| Polylactic acid (PLA) foam is a new and versatile material, which consists of a matrix phase of biodegradable PLA resin and a dispersion phase of gas. Since it has many advantages, such as excellent heat insulation property, high impact resistance, low density, and biodegradability, PLA foam has been widely used in food packaging, biomedical device, and transporting fields. The developments of PLA foams will efficiently solve the problems of a continuous increase in oil price and the environmental concerns about the use of common petroleum-based plastics today. Despite PLA foam has attractive properties and brightness future, the prepareation of high performance PLA foam is difficult due to the poor melt elasticity and the sensitivity to hydrolytic and thermal degradation during foaming process. These factors limit the possibility of manufacturing PLA foams.In order to prepare high performance PLA foam, in this thesis, PLA molecular structure modification, nano-particle filling, and polymer blending methods were carried out. The inorganic particles/polymer interface and polymer/polymer interface were also induced into PLA matrix through these methods. The effects of interface morphology on the crystallization, rheological properties, and cellular structure of PLA were systematically studied. The main achievements and conclusions are shown as following:1. Using a multi-epoxy group compound as chain extender for common PLA, a series of branched structure PLAs was prepared through reactive extrusion. The molecular structure, crystallization, rheological, and cellular structure were characterized. And the results showed that the branching degree of PLA was greatly depended on chain extender content. Subsequently, the crystallization and rheological properties were then affected by the inducing of branched structure. Branched PLA exhibited a faster crystallization rate, a higher crystallization degree, and improved melt elasticity than that of linear PLA. Hereafter, the linear and branched PLAs were foamed through solid-state foaming under130℃by using supercritical CO2as blowing agent. The expansion ratio for branched PLA could reach10.63, which much higher than that of4.28for the linear PLA. The cell density was also increased from3.21×106cell/cm3to7.36×107cell/cm3with the incrasing content of chain extender. Because the branched structure would prevent the cell collapse and maintain the stability during cell growth, which enabled the branched PLA more suitable for foaming purpose.2. While branching structure was formed by adding chain extender, various nanoclay contents were then added into an optimal branched PLA sample. The organoclay was organics-treated by an organic modifier containing quaternary ammonium ion. Competitive reactions would occur among PLA, chain extender, and the organic modifier, which resulted a slightly decrase in molecular weight of chain extended PLA. With the dispersion of nanoclay, the PLA chains were intercalated into nanoclay layers and held by their platelets, which acted as physically tangled knots. This structure endowed PLA with enhanced melt elasticity. Consequently, the expansion ratio of PLA/nanoclay foam increased to11.53, which higher than8.65for PLA foam without adding nanoclay. Due to geometric property of nanoclay with enormous special surface area, a mount of inorganic particles/polymer interface was induced into PLA matrix. In this thesis, the effect of interface morphology on the cellular structure of PLA/nanoclay foams was studied. The cells nucleated much more easily in the PLA/nanoclay interface, thus the amount of cell nucleating points greatly increased with the increasing of interface area. The cell density could reach7.35×108cell/cm3by inducing the nanoclay.3. In order to induce polymer/polymer interface into foaming PLA matrix, PL A/poly (ethylene terephthalateco-1,4-cylclohexylenedimethylene terephthalate)(PLA/PETG) blends were prepared by melt mixing method. A epoxy reactive compatibilizer was used to control the interface morphology of PLA/PETG blends. The effect of interface morphology on the cellular structure of PLA/PETG foams was researched. The results showed that the activity energy of cell nucleation was much lower in the PLA/PETG interface than that of in a homogeneous PLA matrix. The value of cell density associated with the cell nucleation efficiency was shifted to over108cell/cm3decade, indicating a positive effect of the existing of polymer/polymer interface on the cellular structure.4. Since branched structure and polymer/polymer interface could improve the foamability and cellular morphology, maleic anhydride grafted polypropylene (MAPP) was induced into PLA matrix to prepare PLA/MAPP blends. Maleic anhydride groups on MAPP would react with hydroxyl groups on PLA chains, thus a branched structure was then formed. Due to the thermal incompatibility of the branched polymer and the PLA matrix, dispersion/matrix morphology with enormous interface area was also formed. The results showed that the branched structure could prevent cell collapse during foaming, thus increased the expansion ratio. And the enormous interface of PLA/MAPP blends tended to increase cell nucleation mount, which led to a fine cellular structure with higher cell density. |
PDF Full Text Request