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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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”).
The water break test is 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.
The existence of a quantitative relationship between wetting and surface energy is demonstrated for a range of treatments.
These measurements uncovered variation in surface energy of both treated and untreated material along the machine direction of the web, suggesting that more precise control of treatment level could result in a more uniform product.
- A frequent question from people who use contact angle measurements to characterize surfaces and control manufacturing processes is, “What effect does surface roughness have on these measurements?”[spacer height=”10px”]This is a valid question, especially when dealing with surfaces that have a range of textures and roughness levels resulting from variability in molding, casting, machining, and abrasion processes.[spacer height=”10px”]The answer is, “it depends.” It depends on the magnitude of the roughness, and it depends on the way in which the liquid drop is deposited on the surface, and it can depend on the contact angle range being measured.[spacer height=”10px”]However, in most situations where one is using contact angle measurements to control surface cleaning and treatment processes, roughness effects can be ignored. In this white paper we explain and then demonstrate why this is so.
Composites consisting of polymer matrices reinforced with graphite fibers are attractive materials for structural applications in the aerospace industry because they are light in weight and have a high strength-to-weight ratio.
Adhesive bonding is the preferred method for joining composites:
• Eliminates the need for cutting holes in the composites that would damage load-bearing fibers
• Eliminates the stress concentrations that would be associated with mechanical fasteners
• Enables stresses to be distributed over large areas
In addition, adhesively bonded structures frequently have lower weight than similar structures that are assembled using mechanical fasteners.
Adhesives used for joining composites fall into two broad classes: high-temperature curing film adhesives and room-temperature curing paste adhesives.
Although high-temperature curing film adhesives are less sensitive to composite surface preparation than room-temperature curing paste adhesives, the surface-preparation procedure is always critical to achieving a strong bond. Composite surfaces are frequently contaminated with release agents that are used to prevent adhesion of the composites to the molds in which they are cured. The presence of a release agent on the surface lowers the surface energy of a composite and makes it difficult for the room-temperature cure systems to wet the surface.
As a result, surface engineering processes must frequently be applied to polymer composites to remove mold release agents from their surfaces and to increase their surface energies. These surface engineering processes may be as simple as rinsing the surface with a solvent. However, they may be much more complex, removing release agents, roughening the surface to introduce topographical features that allow for mechanical interlocking between an adhesive and the surface, and changing the surface chemistry of the composites by introducing new functional groups.
A wide variety of plasma treatments was performed on polyethylene surfaces, resulting in a wide range of total surface energies.
The linear correlation of with cos θ was discussed in light of the Young–Dupré equation. One-hundred percent of the surface energy variation was accounted for by the polar component of surface energy; the dispersive component was not affected by surface treatment.
These data show that for this polymer, the contact angle of a single polar liquid can be used as a robust quantitative indicator of treatment level, and because of its excellent linear correlation with total surface energy for this system, polar contact angle can be used as a quantitative measure of total surface energy.
To have predictable strength and therefore to be useful in structural applications, adhesive bonds must not fail interfacially. Failure must be cohesive in the substrate or in the adhesive. This means that the interfacial fracture toughness must be maximized.
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™.
Surface preparation is a critical manufacturing process that enables sealing, bonding, painting, coating, printing, and cleaning. By utilizing appropriate surface preparation processes medical device manufacturers can speed production, ensure product safety, and decrease the likelihood of recalls.
Despite the critical nature of this process the majority of manufactures are still relying upon outdated surface evaluation methods such as dyne and water break. These current methods lack precision, allow for subjective interpretation, and are often destructive.
BTG Labs sat down with the Generis Group to discuss the challenges of surface preparation and how manufacturers can overcome them.
There are fundamental differences between an NRL-style goniometer and the Surface Analyst, most of which contribute to the value of the measurement for development and control of surface sensitive manufacturing processes.
These include the method of liquid deposition and the method of contact angle calculation once the liquid drop is deposited.
The original motivation for these differences was to allow for a more compact and convenient instrument that could be easily handheld. However, they also significantly improve the speed and accuracy of the measurement as well as the flexibility of the types of surfaces that can be measured.
Metals present extremely high energy, reactive surfaces to the environment. When mechanically or chemically cleaned, they rapidly oxidize and adsorb contaminants such as organic vapors. Polymers present surfaces that are less reactive towards their surroundings. When cleaned by abrasion processes they also show rapid changes due to oxidation and adsorption, but these changes tend to be of lower magnitude.Successful bonded repair of aircraft structures involves creating a small area of carefully controlled surface composition on metallic or polymeric surfaces. This area to be bonded is located within a larger area of material that may be contaminated with a variety of soils picked up during normal aircraft operation: organic and inorganic soils, fuel, hydraulic fluids, etc.Because of the reactivity of freshly prepared surfaces and the proximity and mobility of contaminants in the surrounding area, cleaning of these surfaces sufficiently to obtain reliable adhesive bonds can be particularly difficult in field situations. Furthermore, because the difference between a well-cleaned surface and a contaminated one may only be a few molecular layers, it can be difficult for the technician to establish when the surface has been properly prepared.Measurement of the geometry of a liquid drop deposited onto the surface can be done extremely rapidly and form the basis of a sensitive check of surface cleanliness and consistency in a repair depot or in challenging field situations. This paper discusses the use of these rapid wetting measurements for quality assurance of surface treatments for adhesively bonded repairs.