J.H. Johnston, B. Mesic, M.J. Cairns
Victoria University of Wellington,
New Zealand
Keywords: nanocomposite, polymer, nanosilica, TEOS, dispersion coating, paper, packaging, water vapour transport rate, cobb
Summary:
We present the development and application of a novel proprietary approach to produce nanocomposite silica-latex polymer dispersion coatings with excellent water repellent and water vapour transport resistant properties for use in paper coatings, as a move towards reducing plastic in food and other packaging applications. The cellulose fibre nature of paper means it readily absorbs water in the liquid and vapour phases, whereby its strength is reduced considerably and the mechanical properties adversely compromised. Also, the transport of water vapour through the paper means it cannot be used effectively to package moisture sensitive produce. The water barrier performance of a sheet material is characterised by the rate of water vapour transport (WVTR) across it, and the direct liquid water absorption by the Cobb test. Paper sheets and packaging have a high WVTR, typically up to 200 g m-2 day -1, and Cobb water absorption values of 20-40 g m-2. These restrict the use of paper in packaging where water repellent and water vapour resistant properties are required. With the move towards reducing plastic packaging material and the environmental consequences, there is a need to provide a paper-based alternative packaging material with improved WVTR and Cobb performance. Our proprietary new nanocomposite silica-latex polymer dispersion coatings technology developed and presented here, provides an attractive opportunity for using paper in packaging applications. The technology involves the hydrolysis of tetraethylorthosilicate (TEOS) in the water-based styrene-butadiene latex coating dispersion and controlling the ensuing silica polymerisation and interaction with the latex particles to form an impervious film on a paper sheet upon drying. The slightly hydrophobic TEOS is first adsorbed onto the surface of the spherical 80–30 nm diameter latex particles. Hydrolysis and polymerisation of TEOS takes place in the interstitial sites of the close-packed latex spheres filling these voids, while still enabling crosslinking and further polymerisation to take place between the latex particles during drying and film formation. The TEOS content and pH of the formulation are important here. Synchrotron-based small angle X-ray scattering (SAXS) and electronmicroscopy suggests the silica particles are about 6 nm in diameter. Infra-red spectroscopy suggests these particles adopt a silica gel-like structure that blocks the voids in the coating film. This coalescence of the polymerised styrene-butadiene latex particles and the silica gel results in significant improvements in the WVTR performance of the nanocomposite coated paper sheet. The WVTR of the sheet of 20 g m-2 day -1 of water vapour is substantially improved over the uncoated sheet of 121 g m-2 day-1. Similarly, the water barrier performance (Cobb) is improved from 25 g m-2 for the uncoated paper, to less than 2 g m-2 for the coated paper. The development work has been carried on a laboratory scale and has been implemented at a pilot scale operation. This, together with the excellent liquid water and vapour barrier properties, provide the attractive opportunity to use paper coated with this proprietary nanocomposite silica-latex polymer formulation in packaging applications, to reduce plastic demand and the undesirable environmental effects.