TPU: the performance elastomer.

The thermoplastic elastomer industry has done an exceptional job of developing new products and even new classes of elastomers. There are thermoplastic elastomers (TPEs) based on styrene, polyvinyl chloride, polypropylene, urethane, polyester, polyamide and blends with several thermoset rubbers.

This assortment offers the product designer or material specifier a wide variety of choices. It sometimes also presents a dilemma as to which TPE to choose for an application.

The selection process for elastomers is made more easily and reliably when a methodology is used to examine the application requirements. As design criteria move from engineering driven to how people use things, from rigor to relevance, our understanding of how the various TPEs perform becomes more important.

Thermoplastic elastomers offer many of the properties associated with traditional rubber materials with the advantages of thermoplastic processing. In general, thermoplastic elastomers offer high elasticity, with performance at low temperatures, good resilience and flexibility, moderate heat resistance, good compression set and a range of hardnesses. These are common properties in almost all categories of thermoplastic elastomers.

Many applications do not require resistance to broad temperature ranges or harsh hydrocarbon environments. They may not require high strength or abrasion resistance. For many end uses, flexibility or resilience or a soft touch is the principal design criterion. There are any styrenic or olefinic elastomers that will perform quite adequately and cost effectively.

When a TPE part is to be used in a demanding application or is required to perform as a critical component in a larger assembly, additional design criteria are required. In these cases, selection of the proper TPE focuses more on performance capabilities. With more than 30 years of commercial product development, one thermoplastic elastomer clearly stands out for its ability to provide many and unique combinations of performance properties for a wide variety of applications - thermoplastic polyurethane (TPU).

Properties such as clarity and tear strength, abrasion and hydrocarbon resistance, load bearing and tensile strength combine to produce tough, environmentally resistant elastomers which meet the needs of the most demanding applications.

This article will cover three areas: Performance aspects of thermoplastic polyurethane, durable goods applications using TPU and applications in which TPU is used in combination with other materials and substrates.

Thermoplastic polyurethane - the material

The broad diversity of products available within the TPU family is made possible by the variety of raw materials that can be used. TPU is a linear segmented polymer containing hard and soft segments. The hard segment is typically based on methylene diisocyanate (MDI) and 1,4 butanediol. When color retention under UV exposure is critical, an aliphatic diisocyanate is substituted for MDI. The soft segment is the reaction product of MDI and a macroglycol.

Performance properties arise to a large extent from the choice of macroglycol or soft segment. The two broad categories of chemistries are polyesters and polyethers. Polyesters are more resistant to hydrocarbons. Polyethers are more resistant to water. Among polyesters, those with fewer ester linkages, such as polycaprolactone, have greater hydrolytic stability. Other properties of polyester and polyether TPUs are generally similar for a given hardness of resin but some distinctions, as shown in table 1, can be made when narrowing the selection to a single grade (ref. 1 ).

Molecular weight of the glycol and the stoichiometric ratio for isocyanate to glycol also influence the processability and mechanical properties of a thermoplastic polyurethane. Other variations of resin can be made by combining both polyester and polyether glycols.

Thermoplastic polyurethane - applications

One of the best means of describing the fit of TPU to the functional requirements is to discuss some applications. Applications for TPU can be found in automotive, medical, industrial and recreational settings.

A useful method of translating end user requirements into quantifiable criteria is to relate performance needs to physical properties and examine the influence of each key property on various functional aspects. There are always trade-offs. The challenge is to satisfy the primary design requirements and adequately address secondary considerations Having a thermoplastic elastomer like TPU with is proven performance in a wide range of related applications allows material selection decisions to be made more confidently. Industrial closure We are all familiar with Ziploc sandwich bags. Imagine the same type of closure system used on covers and shelters large enough to store military equipment such as engines, tanks and aircraft. The material requirements for such a zipper certainly are more demanding.

One of the first design criteria for a material to be used in this application is its ability to withstand contact with the variety of oils and fuels used in military operations. Thermoplastic polyurethane meets this criteria. Additionally, TPU remains flexible over extended temperature ranges and retains tensile strength and chemical resistance in a manner superior to other materials evaluated by the manufacturer. All this adds up to greater reliability and a considerably longer anticipated service life than conventional PVC zippers.

