Fully vulcanized EPDM/PP TPV developments in automotive, building and construction materials. (Tech Service)(Cover Story).

Automotive weatherseals are undergoing materials and processing shifts from thermoset elastomer compounds like ethylene-propylene-diene monomer (EPDM)-based products to EPDM/PP-based thermoplastic vulcanizates (TPVs). EPDM thermoset compounds traditionally are used in weatherseal applications. Going forward, trends indicate that TPVs are gaining momentum, are being applied successfully in building and construction weatherseal applications, and now, in the automotive industry as well. Historically, the evolution of automotive weatherseals materials has been:

Flocked cloth [right arrow] latex [right arrow] SBR [right arrow] EPDM [right arrow] TPV

The key reasons for the latest TPV trends are:

* Lower part fabrication costs;

* 100% recyclability in areas where conventional materials have displayed shortfalls;

* design flexibility (possibility to combine with other thermoplastic materials and elimination of metal inserts);

* weight reduction;

* aesthetics (going beyond "Model T" black and diversifying TPVs by adding color to the seals).

The mechanical properties, benefits, efficiencies and processing solutions of TPVs will be discussed, focusing on weatherseals for automotive, building and construction use. In these segments, applications are diverse and range from: glass run channels, hood-to-cowl seals, hood-to-radiator seals, belt line seals, roof gutter seals and glass encapsulation applications. Presently, most static and semi-dynamic seals are strong candidates for EPDM-based TPVs. The benefits of switching to TPVs will be explored, including cost savings, processing ease, fulfilling "green" recycling goals, offering design flexibility, weight reduction and meeting aesthetic goals.

Initially, this article will outline improvements made with TPVs which address some of the present constraints found in the automotive sealing systems segment. Then, the advantages that this thermoplastic material offers will be detailed. And finally, a convincing business case history presenting bottom line cost savings of the TPV systems versus more traditional systems will be presented.

Lifting present TPV constraints

In the ongoing search to replace some thermoset rubber materials in weatherseal applications, TPVs are being used in a growing number of industries and applications (including automotive under-the-hood rubber components). Yet, while TPVs have successfully displaced PVC, EPDM and poly-chloroprene rubbers, the physical properties of the current crop of commercially available TPVs have not yet achieved their full potential in the marketplace. Recent innovations in 2001 have made strides to drive TPV performance to new levels, while addressing previous limitations. TRS believes they have made significant advancements with their 1000 series TPV product line to improve both mechanical properties and fogging resistance, addressing the shortfalls with a patent pending technology.

Improved mechanical properties

As compared to low hardness (durometer 55A to 75A) compounds, EPDM thermoset compounds typically have exhibited higher tensile and tear strengths than TPV compounds. Recent advancements in TPV technology have made it possible to show improvements in the mechanical strength in the durometer 40A to 50D hardness ranges.

Table 1 outlines improvements in tensile strength, showing gains of at least 10% in low hardness grades. In addition, significant improvements in tear strength are shown (approximately 30%). Tear strength improvements represent a major advance for TPV weatherseals. Higher tear strength helps the handling phase of the total fabrication process, and in overall seal durability.

Fogging resolved

One of the complications in using TPVs for weatherseal applications is the fogging that can occur on clear glass due to the process oil that leaches out of the compound. Additionally, and specific to the automobile marketplace, fogging is an issue for soft-touch interior components like grips, cup holders, handles, bin liners and shift lever seals applications. Consumers and manufacturers both are dissatisfied with seals that contribute to fog on the glass. The 1000 series general-purpose TRS TPVs use a proprietary formula technology that resists oil migration. Other industry manufacturers have developed solutions to this problem in their specialty grade product lines. This year, all of our low hardness TRS TPV compounds offer the low-fog benefit to users, regardless of grade or formulations. Latest tests indicate that these new grades of TPVs yield a weight loss as low as 1.2 [+ or -] 0.2 milligrams at 100 [degrees] C for 16 hours as per the SAE 1756 gravimetric method. The weight loss is roughly half that which traditional general purpose TPVs experience.

In summary, the latest TPVs provide greater flexibility to manufacturers than ever before. The greatest challenge to production is cost; the low friction technology benefit of the latest TPV compounds competes with very expensive flocking and coating technologies used in thermoset systems.

Low friction surface technology

Weatherseals such as glass run channels and belt line seals are designed for noise damping and to provide sealing against air, dust and water. Characteristically, seals require a low friction surface in areas where the seal meets the door glass. The reason for the low friction surface is to provide resistance against wear. In the last few years, a coextruded TPV durometer 75A glass run channel with a TPV 50D durometer slip coat has been used. For stringent wear resistance requirements, it has been determined that a TPV 50D durometer is inadequate. Recent developments (patent pending) have determined that a high density moisture curable polyethylene - like XL-HDPE NexCoat - is a cost-effective and viable alternative for superior wear resistance against the door glass. This technology is based on silane pre-grafted onto the backbone of polyethylene. The pre-grafted resin is then mixed with a catalyst masterbatch just before processing. The compound is then extruded on a conventional thermoplastic extruder. This is a viable alternative to ultra high molecular weight polyethylene, which has exceptional wear resistance but cannot be easily coextruded using thermoplastic processing without sacrificing wear resistance. Surprisingly, XL-HDPE can be coextruded onto TPVs as a slip coat to provide exceptional results. Abrasion resistance testing based on GM9909P (General Motors) specifications yielded 10,330 cycles per 5 kg, where the minimum requirements are 10,000 cycles per 3 kg load. This also yields a coefficient of friction (COF) of 0.3. Using this optimized process, a product can be manufactured with a COF in the range of 0.1 to 0.3, using a clean technology. In contrast, a 50D durometer TPV yields a COF of 0.6, thus inadequate wear resistance. Another benefit is that the scrap of the coextruded 75A durometer and the XL-HDPE slip coat can be reground and utilized, addressing recycling issues.

