Material requirements for composite manufacturing

Grant Agreement number: 101058371 Project acronym: ESTELLA Project title: Design of bio-based thermoset polymer with recycling capability by dynamic bonds for bio-composite manufacturing DELIVERABLE 1.2 Material requirements for composite manufacturing Contractual Date of Delivery: 31/08/2022 Actual Date of Delivery: 30/09/2022 Lead contractor for this deliverable: CIDAUT Author(s): Julia Guerrero Participants(s): Filipa A. Vicente (NIC), Damian Kiełkiewicz (ICSO), Wouter Post (WUR), Ghazal Tavakoli (WUR) WP contributing to the deliverable: WP1 Nature: Public Version V. 3 Grant Agreement 101058371 – Project ESTELLA ESTELLA_D1.2 ©ESTELLA - This is the property of ESTELLA Parties: shall not be distributed/reproduced without formal approval of ESTELLA General Assembly. This reflects only the author’s views. The Community is not liable for any use that may be made of the information contained therein. REVISION TABLE Document version Date Modified sections - Details V1 16.09.2022 First draft V2 22.09.2022 Revised by NIC and WUR V3 30.09.2022 Final version for submission after Content & Quality check Grant Agreement 101058371 – Project ESTELLA ESTELLA_D1.2 ©ESTELLA - This is the property of ESTELLA Parties: shall not be distributed/reproduced without formal approval of ESTELLA General Assembly. This reflects only the author’s views. The Community is not liable for any use that may be made of the information contained therein. Table of Contents ABBREVIATIONS .................................................................................................................................... 4 EXECUTIVE SUMMARY ........................................................................................................................... 5 1. INTRODUCTION ............................................................................................................................ 6 2. REQUIRED PROPERTIES OF THE RAW MATERIALS ......................................................................... 7 2.1. MATRICES - EPOXY ...................................................................................................................... 7 2.1.1. ESTELLA epoxies ................................................................................................................. 8 2.2. REINFORCEMENTS .......................................................................................................................... 13 2.2.1. (Ligno)cellulose (discontinuous fibre) ............................................................................... 14 2.2.2. Hemp (continuous fibre) ................................................................................................... 16 3. RAW MATERIAL PROCESSING CAPACITY ..................................................................................... 17 3.1. AUTOCLAVE MOULDING .................................................................................................................. 17 3.2. OOA PROCESSES ........................................................................................................................... 17 3.2.1. Vacuum Bagging .............................................................................................................. 17 3.2.2. Vacuum Bag Only/Oven Cure ........................................................................................... 18 3.2.3. Resin Transfer Moulding (RTM) ........................................................................................ 18 3.2.4. Vacuum Assisted Resin Transfer Moulding (VARTM) ....................................................... 19 3.2.5. Seeman Composite Resin Infusion Moulding Process (SCRIMP) ....................................... 20 3.2.6. Compression Moulding ..................................................................................................... 21 3.3. PROCESSABILITY REQUIREMENTS ....................................................................................................... 21 BIBLIOGRAPHY ..................................................................................................................................... 23 Grant Agreement 101058371 – Project ESTELLA ESTELLA_D1.2 ©ESTELLA - This is the property of ESTELLA Parties: shall not be distributed/reproduced without formal approval of ESTELLA General Assembly. This reflects only the author’s views. The Community is not liable for any use that may be made of the information contained therein. Abbreviations CAN: Covalent Adaptive Network DA: Diels-Alder DGEBA: Bisphenol A diglycidyl ether DGEBF: Bisphenol F diglycidyl ether OOA: Out-ofAutoclave RTM: Resin Transfer Moulding SCRIMP: Seeman Composite Resin Infusion Moulding Process TEMPO: 2,2,6,6-Tetramethylpiperidinyloxy or 2,2,6,6-Tetramethylpiperidine 1-oxyl VARTM: Vacuum Assisted Resin Transfer Moulding VBO: Vacuum bag-only curing Grant Agreement 101058371 – Project ESTELLA ESTELLA_D1.2 ©ESTELLA - This is the property of ESTELLA Parties: shall not be distributed/reproduced