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“As Received” and Adhesive Performance

Posted on 7/10/2017 8:46:57 AM By Jim Swope
  

In my early days as a supplier of adhesives to automotive assembly operations, I dreaded the words “as received” on a specification because its implied meaning could affect adhesive performance (light mill oil commonly used as a rust preventative by parts suppliers is often the culprit).

Mill oils may not present major problems in assemblies where the adhesive process is augmented by mechanical methods such as spot welds or the adhesive process displaces the oil.  However, when the adhesive is carrying the full load (no mechanical augmentation) or the operating conditions are extreme, surface contamination will present issues. A clean surface is commonly understood as being critical to repeated success in bonding. Sometimes the parts supplier would insist their parts are cleaned prior to shipping but further probing would reveal they added a rust preventative in the final rinse or as a subsequent process. The presence of a rust preventative can retard or arrest chemical cures or act as a barrier to proper wetting of the surface.The outcome of which is classic weak link theory: The assembly fails due to weakening of a critical component – the adhesive interface to the substrate.

What steps can be taken to assure that we avoid failure in critical assemblies? How do we balance that against cost pressures?  Who bears the cost burden of proper surface preparation?

The answer lies with the specification writer who ought to own not only the bonding specification but the condition of the substrates that are critical to the assembly. Fortunately, as adhesives have evolved in both their effectiveness and acceptance so have the tools that enable their use. Wettability is a key factor in effecting a good bond. Wetting is the ability of liquids to form interfaces with solid surfaces. To determine the degree of wetting, the contact angle (q) that is formed between the liquid and the solid surface is measured. The smaller the contact angle and the smaller the surface tension, the greater the degree of wetting.

Something as simple as dyne test inks can provide information on change in wettability of the substrate. While dyne tests are technique sensitive and subject to operator errors caused by readings prone to change (as the result of carrier solvent evaporating from an open package) in less critical applications these can serve an educated user well.

A contact angle measurement tool, which is more sophisticated than dyne inks, can establish the necessary level of wettability for your application (factoring in adhesive and substrate types). Once an acceptable level is established the tool aids in measurement and recording contact angle data in some cases. Laboratory tools are better suited to an SPC measurement or incoming inspection approval but new tools allow in-production measurement of every part if desired.  The ability to easily log data can serve as both a preventive measure in spotting changes in conditions and in forensic analysis when conditions lead to failures.

I have a personal story of how unmeasured varying wettability created havoc during production ramp up. The customer was bonding a powder-coated steel mounting flange to a powder-coated, deep-drawn steel motor housing. Suddenly, we began experiencing adhesive failures to the motor housing, testing with press values at approximately 70% of specification. The adhesive was coming off clean from the housing even with no variations in the powder coat or part vendor. After eliminating all possible in-house variables, we spent a day at the powder coating shop watching how they ran parts. The powder coat had a minimum temperature and duration-at-temperature specification but no upper limit. The coater designed the process to run at a steady state with the oven set to hit the minimum time/temperature specification at maximum oven load. Since this vendor ran mostly automotive suspension parts their normal load was heavy. Rarely, they ran light parts like our motor housings and the cure temperature would spike over 120°F above the minimum. The result was a glass-like finish that passed salt spray, the only specification they had been given, with relatively poor wettability. The vendor was very cooperative – we instituted a dyne test at their facility that prevented future wettability issues.  Later, they shared that our issue helped them to isolate a secondary problem with chipping related to the glass-like cure.

Wettability tests also solve issues with plastic parts such as plasticizer migration, mold releases, and resin-rich surfaces where fillers were specified for adhesion, to name a few. More vexing are those surfaces that exhibit good wetting but still produce poor results. Examples range from finishes we design into the process to those that nature produces and surprise us. Conversion coatings like chromate are common bases for paint, so adhesion would seem to be a given but adhesive performance requires resistance to greater forces. Changes in the coating thickness, hardness, moisture content, and acidity can adversely affect bond strength and are much harder to troubleshoot. Nature provides oxygen for our continued survival and to vex us with metal oxides. Fortunately, there are many analytical tools available and companies that provide analytical services such as EAG (see Figure 1).  Once the source of failure is determined you can select a process monitoring tool appropriate to the variable. Companies like EAG and our own ChemQuest Technology Institute can help in that selection.  These may range from a cross hatch adhesion tester to in-process surface analytical tools like the Optically Stimulated Electron Emission (OSEE) tester.

“AS RECEIVED” whether written or implied is like leaving 1% of your bill of materials unspecified but expecting consistent performance from the assembly.  Many adhesive joints are way too critical to leave the surface to chance.

Figure 1: Analytical Resolution vs. Detection Limit


                 (click to enlarge)