Effect of fiber content, length and arrangement on the mechanical modulus of PHBHV/Miscanthus fiber composites: contribution of a finite element model

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PRINCIPAL CHARACTERISTICS OF VEGETAL FIBERS

The physical structure and the chemical composition of vegetal fibers suggest that they can be considered already as advanced composite materials. The conditions of growth, the type of soil, climate and the aging conditions influence the chemical composition of a plant, playing an important role in the final properties of the fibers. Vegetal fibers are constituted of three main constituents, cellulose, hemicellulose and lignin, that are present in all plants but in different proportion as showed in table I-1.

Synthesis of poly(lactic acid) (PLA)

The poly(lactic acid) is commonly made from α-hydroxy acids and we can consider that lactic acid (2-hydroxy-propionic acid) is the basic building block of this polymer. Lactic acid was produced by petrochemical route and since 1990 by polysaccharides and sugar fermentation, this last being a more eco-friendly approach [121]. Lactic acid exists in two optical isomers, defined as L and D-lactic acid. Actually, by the petrochemical synthesis, the produced lactic acid is a 50/50 mixture of the two isomers, while by the fermentation route, the content of L-isomer is predominant (99.5% of L-isomer and 0.5% of D-isomer) [122]. There are two main ways to produce high molar mass PLA as shown in scheme I-1.
Scheme I-1. Synthesis methods for high molecular weight PLA: condensation/coupling, azeotropic dehydrative condensation and ring-opening polymerization of lactide (adapted from [121]). Starting from lactic acid, PLA can be produced by condensation resulting in low molar mass. This last can be improved only by the use of coupling agents, esterification-promoting adjuvants or chain-extending agents, these last increasing the cost and the complexity of the process. The azeotropic dehydrative condensation is a second route producing a PLA with high molar mass without the use of adjuvants or coupling agents. However, this way requires the use of high quantities of catalysts to favor the reaction rate, whose residues can cause many problems in further processing such as undesired degradation or uncontrolled hydrolysis. For all these reasons the ring opening polymerization of lactides, demonstrated for the first time by Carothers in 1932 [123], is considered as the best way to produce high molar mass and pure PLA. The process starts from lactide which is obtained by depolymerization of low molar mass PLA and which results in a mixture of L-lactide, D-lactide and meso-lactide. The ring opening polymerization can be cationic or it can be anionic [121]. Actually, the commercial PLA is a combination of various copolymers, such as the poly (L-lactic acid) (PLLA), the poly(D-lactic acid) (PDLA) and the poly(DL-lactic acid) (PDLLA), synthesized from L, D and DL-lactic acid monomers respectively whose stereoforms are showed in figure I-19 [124, 125].

Synthesis of poly(ε-caprolactone) (PCL)

Poly(ε-caprolactone) is constituted by hexanoate repeat units. Similarly to poly(lactic acid), PCL can be synthesized by different routes: the condensation of the 6-hydroxyhexanoic acid and the ring opening polymerization of lactones (ROP) [126]. The second route to synthesize PCL, via ring opening polymerization of lactones is undoubtedly the most used. The possibility to work under mild conditions results in polyesters with high molar mass obtained in a shorter time than polycondensation. The ring-opening polymerization can be performed in bulk, in solution, in emulsion or dispersion and to start the process the use of an initiator is necessary, an active species that react with the monomer to give the polymer. This kind of reaction is classified depending on the catalyst used: metal-based, organic and enzymatic. We can distinguish anionic, cationic, monomer-activated and coordination-insertion ROP. Among these four categories, the coordination-insertion showed in scheme I-2 is the most common. In this process, the propagation proceeds through the coordination of the monomer to the catalyst with the subsequent insertion of the monomer into the metal-oxygen bond of the catalyst [127].
Another way to synthesize PCL is the use of enzymes under mild conditions. This route can be considered as a green way to synthesize these biodegradable polymers because of the absence of toxic reagents and it includes also the possibility to recycle the catalyst [126, 128, 129]. One of this mechanism is proposed in the scheme I-3 and it concerns a ROP using lipase, in which a lipase-activated monomer complex, formed after the reaction between the lipase and the lactone, reacts with an alcohol to form the polymer [126, 130, 131].

