RTM Study on High Performance Epoxy Resin System for RTM

 

Resin transfer Molding (RTM) is a process method in which resin is injected into a closed mold to infiltrate the reinforcing material and solidify. It is a molding process that has developed rapidly in recent years and is suitable for the production of multi-variety, medium-batch, high-quality advanced composite products.

 

Due to its characteristics of extremely small tolerance of parts, excellent surface quality, low porosity, short production cycle, strong adaptability to production process automation, low investment and high efficiency, and high fiber volume content of parts, it is widely used in aerospace, automobile, machinery, electronics and construction fields [2]. The key to RTM technology is to develop an ideal resin system.

 

The RTM process has the following requirements for the resin system: low viscosity (<1000mPa·s) at room temperature or process temperature; a certain long pot life; good wettability, matching and adhesion of the resin to the reinforcing material; good reactivity of the resin at the curing temperature, and the post-processing temperature should not be too high (such as T <200℃), short curing cycle and small reaction exotherm [2~5].

 

Epoxy resin is currently the most widely used high-performance composite matrix resin due to its good processability and low requirements for molding temperature and molding pressure [3,6]. However, the high viscosity of epoxy resin currently used limits its application in RTM molding.

 

Aiming at the requirements of RTM process for resin, this paper developed a medium-temperature curing epoxy resin system suitable for RTM by adding self-made high-performance epoxy resin to ordinary bisphenol A epoxy resin, and studied its viscosity, curing, mechanical and dynamic mechanical properties.

 

1 Experimental Part

 

1.1 Raw Materials and Instruments

The raw materials used in the experiment are epoxy resin CYD-128, with an epoxy value of 0.51~0.54, produced by Baling Petrochemical Co., Ltd.; glycidyl ether type epoxy resin A, homemade; curing agent B, liquid amine, homemade.

 

The instruments and equipment used in the experiment are DSC-7 from Perkin-Elmer, USA; DV-III+ programmable rheometer from Brookfield Engineering Laboratory, USA; DMA2900 from TA, USA.

 

1.2 Preparation and performance test of casting body

Add a certain amount of CYD128 and epoxy resin A to a beaker, mix well, then add curing agent in the corresponding proportion and stir thoroughly. Pour the mixed resin system into a mold pre-coated with a release agent, vacuum degas for 30 minutes, and finally put it in an oven for curing according to its curing process. The tensile and bending properties of the resin castings were tested in accordance with GB/T 2568-1995 and GB/T 2570-1995, respectively.

 

2 Results and Discussion

 

2.1 Study on compatibility of resin matrix

CYD128 and epoxy resin A were mixed in different proportions (1:9, 1:1, 9:1) and stirred evenly. They were left to stand for one week at room temperature. It was observed that the mixed resin remained clear, and no stratification or crystallization was found. This indicates that the two resins have good compatibility.

 

2.2 Study on viscosity characteristics of resin system

The viscosity characteristics of the resin system are one of the important indicators of RTM process parameters. Figures 1 and 2 show the viscosity-temperature and viscosity-time variation relationships of the blended resin system, respectively.

 

As shown in Figure 1, the viscosity of the resin system is only 255cps at 30℃. Between 30 and 89℃, the viscosity of the resin system first decreases with increasing temperature, and when the temperature is 89℃, the viscosity reaches a minimum of about 90cps, and then the viscosity of the system gradually increases with increasing temperature.

 

The increase in temperature has the following two effects on the viscosity of the resin [7,8]:

On the one hand, the increase in temperature increases the molecular motion activity, thereby reducing the viscosity of the resin system;

On the other hand, the curing reaction forms a cross-linking network, which restricts the movement of molecules and increases the viscosity. In the initial stage, the reaction degree is low, so the effect of temperature increase on viscosity decrease is the main one; when the temperature increase and curing cross-linking have equal effects on viscosity, the viscosity is the lowest; when the temperature rises further, the reaction degree increases, and curing cross-linking causes the viscosity to rise rapidly.

 

Figure 2 shows the viscosity-time curve of the resin system at room temperature. As can be seen from Figure 2, the viscosity of the resin system can be maintained at 400cps for about 100min; then, as time increases, the viscosity rises rapidly, and the viscosity of the resin system reaches 1000cps around 200min.

     

2.3 Study on the curing process of the resin system

CYD128, epoxy resin A and mixed resin are added to curing agent B in proportion, and a DSC experiment is performed after mixing (as shown in Figure 3), and the reaction characteristic temperature analysis results are obtained as shown in Table 1.

 

As can be seen from Figure 3, the mixed resin system is similar to epoxy resin A and CYD128, and both have only one reaction peak, indicating that the two resins have good compatibility when mixed in this ratio. This conclusion is consistent with the previous compatibility verification conclusion. Due to the high content of CYD128 in the mixed resin, the reaction peak of the mixed resin is close to the reaction peak of the CYD128 system.

 

As can be seen from Table 1, whether it is pure resin or mixed resin, the starting temperature of the reaction is around 73°C, indicating that the two components of the mixed resin can undergo a co-curing reaction at the same temperature. From the DSC curve, it can be seen that the mixed resin system is a medium-temperature curing system, the system exothermic temperature range is 54°C, and the exothermic temperature is moderate. Using the T-β diagram extrapolation method, the gelation temperature Tgel of the mixed resin system is 70°C, the curing temperature Tcure is 96°C, and the post-treatment temperature Ttreat is 118°C.

2.4 Dynamic thermomechanical properties of the resin system

DMA can reflect the change of the dynamic mechanical properties of the test material with temperature under forced vibration, and can be used to test the Tg and high-temperature mechanical properties of the material. Figure 4 shows the DMA curve of the mixed resin system under room temperature curing for 24 hours. As can be seen from Figure 4, under room temperature curing, the peak temperature of the loss factor tanδ of the mixed resin system, i.e., Tg, is about 72°C, and the maximum value of the system loss factor is 1.142, indicating that the resin system has good damping properties.

 

 

 

As can be seen from Table 2, the mixed resin system has high strength, modulus and toughness, which can basically meet the mechanical property requirements of the RTM resin system.

 

3 Conclusion

(1) CYD128 and self-made high-performance epoxy resin A were blended and modified, and a resin system suitable for RTM was obtained by adding liquid amines as curing agents;

 

(2) The viscosity of the resin system at 30℃ was only 255cps, and the viscosity was kept below 1000cps at room temperature for 200min;

 

(3) CYD128 and epoxy resin A can undergo co-curing reaction at the same temperature. The mixed resin system is a medium-temperature curing system, the exothermic temperature range of the system is 54℃, and the exotherm is moderate;

 

(4) The resin system has good damping properties, and its maximum loss factor is 1.142;

 

(5) The resin system has high strength, modulus and toughness, which can basically meet the mechanical property requirements of the RTM resin system.