Engineering Thermoplastic Polymers Research Articles

Closing the loop in space 3D printing: Effect of vacuum, recycling, and UV aging on high performance thermoplastics produced via filament extrusion additive manufacturing

The outlook for space exploration, and the long-term sustainable presence of humans in space, rely upon the advancement of in-situ manufacturing capabilities. In-space additive manufacturing (AM) has gained significant attention due to its potential to produce spare parts in-situ and facilitate repairs for extending space missions. However, to date, there is no AM system capable of processing high-performance engineering thermoplastic polymers within uncontrolled space conditions (i.e., reduced gravity, vacuum, and radiation environment). Additionally, the effect of ultra-violet (UV) radiation on printed parts has remained unexplored. In this work, a custom-built filament extrusion AM breadboard has been developed and tested enabling the fabrication of engineering thermoplastic polymers, whereby the machine was placed inside a vacuum chamber to partly mimic the harsh space environment. Mechanical (tensile and flexural) and thermal (thermogravimetric analysis, differential scanning calorimetry and dynamic mechanical analysis) tests were conducted on the printed specimens before and after exposure to UV aging. Moreover, the printed geometries underwent a recycling process involved the shredding of the pristine 3D-printed polymeric samples, the drying of the obtained granulates and finally the extrusion of 3D-printable filaments. The polymeric recycled 3D-printed specimens were then subjected to further mechanical and thermal testing, offering valuable insights into the feasibility of in-space AM and the potential to augment the circularity of the printed parts. The results of the mechanical and thermal tests, together with the effect of vacuum printing, UV aging, and recycling, are provided within the manuscript. This work cements the basic building blocks to sustainably advance on-orbit and in-space infrastructure, and paves the way for long-term human space missions.

Thermal, Morphological, Electrical Properties and Touch-Sensor Application of Conductive Carbon Black-Filled Polyamide Composites

Polyamide 66 (PA66) is a well-known engineering thermoplastic polymer, primarily employed in polymer composites with fillers and additives of different nature and dimensionality (1D, 2D and 3D) used as alternatives to metals in various technological applications. In this work, carbon black (CB), a conductive nanofiller, was used to reinforce the PA66 polymer in the 9–27 wt. % CB loading range. The reason for choosing CB was intrinsically associated with its nature: a nanostructured carbon filler, whose agglomeration characteristics affect the electrical properties of the polymer composites. Crystallinity, phase composition, thermal behaviour, morphology, microstructure, and electrical conductivity, which are all properties engendered by nanofiller dispersion in the polymer, were investigated using thermal analyses (thermogravimetry and differential scanning calorimetry), microscopies (scanning electron and atomic force microscopies), and electrical conductivity measurements. Interestingly, direct current (DC) electrical measurements and conductive-AFM mapping through the samples enable visualization of the percolation paths and the ability of CB nanoparticles to form aggregates that work as conductive electrical pathways beyond the electrical percolation threshold. This finding provides the opportunities to investigate the degree of filler dispersion occurring during the transformation processes, while the results of the electrical properties also contribute to enabling the use of such conductive composites in sensor and device applications. In this regard, the results presented in this paper provide evidence that conductive carbon-filled polymer composites can work as touch sensors when they are connected with conventional low-power electronics and controlled by inexpensive and commercially available microcontrollers.

4D printing high temperature shape-memory poly(ether–ether–ketone)

