Thermal,mechanical,and morphological properties of aerospace clip panels using woven carbon fibre reinforced PPS

Abstract

The use of advanced thermoplastics in aircraft applications is steadily increasing due to their excellent mechanical strength and thermal stability. This study investigates Carbon Fibre Reinforced Polyphenylene Sulfide (CF/PPS) composites utilising a 5-Harness Satin (HS) fabric. Quasi-isotropic laminates with two different thicknesses underwent a thermoforming process at varying tool temperatures. The objective is to evaluate the effect of the degree of crystallinity (DOC) on the mechanical properties, specifically Impact Strength and Interlaminar Shear Strength (ILSS), as well as on thermal properties through Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) analysis. The results show that the DOC of CF/PPS in both 6- and 8-ply samples led to an improvement of over 50% in impact strength. Furthermore, ILSS increased by 15.3% and 10.38% at tool temperatures of 170°C and 160°C for the 6 plies and 8 plies laminates, respectively. TGA analysis after stamping indicated improved thermal stability with higher residual content, suggesting enhanced char formation. Lastly, for DSC results, DOC demonstrated 22.8% crystallinity after the thermoforming process, further supporting the correlation between DOC and mechanical performance.

Keywords

CF/PPS composites DSC ILSS impact TGA thermoforming

Introduction

Poly(phenylene sulphide) (PPS) is a semi-crystalline thermoplastic polymer synthesised via polycondensation of 1,4-dichlorobenzene with sodium sulphide in polar solvents at elevated temperatures, resulting in alternating phenylene–sulphide backbones.¹ PPS is recognised for its mechanical strength, chemical resistance, and thermal stability, enabling reliable performance under harsh conditions such as elevated temperature and corrosive environments.^2,3 As a high-performance engineering polymer, it has been widely employed in aerospace, automotive, electronics, and precision instruments.^3,4 Despite challenges associated with its high melting point and specialised processing, PPS remains an attractive material for advanced composites.⁵

According to Hamdan et al.,⁶ the reinforcement of PPS with carbon fibres enhances stiffness, toughness, and fatigue resistance, making CF/PPS one of the most promising thermoplastic composites for structural aerospace applications. Recent studies emphasised its suitability for out-of-autoclave processing,⁷ thermoforming,^5,6,8 resistance welding,^7,9 and additive manufacturing^10–13 due to its melt reprocessability and dimensional stability.¹⁴ These composites are increasingly being explored for aircraft primary and secondary structures, where high strength-to-weight ratios and recyclability are critical.^3,14–16

Thermoforming is a process used to shape thermoplastic materials from flat panels into complex 3D geometries.¹⁷ The thermoplastic is heated to its processing temperature until it becomes pliable, then formed into the desired shape using a mould or tool. To ensure optimal part quality, the forming tool is also heated to maintain consistent temperature and improve material flow during stamping.^6,16–21 The example of a product that can be stamped via thermoforming is a clip.^22,23

For the mechanical properties of CF/PPS, Hu et al.²⁴ conducted tests on a carbon fibre plain woven fabric (T300 3K) with a thickness of 0.15 mm and a PPS film with a thickness of 0.05 mm using the interlaminar shear strength (ILSS) method according to ASTM D2344. The study found that failure modes of CF/PPS composites occurred due to delamination under flexural stress and shear loads between layers. Additionally, it was observed that the interfacial strength of the fibre matrix could be improved to enhance the chemical bonding of the composites by introducing Silane Coupling Agents (SCAs), which resulted in an increase in strength by 21.73%. Meanwhile, Chen et al.²⁵ tested unidirectional CF/PPS using the ILSS (ASTM D2344) and reported that ILSS strength increases with increasing DOC, particularly at low cooling rates. Similarly, Lona et al.²⁶ evaluated CF/PPS (comprising laminates of carbon fibre plain weave fabric and thick PPS films) using ILSS (ASTM D2344) and concluded that ILSS increases as crystallinity increases.

