Effects of Gamma Irradiation on Solid Propellant Conventional and UV-Cured Binders

Abstract

Ionizing radiations are responsible for bond scission, radical formation, and oxidative degradation of polymer matrices. This study focuses on the effects of gamma irradiation on solid propellant binders, targeting a comprehensive chemical and mechanical characterization of different formulations. Samples were produced either by conventional methods based on hydroxyl-terminated polybutadiene and standard polyaddition reaction using isocyanates, or innovative approaches involving UV-driven radical curing. The samples were irradiated for comparison and to study their evolution as a function of three absorbed doses (25, 45, 130 kGy) for preliminary characterization studies, using a 60-Co gamma source. Samples were irradiated in air at uncontrolled room temperature. The coupling of spectroscopy techniques (Fourier transform infrared—FTIR, Raman and electron paramagnetic resonance—EPR) and dynamic mechanical analysis (DMA) highlighted the key role of antioxidant agents in tailoring mechanical changes in the binder phase. The absence of antioxidants enhances radical formation, oxidation, and cross-linking. These processes lead to progressively increased rigidity and reduced flexibility as a function of the absorbed dose. Complex interactions between photocured components largely influence radical stabilization and material degradation. These findings provide valuable insights for designing novel radiation-resistant binders, enabling the development of solid propellants tailored for reliable, long-term permanence in space, and advancing the knowledge on the applicability of 3D-printed propellants.

1. Introduction

Solid propulsion systems are key technologies in rocket science, thanks to their simplicity, reliability, and large thrust-over-weight ratio. Indeed, in the last 70 years, solid propulsion has been successfully used in small launch vehicles, first stages, or boosters for heavy launchers with low risk, high performance, and competitive cost. Unlike liquid rocket engines, which require intricate plumbing and tanks, solid propulsion relies on a pre-mixed composition of solid ingredients that are ready to ignite, when needed. High energy density, simplicity, storability, and readiness make this technology ideal for applications requiring more compact and robust design solutions, such as mission-critical defense applications or apogee kick motors. More recently, several trade-off studies have shown the potential of solid rocket propulsion for innovative in-space applications, spanning from end-of-life missions to CubeSat propulsion [1,2,3,4]. An extensive review on solid rocket motors (SRMs) for de-orbiting applications can be found in Ref. [3]. Additional applications have been suggested by Guery et al. [5], who underlined the possibility of using solid propulsion in combination with attitude control systems for exploration missions, such as Mars or Moon insertion and soft planet landing. Similarly, Sathiyanathan et al. [6] presented SRM miniaturized systems for high-accuracy attitude control and formation maintenance.

Compared to the traditional use of SRMs for access to space, all these potential scenarios are characterized by new design requirements. Indeed, traditional boosters or first-stage motors are designed to deliver high thrust, ensure reliability during the launch phase and require long-term storage inside a monitored on-ground environment. On the contrary, in-space operations and de-orbit maneuvers require a thrust profile granting accuracy and versatility in the control of spacecraft accelerations and loads, and long-term reliability and operational life in harsh conditions. Even though detailed specifications are strongly mission-dependent, these general requirements align well with current innovative developments in the field of solid propellants, such as the state-of-art investigation of 3D-printed solid propellants [7,8,9,10,11,12,13]. Indeed, controlling local shapes and compositions in solid propellant grain is a technical capability that enables totally new mission architectures. In fact, this approach enables fine-tuning of thrust profiles and burning rates to meet specific performance characteristics, overcoming the limitations imposed by the current grain geometry technology [14].

To date, industrial production and development have been constrained to conventional manufacturing techniques, based on extrusion in case of thermoplastic polymers, or mix-cast-cure for thermosetting propellants. When the focus is on propellants for space applications, conventional formulations consist of heterogeneous mechanical mixtures of oxidizer and fuel powders (e.g., ammonium perchlorate and aluminum, respectively) kept together by an elastomeric binder. The polymeric elastomer is typically built from functionalized liquid oligomers. A common curing process is driven by hydroxyl-terminated polymeric precursors (e.g., hydroxyl-terminated polybutadiene) and isocyanates (e.g., hexamethylene diisocyanate—HMDI, toluene diisocyanate—TDI, or isophorone diisocyanate—IPDI), through polyaddition. These conventional solid propellant formulations are not suitable for additive manufacturing. Although curing times can be tuned by the choice of the isocyanate, the catalyst, and the thermal process, the reaction is not suitable for an accurate 3D printing [15]. If the curing reaction is kept slow, creeps and deformations of the printed layers are obtained [8]. Even with high temperatures and fast-curing isocyanates, the binder does not cure fast enough to retain the shape of the original printed material. Rather, faster reactions can clog the printing device or can induce anticipated curing and short pot life.