without formal approval of ESTELLA General Assembly. This reflects only the author’s views. The Community is not liable for any use that may be made of the information contained therein. Executive Summary The sustainable and recyclable composites to be developed in the ESTELLA project are made up of matrices and fibres that must meet certain requirements. On the one hand the mechanical and thermal requirements that will confer the desired properties to future applications. And on the other hand the requirements to be processed by out-of-autoclave (OOA) processes. This deliverable compiles the study carried out by NIC, ICSO, WR, CEL, WK+ and CID on the analysis of commercial materials currently used in the manufacture of thermoset composites. Based on this study, and their previous experience, this deliverable presents the specific requirements to be met by the materials developed in ESTELLA. The result of this deliverable will feed the work packages 2 and 3 dedicated to the design of the materials and the manufacture of the composites respectively. This deliverable D1.2 has fully met its objectives as set out in the Grant Agreement. It has suffered a slight delay in time due to vocational season but this has not had any impact in the progress of the project in terms of content or impacts. Grant Agreement 101058371 – Project ESTELLA ESTELLA_D1.2 ©ESTELLA - This is the property of ESTELLA Parties: shall not be distributed/reproduced without formal approval of ESTELLA General Assembly. This reflects only the author’s views. The Community is not liable for any use that may be made of the information contained therein. 1. INTRODUCTION Polymer composite materials or composites are widely used in sectors of high interest for the international economy. So much so that the European market for polymer composites reached a value of 16.6 billion dollars in 2018 and is estimated to increase by 7.5% by 2025(1) due to the growing demand for lightweight materials in strategic sectors such as wind energy, automotive, aeronautics and defense. In addition, their use is expected to expand to other sectors with lower profit margins such as leisure. The importance of these materials lies in the excellent properties that can be achieved, allowing them to replace metals reducing weight and emissions. For example, they can be found in structural elements of an aircraft or an automobile or in construction. The main composition of these composites is thermosetting resin (mainly epoxy) and carbon or glass fibre. Figure 1 shows the use of epoxy composites in different sectors. Figure 1. Epoxy composite on the market Therefore, the advantages of using composites are clear, especially from an environmental point of view. However, despite the advantages of using composites, they have not been fully implemented in most sectors and this is due to its low recyclability rate. As a result, the ESTELLA project was created with the main objective of improving the recyclability of these composites; by acting on the whole value chain of the materials for that purpose. Inherently recyclable polymer resins will be designed and developed through the introduction of reversible covalent links between the polymer chains, which provide the material with the ability to return from the processed state to the original monomers. Furthermore, the manufacturing processes of epoxy resins will be conveniently adapted, in order to maximize the economic revenue and the eco-efficiency along the value chain. Grant Agreement 101058371 – Project ESTELLA ESTELLA_D1.2 ©ESTELLA - This is the property of ESTELLA Parties: shall not be distributed/reproduced without formal approval of ESTELLA General Assembly. This reflects only the author’s views. The Community is not liable for any use that may be made of the information contained therein. The design of the new material requires some important aspects to be taken into account and their due fulfilment will allow the ESTELLA project objective to be achieved. These aspects are: 1) The final properties of the raw materials (matrices and fibres). 2) The ability to be processed by OOA techniques. 3) Recyclability (ESTELLA project objective). The final properties of the raw materials and their processability are objectives of this report (D1.2) and the recyclability properties are covered in D1.4. 