EuroWire November 2018

Technical Article

very slow rate to ensure equilibrium, the graphic representation of this cycle reveals any differences between initial and subsequent cycles. These differences indicate if changes need to be made in the processing techniques. When LSHF compounds are processed with an eye toward achieving equilibrium, cracking can be avoided. analysis example Mexichem was recently asked by a cable manufacturer to analyse a cable that was installed in a harsh outdoor environment and exhibited cracking. of troubleshooting the cracking problem included analysing compound production records to ensure proper ingredients and procedures had been utilised. The company combined efforts to explore the history of production, both at the cable producer and in the compound manufacturing environment. Although this manufacturer had been successfully making LSHF cables for years, it was decided to conduct DSC analysis with cable samples as follows: • Cable 1 : “Good” cable, aged, with smaller diameter and thinner wall. They had not had a failure with this design in over four years The traditional methods 3 Cracked cable

▲ ▲ Figure 3 : Cable 3 DSC curve

▲ ▲ Figure 4 : Cable 4 DSC curve

Looking at the DSC curve for Cable 3, the “failed” cracked cable, reveals a very large difference between the initial heating curve (dashed line) and the second heating (solid line). The true melting behaviour of the jacket compound is revealed in the second heating after slow cooling where the jacket compound reaches equilibrium. The first heating curve appears to have multiple false melting points and large in enthalpy (area of curve). This is an indication that the cable jacket contains a significant amount of frozen in stress. Based on the DSC analysis, Mexichem concluded that the “failed” cable that had cracked in the field exhibited frozen-in stress during processing which led to cracking. Processing recommendations were made to cool the cable as slowly as possible during production to minimise any frozen-in stress. The process engineer for the cable manufacturer noted that just prior to the cable failure, a water chiller had been installed to boost line speed. In order to achieve commitment to higher line speed, an additional water trough was installed so the cable would run through heated water to cooler water to achieve gradient cooling. After the production line was modified, the same lot of compound utilised to make the “failed” cable was run at the faster line speed and sent to Mexichem Specialty Compounds for analysis. This is referred to as Cable 4. Cable 4 was analysed by DSC to determine if the stress level had been reduced. Figure 4 shows that Cable 4 has no significant frozen-in stress, as the first and second heating curves are very close. Cable 4 was then subjected to a thermal cycling test to determine if it would crack if exposed to harsh environment.

• Cable 2 : “Good” cable with a larger diameter, thicker wall, and no cracking • Cable 3 : “Failed” cable that had cracked in the field. It had the same diameter and wall thickness as Cable 2 DSC analysis was performed on the failed cable and both “good” control cables utilising a heat-cool-heat test cycle. For the first heat cycle, the temperature started at 25°C and moved to 200°C at a heating rate of 10°C per minute. The next cycle, a slow cooling process, moved the temperature from 200°C to -25°C at a rate of 5°C per minute. The second heat cycle moved the temperature back up from -25°C to 200°C at a rate of 10°C per minute. See Figure 1 . In reviewing the DSC curve of Cable 1 in Figure 1 , it is noted that the first heating curve (dashed line) is not much different that the second heating (solid line). This shows that the material was processed well and did not contain significant frozen-in stress levels. This outcome was expected as this was a “good cable”. The smaller diameter and thinner wall made it easier to cool slowly during processing, which allowed the polymers to achieve equilibrium, free of significant stress levels. The Cable 2 DSC curve in Figure 2 shows a very similar behaviour. The first heating curve (dashed line) was not much different than the second heating (solid line). Again, this indicated a cable that was processed well and did not contain significant frozen-in stress levels. Although the manufacturer labelled this a “good” cable, the graph shows opportunities for process optimisation. A thicker jacket wall is more difficult to cool slowly (this can be achieved with hot water in the water cooling bath). Figure 2 shows that the material is capable of being produced with minimal frozen-in stress to the compound.

▲ ▲ Figure 1 : Cable 1 DSC curve

▲ ▲ Figure 2 : Cable 2 DSC curve

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November 2018