Sleeping shelter

In order to optimize sleeping comfort for campers summer through winter, a means of controlling the loft in down-filled sleeping bags was needed. The answer came in the form of a separately inflatable blanket and mattress. Depending upon the amount of air used, the shelter provides the appropriate amount of insulation for temperatures ranging from 70 degrees F down to minus 45 degrees F.

The key to this sleeping system is the use of TPU in the inflation system. TPU was chosen for its optimum combination of strength, durability and flexibility and its ability to maintain these properties over a wide service temperature range.

The inflation requirement added two additional considerations - heat sealability and air barrier. The heat sealing characteristics of TPU met the production process needs and produces a leak-free enclosure. By proper grade selection, the desired degree of gas barrier was achieved. As shown in table 2, ester-based TPU can provide a high level of oxygen barrier.

Air tool hose

From automated robotics and dental equipment to industrial staplers and power wrenches, the operation of air-powered tools relies on the durability and efficiency of a length of straight or coiled tubing to provide consistent power.

Users of coiled air hose made from TPU note its good memory and ability to spring back from kinks and bends without permanent damage. This can ultimately help reduce costs associated with downtime and replacement of damaged hoses. In addition, the flexibility allows the operator to use tools more comfortably, reducing physical strain and fatigue that can result from repetitive actions. TPU tubing is well suited to outdoor applications, such as diesel mechanic work, because the tubing resists cracking in cold weather if it gets run over.

In fight spots, a conventional air hose can be difficult to bend or twist into an effective working position. TPU hose is more flexible, making it easier to work around a corner or in close quarters, such as a wheel well. TPU tubing bends easily without cutting off the air supply allowing mechanics to use air tools in places that otherwise would require manual tools.

Polyether-based TPUs are selected for their superior resilience, hydrolytic stability and resistance to attack by microorganisms. These TPUs feature good balance of resistance to fuels, oils and most nonpolar solvents often found in industrial work environments, as well as high compressive strength, low compression set, low gas and vapor permeability and low extractability levels.

The ability of TPU to be easily colored is a distinct advantage over thermoset rubbers. Color is an effective way to code work stations for repetitive operations and to improve safety and efficiency in many coil tubing applications. Bright colors are often used to identify emergency cords, hazardous equipment or high-pressure dangers.

The clarity of most TPUs gives TPU an advantage over many other thermoplastic elastomers because it allows visual verification of the lubrication line to air tools. For example, on automotive assembly lines, oil is dripped through the hose to lubricate the air tool. If nylon hose is used and becomes kinked at any time, the oil line may also kink and cut off the necessary lubrication from the air tool. This can't be seen through nylon hose and will go undetected until the tool runs dry and bums out. With transparent TPU hoses, the lubrication line is easily seen. As a result, the manufacturer saves on maintenance, as well as the cost of replacing expensive air tools.

Hydraulic hose jacketing

TPU offers the manufacturer of hydraulic hose an excellent combination of properties - cut growth resistance, abrasion resistance, good adhesion to polyester and nylon braiding and resistance to oils, fuel, water vapor and ozone.

Table 3 quantifies environmental resistance for two types of TPU (ref. 2). Hydrolysis testing exposed test strips suspended over water at 70 degrees C to 95% relative humidity for 14 days. Resistance testing for ASTM oils #1 and #3 and hydraulic mineral oil was done at 100 degrees C or 72 hours.

Catheter tubing

Elastomers considered for medical tubing applications must possess a diverse array of properties, a well as meet a variety of demanding cost and performance standards. Percutaneous transluminal coronary angioplasty (PTCA) catheter tubing is a particularly demanding application.

PTCA tubing must be made in several sizes and configurations. Some products contain as many as ten lumens, while others range in size from approximately 0,060 to O. 111 inch. It is important than the material maintain different geometric configurations (e.g. triangular, elliptical) of the lumen while maintaining the roundness of the overall tubing. This requires exceptional consistency to hold back pressures and control the material in the extrusion die without fluctuation. Processing parameters such as melt stability, melt flow characteristics, draw down properties and melt strength over relatively broad temperature ranges lead to direct cost benefits in the design and fabrication of die tooling.

