Curing behavior of polyurethane as a binder for polymer-bonded explosives

Influence of Fluorinated Polyurethane Binder on the Agglomeration Behaviors of Aluminized Propellants

In this study, fluorinated polyurethane (FPU) was prepared from dialcohol-terminated perfluoropolyether as a soft segment; isophorone diisocyanate (IPDI) as a curing agent; 1,2,4-butanetriol (BT) as a crosslinker; and 1,4-butanediol (BDO) as a chain extender. Fourier transform infrared spectroscopy (FTIR) and ¹H NMR were used to characterize the structure of the FPU. The mechanical properties of the FPUs with different BDO and BT contents were also measured. The tensile strength and breaking elongation of the optimized FPU formula were 3.7 MPa and 412%, respectively. To find out the action mechanism of FPU on Al, FPU/Al was prepared by adding Al directly to FPU. The thermal decomposition of the FPU and FPU/Al was studied and compared by simultaneous differential scanning calorimetry-thermogravimetry-mass spectrometry (DSC-TG-MS). It was found that FPU can enhance the oxidation of Al by altering the oxide-shell properties. The combustion performance of the FPU propellant, compared with the corresponding hydroxyl-terminated polyether (HTPE)-based polyurethane (HPU) propellant, was recorded by a high-speed video camera. The FPU propellants were found to produce smaller agglomerates due to the generation of AlF₃ in the combustion process. These findings show that FPU may be a useful binder for tuning the agglomeration and reducing two-phase flow losses of aluminized propellants.

Analytical Methods for the Assessment of Curing Kinetics of Polyurethane Binders for High-Energy Composites

Polyurethane (PU) elastomers are largely utilized in the field of high-energy composites such as polymer-bonded explosives (PBXs) and solid rocket grains, due to their distinguished characteristics. Curing kinetics assessment is a crucial parameter to take into account to comprehend the processes to develop new high-energy composites. A comprehensive analytical characterization of curing kinetics is of fundamental importance to produce PU polymers with the most suitable and attractive properties. Moreover, to attain the optimal curing conditions, accurate analytical techniques, and strategies are essential to effectively evaluate their kinetic properties. This paper gives an overview on experimental tools, which can be used for a convenient analysis of kinetic behavior of these binders. The employment of each tool is showed and discussed by appropriate examples from the literature.

Curing Reaction Kinetics of the EHTPB-Based PBX Binder System and Its Mechanical Properties

In this research, differential scanning calorimetry (DSC) was employed to compare the curing reaction kinetics of the epoxidized hydroxyl terminated polybutadiene-isophorone diisocyanate (EHTPB-IPDI) and hydroxyl terminated polybutadiene-isophorone diisocyanate (HTPB-IPDI) binder systems. Glass transition temperature (Tg) and mechanical properties of the EHTPB-IPDI and HTPB-IPDI binder systems were determined using the DSC method and a universal testing machine, respectively. For the EHTPB-IPDI binder system, the change of viscosity during the curing process in the presence of dibutyltin silicate (DBTDL) and tin 2-ethylhexanoate (TECH) catalysts was studied, and the activation energy was estimated. The results show that the activation energies (Ea) of the curing reaction of the EHTPB-IPDI and HTPB-IPDI binder systems are 53.8 and 59.1 kJ·mol−1, respectively. While their average initial curing temperatures of the two systems are 178.2 and 189.5 °C, respectively. The EHTPB-IPDI binder system exhibits a higher reactivity. Compared with the HTPB-IPDI binder system, the Tg of the EHTPB-IPDI binder system is increased by 5 °C. Its tensile strength and tear strength are increased by 12% and 17%, respectively, while its elongation at break is reduced by 10%. Epoxy groups and isocyanates react to form oxazolidinones, thereby improving the mechanical properties and thermal stability of polyurethane materials. These differences indicate that the EHTPB-IPDI binder system has better thermal stability and mechanical properties. During the EHTPB-IPDI binder system’s curing process, the DBTDL catalyst may ensure a higher viscosity growth rate, indicating a better catalytic effect, consistent with the prediction results obtained using the non-isothermal kinetic analysis method.

Cure Kinetics of an Optical Polythiourethane with Amine Catalyst by IR Analysis

An optical polythiourethane based on m-xylylene diisocyanate (XDI) and 4-mercaptomethyl-3,6-dithia-1,8-octanedithiol (BES) has been studied. Triethylamine was adopted as a catalyst, and the solid-state isothermal cure reaction was carried out using FTIR spectroscopy, in the temperature range of 75°C-105°C. The -NCO absorption band of XDI was used to monitor the conversion of diisocyanate into polythiourethane. The reaction rate enhanced with an increase in the content of the catalyst, and the gel time determined by swelling test was shorter for the system with higher catalyst content. Kinetic parameters were calculated from the infrared spectrum data, and the results showed that the curing reaction of polythiourethane accords with first-order kinetic characteristics. The activation parameters obtained from the evaluation of kinetic data were △H∗=97.22 kJ mol−1, △S∗=−6.77 J K−1 mol−1, and Ea=100.23 kJ mol−1. The observed negative entropy of activation value supported the formation of a transition state in the cure reaction.