Next, Hughes²⁷ investigated the impact strength of CF/PPS laminates using the Izod Impact method in accordance with ASTM D256. The study found that 0° samples absorbed more energy than 90° samples, indicating a directional dependence in energy absorption. Similarly, Zhao et al.²⁸ studied CF/PPS composites fabricated using PPS granular slicing and PPS nonwovens reinforced with carbon fibre fabric (CFF, 3K-T300), tested using the Izod impact method. Their findings highlighted that unidirectional fibre laminates exhibited weakness when loaded at 90° to the fibre direction. Moreover, the study observed that composites with 60–85 wt% CFF content show significant improvements in mechanical properties, particularly impact strength.

For thermal analysis, Yeole et al.²⁹ examined the thermal stability of CF/PPS using Thermogravimetric Analysis (TGA). Their results showed that the TGA curve exhibited less than 1% weight loss up to 400°C, indicating good PPS stability and defining its upper processing temperature limit.³⁰ Degradation for PPS was completed at 700°C, where the composite retained 74 wt% of its mass, suggesting that the presence of carbon fibre (CF) contributes to an increase in carbonaceous residue. Meanwhile, Kumar et al.³¹ conducted a comparative study on PPS compounded with 50 wt.% CF, PPSU with 25 wt.% CF, and ABS pellets compounded with 20 wt.% CF. The degradation onset temperature at 5% weight loss (T-Onset) was recorded as 525°C for PPS, 545°C for PPSU, and 350°C for ABS. At 900°C, the residual content was highest for PPS at 71.16 wt.%, followed by PPSU at 53.80 wt.% and ABS at 21.24 wt.%. The increased residual content was attributed to the presence of carbon atoms in PPS and PPSU converted into carbonaceous residue at elevated temperatures.

Yeole et al.²⁹ investigated the thermal behaviour of CF/PPS pellets using Differential Scanning Calorimetry (DSC). The results showed that PPS began to recrystallise during cooling at approximately 250°C. During the final heating cycle, the glass transition temperature was observed between 85 and 90°C, while the melting point ranged from 280–285°C. This melting point plays a critical role in defining the lower processing temperature limit for CF/PPS composites. Next, Hassan et al.³² studied CF/PPS composites composed of 3K-T300-5HS carbon fibres with a density of 1.27 g/cm³ and PPS resin films arranged in 8 plies. Their findings revealed that crystallisation began at a relatively high temperature when the cooling rate was fast, shortening the crystallisation time. The study suggested that under such conditions, the polymer did not have sufficient time to crystallise, leading to the formation of smaller crystals and a higher number of defects.³³

Although previous studies commonly evaluate CF/PPS, there is a lack of research examining properties from specific geometric regions of thermoformed components, particularly those that appear flat but structurally belong to a shaped part. In a thermoformed clip panel, the long flange is directly influenced by forming-induced effects such as temperature gradients, local fibre realignment, and matrix redistribution. The existing literature does not address how these subtle yet critical thermoforming histories affect the mechanical, thermal, and morphological responses of these regions. Therefore, sampling along the long flange fills an important research gap by capturing geometry-dependent forming variations.

Therefore, in this study, the mechanical and thermal properties of CF/PPS are highlighted in the long-flange region. Not only that, but the specimens’ characterisation also meets the requirements of the aerospace industry, focusing on mechanical testing, specifically impact and interlaminar shear strength (ILSS). At the same time, thermal properties were examined through TGA and DSC analyses. Additionally, morphological analysis was conducted to understand the bonding between CF and PPS, particularly after the thermoforming process for the fabrication of the pressed-formed clip panel.

Materials and methods

Materials

The thermoplastic composite used in this study was carbon fibre reinforced polyphenylene sulphide (CF/PPS) woven laminate of 5-Harness Satin (HS), with material specifications based on the TenCate Cetex® TC1100 PPS resin system. The CF/PPS was purchased from Toray Advanced Composites (formerly TenCate Advanced Composites) in the Netherlands. The CF/PPS panel came with 3.66 m × 1.22 m pre-consolidated reinforced laminate (RTL). The material specification of CF/PPS is as shown in Figure 1 and Table 1.

Table 1.

