Natural fiber composites were designed and optimized to achieve good mechanical properties and resistance to growth of living organisms. Composite materials were prepared from poly(lactic acid) (PLA) with flax fibers, where the flax fibers had been subjected to interstitial polymerization to replace the water in the cellulose fibers. Before polymerization, the flax fibers were extracted with sodium hydroxide and acetone to remove lignin, pectin, and waxes from the cellulose. Differential scanning calorimetry was used to study the crystallization and melting of the composites as compared with pure PLA. The surface wetting of the fibers and morphology of the composites were studied by scanning electron microscopy and optical microscopy. Mechanical properties were studied using dynamic mechanical analysis. The influence of the interstitial polymerization on the dynamic storage modulus was found to be significant. The composites of polymerization treated flax, with acetone washed fibers, had higher storage moduli than the unwashed fiber composites, which suggested adhesion between flax fibers, and the matrix was improved by the treatments. The composites were subjected to moist environmental conditions to test for development of mold and fungi, and the acetone washed polymerization treated flax composites were resistant to these growths.

A novel composite material consisting of polypropylene (PP) fibers in a random poly(propylene-co-ethylene) (PPE) matrix was prepared and its properties were evaluated. The thermal and mechanical properties of PP-PPE composites were studied by dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) with reference to the fiber concentration. Although, by increasing PP fiber concentration in PPE, no significant difference was found in melting and crystallization temperatures of the PPE, the storage, and the tensile and flexural modulus of the composites increased linearly with fiber concentrations up to 50%., 1.5, 1.0, 1.3 GPa, respectively, which was approximately four times higher than that for the pure PPE. There is a shift in glass transition temperature of the composite with increasing fiber concentration in the composite and the damping peak became flatter, which indicates the effectiveness of fiber-matrix interaction. A higher concentration of long fibers (> 50% w/w) resulted in fiber packing problems, difficulty in dispersion, and an increase in void content, which led to a reduction in modulus. Cox-Krenchel and Haplin-Tsai equations were used to predict tensile modulus of random fiber-rein forced composites. A Cole-Cole analysis was performed to understand the phase behavior of the composites. A master curve was constructed based on time-temperature superposition (ITS) by using data over the temperature range from -50 to 90 degrees C, which allowed for the prediction of very long and short time behavior of the composite.

Crystal structure and mechanical properties of cellulose fibers were studied to investigate the effect of chemical treatment on the fiber. Pretreatment by acetone extraction, mercerization with 3-20% wt/v sodium hydroxide (NaOH), and acrylonitrile (AN) grafting initiated by azo-bis-isobutylonitrile were performed. From Fourier transform infrared spectroscopy and wide-angle X-ray diffraction quantitative measurements, the pretreated fibers showed an induced slight decrease of crystallinity index. The structural transformation of the fibers from cellulose I to cellulose 11 was observed at high NaOH concentration of 10-20% wt/v. The amount of grafting, 1.56, 2.94, 6.04, 8.34, or 10.46%, was dependent upon the initiator concentration and the volume of monomer in the reactor. The AN grafted fibers had no transformation of crystalline structure as observed after mercerization. Only a variation of X-ray crystallinity index with grafting amount was observed. Moisture regain of pretreated and modified fibers depended on the structure of the fiber and the amount of grafting. The mechanical properties performed by a single fiber test method were strongly influenced by the cellulose structure, lateral index of crystallinity, and fraction of grafting. Scanning electron microscopy was used for analysis of surface morphologies of treated fibers.

Natural fiber-biopolymer composites have been prepared using flax and poly(3-hydroxylbutyrate) bipolyesters (PHB). The biopolyesters consist of the homopolyester PHB and its copolymers with 5 and 12% 3-hydroxyvalerate (PHV). These biopolymer-natural fiber composites provide structures totally composed of biodegradable and renewable resources. The adhesion between the fibers and the polyesters was better than for analogous polypropylene composites. Wetting of the fibers by the polyesters was observed using scanning electron microscopy. The composites were limited by the properties of the polyesters. PHB is a brittle polymer though flexibility is improved in its copolymers with PHV, but at the expense of crystallization rate. Nucleation was increased by the fibers and silane coupling agent used as adhesion promoter. The melting temperature was influenced by the promoted adhesion and copolymerization. The bending modulus was increased in the composites and dynamic mechanical analysis provided storage modulus of as much as 4 GPa at 25degreesC with a smaller component as the loss modulus. The maximum in the loss modulus curve was taken as the glass transition temperature, and this increased in the composites. The influence of silane coupling agent was found beneficial for the material properties of the biopolyester-flax composites.