Photo-curing is a flexible polymerization methodology based on compositions that produce thermosetting polymers when exposed to radiation of a specific wavelength [16]. This approach enables rapid, localized solidification, allowing each printed layer to retain its shape without deformation. By avoiding premature curing and ensuring fast cross-linking only where ultraviolet (UV) light is applied, UV-curing overcomes the limitations of a slow and uncontrolled reaction. Within this frame, the use of UV sources is well established, and has been applied to different manufacturing techniques, such as stereolithography (SLA) or digital light processing (DLP) [17]. The present work is based on a recently patented process aimed at enabling the photocuring of composite solid rocket propellants and, building upon this technology, developing a new additive manufacturing capability [10]. After an initial screening, polybutadiene was selected for technology consolidation, thanks to thermomechanical performance compatibility with propellant requirements [12]. Interestingly, this binder composition is based on a polymer insensitive to radiation, but it can cure under UV light thanks to additives activating a thiol-ene reaction [18].

Given the growing demand for versatility of SRM propulsion for systems of different type, size, and application, it is crucial to explore the long-term reliability in space of the propellants resulting from photochemical polymerization. The space environment presents significant challenges, such as extreme temperature and high levels of radiation. Thus, a mandatory requirement stemming from some of the aforementioned novel applications is the long-term survivability and radiation tolerance of these novel grains.

In this context, this study aims to provide an initial characterization of the impact of gamma irradiation on three solid propellant binders, cured using either UV-based or isocyanate-based curing processes. The binders share the same core of the prepolymer (i.e., butadiene monomers). The photocured binder system is based on polybutadiene (PB), while the standard formulation is prepared using hydroxyl-terminated polybutadiene (HTPB). A second version of the standard formulation includes an antioxidant agent, aiming at improving the radiation tolerance of the formulation. Samples were irradiated using 60-Co gamma radiation, in air and at room temperature, at total absorbed doses ranging from 25 kGy to 130 kGy, to evaluate the radiation-induced effects as a function of the absorbed dose and obtain valuable insights for binder formulation improvement. Indeed, an initial assessment based on selected total absorbed doses in air is commonly adopted in the open literature for preliminary irradiation studies, as thoroughly discussed in Refs. [19,20,21]. The samples were analyzed by optical, mechanical, and spectroscopic techniques, before and after irradiation. Morphological, structural, and compositional properties were investigated by means of micro-Raman and Fourier transform infrared (FTIR) spectroscopies. Post-irradiation changes were studied by coupling FTIR and electron paramagnetic resonance (EPR) spectroscopies with dynamic mechanical analysis (DMA) to evaluate chemical and structural alterations, radical formation, and radiation-induced change of the mechanical properties. This study highlighted the effectiveness of antioxidants in reducing oxidative degradation and radical formation, as well as the complex interplay of photocured components in stabilizing radicals. These findings provide valuable insights for the development of radiation-resistant solid propellant binders tailored for innovative in-space applications. The paper is organized as follows. Section 2 provides an overview of the space environment and reviews relevant studies in the field of solid propellant irradiation. Section 3 details the materials and methodologies previously mentioned, while Section 4 presents the most important results and conclusions are provided in Section 5.

2. Background

The radiation environment a spacecraft must face is complex and potentially hazardous for its mission. Space radiation interacts with materials, depositing dose and possibly leading to their degradation, or even catastrophic failures of the spacecraft. Expected space radiation conditions vary significantly depending on mission parameters such as orbit, mission duration, and solar activity [22]. Space radiation is influenced by various sources, such as galactic cosmic rays, solar wind, and trapped particles [23,24,25], that create a highly variable and complex environment. In addition, the deposited dose in specific elements included in a space mission is expected to highly depend on the used shielding, on the material composition, and on geometry. Due to these factors, quantifying the expected radiation doses for specific missions is not straightforward.

