Development Of High Char Yield Resole Phenolic Resin For Transfer Molding Resin Process

To developweapons of hypersonic speed,high penetration and high precision,high-performance solid rocket motor（SRM）is required to meet the high pressure,long duration and high temperature needs.Therefore,the ablation-resistant parts of nozzle need to have high strength interlayer,be resistant to erosion and of light weight.Traditional ablation-resistant parts are fabricated via the tape overlap wrapping process.The overall interlayer strength of ablation-resistant parts of nozzle fabricated by this method is low,and its anti-erosion property is poor.In addition,the interlayer is prone to peelingunder the condition of high temperature,high pressure,and long high-energy particles and combustion gas stream denudation.In order to prevent ablation and erosion,the most widely used approach is to increase the winding layer thickness.However,the inertia of engine increases and the overall performance decreases.Highly ablation-resistant nozzles are needed to make high-performance SRM of more power and longer life time.To meet these demands,new process and materials must be developed.One promising strategy is by the resin transfer molding（RTM）process using high char yield phenolic resin.RTM process is a cost-effective and efficient molding method for advanced composites,especially in large and complex structures.Compared with other composite molding process,RTM possesses obvious advantages.US,France and Russia have adopted the mature RTM process to fabricate dimensional reinforcing fabric/phenolic resin composites to be used in a variety of strategic and tactical weapons and ablation-resistant nozzles of rocket engines.In the domestic aerospace research,many groups are developing and utilizing RTM technology.However,there is still a large gap between the developed Countries and China in terms of matrix material used in ablative part,especially phenolic resin.The main objective of this report is to describe our research in the development of high char RTM resin for ablation-resistant nozzles.We performed the design of molecular structure,optimization of synthesis conditions and process,structure and property characterization,and evaluation of RTM process.In Chapter 2,we describe the design of molecular structures based on the resin properties.A two-step catalytic synthesis was used and high ortho position and regular structure resin has been synthesized,which showed high crosslinking density after curing.By introducing a polycyclic aromatic compound,the heat resistance of the resin was effectively improved.By adding a char-forming agent in the late synthesis process,the thermal decomposition of phenol resin was decreased.By blocking end groups to diminish the activity of the phenolic resin,longer life was achieved under the high infusion temperature.In Chapter 3,the synthesis conditions were optimized,including the raw materials phenol and formaldehyde ratios,catalyst type and adding method,and types of polycyclic aromatic compounds.We ultimately determined the starting formaldehydeto phenol ratio of1.2:1,adding catalyst bythe An-Baway,using Ar-1 polycyclic aromatic compounds as modifiers,and the methods of adding formaldehyde.The optimal process was defined for the preparation of high char yield phenolic resin.In Chapter 4,we employed a variety of characterization methods to analyze the structure and properties of high char yield RTM phenolic resin.It was found that introducing polycyclic aromatic compounds increases the ratio of high molecular weight resin.The modified maximum curing temperature and char yield at 800℃were significantly improved,reaching 200℃and 66.9%,respectively.In Chapter 5,we used different methods to analyze the thermal dynamics of the resin solution.Kissinger method was used to calculate the pre-exponential factor A and activation energy E in threestages,and the decomposition difficulty of three stages gradually increased.The activation energy of the phenolic resin in the pyrolysis process was investigated by Ozawa formula and utilization rate conversion method.It was found that the activation energy did not increase with the increase of conversion rate.In Chapter 6,we utilized the double Arrhenius model equation to simulate the rheological properties of resin,which showed good consistency with the experimental results.Based on prediction model equations and the established process window analysis for VARTM resin molding process optimization,the optimum temperature range of the resin was determined to be 70⁸0℃.In Chapter 7,we used different material systems to optimize the processing conditions.The effect of injection temperature,injection pressure and fiber content on the composite porosity was assessed and the parameters to achieve minimum porosity were determined.Comparative studies of mechanical properties,thermal properties,ablation performance of composites reinforced by the different material fabric patterns were performed.The results showed that the fabric reinforced materials have excellent mechanical properties,especially interlayer shear properties,comparing with the silica phenolic composites prepared with tape wrapping process.2.5D carbon fiber fabric or carbon felt needle ablation has excellent ablation performance and the interlayer performance has met the anti-erosion and shear strength of high performance nozzle.In summary,phenolic resin suitable for RTM process was developed.The resin possesses the following features:low viscosity with an initial viscosity of≤400mPa·s at80℃,long pot life with a process pot life between 2h and 5h at different RTM process temperatures from 65℃to 85℃and the viscosity was less than 1000 mPa·s,and a high char yield of>66%under N₂ at 800℃.