2. REQUIRED PROPERTIES OF THE RAW MATERIALS The composite materials to be developed in the ESTELLA project consist of the elements shown in the Figure 2: Figure 2. ESTELLA composites manufacturing The nature of the matrices will be epoxy resins, one of which is a modified commercial epoxy and the other two will be synthesised using biomaterials. The nature of both reinforcements is biological and the major difference between them is the morphology: cellulose is a discontinuous fibre and hemp is continuous. The required properties of each material are described below: 2.1. MATRICES - EPOXY The epoxy resin consists of a broad group of reactive compounds that are characterized by the presence of an oxirane or epoxy ring. This is represented by a three-member ring containing an oxygen atom that is bonded with two carbon atoms already united in some other way (Figure 3). Grant Agreement 101058371 – Project ESTELLA ESTELLA_D1.2 ©ESTELLA - This is the property of ESTELLA Parties: shall not be distributed/reproduced without formal approval of ESTELLA General Assembly. This reflects only the author’s views. The Community is not liable for any use that may be made of the information contained therein. Figure 3. Epoxy molecular scheme Hence, the presence of this functional group defines a molecule as an epoxide – where the molecular base can vary widely resulting in various types of epoxy resins. And they are successful because they offer the diversity in molecular structure that can be produced using the same chemical method. Further, epoxy resins can be combined with varied curing agents, modifiers to achieve the properties required for a specific application. The types more common in composite applications are: phenolic glycidyl ethers, aromatic glycidyl amines, and cycloaliphatics (2). The most relevant properties of Epoxy resins are: - High strength - Low Shrinkage - Excellent adhesion to various substrates - Effective electrical insulation - Chemical and solvent resistance The ranges of mechanical properties are shown in Table 1. Table 1. Commercial Epoxy mechanical properties range Property Range Tensile Strength (MPa) 90 - 120 Tensile Modulus (MPa) 3100 - 3800 Glass transition temperatures, Tg (ºC) 150 - 220 Aside from the properties mentioned above, epoxy resins have two main drawbacks which are their brittleness and moisture sensitivity. 2.1.1. ESTELLA epoxies The ESTELLA project, in order to complete the objective, will develop three epoxy resins with specific properties. Figure 4 shows the nature (origin), the type of covalent adaptive network (CAN) and its recyclability and the ESTELLA partner involved. Grant Agreement 101058371 – Project ESTELLA ESTELLA_D1.2 ©ESTELLA - This is the property of ESTELLA Parties: shall not be distributed/reproduced without formal approval of ESTELLA General Assembly. This reflects only the author’s views. The Community is not liable for any use that may be made of the information contained therein. Figure 4. ESTELLA matrices summary WR will develop a resin that is strictly speaking not an epoxy resin. Although the final mechanical properties of typical cured epoxy resins are pursued, the curing of the reactive components is not based on epoxy groups, but on unsaturated groups susceptible to Diels-Alder (DA) cycloaddition. The reprocessability of Diels-Alder resins is based on the fact that the DA reaction is reversible; at high temperatures, chemical bonds formed by the DA reaction break (‘retro-DA’) to give back the original reactive groups. Similar to most epoxy resins, the DA resin is a two-component resin, which means that both reactive components need to be mixed prior to application. One of the aims of WR is to design a ‘one-pot’ version of the resin. This reduces processing times and handling difficulties. In the Estella project, NIC will design a lignin bio-based epoxy resin. Lignin products will be prepared with appropriate reactive sites (hydroxyl and carbonyl groups). In order to form a resin having a covalent adaptive network, the lignin products can be functionalised, for example, by the synthesis of glycidylated lignin products or formation of imine bonds. The use of lignin itself as curing agent with or without the presence of a co-curing agent will be evaluated in order to reduce the complexity of the system. The ICSO concept consists of imparting of CAN into fossil epoxy resin cured with selected carboxylic acids and/or anhydrides, to obtain polymers with inherent recyclability. Transesterification among hydroxyester bonds present in the obtained resins will allow for reaction mechanism comprising diffusion of solvent molecules into the network, breakage of polymer chain, diffusion of chain segments into the solvent, and further repolymerization. Viscosity of resins compositions will be adjusted by epoxidized plant oils serving as a reactive diluents to provide optimal processability in the composite production. In order to establish the properties that can be achieved with the designed resins, a thorough review of the state of the art has been carried out. 2.1.1.1. ESTELLA Lignin-based epoxy resins Table 2 shows the most relevant information obtained from the review related to resins. Grant Agreement 101058371 – Project ESTELLA ESTELLA_D1.2 ©ESTELLA - This is the property of ESTELLA Parties: shall not be distributed/reproduced without formal approval of ESTELLA General Assembly. This reflects only the author’s views. The Community is not liable for any use that may be made of the information contained therein. 