Structure-property relationship of HTPB-based propellants. II. Formulation tailoring for better mechanical properties

There has been a constant endeavor to improve the mechanical properties of hydroxylterminated polybutadiene (HTPB) -based composite solid propellants. A systematic study has been conducted on different batches of HTPB resins with varying molecular weights and hydroxyl values. Propellant formulation experiments were conducted wherein the ratio of chain extender to crosslinker was systematically varied, with a view to achieve the maximum possible strain capability and moderately high tensile strength, keeping all other parameters constant. The influence of increasing hydroxyl content from trimethylolpropane at the expense of hydroxyl content from butanediol, on the mechanical properties of the finished propellant, has been depicted on 3-dimensional graphs. The isoproperty lines, plotted as a triangular chart with the percentage hydroxyl contents from the three constituents, can be used to arrive at the suitable formulation for a specified application depending upon the OH value of the resin. HTPB resins with high molecular weight, low functionality, and low hydroxyl value require higher levels of trifunctional curing agent and higher NCO / OH ratios to obtain outstanding mechanical properties, especially elastic properties, compared to low molecular weight, high functionality resins. The impact of hard and soft segment domain structure on the mechanical behavior of the cured systems is more pronounced in the low molecular weight resin formulations due to the higher hard segment content compared to those attainable in high molecular weight resin formulations. © 1993 John Wiley & Sons, Inc.     

HTPB-based polyurethanes for inhibition of composite propellants

Polyurethanes were synthesized by the reaction of hydroxyterminated polybutadiene (HTPB) and diisocyanates such as toluene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), hexamethylene diisocyanate (HMDI), and isophoron diisocyanate (IPDI). A number of fillers such as carbon black, kaolin, and antimony trioxide (Sb₂O₃) were used in these formulations. Mechanical and thermal stability of these polyurethanes were studied. Based on its properties, HTPB-MDI derived filled polyurethane was selected and evaluated as an inhibitor for a composite propellant. © 1995 John Wiley & Sons, Inc.     

Structure–property relationship of HTPB-based propellants. III. Optimization trials with varying levels of diol–triol contents

A systematic study has been conducted on a composite solid propellant formulation using hydroxyl-terminated polybutadiene (HTPB) prepolymer with varying molecular weights and hydroxyl values. Fairly extensive regions of resin parameters have been studied. Contours of important propellant properties have been laid down. In this set of experiments, varying levels of diol and triol contents were used at two different NCO/OH ratios to arrive at the optimum level needed for different grades of HTPB resin. It is seen that different grades of HTPB resin require varying levels of diol–triol contents to give similar properties for the end product. Also, for the best performance, varying the diol–triol ratio at the optimum level of the diol–triol content is necessary. © 1994 John Wiley & Sons, Inc.     

HTPB-based polyurethanes for inhibition of composite-modified double-base (CMDB) propellants

Polyurethanes were synthesized by the reaction of hydroxy-terminated poly-butadiene (HTPB) and diisocyanates such as toluene diisocyanate (TDI), diphenyl-methane diisocyanate (MDI), hexamethylene diisocyanate (HMDI), and isophorone diisocyanate (IPDI). Carbon black and antimonytrioxide (Sb₂O₃) were incorporated into these formulations as fillers. Dioctyladipate (DOA) and trimethyllolpropane (TMP) were also added as a plasticizer and crosslinker, respectively. These polyurethanes were investigated as inhibitors for composite-modified double-base (CMDB) propellants. Due to superior mechanical properties, thermal properties, and low nitroglycerine absorption, HTPB–MDI–TMP-derived filled polyurethane was selected and evaluated as an inhibitor for a CMDB propellant. © 1997 John Wiley & Sons, Inc. J Appl Polm Sci 65:355–363, 1997     

Radiation curing of AP/HTPB-based energetic composites. I. Effect on physico-mechanical properties

The use of gamma radiation has been successfully demonstrated as a powerful tool for curing of formulations of ammonium perchlorate and hydroxy-terminated polybutadiene. This is not possible by conventional methods. The porosity observed in the composite is attributed to the shrinkage of matrix localized around surface of AP particles which creates voids in the bulk. Occurence of shrinkage is in the interphase zone and is due to excessive crosslinking of polybutadiene caused by radiolytic products of AP. Incorporation of toluene diisocyanate and ferric acetyl acetonate showed loss of radiation induced porosity and increase in density, tensile strength and elongation.     

