Periodic Reporting for period 1 - HipPEEK (Hierarchical porous PEEK via combined physical foaming and additive manufacturing: bringing circularity to advanced engineering materials)
Periodo di rendicontazione: 2023-09-01 al 2025-08-31
A number of engineering applications require lightweight structural materials providing a good combination of stiffness, toughness and strength, including transportation, buildings and energy storage and conversion. Fibre reinforced polymers (FRP) in the form of monolithic and sandwich structures are nowadays mostly used in lightweight aerospace applications. However, FRP’s present an intrinsic sustainability hotspot, given the difficulty on their recycling.
High performance thermoplastic cellular solids (HPTC) are excellent candidates for those applications requiring a compromise between mechanical and functional properties ensuring at the same time recyclability by using thermoplastic polymers. The main intrinsic advantage of cellular solids is the ability to tailor their properties by tuning the microstructure (cell size, wall-thickness, and distribution). However, conventional foaming techniques such batch foaming, extrusion foaming, or foam injection moulding do not allow for precise control of the obtained cellular morphology, production of cell size gradient or the manufacturing of hierarchical macro/microporous structures.
HipPEEK aims at developing a processing strategy towards the manufacturing of multiscale cellular structures focusing on Poly(ether-ether-ketone) (PEEK). PEEK is a high-performance engineering thermoplastic with high strength to weight ratio, reported as an excellent candidate for metal replacement in a variety of applications including space, aerospace automotive, medical and dental.
Multiscale celular PEEK will be accomplished through the combination of environmentally friendly physical foaming with inert gases with the production of three-dimensional scaffolds through resource efficient AM, which will allow the production of controlled multiscale hierarchical structures on the micro and meso scales, together with optimized topology at the macro scale.
High performance thermoplastic cellular solids (HPTC) are excellent candidates for those applications requiring a compromise between mechanical and functional properties ensuring at the same time recyclability by using thermoplastic polymers. The main intrinsic advantage of cellular solids is the ability to tailor their properties by tuning the microstructure (cell size, wall-thickness, and distribution). However, conventional foaming techniques such batch foaming, extrusion foaming, or foam injection moulding do not allow for precise control of the obtained cellular morphology, production of cell size gradient or the manufacturing of hierarchical macro/microporous structures.
HipPEEK aims at developing a processing strategy towards the manufacturing of multiscale cellular structures focusing on Poly(ether-ether-ketone) (PEEK). PEEK is a high-performance engineering thermoplastic with high strength to weight ratio, reported as an excellent candidate for metal replacement in a variety of applications including space, aerospace automotive, medical and dental.
Multiscale celular PEEK will be accomplished through the combination of environmentally friendly physical foaming with inert gases with the production of three-dimensional scaffolds through resource efficient AM, which will allow the production of controlled multiscale hierarchical structures on the micro and meso scales, together with optimized topology at the macro scale.
HipPEEK project is truly multidisciplinary. The main activities performed in the project and key achievements include:
- Exhaustive property mapping of Triply Periodic Minimal Surface (TPMS) based lattices. TPMS are surfaces with zero mean curvature and a periodic pattern which repeats itself in the 3D space. These geometries have been found in nature, as in the Rubi butterfly wings or in the sea urchin skeleton. TPMS based lattices have been found promising for multifunctional applications due to their curvy nature. In this project, we carried out the most thorough characterization so far, to the best of our knowledge, including Young, shear and bulk modulus, compressive and shear strength and sound absorption. This large-scale characterization, through experiments and numerical simulations, provides a comprehensive mapping of the design space for TPMS, which will serve as scafolds for our hierarchical PEEK structures.
