The Thermo-Mechanical Properties of Carbon-Fiber-Reinforced Polymer Composites Exposed to a Low Earth Orbit Environment

Abstract

In this study, we focus on 3D-printed PEEK/CFRTP (Carbon-Fiber-Reinforced Thermoplastic) and PEEK (Polyether Ether Ketone) materials as new space materials. In space, there are intense ultraviolet (UV) rays that are weakened by the atmosphere on Earth, so it is essential to understand the degradation of materials due to UV rays in advance. Therefore, we developed a materials science experiment called the Material Mission, which will be carried out on board Ten-Koh 2. This mission measures the coefficient of thermal expansion (CTE) of the CFRTP samples and the PEEK samples in LEO without recovery. So, we developed a thermal expansion observation system to be installed on the Ten-Koh 2 satellite. In addition, UV irradiation tests simulating the UV environment in LEO were conducted as ground tests. From the results of the ground tests, it was possible to determine in advance the degree of degradation of each material in the UV environment, even up to 100 ESD. By utilizing these results in mission operations, more meaningful measurement results can be obtained, and this mission development can contribute greatly to developing new space materials in the future.

1. Introduction

In recent years, interest in space has grown both domestically and internationally, and this has led to an increase in the number of companies planning businesses that utilize satellites [1]. The threshold for space development has been lowered, and as a result, not only universities but also high schools and venture companies are developing satellites. In particular, the development of small satellites weighing around 300 kg, including CubeSats, has recently attracted attention for several reasons [2]. First, these small satellites have the advantage of being developed at a lower cost than conventional large satellites. This makes it possible to send new satellites into space even with limited budgets and resources, making them an easy option for many institutions and companies. In addition, the development of small satellites can be accomplished in a short period, which contributes to the rapid implementation of space projects [3].

Today, satellites and other spacecraft are being developed, but these spacecraft must be designed to operate in the space environment properly at launch. To this end, the selection of reliable materials to be used in spacecraft is essential, and several important conditions must be met. Specifically, the materials used must be highly reliable and safe. At the same time, they must have excellent mechanical properties; in addition, being lightweight is also important. Spacecraft are exposed to harsh environmental conditions, so their materials must be durable and highly reliable. The mechanical properties of the materials must also be optimized, as they will be deployed on a wide variety of missions, such as space exploration and communication satellites. At the same time, being lightweight is a very important requirement for reducing launch costs and for efficient space exploration.

Widely used in space, CFRP (Carbon-Fiber-Reinforced Plastic) is lightweight, strong, and resistant to corrosion. However, there are several problems with this material. First, thermoset resin is required to be stored in a refrigerator. The resin will age when stored outside of a refrigerator, resulting in a loss of tack and of the stiffness of the prepreg [4]. Second, CFRP processing requires a long curing time to account for the chemical reactions, which can increase the molecular weight and crosslinking [5]. Third, manufacturing CFRP requires large equipment, such as a large autoclave to fabricate complicated shapes [6]. These are issues that must be resolved when considering its future use in situations where space and resources are limited, such as on the International Space Station (ISS) and a manned lunar orbiting base (Gateway). To address the aforementioned issues, I have focused on 3D-printed CFRTP (Carbon-Fiber-Reinforced Thermoplastic) as a material. This material has features that can solve the aforementioned issues. Furthermore, the fact that it is a 3D-printed material leads to a shorter development time and lower costs [7]. However, 3D-printed materials have recently begun to attract attention, and since there is no track record of their use in a space environment, it is not possible to understand how materials deteriorate in a space environment. In the future, when considering the use of materials in space, it is necessary to have actual results on their use in a space environment, in addition to simulated tests on the ground.

In addition, a space environment exposes them to harsh conditions such as temperature cycles, ultraviolet (UV) light, atomic oxygen (AO), radiation, and high vacuum environments [8]. Among them, polymer materials such as CFRTP are known to be degraded mainly by UV rays. In space, there are also intense UV rays below 300 nm, which are weakened by the atmosphere on the ground [9]. UV rays with wavelengths of 200–400 nm have energies equivalent to 301–598 kJ/mol. On the other hand, the important molecular structure of resin used for CFRTP is C-C, and its binding energy is 344 kJ/mol. Therefore, UV light in the wavelength range of 200–400 nm has the potential to break the bonds in the material [10,11].

