High Temperature Thermoplastics Polyurethanes for Cable Jacketing Applications

The temperature rating of jacketing material is very important for cable applications. Temperature rating helps users determine operating range of cable products. There are several ways one can determine temperature rating of cable products. One of the widely used methods is to assign temperature rating according to UL 1581. Until recently, TPUs can only be rated at a maximum 80°C via short term aging as per UL 1581. As such, TPUs were not considered for higher temperature applications. Due to Lubrizol’s proposal, the revision of UL 1581 dated January 30, 2013 enables TPU materials to be rated for 90°C and 105°C via simple 7 days aging test.

In this paper, data are presented for thermal aging of two TPU grades for 90°C and 105°C rating as per UL 1581. Effects of processing variables such as RPM and temperature on mechanical properties of two TPU grades are reviewed. Additionally, post extrusion shrinkage data and effect of processing variables on shrinkage of two TPU grades are presented. At the end, new high temperature TPU technology with ability to get 125°C rating is reviewed.

1. Introduction

Every polymer has its own thermal aging characteristic depending upon chemistry of the polymer and additives incorporated into the product. Generally, polymers age by gradual loss of its ultimate elongation. For wire and cable applications, it is commonly considered that a minimum 50% ultimate elongation of sheathing material is necessary to ensure cable functions without cracking. Many flexible jacketing polymers obtain flexibility by addition of plasticizers and/or oils. Specific examples include flexible PVCs and some TPEs. Upon thermal aging, these compounds gain tensile strength by loss of oil/plasticizer. However, their ultimate elongation reduces because they become brittle by loss of plasticizing additives. TPUs on the other hand behave the opposite way after thermal aging. Wire and cable TPU compounds are flexible by virtue of their inherent chemistry. No oil or plasticizing additives are added to TPUs to obtain lower hardness and flexibility. After thermal aging at temperatures for 90°C and 105°C rating, most polyether TPUs’ tensile strength reduces more than 25%. However, they still maintain absolute tensile strength well above the functional requirements for cable jacketing applications. Because of original tensile strength loss of more than 25%, most of polyether TPUs cannot be rated 90°C and higher under TPE categories as per short term aging criteria specified in UL 1581.

As a result of Lubrizol’s initiative, the latest revision of UL 1581 dated January 30, 2013 includes two new tables 50.227.1 and 50.227.2 for 90°C and 105°C TPUs respectively. For 90°C, the rating table specifies to maintain 65% of unaged tensile strength and 70 % of unaged ultimate elongation. For 105 °C rating it is required to maintain 35% of unaged tensile strength and 85% of unaged ultimate elongation. These new requirements reflect aging behavior of polyether TPUs.

2. Experimental Materials

TPUs used in the study were RoHS and REACH complying commercial Estane® TPU grades. TPU-A and TPU-B were flame retardant 85A hardness polyether and non-flame retardant 87A hardness polyether respectively. TPU-C was newly developed TPU for high temperature application capable of achieving 125°C rating.

3. Results and Discussion

3. 1 90°C and 105°C TPUs

Table 1 shows the unaged and aged tensile properties of TPU-A and TPU-B. For 90°C rating oven aging was conducted at 121°C temperature. It can be seen that both TPUs easily passed new requirements of UL1581 for tensile strength. Ultimate elongation values after thermal aging were even higher than their unaged values.

Table 2 shows physical properties of TPU-A and TPU-B for 105 °C rating. UL 1581 only requires maintaining 35 % of original tensile strength after thermal aging for 105°C TPU. However, original and aged ultimate elongation requirements are very high. Both TPUs tested exhibit typical polyether TPU aging behavior and passes the requirements for 105°C rating. It can be seen that there was significant reduction in tensile strength after aging at 13°C but ultimate elongation was virtually unaffected.

3.2 New 125°C High Temperature TPU

Lubrizol has developed new TPU to help cable manufacturer expand their ruggedized cable product solutions at higher temperatures. New TPU has excellent abrasion and cut through resistance, high flex life, and low temperature flexibility typical of standard TPUs. It improves temperature and chemical resistance performance significantly over conventional TPUs making it highly suitable for demanding applications requiring armor against high temperature, physical abuse and harsh chemical exposures.

Table 4 shows thermal aging data for TPU-C for 125°C rating on tubular specimen extruded on crosshead die. It can be seen that tensile strength and ultimate elongation for TPU-C was virtually unaffected after thermal aging. TPU-C maintained more than 90% of original tensile strength at 158°C for 7 days.

Long term heat aging and hydrolysis performance for TPU-C is shown in Figure 5 and Figure 6 respectively. Tensile properties were measured on die cut specimens from 1mm thick extruded strip. After 3000 h of thermal aging, TPU-C maintained ultimate elongation well above 50%. For comparison, conventional polyether TPU would lose ultimate elongation completely and become brittle after 6-12 weeks of exposure at 150°C.

There was no negative effect on ultimate elongation and marginal drop in tensile strength of TPU-C after 3000 h hydrolysis at 85°C. Table 5 shows excellent oil aging performance of TPU-C.

4. Conclusions

TPU-A and TPU-B are suitable material for 90°C and 105°C rated cable applications specifically if applications involves abusive physical or harsh chemical exposures. In processing study, high rpm and high temperatures resulted in slightly reduced tensile strengths of TPUs studied. Both TPU-A and TPU-B showed extremely low post extrusion shrinkage making them suitable for optical cable jacketing applications. New TPU-C, Estane® TS92AP7 NAT 021, is a higher temperature TPU with its 125°C rating, truly is the next generation of high-temperature-resistant, easy to process TPUs. Because it is an Estane Engineered Polymer, wire & cable manufacturers aren’t forced to sacrifice performance. Estane TS92AP7 NAT 021 offers industry leading toughness and durability, hydrolysis resistance, chemical resistance and low-temperature flexibility.

References

[1] UL 1581, Reference standard for electrical wires, cables, and flexible cords.

[2] UL 2556, Wire and Cable test methods.

.**This article is authored by Mr. Chetan Makadia,
(Sr. Applications Engineer), Mr. Joe Vontorcik, (Americas Applications R&D Technology Manager), Mr. Viviana Wilson, (Regional Market Segment Manager – Wire & Cable) from Estane® Engineered Polymers, Lubrizol Advanced Materials, Inc.