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The heating and extrusion process in fused filament fabrication (FFF) is significantly shorter than the conventional extrusion process where longer heating times and significant pressure are applied. For this reason, it is important to understand whether the crystal history of the feedstock is fully erased through the FFF process and whether the FFF process can be tailored further by engineering the crystallization of the feedstock filaments. In this context, a methodology for evaluating the influence of morphology and mechanical properties on different feedstock and extruded filaments is proposed. Filaments with three different PEEK 450G crystalline structures (standard crystallinity, drawn filament and amorphous filament) were selected and evaluated, before and after free extrusion. The resulting morphology, crystallinity and mechanical properties of the extruded filaments were compared against the feedstock properties. X-ray diffraction (XRD), transmission electron microscopy (TEM), differential and fast scanning calorimetry (DSC/FDSC) and tensile test were the techniques used to evaluate the materials. The results showed clear differences in the properties of the feedstock materials, while the analysis of the extruded filaments points to a homogenization of the resulting material producing mostly similar mechanical properties. However, the use of the drawn filament highlighted a statistically significant improvement in crystallinity and mechanical performance, especially in strain values. This conclusion suggests the innovative possibility of improving the quality of manufactured parts by tailoring the microstructure of the feedstock material used in the FFF process. Graphical abstract

Laser sintering (LS) of composites is an attractive prospect for industries with an interest in complex structures of enhanced mechanical performance. Unfortunately, dry blends of carbon fibre (Cf) and polymeric powders are known to lead to poor mechanical properties. This paper brings evidence that Cf incorporated within the polymer grains (referred in the paper as composite grain powder) can enhance mechanical performance and highlights areas within the powder preparation stages where careful powder examination is critical. An in-depth powder analysis was performed on a dry blend of Cf/PEK powder and several composite grain powder grades obtained from milling, both at 30 wt% of Cf. The results showed that characterisation of the milling fraction is very important and only certain composite grain grades maintain expected powder properties for LS as well as wt% of Cf. The SEM images of laser sintered films provided a clear insight into what happens in the laser sintering of dry blends versus composite grain powders. The complex moduli (E*) obtained from the DMTA tests presented an increase in the samples manufactured using composite grain powders over the dry blend powder and approximately double values when compared with plain polymer HP3 PEK.

As the field of macromolecular science celebrated its 100th anniversary in 2020, we organised a special issue of Polymer International devoted to the synthesis, processing and applications of polyaryletherketones (PAEKs). Developed over 40 years ago, PAEK polymers and their composites experienced a steady increase in use and applications due to their unique characteristics: high mechanical performance, high toughness, high operating temperatures, low wear, non-flammable and chemically resistant as well as their function as a class of bioinert, robust biomaterials. The contributions to the special issue engage the materials science community and include a mixture of spotlights, perspectives, reviews, and research papers, spanning the breadth of basic science and engineering to applied technical/medical science, with authors from academic and industrial backgrounds, which reflects some of the exciting recent developments in PAEK-based materials that is of interest to a broad readership. A further area of research not particularly highlighted within this review; but very much an active concern; is the development of PAEK-based thermoplastic composites for application in the aerospace and medical sectors.

An experimental study on the melt and coalescence of different Poly(aryl ether ketones) (PAEK) powders was carried out. The study consisted of evaluating individual particles and neck growth using a hot stage platform. Three different methods of assessing particles dimensions were evaluated, and the coalescence was compared for the grades on three different substrates: glass, glass coated silicone and amorphous PEKK films, the last substrate with the intent to relate with the laser sintering process. It was found that all individual particles shrink prior to melting, achieving up to 30% reduction in perimeter at temperatures just above melting. This shrinkage is followed by an increase in size, mostly driven by viscous flow. Shrinkage is expected to affect neck growth by delaying coalescence, as particles are pulled away whilst neck increases. This is a plausible explanation for why neck growth occurs at a lower rate than predicted by most coalescence models. The use of different substrates does not seem to affect particle coalescence or change in dimension. Viscosity and particle size influence overall particle behaviour and melt, whilst morphology and porosity do not have a great impact in particles coalescence.

