Advanced lightweight materials for Energy-efficient Structures

1. Introduction

Transport sector accounts for more than fifth of the greenhouse emissions in Europe [1] being an expanding target towards decarbonisation. Despite the largest ever decline in global emissions due to the economic and social shutdowns during the covid-19 crisis, the transport sector rebounded in 2021 to their highest level in history [2]. Furthermore, the fast-growing transport demand is expected to generate a large increase in emissions if no actions are taken [3]. Long term low greenhouse gas emissions development strategies are key enabling instruments to reconciliate near-to-medium term action with the European long-term objectives [4] to become climate neutral by 2050 [5]. In this context, FOREST project proposes a combination of three key drivers for the future of the transport sector decarbonisation (figure 1): Reduce, Recovery, Reshape.

Reduce: structural weight reduction and fossil dependency. The growing environmental concern and demand for light weight materials in the transport sector makes attractive opportunities in European composite market, which is expected to grow at a CARG of 8.3% from 2021 to 2026[6]. Fibre reinforced polymers (FRP) have emerged as promising materials for vehicle weight reduction. This contributes to decrease the emissions of internal combustion engines (ICEs) and increase the efficiency of battery electric vehicles (EVs) and fuel cell vehicles (FCVs) in order to meet the EU net-zero greenhouse gas emissions challenge by 2050 [7]. In terms of life cycle assessment (LCA), the use of light FRP structures may lead to lower the environmental impact due to the reduced fuel consumption. However, there is an environmental concern about the manufacturing and end-of-life (EOL) phases of FRP, since is more energy intensive and less recyclable because of the synthetic nature of most of the composite materials used in transport sector. Hence, the Circular Economy (CE) concept in composite cannot be totally adopted. Moreover, the value of the global consumption of plastics was rated at USD 568.9 billion in 2019 and is estimated to increase by 3.2% annually in the next seven years [8], which is triggering the depletion of fossil sources and delaying the bio-based materials certification.

Recovery: carbon fibre. On the other hand, the current society requirements aligned with new materials sustainability involve a deeper awarness on the environmental issues as well as the customer satisfaction. That is why transport industry is switching from virgin carbon fibres (vCF) to recycled carbon fibres (rCF) to deal with the waste increase from associated manufacturing processes and EOL products. Especially, the global market of carbon fibres estimated at US$3.8 Billion in the year 2020, is projected to reach a revised size of US$7.2 Billion by 2027, growing at a CAGR of 9.5% over the analysis period 2020-2027. The aerospace market is one of the main segments to project a higher carbon fibre (CF) demand by 2027, whereas in the automotive industry the use of CF is emerging as a material of choice replacing steelsteel replacement to lighter materials. Therefore, due to the future significant growth of the CF demand in transport sector, novel solutions are being incorporated to increase the recovery rates for manufacturing and waste disposal at EOL (EU Directive 1999/31/EC on Landfill waste [9] to develop more environmentally friendly products.

Reshape: multifunctionality. The use of lighter, bio-based and recycled materials is an attractive solution to decarbonise the transport sector. However, these sustainable materials must guarantee component stability and safety to be considered a viable solution in the transport sector. It is a major challenge to maintain the same high properties than conventional materials: high mechanical performance (structural functions), safety and reliability in terms of fire retardancy and electromagnetic wave absorption (non-structural functions). Although multifunctional composite materials (MFCMs) display remarkable properties, still can be improved by incorporating other practical properties, such as EMI shielding.

Considering all the challenges associated to the FRP structures (lightening, strong dependency of fossil fuels, high CF waste, and the required multifunctional properties), there is a strong interest in the usage of alternative lightweight biocomposites with the combination of functional materials with different mechanical, fire-retardant and EMI-shielding properties to obtain an enhanced material with a high grade of homogeneity, aligns with actual trends of economical and environment global strategies [10].

