Biocomposites Qualifying
Biocomposites Qualifying FDCA for thermoset applications By: Daniel Zehm, Antje Lieske, Brijesh Savalia Fraunhofer Institute for Applied Polymer Research IAP Potsdam-Golm, Germany and Michael Rabenstein Synthopol Chemie, The Resin Company Buxtehude, Germany For some time now, much has been told about the potential the biobased building block 2,5-furandicarboxylic acid (FDCA) holds for the production of aromatic polyesters [1]. This is particularly true for the new thermoplastic polyethylene 2,5-furandicarboxylate (PEF): When processed as thin film, PEF exhibits superior gas barrier properties than the competing polyethylene terephthalate (PET) [2], which provides a unique selling point and eases invest decisions devoted to enter the huge film and bottle market. However, bringing PEF to the market appears to be a formidable challenge, as decisions of some market participants suggest us [3]. Whether PEF will be a success story is still uncertain. Should this uncertainty prevent us to continue evaluating FDCA-based materials? Would it be helpful for decision makers if we look into different markets in which aromatic polyesters are also dominating? Surely, producing films and bottles based on PEF appears to be highly lucrative, considering the sheer production volume of PET on world scale. Yet, FDCA is presumably not only suited to make PEF, as purified terephthalic acid (PTA) is not exclusively used to produce PET. Following these thoughts, a research project that was devoted to develop partially biobased thermosets for fibre reinforced composites was initiated [4]. More specifically, unsaturated polyester resins (UP resin) were targeted, for which phthalic and isophthalic acid are essential components to achieve a suitable property profile (see Figure 1). The use of FDCA in this context is worth evaluating, since the chances of producing both phthalic and isophthalic acid from biomass using pyrolysis are currently very uncertain. Synthesis of FDCA-based unsaturated polyester resins The preparation of UP resins involves the melt polycondensation of diacids, diols and maleic anhydride or fumaric acid, respectively. This leads to polyesters possessing reactive sides along the polymer main chain which react during curing with a reactive solvent such as styrene, in which the polyester is typically dissolved. The project aimed on FDCA based replacements for two typical compositions of unsaturated polyester resins available on the market: one using propane-1,2-diol, the second one using diethylene glycol as diol component. To evaluate these biobased polyester resins more seriously, analogous petro-based unsaturated polyesters were prepared and characterized. Phthalic and isophthalic acid were used for these UP resins. Curing of FDCA-based reactive resins The curing of the petro- und biobased reactive resins was carried out at 23°C (using methyl ethyl ketone peroxide as hardener and cobalt as accelerator) and 80°C (hardener benzoyl peroxide). At 23°C gel times of approximately 30 and 15 min were observed for the biobased resins based on propane-1,2-diol or diethylene glycol, respectively, which are slightly shorter than for the analogous petrobased resins and presumably due to moderately higher resin viscosities rather than a consequence of the FDCA structure. In contrast, almost identical gel times of approximately 5 min were measured at 80°C for both petro- and biobased resins. Properties of FDCA-based reactive resins The resin based on FDCA und propane-1,2-diol led to thermosets, which fulfilled the requirements according to DIN 16946-2 and EN DIN 13121-1 (resin viscosity: 1000 mPas at 23°C). Even partially improved properties were achieved (see Figure 2a). For example, the modulus for the biobased resin was 4050 MPa (tensile test according to EN ISO 527-1, not shown), which was approximately 300 MPa higher than for the petro-based phthalic acid resin. Further, the tensile strength was measured to be 70 MPa and thus approximately 10 MPa greater than for the petro-based resin. Finally, the heat deflection temperature determined according to ISO 75 was 92°C. Thus, thermosets containing FDCA und propane-1,2-diol are perfectly suited for construction purposes such as structural parts of boats, and windmill nacelles. The general purpose resin based on FDCA und diethylene glycol also fulfilled the requirements according to DIN 16946-2 and EN DIN 13121-1 (see Figure 2b). The modulus of the cured biobased resin was about 3400 MPa according to the tensile test (not shown), while the tensile and flexural strength of this particular resin was measured to be 86 MPa and 146 MPa (resin viscosity: 1500 mPas at 23°C), which are clearly larger than for the petro-based resin based on isophthalic acid. Such general 34 bioplastics MAGAZINE [05/21] Vol. 16
standard bio-based Biocomposites cross-linking points replaced component Figure 1: Reactive polyester resins. purpose resins are typically used for glass reinforced parts which require excellent resistance to hydrolysis (e.g. tanks, pipes and boats), but are potentially also suited for SMC (sheet moulding compounds) and BMC (bulk moulding compounds) applications. Finally, cold cured glass-fibre reinforced composites were prepared by hand lay-up using 6 layers of chopped strand mats (dimensions: 20 x 30 cm, thickness: 5 mm). Subsequently, the Barcol hardness was determined, which is a measure to evaluate how much a resin has cured. The hardness test revealed values in the range of 50 to 60 (see Figure 2c), which demonstrates that complete curing was achieved for both petro- and biobased resins (24 h cold cure and 24 h at 80°C). Structural differences can hardly be deduced from these measurements. However, they show that the biobased reactive resins are suitable as matrix resins for the preparation of glass-fibre reinforced composites. Future research FDCA shows some beneficial properties as biobased building block for the preparation of reactive polyester resins (see Figure 3). Having evaluated the basic property profile of such thermosets, future investigations are devoted to the adaption of the processing characteristics and to SMC manufacturing. Further studies might also include the evaluation of biobased propane-1,2-diol, which would yield resins with a biobased content of up to 46 %. Acknowledgement The authors acknowledge financial support of the German Federal Ministry for Economic Affairs and Energy (IGF: 19804 BR/1). The responsibility for the content of the publications lies with the authors. References [1] N.N.: bioplastic MAGAZINE Vol. 10, issue 04/2015. For a more recent review see: X. Fei, J. Wang, J. Zhu, X. Wang, and X. Liu ACS Sustainable Chem. Eng. 2020, 8, 8471–8485. [2] S. K. Burgess, J. E. Leisen, B. E. Kraftschik, C. R. Mubarak, R. M. Kriegel, and W. J. Koros Macromolecules 2014, 47, 1383–1391. [3] N.N.: bioplastic MAGAZINE Vol. 14, issue 01/2019. [4] The results presented were obtained in the research project „Herstellung und Evaluierung biobasierter Reaktivharze für den Einsatz in den Bereichen Kanalsanierung und Yachtbau“, which was funded by the German Federal Ministry for Economic Affairs and Energy (grand number: IGF 19804 BR/1). www.iap.fraunhofer.de | www.synthopol.com Figure 2: Petro-based in orange, biobased reactive resins in green. Tensile strength (dashed; according to EN ISO 527-1), flexural strength (unfilled bar; according to EN ISO 178). Figure a) resins based on phthalic acid (A8) and FDCA (A15 and A16), both copolymerized with propane-1,2-diol, b) general-purpose resin based on diethylene glycol and isophthalic acid or FDCA, respectively. DIN EN 13121-1 requires tensile and flexural strength of greater than 50 MPa and 75 MPa. c) shows Barcol hardness after curing using methyl ethyl ketone peroxide at 23°C (24 h cold cure and 24 h at 80°C) is shown in Figure c). bioplastics MAGAZINE [05/21] Vol. 16 35