Self-folding behavior is an exciting property of weft-knit fabrics that can be created using just front and back stitches. This behavior is easy to create, but not easy to anticipate and currently cannot be predicted by the existing computer-aided design software that controls industrial knitting machines. This work identifies the edge deformation behaviors that lead to self-folding in weft knits, and methods to characterize the mechanical forces driving these behaviors with regard to chosen manufacturing parameters. With this data and analysis of the fabric deformations, the self-folding behavior was purposely controlled using calculated scaling factors. Furthermore, theoretical equations were developed to mathematically predict these scaling factors, minimizing the trial and error required to design with self-folding behavior and create textiles with novel engineered properties. By understanding the mechanisms responsible for creating these three-dimensional self-folding textiles, they can then be designed in a programmable manner for use in technical applications.

High temperature laser sintering is an Additive Manufacturing (AM) process that allows fabrication layer by layer of complex structures in high performance biomaterial polymers such as Poly Ether Ether Ketone (PEEK) with high design freedom and short lead time. While studies on new material candidates for HT-LS are increasingly appearing, the quality standards of the parts obtained in HT-LS with special attention to the effects of the build orientation are less explored, thus limiting the implementation of AM PEEK in the healthcare sector. This study investigates the effect of four manufacturing build orientations - horizontal, oblique, vertical and inverted horizontal - on the properties of patient-specific laser sintered PEEK cranial implants. The mechanical performance in uniaxial static compression mode, the density variation and dimensional accuracy compared to the original design model are discussed. The implants manufactured in the horizontal and inverted horizontal orientations were found to show the least deviation from the design model and the highest compressive strength resistance. The vertically built implant showed the lowest accuracy in design and more of 70% lower first failure compared to the inverted horizontal implant. Keywords: PEEK, Laser sintering, Cranioplasty, Medical, 3D printing

Poly Ether Ether Ketone (PEEK) is a high temperature polymer material known for its excellent chemical resistance, high strength and toughness. As a semi-crystalline polymer, PEEK can become very brittle during long crystallisation times and temperatures helped as well by its high content of rigid benzene rings within its chemical structure. This paper presents a simple quench crystallization method for preparation of PEEK thin films with the formation of a novel fibre-like crystal structure on the surface of the films. These quenched crystallised films show higher elongation at break when compared with conventional melt crystallised thin films incorporating spherulitic crystals, while the tensile strength of both types of films (quenched crystallised and conventional melt) remained the same. The fracture analysis carried out using microscopy revealed an interesting microstructure which evolves as a function of annealing time. Based on these results, a crystal growth mechanism describing the development of the fibre-like crystals on the surface of the quenched crystallised films is proposed.

The Fused Deposition Modelling (FDM™) process owes its popularity to its hardware versatility, low cost and wide range of materials (and colours) available. In this study, PEEK was produced with 1% and 5% carbon nanotubes (CNTs) and processed in a modified FDM system able to operate with high temperature polymers. The tensile strength, layer bonding and microstructure of the plain and CNT loaded PEEK samples were investigated throughout the three steps of manufacturing: compounded composite feedstock filaments, single FDM deposited layers and fabricated test specimens. Interestingly, every step of processing seems to fabricate structures of lower performance. As part of the characterisation of the FDM structures, short shear beam tests were used as a new method to assess layer-to-layer bonding.

This paper introduces a novel extrusion process for manufacturing helical auxetic yarn (HAY). A range of semi-coextruded HAYs have been manufactured in a cost-effective, consistent and readily usable form. The semi-coextruded HAYs were compared to the conventional spun yarns in terms of tensile properties and auxetic behavior. The results show the presence of the auxetic effect in newly fabricated semi-coextruded HAYs. Similar to the traditional spun HAYs, the new HAYs are sensitive to parameters such as the initial wrap angle, the core/wrap diameter ratio and component moduli. Importantly, a few new manufacturing parameters have been identified for tailoring the auxetic behavior of the semi-coextruded HAYs. The semi-coextruded HAYs are auxetic when an instantaneous true Poisson’s ratio analysis method is applied. The semi-coextruded HAYs give a larger maximum negative Poisson’s ratio than the conventional HAYs due to the advantages of the pre-formed helical wrap structure.