- Optimization of PEEK fused filament fabrication (FFF) print parameters. It is well known that PEEK is challenging to print, due to its semicristalline nature, high melting temperature and fast crystallization kinetics. This results in warping, layer debonding and dimensional fidelity loss of additive manufactured PEEK parts. Thus, to harness PEEK's outstanding mechanical properties and translate them to HipPEEK's hierarchical cellular structures, it is fundamental to fully understand the effects of the different print parameters involved and optimize them. This was rigurously undertaken through a design of experiments coupled with deep microstructural characterization of PEEK printed specimens. Main ourcomes of this work are the determination of the contribution of each process parameter to the mechanical properties of FFF PEEEK, which has highlighted the importance of printing at environmental temperatures around the polymer's Tg, thus the importance of a heated chamber. A further outcome is a general model linking print parameters to mechanical properties. Moreover, it has been found that crystallinity alone does not fully explain the mechanical behaviour of PEEK - short range order of the amorphous phase appears to play a significant role.
- Quantification of the plastitization effect of CO2 in PEEK, as well as sorption and desorption kinetics. The fabrication of HipPEEK's hierarchical cellular structures involves the combination of physical foaming with CO2 and additive manufacturing through FFF. It is a two step process, which involves the saturation of PEEK filament with CO2 in an autoclave, and its in-situ foaming and printing in a single step. Thus, it is fundamental to determine the soprtion and desorption kinetics of CO2 in PEEK, in order to ensure its saturation and determine the saturated filament's shelf life. Further, in order to correctly establish the processing window for the combined foaming and printing step, the cuantification of the plastitization effect of CO2, in terms of depression of the transition and melting temperatures, is required. This was undertaken within the project.
-Design, Fabrication and Characterization of PEEK TPMS hierarchical lattices. Based on the ourcomes of the previous tasks, PEEK hierarchical cellular structures, with TPMS-based meso-structures, from which the struts are composed by foam microstructure.Three different TPMS geometries (IWP (+), OCTO (+) and SYxxx (+) ) were selected and fabricated, as:
- representative examples of rigid (IWP (+) , compliant (SYxxx (+) and mixed (OCTO (+) architectures in terns of compressive behaviour.
- representative of resilient (IWP (+)) and sacrifical (OCTO (+) ) in terms of penetration impact behaviour.
- representative of highly resonant and sound absorbing (OCTO (+) and highly tunable (SYxxx (+) and IWP (+) architectures in terms of their sound absorption behaviour.
It has been demonstrated that hierarchical porosity significantly enhances toughness and ductility of the lattices, paving the way for bio inspired damage tolerant PEEK lightweigth multifunctional materials.
- Exhaustive property mapping of Triply Periodic Minimal Surface (TPMS) based lattices. TPMS are surfaces with zero mean curvature and a periodic pattern which repeats itself in the 3D space. These geometries have been found in nature, as in the Rubi butterfly wings or in the sea urchin skeleton. TPMS based lattices have been found promising for multifunctional applications due to their curvy nature. In this project, we carried out the most thorough characterization so far, to the best of our knowledge, including Young, shear and bulk modulus, compressive and shear strength and sound absorption. This large-scale characterization, through experiments and numerical simulations, provides a comprehensive mapping of the design space for TPMS, which will serve as scafolds for our hierarchical PEEK structures.
- Optimization of PEEK fused filament fabrication (FFF) print parameters. It is well known that PEEK is challenging to print, due to its semicristalline nature, high melting temperature and fast crystallization kinetics. This results in warping, layer debonding and dimensional fidelity loss of additive manufactured PEEK parts. Thus, to harness PEEK's outstanding mechanical properties and translate them to HipPEEK's hierarchical cellular structures, it is fundamental to fully understand the effects of the different print parameters involved and optimize them. This was rigurously undertaken through a design of experiments coupled with deep microstructural characterization of PEEK printed specimens. Main ourcomes of this work are the determination of the contribution of each process parameter to the mechanical properties of FFF PEEEK, which has highlighted the importance of printing at environmental temperatures around the polymer's Tg, thus the importance of a heated chamber. A further outcome is a general model linking print parameters to mechanical properties. Moreover, it has been found that crystallinity alone does not fully explain the mechanical behaviour of PEEK - short range order of the amorphous phase appears to play a significant role.
