Zhang Chongheng1, Hu Daning2, Shen Hongxin1, Geng Zhi-gang1, Bao Le1, Wang Jixi1Zhang Quan1, Monday, Bob1
(1. Xi'an Changfeng Machinery Research Institute, Xi'an, Shaanxi Province, 710065; 2. Military Representative Office of the Army, Gansu Province, Tianwei, 740000)
Abstract:This article introduces the technical requirements, formulation characteristics, performance, bonding process, and application of a mid-curing, heat-resistant phenolic epoxy adhesive.
Keywords:Bisphenol A epoxy resin; adhesive; bonding; long-tail spray nozzle; solid rocket engine
The bonding of solid rocket engine long nozzles includes the bonding of a resin-based carbon fiber composite lining and a metal shell. The bonding performance is mainly determined by the surface condition of the bonded materials, the properties of the adhesive, and the characteristics of the interface layer. Bisphenol A epoxy resin has good bonding performance and thermal resistance due to the presence of polar epoxy groups and benzene rings, and also has good wetting properties with the bonded materials, so it is widely used. The bonding surface of the metal shell is mainly ensured by mechanical sandblasting, and the bonding surface of the lining is mainly ensured by the roughness of the machine-processed surface after heat pressing. The resin-based lining material, which contains barium-phenol-formaldehyde resin, has good bonding performance with the polar epoxy resin adhesive due to the presence of polar groups. The metal shell, which is made of ultra-high-strength steel material, forms a coordination complex with the epoxy group of the epoxy resin in the adhesive due to the d-orbital of the transition metal iron atom and the lone pair electrons of the oxygen atom, so the bonding performance is also good. In addition, the epoxy adhesive has a certain shrinkage during curing, so for bonding interfaces with large gaps, it is generally necessary to increase the amount of adhesive to ensure that the adhesive is evenly and densely filled into the bonding gap. The bonding surface temperature of the nozzle heat components can reach 50-60°C during the operation of the engine, and the Bisphenol A epoxy adhesive can fully meet the requirements.[1].
1. Technical Requirements for Adhesives
(1) Suitable for application temperatures between -20°C and 150°C.
(2) Exhibits excellent bonding strength with metals and heat-resistant linings. Aluminum-aluminum shear strength ≥ 15 MPa, tensile strength ≥ 20 MPa.
(3) Excellent processing performance, with good flowability and excellent penetration properties for the adhered material. It can be cured at room temperature and moderately low temperatures without the need for complex heating or pressure equipment.
(4) Should not cause corrosion or erosion of the adhered material.
(5) Exhibits sufficient aging resistance, maintaining at least 80% of its strength after storage for 10 years.
2. Composition and characteristics of adhesive formulations
Due to the reactivity of epoxy resins with many materials, the curing reaction is a nucleophilic addition reaction, does not release small molecules, has a low shrinkage rate, and does not require high pressure during curing; only a certain contact pressure is needed, making the bonding forming process simple. based on this, selecting the appropriate formulation is also relatively easy. based on the curing temperature, select the corresponding curing agent while also considering the application period and the properties of the adhesive after curing. Considering the flowability of the adhesive, it is necessary to reduce the viscosity to fill the gap between the bonding components (typically 0.05mm～0.20mm). In addition, adding an active diluent is necessary in the formulation components. When using, it is necessary to add heat-resistant inorganic fillers to consider the use temperature. When the temperature reaches 60°C during the curing of the adhesive, the flowability of the adhesive increases. Therefore, adding a certain amount of thixotropic agent is beneficial for filling the large gaps of the long-nozzle spray gun during bonding, reducing the outflow of the adhesive during curing, and causing voids on the bonding surface, resulting in failure in ultrasonic inspection.
The typical formulation for the adhesive used in solid rocket engine tail pipe bonding is shown in Table 1.
Table 1: Composition of long-tail spray nozzle adhesive.
3. Adhesive Performance
Long-tail spray nozzle adhesive, see Table 2.
