Mechanical characterizations on biobased FMLs, being developed for battery boxes, before and after ageing

This paper reports the activities carried out under project FENICE (Upscaling, KAVA 9, EIT Raw Materials, www.Fenice-composites.eu, 2022-2025) about improving fire resistance of electric cars batteries, in particular through the development of Fiber Metal Laminates (FML) battery boxes. The aim of this study is to analyse the corrosion behaviour of FML based on bio-based and closed loop recyclable prepregs, in typical ageing conditions (thermal ageing, neutral and cupric acetic salt spray). FML degradation, evaluated in term of residual tensile strength after ageing, along with surface corrosion is remarkably low, and also resistance to humid environment seems satisfactory. However proper inorganic surface finishing is needed, since otherwise typical automotive tests, such as salty spray chamber ageing, cannot be passed in term of corrosion resistance requirements. The project is studying surface vitrification with sol-gel formulations, since traditional painting, based on organic polymers could ensure aluminium corrosion resistance, but would inevitably decrease fire resistance, while Cr-plating is not environmentally friendly, because of its higher carbon footprint and since it would make direct aluminium recyclability impossible, so. These advanced treatments were, up to now, not compatible with the adoption in the mass production, but things are rapidly changing, following the green transition that is currently bringing a number of small revolutions in the manufacturing sector.


Introduction
Nowadays, current challenges in the automotive sector are the reduction of fuel consumption and CO2 emission of future car generation.To this ends, it is important to use alternative solutions based on secondary raw materials that ensure: 1) an appropriate reduction in the total weight of the final products in order to reduce the costs related to the use of energy and the increase in the useful life of the finished component; 2) maintain or even improve physical, chemical and mechanical performance increasing safety compared to the currently used solutions and 3) increasing the sustainability of both materials and production processes (zero scrap, recyclable, low cost and low C-footprint solutions for multisectoral applications) [1].The work that is taking place within the FENICE project is concerned with the development of a new FML (Fiber Metal Laminate) system capable of replacing the current steel battery box for transport vehicles.FMLs are hybrid structures based on alternating layers of PMC (Polymer Matrix Composite) reinforced with fibers and layers of metal alloys, where the final properties depend on the synergy of the initial materials considering aspects such as the nature of the composite and metallic layers and any metal pre-treatments [2].One of the most critical parts in the development of this new battery box concept, concerns the cell separators inside the box itself.The criticality of this solution concerns aspects such as the need to obtain a final component resistant to fire and corrosive phenomena that may occur in the environment used.In the literature, there are several studies in which the problem of FML corrosion is addressed [3][4][5][6].In this work, the selected raw materials were: 1) a bio-based resin Polyfurfuryl alcohol (PFA) also developed within previous EIT Raw Material Upscaling KAVA 5 -C2CC project [7], and 2) a PET-derived commercial closed loop recyclable resin (prepreg Crosspreg® developed by Crossfire), in association with basalt or glass fibers in order to avoid the use of the less recyclable and less ecofriendly carbon fiber.To achieve the fire resistance required by the automotive standards, the project proved that the application of a non-structural thin layer of aluminum above the PMC is enough to guarantee the expected performances for the application.Other fundamental parameters that were kept in mind in making this choice of fibers and resin systems were: • sustainability of composites in mass applications is measured quantitatively by LCA and is based on a number of factors, with a particular focus on embedded energy and C-footprint.The most promising system must therefore consider the following aspects: fast processing with energy-efficient processes (RT storage); • easy to automat, easy to be closed loop recyclable or reused; • biobased or properly exploiting secondary raw materials; • water-based or VOC (Volatile Organic Compound)/solvent free; • critical raw materials use should be as limited as possible.Another innovation proposed in this paper is a solution to avoid conventional painting on composites and aluminium surfaces, beneficial in term of fire resistance.This substitution is also environmentally friendly since it has been proved paints are responsible, by the erosion phenomena, of 58% of microplastics into the oceans [8] beyond interfering with composites and aluminium direct recycling.All these negative effects can be avoided by adopting sol-gel precursors and use them as inorganic paints.This reduces paint weight of about an order of magnitude, with protection performances higher than conventional painting and higher of all other possible surface finishing treatments, including Cr (VI) plating.Specifically, Nanoprom demonstrated Cr-plating substitution can be substituted by a surface vitrification (www.nalucoat.it),with several environmental benefits, considering Cr (VI) detrimental effect on human health.In addition, adopting Nalucoat surface finishing, C-footprint is reduced to 20% of the original value and there is no interference on Al direct recycling (which is impossible with Cr-plated one).Summarizing, the aim of this paper is to demonstrate the feasibility of these materials and innovative processes for the application to battery boxes for electric and hybrid cars.