Shape-memory polymers have exhibited extensive attractive properties for practical utility and applied potential, such as in clothing industry and for medical implants, which demands soft materials amenable to change shapes at low temperature. Whereas, potential applications in aerospace engineering require materials with higher modulus and higher transition temperature to ensure their effectiveness under harsh environments. As an engineering thermoplastic polymer material, poly(ether–ether–ketone) (PEEK) is a promising candidate that meets the stringent requirements for aerospace applications. Yet, additive manufacturing of PEEK polymers, to bring out the full potential of the materials, remains challenging and unfulfilled because of the intrinsic very high processing temperature and high modulus. In this paper, we present that 4D printing of the high temperature PEEK polymer with outstanding shape memory properties. We systematically investigate the effect of different printing parameters and heat treatment conditions on the corresponding shape memory performance, which has been rarely reported. The highest shape fixity ratio and highest shape recovery ratio that we can achieve in the 3D-printed PEEK samples is 99% and 87.8%, respectively, with Young’s modulus of 3.23 GPa and good thermal stability below 520 °C. Furthermore, the incomplete shape recovery processes have been exploited to fabricate more complex programming of shape changes and to demonstrate the potential for practical applications, and models of PEEK have been designed and printed to demonstrate the outstanding shape memory properties and for potential applications in high temperature environment.

Metallization of polymers by cold spraying with low melting point powders

Polymers and polymer composites have been increasingly used due to their low density and high specific strength; however, their low electrical conductivity has limited their further application. Cold spray has recently been proven to be a feasible approach to metallize polymers. Previous results have shown that tin can be cold sprayed onto polymeric substrates when spraying with a low-pressure cold spray system at gas temperatures close to the melting point of tin. The successful deposition of tin was partly attributed to the tin reaching its incipient melting point. In this work, three low melting point metal powders, namely, Sn, Sn-Zn alloy, and Sn-Bi alloy, were cold sprayed onto five widely used engineering polymers . Different combinations of gas temperature and gas pressure were assessed. Results show that decreasing the melting points of the feedstock powders can improve their deposition efficiency on polymeric substrates, due to a higher degree of particle melting. Among the five polymers, the high-toughness ultra high molecular weight polyethylene and polycarbonate were relatively difficult to metallize. • SnZn and SnBi powders with lower melting points than Sn were cold sprayed on polymeric substrates. • Five common engineering thermoplastic polymers (ABS, Nylon, PC, PE, and PP) were metallized using low melting point powders. • Particle melting facilitated the deposition and inter-particle bonding, resulting in high deposition efficiency. • SnBi coatings exhibit higher adhesion to ABS than Sn when spraying at 300°C and 60 psi.

Fused Filament Fabrication of PEEK: A Review of Process-Structure-Property Relationships.

Poly (ether ether ketone) (PEEK) is a high-performance engineering thermoplastic polymer with potential for use in a variety of metal replacement applications due to its high strength to weight ratio. This combination of properties makes it an ideal material for use in the production of bespoke replacement parts for out-of-earth manufacturing purposes, in particular on the International Space Station (ISS). Additive manufacturing (AM) may be employed for the production of these parts, as it has enabled new fabrication pathways for articles with complex design considerations. However, AM of PEEK via fused filament fabrication (FFF) encounters significant challenges, mostly stemming from the semi crystalline nature of PEEK and its associated high melting temperature. This makes PEEK highly susceptible to changes in processing conditions which leads to a large reported variation in the literature on the final performance of PEEK. This has limited the adaption of FFF printing of PEEK in space applications where quality assurance and reproducibility are paramount. In recent years, several research studies have examined the effect of printing parameters on the performance of the 3D-printed PEEK parts. The aim of the current review is to provide comprehensive information in relation to the process-structure-property relationships in FFF 3D-printing of PEEK to provide a clear baseline to the research community and assesses its potential for space applications, including out-of-earth manufacturing.

Optimization of Cutting Parameters in Machining Polyoxymethylene Using RSM

Polyoxymcthylene (POM) is an extensively used engineering thermoplastic polymer. Owing to its superior properties and potential applications in numerous turfs of structural machineries, it is essential to probe the machining of POM. The economy of machining operations plays an imperative role in increasing productivity and effectiveness. The persistence of optimal cutting parameters, such as cutting speed, feed and depth of cut, which are valid for given cutting tools, is one of the dynamic segments in process planning of metal parts. This research work concerns an experimental study dealing with cutting parameters and its effects on surface roughness (Ra) and material removal rate (MRR) during the turning of POM. The approach is based on Response Surface Technology (RSM). Experiments are designed using central composite design and second-order quadratic models are developed to define the optimal machining parameters. These optimized parameters are validated experimentally.