Synthesis of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBHV)

The synthesis and the properties of the poly(3-hydroxybutyrate-co-3-hydroxyvalerate) are strictly related to those of poly(3-hydroxyalkanoates), the family of polymers to which PHBHV belongs. The PHAs possess many linear carbon side chains (renamed here R) with different lengths depending from the carbon source and the nature of the bacteria (see figure I-20).
In this context, three types of PHAs can be differentiated: short-, medium- and long-chain. The first type of PHA, the so-called PHA scl (short chain length) has a lateral chain constituted by 1 to 3 carbon atoms (R=CH3 to C3H7), the PHA mcl (medium chain length) has a number of carbons atoms in the lateral chain from 4 to 9 (R=C4H9 to C9H19) and at last the PHA lcl from 10 to 14 (C10H21 to C14H29). The poly (3-hydroxybutyrate) (PHB) and poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBHV), whose properties will be also investigated in the experimental part, belong to the P(3-HA)s group [132].
The synthesis of the PHBHV showed in scheme I-4 consists in three fundamental steps based on the chemistry of the acetyl-CoA as precursor, this last being produced by the oxidation of fatty acids and sugars. This route is the most common metabolic way to produce this copolymer. At first the enzymatic activity of the β-ketothiolase allows to the condensation of two molecules of acetyl-CoA to form acetoacetyl-CoA and the condensation of acetyl-CoA with propionyl-CoA to form β-ketovaleryl-CoA. In a second moment, the formed products are converted into the polymer by the activities of the (acetoacetyl-CoA) reductase and of the PHB synthase. The quantity of HV units are dependent from the carbon source; for example, the addition of glucose to the propionic acid results in a great variability of HV units from 0 to 57%.
Scheme I-4. Pathway for the production of PHBHV from acetyl-CoA and from propionyl-CoA (adapted from [133]).

THERMAL AND MECHANICAL PROPERTIES OF ALIPHATIC BIODEGRADABLE POLYESTERS

Polyesters previously described can be considered as thermoplastic polymers with a semicrystalline behavior. This means that a glass transition and a melt temperature characterize them.
Although PLA, PCL and PHBHV have these common characteristics, the range of temperatures at which these polymers can be used and processed is not the same as showed in figure I-21. Properties of PLA change with the content of L-isomer and in the case of PHBHV the variability in the content of the HV units caused different mechanical and thermal properties.
The way of synthesis of PLA and PCL influences their mechanical and thermal properties. In the case of poly(lactic acid), the content of L-isomer that characterize PLA produced from renewable source influences preferentially the crystallinity. More precisely, high content of L-isomer induces crystallinity while low content results in an amorphous PLA, decreasing also the melting and the glass transition temperatures [105, 134, 135]. While, the melting temperature of the polymer is mainly dependent on the optical purity of PLA and it can vary from a maximum of 180°C to 120°C depending on the amount of D-lactide incorporated, the glass transition one is dependent also from the thermal history of the polymer. The mechanical properties of PLA are variable and strictly related to the crystallinity behavior of the polymer. Semicrystalline PLA has an elastic modulus of around 3 GPa, a tensile strength that varies from 50 to 70 MPa and an elongation at break of about 4% [136, 137]. Due to these properties it is considered as a brittle material. For high mechanical performances, semicrystalline PLA is preferred to an amorphous one. Mechanical properties of PLA are related to the molecular weight, to the presence of the L or D isomers and also to thermal treatment such as annealing as showed in previous works [138, 139].
Similarly to PLA, high molar mass poly(ε-caprolactone) can be obtained by ring opening polymerization of lactones and also in this case thermal and mechanical properties of this polymer are dependent from its molar mass and its crystallinity. PCL cannot be considered as a brittle material, exhibiting a low tensile strength (approximately 23 MPa) and high elongation at break (>770%). It has the great advantage to possess lower density than the others, to be miscible with a lot of other polymers and to be mechanically compatible with polyethylene, polypropylene and rubber [117, 126, 140]. It is characterized by a melting temperature around 60°C and a glass transition temperature of around -60°C [141, 142].
Failure stress and Young Modulus of the homopolymer PHB are similar to those of polypropylene but the elongation at break of this last is higher (400%) [143, 144]. The introduction of a co-monomer into the polymer backbone like 3-HV causes a change in thermal properties and a consequent change in mechanical properties. In particular, the final copolymer exhibits an increase in flexibility and toughness, but at the same time a reduction in polymer stiffness. Compared to its equivalent homopolymer, the PHB, that is highly crystalline and brittle, the introduction of 3-HV units allows a decrease in glass transition temperature (Tg) and melting temperature (TM) without significant changes in the crystallinity [114, 143, 145-148]. This weak change is due to a phenomenon of co-crystallization of the two-monomer units (HV and HB) that rearrange creating an intermediary structure and preserving the crystalline character. One of the problems of PHBHV is the presence of a secondary crystallization of the amorphous phase, which occurs during storage time at room temperature. Several authors studied this phenomenon, so a sample stored at room temperature for 60 days have lower values for elongation at break than samples stored for 30 days [149-151]. The range of principal thermal and mechanical properties of PLA, PHBHV at different compositions and PCL are listed in the table I-4.