Therefore, in this study, first, we developed a thermal expansion observation system to observe how materials degrade in a space environment by installing a material degradation observation mission (Material Mission) on the Ten-Koh 2 low Earth orbit environment observation satellite developed by the Okuyama Laboratory of Nihon University. Second, ground tests simulating the UV environment in space were conducted to understand the degradation caused by UV rays because it is very important to conduct ground tests that simulate a space environment in advance.

1.1. Ten-Koh

The LEO environment observation satellite “Ten-Koh” is a quasi-spherical nano-satellite with a diameter of 500 mm developed by Okuyama Laboratory at the Kyushu Institute of Technology. It was launched into a Sun-synchronous sub-recurrent orbit at an altitude of about 600 km using the H-IIA rocket F40 in October 2018 as a sub-payload to JAXA’s Greenhouse gases Observing Satellite-2 (GOSAT-2). Ten-Koh’s Flight Model (FM) is shown in Figure 1 [12,13].

1.2. Material Mission (Ten-Koh)

1.2.1. Material Mission Overview

The main mission of Ten-Koh was to observe the material degradation of PEEK/CFRTP materials in a LEO environment. The objective is to measure the change in the CTE of the samples in a LEO environment. To calculate the CTE in orbit, a thermal expansion observation system was developed without sample recovery, equipped with strain gauges and temperature sensors.

The thermal expansion observation system consists of two boards: a PCB to fix the exposed samples (External PCB) and a circuit PCB around the sensors (Internal PCB). Overviews of the PCBs for Material Mission are shown in Figure 2.

This mission installed the following samples to observe their degradation. The size of all the samples is 50 mm long, 10 mm wide, and 2 mm thick.

• Sample 1: PEEK/CFRTP*¹ with no coating.
• Sample 2: PEEK/CFRTP*¹ with a coating to protect against AO*².
• Sample 3: PEEK/CFRTP*¹ with a coating to protect against UV*³.

*¹ PEEK/CFRTP: The CF was made from plain-woven carbon fabric manufactured by Toray with a 0/90° pattern. The PEEK resin was manufactured by Victrex.

*² Coating to protect against AO: Yttrium oxide (Y₂O₃) made by TOCALO.

*³ Coating to protect against UV: Silsesquioxane (RSiO_3/2) made by TOAGOSEI.

1.2.2. Material Mission Observation Results

The Material Mission observation results are shown in Figure 3.

The Material Mission observation results show that the CTE has a temperature correlation. The CTE changes markedly on the high-temperature side, while no change is observed on the low-temperature side. The change in the CTE is almost the same for the first 120 days of observation. The results show that it is possible to observe the CTE in a space environment and that material degradation can be observed in real time by measuring the CTE. The PEEK/CFRTP materials were not degraded after 120 days in space [12,13].

1.3. Ten-Koh 2

The LEO environment observation satellite Ten-Koh 2 is the successor of Ten-Koh, launched in 2018. It is a satellite 6 U in size with a mass of about 7.0 kg that will be launched at about a 500 km orbit using JAXA’s next-generation H3 launch vehicle and released using the new HTV-X ISS resupply vehicle [14]. Ten-Koh 2’s FM is shown in Figure 4.

2. Methods

2.1. Material Mission (Ten-Koh 2)

The purpose of this mission is to develop a thermal expansion observation system to evaluate material degradation and to observe the CTE of 3D-printed PEEK/CFRTP materials and 3D-printed PEEK materials in LEO.

The thermal expansion observation system consists of two boards: a PCB to fix the exposed samples (External PCB) and a circuit PCB around the sensors (Internal PCB). The thermal expansion observation system is shown in Figure 5.

2.1.1. Observation Method

The Ten-Koh 2 Material Mission uses the CTE of material degradation to make observations. The calculation of the CTE is based on the following equation [13]:

$$\alpha =\frac{L-L_0}{L_0}\frac{1}{T-T_0}=\frac{∆L}{L}\frac{1}{∆T}=\frac{\epsilon }{∆T}$$

(1)

where $\alpha$ is the CTE, $L$ is length, $L_0$ is initial length, $T$ is temperature, $T_0$ is initial temperature, $∆L$ is length change, $∆T$ is temperature change, and $\epsilon$ is strain.

Material Mission is equipped with two main sensors to measure the CTE: strain gauges and temperature sensors. In this mission, they are mounted to measure the intensity of the UV rays to evaluate the degradation of the materials due to UV rays.