In this paper, the effects of particle shape and viscoelasticity on the binary coalescence rates of a pair of PEK HP3 particles in the Laser Sintering (LS) process are investigated in detail. PEK HP3 powder was characterised and the sintering rates were determined using Hot Stage Microscopy (HSM). On average, the neck growth rate shows a slower dynamic compared to the theory. Furthermore, in some of the trials an initial delay in growth and/or a contraction in the neck size were observed. To understand this behaviour, the dynamics of individual non-spherical particles were studied under similar conditions. The overall perimeter of the particles shrank and they gradually attained a final spherical state under the effect of surface tension forces, which indicates that the shape of individual coalescing units influences the sintering rates. In addition, the effects of viscoelasticity on the sintering rate were investigated using the available theoretical approaches and results show that viscoelasticity can change the rate of coalescence but the neck growth rate shows a strictly increasing trend even for very large Deborah numbers. To investigate the shape effects, volume-of-fluid (VoF) simulations of elongated ellipsoidal particles in head-to-head contact were performed and the neck growth rates were determined. These analyses show that the contraction of the neck size can occur purely due to geometrical effects and local surface curvatures of the coalescing particles. In addition, a critical aspect ratio exists beyond which the coalescence does not complete and the contraction continues until the pair of coalescing particles eventually separate forming two independent spheres. To explain this phenomenon, the local curvatures along the surface of coalescing particles were calculated and the results show that local minima in the curvature at the neck's contact points with the particles' main body form which is believed to drive the separation.

This paper discusses various methods of fabrication of plain and carbon-reinforced composite powders, as well as a range of powder characterisation test methods suitable for defining powders for laser sintering. Two milling processes (based on disc blades and rotatory cutting knives) were used as methods of fabrication of powders, starting from injection moulding granule grades, for comparison with current powders obtained directly from polymerisation processes. It was found that the milling process affects the particles properties. The rotary milling produced powders with superior properties in comparison with the disc milling method. Tests including particle size distribution, angle of repose, aspect ratio, sphericity and roundness of particles were employed to compare and assess the suitability of powders for laser sintering. The Brunauer-Emmett-Teller test was identified as a useful method to define surface roughness and porosity of the particles. The carbon fibre (Cf) Poly Ether Ketone (PEK) granules milled well and after an additional sieving process created a good quality powder. This is the first attempt to investigate properties of PEK powder with encapsulated Cf and follow their sintering profile through hot-stage microscopy. It is expected that this type of composite powder will create isotropic structures in comparison with the highly anisotropic properties given by the known dry mix composite powders, currently used in laser sintering.

Powders used in additive manufacturing (AM) are spread onto a compact layer of particles for sintering and this process is repeated layer by layer to form the final products. Spreading of rod-shaped particles in realistic AM settings is simulated using the discrete element method (DEM) to investigate the effects of particle shape and operating conditions on the bed quality, characterised by its surface roughness and solid volume fraction. It is discovered that larger particle aspect ratios, Ar, or higher spreader translational velocities result in a lower bed quality, i.e. a larger surface roughness and a smaller volume fraction. The surface roughness increases monotonically with Ar. However, the volume fraction exhibits a maximum at Ar = 1.5 for randomly packed powder beds that are formed by the roller type spreaders moving at low translational velocities. It is also found that a roller outperforms a blade spreader in terms of the quality of the prepared bed at the same operating conditions. The micro-structural analysis of the beds also shows particle alignment in response to the induced flow, which is qualitatively confirmed by a set of purposely-designed experiments. In addition, a shape segregation is documented for powders with mixed aspect ratios (Ar) such that particles with larger Ar tend to accumulate on the upper layers of the bed.