However, radiation testing is part of the standards applicable to the process of management, engineering, and product assurance for space missions, as indicated by the European Cooperation for Space Standardization (ECSS) norms [26]. Although the application of said norm to this case study is premature because of lack of specific mission details (as requested by the norm itself), it is fundamental to properly address the sensitivity of these polymeric materials to space radiation environment. As a general reference, critical doses for organic materials for use in space applications have been roughly estimated to range up to 100 Gy–100 kGy over the entire mission time, as order of magnitude (e.g., refer to Figure 1 of Ref. [19]). Accelerated testing methods like gamma irradiation provide valuable insights into material degradation trends, enabling to perform parametric experimental campaigns to assess the general behavior of the investigated samples [19]. These methods allow the radiation tolerance of the materials in a specific set of irradiation conditions to be experimentally evaluated, envisaging applicability, potential improvements, or shielding requirements.

In the context of solid propulsion, the element most susceptible to changes from radiation is the polymeric binder that serves as the matrix for energetic materials. The binder not only provides mechanical integrity but also influences the thermal and chemical properties of the propellant. For systems based on a typical inert binder (such as polybutadiene), its aging leads to loss of propellant grain consistency, causing shrinkage, debonding, dewetting, crack generation, and eventually reducing the material toughness [27,28].

At the molecular level, ionizing radiations can induce bond scission, radical formation, and oxidative degradation of polymer matrices [19,29,30,31]. Specifically, the formation of radical species during polymer irradiation leads to interactions among radicals with oxygen, if present, and with the unsaturations within the polymer chains. The interplay of these effects enhances the degree of oxidation in the final materials and can promote cross-linking between various polymer molecules. These radiation-induced processes can result in mechanical property changes as well as chemical composition, molecular structure, and color alterations. The majority of the available studies focus on thermosetting polymer degradation under gamma radiation exposure. A few studies investigate the behavior of UV-cured polymers based on acrylate systems, mostly in the fields of general materials and medical science. The results highlight changes in the microstructure of the materials due to cross-linking and chain scissions and the importance of inhibiting radical propagation reactions and stabilizing the UV-reactive matrix [32,33].

In the context of solid propulsion, the aforementioned changes due to gamma irradiation might lead to cracks and voids in the propellants and, potentially, radiation-induced alterations in the ballistic properties, leading to unpredictable changes of the pressure and thrust levels, unwanted thrust characteristics, and, eventually, in the worst case scenario, motor explosion. The current literature highlights the lack of substantial information on the effects of radiation on solid rocket propellants and, in general, on traditional binders based on polybutadiene. Few studies are available in the open literature, generally focused on the characterization of the macroscopic mechanical properties before and after irradiation tests, disregarding chemical composition changes. Often, the details of the binder formulation were undisclosed. In this respect, lack of details about stabilizers and cross-linking agents prevented a full understanding of the irradiation outcomes, since binder composition could play a key role in determining the effects induced by radiation on the material. Finally, to the authors’ knowledge, no studies have specifically addressed UV-cured polybutadiene systems.

In 1962, Gardner [34] investigated the effect of radiation on the tensile properties of different propellants. He irradiated PBAA (polybutadiene acrylic acid)-ammonium perchlorate-aluminum propellant, an aluminized double-base propellant, and a polyurethane-ammonium perchlorate (AP)-aluminum propellant with gamma-ray doses up to 148 kGy in air at atmospheric pressure. The double-base and polyurethane propellant showed a drop in ultimate tensile strength for doses larger than 40 kGy, while the PBAA-based propellant demonstrated a slight linear decrease in the maximum tensile strength. However, significant differences were observed in strain at maximum stress, secant modulus, and ultimate elongation, highlighting the central role of the binder. Indeed, PBAA-propellant demonstrated a more stable behavior under gamma irradiation, while the double-base propellant was shown to become stiffer but more brittle and the polyurethane-based one more flexible. Additionally, all the propellants showed minor modifications in the burning rate, although to a different extent. Palopoli et al. [35] exposed propellant samples to high-energy electron radiation at NASA MSFC at total absorbed doses of 30 and 60 kGy. Their results showed that radiation induced hardening and increased stress while decreasing strain, advancing the propellant (i.e., the AP) decomposition temperature to 243 °C, which was found to be coincident with the AP orthorhombic-to-cubic phase transition. Similarly, Dedgaonkar et al. [36] observed that, for a HTPB-based propellant exposed to radiation doses up to 720 kGy, the tensile stress progressively increased, while the strain decreased, as either the AP content or the radiation dose was increased. Finally, Holler et al. [37] tested the HTPB-based propellant RESI 172, exposing it to total absorbed doses between 0.025 kGy and 18.4 kGy. Minor changes in the mechanical properties were observed, inducing moderate hardening of the material. No change in the ballistic behavior was highlighted, even though change in the decomposition process of AP was observed, leading to an overall decrease in the decomposition onset temperature of the propellant from 330 °C to 262 °C. This occurrence was previously highlighted also by Palopoli et al. [35].

