Microcellular Foaming Of Isocvanate Functionalized Polvpropvlene And Its Composites With Graphene Oxide

Polypropylene (PP) has a wide application prospect and market demand in fields such as the household, package, transportation, and architecture due to its excellent mechanical and thermal properties. However, PP as a crystalline polymer is very difficult to foaming because when the foaming temperature is lower than its melting temperature, it is solid so that it cannot be expanded; while once above its melting temperature, its melting strength decreases dramatically so that the cells will coalesce and rupture. These two reasons result in a narrow window of foaming temperature. Therefore, the current researches on the preparation of PP microcellular foam via supercritical carbon dioxide are mainly focused on how to improve its foaming behavior.This thesis improved the foaming behavior of PP by the following methods:one is to improve its crystallization and melting behaviors by grafting isocyanate functional groups and graphene oxide (GO) onto PP chain, the other is to enhance the solubility of carbon dioxide (CO2) in PP via mixing with polydimethylsiloxane (PDMS). Moreover, the effects of those factors on the foaming behavior of PP were investigated. Based on the above studies, some results were obtained as below.(1) 3-isopropenyl-α, α-dimethylbenzene isocyanante (TMI) was grafted on PP through melting grafting reaction, which can increase the crystal density and decrease the crystal size. The formed crystal can not only act as a strong heterogeneous nucleation for PP foaming but also hinder the growth of bubbles so that a microcellular PP foam with average cell diameter of 8.7 μm, and cell density of 7.6× 109 cells/cm3 can obtained under 120 ℃ and 20 MPa. Moreover, the diffusion coefficient of CO2 in PP matrix was low due to the polarity effect between NCO and CO2, which widened the foaming temperature to 160 ℃.(2) Using the reaction between hydroxy in GO and NCO function group in TMI, GO was grafted onto PP-g-TMI to form the PP-g-TMI/GO composite via a solution mixing method. The introduction of GO could further improve foaming behavior of PP-g-TMI due to an increase in the crystal density and decrease in the crystal size. For example, the average cell size dropped from 8.7 μm to 5.6 μm after 0.5 wt% GO was added into PP-g-TMI. Moreover, the cell size would have a further drop along with the enhancement of GO concentration. Besides, PP-g-TMI could be foamed at a much higher temperature of 170℃, which was attributed to the relative high melting strength resulting from the combined effect of GO and PP-g-TMI.(3) PP-g-TMI can act as an efficient compatibilizer to decrease the PDMS dispersed phase size in PP matrix so that the PP matrix has a uniform CO2 concentration which is benefit for the acquisition of microcellular foam with small and uniform cells. The foaming temperature can broaden to 170℃ due to the high CO2 solubility. Additionally, the expansion ration will be as high as 20 times when the weight fraction of PDMS increases to 22%.