Flinn Plastic Embedding

Introduction

Imagine a bug embedded in plastic! Plastic embedding allows the preservation of unique specimens for permanent display. A “fixed” specimen is easily embedded with Flinn Embedding Plastic.

Background

Embedding plastic is a thermosetting, clear, polyester resin with a syrupy consistency. The liquid plastic may be kept indefinitely when stored at 40 °F or lower. It becomes unstable and solidifies when mixed with a peroxide catalyst or if exposed to sunlight. When exposed to light or mixed with the catalyst, the plastic polymerizes with the exothermic release of heat. When slowly cured the plastic becomes hard and very durable. The cured plastic can be sanded and buffed to a clear, lustrous finish.

Materials

Safety Precautions

Embedding plastic vapors are toxic and flammable. Work in a well-ventilated area or in a fume hood. Be sure that the work area is void of sparks or other open flames. The catalyst is toxic by ingestion and the fumes are toxic. This activity requires the use of hazardous components and/or has the potential for hazardous reactions. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The embedding plastic may be disposed of according to Flinn Suggested Disposal Method #18b. The embedding plastic catalyst may be disposed of according to Flinn Suggested Disposal Method #22b.

Procedure

1. Preparing Transparent Specimens

Specimens should be mounted in a transparent format if internal structures are of importance. Consult a histology textbook if staining procedures are necessary to show internal structures.

Specimens to be embedded must first be “fixed” or preserved to stop all bacterial action in the tissue. The plastic embedding process will not stop all bacterial activity if the specimen is too thick. For most insects and larvae 70% alcohol is an adequate fixative. For larger specimens, however, 10% formalin is needed and may require special injection techniques to adequately “fix” the tissue.

After fixation for 24 hours or longer, wash the specimen to remove the fixative and then dehydrate the specimen in a standard histological alcohol series. Usually a series by 10% increments is used (i.e., 70%, 80%, 90% and 100%). After complete dehydration, the specimen must be cleared with a clearing agent such as xylol, methyl salicylate or other histological clearing agent. Consult a histology technique book for the specific species being prepared. The specimen should then be soaked in uncatalyzed plastic. This will replace the clearing agent and render the specimen transparent.

When the specimen is in uncatalyzed plastic, it may stand for a week or longer to get rid of any air bubbles.

2. Preparing Opaque Specimens

Opaque specimens are embedded when the external characteristics are of prime interest. Opaque specimens must be fixed in the appropriate manner for the size and nature of the specimen. The specimen should be removed from its fixative fluid and air dried until all the surface moisture has evaporated. The specimen may be dried carefully with paper towels if it is tough enough to withstand the rubbing.

Some opaque specimens may be dry (e.g., bones, leaves, plants, rocks, minerals, shells). These specimens may be wetted with uncatalyzed plastic and placed directly into the mounting plastic.

3. Selecting the Mold

A mold should be selected based upon the size and nature of the specimen. The smallest mold possible should be used so that the least amount of plastic is used while still displaying the entire specimen. Molds may be of any smooth glass, metal or ceramic. The mold must have a smooth interior surface. If the interior surface is smooth and not much larger than the specimen, then little sanding and sawing will be required later in the procedure.

Various releasing agents can be applied to the inside of the mold to facilitate removal of the casting. Mineral oil or a vegetable cooking spray is very good for this purpose. All excess lubricant must be carefully removed and only the thinnest film left inside the mold.

4. Hardening the Plastic

The changing of the embedding plastic from a liquid to a solid state is a two-step process. First the liquid plastic is gelled with the hardening catalyst. This is followed by a curing process to completely harden the plastic.

When the peroxide catalyst is added to the plastic, a chemical reaction occurs which changes it to a stiff, rubbery mass with the liberation of heat. The reaction (or change) will take 30 minutes to 3 hours and is determined by the ratio of catalyst to plastic. When a high proportion of catalyst is used, gelling occurs faster than when a small amount of catalyst is used.