Therefore, it is clear that the investigation of radiation-induced chemical and mechanical changes is fundamental to assess new propellants applicability. Their formulation and, in particular, the binder composition strongly affect the aging of the propellant grain, demanding an in-depth analysis to properly investigate improvements and to understand the requirements on the mission design. Studies on radiation effects typically involve initial test campaigns based on gamma irradiation at variable doses, thus allowing for a parametric analysis of the material response to radiation. Typical absorbed doses used in the previously discussed studies range from a few kGy up to several hundred kGy. On one hand, this wide range is fundamental to obtain a broad understanding of how propellants behave under different levels of radiation exposures; on the other, the dose variability underlines the absence of a standardization for this kind of experimental studies due to the difficulty in quantifying the expected total doses for specific missions.

3. Materials and Methods

3.1. Materials

In the present study, three different binders were investigated: two were produced using conventional polyaddition reactions with isocyanates, thus generating urethane bonds, while the third one was obtained through UV-polymerization. The standard binders (referred to as H and Ha) were prepared from hydroxyl-terminated polybutadiene (HTPB)—R45 resin cured by isophorone diisocyanate (IPDI—CAS Number: 4098-71-9), using dioctyl adipate (DOA—CAS Number: 103-23-1) as plasticizer and dibutyltin diacetate (TIN—CAS Number: 1067-33-0) as curing catalyst. The Ha samples included an antioxidant additive (AA). The specific type and amount of antioxidant cannot be disclosed due to confidentiality restrictions with the industrial supplier. A typical UV-curing process involves a resin, a photoinitiator, and possibly, other processing agents. In the present work, polybutadiene from Sigma-Aldrich (CAS Number: 9003-17-2, average molecular weight $M_w$ = 5000 g/mol) was mixed with 4 phr (parts per hundred rubber) of phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (BAPO), supplied by Sigma-Aldrich (CAS: 162881-26-7, 97% purity) and thiol (Pentaerythritol tetrakis(3-mercaptopropionate) from Sigma-Aldrich, CAS: 7575-23-7, >95% purity). The UV-based curing process is based on thiol-ene photopolymerization, where the thiol groups react with the unsaturated carbon–carbon bonds of the polybutadiene. The BAPO is a photosensitive component that is converted into a radical upon exposure to UV light, thus triggering the cross-linking reaction. The binder compositions are available in

Table 1. Standard (H and Ha) and UV-cured (PB) binders: weight fractions.

	H and Ha	PB
HTPB	79.21	-
IPDI	7.68	-
DOA	13.11	-
PB	-	84.75
BAPO	-	3.39
Thiol	-	11.86

.

The binders were initially mixed by hand to wet the powder, if present, and then further mixed with a Resodyn LabRam resonant mixer while under vacuum. Binder components were mixed at 50–80 g for 5 min and then the standard and UV-cured fuels were degassed for 5 min and 20 min afterwards, respectively. The isocyanate-based binder was then poured into the molds and cured for 2 days in an oven set at 60 °C. The photosensitive binder was poured into silicon molds and placed inside the cast-cure UV illumination prototype [12] to be irradiated by 365 nm UVA LEDs. The samples were exposed for at least 600 s in an area with 97% irradiance uniformity and a mean irradiance equal to 31.87 W/m². Swelling tests were performed to evaluate the curing degree of the UV-cured binders. The degree of cross-linking determined through the Flory–Rehner method was 97%, which represents an asymptotic value for the curing efficiency/time correlation [38,39,40].

