# Anti-Frothing Effect of Poultry Features in Bio-Based, Polycondensation-Type Thermoset Composites
## 1 Introduction  
Thermost resins are an important class of materials for adhesives [1] and load-bearing matrices in fibre-reinforced composites [2]. Typical examples are phenolic and epoxy resins, polyurethane, unsaturated polyesters, or silicone rubbers [3]. However, the main source for these polymers is petrochemical feedstock and only very few attempts at a partial substitution by renewable materials, such as epoxidised castor and soybean oils [4], or bio-based polyols [5], have received wider attention. On the other hand, the growing environmental awareness and recent governmental regulations have increased the pressure on industry and academia to focus more on renewable and sustainable materials, closed material cycles, and materials with low or, ideally, zero carbon footprints. One reason for the difficulties in replacing petrochemical resources with natural ones, when it comes to thermoset resins, is the required high reactivity. Nature usually circumvents highly reactive functional groups, such as isocyanate and epoxy, by using highly specialised enzymes as catalysts in condensation reactions. As a result, bio-based feedstock requires in vitro reaction conditions not directly compatible with the requirements for thermoset resins. Thus, it seems that natural building blocks would not be suitable for making thermoset resins. However, the vast amount of natural or bio-based polyacids and polyolsmerits a closer look: by esterification, which is generally a rather slow, only slightly exothermic reaction type generating a lot of water as by-product, bio-based seminal thermoset materials were successfully generated, as presented in this paper. Quite a common example of such an esterification is poly(butylene succinate), which can be synthesised from succinic acid and 1,4-butanediol via polycondensation and goes back to the work of [PERSON] [6]. Ever since, the polycondensation reaction has been refined and optimised, in particular to include new monomers and more active catalysts [7]. A recent example is the synthesis of a thermoset resin from lactic acid and glycerol, which was used to impregnate regenerated viscose fibre and compression-moulded to form a thermoset composite [8]. Naturally available di- and multifunctional carboxylic acids have been used to cross-link epoxidized sucrose soyate in order to produce a thermoset polyester that rapidly degrades in dilute sodium hydroxide [9]. A promising material is the combination citric acid and glycerol. Polymerised in solution at molar ratios from 0.75 to 1.5, the material obtained was initially an insoluble amorphous solid, that dissolves in water within 8 to 10 days and can be bio-degraded by _Aspergillus niger_ and _E. coli_[10]. A material produced from citric acid and glycerol without a catalyst was used as a degradable thermoset polymer for drug delivery applications [11]. However, when prepared in bulk, the material turns out liquid, gel-like, or as an inhomogeneous foam due to the evaporation of substantial amounts of water as steam [12]. To the best of our knowledge, a catalysed method to produce compact solids from this mixture is not known. Another aspect of substituting petrochemical feedstock with renewable resources is the exploitation of natural waste materials. Among these, poultry feathers are an interesting class, since they have one of the highest fracture toughnesses in nature [13] and make up approximately 10% of the weight of poultry [14]. This is a problem in poultry farms, because huge amounts of feather waste are generated, which are taken to landfills for disposal. The estimated global annual amount of feathers produced in poultry farms is in the order of 4 million tons [15]. Consequently, several research groups have studied the re-use of poultry feathers, e.g., in composite materials. [PERSON] reported lightweight, biodegradable composites of polylactic acid or poly(butyrate adipate terephthalate) with up to 60 wt% poultry feathers, which were proposed as an insulation material [16].