Sacrificial soils are materials added to a soiled surface to help that surface become less soiled. Commonly used in non-critical applications such as home carpet cleaning (in which colloidal silicates and acrylics are removed when the carpet is vacuumed) or in commercial cleaning of textile garments (fluorocarbon polymeric materials or hydrocarbon polymeric materials as claimed in U.S.P. 5,876,461).
But sacrificial soils are not commonly found in critical cleaning processes. This is because more soil is normally the last thing users want to see in a critical cleaning bath.
MORE IS NOT WORSE
The presence of a sacrificial soil makes it easier to remove soil already present on a surface from that surface. This happens because sacrificial soil enhances the removal/recovery of all soil on a surface.
Normally, one thinks that more cleanliness (more removal of more soil) requires more cleaning work (for example, time) as shown below.
The reason the addition of more soil improves affairs is that the added soil is not the same material as already present on the surface. So a different mechanism of soil removal must be implemented. That mechanism allows removal of both types of soil present, the one previously inhabiting the surface and the one added.
DECONTAMINATION WITH A SACRIFICIAL SOIL
We consider critical cleaning to be cleaning in which the presence of retained soil produces critical negative outcomes. Failure to remove radioactive residues, for example, would certainly be a negative critical outcome. So thought the inventors of U.S.P. 8,070,881, issued in December 2011.
The inventors claim removal of contaminated radioactive material from porous surfaces (cement, asphalt, tile, granite, marble, and other stone materials) by addition of a sacrificial soil. The sacrificial soil is a polyphosphazine-based polymer material having polyphosphazine backbone segments and side chain groups that include selected functional groups.
The contaminated surface is irradiated with substantially coherent electromagnetic radiation from a laser which causes the radioactive material to migrate to the sacrificial soil, where it is physically removed by a spray device operated on a robotic arm.
The value of the sacrificial soil in this application is threefold, in that: (1) it can be relocated by the electromagnetic radiation to be within the porous structure, (2) the radioactive residue can be cross linked by the electromagnetic radiation to be bonded to the polymeric sacrificial soil, and (3) it can be completely dislodged by spray effects.
PARTICLE REMOVAL WITH AN SS
Another U.S. patent (6,368,414) is supplemented by the use of a sacrificial soil.1 The patent claims are for “irradiation” of soiled surfaces by close-focused pressure waves generated by ultrasonic transducers. No cleaning chemistry is required for cleaning, making systems based on this patent ideal for critical cleaning by not leaving residues of cleaning chemistry.
But a limitation of the claimed technology is that particles are not well-removed. Spray application of natural water-displacement oil apparently wets the particle-laden surface. This causes the particles to be lifted from the surface so they can be removed with the sacrificial soil by the pressure waves.
In this application, the use of a sacrificial soil, where no other cleaning chemistry is used, makes it possible to remove soils (particles) which could otherwise not be removed.
ETCHING REMOVES SACRIFICIAL SOILS
The manufacture of microelectromechanical systems (MEMS) involves removal of a sacrificial layer of silicon oxide from the base Silicon (as in U.S.P. 8,071,486). The manufacture of semiconductor devices involves deposition of a sacrificial film, also of silicon oxide, to allow various regions to be implanted with different types of impurity ions, with the film being later removed by etching (as in U.S.P. 8,071,447). Print heads are manufactured to close tolerances by depositing sacrificial polymer films which are then etched by plasma to achieve certain dimensions (as in U.S.P. 8,070,260).
None of these manufacturing techniques can be effective if the sacrificial soil material is not removed to exactly the extent and location desired.
CLEANING IS SOIL MANAGEMENT
Though we think of critical cleaning as an exercise in soil removal, it’s actually more than that: it’s soil management. In some operations, critical cleaning must also be an exercise in selective application of materials which must later be removed as soils. So critical cleaning, as all cleaning, is an exercise in management of soils both removal of those that are unwanted and application of others that aid in their removal.
- Surface Contamination and Cleaning: Volume 1., Edited by K.L. Mitral, March 2003, ISNB 9067643769
John Durkee is the author of the book Management of Industrial Cleaning Technology and Processes, published by Elsevier (ISBN 0- 0804-48887). He is the author of the forthcoming book Solvent Cleaning for the 21st Century, also to be published by Elsevier, and is an independent consultant specializing in critical cleaning. You can contact him at PO Box 847, Hunt, TX 78024 or 122 Ridge Road West, Hunt, TX 78024; 830-238-7610; Fax 612-677-3170; or firstname.lastname@example.org.