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October 2016

Sipolprene Offers Greater Resistance To Polymer Degradation For Cable Jacketing Applications.

MACPLAS INTERNATIONAL K2016

Sipolprene range by sipol includes different tpc-et grades, often used for cable jacketing applications, developed by the company throughout the years. A stabilized grade has been obtained, offering greater resistance to polymer Degradation, thus improving the performance of the polymer itself.

In the cable industry, many different thermoplastic elastomer materials are used for jacketing operations. Both electrical flow properties (intensity, voltage etc.) and peculiarities of conductive materials identify the specific insulation needed for cables. When selecting the most suitable thermoplastic polymer for cable jacketing, several properties have to be taken into consideration, bearing in mind the electrical, mechanical, thermal characteristics of the polymer as well as its chemical resistance.

Being insulation the primary requirement for a cable jacket, the electrical properties of the polymer - dielectric constant, insulating capacity to the power factor and arc resistance - need to be evaluated first.
Besides, jacketing has to take into account working conditions and regulatory requirements for the specific application. In the electronic adjustment of car reflectors, for example, the fogging behaviour of the polymer used is the main characteristic to be considered when choosing the polymer for the cable jacketing of such systems. Instead, in complex cable systems for undersea exploration, fogging becomes negligible as the focus is on hydrolysis resistance.

A DEMANDING APPLICATION
Cable jacketing is obtained through a traditional co-extrusion process where the cable core (usually copper or optical fibre) is insulated from the external environment by the thermoplastic polymer.

PVC is still the most widely used polymer for protective jackets, especially in electric applications, due to its excellent cost/performance ratio. The variety of technical requirements to be satisfied have led to a rapid growth in the use of specific engineering plastics, where the main are HDPE, PEX, TPU, PBT, TPV, PA, fluoropolymers and TPC-ET.

Sipol, an Italian company specializing in manufacturing co-polyesters and co-polyamides, has been developing, for years, TPC-ET (polyether-polyester based thermoplastic elastomers), known as Sipolprene and commonly used in cable jacketing.

In highly variable working environments, the Sipolprene range is able to put together in a single kind of polymer, high dielectric strength characteristics (with values between 22 and 27 kV/mm as per IEC 60243), the mechanical characteristics of a thermoplastic elastomer and the typical high chemical resistance of a polyester. TPC-ET, thanks to their block structure with a combination of “hard” and “soft” segments, maintain their mechanical properties of flexibility and creep resistance at temperatures between -30°C and 100°C. In addition, all Sipolprene products do not contain halogens, phthalates and heavy metals, and are suitable for direct food contact.

The interaction between metals and polymers, at high temperatures and with long contact times, leads to thermo-oxidative degradation effects due to the presence of free radicals. Studies have widely demonstrated that some metals, such as copper, at high temperature, also cause a catalytic action that speeds up the polymer degradation process.

Sipol has therefore developed a range of stabilized products (identified with the suffix MD), like Sipolprene 72220 MD, in which the metal catalytic action is inhibited by using a stabilization package that guarantees improved resistance to degradation.

COLLABORATION AND TESTS
The collaboration between Sipol and the Laboratory of Materials and Polymers (LaMPo) of the University of Milan, has provided the demonstration of the advantages achieved in terms of copper stabilization.

Thanks to a comparative study between Sipolprene MD 72220 (stabilized version) and Sipolprene 72220 (standard version), carried out in extreme conditions (high temperature and oxidizing atmosphere), some stability differences have been demonstrated.

Tests to evaluate the MD stabilization package performances were run in the presence of metallic copper on copper wire jacketed with both stabilized Sipolprene 72220 MD and standard Sipolprene 72220 (method prescribed by the IEC 60811-410 regarding the evaluation of oxidation resistance catalyzed by copper of polyolefin-insulated conductors).

From an analytical point of view, tests were run with DSC equipment (Differential Scanning Calorimetry). The DSC analysis is generally used to evaluate the material thermal transitions, such as Tg glass transition temperature, melting temperature and crystallization temperature. In this specific case, the DSC equipment has been used to measure the so-called OIT (Oxidation Induction Time), which is the time that elapses between the end of melting of the material and the beginning of its decomposition, under isothermal conditions of high temperature and in an oxidizing atmosphere with pure oxygen.

The greater the oxidation stability of the material, the higher the OIT value. The samples were heated up to 300°C in an inert atmosphere and then kept at the same temperature for 30 minutes in a pure oxygen atmosphere. Thermograms comparison (figure 1) shows that, while the cable covered with standard Sipolprene 72220 (blue curve) has an OIT of about 9 minutes, the cable covered with Sipolprene 72220 MD (pink curve) shows no evidence of degradation for the entire duration of the test.

The superior behaviour of the MD series has been proved for all those applications requiring extremely high working temperatures. The MD stabilization is available, on request, on all Sipolprene grades with hardness between ShD 25 and ShD 72.

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