Carboxy-terminated Polybutadiene Research Articles

Analysis of viscoelastic behavior of a filled elastomer under action of different loads

Mechanical properties of viscoelastic filled polymers strongly depend on temperature and strain rate and vary for several orders of magnitude. During service life, a viscoelastic body, especially carboxy-terminated polybutadiene (CTPB) composite solid rocket propellant grain, is subjected to many stress-inducing loads. Its structural integrity analysis (hereafter: ?structural analysis?), unlike elastic bodies, is quite complex and sometimes impossible under the action of just a single load. An even greater problem occurs when multiple different types of loads act simultaneously. This study is based on a complete uniaxial mechanical characterization of a viscoelastic CTPB composite rocket propellant, made in MTI- -Belgrade, whose results were used for the analysis of the propellant grain reliability. Through an example, this paper shows a behavior of the viscoelastic propellant grain when it is subjected to extremely different environmental loads at the same time. Similar explicit examples are difficult to found in the literature, except in the form of recommended principles for analysis. It is shown that the tensile strength under the action of fast load due to the pressure may be almost 20 times greater than the tensile strength under the slow temperature load. A probabilistic approach is presented in evaluation the reliability and service life. An example is shown for a rocket propellant grain as a viscoelastic body. The presented principles of the analysis can be applied to any arbitrary viscoelastic body in other areas.

Failure analysis of rubber toughened epoxy resin

AbstractFailure of the blends of epoxy cresol novolac resin (ECN) with varied proportions of carboxy terminated polybutadiene (CTPB) liquid functional rubber was studied. Addition of CTPB improves toughness as reflected in the improvement of tensile, flexural, and impact properties. However, 10 wt % of CTPB was the optimum concentration beyond which a rapid fall of properties, in all cases, was observed. Surface topography of the fractured surfaces, studied by scanning electron microscopy and atomic force microscopy, revealed marked changes in the phase morphology due to addition of rubber and also accounted for the variation of the strength properties. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 861–868, 2003

Some Aspects of Time-Temperature Superposition Principle Applied for Predicting Mechanical Properties of Solid Rocket Propellants

Optimal test conditions for determining the mechanical properties of rocket propellants (temperatures and strain rates ranges) for delivering master curves were investigated. From master curves it is possible to predict the modulus, maximum stress and maximum strain in vide intervals of temperatures and strain rates, and especially the existing conditions during the ignition of rocket motor. Using the control experiments, at high strain rates, the good agreement between the results obtained from master curves was shown. The obtained results for composite rocket propellants (with carboxy-terminated polybutadiene, CTPB, as a binder), point out the drastic decreasing of maximum strain at high strain rates and low temperatures. Einige Gesichtspunkte der Zeit/Temperatur-Überlagerung zur Anwendung bei der Voraussage der mechanischen Eigenschaften von Raketenfesttreibstoffen Optimale Prüfbedingungen zur Bestimmung der mechanischen Eigenschaften von Raketentreibstoffen (Temperaturen und Dehnungsgeschwindigkeitsbereiche) zur Entwicklung von Masterkurven wurden untersucht. Aus Masterkurven ist es möglich, den E-Modul, die maximale Spannung und Dehnung in weiten Bereichen der Temperatur und Dehnungsgeschwindigkeit und besonders die Bedingungen während der Zündung von Raketenmotoren vorauszusagen. Bei Verwendung von Kontrollversuchen wurde gute Übereinstimmung bei hohen Dehnungsgeschwindigkeiten mit den erhaltenen Ergebnissen aus den Masterkurven erreicht. Die Ergebnisse bei Kompositraketentreibstoffen (mit carboxyterminierten Polybutadienen, CTPB, als Binder) zeigten eine drastische Abnahme der maximalen Dehnung bei hohen Dehnungsgeschwindigkeiten und niedrigen Temperaturen. Quelques apects de la superposition temps/température appliqués à la prédiction des propriétés mécaniques de propergols solides de fusée On a étudié des conditions de test optimales en vue de déterminer les propriétés mécaniques de propergols de fusée (température et domaine de vitesse de déformation) en vue de développer des courbes principales. Les courbes principales permettent de prédire le module d'élasticité, la tension et l'élongation maximales dans de vastes domaines de température et de vitesse de déformation et, en particulier, les conditions pendant l'amorcage de moteurs de fusée. En utilisant les expériences de contrôle, on a obtenu, à des vitesses de déformation élevées, un bon accord avec les résultats provenant des courbes principales. Les résultats obtenus pour les propergols de fusée composites (avec des polybutadiènes à terminaison carboxy, CTPB, en tant que liant) ont montré une forte réduction de la déformation maximale à vitesses de déformation élevées et faibles températures.