Figure 1. FOREST aligned with Mobility Decarbonisation

2. Project development

FOREST aims at providing new and innovative green composites for sustainable and safety transport applications by combining the development of bio-based polymers & additives, recycled fibres with increased resource efficiency and particles to avoid EMI interferences, which is fully aligned with the EU 2030 Climate and Energy challenges [11] (figure 2).

Figure 2. Biopolymers, additives and fibres employed to produce demonstrators via OOA technologies for bus, plane and electric vehicle applications.

2.1. Sustainable raw materials development

FOREST will lead a sustainable strategy with the development of bio-based and recycled materials.

Nowadays, commercially available thermoplastic acrylic resins provide a range of attractive properties to the performance and can be produced by OOA processes. However, the bio-based version has not been developed widely. Likewise, commercially available bio-polyamides lack of cost effective structures and therefore they are only established in niche applications. There is a limited available bio-based benzoxazine resins with low variety of bio-based monomers that match fire, smoke and toxicity (FST), mechanical and thermal requirements. Moreover, polymeric reaction costs are too high to be a profitable product in the market. Within FOREST project, green and fast chemistries, for thermoplastic acrylic resins, polyamides and benzoxazine, will be formulated to increase the bio-based content and to accelerate the curing of the reactive resin systems.

However, one of the main drawbacks of biocomposites is their low fire-retardant properties. One of the current possible solutions consists of introducing fossil-based fire retardant as powder additive in around 25%. However, this additive has a negative effect on the impact strength, spinnability and transparency. Usually, inorganic compounds are the halogen free alternative, such as phosphorous and nitrogen. Inorganic flame retardants are aluminium hydroxide and magnesium hydroxide. These additives are effective when introduced in high ratios (around 50-60% w/w), which have a negative effect on the mechanical properties of the polymer. Phosphorous-based flame retardants, and their combinations with nitrogen compounds to generate intumescent systems, are the most effective halogen-free solution up to date with addition ratios between 20-25% w/w. These flame-retardants could also be interesting for conventional oil-based polymers as an alternative to flame retardants obtained from fossil raw materials. Focusing on fossil based phosphorous as flame retardant could become a problem for UE industry as it is considered a critical raw material [12]. The lack of phosphate rock makes interesting the development of alternative bio-based flame retardant to increase Europe competitiveness. Biomass chemical composition include carbon, hydrogen, nitrogen and phosphorous which can interact improving the flame-retardant effect of polymers [13]. Four main families of compounds are candidates for flame retardant applications: carbohydrates, proteins, lipids and phenolic compounds. Some of the most interesting compounds regarding the state of art are phytic acid from plant seeds (28% of phosphorous in the structure and reactive -OH groups), Chitosan from crustaceans (polysaccharide backbone with -OH and NH2 groups), deoxyribonucleic acid (DNA) (high content of nitrogen and phosphate groups) and phenolic compounds like lignin or tannins. The basic criteria to select the proper flame-retardant material is [14] the thermal stability: the decomposition temperature of the compound must be high enough to withstand polymer processing temperatures, high charring ability, presence of reactive chemical groups such as hydroxyl, carboxylic acid, amine or double bonds and presence of phosphorous, nitrogen or silicon with flame retardant properties. FOREST project proposes phytic acid and chitosan for the creation of a bio-based flame-retardant system. These molecules can create ionic interactions in water solution to produce a polyelectrolyte complex (PEC).

In the framework of FOREST project, the reactive extrusion process to obtain PEC through mechanochemical synthesis will be simulated and optimized. This process provides a method to reduce or eliminate the use of solvents by conducting reactions through the grinding of reagents. The solvent free synthesis method will be studied in organic molecules based on a natural intumescent system already developed by AIMPLAS with conventional batch synthesis. A proof of concept has been already carried out at a laboratory scale going from 24 h for the traditional synthesis to 1-2 min synthesis by mechanochemistry with a narrow particle distribution. The fire-retardant chemistry is based on the self-assembly reaction of a polyelectrolyte composed by chitosan and phytic acid which are highly available and cost-effective.

