Radiation Modified LDPE/EVA Blend And Its Foamiability Using Supercritical Carbon Dioxide

Microcellular polymer are famed as"novel materials for the 21st century", which exhibits many advantages over traditional foams and unfoamed polymers such as lower lightweight, higher thermal stability, superior mechanical properties and good fatigue life.It is widely used in various fields. However, the industrial production for the foaming of low density polyethylene(LDPE)/ethylene vinyl acetate copolymer(EVA) blends is application of chemical foaming agent and chemical crosslinking agents with nonhomogeneous cell structure, foaming agent residues and the environment unfriendly and so on. In this paper, an environment-friendly supercritical carbon dioxide foaming techniques integration with 60Coγ-ray irradiated LDPE/EVA blends are used. This paper sudied on the radiation effects on the structure and properties of the LDPE/EVA blends as well as the foaming behavior of the blend. The foaming process factors effects on the morphology of the LDPE/EVA blends foams also be researched in detail. The results are summarized as follows.The effect ofγ-ray irradiation on the structure and mechanical properties of the LDPE/EVA blends was studied. The samples were characterized by gel measurement, Differential Scanning Calorimetry (DSC), advanced rheology expand system(ARES),thermal gravimetric analysis(TG),Fourier Transform Infrared spectroscopy (FT-IR) .The results showed that, the gel content of the blend increased with an increase in radiation dose. Increase in gel content in blends was enhanced by increasing EVA content. The melting point and the crystallinity of LDPE/EVA blend slightly decreased after irradiation. The viscosity of the crosslinked blend was also improved and was higher than the original blend. The thermal stability of the blend increased. However, the radiation crosslinking had a little influence on the main chemical structure of the blend.The effect ofγ-ray irradiation on the morphology of the LDPE/EVA blends was studied. Cell morphology and volume expansion ratio of the blends foam were characterized by scanning electron microscopy (SEM) as well as other testing methods. The results showed that, the crosslinked LDPE/EVA blend foams had better morphology than the uncrosslinked LDPE/EVA blend foams. Crosslinking also widened the foaming temperature range in which a stable and good cell structure could be produced at 130 oC. A smaller average cell size and higher cell density foams could be obtained at appropritate degree of crosslinking; otherwise, cells with bimodal cell size distributions would be generated in the excessive crosslinking. In particular, when EVA content was 10 %, the blend foams had the smallest average cell size. When the addition of EVA content was up to 30%, the foam volume expansion reached the maximum.The effect of the foaming process factors on the morphology of the LDPE/EVA blends was studied. The results showed that the cell morphology of a foamed blend was significantly impacted by foaming temperature and saturation pressure. A lower foaming temperature and a higher saturation pressure were more favorable for leading to a decrease in cell size and an increase in cell density. Different cell size of microcellular polymer could be produeced by optimizing the technological parameters of the foaming conditions. At 105oC and 22 MPa, the value of cell density was more than 10⁸ cm^-3.