New materials for laser sintering (LS) are usually developed using a trial and-error approach that consists of a series of builds within LS systems. This strategy is time consuming, costly and focuses only on the optimisation of the processing parameters, ignoring the powder properties of the materials under examination. Being able to predict processing parameters on the basis of the powder material properties would enable a faster development of new materials and new applications, while acknowledging a more in-depth understanding of the mechanisms involved in LS. This paper provides new results into the prediction of processing conditions from the material properties. It is here shown that high temperature polymers such as poly ether ether ketone (PEEK) and poly aryl ether ketone (PEK) can be successfully used in LS despite the lack of a super-cooling window. The evaluation of the stable sintering region of PEEK 450PF and the application of the energy melt ratio parameter in relation to the mechanical performance of laser sintered PEEK samples are also provided. Lastly, a new method for estimating the powder bed temperature is proposed. Keywords: PEK HP3, PEEK, Laser sintering, Energy melt ratio, Stable sintering region, Super cooling window

This paper presents a systematic study of the helical auxetic yarn (HAY) via careful in-house fabrication and characterisation of a wide range of polymeric fibres and yarns. It provides a better understanding of the auxetic behaviour of the HAY in order to tailor their properties for specific applications. The study focused on three parameters: component moduli, the core/wrap diameter ratio and the initial wrap angle. The results show that a larger difference in component moduli, a higher core/wrap diameter ratio and a lower initial wrap angle can produce a larger maximum negative Poisson’s ratio value and thereby a better auxetic performance for HAYs. All three parameters could be carefully utilised when in combination to achieve the required auxetic behaviour of HAYs. Moreover, the instantaneous true Poisson’s ratio analysis accurately presents the instantaneous behaviour of highly strain dependent HAYs.

Poly aryl ether ketone (PAEK) polymers are gaining interest in 3D printing for their good mechanical properties and high service temperatures. The aim of this study was to compare the crystallisation kinetics, morphology, and mechanical properties of two different PAEK polymers used in fused filament fabrication (FFF), i.e. the fast crystallising PEEK151 (poly ether ether ketone) grade originally designed for injection moulding and the slow crystallising AM 200 grade tailored specifically for FFF. The crystallisation kinetics of both grades were examined across a wide temperature range. A method to select annealing temperatures and annealing times based on the intrinsic crystallisation behaviour of each polymer was proposed. The dual-Avrami model highlighted a different crystallite growth for AM 200 in comparison with PEEK151 with a higher rate of secondary crystallisation. Lamellar thicknesses were measured by SAXS and calculated via the Thomson-Gibbs equation. The lamellar thicknesses of primary and secondary crystallisation for AM 200 showed a stronger temperature dependence with steeper slopes when increasing the isothermal temperature. The benefit of using a slow crystallising PAEK polymer over the conventional fast crystallising grades is evidenced by the improvement in Z strength which enhances the overall isotropy of printed parts.

This paper demonstrates the utilisation of in-situ synthesised novel metal organic framework (MOF)-polymer nanocomposite laser-sintered parts with enhanced CO2 adsorption properties. Making use of polyamide PA12, one of the most common materials in powder bed fusion process as the base polymer, an in-situ synthesis of nanofiller ZIF-67 crystals on the surface of polyamide polymer particles was proposed to allow the fabrication of a nanocomposite powder with a good dispersion, reducing any health and safety handling issue arising from use of loose nanoparticles. This in-situ synthesis method allowed a maximum exposure of the ZIF-67 nano-porous sites. Laser sintering was used to fabricate porous structures with additional macro-pores and controlled cavities to increase the surface area. The laser-sintered ZIF67-PA12 part at only 2.6% wt ZIF-67 concentration exhibited a CO2 capacity of 3.02 and 4.89 cm3/g at 298 K and 273 K at 1 bar. This in-situ synthesis method of making ZIF67-PA12 powders combined with the design freedom and the ease of fabrication of parts opens opportunities in a wider range of applications for MOFs such as energy storage and conversion.

Laser sintering (LS) is one of the most popular additive manufacturing (AM) techniques as it produces parts of complex geometry with high dimensional accuracy and good mechanical strength. However, the nature of the LS process often leads to brittle behavior characterized by a low elongation at break if compared to conventional polymer processing techniques, e.g., injection molding (IM). For poly(ether ketone ketone) (PEKK), such elongation is currently below 3%. This study determines and then optimizes the relationship between cooling time and crystallization of PEKK during LS and the resulting elongation at break. The elongation at break of PEKK was successfully improved by using shorter cooling times. The combination of the slow crystallization kinetics of PEKK and a short cooling time of 1 h increased elongation at break to 14%; this is a striking result never achieved for PAEKs in LS before. A calibration curve was developed that can be used to correlate PEKK structure and mechanical properties to cooling conditions according to the application. This methodology can also be applied to select and optimize the mechanical properties of other LS polymers sharing similar kinetics of crystallization and processing temperatures. This work suggests a great potential for a wide range of “post-processing” heat treatments to be used in AM to tailor the ultimate mechanical properties.

