4.7.
METHACRYLATE EMBEDDING
99
failure. Ironically, it seems to be the cheapest sort of fluorescent lamp
that is most effective, while a high quality arc or mecury vapor lamp
often will not induce polymerization. These apparently produce radiation
of too short a wavelength. Polymerization seemingly is carried on most
efficiently by quite long wavelength ultraviolet light.
Weinreb (1955) suggested the use of Westinghouse Fluorescent Sun
Lamp Bulbs, #FS, 20T12, which seem to give completely satisfactory
and consistent results. This is a low-cost, 24-in. tube which fits into
standard fluorescent fixtures. The bulbs have a low current consumption, do not liberate much heat, and have a long life. They can be run
continuously for many days. Some other types of fluorescent "black light"
bulbs will work equally well.
Loaded capsules to be polymerized are simply lined up in a rack about
1 in. away from the light source. Naturally, the rack must be designed
so that most of each capsule (particularly its base) receives direct radiation. One can expect much overnight polymerization even though the
"inhibitor" is left in the methacrylate. But 2 days should be allowed for
complete polymerization.
The amount of catalyst is not particularly critical when ultraviolet
polymerization is employed. In this writer's laboratory, it is our practice
simply to add a few grains (8-12) of benzoyl peroxide to each loaded
capsule before sealing it. These grains are picked up on a forcep's tip
previously wet by dipping in methacrylate. It is necessary to mix the
contents of the capsules after the catalyst has dissolved or layering may
result in uneven polymerization.
There are several reasons why this writer now prefers ultraviolet to
heat polymerization. It is his experience that it produces a substantially
superior block. Clearly, the physical properties of the final plastic are
somewhat different after the two treatments. After exposure to ultraviolet radiation the plastic is more "rubbery." This is a desirable feature.
The blocks are softer, but this can be compensated for by slightly increasing the proportion of ethylmethacrylate. Blocks are also more uniform. (With heat there is apt to be considerable variation for unknown
reasons, and one sometimes gets a nonuniform, almost granular polymerization that is almost impossible to section. We have not had this
happen when using ultraviolet radiation.) Trapped bubbles not infrequently mar heat-polymerized blocks, but ordinarily are not a serious
problem when ultraviolet polymerization is used.
If one does experience serious numbers of bubbles trapped around
METHACRYLATE EMBEDDING
99
failure. Ironically, it seems to be the cheapest sort of fluorescent lamp
that is most effective, while a high quality arc or mecury vapor lamp
often will not induce polymerization. These apparently produce radiation
of too short a wavelength. Polymerization seemingly is carried on most
efficiently by quite long wavelength ultraviolet light.
Weinreb (1955) suggested the use of Westinghouse Fluorescent Sun
Lamp Bulbs, #FS, 20T12, which seem to give completely satisfactory
and consistent results. This is a low-cost, 24-in. tube which fits into
standard fluorescent fixtures. The bulbs have a low current consumption, do not liberate much heat, and have a long life. They can be run
continuously for many days. Some other types of fluorescent "black light"
bulbs will work equally well.
Loaded capsules to be polymerized are simply lined up in a rack about
1 in. away from the light source. Naturally, the rack must be designed
so that most of each capsule (particularly its base) receives direct radiation. One can expect much overnight polymerization even though the
"inhibitor" is left in the methacrylate. But 2 days should be allowed for
complete polymerization.
The amount of catalyst is not particularly critical when ultraviolet
polymerization is employed. In this writer's laboratory, it is our practice
simply to add a few grains (8-12) of benzoyl peroxide to each loaded
capsule before sealing it. These grains are picked up on a forcep's tip
previously wet by dipping in methacrylate. It is necessary to mix the
contents of the capsules after the catalyst has dissolved or layering may
result in uneven polymerization.
There are several reasons why this writer now prefers ultraviolet to
heat polymerization. It is his experience that it produces a substantially
superior block. Clearly, the physical properties of the final plastic are
somewhat different after the two treatments. After exposure to ultraviolet radiation the plastic is more "rubbery." This is a desirable feature.
The blocks are softer, but this can be compensated for by slightly increasing the proportion of ethylmethacrylate. Blocks are also more uniform. (With heat there is apt to be considerable variation for unknown
reasons, and one sometimes gets a nonuniform, almost granular polymerization that is almost impossible to section. We have not had this
happen when using ultraviolet radiation.) Trapped bubbles not infrequently mar heat-polymerized blocks, but ordinarily are not a serious
problem when ultraviolet polymerization is used.
If one does experience serious numbers of bubbles trapped around