Mechanical and Acoustic Performance of Compression- Molded Open-Cell Polypropylene Foams Joe D. McRae, 1 Hani E. Naguib, 1 Noureddine Atalla 2 1 Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, Canada 2 Groupe d’Acoustique et de Vibrations de l’Universite ´ de Sherbrooke, Canada Received 17 March 2009; accepted 11 October 2009 DOI 10.1002/app.31581 Published online 17 December 2009 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: Open-cell materials are lightweight and multifunctional capable of absorbing acoustic energy and supporting mechanical load. The acoustic and mechanical performance of open-cell materials can be optimized through processing. In this article, the relationships between processing parameters and acoustic and mechani- cal performance are shown for polypropylene (PP) foams. PP foam samples are fabricated using a combined com- pression molding and particulate leaching process. The results from a parametric study showed that both salt size and salt to polymer ratio affect the acoustic and mechani- cal performance of open-cell PP foams. As salt size increases, cell size increased and cell density decreased. The salt to polymer ratio had opposite affect on cell den- sity, and increasing the salt to polymer mass ratio increased the open-cell content. The airflow resistivity decreased significantly by increasing the cell size, which means that foam samples with smaller cell size have better sound absorption. When foam samples were thin, smaller cell sizes produced better sound absorption; however, as thickness of the sample increases, medium cell size offered the best acoustic performance. The compressive strength of the foams was increased by increasing the relative den- sity. Acoustic performance results from the parametric study were compared to the Johnson-Allard model with good agreement. Finally, optimal cellular morphologies for acoustic absorption and mechanical performance were identified. V C 2009 Wiley Periodicals, Inc. J Appl Polym Sci 116: 1106–1115, 2010 Key words: open-cell foam; polypropylene (PP); sound absorption; compression strength; computer modeling INTRODUCTION Compression molding has been widely used in the plastic industry to manufacture large parts such as automotive hoods, fenders, and spoilers. 1,2 The pro- cess is relatively inexpensive because of its simplic- ity and low waste. 3 The process is also capable of manufacturing large intricate parts with good sur- face finish. Compression molding has been used to create open-cell polymer foams by many researchers. Fos- sey et al. combined solvent blending and particulate leaching with compression molding to create poly- ethylene foams. 4 Here, the solvent blending was crit- ical to evenly disperse polyethylene throughout the salt particles. More recently, compression molding has been used to create open-cell bioscaffolds for tis- sue engineering. 5,6 Mooney and coworkers used a combined compression and gas foaming process to produce PLGA bioscaffolds. 5 In this process, sam- ples are saturated with gas in a foaming chamber. When the pressure is released, a thermal instability is created in the polymer matrix and cells nucleate as gas escapes. Mooney and coworkers has shown that using carbon dioxide instead of nitrogen creates foams with greater porosity. 7 In the noise control industry, three major methods are used to control unwanted noise: reducing sour- ces of noise and vibration, using barriers to prevent sound and vibration from entering a controlled space, and applying sound absorbing materials to dissipate unwanted sound energy. The materials investigated in this article fit into the third method, dissipating unwanted sound energy. It is critical that these materials have open-cell networks that are interconnected with the ambient environment to absorb sound energy. The acoustic performance of a material is meas- ured by determining the fraction of energy absorbed by a sample when a plane wave is incident on its surface. This measurement, which can be conducted Correspondence to: H. E. Naguib (naguib@mie.utoronto. ca). Contract grant sponsor: Networks of Centers of Excellence of Canada; contract grant number: AUTO21. Contract grant sponsors: Natural Science and Engineering Research Council of Canada (NSERC), Canada Research Chair (CRC), Canada Foundation of Innovation (CFI). Journal of Applied Polymer Science, Vol. 116, 1106–1115 (2010) V C 2009 Wiley Periodicals, Inc.