Impact of fibre moisture content on the structure/mechanical properties relationships of PHBV/wheat straw fibres biocomposites M.-A. Berthet, N. Gontard, H. Angellier-Coussy * Unit e Mixte de Recherche « Ing enierie des Agropolym eres et Technologies Emergentes », INRA/UM/Montpellier SupAgro/CIRAD, Pl. Viala, 34060 Montpellier, Cedex 01, France article info Article history: Received 23 March 2015 Received in revised form 23 July 2015 Accepted 27 July 2015 Available online 30 July 2015 Keywords: Short-fibre composite Polymer-matrix composites (PMCs) Hygrothermal effect Mechanical properties Extrusion abstract The objective of the present work was to investigate the impact of fibre moisture, at the moment of the manufacturing process, on the relationships between the structure of PHBV/wheat straw fibres com- posites and their final mechanical properties, with a focus on the interphase role. For that purpose, wheat straw fibres were equilibrated at extreme relative humidity conditions before extrusion with PHBV, reaching moisture contents of 2 wt% w.b. and 14.2 ± 0.4 wt% w.b. at 0 and 98% RH, respectively. The structure and mechanical properties of resulting composites (prepared with a fibre content of 20 wt%) were compared to those of materials filled with fibres classically dried (moisture content of 3 wt% w.b.). It was shown that the fibre/matrix adhesion was visually not affected by the initial fibre moisture content. However, PHBV molecular weight decreased significantly with increasing fibre moisture content, which was attributed to hydrolysis reactions induced by residual water molecules. The increase in crystallinity along with initial fibre moisture content was put in relation to the observed decrease of molecular weight. In spite of structural differences, the tensile properties were similar for all composites, leading to the conclusion that the initial fibre moisture content was not a predominant factor for controlling the mechanical properties of PHBV/wheat straw fibres composites. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Natural fibres are intrinsically hygroscopic due to the polar character of their main constituents, i.e. cellulose, hemicellulose & lignin, and are found to contain from 8.8 to 15 wt% d.b. of water (for equilibration at 65% RH at 21  C) depending on their botanical origin [1]. A major limitation of using natural fibres in durable composite applications is their high moisture absorption or release (depending on environmental conditions) and poor dimensional stability (swelling). Thus, one recommendation commonly done is a cautious drying of lignocellulosic fibres before using them as fillers in composites [1]. Numerous studies have been devoted to study the water dam- age of natural fibre composite materials [2e4]. When fibres are exposed to mild humidity, water molecules can induce a plasti- cizing effect, thus affecting the strength and the rigidity of fibres. Studied have also evidenced that moisture can seriously jeopardise the fibreematrix adhesion, leading to the deterioration of the stress transfer efficiency from matrix to reinforcement [3,5]. The degra- dation process starts with the swelling of the cellulose fibres, causing micro-cracking of the matrix around the swollen fibres [3,6]. Main results displayed a decrease of flexural strength and modulus, in relation to a decrease of interfacial shear strength, which was attributed to the poor interfacial bonding formed at the high relative humidity conditions [7,8]. It is worth noting that fibre treatments were widely and successfully experimented in order to reduce both the moisture level and the rate of absorption very significantly [9e13]. Nevertheless, few authors took interest in the influence of fibre moisture content at the moment of composites processing and its resultant impact on structure and properties [7,8]. Another possible impact of moisture is the hydrolytic degrada- tion of the polymer matrix, which has been already largely described in the case of hydrolysable polymers such as poly- carbonates [14,15], nylons [15,16], and polyesters such as PET [15,17] and PHBV [18]. PHBV is a biodegradable bacterial polymer extensively studied these last years, because of its promising properties. However, its rigidity and brittleness are exacerbated by fibre introduction, limiting its potential application as bio- composites [19,20]. PHBV hydrolysis could be turned into an * Corresponding author. E-mail address: helene.coussy@univ-montp2.fr (H. Angellier-Coussy). Contents lists available at ScienceDirect Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech http://dx.doi.org/10.1016/j.compscitech.2015.07.015 0266-3538/© 2015 Elsevier Ltd. All rights reserved. Composites Science and Technology 117 (2015) 386e391