Sample Preparation


Sample Preparation of Plastics

Plastic samples present some special challenges since they come in numerous forms, powders, pellets, fibers, sheets, and variously shaped solids, and cannot be prepared by most of the usual methods.

Plastic Powders:
Plastic powders like other powders can be presented in a sample cup either loose or manually pressed. Plastics are resistant to hydraulic pressing, unless heated to 100 to 150C before pressing or pressed using heated platens. Manually pressed samples are recommended for most applications.

Plastic Pellets, Beads, or Regrind:
Plastic Pellets can also be analyzed loose in a sample cup as long as the pellets are fairly uniform in shape. Using a sample spinner helps with the analysis. It is also possible to heat pellets to 100 to 150C and press them into a pellet cap, or press them by normal methods. For most applications loose pellets presented in a spinning sample cup will give good results. Coarse regrind material may also be analyzed in this manner.

Plastic Fibers:
Fibers are particularly tricky to work with. They can be coiled or balled up in a cup, but the reproducibility is generally not very good. It helps somewhat to insert a weight in the cup on top of the sample. An alternative approach that often works better is to win the fibers on a spool. If the wound spool is thick and fairly uniform the results may be satisfactory. Ultimately most fibers must be ground to a powder using a freezer mill or other suitable plastic grinding apparatus and presented in powder form.

Plastic Films, Sheets and Fabric:
Plastic films, sheets and fabric, can often be analyzed simply by cutting them to size and placing them in the analysis position. Several layers of thin films and sheets can measured to increase the net counts for elements with higher energy x-rays. It is also useful to weight some types of films or fabrics with a flat piece of solid metal or a metal ring, provided a metal is selected that doesn’t interfere with the analysis. It is important that the part of the sample in the analysis region be uniformly flat and smooth, or else there will be reproducibility errors.

Solid Plastic Samples:
Solid plastic parts usually make excellent samples. It is only important to remember that the surface must be uniformly flat and smooth in the analysis area. Many plastics shrink during curing plastic pieces giving them a concave surface. As long as all the parts have the same shape and can be position reproducibly, it may still be possible to analyze them without flattening the surface in some way.
It is important to note that the composition of the surface skin is sometimes slightly different than in the interior of the part, since the molding process forces some additives to the surface. To measure the average interior composition it may be necessary to grind or sand off the surface layer. It is then important to make sure that the surface finish is repeatable.