2.1.2. Measurement Method

In the measurement of the strain using strain gauges, the voltage change caused by a change in the shape of the strain gauge is measured using a Wheatstone bridge and converted into strain. The relationship between strain and change in resistance is shown in Equation (2) [13].

$$\frac{∆R}{R}=K_s\epsilon$$

(2)

where $R$ is the resistance, $∆R$ is the change in resistance, $K_s$ is the gauge factor, and $\epsilon$ is the strain. If the change in resistance is less than $R$, the output voltage $e_0$ is as shown in Equation (3) [13].

$$e_0=\frac{∆R}{4R}E=\frac{1}{4}{K}_s\epsilon E$$

(3)

where $e_0$ is the output voltage and $E$ is the input voltage of the bridge. Therefore, using Equation (4) [13], strain and temperature are measured, and a change in the CTE is observed.

$$\epsilon =\frac{4e_0}{K_{s}E} ,\hspace{1em} \alpha =\frac{∆L}{L}·\frac{1}{∆T}=\frac{\epsilon }{∆T}$$

(4)

The UV sensor generates an electric current when the sensor is irradiated by UV light. This current is too small to be measured as it is. Therefore, an amplifier is used to convert the current into a voltage value that can be measured. The data obtained from the UV sensor are the voltage produced by the UV radiation. Therefore, it is necessary to conduct preliminary tests to understand the relationship between the intensity of the UV light and the voltage measured.

2.1.3. Temperature Sensors

The temperature sensors used in this observation system are AD590KRZ-ND sensors. This temperature sensor is the same as the one used in the OBC of Ten-Koh 2. The reason for this selection was to reduce the number of library functions for each sensor as much as possible and to create a program that fits within the limited program memory. The same temperature sensor is also used in Ten-Koh, its predecessor, and has a proven track record of use. A cross-section of the Eternal PCB for Material Mission is shown in Figure 6.

Generally, a thermocouple must be attached directly to the sample to measure the temperature of the sample. However, the adhesive may vaporize and peel off in a high vacuum environment, resulting in the system described above. To ensure that the heat from the sample is efficiently transferred to the temperature sensor, thermal grease and Lambda GEL are sandwiched in between for heat dissipation.

2.1.4. Strain Gauge

This observation system used two types of strain gauges, ZFCAL-1-11 and BFCAB-2-8. The reasons for selecting the strain gauges are as follows.

• The CTE value of the strain gauge itself is close to the CTE value of the sample.
• The difference between the longitudinal-direction and short-direction data is large and easily distinguishable.
• A larger change is seen when the temperature and strain data are graphed (the slope of the graph is larger).

As for the strain gauges, the strain gauges are inserted between the samples. The samples to be mounted in this mission are all made using 3D printers. Therefore, during the manufacturing process, the equipment is stopped once when the sample is stacked to half its thickness, the strain gauge is placed on the sample, and stacking is restarted to sandwich the strain gauge into the sample. An X-ray image of the sample is shown in Figure 7.

2.1.5. UV Sensor (Photodiode)

The photodiode used in this observation system is equipped with five types of photodiodes that can detect UV wavelengths of UVC. The photodiode’s photosensitive surface is made of glass, so it was necessary to consider a design that would withstand vibrations during launch. As a design solution for withstanding vibrations, a silicone material that can absorb vibrations was inserted between the PCB, bottle nut, and spacer to prevent the glass in the photosensitive area from breaking. The vibration tests were conducted under the test conditions required by JAXA, and the design was confirmed to be durable. The Material Mission External PCB for the EM (Engineering Model) is shown in Figure 8.

2.1.6. Sample

For this mission, three 3D-printed materials were mounted. A Material Mission sample overview is shown in

Table 1. Material Mission sample overview.

	Material	Length [mm]	Width [mm]	Thickness [mm]
Sample 1	3D-printed PEEK/CFRTP*1	50	12	3.0
Sample 2	3D-printed PEEK/CFRTP*1	50	12	1.5
Sample 3	3D-printed PEEK*2	50	12	1.5

*¹ PEEK/CFRTP: Apium PEEK CFR 4000. *² PEEK: Apium PEEK 4000 Natural.

.

2.2. UV Irradiation Test (Ground Test)

As a ground test for the Material Mission, a UV irradiation test was conducted to simulate UV rays in a space environment. This test was conducted using the UV irradiation equipment (NASDA-PSPC-7493 ESS-7000 Unit 1) at JAXA. In addition, only the 3D-printed PEEK materials with large color changes were measured for their solar absorptance, where the color change can be numerically checked. The solar absorptance was measured using PM-A2 and PM-A0, owned by Koei Co. The objectives of this test are as follows.

