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GRAY-STUART ET AL .
packaging weight) on the boxes on the bottom layer of a pallet. It is important to acknowledge that the load distribution on a box in creep and compression tests is simplified compared with a real-world sce- nario. The stacking configuration, box misalignment, and the pallet itself all influence the load distribution. 29 – 31
period. Secondary creep rate is given by the absolute value of this gra- dient divided by the height of the boxes (295 mm). Figure 3 illustrates peak detection method used to calculate the secondary creep rate. For boxes which failed, a numerical differentiation method was used to see when the steepest gradient occurred and thus give the time at which a box failed. The secondary creep rate for boxes which failed used the same peak detection method; however, 10% to 90% of the time period was used in order to get a suitable secondary creep rate value as the time period to failure was often much shorter than boxes which did not fail.
3.2
Moisture content
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The moisture content of the board at the end of the creep tests is given in Table 2. Across the four different conditions used, the aver- age moisture content was highest for the filled boxes followed by the control, the nonfoil side from the foil covered box and the foil covered panels. Aluminium foil provides a complete barrier to moisture and restricts the direct vapour access to the covered panels, so the mois- ture content will be lowest for the covered panels. It takes a long time for the board to reach equilibrium moisture content as can be seen from the difference between the constant 90% RH condition and the two cycling trials where the boxes were only held at 90% RH for 12 h and 5 days, respectively. From the sorp- tion and desorption isotherms for corrugated fibreboard presented in the literature, 3,32,33 the equilibrium moisture content for sorption at 90% RH and desorption at 70% RH are similar. We hypothesise that even during the cycling RH in trial A the mean moisture content of the panels has a narrower range than the equilibrium moisture con- tent at 70% and 90% RH, respectively. In all trials, the highest moisture content was measured for the filled boxes (Table 2). This indicates that the actual moisture content of boxes in the supply chain could be higher than what is measured from empty boxes in compressive creep tests. However, this differ- ence may not be significant enough to influence box behaviour. Hav- ing product inside the box might result in the inner liner having a higher moisture content. In an empty box, moisture from the inner liner desorbs into the air space and is adsorbed by the top and bottom
2.6
Moisture content
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At the end of the trials, the moisture content of the boxes was mea- sured. The trials concluded with the 90% RH cycle apart from the con- stant 70% RH trial. The four side panels from each box were cut out and weighed immediately after the test; they were then dried at 105 C for 72 h to obtain the dry weight. The % moisture content of the board at the end of the trial was given by (wet weight-dry weight)/wet weight.
3
| RESULTS AND DISCUSSION
3.1
Box compression tests
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The mean BCT value measured was 4.41 kN (450 kg force) with a standard deviation of 0.186 kN (18.9 kg force); 30% of the BCT value was chosen for the creep tests as it gives a load of 1.32 kN (135 kg force), which is close to the gross maximum load that these boxes would experience when palletised in the supply chain. The boxes used in this study are typically palletised with an interlock pattern six high, which is an equivalent total load of 1.23 kN (125 kg force plus
FIGURE 3 Example of box displacement data over time for a box from trial A. The black dots indicate peaks and the solid grey lines are linear regressions through the peaks and a rolling mean respectively, taken within the second and eighth data quantiles represented by the outermost dotted vertical lines
TABLE 2 Moisture content (% wet basis) of the board at the completion of the trials, mean with standard deviation in brackets
Trial
Control
Filled boxes
Foil sided bare
Foil sided covered
13.4 (0.25) b
14.0 (0.34) a
13.3 (0.32) b
12.2 (0.27) c
A
14.2 (0.28) a,b
14.5 (0.22) a
14.1 (0.36) b
13.1 (0.31) c
B
14.9 (0.25) a
15.0 (0.40) a
14.7 (0.59) a
14.7 (0.55) a
C
11.0 (0.21) a,b
11.1 (0.13) a
11.0 (0.20) a,b
10.8 (0.14) b
D
Note : For each trial, means followed by a common letter are not significantly different by the HSD test at 5% level of significance.
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