2.2.2. Hemp (continuous fibre) Hemp fibre-reinforced composites have been successfully used for lightweight and low-cost applications in recent years, but significant barriers for structural applications of these composites still exist. These barriers include lack of confidence in the use and performance of natural plant fibres and their composites limited understanding of diffusion behaviour and poor resistance to moisture(22)(23). Table 8 shows the most relevant information obtained from the literature review related to hemp. Table 8.Continuous hemp fibres properties Type Tensile strength (MPa) Modulus (GPa) Density (g/cm3) Thermal decomp. (ºC) Stiffness (GPa) Failure to strain (%) Ref. Hemp fibre 900 70 --- --- --- --- (24) 20-1000 --- 1.4-1.6 --- 17.6-35.5 2.1-2.6 (25) --- --- --- 280–500 --- --- (26) --- --- --- 250-320 --- --- (27) Making continuous reinforcement using hemp fibre is quite challenging and it will be tried in this ESTELLA project. Before that as a reference material another bast fibre flax will be used for the initial testing purpose. As Table 8 shows, the mechanical properties of the hemp fibre are higher than other plant reinforcement. The ESTELLA hemp will be developed in order to achieve a tensile strength around 600-900 MPa, the flax will be between 3501030 MPa and the modulus between 20-60 Gpa. These properties are related with the morphology of the fibres and the pre-treatment. The ESTELLA hemp has inherent hollow microstructure and the presence of cellulose or lumen shows extremely lower thermal conduction than conventional fibres. The diameter of the elementary fibre will be between 10-45 μm. Other relevant property is that it will have a density range between 1-45-1.55 g/cm3. As mentioned earlier, the hemp yarns from bast is a demanding task hence the plan is to buy the commercially available yarn and to weave them in to a fabric by applying different patterns (Plain and Twill) for the composite application. The surface modification will be carried out to improve the surface morphology and to remove the hemicellulose. The chemical modification methods like NaOH treatment, peroxide and anhydride treatment will be utilized. The atmospheric plasma treatment will be utilized to improve the wetting behavior and the surface morphology. In addition, this modification will improve the flexibility as well as the thermal stability. The initial decomposition due to humidity is up to 150°C and the first stage of decomposition starts around 200°C hence the natural fibre should Grant Agreement 101058371 – Project ESTELLA ESTELLA_D1.2 ©ESTELLA - This is the property of ESTELLA Parties: shall not be distributed/reproduced without formal approval of ESTELLA General Assembly. This reflects only the author’s views. The Community is not liable for any use that may be made of the information contained therein. withstand minimum of 200°C. Thermal characterization will be utilized to assess the thermal stability of the flax or hemp fibre. Table 9 shows a summary of ESTELLA cellulose targeted properties: Table 9: Summary of ESTELLA cellulose nanofibres properties Tensile strength (MPa) Modulus (GPa) Density (g/cm3) Diameter (µm) Thermal decomposit ion Tª (ºC) ESTELLA Hemp 350-1030 20-60 1.45-1.55 10-45 200 3. RAW MATERIAL PROCESSING CAPACITY Thermoset composites could be processed by several techniques(28) depending on the morphology of fibres, the product geometry,… They could be classified in two main types: autoclave moulding and OOA processes. 3.1. Autoclave moulding Autoclave moulding is one of the open moulding processes where the moulded part is cured by application of the vacuum, heat, and pressure of the inert gases. The moulded part (or piece) is placed in a plastic bag, where the air is exhausted by a vacuum pump. This removes air inclusions and volatile products from the moulded part. Then, heat and inert gas pressure are applied in the autoclave causing curing and densification of the material. Finally, autoclave curing enables fabrication of consistent homogeneous materials. The method is relatively expensive and is used for manufacturing high-quality aerospace products. This process has some advantages such as the pressure that helps bond composite layers, the ability to manufacture pieces with high fibre loads, as well as high-quality products. However, it requires long cycle times and high energy and environmental costs. For this reason, composite materials manufacturing processes are nowadays aiming to avoid this process (29). 