Barrier May Mitigate Dry Ice Supercooling Effects in Transit

Key takeaway: The findings revealed the potential of CO₂ barriers in managing supercooling. "By using a CO₂ barrier to control gas flow, we can reduce temperature drops and mitigate the rate of dry ice sublimation,” Deltano said.

In temperature-sensitive pharmaceutical packaging and transport, the use of dry ice is a common practice for maintaining low temperatures.

Unlike traditional ice, dry ice doesn’t melt—it sublimates, meaning that it changes from solid to gas without ever entering a liquid state. However, the phenomenon of supercooling, where dry ice drops below its typical sublimation point of -78.5°C, poses significant challenges and can lead to temperature excursions that can compromise product integrity.

Could applying an outer barrier to the shipper mitigate these risks? At ISTA’s TempPack, Kristin Deltano, senior product development engineer at Cold Chain Technologies, LLC, presented early data on CO₂-impermeable, vented barriers around shippers. Results showed promise in lessening temperature excursions and tests are ongoing.

Causes of supercooling 

Deltano explained that the sublimation point of dry ice can change depending on the partial pressure of the CO₂ in the shipper. Partial pressure is the pressure exerted by one gas in a mixture of many gasses. If this partial pressure drops, for example when CO₂ gas escapes from gaps in the shipper—then so, too, will the sublimation point of the dry ice. This can result in multiple issues for a manufacturer:  

·      When dry ice in a shipper sublimates faster than expected, the system may supercool below the temperature a pharmaceutical product can withstand.

·       The system may not meet the expected duration of the shipper because the dry ice sublimates too quickly in the shipment.

Research into supercooling has looked at dry ice surface area, vibration, pressure, convection, and shipper orientation as possible causes. Among these, convection, dry ice surface area, and shipper orientation were the biggest factors leading to this phenomenon.

As a refresher, convection is the heat transfer process where warmer substances rise and colder, denser substances sink. This phenomenon causes dense CO₂ gas to escape from lower gaps in the shipper, and this can lead to a drop in pressure and subsequent supercooling.

Why orientation matters

It’s commonly understood that a “This Side Up” label is not a guarantee that a package will remain upright in the unpredictable world of shipping. Packages may fall over, or they may be stacked sideways or upside down for long durations, and this can impact shipper performance. But why would a shipper supercool faster on its side?

CO₂ gas escapes from gaps in the shipper, leading to a drop in pressure of the system. Deltano explained that for a multi-panel shipper, using a six-piece EPS cut sheet or vacuum insulated panels (VIPs), gas can escape through the gaps in any orientation resulting in supercooling risk. No matter how tightly the panels are packed together, there is still some form of a gap that gas can move through.

For a molded shipper with its only gaps at the top—between the lid and the base—CO₂ gas can prematurely sneak out of the gaps in the top when it is laid on its side or upside down, inducing supercooling. “Upright, the gas can just build up within the system until it meets that lid gap. On its side, that gap is now oriented towards the ground and the CO₂ is going to flow right out of it,” said Deltano. 

Cold Chain Technologies, LLC

So, molded shippers will see their worst supercooling when placed on their side or upside down because of the gap between the lid and the base. Multi-paneled shippers, like molded shippers, will see the worst supercooling on their side or upside down because the gap between the lid and the base is larger than the gap between the side panels that have been tightly pressed together. "Most excursions below -90°C were actually caused by an upright shipper falling to its side during transit, which is an incredibly common occurrence," she said.

Testing barriers

A team at CCT conducted a series of tests to explore how different configurations of CO₂ barriers could mitigate this effect. Deltano said that while it may be tempting to simply plug up all the gaps to prevent CO₂ leakage, she noted that dry ice is a hazardous material that typically requires specialized handling. The system must have some ability to vent when needed to prevent a dangerous buildup of pressure that can lead to explosion.

They tested a CO₂ impermeable barrier—a polyethylene bag—around shippers built with insulated panels, which would block any gaps in the system, with a vent opening at the top. 

Cold Chain Technologies, LLC

The experiment involved three identical six-paneled insulated shippers. As alluded to above, multi-paneled shippers typically experience more supercooling than molded shippers, because molded containers will only see supercooling on their side or upside down. Multi-paneled shippers can see supercooling no matter the orientation.

·      The first, a control shipper, had no barrier.

·       The second, labeled the Base Sleeve, covered five sides but left the top open.

·       The third was the Base Sleeve + Overhang, encasing all six sides except for a small vent hole at the top.

The shippers were filled entirely with dry ice and left in the lab’s ambient conditions at 20°C. Thermocouples were placed on the same inner sidewall to record temperatures throughout the test duration.

Seventeen hours into the test, they tipped the shippers onto their sides so that the side with the thermocouple became the bottom. “Then, 40 hours into the test, we shook the shippers. We took weights at three points: the beginning, right before tipping, and at the end,” she said.

Results

The control shipper never recovered from its initial supercooling, dropping 10°C below the barrier samples to approximately -90°C, indicating a clear difference in performance due to the presence of CO₂ barriers. Weight loss rate showed that the control shipper had a higher rate of sublimation, implying a shorter duration.

Cold Chain Technologies, LLC

Between the two samples with barriers, differences were minimal initially. However, tipping the shippers on their sides introduced new dynamics. With its open top, the Base Sleeve saw a significant temperature drop as CO₂ escaped easily.

In contrast, the Base Sleeve + Overhang maintained better pressure and experienced less temperature fluctuation with its higher vent hole from the ground.

"Both sleeve samples saw a dip in temperature when tipped," explained Deltano. "The Base Sleeve saw a larger dip because the CO₂ could escape easily, whereas the Overhang sample maintained some pressure due to its vent hole placement."

The findings revealed the potential of CO₂ barriers in managing supercooling. "By using a CO₂ barrier to control gas flow, we can reduce temperature drops and mitigate the rate of dry ice sublimation,” Deltano said.

This early research highlighted that both placement and size of vent holes make a difference in maintaining desired temperatures and durations. The Base Sleeve + Overhang, with its strategically placed vent, outperformed the base sleeve in shipper duration aspects.

When asked if there are concerns that the vent could be blocked in transit via misapplication or neighboring shippers and create an overpressure situation, Deltano noted that they are still looking into particular vent sizes and blockage potential. If the vent is blocked by corrugate, gas can still escape because that material is CO₂ permeable.

Research remains ongoing with different materials. A paper barrier was shown to have a small effect on supercooling mitigation, but the effect was not as pronounced as the polyethylene bags used in this study. 

Across the industry, manufacturers are looking to shift to sustainable transport packaging materials. The addition of new materials should always be weighed with the risks of product damage and waste.