Characterisation of UHMWPE Polymer Powder for Laser Sintering

Effect of Activator and Outgoing Ligand Nature on the Catalytic Behavior of Bis(phenoxy-imine) Ti(IV) Complexes in the Polymerization of Ethylene and Its Copolymerization with Higher Olefins

A series of bis(phenoxy-imine) (FI) titanium(IV) and zirconium(IV) complexes have been synthesized. The effect of the nature of the activator (MAO, combinations EtnAlCl3-n + Bu2Mg and iBu3Al + [Ph3C]+[B(C6F5)4]-) on the catalytic activity and properties of the resulting polymers was studied. It was found that Ti-Fi complexes, despite the nature of the outgoing ligands (Cl or iPrO) in the presence of Al/Mg activators, effectively catalyze the polymerization of ethylene (with the formation of UHMWPE); copolymerization of ethylene with 1-octene (with the formation of ultra-high molecular weight copolymers); and the ternary copolymerization of ethylene, propylene and 5-vinyl-2-norbornene (with the formation of polyolefin elastomers). It has been shown that Zr-FI complexes are not activated by these Al/Mg compositions. The resulting UHMWPE can be processed by a solventless method into high-strength and high-modulus oriented films; however, their mechanical characteristics do not exceed those obtained using MAO.

Investigating the Feasibility of Processing Activated Carbon/UHMWPE Polymer Composite Using Laser Powder Bed Fusion

Activated Carbon (AC) is widely available at a relatively low cost, has a high porosity and is commonly used as a filter material for a range of applications. However, it is a brittle and friable material. Ultra-High Molecular Weight Polyethylene (UHMWPE) polymer is a tough engineering plastic that has been used as a binder. The traditional method used in manufacturing AC/UHMWPE filters involves compressing AC/UHMWPE composite powder during heating in a mould. This process compresses the particles together and the materials undergo sintering. This process results in a low pore interconnectivity, which has a considerable impact on the filter's efficiency. Selective Laser Sintering is a laser powder bed fusion additive manufacturing technique for polymers. This has a number of advantages compared to the conventional technique and produces a porous structure with improved filtration efficiency. We propose that this is due to the greater pore interconnectivity. In this work, AC/UHMWPE powdered composites were prepared with different AC and UHMWPE ratios. The structure and properties of the AC/UHMWPE composite were investigated and characterised to assess their suitability for selective laser sintering. Particle size and morphology analysis were conducted, as well as density measurements, powder flow, thermal analysis, and crystallinity measurements. The results reveal that the addition of AC improves the UHMWPE flow. The thermal analysis results show that the intrinsic thermal properties of UHMWPE powder are not significantly affected by the introduction of activated carbon. However, thermal gravimetric analysis revealed that the onset of mass loss is considerably shifted (20 °C) to higher temperatures for the AC/UHMWPE composites, which is favourable for laser sintering. Additionally, the change in the composition ratio of untreated composite does not have a significant effect on the degree of crystallinity. Laser-sintered AC/UHMWPE parts were successfully manufactured using a commercial laser-sintering machine.

Deposition of Biocompatible Polymers by 3D Printing (FDM) on Titanium Alloy

Nowadays, the replacement of a hip joint is a standard surgical procedure. However, researchers have continuingly been trying to upgrade endoprostheses and make them more similar to natural joints. The use of 3D printing could be helpful in such cases, since 3D-printed elements could mimic the natural lubrication mechanism of the meniscus. In this paper, we propose a method to deposit plastics directly on titanium alloy using 3D printing (FDM). This procedure allows one to obtain endoprostheses that are more similar to natural joints, easier to manufacture and have fewer components. During the research, biocompatible polymers suitable for 3D FDM printing were used, namely polylactide (PLA) and polyamide (PA). The research included tensile and shear tests of metal–polymer bonds, friction coefficient measurements and microscopic observations. The friction coefficient measurements revealed that only PA was promising for endoprostheses (the friction coefficient for PLA was too high). The strength tests and microscopic observations showed that PLA and PA deposition by 3D FDM printing directly on Ti6Al4V titanium alloy is possible; however, the achieved bonding strength and repeatability of the process were unsatisfactory. Nevertheless, the benefits arising from application of this method mean that it is worthwhile to continue working on this issue.

