198
6. SECTION MOUNTING
0.5% amyl acetate solution on a large clean water surface. Surface tension
spreads the solution before evaporation sets the film. Films made in this
way look beautiful, but they do not seem to be as strong proportionately
as films cast on glass, and therefore this method is not recommended.
6.3. Formvar Films
Formvar is the trade name of a polyvinyl formal plastic manufactured
by the Shawinigan Products Co., 350 5th Ave., New York 1, New York.
Formvar E is the desirable grade, marketed in England as Grade 15/95.
Formvar supporting films are generally regarded as being slightly
stronger than Parlodion ones, and therefore have been the films of choice
in many laboratories in spite of the fact that they are a little more difficult to prepare, and stock solutions do not keep well. If one is going
to use carbon to stabilize a plastic film (Chapter 6.7) or uses a plastic
film simply as a substrate upon which to build a carbon film, differences
are negligible and the more easily handled Parlodion seems preferable.
In principle, Formvar films are made just like Parlodion ones. However, there is no solvent for Formvar which behaves as nicely as amyl
acetate for Parlodion. Most commonly, ethylene dichloride has been used
as the solvent for Formvar, and the stock solution is usually 0.2-0.5%.
In this writer's experience, film quality may deteriorate
noticeably
within days after a fresh stock solution in this solvent has been prepared, although its life may be lengthened by storing in a refrigerator.
Chloroform is used in some laboratories as the solvent of choice, and
these solutions seem to be more durable. Chloroform evaporates so
rapidly, however, that it is difficult to drain the slide upon which the
film is being cast in a uniform and reproducible manner, and the cooling which occurs with evaporation may cause water condensation. One
can minimize this difficulty by draining the slide in an atmosphere nearly
saturated with chloroform vapor. If the dipping solution fills only the
bottom of a tall container, the upper part of the container will be filled
with chloroform fumes, and the slide, after dipping, can be held there
until it is drained. It should then be withdrawn sufficiently slowly so
that evaporation does not chill the slide to the point where water condenses upon the still liquid film.
After the Formvar film has dried upon the glass slide it is stripped
just as a Parlodion film would be. However, it is commonly somewhat
6. SECTION MOUNTING
0.5% amyl acetate solution on a large clean water surface. Surface tension
spreads the solution before evaporation sets the film. Films made in this
way look beautiful, but they do not seem to be as strong proportionately
as films cast on glass, and therefore this method is not recommended.
6.3. Formvar Films
Formvar is the trade name of a polyvinyl formal plastic manufactured
by the Shawinigan Products Co., 350 5th Ave., New York 1, New York.
Formvar E is the desirable grade, marketed in England as Grade 15/95.
Formvar supporting films are generally regarded as being slightly
stronger than Parlodion ones, and therefore have been the films of choice
in many laboratories in spite of the fact that they are a little more difficult to prepare, and stock solutions do not keep well. If one is going
to use carbon to stabilize a plastic film (Chapter 6.7) or uses a plastic
film simply as a substrate upon which to build a carbon film, differences
are negligible and the more easily handled Parlodion seems preferable.
In principle, Formvar films are made just like Parlodion ones. However, there is no solvent for Formvar which behaves as nicely as amyl
acetate for Parlodion. Most commonly, ethylene dichloride has been used
as the solvent for Formvar, and the stock solution is usually 0.2-0.5%.
In this writer's experience, film quality may deteriorate
noticeably
within days after a fresh stock solution in this solvent has been prepared, although its life may be lengthened by storing in a refrigerator.
Chloroform is used in some laboratories as the solvent of choice, and
these solutions seem to be more durable. Chloroform evaporates so
rapidly, however, that it is difficult to drain the slide upon which the
film is being cast in a uniform and reproducible manner, and the cooling which occurs with evaporation may cause water condensation. One
can minimize this difficulty by draining the slide in an atmosphere nearly
saturated with chloroform vapor. If the dipping solution fills only the
bottom of a tall container, the upper part of the container will be filled
with chloroform fumes, and the slide, after dipping, can be held there
until it is drained. It should then be withdrawn sufficiently slowly so
that evaporation does not chill the slide to the point where water condenses upon the still liquid film.
After the Formvar film has dried upon the glass slide it is stripped
just as a Parlodion film would be. However, it is commonly somewhat