Influence of moisture on the performance of polyethylene coated solid fiberboard and boxes

Abstract

Effects of moisture on the mechanical properties of one commercial solid fiberboard grade and on the compression strength of mill-fabricated transport boxes made from this material were studied. The solid fiberboard (1220 g/m2 ) has four middle layers made of old corrugated container (OCC). The middle layers are strongly internally sized with rosin sizing. The outer layers on both sides are made of bleached machine finished kraft paper. The kraft paper has extrusion coated low-density polyethylene (LDPE) doublelayer as moisture barrier. The six paper and paperboard layers are glued together in an industrial lamination process. The boxes have unusual design including double-panel walls and webbed corners, and they are used in wet and humid conditions for transporting fresh round fish. The transportation takes up to eight days at the ambient environment of 4°C and 90-100% RH. These conditions are taken as the framework of the study.

Results show that the LDPE coating considerably slows down the transverse moisture penetration. The in-plane diffusivity (5.9·10-10 m2 /s, 27°C, 50/90% RH) was determined with an integrated unsteady state moisture transport model and was found to be nine times higher than the transverse diffusivity through the PE layer. Based on the diffusivity, water vapor can theoretically affect 80 mm from the open unsealed board edge during transport. Experiments show that liquid water penetrates 40-50 mm during eight days. In the middle of the sheet the moisture content increases moderately, approximately 0.3 percentage points during transport. The packaging producer’s main concern is thus to ensure that the open material edges are at least 80 mm away from the load bearing sections of the box. Due to uneven moisture penetration into the boxes, the average moisture of a box is a questionable measure of moisture pick-up. Instead one should consider the moisture content of the load bearing parts.

The role of the adhesion layers in bending stiffness of the solid paperboard was modeled with laminate models. Results show that the adhesion layers affect the mechanical properties of the combined board and need to be addressed in the models. Best agreement with the measured bending stiffness values was obtained by a 11-layer model, where the properties of the adhesion layers are taken from the paper-glue-paper sandwich tests. The PE and kraft paper are regarded as one layer in the model. With this model, the calculated bending stiffnesses are close to the measured reference at 50% RH in MD (difference 1%). In 90% RH and in MD the results are 14% lower. Neglecting the adhesion layers gives 5% lower values in 50% RH and 18% in 90% RH compared to the reference measurement in MD. In each of the cases, the difference from reference is larger for the CD material direction. Three thickness measurement techniques (ISO, STFI, and SEM) were used to gather input data for the bending stiffness model. In this application, the ISO technique provided best results compared to the other techniques.

It was found that the top-to bottom failure of a box can be expressed as a critical vertical displacement that is independent of moisture content. Similar results have been previously discovered when studying simpler structures like corrugated board panels and regular slotted containers (RSC). This research indicates that the strain dependent failure criterion also applies on more complex box structures.

Keywords: paper laminates, solid fiberboard, moisture, mechanical properties, diffusivity, hygroexpansion, bending stiffness, box compression.