molecules Review Phosphorus-Containing Flame Retardants from Biobased Chemicals and Their Application in Polyesters and Epoxy Resins Jacob Sag 1, Daniela Goedderz 1,2, Philipp Kukla 1, Lara Greiner 1, Frank Schönberger 1 and Manfred Döring 1,* 1 Fraunhofer Institute for Structural Durability and System Reliability LBF, D-64289 Darmstadt, Germany; [email protected] (J.S.); [email protected] (D.G.); [email protected] (P.K.); [email protected] (L.G.); [email protected] (F.S.) 2 Ernst-Berl Institute for Chemical Engineering and Macromolecular Science, Technische Universität Darmstadt, D-64287 Darmstadt, Germany * Correspondence: [email protected] Academic Editors: Rodolphe Sonnier, Laurent Ferry and Henri Vahabi Received: 30 September 2019; Accepted: 16 October 2019; Published: 17 October 2019 Abstract: Phosphorus-containing flame retardants synthesized from renewable resources have had a lot of impact in recent years. This article outlines the synthesis, characterization and evaluation of these compounds in polyesters and epoxy resins. The different approaches used in producing biobased flame retardant polyesters and epoxy resins are reported. While for the polyesters biomass derived compounds usually are phosphorylated and melt blended with the polymer, biobased flame retardants for epoxy resins are directly incorporated into the polymer structure by a using a phosphorylated biobased monomer or curing agent. Evaluating the efficiency of the flame retardant composites is done by discussing results obtained from UL94 vertical burning, limiting oxygen index (LOI) and cone calorimetry tests. The review ends with an outlook on future development trends of biobased flame retardant systems for polyesters and epoxy resins. Keywords: biobased polymers; polyester; epoxy resin; flame retardant; phosphorus-containing flame retardants 1. Introduction Regarding the definition of the term “biobased” by IUPAC, a biobased material is comprised partly or wholly of biological products generated from biomass. Biobased materials must not necessarily be biodegradable, biocompatible or environmentally friendly, particularly when they are used to replace their counterpart polymer derived from petrochemical resources [1]. In terms of research and development, a growing interest has emerged of new polymers derived from biobased chemicals or biomass. There is also an increasing interest in a greener synthesis process of flame retardants using sustainable substances or plant-derived chemicals, particularly when considering that petroleum resources are finite. This would also lower the environmental impact. Plant proteins, mainly derived from wheat, soybean and corn or plant oils like soybean, corn and flax oil or plant starches like carbohydrate polymers and cellulose are plant-derived products from renewable sources that can be used neat or modified for the replacement of petrochemicals in certain applications. Biobased polymers or flame retardants can be made partially or totally from renewable sources. The biobased content in materials is determined by using the standard ASTM D6866-11 test [2] by radiocarbon analysis which quantifies the total organic carbon content in the product. For the utilization Molecules 2019, 24, 3746; doi:10.3390/molecules24203746 www.mdpi.com/journal/molecules Molecules 2019, 24, 3746 2 of 31 Moleculesof substances 2019, 24, derived x FOR PEER from REVIEW renewable resources it is imperative to fulfill the following requirements:2 of 31 sustainability, affordability, compatibility and durability [3–5]. Nowadays,Nowadays, there there are are many many processes processes available available fo forr the the production production of of biobased biobased chemicals chemicals in in biotechnicalbiotechnical processesprocesses involvinginvolving the the extraction extraction and an modificationd modification of biomass of bioma andss fermentation and fermentation of natural of naturalproducts products like sugars like or plantsugars material. or plant The material. development The development of biobased flame of biobased retardant formulationsflame retardant as a formulationswhole includes as biobaseda whole flame includes retardants biobased and biobasedflame retardants polymer matrices.and biobased Engineering polymer plastics matrices. like Engineeringpoly(ethylene plastics terephthalate) like poly(ethylene (PET), poly(butylene terephthalate) terephthalate) (PET), poly(butylene (PBT), etc. and terephthalate) biobased polymers (PBT), etc. like and(poly(lactic biobased acid) polymers (PLA) canlike be(poly(lactic produced acid) by using (PLA) biobased can be chemicals.produced by Products using biobased made from chemicals. biobased Productsmaterials made are not from necessarily biobased