Modeling and rheology of HTPB based composite solid propellants

The propellant with the minimum viscosity required for a defect-free casting can be obtained by proper selection of the size and fractions of solid components leading to maximum packing density. Furnas' model was used to predict the particulate composition for the maximum packing density. Components with certain size dispersions were combined to yield a size distribution that is closest to the optimum one given by Furnas for maximum packing. The closeness of the calculated size distribution to the optimum one was tested by using the least square technique. The results obtained were experimentally confirmed by viscosity measurement of uncured propellants having HTPB binder and trimodal solid part accordingly prepared by using aluminum (volumetric mean particle diameter of 10.4 μm) and ammonium perchlorate with four different sizes (volumetric mean particle diameters: 9.22, 31.4, 171, and 323 μm). The experimental measurements showed that the compositions for the minimum viscosity are in good agreement with those predicted by using the model for maximum packing. The propellant consisting of particles with mean diameters of 10.4, 31.4, and 323 μm was found to yield the minimum viscosity. This minimum viscosity was observed when the fraction of the sizes with respect to total solids was 0.141, 0.300, and 0.559, respectively.     

Thermoanalytical Investigations on the Effect of Atmospheric Oxygen on HTPB resin

TG-DSC studies, carried out on hydroxy-terminated polybutadiene (HTPB) in air and nitrogen atmospheres show three transitions which give rise to (1) an exothermic DSC peak and mass gain in TG at 170°C–240°C, seen only in air (2) exothermic peak and low mass loss, both in air and nitrogen, due to depolymerization, cyclization and cross linking, and (3) final mass loss corresponding to pyrolysis in nitrogen and combustion in air, appearing respectively as an endothermic peak and as a sharp exothermic peak in the two atmospheres. The FTIR spectrum of the product isolated from TG after the mass gain step shows addition of oxygen to the butadiene back bone. Arrhenius activation parameters (E and A) were computed for the exothermic oxygen addition reaction. The Ozawa method refined by MKN two-term approximation for p(x) function gave results quite comparable to those from Kissinger method.     

Cooking schedule of alkyd resin preparation. I. Effect of cooking schedule on viscosity-molecular weight relationship

The two constants in the equation log η = A + C′ M?_n^1/2 (η is the viscosity of molten alkyd and M?_n the number average molecular weight) were determined at 110°C for two kinds of alkyd resin prepared with the same formulation but with different cooking schedules. It was found that the magnitude of the slope C′ was larger for the alkyd which was prepared by raising the reaction temperature directly up to 230°C, in comparison with that of the alkyd which was prepared by maintaining the temperature at 170°C for an hour and then raising it up to 230°C. Measurements of η and M?_n were carried out until the gelation occurred. Both upward and downward breaks were observed in log η vs. M?_n^1/2 plot. Based on these viscometric data, the gel point mechanism was discussed. Disagreements between the molecular weight observed and that calculated from the Flory's theoretical equation became more remarkable as the esterification proceeded. This suggested that a large extent of intraesterification reaction is taking place in alkyd synthesis.     

Estimation of ammonium perchlorate in HTPB based composite solid propellants using Kjeldahl method

Kjeldahl method has been applied to determine the percentage of ammonium perchlorate (AP) in hydroxy-terminated polybutadiene (HTPB) based composite solid propellants. It has been found that in pastes containing aluminium (Al) it is important to eliminate all the Al before starting the actual reaction (distillation). The results obtained are in the range of ±0.05% for pastes with and without Al whereas the range is ±0.02% for pure AP. End points are confirmed using pH meter.     

