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”).
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.
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 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.
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.
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.
This paper reviews recent research progress in the detection of contamination on surfaces for bonded composites.
Results to date indicate that it is possible to use a simple handheld instrument to determine if a composite surface is in such a state that a durable bond can be achieved.
This study examined both airborne and contact contamination and found that contact contaminants can originate from unexpected sources. Monitoring of airborne contaminants in various manufacturing locations indicated that discrete contamination events can occur that are potentially detrimental to adhesion.
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.
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.