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

In fibre-based packaging, barrier resistance against water vapour, oxygen and grease has become of increasing importance. The use of mineral fillers in dispersion coatings to enhance water vapour barrier properties has been extensively studied in recent years [1-11]. Barriers can be applied on a fibrous substrate such as linerboard using extrusion, lamination or dispersion coating techniques. The latter has several advantages, most notably higher machine speeds and fewer processing steps than is the case for extrusion coating. This work has investigated the effect on barrier properties of applying a pre-coat containing starch, AKD and kaolin clay and investigating substrates of differing porosity. All coatings were applied to an internally sized commercial kraft linerboard (200 g/m², 54% pine and 46% eucalypt) using a DOW laboratory coater and tested for Water Vapour Transmission Rates (WVTR) and Cobb (30 min). The uncoated liner exhibited a WVTR of 2365 g/m²/day at 38°C and 90% relative humidity (RH) which did not change appreciably on application of 6 g/m² wheat starch. Adding hydrophobic AKD to the starch in equal ratio on a solids basis improved WVTR by up to 8%. Scanning electron micrograph of the coated surface as shown in Figure 1 showed both the AKD to be non-uniformly distributed and the presence of pinholes, which offers an explanation for the limited improvement in WVTR observed. The importance of film coverage on WVTR has been demonstrated by applying to the linerboard PE film with different levels of open area. It has been shown that a conventional barrier coating alone gives a WVTR of 203 g/m²/day at 38 °C and 90% R.H. at a coat weight of 21 g/m². Applying a 6 g/m² pre-coat (starch/AKD in equal ratio on a solids basis) to the barrier coat did not change WVTR within the precision of the measurements. This suggests that the high barrier coat weight may be masking any potential improvement from the pre-coat as described above. Modifying the pre-coat to include 10% kaolin clay improved both WVTR and Cobb by 10%.