Structure-property relationships in thermoplastic elastomers: I. Segmented polyether-polyurethanes

Segmented poly(ether-b-urethanes) have been synthesized with 2000 MW polypropylene oxide coupled with diisocyanates and diol type chain extenders. The diisocyanates used were symmetric rigid 4, 4′-diphenylmethane diisocyanate (MDI), linear aliphatic hexamethylene diisocyanate (HDI), and unsymmetric rigid toluene-2, 4-diisocyanate (TDI). The chain extenders were symmetric N, N′-bis(2-hydroxyethyl) terephthalamide (BT) and N, N′-bis(2-hydroxyethyl)-hydroquinone (BH) unsymmetric N, N′-bis(2-hydroxyethyl)isophthalamide, and linear aliphatic 1, 4-butanediol (B). Hard segment contents ranged from 20 to 40 wt percent. The thermal behavior of these materials is consistent with phase separation into separate hard and soft domains, In order of increasing temperature above the soft segment T_g, there are transitions which occur in the regions ?56 to ?36°C (T_a), 70 to 90°C (T_b), and 138 to 168°C (T_m). The former is probably associated with soft segment change from a viscoelastic to an elastomeric state. Values of T_a are ～ ?51 C and ?56°C for the MDI-BT and HDI-BT polymers, respectively, and are independent of hard segment content. Microscopy showed that the former polymers have spherulitic morphology, so these materials have good microphase separation and exhibit crosslinked elastomeric properties. The TDI-BT or BI and MDI-B polyurethane have composition-independent T_a values of ?41 and ?36°C, respectively. These materials probably have considerable “domain-bound-ary-mixing”. At low hard segment content the MDI-B polymers behave as non-crosslinked elastomers. Only the MDI-BI polymers have _Ta values, which are strongly affected by composition, increasing in magnitude with increasing of hard segment content. This is interpreted as significant “mixing-in-domains” and is supported by morphology observed by microscopy. The next higher transition, T_b, probably involves dissociation of interdomain hydrogen bonding. In the case of the MDI-BT polyurethanes, the spherulites associated with the hard domains had disappeared at 141°C and the few small spherulites in the MDI-BI polymers disappeared at 130°C. The T_b values are 70, 83 to 90, and 100°C for the MDI-B, HDI-BT, and HDI-BI polymers, respectively. The melting transitions occurred between 138 to 168°C for the various polyurethanes except for the MDI-BT systems which decompose before melting. Thermal decomposition is a two-stage process. Hard segments decompose between 200 and 300°C. The initial decomposition temperatures are lowered in the presence of strong acid. Soft segments decompose at higher temperatures. The mechanical properties of the MDI-BI polyurethanes are charateristic of crosslinked elastomer, the results of which will be presented in a subsequent paper.     

Effect of reactivity of different types of hydroxyl groups of HTPB on mechanical properties of the cured product

The amount of three types of hydroxyl functional groups (G_OH, H_OH, and V_OH) in hydroxyl terminated polybutadiene (HTPB), remaining in samples acetylated to the same extent under two different conditions, viz., fast acetylation using acetyl chloride in presence of N-methyl imidazole catalyst and slow acetylation by acetic anhydride, differed significantly. For the fast acetylation there is a uniform reduction in all the three types of hydroxyls, probably because the reaction becomes random at rapid rates and the reactivity of the different types of hydroxyls does not play a major role. However, in the slow reaction, the reduction of G-type hydroxyls was 30% more than the expected value and there was a corresponding increase in the amount of V-type hydroxyls remaining in the acetylated product, showing reactivity of OH in the order of G > H > V. When the reaction is slow, it becomes selective and the change in reactivity of the three types of OH groups is reflected in the extent of conversion. The mechanical properties and the crosslink density data show a reduction in the samples containing lesser amounts of G_OH, confirming the branching nature of G_OH, which is involved in the crosslinking reaction. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 1313–1320, 1997     

Curing reactions in plastic prepolymers and propellants. I. Polystyrene

Curing reactions of the viscous PS prepolymer and PS/AP propellant slurry have been studied. The molecular weight of the binder (separated from the propellant) and the prepolymer was found to increase to a maximum value, remain constant for some time, and then fall off between 50–125°C. The molecular weight of the binder was found to be less than corresponding prepolymer between 100–150°C but at lower temperatures (50–75°C) the reverse was found to be true. The increase in the molecular weight during curing at lower temperatures has been explained on the basis of Trommsdorff effect which gets support from the estimated activation energy (9 kcal mole^?1) for the curing process. Curing was recognized as chain extension where the rate of polymerization becomes diffusion controlled below 75° C.     

