By Evan Laskaris, Vice President of Operations for RXO Managed Transportation
Every year, the U.S. freight industry transports billions of tons of goods. Yet, a persistent challenge remains: cargo damage. While load securement might seem simple—strap it down or block it in—the physics of transportation are complex, especially when a truck travels from sea level to over 11,000 feet in elevation.
At RXO, we believe that reducing freight claims and ensuring safety requires more than just best practices; it requires hard science. That is why we are proud to have funded a groundbreaking study at Clemson University, resulting in the thesis: “Effects of Variation in Altitude and Temperature on the Restraining Force and Internal Gauge Pressure of Constrained Dunnage Airbags.”
This research sheds new light on how dunnage airbags behave under extreme conditions, providing the industry with the data needed to protect cargo better than ever before.
The Problem: The Eisenhower Tunnel Effect
Dunnage airbags are inflatable bladders placed between pallets to prevent movement. They are essential for LTL (Less-Than-Truckload) and full truckload shipping. However, dunnage airbags are sensitive to their environment.
The study focused on a real-world scenario that plagues many shippers: a truck traveling from a low elevation (like Clemson, SC) to a high elevation (crossing the Continental Divide at the Eisenhower Tunnel in Colorado).
As a truck climbs, atmospheric pressure drops, causing the air inside the dunnage bag to expand. If the bag is too stiff, it can crush the cargo. If the bag stretches too much to accommodate the pressure, it may overly deform and lose its ability to secure the load once the truck descends back to lower elevations.
The Study: Putting Materials to the Test
The research team, led by Dr. Greg Batt, Professor at Clemson University, tested dunnage bags made from the three most common materials under simulated transit conditions:
- 4-Ply Kraft Paper: The industry standard for rigidity.
- Woven Polypropylene (WPP): A strong synthetic fabric.
- Polyvinyl Chloride (PVC): A flexible, film.
They subjected the airbags to variable atmospheric pressures using a vacuum chamber and variable temperatures using an environmental chamber.
Key Findings
The study revealed a critical trade-off between stiffness and ductility:
- Altitude Impacts Pressure More Than Temperature: The study proved that while temperature changes do affect pressure, altitude changes (like driving over a mountain pass) cause pressure swings that are an order of magnitude greater.
- Kraft Paper and WPP Performance: Because Kraft paper is stiff, it converts altitude-induced pressure almost entirely into restraining force. WPP airbags exhibited similar properties. This makes both types of airbags excellent for holding heavy loads, but they carry a risk: if over-inflated at sea level, they can exert enough force at high altitudes to damage cargo.
- The PVC “Stretch”: PVC bags are ductile. When they hit high altitudes, they expand in volume (stretch) to relieve the pressure. However, the study found that after this expansion, the bags often lost significant restraining force when returning to normal conditions. In some cases, PVC bags even burst when re-pressurized at altitude.
- Predictability: The study found that Kraft and WPP bags behave in a predictable way during altitude changes, whereas PVC behavior is harder to model due to material deformation.
What This Means for the Freight Industry
This research moves load securement from a guessing game to a science. Here is how these findings translate into value for shippers and carriers: