Modification de la structure chimique du bois par des alcoxysilanes diversement substitués

Une étude fondamentale des phénomènes au niveau moléculaire lors de la modification chimique du pin maritime (Pinus pinaster) par des trialcoxysilanes modèles diversement substitués a été entreprise au cours de ce travail de thèse. Dans un premier chapitre, la modification chimique du bois à partir du 3- isocyanatopropyltriéthoxysilane (IPTES) et son couplage ultérieur avec le méthyltriméthoxysilane (MTMS) ont été étudiés. La réaction avec IPTES a été réalisée en présence de dilaurate de dibutylétain (DBTDL) et a conduit au greffage de fonctions triethoxysilanes réactives à l’intérieur des parois cellulaires. Des polymères de MTMS ont ensuite été fixés durablement à l’intérieur du bois, par l’intermédiaire de ces fonctions . Dans un deuxième chapitre, les interactions entre les polymères lignocellulosiques et la fonction trialcoxysilane ont été plus particulièrement étudiées, grâce à des méthodes d’investigation moléculaires comme l’IRTF ou la RMN du solide. La condensation de la fonction triméthoxysilane avec les groupements hydroxyles du bois a été confirmée en milieu anhydre (en présence d’éthylamine) mais les liaisons formées se sont avérées hydrolysables. En présence d’eau (et de DBTDL), la fonction triméthoxysilane a été hydrolysée et condensée à l’intérieur du bois, sous forme de polysiloxanes stables. Un greffage durable et significatif a alors été obtenu pour une teneur en eau contrôlée de 8% à l’intérieur du bois.A fundamental study of the molecular phenomena occurring during the chemical modification of maritime pine (Pinus pinaster) with trialkoxysilane model compounds has been carried out in this thesis. In a first chapter, the chemical modification of wood with 3- isocyanatopropyltriethoxysilane (IPTES) and its subsequent coupling reaction with the methyltrimethoxysilane (MTMS) were studied. The reaction with IPTES was performed in the presence of dibutyltin dilaurate (DBTDL) and led to the grafting of reactive triethoxysilane functions within the wood cell walls. Polymers of MTMS were then durably fixed in wood via these functions. In a second chapter, the interactions between lignocellulosic polymers and the trialkoxysilane function were particularly studied, with the help of molecular techniques such as FTIR or solid state NMR spectroscopy. In anhydrous conditions, the condensation between the trimethoxysilane function and wood hydroxyl groups was confirmed (in the presence of ethylamine), but the bonds formed with wood were found to be hydrolysable. In the presence of water (and with DBTDL), the trimethoxysilane function hydrolysed and condensed in wood, in the form of stable polysiloxane polymers. A significant and durable grafting was then obtained, with a controlled moisture content in wood of 8%

Experimental and statistical study of three adherence tests for an epoxy-amine/aluminum alloy system: Pull-Off, Single Lap Joint and Three-Point Bending tests

The mechanical resistance of a bonded joint depends on the adhesive interaction onto the substrate and the mechanical properties of the adhesive itself. Many existing tests can be useful for measuring the adherence or evaluating mechanical adhesive response. All these tests do not provide the same information: in particular, adherence measurements can be split into initiation tests and propagation ones. In this paper, three adherence tests have been considered for the evaluation of the fracture initiation between a poly-epoxide adhesive (a mixture of pure epoxy and amine) and an aluminum surface (AA 2024-T3), namely the Pull-Off, Single Lap Joint (SLJ) and Three-Point Bending tests. Various surface preparation protocols before bonding have been tested and optimized for aluminum substrates, including mechanical and chemical surface treatments, followed by the application of an appropriate primer before bonding. This study paves the way for the future development of adhesive systems as it provides reliable surface preparation protocols for aluminum surfaces and gives an insight into the choice of an adequate adherence test dedicated to high-performance adhesives. The load at break (FMax), the experimental error, the failure mode and statistical studies according to the Weibull model and Principal Component Analysis (PCA) were studied on each surface preparation configuration. It has been shown that the application of a primer, especially a sol-gel product increases the load at break and provides more reliable results. Then, this paper shows that the two tests can quantify the failure initiation and distinguish the different surface preparation efficiency, are the Single Lap Joint test (mode II or mode I + II) and the Three-Point Bending test (mode I), with an increase of the results reliability with the latter one. The Pull-Off test (mode I) is useful as a routine checking, and particularly interesting because its response does not depend on the substrate thickness, even though it cannot highlight the difference between all surface preparations