The mechanical and structural properties of a composite consisting of polypropylene fibres (PP) in a random poly(propylene-co-ethylene) (PPE) has been prepared and its properties evaluated. The mechanical properties of PPE laminates were largely determined by the presence of a complex fibre orientation distribution in the composite. The results showed that all-PP composites demonstrated enhanced stiffness and creep resistance with decrease in the orientation angle, θ, between the fibre axis and the load direction; the friction coefficient decreased linearly as θ increased from 0° to 90°. Composites with zero angle (unidirectional composites) between fibre axis and applied load (θ = 0°) possess the highest stiffness, because the fibre efficiency is inherently strong for this system as all the fibres are able to contribute to the composite stiffness and to carry the load. An increase in θ leads to a decrease in composite stiffness, which indicates that by increasing θ, a lower proportion of the applied load is transferred to the fibres and thus it is not completely distributed among the fibres. The composite with θ = 90° showed the highest relative creep, whereas the composite with θ = 0° displayed the lowest creep. In general, the relative creep increased steadily with increasing θ; also the increment in θ produced a decrease in the creep modulus, and it was observed up to 90°. Composites with woven fabric showed the best properties, after the unidirectional composites, due to the interlaced structure of the woven fabric. The bundles of plain fibre cloth restrict displacement in each other and result in high stiffness. In addition the mechanical properties of the composite with random fibres are somewhat between the composite with 0°< θ < 90°, due to alignment of fibers along any direction in the composite.

The effect of modified flax fibers by plasticizer absorption in poly(L-lactic acid) composites was investigated. The plasticizes chosen were triethyl citrate (TEC), tributyl citrate (TBC) and glycerol triacetate (GTA), which were derived from natural sources. Characterization was performed by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The morphology was examined from scanning electron microscopy (SEM) and optical microscopy (OM). The results showed that the plasticizer caused a marked increase in the storage modulus of the composites, which could be due to an improvement in the morphology of the matrix and a smoother surface coverage of the fibers by the matrix. The thermal properties were also affected, in which the glass transition temperature (Tg), the crystallization temperature (Tc) and the crystallinity (Xc) were reduced depending on the plasticizer. The citrate esters revealed to be the most effective plasticizers of those tested.

Natural fibre-biopolymer composites have been preparred from flax and polyhydroxybutyrate (PHB). The flax was modified by drying, followed by plasticiser absorption to replace the water lost to prevent embrittlement. This protects the fibres from problems associated with their water content and changes in water content due to equilibration with the environment. Flax and PHB showed good interfacial adhesion, which was decreased when plasticisers were present. Some plasticiser migrated from the flax to PHB and caused complex changes in the glass transition, crystallisation and crystallinity of the PHB. Morphology of the composites was examined by scanning electron microscopy (SEM) and optical microscopy (OM), SEM provided information on the interfacial adhesion through fractography. OM showed extensive transcrystallinity along the fibre surfaces. Dynamic mechanical analysis was used to measure elastic and damping characteristics and their relation to composition and morphology.

Composite materials have been prepared from hydrophobic and hydrophilic polypropylene with flax fibres, where the flax fibres have been used with and without silane surface treatment. Other comparable composites have been prepared from poly(lactic acid) and poly(ethylene succinate), recently developed biodegradable polymers with promising mechanical properties. The polar polymers were found to provide better wetting of the surface of the flax, regardless of the surface treatment. Differential scanning calorimetry (DSC) was used to study the crystallisation and melting of the composites compared with the pure polymers. The surface wetting of the fibres and morphology of the composites was studied by scanning electron microscopy (SEM) and optical microscopy. Mechanical properties were studied using dynamic mechanical analysis (DMA). It was found that the relatively low density flax fibres provide efficient reinforcement compared with that expected from analogous glass fibre composites. The influence of the transcrystalline interphase on the dynamic storage modulus was found to be important.

The microdroplet test has been used to characterize the interfacial bond in fibrous composite materials. The specimen consists of a length of fibre with a drop of resin cured on it and pulled out while the drop is being supported by a platinum disc with a hole. A properly tested specimen fails at the droplet's tip-fibre interface, revealing the ultimate interfacial shear strength. The microdroplet test is carried out under optimal testing conditions, established from our previous work, in order to obtain fully debonded specimens for the interfacial shear stress analysis of the fibre-matrix bond. Three thermosetting resins are applied on silane coated and acetone washed (silane removed) glass fibres in order to investigate the influence of the silane coupling agent on the fibre-matrix bond. Also, these systems were aged in water at room temperature for periods of 3, 6 and 10 weeks and the influence of water degradation on the fibre-matrix bond is investigated for both groups of glass fibres (silane coated and acetone washed). The group of glass fibres washed in acetone produced weaker fibre-matrix bond and accelerated water degradation of the interfacial bond. It is found that the influences of two dominant geometric parameters (conus angle and droplet length) on the test results are reciprocally opposite, generating scatter in the experimental results. It is shown in this work that the microdroplet test can be used as a reliable tool for measurement of the fibre-matrix adhesion.