Figure 5.4. Brief summary of colour development and cou-pling chemistry. The reaction depicted at the top corresponds to the two-equivalent development of silver halide, AgX, by a paraphenylenediamine (PPD) developer species. The latter reduces two silver ions to elemental silver and is transformed by a two-electron oxidation to quinonediimine, (QDI). The R substituent may be the same or different in the develop-ers and in the coupldevelop-ers. In addition, the PPD may have other ring substituents. The reactions depicted at the bottom illus-trate indoaniline image-dye formation from QDI and various classes of couplers. The middle coupling examples represent coupling with phenols or naphthols to produce cyan image dyes. In this example, there is a Cl coupling-off group at the para position. The coupler is activated by hydroxide, thus ion-izing the hydroxyl group, and then reacts with one QDI to form a cyan image dye. The yellow (upper) and magenta (lower) dye-forming reactions are examples of four-equivalent cou-plings. The corresponding couplers do not have coupling-off groups. After activation by hydroxide and ionization, reaction with QDI produces a leuco-dye intermediate. The latter under-goes a two-electron oxidation by another QDI to produce the final dye molecule
must then be placed in mechanical registry to produce a final colour print of the original scene. Such pro-cesses are still pursued for certain commercial appli-cations, such as in proofing for colour advertising. The old Kodachrome® process was based on some related chemistry, but relied on carefully timed and diffusion-controlled processing to produce such separations in an integral multilayer pack.
Colour instant photography has much in common with colour negative photography. There are 50-100%
more layers in a colour instant element. Some of the additional layers required include base releasing layers, timing layers to release pH-adjusting chemicals to shut down development chemistry, dye-permeable opacify-ing layers, mordant layers (to bind diffusible dyes) and incorporated developer layers. Some colour instant sys-tems have been built by using colour developers such as PPD. More common is the use of dye-developers or so-called redox dye releasers. Dye-developers dif-fuse to developable silver halide, and upon reduction of the latter they release a diffusible image dye. This dye will then diffuse through an opacifying layer to a mordant.
Heat-developable silver halide elements make it pos-sible to avoid wet-development processing, and the elimination of wet-processing chemistry has some envi-ronmental motivation. The only commercially available heat-developable products are microfilms. There is a large patent literature on heat-developable materials. The key limitation to the commercialization of colour heat-developable systems is the problem of getting rid of undeveloped silver halide. In normal wet-developable systems, fixing agents such as thiosulfate (hypo) are used to dissolve undeveloped AgX. Silver halide microcrys-tals typically provide a lot of scattering, so it is generally necessary to fix out such AgX and to bleach (in the case of colour systems) the silver image.
2 SURFACES AND PARTICLES IN
et al. (Chapter 7) for more information, although cer-tain critical preparative treatments are described below.
Alternative reflection base materials may perhaps be described as synthetic papers, in that the inherent reflec-tion properties are achieved solely from coated pigment layers or from processes producing voids in polymeric layers, so as to produce scattering (reflecting) layers from synthetic polymeric compositions. Transparent film support materials are typically manufactured in coating mills approaching two metres in web width. A detailed discussion of the manufacture of such coating supports is beyond our present scope, but physical and chemical surface treatments are very important, as was discussed earlier in the application of subbing layers. Chemical and surface treatments of coating supports are critical for the successful coating of chemistry-containing layers, for the storage of such coated elements (e.g. to prevent coiled and stacked layers from adhering strongly to one another during storage and for the dissipation of static electrical energy), and to maintain dimensional stabil-ity and structural integrstabil-ity during processing and long-term storage. While the width (> metres) and length (103 metres) scales are certainly macroscopic, the thick-ness of these supports typically is microscopic and in the range of 80-200 urn.
2.2 Particulate materials
The bulk of chemicals incorporated into photographic elements is in particulate form. This can be illustrated by reviewing Table 5.1, where classes of various par-ticulate materials are listed. Silver halide crystals cover
Table 5.1. Particulate materials in photographic elements
Inorganic Silver halide Silver
Antistats (oxides)
Opacifying materials (TiO2, BaSC^) Binders (SiO2)
Organic Couplers
Absorber dyes, filter dyes, antihalation dyes Sensitizing dyes
Oxidized developer scavengers Developers
Dye-Developers Antistats (latexes) Mordants (latexes)
Opacifying materials (hollow polymer spheres) Binders (gelatin, latexes, microgels)
a wide range of sizes. Those used in light capture as three-dimensional grains are typically in the range of 300 nm to 3 urn in the largest dimension. Tabular high-aspect-ratio grains may have their largest dimensions closer to 5 urn. The so-called Lippmann emulsions are just a few hundred angstroms in diameter, and are used for astronomical plates where high resolution is required.
