# Regolith-Rich PEEK Composite Bricks: Steps Towards Space-Ready Lunar Construction Materials
## 1 Introduction
### Organic Phase Selection  
This subsection provides a comparative analysis of high-performance thermoplastics frequently considered for space-grade applications, including polyetheretherketone (PEEK), thermoplastic polyimide (TPI), polyphenylene sulfide (PPS), polytetrafluoroethylene (PTFE), and polyetherimide (PEI). It examines their thermal and mechanical properties, as well as the effects of radiation and UV exposure. Notably, all materials considered meet the requirement for Total Mass Loss (TML) below 1% [27]. Based on reference values from the literature [28], these materials can be ranked from most to least performant as follows: PTFE, PPS, PEEK, TPI, and PEI. The melting temperature is a critical parameter for processing, where, in principle, a lower melting temperature is advantageous for easier handling. However, both the melting and glass transition temperatures must be sufficiently high to withstand the Moon's extreme temperatures. PEEK has a melting point (T\({}_{m}\)) of 343 \({}^{\circ}\)C and a glass transition temperature (T\({}_{g}\)) of 143 \({}^{\circ}\)C [29]. In comparison, TPI exhibits a significantly higher T\({}_{g}\) of 250 \({}^{\circ}\)C and T\({}_{m}\) of 388 \({}^{\circ}\)C [30]. PPS, lower in thermal resistance, has a melting point of 280 \({}^{\circ}\)C and a glass transition temperature of 85 \({}^{\circ}\)C [31]. PTFE presents a unique case; while its glass transition temperature remains debated [32], it has a melting point of 260 \({}^{\circ}\)C. Lastly, PEI offers a T\({}_{g}\) of 215 \({}^{\circ}\)C; as an amorphous thermoplastic, it lacks a distinct melting point, but processing temperatures between 340 \({}^{\circ}\)C and 400 \({}^{\circ}\)C are generally used to achieve a molten state [33]. Another important thermal property to consider is the Coefficient of Thermal Expansion (CTE), as it indicates the material's tendency to deform with temperature changes. Among the materials under consideration, PEEK performs best, with the lowest CTE at 46 \(\times\) 10\({}^{-6}\) 1/\({}^{\circ}\)C, followed closely by PPS, TPI, and PEI, with values of 52, 55, and 65 \(\times\) 10\({}^{-6}\) 1/\({}^{\circ}\)C, respectively. In contrast, PTFE is more affected, with a higher CTE around 124 \(\times\) 10\({}^{-6}\) 1/\({}^{\circ}\)C.  
Beyond mere survival, the material must retain key properties under the temperature fluctuations to be expected on the Moon. Thus, evaluating how they affect mechanical properties is essential. The tensile strength, for which data are available in the literature across a wide temperature range, can be considered as an indicator. At room temperature, PPS, TPI, PEEK, and PEI exhibit strong performance for structural applications, with yield strengths of 93, 92, 90, and 86 MPa, respectively. PTFE, however, shows a significantly lower yield strength of 21 MPa. At low temperatures (\(-\)196 \({}^{\circ}\)C), PPS stands out with the highest yield strength at 260 MPa, while PEEK and TPI also maintain structural stability with yield strengths of 200 MPa and 189 MPa. Although lower, PTFE and PEI still offer essential values for low-temperature applications, with 140 MPa and 101 MPa, respectively. At high temperatures (150 \({}^{\circ}\)C), TPI leads with a yield strength of 58 MPa, followed by PEEK at 35 MPa and PEI at 17 MPa. PPS and PTFE, however, experience significant declines at high temperatures, dropping to 3.5 MPa and 2 MPa, which limits their suitability. The degradation of mechanical properties due to ionizing radiation varies significantly across different materials. In general, when polymers are irradiated, the breaking of the main molecular chains might occur [34], contributing to the decline in the material's overall performance. When it comes to mechanical performance, residual tensile strength after irradiation is a good parameter for assessing material degradation. However, direct comparisons between different polymers are challenging due to the substantial variation in their initial properties.