPO. In this instance. any suitable curing agent may be used instead of or in combination with untreated MAPO when, for example. the  
 required total amount of MAP0 is relatively small. The reaction of a monocarboxylic acid with an aziridine ring of MAP0 occurs easily at room temperature to a slightly raised temperature of about C.  
  To further illustrate the invention, the following nonlimitative examples are presented.  
 EXAMPLE I A CTPB of about 3000 M.W., MAP0 and caprylic acid were mixed all at once at room temperature in a molar ratio of 1 0.933 0.8.  
  A series of experiment was carried out to compare the burning rate and physical properties of a composite propellant prepared from the obtained fuel binder with those of similar propellants from the prior art fuel binders. The propellants were prepared by mixing the fol- H C-CH-CH a c-ca-cn a c I -ca H2C\N i= N/CFL-CHS n-coon l/N If 3 I l C-HC o a ca -o-c-a n ca 3 ll a e-ac o 2 o lowing&#39;constituents followed by heat curing at 60C for 96 hr. CTPB fuel binder 19 parts by weight.  
 B-l A mixture of a CTPB. MAP and a bifunctional epoxy compound in a molar ratio of l 0.5 67 0.15.  
 &#39; 3-2; A mixture Ofa CTPB, and MAP0 in molar ratio Fuel hinder 8-1 8-2 8-3 Tensile strength kg/cm 8.5 15 9.0  
 Elongation Young&#39;s modulus v kg/cm 40 90 60 Burning rate mm/sec 6.0 10.3 5.0 i 0.3 4.0 t 0.3  
 Burning rates were measured at 50 kg/cm The advantage ofa fuel binder of the invention (3-3) will be apparent from a comparison with 8-2 (a prior art fuel binder for a low burning rate). When 3-2 was used. the propellant became extraordinally rigid despite only about l5 percent reduction in the burning rate from a standard fuel binder 8-]. On the other hand. B-3 brought about a burning rate reduction of more than 30% with only a slight change in elasticity.  
 EXAMPLE 2 MAP0 and valeric acid were mixed equimolarly and heated at 60C for 3 hrs. The CTPB of Example 1, untreated MAP0 and the valeric acid added MAPO were mixed in amolar ratio of 10 6 l. The properties of a propellant prepared with the same composition as in Example I using the fuel binder of Example 2 were comparable with the values shown in the column 8-3 of the above table.  
 EXAMPLE 3&#34; The steps of Example 2 were followed except that propionic acid was &#39;used in lieu of valeric acid.  
 I EXAMPLE 4 Example 2 was again repeated using linoleic acid in lieu of valeric acid.  
  The burning-rate and physical properties of propellants of the aforementioned composition prepared from the fuel binders of Examples 3 and 4 were substantially similar to those of the propellant from 8-3 binder of Example I in spite of considerable differences in the chain length and bond structure of the fatty acids.  
 What is claimed is:  
  1. In a fuel binder for composite propellants, having a carboxyl-terminated polybutadiene and tris 1-(2- methyl) aziridinyl phosphine oxide. the improvement comprising at least a portion of said tris l-(2-methyl) aziridinyl phosphine oxide being a bifunctional derivative thereof having the formula:  
  H C-Ci7H a c l N-1 --N-- ca- CH3 11 0-30 a a CHz-O-C-R monocarboxylic acid has less than 20 carbon atoms.