# Impact of Temperature and Humidity on Key Mechanical Properties of Corrugated Board
## 2 Materials and Methods  
The primary objective of this study is to investigate the effect of varying temperature and humidity conditions on the mechanical properties of 3- and 5-ply corrugated cardboard. The selection of specific board types was made after consultation with one of the biggest cardboard manufacturers in Poland based on the most commonly used cardboard sheets. In the first stage, six corrugated cardboard types were selected, including three 3-ply and three 5-ply cases, manufactured from 24 different papers. Detailed information on the corrugated cardboards and papers considered in this study is provided in Table 1.  
Next, a range of temperature and humidity combinations was selected to condition the corrugated cardboard samples in a climate chamber. The relative humidity settings for the samples ranged from 30% to 90%, in 10% increments. The temperature settings ranged from 10 \({}^{\circ}\)C to 60 \({}^{\circ}\)C, in 10 \({}^{\circ}\)C increments. A set of samples was also tested under standard laboratory conditions (23 \({}^{\circ}\)C and 50% relative humidity) according to relevant standards [37; 38]. Each humidity-temperature combination was maintained for 5-8 h (according to ISO 187:2022 [37]) in KK750 climate chamber. The mass change in the samples was periodically checked and at the end of conditioning the hourly mass increase did not exceed 0.25% [37].  
In the next stage, samples were prepared for testing on a strength testing machine. All samples were cut using a Computerised Numerical Control (CNC) laser device. The Box Strength Estimation (BSE) machine (Figure 1), part of the Box Strength Estimation System, was used for mechanical testing [39]. Various mechanical tests were conducted including:  
* the Bending stiffness Test (BNT) in both the Machine Direction (MD) and the Cross-machine Direction (CD), using a 250 \(\times\) 50 mm sample [40; 41];
* the Edge Crush Test (ECT), using a 100 \(\times\) 25 mm sample [42; 43; 44; 45];
* the Shear Stiffness Test (SST), using a square 85 \(\times\) 85 mm sample;
* the Torsion Stiffness Test (TST) in both MD and CD, using a rectangular 150 \(\times\) 30 mm sample.  
The bending and torsion tests were carried out in the machine direction, i.e., along the board flute, and in the cross-machine direction, i.e., across the flute. The testing speed in the ECT was equal to 12.5 mm/min, according to the EFECO Testing Method No. 8 [44], while equal to 0.03 rad/s in the TST and 37.5 mm/min in the BNT and SST.  
Figure 2 schematically represents the loading and boundary conditions applied on corrugated board samples for each test setup. Moreover, the pictures shown in Figure 1 depict samples in the measurement sockets of the BSE machine.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Cardboard** & **Grammage** & **Grammage of Papers** & **Thickness** \\  & **(g/m\({}^{2}\))** & **(g/m\({}^{2}\))** & **(mm)** \\ \hline E flute & 508 & 170–135–170 & 1.73 \\ B flute & 415 & 125–120–125 & 2.97 \\ C flute & 456 & 125–130–130 & 4.02 \\ EE flute & 653 & 125–120–80–120–125 & 2.77 \\ EB flute & 578 & 125–90–80–90–125 & 4.19 \\ BC flute & 803 & 170–135–80–135–170 & 6.72 \\ \hline \end{tabular}
\end{table}
Table 1: Specifications of corrugated cardboards used in this study.  
of cardboard were captured. Tests on the strength machine were conducted simultaneously for samples in a given direction. For each temperature-humidity combination and type of cardboard, five tests were performed on identical sample sets to obtain statistically representative results. Figure 3 shows a single sample set. A total of 1290 individual tests were conducted--namely, 6 types of cardboard \(\times\) 5 samples \(\times\) (7 humidity levels \(\times\) 6 temperature levels + 1 laboratory standard).