羟基硅油改性HTPB型聚氨酯的合成与性能

以端羟基聚丁二烯(HTPB)为软段、异佛尔酮二异氰酸酯(IPDI)和3,3′-二氯-4,4′-二苯基甲烷二胺(MOCA)为硬段、羟基硅油(PDMS-OH)为改性剂和硅烷偶联剂(DB-550)为交联剂,合成了硅氧烷封端的HTPB型聚氨酯(PU)。采用L9(34)正交试验法优选出最佳工艺条件,探讨了PDMS-OH用量对改性PU的力学性能、耐水性能的影响,并对改性PU的耐老化性能进行了测定。结果表明:加入DB-550后,可以降低相分离程度;当R=2.5、w(HO-PDMS)=9%和n(MOCA):n(DB-550)=9:1时,采用直接法合成的改性PU具有优异的力学性能,并且其吸水率较低、耐老化性能较好。     

Rheology of HTPB propellant. I. Effect of solid loading,oxidizer particle size,and aluminum content

Composite solid propellants based on hydroxyl-terminated polybutadiene (HTPB) have become the workhorse propellants in the present-day solid rocket motors. The other major ingredients of a composite propellant are the crystalline oxidizer and metallic fuel. As the solid loading of such propellants is as high as 86–90%, their rheological behavior is very complex. The propellant slurry needs to have reasonably low viscosity and a long pot life for better casting and, hence, for a defect-free rocket motor. The primary factors affecting the solid propellant viscosity are solid content, particle size, shape, and distribution. The present study concerns the variations of solid loading from 80 to 89% at constant aluminum cotent, variation of aluminium from 0 to 22% at constant solid loading, and the coarse-to-fine ratio of the oxidizer. The plots of yield stress, consistency index, pseudoplasticity index, and thixotropic index at different time intervals are drawn for all these parametric changes. Based on these rheological studies, the optimum ratio of oxidizer coarse-to-fine ratio, aluminum content, and level of solid loading have been determined.     

水性羟基丙烯酸树脂的制备与研究

以甲基丙烯酸甲酯(MMA)、苯乙烯(St)和丙烯酸丁酯(BA)等为主要单体,引入丙烯酸(AA)、丙烯酸羟基乙酯(HEA)与甲基丙烯酸异冰片酯(IBOMA)等作为功能单体,通过半连续溶液聚合工艺,最后加水分散制得水性羟基丙烯酸树脂。利用FT-IR、透光度、粘度分析研究了单体配比、引发剂(BPO)用量、温度、链转移剂(DDM)用量、功能单体用量等因素对树脂性能的影响。结果表明,当AA、HEA、IBOMA、BPO和DDM的质量分数分别为3%、12%、10%、3%和2%,聚合反应温度100℃时可获得粘度为5 Pa.s,固含量约45%的水性羟基丙烯酸树脂。     

活性炭的微结构与超级电容器性能的构效关系

以生物质柞木为原料,采用不同活化法制备具有不同结构特征的柞木基活性炭,利用N₂吸附、FT-IR、XPS、XRD、Raman光谱等表征手段对活性炭的微结构特性进行解析,探究活化方式对活性炭微结构性能的影响;微结构与超级电容器性能的构效关系。研究表明： KOH和H₃PO₄-KOH法制备的活性炭微孔发达,炭结构表面缺陷位与杂原子丰富,在低电流密度下表现出更高的比电容;H₃PO₄-KOH法制备的活性炭具备更宽的微介孔分布与孔道连通性,使其具有更好的电容保持率;CO₂、H₃PO₄和H₃PO₄-CO₂法制备的活性炭介孔发达,微孔体积小,孔道连通性差,炭结构相对完整,裸露于炭结构表面的缺陷与杂原子相对较少,尽管电容保持率较高,但比电容较低。因此,高性能的超级电容器活性炭电极应具有发达的微孔结构、较宽的微介孔分布、通畅的微介孔连通结构,同时含有更多的裸露于炭结构表面的结构缺陷与杂原子基团,从而提高超级电容器的能量密度。     