Wide-angle X-ray scattering, Fourier transform infrared spectroscopy, and scanning electron microscopy studies on the influence of the addition of liquid functional rubber into epoxy thermoset

Sophisticated analytical methods (viz. wide-angle X-ray scattering, Fourier transform infrared spectroscopy, and scanning electron microscopy have been applied to investigate the mechanism of toughening of epoxy cresol novolac resin due to the addition of carboxy-terminated polybutadiene (CTPB) liquid functional rubber. The average molecular interchain spacing (R) in Angstroms of neat epoxy and epoxy-rubber blends were calculated from the strong maximum in the diffraction scan using established equations. The half-width (HW) of the maximum was used to qualitatively describe the distribution of (R). An increase in (R) value signifies formation of a separate packing order, as well as an increase in the free volume which, however, varies with the extent of compatibilization between epoxy cresol novolac and CTPB. Fourier transform infrared studies convincingly establish the crosslinking between the oxirane group of epoxy and the carboxyl group of CTPB as reflected in the characteristic peak shifts in the blends, compared with individual polymers. The merger of several peaks of individual polymers, as well as the appearance of minor peaks elsewhere, were also evident. Scanning electron microscopy studies have also been undertaken to study the phase morphology development, as well as changes in the fracture surface topography with varied CTPB content.

Cure characterization of rubber modified epoxy novolacs

A new type of toughened epoxy polymer based on epoxy cresol novolac resin (ECN) and carboxy terminated polybutadiene (CTPB) liquid functional rubber has been studied. ECN has been synthesized in the laboratory and CTPB used was also of indigenised origin. Rubber modified epoxies were characterized with the help of differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and scanning electron microscopy (SEM) techniques.

Influence of structure of polymeric fuels on the combustion behaviour of composite solid propellants

This is a first report in which the influence of the structure of polymeric binders, such as molecular weight ( M), end-group functionality and cis-trans proportion, on the combustion behaviour of composite solid propellants (CSP) has been examined. In admixture with ammonium perchlorate (AP), dicarboxylic acids were used to simulate the commonly used telechelic polymers such as carboxy-terminated polybutadiene (CTPB) fuels. The burning rate ( r ̇ ) varies exponentially with the M of the acids and embraces the values for CTPB. The end-group effect diminishes at higher M due to dilution of the acid groups. The dependence of r ̇ on M is expressed as a newly defined molecular weight sensitivity of burning rate ( σ m). The σ m values can be divided into three regimes: I (118 < M < 150), II (150 < M < 200) and III ( M > 200) for polymeric fuels. The σ m values in regimes I and II are explained by the selective tendency for anhydride formation; in regime III, fragmentation of the polymeric fuel occurs. In regimes I and II, volatilization of the acids occurs at lower temperatures than for their neat form and in regime II this volatilization is limited by the phase transition of AP. The volatilization rate decreases with increase in M, but the ratio of the condensed-phase to gas-phase heat release increases with M.

Some Observations on the Ignition of Composite Solid Propellants

Heat-up times derived from studies on the ignition characteristics of a few model composite solid propellants, containing polystyrene, carboxy-terminated polybutadiene, plasticised polyvinyl chloride and polyphenol formaldehyde as binders, show that they are directly proportional to the mass of the sample and inversely proportional to the hear flux. Propellant weight-loss prior to ignition and high pressure ignition temperature data on the propellants, ammonium per chlorate, and binders show that the ignition is governed by the gasification of the binder pyrolysis products. The activation energy for the gasification of the pyrolysed polymer products corresponds to their ignition behaviour suggesting that propellant ignition is controlled by the binder.