Dispersion and Reinforcing Potential of Carboxymethylated Nanofibrillated Cellulose Powders Modified with 1-Hexanol in Extruded Poly(Lactic Acid) (PLA) Composites

Bionanocomposites of poly(lactic acid) (PLA) and chemically modified, nanofibrillated cellulose (NFC) powders were prepared by extrusion, followed by injection molding. The chemically modified NFC powders were prepared by carboxymethylation and mechanical disintegration of refined, bleached beech pulp (c-NFC), and subsequent esterification with 1-hexanol (c-NFC-hex). A solvent mix was then prepared by precipitating a suspension of c-NFC-hex and acetone-dissolved PLA in ice-cold isopropanol (c-NFC-hexsm), extruded with PLA into pellets at different polymer/fiber ratios, and finally injection molded. Dynamic mechanical analysis and tensile tests were performed to study the reinforcing potential of dried and chemically modified NFC powders for PLA composite applications. The results showed a faint increase in modulus of elasticity of 10% for composites with a loading of 7.5% w/w of fibrils, irrespective of the type of chemically modified NFC powder. The increase in stiffness was accompanied by a slight decrease in tensile strength for all samples, as compared with neat PLA. The viscoelastic properties of the composites were essentially identical to neat PLA. The absence of a clear reinforcement of the polymer matrix was attributed to poor interactions with PLA and insufficient dispersion of the chemically modified NFC powders in the composite, as observed from scanning electron microscope images. Further explanation was found in the decrease of the thermal stability and crystallinity of the cellulose upon carboxymethylatio

Reinforcing effect of carboxymethylated nanofibrillated cellulose powder on hydroxypropyl cellulose

Bionanocomposites of hydroxypropyl cellulose (HPC) and nanofibrillated cellulose (NFC) were prepared by solution casting. The various NFC were in form of powders and were prepared from refined, bleached beech pulp (RBP) by mechanical disintegration, optionally combined with a pre- or post mechanical carboxymethylation. Dynamic mechanical analysis (DMA) and tensile tests were performed to compare the reinforcing effects of the NFC powders to those of their never-dried analogues. For unmodified NFC powders an inferior reinforcing potential in HPC was observed that was ascribed to severe hornification and reagglomeration of NFC. In contrast, the composites with carboxymethylated NFC showed similar behaviors, regardless of the NFC suspensions being dried or not prior to composite preparation. SEM characterization confirmed a homogeneous dispersion of dried, carboxymethylated NFC within the HPC matrix. These results clearly demonstrate that drying of carboxymethylated NFC to a powder does not decrease its reinforcing potential in (bio)nanocomposite

Hydrophobic cellulose nanopaper through a mild esterification procedure

Films of cellulose nanofibrils (CNF) (referred to as nanopaper) present a great potential in many applications due to the abundance, low environmental impact, excellent oxygen barrier properties and good mechanical performance of CNF. However, the strong hygroscopic character of the natural nanofibers limits their use in environments with high relative humidity. In this paper, we introduce a simple route for the esterification and processing of CNF with the aim of reducing their hydrophilicity, and producing hydrophobic cellulose nanopaper with reduced moisture sensitivity. The preparation steps of hydrophobic nanopapers involve vacuum filtration, solvent exchange from water to acetone, and reaction with anhydride molecules bearing different hydrophobic alkyl chains by hot pressing. Porous films having a surface area between 38 and 47g/m2 and pore sizes in the 3-200nm rangeare obtained. This method preserves the crystalline structure of native cellulose, and successfully introduces hydrophobic moieties on CNF surface as confirmed by FTIR, XPS and elemental analysis. As a result, modified nanopapers have a reduced moisture uptake, both higher surface water contact angle and wet tensile properties as compared with reference non-modified nanopaper, thus illustrating the benefit of the modification for the use of cellulose nanopaper in humid environments

