Sat Nistala, Larry Hewitt, Jacques Horrion and Santosh Mishra
Thermoplastic polyurethanes (TPU) are a class of polyurethane plastics with many exceptional properties including elasticity, high tear strength, and resistance to oil, grease and abrasion. Technically, they are elastomers of linear segmented block copolymers composed of hard and soft segments. TPUs were first developed by The Lubrizol Corporation (then The BFGoodrich Corporation) in the 1950s; the Estane® brand TPUs were patented and introduced commercially in 1959.
Unlike many commodity elastomers such as polyethylene PE, polypropylene PP, nylon 6, and PVC, the chemistry of TPU allows the creation of a wide range of products with hardness ranging from about 65 Shore A to 85 Shore D. Nearly all TPUs are pure polymers and achieve their range of behavior without the use of additives such as plasticizers, fillers or ‘blending’ resins. In addition to hardness, properties such as flexibility and low temperature performance cover a wide range. However, key TPU expectations such as elasticity, toughness and abrasion resistance are essentially retained. Unlike most rubbers and cast polyurethanes, TPUs are not cross-linked and not highly crystalline, which allows them to be readily melted and processed by conventional injection molding and extrusion equipment. The thermoplastic nature of TPUs allows for the reuse of reclaimed TPU from, for example, injection molded parts with a size defect or cable jackets from cables that did not meet a diameter requirement.
The chemistry and polymeric behavior of TPUs has been published in many journals and review articles. Recent articles1 “Thermoplastic Polyurethane (TPU) in Wire and Cable Applications,” covered basic TPU chemistry and focused on properties that are especially important for wire and cable application. Information in the reference will be useful to select a TPU grade and to process it on an extruder successfully. The characteristics of the two main chemical classifications within TPU – polyester TPUs and polyether TPUs are discussed in the article.
1. Sat Nistala, Larry Hewitt, Santosh Mishra and Ed Godlewsky, Wire and Cable India, Nov-Dec, 2011 Page No: 50.
Comparison of TPUs to Other Elastomer Groups
When confronted with several material choices for a wire and cable application, it is helpful to compare and contrast the behavior of TPUs to the behavior of other thermoplastic materials. Figure 1 demonstrates the tensile / elongation behavior of several classes of elastomeric materials. While most are considered thermoplastic, materials such as PP + EPDM and SBR require curing steps after an extrusion process to create a cable jacket, for example. The wide product range of Lubrizol’s TPU, demonstrated below, provides high tensile strength and high elasticity for diverse applications.
Figure 2 contrasts the E-modulus (measure of stiffness) of the various classes of elastomers. By this criterion, the Lubrizol’s TPU overlaps many material groups. The complete Lubrizol TPU family of products is shown in the figure 2; included is the Estane® TPU product line with the Isoplast® TPU grades at the high end of the modulus range. Estaloc® RETP and LGF Isoplast TPU are reinforced grades that are frequently considered as metal replacements.
TPU Products in Typical Wire and Cable Applications
While TPU products encompass both polyester and polyether types, polyether grades are used nearly exclusively for wire and cable (W&C) applications; indeed, many of the polyether TPU grades were created originally for W&C applications. Most uses of polyether TPU in W&C today involve use as a protective outer jacket or sheath. The main function of the jacket is to protect the primary insulation from environmental damage, be it weathering, hydrolysis, or physical abuse. TPU rarely is used as a primarily electrical insulator although there are recent developments in under-the-hood cables.
The jacketing typically uses TPU in the Shore A 80 to 90 range. Although there are grades that are harder, they also impart added stiffness. TPU is usually used in cables with dynamic applications. Most TPU jacketed cables are in frequent or continuous motion, and those in the geophysical industry may take a brutal treatment. TPU also provides important protection during the installation process for cables in static environments. Cables jacketed with the high volume commonly used materials – polyolefin (PE) or PVC – cannot withstand the abuse of these applications.
Unique TPUs for Optical Fiber Cables
Fiber optic cable constructions present different challenges for elastomers. The cables are typically not passing electrical current; hence, there is no electrical insulation aspect. However, the relatively fragile glass fibers require extensive protection to prevent breakage, and encasing the fibers within a tough material is essential. Historically, materials such as nylon 12 (PA12) have been frequently used as components within a fiber optic constructions even though TPUs offer better abrasion and puncture/cut resistance properties. However, unique TPUs from Lubrizol Corporation are getting popular in fiber optical cable applications due to recent shortage of PA 12. TPU considered being relatively cost effective compared to PA 12. Much of the remainder of this article will focus on TPU as a candidate for fiber optic cable. In the example, TPU would function as the coating directly on the glass fiber. The same or different TPU could provide the outer jacket. Finally the material separating the coated fibers could also be a TPU.
Lubrizol has developed and commercialized a line of TPUs that can match and even exceed the same functions as PA12, PA11, copolyester elastomers (COPE) and copolyamide elastomers (COPA). The Lubrizol ETE (Easy-to-Extrude) TPUs are hard (>50 Shore D), high modulus compounds. They not only have most physical properties and behaviors comparable to PA12, the ETE TPU grades are relatively easy to melt process in single screw extruders.