Composite materials specification.

Material	Thickness	Fibre	Resin content (%)	Orientation	$T_{g}$ (°C)	$T_{m}$ (°C)	$W_{f}$ (%)
5-Harness satin	1.90 mm (6 plies)	T300 J 3K	43	[(0,90)/(±45)/(0,90)]_S	93.75	283.68	58.4
5-Harness satin	2.52 mm (8 plies)	T300 J 3K	43	[[(0,90)/(±45)]₂]_S	93.94	283.61	58.2

Experimental methods

The thermoforming process was performed at the Smart Manufacturing Technology Centre (SMTC), UTeM, Ayer Keroh, Melaka, Malaysia using the Integrated Hot-pressed Machine. Before thermoforming, the simulation analysis of the thermoforming process was conducted using ANIFORM software. The drapeability, tensioner (spring), gripper element, adjuster, and spring location were simulated as shown in Figure 2.⁷ In this project, the chosen spring with a stiffness of 0.62 N/mm demonstrated good formability of the panel during thermoforming, with no wrinkles or tears. Next, the auxiliary equipment from the simulation was translated into the experimental process.

The thermoforming of the press-formed clip panel began with installing the RTL into the blank holder, with 4 AUY12-50 tensioners/springs (0.62 N/mm) at each edge. Next, the RTL underwent a heating process to soften the material below the degradation temperature, but higher than $T_{m}$ for the panel to be processed, allowing it to be pressed more easily during the stamping process. It was recorded that the 6 plies required 120 s to reach the processing temperature of 320°C, while the 8 plies needed 150 s.

Prior to this, the tool was set up to the different temperatures of 150, 160, 170, 180, and 195°C. The consolidation time was about 135 s, with a pressure of 12 bar during the thermoforming process, resulting in full consolidation. The entire methodology for the thermoforming process and its characterisation by thermal testing, mechanical testing, and morphological analysis is shown in Figure 3. Finally, the demoulding process took place, and the final product was cut accordingly to the testing standard dimensions using a waterjet, after cooling to room temperature as shown in Figure 4.

The extraction of specimens for testing purposes was fully covered by the consolidated area from the tool along the long flange. The mapping is as shown in Figure 5.

Thermal testing

Thermogravimetric analysis (TGA) (ASTM E1131)

For TGA, the machine used was TGA Perkin Elmer STA 6000 (USA). The sample size was 10 mg. 10 mg of the samples were heated from 30 to 992°C at a heating rate of 10°C/min under a nitrogen atmosphere with a flow rate of 100 mL/min, according to the test standard ASTM E1131.³⁴

Differential scanning calorimetry (DSC) (ASTM D3418)

The procedure for DSC is specified in ASTM D3418 standard.³⁵ TA Instruments Q2000 (USA) was used to conduct this test. The sample size of 10 to 15 mg was crimped into an aluminium pan and tested over a temperature range of 30 to 330°C at a scanning rate of 10°C/min under a nitrogen atmosphere. The DOC was calculated by using the following equation (1):

X = \frac{Δ H}{{Δ H}_{100 %} (1 - W_{f})} x 100 %

(1)

where X is the DOC, ΔH is the enthalpy from the melt peak from the DSC curve,

{Δ H}_{100 %}

is the enthalphy of melting a theoretical 100% crystalline polymer (150.4 J/g), and

W_{f}

is the fibre content as a percentage of an initial mass from the acid digestion test (0.58).

Mechanical testing of CF/PPS composites

Mechanical testing for the CF/PPS encompassed: interlaminar shear strength and impact strength. For each test, five samples were tested, and the average values were recorded.

Izod unnotched impact test (ASTM D256)

Izod Unnotched Impact Test was conducted by using the Motorised Pendulum Impact Tester CEAST 9050. The pendulum hammers were set to 22 J of force at an impact speed of 3.46 m/s, with a starting angle of 150° and a length of 326.8 mm. The dimension of the specimen was 64 mm × 12.7 mm based on ASTM D256³⁶ as shown in Figure 6.

