Giana Almeida

Changes in wood-water relationship due to heat treatment assessed on micro-samples of three Eucalyptus species.

Abstract Wood-water relationship of untreated and heat-treated wood was studied. Specimens of Eucalyptus grandis, E. saligna, and E. citriodora were submitted to five conditions of heat treatment: 180°C and 220°C with air; 220°C, 250°C, and 280°C with N2. The wood-water relationships were accurately studied in a special device, in which the moisture content (MC) of the sample was measured with a highly sensitive electronic microbalance placed in a climatic chamber. The dimensions of the sample were collected continuously without contact by means of two high-speed laser scan micrometers. Sorption curves and shrinkage-MC relationships were observed. To study the effects of heat treatment, the following parameters were also determined: fiber saturation point (FPS), wood anisotropy (T/R ratio), shrinkage slope, reduction in hygroscopicity, and anti-shrink efficiency (ASE). The physical properties were significantly affected only at 220°C and above. At heat temperature levels higher than 220°C, the reduction in hygroscopicity and ASE are higher than 40% and continue to be reduced with increasing temperature level. This work also demonstrates that heat treatment does not change the slope of the curves “shrinkage vs. MC”, proving that heat treatment affects the domain of alterations in wood properties, but not the behavior within this domain.

New modelling approaches to predict wood properties from its cellular structure: image-based representation and meshless methods

Key messageThe real tissue structure, including local anisotropy directions, is defined from anatomical images of wood. Using this digital representation, thermal/mass diffusivity and mechanical properties (stiffness, large deformation, rupture) are successfully predicted for any anatomical pattern using suitable meshless methods.IntroductionWood, an engineering material of biological origin, presents a huge variability among and within species. Understanding structure/property relationships in wood would allow engineers to control and benefit from this variability. Several decades of studies in this domain have emphasised the need to account simultaneously for the phase properties and the phase morphology in order to be able to predict wood properties from its anatomical features. This work is focused on the possibilities offered by meshless computational methods to perform upscaling in wood using actual tissue morphologies obtained by microscopic images.MethodsAfter a section devoted to the representation step, the digital representation of wood anatomy by image processing and grid generation, the papers focuses on three meshless methods applied to predict different macroscopic properties in the transverse plane of wood (spruce earlywood, spruce latewood and poplar): Lattice Boltzmann Method (LBM) allows thermal conductivity and mass diffusivity to be predicted, Material Point Method (MPM) deals with rigidity and compression at large deformations and peridynamic method is used to predict the fracture pathway in the cellular arrangement.ResultsThis work proves that the macroscopic properties can be predicted with quite good accuracy using only the cellular structure and published data regarding the cell wall properties. A whole set of results is presented and commented, including the anisotropic ratios between radial and tangential directions.

Energy Consumption in the Convective Drying of Timber Analyzed by a Multiscale Computational Model

Energy consumption during timber drying has become an increasingly important issue, alongside conventional concerns such as quality, cost, and drying time. This paper proposes a detailed analysis of the energy consumption evolution in conventional kiln drying using a sophisticated modelling approach. The formulation of energy consumption proposed by Perré et al.[ 1 ] was embedded in a multiscale computational model to consider kiln drying of a stack of boards. The drying of each board in the stack is simulated using a full version of a heat and mass transfer code (the 1-D version of TransPore), which allows all boards to be different and facilitates consideration of wood variability, in addition to the effect of position within the stack. For simple configurations, the energy consumption predicted by this complex computational tool is in agreement with the global approach.[ 1 ] Moreover, this sophisticated model is able to account for the standard deviation of the final moisture content (MC), which allows a compromise to be struck between quality, drying time, and energy consumption. In addition, this model can also be applied to test innovative, energy-saving drying strategies with an equivalent drying quality, i.e. sorting boards of similar properties before drying, or sorting boards at the end of drying to re-dry those boards that have a final MC that is still too high.

Influence of Particle Size and Concentration on Rheological Behaviour of Reconstituted Apple Purees

This work investigates the impact of structural parameters on the rheological behaviour of apple purees. Reconstructed apple purees from 0 g/100 g up to 2.32 g/100 g of insoluble solids content and varying in particle size were prepared. Three different particle size distributions were obtained by mechanical treatment only, to modify both size and morphology of the particles without modifying the intrinsic rigidity of the cell walls. Rheological measurements showed that the insoluble solids content have a first order effect on the rheological behaviour of the suspensions: three concentrations domains were observed in both dynamic and flow measurements. A model is proposed for each domain. The existence of a weak network between particles is clearly shown over a critical concentration of insoluble solids (cell walls) depending on particle size distribution (semi-diluted domain). In a concentrated domain, particles are on close packing conditions and their apparent volume begin to shrink. Particle size and shape also play an important role on the rheological behaviour of reconstructed apple puree. Due to their irregular shape, cell clusters clog the medium at lower concentration compared to individual cells.

Cellulose nanocrystal surface functionalization for the controlled sorption of water and organic vapours

The surface grafting of cellulose nanocrystals (CNC) is a valuable tool to increase opportunities for their application. This work had several goals designed to improve CNC: reduction of hornification, increased re-dispersibility after CNC drying, and tuning of the surface graft to enhance the adsorption of particular molecules. To achieve this, the CNC surfaces were modified chemically with aromatic surface grafts using widely employed methods: the creation of urethane linkages, silylation and esterification. Even a low degree of grafting sufficed to increase water contact angles to as much as 96°. The analysis of water sorption isotherms showed that at high water activities, capillary condensation could be suppressed and hysteresis was decreased. This indicates that hornification was significantly suppressed. However, although the contact angles increased, the water sorption isotherms were changed only slightly because of reduced hysteresis. The grafts were not able to shield the surface from water vapour sorption. A comparison of the sorption isotherms of anisole and cyclohexane, sorbates with a similar surface area, showed that the sorption of anisole was three times higher than that of cyclohexane. The specific sorption of aromatic molecules was achieved and the most efficient methodology was the esterification of CNC with carboxylic acids containing a flexible linker between the aromatic moiety and ester bond.

