New TPE Developments For Automotive Weather Seals

By Jurgen Glockler,
Advanced Elastomer System, N.V./S.A.

Introduction

Thermoplastic Elastomers (TPEs) are group of materials which bridge the gap between thermoplastics and thermo set rubber(1) TPEs are sold a ready to use materials in free flowing pellets and are available in hardness ranges from 35 shore A to 50 Shore D.

These materials exhibit significant higher average growth rates since their introduction in the late 1970’s, early 1980’s. According to a market research study conducted by the international Institute of Synthetic Rubber Producers in 1996, the consumption of TPEs in 2005 will be close to 2000 KMT worldwide. Figure 1 shows the estimated growth with a split of North America and Europe.

The automotive market segment is the most important one for TPE materials. Various engineering applications like blow moulded rack and pinion boots, airducts, injection moulded seals and gaskets, plugs and grommets and car body sealing systems – weatherseals are ideally suited to be designed around and manufactured with high performance thermoplastic elastomers.

New requirements of automotive industry Original Equipment Manufacturers (OEMs), like weight reduction, system cost reduction, design flexibility, colourability, aesthetics and recyclability, favour the use of TPEs instead of traditional thermoset rubber.

More and more OEMs are increasing their global activity and are designing, sourcing and producing parts and vehicles in several world areas as a way of improving their overall cost structure and profitability. This trend also support the use of engineering thermoplastics produced according to worldwide specifications, thereby ensuring global supply of an uniform material.

Even though TPEs are more expensive raw materials in comparison to thermoset rubber compounds, finished parts or integrated systems with TPE as sealing component offer cost saving potentials thanks to the more economical thermoplastic processing technology.

Savings are realized through lower part weight, shorter cycle time in processing, lower energy usage, less scrap, recyclability of scrap and design/system, flexibility. On top of all these benefits, TPEs are environmentally friendly materials.

Key Features of TPEs and classification

Almost all commercial TPEs have one feature in common: they are phase separated systems, in which one phase is hard at room temperature, while another phase is soft and elastomeric. The harder phase gives TPEs their strength and, when melted in a thermoplastic machine, their processability. The soft phase gives TPEs their elasticity and rubber performance².

The ability of TPEs to repeatedly become fluid when heated, and solid after cooling, gives manufacturers the ability to produce, on conventional thermoplastic machines, rubber-like articles. Scrap can usually be reground and recycled key advantages for the manufacturer are using fast processing thermoplastic equipment by feeding free flowing pellets, short cycle times, low scrap rates, high automation, lower labour cost.

In the case of thermoset rubber replacement, the part is redesigned to leverage and maximize the physical properties and processing characteristics of TPEs. In the marketplace, many different families of TPEs are known. Therefore a classification based on the chemistry is helpful.

TPEs can be classified into blockcopolymers and electromeric alloys. Figure 2 gives an overview of the materials included in these two categories.

The most important group of TPEs are thermoplastic vulcanisates, based on fully crosslinked EPDM dispersed in a polypropylene (PP) matrix. Crosslinking of the rubber phase in PP is usually done by dynamic vulcanization. Fine dispersion of the elastomeric phase in the hard PP-phase results in an excellent balance of rubberlike properties and good thermoplastic processing. Products based on fully crosslinked EPDM are also known as EPDM-X + pp (TPE-V, TPV, EA) and are the closest to be able of substituting classical thermoset rubber applications in the automotive industry like the highly demanding weatherseals applications.

AUTOMOTIVE WEATHERSEALS- COMPONEMNTS

Weatherseals or car body sealing systems are used in the car as sealing elements against water, air and noise damping, as sliding guide for movable windows. A profile systems also provides an aesthetic and cosmetic aspect in-line with the overall car design.

The most important elastomer for current thermoset rubber based weatherseals in EPDM. Market studies from EPDM raw material suppliers estimate that around 400 KMT of EPDM is consumed for automotive weatherseal systems¹. In order to understand which one of the various thermoset rubber weatherseal systems in the car can be replaced by engineered TPEs, one has to classify all applications. Figure 3 shows the different applications, split between static and dynamic. The different, applications are listed on the X-axis from low to high complexity. The lowest number of requirements apply to static sealing profiles, while the best sealing performance and surface aspects are needed for dynamic applications. Particularly for foamed profiles used as doorseals, trunkseals and tailgate seals.