Biocompatibility is an essential factor in elastomer selection, Medical grade resins are tested for compatibility with blood, body liquids and fluids. Compatibility with therapeutic drags is also a factor for resin selection as well as stability to ethylene oxide gas, gamma radiation and electron beam sterilization procedures and an optimum balance of stiffness and flexibility.

Athletic shoes

The sports and recreation market utilizes many thermoplastic elastomers. As products become increasingly performanceoriented, the need for TPEs with higher capabilities has increased. Comfort and fit of athletic shoes have been significantly improved with inflatable devices made from TPU.

The average runner strikes the ground with a force three times his or her body weight. One means of cushioning this impact is an air bladder made from TPU. Another cushioning system includes a fluid-filled pad in the heel and forefoot portions of the shoe. TPU is able to withstand pressure surges due to repeated loading on the pad and meet the overall strength demands for reliable performance. To achieve the effect of a self-adjusting hydraulic shock absorber, fluid flow needs to be controlled through orifices formed during the sealing process. TPU allows such high integrity seals to be made consistently.

Combinations

TPU also shows its versatility by its uses in combination with other resins and substrates. It may be used as a distinct layer in an injection molded part, or be used as a functional coating over fabric, or be mixed with reinforcing filler, or be blended with another thermoplastic resin.

Co-molding

In some cases, a thermoplastic elastomer alone cannot meet the requirements for a product. For example, in a structural part where ruggedness and tactile feel are important, the answer can be co-molding of an engineering thermoplastic with TPU.

In one study, TPU was coinjection molded with ABS to yield a structural material that breaks in a ductile rather than brittle manner at relatively high impact speeds. In addition, the appearance surface has the benefit of the softer, more tactile feeling TPU.

As seen in the instrumented impact test data in figure 1, the resulting opposite structure demonstrated high energy absorption and it did not crack.

In developing the first-of-its-kind wearable computer, GRiD Systems Corp. wanted the computer housing to be rugged yet provide a high degree of comfort for the user. A polycarbonate/ABS resin provided the required structural properties while TPU provided the desired feel for the user. In this application, TPU is utilized as the outer skin of the unit where users grip the product, notably on the curved back and sides. In addition, TPU also offers considerable shock protection, allowing the unit to withstand three-foot drops onto concrete.

Insert molded TPU typically bonds well without adhesives to such engineering and structure plastics as polycarbonate, ABS, acetal and PVC. When the structural plastic is an olefin such as polypropylene, an olefinic thermoplastic elastomer may be the appropriate material choice. However, if the application has a prior performance criteria in which TPU excels, polypropylene/TPU combinations can be made. One such example is shopping cart wheels and castors where abrasion resistance is important. A polypropylene hub is molded with flow through channels to allow mechanic locking for the overmolded TPU.

Coated fabrics

The coated fabrics used for inflatable rafts demand reliable performance. For military assault and reconnaissance mrs, the requirements are even higher. Resistance to punctures, abrasion and the ability to withstand temperature extremes are all important. Additionally, the rafts are required to be light in weight, as well as flexible, and to facilitate efficient manufacture.

Starting with a specialty grade of nylon fabric, TPU is extrusion coated on both sides to produce a 16 ounce fabric with a total thickness of approximately 20 mil. The threeman raft is constructed from the coated fabric. The average tensile strength of this fabric is 360 lbs. with a tear strength of 25 lbs. A 33 ounce fabric used in the seven and fifteen man assault rafts has a tensile strength of 1,600 lbs. and a tear strength of 125 lbs. This raft can accommodate 2,100-4,500 lbs. In tests conducted by the military, TPU coated fabric was found to provide greater durability, abrasion resistance, a longer life span and better overall performance characteristics than a robber-coated fabric.

While TPU is more expensive than thermoset robber on a per pound basis, less TPU is required. This results in a lighter weight fabric which is easier to handle during both assembly and field use. Total assembly time of the raft takes significantly less time with TPU coated fabric since it eliminates the need for extended curing steps that are common with rubber coatings. Overall, the final part cost using TPU is economically competitive.

