Journal of Energetic Materials

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In order to increasing of hydroxy functionality of HTPB an in-situ nitroxide mediated polymerization (NMP) of 1,3-butadiene was carried out in presence of TEPA as a polyfunctional nitroxide precursor. The multi-modal molar mass distribution of the product was elucidated by reductive hydrolyzation of the oligomeric product. The new polyfunctional hydroxyl-terminated polybutadiene (PFHTPB) was characterized and used in preparation of an HTPB-based polyurethane. The effects of this new hydroxyl-terminated polybutadiene on mechanical properties and thermal stability of its corresponding polyurethane elastomer were studied. The mechanical and thermal stability characterization of the elastomer were evaluated by tensile and TGA tests. Due to an efficient increase in crosslink density of the PFHTPB-based polyurethane elastomer, a significant enhancement in mechanical properties (more than 2.5 fold in tensile strength with small increase in elongation at break) was achieved. Furthermore the thermal stability of PFHTPB-based PU was comparable with its corresponding HTPB-based polyurethane elastomer.

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Tetrazoles are cyclic aromatic compounds that include four nitrogens and one carbon atom in one ring .These compounds have many applications in civil and military industries. These compounds can produce great amount of N2 after burning and for this reason they have a little environmental contamination and are considered as green explosive materials. Introduction of tetrazoles in polymeric chain structure can enhance the energetic properties of polymers. So far various methods were reported for the synthesis of polytetrazoles .In the most of them, tetrazoles linked to the main chain as suspension branches. By using the functionalized tetrazole rings as monomer, we can insert these rings into the main chain structure. By controlling and improving the mechanical properties of polytetrazoles, these polymers due to their special impulse and good termal stability, have the capability to substitute with some high energetic polymers such as GAP. In this paper, the synthesis of various tetrazole polymeric systems are investigated.

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In order to increasing of hydroxy functionality of HTPB, an in-situ nitroxide mediated polymerization (NMP) of 1,3-butadiene was carried out in presence of TEPA as a polyfunctional nitroxide precursor. The effects of this new hydroxyl-terminated polybutadiene on thermal and dynamic-mechanical properties of its corresponding polyurethane elastomer were studied by DSC and DMTA tests. A significant enhancement in crosslink density of the elastomeric product with small increase in glass transition temperature was achieved using PFHTPB. Also the absence of any endothermic peak in DSC thermogram was an indication of no crystalline domain existing in these samples.

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For improving the mechanical and thermal properties of polyurethanes based on glycidyl azide polymer (GAP), a series of polyurethane elastomers with two soft segments were prepared from hexamethylene diisocyanate (HDI) / (GAP) / (PPG) /TMP and isophorone diisocyanate (IPDI) / GAP/ PPG/ TMP formulation. The weight ratio of soft segments GAP/PPG in polyurethane samples is changed as following order: 0:100, 30:70 and 50:50. For the study the structure – property relationships of polyurethane samples, the average molecular weight between cross-link junctions and effective cross-link density (νe) of samples were calculated from swelling and dynamic mechanical analysis tests. Both tests have shown that, as the GAP contents of copolyurethane samples are increased, molecular weight between cross-links (Mc) is increased. Also, dynamic mechanical analysis results of polyurethane samples have shown that by increasing of GAP ratio in the soft segment of polyurethane, the storage modulus at elastic region decreased and the glass transition temperature (Tg) have increased. As well as, hardness of samples is measured and indicated that by increasing the GAP extent of polyurethane samples, the migration of soft segment to the surface of polyurethane is improved and causes the dropping of the sample hardness.