Prior to ETE TPUs, >50 Shore D TPUs were essentially melt processable only by high shear injection molding. Extrusion of typical hard TPU grades was near impossible due to their high crystallinity which resulted in a narrow melt range, very fast crystallization and tendency to freeze within the extruder. ETE TPUs are a unique patented technology that overcomes the extrusion difficulties common to hard TPUs. Among the ETE TPUs (Table 1), the two hardest grades exhibit the polymer behaviors that make them best candidates for PA12 replacement. The properties of ETE Estane® 70DT3 (70 Shore D) and 75DT3 are highlighted in Table 2 (the extrusion processing aspects of ETE TPU will not be covered in this article).
During bend testing of small diameter tubing made from PA12 and the two hardest ETE grades, it was noted that TPU resists kinking better than PA12 at small bend radii. This behavior may be important in some fiber optic cable uses.
PA12 in general exhibits resistance to a variety of chemicals. ETE 70DT3 and 75DT3 TPUs exhibit high resistance as well. Table 2 demonstrates the effects of 1-week exposures of each material to water at 80°C, IRM 902 oil at 100°C and heat aging in air at 110°C.
PA12 is generally considered as an acid-resistance material while much literature notes the unsuitability of TPUs with acids. Polyether TPUs inherently have good resistance while the ETE grades exhibit exceptional behavior. The results of one week acid exposures on tensile / elongation retention are shown in Table 3. ETE 70DT3 appears to be the most affected by the 10% HCL solution; however, the tensile retention is still over 80%.
Requirements for Fiber Optic Cables
Specifications, standards, and test procedures for fiber optic cables are issued by several global and regional organizations. The Fiber Optic Association (FOA, www.thefoa.org) maintains a large database listing of technical documents (or links to) such as the FOTPs (Fiber Optic Test Procedures) and TIA (Telecommunications Industry Association) Standards. FOTPs are issued by the TIA. Because of the large number of uses of cables, one example of use in underground installation is selected. General expectations of FO cables for underground installation are grouped below as Functional Requirements.
Functional Requirements
1. The design and construction of Optical fiber cable shall be inherently robust and rigid under all conditions of installation, operation, adjustment, replacement, storage and transport.
2. The Optical fiber cable shall be able to work in a saline atmosphere in coastal areas and should be protected against corrosion.
3. Life of cable shall be at least 25 years. Necessary statistical calculations shall be submitted by the manufacturer, based upon life of the fiber and other component parts of the cable. The cable shall meet the cable aging test requirement.
4. It shall be possible to operate and handle the optical fiber cable with standard tools. If any special tool is required for operating and handling the optical fiber cable the same shall be provided along with the cable.
5. The optical fiber cable supplied shall be suitable and compatible to match with the dimensions, fixing, terminating & splicing arrangement of the splice closure. The cable supplied shall also meet other requirement of splice closure.
6. The manufacturer shall submit an undertaking that the optical and mechanical fiber characteristics shall not change during the lifetime of the cable against the manufacturing defects.
Flexible (approximately 80 to 90 Shore A hardness) Estane® polyether TPU grades have been used for nearly 50 years as cable jacketing and have met requirements for a large variety of cable constructions and uses including underwater installations. Accelerated high temperature water immersion test data concludes that typical polyether TPUs have tensile half lives > 25 years submerged. As outer jacket materials, 70D and 75D hardness Estane ETE TPUs represent the more durable end of the TPU line. While some of the expectations are dependent mostly upon the cable design and construction, ETE TPUs are expected to contribute to meeting the requirements.
Fiber optic cable specification IEC-60794 (consisting of several parts) details the specific performance requirements. The characteristics of materials used in the jacket play a key role in whether the cable will pass the particular test. Several examples of critical tests within IEC-60794:
Abrasion Test : As per IEC-60794-1-2-E2 or by any other international test method.
Crush Test (Compressive Test) : IEC 60794-1-2-E3.
Impact Test : IEC 60794-1-2-E4.
Repeated Bending : IEC 60794-1-2-E6
Torsion Test : IEC 60794-1-2-E7.
Cable Bend Test : IEC 60794-1-2-E11 (Procedure-I).
Temperature Cycling (Type Test) : IEC 60794-1-2-F1
Test of Figure of 8 (Eight) on the cable (Type Test)
While the tests are performed on finished cables, the jacket would provide a major contribution to the test results. The cable test equipment is quite different from elastomer-only test equipment such as used for tensile measurement. Material test data such as provided in the earlier tables can provide a guideline for use. Indeed, fiber optical cables for tactical applications have been constructed similar to those in the earlier cross section above have been commercialized employing Estane® ETE 70 DT3 and ETE 75DT3. Typical cross section of fiber optical made using Estane TPUs shown below (figure 4).
The above cable integrity has been extensively tested along with several properties such as cable fiber attenuation, fiber dimension, dimensional & weight check, tensile strength test, crush load test and bend test. The termite resistant test for FO cables is currently underway. However, TPU is considered being termite and rodent resistant (not chewable) due its excellent abrasion and cut resistant properties. While extrusion processing conditions for ETE products are provided in Technical Datasheets, it is recommended that customers planning wire and cable extrusion to consult with technical service representatives.