Facilitating the additive manufacture of high-performance polymers through polymer blending: A review

Fused Filament Fabrication (FFF, a.k.a. fused deposition modeling, FDM) is presently the most widespread material extrusion (MEX) additive manufacturing technique owing to its flexibility and robustness. Nonetheless, it remains underutilized in load-bearing applications, as often seen in aerospace, automotive and biomedical industries. This is largely due to the processing challenges associated with high performance polymers (HPPs) like poly-ether-ether-ketone (PEEK) or polyetherimide (PEI). Compared with commercial-grade plastics such as polylactic acid (PLA), parts produced with HPPs have outstanding mechanical properties and thermal stability. However, HPPs have bulkier chemical structures and stronger intermolecular forces than common FFF feedstock materials, and this results in much higher printing temperatures and greater melt viscosities. The demanding processing requirements of HPPs have thus impaired their adoption within FFF. Polymer blending, which consists in properly mixing HPPs with other thermoplastics, makes it possible to alleviate these printing issues, while also providing additional advantages such as improved tensile strength and reduced friction. Further to this, manipulating the crystallisation processes of HPPs mitigates distortion or warping upon printing. This review explores some emerging trends in the field of HPP blends and how they address the challenges of excessive melt viscosity, polymer crystallization, moisture uptake, and part shrinkage in 3D printing. Also, the various structural/mechanical/chemical enhancements that are afforded to FFF parts through HPP blending are critically analysed based on recent examples from the literature. Such insights will not only aid researchers in this field, but also facilitate the development of novel, 3D printable HPP blends

Manufacturing Processing and Properties Manipulations of Thick Advanced Performance PEEK Polymer and Composites for Biomedical and Extremely Harsh Environments

This study’s aim is to investigate manipulating the compression molding manufacturing process to influence morphology and mechanical properties of thick wall and tall advanced performance thermoplastic polymers, as well as to highlight the mechanisms that cause property deterioration in those products. Two advanced performance polymer systems, neat poly(etheretherketone) (PEEK) and its composite (CF/PEEK), were considered as model systems to fundamentally understand structure-property relationships in thick wall advanced polymeric materials. An instrumented compression molding setting with thermal control and 3D embedded thermocouples is designed and fabricated to produce thick polymer parts and investigate how altering processing procedures influences properties. A novel hybrid sealing method is invented to enhance compression molding quality and avoid leaking issues associated with this process. The temperature distribution profiles throughout the compression molding and the bushing are collected during heating and cooling processes. The resultant temperature profiles are analyzed to further understand the compression molding process behavior, and thus adjust the processing procedure to enhance products morphology and properties. Crystal structure formation is controlled via templating material manufacturing cooling process. The influence of holding temperature at the crystallization temperature while increasing the hold time is examined by characterizing samples throughout bushings processed using various strategies. Manipulating the cooling is expected to guide the polymer amorphous arrangement toward a uniform crystal structure and grow this structure equivalently throughout the thick cross-section and the extended length of the final product. Remarkable crystallinity improvement with adequate consistency was achieved throughout thick wall and tall compression molded PEEK bushing that improved the compression molding product properties. Carbon fiber reinforcement’s influence on crystal morphology and mechanical properties of thick products is addressed in this dissertation. Different techniques and tests are used to investigate the bushings produced using different processing strategies such as dynamic scanning calorimetry (DSC), dynamic mechanical analysis (DMA), scanning electron microscopy (SEM), wide angle x-ray scattering (WAXS), polarized optical microscopy (POM), compression test, and 3-point bending test. Those techniques assisted in establishing correlation between the morphology modification and the material properties response. Predictive numerical models are developed to simulate the compression molding heating process. Experimental validations provide beneficial tools to predict the heating time required for various thick compression molded materials. The predictive models established in this study can substitute building an expensive thermal control system and performing compression molding with embedded thermocouples to estimate material processing time. These models can provide a great assist for industrial applications. This study highlights an intelligible processing procedure for developing thick compression molding bushing with consistent crystallinity and enhanced mechanical properties. The processing protocol introduced in this study acquired based on analyzing compression molding temperature profiles and studying the possibility of using different methods to control the process during the cooling stage to produce neat and composite polymers with better properties. The produced products can be used for many applications such as aerospace, biomedical, automotive, food processing, oil and gas industry, etc

Engineering the Crystalline Architecture for Enhanced Properties in Fast-Rate Processing of Poly(ether ether ketone) (PEEK) Nanocomposites.

