Fabrication of High Expansion Microcellular Injection-Molded Polypropylene Foams by Adding Long-Chain Branches

Abstract

Herein, 10-fold expansion ratio foams were successfully fabricated for the first time by a core-back foam injection molding (FIM) technique using a newly developed long-chain branching polypropylene (LCBPP). Different rheological curves were investigated to distinguish the strain hardening behavior and melt strength of the LCBPP from isotactic polypropylene (iPP). Fast scanning chip calorimetry (FSC) results revealed that the long-chain branches could increase the crystallization temperature even at fast cooling rates. The FIM of LCBPP could notably not only reduce the cell sizes but also increase the cell densities through the capability of LCBPP to stabilize cells and prevent cells from coalescence. Furthermore, the foam processing window of manufacturing 10-fold expansion foams was significantly broadened for LCBPP foams. This study revealed that the introduction of longchain branches is a good approach to fabricate high expansion microcellular foams for different potential applications such as construction and transportation.

Evolution of cellular morphologies and crystalline structures in high-expansion isotactic polypropylene/cellulose nanofiber nanocomposite foams

“…1b, relatively large cells inevitably appeared in the core layer of the pure iPP foams due to its low melt elasticity and poor melt strength. 22,32,33,43 The calculated average cell size and cell density of the 2-fold iPP foams were approximately 52 mm and 3.8 Â 10 6 cells per cm 3 , respectively, which was consistent with the results of our previous work. reaching an increase of approximately 120 times compared with that of neat iPP.…”

“…2a, cells in the 5-fold expansion iPP foams became elliptical and deformed along the core-back direction, which was attributed to the extensional stress induced during the mold-opening operation. 32,33,43 With increasing expansion ratio, the cells were further deformed and oriented along the coreback direction. The cells were greatly stretched in the 10-fold iPP foams.…”

“…This secondary nucleation was due to the rupture of the thinning cell wall during the extended stretching process. 22,32,33 Interestingly this core-back operation could transform the closed cells of the low-expansion foams into open cells in the high-expansion foams.…”

“…Owing to the weak melt strength of iPP, several approaches such as cross-linking, 34 branching, 28,32,35 blending, 23 and nanoparticle addition 22,24,33,36 have been investigated as ways to modify iPP to possess adequate melt strength and foaming processability. Cellulose nanober (CNF), 37 which is derived from abundant and renewable resources, has highly promising applications as a cell nucleating agent and a reinforcing agent for plastic foams.…”

“…The details of core-back FIM operation have been described previously. 26,32,43 The optimum dwelling time (coreback timing) was individually obtained for the neat iPP and PP/CNF composites for the studied processing conditions listed in Table 1.…”