This paper presents a comprehensive study in the fabrication and safety of new nanocomposite powders for laser sintering. The nanocomposite powder is based on a core-shell structure where nanoparticles (graphene nanoplatelets, GNP) are encapsulated on the surface of the polymeric particles (polyamides PA12) in a thin layer of poly(vinyl alcohol) (PVA). Powder rheology data as well as SEM and TEM showed that GNP were dispersed well in the PVA coating and improved flow. Half time crystallisation kinetics was used to determine differences induced in the polymer and the laser sintering process by the presence of GNP. Nanosafety aspects, critical in a manufacturing environment, are also considered here and exposure monitoring tests were carried out. Results confirmed a low nanoparticle air exposure and therefore confirmed the successful surface encapsulation of the GNP in nanocomposite powders. The laser-sintered 0.1GNP/PVA-PA12 parts showed enhanced mechanical properties in tensile, compression and 3-point bending test.

Additive manufacturing (AM), otherwise known as three-dimensional printing (3DP), is a growing technology area comprising a spectrum of processes that allow production of solid objects of virtually any shape from information obtained from a digital object. These days, AM processes drive major innovations in engineering, manufacturing, art, education, and medicine. However, most AM processes are not necessarily new. Introduced commercially in the 1990s mainly through prototyping efforts for the manufacture of complex metal parts, AM processes have almost a 30-year history for plastics, and have driven the development of multiple commercial products through manufacturing techniques ranging from stereo-lithography to laser-based powder fusion processes. A growing number of polymeric resins intended for AM have become available in recent times due to developments of new processes and technological advancements. Of these, high-performance thermoplastics such as the polyaryletherketones (PAEKs) are perhaps the most promising candidates for demanding engineering applications. Polymers such as polyetheretherketone (PEEK), polyetherketoneketone (PEKK), and polyetherketoneetherketoneketone (PEKEKK), could revolutionize and enable the use of additively manufactured plastic parts in critical environments. However, despite their similarity in terms of chemistry and composition, commercially available PAEK resins show varying physical properties due to their molecular size-dependent structural differences that make them function differently in common AM processes. This chapter describes some of the advancements and opportunities for PAEK polymers in AM processes, as well as the relationships between structure and property and the morphological changes observed in these materials when subjected to conditions typically found in common AM processes.

The crystal structure of Poly Aryl Ether Ketones (PAEKs) fabricated using a range of manufacturing processes has been investigated by high resolution electron microscopy. The study has revealed a hierarchical spherulitic crystal structure, different from the common spherulite morphology with lamellae grown in a radial direction. The results show that spherulites in laser sintered PAEKs are built of granular crystal blocks of 75–145 nm in size self-assembled along the radial direction. The study also showed that the granular crystal blocks are composed of small primary crystals approximately 20–30 nm in size combined together with a secondary structure. The secondary structure was found to be less ordered and was preferentially removed during chemical etching whereas the primary crystals remained intact.

The properties of graphite platelet reinforced Poly Ether Ether Ketone (PEEK/GP) composites from powder to laser sintered parts were investigated in this study. The flowability, particle size and laser absorption characteristics of PEEK/GP powders with various graphite loadings were studied. It was found that the addition of graphite improved laser absorption; however, the flowability of powder was reduced. Micro-CT scanning was used to study the distribution, dispersion and the orientation of graphite platelets as well as the porosity and maximum pore size of laser sintered PEEK/GP composites. The graphite platelets were observed to be distributed evenly in the structure without significant agglomeration. Most of the graphite had their in-plane surface orientated in the X–Y plane of fabrication, which increased the tensile strength of the composites incorporating 5 wt.% graphite. The investigation also demonstrated that the porosity and maximum pore size increased with increasing amounts of graphite. A significant increase in porosity and pore size was found in PEEK/GP composites with 7.5 wt.% graphite, and it is believed to be responsible for the drop in tensile strength. DMA analysis showed no reduction of the damping properties in the composites incorporating up to 5 wt.% graphite, whereas the composites with 7.5 wt.% graphite showed increased stiffness. Keywords: Laser sintering, PEEK, Composites, Thermomechanical properties, Micro-CT, Microstructure characterization

