What if the Adhesive was Part of the Substrate in the First Place?

Posted on 4/2/2018 4:25:35 PM By Dr. Cynthia Gosselin (guest blogger)
  

My last blog post focused on providing the automotive industry with a way to obtain a pristine metal surface for adhesive bonding and painting by coating coils with a thin film acrylic that could be easily removed during the standard alkaline cleaning process.  Prior to removal, the coating protected the surface from soils that, if not properly removed, could inhibit adhesive, sealant and paint adhesion and/or field durability.  Taking this concept one step further, what if the adhesive was actually part of the substrate in the first place?

There were (and still are) applications dating back to the late 1970’s wherein adhesives were successfully applied to coils in automotive applications.  One example is the use of phenolic adhesives on steel for transmission rings.  In this case, these coated substrates not only had to withstand a flat stamping operation but subsequently had to bond the transmission rings to specialized papers using induction heating.  The B-staged strong (albeit brittle phenolic) adhesive was a good fit because the coated system did not undergo a forming or drawing (i.e. stretching) operation.  The advantage of this product was that the pretreatment and adhesive application was uniform on both sides across the entire width of the strip and the B-staged cure was consistent. An added advantage was that coil-coated pretreatment is specially formulated to provide excellent cut edge corrosion protection.

Another successful example of adhesive-coated steel is currently in use for automotive trim.  The substrate is generally bright-annealed Type 304 stainless steel which does not inherently exhibit good adhesion characteristics without specialized surface preparation. In this case, the stainless steel is pretreated and primed and the adhesive is applied on thin-width coils.  This formable two-coat system improves roll forming and has inherent over-bake resistance properties.  A wide variety of EPDM and SBR rubber compounds are bonded to these systems and used on many automotive window trim applications.  This coil-coated product is an asset to Tier 1 assembly operations in streamlining the manufacturing of components requiring a high level of quality in this very visible application.  The fact that coils, rather than individual formed parts are coated, ensures a consistently pretreated and coated end product on a difficult-to-bond substrate. 

During the late 20th century, coil coaters provided trials with adhesive-coated steels that exhibited formability, drawability, could be spot welded and exhibited corrosion resistanceon par with e-coated products.  Components that demonstrated these characteristics included doors, wheel wells, deck lids and hoods.  In these cases, coils were coated with 0.5 – 0.8 mils of various B-staged adhesives that were cured enough to be stamped, formed and assembled, yet retained enough activation potential to bond together during an off-line or e-coat curing process.  The concern around added weight was mitigated because these adhesive coatings also provided corrosion resistance equivalent to or better than e-coated substrates.  For inner applications (where e-coatings were traditionally thinner), these “full-bodied” adhesive coatings that did not accept e-coat proved to be an excellent substitute that did not contribute to weight gain. In all cases, the cut edge exhibited excellent corrosion resistance while allowing for a uniform adhesive coating within a hemmed area.  Sandwich laminates manufactured from thinner gauge hot dip galvanized precoated on one side with an adhesive and subsequently stamped and bonded together provided sound and vibration damping components without materially changing the gauge of the final product.  Full cure of the precoated adhesive was achieved during e-coat curing by advancing the curing mechanism of the adhesive.  Even though these trials demonstrated that the concept was viable, for a variety of reasons, it remained on the drawing board.

Today there are a variety of avenues to validate and apply adhesive-coated metals.   For example, the use of aluminum for a wide variety of automotive panels would benefit from adhesive coating in coil form.  It is much easier to pretreat and coat aluminum alloys in flat strip coil than on variously contoured body-in-white panels.  I think that theswitch to aluminum on the F-150 would have been less costly had coil coaters been involved in applying adhesives (and maybe even paint primers) to substrates.  Pretreatment of aluminum substrates is critical, whether for painting or adhesive bonding. The consistent ability of coil coating to achieve uniform surface cleanliness and pretreatment significantly enhances the material in final assembly.

Further rumination on the concept of an adhesive as an integral component of a metal substrate leads to considerations of custom products.  An alternative to fully coated coils could be the application of adhesives to blanks – whether tailor-welded/formed or traditional component blanks.  Consider blanking operations that employ plasma-driven cleaning and pretreatment steps before pressure-sensitive adhesive tape applications are applied in targeted patterns.  Tailor-welded blanks could provide, not only formed part precursors, but delineated adhesive applications for ease of assembly. Moreover, every adhesive formulator has product hidden on a back shelf or on the drawing board that could accelerate the use of precoated adhesive-coated steels, whether on coil lines or targeted blanking operations. 

Using out-of- the-box “Imaginarium Science” combined with today’s 3-D computer modeling techniques, there is no shortage of applicable concepts that would allow adhesives to be applied to pristine substrates.  Variously targeted chemistries could facilitate forming, reduce panel movement within an e-coat oven and provide adhesive strength levels combined with superior corrosion resistance, even by modern engineering standards.  With the manufacturing economy perking up and more resources available for driverless vehicles, it may be time to re-evaluate the adhesive-coated coil-coated metal substrate’s value proposition for advancing vehicle design and assembly.

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