Thermal Analysis of Heat Shrink Tubing

Heat shrink tubing appears simple at first glance: wrap it around your wires, apply a heat gun, and voila – perfectly insulated wires every time. But these tubes are more complex than they seem.

Almost all materials, from rubber and metal to water and glass, expand when heated. Heat shrink tubing is uniquely designed to shrink at an appropriate temperature: not so low that it may accidentally shrink in transport or storage, yet not so high that it risks damaging enclosed wiring and nearby electronics. Furthermore, most heat shrink tubing is also flame-retardant to protect delicate wires, requiring further material optimization.

While heat shrink tubing is already an impressive feat, the market is growing even more demanding with the rapid expansion of AI data centers. For decades, data centers have used heat shrink tubing to label, bundle, and insulate their cables from damage and heat. New AI data centers generate more heat from high-power hardware, requiring sophisticated thermal management systems. Heat shrink tubing must not only meet current application expectations but now accommodate increasingly challenging environments such as these higher-heat data centers.

How do material engineers strike this fine balance between thermal stability and heat-induced shrinking in wire insulation? We studied commercially available heat shrink tubing with three thermal analysis techniques to better understand their properties and demonstrate how manufacturers can benefit from complete characterization of their materials‘ behavior.

Differential Scanning Calorimeters (DSC) measure temperatures and heat flows associated with thermal transitions in a material. In heat-shrink tubing, DSC is ideal for identifying endothermic changes during heating.

Our DSC temperature ramp revealed unexpected endothermic transitions at 50 °C and 88 °C, both significantly below this tubing’s specified 125 °C melt point. The first heating ramp is representative of the sample’s thermal history and evaluates the material „as received“.

The early peak at 50 °C may indicate a relaxation or recovery process rather than melting since it is not present when reheating the material. This transition can be further investigated with Modulated DSC (MDSC) – a feature included on every TA Instruments Discovery DSC – or by using the optional DSC Microscope Accessory. Modulated DSC would help clarify this by separating reversing (e.g., melting, heat capacity) from non-reversing (e.g. kinetic) events.

The DSC Microscope was used to capture video for the entire heating ramp, along with snapshots every 30 seconds. From the images and video, we see there are only small changes at temperatures below 80 °C and the shrinkage is relatively rapid from 88 to 103 °C. DSC provides this initial investigation into thermal events, and our subsequent TMA moves further to characterize the mechanical changes taking place at these temperatures.

Thermomechanical Analysis (TMA) measures changes in the dimensions of a sample as a function of time, temperature, and force in a controlled atmosphere. In our analysis of heat shrink tubing, TMA is especially valuable in understanding how the material shrinks and identifying its shrinkage temperature.

Our TMA test shows the heat shrink tubing expanding until about 88 °C. Most materials expand with heat, so this is expected. Then at 88 °C, heat shrink tubing undergoes its shrinking onset and shows a very sharp change in its dimension.

This shrinking onset aligns with the large endothermic heat flow revealed in our DSC analysis above. Again, an 88 °C shrinking onset is below the manufacturer’s specified 125 °C shrinking point, but the material is shown to finish its shrinking around 125°C. In this case, the manufacturer is reporting the temperature required for complete shrinkage – guiding users to successful wire insulation.

From the DSC images above, we saw that most of the shrinkage is in a single direction. TMA can be particularly helpful when investigating materials that behave differently depending on their orientation. Materials with fillers or layers are commonly placed in the TMA in different directions, aligning the material along the x-, y-, or z- axis for independent measurements. The expansion (or contraction) can be quantified in multiple orientations.

Thermogravimetric analyzers (TGA) measure weight change (loss or gain) and the rate of weight change as a function of temperature, time, and atmosphere. TGA is especially useful for detecting thermal degradation at high temperatures, helping to ensure the thermal stability of heat shrink tubing.

Our initial TGA weight change measurements are typical: we heat the material from 25 °C to 1000 °C and see a weight loss signaling degradation at approximately 325 °C. This temperature is far above what heat shrink tubing should be exposed to in regular use.

If this was a simple polyolefin, the derivative curve would simply show one full weight loss. However, the derivative TGA weight curve shows more than one weight change. This complexity suggests that the tubing is made of more than one material. The precise sensitivity of the TA Instruments Discovery TGA 5500 enables us to see this hidden behavior. Advanced features like Hi-Res TGA methods can be used to help separate overlapping transitions with ease by automating adjustments to the heating rate based on the sample weight loss profile.

Material engineers would benefit from precise TGA analysis to reveal material degradation as well as the presence of complex formulations in seemingly simple products. This tubing likely has a slew of additives, such as antioxidants, UV stabilizers, fillers, processing aids, and flame retardant mixed with the polyolefin.^{1, 2} Material manufacturers and engineers use TGA to quantify additives and assess how these affect end-use properties and material degradation. The TA Instruments Discovery TGA includes two integrated mass flow controllers for automated switching between inert and oxidative gases for simplicity in testing.

Combining three thermal analysis techniques allowed us to gain a broader view of heat shrink tubing’s properties, uncovering some interesting connections and results:

• DSC shows endothermic transitions at 50 °C and 88 °C, far below the specified melt temperature
• TMA shows shrinking begins around 88 °C, lining up with DSC’s endothermic transition
• A TGA derivative curve shows complex degradation, revealing the presence of additives altering the polyolefin’s behavior

Learn more about our analysis of heat shrink tubing and see the full experiments in an application note coming soon – keep an eye on TA Instruments‘ LinkedIn for it to be published.

This analysis was made possible by TA Instruments‘ advanced engineering, delivering the most accurate and reliable thermal characterization. Explore TA Instruments‘ complete line of industry-leading thermal analyzers and contact us for personalized recommendations to enhance your workflow.

1. TA Instruments. Use of TGA to Distinguish Flame-Retarded Polymers from Standard Polymers. https://www.tainstruments.com/applications-notes/use-of-tga-to-distinguish-flame-retarded-polymers-from-standard-polymers/
2. TA Instruments. Ash Testing of Formulated Products Using Thermogravimetric Analysis. https://www.tainstruments.com/applications-notes/ash-testing-of-formulated-products-using-thermogravimetric-analysis/