Poly(styrene)-supported N-heterocyclic carbene coordinated iron chloride as a catalyst for delayed polyurethane polymerization

An advanced organometallic catalyst based on N-heterocyclic carbene (NHC) coordinated FeCl3 has been synthesized and used to control the reaction rate in polyurethane (PUR) polymerization. The imidazolium (Im)-based NHC was functionalized on the surface of the supporting material of bead-type chloromethyl polystyrene (PS) resin. The PS-Im-FeCl3 catalyst was synthesized through the coordination reaction between Im and FeCl3. The successful formation, functional groups, structure, and geometry of the PS-Im-FeCl3 catalysts were confirmed by Fourier transform infrared and X-ray photoelectron spectroscopy techniques. A thin layer of Im was observed to be coated uniformly on the PS bead surface and FeCl3 nanoparticles were observed to cover the coating layer homogeneously, as determined by field-emission scanning electron microscopy, transmission electron microscopy, and energy dispersive X-ray spectroscopy measurements. The PUR polymerization reaction was investigated through viscosity measurements and non-isothermal activation energy calculations by differential scanning calorimetry analysis. Based on the viscosity measurements, delayed PUR polymerization was achieved using the PS-Im-FeCl3 catalyst system. The highest viscosity (6000 cP) was achieved without any catalyst, with triphenylene bismuth, and with the PS-Im-FeCl3 catalyst after 23, 5, and 25 h of reaction time, respectively. Furthermore, the calculated activation energies (E a) were 27.92 and 36.35 kJ mol-1 for the no-catalyst and the PS-Im-FeCl3 systems, respectively. Thus, the viscosity measurements and DSC analyses confirm that the PS-Im-FeCl3 catalyst considerably increases the PUR reaction time. The Im-FeCl3 catalyst supported by CMPS can control the reaction rate in PUR synthesis because of its high activity. Thus, the PS-Im-FeCl3 catalyst can be used as a curing retardant in the PUR industry.

Properties related to linear and branched network structure of hydroxyl terminated polybutadiene

The correlation between properties and the network structure of hydroxyl terminated polybutadiene (HTPB) based polyurethanes (PUs) was studied through linear and branched structure polymer matrixes formed by toluene diisocyanate (TDI) and an aliphatic polyisocyanate curing agent (N100). The curing reactions were monitored using differential scanning calorimetry (DSC) and viscosity build-up. The swelling capacity of PUs decreased with the increase of crosslink density with a stable solubility parameter according to the equilibrium swelling test. Tensile properties of PUs cured by TDI and N100 in different stoichiometric ratios of NCO/OH groups were tested. Both breaking elongation and tensile strength were remarkably improved by N100. The thermal decomposition processes of HTPB/TDI and HTPB/N100 indicated that a branched structure has higher depolymerization temperature, and hence, improved thermal stability. In addition, PU with a branched network prevented the migration of the plasticizer during isothermal accelerated aging due to the higher crosslink density.

Molecular dynamics simulations on ε-CL-20-based PBXs with added GAP and its derivative polymers

Molecular dynamics simulations have been employed to study the ε-CL-20-based PBXs under COMPASS force field.

Kinetic studies on the cure reaction of hydroxyl-terminated polybutadiene based polyurethane with variable catalysts by differential scanning calorimetry

This paper employs differential scanning calorimetry (DSC) to investigate the reactions of hydroxyl-terminated polybutadiene (HTPB) binder and isophorone isophorone diisocyanate (IPDI) with two different cure catalysts, namely, dibutyl tin dilaurate (DBTDL) and stannous octanoate (TECH). This study evaluates the effects of two cure catalysts (i.e. DBTDL and TECH) on rate constants of the polyurethane cure reactions. Throughput the study, the kinetic parameters and the curing reaction rate equations are obtained. The present work concludes that both catalysts had a catalytic effect on the HTPB-IPDI system, but that the catalytic effect of DBTDL was higher than that of TECH. The binder system with the TECH catalyst displayed a longer pot-life and lower toxicity compared with the DBTDL. Additionally, this study investigates the binder system’s viscosity build-up at 35°C and the viscosity build-up results were in agreement with the DSC analysis results.

Polyurethane types, synthesis and applications – a review

Polyurethanes (PUs) are a class of versatile materials with great potential for use in different applications, especially based on their structure–property relationships.