Recycled Composites Research Articles

Multiscale Manufacturing of Recyclable Polyimide Composite Aerogels.

Mitigating embodied emissions is becoming increasingly crucial as the energy supply shifts toward more sustainable sources. Bio-based materials present a potentially more sustainable alternative to synthetic polymers; however, it often do not yet match the performance of synthetic materials. Given the ongoing reliance on high-performance, high-environmental-impact materials, it is essential to ensure their complete recyclability. Aerogels, recognized by IUPAC as one of the top ten emerging technologies, are witnessing rapid market growth in thermal insulation and thermal protection applications. In certain applications, synthetic and composite aerogels exhibit superior performance, particularly under high temperatures. Here, molecular simulation tools are employed to elucidate the interaction forces between polymers and solvents, develop a recycling strategy for polyimide-based aerogels, and demonstrate their application in thermal protection for firefighter textiles and thermal runaway protection for Li-ion battery packs. These composites are engineered for disassembly, allowing for the complete recovery of starting materials without any degradation of components after multiple recycling cycles. The recyclable composites can be fabricated using various manufacturing techniques to produce fibers (1D), membranes (2D), and complex structures (3D). This unique combination of outstanding performance and excellent recyclability facilitates the sustainable utilization of aerogels in protective clothing, electric mobility, consumer goods, and aeronautics.

Life Cycle Assessment and Experimental Mechanical Investigation of Test Samples for High-Performance Racing Boats

This paper investigates the environmental impact and mechanical performance of two composite sandwich structures, named Series 1 and Series 2, used in high-performance racing boats. Mechanical tests, including four-point bending and drop impact tests, were performed. It was found on a general basis that Series 2 has higher load-bearing capacity and limited deflection. Series 1, which has a higher density, was able to absorb more impact energy but was more susceptible to damage. A Life Cycle Assessment (LCA) was conducted to evaluate the environmental impact associated with the materials, considering also the testing phase, which plays an important role in the life cycle of materials and structures for advanced marine applications. In addition, two performance indexes were introduced to correlate the mechanical and environmental properties of the analyzed materials. This study emphasizes the importance of considering the testing phase in LCA, as the energy-intensive nature of mechanical testing contributes significantly to the overall environmental impact. The introduced indexes allow for a more comprehensive understanding of the balance between mechanical performance and environmental sustainability. The findings suggest a trade-off between mechanical performance and sustainability, calling for further research into recyclable composites and greener manufacturing processes to balance these competing priorities.

In-Plane Cyclic Mechanical Properties of CF/PEEK/TPU Flexible Composite with Zero Poisson Ratio.

This paper presents the in-plane deformation and cyclic mechanical properties of CF/PEEK (Carbon Fiber-Reinforced Polyetheretherketone)-reinforced TPU (thermoplastic polyurethanes) flexible composites with a zero Poisson ratio. A novel CF/PEEK honeycomb reinforcement with a zero Poisson ratio was fabricated by using 3D-printing technology. Then, TPU was bonded in the two sides of the CF/PEEK honeycomb reinforcement. The in-plane deformation ability and cyclic mechanical properties were evaluated. The results show that the zero Poisson ratio flexible composite can achieve a large in-plane plastic deformation of more than 50% and can better maintain the zero Poisson ratio superstructure. By collecting and comparing the mechanical characteristic values of the CF/PEEK flexible composite under a cyclic load, the CF/PEEK flexible composite MH22-t0.6-CT has the best structural stability. The length of the structure was increased by about 12.53%. By studying the deformation mechanism and failure mechanisms of the flexible composites, the in-plane recyclability of the flexible composites was evaluated, which provides the basic research basis for large-scale in-plane deformation composites.

Composite Recycling with Biocatalytic Thermoset Reforming.