Effect of Borpolymer on Mechanical and Structural Parameters of Ultra-High Molecular Weight Polyethylene

The paper presents the results of studying the effect of borpolymer (BP) on the mechanical properties, structure, and thermodynamic parameters of ultra-high molecular weight polyethylene (UHMWPE). Changes in the mechanical characteristics of polymer composites material (PCM) are confirmed and complemented by structural studies. X-ray crystallography (XRC), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and infrared spectroscopy (IR) were used to study the melting point, morphology and composition of the filler, which corresponds to the composition and data of the certificate of the synthesized BP. Tensile and compressive mechanical tests were carried out in accordance with generally accepted standards (ASTM). It is shown that BP is an effective modifier for UHMWPE, contributing to a significant increase in the deformation and strength characteristics of the composite: tensile strength of PCM by 56%, elongation at break by 28% and compressive strength at 10% strain by 65% compared to the initial UHMWPE, due to intensive changes in the supramolecular structure of the matrix. Structural studies revealed that BP does not chemically interact with UHMWPE, but due to its high adhesion to the polymer, it acts as a reinforcing filler. SEM was used to establish the formation of a spherulite supramolecular structure of polymer composites.

Main 3D Manufacturing Techniques for Customized Bone Substitutes. A Systematic Review

Clinicians should be aware of the main methods and materials to face the challenge of bone shortage by manufacturing customized grafts, in order to repair defects. This study aims to carry out a bibliographic review of the existing methods to manufacture customized bone scaffolds through 3D technology and to identify their current situation based on the published papers. A literature search was carried out using "3D scaffold", "bone regeneration", "robocasting" and "3D printing" as descriptors. This search strategy was performed on PubMed (MEDLINE), Scopus and Cochrane Library, but also by hand search in relevant journals and throughout the selected papers. All the papers focusing on techniques for manufacturing customized bone scaffolds were reviewed. The 62 articles identified described 14 techniques (4 subtraction + 10 addition techniques). Scaffold fabrication techniques can be also be classified according to the time at which they are developed, into Conventional techniques and Solid Freeform Fabrication techniques. The conventional techniques are unable to control the architecture of the pore and the pore interconnection. However, current Solid Freeform Fabrication techniques allow individualizing and generating complex geometries of porosity. To conclude, currently SLA (Stereolithography), Robocasting and FDM (Fused deposition modeling) are promising options in customized bone regeneration.

UHMWPE/CaSiO3 Nanocomposite: Mechanical and Tribological Properties

This paper studied the effect of additives of 0.5-20 wt.% synthetic CaSiO3 wollastonite on the thermodynamic, mechanical, and tribological characteristics and structure of polymer composite materials (PCM) based on ultra-high-molecular weight polyethylene (UHMWPE). Using thermogravimetric analysis, X-ray fluorescence, scanning electron microscope, and laser light diffraction methods, it was shown that autoclave synthesis in the multicomponent system CaSO4·2H2O-SiO2·nH2O-KOH-H2O allows one to obtain neeindle-shaped nanosized CaSiO3 particles. It was shown that synthetic wollastonite is an effective filler of UHMWPE, which can significantly increase the deformation-strength and tribological characteristics of PCM. The active participation of wollastonite in tribochemical reactions occurring during friction of PCM by infrared spectroscopy was detected: new peaks related to oxygen-containing functional groups (hydroxyl and carbonyl) appeared. The developed UHMWPE/CaSiO3 materials have high wear resistance and can be used as triboengineering materials.

Hierarchical Structural Engineering of Ultrahigh-Molecular-Weight Polyethylene

Nature has inspired the design of next-generation lightweight architectured structural materials, for example, nacre-bearing extreme impact and paw-pad absorbing energy. Here, a bioinspired functional gradient structure, consisting of an impact-resistant hard layer and an energy-absorbing ductile layer, is applied to additively manufacture ultrahigh-molecular-weight polyethylene (UHMWPE). Its crystalline graded and directionally solidified structure enables superior impact resistance. In addition, we demonstrate nonequilibrium processing, ultrahigh strain rate pulsed laser shock wave peening, which could trigger surface hardening for enhanced crystallinity and polymer phase transformation. Moreover, we demonstrate the paw-pad-inspired soft- and hard-fiber-reinforced composite structure to absorb the impact energy. The bioinspired design and nonequilibrium processing of graded UHMWPE shed light on lightweight engineering polymer materials for impact-resistant and threat-protection applications.

A Review of Methods Used to Reduce the Effects of High Temperature Associated with Polyamide 12 and Polypropylene Laser Sintering

The polymer laser sintering (PLS) process is one of the most promising additive manufacturing (AM) technologies for polymeric materials. However, the technique has challenges because the physical, mechanical, and chemical properties of the polymeric powder deteriorate due to the high temperatures prevailing in the build chamber during manufacture. These high temperatures cause agglomeration of powder, which leads to a decrease in the flowability of powder. There is also a related drop in the coalescence of the powder granules during PLS, which results in porosity that undermines the mechanical integrity of printed parts. Moreover, the viscosity of the melt increases due to cross-linking of molecular chains. This, in turn, increases the tensile strength of the printed components at the expense of the percentage elongation at break. Thus, high prolonged processing temperatures decrease the reusability of polymeric materials used in PLS. In this paper, a review of the studies conducted to investigate ways of reducing the effects of high temperature on polymeric powders is presented.