Packaging-related mechanical and barrier properties of pulp–fiber–chitosan sheets

In order to investigate ways of incorporating an oxygen- or air-barrier component, in this case chitosan or chitosan-acetic acid salt (chitosan salt), at an early stage in the paper-making process, sheets of chitosan and pulp fiber were produced in a hand sheet mould or solution cast in petri dishes. Some sheets were buffered in an alkaline solution, in order to reduce the moisture sensitivity. The sheets were characterized with respect to air permeance, oxygen permeability, fracture stress and strain, youngʹs modulus and moisture content. The addition of the chitosan solution to the pulp slurry led to a substantial loss of fiber and chitosan through the wire screen and consequently a low grammage and high sheet porosity and air permeance. Forming a wet pulp fiber sheet before introducing the chitosan solution increased the grammage and the amount of remaining chitosan, the latter revealed by infrared spectroscopy. In addition, pressing the sheets before drying reduced the porosity and the air permeance. Air mixing, included in conventional laboratory sheet making, yielded a more heterogeneous sheet and inferior mechanical and barrier properties. In general, the paper sheets prepared in the hand sheet mould became weaker, softer and less ductile with increasing content of chitosan. Only solution casting in petri dishes resulted in good barrier properties. Scanning electron microscopy showed that holes were absent in this case. In addition, optical microscopy and infrared spectroscopy revealed that the chitosan-salt formed a continuous phase and that it was uniformly distributed in the sheet. Interestingly, the problem of shrinkage when chitosan salt sheets are buffered for improved high-moisture gas-barrier properties, was reduced significantly by the restraining action of the pulp fibers.