Effect of the physical state of the activators on the combustion of composite rocket propellants. I. The applied catalysts

The effects on the burning and thermal decomposition of composite rocket propellants, based on ammonium perchlorate and butyl rubber, of oxide coated catalysts applied to tge surface of the ammonium perchlorate crystals and introduced into the propellant in the form of a colloidal suspension are investigated. It is shown that the possibility of changing the burning rate by means of applying the catalyst on the oxidizer crystal surface is determined by the chemical nature, the content of the compounds deposited on the oxidizer surface, and by the structure of the coating formed on the ammonium perchlorate surface. Excluding the agglomeration of the catalytic additives using the developed methods, the variation in their dispersivity and the nature of localization in the propellant are the indicators of the propellant's performance efficiency within the region of small additive concentrations (up to 0.5%) in the propellant. Scientific Research Institute for Physicochemical Problems, Belorus State Univ., 220050 Minsk. Translated from Fizika Goreniya i Vzryva, Vol. 31, No. 6, pp. 82–88, November–December, 1995.     

超低分子质量羟基丙烯酸酯树脂合成方法研究

通过自由基聚合反应制备得到了羟基丙烯酸酯树脂,探讨了聚合时间、聚合温度、引发剂、反应溶剂等影响因素对分子质量、分子质量分布和固含量的影响。通过优化合成条件,制备得到了超低分子质量的羟基丙烯酸酯树脂,其数均相对分子质量小于4 000,分子质量分布≤3.0,高固低黏特征显著。     

Viscoelastic properties of epoxy resin. I. Effect of prepolymer structure on viscoelastic properties

The intermolecular cyclization reaction is investigated in highly crosslinked epoxy systems, where diepoxides with different mobilities between terminal epoxy groups were crosslinked with ethylene diamine. Based on the measured values of the Clash-Berg 10-sec modulus in the rubbery region, the correlation between the mobility and the cyclization reactivity of the diepoxides is discussed. The epoxide with higher mobility is found to have a higher rubbery modulus than that with lower mobility, as was expected. This is tentatively explained by the difference in the reactivity of the formation of the 11-membered ring. Dynamic mechanical measurements were also run on a forced vibration apparatus. The higher β-transition peak of the polymer of bisphenol-A diglycidyl ether was interpreted in terms of its higher free volume as well as lower density and lower glassy modulus. The higher modulus in the glassy region of ethylene glycol diglycidyl ether–ethylenediamine was explained on the basis of hydrogen bonding.     

Structure-property relationship of thermotropic liquid-crystalline vinyl polymers containing no traditional mesogen

A series of non-mesogenic vinyl monomers, 2,5-bis(alkoxycarbonyl)styrene, were synthesized and polymerized via free radical polymerization. The alkoxy groups were systematically varied to investigate the effects of their size and architecture on the thermotropic liquid-crystalline properties of the resultant polymers. Although no traditional mesogen was present in the macromolecular structure, all the polymers revealed stable hexagonal columnar liquid-crystalline phase at the temperatures well above their glass transitions when the molecular weights were high enough, as evidenced by a combinatory analysis of differential scanning calorimetry, polarized light microscopy, and one- and two-dimensional wide-angle X-ray diffraction techniques. For the polymers containing five carbon atoms in the alkoxy terminals, the temperatures of glass transition and mesophase formation decreased and the d-spacing value of mesophase increased as the methyl substituent moved away from the connecting phenyl ring. Increasing the number of methyl group or the size of the substituent had the same effect. In the case of polymers with Y-shaped alkoxy terminals, the larger the side groups, the lower the glass-transition temperature and mesophase formation temperature and the larger the d-spacing value of mesomorphic structure.