Interlaminar shear strength ILSS (EN 2563)

The ILSS test was performed using a Universal Testing Machine 5 (UTM5)/Instron 5967 according to EN 2563.³⁷ The specimen dimensions were 20 mm × 10 mm, with a span length of 10 mm and a speed of 1 mm/min. The loading nose and support radii were 3 mm, as shown in Figure 7.

Characterisation

Morphological analysis

For morphological observation, a JEOL6010 PLUS-type SEM was used. The fracture surfaces were sputter-coated with platinum to provide electrical conductivity to the specimens, enabling better observation of the surface morphology of CF/PPS. The test setup is shown in Figure 8.

Results and discussion

Thermogravimetric analysis (TGA)

Thermal analysis is the general term given to a group of analytical techniques that measure the properties of a material, and one of them is TGA. It is a method that measures weight changes in materials as a function of temperature or time in the presence of inert or reactive gases, such as nitrogen, following ASTM E1131.³⁴ The materials degrade as the temperature increases.³⁸ As it degrades, the weight of the sample is reducing on the balance.³⁹ The main objective of TGA is to measure a material’s thermal stability and its composition, which helps to understand the safe service temperature of the material. The TGA results of both plies are shown in Figure 9 and Table 2.

Table 2.

Data residue of CF/PPS composites at different tool temperatures.

Ply	Tool temperature (°C)	Control sample residue (%)	Final tool temp residue (%)
6	150	46.92	70.72
	160		71.07
	170		72.85
	180		74.74
	195		73.46
8	150	43.93	69.95
	160		73.55
	170		72.80
	180		71.32
	195		73.94

In this study, the initial degradation temperature (IDT) of the material started to degrade at 453.20°C and 347.95°C for 6 and 8 plies, respectively. Whereas, according to Rahate et al.,⁴⁰ the breakdown of pure PPS and composite begins at around 450°C and 465°C, respectively. Thus, the 6 plies recorded a higher IDT compared to 8 plies. The residual weight of the 6-ply CF/PPS was 46.92%, whereas the 8-ply CF/PPS was 43.93% for both control samples.

According to Khan et al.,³⁰ two degradation phases were observed in the thermograms of pure PPS and PPS/CF composites during thermogravimetric analysis (TGA). The degrading section was originally attributed to scission of the PPS polymer chains, and then to depolymerisation from radical chain ends, disproportionation, and cyclisation. The addition of CF delayed matrix deterioration and enhanced thermal stability by lowering the onset degradation temperature, indicating that most of the degradation was due to random scission of polymer chains.

After the thermoforming process, the results were totally different in the thermal residual balance. The final residue was recorded at around 70–75 %. The lowest final residue was recorded in both plies at a tool temperature of 150°C, with 6 plies and 8 plies recording 70.72% and 69.95%, respectively. However, the final residue of the stamping data is less significant when comparing them. The data obtained show that thermal stability was improved after the stamping process.

In this study, the Differential Thermogravimetry (DTG) value was observed near 540 to 550°C. This result also show good agreement with previous researchers, as reported by Díez-Pascual et al.,⁴¹ who stated that the initial decomposition of 2% and 10% occurred at 502°C and 546°C, respectively. This also aligns with Carpier et al.,⁴² who reported that IDT occurred at 483°C for plain PPS and 503°C for CF/PPS. In contrast, Schuhler et al.⁴³ found that thermal deterioration begins at 480°C, with considerable mass loss occurring around 550°C for PPS-based composites. This is due to PPS exhibiting good dimensional and thermal stability, owing to its ordered alternating arrangement of phenylene and sulphide atoms.⁵

In PPS-based composites, thermal deterioration began between 350 and 500°C, and considerable mass loss occurred between 500 and 600°C (first decomposition step). The primary mechanism in the first breakdown stage is thought to be random chain scission. In contrast, the second decomposition process is believed to be the oxidation of the carbonaceous char created in the first decomposition phase.⁴⁴ Not only that, previous studies on residual weight of the reference PPS/CF recorded 76% in their findings.⁴⁵

Compared to the control samples, the increase in residue for 6 plies occurred at 180°C (+59.29%) and 195°C (+68.31%) for 8 plies. The increase in TGA residue due to thermoforming was primarily attributed to thermal-history effects on the PPS matrix, rather than inhomogeneity at the laminate level. This is further influenced by the recrystallisation of PPS crystals, as well as its inherent dimensional and thermal stability due to the ordered alternating structure of phenylene and sulphide units. Therefore, it can be concluded that the thermal stability of CF/PPS is strongly influenced by the pressing temperature.