New material developments and fabrication technologies for TPEs, especially EPDM-x+PP like santoprene® thermoplastic rubber, have been developed to meet all requirements for these applications. The “All- TPE Weatherseal Vehicle” is now is sight.

In the following chapters we will describe the status of the successful penetration of these applications with engineered TPEs like santoprene® thermoplastic rubber.

STATIC SEALING APPLICATIONS

Static weatherseals are applications which provide a constant sealing over the lifetime of a car. The most important parameter is the constant stress over time. In order to design the optimized profile geometry relaxation measurements from TPEs are taken. With the introduction of flush glazing and zero gap designs TPEs are preferred materials as trimseals and gapfillers.

The late eighties saw the commercialization of the first static sealing systems made with santoprene rubber. Most of these are coextruded profiles, with a rigid food out of PP and a sealing lip based on EPDM-X+PP. This combination is economical in comparison to a thermoset rubber EPDM system because of the effective synergy between a commodity plastic (PP) for the solid foot, and a TPE for the sealing lip, in an optimized design. Figure 4 illustrates a typical hard/soft static profile system.

Another main growth opportunity in static sealing systems in glass encapsulation. For the encapsulation of glass, two processes can be used: injection moulding or robotic extrusion. Both processes require speciality products to ensure the best balance of processing, lowest glass breakage and good sealing. Figure 5 shows the glass encapsulated quarter-light of the Mercedes-M Class, which uses a santoprene rubber M 100 series grade.

Direct extrusion of profiles onto glass for large rear and front windows are also commercial using a patented robotic extrusion system together with a special great of Santoprene rubber. Sekurit Saint-Gobain. Deutschland GmbH & Co K G has developed this technology in Co-operation with advanced Elastomer Systems³. Its main advantage is the possibility to produce many different profile designs including bulb seals and the robotic system allows to seal large windows. Figure 6 shows two Windows with profiles based on a robotically extruded special grade of santoprene rubber.

DYNAMIC SEALING APPLICATIONS

Dynamic sealing application are still mainly based on thermoset rubber EPDM compounds developed and manufactured by thermoset rubber companies. Solid (dense) belt line seals, glass run channels or sponge weatherseal systems like door and trunkseals are the most critical applications with the highest level of performance requirements. Dynamic sealing systems are characterized by a balance of deformation and relaxation. A key variable of a good dynamic sealing profile is the time in which a sealing lip is moving back to the original shape after deformation. The classifclip is to measure its compression set at different temperature.

Lab tests comparing engineerd TPEs with EPDM have shown that, especially at higher temperatures like 70ºC to 100ºC, fully vulcanized TPEs (EPDM-X+PP) show comparable compression set data. With the ageing of sulfur cured EPDM thermoset rubber profiles as a well known weakness, engineered TPEs based on EPDM represent an ideal alternative to overcome this deficiency of traditional thermoset rubber systems.

In parallel measurements, it has been demonstrated that EPDM-thermoset rubber profile design can be optimized around TPEs to achieve maximum performance for dynamic weatherseals.

In a belt line seal application, the effect on compression set of varying the design, with different types of santoprene rubber, has been studied4. Two commercial grades with a hardness of 58 shore A and 67 shore A, and a developmental grade with optimized elastic recovery have been evaluated. Figure 7 clearly shows that an optimised lip design incombination with an optimised elastic recovery have been evaluated. Figure 7 clearly shows that an optimised lip design incombination with at optimised elastic recovery product gives the best result. These tests were after ageing at 70ºC, for 94 hours and after 1 hour relaxation before measurement.

Another interesting comparison of an EPDM rubber based belt line seal with lip designs made with Santoprene rubber relates to ageing. Figure 8 shows the permanent
deformation/set after ageing at 70ºC. This confirms the lab data, in that profiles made with santoprene rubber have a more constant ageing behaviour after 6000 hours at 70ºC,, the plot indcates that EPDM ageing is by far the weakest, less predictable or consistent over time.

This study clearly demonstrates that high performance engineered TPEs like Santoprene thermoplastic rubber are ideal sealing materials for belt line seals.

