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was used for this work. Four different compressive creep conditions were performed for this study as detailed in Table 1. Boxes were sub- jected to 30% of the BCT for in all trials to represent highly loaded boxes, for example, those on the lowest layer of a pallet. The box conditioning protocol used by Hussain et al. 3 was employed for this study. Prior to preparing the boxes, they were con- ditioned for 48 h at 23 C and 50% RH as they would be for BCT. Once the boxes had been prepared, they were placed in a press in the test room. All boxes were initially subjected to two 24-h humidity cycles, consisting of 12 h at 70% RH and then 12 h at 90% RH with no load applied. Each press was equipped with a load cell and linear variable differential transformer (LVDT) which enabled the applied load and displacement to be measured, while the relative humidity and temperature in the test room was measured and recorded using multiple sensors. All data were recorded at 5-min intervals. Table 1 outlines the RH conditions for the four trials. Trials (A) and (C) were stopped at the end of a 12-h 90% RH cycle, and trial (B) was stopped at the end of a 5-day constant 90% period. This was to ensure the moisture content of the boxes would be at their maximum for the respective trials. All trials were run for between 21 and 25 days.
highest load prior to failure was recorded as the BCT load. Ten repli- cate samples were used for the BCTs, and boxes were conditioned for 48 h at 50% RH and 23 C prior to testing.
2.3
Box preparation
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Three preparations of boxes were used in this study: control, filled, and foil boxes. Empty boxes were used as the control. The filled boxes contained 20 ± 0.2 kg of Chelsea ® standard granulated white sugar. Sugar was chosen as it has a similar bulk density (900 kg m 3 ) to the product normally contained in the boxes and is shelf stable. This left a headspace of ≈ 80 mm between the product and the top of the box that ensured that the box would take the applied load and not the contents. Sugar is hygroscopic; to prevent unintended issues with moisture, the sugar was packed in two black plastic (LDPE) rubbish bags which were then sealed with foil tape. The foil boxes had two adjoining external side panels, without the manufacturer's joint, cov- ered in aluminium foil to represent a box on the corner of a pallet. The foil was secured with aluminium tape. All boxes were sealed with standard brown packing tape holding the top and bottom flaps in place. The test facility had 24 presses available and for each trial eight replicates of three different box preparations were set up. Figure 2 shows how the boxes looked with the foil coating applied to two adja- cent panels.
2.5
| Box lifetime and secondary creep rate
The R code developed by Hussain et al. 3 was used to determine sec- ondary creep rate and box lifetime. Briefly, a method was developed to detect the peaks in the box displacement versus time data. For boxes which did not fail, a linear regression was fitted through the peaks of the data between 20% and 80% of the experimental time
2.4
Creep tests
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Cyclic RH creep tests often use a range of at least 40% RH, with cycling between 50% and 90% of RH being commonly used. 6,15,26 In these trials, low and high RH levels of 70 and 90% at a temperature of 4.5 C were chosen. The rationale for this approach is that refrigerated storage and transport is particularly common for horticultural and agri- cultural exports. RH in refrigerated conditions is typically between 70% to 90% and the typical temperature range is 0 – 8 C and product dependent 27,28 ; the boxes used in this study are made for refrigerated product. The WHITE (Weight Humidity Intervals Temperatures Exper- iments) room test facility at the Scion Te Papa Tipu Innovation Park
TABLE 1
Creep testing conditions
Trial RH conditions A Cycling; 24 h cycles (12 h at 70% RH, 12 h at 90% RH) B Cycle/interval; 5 times 24 h cycles (12 h at 70% RH, 12 h at 90% RH), 5 days constant RH at 90%
C Constant RH at 90% D Constant RH at 70%
FIGURE 2 Assembled box with two normal panels (A) and aluminium foil covering two adjacent panels (B), the top flaps of the box were sealed with tape after the aluminium foil was applied
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