EuroWire November 2018

Technical Article

Halogen-Free Compound Solutions to Address Thermal Stress Cracking in Extreme Conditions By Mark A Jozokos and Tanya Artingstall, Mexichem Specialty Compounds

Abstract Low-smoke halogen-free (LSHF) com- pounds are used in many cable jacket applications. The majority of LSHF compounds are semi-crystalline polymer systems that are highly filled with metal hydrate flame-retardants. Due to the crystalline nature of these compounds, they are more susceptible to cracking in storage or in the field. Mexichem Specialty Compounds has established a systematic analysis to determine if the cracking failure is due to compound performance/capability or if the cracking could be avoided through improved processing techniques. Besides analysing and helping to pinpoint necessary processing techniques for these polymer types, Mexichem designs low-smoke halogen-free (LSHF) materials that are more resistant to thermal stress cracking. These compounds are formulated to be more robust, making them a higher- performance solution for speciality cables used in harsh environmental conditions. This paper will review the company’s process for analysing frozen-in stress via DSC analysis and presenting solutions both through processing techniques and with higher-performance materials. 1 Introduction As the technical requirements of our wired global community expand, so do the

To answer these challenges, Mexichem Specialty Compounds has established a systematic analysis to determine if the cracking failure is due to compound performance or if the cracking could be avoided through different processing techniques. 2 Troubleshooting a cracked cable When presented with a cracked cable, Mexichem Specialty Compounds analyses the cable using differential scanning calorimetry (DSC) analysis. As most LSHF compounds are based on polyolefin polymer systems which are crystalline in nature, they are prone to frozen-in stress if cooled too quickly. Via this thermal analysis technique, the frozen-in stress is revealed as false melting peaks in a heat-cool-heat cycle. If cooled too quickly, the jacket is prevented from shrinking and obtaining the per cent crystallinity the polymer wants to achieve (equilibrium). The polymer shrinks with the chain mobility trying to reach its equilibrium state, and the stress level in the jacket exceeds the strength of the polymer which leads to cracking. In other words, equilibrium is prevented by the lack of chain mobility at temperatures below its melting point. Via a process of heating the cable material beyond the melting points of the polymer and then cooling at a

variety of materials we work with. Some of these materials, such as low-smoke halogen-free (LSHF) compounds, are unique in the way they behave. From the way they are processed to achieve optimum performance to the way they are tested to survive demanding environments, LSHF cables often need a different approach than we apply to more traditional materials such as PVC. Depending on the end-use environment of an LSHF cable, choosing a high-performing LSHF compound is important. For example, cables that are intended to survive in arid and/or humid spaces will require compounds formulated to be more robust. In a desert environment, thermal stress cracking is of concern, so choosing a compound with improved thermal stress crack-resistant properties is important. best-suited compound, the manner in which these materials are processed is another deciding factor in the optimum per- formance of the end product. One of the challenges many cable producers face is manufacturing a LSHF compound with equipment and processes designed for more traditional materials such as PVC. Being an amorphous material, PVC compounds have a much wider processing window as compared with LSHF compounds. Using extrusion practices built around PVC materials often results in creating frozen- in stresses on the LSHF cable which leads to cracking. Besides choosing the

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November 2018