Thermoplastic elastomers for automotive applications -- past, present and future.

A thermoplastic elastomer (TPE) can be characterized as a material that processes like a plastic, but has the physical properties of a rubber. It is important to recognize that there are various types of TPEs. Several of the major classes are as follows:

* Styrenic based;

* polyolefin blends;

* thermoplastic polyurethanes;

* thermoplastic vulcanizates (TPVs);

* copolyesters.

This articles focuses on the TPV class of TPEs. A TPV is defined as TPE material which has been manufactured by a dynamic vulcanization process. This process is based on the principle of in-situ vulcanization or curing of thermoset rubber during the mixing with thermoplastics during the melt state. This process produces TPEs with a unique morphology and with the performance characteristics of thermoset rubbers. The morphology of these vulcanizates is illustrated in figure 1. It is important to note that TPEs produced by this process have superior performance properties when compared to TPEs comprised of mechanical blends, as shown in table 1. It is also important to note that TPVs containing fully crosslinked rubbers are superior in performance to TPVs which contained a partially crosslinked rubber phase. TPVs referred to here contain a fully crosslinked rubber phase.

Table 1 - comparison of uncrosslinked and highly crosslinked EPDM rubber/polypropylene compositions

Property                         Uncrosslinked   Crosslinked

Hardness, Shore A                     81             84
Ultimate tensile strength, MPa        4.0            13.1
Ultimate elongation, percent          630            430
Stress at 100% elongation, MPa        2.8            5.0
Compression set, percent              78             31
Tension set, percent                  52             14
Swell in ASTM #3 oil, percent         162            52

Parts by weight: EPDM rubber, 91.2; polypropylene 54.4; extender oil 36.4; carbon black 36.4

[Figure 1 ILLUSTRATION OMITTED]

The automotive industry has chosen to utilize TPVs in lieu of thermoset rubber and other TPEs because they result in a variety of important benefits. Opportunities to reduce cost are paramount. Part cost savings are realized as the result of faster production cycle times, lower part weight due to the material's specific gravity, elimination of scrap through the use of regrind and elimination of material mass by redesigning parts to take full advantage of the physical properties and processing capabilities of the TPV. Environmental impact is another benefit that is growing in importance. The relatively low specific gravity of TPVs provides opportunities to reduce part weight which contributes to improved fuel economy. This is very important as government corporate average fuel economy (CAFE) requirements become more demanding. The automotive OEMs are also becoming more sensitive to the landfill issue, and the use of TPVs provides opportunities to recycle part materials after the useful vehicle life.

Past developments

Early successful automotive innovations with TPVs were in the critical underhood environment and component parts of steering systems where properties and performances are demanded from -40 [degree]C to 150 [degrees]C. Not only temperature demands must be met for these applications, but resistance to a variety of fluids, such as engine oils, gasoline, brake fluids, power steering fluids, greases, etc., is required. Other applications were developed for interior and exterior components where resistance to UV was needed. Some of the early commercial applications for TPVs include:

* Rack and pinion boots;

* air management ducts;

* tie rod end seals;

* brake cable covers;

* steering column covers;

* distributor wire boots;

* seat belt sleeves;

* windshield washer reservoir seals.

While these illustrate only a few of the early commercial applications, they do represent the variety of processes which can be employed for manufacture of TPV components. Injection molding, blow molding and extrusion operations were used to produce the parts listed.

Although these early applications may be deemed past, one cannot make a clear distinction between past, present or future applications. These early applications continue to play an important role in providing today's automotive engineers with cost-effective, high-performance parts. What does distinguish the early applications listed above is that the TPVs used were not specifically tailored for automotive use only. These materials are used across many market areas such as construction, appliance, fluid delivery, etc. In other words, off-the-shelf TPV grades served the automotive market very well in terms of delivering material which met the required performance.

Present developments

As TPVs continue to evolve through the product life cycle, manufacturers of these materials have had to adjust their product and market development strategies to meet the demanding needs of the automotive market. Automotive OEMs continue to demand lower costs, higher quality and more complete systems entering their assembly plants. The latter has resulted in Tier 1 suppliers that have a greater role in the design and engineering of major systems and their integration into the assembly process of the vehicle. As the Tier I suppliers' roles and responsibilities have increased, so has their authority to make material selections. The changing dynamics within the automotive industry have required TPV suppliers to focus on individual application and customer requirements.

Although TPVs have been utilized for glass run channel applications since the mid-1980s, weatherseal applications have not been deeply penetrated by TPEs. New products and technologies have been developed, however, which have increased commercial successes. Such new products are:

* TPV that bonds to EPDM (ethylene propylene-diene-terpolymer) robber;

* TPV slipcoat;

* new water foaming TPV.

TPVs which bond to EPDM offer weatherseal suppliers the opportunity to utilize processing advantages of TPVs for applications requiring molded details. These applications would include molded corners in glass run channels and molded details on extruded secondary seals such as B pillar and lower door seals. These materials offer advantages over thermoset robbers such as reduction in cure/set times, elimination of trimming operations and potential for less tooling. Overall, for many systems a cost reduction is achieved with equivalent part performance. Some watchouts to consider for these TPVs are: Compression set values are higher than general purpose TPVs; color and gloss match to EPDM profiles should be considered in the same manner as molded EPDM to extruded EPDM; and thermoplastic type molding equipment and tools are necessary.