3.2. Methods

3.2.1. Irradiation Tests

Gamma irradiation tests were performed at the Calliope gamma irradiation facility, which is a pool-type irradiation facility equipped with a cobalt-60 radio-isotopic source array emitting two photons in coincidence with a mean energy of 1.25 MeV [41,42,43]. The samples were irradiated at 25, 45, and 130 kGy absorbed doses, with a dose rate value of approximately 1 kGy/h at room temperature in air. All absorbed doses and dose rate values are referred to water. The latter are experimentally determined using the alanine-EPR dosimetry system. To the authors’ knowledge, no standardized methodology currently exists for irradiation studies on polymers intended for use in space applications. Thus, this test campaign follows widely accepted irradiation guidelines (as described in Refs. [19,20,21,29]) which identify gamma irradiation in air and at uncontrolled room temperature as the first step in irradiation studies. In this way, a preliminary characterization of the materials can be performed, to better target, if needed, further investigations on the dependence of radiation effects on specific parameters of interest for specific applications, such as high temperature, or vacuum conditions. Finally, it is worth noting that the chosen absorbed doses are consistent with values reported in the existing literature (refer to Section 2) and with the expected ones in space missions for organic materials [19], while granting a systematic evaluation of the materials degradation.

3.2.2. Characterization Techniques

A Horiba XploRA Plus micro-Raman spectrometer was employed to obtain optical microscope images and Raman spectra of samples. The images were acquired through a microscope at 10X objective in transmittance mode under 80% of illumination. Raman spectra were recorded with a 785 nm laser excitation for 10 s by setting the laser power at 50 mW, diffraction grating at 1200 gr/mm, and objective magnification at 10X. Prior to analysis, the Raman spectra were background-subtracted and smoothed. A Spectrum 100 Perkin-Elmer FT-IR spectrometer was used to collect the FTIR spectra in the range 650–4000 cm⁻¹, before and after irradiation, under attenuated total reflectance (ATR) mode by using a ZnSe crystal. To perform the analysis of each spectrum, the background (i.e., air) was subtracted, and a correction of the baseline was performed. For each sample, three independent spectra were recorded, and the mean values of the analyzed peaks’ parameters were used. Principal component analysis (PCA) was used to disclose key differences in chemical composition between the samples before irradiation. The analysis was conducted by normalizing to one the data before computation. The first two principal components (PC1 and PC2) were derived as the major contributions to variance, providing insights into the unique characteristics of each polymeric blend before undergoing irradiation [44]. The influence of irradiation on the oxidation of the polymeric matrices was assessed by calculating the carbonyl index (CI) [45,46]. The latter is determined by the ratio of the area of the FTIR spectrum peak at 1730 cm⁻¹ ($A_{1720}$) to the area of the peak at 2900 cm⁻¹ ($A_{2900}$), as detailed in Equation (1):

$$CI={\displaystyle \frac{A_{1720}}{A_{2900}}}$$

(1)

Specifically, for these measurements, the frequency was 9.4 GHz, the microwave power 0.14 mW, and the magnetic field varied within the range of 3400–3580 G. The samples were placed in a conventional quartz tube. All the EPR spectra reported in Section 4 were normalized to the sample mass, with each sample weighing approximately 80 mg. Dynamic mechanical analysis (DMA) measurement tests were performed to obtain the elastic ($E^{\prime }$) and loss ($E^{″}$) modulus of the materials. A DMA-2980 analyzer from TA Instruments was employed in flexion mode with single cantilever clamps. Samples of 35 × 15 × 5 mm were tested at 30 °C at frequencies of 0.1, 5, and 10 Hz with an oscillation amplitude of 15 $\mathsf{\mu }$m. For statistical significance, at least three tests were conducted for each sample. For each sample type and irradiation condition, three independent replicates were produced and tested. The error bars shown in the figures, expressed as standard deviations, are calculated based on measurements performed on these three distinct samples for each condition.

4. Results and Discussion

4.1. Unirradiated Samples

For each type of polymeric blend, 3D samples with cubic, parallelepiped, and cylindrical architectures were produced (see Supporting Material for details). Representative pictures, optical microscope images, and deconvolved Raman spectra of reference HTPB (H samples), HTPB containing antioxidant (Ha samples), and polybutadiene loaded with thiol molecules (PB samples) before irradiation are shown in Figure 1.

