Thermoplastic Composites

The growing concerns about the environment and energy have driven the widespread adoption of new lightweight materials such as polymer composites in many industrial sectors, crucial issues being performance, cost efficiency, and multifunctionality (Krawczak, 2019). Among them, thermosetting matrix composites offer not only many advantages such as high specific stiffness and strength and their consolidated use in primary aircraft structures but also technical issues such as long processing times, limited fracture toughness, reduction of mechanical properties in the presence of water and solvents, and difficulty of end-of-life treatment (Krawczak, 2011). A key cultural problem is associated with the need of a chemical reaction during part fabrication performed in mechanical/aerospace/naval industries.

Although the use of thermoplastic matrix composites in aeronautic structures has been proposed since the second half of the 80s (Nguyen and Ishida, 1987; Maffezzoli et al., 1989), their first application was at the beginning of the third millennium with the wing leading edge of Airbus A380 (Offringa, 2005). Their demand is continuously increasing (JEC Observer, 2020) since they offer many advantages such as high toughness, long storage time, easy repairing and recycling, higher performances in hot-wet conditions, and ability to be thermoformed and heat-welded. However, manufacturing issues such as high processing temperature, fiber impregnation, and warpage still need a deeper knowledge (Advani and Hsiao, 2012). Thermoplastic composites can play a key role in the 21st century industry as new materials are emerging day by day, as manufacturing processes are evolving to meet the severe industrial requirements of performance, production, and multifunctionality (Advani and Hsiao, 2012; Boisse, 2015), and as sophisticated numerical simulation tools are making virtual prototyping and engineering more reliable than ever (Binétruy et al., 2015; Boisse, 2015). In that field, innovation is multifaceted and may come either from the constitutive materials (high-performance engineering thermoplastic matrices, commingled semipreg of fibers/thermoplastics, self-healing, bio-based or biodegradable matrices, natural or thermoplastic fibers, 3D textile reinforcement…) or from the manufacturing processes (automated out-of-autoclave processes, hybridization/coupling of technologies, additive manufacturing, hybrid welding…). Moreover, thermoplastic composites allow for automated, high-speed mass production with reduced need for secondary operations (Mafeld, 2014), creating increased interest in the industry for various applications: aerospace and defense, automotive and rail transportation, construction and building, energy, oil and gas, sporting goods, drones, etc. Also, thermoplastic composites offer an interesting recycling potential using mechanical, thermal, or chemical processes (Oliveux et al., 2015). As thermoplastics, it is quite easy to recycle them by regrinding, compounding/blending, and reprocessing; additionally, they are at the forefront of current industrial scale-up attempts of chemical recycling, making it possible to recover not only fibers but also feedstock or original monomers which can eventually be re-polymerized to produce new polymers.
This Research Topic focuses on the latest advances and developments of fiber-reinforced thermoplastics, mainly referring to continuous fiber-reinforced composites and of related processing technologies. A collection of seven articles addresses some challenging issues related to advanced manufacturing processes, such as consolidation by compression molding (Ayadi et al.), injection-overmolding (Akkerman et al.), reactive pultrusion (Zoller et al.), joining by ultrasonic (Villegas) or induction (Scarselli et al.) welding, as well as solid-state drawing (Walker et al.) or electrospinning (Sessini et al.) of fibers. Numerical modeling and simulation, and performance assessment through advanced characterization techniques, correlating microstructure and properties, are also addressed. These papers highlight how thermoplastic matrix composites can be processed, adopting a completely different paradigm in comparison to thermosetting matrix ones, the former not requiring a reaction to set the properties but complex thermal cycles, often performed in a short time span.
The Guest Editorial team is confident that this collection of papers will benefit a large community in research and application of advanced thermoplastic composite materials so as to further advance along the pathway toward sustainability, performance, and competitiveness.

The injection over-moulding of 30wt% short glass fibre reinforced PA6 (SGF from Solvay Engineering Plastics) onto consolidated unbalanced (87/13) 70wt% glass fabric reinforced PA6 (Continuous Fibre Reinforced Thermoplastic (CFRT) from Solvay Engineering Plastics) was investigated with the objective to optimise the flexural and interlaminar shearing of the complex. Among the processing parameters, the temperature of the fabric before injection and the over-moulded melt temperature associated to the mould temperature (cooling rate of the complex) were revealed as the main parameters directing the mechanical properties of the complex. Moreover, the flexural modulus and the apparent interlaminar shear strength fall down critically in the main direction (chain direction of the fabric) under a CFRT temperature of 150°C. The effect of the SGF/CFRT interface was quantified in term of quadratic distance of diffusion through the interface. First, the 1D cooling of the complex was simulated ac...