Table 2: Performance of long-tail spray pipe adhesive
4 Adhesive Bonding Process
4.1 Adhesive Curing Procedure Established
The reaction between E-51 epoxy resin and 2-ethyl-4-methylimidazole involves three basic reactions: firstly, the reaction between the epoxy group and the active hydrogen on the imidazole ring, secondly, the generation of an oxygen anion, and thirdly, the chain growth reaction of the oxygen anion's active center. General kinetic studies have not been able to distinguish between these reactions, resulting in significant variations in the results. Some researchers have determined the kinetic parameters of the first step reaction using DSC extrapolation, by increasing the amount of 2-ethyl-4-methylimidazole to stop the reaction only at the first step. The resulting kinetic equation is:
=1.5×109e- Putty powder- Ready-mixed putty paste- Peeling- Detachment- Substrate- Interface treatment agent(1-α)s-1
In the equation: α represents the percentage of epoxy reaction, meaning the first step involves a first-order reaction proportional to the epoxy concentration. However, the amount of 2-ethyl-4-methylimidazole is typically small, resulting in a smaller proportion of the first step reaction and a smaller proportion of the second step reaction. Therefore, the third step reaction is the primary one. This corresponds to the observed kinetic behavior. The observed kinetic equation for the third step is:
=3.49×1027e-putty powder-ready-mixed putty paste-peeling-detachment-substrate-interface treatment agents-1
This refers to a zero-order reaction, which is independent of the epoxy concentration. Using this formula, the complete curing time at a specified temperature can be calculated. For example, when curing at 80°C, the calculated time (t∞) is 4.44 h. Considering the influence of heat transfer from the workpiece size, a curing time of 6-8 hours at 80°C is generally adopted. The theoretical calculation is essentially consistent with practice. This reaction is the only curing reaction with a clear reaction mechanism and can be theoretically calculated.[2,3].
4.2 Adhesive Bonding Process for Long-Tail Spray Nozzles
A solid rocket engine's long nozzle is bonded using a snap-fit molding process. The bonding process flowchart is shown in Figure 1, and the schematic diagram of the long nozzle is shown in Figure 2. Due to the common drawbacks of room-temperature curing epoxy adhesives, such as slow curing speed, low strength, and poor thermal resistance, this study employs a mid-temperature curing epoxy resin system. Through long-term production practice, this structural adhesive has been proven to have stable performance, feasible processing, and product quality that fully meets the requirements of engine use.
Figure 1: Process flow diagram for bonding with long-tail nozzles.
Figure 2: Schematic diagram of a long-tail spray nozzle.
5. Application of adhesives in the bonding of long-tail nozzles for solid-propellant rocket engines
E-51 epoxy resin and 2-ethyl-4-methylimidazole-based adhesive system belongs to a mid-temperature curing epoxy resin structural adhesive. based on long-term batch production and usage, it can meet the bonding requirements between the metal shell and glass fiber lining of tactical missile solid-propellant rocket engines, particularly for long nozzles. The adhesive's performance indicators, forming process, quality stability, and reliability fully meet the product usage requirements. This adhesive can be referenced or promoted for use in similar products.
[Author Biography] Zhang Chongheng, male, Researcher at the Composite Materials Research Center of the 210th Institute of Aviation. He has been working at the 210th Institute since 1992, primarily focusing on solid rocket engine materials and processes. Contact phone: 13636701310, email: [email protected].
References
[1] Qiu Zheming. Solid Rocket Engine Materials and Processes [M]. Beijing: Aerospace Press, 1995.
[2] Zhang DuoTai. Determining kinetic parameters of complex reactions using the force-out method (DSC) – A study of EMI-2,4 epoxy resin [J]. China Adhesives, 1997, 6(5): 38-43.
[3] Zhang Duo Tai. Determining the kinetic parameters of EMI-2,4 epoxy resin using DSC curves [J]. Aerospace Materials Technology, 1994, (5): 31-33.
Article source: China Epoxy Resin Application Technology Society WeChat official account