As shown in Figure 1a, the 3D objects exhibit well-defined shapes and geometries. In particular, the topography of H, Ha, and PB samples is characterized by a striated texture at the microscale level introduced by the mold during processing—refer to Figure 1b. It is worth noting that the samples before irradiation exhibit different colors, likely due to varying chemical formulation of the different blends used to produce the 3D objects. Similarly, these differences can be observed in the Raman spectra, revealing that the chemical formulations of the polymeric blends is crucial for tailoring the structural and appearance features of the final materials. While all the Raman spectra (Figure 1c) show typical signals of the polybutadiene phase [47,48]—refer to the Supporting Material for details—the PB samples also exhibit a band around 850 cm⁻¹, which is plausibly attributable to the bending modes of SH groups. In addition, the signal associated with C=C vinylic functional groups is clearly distinguishable in the spectra of H and Ha samples, while this peak is weaker in the PB samples spectra. Finally, the ratio between the sum of the integrated intensity values of C=C and C-C signals is 2.2, 2.3, and 1.8 for H, Ha, and PB samples, respectively. According to these observations, Raman spectroscopy analysis indicates that the structural features of the H and Ha samples are highly similar, while the chemical formulation and the high cross-linking degree achieved through the photo-reticulation process result in a significantly lower degree of unsaturation for the PB samples. Additional information on the chemical composition of H, Ha, and PB samples is provided by FTIR-ATR measurements and principal component analysis, as detailed in Figure 2.

In agreement with Raman spectroscopy analysis, all FTIR spectra show the signals attributable to polybutadiene-based plasticizers, such as the stretching vibration bands of C-H on saturated carbon atom between 3100 and 2800 cm⁻¹, the CH=CH₂ stretching bands, and the CH wagging bands of trans-1,4, vinyl and, cis-1,4 moieties at 3070, 966 and 911 cm⁻¹, and 725 cm⁻¹, respectively [48,49,50]. The broad OH stretching absorption band between 3600 and 3200 cm⁻¹ can be assigned to alcohol groups of HTPB molecules or to absorbed water. The signals at around 1530 and 1230 cm⁻¹, ascribable to the formation of urethane species, are clearly detectable only for the spectra of H and Ha samples, while these peaks are barely observable in the PB samples spectrum. Analogously, the ratio between the trans-1,4 and vinyl functional groups is similar for the H and Ha samples, while the value is greater for PB samples. All these findings corroborate the different chemical composition of the produced materials, as clearly highlighted in Figure 2b, where the loading plot derived from PCA is shown. As depicted in Figure 2b, the PC1 and PC2 values (i.e., the first two principal components resulting from PCA, which represent the directions of maximum variance in the data) are similar for the H and Ha samples, while both PC1 and PC2 values are different for the PB samples.

The DMA results at 5 Hz are discussed below, while details at 0.1 and 10 Hz are available in the Supporting Material. These additional data do not affect the conclusions derived from the results presented in the main text. Figure 3 shows the DMA complex modulus, defined as $E^{*}=E^{\prime }+i{E}^{″}$. The complex modulus is mainly affected by $E^{\prime }$, which ranges approximately from 1 to 4 MPa, while $E^{″}$ is one order of magnitude lower. Thus, this indicates that the materials mainly exhibit elastic behavior and an extremely low energy dissipation. A different response can be observed between the HTPB-based samples and the 3D-printed UV-cured material due to the different chemical composition discussed in the previous paragraphs. Specifically, H and Ha exhibit an $E^{*}$ of approximately 1 MPa, while PB shows a higher $E^{*}$ of more than 4 MPa. This difference in stiffness—even though minimal if focusing on the absolute values - suggests that the H and Ha samples may exhibit greater deformability than the PB sample. These results are consistent with the structural features observed through Raman spectroscopy analysis, suggesting that the high cross-linking degree obtained through the UV-curing process leads to a greater $E^{*}$ with respect to the H and Ha samples.

4.2. Irradiated Samples

Representative pictures of H, Ha, and PB samples with parallelepiped geometry before and after irradiation at several doses are shown in Figure 4.

As shown in Figure 4, gamma irradiation induces different effects on the appearance of the samples, depending on the absorbed dose as well as on the chemical composition of the samples. For the reference H samples, no visible changes are observed up to an absorbed dose of 45 kGy. However, at 130 kGy, the sample begins to show a distinct yellowing. On the contrary, the Ha samples undergo browning at 25 kGy, while from 25 kGy up to 130 kGy, no further significant color change can be observed. These findings suggest that the presence of antioxidant components within the chemical formulation plays a crucial role in the generation of chromophores after gamma irradiation. In particular, the color change suggests that the involvement of antioxidants mitigates radical-induced reactions due to gamma exposure, preventing significant chromophore formation and thus stabilizing the color at higher absorbed doses. Photopolymerized PB samples do not exhibit significant chromatic variation. This behavior is likely due to the inherent properties of the photopolymerized materials, which appear less prone to gamma-induced chromophore formation under these irradiation conditions. Given such considerations, it is reasonable to argue that the PB chemical formulation (including photosensitive components) supports radical formation but does not lead to visible color changes.

