Thermal Barrier Coatings for Cellulose Substrates: A Designed Molecular Simulation Study of the Effects of Nanoparticles and Porosity on Thermal Diffusivity

Design of optimal thermal barrier coatings for cellulose substrates are of key importance in the food packaging industry. Maintaining the quality of packaged food items hinges upon protecting the food from heat and moisture, especially during transportation. In our research group, we have previously explored the effects of adding cellulose nanocrystals (CNCs) and cellulose nanofibers (CNFs) to a coating formulation, comprised of calcium carbonate (CaCO₃) and a copolymer (binder), i.e., poly(styrene-co-methacrylic acid) (PS-co-PMAA), on its thermal barrier properties when applied to packaging paper.¹ Our experimental observations indicate reduced thermal conductivity in coated paper when adding CNC (or CNF) to the formulation.¹ However, we have also observed different levels of surface porosity in the cellulose nanoparticle-modified versus unmodified coatings.¹ Since the effects of cellulose nanoparticles and porosity on the thermal barrier properties of the coating could not readily be discerned by the experiments, we performed classical molecular dynamics (MD) simulations of different coating formulations with different levels of porosity (represented as different free volume percentages to roughly mimic surface porosity) in a statistical design of experiments framework. For this purpose, we used a Response Surface Methodology (RSM) based design with three factors at two levels, i.e., CNC, CNF, and free volume. The modeled nonporous and porous coatings were comprised of nano-sized aragonite and PS-co-PMAA, CNC, and CNF with different free volume percentages, implemented as different coating densities. For the latter, we established a correlation between density and free volume of the coatings. Our objective was to investigate both the main and interaction effects between the coating constituents (nanoparticles and porosity) on the thermal conductivity and diffusivity (responses) of the resulting coating. For the simulations, we used the polymer consistent force field (pcff) within the open-source LAMMPS software package. We calculated the thermal conductivities using an established method. We validated our force field predictions of the thermal conductivities of pure components, modeled as random PS-co-PMAA, CaCO₃ with the common aragonite crystal structure, and CNC as cellulose Iβ crystal. For example, our predicted room-temperature thermal conductivities of aragonite and CNC were 2.44 and 0.92 W m^-1 K^-1, matching the published data.^2,3 The simulations are still ongoing and preliminary results will be presented.

References:

(1) Hutton-Prager, B.; Khan, M. M.; Gentry, C.; Knight, C. B.; Al-Abri, A. K. A. Thermal Barrier Enhancement of Calcium Carbonate Coatings with Nanoparticle Additives, and Their Effect on Hydrophobicity. Cellulose 2019, 26 (8), 4865–4880.

(2) Robertson, E. C. Thermal Properties of Rocks. Open-File Report 1988. DOI: 10.3133/ofr88441

(3) Dri, F. L.; Shang, S.; Hector, L. G.; Saxe, P.; Liu, Z.-K.; Moon, R. J.; Zavattieri, P. D. Anisotropy and Temperature Dependence of Structural, Thermodynamic, and Elastic Properties of Crystalline Cellulose Iβ: A First-Principles Investigation. Model. Simul. Mater. Sci. Eng. 2014, 22 (8), 85012.