Reliably predicting dye penetrant wettability for fluorescent crack inspections hinges on understanding surface cleanliness prior to penetrant application.
Validating the efficacy of a parts washer using water contact angle measurements is a non-destructive method for quantifying surface cleanliness and can ensure complete wetting of the penetrant across a substrate.
Premise: Surface energies of washed and un-washed aluminum parts were characterized via water contact angle measurements and subsequently correlated with dye penetrant wettability.
Penetrant was applied to the surface via cotton swab and allowed to wet the surface for 15 minutes before obtaining a visual inspection of wetting pattern; a smooth, uniform distribution of penetrant indicated sufficient wettability while a blotchy, non-uniform distribution indicated insufficient wettability.
Freshly washed samples displayed low contact angles and yielded acceptable penetrant wettability while unwashed parts displayed high contact angles and displayed unacceptable penetrant wettability. Water contact angle can be used to determine surface readiness prior to penetrant application.
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A recent concept in automotive light-weighting is that of the ‘joining of dissimilar materials.’ The purpose is to allow tailoring the materials in a structure so as to ensure that each part of the structure has the optimum mechanical properties and minimum weight. An example would be the bonding of aluminum stiffening ribs to a polymeric body panel.
The concept of ‘joining’ has many subtleties. The purpose of the joint (or interface) between the components is to transfer the applied load from one component to the next. If the means of joining is by welding, the stress distribution is evenly distributed throughout the joint.
However, dissimilar materials are almost never able to be welded, and mechanical fasteners (bolts or rivets) are frequently used. All of the transferred stress in a structure joined with mechanical fasteners is concentrated in the fasteners and the holes through which they pass.
To resist fracture, the material must be made thicker and heavier in order to sustain these stress concentrations, which negates much of the advantage to be derived from multi-material structure design.
From a structural standpoint, adhesive bonding provides the advantages of welding with the ability to use multiple materials. Stresses in bonded structures are uniformly distributed and allow the absolute minimum gage materials while retaining excellent mechanical properties such as strength, stiffness, and impact resistance. However, adhesive bonding processes bring a distinct set of challenges to manufacturing.
At first glance, bonding operations appear to be straightforward mechanical processes which involve various combinations of washing or wiping, abrasion, surface treatment, adhesive application, positioning and fixturing components, and curing, perhaps through application of some combination of heat and pressure. The perception of bonding as a mechanical process has resulted in a failure to appreciate the fact that creating a successful bond between an adhesive and a substrate is actually a multistep chemical process. The first step occurs at the manufacturer of the adhesive, where the resin is synthesized. The second step occurs on the shop floor of the end user, where a bonded interface is synthesized from the reactions of the adhesive with the prepared surface. Whereas the bulk properties of the cured adhesive depend on the manufacturer’s control of the quality of the coating or adhesive and on the ability of the technician to properly execute the cure cycle, the properties of the interface are established on the shop floor by the technician during the bonding process. The quality of the established interface depends on generating a prepared surface of identical chemical composition and structure time after time [1-8]. This is more difficult than it may seem at first glance, because the properties of a surface are determined by the composition and structure of only the uppermost 2 to 3 molecular layers. By way of contrast, a fingerprint leaves a layer of oils and fatty acids that is around 1000 molecular layers thick. The residue from a human’s breath is 100’s of molecules thick. What might seem to be insignificant changes in incoming material, storage and handling, processing or environment can actually result in large changes in the properties of a surface, and therefore the properties of an adhesive bond.
The water break test a common pass/fail method used to evaluate surfaces for the presence of hydrophobic contaminants, which can be detrimental to adhesion of paint or an adhesive. It is a qualitative means of evaluating surface energy, which is directly related to surface cleanliness.
In this test, a stream of water is visually evaluated as it flows over a surface:
• If it spreads out into a continuous, unbroken sheet, it indicates that the surface is substantially free of hydrophobic contaminants.
• If the surface is contaminated with low surface energy substances, the flowing water will not sheet uniformly over the surface but rather it will break into rivulets and tend to bead up (termed “water break”).
Water break tests are not ideal as a quality control tool. They are messy: a relatively large amount of water is used which has to be removed and the component must be dried before coating or bonding. Cases of water break tests contaminating sensitive surfaces because of impure water or transfer of contaminants during the drying process are not uncommon. Because the result is only a binary ‘water break free/not water break free,’ it is unknown whether it is too sensitive for some applications or not sensitive enough.
Cleanliness in automotive powertrain manufacture is critical for several reasons:
• Particulates generated in the various casting and machining processes must be effectively removed to prevent both the premature wear of sliding and rotating parts as well as the catastrophic failure of components such as transmission valve bodies.
One of the final steps in component manufacturing is the cleaning process, and current commercial washer systems can be quite effective at removal of particulate contaminants. In fact, washer system performance is traditionally evaluated based on particulate removal efficiency.
Efficiency is typically quantified using tests such as the “Millipore Test.” In this test, a cleaned part is thoroughly rinsed with solvent under pressure, the solvent is collected and then filtered to recover any particulates that may have remained on the cleaned part. The mass of the recovered particulates is measured, as this value is used as a parameter to evaluate washer system performance.
The Surface Analyst™ rapidly obtains contact angles from surfaces via Ballistic Deposition, whereby a small drop of liquid (usually water) is constructed in situ on the surface via a pulsed stream of nanoliter-sized droplets.
The contact angles established in this manner are a sensitive function of surface chemical composition.
In this work, injection molded polypropylene panels were oxidized to various levels via exposure to an atmospheric pressure plasma treatment process. Surface chemical composition was determined via X-ray Photoelectron Spectroscopy (XPS), and the chemical composition was related to the water contact angles determined using a Surface Analyst™.
Solvent wiping and sanding procedures can greatly affect the surface energy of a substrate. To investigate the surface energy differences following different preparation procedures of an epoxy/composite laminate, several different surface conditions were created utilizing different cleaning techniques.
Measurements were obtained using a BTG Labs Surface Analyst™. The Surface Analyst is a fast, easy, accurate and nondestructive instrument that measures the contact angle of water that is applied to the surface in a precise, controlled manner.
This contact angle is determined by how strongly the surface energy of the substrate and the liquid interact with one another. The relationship between this contact angle and surface energy is complex but well understood. More importantly, this relationship correlates with the strength of adhesion of a paint, coating, print or adhesive to the substrate.
While suitable in some cases for estimating surface energy (and therefore cleanliness or treatment level), the imprecision and subjectivity of wetting tension measurements makes them a poor choice for quality assurance and process control of surface cleaning, surface treatment, bonding, coating, and printing operations.Dyne inks are also destructive to the surface being measured. An alternative method for gauging surface condition and consistency is the Surface Analyst™, which provides a rapid, automated measurement of the water contact angle in a precise, controlled manner.This contact angle is determined by the surface energy of the substrate and the liquid and how strongly they interact with each other. This water contact angle correlates very well with the cleanliness and consistency of a surface.