A thermodynamic proposal for the exposition of criticality and other subtle features in self-deflagrating ammonium perchlorate systems

A first comprehensive investigation on the deflagration of ammonium perchlorate (AP) in the subcritical régime, below the low pressure deflagration limit (LPL, 2.03 MPa) christened as régime I', is discussed by using an elegant thermodynamic approach. In this régime, deflagration was effected by augmenting the initial temperature ( T 0 ) of the AP strand and by adding fuels like aliphatic dicarboxylic acids or polymers like carboxy terminated polybutadiene (CTPB). From this thermodynamic model, considering the dependence of burning rate ( ṙ ) on pressure ( P ) and T 0 , the true condensed ( E s,c ) and gas phase ( E s,g ) activation energies, just below and above the surface respectively, have been obtained and the data clearly distinguishes the deflagration mechanisms in régime I' and I (2.03–6.08 MPa). Substantial reduction in the E s,c of régime I', compared to that of régime I, is attributed to HClO 4 catalysed decomposition of AP. HClO 4 formation, which occurs only in régime I', promotes dent formation on the surface as revealed by the reflectance photomicrographs, in contrast to the smooth surface in régime I. The HClO 4 vapours, in régime I', also catalyse the gas phase reactions and thus bring down the E s,g too. The excess heat transferred on to the surface from the gas phase is used to melt AP and hence E s,c , in régime I, corresponds to the melt AP decomposition. It is consistent with the similar variation observed for both the melt layer thickness and ṙ as a function of P . Thermochemical calculations of the surface heat release support the thermodynamic model and reveal that the AP sublimation reduces the required critical exothermicity of 1108.8 kJ kg -1 at the surface. It accounts for the AP not sustaining combustion in the subcritical régime I'. Further support for the model comes from the temperature-time profiles of the combustion train of AP. The gas and condensed phase enthalpies, derived from the profile, give excellent agreement with those computed thermochemically. The σ p expressions derived from this model establish the mechanistic distinction of régime I' and I and thus lend support to the thermodynamic model. On comparing the deflagration of strand against powder AP, the proposed thermodynamic model correctly predicts that the total enthalpy of the condensed and gas phases remains unaltered. However, 16% of AP particles undergo buoyant lifting into the gas phase in the ‘free board region’ (FBR) and this renders the demarcation of the true surface difficult. It is found that T s lies in the FBR and due to this, in régime I', the E s,c of powder AP matches with the E s,g of the pellet. The model was extended to AP/dicarboxylic acids and AP/CTPB mixture. The condensed (∆ H 1 ) and gas phase (∆ H 2 ) enthalpies were obtained from the temperature profile analyses which fit well with those computed thermochemically. The ∆ H 1 of the AP/succinic acid mixture was found just at the threshold of sustaining combustion. Indeed the lower homologue malonic acid, as predicted, does not sustain combustion. In vaporizable fuels like sebacic acid the E s,c régime I', understandably, conforms to the AP decomposition. However, the E s,c in AP/CTPB system corresponds to the softening of the polymer which covers AP particles to promote extensive condensed phase reactions. The proposed thermodynamic model also satisfactorily explains certain unique features like intermittent, plateau and flameless combustion in AP/polymeric fuel systems.

Thermal degradation of functionally terminated polybutadienes: Pyrolysis gas chromatography and thermogravimetric studies

AbstractThermal degradation of functionally terminated liquid polybutadiene polymers, carboxy‐terminated polybutadiene (CTPB), hydroxy‐terminated polybutadiene (HTPB), and poly(butadieneacrylonitrile‐acrylic acid) terpolymer (PBAN) and their cured analogues were studied by pyrolysis gas chromatography in the temperature range 500–850°C. It was found that CTPB, HTPB, and PBAN could be distinguished from one another by the variation in the composition of their pyrolyzates. The differences in the product composition were rationalized by the relative importance of unzipping and hydrogen transfer reactions in the mechanisms of their degradation. The presence of the OH group was found to favor hydrogen transfer reactions whereas the COOH group has only marginal effect. The cyclization reactions were found to predominate in the thermal degradation of cured PBAN and the pyrolysis results were substantiated by thermogravimetric studies at several heating rates. The activation energy for the decomposition of cured PBAN was estimated to be 259 ± 16 kJ mol−1 by Ozawa's method.