- Quantification of the plastitization effect of CO2 in PEEK, as well as sorption and desorption kinetics. The fabrication of HipPEEK's hierarchical cellular structures involves the combination of physical foaming with CO2 and additive manufacturing through FFF. It is a two step process, which involves the saturation of PEEK filament with CO2 in an autoclave, and its in-situ foaming and printing in a single step. Thus, it is fundamental to determine the soprtion and desorption kinetics of CO2 in PEEK, in order to ensure its saturation and determine the saturated filament's shelf life. Further, in order to correctly establish the processing window for the combined foaming and printing step, the cuantification of the plastitization effect of CO2, in terms of depression of the transition and melting temperatures, is required. This was undertaken within the project.
-Design, Fabrication and Characterization of PEEK TPMS hierarchical lattices. Based on the ourcomes of the previous tasks, PEEK hierarchical cellular structures, with TPMS-based meso-structures, from which the struts are composed by foam microstructure.Three different TPMS geometries (IWP (+), OCTO (+) and SYxxx (+) ) were selected and fabricated, as:
- representative examples of rigid (IWP (+) , compliant (SYxxx (+) and mixed (OCTO (+) architectures in terns of compressive behaviour.
- representative of resilient (IWP (+)) and sacrifical (OCTO (+) ) in terms of penetration impact behaviour.
- representative of highly resonant and sound absorbing (OCTO (+) and highly tunable (SYxxx (+) and IWP (+) architectures in terms of their sound absorption behaviour.
It has been demonstrated that hierarchical porosity significantly enhances toughness and ductility of the lattices, paving the way for bio inspired damage tolerant PEEK lightweigth multifunctional materials.
The HipPEEK project has successfully advanced the design, modeling, and additive manufacturing of hierarchical porous PEEK structures, achieving all defined objectives and key performance indicators. It generated the most extensive experimental and numerical mapping to date of triply periodic minimal surface (TPMS) lattices, encompassing 36 geometries and enabling the rational selection of architectures for specific mechanical and acoustic functions. This comprehensive structure–property database, validated through experiments, provides a benchmark for the design of multifunctional lightweight materials. The project has demonstrated, for the first time, tunable sound absorption in TPMS lattices through relative-density grading and identified distinct acoustic regimes governed by architecture and flow resistivity. These findings open new routes for structural–acoustic optimization. Furthermore, the project established the processing–structure–property relationships for FFF-printed PEEK, revealing that amorphous phase ordering—rather than crystallinity alone—governs mechanical performance. The identified CO2 plasticization effects and the associated processing-window shift enable foam printing at reduced temperatures, facilitating the fabrication of hierarchical PEEK foams with controlled cellular morphology. Altogether, HipPEEK delivers a validated scientific foundation for the next generation of high-performance, multifunctional PEEK components combining stiffness, resilience, and acoustic absorption.
The results position the project at the forefront of PEEK additive manufacturing and foam printing, with strong potential for exploitation in aerospace, transport, and biomedicine. To ensure further uptake, key needs include: (i) demonstration at component scale to validate industrial feasibility, (ii) integration of in-printer CO2 saturation for continuous production,, and (iii) alignment with emerging standardisation frameworks for AM of high-temperature polymers. These steps would accelerate commercialisation and strengthen Europe’s technological leadership in advanced polymer additive manufacturing.
The results position the project at the forefront of PEEK additive manufacturing and foam printing, with strong potential for exploitation in aerospace, transport, and biomedicine. To ensure further uptake, key needs include: (i) demonstration at component scale to validate industrial feasibility, (ii) integration of in-printer CO2 saturation for continuous production,, and (iii) alignment with emerging standardisation frameworks for AM of high-temperature polymers. These steps would accelerate commercialisation and strengthen Europe’s technological leadership in advanced polymer additive manufacturing.