Thermal expansion behaviour and mechanical performance of high temperature laser sintered PEK (LS-PEK) and glass bead filled PEK composite (LS-GB/PEK) are reported and discussed for the first time. The laser sintered PEK and its composite show anisotropic thermal expansion properties related with their multilayer structure created by the laser sintering method. Compared with the PEK manufactured by conventional injection moulding (IM-PEK), LS-PEK has a 10% lower tensile strength but a higher hardness due to its greater crystallinity. Addition of glass bead to PEK increases the hardness of PEK without affecting the ultimate tensile strength and also improves the thermal stability of laser sintered parts. The rule of mixtures is applied to simulate the coefficient of thermal expansion (CTE) of the LS-GB/PEK composite structures and compare with experimental results. The measured CTE values match the calculated results below Tg and deviate slightly from the simulated trend line above Tg. Keywords: Laser sintering, Glass, Polyetherketone, Thermal analysis, Thermal expansion, Microstructure

Functional auxetic composite materials can be fabricated from conventional or from auxetic components. The helical auxetic yarn (HAY) is a recently invented auxetic reinforcing component for composite materials. This paper investigates the Poisson's ratio behaviour of a further development of the HAY, needed for many practical applications. The 3-component auxetic yarn is based on a stiff wrap fibre (the first component) helically wound around an elastomeric core fibre (the second component) coated by a sheath (the third component). The resultant structure can overcome problems such as slippage of the wrap and changes in wrapping angles previously encountered during the manufacture and utilisation of the two-component HAY. The mechanical performance of conventional and novel systems is investigated; with emphasis on the differences between the engineering and true Poisson's ratio. The importance of the utilisation of a true tensile modulus and a true Poisson's ratio is demonstrated. This is the first time reported in the literature that an experimental auxetic effect analysis of HAYs was carried out by comparing true and engineering Poisson's ratio. We show that depending on the coating thickness of the third component, the 3-component auxetic system can demonstrate auxetic behaviour, and the coating thickness can be employed as a new design parameter to tailor both the Poisson's ratio and modulus of this novel composite reinforcement for a wide range of applications.

Currently, the HT-LS sector is predominantly based around one commercial Poly Ether Ketone (PEK) polymer. Although the combination of polymer and process works well, a lower melting temperature polymeric material, part of the same Poly Aryl Ether Ketone (PAEK) family would be preferable in certain applications. This study presents the optimisation and characterisation of Poly Ether Ether Ketone (PEEK), a polymer which is part of the PAEK family with a 30 °C lower melting temperature than PEK. The systematic characterisation of laser sintered samples of PEEK revealed a very good overall performance in comparison with the HP3 PEK material, with no change in storage modulus and only 25% drop in tensile strength. The possibility of variable building configurations available within the HT-LS system, i.e. reduced, half and full chamber building modes, is examined in relation to the mechanical performances of the components. The effect of the post sintering time, an additional heating phase supplied to the powder bed at every layer, found only in the HT-LS system EOSINT P 800, is also examined.

The interface between cellulose nanocrystals (CNCs) in thermoplastic matrices is one of the most important issues prohibiting the development of CNC-based polymeric composites prepared via melt processing. In the present work, polyethylene oxide (PEO) was used as a compatibilizer to enhance the interface with a polyethylene matrix. It was found that the composites produced using a PEO compatibilizer possess better overall mechanical properties and a higher degree of crystallinity of the polyethylene than the unmodified samples. An increase in both the tensile strength and modulus of the composites was observed for up to 1.5 wt.% of CNCs; beyond this point no significant increases were observed. When CNCs were added (up to 1.5 wt.%) to the matrix, the crystallization peak of the composites in the DSC thermograms shifted to higher temperatures. The stress-transfer process in the composites was monitored using Raman spectroscopy. Higher Raman band shift rates with respect to tensile strain of a peak corresponding to main chain molecular deformation were observed for the composites produced using the PEO compatibilizer. This demonstrates that stress is transferred from the matrix to the fillers more effectively with the presence of PEO. The simple PEO-modification approach adopted in this study avoids the classical solvent method of production of CNC-based nanocomposites which is not really applicable at an industrial scale.

