When wire or cable is exposed to elevated temperatures—sometimes even as little as two days at 100 °C—the polymer compounds used to make the wire or cable can experience degradation or polymer reordering, which can result in a loss of tear strength. This degradation or polymer reordering may result as cracks in the outer layer in a phenomenon called thermal stress cracking. A product’s thermal stress cracking resistance (TSCR) depends on the polymer compound formulation and the processing conditions under which the wire or cable was extruded, in addition to the temperature of the environment. Thermal stress cracking can be minimized or eliminated by carefully choosing the polymer compound formulation and extrusion conditions.
Why does thermal stress cracking occur?
When a wire or cable is melted, extruded, and then cooled, stresses can be “frozen in” to the product when the polymer is cooled too quickly. When a polymer is exposed to elevated temperatures close to its melting point and above its glass transition temperature, the polymer chains begin to move or reorder crystalline sites from their frozen-in position (in the amorphous state) back to their equilibrium (crystalline) state, which results in shrinkage of the polymer. When the amount of shrinkage exceeds the tensile strength of the polymer, a crack is formed.
The cracking mechanism may also result from the reduced tear strength observed at elevated temperatures. A tear can be initiated by defects on the internal surface of the cable arising from the use of tapes or armour (see Figure 1)
Figure 1 “Sheath internal defects caused by tape wrap”
Tears can also be initiated by defects on the outer surface of the cable caused by poor installation practices. Once a tear is initiated at such a “notch,” it can propagate along the cable and be seen as a crack, as shown in Figure 2.
Figure 2 – “Here the split has followed the line of the armour”
How does polymer compound formulation affect TSCR?
Thermal stress cracking occurs more frequently with low-smoke, halogen-free (LSHF) polymer compounds, which typically contain high levels of minerals in polyolefins like polyethylene (PE), ethylene methacrylate copolymers (EMA), ethylene vinyl acetate copolymers (EVA), and thermoplastic elastomers (TPE). It is easier to “freeze” stresses into these semicrystalline polymers than into amorphous PVC. High mineral filler levels can enhance crack formation by increasing the tensile pressure on the polymers.
Experts in polymer compound formulation can help identify the optimal polymer, flame retardant, and other additives, such as coupling agents, to meet the physical property requirements of an application and minimize stress cracking. “Everything is a balance,” explains Tanya Artingstall, Wire and Cable Development Manager at Mexichem Specialty Compounds (MSC). “A certain polymer compound might be desired to pass a certain flame test, but physical property requirements might call for a slightly different formulation. In some cases, low temperature impact, tensile, and flex strength are important. TSCR is another factor. And all these requirements must also be balanced with cost.”
How does processing affect TSCR?
The conditions used to extrude the wire or cable play a significant role in TSCR. Cooling rate is the most critical factor. Cooling too quickly can freeze more stress into the polymer. The polymer’s crystallinity is one aspect to consider when optimizing the cooling rate; the higher the equilibrium crystallinity of the polymer, the slower the cooling rate will need to be. The part dimensions should also be considered. A product with a thicker wall will need slower cooling than a product with a thinner wall.
Slower cooling typically means slower line speeds. Gradient cooling, which uses progressively cooler water to speed cooling without initially quenching the polymer, can help prevent cracking while maintaining line speeds.
Thermal stress cracking can be simulated in the laboratory using the Environmental Stress Crack test jig described in the British Standards Institution test, BS EN 60811 part 4.1: 2004 (or the equivalent IEC standard). Compression molded plaques are heated in an oven at a defined temperature, and the crack behavior of the samples is monitored over time. This test can be used to compare polymer compounds, but it does not take processing effects into account because samples are molded rather than extruded.
Some cable producers run field tests: they extrude cable, wrap it on a mandrel, and expose it to elevated temperatures to see if it will crack. MSC developed a laboratory test to predict thermal stress cracking resistance on extruded parts made under various extrusion conditions. This test gives more immediate results than field testing and has been shown to be a good predictor of cracking. It can help determine the optimal rate to cool the extrudate efficiently but not freeze in stress.
Improved TSCR formulas
MSC’s MEGOLON® S382 and S384 were developed to improve tear strength at elevated temperatures compared to an existing grade with a Limiting Oxygen Index (LOI) of 35. MSC then developed MEGOLON® S386, which has a LOI of 40 and excellent TSCR compared to a standard grade with LOI of 40. Table I shows the results of the new formula compared to the conventional one at a range of temperatures from 60 to 100 C, using the laboratory thermal stress crack test and the BS EN 60811 jig.