(572g) Evaluating Nitrogen As a Co-Blowing Agent for CO2-Blown Polymeric Nanofoams

The development of low-cost, low-density thermoplastic foams consisting of cells with nanoscale dimensions is desirable for insulation and other applications and to address the global trends of improved energy efficiency and reduced carbon footprint. When the sizes of foam cells approach the mean free path of the gas molecules in the foam (ca. 75 nm), the gas begins to collide more frequently with the foam cell walls than with other gas molecules, interfering with the gas-phase conduction mechanism that dominates the thermal conductivity of conventional insulating foams. A variety of factors contribute to determining the thermal conductivity of such nanofoams including cell size distribution, porosity, polymer thermal conductivity, etc., and they can be difficult to optimize independently (e.g. cell size vs. porosity). In turn, many of those characteristics are determined by factors such as blowing agent solubility, depressurization rate, and other foaming conditions.

To date, most studies of polymeric nanofoams have focused on a single blowing agent with high solubility in the polymer (e.g., carbon dioxide with acrylic copolymers ¹) because a very high solubility is important for generating a sufficiently high nuclei density.  However, in more conventional insulating foams made from polystyrene, for example, mixtures of blowing agents is commonly used (e.g., a hydrofluorocarbons, hydrocarbons,  carbon dioxide,  water etc.). Combinations of blowing agents with low environmental impact, in particular carbon dioxide and nitrogen) have been explored in studies of microcellular foams in the literature as a way to optimize foam characteristics.  It has usually observed that nitrogen enhances nucleation in microcellular foams, thus reducing cell size, while addition of higher carbon dioxide levels led to lower density.² This approach has not yet been explored for nanofoams.

In this paper we study the effects of the addition of nitrogen as a co-blowing agent in carbon-dioxide-blown nanofoams of acrylic copolymers. It appears that nitrogen did not enhance nucleation in nanofoams and failed to provide any clear benefit for optimizing the balance between cell size and porosity. However, the study helps to clarify the different impacts of blowing agent solubility and depressurization rate on those two key metrics of nanofoam morphology.

References

1.            S. Costeux and L. Zhu. "Low density thermoplastic nanofoams nucleated by nanoparticles." Polymer, In Press, Corrected Proof (2013).

               http://dx.doi.org/10.1016/j.polymer.2013.03.052

2.            E. Di Maio, G. Mensitieri, S. Iannace, L. Nicolais, W. Li, and R. W. Flumerfelt. "Structure optimization of polycaprolactone foams by using mixtures of CO₂ and N₂ as blowing agents." Polymer Engineering & Science, 45, 432-441 (2005).

               http://dx.doi.org/10.1002/pen.20289