Carbon fiber reinforced polymers (CFRPs, or composites) are increasingly replacing traditional manufacturing materials used in the automobile, aerospace, and energy sectors. With this shift, it is vital to develop end-of-life processes for CFRPs that retain the value of both the carbon fibers and the polymer matrix. Here we demonstrate a strategy to upcycle pre- and postconsumer polystyrene-containing CFRPs, cross-linked with unsaturated polyesters or vinyl esters, to benzoic acid. The thermoset matrix is upgraded via biocatalysis utilizing an engineered strain of the filamentous fungus Aspergillus nidulans, which gives access to valuable secondary metabolites in high yields, exemplified here by (2Z,4Z,6E)-octa-2,4,6-trienoic acid. Reactions are engineered to preserve the carbon fibers with much of their sizing so that the isolated carbon fiber plies are manufactured into new composite coupons that exhibit mechanical properties comparable to those of virgin manufacturing substrates. In sum, this represents the first system to reclaim a high value from both the fiber fabric and polymer matrix of a CFRP.

Sublimation rate of solid NaCl powders and evaporation rate of liquid NaCl upon heating in vacuum and air

The volatilization of sodium chloride (NaCl) is important for multiple applications, such as energy storage, manufacturing porous parts, and purification and recycling of composites. Until now, only limited information on this feature has been published. To expand the knowledge in this field, we investigated the sublimation and evaporation of two NaCl powders by thermal gravimetry in vacuum and air. The study has shown that the volatilization kinetics of powders with different shapes and sizes are identical. The volatilization of NaCl powders in vacuum occurs mainly by sublimation before melting. In contrast, the volatilization of NaCl in the air is preferentially caused by the evaporation of the melt. In a vacuum, the evaporation of NaCl powder after melting is significantly slower than the sublimation before melting, because of a drastic decrease in the volatilization area. The Hertz-Knudsen relation with an appropriate fitting coefficient satisfactorily describes the mass loss by volatilization in vacuum and air.

Recyclability of Zinc Palmitate-Based Composites in Fatty Acid Methyl Ester Production from Oil Feedstocks at Varied Acidity.

Zinc hydroxide nitrate (Zn5) is one of the layered metal hydroxide materials, which has been extensively reported as an effective bifunctional catalyst for biodiesel production from acidic oils. This report gives a comprehensive summary of the reactivity of Zn5 in the methanolysis of various oil feedstocks, plant oils, free fatty acids, and other acidic oils. Notably, as evidenced by this work, Zn5 is highly effective in converting acidic oils [palmitic acid/palm oil (PO)] to fatty acid methyl ester (FAME or biodiesel) at high yields ranging from 80 to 95%, withstanding acid content up to 10% without soap formation. The high FAME yields result from complex methanolysis and hydrolysis processes, e.g., transesterification of triglycerides in the PO, esterification of palmitic acid, and hydrolysis of the triglycerides. Despite this, Zn5 is nonrecyclable because it is unstable in the reaction media and transforms into zinc hydroxide nitrate/zinc palmitate (Zn5/ZnP) composites. The Zn5/ZnP composites were suitable for use in FAME production from PO at 100 °C for 2 h by using a methanol-to-oil molar ratio of 30:1, yielding high FAME yields of 97 and 70.7% in the first and fourth cycles, respectively. This study added better insight into how to effectively produce FAME from oil feedstocks of varying acidity by using zinc layered hydroxide- or zinc carboxylate-based materials.

Influences of hybrid fiber on functional and drilling characteristics study of polymer hybrid Nanocomposite

The severe environmental pollution related to the manufacturing, recycling and disposal of metal-based and synthetic fiber-based composites has a major impact on human and animal life. A wide range of studies are conducted on alternative materials in order to reduce greenhouse gas emissions. Various industries are focusing on using natural fibers from banana, jute, coir, hemp, pineapple, areca, kenaf, Sterculia foetida and Delonix regia that are the key sources of different components for developing nanocomposites. Drilling is a vital factor in the assembly process of composites. The amorphous structure of the natural fibers causes the formation of cracks at the entry and exit sides of the drilled hole. So, in the present work a nanocomposite was synthesized and optimized the machining parameters, which affect the surface finish of the hole at the entry, exit and inner surface of the drilled hole. The effect of the hybridization of glass fiber with the pineapple leaf fiber and areca fiber was studied in terms of tensile properties, delamination around the hole and surface roughness. The microstructure of the holes was studied by Scanning Electron Microscopy.

Comparison effects of PEF and SC-CO2 treatments on lycopene, β-carotene, lutein, β-cryptoxanthin, total polyphenols values, and antioxidant activity of tomato fruits.