Differential scanning calorimetry (DSC)

In addition to identifying the material through its glass transition temperature (Tg) and melting temperature (Tm), DSC also provides insights into the thermal stability of the material.⁴⁶ According to Hasan,⁴⁷ thermal analysis is the general term for a group of analytical techniques that measure the properties of a material as it is heated or cooled. The DSC curve can also determine the enthalpy, ΔH, of the material in exothermic and endothermic reactions, as the heat energy applied to it in heating conditions or the heat released in cooling conditions, following ASTM D3418.

The DSC results show that thermal properties such as glass transition temperature (Tg) (Figure 10), melting temperature (Tm) (Figure 11), cold crystallisation temperature (Tcc) (Figure 12), enthalpy of cold crystallisation (ΔHcc) (Figure 13), and enthalpy of melting (ΔHm) (Figure 14), were affected by the thermoforming process at different tool temperatures for both 6 plies and 8 plies CF/PPS laminates. For the 6 plies sample, Tg increased slightly from 95.73 to 97.03°C as tool temperature rose from 150 to 170°C, with ΔHm also increasing from 13.53 to 15.05 J/g. Meanwhile, Tcc remained consistent around 142–143°C, but ΔHcc was still high, ranging between 16.5 and 17.1 J/g, indicating that cold crystallisation was still occurring post-pressing. The 8 plies sample show a similar response, where Tg increased from 93.21 to 94.61°C, and ΔHm peaked at 14.47 J/g at 160°C, while Tcc ranged around 141–142°C. At higher temperatures like 180 and 195°C, Tg and ΔHm slightly decreased, and ΔHcc remained high, suggesting that excessive thermal exposure may have limited crystallite formation due to relaxation or minor degradation effects.

From the DSC, it was found that the glass transition temperature, Tg, was recorded as 93.75°C for 6 plies, while 93.94°C for 8 plies. Melting temperature, Tm, was recorded as 283.68°C for 6 plies, and 283.61°C for 8 plies. The values of Tg and Tm were in line with previous researchers on DSC, whether it is neat PPS or CF/PPS. According to Li et al.,⁴⁶ the Tg of PPS was 89°C, and the Tm of PPS was 280.48°C. Meanwhile, Hasan⁴⁷ found Tm between 278 and 283°C and Mahat et al.⁴⁸ obtained a Tm of 290°C.

For the Degree of Crystallinity (DOC), the highest values were recorded at tool temperatures of 160°C for the 6-ply laminate and 170°C for the 8-ply laminate, with both achieving a DOC of 22.8% after the thermoforming process, as depicted in Figure 15. The initial DOC of the control sample was 19.1% and 18.2% for 6 plies and 8 plies, respectively. These data are parallel with the previous researchers⁴⁸ who found DOC between 20 and 60% depending on the cooling rate, annealing, or quenching process. The crystallinity increased when the temperature was slowly cooled and held at the crystallisation temperature. At different tool temperatures, the range of cooling rates was recorded between 30 and 39°C/min for both thicknesses after the thermoforming process.

The significance of the Degree of Crystallinity (DOC) lies in its strong influence on the chemical and mechanical properties of the material. While the crystalline phase enhances stiffness and tensile strength, the amorphous phase contributes to better impact energy absorption.⁴⁹ Mechanical testing demonstrated improvements in stiffness, strength, and fibre/matrix interface for laminates processed at lower cooling rates. This also shows good agreement with Saraiva,⁵⁰ that the high cooling rate during formation reduces the degree of crystallinity of the polymer and increases the void content. Hence, Ste-marie⁵¹ concluded that a high crystallinity is achieved with a high cooling rate, and, accordingly, each polymer has a varied maximum crystallinity.

