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Flow and Infiltration Performance of Epoxy-curing Agent Reactive System in Fiber Preforms

Author: DaiXiaoQing
Tutor: XiaoJiaYu
School: National University of Defense Science and Technology
Course: Materials Science and Engineering
Keywords: Resin system Viscosity Reacting caloric Temperature Curing degree Reacting dynamics Fiber reinforced polymer matrix composites
CLC: TB332
Type: PhD thesis
Year: 2010
Downloads: 206
Quote: 1
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Abstract

Fabrication of fiber reinforced polymer matrix composites is completed by curing of the polymer resins. For the liquid molding process of the composites, fabricating process of the composites would be affected by the flowing and infusing characteristics of the resins in fabric preforms. A reactive resin system is a mixture of resins, curing agents and curing catalysts. Viscosity of the reactive resin system changes as temperature and curing degree of the resins change, and the viscosity of the resin will affect the flowing and infusing behaviors of the reactive resin system and finally affect the quality of the composite materials. Currently, it is difficult to measure real-time viscosity of the resins due to the coupling of the temperature and curing degree of resins, especially to the resins in the fabric preforms.In order to solve the problem on measuring real-time viscosity of the reactive resins, we try to develop a new method to predict the real-time viscosity of the resins. The research focuses on effects of temperature, curing caloric, curing degree and fabric preform on the viscosity of the resins. Based on the elementary principle of the fluid dynamics for porous medium and the Darcy’s law, the flowing and infusing behaviors of the CYD-128/GA-327 of the typical epoxy system in the RTM mold filling process were investigated by the level set method to validate the prediction method. Main contents of the research include:(1) Effects of fabric preform on curing characteristics of the epoxy-curing agent systemCompared with the relevant values of the net CYD-128/GA-327 system, the apparent activation energy, pre-exponential factor, reaction order and reaction rate constant of the resin system in glass fiber or carbon fiber preforms increased with small increment, while peak areas of the exothermal reaction and the reaction caloric decreased, and those effects increased in direct proportion with the mass content of the fiber. The starting temperature T0 and peak temperature Tp of the curing reaction decreased. However, the stopping temperature Tf of the curing reaction increased, and the temperature range (T0~Tf) of the curing reaction broadened. In the same elevated temperature rate, the maximum of the curing reaction rate decreased, and the starting time of the curing reaction became short, while the period of the curing reaction became long.(2) Prediction method of the real-time viscosity of a reactive resin involving curing reaction caloric and curing degreeThe isochronous corresponding relationship of the viscosity in a log scale (lnη) and the curing degree (α) was obtained in the isothermal conditions, and the relationship of lnηand temperature (T) in the conditions of constant-curing-degree was obtained. Consequently, the effect of temperature or curing degree was obtained, and the separation of the chemical mechanism and the physical mechanism of the viscosity was achieved. Based on an assumption of partial adiabatic system, a relationship of temperature and time for CYD-128/GA-327 epoxy resin system at different initial temperature Tinit was obtained. The lnη(T,α)-t relationship of the reactive resin system under the combined effects of the temperature and the curing degree was obtained.Three stages in the lnη(T,α)-t relationship of the CYD-128/GA-327 resin system can be observed. In the first and third stages, the viscosity changed little, while in the second stage, the viscosity increased quickly along with the time. The higher the initial temperature Tinit, the more significant the three-stage will be for the resin system. The maximum of lnη(T,α) reached in the last stage increased with the initial temperature increasing.Under the combined influence of the temperature and the curing degree, the lnηof the resin system decreased before the lowest viscosity, and then increases with the time, whereas this trend becomes unconspicuous along with the increasing initial temperature Tinit. The minimal lnη(T,α) of the resin system is the critical point at which the controlled mechanism changes from the physical mechanism to the chemical mechanism of the viscosity. In the physical mechanism region, the viscosity of the resin system is quite sensitive to the temperature, and the control of the resin viscosity can be carried out by adjusting the temperature. In the chemical mechanism region of the viscosity, the temperature only changes the increasing rate of the viscosity for the resin system. The initial lnη(T,α) and the minimum of lnη(T,α) decrease with the initial temperature increasing, and the relative plateau period of the low lnη(T,α) is shorten with the initial temperature increasing.(3) Prediction method of the real-time viscosity of a reactive resin in fabric preformsA composite mass unit composed of fiber and resin system was used as a model. The relationship of the viscosity vs time for the composite mass unit under the isothermal conditions and the relationship of the viscosity vs temperature under the constant curing degree conditions were obtained, respectively. Furthermore, the separation of chemical and physical mechanism for the viscosity of the resin system in the composite mass unit was achieved. Based on an assumption of partial adiabatic system, a relationship of temperature and time for the composite mass unit at different initial temperature Tinit was obtained. The lnη(T,α)-t relationship of the reactive resin system in the composite mass unit under the combined influences of the temperature and the curing degree was obtained.For CYD-128/GA-327/Gf and CYD-128/GA-327/Cf composite system with resin mass content, Mr, of 0.36968 and 0.42973, respectively, the relationship of the viscosity and time under the combined effects of the temperature and the curing degree was obtained. Results show that the time was delayed for the composite material systems when the viscosity reached the minimum of lnη(T,α), and the relative plateau period of low lnη(T,α) was prolonged, while the minimum of lnη(T,α) was lower than that of the net CYD-128/GA-327 system.(4) Study on the flowing and infusing of the reactive resin in fabric preformsThe level set method was used to simulate the flowing behaviors of the CYD-128/GA-327 resin system injected into the preform composed of 10 layers of 04 orthogonal woven glass fabric under the pressure of 0.1MPa. Results show that the distance of the flow front rapidly increased with the time in the early stage, and the increased rate of the flow front decreased gradually. Near to the position of the inlet, the time for the flow front of the reactive resin reached the same distance at different initial temperature Tinit was almost same. Along with the distance of the flow front increasing, the time reached the same distance decreased with the initial temperature Tinit increasing. Consequently, the flow stopped when the pressure gradient became small and/or the viscosity of the resin became very large. The predicted flow front was coincident well with the experimental data.

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