Reinforcing fillers

The addition of glass fibers turns TPU thermoplastic elastomer into a structural plastic. Flexural modulus increases from about 6 to 2,400 MPa. This has a significant effect on heat resistance as shown in figures 2 and 3. Using heat deflection temperature (HDT) at 455 kPa as an indicator, it rises from 85 degrees C to 120 degrees C as the flexural modulus increases from 400 to 2,400 MPa. Impact strength is also affected.

The combination of TPU and reinforcing fillers makes a resin that is rigid yet maintains the desirable properties of TPU such as paint adhesion using primerless systems. The low viscosity of TPU during the molding cycle allows the glass fibers to orient and thus maximize the resistance to thermal expansion. A coefficient of linear thermal expansion (CLTE) and contraction as low as 2 x [10.sup.-5] mm/mm/degrees C can be achieved. This enables parts to maintain their original length when exposed to extreme temperatures and makes it possible to design parts to much tighter tolerances so they can be produced to almost exact measurements.

The low viscosity during mold filling enables large, complex parts to be filled with a single gate rather than multiple gates. This eliminates weld lines which mar a part's surface appearance and can be structural weakening points. An additional benefit comes from the proportioning that occurs at the surface. The outmost surface is largely free of reinforcing filler and can thus have a desirable "class A" finish. No surface treatment or primer is needed prior to painting, thus decreasing overall part cost.

With its good impact resistance at high and low temperatures, it is not surprising that glass reinforced TPU is found in automotive applications such as body side moldings, door claddings, bumper rub strips, rocker panels and grills.

The large claddings and rocker panels on this automobile utilize several primary features of reinforced TPU - low coefficient of thermal expansion, paintability, good impact resistance and high melt flow rate. Glass reinforced TPU molding has replaced metal and polyurethane construction in some body side moldings. Major cost and weight reductions were realized by going to a one piece injection molded section minus the steel backup.

There are several other application areas in which reinforced TPU is used. Automotive applications are particularly illustrative of its performance properties because of the demanding end use environment coupled with a significant cost-effectiveness driving force.

Blends

By balancing the performance properties of thermoplastic polyurethane and engineering thermoplastics, new combinations of flexibility and toughness are possible. One such combination is a blend of TPU and acrylonitnle-butadienestyrene (ABS).

In one study (table 4, ref. 3), varying levels of ABS were melt compounded with TPU.

These blends have good toughness at low temperatures, higher flex modulus than TPU and better solvent and fuel resistance than ABS. In addition, ease of processing, good mar resistance and paintability without primers makes TPU/ABS blends well suited for automotive applications. With increasing emphasis on the ability of plastics to be recycled, physical property retention of actual painted parts becomes important.

TPU/ABS regrind of commercially molded and painted fascias was mixed with varying levels of virgin resin and molded into standard test specimens As shown in figures 4 and 5, the TPU/ABS blend showed superior retention of tensile, elongation and impact properties compared to TPO. In addition, test specimens containing regrind were painted again and were found to have the same level of gloss and distinction of image (ref. 4).

Conclusion

Thermoplastic polyurethane is a versatile, robust class of thermoplastic elastomers. With many chemical composition options, the extensive experience of suppliers and end-users, and a well-established application history, TPU is able to match design requirements to product performance, particularly in applications having several key design parameters.

With all this successful history, will there by anything new? From our perspective at Dow, the future of TPU has significant potential. Several new applications are under development in automotive, healthcare, consumer and industrial markets. New products that will extend the capabilities of thermoplastic polyurethane even further are moving from the lab to commercial trials with accelerated growth will come the greater use of TPU in combination with other substrates and thermoplastic resins. Yes, the future of TPU looks bright.

References

1. W. Meckel, W. Goyert, W. Wieder, Thermoplastic polyurethane elastomers, in Thermoplastic elastomers, a comprehensive review (New York: Hanser Publishers, 1987) p. 33 and company data.

2. John Penfold, "Thermoplastic polyurethane elastomers as hydraulic hose jacketing material," Rapra Technology Conference proceedings, London (March 25, 1988).

3. A.T. Chen, D.E. 'Henton, F.M. Player, A. McLaughlin, "TPU elastomer/ABS blends have good properties at reasonable cost," Elastomencs, September, 1990.

4. F. Michael Plaver and Robert D. McElhaney, presented at a meeting at the SAE International Congress and Exposition, Detroit, MI, February 25|March 1, 1991 and company data.