In this study, the percentage of DOC recorded was approximately 20%. PPS is a type of semi-crystalline structure. Therefore, it was influenced by the DOC. According to Batista et al.,⁴⁹ a semi-crystalline thermoplastic typically develops spherical structures known as ‘spherulites’ when cooled from the melt state. According to one description, spherulites are roughly radially symmetric semi-crystalline formations. Each spherulite is made up of a radial assembly of thin crystalline lamellae that are separated by amorphous layers when seen from its centre. Whereas the amorphous phase is made up of an assemblage of disordered macromolecules that are structurally limited by the surrounding crystalline lamellae, the crystalline domain is created by regularly arranged molecular chains. Spherulites range in size from micrometres to millimetres, depending on the polymer chain structure and crystallisation parameters such as cooling rate, crystallisation temperature, and nucleating agent concentration.

On the other hand, Mashau⁵² stated in his study that the tool temperature should be set around 200°C ± 20°C, whereas Tencate⁵³ set the tool temperature in a range of 182.5°C ± 7.5°C. In this study, it was found that tooling temperatures from 150 to 195°C produced different DOCs of 20% and 22.8% after the thermoforming process, with high impact strength and more than 50 % improvement.

Mechanical analysis

Interlaminar shear strength (ILSS)

According to Azam et al.,⁴ ILSS is used to evaluate the influence of fibre matrix bonding on ILSS at a laminate level. Short-beam shear testing is a popular method for determining the interlaminar failure resistance of fibre-reinforced composites. The method involves loading a beam with dimensions under three-point bending.⁵⁴ Unlike metals, where failure may occur due to the propagation of a single macroscopic crack, composite materials fail through the accumulation of multiple damage modes. These include matrix-fibre debonding, fibre fracture, transverse-ply cracking, delamination, as well as matrix crazing and cracking under cyclic loading.⁵⁵

The ILSS result for CF/PPS is shown in Figure 16, following the test standard EN 2563. Based on the graph, the ILSS value was recorded as highly significant at a tool temperature of 170°C for 6 plies (86.5 MPa). At a tool temperature of 160°C, the ILSS for 8 plies was 87.2 MPa. ILSS values recorded higher strength at 8 plies than at 6 plies. The control sample show slightly lower strength before the thermoforming process, with 75 MPa and 79 MPa for 6 and 8 plies, respectively. The result of pressed-formed under ILSS reveals slightly greater improvements in strength, with nearly 15.3% for 6 plies and 10.38% for 8 plies, compared to the control sample.

The DOC at different tool temperatures and the ILSS data show the same trend, indicating that ILSS depends on the DOC of the materials. According to Hamdan et al.,⁵⁶ the interlaminar shear strength increased as the material tended to be more elastic in the thick material. ILSS is employed to evaluate both the interfacial adhesion of the matrix and the influence of the binder on the mechanical characteristics of the composite, such as fracture toughness and interlaminar shear strength.⁵⁷ The ‘apparent’ interlaminar shear strength of composite materials is measured using this approach. Thus, the interlaminar shear strength offers information regarding the quality of adhesion at the fibre/matrix interface.⁵⁴ In the present study, the improved ILSS observed after the thermoforming process can be attributed to enhanced fibre wetting and matrix flow during processing. Although the CF/PPS materials were supplied in a pre-consolidated form, the application of heat and pressure during thermoforming promotes further matrix redistribution, improved fibre-matrix contact, and reduction of micro-scale interfacial imperfections. These effects contribute to stronger interlaminar bonding and more efficient stress transfer under shear loading. Therefore, the ILSS results obtained in this study confirm that the thermoforming process effectively enhances the interlaminar performance of CF/PPS composites. Further discussion of the associated damage mechanisms is presented in the morphological analysis section.

Izod unnotched impact strength

The toughness properties of materials are measured using impact tests following ASTM D256. According to Mouritz,⁵⁸ these tests involve measuring the energy required to fracture a material sample when it is impacted at high velocity by a heavy object.