Moreover, FOREST will recover the greatest amount of recycled carbon fibre to develop three semi-finished materials (non-woven, organosheet/SMC and yarn/sliver) which will be used as a reinforcement for the biocomposite obtention.

2.2. Lightweight materials through its functionality for extreme environment (mechanical and electromagnetic interference (EMI) shielding properties).

The electromagnetic radiation generated by external sources affects electronical devices by electromagnetic induction, electrostatic coupling, and conduction, which is known as electromagnetic interference [15]. Insulating from this radiation is gaining importance in mobility due to the increasing use of electronic devices in new vehicles. Thus, the susceptibility to EMI must be considered across the engineering design phase to protect the critical systems, such as flight control in aircrafts, and battery management systems (BMSs) in electric vehicles to avoid any elctromagnetic disturbance. Such electronic devices involving high operating frequencies, miniaturisied electronic design, high component integration, printed circuit board (PCB) size and thickness reduction, and devices [16], are vulnerable to electromagnetic radiation. Currently, two basic approaches are considered to shield the electromagnetic emissions from a device or a system and improve its susceptibility performance. The first one is to shield at the printed circuit level using a proper design. The second one is to place the device or system in a EMI proved case [17]. These design principles will have an effect on the likelihood of generating complex EMI leakage. Additionally, the use of conductive gaskets as a faraday cage leads to a great weight increase. EMI-shielding alternatives should be considered as an intrinsic material property. In this regard, FOREST proposes the modification of polymers studying the addition of electromagnetically-active particles to substitute metals, and thus being of benefit to weight reduction and electromagnetic shielding in safety-critical applications in aeronautics and automotive.

2.3. Manufacturing and Testing of High-Value Components.

FOREST project framework will include the manufacturing and testing of 3 prototypes for multifunctional material structures: a bus roof covering panel, an aero cockpit panel and a battery pack cover. The specific challenges related to efficient operations will be addressed, especially in relation with the cycle time and the energy efficiency. Curing temperature of reactive systems (eg. bioacrylic thermopalstic and biobenzoxazine resins) will be measured upon processing the composite, using dielectric sensors to optimise the curing time and ensure the appropriate composite curing degree. Extreme properties such as fire retardant and EMI-shielding will be tested to fulfill the end-user´s standards improving the mechanical stability and resistance. In order to fulfill the transport challenge proposed ‘Smart, green and integrated transport’ an extensive ongoing research will address the benefits of the biocomposite solutions provided within the FOREST project. Special focus will be put on reducing the environmental impact (efficiency in natural resources, recovery of fibre waste, weight reduction, energy efficiency, suitability for recycling, etc.). Moreover, a basis will be set for a future certification of the FOREST biocomposite parts in the bio-based Circular Economy (BCE).

3. Conclusions and future work.

FOREST will develop novel lightweight multifunctional biocomposites as a competitive alternative to conventional composites. New chemistries will be developed based on bio-based materials (reactive and non-reactive polymeric system and fire-retardant additive) in combination with fully recycled carbon fibre and EMI particles. These biocomposite candidates will be obtained using one-shot manufacturing techniques, involving out-of-autoclave (OOA) processes to build and test prototypes (TRL5) with improved multifunctional properties (mechanical resistance, fire-retardant, EMI-shielding) for transport application. FOREST will focus on the sustainability in circular economy (CE) by investigating effective circularity solutions applied to multifunctional biocomposites constituents based on >50% sustainable materials.

FOREST will lead an intensive study of bio-based economy (BBE) for a better understanding of the sustainability criteria and indicators. Environmental and socio-economic aspects for the impact of each biocomposite product will be conducted by LCA, life-cycle-cost (LCC), social life cycle assessment (SLCA) paying special attention to the EOL. Additionally, policy instruments such as regulation, public procurements, and standardization, will be considered in order to lower the barriers and bring biocomposites into the market.

4. Acknowledgement

The FOREST project is funded by the European Union’s Horizon Europe research and innovation programme under Grant Agreement No. 101091790.

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