Effect of β-nucleating agent and zeolite molecular sieves on supercritical CO2-assisted cell and crystal nucleation during the long-chain-branched polypropylene foaming

It is of great interest and importance to design and fabricate the double crystal melting peak (DCMP) structure for the expanded polypropylene (EPP) bead foams during sintering. Two kinds of methods with different mechanisms were proposed to prepare long-chain-branched polypropylene (LCBPP) samples with a DCMP structure: One was that the DCMP structure was prepared by generating crystals of different perfection degrees and the other one was that the DCMP structure was prepared by adding beta-NA to produce various crystal forms. In addition, zeolite molecular sieve (ZMS) as the synergist was introduced into LCBPP/beta-NA system to increase the efficiency of beta-NA. The crystallization behaviors of various PP and their foams were tested by differential scanning calorimetry and wide-angle X-ray diffraction, as well as the cellular morphologies of various LCBPP foams were observed by scanning electron microscope. The results showed that a DCMP structure was formed in pure LCBPP in the presence of CO2 at 168-172 degrees C. The content of beta-crystal in LCBPP/beta-NA system was the highest when the content of beta-NA optimized as 1.5 phr. The percentage of beta-crystal in LCBPP/beta-NA system was further increased when a small content (0.045 phr) of ZMS was added. Compared with that in unfoamed LCBPP, the percentage of beta-crystal in LCBPP foams was obviously enhanced, due to the coupling effect of crystal nucleation and cell nucleation. In the LCBPP foams, the cell size became smaller and the cell density became larger, owing to the addition of ZMS. (c) 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 48183.