Rapid cooling in fast-rate manufacturing processes such as additive manufacturing and stamp forming limits the development of crystallinity in semicrystalline polymer nanocomposites and, therefore, potential improvements in the mechanical performance. While the nucleation, chain mobility, and crystallization time from rapid cooling are known competing mechanisms in crystallization, herein we elucidate that the crystalline morphology and architecture also play a key role in tuning the mechanical performance. We explore how modifying the spherulite morphology via a cellulose nanocrystal (CNC) and graphene nanoplatelet (GNP) hybrid system in their pristine form can improve or preserve the mechanical properties of poly(ether ether ketone) (PEEK) nanocomposites under two extreme cooling rates (fast -460 °C/min and slow -0.7 °C/min). A scalable manufacturing methodology using water as the medium to disperse the powder system was developed, employing a CNC as a dispersing agent and stabilizer for PEEK and GNP. Despite the expected limited mechanical reinforcement due to thermal degradation, CNCs significantly impacted PEEKs crystalline architecture and mechanical performance, suggesting that surface interactions via lattice matching with PEEKs (200) crystallographic plane play a critical role in engineering the microstructure. In fast cooling, the CNC and CNC:GNP systems reduced the crystallinity, respectively, yet led to minimizing the reduction in the tensile strength and maintaining the tensile modulus at the Neat level in slow cooling. With slow cooling, crystallinity remained relatively unchanged; however, the addition of CNC:GNP improved the strength and modulus by ∼10% and ∼16%, respectively. These findings demonstrate that a hybrid nanomaterial system can tailor PEEKs crystalline microstructure, thus presenting a promising approach for enhancing the mechanical properties of PEEK nanocomposites in fast-rate processes

Tribological and mechanical properties of graphene nanoplatelet/PEEK composites

Poly(ether ether ketone) (PEEK) is a relevant thermoplastic in industry and in the biomedical sector. In this work, the lubricant capability of graphene nanoplatelets (GNPs) is used for improving the PEEK wear properties. Nanocomposites were prepared by solvent-free melt-blending and injection molding at various compositions between 1 and 10 wt. % of GNPs. The Raman G band shows a progressive increment proportional to the bulk GNP percentage. From calorimetric data, the polymer matrix structure is interpreted in terms of a 3-phase model, in which the crystalline phase fluctuates from 39 to 34% upon GNP addition. Thermal conductivity varies in accordance with the polymer crystallinity. Tensile and flexural tests show a progressive increase in the modulus, as well as a decrease in the fracture strength and the work of fracture. Most important, the composite surface undergoes a substantial improvement in hardness (60%), together with a decrease in the coefficient of friction (-38%) and a great reduction in the wear factor (-83%). Abrasion and fatigue wear mechanisms are predominant at the lowest and highest GNP concentrations respectively. In conclusion, GNPs are used without any chemical functionalization as the filler in PEEK-based materials, improving the surface hardness and the tribological properties

Effect of annealing treatment and infill percentage on 3D-printed PEEK samples by Fused Filament Fabrication

A strategy that is gaining momentum in several industrial sectors is metal replacement, which aims to find suitable alternatives for replacing metal components with lighter ones. One possible solution is represented by high-performance polymers (HPP), which are a family of materials with improved thermo-mechanical and functional properties, compared to commodity plastics. Additive manufacturing (AM) is revolutionizing the industrial world due to its high design freedom, dimensional accuracy, and shortened total production time. Thus, combining the use of HPP with AM technologies could lead to innovative results, which could offer new metal replacement solutions through redesign and new material properties. However, HPPs have some manufacturing limitations, for example, they require high processing temperatures, and some of them are subject to significant warping and deformation phenomena. This aspect is particularly significant for semi-crystalline polymers, as in the case of poly(ether-ether-ketone) (PEEK), which is affected by thermal gradients during 3D printing. In this research, an investigation was carried out on the Fused Filament Fabrication (FFF) of different 3D printed PEEK samples, evaluating the effect on final properties not only of various infill percentages (30%, 50%, 70%, and 100%) but also of two different heating treatments. In this regard, a traditional annealing in oven, post 3D printing, was compared to a direct annealing approach, performed during FFF. The mechanical performance of the samples was characterized through tensile and compression tests along with the thermal properties and the thermal stability. In addition, for all different cases, energy consumption was measured, to provide an indication of the sustainability of the presented approaches. The findings suggest that the direct annealing solution holds promise and merits further investigation to bridge knowledge gaps in this domain. This research contributed to advance the understanding of PEEK 3D printing by FFF and played a vital role in the practical implementation of metal replacement as a sustainable strategy across various industrial applications

