Promoting Cross-Link Reaction of Polymers by the Matrix-Filler Interface Effect: Role of Coupling Agents and Intermediate Linkers

The matrix-filler interface effect plays an important role in determining the structural stability and mechanical properties of polymer composite materials, which remain ambiguous and need to be studied. The network-forming dynamics of poly(3,3-bis (azidomethyl) oxetane-tetrahydrofuran) (PBT) at the ammonium perchlorate (AP) surface was studied by using atomistic molecular dynamics simulation, considering the additives of curing agent toluene diisocyanate (TDI), cross-linker trimethylolpropane (TMP), and coupling agent triethanolamine (TEA). The presence of the AP surface promotes chain cross-link reaction, which is attributed to the increased production of intermediate linkers formed by TDI, TMP, and TEA. The intermediate linker has three reactive sites that can react with PBT main chains to form a cross-linked structure. Owing to the strong interaction with the AP surface, the coupling agent TEA plays a dominant role in forming the intermediate linker. At the early stage of network forming (reaction ratio r < 30%), the AP surface adsorbs TEA, which leads to a maximum contact density to PBT. As r increases to 60%, the density of intermediate linkers near the AP surface reaches a maximum value. Consequently, the chain cross-link reactions between the intermediate linker and PBT main chains are enhanced as r > 60%. This work explains the micromechanism of the promotion of chain cross-link reaction by the interface effect and provides important insights on designing polymer materials with high mechanical properties.

Comparative Study on Thermal Response Mechanism of Two Binders during Slow Cook-Off

The HTPE (hydroxyl-terminated polyether) propellant had a lower ignition temperature (150 °C vs. 240 °C) than the HTPB (hydroxy-terminated polybutadiene) propellant in the slow cook-off test. The reactions of the two propellants were combustion and explosion, respectively. A series of experiments including the changes of colors and the intensity of infrared characteristic peaks were designed to characterize the differences in the thermal response mechanisms of the HTPB and HTPE binder systems. As a solid phase filler to accidental ignition, the weight loss and microscopic morphology of AP (30~230 °C) were observed by TG and SEM. The defects of the propellant caused by the cook-off were quantitatively analyzed by the box counting method. Above 120 °C, the HTPE propellant began to melt and disperse in the holes, filling the cracks, which generated during the decomposition of AP at a low temperature. Melting products were called the “high-temperature self-repair body”. A series of analyses proved that the different thermal responses of the two binders were the main cause of the slow cook-off results, which were likewise verified in the propellant mechanical properties and gel fraction test. From the microscopic point of view, the mechanism of HTPE’s slow cook-off performance superior to HTPB was revealed in this article.

Aziridine type bonding agents for composite solid rocket propellant

Bonding agents, as a paramount component in the composite solid rocket propellant, effectively enhance the interactions between polybutadiene matrix and oxidizer. It mainly affects the processing ability, mechanical behavior, and aging. Bonding agent plays the role of an anonymous soldier to defeat stresses produced by loading conditions, changes in the environmental condition, and transportation of composite propellant. The role and the mechanism of aziridine type bonding agents are discussed focusing on a novel MST aziridine bonding agent (Tris-(2-methyl-1-aziridinyl) phosphine oxide (MAPO) plus Stearic acid plus Tartaric acid) (poly (isopropylamine N(2-methyl-1-aziridinyl) phosphine oxide octadecanoic)-co-(isopropylamine tartrate)). Preparation of composite solid propellant samples containing the same weight percentage of different bonding agents is conducted. Mechanical properties and aging behavior for the prepared samples were monitored to extract the impact of bonding agent type. Mechanical properties of the bonding agents were recorded at three different temperatures in order to simulate various environmental conditions. Results of stress-strain responses during uniaxial tensile tests showed that MST bonding agent has the strongest interaction with solid particles reaches a value of 50.4 % strain and 8.052 Kg/cm2 stress at normal conditions. Compared to MT-4 (produced from the reaction of 2 moles MAPO plus 0.7-mole Adipic acid and 0.3-mole Tartaric acid) and MAPO, MST improved the aging behavior.

Synthesis of two novel neutral polymeric bonding agents to enhance the mechanical properties of composite solid propellants

Currently, only a few bonding agents can be utilized efficiently in nitramine filler material-based solid rocket energetic binder systems. Herein, we demonstrate the synthesis and specific characterization of two kinds of neutral polymeric bonding agents (NPBA-1 and NPBA-2) and their application in composite propellants consisting of the nitramine octogen (HMX) and glycidyl azide polymer (GAP). The as-obtained NPBAs were well-coated on the surface of HMX and RDX due to their functionalized groups, and they significantly affected the viscosity of the uncured propellant mixtures and possessed obviously enhanced mechanical properties in the cured AP/HMX/GAP propellant mixtures, even at low concentrations (down to 0.001 wt% of the whole propellant). In addition, because of the existence of an epoxy group and no hydroxyl functionalities, NPBA-2 exhibited improved mechanical strength and glass transition temperature as compared to NPBA-1, which has plenty of reactive hydroxyl groups. The as-synthesized epoxy-modified NPBAs are essential for obtaining NEPE propellants with high bonding and mechanical properties.

The NPBA-2-based composite solid propellant exhibited improved mechanical strength and glass transition due to the strong interactions between epoxy groups and HMX or RDX.

A Review of Polysiloxanes in Terms of Their Application in Explosives

Polysiloxanes are reviewed for their properties depending on the functionalization of a silicon–oxygen backbone chain. Next, the properties were referred to the requirements that polymers used in plastic/polymer-bonded explosive (PBX)-type explosives must meet. Finally, the current state and prospects for the implementation of polysiloxanes in plastic/polymer-bonded explosive (PBX) formulations are presented.

Highly stretchable, high efficiency room temperature self-healing polyurethane adhesive based on hydrogen bonds – applicable to solid rocket propellants

The introduction of weak hydrogen bonds based on the isophorone structure enables the polymer to have high stretchability and self-healing ability at room temperature to heal propellant damage.