Description of measurement techniques
- FTIR (Fourier Transform Infrared Spectroscopy)
- SEM (Scanning Electron Microscopy)*
- ESCA (Electron Scanning Chemical Analysis)*
- TOFSIMS (Time Of Flight Secondary Ion Mass Spectroscopy)*
- AFM (Atomic Force Microscopy)*
- DSC/MDSC (Modulated Differential Scanning Calorimetry)
- TMA (Thermo-Mechanical Analysis)
- TGA (Thermal Gravimetric Analysis)
- DMTA (Dynamic Mechanical Thermal Analysis)
- DETA/DEA (Dielectric Thermal analysis)
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 less than 100 nm depth of surface is involved for the analysis.
SEM (Scanning Electron Microscopy)
SEM is a useful tool for the characterisation of phase morphology in polymer-blends, fracture mode and elemental composition (EDAX surface penetration in µ-region).
ESCA (Electron Scanning Chemical Analysis)
In ESCA ( also known as XPS ), the sample is bombarded with soft X-rays and the photoelectrons emitted are analysed in terms of kinetic energy. For elemental surface analysis in the range of 1-5nm, ESCA has proved to be very useful. In case polymeric materials are subjected to a pre-treatment (corona, plasma or flame), O/C ratio of the polymer-surface before and after the pre-treatment is analysed with the help of ESCA and correlated with contact angle findings.
ESCA is a quantitative surface analytical technique and is very useful in quantifying e.g. silicone by determining the amount of Si-element after Silicone (PDMS) has been detected beyond doubt with the help of TOS-SIMS (see below).
TOF-SIMS (Time Of Flight Secondary Ion Mass Spectroscopy)
The sample is bombarded with primary ions of 15-25 KV and the secondary ions are extracted perpendicular to the sample surface before being deflected to the detector. Whereas ESCA involves characterisation of 1- 5nm surface depth and is mainly helpful in quantitave elemental analysis, TOFSIMS is suitable for surfaces < 1nm and gives information about molecular structure of monolayers at the surface. These two techniques complement each other quite often. TOFSIMS is a very effective method of detecting silicone (PDMS) and other release agents / surfactants 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. Coupling agents like epoxy- and amino silanes are often applied as very thin layers on substrates like steel before a polymeric coating is applied. In many cases, only TOFSIMS is able to characterise the very thin layer of the coupling agent on the substrate.
AFM (Atomic Force Microscopy)
Surface roughness contributes to the adhesion of paints / coatings by way of interlocking. The level of surface roughness needs, therefore, be controlled accurately. Atomic Force Microscopy is being increasingly employed for surface characterisation in sub-micron range after the roughness is induced on a polymeric surface by corona / plasma treatment..
DSC/MDSC (Modulated Differential Scanning Calorimetry)
DSC has been employed for over 3 decades to investigate cure kinetics of adhesives / coatings and characterize / quality control polymeric materials by way of measuring the tg and degree of crystallinity. With rising demands of high performance polymers, the resolution and sensitivity of this conventional technique became inadequate.
MDSC, a recent modification of the 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 that the real glass transition temperature, tg, is determined from the reversing heat flow curve.
TMA (Thermo-Mechanical Analysis)
TMA measures dimension change (linear or volumetric) as a function of time and temperature. The technique is a standard method of determining the coefficient of thermal expansion and is based on a movable core linear variable differential transformer (LVDT), the output of which is proportional to the linear displacement of the core caused by changes (as small as 100 nm) in sample dimensions.
TGA (Thermal Gravimetric Analysis)
TGA, which measures weight changes with temperature, provides information about material composition (filler content, carbon black, hydration water etc.) and thermal stability.
DMTA (Dynamic Mechanical Thermal Analysis)
DMTA 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 (visoelastic) sample, the strain response lags behind the stress applied. E* is then resolved into E' (elastic/storage) modulus and E'' (viscous/loss) modulus components.
The ratio E''/E' = tan d, often reffered to as mechnical loss or damping, is a very useful parameter.
The peak maximum of 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.
DETA/DEA (Dielectric Thermal analysis)
In the presence of polar groups in various coating materials (e.g. epoxy and PU), this techniques 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 dieclectric loss tan d = E''/E' is determined with respect to temperature. The higher degree of cure is manifested in decreased dielectric loss. DETA 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.