An economical alternative to heat-activated pressure-sensitive tapes.

This article outlines a new patented technology for bonding extruded elastomer profiles to generic double-sided pressure-sensitive self adhesive (PSA) tapes in an on-line process to reduce material, labor and production costs. It discusses the challenges faced by engineers requiring high performance seals, profiles or gaskets. The article introduces the Boss Generic Self-Adhesive system (BGSA system), which creates robust bonding between PSAs and custom designed extrusions. The benefits and limitations of the technology are discussed.

Making flexible elastomer seals or profiles adhere to other host surfaces is an established and understood process, but up to now the processing options have been limited by either cost or bond quality.

The BGSA system overcomes the cost and quality issues by using generic double-sided PSAs and a process of securely bonding the tapes to custom elastomer extruded profiles in a high quality one-step process.

The BGSA system applies most proprietary self-adhesive tapes to a thermoset elastomer extrusion inside the tool to create a permanent welded bond. This ensures that the final bond strength between the flexible elastomer material and the host surface is at the standard set by the manufacturer of the self-adhesive tape. The extrusion or molding may be simple or complex, flexible or rigid, soft or hard. Likewise, the host surface may be virtually any material, in any shape or form with or without a surface coating.

This technology is currently being applied in the automotive, construction and white goods industries, where flexible materials, such as thermoplastic elastomers, are bonded to a host surface to seal it against dust, noise, water or wind, or to provide a protective seal or cushion. This technology is particularly effective in high-performance applications requiring robust weather or moisture seals. This patented technology increases design flexibility and is available under license.

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Current seal fixing methodology

Since the application of rubber, and more recently, polymer technology, to creating seals between surfaces, manufacturers have been searching for an attachment process that produces an effective seal at the lowest cost. Among the seal attachment techniques commonly used are:

* Pins, rivets, staples, screws--a mechanical device fabricated from metal or plastic is used to fix the seal in position. This method requires holes to be punched or drilled in the host surface, which provides openings for corrosion. Then, each fastener must be inserted. This fixing method only offers point attachment, is slow, labor intensive and requires additional components, which must be managed in the supply chain.

* Channel or groove--a fixing system in the shape of a channel or groove is specially added to the host surface to hold the seal. The channel or groove may be built into the shape of the host surface, or it may be fabricated and added separately. This technique requires additional components and processes, adds weight and creates a further surface-to-surface joint that must be sealed.

* Friction fit or wire carrier--a friction fit fixing system can present seal retention and sealing problems. The additional metal component required adds weight, cost and corrosion issues.

* Liquid adhesive--a fixing solution uses specially compounded adhesive. Handling and applying the adhesive can be messy and can create occupational, health, safety and environmental issues. The initial bonding strength of the adhesive may be such that the seal can move on the host surface during assembly, which can result in quality or sealing issues.

* Heat-activated pressure-sensitive adhesive tape is a continuous foam tape using a heat activated adhesive on one side of the foam to bond to the elastomer extrusion. On the other side of the foam is a pressure-sensitive acrylic adhesive to bond to the host surface. Special equipment to laminate the adhesive tape to the elastomer extrusion is required, and this step is generally designed into the production process as an "add on" or "post production" operation. While this type of tape has proven effective and is growing in acceptance worldwide, these tapes and the required laminating process are costly and are limited to a relatively thick foam tape.

Challenges to overcome

The three main customer drivers in the development of new technology for high performance seals and profiles were:

* Cost reduction--global competition forces manufacturers to develop and implement the most cost-effective solutions;

* quality--modern consumers demand high product performance standards for components such as seals; and

* aesthetics--contemporary design is clean and sleek--form and fit are integral to achieving the right "look."

The drive for effective low cost seals is greatest in industry sectors producing weather seals, especially the automotive industry. In these sectors, the performance of the seal itself defines the final product.

There are a number of challenges that engineers must overcome to achieve a quality seal. They want an effective seal technology that:

* Does not restrict design freedom;

* achieves strict moisture, wind, dust and noise sealing standards;

* delivers the specified performance cost-effectively;

* minimizes material and supply chain costs;

* minimizes installation labor costs;

* integrates smoothly with the production process and minimizes capital cost;

* eliminates warranty costs;

* minimizes occupational, health, safety and environmental risks;

* uses 100% recyclable materials; and

* minimizes weight.

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The BGSA system

New patented technology securely bonds extruded thermoplastic elastomers to generic double-sided, pressure sensitive self-adhesive tape products. The keys to the technology are designed into the preparation of the generic tape and the profile extrusion tooling. A special extrusion crosshead process brings together the elastomer profile that will become the actual seal, a semi-rigid film interlayer and the pressure sensitive self-adhesive tape that will ultimately bond the seal to the host surface.

