Preparation of ultra-high cell density polypropylene foam

The excellent mechanical properties of high cell density polypropylene foam such as tensile strength and high modulus are the reason for its applications in various fields like heat and sound insulations, automotive and electronic packaging. Usually, polypropylene (PP) foam is prepared at low cell densities and with a broad range of cell size distributions. Moreover, the foaming process of most polypropylene still experiences several difficulties in achieving the desired properties. For example, the nonuniform cell distribution and large cell sizes in foamed PP are not preferred for good mechanical properties due to poor nucleation density.

As a result, various approaches for increasing the nucleation density during the foaming process have been developed. For instance, the addition of nanoparticles can improve the nucleation of PP during the foaming process thereby leading to the formation of high cell density. However, the high cost of using graphene, carbon nanofibers and carbon nanotubes in supercritical carbon dioxide makes it unsuitable for high scale production. This has led to the recent development of virgin polytetrafluoroethylene (PTFE) powder as a nucleation agent for polymer foams. It has good thermal stability and high dispersion property.

Recently, a team of researchers at Shanghai Institute of Applied Physics led by ProfessorGuozhongWu developed a novel and effective method for preparation of ultra-high cell density PP foam by supercritical carbon dioxide (_scCO₂) foaming. The synthesis is done in the presence of low molecular weight PTFE micropowder with nanoscale voids. Variation in the amount of PTFE added to the PP was used to control the cell density. This was in a bid to investigate the cellular morphologies and mechanical properties of the PP and PP/PTFE foam materials. Furthermore, the authors analyzed the foaming behavior of PP/PTFE and the effects induced by PTFE microparticles. Their work is published in the research journal, Industrial and Engineering Chemistry Research.

The authors observed a significant increase in the cell density in the prepared foam as compared to the pristine PP foam. This was attributed to the formation of an extensive number of nucleation sites due to the addition of the PTFE containing voids. The voids were filled with carbon dioxide that resulted to expansion and pressure release making them split into several granules. Consequently, properties of the PP/PTFE foam such as tensile strength and sound absorption were improved. Also, it was possible to control the preparation process of PP/PTFE foam over a wide range of temperatures.

The voids of PTFE microparticleswere filled with _scCO₂, causing them to split into multiple granules due to the force of its expansion during the pressure release, resulting in the formation of a huge number of nucleation sites.The cell density of this foam reached 10¹⁰～10¹¹ cells/cm³, which was 2 to 5 orders of magnitude higher than the pristine PP foam and the cell size declined significantly (from ~70 μm to 7.6 μm) as the loading of PTFE micropowder increased.

In the saturation process, CO₂ can enter into these voids at the preset pressure and temperature. As CO₂ diffuses into the mixed phases and reaches a saturated equilibrium state, the void channels of PTFE microparticles are also filled with pressurized CO₂.The pressure drops rapidly when the saturated CO₂ is released from the void channels of PTFE microparticles, causing nucleation. The low molecular weight PTFE microparticles were split into many nanoscale granules by the force of CO2 expansion, and the nanoscale granules reduced the energy barrier to cell nucleation. The appearance of these nucleation sites then triggered the generation of cells via explosive dispersion and nucleation. During this process, the PTFE microparticles were divided into smaller granules, each smaller granule may act as a nucleation site during the foaming process.

“As the environmentally-friendly or recyclable material, PP foam has attracted more attention to improve the acoustic property, it shows the promise as the substitute of polyurethane foam, which is commonly used as sound insulators. The high cell density of PP/PTFE foam indicated more cell walls existing, which largely increased the vibration, dissipation of the cell walls and the number of sound wave reflections, led to an increase in the absorption coefficient. In addition, the increased number of air interlayers between inside cavities also enhanced the absorption efficiency of PP foam. The PP/PTFE(5.0) foam possessed the maximum noise reduction coefficient(NRC) of 0.59, which was almost twice that of the neat PP foam, and the value of NRC was very close to the commonly used acoustic polyurethane (PU) foam”. Said Professor Guozhong Wu, the senior author.

Generally, the properties of the PP/PTFE foams depended on the cell size, cell density and volume density. On the other hand, high-cell-density PP/PTFE foam can be prepared under different foaming pressures and temperatures. For instance, an increase in foaming temperature increased the cell growth rates and expansion ratios. Consequently, it also resulted in a decrease in the tensile stress. This was due to the parameter morphology of the foamed samples like the volume density. According to the authors, PP/PTFE blend is associated with a broad foaming window and will, therefore, advance its use in various applications and more so the possibility of its application in the extrusion foaming processes.