TPV slipcoats have been developed for use on glass run channels and belt seals. These are materials which can be co-extruded with the TPV profile to modify the surface characteristics of the part. The materials were developed to replace the traditional nylon or polyester flock used for these applications. All TPV glass run channels have been produced by co-extruding two grades of TPVs; an ultraviolet (UV) grade (67 Shore A) with excellent weatherability and compression set and a low friction grade for improved slip performance. The benefits of this new all-TPV glass run channel are: Lower weight and cost; excellent sealability, durability and abrasion resistance; clean, efficient, one-step processing; simplified comer molding; elimination of solvents, lacquers and adhesives; non-staining on painted surfaces; and 100% recyclability.

A new extrusion process has been developed to foam TPVs using water as the foaming agent. This process allows for the foaming of TPV extrusions without the use of harmful chemicals. This process is capable of producing an extruded profile with a density as low as 0.2gr/[cm.sup.3]. The process for water foamed TPV is simpler (elimination of cure cycle) than with cellular thermoset robbers. In addition, processing scrap can be recycled. Products can also be made in color, which may provide an added benefit for designers. Hoodseals have been produced commercially with foamed TPVs and provide the following advantages: Lighter weight; lower part cost; minimal environmental impact during manufacturing; ease of processing; design flexibility; and good sealing and heat resistance.

New higher-flow TPV products have been developed and are being used in existing glass encapsulation applications. These products have flow characteristics which allow the material to be injection molded in thin sections for relatively long distances at moderate injection pressures. These products can be successfully used in conjunction with commercially available adhesive systems. The benefits these products offer are: Excellent sealing and exterior weathering characteristics;) cost and weight savings; excellent ozone resistance; broad processing capabilities; no fading or blooming; and recyclability.

Automotive interiors is a market segment that continues to offer opportunity for TPV applications. OEMs have identified the look, feel and sound within automotive interiors as an effective way to differentiate vehicles. TPVs, because of their soft robbery feel and noise dampening qualities, are well positioned for continued growth in this segment. OEMs and interior systems suppliers are looking for products that can be color matched without painting to the overall color schemes of the vehicle's interior, and be compatible with the rigid plastics currently used in interior systems. Unlike underhood, exterior and weatherseal applications, interior applications are generally not replacement of thermoset robber.

One such application is air bag covers (sometimes referred to as air bag doors). The use of air bag systems has grown tremendously in the last few years, largely due to the U.S. legislating mandatory air bag systems or automatic seat belts. TPEs are becoming the material of choice for air bag covers in place of reinforced urethane. TPVs have successfully performed in air bag cover applications and have met the following performance criteria which are widely accepted throughout the industry:

* The air bag cover must perform predictably over a temperature range of-40 [degrees] C to 100 [degrees] C.

* The air bag must tear through the cover and be fully inflated within about 0.03 seconds after impact.

* No fragments of cover material or paint can come loose during deployment.

* The air bag cover must maintain a high degree of UV stability in terms of both retention of physical properties and resistance to color fading.

* Covers must withstand long term (1,000 to 4,000 hours) heat aging (90 [degrees] C to 120 [degrees] C) with at least 75% retention of properties.

* Cover materials must maintain ductile properties at cold temperatures ([is less than] -40 [degrees] C) in order to prevent brittle failures.

* Lot-to-lot consistency of selected material is vital. Even relatively small variances in material properties can have major effects in deployment performance of air bag covers.

Other applications for automotive interiors for TPVs include mats, cupholders, close-out seals, etc. All of these applications must meet requirements for fogging, UV, flammability, odor and various appearance standards. Parts must be heat and humidity resistant, be suitable for cleaning, as well as exhibit good scuff and mar resistance. TPVs have met the challenge of these OEM requirements and are being specified for numerous interior applications.

TPVs that bond to polyamides (nylon-6 and nylon 6,6) have been developed to offer new opportunities for automotive design engineers. Soft touch and feel can now be added to substrates other than polyolefins. This will open new applications for TPVs and offer yet another avenue for design flexibility.

Future product developments

For future development of TPVs for automotive applications, there will be a need for new materials, as well as fabrication and processing. Areas in which these materials may have an impact include:

* High performance materials for applications such as engine and transmission seals.

* Materials for dynamic applications such as engine mounts and suspension bushings.

* Materials which can be molded in color and meet scuff and mar resistance requirements for interiors, with soft touch aesthetics for air bag cover applications.

* New materials which bond to rigid plastic substrates other than polyolefins and polyamides.

* New materials and technology for development of automotive radiator and heater hoses.

New TPV products are continually being developed to expand the performance range for automotive applications. As vehicle warranties continue to become longer, not only will new products be needed, but continuous improvement of existing products is necessary.

Summary

TPEs have proven themselves in meeting the wide range of demanding engineering requirements for automotive applications. These applications will continue to grow because of the cost savings provided and the performance delivered. New products will continue to be developed which are specifically tailored for the challenging automotive market. TPEs will continue to replace thermoset rubber for applications in which they offer cost advantages and design flexibility to the automotive engineer. Growth will also be realized in automotive applications which have not resulted from replacement of traditional rubber. Cost and performance will be the drivers for continued success of TPEs in automotive applications.3