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

Nano-TiO 2 , 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 (σ c ) 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 °C; k reduced by 0.0142 W/m K; 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 σ c by 13.0 mN/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 σ c 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.