• Evaluation of the degradation of 3D-printed materials under UV rays in a space environment
• Comparison of the degradation according to UV irradiation time

The test conditions are shown in

Table 2. The UV irradiation test conditions.

Item	Value
Wavelength	200~400 nm
UV Strength	10 ESD/day
Days of Irradiation	10~100 ESD
Sample Materials	3D-printed PEEK, 3D-printed PEEK/CFRTP
Sample Temperature During Irradiation	About 30 °C
Sample Size	Diameter 25 mm, thickness 3 mm

.

The daily UV irradiation in the LEO environment is $1.02\times {10}^3 \mathrm{J}/{\mathrm{cm}}^2$, which is defined as 1 ESD.

3. Results

The changes in the sample appearance before and after the UV irradiation test are shown in Figure 9 and Figure 10.

Next, the results of the solar absorptance values are shown in

Table 3. The results of the solar absorptance values.

	0 ESD	10 ESD	20 ESD	30 ESD	40 ESD	60 ESD	100 ESD
αs [-]	0.64	0.73	0.75	0.77	0.77	0.77	0.78

α_s: The solar absorptance values.

and Figure 11.

4. Discussion

4.1. Material Mission (Ten-Koh 2)

In this study, we developed a thermal expansion observation system to observe how materials degrade in a space environment by installing a material degradation observation mission (Material Mission) on the Ten-Koh 2 low Earth orbit environment observation satellite developed by the Okuyama Laboratory of Nihon University. The Ten-Koh 2 Material Mission was developed with reference to its predecessor, the Ten-Koh Material Mission. One of the changes was the addition of photodiodes to measure the intensity of the UV rays. Therefore, this mission can observe the degradation of materials in a space environment from the viewpoint of UV rays, and if it is confirmed to be observable according to actual operations, it can be said that we were able to develop a mission that will greatly contribute to the development of new materials for space use in the future.

4.2. UV Irradiation Test (Ground Test)

CFRP, a material widely used in the space environment, has problems, as described in Section 1. Therefore, this study focuses on 3D-printed PEEK/CFRTP materials. However, since these materials have not been used in the space environment, it was not known whether the materials could be used in the space environment or how they would deteriorate. It is also true that polymer materials such as CFRTP are mainly degraded by UV light. Conducting ground tests that simulate UV rays in the space environment will be a very important concept. Therefore, the purpose of this ground test was to evaluate the degradation caused by UV rays in the space environment. In this study, a UV irradiation test was conducted, and solar absorption rate measurements were taken to evaluate the degradation.

4.2.1. Three-Dimensional-Printed PEEK

In this study, a Material Mission was developed, and a UV irradiation ground test was conducted. In space, because the atmosphere is thin, there is less absorption and scattering as a result of the atmosphere. When ultraviolet rays enter polymeric materials, they turn brown. The materials used in spacecraft, especially on the surface of the fuselage, reflect sunlight, dissipate heat via infrared radiation, and control the flow of heat into and out of the spacecraft. Therefore, a change in the color tone of the material may affect the balance of heat flow [15].

Therefore, I will first use the results obtained and discuss them from the viewpoint of thermal control. The parameters that were important to the thermal control design (α_s/ε_H) calculated from the results of this test are shown in

Table 4. Calculation results for parameters important to thermal control.

	0 ESD	10 ESD	20 ESD	30 ESD	40 ESD	60 ESD	100 ESD
αs [-]	0.64	0.73	0.75	0.77	0.77	0.77	0.78
εH [-]	0.66	0.66	0.66	0.66	0.66	0.66	0.66
αs/εH [-]	0.97	1.11	1.14	1.17	1.17	1.17	1.19

α_s: the solar absorptance value, ε_H: total hemispherical emittance.

.

It is known that the temperature of a sphere is determined by the ratio of the solar absorption rate to the total hemispheric emittance. In other words, this ratio determines the temperature potential of the spacecraft. Therefore, it is important to accurately determine the values of the thermo-optical properties of the materials used for the spacecraft surface [16]. Table 4 shows that the α/ε ratio increases as the number of days of irradiation increases. This suggests that the color change may affect the heat flow balance, albeit in a small way.

Furthermore, Figure 11 shows that the solar absorptance varied significantly. The solar absorptivity value increases logarithmically with the number of days of UV irradiation. As for the results of this study, the change in solar absorptivity is almost saturated after about 30 days of irradiation. In addition, the values changed most significantly from 0 to 10 days. In other words, the same results are expected to be obtained in the Ten-Koh 2 Material Mission, which will be launched soon. Therefore, it is necessary to pay close attention to the operational data for one month and especially one week after the satellite is released. If similar results are obtained on the Ten-Koh 2 Material Mission, this test indicates that the use of these materials in the space environment will require some modification, such as applying a coating to protect against UV rays.

