98
4.
EMBEDDING
deliquescent. It can be stored for many months at room temperature.
Eventual decomposition does occur so that it ceases to be properly effective. Therefore, small quantities should be purchased with the expectation of replacing the catalyst once or twice a year. The main supply
should be kept tightly stoppered in storage, or even in a desiccator, to
increase its life. A small container of less than an ounce will serve daily
use, and its contents should be discarded and replaced when there begins
to be doubt about its efficacy.
Polymerization can be induced either by mild heat or by relatively
long wavelength ultraviolet radiation. This writer much prefers the latter
method for reasons that will become apparent, but heat is the most traditional technique.
When heat polymerization is employed, 2% benzoyl peroxide (or
Luperco CDB) should be fairly accurately weighed and added to the
bulk methacrylate. It dissolves somewhat slowly, and this must be completed before the solution is added to the embedding capsules. Overnight heating at 50°C will start polymerization. Since it may not complete it, it is better practice to allow 2 days for this.
Methacrylate monomer is supplied by the manufacturer with a hydroquinone "inhibitor" in it. It originally was assumed that this had to be
removed before attempting polymerization. This is quite
unnecessary,
however.* The presence of the inhibitor does not affect the final properties of the block, but only influences the rate of polymerization.
Almost from the beginning of methacrylate embedding there have been
occasional comments that polymerization was achieved by ultraviolet
radiation. The nature of the source has rarely been stated, however, and
this writer knows of many investigators who have been discouraged by
* Removing the inhibitor involves shaking the monomer with a small quantity of
30-50% sodium hydroxide solution in a separatory funnel. After two or three treatments the solution should be colorless, and this is followed by a number of washings
with distilled water to remove the alkali. So long as alkali remains, the drain water
will produce a feeling of slipperiness when rubbed between the fingers. It is then
necessary to remove the last water completely. If the methacrylate is filtered through
several layers of paper, all but traces of the water will be absorbed by the paper.
Following this, a drying agent such as anhydrous calcium chloride or sulfate can be
used, or better, Linde, 4A molecular sieve granules. Finally, any particles of this must
be removed by filtration before the monomer is used for embedding. If the water is
not thoroughly removed, serious bubbles may appear during polymerization. Thus,
removing the inhibitor is not only a laborious process, but also a potential source of
trouble.
4.
EMBEDDING
deliquescent. It can be stored for many months at room temperature.
Eventual decomposition does occur so that it ceases to be properly effective. Therefore, small quantities should be purchased with the expectation of replacing the catalyst once or twice a year. The main supply
should be kept tightly stoppered in storage, or even in a desiccator, to
increase its life. A small container of less than an ounce will serve daily
use, and its contents should be discarded and replaced when there begins
to be doubt about its efficacy.
Polymerization can be induced either by mild heat or by relatively
long wavelength ultraviolet radiation. This writer much prefers the latter
method for reasons that will become apparent, but heat is the most traditional technique.
When heat polymerization is employed, 2% benzoyl peroxide (or
Luperco CDB) should be fairly accurately weighed and added to the
bulk methacrylate. It dissolves somewhat slowly, and this must be completed before the solution is added to the embedding capsules. Overnight heating at 50°C will start polymerization. Since it may not complete it, it is better practice to allow 2 days for this.
Methacrylate monomer is supplied by the manufacturer with a hydroquinone "inhibitor" in it. It originally was assumed that this had to be
removed before attempting polymerization. This is quite
unnecessary,
however.* The presence of the inhibitor does not affect the final properties of the block, but only influences the rate of polymerization.
Almost from the beginning of methacrylate embedding there have been
occasional comments that polymerization was achieved by ultraviolet
radiation. The nature of the source has rarely been stated, however, and
this writer knows of many investigators who have been discouraged by
* Removing the inhibitor involves shaking the monomer with a small quantity of
30-50% sodium hydroxide solution in a separatory funnel. After two or three treatments the solution should be colorless, and this is followed by a number of washings
with distilled water to remove the alkali. So long as alkali remains, the drain water
will produce a feeling of slipperiness when rubbed between the fingers. It is then
necessary to remove the last water completely. If the methacrylate is filtered through
several layers of paper, all but traces of the water will be absorbed by the paper.
Following this, a drying agent such as anhydrous calcium chloride or sulfate can be
used, or better, Linde, 4A molecular sieve granules. Finally, any particles of this must
be removed by filtration before the monomer is used for embedding. If the water is
not thoroughly removed, serious bubbles may appear during polymerization. Thus,
removing the inhibitor is not only a laborious process, but also a potential source of
trouble.