Cellulose nanofibers hold much promise for enhancing the mechanical properties of composites owing to their uniquely high stiffness and strength. One major issue limiting this performance however is the dispersion and mixing of cellulose nanofibers within thermoplastic resins. A combination of Raman imaging and chemical analysis has been used to quantify the distribution and mixing of cellulose nanocrystals (CNCs) in a polyethylene-matrix composite. Large area spectral imaging provides information about the effect of a compatibilizer - namely poly(ethylene oxide) (PEO) and maleated polyethylene (MAPE) - on the distribution of CNCs in the thermoplastic matrix. High-resolution images enable quantification of the degree of mixing between the CNCs and HDPE. Lower resolution images, but with greater spatial spread, allow quantification of the distribution of the CNCs. It is shown that the CNCs tend to agglomerate, with little increase in distribution even with the use of the compatibilizer. A shift in the position of characteristic Raman bands indicates the formation of hydrogen bonding between the PEO compatibilizer and the CNCs, which in turn is thought to affect the distribution of aggregates of the reinforcing phase.

This paper proposes a new technique for fabrication of vertically aligned carbon nanotube (VACNT) structures, controlled in shape, height and functionality, through continuous successive growth of VACNT layers by chemical vapour deposition (CVD) combined with patterning strategies. This was achieved by vacuum deposition of additional catalyst material onto the original VACNT “forest” layer. A second forest layer is then observed to grow underneath the first by CVD. It is proposed that the new catalyst material diffuses through the porous nanotube forest to coat the growth substrate underneath. The enhanced height, coating, and vertical alignment of the nanotube forests were verified by electron microscope observation. By repeating this process, aligned nanotube bi-layers and tri-layers were grown, producing a “stack” of nanotube layers. By using a “shadow mask” patterning technique to screen areas of the original forest from catalyst deposition, the growth can be confined to specific areas of the substrate. Potentially, these multilayer nanotube structures would have diverse applications as long composite reinforcements, p–n junctions for electronic devices, or to allow the production of near net shape complex multilayer nanotube structures.

This paper examines the mechanism of infiltration by capillary flow of epoxy resin into vertically-aligned carbon nanotube forests. The resin viscosity during curing was characterized by rheometry. Carbon nanotube forests were brought into contact with resin at a range of times during curing, therefore at a range of viscosities. The penetration of the resin into the forests was measured using electron microscopy, X-ray micro-computed tomography and energy-dispersive X-ray spectroscopy, the latter relying on a chromium-complex dye additive which acts as a marker for the presence of resin. Experimental results were compared to a simulation based on the Implicit Lucas–Washburn equation for capillary flow. It was found that prior to the resin gel point, the resin penetrates through the full height of the forest. Close to the gel point, the flow into the forest ceases, leaving unwetted regions of nanotubes. Understanding the relationship between resin flow in nanotube structures and the resin viscosity and curing has important application in the fabrication of nanocomposite materials. This “partial wetting” effect is a key requirement for a previously proposed method for the fabrication of carbon nanotube composites by additive manufacture (AM) which would provide strong interlayer reinforcement combined with the versatility of AM.

Materials with a negative Poisson’s ratio are referred to as auxetic. One recently invented example of this is the helical auxetic yarn (HAY). This has been proved to successfully exhibit auxetic behaviour both as a yarn and when incorporated into fabric. The HAY is based on a double-helix geometry where a relatively stiffer ‘wrap’ is helically wound around a compliant core fibre. This paper studies the effect of the interaction between the core and the wrap fibre on the auxetic behaviour of the HAY, including the effect of their relative moduli. Assessment of the Poisson’s ratio of the HAYs has revealed that an elevated difference in component moduli causes the wrap fibre embedding itself into the core fibre, thus decreasing the auxetic effect. Careful determination of an optimum core–wrap moduli ratio where the ratio is high enough to yield an auxetic effect and low enough to prevent the core-indentation effect can lead to the fabrication of a yarn with largest negative Poisson’s ratio.

