1. Department of Fibre and Polymer Technology, Royal Institute of Technology, SE-100 44 Stockholm, Sweden, STFI-Packforsk AB, Box 5604, SE-114 86 Stockholm, Sweden, Department of Materials Science, Engineering Faculty, University of Basque Country, 48013 Bilbao, Spain, and Department of Crop Science, The Swedish University of Agricultural Sciences, SE-230 53 Alnarp, Sweden