3.2. OOA processes 3.2.1. Vacuum Bagging The bagging process is a process in which a flexible, transparent film is used to enclose and compact wet laminates that are cured under atmospheric pressure. This method uses a vacuum pump to draw air from inside the vacuum bag and then compresses the part under atmospheric pressure (Figure 7). The resin is squeezed and sucked out of the wet laminate in the bleeder. This is a simple process that can use a wide variety of moulds. However, when it comes to large parts it requires longer set-up times and the manufacturing rate is low (30). Grant Agreement 101058371 – Project ESTELLA ESTELLA_D1.2 ©ESTELLA - This is the property of ESTELLA Parties: shall not be distributed/reproduced without formal approval of ESTELLA General Assembly. This reflects only the author’s views. The Community is not liable for any use that may be made of the information contained therein. Figure 7. A typical vacuum bagging lay-up before and after vacuum is applied 3.2.2. Vacuum Bag Only/Oven Cure Vacuum bag-only curing (VBO) uses the vacuum bag as the previous process and cures inside a non-pressurised oven. In the absence of high pressure, it is important to consider the property of the OOA resin, the architecture of the fibre bed and the prepreg system. Proper fabrication requires the removal of bubbles and volatiles before the gel time in order not to generate voids and thus defects. The mechanical properties of the composites are similar to those manufactured with the autoclave but the manufacturing times are longer. 3.2.3. Resin Transfer Moulding (RTM) The RTM process involves using a closed mold to fabricate a composite part. Fiber preform is cut according to the mold shape and placed in a closed mold cavity. A low-viscosity thermoset resin is injected through the injection port into the mold cavity, usually with a 3.5–7 bar pressure. The injected resin impregnates the preform evacuating entrapped air bubbles until complete wetting is reached. Once the resin starts exiting from the vent ports, the resin injection is stopped, and vent ports are closed. The resin is allowed to cure by heating the mold or the initial addition of inhibitors to the resin system. After the resin is cured, the mold is opened, and the part is de-molded. Some variants of the RTM process are VIPR, FASTRAC, light RTM (LRTM), structural reaction injection moulding (S-RIM), and co-injection resin transfer moulding. Figure 8 shows the scheme of the process. Grant Agreement 101058371 – Project ESTELLA ESTELLA_D1.2 ©ESTELLA - This is the property of ESTELLA Parties: shall not be distributed/reproduced without formal approval of ESTELLA General Assembly. This reflects only the author’s views. The Community is not liable for any use that may be made of the information contained therein. Figure 8. RTM process scheme Some advantages of RTM are that the process can produce parts with close dimensional tolerance and an improved surface finish. Parts made by RTM have a high-volume fraction of about 60–70%. RTM can manufacture complexshaped composite parts. Consistent reproducibility of composite parts can be achieved using the RTM process. Due to high resin pressure and faster mould opening and closing, a fast-manufacturing cycle is reached, further improved by process control. Some drawbacks of the RTM process are the limited size of parts that can be manufactured. Fibre wash can occur due to high resin pressure and loose fibre compaction. Furthermore, improper location of injection gates and vents can lead to a macro void in the composite (31). 3.2.4. Vacuum Assisted Resin Transfer Moulding (VARTM) In the VARTM method, the reinforcement is placed on a one-sided mould and sealed with a vacuum bag to form a closed mould. A vacuum is applied at the vent, which drives the resin under atmospheric pressure to impregnate the reinforcement while evacuating the air bubbles and compacting the fibre preform. The resin flows through the porous preform and arrives at the vent. The injection is closed, but the vacuum is maintained until the part is completely cured and de-moulded. Figure 9 shows the scheme of the process. Grant Agreement 101058371 – Project ESTELLA ESTELLA_D1.2 ©ESTELLA - This is the property of ESTELLA Parties: shall not be distributed/reproduced without formal approval of ESTELLA General Assembly. This reflects only the author’s views. The Community is not liable for any use that may be made of the information contained therein. Figure 9. VARTM process scheme The VARTM process is used to produce large composite parts at a low cost with a low production volume. This process is widely used in the energy, aerospace, marine, defense, and infrastructure building industries. Variations of VARTM have been invented to cater to the manufacturing of complex parts with better quality at a reduced cost. The VARTM process has some advantages: flexibility of mould tooling and selection of mould materials, resin and catalyst can be stored separately and mixed before infusion, low emission of volatile organic compound, and visible inspection of the process to identify and manage dry spot occurrence. However, some drawbacks of this process are that consumables such as sealing tape, peel-ply, and vacuum bags may not be reusable. The low resin injection pressure can limit void compressibility resulting in high void content and low fiber volume fraction. The process may be susceptible to high chances of air leakage, depending on the operator’s skill level (32). 