Inner and outer belt line seals are designed as hard/soft combinations which also points to the use of TPEs. The rigid (hard) piece provides structure for ease of assembling and can be produced either from reinforced PP or ABS. The sealing lip is based on a low modulus, high elastic TPE like Santoprene rubber. Figure 9 shows the latest commercialization example in the case of the Ford Focus. The sealing lip is covered with flock to ensure low friction for sliding the window.

Belt line seals are the fastest growing TPE application in solid weatherseals because of the obvious advantages of using thermoplastic materials. This is due to the potential cost saving and by the elimination of recognized issues with EPDM rubber profiles, like the difficulty to coextrude a very hard compound (>45 shore D) with a softer component (75 shore A), as well as the critical post shrinkage of the profile.

Glass Run Channels are another application where TPEs add value. Most glass run channels are made in one hardness around 75 shore A, and a low friction coating layer is applied onto the thermoset rubber profile lips in contact with glass. In addition, most systems have different designs and profile sections are connected with corners, typically called corner mouldings. The main advantages of using an All-TPE glass run channel are: cost savings through elimination of complex low friction coating technology, corner moulding and weight reduction.

Current EDPM thermoset rubber coating systems for low friction are very expensive and the scrap rate in most cases in higher than 10%. An All-TPE alternative glass run channel system is based on a standard extrusion grade of about 70 shore A and the low friction layer is coextruded.


AES has developed special low friction grades of Santoprene rubber which meet the important balance of low coefficient of friction, good abrasion resistance and good aesthetics. These speciality grades are designed to be coextruded. In the mid-eighties, the first glass run channel based on santoprene rubber has been commercialized on the Rover 800 and full functionality was demonstrated for the full lifetime of the car. Figure 10 shows the latest commercialization example of a glass run channel made with santoprene rubber which fulfills all above mentioned features.

In 1998, Toyota approved santoprene thermoplastic rubber for this application because of a weight saving of around 30% for a 4-door version compared to traditional systems used and because of cost savings resulting from simplified comer mouldings. The TPE glass run channel is promoted in the car advertising brochure as a recyclable, environmentally friendly polyolefin-based material.

FOAMING

Physical foaming of Santoprene thermoplastic rubber has already been described earlier in several publications¹. A basic patent to foam TPEs, by using water as a blowing agent, has been patented by AES in 19915. At the same time, a suitable high performance extrusion line to produce foamed sealing profiles was patented by the German machine manufacturers Berstorff6. The combination of a special foaming grade of santoprene rubber and special berstorff extruder allows to produce foamed sealing profiles was patented by the German machine manufacturer Berstorff6. The combination of a special Berstorff extruder allows to produce foamed sealing profiles. The first water foamed hoodseal was commercialized by Mitsubishi already in 19951,7. The key differences in the process of foaming a TPE compared to EPDM thermoset rubber sponge has been reviewed and the process analysis shows that the TPE Process offers clear recognized advantages1 .

The main advantage of a water foamed TPE is that the process starts with a fully vulcanized TPE material and the only variable to control during extrusion is the blowing/water foaming of the PP-matrix, which is done by injecting water in the melt and by on line temperature control of the extruder. Building on the excellent market feedback with the Mitsubishi hoodseal, Coextruded profiles have now also been developed.

In March 1999, another breakthrough has been announced. In a joint press release, Mitsubish and AES, together with Hiroshima Kasei, reported the first tailgate/trunkseal commercialization based on Santoprene rubber8. Figure 11 shows the coextruded tailgate designed with a metal –reinforced solid foot and the water foamed bulb to provide the sealing performance.

This is the first All –TPE tailgate seal worldwide. The application also uses a new technology of simultaneously co-processing a solid santoprene rubber substrate, a waterfoamed santoprene bulb seal and a light weight metal carrier. The use of santoprene rubber results in a significant weight reduction, increased recycling rates, improved part durability and part cost reductions. Kinking does not occur thanks to the optimised design and the special metal carrier. This part yields an expected 5-10% cost reduction versus an EPDM rubber solution.

