In many sectors of industry the cleanliness of component surfaces after pre-cleaning, intermediate cleaning or final cleaning processes is an important quality feature. Nevertheless, contamination can occur in manufacture despite the latest production techniques. This contamination must be lastingly removed before further process steps are taken.
We identify contaminating films quantitatively and qualitatively.
These are often manufacturing or cleaning residue such as oil, grease, cooling lubricants, cleaning agents, and also residue from corrosion inhibitors, coatings and other agents used in manufacture. Preservatives and adhesives as well as the sweat from hands and finger-marks also occur here. Further process steps such as bonding, welding, painting or assembly are made more difficult or prevented by them. The consequences include leaks or coating adhesion problems.
The methods we use depend on the requirements in the related subsequent processes. The measurement systems and methods cannot be compared with each other. Furthermore, for contaminating films there are no standards or standardised limits for the evaluation of the analysis results. Individual advice and the precise agreement of the goal of the analysis are therefore one of the basic prerequisites for success.
LOCAL AND INTEGRAL MEASUREMENT METHODS FOR CONTAMINATING FILMS:
To check whether a component is suitable for subsequent processes, we analyse it using chemical analytics with various methods and measured variables:
- Determination of the surface tension by means of test ink
- Detection of contaminating films by means of fluorescence measurement
- Qualification using gas chromatography (GC) with flame ionisation detector (FID)
- Material identification using RAMAN and FT-IR spectroscopy
- Gravimetric assessment
Contaminated surfaces are more difficult to wet than clean surfaces. Good wettability is the basic prerequisite for coating with paints, printing inks or adhesives. A simple method for the measurement of the surface tension on a very wide range of materials is to use test inks. For this purpose, we apply test ink to the surface. If the lines remain stable, the surface is easy to wet. The surface of the substrate then corresponds as a minimum to the surface tension of the test ink. If the lines pull together, the check is continued with the next test ink down.
Via the surface tension of the test ink we can determine the surface tension or surface energy on the test piece; a high value signifies good wettability. If the liquid forms beads, it can be assumed there is contamination present.
Organic contaminating films such as greases, oils or wax fluoresce if they are excited using UV light. Materials that do not fluoresce can also be rendered visible by including fluorescing dyes as markers. In this way we can quickly and without contact verify the presence of organic substances on metallic surfaces. We only need an absolutely clean surface as a reference. The higher the fluorescence value measured, the thicker the filmic contamination.
VERIFICATION AND QUALIFICATION USING RAMAN AND FT-IR SPECTROSCOPY
Chemical contaminating films such as oils, greases, cooling lubricants, cleaning agents, preservatives, solvents and more can be analysed directly on the surface of the component using the spectroscopic analysis methods RAMAN and FT-IR. These measurement and analysis methods provide qualitative measured values. This means we can not only verify the presence of contamination, but also identify it unambiguously. Reference databases are required for these near-surface analysis methods.
VERIFICATION AND QUANTIFICATION USING GAS CHROMATOGRAPHY (GC-FID)
For this purpose, we extract the contamination from the component using suitable solvents and analyse the organic residue released using gas chromatography coupled with a flame ionisation detector (GC-FID). As a result, we obtain the sum of the organic contamination.
Here we extract the contamination from the component using suitable solvents and separate the solid residue using filtration. After the evaporation of the solvent, we determine the mass of the organic soluble residue by reweighing a conditioned flask.