Thermoanalytical studies on CTPB‐NH4ClO4 composite solid rocket propellants

AbstractPremature reignition of controllable solid propellant motors, a factor affecting the extinguishment reliability, has been attributed to the first exotherm of NH4ClO4. Detailed studies were made(1) on suppressing the first exotherm of NH4ClO4 with the use of materials like NH4BF4, NH4PF6, (NH4)2TiF6 etc. The present paper deals with the thermal decomposition kinetics of carboxy‐terminated polybutadiene (CTPB)‐NH4ClO4 propellant with NH4BF4 as an additive to suppress the first exotherm of NH4ClO4.The techniques employed in this study include: differential scanning calorimetry (DSC) for the thermal decomposition studies, and strand‐burner for the measurement of propellant burning rates. DSC experiments were conducted on the propellants at pressures of ambient, 17 kg/cm2 and 40.82 kg/cm2; at a constant heating rate under an inert atmosphere of nitrogen. The burning rate of the propellant samples were determined in the pressure range of ambient to 70 kg/cm2 in an atmosphere of nitrogen.The burning rates were found to increase with pressure at each of the concentrations of the additive. The decrease in burning rate with the increase in concentration of the additive, at pressures of 35 kg/cm2 and below is attributed to the excess of ammonia formed during the decomposition of both NH4ClO4 and NH4BF4. At pressures above 35 kg/cm2, the increase in burning rate with the increase in concentration of the additive is attributed to the increased concentration of the metal ion (boron) in NH4BF4. These observations are in agreement with those of Mayer(1) et al., and Glaskova(2).

Aging studies on carboxy terminated polybutadiene (CTPB) binder and propellant

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Curing reactions in elastomers. II. Carboxy‐terminated polybutadiene

AbstractAttempts have been made to understand the curing reactions in carboxy‐terminated polybutadiene (CTPB), which happens to be the most practical binder in advanced solid composite propellants. The curing of CTPB has been studied for different ratios of curing agents (MAPO and Epoxide) by gel content, molecular weight, crosslink density, and penetration temperature measurements, and the optimum composition of curators for effective curing of CTPB has been determined. Activation energy calculations on the curing of CTPB with 9.5% epoxide and 0.5% MAPO in the temperature range 75–100°C gave 14.1 kcal/mol for which a diffusion‐controlled or acid‐catalyzed epoxide ring opening mechanism has been suggested for the curing process in CTPB.

Characterization of two Nitrile-epoxy Structural Adhesives

Abstract High pressure liquid and gel permeation chromatography, together with infrared studies on particular chromatography fractions, has been used to study two nitrile-epoxy adhesives. One of these materials has been available for some fifteen years while the other is a more modern development. A detailed consideration of the distribution of epoxy oligomers has indicated a change from taffy to fusion process DGEBA resins in recent times. Study of the acrylonitrile content of high molecular weight fractions has shown that whereas the older adhesive only contains a carboxylated nitrile rubber, the newer system also contains some carboxy-terminated polybutadiene.

Microbial susceptibility of condensation products of carboxy-terminated polybutadiene and glycerol

Keeping in view the prospects of biodegradable polymers, a polymer was synthesized by the condensation of carboxy-terminated polybutadiene (CTPB) of Mnsim-5000 with glycerol and tested for its microbial susceptibility. The results of end group estimations and viscosity measurements indicated a quantitative reaction between the two reactants under experimental conditions. The clear-zone method was employed in this investigation to test biodegradability. Two strains of Serratia and three strains of Staphylococcus did show a clear zone surrounding the colony. However, the microbial growth was found to diminish after 4 or 5 days.

Calorimetric values of composite solid propellants

The calorimetric values of composite solid propellant based on polystyrene, polyphenolformaldehyde, poly(vinyl chloride) and carboxy-terminated polybutadiene were determined using combustion calorimetry in order to assess the uncertainities in their measurements. The dependence of the calorimetric values on various propellant composition was obtained. The stoichiometry of oxidizer and fuel in the propellant for complete combustion obtained experimentally were compared with the theoretical stoichiometry calculated based on the oxidizer decomposition.

Thermogravimetric and mass‐spectrometric study of the thermal decomposition of PBCT resins

AbstractThe thermal decomposition of three commercial samples of carboxy‐terminated polybutadiene (PBCT) resins was studied by thermogravimetric analysis (TGA) at heating rates varying from 2° to 100°C/min. Kinetic parameters of the decomposition process at different heating rates were evaluated by means of the Fuoss method.1 The decomposition process and the activation energy values are found to be dependent on heating rate. Mass‐spectrometric analysis of the decomposition products shows that the pyrolysis products of PBCT resins are mainly low molecular weight hydrocarbons: ethylene, acetylene, butadiene, propadiene, vinylcyclohexene, etc. The rates of evolution of these hydrocarbon products vary with the carboxy content of the PBCT resin. Based on this, a carbonium ion mechanism has been suggested for the thermal decomposition. The data generated from this work are of importance for a consideration of the mechanism of combustion of composite solid propellants based on PBCT binders.