3.2.5. Seeman Composite Resin Infusion Moulding Process (SCRIMP) The SCRIMP process is a modification of the VARTM process. It is an improved version of the VARTM process to efficiently and effectively distribute resin during impregnation using a distribution media. Therefore, it is used for making highquality and repeatable parts with minimal volatile emissions. Composite parts made by the SCRIMP process have a high fiber volume fraction typical of about 60–75%. The distribution media is a highly permeable material placed between the vacuum bag and the topmost layer of the fabric. It helps to distribute the resin quickly, thereby reducing the fill time. The resin initially flows through the DM layer before wetting the reinforcements through the thickness direction (see Figure 10)(33). Grant Agreement 101058371 – Project ESTELLA ESTELLA_D1.2 ©ESTELLA - This is the property of ESTELLA Parties: shall not be distributed/reproduced without formal approval of ESTELLA General Assembly. This reflects only the author’s views. The Community is not liable for any use that may be made of the information contained therein. Figure 10. SCRIMP process scheme 3.2.6. Compression Moulding Compression moulding is among the oldest materials processing techniques. For plastics, it was one of the first industrial methods, and is also known as matched die moulding. The basic process consists of heating a thermoset resin, under severe pressure, within a closed mould cavity until the resin cures through a chemical reaction of cross-linking polymeric chains. Under pressure, the resin liquefies and flows, taking the shape of the mould cavity, and then hardens into the desired product (Figure 11). Once sufficiently cooled and strong, the part is removed from the mould. The curing reaction continues while cooling to ambient (room) conditions. Figure 11. Compression Moulding process scheme 3.3. Processability requirements The ESTELLA project is committed to the manufacture of composites using environmentally friendly processes, i.e. outside the autoclave. This reduces energy consumption and environmental emissions. The specific process in which they are to be manufactured will be defined when the prototypes are designed, as they depend on their geometric Grant Agreement 101058371 – Project ESTELLA ESTELLA_D1.2 ©ESTELLA - This is the property of ESTELLA Parties: shall not be distributed/reproduced without formal approval of ESTELLA General Assembly. This reflects only the author’s views. The Community is not liable for any use that may be made of the information contained therein. complexity. However, regardless of the selected process, all of them have in common a series of parameters that will make the designed materials capable of being manufactured by OOA processes. The most relevant parameters for the processing of the matrices are: - Viscosity The matrices must penetrate the fibre and be injected or infused easily, so the viscosity of the matrices must be as low as possible. According to commercial resins the optimum viscosity should be 10-600mPa.s. - Curing time The curing curve relates the processing temperature to the time required for all the matrix bonds to be established and to obtain maximum properties. In the case of the resins designed in the ESTELLA project; these are not fast curing resins (cured in a few minutes) as they lack catalyst. The curing time of the resins varies from 30 min to a few hours, usually up to 6 h, and the process follows a multi-step approach at different temperatures, ranging from 40 to 140ºC(34) . Table 10 shows some examples of commercial epoxy rheological properties and Table 11the targeted ESTELLA epoxy properties: Table 10. Commertial epoxies rheological properties Reference Viscosity at 120ºC (mPa.s) Curing time at 120ºC (min) Reference RTM6_HexFlow 30 >240 (35) Resoltech 1500 16 2 (36) Araldite LY3585 40-60 2 (37) Table 11. ESTELLA epoxies properties Viscosity at 120ºC (mPa.s) Curing time at 120ºC (min) ESTELLA bio matrices 10500 360 ESTELLA fossil matrix 10-500 15-90 With regard to fibres, the parameters for their processability are ease of handling and compatibility with resins. Both are achieved thanks to the weaving of the fibres (in the form of fabric for hemp and mat for cellulose) and the compatibilisation pre-treatments described above. Grant Agreement 101058371 – Project ESTELLA ESTELLA_D1.2 ©ESTELLA - This is the property of ESTELLA Parties: shall not be distributed/reproduced without formal approval of ESTELLA General Assembly. This reflects only the author’s views. The Community is not liable for any use that may be made of the information contained therein. Bibliography 1. U.S. Commercial Service. Overview of the European composites market. 2020;(March). 2. Epoxy technical review [Internet]. Available from: https://omnexus.specialchem.com/selection-guide/epoxy-resins-a-to-ztechnical-review-of-thermosetting-polymer 3. Gioia C, Colonna M, Tagami A, Medina L, Sevastyanova O, Berglund LA, et al. Lignin-Based Epoxy Resins: Unravelling the Relationship between Structure and Material Properties. Biomacromolecules. 2020 May;21(5):1920–8. 4. Zhang Y, Wang H, Eberhardt TL, Gu Q, Pan H. Preparation of carboxylated lignin-based epoxy resin with excellent mechanical properties. Eur Polym J. 2021;150:110389. 5. 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