The system eliminates the need for compounding and curing lines, resulting in a simpler, cleaner and faster manufacturing process. Recycling is easier and

Properties	Standard	TPE Profile	EPDM Profile
Density  Water Absorption, % 24 hrs @Rtatmospheric pressure17kpa Compression set, % 22hrs@RT 22hrs@70ºC 22hrs@100ºC 22hrs@-40ºC  Load Deflation, N/100 mm@40% Load aged Deflection After 72 hrs@ 100ºC Load Deflection Change, % Surface roughness, um	ISO 1183  ISO R 62     ISO 815            ISO 3386   ISO 3386     ISO 4288	0.55  1       27 58 75 70    9.7   6.6   -32% 5.4	0.59  2       6 24 51 71    8.9   6.3   -30% 5.3

The profile fulfills the OEM requirement for an environmentally-friendly production. Property have been measured and compared against those of a high performance EPDM trunkseal, as listed in Fig. 12.
If we compared the measured tailgate seal properties of both options, we see that the TPE system has been produced at same density as the high performance EPDM profile. Water absorption measured at room temperature is good. Compression set data show better values for EPDM thermoset rubber, as expected. However, at -40ºC, boot profiles show the same level. One way to optimize compression set for the TPE profile could be the reduction of its foam density.

Load deflection shows a good comparable level even after ageing. However, the optimal load deflection must be designed by choosing a good balance between the shape of the profile and foam density. Surface roughness is similar, but we know that at a density of 0.59, EPDM is really at the limit of acceptable performance, whereas the TPE profile density could be further reduced down to 0.3, while keeping good properties and surface.

This breakthrough development and commercialization demonstrates the potential for TPEs in dynamic weatherseals. New innovative developments for secondary and primary doorseals are now underway with the first commercialization expected in 1999. Special features like easy colouring and significant weight reduction in the order of 1.2 kg/car will be resented with high performance EPDM-X+PP materials9.

CONCLUSION AND OUTLOOK

OEMs have now recognized the potential of engineered thermoplastic elastomers. Especially fully crosslinked EPDM-based alloys (EPDM-X+PP), like Santoprene® thermoplastic rubber, are the preferred family of TPE materials because of their best rubberlike properties, closests to EPDM thermoset rubber. Car manufacturers are requesting further cost savings and innovations. Both criteria cannot be met together by classical EPDM rubber systems. Weight reduction, colouring, design flexibility, recyclability, plus global supply, are additional attributes which clearly point to TPEs.

Advanced Elastomer Systems (AES) has formed a dedicated global team to support the acceleration of TPE penetration in weatherseals. The translation of market requirements into clearly defined product targets with our customers, product development and fabrication/application technology support are the key focus areas of the team to convert all weatherseals systems. Whether static or dynamic, solid or foamed, to Santoprene rubber.

Based on the excellent track record in static hard/soft weatherseals, glass encapsulation, glass run channels and belt line seals, engineered TPEs like Santoprene rubber can now target highly demanding foamed coextruded weatherseals. The “All-TPE Weatherseals Vehicle” will son be a reality and figure 3 shows that doorseals are the only application yet to be commercialized. Developments are underway. Prototyping has been done and, at the end of 1999, this last validation step for santoprene rubber will be achieved.

Reference:

1. J. Glockler, Innovative TPE solutions in automotive weatherseal applications, ETP Conference, ETP’98, Zurich, June 22-24 1998.

2. C.M. McMahan, Thermoplastic elastomers for automotive applications AES’ paper, Rubber Division, American Chemical Society, Indianapolis, Indiana, May 5-8, 19978.

3. AES, St. Gobain Sekurit/Gepoc-Robitic extrusion brochure. AES Marketing/Communications Brochure, 1995

4. T. Burton, H.-J. Holz. Belt line seal development. AES Presentation, March 1999

5. US patent 5070111, Advanced Elastomer Systems L.P., December 3 1991.

6. J. Meike-Phsikalische Verschaumung von TPE mit dem Extrusionsverfahren. SKZ, Wurzburg, 1996.

7. Technical paper presented by Mitsubishi Motor Company at the Society of Automotive Engineers in Japan. AES with Hiroshima Kasei, May 17, 1995.

8. Mitsubishi selects santoprene® thermoplastic rubber for tailgate weatherseal applications. AES press release, March 12, 1999.

9. Draftex- Foamed TPE Profiles. Advertisement, 1998.