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Table of contents :

Chapter I Green Wood Plastic Composites
I-1. An overview on composites materials
I-2. The reinforcing fibers
I-2.1. Natural fibers
I-2.2. Principal characteristics of vegetal fibers
I-2.2.1. Cellulose
I-2.2.2. Hemicelluloses
I-2.2.3. Lignin
I-2.3. Presentation of Miscanthus
I-3. Aliphatic biodegradable polyesters
I-3.1. Synthesis of biodegradable aliphatic polyesters
I-3.1.1. Synthesis of poly(lactic acid) (PLA)
I-3.1.2. Synthesis of poly(ε-caprolactone) (PCL)
I-3.1.3. Synthesis of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBHV)
I-3.2. Thermal and mechanical properties of aliphatic biodegradable polyesters
I-3.3. Biodegradability of aliphatic polyesters
I-3.4. Thermal degradation of aliphatic polyesters
I-4. Toward biocomposites
I-4.1. Treatment of vegetal fibers
I-4.2. Processing methods
I-4.2.1. The compounding processes
I-4.2.2. The molding processes
I-4.3. PLA-based composites
I-4.4. PCL-based biocomposites
I-4.5. PHBHV-based biocomposites
I-5. Micromechanics of heterogeneous materials
I-5.1. Micromechanical framework
I-5.2. First Simple analytical methods
I-5.3. Eshelby approximation
I-5.4. Halpin-Tsai Equations
I-5.5. Mori-Tanaka-Benveniste Model
I-5.6. Numerical Methods: a focus on Finite Element Method
I-6. Conclusions
I-7. References
Chapter II Functionalization of Miscanthus by photo-activated thiol-ene addition to improve interfacial adhesion with polycaprolactone
II-1. Introduction
II-2. Experimental
II-2.1. Materials
II-2.2. PMMS grafting onto Miscanthus giganteus fibers
II-2.3. Composites manufacturing, PCLxMISy
II-2.4. Analytical techniques
II-3. Results and discussion
II-3.1. PMMS photo-grafting on the Miscanthus fibers
II-3.2. Preparation of biocomposites, PCLxMISy
II-3.3. Characterization of biocomposites
II-4. Conclusions and Perspectives
II-5. Appendix
II-5A. Effect of fibers content on PCL
II-5B. Effect of BPO on PCL/MIS biocomposites
II-6. References
Chapter III Study of mechanical properties of PHBHV/Miscanthus green composites combining experimental and micromechanical approaches
III-1. Introduction
III-2. Materials and methods
III-2.1. Materials
III-2.2. Composite processing
III-2.3. Materials characterization
III-2.3.1. Scanning electron microscope (SEM)
III-2.3.2. Mechanical properties
III-2.3.3. Fiber-size distribution
III-2.3.4. Density measurements
III-2.3.5. Differential scanning calometry (DSC)
III-2.4. Modeling
III-3. Results
III-3.1. Mechanical properties of biocomposites
III-3.2. Scanning electron microscopy (SEM)
III-3.3. Fiber-size distribution
III-3.4. Density of Miscanthus and composites
III-3.5. Results of numerical simulation
III-4. Discussion
III-4.1. The mechanical behavior of PHBHV/MIS composites