Further information about the effects induced by gamma radiation is provided by the FTIR-ATR analysis [51,52,53]. In Figure 5, the FTIR-ATR spectra of H, Ha, and PB samples before and after irradiation are reported (Figure 5a–c), along with the percentage variation of the carbonyl index parameter, denoted as “$\Delta$(CI)%” (calculated with respect to the CI of the unirradiated samples), as a function of the absorbed dose (Figure 5d).

The H and PB samples are characterized by a noticeable broadening of the bands associated with C=C unsaturations as the absorbed dose increases—yellow box in Figure 5a,c. Concurrently, the increase in absorbance values corresponding to OH and C=O bonds is consistent with a radiation-induced oxidation phenomenon. In contrast, the FTIR spectra of the Ha samples exhibit minimal changes despite the increase in the radiation doses. The signals corresponding to C=C and OH bonds remain nearly unchanged, while the C=O absorbance slightly increases from 0.59 to 0.67 between 0 and 25 kGy. After 25 kGy, the CI values remain constant (refer to Figure 5b). To better clarify these considerations, the trends of $\Delta$(CI(%)) values are depicted in Figure 5d. For Ha, the $\Delta$(CI(%)) increases to approximately 20% at 25 kGy and remains constant up to 130 kGy. This behavior can be reasonably attributed to the presence of the antioxidant within the polymer matrix, which effectively mitigates the formation of radicals, thereby preventing extensive oxidation and cross-linking. On the contrary, the absence of antioxidant agents in the reference H polymeric matrix results in a different trend. Specifically, the $\Delta$CI(%) increases monotonically from approximately 20% at 25 kGy to 100% at 130 kGy. Due to the different chemical compositions, the most significant increase in $\Delta$CI(%) is observed for the PB samples. More specifically, the $\Delta$CI(%) increases from 100% at 45 kGy to 150% at 130 kGy. Given such considerations, the different molecular structures and chemical compositions of the samples play a crucial role in modulating the effects of gamma irradiation.

Further considerations can be derived from the EPR analyses. As shown in Figure 6a–c, the intensity of the EPR signal normalized to the sample mass increases with the absorbed dose for all samples. More specifically, the peak positions in the spectra reasonably indicate that the radicals formed are organic in nature. In samples H and Ha (Figure 6a and Figure 6b, respectively), the EPR peak shapes are similar, but the intensity of the signal increases more significantly with radiation dose for sample H than for Ha. The behavior of sample PB is notably different. At an absorbed dose of 25 kGy, the EPR signal is already pronounced, with integrated intensity values comparable to those of the H samples at 130 kGy. Moreover, under all tested gamma irradiation conditions, the area of PB signal is more than ten times that of sample Ha. Analogously to the results achieved by FTIR analysis, these observations can be attributed to the initial chemical formulation and the synergistic effects of the components within the polymer matrices, which collectively interact with gamma radiation.

In particular, the interaction of gamma rays with these matrices promotes the formation of free radicals, a key mechanism underlying the observed changes in material properties [54,55]. Specifically, unsaturations, C atoms bonded to N, O, and S atoms, and aromatic molecular structures in the initial mixtures provide sites available for radical formation. Due to the peculiar chemical composition of the Ha samples, the results of color inspection, FTIR, and EPR indicate that the presence of antioxidant agents mitigates radical formation at the tested absorbed doses. Conversely, the absence of antioxidant species leads to a substantially higher quantity of stable radicals within the H sample matrix. At 130 kGy, the significant number of radicals likely facilitates the formation of chromophore groups, which explains the observed yellowing of sample H. A different behavior was observed for the PB samples, where the presence of reactive species, such as photoinitiators, thiols, and photosensitive molecules leads to pronounced EPR peaks even at low doses (e.g., 25 kGy). The shape of the EPR signal peaks further suggests the presence of multiple radical species in this type of sample, highlighting a complex interplay between chemical components and following radical production and kinetics. The substantial modifications observed in the FTIR spectra ($\Delta$CI(%) > 150%) suggest that the radical entities are stabilized within the intricate chemical composition of the PB polymer matrix, reasonably explaining the absence of significant color changes in the PB samples, even at the highest absorbed dose.

