O. Ghita

Unusual crystalline morphology of Poly Aryl Ether Ketones (PAEKs)

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.

High yield growth of patterned vertically aligned carbon nanotubes using inkjet-printed catalyst.

This study reports on the fabrication of vertically aligned carbon nanotubes localized at specific sites on a growth substrate by deposition of a nanoparticle suspension using inkjet printing. Carbon nanotubes were grown with high yield as vertically aligned forests to a length of approximately 400 μm. The use of inkjet printing for catalyst fabrication considerably improves the production rate of vertically aligned patterned nanotube forests compared with conventional patterning techniques, for example, electron beam lithography or photolithography.

Poly Aryl Ether Ketones (PAEKs) and carbon-reinforced PAEK powders for laser sintering

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.

Polymer viscosity, particle coalescence and mechanical performance in high-temperature laser sintering

High-temperature laser sintering (HT-LS) is an additive manufacturing technology whose potential could be limited by the restricted number of materials optimised for the process. Poly ether ether ketone (PEEK) with different melt viscosity values, PEEK 150PF and PEEK 450PF, have been used in parallel with the commercial grade, poly ether ketone (PEK) HP3, to investigate the role of material viscosity on particle coalescence, structure and mechanical performance of components manufactured in HT-LS. The material with lower viscosity, PEEK 150PF, was found to exhibit faster coalescence and lower tensile strength than the grades with higher viscosities, PEEK 450PF and PEK HP3.

Fabrication of three dimensional layered vertically aligned carbon nanotube structures and their potential applications

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.

Control and modelling of capillary flow of epoxy resin in aligned carbon nanotube forests

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.

Large-scale manufacturing of helical auxetic yarns using a novel semi-coextrusion process:

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.

Enhanced Ductility of PEEK thin film with self-assembled fibre-like crystals

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.

Novel Remanufacturing Process of Recycled Polytetrafluoroethylene(PTFE)/GF Laminate

Currently, the PTFE/GF laminate and PTFE PCB manufacturers are under considerable pressure to address the recycling issues due to Waste Electrical and Electronic Equipment (WEEE) Directive, shortage of landfill capacity and cost of disposal. This study is proposing a novel manufacture method for reuse of the mechanical ground PTFE/Glass fibre (GF) laminate and production of the first reconstitute PTFE/GF laminate. The reconstitute PTFE/GF laminate proposed here consists of a layer of recycled sub‐sheet, additional layers of PTFE and PTFE coated glass cloth, also covered by copper foils. The reconstitute PTFE/GF laminate showed good dielectric properties. Therefore, there is potential to use the mechanical ground PTFE/GF laminate powder to produce reconstitute PTFE/GF laminate, for use in high frequencies PCB applications.

In-Line Monitoring of Plastics during Injection Moulding Using near Infrared Spectroscopy:

In recent years, near infrared (NIR) spectroscopy has become a rapidresponse analytical tool and has gained acceptance in different production processes and many industrial sectors such as petrochemical, pharmaceutical, environmental, biomedical, composite and automotive sectors. The aim of the current study is to show, through specific applications, how this technology is useful to injection-moulders. The use of NIR spectroscopy in injection moulding is seen as a promising technique with several potential benefits: (i) productivity improvements, (ii) reduction of costs, (iii) real-time monitoring, (iv) improvements in quality control and (v) health and safety. In order to accommodate the NIR probes, modifications were made to a conventional Battenfeld injection-moulding machine. A detailed description of the injection moulding–optical fibre probes system design was presented in a previous study. The NIR optical fibre sensors had been used for monitoring (i) the concentration of colour and (ii) moisture level, in polymers during injection moulding processing. It was noticed that poor control of material drying, addition of colour or additives would greatly affect either the mechanical properties or the look of the final part. In most of the cases, defect detection and improvements to the quality of parts are done on the finished parts. Currently the optimisation of certain injection moulding parameters is carried out using offline analytical techniques. This type of investigation is costly and time consuming and would lead to huge losses in a manufacturing cycle. Several tons of polymers are sometimes wasted because the products are not fulfilling the quality demanded by the customers. The increasing demands on the quality of the finished parts lead to a need for fast and reliable in-line control monitoring techniques.