Experimental section
Two different FML solutions were developed, with different lay-up structure and prepreg nature: (1) basalt reinforced PFA-based prepregs, 630 gsm basalt, procured from Basaltex (www.basaltex.com)and (2) glass reinforced Crosspreg ® , 400 gsm glass procured from Crossfire (www.crossfire-srl.com)with aluminium sheets (0.1 mm thickness).In term of sustainability, the key point of PFA is being waterbased and biobased [9][10][11][12], while patent pending Crossfire resins are particularly interesting for their closed loop recyclability, their VOC-free nature and the fact they are hybrid, hybrid, meaning they are thermoset, but with a thermoplastic behavioue above Tg, which allow remoulding.Other advantages of Crossfire resin are: (1) high adhesion on aluminium, which does not need special cleaning or pretreatments and (2) remarkable use of secondary raw materials coming from recycled PET.Regarding sol-gel protective coating, several Nanoprom products were tested during FENICE project, among solvent based Polysil ® SC silane-based formulations.Upon curing (which is simply catalysed by environmental humidity, in the range 25-80%, after solvent evaporation) a mostly inorganic surface vitrification is obtained and such coating is highly hydrophobic and other functionalities can be improved through nano-additivation (e.g.graphene addition).The ageing in salty spray environment was carried out in accordance with EN ISO 9227-2022 [13] by means of an ACS dry corrosion test cabinet (Angelantoni Test Technologies).The fully automatic climatic chamber operates between 23°C and 55 °C and between 50 and 99 % RH with the option of accelerated ageing in neutral (NSS), acetic and cupric acetic salt spray (CASS).The apparatus is characterized by a volume of 900 litres and allows testing the ability of materials to resist in aggressive saline environment, acidic or basic solutions.In this work, measures were performed at room T and in neutral conditions (operating conditions: Temperature: 35 °C ± 2 °C, Average collection rate for a horizontal collecting area of 80 cm 2 : 1.5 ml/h ± 0.5 ml/h, Concentration of sodium chloride: 50 g/l ± 5 g/l, pH: 6.5 to 7.2), as requested by the automotive for this application.Thermal degradation of samples was also studied: the test consisted in an isothermal ageing at different temperature (110-150°C) for 120h.Water absorption test has been performed by immerging samples (110*300 mm) in deionized water at 25°C for 120 days.In order to assess the corrosion degradation of composites surface, both a comparison of mechanical behavior first after ageing (through tensile strength tests) and a visual evaluation were employed.The mechanical tests were carried out in ENEA Laboratories using a servo-hydraulic MTS testing machine (67 kN full scale), in accordance with the standard ASTM D3039 [14].

Results and discussion
In Tables 1 and 2 the post-ageing tensile properties (average values) are reported, they refer to FML samples of the two considered types, before and after NSS and thermal ageing, with and without inorganic surface protective coating.No deterioration of the tensile properties of the composite materials was observed after salt spray exposure.However, the absence of a protective film onto the aluminum leads to a corrosion of the surface, not in line with automotive requirements for battery boxes.The accelerated ageing tests showed that the presence of a Polysil® SC coating appears effective in ensuring aluminium surface aesthetical characteristics as new for the battery box expected lifespan.The coating obtained by sol-gel process, grafted to the aluminum surface, inhibits the corrosion phenomenon thanks to its proved high chemical inertia towards solvents, pH between 2 and 11 and chemicals.In addition, the results of thermal ageing suggested that long term exposure (120 h) up to Tg (around 110 ℃) do not reduce the tensile strength, while a limited decrease observed on Crossfire based FML only upon exposure to 130 ℃, that is at a temperature above the glass transition temperature.
The results regarding PFA FML solutions are shown in Table 2, in terms of Tensile Strength and Young's Modulus (average values).Regarding the results of thermal ageing, it is observed that long term exposure (120 h) up to 150 ℃ do not reduce significantly the tensile strength (decrease around 14%).Regarding the results upon the salt spray exposure, Nanoprom coating (Polysil® SC) is confirmed to protect aluminum both in NSS and CASS conditions, although also without a coating, the decrease in term of tensile strength is limited (max minus 19% after 1400 h of NSS).The presence of the coating helps preventing this decrease in terms of tensile strength upon exposure to NSS (around 14% decrease instead of 19% after 1200 h of NSS), beyond being necessary to protect aluminum surface from corrosion as in the previous case.Results reported in Table 3 highlight the positive effect of Polysil® SC also on reduction of the water absorption.This effect is most probably related to the increased hydrophobicity of the coating with respect the bare FML.On the other side, the PFA-FML, with respect the Crossfire-FML, result more prone to absorb water, due to his higher porosity and chemical affinity to water.
In Figure 1, a FML sample is presented before and after exposition to NSS for 700h, showing no corrosion effects and remarkable anti-soiling properties.The reported tensile tests were performed after NSS accelerated ageing test and after cutting standard samples 250 x 25 mm wide.The remarkably positive results obtained in NSS, suggested to try testing also in much harsher condition, corresponding to CASS, described in the same mentioned international standard [14].In figure 2 and 3, the external aluminium surface of an FML coated with Polysil® SC is presented after being exposed to CASS for different length of time: after 48h (Figure 2) a slight surface corrosion can start to be appreciated, but the test is still positive (the evaluation is carried out by visual inspection, evaluating the fraction of corroded area).After 77h (Figure 3) slight pitting is present, but the test is again passed, showing a resistance that is higher than Cr-plating (again, by evaluating visually the fraction of corroded area).