To further support these findings, the mechanical properties of the H, Ha, and PB samples were investigated through dynamic mechanical analysis, providing insight into how chemical structure correlates with macroscopic performance under irradiation. The DMA results at 5 Hz are shown in Figure 7. Broadly speaking, an increase in both the elastic modulus and loss modulus can be observed across the samples, even though to a different extent. The loss modulus remains roughly one order of magnitude lower than the elastic modulus, ranging approximately from 0.1 to 1.5 MPa. Hence, similarly to the original samples, the complex modulus values are mostly dictated by the elastic modulus $E^{\prime }$. Two distinct behaviors can be recognized, depending on whether the antioxidant is part of the formulation. Indeed, Ha shows a relatively stable complex modulus which is consistent with its minimal chemical changes, showing a maximum $E^{*}$ increase of approximately 25% at 130 kGy. These minor modifications highlight that the elastic and viscous responses differ slightly among the samples characterized by different absorbed doses, demonstrating consistent mechanical characteristics. This aspect makes Ha particularly suitable for granting safe and repeatable performance, as well as acceptable mechanical characteristics in solid propellants intended for in-space systems. Indeed, the antioxidant seems to mitigate major changes in the polymers (i.e., avoiding extensive cross-linking or chain scission), thereby resulting in a limited increase in sample rigidity. On the contrary, both H and PB demonstrate a large change in the complex modulus likely due to the substantial chemical modifications indicated by the previous chemical analyses. The H samples show a progressive change in the $E^{*}$, suggesting a gradual increase in stiffness as the absorbed dose increases, while PB samples exhibit large complex modulus values even at 25 kGy and a subsequent 30% increase from 25 kGy to 130 KGy. Interestingly, the elastic modulus trend closely resembles both the variation of EPR signal area and the variation of CI index values (relative to the measurements of the unirradiated samples) as a function of the absorbed dose for H, Ha, and PB samples. This confirms the close relation between the mechanical property changes and chemical formulation modifications induced by the gamma irradiation.

5. Conclusions

Polymeric binders are fundamental components of solid propellants, as they ensure the necessary mechanical and chemical stability in harsh space environments. The effects of gamma radiation on traditional formulations are poorly addressed in the open literature, and even less is known about the materials compatible with additive manufacturing technologies. This study explored the effects of gamma irradiation on three different formulations: a UV-cured binder based on polybutadiene and thiol-ene chemistry (PB), and two traditional HTPB-based systems, one containing antioxidants (Ha) and one without (H). The materials were exposed to absorbed doses of 25, 45, and 130 kGy and characterized using Raman, FTIR, EPR spectroscopy, and DMA techniques. The results revealed significant degradation in both the PB and H samples, with PB showing chemical and mechanical changes even at the lowest dose. These modifications are linked to the presence of reactive species such as thiols and photoinitiators. On the contrary, the Ha samples showed a more stable behavior, indicating the effectiveness of antioxidants in reducing oxidation and radical-driven damage. The EPR measurements confirmed these trends, with lower signal intensity observed in Ha compared to the other formulations. The findings emphasize the importance of binder chemistry in determining radiation resistance and suggest that antioxidants play a key role in improving long-term performance. This work provides a solid basis for the design and optimization of propellants intended for use in space, along with a reliable methodology for future experimental studies under relevant environmental conditions.

Future studies should focus on two main directions. On one hand, the binders should be irradiated in an inert-gas atmosphere and under vacuum, to evaluate the extent to which these conditions modify the degradation processes identified in this work. In particular, irradiation in the presence of oxygen can lead to oxidative degradation mechanisms that are strongly influenced by oxygen diffusion within the polymer matrix. This may result in diffusion-limited oxidation (DLO) [56,57], where oxidation occurs primarily close to the surface, leading to heterogeneous material properties. Moreover, vacuum or inert atmospheres more closely replicate space environments and would minimize oxidative effects, potentially highlighting different degradation reactions—such as chain scission or cross-linking—without the influence of oxygen. Secondly, the high sensitivity of PB samples to gamma radiation demands an in-depth study on antioxidants for UV-curable binders. This may lead to the identification of radiation-resistant antioxidants that are chemically compatible with thiol-ene systems and do not interfere with the photopolymerization process. Additionally, modifications to the binder matrix itself (i.e., adjusting the thiol-to-ene ratio or incorporating more stable backbone structures) may be investigated, while preserving printability. As an alternative approach, the effect of radiation shielding on polymer aging should be explored. Finally, the investigation of the interplay between the binder and oxidizer shall be studied to better simulate real propellant conditions. These directions are essential to enhance the long-term performance of 3D-printed solid propellants in space environments.