|Advanced techniques for characterization of surface / interface and adhesive cure mode.|
By Amir Hussain
In order to understand the adhesion and failure phenomena of adhesive bonds, characterization of the substratesurface, before applying adhesives, and the cure mode of adhesives are very important. For studying the surface roughness in the submicron range, AFM is proving to be increasingly important.
Depending upon the thickness of surface treatment and/or coupling agents, surface analytical techniques like FTIR, ESCA and/or TOFSIMS are almost indispensable.Results of contact angle measurements can be correlated with those obtained from these techniques. Contamination is one of the bigger enemies in surface engineering and ESCA/TOFSIMS are proving extremely helpful in detecting and combating the enemy.
After an adhesive is applied on the substrate, it is imperative to investigate the cure/crosslinking mode of the adhesive-especially for epoxides and PU adhesives - in an accurate way. MDSC, DMTA and DETA/DEA are being employed very successfully as analytical tools for following the degree of cure.
In most composites, two different materials - usually an inorganic substrate and a polymeric material - form adhesive bonds with each other. The durability of a composite is directly related to the quality of adhesion. Chemists tend to associate adhesion with the energy liberated when two surfaces meet to form intimate contact with the formation of an interface. In other words, adhesion may be defined as the energy required to dismantle the interface between two materials. Physicists and engineers usually describe adhesion in terms of forces, with the force of adhesion being the maximum force exerted when two adhered materials are separated. There are many theories regarding the mechanism of adhesion, such as adsorption (van der Waals forces), electrostatic, diffusion (entanglement of polymers with a substrate), chemical bonding, mechanical interlocking, etc., all of which may play a significant role in interracial bonding.
When an adhesive is applied on a substrate (organic or inorganic), a chemical reaction is expected to occur when the surface contains functional groups. It is often desirable to modify the adhered surface to enhance the reactivity at the interface by removing contamination and/ or introducing functional groups. This simplified view of the interfacial or interphase bonding neglects physical forces between two materials, which are influenced by, e.g. surface roughness. For a comprehensive characterization of adhesive bonds, surface analysis of substrates (chemical as well as topographic) and thermal analysis of the cured adhesive bulk is imperative.
Characterization of Adherend Surface
After adhesive is applied on the substrate, it is important to check the
cure/crosslinking mode of the adhesive accurately.
It is almost impossible to list all the factors that may affect adhesion of composite materials because of the broad spectrum of substrates that can be involved with adhesive bonds. Outlines of some contributions of FTIR (Fourier Transform Infrared Spectroscopy), ESCA (Electron Scanning Chemical Analysis), TOPSIMS (Time Of Flight Secondary Ion Mass Spectroscopy), AFM (Atomic Force Microscopy) and contact angle techniques are mentioned here (1):
FTIR (Fourier Transform Infrared Spectroscopy): FTIR is based on the absorption of infrared light as it passes through the sample. The IR spectrum, i.e. the amount of transmitted energy as a function of wave number, is obtained.
Generally, the FTIR surface techniques can be classified in two categories: reflection (ATR: attenuated total reflection) and non-reflection techniques (PAS: photoacoustic spectroscopy).
PAS utilizes the detection by a sensitive microphone of an acoustic signal emitted from a sample after absorption of a modulated radiation. This technique is proving increasingly useful for oxidation depth profiling of polymeric surface after corona or plasma treatment. FTIR has its limits when <5 nm depth of surface penetration is involved for the analysis.
ESCA (Electron Scanning Chemical Analysis): In ESCA (also known as XPS), the sample is bombarded with soft X-rays and the photoelectrons emitted are analyzed in terms of kinetic energy. For elemental surface analysis in the range of 1 to 5 nm, ESCA has proved to be very useful. In Table 2, the O/C ratio of a PP surface is analyzed with the help of ESCA and correlated with advancing and receding contact angle findings.
The O/C ratio of 0.12 was found to be optimal for PP after surface treatment. ESCA proved to be a very useful analytical tool for surface elemental analysis and also for establishing a correlation between the elemental composition and contact angle results.
The sample is bombarded with primary ions of 15 to 25 KV; the generated secondary ions are extracted perpendicular to the sample surface before being deflected to the detector. Whereas ESCA involves characterization of a 1 - to 5-nm surface depth and is mainly helpful in quantitave elemental analysis, TOFSIMS is suitable for surfaces <1 nm and gives information about the molecular structure of monolayers at the surface.
