A Study On The Application Of Thermo-expandable Microspheres In Foaming Of Low Density Polyethylene

Thermo-expandable microspheres are kind of core-shell materials comprising a thermoplastic polymer shell encapsulating a propellant. The propellant is normally a liquid having a boiling temperature not higher than the softening temperature of the thermoplastic polymer shell. With its excellent dimensional stability and elastic properties, thermo-expandable microspheres are used as a foaming agent in many different fields, such as thermoplastic resin, thermosetting resin, underbody coating, printing ink, adhesive, paper and explosives. There are many kinds of commercial available microsphere products with different particle sizes and expandable temperatures for various application requirements.For developing new application of microspheres and constructing special pore structure, we select LDPE as polymer matrix and Expancel, a commercial thermo- expandable microsphere, as blowing agent. We investigated the expanding behavior of Expancel in LDPE compounds and the cellular properties of the LDPE foams produced, and studied the blowing behavior of Expancel combining with chemical blowing agent AC, H and OBSH in LDPE respectively.The components and morphology of Expancel were qualitatively and quantitatively characterized by DSC, TGA, FTIR and SEM. The results show that the shell of Expancel 951DU120 is mainly made of acrylonitrile copolymer, Expansion of Expancel starts at 136℃,maximum expansion is reached at 208℃,and about 60% alkanes with low boiling point losses when the temperature is over 280℃. The diameter of expanded microspheres could reach 110μm, and the volume is 50 times larger than unexpanded particles. These physical properties allow hot-mixing Expancel with LDPE and the following foaming processing.The study of LDPE/Expancel foaming shows that, the effect of temperature on the expansion ratio of LDPE foam is not obvious at a wide temperature range(170-200℃). The LDPE foam density is about 0.2g/cm3 with addition of 10phr Expancel. The LDPE foam is constructed by countless spherical shells dispersing in LDPE matrix. Expansion ratio largely depends on cross-linking degree of LDPE and Expancel contents. The actual density of LDPE foams matches the theoretical calculation, so we can calculate the density of LDPE foams according to Expancel amount used.In the study of the blowing behavior of Expancel combining with chemical blowing agent in LDPE, the results show that, the blowing behavior of Expancel and chemical foaming agent is independent, and the cellular structure depends on the ratio of Expancel/chemical foaming agent. LDPE formed embedded pore structure when Expancel content is more than chemical blowing agent one.In this paper, we combined thermo-expandable microspheres with chemical foaming agent for the first time to produce composite cellular structure in polymer foams, with chemical blown small pores embedded with expanded microspheres. This structural characteristics provides a new method for producing polymer foams with specific cellular structure.