The effect of metal oxides on the oxidation of polybutadiene

Carboxy-terminated polybutadiene (CTPB) was oxidised as a coating on various metal oxides by heating in air on a thermobalance. The oxidation was found to be free from diffusion control and was catalysed by certain of the oxides. The catalysis was shown to be independent of semiconductor type and to be associated with those oxides for which the width of the forbidden zone between the valence and conduction bands (U) is less than 1.9 eV. In general, low values of U result in high catalytic activity. It is proposed that catalysis occurs by a redox mechanism, viz. RO 2H + p → RO 2 . + H +, RO 2H + e → RO . + OH −.

The thermal degradation and oxidation of polybutadiene

Detailed studies have been made of the successive stages in the thermal degradation in air and nitrogen of carboxy-terminated polybutadiene (CTPB). During oxidation at high temperatures, a protective surface film is first formed; this film ruptures at temperatures where pyrolysis leads to the formation of volatile products in the bulk of the polymer. Thermogravimetric curves for the degradation of CTPB in nitrogen are complex in shape; it appears that the free-radical crosslinking and cyclisation reactions cause an increase in the thermal stability of the polymer during degradation.

Effects of copper chromite and iron oxide catalystson AP/CTPB sandwiches

Results are presented of an experimental motion-picture study designed to provide observations of catalyzed composite solid-propellant combustion. Various methods of catalyst addition were incorporated into two-dimensional sandwiches, composed of pressed ammonium perchlorate (AP) oxidizer sections laminated together with a thin layer of carboxy terminated polybutadiene (CTPB) binder. Catalysts investigated were harshaw Catalyst Cu 0202 (copper chromite—CC) and Fe 2 O 3 (iron II oxide—IO). The sandwiches were burned in a window bomb at pressures of 600 and 2000 psig. Cinephotomacrography, at a frame rate of 1600 pictures per second and magnification of 2:1, was used. Copper chromite was found to be a more effective catalyst than IO for AP/CTPB sandwich systems. Copper chromite appeared to: augment the AP-deflagration process; possibly catalyze gas-phase reactions; possibly promote gas-phase reactions in crevices between the solid fuel and solid AP (at the interface); not promote heterogeneous reactions of gases with the solid-fuel binder; not modify the pyrolysis mechanism of the solid-fuel binder. Iron oxide was found to have effects qualitatively similar to CC, with the exception that it inhibited the AP-deflagration process at 600 psig. With the exception of IO at low pressures (600 psig), the addition of CC and IO in the AP was found to be the most effective method of increasing sandwich burn rate; their addition at the AP/binder interface was less effective; and their addition in the binder did not appear to be very effective at all, at least in the sandwich configuration. These findings, concerning CC and IO catalysts, indicate that some of the theories concerning the catalytic mechanisms of CC and IO with AP/binder composite propellants are justified, whereas others are not.

Applied Research and Development of Adhesives for Bonding Filled Carboxyl Terminated Polybutadienes to Various Substrates

Abstract A one-year applied research and development program was conducted on the bonding of carboxy-terminated polybutadiene (CTPB) propellant to various substrate materials encountered in solid propellant rocket motors. Under this program, in addition to CTPB liners, liners were also prepared from polyesters, polyethers, polyurethanes, polyacetal polymers, and epoxy resins. The use of various crosslinkers, emulsifiers, wetting agents, fillers, and stabilizers was also evaluated. Four optimized liner formuations with the best all-round properties were fully characterized. The optimized formulations represented an HC liner formulation with two Ievels of glycerol additive, an HC formulation with a sorbitol additive, and a Butvar polyacetal-type liner. A standard HC-polymer liner formulation, designated as TL-H-304, was used as a control. Unaged liner peel and shear properties were measured at -65°F, 77°F, and 160°F. Samples, aged for 30 days at 160°F, were tested at 77°F only. The liners were tested against propellant, steel, aluminum, magnesium, titanium, epoxy-fiberglass, phenolics, polyisoprene, and butadiene-acrylonitrile as substrate materials. The steel, titanium, and polyisoprene rubber substrates gave the best adhesive results. The substitution of asbestos and Cab-O-Sil for the Thermax filler in the liner gave comparable adhesive results while the substitution of clay fillers gave poor results. This program was performed while at the Elkton Division of the Thiokol Chemical Corporation in fulfillment of the requirement of Contract N123 (60530–53329A) U.S. NAVAL ORDNANCE TEST STATION, China Lake, California, reported previously in U.S. Naval Report NOTS-TP4283.