Polybutadiene binder properties

With only small differences in (Is)max the choice of the binder system is influenced by processability, physical properties, and propellant density. Thus, with the polyether binder an Is of 247 is reached with about 14% binder, but with the polyester the same Is is obtained with 11.5% binder, which is a definite disadvantage in terms of processability and mechanical properties. The higher Is with the polybutadiene binder is realized only at high solids loadings, but owing to its lower density, processability is still satisfactory. 

The compound was examined as a propellant explosive, it proved easily detonable and more sensitive than ammonium perchlorate. A formulation with hydroxyl terminated polybutadiene binder ignited spontaneously at room temperature [1]. Other workers have found it more tractable [2]. Detailed study of properties has been made. It is too friction sensitive for safe transport [3]. 

Corrosive attack by organic carboxylic acids on polybutadiene binder is exhibited by oxidization and depolymerization processes, with the resulting the formation of carboxyl groups COOH. Carboxylic acids generally are weaker than inorganic acids because the speed of these processes is lower and, therefore, the decrease of physical-mechanical properties of RubCon occurs more slowly. The most intensive reduction of RubCon strength properties (compressive strength and modulus of elasticity) due to carboxylic acids attack were noted in the first 90 days. Penetration depths of... 

The analysis of the obtained results shows that fiber-reinforced RubCon is a composite hydrophobic material with a coefficient of water resistance of Kcr = 0.995. Decreasing compressive strength was not observed and water absorption was 0.05% on weighing of samples. The small change of weight is due to the hydrophobic surface of RubCon. This is due to the intrinsic properties of the polybutadiene binder, which is not moistened with water. Furthermore, polybutadiene oligomer is nonpolar liquid. 

Polybutadiene glycol Properties M.w. 1350-2800 Uses Cable joint potting compds. pipe sealants alkyd resin modifier PU adhesives binder rocket fuel binder... 

Another class of hydrocarbon binders used in propints are the carboxy-terminated polybutadiene polymers which are cross-linked with either tris[l-(2-methyl)aziridinyl] phosphine oxide (MAPO) or combinations with phenyl bis [l -(2-methyl)aziridinyl] phosphine oxide (Phenyl MAPO). Phenyl MAPO is a difunctional counterpart of MAPO which makes possible chain extension of polymers with two carboxylic acid groups. A typical propint formulation with ballistic properties is in Table 11 (Ref 83) Another class of composites includes those using hydroxy-terminated polybutadienes cross-linked with toluene diisocyanate as binders. The following simplified equations illustrate typical reactions involved in binder formation... 

Under increasing strain the propint volume increases from the voids created around the unbonded solid particles. Nonlinearities in Young s modulus and Poisson s ratio then occur. Francis (Ref 50) shows this effect for a carboxy-terminated polybutadiene composite propellant with 14% binder as in Figure 12. He concludes that nonlinearities in low-temperature properties reduce the predicted stress and strain values upon cooling a solid motor, and therefore a structural analysis that neglects these effects will be conservative. However, when the predictions are extended to a pressurized fiberglas motor case, the nonlinearities in properties produce greater strains than those predicted with linear analysis... 

Hydroxy-terminated polybutadiene (HTPB) is considered to be the best binder for obtaining high combustion performance, superior elongation properties at low temperatures, and superior mechanical strength properties at high temperatures. This combination of properties is difficult to achieve in double-base propellants. HTPB is characterized by terminal -OH groups on a butadiene polymer. The other type of butadiene polymer used is carboxy-terminated polybutadiene (CTPB), which is cured with an imine or an epoxy resin. It should be noted that CTPB is somewhat sensitive to humidity, which has an adverse effect on its ageing charac-... 

Curing Agents for Carboxyl-Terminated Polybutadiene Prepolymers. The types of curing agents used to prepare binders for CTPB propellants are the same as those for PBAN or PBAA. The bifunctionality of CTPB, however, requires that part of the curing agents be polyfunctional to provide for the formation of the tridimensional network. Almost without exception, the polyfunctional aziridines and epoxides used with CTPB undergo side reactions in the presence of ammonium perchlorate, which affects the binder network formation. Kinetic studies conducted with model compounds have established the nature and extent of the cure interference by these side reactions. The types and properties of some of the crosslinkers and chain extenders used to prepare solid propellants are summarized in Table IV. 

A specific requirement of solid propellant binder polymers is the small tolerance allowed in the reproducibility of the product properties. As a result, some polymers that cannot be specified easily must be adjusted lot by lot in accordance with qualification tests. This is illustrated graphically by the data of Figure 1, where different lots of a carboxy-terminated polybutadiene procured to the same specification are compared with the different equivalents of the BITA (butylene imine adduct of trimesic... 

Composite rocket propellants are two-phase mixtures comprising a crystalline oxidizer in a polymeric fuel/binder matrix. The oxidizer is a finely-dispersed powder of ammonium perchlorate which is suspended in a fuel. The fuel is a plasticized polymeric material which may have rubbery properties (i.e. hydroxy-terminated polybutadiene crosslinked with a diisocyanate) or plastic properties (i.e. polycaprolactone). Composite rocket propellants can be either extruded or cast depending on the type of fuel employed. For composite propellants which are plastic in nature, the technique of extrusion is employed, whereas for composite propellants which are rubbery, cast or extruded techniques are used. 

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. 

Some binder systems have used MAPO as the only curing system for carboxy-terminated polybutadiene polymers. Again a problem in long term stability of physical properties is sometimes encountered since MAPO is hydrolytically unstable. The following reaction is possible ... 

The influence of -( CH2)-x binder content on the theoretical specific impulse of AP composite containing 8, 12 and 16% aluminum reaches a max at binder contents between 10 and 15% as shown in Fig 16, while the max level of acceptable physical properties occurs at the 10—16% level. Most operational proplnts accept a sacrifice in energy and operate at the 14—16% binder level since this normally determines service life. Differences in hydrocarbon binders as typified by polyurethane, polybutadiene-acrylic acid copolymer, polybutadiene-acrylic acid-acrylonitrile terpolymer and carboxy-terminated... 

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