The chemistry and uses of organosilanes in surface modification applications is reviewed with 54 refs. A mechanism for the deposition of organosilanes is proposed. Keywords: Silane Coupling Agents; Surface Modification; Polymer Composites; Enzyme Immobilization; mitochondria; adhesion promotion; contact angle; critical surface tension

In thermoplastic pultrusion process, unidirectional glass fibre bundles are very often used as reinforcement. This study was motivated by the industrial requirement to evolve from low viscosity thermoset resin processes to high viscosity thermoplastic polymers. The key parameters to control the impregnation process are the permeability of the fibrous reinforcement and the capillary pressure. The current study's objectives are threefold: (1) study the fibres arrangement by X-ray tomography, (2) determine the longitudinal and transversal permeabilities as well as capillary pressure to use them as input parameters of impregnation process in pultrusion and (3) carry out sensitivity analyses of the permeability measurements to the perturbation of fibre structure. To reach these purposes, two theoretical models were used to determine both axial and transversal tow permeability. The axial impregnation of aligned fibres was described by Washburn equation applied to 1D infiltration into the bundles. The in-plane impregnation was described by Darcy's law. The elliptical equation for highly anisotropic medium was resolved. Results show that permeability measurements are subjected to large variations, caused by perturbation of the fibre architecture during experiments. Besides, it was shown that the tensile force applied on fibres during pultrusion has a great effect on impregnation. Indeed, the permeability increases with the applied tension.

The static ASTM physical properties, long term flexural creep resistance, impact properties, wear and frictional properties, and environmental resistance of C fiber-reinforced nylon 66 [32131-17-2], poly(butylene terephthalate) [24968-12-5], polysulfone, poly(phenylene sulfide) [9016-75-5], ethylene-tetrafluoroethylene copolymer [25038-71-5], and tetrafluoroethylene-vinylidene fluoride copolymer [25684-76-8] were evaluated, and the carbon fiber-reinforced thermoplastics showed mechanical properties which were improved relative to the unreinforced and glass-reinforced polymers.  Flexural modulus values were about twice those of glass fiber-reinforced plastics, and long term flexural creep was essentially halved.  The thermal conductivity of the carbon fiber-reinforced thermoplastics was 2-3 times greater than glass fiber-reinforced thermoplastics and 3-5 times greater than that of the base resin.  The carbon fiber-reinforced plastics were elecically conductive, and had reduced coefficient of friction and wear and increased limiting pressure velocity values.

Abstract:
The injection over-moulding of 30wt% short glass fibre reinforced PA6 (SGF from Solvay Engineering Plastics) onto consolidated unbalanced (87/13) 70wt% glass fabric reinforced PA6 (Continuous Fibre Reinforced Thermoplastic (CFRT) from Solvay Engineering Plastics) was investigated with the objective to optimise the flexural and interlaminar shearing of the complex. Among the processing parameters, the temperature of the fabric before injection and the over-moulded melt temperature associated to the mould temperature (cooling rate of the complex) were revealed as the main parameters directing the mechanical properties of the complex. Moreover, the flexural modulus and the apparent interlaminar shear strength fall down critically in the main direction (chain direction of the fabric) under a CFRT temperature of 150°C. The effect of the SGF/CFRT interface was quantified in term of quadratic distance of diffusion through the interface. First, the 1D cooling of the complex was simulated according to the heat transfer module of COMSOL Multiphysics® in order to determinate the variation of the temperature field during the cooling stage of process. The calculations were achieved with an initial CFRT temperature of 23, 100, 150 and 200°C, the mould and SGF melt temperatures were kept constant. The diffusion theory has then been applied to calculate the variation of the auto-diffusion coefficient through the thickness during the complex cooling, the diffusion is supposed occurring only at a temperature above the PA6 crystallisation temperature (185°C). The calculation of the quadratic distance of diffusion through the thickness confirmed the mechanical results. Under a CFRT temperature of 150°C, the ability to the molecular diffusion at the interface becomes non-existent. The melt temperature of the SGF PA6 has to be sufficient to melt the CFRT PA6 interface, the time of diffusion directed by both the CFRT and mould temperatures (cooling rate) has to be long enough to allow the molecular diffusion from the material to the other.