biodegradable, materials are not particularly necessarily when biodegradable, they are chemically particularly modified. when Mostthey arepolymers chemically are known modified. to withstand Most polymers degradation are known which to is withstand a desirable degradation property, in particularwhich is a whendesirable they property,are exposed in particular to weathering when such they as are in theexposed case of to wind weathering turbine such rotor as blades, in the etc.case [6 of,7 ],wind but thisturbine resistance rotor blades,to degradation etc [6,7], is but a challenge this resistance for biodegradability. to degradation Müller is a challenge et al. reported for biodegradability. a depolymerization Müller of PET et al. by reportedusing a hydrolase a depolymerization isolated from of ThermobifidaPET by using fusca a hydrolaseresulting isolated in water from soluble Thermobifida oligomers fusca/monomers resulting [8]. inThe water biodegradability soluble oligomers/monomers has to be considered [8]. in theThe development biodegradability of materials has to produced be considered with biobased in the developmentmaterials to generate of materials sustainable produced materials. with Biodegradabilitybiobased materials and to synthesis generate pathways sustainable with materials. biobased Biodegradabilitymaterials are still aand big synthesis challenge inpathways the future, with because biobased due tomaterials the plant are growth still therea big haschallenge to be a balancein the future,between because the application due to the of plant plant growth material there for food has andto be raw a balance materials between for greener the application solutions. Theof plant land materialuse for biobased for food polymersand raw materials (PLA, PHA, for greener PTT, PBAT, solu starchtions. The blends, land etc.) use infor 2017 biobased was 672,000 polymers ha (PLA, which PHA,corresponds PTT, PBAT, to 0.005%. starch The blends, global etc.) agricultural in 2017 was area 672,000 requires ha 5 billionwhich hacorresponds (3.65%). An to increase 0.005%. upThe to global1,038,000 agricultural ha (0.008%) area is predictedrequires by5 billion IfBB for ha biobased (3.65%). polymersAn increase in 2022 up [to3– 51,038,000,9]. ha (0.008%) is predictedBiobased by IfBB polymers for biobased include polymers all polymers in 2022 which [3–5,9]. are made of renewable biosources resulting in biodegradableBiobased polymers or non-biodegradable include all po biobasedlymers which polymers. are made The of biobased renewable non-biodegradable biosources resulting part in of biodegradablebiopolymers still or representnon-biodegradable a greater partbiobased of the poly biobasedmers. polymersThe biobased production non-biodegradable capacity in 2017 part and of biopolymersthere will be still a slight represent change a greater until 2022 part (Figureof the bi1).obased The mainpolymers part producti of biobasedon capacity non-biodegradable in 2017 and therebiobased will polymersbe a slight is representedchange until by 2022 bio-PET (Figure derived 1). The from main plant part material. of biobased The Coca-Cola non-biodegradable Company biobaseddeveloped polymers the PlantBottle, is represented which isby a bio-PET blend of de plant-derivedrived from plant material material. (up to The 30%) Coca-Cola and petrochemical Company developedmaterial in the 2009 PlantBottle, [10,11]. PLA which use asis a blend biodegradable of plant-derived polyester material will rise (up from to 10.5%30%) and in 2017 petrochemical to 18.7% in material2022 [9]. in 2009 [10,11]. PLA use as a biodegradable polyester will rise from 10.5% in 2017 to 18.7% in 2022 [9]. Figure 1. Biobased polymer production capacities for 2017 and 2022 by material type (source: IfBB Figure 1. Biobased polymer production capacities for 2017 and 2022 by material type (source: IfBB 2018 [9]). 2018 [9]). Table 1 summarizes selected chemical building blocks which can be produced by a biotechnical process for certain applications [12,13]. Combustible plastics which are near electrical or heat sources require flame retardant solutions. Due to the development of electric mobility in the last years and limited petrochemical resources biobased polymers will be one of the target materials for the future Molecules 2019, 24, 3746 3 of 31 Table1 summarizes selected chemical building blocks which can be produced by a biotechnical process for certain applications [12,13]. Combustible plastics which are near electrical or heat sources require flame retardant solutions. Due to the development of electric mobility in the last years and limited petrochemical resources biobased polymers will be one of the target materials for the future in this and other segments.