Effect of pre-coat and substrate porosity on conventional barrier coatings performance

The effect of applying a pre-coat containing starch, AKD and kaolin clay on the barrier properties of internally sized, commercial kraft linerboards of basis weight about 200 g/m² was investigated. A DOW laboratory coater was used to apply barrier coatings to commercial linerboards made from either a mixture of 54% pine and 46% eucalypt pulp, or 100% pine pulp. Handsheets (100% pine at 200 g/m²) were also prepared and coated in the same manner. Water Vapour Transmission Rates (WVTR) and Cobb (30 min) tests were conducted. The uncoated liner made from the mixed pine and eucalypt pulps exhibited a WVTR of 2365 g/m²/day at 38°C and 90% relative humidity (RH), compared to 1880 g/m²/day under the same conditions for 100% pine linerboard, a difference of 21%. When a barrier coating was applied, the WVTR of these linerboards was reduced to 200 and 160 g/m²/day respectively. The 100% pine linerboard still showed a 20% better WVTR, indicating that the substrate was still crucial to the overall WVTR even with a 21 g/m², single-layer barrier coating. Handsheet tests suggested that the lower WVTR for the 100% pine linerboard may have been related to the level of beating of the pulp furnishes. WVTR did not change appreciably when a 6 g/m² wheat starch pre-coat was applied. However, adding hydrophobic AKD to the starch in equal ratio on a solids basis improved WVTR by up to 8%. Modifying the pre-coat to include 10% kaolin clay improved both WVTR and Cobb by 10%.