In this research, Izod unnotched impact tests revealed that damage was concentrated in the impact zone, with specimens exhibiting varying degrees of failure, ranging from localised damage to complete fracture, reflecting differences in energy absorption. Complete fractures arose from bending-induced tensile failure at the rear and shear-induced interlaminar cracking at the mid-plane, following progressive damage such as matrix cracking, fibre matrix debonding, and delamination rather than purely brittle failure. In the unnotched condition, failure was governed by microstructural features, with cracks propagating through interlaminar and matrix-rich regions, and was influenced by laminate thickness and fibre-matrix interactions. The results of the impact test are shown in Figure 17.

Next, impact strength show significant improvement after the thermoforming process, with the highest values recorded at a tool temperature of 170°C for the 6 plies laminate (306.67 kJ/m²) and at 160°C for the 8 plies laminate (382.99 kJ/m²), as shown in Figure 18. In comparison, the control samples recorded 181.74 kJ/m² (6 plies) and 241.47 kJ/m² (8 plies). This indicates increases of approximately 68.74% for the 6 plies and 58.61% for the 8 plies, highlighting the effectiveness of the stamping process in enhancing impact performance.

The DOC values after stamping and the impact strength are in line, indicating that higher DOC contributed to greater mechanical strength after pressing. Not only that, the average DOC for 8 plies was higher than for 6 plies, at 21.46% and 20.74%, respectively. Therefore, this study is also in line with previous researchers, who mentioned that the fibres may influence crystallisation.⁴⁹

In this study, 6 plies show lower impact resistance than 8 plies. Similarly, Wang⁵⁹ found that the impact resistance of the composite was increased with thickness. Not only that, the sample with the maximum number of layers absorbed the most energy.⁶⁰ As a result, the impact strength of 8 plies absorbed higher energy. Therefore, Wang et al.⁶¹ agreed that this increase in impact strength was attributed to greater energy absorption in laminates during impact, because the material’s toughness was enhanced as the layers increased.³⁰

Thermoforming contributed to controlled damage propagation, enhancing fibre wetting and matrix continuity, so that even at high impact energies, failure occurred progressively rather than as an abrupt brittle fracture.

Morphological analysis of CF/PPS composites

Throughout the ILSS experiment with SEM validation, it can be observed that 6 plies recorded the lowest ILSS value, with more failures recorded than in the 8-ply case, as shown in Figure 19.

From the results, a typical shear failure mode was observed, with interlaminar cracking in the middle part of the transverse region of the specimen. This shows good agreement with De Paiva et al.⁵⁷ that the test should reveal the shearing that occurred at the centre of the specimen. Therefore, this observation is consistent with previous researchers.⁵⁵

Throughout this research, the crack occurred across the warp fibre. It started to crack and then propagate, breaking the fibre from the warp as it continued to propagate. These weft regions might not be prone to damage initiation and cracks. The damage mode is illustrated in Figure 20. It was due to cracks and damage initiation appearing in transverse fibre bundles, which then propagated across the matrix-rich regions or at the interface between warp and weft (overlapping areas) under axial loading.

To conclude, ILSS failure was dominated by shear-induced interlaminar cracking, primarily initiating at the mid-plane and propagating through matrix-rich and interfacial regions across the warp fibre direction. The weft fibre regions were less susceptible to damage initiation, highlighting the dominant role of interlaminar and matrix-controlled mechanisms in governing failure behaviour. Importantly, the controlled crack propagation and the absence of severe interlaminar defects demonstrated that the thermoforming process had a beneficial effect, enhancing fibre-matrix contact and matrix continuity, thereby improving damage resistance and the structural integrity of the CF/PPS laminates.

Next, for the impact test, the morphological analysis of CF/PPS after the thermoforming process revealed a more ordered and stable microstructure at tool temperatures of 160 and 170°C, indicating a higher degree of PPS crystallinity than at lower or higher temperatures. Within this temperature range, the PPS matrix existed in a semi-molten state, allowing molecular chains to rearrange and form a more well-developed crystalline structure without thermal degradation or excessive matrix flow.

