Thermal barrier enhancement of calcium carbonate coatings with nanoparticle additives, and their effect on hydrophobicity

Abstract

Nano-TiO2, nanoclay, and cellulose nanocrystals (CNC) were each introduced into calcium carbonate coatings common in paper/paperboard applications, to investigate improvements in thermal barrier performance and hydrophobicity. An in-house apparatus was built in which the temperature was measured on both sides of a coated cellulose substrate in the presence of a constant, applied thermal load. Hence, a temperature difference (ΔT) across the coated substrate was recorded for each coated sample. Thermal conductivity (k), contact angle (CA) and critical surface energy (σc) of the coated samples were also measured. In all cases, the presence of the nanoparticle (NP) additives to the calcium carbonate coatings improved the thermal barrier performance (increased ΔT and reduced k), and showed mild enhancement in the CA compared with coated samples that did not have NP added to the coating. Specifically, with the introduction of 2% CNC into the calcium carbonate coating, ΔT increased by 28.3\xa0°C; k reduced by 0.0142\xa0W/m\xa0K; and CA increased by 23°. The effects of thermal load application on the coated sample caused an increase in surface porosity of 7% and a reduction in σc by 13.0\xa0mN/m, potentially indicating a loss of mechanical integrity. Thermal barrier and hydrophobic improvements were less successful with nanoclay additions to the calcium carbonate coatings, however the σc remained constant after thermal load application, indicating a more robust surface against applied heat. This study adds significant information to the little-studied field of thermal barrier improvements to paper coatings for food packaging applications.