The Preparation Of LDPE/PVA Blends With Moisture Absorption Function By Melting Mixing

In this thesis, the Differential Scanning Calorimetry (DSC) and Thermo Gravimetric Analysis instrument (TGA) were used to analysis the thermal stability of LDPE / PVA blends. Then the hygroscopicity, structure and mechanical properties of blends were studied by using Scanning Electron Microscopy (SEM), Electronic Universal Testing machine and Constant Temperature and Humidity Testing machine.Because of its high alcohol degrees, PVA1799 contains a lot of hydroxyls (- OH) and side-(- OH) which are relatively small and easy to form hydrogen bonds and achieve a higher degree of crystallization. So those substances with small molecules, such as glycerin, plasticizers, are difficult to insert into PVA and achieve melting processing. While PVA1788 has low alcohol degrees, there are many types of acetate which has increased the distance between PVA molecules. This is to say, the hydrogen bonds between molecules are weakened and PVA1788 is easy to achieve melting processing by blend with some plasticizers. It is found that the blend PVA-MB with PVA1788 /plasticizer / stabilizer (100/30/2) can be melted easily and then the blends LDPE/PVA with different contents of PVA can be prepared by melting processing under a HAAKE mixer.The morphology of the blends is observed by SEM, which indicated that lots of larger dispersed phase droplets with the size about 20μm can be found due to the poor compatibility of LDPE and PVA. LDPE-g-MAH can effectively improve the compatibility of LDPE and PVA, and decreases the size of PVA droplets size to 2μm. As a result, the mechanical properties of blends are improved by presence of the compatilizer LDPE-g-MAH.The moisture absorption capability of the blends is enhanced by adding PVA, and with the increasing of PVA, the moisture absorption rate of blends increase. On the hand, the moisture absorption rate of blends will be reduced by adding LDPE-g-MAH compatibilizer. It seems that the PVA particles dispersed into the LDPE matrix would be coated by LDPE-g-MAH compatibilizer, and then the moisture absorption function would be affected. The moisture absorption rate of blends is very fast in the first 12 h. With the extension of the time, the moisture absorption rate of blends decreases gradually.The hygroscopicity of blends could be greatly affected by humidity environment. With the increasing of humidity environment, short-term moisture absorption capability of blends doesn't change very much, while the long-term moisture absorption capability enhances more. As the film materials are very thin, the water molecules are easy to access the inside of composite membrane materials. So the particles of PVA can absorb moisture evenly. But the PVA particles which absorb moisture enough are easy to shed from the LDPE matrix and show a smooth interface of the two phases while broken in the liquid nitrogen. Because of the pervasion of water molecules, the interface strength between the PVA particles and matrix is spoiled. So the overall strength of blends decreases.