A New Promising Nucleating Agent for Polymer Foaming: Applications of Ordered Mesoporous Silica Particles in Polymethyl Methacrylate Supercritical Carbon Dioxide Microcellular Foaming

Abstract

The introduction of inorganic particles to improve the cell morphology of polymeric foams has been studied for decades. To this end, identifying an ideal nucleating agent and understanding the methodology to control nucleation have always been the focus of this field. In this study, spherical ordered mesoporous silica (OMS) particles were synthesized and applied as a potential nucleating agent in polymethyl methacrylate (PMMA) supercritical carbon dioxide (scCO2) microcellular foaming. These particles were modified with a silane containing fluorine to enhance their affinity with scCO2. For comparison, solid silica (SS) particles with almost similar particle size and surface treatment have also been studied. It was found that both of them could be well dispersed in the PMMA matrix, and exhibited excellent heterogeneous nucleation performance during the foaming process. The addition of a nucleating agent greatly increased the cell density and decreased the average cell diameter, and more importantly, there was no increase in the bulk density. Compared to SS particles, OMS particles showed higher nucleation efficiency. The addition of 5.0 wt % of OMS particles reduced the average cell diameter of PMMA foam from 1.62 to 0.66 μm and increased the cell density from 2.3 × 1011 cells/cm3 to 3.7 × 1012 cells/cm3, while for the composite foams with a similar content of SS particles, the cell diameter and cell density are 1.19 μm and 6.12 × 1011 cells/cm3, respectively. We speculated that the mesoporous structure of OMS particles might trap the CO2 forming gas cavities. The pre-existing gas cavities resulted in a lower nucleation energy barrier. The superior nucleation effect of the OMS particles became more significant when the foaming process was conducted at low foaming temperature, low saturation pressure, and/or high pressure drop rate.

PMMA-sepiolite nanocomposites as new promising materials for the production of nanocellular polymers

“…The pressure drop rates in these experiments were 15 MPa/s, 56 MPa/s and 100 MPa/s, respectively. For this study, foaming has been [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] carried out at 80°C during 2 min. Secondly the effect of the foaming temperature has been analysed by fixing the saturation pressure to 10 MPa, varying the foaming temperature from 80°C to 110°C, keeping, once again, 2 min as foaming time.…”

“…This trend is a consequence of the plasticization effect of the CO 2 : when the gas diffuses into a polymer, the glass transition temperature drops [49][50][51][52], and the polymer is now characterized by its effective glass transition temperature, T g,eff . The temperature difference between the foaming temperature and the T g,eff determines the relative [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] density obtained, because once the T g,eff reaches the temperature of the thermal bath the polymer is no longer in the rubbery state and has no mobility to grow. At a higher pressure, the T g,eff of the polymer is smaller because of the plasticization effect of a higher amount of CO 2 .…”

“…A wide variety of polymeric matrices and nanoparticles have been used for this purpose. Spherical nanosilicas [22][23][24] and layered montmorillonite-type nanoclays [18,25,26] have been proved to be suitable nucleating agents in different polymeric matrices. For instance, in the work of Siripurapu et al [27] ultraporous PMMA films were produced using silica particles.…”

“…A 12 wt% of these particles led to a reduction in the cell size from 10 to almost 1 μm. Yang and coworkers [24] used spherical ordered mesoporous silica particles to increase the cell density in PMMA. The addition of a 5 wt% of these particles reduced the pore size from the micro (1.62 μm) to the nanometric range (660 nm).…”

A new promising nucleating agent for polymer foaming: effects of hollow molecular-sieve particles on polypropylene supercritical CO2 microcellular foaming

“…Besides their excellent properties, the reduction of raw material cost is a signicant advantage in using these materials. [1][2][3] Polypropylene (PP) foam is one of the most promising materials because of its outstanding overall performance, such as high toughness, low electrical conductivity, excellent chemical resistance, easy recycling, and high thermal stability for a variety of applications. [4][5][6][7][8][9][10] However, owing to its weak melt strength and melt elasticity, the properties of the resulting PP foams are poor.…”