Since the recycling of composites from plant tissues is difficult, extraction of bioactive compounds from plant sources requires pre-treatment by new technology such as pulsed electric fields (PEF). Due to the reduced consumption of organic solvents, the extractive techniques such as using supercritical CO2 (SC-CO2) are of interest to researchers. This work aimed to investigating the influences of different parameters of SC-CO2 (pressure, modifier volume, temperature, and dynamic time) and PEF (frequency and field strength) treatments on the amount extraction of β-carotene, lycopene, lutein, β-cryptoxanthin, total phenol content (TPC), and also antioxidant activity percentage of tomato to obtain the optimum circumstances extraction via PEF and SC-CO2 methods. PEF data showed that treatments with moderate intensity (1 Hz and 0. 25 kV/cm) enhanced the extractability of lycopene (88%), β-carotene (69%), and β-cryptoxanthin (24%). The maximum recovery in total polyphenols was achieved at a 1 Hz and 1.75 kV/cm, leading to a 41.68% growth. The SC-CO2 results showed that extraction at 55°C and 35 MPa, and in a short time of 20 min (without any modifier: methanol) resulted in the highest levels of carotenoids (100% recovery), especially lycopene, and antioxidant activity. Largest value of total polyphenols was obtained at 35 MPa, 35°C, during 30 min, and 250 μL methanol as a modifier (58.79% recovery). Results showed that the extraction of polyphenols, unlike carotenoids, required a modifier. Organic solvents, often called modifiers, are sometimes added to the supercritical fluid to increase the polarity range of the extraction process and to help overcome analyte retention in the matrix. In this study, methanol was used as a modifier in different volumes. Therefore, the SC-CO2 gentle processing conditions, compared with PEF, improved the recovery of tomato bioactive compounds and antioxidant activity. Nevertheless, further studies are needed to optimize such treatments.

Thermal Recycling Process of Carbon Fibers from Composite Scrap—Characterization of Pyrolysis Conditions and Determination of the Quality of Recovered Fibers

In this study, we took a closer look at the thermal recyclability of CFRP composites used in the manufacture of high-pressure cylinders. Thermal analysis was used to determine the minimum temperature at which stable resin decomposition begins. The aim was to find temperature parameters and retention times with which the pyrolysis process is as economically viable as possible, and the recovered fibers retain optimum mechanical properties. The surface morphology of fibers annealed in both inert and oxidizing atmospheres was examined. In addition, the mechanical strengths under static as well as dynamic conditions of the newly manufactured laminates containing the recovered fibers were investigated. During research, it was found that reusing fibers is very difficult. The recycled carbon fibers were successfully compressed in an epoxy matrix in the form of a pre-impregnated carbon mat with the presence of air. The presence of oxygen during the thermal degradation of the composite severely damaged the surface and structure of the carbon fiber, causing composites made from these fibers to be mechanically weaker by more than 247%.

Self‐healing, low oil leakage and recyclable thermally conductive Al2O3/boron nitride@siloxane composites based on reversible dynamic network

AbstractIn order to overcome the commercialized silicone‐based thermal interface materials' issues such as oil leakage and mechanical damage, we developed a thermoset silicone rubber matrix (SR) achieved through the condensation reaction of hydroxyl silicone oil and tetraethyl silicate. Additionally, we constructed a self‐healing and recyclable silicone rubber matrix (SCNR) by incorporating heat reversible ionic and hydrogen bonds between carboxyl and amino functionalized polydimethylsiloxane. Finally, the flexible, low oil leakage, self‐healing, and recyclable composites Al2O3/BN@SR/SCNR was prepared by adjusting the ratio of SR/SCNR and the spherical alumina and boron nitride (BN) fillers. The results indicated that with SR/SCNR mass ratio of 1/0.075 and filler content of 70 wt%, the thermal conductivity of the composite reaches 1.866 W m−1 K−1, 646% increasing over the SR/SCNR matrix. The self‐healing efficiency reaches 97% and the thermal conductivity remains essentially unchanged after being recycled for three times. The composites can be bent and twisted multiple times and still return to their original shape. Notably, the oil leakage of the Al2O3/BN@SR/SCNR composites is significantly lower than that of additive silicone composites. Therefore, the recyclable thermal conductivity composite developed in this study provides a promising solution for improving the durability of heat dissipation materials in electronic devices.

