Issue link: https://cp.revolio.com/i/586959
Further, paper appears to behave like a viscoelastic polymer, which is a result of consisting of both amorphous fibers and crystalline components. This dual nature is reflected in how paper responds to moisture. When absorbing moisture, hygroexpansion occurs, and the paper exhibits an elastic strain response, but also a viscous response – which means the strain response continues long after moisture content equilibrium occurs. The practical outcome of this is that paper strain in a cycling humidity environment exhibits an upward ratcheting affect, providing a notably greater strain response than in a high, but steady state, humidity environment (Padany; 1991). This effect is shown graphically in Figure 2. Elevated strain in a cyclic humidity environment is interesting for three reasons. First, it will accelerate strain (and stress build-up) in component papers, and its distorting effect on corrugated geometric structure, the end result of which is acceleration of strength loss (when compared to steady state conditions). Secondly, cyclic humidity is a common phenomenon in vehicles, particularly for land shipment transit legs, as can be seen in Figure 3. It has been well established that cyclic humidity accelerates strain in paper and corrugated. One implication of the accelerating effects of strain in cyclic humidity environments is that while it is commonly believed that oceans shipments provide an especially destructive environment for box compression strength loss, the reality is that while on the ocean, relative humidity is steady state, while on land, relative humidity varies in 24 hour cycles, due to diurnal temperature cycling. As David Leinberger stated in his well-known presentation on temperature/humidity data collection for land-ocean-land shipments – we need to "focus on the land!" if we want to know where the greatest compression strength loss occurs (Leinberger; 2006). Thirdly, we can use cyclic relative humidity to accelerate testing used to evaluate factors which accelerate loss of box compression strength. 20 ista views • October 2015 • www.ista.org Long Term Corrugated Container Compression Performance > CONTINUED FROM PAGE 19 Figure 2: Deflection Comparison - Steady State Versus Cyclic Relative Humidity. Used with Permission (Chalmers; 2011) Deflection (mm) Figure 3: Note the 24 hour cycles for temperature and relative humidity levels during land transportation. Used with Permission (Leinberger; 2006).