Mechanical Characterizations on biobased Sheet Molding Compound (SMC) developed for battery boxes.

In this work, performed within FENICE project (www.fenice-composites.com), semifinished materials for the mass production of fire-resistant battery box in the automotive. SMC were developed using PFA (polyfurfuryl alcohol) resin as the matrix and both long and short basalt fibers as reinforcement in a SMC and impregnation pilot line. Mechanical characterization is used to compare prepreg based on fabric impregnation and SMC and measure basic data needed for modelling. Short fiber reinforced composites are cheaper and more isotropic than those reinforced with fabrics, making them more interesting for mass market application in the automotive, such as electrical cars. The semifinished material production must be optimised, and basic mechanical data must be measured, to be used as input into FEM of the components to be produced.


Introduction
FENICE -Fire Resistant Environmentally Friendly Composites (www.fenice-composites.eu,EITRM upscaling, KAVA9, 21099, 2022-2025, 2.53M€ funding) -is funded in the framework of the EIT Raw material Programme originates from the urgent need of efficient and affordable fire-resistant battery box solutions for the automotive electrification.FENICE is focusing its attention on current lithium-based battery modules, trying to cope to the potential risks of an increased use in mass production.As other authors in the field, the target is substituting metal structural battery boxes with fire resistant composite ones, applying an approach of equivalent [1].Biobased resins, reinforced with basalt fabrics, were studied in previous works [2], but in order to achieve enough fire retardancy, PFA was chosen by ENEA as the resin of interest in FENICE, and have been studying long fiber reinforced composites obtained from commercial prepregs, by hand lay-up and warm pressing.Microtex in the same project decided to study corresponding Sheet Moulding Compound (SMC) in an Industrial Pilot Line, with the aim to avoid the need of hand lay-up, resulting in the possibility to warm press fire-resistant battery boxes (or battery spacers) in one autonomized step.Current results will be presented, in particular a comparison of the mechanical performances obtained on developed SMC and commercial prepregs, with similar composition and matrix-to-fiber ratio.Other authors have previously discussed the advantage of glass based SMC, compared to metals and carbon based ones, in term of higher deformation at break [3].PFA as a resin have been studied in a number of previous works [4][5][6][7] demonstrating sustainability of the solution, along with validating its performances.In this work, SMC as semifinished products, studying a combination of chemical activators, following the basic chemistry of the system [8][9].The primary objective of the optimisation was making the use of PFA-basalt SMC easy to be used at the industrial level.This paves the way on further optimisations of the system, exploiting, for example, the addition of lignin, known to be able to further improve performances [10].

Experimental description
PFA resin is a biobased material as it is derived from the byproducts of sugarcane processing, by extracting solubilized hemicellulose sugars, converting them in Furfural (figure 1).Furfural is then converted through catalytic hydrogenation into Furfuryl alcohol, a precursor of PFA resin (figure 2).PFA precursor "Furolite 050915 A RF 2ST HV" was procured from TFC (Transfurans Chemicals, www.transfurans.be)with a very low and controlled free furfuryl Alcohol content which is the more health dangerous component in the mixture.TFC also procured the hardeners, namely ATMP [Amino Trimethylene Phosphonic Acid, 50%] and HM 1448 [(2-hydroxyethyl) ammonium nitrate, 65%] being the first active also at room temperature, while the second being activated by high T.At first the ratio used was 4:1 phr (parts per hundred resin), then the ratio was optimised, using viscosimetric and thermal analysis, to be more in line with the production equipment and product target.The impregnation process was implemented in the b-stage SMC line at Microtex Composites, with a production capacity, with a production capacity of 300 tons per year.The fiber of choice was basalt procured from DBF Germany.FENICE project study basalt fiber, being interesting for battery box applications because of cradle-to-cradle closed loop recyclability (with no performance loss upon remelting); low embodied energy and C footprint (compared to carbon fiber); electrical insulation and thermomechanical properties higher than e-glass.