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Epoxy resins are the most important polymeric materials used in the preparation of thermal insulator in double base rocket propellants. In this work, Taguchi statistical method was used for the flexural strength investigation of epoxy resins (after curing). The effects of the type of resin, concentration of diluent (phr), type of curing agent and concentration of curing agent were investigated on the flexural strength and data given of ANOVA showed that the type of curing agent with 46.9 percent has the highest influence on the flexural strength of the epoxy insulator. The data given of ANOVA showed that the best flexural strength of 96.6 ± 13.3 MPa was obtained for the optimum conditions of MANA POX-102 as epoxy resin, H-35 as curing agent with 57 phr. In comparison, practical result of flexural strength with three replicate runs was obtained 92.3 ± 3.8 MPa. Finally, adhesion strength for optimal sample on the surface of a double base propellant by Pull-off test with three replicate runs was resulted as 2.2 ± 0.1 (n=3) MPa.

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The reversibility of the Diels-Alder reaction can be used advantageously in polyurethane materials to endow them with recyclable, thermally reversible properties and self-healing ability to expand their application range. In this paper, linear and crosslink polyurethanes based on HTPB, which exhibit thermal reversibility through Diels-Alder and retro Diels-Alder reactions, were prepared and characterized from reaction of bismaleimide and furan terminated polybutadiene. Also tetrafuran cross linker is synthesized and characterized by FT-IR and NMR for preparation of Diels-Alder network. Reversibility of Diels-Alder reaction was investigated through heating and cooling cycle by using differential scanning calorimetry (DSC). The results have shown that the retro Diels-Alder reaction is occurred at 150°C. Evaluation of healability of prepared polyurethane was done by cutting of polymer and recovering of its initial shape after reheating at 150°C.

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The mechanical properties of the final energetic composite depend mainly on the HTPB prepolymer microstructure; thus, the prepolymer microstructure of HTPB is one of the important factors influencing the mechanical properties of high-energy composite based on HTPB resin. In this paper, the effect of microstructure parameters of HTPB on the behavior and mechanical properties of its high-energy composite has been investigated. The difference in the production of these polymers, as well as their variability, leads to the formation of various polymers in the microstructure parameters of HTPB (molecular weight distribution and factor distribution). These differences often affect the processability, lifetime of the appliance, the behavior of cooking, and especially the mechanical properties of high-energy composites. Using various grades of HTPB has various microstructural parameters, in order to achieve the optimal mechanical properties with high bar resistance (with proper life jacket), according to the results, there is a need to understand the microstructure of the binder, namely, pre-polymer. The studies carried out in this paper show that with increasing molecular weight of HTPB, tensile strength, modulus, rupture resistance and hardness decrease, and by decreasing the high molecular weight component of HTPB, the exponential and high-energy composite modulus increases. There is a linear relationship between the amount of hydroxyl and the mechanical properties of the tensile strength and modulus.

 

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Nowadays solid propellant motors (SRMs) are widely used because of simple structure and reliability. One of the requirements in SRMs is high tensile and bonding strength of liner for integrity of grain structure and neutralizes stresses applying to motor. In this research, effect of curing agent ratio (NCO/OH) in the range of (R=0.9-1.4) on tensile strength of liner and bonding strength of liner-propellant was investigated. Results showed, increase in isocyanate which grows crosslinking, increase the tensile strength and decrease elongation at break. In addition, with increase of NCO content, the ability urethane linkage formation in liner - propellant interface grows, therefore bonding strength increases. Finally, with considering of design requirements, processability, costs and safety the optimized NCO/OH ratio was selected.

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Solid propellant systems are basically two types: case bonded and cartridge. Liner, a polymeric glue, is responsible for bonding propellant to casing in case bonded ones. Therefore, it must have sufficient mechanical and adhesive strength. The type and amount of each component of polymeric composition plays an important role in liner strength. In this research, effect of chain extender content (4-6 percent) and fillers (9-17 percent) on tensile strength of liner based on HTPB and bonding strength of liner-propellant was investigated. The result showed that with increase in amount of chain extender, bonding strength between liner and propellant, tensile strength and elongation at break liner was increased. In addition, it was found that increase in fillers content up to 15 percent improve tensile strength and elongation at break, whiles increase in fillers, decrease bonding strength through decrease in permeability of liner into propellant surface.