Free shrinkage of wood determined at the cellular level using an environmental scanning electron microscopE

The shrinkage at the cellular level was determined in the transverse plane of wood using an environmental scanning electron microscope (ESEM). The whole procedure is based on pairs of images grabbed with the same imaging conditions (magnification, working distance, voltage) without removing the sample from the chamber: one image collected at saturated conditions (1067 Pa, 1°C) and the other at air-dry state (533 Pa, 16°C). A closed chain of the same reference points chosen from the anatomical markers was defined on both images at the external part of the zone of interest. A custom software program, called MeshPore, allowed the shrinkage coefficients to be extracted from the slight difference of shape between these two chains. Measurements of transverse shrinkage were performed on earlywood and latewood zones from Norway spruce (Picea abies), only normal wood was studied. The interested zones were isolated from the rest of the annual ring with the aid of a microtome blade, insuring the observations to be done under free shrinkage. As additional features, the changes of the cellular morphology and the cell wall thickness were also evaluated thanks to the resolution provided by the ESEM.

Designed cellulose nanocrystal surface properties for improving barrier properties in polylactide nanocomposites

Nanocomposites are an opportunity to increase the performance of polymer membranes by fine-tuning their morphology. In particular, the understanding of the contribution of the polymer matrix/nanofiller interface to the overall transport properties is key to design membranes with tailored selective and adsorptive properties. In that aim, cellulose nanocrystals (CNC)/polylactide (PLA) nanocomposites were fabricated with chemically designed interfaces, which were ensuring the compatibility between the constituents and impacting the mass transport mechanism. A detailed analysis of the mass transport behaviour of different permeants in CNC/PLA nanocomposites was carried out as a function of their chemical affinity to grafted CNC surfaces. Penetrants (O2 and cyclohexane), which were found to slightly interact with the constituents of the nanocomposites, provided information on the small tortuosity effect of CNC on diffusive mass transport. The mass transport of water (highly interacting with CNC) and anisole (interacting only with designed CNC surfaces) exhibited non-Fickian, Case II behaviour. The water vapour caused significant swelling of the CNC, which created a preferential pathway for mass transport. CNC surface grafting could attenuate this phenomenon and decrease the water transport rate. Anisole, an aromatic organic vapour, became reversibly trapped at the specifically designed CNC/PLA interface, but without any swelling or creation of an accelerated pathway. This caused the decrease of the overall mass transport rate. The latter finding could open a way to the creation of materials with specifically designed barrier properties by designing nanocomposites interfaces with specific interactions towards permeants.

Physical behavior of highly deformable products during convective drying assessed by a new experimental device

ABSTRACT This article presents an experimental device specially designed for the continuous measurement of mass and dimensions of samples submitted to convective drying. This experimental device consists of a magnetic suspension balance and an image acquisition system. The sample deformation is determined by image correlation using a custom software. The entire system is able to achieve accurate mass and dimension measurements over a wide range of temperature and relative humidity. To present the potential of this equipment, experimental data of highly deformable food products (potato and apple) submitted to several drying conditions are presented. The obtained results confirm that it is required to consider the actual sample surface to determine the mass flux. This allowed a first drying stage to be observed for potato. Clear differences in physical behavior of these two vegetable products were also observed, for example, an isotropic shrinkage behavior for potato and an anisotropic behavior for apple. These results were explained by the anatomical structure of the products.

Design of Model Apple Cells Suspensions: Rheological Properties and Impact of the Continuous Phase

The objective of this work is twofold: to develop a relevant model system to study plant cells suspensions’ rheology and to evaluate the impact of the continuous phase composition and viscosity on the rheological behaviour of apple cells suspensions. Model suspensions of individual or clustered apple cells were developed. Rheological behaviours of both type of suspensions were observed separately, suspending from 0.145 g/100mL to 3.48 g/100mL of particles in five model media and in the original apple serum. Our results show that model suspensions successfully reproduce the rheological behaviour of apple purees, following three concentration domains. In particular, cell clusters greatly reproduce the behaviour of bimodal apple purees, suggesting that clusters dominate the rheological behaviour of the whole puree. One of our main result is that continuous phase does not affect elastic properties of suspensions in the concentrated domain since they are essentially governed by particle interactions: G’ values are similar whatever the continuous phase. If the continuous phase has the main impact on diluted suspensions’ viscosity, its effect becomes smaller as particle concentration increases. A lubricating effect was observed in the concentrated domain for continuous phases containing polymers. Presence of polymers may help in structuring the network in the intermediate domain.

Mass diffusivity of low-density fibreboard determined under steady- and unsteady-state conditions: Evidence of dual-scale mechanisms in the diffusion

Abstract The mass diffusivity of low-density fibreboard (<300 kg m−3) was measured under steady-state conditions (cup method) and under unsteady-state conditions (sorption in the relative humidity range 37–71%). The diffusivity was measured parallel and perpendicular to the sheet plan, for different sample thicknesses (~24, 20, 10, 4 and 2 mm) and different densities (160 and 270 kg m−3). The change in mass was measured with a conventional balance for thick samples and a magnetic suspension balance for thin samples. The results indicated that the diffusivity of fibreboard thicker than 20 mm was similar in steady- and unsteady-state measurements. For boards less than this dimension, diffusivity in transient sorption (unsteady state) depended on the sample thickness. In this case, the simplified continuum representation of the physical medium must be discarded. The discussion emphasizes the effect of the double porosity of the material to explain this diffusion behaviour.