Dyes are chemically of two different types, acid and basic; so-called
acid dyes are the alkali salts of organic acids, while basic dyes are
the chlorides, sulphates, etc., of organic bases.
For the tinting of film only “acid” dyes have been considered
since it is not possible to make a complete selection from basic
dyes alone, which would necessitate the use of acid and basic dyes
in admixture—a procedure highly undesirable and in many cases
impossible.
Any dye suitable for admixture with others to produce intermediate
tints should possess the following properties:
_A._ The dye should be inert and not attack the gelatine or support.
This is of fundamental importance as the gelatine coating of dyed
film in many cases has a tendency to lose its flexibility, giving
rise to what is known in the trade as “brittleness.”
Several dyes when employed at a concentration of 1% attack gelatine
readily at 70 degrees F. and vigorously at 80 degrees F., especially
in presence of small amounts of acid, producing a marked softening
and often partial dissolution of the film. The effect is roughly
proportional to the concentration of the dye and to the temperature,
and varies with each individual dye.
Experience has shown that the gelatine coating of film which has
been softened in this way by the dye becomes “brittle” on subsequent
projection.
The actual factors in the production of brittleness are:
1. The hydrolysing action of acid which in many cases is added to
assist dyeing. In case a solid acid is employed the heat encountered
during projection will greatly accelerate this hydrolysis.
2. The corrosive action of the dye itself. Dyes vary considerably in
this respect according to their particular composition. So far, it
has not been possible to make any general classification of dyes in
this connection, though nitro compounds appear to be particularly
corrosive in their action.
3. The presence of impurities in the dye. These take the form of
excessive amounts of loading material such as sodium sulphate or
chloride, or small traces of iron, the latter having a tendency to
harden the film considerably.
In all the above cases, the physical nature of the gelatine is
altered, whereby it loses its property of remaining resilient
under normal conditions of temperature and humidity, resulting in
“brittleness.”
A suitable test as to whether a dye has any propensity to produce
brittleness is to incubate a sample of film, half of which has been
dyed, for about 48 hours at 212 degrees F. If any difference in
brittleness is noticeable between the dyed and the undyed portions
so treated after the film has been allowed to stand in the air for
some time, the dye is unsuitable for tinting.
Apart from the above, most dyes when used at a concentration of 1%
and at 80 degrees F. produce more or less softening of the gelatine,
which may be prevented by:
(a) Use of only dilute solutions. Except in very special cases, a dye
solution stronger than 0.5% is seldom required, the usual strength
employed being about 0.2%, at which concentration softening is
unusual.
(b) Use of a minimum amount of volatile acid in the dye-bath or
omission of the same whenever possible.
(c) By working at temperatures not higher than 70 degrees F.
(d) By slight hardening of the film before dyeing and subsequent
softening by glycerine, as described below:
_B._ The dye should not “bleed” to any considerable extent when the
film is washed; in other words, the rate of removal of the dye should
be slow and only a slight amount should wash out in a period of, say,
five minutes.
In tinting, bleeding is of very considerable importance, since,
during the periods between rinsing after dyeing and the placing of
the film on the drying rack, any drops of water on the surface of the
film become more or less saturated with dye, and these after drying
remain as spots and irregular markings which are very apparent on the
screen.
It is possible in some cases to modify this bleeding by an acid “stop
bath,” or by adding acid to the dye-bath; though it may be considered
a general rule that the bleeding of a dye is a property peculiar to
itself. In making a selection of dyes therefore, it is necessary to
choose only those whose propensity for bleeding is a minimum.
_C._ The rate of dyeing should be only slightly affected by the
addition of acid to the dye-bath, although most acid dyes are more or
less sensitive to acid.
In case the rate of dyeing is appreciably affected by the addition
of acid it is possible to take advantage of this fact since a much
weaker dye solution may be employed to obtain a given tint, while an
apparently exhausted bath may be revived by adding a small trace of
acid.
Only a volatile acid such as acetic acid should be used since this
will evaporate on drying, while the strength of the acid in the
dye-bath should not exceed .05% or one part in 2000, otherwise
softening of the gelatine is liable to occur, especially if the
temperature exceeds 70° F.
In the case of a bath containing a mixture of dyes, owing to the fact
that acid affects some dyes more than others, as the dye-bath becomes
exhausted and the acid content changes, the tint obtained will
gradually change also. It is only possible therefore to use acid with
any degree of certainty in the case of dye-baths containing a single
dye.
_D._ The dyes should not fade on exposure to heat or light and should
not be “dichroic” or change color on dilution.
_Moreover, the wear and tear of the film should not be impaired in
any way after dyeing, and even after incubating for 48 hours at 212
degrees F., afterwards allowing the film to humidify, no difference
should be discernible between dyed and undyed film so treated._
_E._ The dye should not be affected by the acid fixing bath since any
fixing solution accidentally splashed thereon, would destroy the dye
immediately.
In view of the large number of tints required in commercial work, it
is undesirable to keep a separate dye-powder for the preparation of
each particular bath, but rather to prepare the same by admixture
of three or more dyes. If three only are employed, mixing must be
conducted with great precision in order to reproduce any given tint,
but this difficulty is removed by the use of intermediate colors.
The following six standard dyes have been chosen as fulfilling the
above conditions as nearly as possible, and by suitably mixing
solutions of these, almost any desired tint may be obtained.