| Foamed natural and wood fiber-plastic composite (WPC) with a fine cellular structurecan overcome some shortcomings such as high density and low toughness of non-foamedWPC and can achieve outstanding cost-to-performance ratio and has a favorablestrength-to-weight ratio compared to its non-foamed analogue. Due to these unique properties,foamed WPC has widely used in building materials, decoration, packaging, especially in theconstruction and automotive industries and etc. In this paper bamboo powder reinforcedpolypropylene (PP) foaming composites were developed with bamboo powder asreinforcement, PP and high melt strength polypropylene (HMSPP) blends as matrix, modifiedazodicarbonamide (AC) as foaming agent and the composites were prepared by injectionmoulding.In the first interest for this study, effects of different proportions of PP and HMSPP onthe physico-mechanical properties of foaming composites were investigated and theappropriate ratio of PP and HMSPP was fixed to80︰20. Thermo-decomposition behaviorsof three types of chemical foaming agents (CFA) such as endotherimic, exo-endothermic andexothermic CFA were carried out and the suitable type of CFA was selected. Then, effects ofexo-endothermic FA and modified AC (MAC) on the physico-mechanical properties, cellmorphology, processing performance and thermal stability of foamed composites werecomparative analyzed. The mechanical measurements and ESEM test results indicated thatthe foamed composite with1%MAC had better physico-mechanical properties and smallercell size and better cell distribution compared with endothermic FA. And the processing andrheological performance of foamed composite adding1%MAC improved; and thedependence of apparent viscosity and shear rate got smaller; but dependence of apparentviscosity and temperature got larger and the viscous activation energy of the foamedcomposite with1%MAC was46.41KJ mol-1. This was an increase of8.9%compared withthat of the non-foamed analogue. In addition, the thermal stability of this foamed compositeslightly decreased.Second focus of this work was to study the effects of maleic anhydride modifiedpolypropylene (MAPP), silane KH570; aluminum zirconium TL-4and titanate DN301coupling agents on the mechanical properties, water absorption, surface wettablity,processing performance and thermal stability of foamed composites. With addition of coupling agents, the mechanical properties, water resistance and surface wettability allenhanced. The optimal amounts of these coupling agents were9%MAPP,2%KH570,1%TL-4and2%DN301. And the foamed composite with9%MAPP showed best comprehensiveperformance； although its thermal stability had a slightly decrease but the specificbending,tensile and the notched impact strength increased by23.1%,9.9%and47.8%andthe melt index (MI) increased from2.79g·10min-1to3.61g·10min-1. Followed with1%TL-4,the corresponding specific strengths increased by23.1%,9.9%and47.8%respectively andthe MI increased to4.50g·10min-1and the thermal stability kept fine. Next was2%KH570,its corresponding specific strengths improved by7.1%,5.8%and24.9%respectively; but MIdecreased to2.55g·10min-1and the thermal stability reduced. Last was2%DN301,itscorresponding specific strengths only raised by9.6%,6.8%and11.8%respectively; but MIincreased to3.99g·10min-1and the thermal stability had a little enhancement.Moreover, in this part the influences of the type and dosage of coupling agent onthe the rheological properties about the bamboo powder/PP foamed composites were alsocarried out. By calculating the lgG'-lgf curve slopes in the low frequency region of thefoamed composites, the homogeneous degrees of composites before and after modificationwere measured. The relationship between viscosity and frequency of the foamed compositecould be fiitted by using the Cross model and the linear viscoelastic conversion betweendifferent solid particle fillers could be achieved through translation factor α(wt%). And thechanges of the internal structure and relaxation process of foamed composites before andafter treatment were reflected by Han curve and the Cole-Cole diagram. Furthermore, thenon-Newtonian index of foamed composites was calculated using power law equation. Thestorage, loss modulus and complex viscosity all increased first and then decreasedwith the addition of coupling agents and in overall declined; and the homogeneous of foamedcomposites after modification increased. The lgG'-lgf slopes of nontreated composites was0.709and with addition of9%MAPP,2%KH570,1%TL-4and2%DN301the slopesincreased to0.782,0.758,0.842and0.811respectively. The coupling agents did notchange the non-Newtonian pseudoplastic characteristics of bamboo powder/PP foamedcomposite, but the Newton behavior enhanced with the increase of MAPP amount and thepseudoplastic behavior enhanced with the increase of KH570dosage and thenon-Newtonian index firstly increased and then decreased with the TL-4dosageincreased; while DN301had little effect on the non-Newtonian index of the foamedcomposite. The interface modification and chemical bonding mechanism of the foamed compositewas studied. Microscopic structures of foamed composites before and after modification wereanalyzed using environmental scanning electron microscopy (ESEM); and molecular groupsand molecular structure of the untreated and treated bamboo powder were analyzed usingfourier transform infrared spectroscopy (FTIR); and the surface element and chemicalbonding of foamed composites were investigated by X-ray photoelectron spectroscopyanalysis (XPS) and the crystal morphology of matrix and crystallinity of composites werecompared using X-ray diffraction analysis (XRD).The maleic anhydride in the MAPP can etherified with the hydroxyl of the bamboopowder and thus the interfacial bonding force of bamboo powder and PP enhanced. Thesilanol of KH570and bamboo powder can formed-Si-O-C bonds during heating and henceinterfacial strength between bamboo powder and PP improved. TL-4with the hydroxyl of thebamboo powder had the chemical reaction and produced carboxyl and other organicfunctional groups and thus played an effective role in modifying and coating the surface ofthe bamboo powder. XRD results indicated that coupling agents did not affect the crystallinestructure of the matrix and the matrix of composites was all the α-crystalline PP; but thecrystallinity index of composites increased. The crystallinity degree of foamed compositewithout modification was21.05%; and increase to31.52%when adding9%MAPP.The third aspect was to optimize the injection molding technology and the amount ofbamboo powder by using a single factor experiments. The optimal injection parameters werethat the injection temperature of175℃-190℃, mold temperatureof90℃, injection pressureof4.5MPa-6.5MPa, injection speed of40mm/s-70mm/s. The best amount of bamboopowder was50phr, which was33wt%, for its best comprehensive performance and lowermaterial price. When the bamboo powder was50phr, adding9%MAPP and1%MAC, theaverage cell diameter of the foamed composite was81.3μm and the cell density was2.66×107cell/cm3which was close to requirement of microcellular foaming. And the densitywas0.845g·cm-3, the bending modulus of2813.31MPa, the specific flexural, tensile andnotched impact strength of47.59MPa,26.12MPa and7.22KJ·m-2respectively. Comparingto its non-foamed composite, the density decreased by16.1%and the specific strengthsenhanced by16.8%-40.2%and the heat resistance also slightly improved. At the same time,the surface wettability of foamed composites was characterized by dynamic mathematicalmodel and the influences of coupling agents and bamboo powder amounts on the surfacewettability of foamed composites were investigated by using the coefficient K. When theamount of bamboo powder was50phr and with addition of9%MAPP, the initial contact angle of foamed composite in distilled water and glycerol were88.5°and91.5°respectivelyand the declining rate of contact angle that was K value of0.0071and0.0119respectively;and the surface free energy was34.19mJ·m-2.In the third part also research the impact of the bamboo powder content on therheological behavior of bamboo powder/PP foamed composite using a rotational rheometer.The foamed composite in the low frequency exhibited "solid-like behavior" and the storagemodulus, loss modulus and complex viscosity increased significantly with the bamboopowder content increased. The loss tangent (tanδ) slightly increased when thebamboo powder amount was10phr and30phr and following gradually increased with theincrease of the bamboo powder content. Both the zero shear viscosity and theCross characteristic relaxation time (λ) of the foamed composites increased with theincrease of bamboo powder content；while the Cross rate constant and non-Newtonianindex basically showed a decreasing trend.The forth aspect was to research on the influences of three different toughening fillerssuch as talc, POE and POE-g-MAH on the physico-mechanical properties, processingperformance, rheological properties and thermal stability of foamed composites. Whenadding8%POE-g-MAH, the33wt%bamboo powder/PP foamed composite showed the bestperformance and could replace PP in the field of automotive interior parts. At this point, thedensity of toughened foaming composite was0.849g·cm-3and the specific bending strengthof45.72MPa, flexural modulus of3019.91MPa, the specific tensile strength of25.22MPaand the specific notched impact strength of10.07KJ m-2and the vicat softening pointtemperature of155.8℃; and the MI was3.69g·10min-1and the apparent viscosity was moresensitive to the shear rate; and had no effect on the thermal stability of foamed composite.The last aspect was to analyze the effects of the artificial accelerated aging and exteriornatural weathering on the bamboo powder/PP foamed composites with and without agingagents in view of color, thickeness, weight, flexural properties, tensile properties, notchedimpact strength and creep performance. And the short-term creep behavior of foamedcomposites before and after aging at25%,50%and75%stress levers was simulated withfour-component model and six-component model. By adding anti-oxidant B225and anti-UVagent UV770, the aging resistance of the foamed composite improved. However, afteraccelerated aging and natural aging for2160h, the color change△E*still were47and2.8;and the impact strength decreased by25.3%and9.2%, and the tensile strength slightlyenhanced by1.3%and6.0%respectively. The bending properties decreased by around13% after accelerated aging for2160h, and natural aging had little effect on the flexuralproperties. And after5400s creep test, the creep strain increased by1.2times; and theresidual retention of flexural performance were91.97%-97.70%and92.66%-98.87%,respectively.
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