Development and Manufacturing of a Fibre Reinforced Thermoplastic Composite Spar Produced by Oven Vacuum Bagging.

The limited recyclability of fibre-reinforced thermoset composites has fostered the development of alternative thermoplastic-based composites and their manufacturing processes. The most common thermoplastic-based composites are often costly due to their availability in the form of prepreg materials and to the high pressure and temperatures required for their manufacturing. Yet, the manufacturing of economic and recyclable composites, made of semi-preg composite materials using traditional composite manufacturing technologies, has only been proved at a laboratory scale through the manufacturing of flat plates. This work reports the manufacturing of a real structural part, a wing spar section with complex geometry, made of commingled polyamide 12 (PA12) fibres and carbon fibres (CFs) semi-preg and by oven vacuum bagging (OVB). The composite layup was studied using finite element analysis, and processing simulation assisted in the determination of the PA12/CF preform for OVB. Processing of two forms of semi-preg materials was first evaluated and optimised. The material selection for part manufacturing was mainly defined by the materials' processability. The spar section was manufactured in two OVB stages and was then mechanically tested. The mechanical test showed a linear strain response of the prototype up to the maximum load and validated the optimised layup configuration of the composite structure.

Packing simulation and analysis applied to a thermoplastic composite recycling process

A numerical model of packing applied to rigid objects is presented. It aims at describing a random stack of polymer composite chips in order to model the packing step of an existing recycling technique. The geometric properties of the stack play a major role in the mechanical properties of the recycled products. Short, simple and effective geometric descriptors of the stack are proposed. Their ability to differentiate random stacks is illustrated with an example. Then, a validation is proposed based on experimental data obtained from a bench specially designed for this work. The tests consist in the free fall of square chips. Finally, the developed model is compared to other models (free fall and packing of fibers) in order to enforce its relevance in the simulation of packing of rigid objects.

Advances in mechanical and physicochemical performances of recycled carbon fiber reinforced PEKK composites

In recent years, waste storage and recycling have been important issues. All efforts shown in these fields aim to help resolve concerns related to climate change. In this study, recycled CF/PEKK thermoplastic composite materials were reviewed to reduce the environmental impact of materials during the production phase as well as economic costs and ensure product continuity. Comparisons between unprocessed or virgin material and recycled materials were performed by bending and interlaminar shear strength (ILSS) analyses. In addition to mechanical tests, physicochemical tests such as differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and fiber void ratio (FVC)-porosity tests were conducted. Images of the samples were observed by scanning electron microscopy (SEM) to interpret changes in the structure before and after the recycling. According to the experimental results, it was found that the virgin thermoplastic composite showed 68.05% higher flexural strength and 7.85% higher flexural modulus compared to the recycled thermoplastic composites. The average ILSSs were measured as 81.8 MPa and 64.7 MPa for the virgin and the recycled thermoplastic composites, respectively. Hence, it is obvious that the recycled composites could be used for applications that require less strength and durability. Finally, it was concluded that recyclable materials can be reused in the aero structures.

Recyclable azine polyurethane thermosets and carbon fiber reinforced composites with excellent thermostability and mechanical properties

Carbon fiber reinforced composites (CFRCs) have received increasingly concerns and widely used in high-end applications, owing to their outstanding mechanical properties. Most of the polymers used in CFRCs were thermosets. However, a large number of applications brought enormous wastes due to the stable structure of thermosets which is difficult to degrade or recycle. CFRCs based on dynamic covalent polymers (DCPs) were developed to resolve the recycling problem. Polyurethanes were used for fabricating CFRCs applied in energy absorbing materials, vehicle interiors, sporting goods, and constructing materials, owing to excellent toughness, bonding abilities and low VOCs. Although some recyclable PU CFRCs were developed to resolve the recycling of CFs, there remain the problems of excess catalysts, low chemical resistance or thermostability due to the dynamic feature of DCPs. Herein, a series of azine polyurethane thermosets (APUTs) were prepared from hexamethylene diisocyanate (HDI) trimer and dihydroxy azines with different lengths of carbon chains. The resulting APUTs displayed excellent thermal stabilities with all Td, 5 % exceeding 320 °C, owing to high exchange temperature of azine moieties and high crosslinking density. APUTs-6 showed the highest tensile strength of 36.72 ± 1.18 MPa. APUTs-6 could be easily degraded in 0.1 M acetone/water solution at 50 °C. APUTs-6 showed excellent reprocessing properties and tensile strength could maintain 83.1 % of its initial one. The APUTs-6/CFs composites exhibited excellent mechanical properties, chemical resistance and recyclability. CFs could be readily recycled by acid degradation and maintain excellent mechanical properties. The tensile strength of recycled composites could recover 94.36 % of the original one after two times of recycling. The recovery ratio of tensile strength of recycled CF filaments was 95.7 %. The application of phenyl azine provided a feasible solution for the industrialization of recyclable CFRCs.