BN-MWCNT/PPS core-shell structured composite for high thermal conductivity with electrical insulating via particle coating

Abstract Polyphenylene sulfide (PPS) is a promising material as an important engineering thermoplastic polymer for various industrial applications. Here we report a highly thermally conductive PPS composite containing boron nitride (BN) and multiwalled carbon nanotubes (MWCNTs) as a thermally conductive filler. The composite was prepared by the particle coating method into polymer pellets in order to obtain high thermal conductivity with low filler concentration, and thus overcome the drawbacks of melt mixing. A comparison between conventional melt mixing and the proposed method showed that the continuous connection of particles at the surface of each pellet led to great enhancement of thermal conductivity. Moreover, the electrically insulating BN layer effectively interrupted electron conduction, which is the major problem with using MWCNTs as a thermally conductive filler, despite their superior thermal conductivity. Our results show that the particle coating method may be used as a promising composite fabrication method for engineering plastics-based thermally conductive materials.

Structural and Thermo-mechanical Evaluation of Two Engineering Thermoplastic Polymers in Contact with Ethanol Fuel from Sugarcane

Special polymers have been used in the manufacture of storage structures and pipelines avoiding corrosive processes during ethanol fuel transport/storage. Therefore, this work investigated comparatively the effects of the ethanol on the physical-mechanical properties of poly (ether ether ketone) (PEEK) and polyamide 11 (PA-11) based on ageing tests. The WAXD and DSC results demonstrated slight reductions on the crystallinity degree of the aged PEEK, contrariwise to what happened with PA-11, where Xcincreased after ageing. However, the results of thermal, thermomechanical and mechanical analysis (TGA, DMTA, tensile and micro-IITs) demonstrated that PEEK is stable and no significant changes were observed in its elastic modulus (Ey≈ 3.4 GPa, E’ andEit≈ 3.7 GPa) or glass transition temperature. PA-11, conversely, was sensitive to ethanol fuel and expressive changes of its physical-mechanical properties were verified. For both materials, a reasonable correlation between crystallinity and mechanical properties was established.

Novel synergists for flame retarded glass-fiber reinforced poly(1,4-butylene terephthalate)

Poly(1,4-butylene terephthalate) (PBT) is an important engineering thermoplastic polymer which is very often reinforced by glass fibers (GF) to improve its mechanical properties. Applications of GF-PBT could be limited due to its high flammability and flame retardants (FRs) must be incorporated into the polymer. In this work, the selected FRs are aluminum diethylphosphinate (OP 1240) and a combination of melamine cyanurate and of aluminum diethylphosphinate (OP 1200). Synergists are investigated in formulations of GF-PBT containing OP1200 or OP1240 according to different fire scenarios (limiting oxygen index (LOI), UL-94 and mass loss calorimetry). It is found that polyhedral silsesquioxanes (POSS)-based molecules are synergists of interest combined with OP1200 and OP1240. The combination of the layered potassium acetate modified kaolin with OP1200 also appears to be beneficial. Introduction Engineering thermoplastic polymers such as polyamides, polycarbonates, semicrystalline aromatic polyesters and their blends are widely used in electrical and electronic appliances [1]. Poly(1,4-butylene terephthalate) (PBT) is one of them and it is a very important engineering thermoplastic polymer since it combines several desirable properties such as high strength and rigidity, low moisture absorption, excellent electrical properties and chemical resistance, rapid molding cycles and reproducible mold shrinkage [2]. It is very often reinforced by glass fibers to make various composites with high performance. Glass fibers are incorporated into PBT in order to improve properties such as stiffness, strength, creep and to increase the dimensional stability of the polymer [3]. The amount (20-50 wt.-% typically) and length of fibers as well as their orientation in PBT are governing factors that enhance

Layered double hydroxides used as flame retardant for engineering plastic acrylonitrile–butadiene–styrene (ABS)

