A manufactured product may require components to have a certain maximum level of surface contaminants such as particles and oils to enable wetting, bonding, or machining operations, or to enable assembly, testing, or functional performance of the product. The ability of surface to be cleaned to a pre-determined and desired cleanliness level, efficiently and without damage to the surface or product, is termed cleanability.
Cleanability is a measure of the nature and extent of surface contamination reductions achieved over a predetermined amount of time and for a certain quantity of surface area, for example, non-volatile residue level (NVR, µg/cm2), particle level (count, size, area), total mass loss (TML, %), and collectable volatile condensable matter (CVCM, %). Cleanability relates to several interdependent chemical and physical surface, contaminant, and cleaning agent phenomenon. Key cleanability phenomenon relevant to CO2 processing technology include, for example:
- Cohesion Chemistry of a Surface – Swelling, penetration, and wettability of a contaminated surface by a CO2 cleaning agent.
- Cohesion Chemistry of a Contaminant – Swelling, ionization, or solubility of surface contaminants by a CO2 cleaning agent.
- Shear Stress – Cavitation and shearing actions (cleaning energy) imposed on a surface and surface contaminants by a CO2 cleaning agent.
Contamination levels present on a surface are determined using a variety of analytical techniques. For example, particle level measurements are typically performed indirectly and offline, and employ a solvent (i.e., isopropyl alcohol or deionized water) soak or flush of an entire part or surface portion (having a known or estimated surface area) to remove and suspend microscopic particles followed by a laser-based particle measurement to determine particle quantity and size (i.e., 0.3 µm, 0.5 µm, 1 µm, 5 µm). More elaborate and offline analytical methods for determining surface cleanliness include, for example:
- X-ray Photoelectron Spectroscopy (XPS) for quantifying inorganic surface contamination such as metallics, carbon, oxygen, fluorine, and silicon levels;
- Fourier Transform Infrared Spectroscopy (FTIR) for measuring organic contamination; and
- Scanning Electron Microscopy (SEM) for determining changes in surface morphology or determining the chemical and physical nature of surface contaminants.
Finally, lower cost and less complicated analytical techniques may be used which lend themselves to on-line or real-time utilization include Drop Shape Analysis (DSA), Optically Stimulated Electron Emission (OSEE), and Airborne Particle Counting (APC) within a confined CO2 spray cleaning cell. All of these methods are very useful for CO2 processing for cleanability, and are discussed in more detail in the CO2 Mechanics blog.
CO2 processing technology provides a number of state-of-the-art methods and processes to increase cleanability. These processes include spray treatments, immersion treatments, plasma treatments, and hybrids of same. CO2 processing is robust, 100% dry, selective, and uniquely adaptable to manufacturing processes, tools, and lines.