From waste to resource: Exploring the recyclability and performance of gypsum-graphene nanofiber composites

Gypsum is a widely used building material with a variety of benefits, including good fire resistance, sound insulation, and environmental friendliness. However, gypsum disposal is commonly associated with significant environmental risks, including unhealthy gas emissions and land degradation. Gypsum recycling can help to reduce these impacts, but it can also lead to a decrease in the performance of the recycled gypsum products. This study investigated the effects of graphene nanofibers (GNFs) on the recyclability and properties of gypsum plasters. The results showed that test samples containing 1 % of GNFs exhibited improved mechanical response, with flexural and compressive strength exceeding those of neat gypsum by 30 % and 60 % respectively. This improvement is attributed to the retained functional properties of GNFs during the recycling process. Recycled gypsum pastes containing GNFs presented a more uniform and denser matrix with longer crystals, enhanced bonding, and reduced porosity. These findings suggest that GNFs can be used to facilitate the recycling of gypsum waste and to produce recycled gypsum components with improved properties. Overall, this study demonstrates the potential of GNFs to improve the sustainability and performance of recycled gypsum plasters.

Evaluating the impact of recycling on polymer of 3D printing for energy and material sustainability

This research explores the sustainability of recycling polymer composites using fused deposition modelling (FDM). The objective was to assess how different recycling cycles affect the mechanical integrity and energy efficiency of recycled polymers. The study employed quantitative assessments of tensile strength, energy consumption, and carbon emissions across multiple recycling cycles. Recycled materials were compared with virgin materials to establish a baseline for degradation and efficiency. Various additives were tested to evaluate their ability to stabilise material properties. Significant findings indicate that recycled polymers retain up to 90 % of their original tensile strength after the first cycle, declining to 80 % after three cycles. Energy usage during the recycling process decreased by 30 %, while the carbon footprint was reduced by 25 %, showcasing notable environmental benefits. The study confirms that FDM recycling of polymer composites can be optimised to achieve substantial sustainability benefits in terms of environmental impact and material preservation.

Polyurethane Composites Recycling with Styrene-Acrylonitrile and Calcium Carbonate Recovery.

The glycolysis process of flexible polyurethane foams containing styrene-acrylonitrile and calcium carbonate as fillers was explored in detail. The use of DABCO as a catalyst allowed us to reduce the catalyst concentration and the polyurethane-to-glycol mass ratio to 0.1% and 1:1, respectively. The glycolysis process allowed us to obtain a high-purity polyol (99%), which can totally replace raw polyols in the synthesis of new flexible polyurethane foams, maintaining the standard mechanical properties of the original one and modifying the ratio of isocyanates employed to correct the closed cell structure caused by the impurities present in the recovered polyol. This isocyanate mixture was also optimized, resulting in a ratio of 30 and 70% of the isocyanates TDI80 and TDI65, respectively. Additionally, the fillers incorporated in the glycolyzed foams were recovered. Both recovered fillers, styrene-acrylonitrile and calcium carbonate, were fully characterized, showing a quality very similar to that of commercial compounds. Finally, the replacement of commercial fillers by the recovered ones in the synthesis of new polyurethane foams was studied, demonstrating the feasibility of using them in the synthesis of new foams without significantly altering their properties.

