Study On Preparation And Properties Of Polyvinyl Alcohol Foams

Polyvinyl alcohol?PVA?is a biodegradable material obtained by alcoholysis of polyvinyl acetate?PVAc?,and has wide application in various fields due to its excellent properties.Supercritical carbon dioxide?scCO2?is an environmentally friendly and inexpensive foaming agent,so the application of supercritical foaming technology to polyvinyl alcohol is in line with the concept of green environmental protection.In this paper,PVA foam was prepared by compression foaming method and the advantages and disadvantages of preparing PVA foam by solution and thermoplastic processing method were compared.In order to further improve the foaming performance of the material and obtain PVA foam with high foaming ratio and stable cell structure,PVA/SiO2 nanocomposites and PVA/PLA blend foams were preared and the effects of different parameters on the foaming behavior and mechanical properties of material were systematically studied.In the first part of the study?chapter 2?,PVA foam precursor was prepared by aqueous solution method and thermoplastic processing method respectively.PVA foam was prepared by mould foaming method with supercritical CO2 as foaming agent..The effects of processing parameters?foaming temperature and saturation pressure?on the foaming behavior of PVA foam were studied.The solution method mainly took advantage of the excellent water solubility of PVA and PVA hydrogel was prepared by physical crosslinking method.After the process of oven drying and alcohol drying,the density of PVA dry gel ranging from 1.150 g/cm³ to 1.170 g/cm³ was obtained.Thermoplastic processing method used glycerin and ethylene glycol as the compound polyol plasticizers to destroy the intra-and inter-molecular hydrogen bonding within PVA,which resulted in a wider melt processing window.Compared with thermoplastic processing method,PVA foam prepared by the solution method and mould foaming method had a higher cell density,which may be due to the lower foaming temperature and the increase of the solubility of CO₂ in the system.However,the solution method was time conuming and water was unstable in the system,and foaming performance was also relatively poor.In the second part of the study?Chapter 3?,nano-silica was introduced to prepare the PVA/SiO2 nanocomposite foam based on the first chapter of thermoplastic processing method.TG and SEM results showed that the prepared SiO₂ with a hydroxyl group content of 5.35 wt%had a mean diameter of about 150 nm and exhibited excellent uniformity without obvious agglomeration.PVA/SiO2 nanocomposite foams had smaller average cell size and larger cell density and relative density than the PVA foams under the same foaming conditions.A low foaming temperature and a high saturation pressure contributed to high cell density.SiO₂ had a reinforcing effect on PVA which was confirmed by both tensile and DMA tests.It was found that Tg shifted toward higher temperatures and the peak height of tan?lowered with the incorporation of SiO₂.Wide angle X-ray diffraction results showed that the addition of nano silica enhanced the degree of orientation of PVA crystals in the flow direction but reduced the crystallinity.In order to further improve the foaming performance of PVA,the PVA/PLA blend foam was prepared by melt blending in the third part of the study?Chapter 4?.The results of DMA and FTIR indicated that the compatibility of the blend was good and the reason may be that the addition of stannous octoate caused the esterification reaction of PVA and PLA to change the blend from phase separation to compatibility.When the mass of stannous octoate exceeded 1 g,the foaming performance of the blend was excellent.This may be due to the esterification reaction of PVA and PLA when the stannous octoate exceeded 1 g,which improved the foaming performances of the blend.The tensile results showed that the tensile strength of the PVA/PLA blend was almost constant and the elongation at break was almost doubled compared to PVA.For blend foams with different relative densities,as the relative density decreased,the tensile strength and elongation at break of the blend foam decreased.