Background

Why separate heavy particles?

In December of 1985, when the Materials Laboratory at Wright-Patterson Air Force Base issued its report titled “Evaluation of The effects of a Plastic Bead Paint Removal Process on Properties of Aircraft Structural Materials”, it appeared that the end of plastic media blasting as a paint removal process was at hand.

In July 1986, Battelle Columbus Division issue its final engineering report on PMB title “Plastic Bead Materials Characterization Study”, which cast the majority of the blame for poor fatigue life tests on the suspected use of contaminated plastic media. This was later verified by another Battelle Columbus Division engineering report titled, “Plastic Bead Blasting Materials Characterization Study Follow-up Report,” which was published November 13, 1987.

The contamination this report centers on is not the normal type of coating residue or plastic fines contamination normally associated with the use of recoverable plastic or starch abrasive media. These contaminates are readily removed by the use of a good cyclone separator and a properly adjusted and maintained pneumatic media recovery system.

The contamination causing the problem is referred to as “High Density” or “Heavy Particle Contamination” and is made up of a type of contamination which is of a higher specific gravity than the plastic blast media. To 1-1-8 defines the various types of high density contamination as follows:

1. Sand, glass and other silicate based materials.
2. Aluminum, magnesium, iron, and zinc based metals.
3. Paints and sealants.
4. High density plastics.

Although all contaminates should be eliminated as much as practicable, the contaminates in categories 1 and 2 above are particularly harmful to aircraft structures, especially thin aluminum or composite surfaces.

In the latest version of TO 1-1-8 two chemicals are used to separate samples of plastic media for determination of contamination levels. The first chemical, Trichloro-Triflouro-Ethane (TTE) is used to separate all contaminates in the flour categories listed above. Plastic or starch media will float in this particular chemical while materials of a higher density will sink. By using a device such as a separatory funnel, where there is a method of draining off the heavy contaminates from the bottom after settling has taken place, these contaminates can be easily isolated and removed from the plastic media, allowing a comparison of weights.

The first separation provides information on the total contamination level of a particular plastic or starch media sample. However, of major importance is the fraction of that total contamination which is of the “high density” variety in categories 1 and 2 above. This requires an additional step.

Per TO 1-1-8, the total contamination which has settled to the bottom using the chemical TTE above must now be put into another separatory funnel. This time the chemical Bromo Chloro Methane (BCM) is added. Some of the contaminates which sank in the first separation will now float. The contaminates which still sink are the higher density variety, and it is the ratio of weight between these high density contaminates and the original media sample which determines the usefulness of the media for aircraft applications.

High density contamination levels greater than .02% or 200 parts per million, by weight, demand, per TO 1-1-8, that the media be removed from aircraft stripping applications.

These high density contaminates are, many times, introduced into the blast room by improperly prepared work pieces or by insufficient procedures controlling entry into the blast room.

Aircraft wheel and flap wells, as well as tires must be inspected and pressure washed to remove sand and other high density contaminates prior to entry to the blast area.

Operators should enter through an airlock or, at the very least, through a designated and controlled entry room, which allows the removal of sand or other high density grit from clothes and shoes prior to entry into the blast room. This type of room or airlock would also facilitate the removal of dust and plastic media from the worker prior to leaving the blast area for the day.

Additionally, blast areas should be just that. All pre-blast preparation and post-blast cleanup should be accomplished outside the blast area, with access into the blast area being opened only during the movement of work pieces into or out of the blast area.

Absence these types of controls, the question becomes; How do we remove high density contamination from blast media already in use?