| Microcellular polymer is a new kind of macromolecule material with the cell diameter of 10μm or less and the cell density of more than 108 cm-3. Compared with dense polymers, microcellular foams provide a substantial reduction in weight combined with superior mechanical properties and good fatigue life. Therefore, such materials are extensively used on many purposes and hailed as"novel materials for the 21st century". Many specialists are greatly interesting in developing environmentally friendly physical blowing agents, for foaming agents such as chlorofluorocarbons and hydrochlorofluorocarbons cause destruction of the ozone layer in the atmosphere. Supercritical carbon dioxide (scCO2) is a green blowing agent with non-toxicity and non-contamination. There are some remarkable advantages using scCO2 as a blowing agent such as inexpensiveness, broad resource, recycle, high foaming efficiency and so on. ScCO2 is regarded as one of the most potential replacements of the blowing agents such as chlorofluorocarbons. It is highlighted in the field of microcellular polymer.In this paper, the microcellular cross-linked LDPE (x-LDPE) foams were successfully prepared by a combination of radiation cross-linking and the scCO2 foaming process. Two processing protocols were used to prepare x-LDPE sheets: (1) irradiation followed by screw extrusion and hot pressing (SA), and (2) hot pressing followed by irradiation (SB). The x-LDPE sheets were foamed by using scCO2 as a blowing agent at a batch scale. Melt temperature, crystallization, gel content of x-LDPE sheets and cell morphology, volume expansion ratio of x-LDPE foams had been characterized by some testing methods, such as differential scanning calorimeter (DSC), X-ray diffraction (XRD), Fourier transform infrared spectrometer (FT-IR) and scanning electron microscopy (SEM).The effects ofγ-irradiation on melt temperature, crystallization and gel content of LDPE were studied. The foaming behavior difference between cell size distribution, cell size, cell density and volume expansion ratio of SA and SB sheets was confirmed. The influences of radiation dose, processing order of x-LDPE sheets, foaming temperature and pressure on morphology of the x-LDPE foams were investigated systemly, too. The results indicated that SA and SB sheets possessed the molecular chain backbone and the crystalline structures of LDPE. Irradiation made the molecules of LDPE mainly cross-link under the limited oxide condition. The cross-linking degree increased and the crystallinity decreased in LDPE with increasing radiation dose. The foaming behavior of the SA sheets was different from that of the SB sheets, namely the SA foams had larger cell size, lower cell density, higher volume expansion ratio and higher expansion efficiency under the same conditions. A continuous manufacture about x-LDPE foam may be actualized by the SA method. At 50 kGy, cells of x-LDPE foams were finest and the volume expansion ratio was highest. At 100°C, 50 kGy and 22MPa, the cell size of x-LDPE foam was less than 10μm, and the value of cell density was more than 109cm-3. Furthermore, The thermal stability of the x-LDPE was significantly improved and the foaming temperature range wided in comparison with LDPE. |
