Preparation of clay-supported sodium benzoate and its effect on the rheology, crystallization and foaming of long chain branched polypropylene

The foaming of PP has encountered challenges because of its low melt strength and semi-crystalline characteristics. Introducing long chain branching (LCB) onto the PP backbone and the addition of particles as cell nucleating agent are the commonly used methods to improve the cell structure of PP foams. In this paper, clay-supported sodium benzoate (NaB-OMMT) was prepared from Na-montmorillonite (Na-MMT) via the partial ion exchange reaction with surfactant and subsequent reaction with benzoic acid. The as prepared NaB-OMMTs were then applied to prepare LCBPP/NaB-OMMT nanocomposites and their foams. The dispersion state of clay, crystallization behaviors and rheological properties of the nanocomposites, as well as the cell structure and compression properties of the foams were investigated. The results show that clay layers are mostly exfoliated in the nanocomposites with low NaB-OMMT content, while intercalated layers dominant with the content reaches 2% or higher. The melting temperature of the nanocomposites decreases and the crystallinity increases, while the spherulite size decreases compared to LCBPP. The melt viscosity and elasticity of nanocomposites increase significantly when the content of NaB-OMMT reaches 2% or higher. LCBPP/NaB-OMMT nanocomposites foams exhibit higher cell density, smaller cell size and its standard deviation. Therein, the foam with 1% of NaB-OMMT shows the maximum cell density and the minimum cell size. It is concluded that the cell structure of the foam is highly dependent on the content, dispersion state and structure of the cell nucleating agent. When NaB is supported on well dispersed and exfoliated clay layers, the cell nucleation efficiency is significantly enhanced. The compression properties of the foams depend on their cell structures and exhibit the similar variation trend with the latter with the content of NaB-OMMT increasing.