The gelling period is the most critical in the entire embedding process. Two pieces of information are required: The depth of the mount and the appropriate volume of the liquid Flinn Embedding Plastic. Care must be taken to determine the depth of the mount prior to casting it. The proper amount of catalyst must be used. The ratio is determined by the thickness of the mount and by the volume of the plastic used. Note that the ratio is an inverse one.

The following table is based on mixing at normal room temperature of 68–72 °F.

5. Embedding the Specimen

For best results, embedding the specimen is done in two parts, hardening the base layer and then the specimen layer.

The following is the basic procedure for a typical specimen:

1. Select a mold appropriate for the specimen.
2. Determine the thickness of the finished mount. Add about 3 mm on each side of the specimen (thickness of specimen + 6 mm = thickness of the final mount).
3. Determine the volume of liquid needed to fill the mold about 3 mm deep.
4. Coat the mold with a very thin layer of releasing agent.
5. Catalyze enough embedding plastic necessary to fill the mold up to 3 mm. Do this by adding the catalyst one drop at a time and stirring thoroughly.
6. Pour the catalyzed plastic into the mold.
7. Set the mold aside and cover with waxed paper until the plastic has gelled. (This can be determined by gently tilting the mold.) It could be hours before it gels depending upon the size of the mold.
8. When the base layer has gelled, measure and catalyze enough plastic for the specimen layer.
9. Place the specimen on the base layer. Pour the specimen layer over the specimen. Use care to prevent trapping air bubbles under the specimen. Dissecting needles might be helpful in positioning the specimen and moving air bubbles. Be sure the specimen is covered by 3 mm of plastic on the top side of the mold.
10. Allow the entire mold to gel overnight. Cover the mold with waxed paper and set overnight in a cool place (70 °F or less). Note: Some opaque or dried specimens might float when plastic is poured in the second layer. To overcome this, the plastic can be poured in three layers instead of just two—the third layer added on top of the specimen after the specimen layer has gelled.

6. Curing the Mount

If time allows, the mount should be cured in the mold by simply letting it set at room temperature for a week. If time is critical, the mold can be cured with heat. This can be done in a hot-air oven or a water bath. Either way the temperature must be carefully regulated to avoid cracking during the curing process.

If heat is going to be applied to the mold, it should be placed in an oven or water bath at room temperature. Gradually raise the temperature a few degrees at a time over a 3-hour period until 120 °F is reached. Maintain this temperature for at least one hour. (More if time allows.) After this time, turn the heat source off, do not remove the mold, and let it cool back down to room temperature. When the mount has cooled, it can be removed from the mold.

7. Finishing the Mount

If the mold has a smooth inner surface, only the exposed surface will need sanding and buffing. Sanding the surfaces of the finished mount is done by using successive grades of wet sandpaper or abrasive sanding belts if they are available. (Your school shop area might be of assistance at this step.) Work up through various grits of sandpaper until 600 grit is achieved. At this point a buffing compound or buffing wheel can be used to produce a final perfect luster. Continue polishing the mount until it is free of all scratches. If necessary, mounts may be sawed with a hacksaw or a bandsaw.

Teacher Tips

• Labels can be imbedded along with specimens. Most writing and typewriter inks will dissolve in the plastic and blur. India ink is the desired ink for labeling in the plastic. Labels can be placed on the base layer along with the specimen. Check during the gelling process that the label and specimen stay in their desired location.

• Never allow catalyzed plastic to gel in direct sunlight. The sunlight speeds gelling and elevates the temperature too quickly which may cause cracking.
• Mounts that are 50 mm or more in thickness require special care in casting. They need to be cast in several layers until the total thickness is reached. Each layer must be catalyzed, gelled and cooled completely before the next layer is poured. Thick casts should be done by reducing the quantity of catalyst by one-half and thus having longer gelling times. The casts can also be made in cooler environments.
• Utensils used for embedding may be cleaned by boiling in very strong soapy water for about a half an hour. Soaking in acetone can also clean utensils. Take appropriate precautions when using acetone since it is flammable and toxic by ingestion.
• Using disposable mixing containers and stirrers can make cleanup much easier. Recycled glass jars and old stirring sticks can be utilized.