90
4.
EMBEDDING
From the point of view of preparing uniform blocks, the principal
hazard is that the resin mixtures will not be adequately mixed when they
are used. This is a problem which cannot be overemphasized. Above all
else, the accelerator must be uniformly distributed throughout. If there
are local concentrations of accelerator, this will produce plastic that is
much too hard, and, conversely, too little will prevent adequate curing.
The one thing that we have learned not to do is to rely upon an ordinary
electric or magnetic stirrer. These do not adequately disturb the layering
of the plastic components which invariably results when they are added
one by one to the container. It is absolutely essential to create some sort
of a pumping or tumbling action when these components are mixed.
In actual practice, we ordinarily use graduated cylinders of the short,
stubby shape as mixing vessels. We stir the contained mixture by hand
with a glass rod approximately i/ 2 inch in diameter, with a major emphasis on pumping the rod up and down vertically, to break up the layering. This is kept up for 5-10 minutes, much longer than the novice is
apt to regard as being necessary. Recently, when feasible, we have resorted to using disposable polypropylene beakers with lids as mixing
containers. The mixture then can be sloshed, rolled, and tumbled in all
directions. Undoubtedly much failure with sectioning is attributable to
inadequate mixing, which in turn results in blocks with undesirable
characteristics.
Most laboratories prepare resin mixtures volumetrically. In pouring
highly viscous material into a graduated cylinder, it is important that
it go down the center and not land on the side walls. Accurate measurement becomes impossible if the side walls become coated with an indefinitely thick layer of highly viscous material. Some laboratories use
gravimetric procedures to measure components and thus avoid these
problems.
It is particularly important that the accelerator be measured fairly
accurately. DMP-30 is the one ordinarily used with epoxy resins, and,
fortunately, this is not sufficiently viscous to cause a serious problem.
A 1 or 2 ml syringe should be used for this purpose. In many laboratories
such a syringe is kept with the bottle of accelerator and used over and
over again without cleaning it. In some laboratories a fairly exact amount
of accelerator is added, one drop at a time, while the mixture in its
container is being weighed on a balance. The worst way to try to add
accelerator is to use an ordinary volumetric pipette. DMP-30 is sufficiently
4.
EMBEDDING
From the point of view of preparing uniform blocks, the principal
hazard is that the resin mixtures will not be adequately mixed when they
are used. This is a problem which cannot be overemphasized. Above all
else, the accelerator must be uniformly distributed throughout. If there
are local concentrations of accelerator, this will produce plastic that is
much too hard, and, conversely, too little will prevent adequate curing.
The one thing that we have learned not to do is to rely upon an ordinary
electric or magnetic stirrer. These do not adequately disturb the layering
of the plastic components which invariably results when they are added
one by one to the container. It is absolutely essential to create some sort
of a pumping or tumbling action when these components are mixed.
In actual practice, we ordinarily use graduated cylinders of the short,
stubby shape as mixing vessels. We stir the contained mixture by hand
with a glass rod approximately i/ 2 inch in diameter, with a major emphasis on pumping the rod up and down vertically, to break up the layering. This is kept up for 5-10 minutes, much longer than the novice is
apt to regard as being necessary. Recently, when feasible, we have resorted to using disposable polypropylene beakers with lids as mixing
containers. The mixture then can be sloshed, rolled, and tumbled in all
directions. Undoubtedly much failure with sectioning is attributable to
inadequate mixing, which in turn results in blocks with undesirable
characteristics.
Most laboratories prepare resin mixtures volumetrically. In pouring
highly viscous material into a graduated cylinder, it is important that
it go down the center and not land on the side walls. Accurate measurement becomes impossible if the side walls become coated with an indefinitely thick layer of highly viscous material. Some laboratories use
gravimetric procedures to measure components and thus avoid these
problems.
It is particularly important that the accelerator be measured fairly
accurately. DMP-30 is the one ordinarily used with epoxy resins, and,
fortunately, this is not sufficiently viscous to cause a serious problem.
A 1 or 2 ml syringe should be used for this purpose. In many laboratories
such a syringe is kept with the bottle of accelerator and used over and
over again without cleaning it. In some laboratories a fairly exact amount
of accelerator is added, one drop at a time, while the mixture in its
container is being weighed on a balance. The worst way to try to add
accelerator is to use an ordinary volumetric pipette. DMP-30 is sufficiently