PWIS (Paint Wetting Impairment Substances) Free Analysis

Paint wetting degradation substances (PWIS) have been a prominent topic in the automotive industry for years, but of course this issue applies to all painting companies. In this regard, operators of painting plants control the materials and components used and set requirements for PWIS-compliant products for their suppliers. Paint wetting degradation substances (PWIS) can cause coating defects even in very small quantities and thus affect the quality of the coating.

Whether a substance causes paint wetting failure depends on its interaction with the paint system. The PWIS compatibility statement defines the substance-independent requirement. Depending on where a product is used, there may be different requirements for PWIS compatibility.

Paint wetting interfering substances are ubiquitous in everyday life and come into contact with these substances on a regular basis. Therefore, special precautions must be taken to prevent contamination with substances that interfere with paint wetting.

Typical PWIS sources in manufacturing include:

• Lubricants (moving parts of equipment)
• Release agents and plasticizers from plastic components used in plant engineering or workpieces
• Residues of drawing agents and lubricants
• Hoses and sealing compounds
• Contamination distributed via room air technology, process air technology or compressed air system
• Transported from other production areas
• Contaminated paint material (e.g. due to improper handling or storage or unsuitable additives, solvents)
• Operating and auxiliary materials such as cleaning agents, cleaning cloths, abrasives, polishing agents, adhesive tapes, gloves, work clothes or assembly devices

PWIS contamination can cause coating defects in the form of craters and large-area wetting defects on the painted surface, leading to rework or rejection and, if necessary, production shutdowns.

The reason for the occurrence of paint wetting disorders is that the surface to be painted, the paint or the paint supply system is contaminated with inhibitory substances above the paint wetting limit concentration, which causes the wetting disorder to be effective.

For a paint to completely wet a substrate, the surface tension of that substrate must be higher than that of the paint. Substances that inhibit paint wetting have lower surface tension than that of the paint.

Surfaces contaminated with PWIS cannot therefore be coated with paint or perfectly coated. A practical example of the influence of different surface tensions in everyday life is the completely wetted water film on the bottom of a stainless steel pan and the non-wetting of the water film on the bottom of a non-stick coated pan.

Whether a substance has a disturbing effect on paint wetting depends on its interaction with the coating system, the surface to be painted and the ambient conditions.

Because silicones were originally known primarily as substances that interfere with paint wetting, the term silicone-free is still used today in error. Silicone-free status is not equivalent to PWIS compliance. It is not possible to produce a generally valid negative list of substances that interfere with paint wetting.

It is often requested that a product be PWIS free as a product feature. However, this basic requirement is not always technically feasible. For example, many plastics contain plasticizers or release agents that impair paint wetting. Such materials can never be PWIS free, but can be PWIS compliant depending on the application or field of application. For these, testing is required on a case-by-case basis for their use in PWIS-critical areas. PWIS compliance describes the status of a material or test sample tested according to the standard “VDMA 24364:2008-05 Test for substances that impair paint wetting (PWIS compliance)” developed by the Association of German Engineers (VDI). However, different test methods and coating systems can lead to different results.

The procedure for testing paint wetting degradation substances (PWIS) compatibility (PWIS free analysis) includes the following steps:

• Selection of product group and test class: The product group is selected according to the intended use of the product and its zone classification. It forms the application reference for the PWIS test and thus forms the basis for the selection of the test class. The test class should be selected according to the exposure when the product is used as intended. The relevant surfaces of the product should be taken into account. Relevant are those surfaces that may release or transfer paint wetting impairing substances to a PWIS zone when the product is used as intended.
• Testing: All tools and work materials for testing must be PWIS compliant so that they do not falsify the test result. Aids contaminated with PWIS may cause paint wetting discomfort during testing. For comparable test results, the same paint and the same solvent must always be used. A zero test must be performed for each type of sample extraction before testing is performed. This test ensures that the test environment, tools and work materials are PWIS compliant during testing. The test requirements depend on the selected test class. The test class is derived from the product group resulting from the intended use of the product in the PWIS region.
• Assessment: This process is carried out on the cured paint film in a shiny and non-glare location. The EN ISO 13076 standard applies for information on the procedures for assessing surfaces and illumination for visual inspection of coating defects. Assessment is carried out visually, without optical aids for magnification. If there is uncertainty about the type of defect (e.g. craters or pinholes), a magnifying glass can be used. The coated test plate is visually checked for surface defects. The inspector must be able to assign the types of defects to the relevant causes. The test result does not allow any quantification, only a decision “WIS compliant” or “PWIS non-compliant”.
• Documentation: It is recommended that the test report include the following information:
  • Auditor
  • Test date
  • Identification of the test sample (e.g. material number, manufacturer)
  • Intended use of the test sample
  • The region assignment and test class where the test is performed
  • Sample extraction method
  • Set parameters
  • Full description of solvent used
  • Full description of test dye used
  • Type of test plate
  • Evaluation time (paint flash time) and paint film condition
  • For larger samples, which areas of the product were tested (with photographic documentation if necessary)
  • Extraordinary observations
  • Test result

The VDMA 24364:2008-05 standard divides surfaces into the following zones in terms of substances that cause paint wetting failure:

• Zone I - PWIS high relevance zone: All pre-treatment, paint application and drying areas and exposed surfaces
• Zone II - PWIS relevance medium zone: This zone is the zone within the production area where paint is used but is not included in Zone I.
• Zone III - No PWIS relevance: This zone is within the company but outside the paint production area.

The results of PWIS compliance tests performed according to different test standards are not in principle comparable. An understanding of the comparability of tests performed according to different test standards can be obtained by a series of tests performed in parallel according to the test standards to be compared. In order to assess the comparability of tests according to the test standards to be compared, the test results must be evaluated by several people.

Paint wetting agents and non-PWIS compliant products must be banned from the supply chain and production. In the following cases, PWIS compliance must be tested (PWIS free analysis must be performed)

• Materials (mostly plastics and elastomers)
• Excipients and materials
• Work Equipment
• Transport containers
• Care products provided by the company (soap, skin protection cream)
• Work clothes and personal protective equipment

Products contaminated with PWIS (e.g. products contaminated with release agents) can be cleaned with wet chemical or plasma. Products containing PWIS (e.g. seals containing elastomers) can be temporarily conditioned to PWIS compliance.

If the use of substances containing PWIS is absolutely necessary, it is necessary to separate this production area from the PWIS-compliant production area in such a way that transport (e.g. by production processes, employees) is not possible. If transport by air is not possible, spatial separation by partitions or fences is sufficient.

Production areas should be marked as “PWIS compliant” or “non-PWIS compliant.” For PWIS compliant production areas, transport containers, tools, test equipment, and similar equipment should be provided.

Our organization has a strong staff that closely follows the developments in the world in the field of science and technology and constantly improves itself. Among the numerous test, measurement, analysis and evaluation studies provided for businesses in various sectors, there are also PWIS (paint wetting impairment substances) free analysis services.