Results and Discussion
TFC suggests obtaining b-stage of PFA based prepregs by a thermal treatment.These indications were followed in formulation "PRO 3115" (Furolite = 100, HM 1448 = 4, ATMP = 1) whose DSC are reported in Figure 3 (b-staging of 2 min at 150℃).From the obtained curves (Figure 3) for formulation "PRO 3115", endothermic phenomena can be observed that make the study of the exothermic crosslinking peak challenging.This phenomenon is more pronounced in the resin analysed at once after the mix, probably due to the production of water during crosslinking.This explanation seems consistent with the reduction of this phenomenon in the resin after the B-stage (Figure 4), as in this process partial crosslinking occurs.In the next thermograms, endothermic phenomena were reduced using highpressure pans.This first formulation showed limited latency at room temperature (72h after B-stage) making it not easy to be used into a mass production by compression moulding.Subsequently developed formulations optimised b-staging process at room temperature.From the following figures, it is possible to appreciate how the onset of the crosslinking peak in "PRO 4319" (Figure 4) compared to "PRO 3115" (Figure 3), which typically indicates greater reactivity at low temperatures.However, the prepreg with "PRO 4319" matrix exhibits a higher latency at 21°C.This is attributed to two factors: firstly, the b-stage process conducted at a lower temperature, and secondly, the reduced quantity of hardeners in the formulation.Having obtained a stable formulation for our process, we conducted resin curing tests.From following Figure 5, it can be observed a DSC analysis conducted on the cured resin alone for 30 minutes at 140°C.As it can be seen, the cross-linking peak has completely disappeared, confirming the complete curing of the resin.The "PRO 4319" matrix was thus used in the pilot production to obtain laminates for mechanical testing.
From the obtained laminates, in addition, a thermomechanical DMA analysis was also conducted.The DMA profiles (Figure 6) are unusual for a composite material of this type.The storage modulus shows no significant losses, tanδ does not exhibit clearly readable peaks.The only data that is more clearly readable is that of the Loss modulus, which shows a peak around 125°C.Speaking with the resin manufacturer and searching the literature, it becomes clear that this is a characteristic behaviour of the resin.This is also connected to the absence of a glass transition in the DSC analysis of the cured resin (Figure 5), where the energy variation during the glass transition is greatly reduced in this chemical system.
Based on laminate obtained from basalt SMC and fabric prepreg, the following mechanical data were registered, comparing SMC and fabric reinforced prepreg (table 2).There is no need to discuss the data in term of specific mechanical properties, since the data were collected by considering composites materials with the same composition, density and fiber-to-matrix ratio.Same fiber-to-matrix ratio was indeed optimised by trial and error, so that density was the same for SMC and prepreg derived composites (last line of table 2).8) not perfectly impregnated fibres can be observed, making clear these composites can still be optimised, by improving the impregnation process.In addition, observing the cross-sectional cut of an SMC laminate (figure 9), and broken samples after tensile and compression tests (both obtained from SMC and long fibers prepregs), separation and delamination phenomena between the resin and the fibres can be noticed.Comparison with SMC cross-sections (an example in figure 10) suggests that improving the fibre/matrix adhesion and affinity would be the path to improve the mechanical performances.

Conclusions
As expected, the mechanical properties of the SMC laminate are lower than the long fibre laminate.However, considering the ease of press moulding of SMC material and the potentials for mass production of components, it is worth investing in the improvement of this semifinished material.Prepreg and SMC were compared keeping the fiber-to-matrix ratio fixed and optimising b-stage, to make stability at room temperature high enough.Regarding the achieved performances, the resin is intrinsically fire retardant and can be used to produce biobased fire-resistant composites.The solution is more performing than thick aluminium battery boxes in term of fire resistance and allows weight reduction compared to steel.Economic and environmental sustainability are ensured by the facts (1) the resin is biobased and already industrialised for mass production and (2) basalt fibers have higher tolerance to high temperature than E-glass and lower carbon footprint.Investigations will be continued on basalt-PFA SMC in the "engineering against failure" direction, to further optimise mechanical properties and study the types of damage/failure modes.The performed preliminary observations on broken samples suggest failure starts from the resin, which shows limited adhesion to the fibers.

Figure 1 .
Figure 1.First steps of TFC Furolite production, starting from sugarcane.

Figure 10
Figure 10 Cross-sectional on a SMC laminate.

Table 2 .
PFA based composites mechanical properties.