Australian cold chain professionals obsess over the conductive heat transfer properties of their insulation materials — and rightfully so. But there is a second heat transfer mechanism that receives far less attention and causes far more cold chain failures during Australian summer: thermal radiation. On a 40°C January day in Brisbane, Sydney or Perth, the radiant heat load on an exposed cold chain package can match or exceed the conductive heat ingress through the packaging walls. Ignoring it is an engineering mistake.
What Is Thermal Radiation?
Thermal radiation is electromagnetic energy emitted by any object above absolute zero. Unlike conduction (which requires physical contact) and convection (which requires a fluid medium), radiation travels through a vacuum and requires no physical medium. The power radiated by a surface is described by the Stefan-Boltzmann Law:
P = ε × σ × A × T⁴
Note the fourth power dependence on temperature: doubling the absolute temperature of a surface increases radiated power by a factor of 16. This is why hot surfaces are such intense heat sources.
Australian Surface Temperatures in Summer
| Surface / Location | Peak Summer Temp (°C) | Risk Level |
|---|---|---|
| Shaded concrete dock | 28–35 | Moderate |
| Exposed asphalt (depot/parking) | 55–70 | High |
| Delivery van steel floor (parked) | 60–70 | Very High |
| Direct sunlit carton surface | 40–55 | High |
| Airport tarmac (Darwin, July) | 65–75 | Extreme |
| Unshaded loading dock roof | 70–80 | Extreme |
A delivery van floor at 65°C radiates approximately 900 W/m² toward any package sitting on it. A standard cardboard-outer carton (emissivity ~0.90) absorbs ~810 W/m² through its base alone. For a package base of 0.04 m², that is 32 W of radiant heat ingress through the base alone — comparable to the total conductive heat ingress through all four walls of a well-insulated carton.
Emissivity: The Key Property
Emissivity (ε) describes how efficiently a surface absorbs thermal radiation. A surface with ε = 0.90 absorbs 90% of incident radiant heat. A surface with ε = 0.05 absorbs only 5% — reflecting 95%.
| Outer Packaging Surface | Emissivity | Radiant Heat Absorbed | Australian Summer Suitability |
|---|---|---|---|
| Plain cardboard | 0.90 | 90% | Poor |
| White-painted surface | 0.90 | 90% | Poor — colour matters for visible light, NOT infrared |
| Bubble-foil (genuine aluminium side out) | 0.05–0.10 | 5–10% | Good |
| MPET metallised liner | 0.03–0.05 | 3–5% | Excellent — near-perfect IR reflector |
| Plain white HDPE | 0.85–0.90 | 85–90% | Poor despite light colour |
Critical note: White paint absorbs approximately 90% of far-infrared thermal radiation — essentially the same as black paint. The property that matters is emissivity at infrared wavelengths, not visible colour. Many buyers assume their “white” outer packaging reflects heat. It does not.
The MPET Solution
The engineering solution is to place a low-emissivity surface on the exterior of the insulated package. MPET (metallised PET) insulated liners and mailer systems have a metallised aluminium surface that reflects over 95% of incident infrared radiation before it can enter the insulation layer.
Field testing in Australian summer conditions (38°C ambient, vehicle floor at 65°C) has shown MPET packages maintaining target temperature up to 40% longer than standard EPS-only boxes of equivalent wall thickness. This improvement comes entirely from eliminating the radiant heat pathway — conductive performance is identical.
Practical Recommendations
- Always use MPET-lined packaging for shipments in November–March, particularly in Queensland, NT, WA, and outback NSW/SA.
- Never leave packages exposed to direct sunlight during loading or delivery. Even 10 minutes can represent a significant fraction of your refrigerant capacity.
- Confirm genuine MPET content — request the emissivity specification. Genuine MPET will be ε ≤ 0.05. Plastic-printed “silver” surfaces are often ε = 0.85–0.90, providing almost no radiant protection.
- Use validated qualification tests at summer peak ambient temperatures for pharmaceutical shipments — not average annual temperatures.
Conclusion
Radiant heat is the hidden variable that causes Australian cold chain systems to fail in summer when they performed perfectly in cooler months. The investment in genuine MPET-lined packaging is invariably recovered through refrigerant savings and reduced failure rates. Explore Dry Chill’s range of insulated packaging with MPET technology engineered for Australian conditions.