The concept for the process was developed following a customer brief for a custom automotive weather seal. The customer was seeking an extruded seal for an automotive ornament. The seal specified was delicate, of a low hardness elastomer, with automotive approved PSAs on one surface and had to provide an effective seal between the ornament and the boot (trunk) panel.

An attempt was made to extrude heat activated adhesive onto Santoprene thermoplastic elastomer. However, to insure the bond between the Santoprene and the PSA, the resulting adhesive coated extrusion needed to be of a thicker cross-section and an 80 durometer A rubber hardness to accept the heat-activated system. The resulting extrusion was too stiff to make a good seal and the heat-activated adhesive was too thick and bulky, making the profile unsuitable.

The company applied its expertise of cross-head bonding of rigid to flexible thermoplastic elastomers and ran trials on an innovative multi-extrusion process that lead to the solution:

* The core innovative step was introducing an interlay between the pressure-sensitive self-adhesive tape and the flexible elastomer profile. In this case, the interlay was a 0.3 mm thick corona-treated polypropylene film.

* Extruding the thermoplastic elastomer profile onto the polypropylene film; and

* laminating the pressure-sensitive adhesive tape to the polypropylene film to achieve maximum bond strength.

This technology enables a manufacturer to custom design seal profiles of virtually any solid or sponge thermoplastic material to bond to almost any host surface, and to apply the seal in a clean and simple one-step roll-on process. The bond strength of the seal to the host surface is limited only by the adhesion standards of the generic self-adhesive tape purchased for the application.

The bond between the interlay film (in this case corona-treated polypropylene) and most generic PSAs is permanent and strong, with bonding strengths published by the tape supplier.

On the other side of the interlay film, the elastomer seal is a "weld" produced at the extruder head. The industry accepted strength for the weld is approximately 80% of the strength of the seal elastomer itself. The interlayer film also acts as an interface/bridge/platform to distribute tensile and shear forces exerted by tensile or compressive forces applied to the elastomer seal. This is the key to why the system performs so well and why foam tapes are optional with this technology. The interlayer also plays an important role when the seal is applied to the host surface by resisting seal profile stretch and flex.

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Independent bond strength tests have confirmed that the limits of adhesion for the BGSA system are determined by the specification of the pressure-sensitive tape holding the profile to the host surface. Tensile test results are reported in figure 7, revealing that after applying 3,000 Newtons of force to stretch a Santoprene profile 68 mm beyond its original 60 mm height, failure occurred first between the tape's self-adhesive surface and the alloy plate surface. The weld between the tape and the extruded profile proved strongest.

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This technology has been developed over four years, and it has been fully put into production with extruders running at maximum extruder output. More importantly, the technology allows the type and grade of pressure sensitive self-adhesive tape to be selected according the customer's performance and cost requirements. Most double sided acrylic PSA, reinforced, non-reinforced and foam tapes are suitable for the BGSA system, and are available from most tape manufacturers.

We have found that double-coated acrylic foam tapes offer exceptional adhesion to most surfaces, especially surfaces that are uneven or contoured. Most tapes are designed for external use, the liner side to adhere to automotive paint and the other side to low surface energy plastics. This is ideally suited to our polypropylene interlayer, sponge/foamed Santoprene rubber and automotive grade weather seals.

By using other specialty tapes, we have had exceptional results bonding Santoprene rubber seal profiles to surfaces which are difficult to adhere to, such as silicone coated polycarbonate used in automotive headlights, as well as other demanding low energy surfaces.

Of the generic tapes suitable for the BGSA system, our investigations have shown that 3M VHB double-coated acrylic foam tapes deliver excellent performance at about 60% of the purchase cost of heat-activated tapes. For a lower performance requirement, such as Australian GM Holden automotive approval HN1555, a non-foamed tape such as Avery Dennison's FT 2161 offers exceptional adhesion to most smooth, even surfaces at about 9% of heat activated tape price.

Based on our experience to date, we believe the only PSAs that perform as well as our technology are the heat activated range of tapes. We believe these tapes are currently only available from one manufacturer, and the range is limited. These heat-activated pressure-sensitive adhesive tapes are a premium cost tape and require substantial investment in capital equipment and unique profile tooling to laminate the tape to the profile in an add-on or post-production operation.

There are now many proven automotive and building industries applications using the BGSA system, including:

* Automotive seals for aerodynamic body skirts and moldings, utility vehicle cargo space liners and covers, wheel arch flares, rear glass canopy doors and a bull bar to body spacer seal;

* building door and window seals; and

* concrete construction expansion joints, control joints and curtain wall seals.

System benefits

With a range of successful applications in the marketplace, the BGSA process has proven its benefits over traditional seal fixing processes. This process greatly reduces material costs, labor and production time involved in making consistently high quality specialist extruded profiles, and eliminates the add-on or post-production processes previously required to fabricate components, fit them to the host surface and then finish off the aesthetic detail.