Acrylonitrile–butadiene–styrene (ABS) resin is widely used as an important engineering thermoplastic polymer in various industrial applications, but suffers from easily burning and generating a large amount of smoke and toxic gases. Here we report a utilization of hydrotalcite-like MgA1- and ZnMgA1-layered double hydroxides (MgA1- and ZnMgA1-LDHs) as an inorganic flame retardant to ABS resin. The LDHs were prepared by a scalable method involving separate nucleation and aging steps (SNAS). The performances of the LDHs/ABS composites were evaluated by measuring limiting oxygen index (LOI), smoke density (Dm), heat release rate (HRR), and average mass loss rate (av-MLR). The results obtained show that both LDH/ABS composites exhibit higher LOI and lower Dm values, lower values of pk-HRR and av-MLR, and a prolonged combustion time, in comparison with the pristine ABS. Comparison between MgAl- and ZnMgAl-LDH-containing composites shows that the introduction of Zn2+ is able to facilitate flame retardance, smoke suppression efficiency, and tensile strength elongation rate of the ZnMgAl-LDH/ABS composite. Our results show that LDHs may be used as a type of promising inorganic flame retardant to enhance smoke suppression and flame retardant for ABS resin.

Glow wire ignition temperature (GWIT) and comparative tracking index (CTI) of glass fibre filled engineering polymers, blends and flame retarded formulations

Engineering thermoplastic polymers such as polyamides, polycarbonates, semi-crystalline aromatic polyesters and their blends are widely used as insulating materials in electrical and electronic appliances. Flame retardants are often employed in the formulation of these materials, since good performance in terms of ignition and tracking resistance, evaluated by Glow Wire Tests (GWIT) and Comparative Tracking Index (CTI) are required in these applications. In this paper, a comparative evaluation of GWIT and CTI performances has been simultaneously performed for a wide set of glass fibre filled materials chosen among engineering thermoplastics and their blends. Some flame retarded formulations have been also tested, in order to screen the effects of various additives. Useful indications have been obtained on the effect of each polymer and additives on GWIT and CTI properties. In addition, interesting synergies have been observed, especially by blending polyesters and polyamides. Thermogravimetric measurements of char yields have been successfully related with CTI behaviour. The presence of additives changes the structure of the carbonaceous residue, hence the conductivity of the tracks. Neat polycarbonate passed the GWIT test but not CTI, while poly(butylene terephthalate) showed the best balance of GWIT and CTI performance among the pure resins tested. Blending polycarbonate with polyester did not improve significantly GWIT performance, but had a negative effect on tracking resistance. Polyesters/polyamide blends were dominated by polyester in GWIT, but they showed synergistic effects in CTI.

An investigation on low stress abrasive wear characteristics of high performance engineering thermoplastic polymers

► Low stress abrasive wear on various unreinforced thermoplastic polymers. ► Abrasive wear rates strongly influenced by the applied load and type of polymeric materials. ► Semicrystalline polymers reflected ductile failure mode and amorphous polymers by brittle failure mode. ► A correlation between wear rates of polymers with various combinations of mechanical properties. Low stress (three-body) abrasive wear tests have been carried out on various unreinforced thermoplastic polymers using a rubber wheel abrasion test (RWAT) rig. Wear studies have been carried out using angular silica sand particles of size ranging between 150 and 250 μm and used as dry and loose abrasives. Abrasive wear studies was carried out at a constant sliding velocity ( v = 2.4 m/s) of rubber wheel and at different loads (5–20 N). The results showed that abrasive wear rates were strongly influenced by the applied load and type of polymeric material. The worn surfaces have been observed using optical microscope and scanning electron microscope (SEM) to understand the possible wear mechanisms involved during material removal processes. It was observed that semicrystalline polymers reflected ductile failure mode whereas amorphous polymers dominated by brittle failure mode. Efforts were made to correlate the abrasive wear rates with various combinations of mechanical properties. Possible correlations between abrasive wear rates with mechanical properties are reported.