SEM images at 160 and 170°C revealed cleaner fracture surfaces with a significant reduction in fibre pull-out compared to 150°C, suggesting improved fibre matrix interfacial bonding. This behaviour is closely associated with the increased crystallinity of PPS, where a more developed crystalline phase contributes to higher matrix stiffness and more efficient load transfer between the fibres and the matrix.

In addition, at these temperatures, matrix cracking and interfacial debonding remained relatively controlled when compared to higher tool temperatures (≥180°C), at which the PPS matrix becomes excessively softened and more susceptible to plastic deformation under impact loading. This indicates that optimal levels of crystallinity at 160 and 170°C provide a balanced combination of matrix strength, stiffness, and toughness, thereby enhancing the overall impact response of the laminates (Figure 21).

In contrast, at the lower tool temperature of 150°C, PPS crystallinity was not fully developed due to limited molecular chain mobility, leading to weaker fibre-matrix interfacial bonding and a dominance of failure mechanisms, such as fibre pull-out. At higher tool temperatures (≥180–195°C), although laminate consolidation improved, excessive crystal growth and possible matrix relaxation led to increased localised matrix cracking and interfacial debonding, as evidenced by the SEM observations. As a result of the impact testing, typical failure modes such as fibre fracture, fibre pull-out, and fibre breakout were observed. SEM results show CF/PPS for both 6 plies and 8 plies after impact tests, with 6 plies showing much greater fibre pull-out.⁵⁶

Conclusions

The thermoforming process is indispensable for CF/PPS composites because pre-consolidated materials supplied by the manufacturer are not immediately suitable for structural panel fabrication. Thermoforming ensures full consolidation, accurate geometry, and strong fibre-matrix interfacial bonding, all of which are essential for reliable mechanical and thermal performance. Examining the composite behaviour after thermoforming addresses an important research gap, as it verifies the suitability of the material under actual processing conditions and reveals how thermal history influences structural integrity. Mechanical characterisation through impact testing and interlaminar shear strength (ILSS) evaluation provides direct insight into the load-bearing capability of the formed laminates, while thermal analyses using TGA and DSC clarify changes in thermal stability, decomposition behaviour, and crystallinity development. The results show clear improvements after thermoforming, where impact strength reaches 382.99 kJ/m² at 160°C for the 8-ply laminate and 306.67 kJ/m² at 170°C for the 6-ply configuration. ILSS also increased to 87.2 MPa at 160°C for the 8-ply laminate and 85.5 MPa at 170°C for the 6-ply laminate. The TGA residue rose substantially from approximately 44 to 47% to around 72%, demonstrating enhanced thermal stability of the PPS matrix. These improvements correlated strongly with the increase in crystallinity, which reached approximately 22.8%, indicating that thermoforming promoted more ordered molecular alignment and strengthened the fibre-matrix interface. The combined outcomes of TGA and DSC confirm that thermoforming fundamentally modified the thermal stability and microstructural morphology of the PPS matrix through recrystallisation, reduction of volatile components, and the development of more organised crystalline lamellae. This refined microstructure becomes more resistant to chain scission, improves stress transfer, minimises void formation, and delays interfacial failure. As a result, the improvements in ILSS and unnotched impact strength can be directly attributed to the enhanced thermal stability and increased crystallinity produced by the thermoforming process. Overall, these findings demonstrate that thermoforming is essential for producing aerospace-grade CF/PPS panels and for establishing the structure–property relationships that govern their performance under realistic processing conditions.

Footnotes

Acknowledgements

Special thanks to the Kesidang Scholarship (UTeM) for the support in this study. The authors gratefully acknowledge UTeM, Malaysia, for providing access to its advanced laboratory facilities, which were instrumental in conducting this study.

ORCID iDs

Nadlene Razali

M. Sapuan S

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work were supported by Kesidang; Kesidang Scholarship (UTeM) and IMAP/2024/FTKM/AEROSPACEMALAYSIA/IM0002.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Data Availability Statement

Data will be made available on request.*

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