Fast polymerization at low temperature of an infrared radiation cured epoxy-amine adhesive

In the industry, the cure time of two-component adhesives is very important for a cost-effective manufacturing. Too fast, it does not favor the application of the product and the control of bonded joints. Too slow, it leads to long process times and too high process costs. The best compromises are two-component adhesives that cure slowly at room temperature and can reach full polymerization in minutes, on demand. In this paper, the curing behavior of a model poly-epoxide adhesive (a stoichiometric mixture of a pure epoxy and amine) polymerized with infrared radiation will be studied. The kinetic follow-up of this polymerization will be carried out by thermal analysis (determination of the residual heat peak by Differential Scanning Calorimetry-DSC). This study paves the way to a cold and universal cure-on-demand process, which means achieved in few minutes at low temperature without any initiators, catalysts or accelerators. Basically, infrared curing can be possible thanks to an increase in temperature (called thermal effect). But it has been shown that a “non-thermal effect” could also be involved in accelerating kinetics with infrared. This increase due to a non-thermal effect, suggested as a function of the infrared radiative flux, has been shown to be possible thanks to the absorption of infrared radiation, leading to a reduction in the energy barrier of the primary epoxy/amine reaction

Hydrophobic cellulose nanopaper through a mild esterification procedure

Films of cellulose nanofibrils (CNF) (referred to as nanopaper) present a great potential in many applications due to the abundance, low environmental impact, excellent oxygen barrier properties and good mechanical performance of CNF. However, the strong hygroscopic character of the natural nanofibers limits their use in environments with high relative humidity. In this paper, we introduce a simple route for the esterification and processing of CNF with the aim of reducing their hydrophilicity, and producing hydrophobic cellulose nanopaper with reduced moisture sensitivity. The preparation steps of hydrophobic nanopapers involve vacuum filtration, solvent exchange from water to acetone, and reaction with anhydride molecules bearing different hydrophobic alkyl chains by hot pressing. Porous films having a surface area between 38 and 47g/m2 and pore sizes in the 3-200nm rangeare obtained. This method preserves the crystalline structure of native cellulose, and successfully introduces hydrophobic moieties on CNF surface as confirmed by FTIR, XPS and elemental analysis. As a result, modified nanopapers have a reduced moisture uptake, both higher surface water contact angle and wet tensile properties as compared with reference non-modified nanopaper, thus illustrating the benefit of the modification for the use of cellulose nanopaper in humid environments

Influence of moisture on the vibro-mechanical properties of bio-engineered wood

In this study, changes in the vibro-mechanical properties of fungi-treated wood, during sorption and desorption at different humidity levels, were investigated. Norway spruce resonance wood (with uniform narrow annual rings and high tonal quality for musical instrument craftsmanship) was incubated with Physisporinus vitreus for 36weeks. Stiffness, internal friction, and tonal performance indices of control (untreated) and fungi-treated wood were compared after exposure to a stepwise variation of relative humidity. It was demonstrated that fungal treatment increased the internal friction and decreased the specific modulus of elasticity, during reduction of wood density. Internal friction of both control and fungi-treated wood significantly increased during dynamic sorption, especially during early stages (hours) of each humidity change step. Both specific modulus of elasticity and internal friction showed a hysteretic behavior during humidity variation cycles. Hysteresis was smaller in fungi-treated wood. Also, tonal performance indices were improved after fungal treatment and showed a reduced variation at different relative humidity conditions. Dynamic vapor sorption tests and FT-IR microscopy studies revealed changes in hygroscopicity and the supramolecular structure of wood, which may explain the observed vibrational behavior. Less dependency of wood vibrational properties to the variation of the ambient humidity is important for the acoustic performance of string instruments