Study of crystallization behavior of poly(phenylene sulfide)

Poly(phenylene sulfide) (PPS) is an engineering thermoplastic polymer that presents high temperature resistance (glass transition temperature around 85 ºC and melting point at 285 ºC). These properties combined with its mechanical properties and its high chemical resistance allows its use in technological applications such as molding resins and as matrix for structural thermoplastic composites. During the manufacture of thermoplastic composites, the polymer is exposed to repeated melting, quenching and crystallization processes. The properties of semicrystalline polymers, such as PPS, depend on its crystallization behavior. This work deals with the PPS crystallization kinetics under different thermal cycles. This study was performed under isothermal conditions in a differential scanning calorimetry (DSC), coupled to Perkin Elmer crystallization software referred to as Pyris Kinetics - Crystallization. The results were correlated with microscopic analyses carried out in a polarized light microscope, equipped with a controlled heating and cooling accessory. In this case, the experimental conditions were the same adopted for the DSC analyses. From the results, parameters could be established to be used in the composite manufacture.

Friction and wear performance of some thermoplastic polymers and polymer composites against unsaturated polyester

Wear experiments have been carried out with a range of unfilled and filled engineering thermoplastic polymers sliding against a 15% glass fibre reinforced unsaturated polyester polymer under 20, 40 and 60 N loads and 0.5 m/s sliding speed. Pin materials used in this experimental investigation are polyamide 66 (PA 66), poly-ether-ether-ketone (PEEK) and aliphatic polyketone (APK), glass fibre reinforced polyamide 46 (PA 46 + 30% GFR), glass fibre reinforced polytetrafluoroethylene (PTFE + 17% GFR), glass fibre reinforced poly-ether-ether-ketone (PEEK + 20% GFR), glass fibre reinforced poly-phylene-sulfide (PPS + 30% GFR), polytetrafluoroethylene filled polyamide 66 (PA 66 + 10% PTFE) and bronze filled pofytetrafluoroethylene (PTFE + 25% bronze) engineering polymers. The disc material is a 15% glass fibre reinforced unsaturated polyester thermoset polymer produced by Bulk Moulding Compound (BMC). Sliding wear tests were carried out on a pin-on-disc apparatus under 0.5 m/s sliding speed and load values of 20, 40 and 60 N. The results showed that the highest specific wear rate is for PPS + 30% GFR with a value of 1 × 10 −11 m 2/N and the lowest wear rate is for PTFE + 17% GFR with a value of 9.41 × 10 −15 m 2/N. For the materials and test conditions of this investigation, apart from polyamide 66 and PA 46 + 30% GFR polymers, the coefficient of friction and specific wear rates are not significantly affected by the change in load value. For polyamide 66 and PA 46 + 30% GFR polymers the coefficient of friction and specific wear rates vary linearly with the variation in load values.

Blending thermotropic liquid crystal and thermoplastic polymers for microreinforcement

A new process to obtain fibrous shaped thermotropic liquid crystal domains in bulk forms by proper blend processing with engineering thermoplastic polymer systems is presented. Normally, unless such a blend has experienced high elongational flow, the liquid crystalline domains are globular in shape and provide little or no improvement in tensile mechanical properties. One solution to this dilemma was to make the liquid crystalline domains small enough so that their size approaches that of microvoids in the thermoplastic polymer system. Preliminary investigations were made to demonstrate the process. It was then successfully applied to a thermoplastic polymer for use in aircraft interior panels. Besides the tensile mechanical property improvement, several other properties were greatly improved. One of these was fracture toughness, which doubled in value.

Fabrication of carbon-carbon composites by using hot isostatic pressing technology and novel precursor materials

Hot isostatic pressing (HIP) is a well established tool in the areas of powder metallurgy and the healing of castings, the use of which is being continually extended into an increasing range of materials processing. A good example of this new technology is the densification of carbon-carbon composites. The general principles, advantages and disadvantages of HIP fabricated carbon-carbon are described along with novel polyaromatic engineering thermoplastic polymer precursors.