Conclusions
Composites recycling is an open issue.Trans-sectorial and trans-disciplinary innovation is needed, applicable (in synergy) to mass production in automotive, marine and renewable energy production, considering low cost and sustainable EU raw materials and low energy demanding processes (both in production and recycling).It is crucial to focus on zero scrap and fully recyclable raw materials (from biobased & secondary raw materials) with lower cost and lower C and environmental footprints.Nonconventional heating (exploiting renewables) is another priority.Thanks to thin films it is possible to increase durability and performance over time, and prevent microplastics production and keep direct recyclability of aluminium and composites.By exploiting recyclable and rPET-based hybrid resins, developed and produced by Crossfire, it is possible to avoid glues and solvents, reduce production and maintenance time and costs, while increasing specific strength.Fiber Metal Laminates are a relevant class of composites materials, even more challenging than composites in term of component production, reliability and recycling.In this work, they were produced starting from PFA and Crossfire resins, both associated with an external layer of surface aluminium.Salt spraying was employed as accelerated ageing conditions, measuring mechanical properties after 700 h and 1400 h.All FMLs, regardless composite layer nature, showed perfect resistance.External aluminium layer also demonstrated perfect resistance, thanks to a spray coating with Nanoprom sol gel formulations.CASS is also passed up to 77 h (performances higher than Cr plating).The choice of the best composite layer is to be done based primarily on the expected operative temperature for the battery modules and a number of other criteria (production costs, mechanical and thermophysical performance, reliability in expected working conditions, tolerance to fatigue, C-footprint, recyclability and sustainability), all being considered optimised in EU FENICE project.Regarding FML tolerance to accelerated ageing conditions, the following conclusions can be summarised: (1) all solutions withstand thermal ageing of 120 h up to Tg (110 ℃ in the case of Crosspreg® and 150 ℃ in the case of PFA based prepregs) without a significant decrease in tensile strength; (2) Crosspreg® can withstand exposure to temperature higher than Tg for 120 h with very limited tensile strength decrease (13% at 130 ℃); (3) at the same temperatures no decreasing in tensile strength is observed in the case of PFA based prepreg; (4) Exposure to NSS environment lead to limited decrease in the case of PFA (around 19%) which seems to be reduced upon the application of a hydrophobic protective coating (Polysil® SC) on aluminium (decrease around 14%) associated with a water absorption around 3.5 %w/w; (5) Crosspreg® do not present a decrease in term of tensile strength after 700h and 1400h of NSS both with and without Polysil® SC; (6) in the same conditions, Crosspreg® absorbs a much lower quantity of water (around 1%w/w) which can be limited down to 0.7%w/w through the application of the considered hydrophobic protective coating; (7) in all cases, inorganic Polysil® SC surface finishing was proved to be effective in avoiding aluminium corrosion phenomena, including in accelerated NSS and CASS conditions, and the presence of coating does not worsen significantly fire tests results, due to mostly inorganic nature of the protective coating.

Table 1
Crosspreg® based FML mechanical properties (average values) after ageing in relevant conditions ® FML solutions are shown in Table1, in terms of Tensile Strength and Young's Modulus (average values).

Table 3
Mass decrease (water absorption of FML plates around 110x300 mm, before cutting for tensile tests) upon 120 day of water immersion