Adhesive Wear Phenomena in High Performance Polyaryletherketones (PAEK) Polymers

Adhesive wear is one of the most difficult types to study and is especially challenging for polymers. Such wear processes involve the mutual sticking of surface asperities followed by removal of debris from the bulk. This differs from abrasive wear in which debris is formed due to the penetration of hard rough asperities into the softer surface. Such descriptions have served the polymer tribology community for decades and are well suited for post-mortem analysis of wear surfaces. For instance, the presence of rippled features on the wear surface and large flake shaped debris are typical indicators of adhesive wear. However, this approach offers little insight into the underlying physics that occur at the interface. The overall objective of this research is to gain fundamental knowledge of adhesive wear phenomena in polyaryletherketone (PAEK) polymers. Ultimately, the hope is to correlate the observed surface damage and friction response with material science based explanations. Since no true adhesive wear test configuration exists, a top down approach was used in designing a set of experimental conditions. This was done with a multi-axis tribometer capable of being programmed to a wide array of displacements and trajectories. A catastrophic form of adhesive wear is termed fretting and results from the repeated slip of mutually loaded contacts. Using the multi axis tribometer PAEK polymers were studied in both multi directional sliding and fretting configurations with varied environmental conditions. An important aspect of PEEK tribology is the surface temperature reached during sliding. Infrared thermography was used to observe the full field temperature map of PEEK during ball-on-disc sliding. Additionally, friction studies were performed with steel and sapphire counterfaces. The results of this study illustrate the important role transfer films play in determining both the friction and temperature response of the PEEK wear interface. The formation of transfer films resembles a unidirectional drawing process. Polarized FTIR-ATR measurements were used to assess chain orientation in the friction formed PEEK on steel transfer films. The results of these studies serve to better elucidate underlying mechanisms involved in adhesive wear of PAEK polymers

Manufacturing Processing and Properties Manipulations of Thick Advanced Performance PEEK Polymer and Composites for Biomedical and Extremely Harsh Environments

This study’s aim is to investigate manipulating the compression molding manufacturing process to influence morphology and mechanical properties of thick wall and tall advanced performance thermoplastic polymers, as well as to highlight the mechanisms that cause property deterioration in those products. Two advanced performance polymer systems, neat poly(etheretherketone) (PEEK) and its composite (CF/PEEK), were considered as model systems to fundamentally understand structure-property relationships in thick wall advanced polymeric materials. An instrumented compression molding setting with thermal control and 3D embedded thermocouples is designed and fabricated to produce thick polymer parts and investigate how altering processing procedures influences properties. A novel hybrid sealing method is invented to enhance compression molding quality and avoid leaking issues associated with this process. The temperature distribution profiles throughout the compression molding and the bushing are collected during heating and cooling processes. The resultant temperature profiles are analyzed to further understand the compression molding process behavior, and thus adjust the processing procedure to enhance products morphology and properties. Crystal structure formation is controlled via templating material manufacturing cooling process. The influence of holding temperature at the crystallization temperature while increasing the hold time is examined by characterizing samples throughout bushings processed using various strategies. Manipulating the cooling is expected to guide the polymer amorphous arrangement toward a uniform crystal structure and grow this structure equivalently throughout the thick cross-section and the extended length of the final product. Remarkable crystallinity improvement with adequate consistency was achieved throughout thick wall and tall compression molded PEEK bushing that improved the compression molding product properties. Carbon fiber reinforcement’s influence on crystal morphology and mechanical properties of thick products is addressed in this dissertation. Different techniques and tests are used to investigate the bushings produced using different processing strategies such as dynamic scanning calorimetry (DSC), dynamic mechanical analysis (DMA), scanning electron microscopy (SEM), wide angle x-ray scattering (WAXS), polarized optical microscopy (POM), compression test, and 3-point bending test. Those techniques assisted in establishing correlation between the morphology modification and the material properties response. Predictive numerical models are developed to simulate the compression molding heating process. Experimental validations provide beneficial tools to predict the heating time required for various thick compression molded materials. The predictive models established in this study can substitute building an expensive thermal control system and performing compression molding with embedded thermocouples to estimate material processing time. These models can provide a great assist for industrial applications. This study highlights an intelligible processing procedure for developing thick compression molding bushing with consistent crystallinity and enhanced mechanical properties. The processing protocol introduced in this study acquired based on analyzing compression molding temperature profiles and studying the possibility of using different methods to control the process during the cooling stage to produce neat and composite polymers with better properties. The produced products can be used for many applications such as aerospace, biomedical, automotive, food processing, oil and gas industry, etc