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Development of an innovative bio-based resin for aeronautical applications

Periodic Report Summary 2 - BME CLEAN SKY 027 (Development of an innovative bio-based resin for aeronautical applications)

Project Context and Objectives:
The development tendency of the replacement of traditional mineral oil based plastics with innovative bio-based resin systems is nowadays characteristic for many segments of the industry; however, for aeronautical applications (interiors and/or structures) the challenge is much larger than elsewhere. To meet this challenge, a flame retarded special thermosetting polymer system was planned to be prepared using bio-based components.
The project aims at providing replacement for conventional petroleum-based plastics with new and innovative bio-based composites by development and synthesis of their components for aeronautical applications (internal and external elements as structural material). Because of extreme working conditions in aeronautical field, the quality and safety requirements are considerably high, therefore researchers face major challenges in fulfilling these requirements.
In the frame of this project a new type of epoxy bio-materials are produced that can meet the high requirements of aviation industry. During the synthesis different types of sugars are used as starting materials, which are renewable bio-materials and do not compete with the food industry due to the large oversupply of sugar in the recent decades. Innovative chemical methods are used for manufacturing resin components in order to improve the properties of the resin, including mechanical characteristics and reduced combustibility. The component synthesis process design is carried out considering the principles of green chemistry, energy efficiency, opportunities to scale up, environmental and health protection. Qualification and selection of the right combination of compounds was verified based on complex criteria including chemical, physical and mechanical properties, thermal stability and flammability. The synthesis of the chosen optimal bioresin component was planned to be up-scaled using advanced reactors, such as a computer-controlled reactor equipped with online Raman feedback and microwave heating.
In order to achieve improved mechanical strength necessary for application as a structural material, natural fibers were planned to be used. By combining bio-based resins and natural fibers the aim was to prepare fully bio-based composited meeting the requirements of the aeronautical sector as well. To reach this goal the adhesion between the matrix and the fibre had to be improved and the flammability requirements had to be reached by surface treatment elaborated during the project.
After preliminary experiments, the impregnation of the chosen natural reinforcing fabrics with the synthesized bioresin was carried out and samples for detailed evaluation were manufactured. After selecting the optimal resin/fibre systems (composition, treatment and structure) for industrial evaluation, the bio-composites prepared were subjected to extensive testing including dynamic mechanical analysis, tensile tests, three point bending tests, limiting oxygen index measurement (LOI), UL-94 and cone calorimeter tests, evolved gas analysis laser pyrolysis coupled with Fourier transformation infrared spectrometer. The most appropriate biocomposite system is planned to be tested in a form of sandwich composite using polymethacrylimide foam cores, aiming at application as internal floor panels in aircrafts.

Project Results:
New epoxy components and flame retardant curing agents were developed and selected. Starting out from glucose (sugar), bi-, tri and tetrafunctional epoxy components were prepared. Based on the synthesis yield and the achieved glass transition temperature, the two trifunctional ones, a glucopyranoside (GPTE) and a glucofuranoside-based one (GFTE) were chosen for up-scaling.
In order to improve the flame retardancy of the bioresins, three types of P-containing amines (an aliphatic and two aromatics) were synthesized, which can act both as reactive flame retardants and crosslinking agents. For their preparation, an environmentally friendly reaction way was established with improved atomic efficiency, starting out from a non-halogenated phosphorylating agent (triethyl phosphate) and producing non-harmful by-product (ethanol).
Four epoxy resin systems were characterized: First, the applicability of the currently most used bio-based epoxy resin matrix material, epoxidized soybean oil in DGEBA based anhydride cured epoxy resin was studied. After that effect of epoxidized soybean oil in different aromatic and aliphatic epoxy resins was thoroughly examined, with special emphasis of aliphatic resins which can be potentially synthesized from renewable biomaterials. Characterization of sorbitol based flame retarded epoxy resin system containing reactive flame retardant synthesized in the frame of was also carried out. Finally, screening of bio-based epoxy resin matrix materials was performed in order to be able to choose the appropriate bio-resin component for up-scaling. Based on results of the test screening of the synthesized bio-based epoxy resins (curing behaviour, gelling, dynamic mechanical analysis, tensile testing, thermogravimetrical analysis, fire retardancy tests) a furanoside-based bioresin, GFTE was chosen as matrix material for final testing and composite preparation.