[FIGURE 1 OMITTED]

Also, our latest developments impart the benefits of reduced glare and a dry, non-greasy look and feel in a lightweight TPV compound. Of course, there is more to TPVs than low friction technology.

Adding up TPVs

Every manufacturing business is under pressure to keep their operation lean and as productive as possible. Nowhere is this truer than in the automotive business and construction markets. Generally, due to the high content of the pure EPDM in TPV-material EPDM/PP products, it is more expensive than traditional materials. The upside to using TPVs is that manufacturers see quantitative savings in the finished product; typically, the cost per part is reduced by 10-30% due to efficient thermoplastic processing, waste reduction and the ability to recycle waste materials. EPDM/PP based TPVs exhibit a two-phase morphology. The PP is the continuous phase component in the morphology, and the highly vulcanized rubber particles (approximately 1 to 2 microns) constitute the dispersed phase. Even though the concentration of rubber is much higher than the PP in TPVs, the PP provides ease of processing. Thus, TPVs are ideal for injection molding, blow molding and extrusion operations.

An all TPV glass run channel and belt line seal is the "best-fit" automotive weatherseal application. It benefits most from tremendous cost savings (30%) through the elimination of long cycle times for corner molding and complex traditional flocking processes. In addition to the cost savings, there is also an approximately 22% weight savings by using an all TPV seal.

A cost comparison chart is shown in table 2. This is where the "robber meets the road."

Summary

The latest generation of EPDM/PP fully vulcanized TPVs is not restricted to "old materials paradigms." By directly addressing the needs of the marketplace, newly formulated TPV compounds deliver low fogging, higher tear strength, higher tensile strength and more environmentally friendly materials to aid in the goal of creating fully recyclable vehicles. Enhanced aesthetics and colorability, and UV resistance coupled with ease of processing position the latest TPV grades for expansion into other areas beyond secondary weatherseals for the automotive, building and construction markets. In the future, a trend of system integration like combining polycarbonate windows with an integrated TPV seal is a possibility.

Going forward, EPDM based TPVs are positioned to expand into the 500 km ton plus automotive weatherseal EPDM compound market. TPVs offer viable alternatives to markets that strive to overcome the limitations of thermoset rubber, while maintaining their benefits like cost, weight reduction and recyclability.

Table 1 -- mechanical properties

Property                Test       Units       Durom. 55A
                       method                TPV-   Nexprene
                       (ASTM)                 A      1055A

Specific
 gravity               D-792                 0.97     0.97
Tensile
 strength              D-412        MPa      4.4      6.6
Tear
 strength
 ("X flow")            D-624        KN/m      19       31
Compression
 set (22 hrs.          D-395         %        27       27
 @ 70 [degrees] C)   (method B)

Property                   Durom. 64A          Durom. 73A
                        TPV-      Nexprene   TPV-   Nexprene
                         A         1064A      A      1075A

Specific
 gravity                0.97        0.97     0.97     0.97
Tensile
 strength               6.9         7.6      8.5       10
Tear
 strength
 ("X flow")              25          35       28       47
Compression
 set (22 hrs.            31          32       35       35
 @ 70 [degrees] C)

Property                   Durom. 80A          Durom. 87A
                        TPV-      Nexprene   TPV-   Nexprene
                         A         1080A      A      1087A

Specific
 gravity                0.97        0.97     0.96     0.96
Tensile
 strength                11         12.4     15.9     16.9
Tear
 strength
 ("X flow")              34          57       49       69
Compression
 set (22 hrs.            41          41       45       45
 @ 70 [degrees] C)

Note: TPV--A isa commercially available fully vulcanized TPV
Table 2

Cost                      All TPV glass    Traditional thermoset
                           run channel    EPDM compound + flocking

Processing
Rubber compounding              0                     5%
Extrusion                  10% (co-ex)       15% (vulcanizing)
Flocking                        0%                    6%
Cutting                   0% (in-line)                4%
Corner molding                 15%                   25%
Flash/Scrap                     1%                    5%
Total processing cost          26%                   60%

Raw material
Extrusion material             40%                   20%
Corner molding material         4%                    2%

Flocking material
Primer                          0%                    4%
Adhesive                        0%                    6%
Polyester fiber                 0%                    8%

Total raw material cost        44%                   40%
Total part cost                70%                  100%

Jonas Angus is president and CEO of Thermoplastic Rubber Systems, a company he co-founded in 2000. He began his career at Polysar and moved to DSM when that company purchased the Sarlink business. He has also worked for Ticona.