The processing cost reduction and production advantages of the BGSA system include:

* Material savings--no pins, channels, grooves, clips, carriers, etc., are required to fit the seal profile to the host surface. This eliminates supply chain management of components and saves workshop space. High cost heat-activated adhesive tape is not required. Low cost proprietary or generic tape is used. Special equipment to laminate the profile to heat-sensitive tape is eliminated.

* Labor savings--the self-adhesive tape is bonded to the profile inside the tool as part of the extrusion process. This process becomes the extruder operator's responsibility, eliminating additional labor required for downstream or post-production operations.

* Capital equipment savings--the technology requires minimal additional tooling on existing equipment. The imported laminating machines required to bond heat-activated tape to the profile are eliminated, together with the related lengthy lead and learning times. Dedicated jigs and tools to guide the extrusion through the heat-activated process are not required.

* Maximum output--this production proven process facilitates maximum extruder output, line speed and bond quality/consistency.

* Multiples--for high volumes, tape can be applied across multiple profile sections on the one extrusion at the same time.

* Accuracy--applying the tape at the extruder head allows accurate positioning on the profile.

* Flexibility--virtually any self-adhesive tape can be applied to a wide range of extruded profiles--delicate, soft, solid or sponge profiles. This means the customer can specify a seal bonding performance level tailored to the product, its projected life cycle and its market price point.

* Occupational, health, safety and environment--no chemicals are required.

* Adhesion quality and process control--a strong and consistent bond is achieved with most thermoplastic materials. Because of this, and the technology employed, the system requires minimal quality control supervision.

Known limitations of the technology

The BGSA system is limited to thermoplastic polymer materials. Another limitation, compared to the heat-activated tape process, is the fact that the tape system requires a semi-rigid film interlayer. This film can limit the profile bending radius on the adhesive plane only (there is no limitation bending at right angles to the adhesive plane). The tendency for the profile to lay straight and resist stretch is a major advantage for most installations. However, on other applications that require a sharp installation radius, a different design approach is required.

Profile shape, elastomer selection, adhesive tape width, type of adhesive and the contour of the host surface plane are the major factors impacting the profile's installation radius. However, if there is no other option, slit/notching the tape is a proven process.

Successful trials were conducted to overcome this limitation. The trials showed that the BGSA system was capable of bonding to a flat plane to a similar radius as that of the heat-activated tape system. The trials involved mechanically de-laminating the protective release liner prior to the extruder head, then slit/notching the adhesive tape from either side and re-laminating the protective release liner. The degree of bend radius this system is able to achieve is relative to the depth, width and frequency of the slit/notching.

Foamed TPEs

There is ever-greater pressure on manufacturers to reduce scrap and increase the ability to recycle. To meet this requirement in the automotive industry, engineers are increasingly specifying readily recyclable thermoplastic elastomer parts to replace components manufactured from thermoset rubber, which is more costly to recycle.

For example, in the automotive weatherseal market, ExxonMobil's Santoprene rubber and Trexel's Mucell process are leading this trend away from EPDM thermoset rubber. The Mucell process allows Santoprene rubber to be foamed to a microcellular structure that exhibits similar physical properties as the traditional EPDM, with the added benefits of a thermoplastic. For further details on the Santoprene/ Mucell process, refer to Rapra TPE 2005-Berlin.

Multi component profiles

With the BGSA system, engineers now have a low cost method of combining a number of materials in a single extrusion process. This offers unlimited opportunities for the development of profiles, seals and gaskets for new applications. The engineering flexibility of the BGSA system delivers cost advantages, which can now make new applications viable that previously were not cost-effective.

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The relatively new Santoprene/Mucell process and other foaming processes now available open new opportunities. The fact that foamed Santoprene can be co-extruded with other compatible thermoplastic materials, such as soft or hard elastomer and/or rigid or flexible polymers, along with other additions, such as substrates and low friction surface technology, all present new product development challenges.

Until now, cost has limited the number of fabricators able to enter the growing high-performance weather seal and profile markets. Application of specialty adhesive tapes to profiles has been restricted to companies and markets able to afford expensive laminating machines and heat-activated tape. The reduced costs of this technology makes it a real alternative, offering all extruding companies the opportunity to expand into new markets with existing equipment and minimal capital outlay.

New markets to explore include:

* Automotive--this industry has major potential, as auto companies in the U.S., Japan and Europe are beginning to replace traditional fixing systems with self-adhesive heat-activated profiles;

* building industry--seals for heating and ventilation equipment;

* construction and mining industries--seals for expansion joints;

* communications and medical industries--small, intricate seals for equipment.

* transport industry--"Stick on" bumpers to protect edges, corners, etc., on buildings, equipment, vehicles and rolling stock.