The Arrhenius reaction equation is also used for further discussion. The Koike–Tanaka model, which predicts the degradation rate based on environmental factors and the elapsed time, is used as a reference for discussion [17]. The Arrhenius reaction equation used in this study is as follows [17].

$$\ln \left(\frac{\alpha }{{\alpha }_0}\right)=Cu{\left(Ut\right)}^A\exp \left(-\frac{E}{RT}\right)$$

(5)

The parameters used are shown in

Table 5. Parameters used in the Arrhenius reaction equation.

U	T	A	E	R	${\mathit{\alpha }}_0$
$1020 [\mathrm{J}/{\mathrm{cm}}^2$]	303.15 [K]	1.0 [-]	$25.6 [\mathrm{kJ}/\mathrm{mol}$]	$8.314 [\mathrm{J}/\mathrm{mol}·\mathrm{K}$]	0.64

U: amount of UV rays, T: temperature, A: UV degradation characteristic value, E: activation energy, R: gas constant, ${\alpha }_0$: 0 ESD solar absorptance, $\alpha$: solar absorptance, t: time.

.

From the above parameters, I calculated Cu in Equation (5). Figure 12 shows a graph for calculating Cu.

In this study, it was found that there was rapid degradation between 0 and 10 ESD. Therefore, Cu (UV degradation characteristic value) was calculated from the graph of 0~10 ESD in Figure 12.

$$Cu=0.0067 \left[{\mathrm{cm}}^2/\mathrm{J}·\mathrm{ESD}\right]$$

(6)

The degradation of solar absorptivity due to UV rays can be predicted from the UV degradation characteristic values of Cu obtained in this study. Future comparisons with in-orbit experiments should be conducted to determine whether similar values can be obtained in actual orbit.

The results of this study will be discussed in terms of the points to be considered in the future. The results on the sunlight absorption rate confirmed the change in the appearance of the 3D-printed PEEK material, especially the color change. This confirms the deterioration on the surface. Therefore, it is necessary to conduct experiments to determine the extent to which degradation due to UV rays reaches the interior. Elastic modulus measurement using the ultrasonic method, which is a non-destructive test, and the Raman scattering test are considered to be reliable methods for investigating internal degradation. In addition, it is necessary to conduct in-orbit experiments to determine whether the calculated Cu values can be used for prediction.

4.2.2. Three-Dimensional-Printed PEEK/CFRTP

As shown in Figure 10, there was no change in appearance or change in color for the PEEK/CFRTP. Therefore, it is not necessary to consider the thermal control system, which should be considered for 3D-printed PEEK materials. In this study, UV irradiation tests were conducted as ground tests, and no signs of degradation were observed during the experiments. However, further space demonstration tests are needed to confirm whether these results are equally applicable in an actual space environment.

The results of the appearance changes confirm that there are no signs of deterioration on the object’s surface. However, we cannot assert that there is no internal degradation just because there is no change in the external appearance. Therefore, future experiments are needed to investigate the internal degradation and the 3D-printed PEEK materials.

5. Conclusions

In this study, we focused on PEEK/CFRTP and PEEK materials made using a 3D printer as new space materials and investigated their degradation due to UV light. To this end, this study developed a Material Mission and conducted UV irradiation tests simulating the UV environment in LEO. The following important conclusions were obtained from this study.

• UV light in a space environment causes color changes in 3D-printed PEEK materials.
• The change in the solar absorptivity of the 3D-printed PEEK materials with UV irradiation time shows a logarithmic increase.
• From the results on solar absorptivity, it was found that the value of Cu required for prediction is 0.0067 [cm²/J·ESD].
• The 3D-printed PEEK/CFRTP material did not degrade in this ground test.
• This study developed a “Material Mission” to measure the CTE of the 3D-printed PEEK/CFRTP and PEEK samples in LEO without recovery.
• Based on the above, the following points should be considered in the future.
• Conduct tests to confirm the internal degradation of each material.
• Evaluate the results of the ground tests according to the operation of the Ten-Koh 2 LEO environment observation satellite.

This study shows that we should focus on the first month or so after the satellite’s release, especially the 10th day after. By confirming the Cu values calculated in this study in in-orbit experiments, it will be possible to predict the degradation of PEEK materials due to UV light.