A Novel Approach to Surface Cleanliness for Painting and Adhesive Bonding in Vehicle Production

Posted on 1/17/2018 6:51:47 AM By Dr. Cynthia Gosselin and Dan Daley

When paint delamination or adhesive bonding failures occur, a whole host of suppliers involved in the manufacture of the vehicle are mobilized to determine what part of the system is the root cause of the failure.  Substrate producers provide reams of data to prove that their product met material specifications.  Pretreatments are vetted for proper bath chemistry and paints and adhesives are scrutinized for defects and material incompatibility, while plant specifications are reviewed for potential errors.  When vehicle OEM vendors can’t agree on the root cause (usually locus) of the failure, a third-party forensics analysis is sometimes implemented. Independent studies frequently cite cleanliness of the substrate surface as the root cause of coating and adhesive failures, as opposed to defective materials (substrates, paint & adhesive products), material incompatibility, or even improper application and curing methods.  In fact, cleanliness of the substrate surface is arguably the most overlooked factor in promoting good adhesion and durability.

Within the automotive industry, cleaner baths have evolved to combat most organic soils that are present on vehicle substrates.  Corrosion inhibiting oils on metal substrates are specified not only to prevent flash rusting and transit corrosion but also to ensure complete removability in cleaning baths, and compatibility with adhesives that are applied prior to cleaning.  However, soils can remain undetected on the substrate surface despite a preventive engineering protocol to ensure a clean surface for the following reasons:

  • Cleaner baths can lose a level of efficacy during the manufacturing cycle.
  • Heavier molecular weight prelubricating oils (prelubes) while more effective for stamping parts that are difficult to form, are also applied at higher thicknesses and designed to adhere to the surface a bit better than corrosion inhibitors.  These factors make prelubes more difficult to remove within some alkaline cleaner concentrations.  If prelubes are acknowledged during a production run, cleaner bath chemistries or time in the cleaner bath can be modified to accommodate these more difficult-to-remove solutions.  If not, there have been many instances where residue has been left on the surface which inhibited pretreatment deposition.  This is especially important in porous metallic coatings such as electrogalvanized steel.
  • Outer body components for new vehicle models manufactured ahead of a “Job 1” production schedule may be stored in hot warehouses over an entire summer – allowing even benign corrosion inhibiting oils to polymerize onto the surface – negating removal in the cleaner baths. 

Any experienced automotive engineer has a story involving at least one of these examples that required long hours to resolve.  Left unchecked, undetected soils on the substrate surface after cleaning will inhibit pretreatment deposition. At worst, this could lead to field failures in the form of paint delamination after the customer has purchased the vehicle.  And, manufacturing schedules can be upended when polymerized oils not removed during cleaning cause severe cratering defects on an electrocoated body-in-white – leading to, at minimum,significant reworking, or worst case, scrapping the vehicle.  The adage, “cleanliness is next to Godliness” is truly one of the preeminent principles when the bonding or painting of substrates.

In the early 1990s, a few suppliers and automotive manufacturers found a way to ensure that soils truly “come off in the wash” on steel substrates.  Instead of applying corrosion inhibiting oils, coil production lines would coat coils with a removable thin film acrylic that inhibited shipping corrosion and remained intact during the component stamping process.  In many cases, the acrylic film also eliminated the need for forming lubricants - a main source of organic soils.  Equally important, the acrylic film provided a constant low coefficient of friction throughout the forming die.  This led to sharper feature lines, more consistent parts, and less breakage. 

Coefficient Table_CAG
From:  Coil Coating for Automotive – ZincrometalTM to Thin Film Organics.  C. A. Gosselin, 1998

In some instances, it was even possible to use a less formable base steel because the thin film organic coating allowed for such a consistently low coefficient of friction irrespective of surface roughness that the steel was able to move through die more efficiently.  Unlike oil, the coating did not age, so storing blanks for as long as two years (in high temperature and humidity environments) didn’t interfere with the removal process.

The most important attribute of this thin organic coating was that it dissolved in the alkaline cleaner even after two years, taking with it any soils, fingerprints or dirt, which resulted in a pristine active surface for pretreatment deposition that ultimately promoted paint durability.  Phosphates deposited uniformly, leading to small, acicular crystals on variously galvanized materials that completely covered the entire substrate surface.  This led to an excellent surface for electrocoat deposition – eliminating the concern of undetected organic soils contaminants remaining trapped beneath the paint surface.

Phosphate photo_CAG
Phosphating result two years after acrylic film coating.  (1000x)

A second important characteristic of this thin film was that it was found to be compatible with the adhesives that were commonly used in vehicle assembly.  Extensive testing with actual adhesives from each assembly plant verified compatibility.  Durability with those systems was also validated by subjecting lap shear joints to the accelerated environments as shown in the chart below.  Notwithstanding these results, it is important to always validate adhesive compatibility with a plant trial.

Lap Shear Strength_CAG  
(click to enlarge)

Groups 1 - 5 image_CAG

Even today, analyzing and correcting paint delamination and adhesive bonding failures is still common (and costly) in vehicle manufacturing.  Considering that the primary source of material failures is undetected soilon the surface of the substrate, an economical solution would be to apply a removable thin film organic coating to the substrate surface.  This would not only ensure lower costs associated with the overall manufacturing process, but also the elimination of the largest root cause of interfacial bonding and delamination failures.  Perhaps it is time to resurrect this cost-effective product development.