Examples from practice
The following selection of specific examples demonstrates the contribution that surface and materials analysis makes to solving problems in industrial practice.
Checking the corona treatment of films
Surface treatments to improve adhesion properties play an important role in many areas of polymer technology. Flame treatments or corona treatments are used to increase the surface energy of injection-molded parts or films and thus influence properties such as printability or static charge. Checking the quality of such treatments using simple test methods such as test inks is not always reliable.
Here, X-ray photoelectron spectroscopy (XPS / ESCA) can provide important information about the surface condition of the polymer. In addition to providing information about the quality of the treatment carried out, the measurement also allows any contamination on the surface to be detected.
X-ray photoelectron spectrum of a PET film (right) after corona treatment. By determining the ratios of the characteristic oxygen and carbon lines of the PET on the surface, the quality of the corona treatment and its ageing can be investigated.
A PET film shows poor processing properties after corona treatment (static charge, poor printability). It is suspected that the corona treatment may have penetrated to the unprinted outer side. X-ray photoelectron spectroscopy is used to determine the percentage ratio of oxygen and carbon in atomic % based on the intensity of the O1s and C1s signal lines on the unprinted side. These values are compared with those of an untreated film. In this way, corona treatment of the examined side of the film could be ruled out.
Screen printing: Lack of ink adhesion
Ink applied to PVC films using the screen printing process does not adhere sufficiently to films from certain batches. It is known that adhesion problems of this kind can be caused by trace impurities on the polymer surface, so it is necessary to analyze the chemical composition of the outermost atomic or molecular layers of the film surface in order to find the exact cause of the differences in printability. The static SIMS method makes it possible to detect contamination of polymer surfaces or additives that have migrated to them with high sensitivity.
Pos. TOF-SIMS spectrum of a poorly printable PVC film (=> detection of lubricant additives)
Example: When analysing the surface composition of a poorly printable PVC film, the plasticizer dioctyl phthalate (® DOP), the PVC stabilizer barium stearate (® BS) and a lubricant additive called ethylene bis-stearamide (® EBS) are detected on the film surface.
Comparative measurements on easily printable PVC films show that these surfaces have significantly lower concentrations of the lubricant additive EBS. It was possible to prove that the poor printability of the films was directly caused by the increased concentration of the additive on the surface.
Adhesion problems with metallic inspections
The quality of paintwork and metallization on plastic surfaces depends largely on the properties of the surface to be coated.
Residues of demolding agents and cleaning fluids…
Migration of plasticizers to the surface or…
Trace contamination with lubricants…
…even the smallest traces can lead to considerable adhesion problems and even large-scale delamination. Using highly sensitive surface analysis techniques (such as the TOF-SIMS method), such trace contaminants can be detected and identified. In this way, sources of error can be quickly localized and eliminated.
TOF-SIMS spectrum recorded in the defect of a chrome plating on an injection molded part
Example: Furniture knobs and cabinet handles are often made of plastic and then electroplated with a decorative metal coating. The metal layer always peels off due to insufficient adhesion to the plastic surface. For the manufacturer, the question now arises as to whether the fault lies in incorrect treatment during metallization or is caused during the production of the blanks. A TOF-SIMS microanalysis within the defect (Fig. top right) detects contamination with release agents (EBS / PDMS). As these are only used during demoulding, the fault could be traced back to inadequate cleaning before metallization. The error was permanently eliminated by making minor changes to the production process.
Characterization of lubricants: Perfluoropolyethers
Perfluoropolyethers (PFPEs) are liquid polymers with good lubricating properties at room temperature. They have high viscosities and boiling points as well as low surface energies. PFPEs are used in almost all areas of industry, e.g. the electronics industry, aerospace and the semiconductor industry. For example, hard disks in computers are coated with a thin layer of perfluoropolyether to prevent friction losses or welding when the read/write head comes into contact with the hard disk surface. Or: Greases based on perfluoropolyethers are used to lubricate highly loaded ball bearings in conveyor belts in paint shops and ball bearings in ovens.
The physical properties of the individual PFPE products (trade names: e.g. Krytox, Demnum, Fomblin) depend on the structure of the polymer backbone, the molecular weight and the width of the molecular weight distribution.
These values can be easily determined using the TOF-SIMS method (see figure). In addition, the method enables trace detection and the exact identification of PFPEs in the area of defect analysis. The TOF-SIMS therefore plays an important role in the search for the source of PFPEs in painting processes that lead to defects (e.g. craters) in paint layers.
Figure 1: Positive TOF-SIMS spectrum of a thin Krytox film on silver substrate. An average molecular weight of approx. 7000 u corresponding to n = 47 repeat units is measured (for an explanation of the individual mass signals, see H. Feld et al., Analytical Chemistry, 65 (1993) 1947-1953).
The TOF-SIMS method enables the highly sensitive detection of perfluoropolyethers (sub-ppm range) even in microscopically small areas, the exact determination of the masses of their repeating units (R) and polymer end groups as well as the width and average molecular weight of the oligomer distribution (polymer dispersion).
Inclusions in painted surfaces
When painting and coating components, even the smallest surface defects such as craters, specks or bubbles can massively impair the visual impression. While paint craters are generally caused by trace contamination with grease, oil or release agents, spots, pimples or other localized elevations in the paint surface are often caused by the inclusion of foreign particles such as dust particles, metal abrasion, hair or fibres. To reliably avoid such errors, it is first necessary to characterize the shape and composition of the trapped foreign particles as precisely as possible. Together with suitable sample preparation, microanalytical methods such as SEM/EDX, FTIR microscopy or TOF-SIMS can provide valuable information.
Example: On powder-coated aluminum profiles, pimple-shaped elevations are occasionally observed in the paint surface. SEM images of a cross-sectional area through a typical defect show that a foreign particle is trapped in the paint layer, which extends to the paint surface but has no contact with the base material. EDX element analyses only detect aluminum in the area of the inclusion, while the intact coating layer only contains the typical binder elements carbon and oxygen.
Overall, the investigations show that the paint defects are caused by small aluminum chips, which presumably only reach the surface during the coating process and are incorporated into the not yet cured paint layer.
Checking the coating structure
Surface coatings specially optimized for the respective practical requirements are of decisive importance for the function, durability and decorative appearance of technical products. The spectrum ranges from hot-dip galvanizing and multi-layer coating systems to complex thin-film systems that are used as optical coatings on glass.
The function of the coating depends, among other things, on the layer sequence, the layer thicknesses and the chemical composition of the individual layers and interfaces. Depth profile analyses, e.g. using the SNMS or GDOS method, provide essential information on the layer parameters mentioned. The coating is continuously removed by ion bombardment and the element composition is recorded as a function of the coating depth.
Example: Stainless steel trim panels are chrome-plated in two different processes for decorative reasons. As part of quality assurance, the aim is to check whether the layer structure and composition of the current batches correspond to the reference samples previously analyzed as a quality standard.
GDOS depth profile analyses show (see figure above):
1. sample 1 is a simple chrome plating layer.
In sample 2, an additional nickel intermediate layer was deposited between the steel and the chrome layer. The total layer thicknesses are approx. 500 and 300 nm respectively.
2. no interfering foreign element accumulations are found in the entire layer structure.
3. the base material is a Cr/Ni steel with approx. 20% Cr and 8% Ni.
Overall, the coating parameters are within the tolerance values. Based on the analyses, the coatings are expected to function perfectly in practical use.
