| The microcellular foams have a new closed-cell structure whose cell diameter is between1～10μm and cell density can be larger than109cells/cm3. Compared with the unfoamed materials, the microcellular materials have higher impact strength, better toughness, higher strength, longer fatigue life, lower dielectric constant, lower thermal conductivity, and so on. As a result, microcellular foams have very broad market prospects and application values, known as the "21st century new material". Because of the good thermal stability, high mechanical properties, good resistance to degradation and chemical stability of PP foams, the other foam materials have been replaced by PP foams which have been widely used. If the microcellular PP foams can be well prepared, it will enlarge the potential applications. However, due to the inherent structural characteristics of PP, it is very difficult to control the microcellular foaming process, thus affecting the industrialization. Hence, researches on microcellular PP foams have become a very important topic in the field of plastics processing.Using supercritical CO2(scCO2) as the physical blowing agent, the carbon fibers (CFs) or bamboo fibers (BFs) was attempted to add in PP to improve the foamability. The incorporation of fibers can not only improve the viscoelasticity but also change the way of cell nucleation during the foaming process. In addition, the foamed composite will have better properties due to the addition of fibers, especially the great increment of mechanical properties, which can enlarge the potential industrial applications. The main research contents and results are summarized as follows:1First of all, the surface of CFs was pre-treated to remove the impurities and increase surface roughness which led to enhance the nucleation efficiency. In reference to the way of papermaking, uniform distribution of CFs in the CFs/PP composites was obtained and the basic properties were studied. Latter, gravimetric method was used to measure the CO2solubility in the composites with various processing conditions and different CFs content. It was found that180℃was suitable as foaming temperature using temperature-induced foaming method to prepare foamed CFs/PP composites. The effects of CFs content and the processing conditions on the cell morphology were also investigated. The results showed that heterogeneous nucleation was dominated in the foaming process which led to the improvement of foamability. Owing to the weak viscoelasticity, most of the cells showed open-structured when pressure-induced foaming method was applied. As CFs contents increased, the mean cell diameter increased and cell density decreased. The effect of processing conditions on the cell morphology of the foamed composite was also studied. When the CFs content was15wt%, the mean cell diameter of foamed composites decreased as the saturation pressure increased or the foaming temperature decreased while the cell density showed the opposite trend. It was also found that the depressurization rate had little effect on the cell morphology.2In order to improve the adhesion between BFs and PP, the surface of BFs was firstly alkali pre-treated to remove the impurities and weaken its hydrophilicity. The BFs/PP composite was prepared in reference to the way of papermaking. Gravimetric method was also used to measure the CO2solubility in the composite and the effects of processing conditions and BFs contents on the CO2solubility were investigated. When the BFs/PP composite was foamed using temperature-induced foaming method, there were few cells in the foamed composite which may be ascribed to the low nucleation efficiency of BFs and poor heat transfer performance. However, using pressure-induced method, the foamed BFs/PP composites with good cell morphology could be obtained. The effect of processing conditions on the cell morphology was also studied.3The properties of foamed materials strongly depend on the cell parameters, especially the mechanical properties. In order to improve the performances of the foamed materials, some nano-CaCO3was added to serve as nucleating agent in the CFs(BFs)/PP composites. The nano-CaCO3was firstly treated to inhibit the aggeration. When the nano-CaCO3/CFs/PP nano-composite was foamed using temperature-induced or pressure-induced foaming method, nano-CaCO3can serve well as the heterogeneous nucleating agent to reduce the energy barrier for cell nucleation. As a result, the mean cell diameter of the foamed nano-composite decreased and cell density increased compared with that of the foamed composite. When the nano-CaCC>3was2.9wt%, the foamed nano-composite had the minimum mean cell diameter and the largest cell density under the same foaming processing conditions using temperature-induced method. Meantime, when the saturation pressure was15MPa and foaming temperature was160℃, the foamed nano-composite with4.8wt%nano-CaCO3had the minimum mean cell diameter and the largest cell density using pressure-induced foaming method. When the temperature-induced foaming method was used to foam the nano-CaCO3/BFs/PP nano-composite, it was strange that there were no regular cells, only appearing some big cells or holes, no matter how to change the nano-CaCO3content and foaming processing conditions. However, the foamed nano-CaCO3/BFs/PP nano-composites with good cell morphology were obtained using pressure-induced foaming method because nano-CaCO3can sever as as nucleating agent well. In addition, the effects of processing conditions on the cell morphology of the foamed nano-CaCO3/CFs(BFs)/PP nano-composites were also studied.4In the final part of this thesis, the CFs/PP composite and nano-CaCO3/CFs/PP nano-composite were foamed using temperature-induced foaming method in the constrained mould. Owing to the presence of constrained mould, the traditional cell model or island model could not be applied when the cell grew. The results showed that due to the weak melt viscoelasticity of the samples, compared with the cell parameters of free foaming, the mean cell diameter in the constrained condition became very larger and cell density very lower. Moreover, the relationship between the unnotched impact strengths, thermal properties and the cell morphology of the foamed samples in the constrained condition were investigated. The resulted showed the unnotched impact strength strongly relied on the cell parameters. The smaller the mean cell diameter and the larger cell density, the higher the unnotched impact strength was. When the cell diameter was large, it would become the stress concentration point during the impact process which caused the decrease of unnotched impact strength. Compared with the unfoamed, the crystalline of foamed composite in the constrained condition became a little higher. This is because during the constrained foaming process, the shear force of growing cells acted on the molecular chain around the cells which led to the orientation of the molecular chain. As a result, the regularity of the molecular chain was improved and the crystalline became larger. |
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