High‐performance, semi‐bio‐based degradable epoxy resins and their application to recyclable carbon fiber composites

AbstractEpoxy resins containing dynamic covalent networks enable recycling of carbon fiber composites. However, the simultaneous realization of high performance and mild condition recycling of composites is still a challenge. In the present work, we mixed citric acid epoxy resin with bisphenol A epoxy resin to form a hybrid resin (DER) that meets the requirements of medium temperature curing epoxy resin, and prepared a carbon fiber reinforced resin composite material (DER@CF) that can rapidly degrade and recycle carbon fibers under mild conditions. The Tg of the DER was 125.67 °C, and the tensile strength (79.63 MPa) was comparable to that of the bisphenol A epoxy resin (79.90 MPa). It degraded rapidly after 2 h in ethylene glycol solution at 120 °C. The tensile strength of DER@CF was 825 MPa, and clean fibers could be obtained after 8 h of treatment in ethylene glycol solution at 120 °C. The results of SEM, Raman and tensile tests showed that the recycled fibers were similar to the original fibers in terms of morphology, chemical structure and mechanical properties. © 2024 Society of Chemical Industry.

Optimising Recycling Processes for Polyimine-Based Vitrimer Carbon Fibre-Reinforced Composites: A Comparative Study on Reinforcement Recovery and Material Properties.

We investigated the recycling process of carbon fibre-reinforced polyimine vitrimer composites and compared composites made from virgin and recycled fibres. The vitrimer matrix consisted of a two-component polyimine-type vitrimer system, and as reinforcing materials, we used nonwoven felt and unidirectional carbon fibre. Various diethylenetriamine (DETA) and xylene solvent ratios were examined to find the optimal dissolution conditions. The 20:80 DETA-xylene ratio provided efficient dissolution, and the elevated temperature (80 °C) significantly accelerated the process. Scaling up to larger composite structures was demonstrated. Scanning electron microscopy (SEM) confirmed effective matrix removal, with minimal residue on carbon fibre surfaces and good adhesion in recycled composites. The recycled nonwoven composite exhibited a decreased glass transition temperature due to the residual solvents in the matrix, while the UD composite showed a slight increase. Dynamic mechanical analysis on the recycled composite showed an increased storage modulus for nonwoven composites at room temperature and greater resistance to deformation at elevated temperatures for the UD composites. Interlaminar shear tests indicated slightly reduced adhesion strength in the reprocessed composites. Overall, this study demonstrates the feasibility of recycling vitrimer composites, emphasising the need for further optimisation to ensure environmental and economic sustainability while mitigating residual solvent and matrix effects.

Mechanical recycling process: An alternative for a viable disposal of carbon fiber‐reinforced thermoplastic waste

AbstractConcern for the environment has increased the interest of the automotive and aerospace industries in evaluating the feasibility of mechanical recycling of waste carbon‐fiber‐reinforced polymer composites (CFRP) and waste from protective films used during the transport and storage of thermosetting composites, which are removed and discarded during manufacturing. Therefore, this work aimed to develop a recyclable composite from protective film waste by incorporating carbon fiber/poly(aryl ether ketone) (CF/PAEK) composite particles obtained through grinding in knife mills, followed by an analysis of the particle size distribution. Protective film residues (PEres) were cut to facilitate processing. A mixture of low‐density polyethylene and PEres (50/50) was used as the composite matrix, and 5, 10, 15, and 20 wt% CF/PAEK composite particles were added as the reinforcing agents. Then, these recycled composites were prepared by melt mixing using a twin‐screw extruder and injection molding to obtain standardized samples. Mechanical, thermal, electrical, and electromagnetic properties were evaluated. It was possible to observe that the increase in the addition of CF/PAEK composite particles contributed to a 290% increase in elastic modulus values and a 9.1% increase in Shore D hardness, electrical conductivity, and attenuation of the electromagnetic wave. Furthermore, satisfactory adhesion between filler and matrix contributed to a good interface and reinforcement of the final material.Highlights Mechanical recycling by extrusion of waste from the automotive and aerospace sectors Grinding of carbon fiber/poly(aryl ether ketone) composite waste to be used as filler Protective film waste used as matrix Composites developed with waste with good electrical and electromagnetic properties The addition of thermoplastic composite residues does not affect thermal and mechanical properties