These two techniques complemenet each other quite well.TOFSIMS is a very effective method of detecting silicone (PDMS) and other release agents/suffactants as contamination on metallic and polymeric substrates. Surface contamination is the most important enemy of surface engineering. What is more important is not a clean surface, but a controlled one. Surface analysis is the most effective weapon against the enemy. ESCA helps in quantifying silicone by measuring Si-content as a complementary technique to TOFSIMS.
Coupling agents like epoxy and amino silanes are often applied a very thin layers on substrates like steel before a polymeric coating is applied. In many cases, only TOFSIMS is able to characterize the very thin layer of the coupling agent on the substrate. ESCA and TOFSIMS analyses have to be carried out at a high vaccum, which is a disadvantage.
AFM: Surface roughness contributes to the adhesion of paints/coating by way of interlocking. The level of surface roughness needs, therefore, to be controlled accurately. Atomic Force Microscopy is being increasingly employed for characterization in submicron range.
Characterizazion of Adhesives Cure Mode
The strength and durability of bonded structures is a combined effect of interfacial and cohesive factors. Besides the interface, cohesive properties of adhesive bulk are very important.
The cohesive energy is determined largely by the molecular structure arising from careful curing of adhesive; cure reactions can be controlled by advanced techniques of thermal analysis(2).
DSC has been employed for more than two decades for investigating cure kinetics of various adhesives and coatings. With rising demands of high-performance adhesives, the resolution and sensitivity of this techniques became inadequate.
MDSC (Modulated Differential Scanning Calorimetry), a recent modification of a conventional DSC, has gained importance by overcoming the limitations of the latter. Specifically, a sinusoidal modulation of the heating profile is overlaid on the conventional linear heating. The net effect, i.e. increased resolution and sensitivity, is the same as if two experiments were run simultaneously. MDSC separates reversing and non-reversing components of the heat flow, so the real glass transition temperature, Tg, is determined from the reversing heat flow curve.
DMTA (Dynamic Mechanical Thermal Analysis) defines the state of cure and hence the molecular architecture by measuring modulus and mechanical damping or loss with respect to temperature and frequency. When a sinusoidal stress is applied to the (viscoelastic) sample, the strain response lags behind the stress applied. E* is then resolved into E' (elastic/storage) modulus and E" (viscous/loss) modulud components. The ratio E"/E'=tan d, often referred to as mechanical loss or damping, is a very useful parameter.
The peak maximum of a tan d is defined as Tg of the cured adhesive/coating. DMTA (also known as DMA) is an extremely sensitive and useful technique, but has the obvious disadvantage of sample geometry and preparation.
In the presence of polar groups in the adhesive materials (e.g. epoxy and PU), this technique has proved to be the most sensitive of the three techniques mentioned here. A sinusoidal electric field is applied to the sample and the electric displacement followed.
The complex dielectric permitivity e* obtained can be resolved into the storage component e` and loss component e". The dielectric loss tan d = e "/e´ is determined with respect to temperature. The higher degree of cure is manifested in decreased dielectric loss. DETA (Dielectric Thermal Analysis) is very sensitive for detecting moisture absorption at the interface and in the polymeric bulk material. This technique has also the disadvantage of sample geometry and preparation.
The strength and durability of adhesive bonds depend largely on the integrity of the interface and structure of adhesive bulk after cure reactions. For high-performance bonded products, an accurate characterization of the substrate-surface and cure mode of adhesives is imperative.
A combination of modern analytical techniques ESCA/TOFSIMS is proving increasingly useful and essential for understanding the role of the interface. Besides the interface, the other important step is to look at the cohesive properties of the adhesive bulk.
MDSC and DMTA are emerging as very powerful complementary techniques and responding well to the rising demands for characterization of the cure mode of adhesive materials.
MDSC, DMTA and DETA/DEA experiments were carried out at Comtech GmbH for which the author is thankful to Christa Pflugbeil. The ESCA and TOFSIMS analyses were conducted by our cooperation partners on a contract basis.
This paper was presented at FEICA's World Adhesives Conference last September in Barcelona, Spain.