Moisture-induced effects on stacking strength of moulded-fibre packaging in varying environmental conditions

Abstract

Stacking strength of moulded-fibre trays was investigated as both compressional creep and static compression strength at constant and varying humidity conditions. The compressional creep behaviour resembled that of other paper and board containers and was accelerated at humidity cycling between 33% and 94% relative humidity (r.h.) for 18 days compared to constant humidity conditions 91% and 94% r.h. Although the moulded-fibre trays did not experience failure, secondary creep rate was accelerated by a factor of 10–20 and total creep strain by a factor of 1.3–1.6. Compressional creep was thus affected by mechanosorption, whereas static compression was found not to respond to cycling of environmental humidity. Static compression strength was merely determined by the moisture content of the moulded-fibre material. The effect of varying temperature on tray moisture content was examined by transfer of the moulded-fibre tray from preconditioning at cold storage (5°C, 59% r.h.) to ambient conditions (25°C, 54%). When a food simulant [agar gel, water activity (aw) ∼1] was sealed inside the moulded-fibre tray, moisture condensed on the tray outer surface (moisture gain 1.4 g/100 g dry fibre) within 40 min of transfer, contrary to when empty moulded-fibre trays were exposed to same conditions. Condensation could thus potentially induce a large initial creep deformation of the moulded-fibre tray. Copyright © 2004 John Wiley & Sons, Ltd.