The Development of Barrier Coatings for Paperboard Packaging with Low Water Vapour Transmission Rates

New Zealand exports fruit worth NZ$1343 million p.a. to international  markets notably Australia, UK and Europe, much of which has to be  chilled and packaged to prevent moisture loss. The paperboard packaging  itself needs to be strong, durable and also possess excellent water  vapour barrier properties to prevent moisture loss from the fruit in  transit. Currently non-recyclable materials are used to fulfil this  task. The aim of this project is to produce a new cost effective  recyclable paper coating with an improved water vapour transmission rate  (WVTR) of less than 5 g m-2 24 h-1, for potential application in the  food industry. Silica nanospheres were produced via the Stöber method  and then incorporated into a latex/clay formulation to increase the  physical barrier and effectively lower the WVTR. Different combinations  of single and double coated paper, with and without the addition of wax  were tested. Additionally, two different drying methods were  investigated. A WVTR of 10 g m-2 24 h-1 and a corresponding Cobb120  value of 0.1 g m-2 was the best result achieved. The reason for this low  WVTR was attributed in part to hydrolysis of unreacted  tetraethylorthosilicate (TEOS) in situ to form silica nanoparticles. The  simplification of the manufacturing process and the cutting of costs  led to the testing of silica spheres that were produced from sodium  silicate and Klebosol, a commercial equivalent. The resulting WVTR and  Cobb120 values were found to be higher than the coatings produced from  TEOS. Improvements on the WVTR and Cobb120 values led to the  investigation into the unreacted TEOS, which subsequently led to the use  of the Stöber method in situ in the latex. Varying the concentrations  of the TEOS led to a WVTR of 15 g m-2 24 h-1 and a Cobb value of 0.2 g  m-2. These were promising results for the draw-down coatings via a bird  bar. Similarly, the addition of both clay-slurry and wax was  investigated but the change was insignificant. A lower concentration of  clay was added to the latex without the use of a dispersant and a WVTR  of 15 g m-2 24 h-1 and a Cobb120 value of 0.1 g m-2 was the lowest value  achieved this way. The reason for these significant improvements was  attributed to the formation of silica particles in situ, that were small  enough (~7-10 nm) to move in the pores of the latex and within the  latex particles themselves, inducing swelling that resulted in a tightly  packed coating and an observed increase in surface roughness. In  addition, it was proved that the use of dispersants within the latex and  also as an addition to disperse the clay does effect both the WVTR and  Cobb120 values. Although the target value was not quite achieved, the  method is promising and there is still room for improvement.