Absorption-based microcomputed tomography (µCT) scans using polychromatic X-ray sources are frequently used to analyse the microstructure of polymer-matrix composites. Assuming the proportionality between linear X-ray attenuation coefficients and gray levels in the reconstructed µCT images, segmentation techniques can be used to conduct qualitative and quantitative analyses. Nevertheless, the reliability of such analyses is limited in partially consolidated composites formed of commingled yarns due to low contrast induced by beam hardening and partial volume effect. This paper aims at investigating the possibility of using low-contrast µCT images to analyse fiber bed deformation and impregnation mechanism by staged consolidation of a weft-knitted fabric of commingled thermoplastic/glass yarns. The experimental work is focused on the effect of the compaction ratio of the consolidation cycle. Absorption-based µCT analyses are conducted for representative samples using a voxel size of 10 µm and a constant energy-level of the X-rays source. Preliminary qualitative analyses of the raw reconstructed images indicate that beam hardening is the most significant in the case of the non-consolidated sample and that partial volume effect makes it difficult to visually distinguish between dry and impregnated zones inside the yarns at a compaction ratio of 63%. Thus, two image-quality descriptors, viz. contrast and signal-to-noise ratio are evaluated based on the K-means clustering method to follow their variation for four consolidation levels. The corresponding volume fractions of glass fibers, thermoplastic matrix, and voids are compared with the results of a second weight-controlled thresholding method. Considering that the weight-controlled thresholding method is less sensitive to beam hardening, it is confirmed that obtaining reliable segmentation results based on the K-means method requires a contrast around 0.5 and signal-to-noise ratio lower than 3. The results from the comparison with the classical consolidation scenario of commingled yarns confirm the potential of using µCT images of different qualities to characterize partially consolidated weft-knitted fabrics.

Presentation of dat'a for thermoplastic composites possessing specific utility in magnetic and electricaL service. Thermoplastic composites which are permanent magnets and electrically conductive composites will be characterized. These new materials offer vastly superior processing capahi lities when compared to the conventional "ceramic" processing methods in use today for preparing permanent magnets.

Comparative data are presented on the chemical resistance of eleven high temperature resins under various conditions of thermal exposure and under exposure to eleven different solvents. A generalized conslusion indicates that, if the chemical resistance of a material is judged excellent or acceptable when unstrained, the additional tensile strength loss for the material under strain is generally from 0 to 5%. Thus, most glass-fortified thermoplastics judged excellent or acceptable unstrained, remain excellent or acceptable strained. On the other hand, thermoplastics with marginal or unacceptable resistance generally lose an additional 5 to 15% of tensile strength when strained.

Results of an experimental study are presented which show that the addition of a modified PTFE lubricant to thermoplastic resins results in improved wear resistance, increased PV (pressure-velocity) limits, and lower coefficient of friction. The addition of glass fiber reinforcement results in increased physical properties and greater dimensional stability. Increases in ambient temperature result in increased wear rate and decreased limiting PV. addition of a migratory internal lubricant (silicone) to a PTFE lubricated thermoplastic can further reduce wear by 50%.

The demand for thermoplastic composites is continuously increasing because these materials offer many advantages over their thermoset counterparts, such as high toughness, long storage time, easy repairing and recycling, and ability to be thermoformed and heat-welded. However, the manufacturing of thermoplastic composite parts using liquid composite moulding techniques (e.g. resin transfer moulding, vacuum assisted resin transfer moulding …) is often tricky in the case of melt processing where high temperature and pressure should be chosen to impregnate the fibre reinforcement because of the high melt viscosity of thermoplastics. These issues may be overcome by means of reactive processing where a fibrous preform is first impregnated by a low viscosity mono- or oligomeric precursor and the polymerization of the thermoplastic matrix then occurs in-situ.
This article draws a state of the art on the manufacturing characteristics of continuous fibre reinforced acrylic-based reactive thermoplastics (e.g. polymethymethacrylate (PMMA) such as Elium), which are becoming more and more popular compared to other fast curing thermosets and thermoplastics for in-situ polymerization. Techniques for the in-situ polymerization of methymethacrylate monomers, characterization and modelling of the rheological properties and polymerization kinetics, and some manufacturing related issues such as polymerization shrinkage are reviewed. Particular features of the use of reactive PMMA in different manufacturing techniques of continuous fibre reinforced composites and potential industrial applications are also introduced. Finally, some perspectives for the academic research and industrial development are proposed.

The objective of coupling agents as components of composite plastic systems is to effect the greatest finished-product strength. Assuming that the optimum reactive groups for substrate and matrix can be selected, the objective becomes to maximize interaction. This may be addressed by reacting the maximum number of active substrate surface sites in order that the reacted surface may in turn present a firmly bonded film with high organic-reactive site density to the matrix. These products contain recurrent dialkoxy groups; compared with monomeric silanes and polymerics with trialkoxy groups, they possess superior substrate reactivity and provide performance improvements.

Eleven resins and composites (glass-fiber reinforced resins) were selected for high temperature testing. Comparative data are presented on the room-temperature properties, the tensile strength at elevated temperatures and the stress-relaxation behavior of the eleven materials, which include polyimide, ETFE, FEP, polyphenylene sulfide, polyethersulfone, nylon 6/6, polyester, polysulfone, polyarylsulfone, poly-p-oxybenzoate and polyamide-imide.

