Experimental study on the combustion characteristics of carbon fiber composite materials coated in different coatings

The special environment in which the aircraft operates in terms of flight and safety assurance has special requirements for coating performance. Based on the combustion characteristics of four kinds of high-performance aviation coatings coated with carbon fiber composites, for parameters such as average ignition time and heat release rate of the composite, the results show that the carbon fiber composite has the longest average ignition time, Minimum heat release rate, while the 2-ethyl-4-Mylimidazole cured epoxy coated fiber composite had the shortest average ignition time. The maximum heat release rate was observed.


Introduction
Conical calorimetry is a commonly used method to determine the combustion performance of materials.The conical calorimeter can determine the ignition time (TTI), heat release rate (HRR), and other combustion performance parameters of materials.Usually, the shorter the average ignition time of the material, the greater the heat release rate, and the shorter the time to reach the maximum heat release, the greater the fire hazard.Because the test environment of the cone calorimeter is close to the real combustion environment of the fire material, the measured parameters used to evaluate the combustion behavior of the material in the fire are more accurate [7][9] .Its application mainly includes the evaluation of the combustion hazard of the material, studying the flame-retardant mechanism and the fire model research.
This project with T700 carbon fiber plate as the matrix, with 650 low molecular polyamide curing epoxy resin coating, 2-phenol-propane epoxy resin coating, 2-ethyl-4-4-methyl imidazole curing epoxy resin coating, double maleimide resin coating, and other four aviation high-performance coating as the analysis object, respectively determine the combustion characteristics of carbon fiber composite plate after coating different coatings [5][8] .

Materials and Methods
In this experiment, the conical calorimeter was used as the experimental instrument [4] [10] .The T700 carbon fiber composite plates coated with four kinds of aviation coatings were selected to determine the combustion performance of conical calorimetry.The test scheme is shown in Table 1:

Determination of average ignition time
Ignite is an important property in describing the risk of composite fire, and the ease of ignition is usually characterized by ignition time.Ignite time refers to the time taken from heating to continuous combustion on the surface of the test material under a specific thermal radiation intensity.This time can be used to characterize the fire risk and fire resistance of the material [11][12] .During the experiment, using the time recording tool of the software system, after the discharge brush is opened and the shield is opened, it is counted as 0 o'clock at the start time.When the test features, we manually click the time recorder and record the ignition time t1.After the flame disappeared, we record the burning time t2, and three groups of the experimental tests to obtain the average.The test results are shown in Table 2.As can be seen from Table 2, under the same thermal radiation power conditions, the lighting time for the resin is up to 149 s, followed by 650 low molecular polyamide cured epoxy resin 141 s and 2phenolic propane epoxy resin 104 s, while the shortest lighting time for 2-ethyl-4-MMG is only 86 s.From the perspective of ignition time, the flame-retardant performance of carbon fiber composite coated with double maleimide resin coating is better.In contrast, the combustion risk of carbon fiber composite coated with 2-ethyl-4-methimidazole cured epoxy resin coating is greater.

Measurement of the heat release rate
The heat release rate (HRR) is one of the most important fire response characteristics because the heat released by the material combustion provides the additional heat needed for the fire to grow and spread [1][3] .Furthermore, other fire response properties, such as flame surface diffusion, smoke generation, and CO emissions, depend on or are associated with the HRR [6] .
The heat release rate is the heat energy generated by the product of decomposition after combustion per unit area surface in fire or under heat flow near the material.The heat release rate peak (PkHRR) occurs in a very short period, usually shortly after ignition, and generally very well indicates the maximum flammability of the material [2][3] .The average heat release rate is the total heat released, averaged during combustion, and is considered the most reliable measure of heat released in a sustained fire.
Figures 1(a)-(d) show the change in the heat release rate of a carbon fiber sheet coated with four coatings over time.
Figure 1 Heat release rate of different coatings over time.As can be from Figure 1(a), the combustion and heat release process of 650 low molecular polyamide coated carbon fiber board with epoxy resin is generally divided into two stages.Figure (b)  shows similarities between the heat release rate of 2-phenolic propane epoxy resin coated with time and the heat release rate of 650 low molecular polyamide cured epoxy resin.There are two combustion and heat release processes.Figure (c) shows that the heat release rate of 2-ethyl-4-methimidazole cured epoxy resin coated with carbon fiber board only showed one significant peak.Figure (d) shows that during the entire combustion process, the peak and mean heat release rate of 2-ethyl-4-Mimidazole cured epoxy coating are both at a very low level.
After experimental determination, Table 3 shows the mean value of the peak and the time of the peak under different coating conditions.As can be seen from Table 3 that the average heat release rate of 2-ethyl-4-MIMazole is 113.3 kW • −2 .The average heat release rate of 2-phenolic propane epoxy resin coating is 109.1 kW • −2 , taking the second place 650.The average heat release rate is 68.7 kW • −2 , which is the lowest.From the experiments measuring the heat release rate, the average heat release rate of 2-ethyl-4-Mimidazole cured epoxy resin coated carbon fiber board is the greatest among the four composites, the fire hazard is the greatest at 650.With the smallest average heat release rate of LPRP coated with epoxy resin, the fire is the least harmful.

Conclusions
The results show that the average ignition time of double maleimide epoxy resin coating is the longest and the lowest heat release rate, and the best fire performance of the 2-ethyl-4-MIMM has the highest average ignition time, the largest heat release rate, and the worst fire performance of the coatings.

Table 1
Combustion performance determination scheme by conical calorimetry.

Table 2
Average ignition time of the four test samples.

Table 3
Heat release rate peak, mean and peak occurrence time of carbon fiber board.