Glossary of surface technology


Paint wetting impairment substances, or PWIS for short, cause clearly visible defects in the end product by preventing uniform wetting of the surface to be painted. Funnel-shaped impurities and craters appear in the paint layer. These substances can be for example silicones, fluorinated (PTFE) substances, certain oils and greases. In the automotive industry, for example, a growing environmental awareness combined with constantly increasing requirements and regulations in the field of pollutant emissions has led to the development of solvent-free (low-solvent) paints and varnishes. In order to achieve a good wettability of the material surface by paint or adhesive, the surface must be clean, dry and free of oil and silicone. "Silicone-free", however, is often not sufficient if the components are used for example in medical technology, highly automated painting technology or sealing technology. Here, the components used must be PWIS-free so that no imperfections occur in the end product. The contamination of components with silicone-containing substances can have numerous causes. It can for example be due to internal or external separating or anti-friction agents used in the extrusion of elastomer profiles. Pigments and fillers are treated with emulsions containing silicone. Also, the components in a production process may be contaminated by lubricants. The people handling the parts are another source of contamination which should not be underestimated. They are often unaware that contamination can be caused by waterproofed working clothes, wearing siliconised rubber gloves, use of cosmetics, hairdressing products or scent. It is impossible to rule out all sources in the production chain. If PWIS-free components are specified, appropriate cleaning is required. Cleaning with solvents is a method that cannot be favoured, neither from an occupational health nor an environmental point of view. The innovative process of cleaning with plasma can be the alternative. The principle of plasma cleaning is based on:

(-CH2-CH2-)n + 3nO2  → 2n CO2 +2n H2O

The plasma process permanently removes all paint wetting impairment substances from the surface and from the elastomer itself.

Although silicone-containing release agents have already been replaced by other products in many areas, contamination by them is always an issue

because it considerably disrupts subsequent painting processes. Here, cleaning with low-pressure plasma presents itself as a reasonably priced, environmentally compatible process which meets these requirements. However, since this plasma process is intended for ultra-fine cleaning and can remove only contamination layers of a few nm, precleaning is required in many cases. If heavily contaminated parts come into direct contact with the plasma, resinifying of the silicone oils may occur, forming a layer which is even harder to remove. After cleaning, the components undergo plasma treatment for up to one hour, depending on their degree of contamination. To confirm the treatment success and thus the PWIS-free state, a PWIS test in compliance with the Volkswagen Test Regulation PV 3.10.7 is carried out after plasma treatment; it is a rapid test for the detection of residual silicones. All you need is a clean glass plate, isopropanol, and a commercially available spray paint which obviously must be free of silicones. The colour white has proven to be particularly well suited for this test. For the test, the material is placed on the glass plate and rinsed with isopropanol. After venting of the isopropanol, the glass plate is sprayed crosswise with the spray paint. After drying of the paint, it is clearly visible whether there are silicone residues on the surface. In these spots, the surface is not wetted by the paint, and so-called crater formation is observed.

The Diener electronic PWIS test set, which comprises all items required for testing, you can test rapidly and reliably whether the components are PWIS-free. This test is very interesting for the incoming goods inspection because it can be determined quickly and reliably whether the parts declared PWIS-free meet the specifications.

If the PWIS test shows no craters, the parts must be packed contamination-free. Utmost care must be taken in the subsequent handling of the components. Do not remove them from the plasma chamber with your bare hands but wear PWIS-free gloves to prevent re-contamination. To ensure that the components are not accidentally re-contaminated, the parts can for example by sealed in a specific protective film, and the package is marked with a label identifying them as PWIS-free parts. PWIS-free products must remain permanently protected from external influences. If factories use for example silicone as release agent on their shop floor, it is mandatory that the cleaned components are assembled in a separate, silicone-free hall. Cleaning is possible for components made of a wide range of materials such as PVC-U, PVC-C, PP, PE, ABS and PVDF, and metallic components. By means of special processes, plasma cleaning is also suitable for treating silicone materials. Even silicone rubber can be made PWIS-free. Using innovative and environmentally compatible low-pressure plasma technology, PWIS substances can be removed from the surfaces of components which are to be coated. This solves a problem which is becoming more and more pressing. A plasma cleaning process integrated in the production chain has a number of benefits, such as:

  • It reduces rework rates
  • It reduces scrap rates
  • It prevents complaints
  • It increases the product reliability

A major starting point for fulfilling the customer’s quality requirements is the individually tailored control of the units. All systems are equipped with a USB and LAN connection and a database connection to ensure traceability by way of printed-out process records. A label printer can be connected as well. The labels are attached to the component packaging. This data allows you to check which batch is affected and when it was manufactured; thus, the log can be unambiguously assigned to the respective products, ensuring the proof of a complete documentation. A validated process software which records all relevant data, generates error messages in case of deviations and stores them in multiple sites makes this possible. Diener electronic also offers this process as a surface treatment service. Several plasma systems as well as skilled and experienced staff are at your disposal. Thus, we can ensure optimum surface quality of your components and parts.

Video of PWIS test

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