Thermoplastic resin composites reinforced with glass fibers and having improved mechanical and high tempera ture properties may be made by incorporating a small per centage of a metallocene coupling agent in the composite. Suitable metallocenes include titanocene, hafnocene and zirconocene dihalides; titanocene disulfonates; and stable derivatives thereof, any of which may be used with or without a conventional silane coupling agent. Stable ferrocene derivatives may also be used in combination with a reactive silane. The metallocene coupling agent may be applied directly to the glass ?bers, or mixed with the thermoplastic resin or composite blend. The present invention relates to glass reinforced ther moplastic compositions and methods for making the same. More particularly, the present invention is directed to thermoplastic composites reinforced with glass fibers and provided with metallocene coupling agents. The use of composite thermoplastics, particularly those containing chopped glass ?bers is expanding rapidly. More specifically, thermoplastic composites are penetrating en gineering applications in which strength and high tempera ture properties are of great importance. Such applications include tool and machine housings, gears and connecting blocks, and many others.

The effect of a variety of environments on the physical properties and performance of a series of glass fortified thermoplastics is detailed. The performance of glass fortified thermoplastic resins and the respective base resins are correlated for thermal, chemical, and solvent resistance testing. Keywords: Environmental Resistance; Thermoplastic Composites; Heat Aging; Fatigue Endurance; Weathering Resistance

The effect of sterilization on the structural integrity of the thermoplastic matrix composite polyetheretherketone (PEEK) reinforced with carbon fibers (CF) is investigated by nanoindentation and nanoscratch tests. The use of the material as a medical implant grade requires a detailed understanding of the micromechanical properties which primarily define its in vivo behavior. Sterilization is a mandatory process for such materials used in medical applications like bone implants.

ABSTRACT: Impact fracture toughness of polypropylene (PP) blends, high density polyethylene (HDPE) and rubber toughened polymethylmethacrylate (RTPMMA) has been studied by means of three-point bending falling weight impact testing at different temperatures ranging from –60ºC to room temperature using the cleavage fracture toughness, JC parameter [ASTM E1820-99a]. The latter Fracture Mechanics methodology was chosen due to its simplicity [Fasce et al., 2003]. Traces of the impact tests were analyzed using an inverse methodology just proposed by Pettarin et al. (2003). This methodology makes it possible to obtain from a three-point bending instrumented impact test the mechanical response of the material, discarding the dynamic effects associated with the test. The results show that the average JC values calculated with treated and untreated data are similar for a given material, while the standard deviations are larger when the calculations are made with the untreated data. It is clea...

The effect of sterilization on the structural integrity of the thermoplastic matrix composite polyetheretherketone (PEEK) reinforced with carbon fibers (CF) is investigated by nanoindentation and nanoscratch tests. The use of the material as a medical implant grade requires a detailed understanding of the micromechanical properties which primarily define its in vivo behavior. Sterilization is a mandatory process for such materials used in medical applications like bone implants. The steam and gamma radiation sterilization processes employed in this study are at sufficient levels to affect the micromechanical properties of some polymer materials, particularly in the interphase region between the polymer matrix and the reinforcing fibers. Nanoindentation and nanoscratch tests are used in this work to reveal local gradients in the hardness and the elastic properties of the interphase regions. Both methods help to explore microscopic changes in the hardness, reduced stiffness and scratch resistance in the interphase region and in the bulk polymer matrix due to the different sterilization processes employed. The results reveal that neither steam nor gamma radiation sterilization entails significant changes of the reduced elastic modulus, hardness or coefficient of friction in the bulk polymer matrix. However, minor material changes of the PEEK matrix were observed in the interphase region. Of the two sterilization methods used, the steam treatment has a more significant influence on these small changes in this region and appears to increase slightly the thickness of the interphase zone.

Data on flammability resistance, weatherability, and effects of water on glass-reinforced thermoplastics is presented. Performance data of thermoplastic composites of polypropylene, modified polyphenylene oxide, polystyrene, polysulfone and various polyamides are provided.

These plastics are blends of a thermoplastic resin and a small percentage of silicone. The silicone migrates to the surface of a molded or extruded part, thus generating a polymer film which serves as a boundary, or mixed-film lubricant. Hence, such materials are called migratory internally lubricated thermoplastics. They provide lower friction and wear than earlier internally lubricated thermoplastics and they eliminate the run-in period to bring the lubricant to the surface. A further enhancement of wear and friction properties has been achieved by adding both migratory-type polymer lubricants (silicone) and particle-type internal lubricants (such as polytetrafluorocarbon) . As these materials combine at the wear surface, they form a high-temperature grease. The PTFE acts both as a thickener and an extreme-pressure additive, and the silicone lubricant ensures continuous lubricity, both at startup and at high speeds. Tabulations of wear and friction properties and of physical and mechanical properties of acetal copolymer, nylon, polycarbonate and polystyrene with and without lubricant additives are presented. Keywords: Polymer Alloys; Tribology; Gear and Bearing Design; Silicone Fluids; Friction and Wear