III-4.2. Internal morphology and density of the bio composites
III-4.3. Numerical simulation
III-5. Conclusions and perspectives
III-6. Appendix
III-6A. Identification of the Young modulus of the matrix
III-6B. Estimation of volumetric fraction of fibers in the specimens
III-6C. Effect of time on mechanical and thermal properties of PHBHV-based composites…
III-7. References
Chapter IV Effect of fiber content, length and arrangement on the mechanical modulus of PHBHV/Miscanthus fiber composites: contribution of a finite element model
IV-1. Introduction
IV-2. Experimental investigation
IV-2.1. Materials and processing
IV-2.1.1. Materials
IV-2.1.2. Composite processing
IV-2.2. Materials characterization
IV-2.2.1. Scanning electron microscopy
IV-2.2.2. Morphology of the fibers
IV-2.2.3. Mechanical properties of the composite
IV-2.3. Experimental results
IV-2.3.1. Scanning electron microscopy (SEM) and fibers characterization
IV-2.3.2. Mechanical properties of the biocomposites
IV-3. Numerical investigation
IV-3.1. Finite element models
IV-3.1.1. 2D FE models
IV-3.1.2. 3D FE models
IV-3.2. Homogenization models
IV-3.3. Numerical results
IV-3.3.1. Tensile modulus
IV-3.3.2. Stress distribution
IV-4. Discussions
IV-4.1. Effect of fiber length and content on the mechanical behavior of the biocomposites
IV-4.1.1. Tensile modulus
IV-4.1.2. Tensile strength
IV-5. Conclusions
IV-6. Appendix
IV-6A. Processing parameters during extrusion and injection molding
IV-7. References
Chapter V Biocomposites based on Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBHV) and Miscanthus giganteus fibers : multiphase modeling of the effective mechanical behavior of biocomposite with improved fiber/matrix interface
V-1. Introduction
V-2. Experimental
V-2.1 Materials
V-2.2 Chemical treatment of Miscanthus giganteus fibers
V-2.3 Composite Manufacturing
V-2.4 Materials characterization
V-2.4.1 Gel fraction
V-2.4.2 Mechanical testing
V-2.4.3 Scanning electron microscopy (SEM)
V-2.4.4 Fourier Transform Infrared Spectroscopy (FTIR)
V-2.4.5 Differential Scanning Calorimetry (DSC)
V-2.4.6 X-ray Diffraction (XRD)
V-3. Results and discussion
V-3.1 Evaluation of PHBHV grafting onto MIS surface during processing evaluated by FTIR-ATR analysis
V-3.2 Tensile properties
V-3.3 Fracture facies Morphology
V-3.4 Characterization of biocomposites by DSC and XRD analyses
V-3.5 Analytical and numerical models
V-3.5.1 Use of a model involving three phases
V-3.5.2 Evaluation of EG and ΦG by a mathematical approach
V-3.5.3 Results of numerical and mathematical approach
V-4. Conclusions
V-5. Appendix
V-5A Realization of specimens of PHBHV90MIS10 (2.2% DCP)
V-6. References

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