As for the reinforcement, different natural fabrics (3 types of hemp, 3 types of jute, 2 types of linen and a hemp/linen differently woven fabrics) were subjected to strip tensile tests. Taking into account both the properties of the reinforcement and the commercial availability a plain jute fabric was chosen as fibre reinforcement for composite preparation.
The flammability of the natural fabrics represents a crucial issue, which can be reduced by surface treatments. The first treatment consisted of filling the capillaries with ammonium phosphate, the second one was a sol-gel with aminosilane and the combination of this two treatments. Both the thermal stability and the flame retardancy reached the best values when the combined treatment was applied.
During the manufacture of the composites it was proven that the developed bio-based epoxy resin is suitable for composite matrix application. From the mechanical tests of the neat matrices it could be declared that the mechanical properties of the developed bio-based systems are adequate, slightly lower than the ones of the synthetic resins. While the results of the jute fiber reinforced composites were mixed, from the results of the carbon fiber reinforced composites it can be clearly seen that the developed matrix material can already replace the reference synthetic epoxy resin matrices.
This was supported by the preparation of the sandwich composite panels. The results of the sandwich composites also support the applicability of the developed bio-based matrix system as a high performance matrix material. Based on the three point bending test results and the specimen production experience of the synthetic foam core sandwich composites it can be declared that the developed bio-based GFTE matrix sandwich system can not only successfully substitute the presently used synthetic DGEBA matrix sandwich structures, but also significantly outperforms it, opening a wide range of aircraft interior applications for the developed GFTE matrix composite system.

Potential Impact:
Decreasing amount of mineral oil stock, increasing environmental awareness and legislations aiming at fostering the use of renewable resources all supported the partial or full replacement of the synthetic epoxy resin components with renewable sourced ones in polymer composite applications. However, for aeronautical applications (interiors and/or structures) the challenge is much larger than elsewhere due to high quality and safety requirements. To meet this challenge, this project aimed at synthesis and development of new and innovative bio-based epoxy resin components and preparation of flame retarded thermosetting polymer system using bio-based components.
In the frame of this project were during the synthesis of new bio-based epoxy resin components different types of sugars were used as starting materials, which are renewable bio-materials and do not compete with the food industry due to the large oversupply of sugar in the recent decades. Innovative chemical methods were used for manufacturing resin components in order to improve the properties of the resin, including mechanical characteristics and reduced combustibility. The component synthesis process design was carried out considering the principles of green chemistry, energy efficiency, opportunities to scale up, environmental and health protection. Qualification and selection of the right combination of compounds was verified based on complex criteria including chemical, physical and mechanical properties, thermal stability and flammability. As epoxy resin components the newly synthetized glucofuranoside-based structure (GFTE) proved to be an effective composite material suitable for industrial applications, because it provides numerous advantages as glass transition temperature above 180°C , higher synthesis yield, liquid state, consequently easier processing and composite preparation and lower postcuring temperature. The synthesis of the chosen optimal bioresin component was up-scaled using advanced reactors, such as a computer-controlled reactor equipped with online Raman feedback and microwave heating.
In order to improve the flame retardancy of the bioresins, three types of P-containing amines (an aliphatic and two aromatics) were synthesized, which can act both as reactive flame retardants and crosslinking agents. For their preparation, an environmentally friendly reaction way was established with improved atomic efficiency, starting out from a non-halogenated phosphorylating agent (triethyl phosphate) and producing non-harmful by-product (ethanol). By application of P containing flame retardant curing agent and/or surface treated natural fibre, the developed epoxy composites can fulfil the requirements of aircraft interiors FST (Fire, Smoke and Toxicity). No environmental issues are known concerning the used P and Si containing flame retardants.
During the preparation of natural fiber reinforced composites 60 mass% fibre content could be achieved by hot pressing, leading to appropriate mechanical properties, consequently the so prepared completely bio-based composites are a possible candidates for the replacement of currently used carbon fibre reinforced synthetic epoxy resin composites in some interior applications areas in the aircraft industry.
The most appropriate biocomposite system (consisting of glucofuranoside-based epoxy resin componens (GFTE) cured with anhydride type curing agent and jute fabric as reinforcement) was tested in a form of sandwich composite using polymethacrylimide foam cores. The preparation and testing of sandwich composites was carried out in order to determine the applicability of the developed system as internal floor panels in aircrafts. It can be declared that the developed bio-based GFTE matrix sandwich system significantly outperforms the presently used synthetic DGEBA matrix sandwich structures, enabling a variety of aircraft interior applications from the developed biocomposite system.

List of Websites:

http://ch.bme.hu/en/research/details/project/2/