One of our areas of specialized expertise has long been the analysis of adhesive bonding. Adhesive failures are commonly said to be either cohesive failures or adhesive failures. A cohesive failure is a failure in the bulk layer of the adhesive and is usually the desired mode of failure. An adhesive failure occurs at the interface between the adhesive and the adherend, or the material to be bonded. Such disbonds at the interface with all of the adhesive on one side of the separated materials is desired in some cases, such as in use with temporary notes, a peel-ply adhesive, or a backing on a roll of tape.
In many adhesive bonding failures, the reality is actually more complex. Failure which appears cohesive may occur due to degradation of the adhesive itself, such as by water or excessive heat or radiation. Apparent interfacial failures may be due to the prior excessive hydration of the adherend, whether it is metallic or an inorganic particle. It might be due to a surface being excessively basic, since good adhesive bonding usually favors a slightly acidic surface. A classic case Dr. Anderson diagnosed was the excessively alkaline surface of glass microspheres in the thermal insulation of the Space Shuttle external fuel tank. Most adhesives have a number of low concentration additives, which may segregate strongly to the interface forming a very thin layer at the interface of a highly different composition from the bulk adhesive. This thin and invisible layer may be desirable to strengthen the bond to the adherend surface, but improper application conditions may cause this thin interfacial layer not to form or to form incorrectly. Some contaminants may be particularly capable of interfering with the formation of this needed thin layer. Degradation of the adhesive and/or the adherend/adhesive interface chemistry may precede an interfacial or adhesive failure if the ingress of water to the interface occurs. Adhesive or interfacial failures also can occur at plastic surfaces due to migration of a plasticizer to the interface.
Adhesive failures often require sophisticated analysis to understand. The location of the failure may be in a very thin layer at an interface. The interface may have become contaminated during any of the processes used to prepare the adherend surface or afterwards. Fatal contamination layers are often less than 3 nm thick! One classic example is a silicone or dimethyl siloxane release agent or lubricant, which is what caused the failures at the frit interface in the above image. This thin layer of dimethyl siloxane was not detected by two prior laboratory investigations, but was readily detected in our laboratory using XPS surface analysis. Adhesives and organic coatings which are to adhere to surfaces are usefully characterized with FTIR analysis. Sometimes FTIR is even useful for detecting relatively thin layers of organic material that may either cause unintentional adhesion, such as in aluminum honeycomb processing, or in detecting thicker organic contamination layers that interfere with adhesive bonding.
In other cases, a metal surface may be treated to promote adhesion with a high surface area, then treated to retard corrosion with a very thin layer of solid particles, and then treated with a silane adhesion promoter which may be thinner than 10 nm thick. Many things may go wrong in these processes. The adhesive itself may have numerous component ingredients and some may be at very low concentrations in the bulk of the adhesive, but are supposed to migrate to the interface to promote adhesion. Sometimes this migration process does not work right. Sometimes an impurity migrates to the interface instead. Exposure of a metal surface to humidity may also cause interfacial or adhesive bond failures in the metal surface.
Bonding to plastic, glass, ceramics, inorganic particles, or composite materials may suffer failures at or near interfaces for many reasons. Inorganic particle filler materials are often given treatments to promote polymer resin wetting, whether in composite materials, paints, or sealants. When that fails in a composite material, the material is greatly weakened. In a paint or sealant, they may agglomerate, which may allow degradation due to UV exposure or it may cause the paint or sealant to peel off the adherend. Or, such inorganic particles may lose bonding due to excessive hydration. Glass surface bonds may fail if the glass surface was too alkaline when bonded, as Dr. Anderson demonstrated was the cause of Space Shuttle External Fuel Tank failures in the early 1990s. Metal surfaces, especially those with +2 valence states are inclined to suffer from excessively alkaline chemistries at surfaces also. Plastic bonding may fail due to migration of plasticizers to the surface or residues of mold release agents.
Determine surface and interface chemistries using XPS surface analysis:
- Surface analysis of adherend or substrate material as prepared for bonding
- Surface analysis of both sides of peeled or failed interfaces to find locus of failure, identify chemistry, and identify any contaminants
- Correlate surface chemistry of samples tested to failure with measured strength
- Examine aging and degradation effects on the chemistry at the interface as a function of time and exposure environment
- Determine differences of bondline adhesive chemistry from the bulk adhesive chemistry, since distinct and often radically different interfacial adhesive interphase layers are common
- Identify interfacial contaminants due to hydrolysis, silicones or siloxanes, plasticizers, surfactants, hand lotions, cutting oils, hard water deposits, solder fluxes, insufficient rinse of pretreatment acids or bases, mold release agents, and photoresists
Prepare very clean adherend surfaces for reference to mechanically tested bond strengths of manufactured or prototype bonded materials.
Microscopy – SEM and Optical – Failure Analysis:
- Locates failure plane
- Indicates type of failure
- Reveals gas bubbling effects due to contaminants or improper 2-part adhesive mixtures
Failed lap shear tested coupon for analysis to determine the cause of adhesive failure.
Honeycomb skin with poor adhesive bond in some areas for analysis to determine the cause of failure.
- TMA to determine differences of thermal expansion of adherends and adhesive since such differences may cause tremendous forces capable of ripping the adhesive bonds apart
- TMA to indicate whether thermoplastics have too much crystallinity, which may produce a large thermal expansion spike at the crystalline melting temperature
- TMA and DSC to determine maximum temperatures appropriate for curing adhesive bondlines or to prevent phase changes and degradation processes in use
- DSC to determine good adhesive curing conditions or problems due to improper mixtures of 2-part adhesives
- DSC to measure the degree of cure
Electrochemistry – Electrochemical Impedance Spectroscopy (EIS):
- Measures water concentration buildup in organic coatings and adhesives which can cause hydrolysis degradations of either the adhesive or the adherend surfaces
- Measures other ion concentration increases in coatings and adhesives over time that may indicate the corrosion of metal surfaces at the adhesive bondline with subsequent adhesion failures
- Measures effect of adhesive or coating pores, holidays, and fisheyes upon water reaching interface to cause hydrolysis attack of bondline
- Identify chemical differences at bondline interface relative to the bulk of the adhesive
- Contrast chemistry of 1 μm depth to that of the 8 nm measured by XPS
- Identify plastic, rubber, resin, and laminate materials being bonded
- Identify the adhesive and/or the primer to which adhesive is applied, as it is in aerospace adhesive bonds to aluminum surfaces
- Very useful in determining which areas to further analyze with the above techniques
- Used to document the microscopic appearance associated with the subsequent results.