Thermoplastic Polyurethane (TPU) in Wire and Cable Applications
Thermoplastic polyurethanes (TPU) are a class of polyurethane plastics with many useful properties, including elasticity, transparency, and resistance to oil, grease and abrasion. Technically, they are thermoplastic elastomers consisting of linear segmented block copolymers composed of hard and soft segments.
Who invented TPU?
Commercially, polyurethane rigid and flexible foams are the dominant market products and were developed in the late 1930s. Thermoplastic polyurethane was developed by Lubrizol (then BFGoodrich) in the 1950s; Estane® brand TPUs were patented and introduced in 1959. TPUs differed significantly from the earlier polyurethane materials in being thermoplastic rather than thermoset. Although they are manufactured from the same classes of raw materials, the polymers that are formed are linear, not cross-linked, are not highly crystalline and can be melted at temperatures common to many thermoplastic materials. Consequently, they are soluble in many solvents and can be melted and formed with conventional injection molding and extrusion equipment.
Early uses of TPU took advantage of this linear polymer structure when they were used solution-based formulations for creating durable industrial coatings (fabric, cables, tank liners, etc.) and adhesives.
Chemistry of TPU
Thermoplastic polyurethanes are manufactured primarily using three basic raw materials:
1. Diisocyanates such as diphenylmethane diisocyanate (MDI)
2. Difunctional polyols with lower molecular weights, and
3. Lower molecular weight diols commonly called chain extenders.
The diisocyanate chemical group -NCO reacts with the hydroxyl groups -OH on both the polyol and diol to form urethane groups. The reaction of the three ingredients leads to the formation of a linear block copolymer. The diisocyanate and polyol react randomly to form soft segments while the diisocyanate and diol form hard segments. The entire TPU polymer chain then consists of alternating sequences of hard and soft segments. The average lengths of the hard and soft segments are governed by the starting amounts of the three ingredients.
In a solubilized or melted state, TPU polymers are basically just entangled nearly independent chains (Figure 1 model). However, in the solid state resulting after removal of the solvent or cooling of the melt, the chains re-organize and start to interact. The chemical structures of the hard segments are more polar and phase segregate from the soft segments. The segregation leads to the formation of hard segment domains consisting of many hard segments organized in a somewhat spherical shape. The polymer chain portions not in the hard segment domains are rich in soft segments.
While the chemical makeup of the TPUs is partly responsible for the TPU behavior it is the polymer morphology of hard segment domains within the soft-segment rich matrix that creates the unique properties. The hard segment domains act as “virtual crosslinks” that act to reinforce the polymer structure. This structure is depicted in Figure 2.
The polyols are generally of two chemical classes: polyester polyols and polyether polyols. The choice leads to the formation of the two major TPU classes: polyester TPUs and polyether TPUs. Within each class, selecting the specific diol and adjusting the ratios of the three ingredients leads to the creation of product lines of polyester and polyether TPUs ranging in hardness from about 70 Shore A to 70D. Flexural moduli range from about 1000 psi to 70000 psi (7 to 490 MPa).
Basic Properties of TPU
Due to the inherent chemistry and phase segregated (hard segment domains) structure of TPUs; many physical properties of the TPUs are significantly enhanced over polymers that lack this “virtual crosslinking”. The table below highlights several key physical properties of TPUs separating them by polyester and polyether types. Within the two main classes (polyester and polyether), most properties improve as the hardness increases except flexibility. For example, the flexibility of either type decreases with increasing hardness (Table 1).
Table 1: Properties comparison of polyether TPU vs. polyester TPU
TPUs in Wire and Cable Applications
Evaluations of TPU in the wire and cable occurred in the early 1960s. TPUs were selected based on need for one or more improvements in properties noted in the table. Property enhancements were sought over those provided by PVC, Nylon and several other rubbers despite the increased cost of TPU (Table 2 and Figure 3).
Table 2: Comparison of TPU vs. other plastics and rubbers
Early TPU coatings were conducted with solution-based coatings. Generally no more than a few mils (20-60 micron) thickness could be achieved unless multiple layers were applied. While tough, flexible coatings resulted, it was soon found that the durability of polyester types was deficient. In humid, moderately hot environments the polyester TPU coatings deteriorated – micro crazing, cracking, and overall loss of physical properties occurred particularly as cables were flexed. Consequently, successful wire and cable jacketing adopted polyether grades with their inherent hydrolysis resistance. This trend has continued: 90+% of the market for TPUs in wire & cable use polyether grades.
The main function of a wire and cable jacket is to protect the primary insulation from environmental damage, be it weathering, hydrolysis, or physical abuse. Nearly all uses of polyether TPU today involve use as a protective outer jacket or sheath. Rarely is TPU used as a primarily electrical insulator although there are recent developments in under-the-hood cables.
To meet these functions, the Lubrizol Advanced Material group developed a broad selection of polyether Estane® TPU optimized for use in wire and cable applications. The attributes include toughness and elasticity with excellent processability; countless W&C uses have selected these TPUs for performance and durability.
The jacketing typically uses TPUs in the Shore A 80 to 90 range. Although there are grades that are harder, they also impart added stiffness. TPUs are used in cables that are usually in dynamic applications. The cables are in frequent if not continuous motion. In seismic exploration uses the cables are periodically dragged into different positions. While the overall cable design governs the overall stiffness, the jacket material can play a significant role depending on the thickness. Most cables with TPU jacketing need to be moderately flexible at ambient conditions.