In an attempt to expand the range of engineering polymers used for laser sintering, this paper examines the morphology, flowability and interparticle interactions of two commercially available Poly (ether ether) ketone (PEEK) powders, not yet optimised for the LS process, by comparison with the LS optimised Polyamide (PA) and Polyetherketone (PEK) powdered polymers. The effect of incorporating fillers and additives on the flow behaviour is also analysed. The Particle Size Distribution (PSD) results alone do not allow ranking the powder materials in relation to the flow behaviour. The particle morphology has a stronger influence on the flow characteristics for materials with similar PSDs. The work also provides additional characterisation parameters to be considered when analysing LS powders.

In-line spectroscopy is a useful tool for monitoring processing conditions and polymer quality in the melt state in processes such as extrusion and injection moulding. This study extends the scope of this technique to the quantitative detection of poly(lactic acid) (PLA), which is a contaminant of concern in recycled poly(ethylene terephthalate) (PET). The near-infrared spectra of thermally treated PLA were studied for evidence of spectral changes which could be used as markers for PLA in PET, and these are discussed in detail. Several chemometric models for quantifying PLA in PET were developed and the influence of chemical or physical changes on the performance of the models is discussed. The selection of the final model was based on criteria aimed at eliminating interference due to spectral changes resulting from physical processes, to give a robust model based specifically on the chemistry of the polymers studied. Finally, the models were tested against a new test mixture of known composition and the outputs are discussed.

Laser sintering (LS) of polymers has high potential for growth as a manufacturing technique into a wide range of applications provided the range of engineering polymers available for LS expands and machines and LS process conditions are optimised for such materials. This study is the first investigation into laser sintering of both virgin and used polyether ketone (PEK) powder using a bespoke high temperature (HT) polymer laser sintering machine (known commercially as EOSINT P800). The physico-chemical results reveal that, in spite of polymer degradation, used PEK has a viable processing window for LS manufacturing which, combined with optimisation of specific parameters can successfully lead to manufacture of good quality parts. The proposed sintering mechanism of both, virgin and used powders is supported by the experimental data. The incorporation of 30% used HP3 PEK powder led to an approximately 17% drop in tensile strength.

Shrinkage is extensively mentioned in the literature as one of the main causes for dimensional instability or poor performance in Laser Sintering (LS). This study proposes and examines a methodology to describe shrinkage in cooling from a material perspective. Thermal behaviour and crystallisation effects were measured to determine the influence of powder structure and density on the overall LS shrinkage. PEKK and PA12 powders were used to assess such behaviour. The shrinkage parameter associated with powder bulk properties has the greatest impact in PEKK, contributing to 57% of the total shrinkage observed in cooling for this material as supported by the low values of bulk density, irregular morphology and internal porosity observed for these particles. For PA12, crystallisation is responsible for 60% of the overall shrinkage observed. In LS, PEKK shows an overall shrinkage approximately 30% lower than PA12, which makes it a promising material for maintaining final part dimensions. Keywords: Laser sintering, Shrinkage, Powder bulk density, Crystallisation, PEKK, PA12

This is the peer reviewed version of the following article: Wang Y., Ghita O., Kavanagh D., and Chandler D. (2014), A study of interface adhesion between polyamide 6 (PA6) and nitrile rubber (NBR), Surf. Interface Anal., 10-11, pages 1000–1004, which has been published in final form at 10.1002/sia.5488. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving: http://olabout.wiley.com/WileyCDA/Section/id-820227.html#terms

Whilst demonstrating desirable mechanical properties, corrosion resistance and the ability to retain structural integrity over extended temperatures, PEEK (poly(ether ether ketone)) remains expensive, restricting broader usage. The reuse and recyclability characteristics of PEEK are therefore commercially important, where the most prevalent manufacturing process for PEEK is injection moulding. This study comments on the reprocessability of PEEK specifically applied to the injection moulding process, comparing the effect of repeated reuse on mechanical properties. Recycled PEEK retains its tensile properties through at least three moulding and regrinding cycles. XRD and DSC measurements confirmed that reused PEEK shows no degradation in crystallinity.