Jackets made from Estane TPUs can provide the long lasting protection that wire and cable applications require. The jackets are tough enough to take the most brutal treatment. In applications subject to rugged teratment such as geophysical cable jackets Estane TPUs consistently outperform conventional rubber compounds. Its remarkable abrasion resistance makes Estane TPU superior to copolyesters, polyolefins, and PVC.
TPUs used in the W&C jacketing are affected by sunlight and other UV light sources. Unprotected natural, clear grades will yellow upon exposure to sunlight. Prolonged exposures will lead to hardening of the surface, crazing, eventual cracking when cables are flexed and gradual overall loss of properties. Nevertheless, many TPU jacketed cables are used extensively in outdoor applications. Nearly all Estane TPU grades for W&C contain optimized stabilizer packages to inhibit the changes. In addition, many pigment masterbatches used to color TPU jackets during the extrusion process can be formulated with additional stabilizers optimized for the particular color. Black pigmented W&C TPUs have superior UV resistance as carbon black itself is an excellent UV absorber. Based on outdoor aging studies such as depicted in Figure 4 and customer feedback of actual use, black polyether TPU cable jacketing in many applications can perform successfully for year outdoors. Lubrizol provides a limited number of W&C TPUs precolored black.
For high load-bearing wire and cable jacketing designed for long-term service, the wear resistance and durability of Estane® TPU provides superior protection from physical damage. These TPUs are strong enough for the most rugged uses. A wire or cable jacket that deteriorates in rugged environments is costly to replace. Estane TPU is one of the highest rated elastomeric materials for tear strength. It outperforms low-density polyethylene and plasticized PVC, while providing cut-resistance in many ruggedized applications.
Processing TPU for Wire and Cable Jacketing
Estane compounds are fully reacted TPU materials and are low in moisture when packaged. However, all polyurethanes, as well as many other plastic materials such as nylon, are hygroscopic and will absorb moisture from the atmosphere. Therefore, they must be dried at the time they are extruded. Specific conditions are provided in Technical Datasheets.
The non-flame retardant (FR) polyether TPUs usually exhibit a high gloss as extruded jackets. Process conditions can be altered to impart reduced gloss. Proper cooling such as cooled water bath is required to reduce the inherent tackiness of hot TPU cable jackets. Antiblocking behavior has been incorporated in all of the Estane W&C products to virtually eliminate sticking to themselves and cables will unreel easily.
However, most of the extruded FR cable jackets exhibit an attractive dull matte finish that can be controlled by altering processing conditions. The matte finish provides inherent antiblocking as well. Most of the TPU FR grades also result in opaque jackets.
Most FR TPU is processed by single screw extrusion. Although FR TPU can be injection molded, it is recommended to use conditions that keep melt temperatures under 400º F with short residence times. The FR technology in both halogen and non-halogen types is more heat sensitive than the basic TPUs.
Procedure: The process of coating wires and cable by extrusion is diagrammed in Figure 5. Estane® TPU pellets are compacted and fluxed in the extruded barrel. The molten material is extruded in the crosshead at which point the direction of flow is changed 900. It is in the crosshead that the wire, coming from the unwind and pre-heater, comes in contact with the molten Estane® TPU. The crosshead also holds the guide-tip and the wire die. The guide tip keeps the wire centrally located in the molten insulation and the properly selected die controls the wall thickness of the final construction.
The driven capstan pulls the hot coated wire through the water cooling trough and the high voltage spark tester. The choice of die opening, capstan speed, and screw RPM are all variables that determine the dimensions of the coated wire.
Unwind: Very small single conductor running at high linear velocities at 4,000+ feet/minute are paid off from stationary reels similar in action to that of a spinning reel used for fishing. Larger wires and multi-stranded wires where even slight twisting during the unwind process cannot be tolerated are normally paid-off from rotating reels. The payoff reels are usually installed in pairs so that as one reel is emptied the other can be hooked in by splicing on the fly without the need for lengthy shutdowns.
Pre-heater: Preheating of the conductor prevents stresses that may occur in the jacket due to premature chilling of hot plastic from the relatively cold conductor. In the case of small conductors, this can be accomplished by using a low voltage resistance applied between two properly insulated metallic rolls placed just before the bare wire goes into the crosshead. In larger diameter conductors, and for secondary jacketing operations, the pre-heating can be done with either a gas flame or water cooled quartz pre-heater tunnels.
Dies: The two basic types of dies are ‘pressure’ dies and tubing dies. In both types, the wire is led into the die opening through a guide-tip. In order to maintain concentricity, the clearance between the wire and the tip is minimal. In order to minimize the abrasion that occurs between the wire and the inside of the guide, the guide-tip is made from a very hard metal such as Carbaloy.
In the pressure die, Figure 6, the plastic is still under some pressure inside the die when it contacts the conductor. As the conductor emerges from the die, it is coated.
The tubing die, Figure 7 extrudes plastic tubing concentrically around the emerging conductor. The tubing collapsed onto the conductor just after the die face by controlled vacuum drawn from behind the crosshead and through the same passage in which the conductor travels. We recommend hard chrome plated dies for Estane® TPUs.
Cooling Trough: All thermoplastic covered wire is cooled by passing through a water trough. Sufficient immersion time is needed to allow cooling of the coated product without distortion of the jacketing.
Take-Up: The wire or small cable is pulled through the line by a capstan puller or, for large diameter cables, caterpillar capstans that are basically the same type of haul-off as rigid PVC pipe pullers. From the pulling capstan, the wire is taken up on reels for storage. Frequently, a festoon system is installed with several adjustable loop lengths of cable in inventory which can allow sufficient time for changing reels without interruption of the extrusion process.
While extrusion processing conditions are provided in Technical Datasheets, it is recommended that customers planning injection molding to consult with technical service representatives.