Silver halides, therefore, span two order of magnitude in their largest dimension. Colloidal silver can be pre-pared as monodisperse dispersions varying in colour from blue to yellow, depending on the particle size (e.g.
7-12 nm). Yellow silver dispersions are used in filter layers to absorb blue light. The small particle size makes them insignificant as scattering centres. Semiconducting oxides such as tin oxide are useful as antistats. The pre-ferred sizes are less than 10 nm, so that the particles do not contribute to light scattering. Opacifying pig-ments, such as titania and barium sulfate, are typically dispersed as highly scattering particles, ranging in sizes from 0.5 to 5 um in their largest dimension. Such inor-ganic pigments are often coated on photographic papers to increase opacity and diffuse reflectivity. Most of the binders used in photographic elements are organic, but nanoparticulate silica (20-30 nm) has been used as a binder in certain layers, usually with a small amount of gelatin to provide cross-linking sites.
Many water-insoluble organic materials are also incorporated as particulates. Foremost in colour pho-tography are photographic couplers. They are typically prepared as amorphous dispersions by an emulsification process (discussed below), with or without various plas-ticizers (coupler solvents). They also can be prepared as nanocrystalline dispersions by comminution processing, and as metastable mixed micelles by precipitation from homogeneous solution. Coupler particles are also pre-pared as latexes, where coupler monomers are copoly-merized with other monomers, typically using emulsion polymerization, to produce submicron water-borne dis-persions. Such polymeric couplers can be attached per-manently to the polymeric backbone, or attachments can be made through a coupling-off group. In the latter case, once the oxidized developer has reacted with the cou-pling moiety, the resulting image dye is separated from the polymeric backbone and is free to diffuse if soluble or mobile in the continuous phase. Such formulations are useful in diffusion-transfer imaging elements. Parti-cle sizes range from 10 to 30 nm for the mixed micelle formulations to fractions of a micron for dispersions pre-pared by comminution or by emulsification.
Absorber, filter and antihalation dyes can be pre-pared by several different processes. All three types can be prepared as dispersions by the emulsification
process discussed below for couplers. They also can be imbibed into swollen latexes (latex loading - also discussed below), made part of a latex through copoly-merization, and dispersed as small solid particles by using comminution processing. While sensitizing dyes have historically been added to finishing processes as solutions, they more recently have been dispersed as submicron-sized solid particle dispersions prepared by comminution. Their small aqueous solubility is generally sufficient to afford diffusion transfer to silver halide sur-faces. Oxidized developer scavengers and incorporated developers and developer precursors can be dispersed by emulsification and by comminution. Dye-developers are important molecules in colour instant processing, and are also dispersed by emulsification and by comminution.
Traditional polymeric antistats relying on ionic con-ductivity and comprising quaternary halide groups or acidic groups can be prepared as latexes. Electrically conducting polymers are also typically prepared as aque-ous latexes. In dye-diffusion-transfer processes, espe-cially for anionically charged dyes, mordent polymers are also prepared as latexes.
Organic light scattering pigments produced as hol-low spheres in the submicron and larger size ranges have been used as alternatives to inorganic pigments such as titania. Various polymers have been evaluated as binders, but gelatin is still the most widely used material for this application. Various latexes, as small as 40 nm and less, have been used as binders, usually in conjunc-tion with gelatin. Microgels of water-swellable polymers are now beginning to be used as binder components.
Matte beads in the range of 1 to 10 um are used to guard against layer-to-layer adhesion and are typically made from polystyrene, poly(methyl methacrylate) and related polymers. Such beads may be chemically cross-linked, for example, by incorporating divinyl benzene in the monomer mix.
This range of scales may be qualitatively considered in terms of the transmission electron micrograph (TEM) shown in Figure 5.5, where a thin cross-section of an amateur colour-negative film is depicted. Thirteen distinct layers are evident, as is the particulate nature of almost every layer. Starting from the right-hand side of the figure, one can identify the following layers: an overcoat layer, a UV-filter dye layer, fast blue, slow blue, two distinct interlayers, fast green, slow green, an interlayer, fast red, slow red, antihalation layer and PET support, plus a subbing interlayer (which is not visible).
With the exception of some lengthy artifacts spanning two or more layers and arising from the formation of folds in the very thin (100 nm) cross-section used in this TEM, the black particles are silver halide or
Figure 5.5. Transmission electron micrograph of a thin (~ 100 nm thick) cross-section of commercial VR100 amateur colour-negative film. The top overcoat layer is at the far right.
The next is a UV layer with a Lippmann emulsion, followed by the fast and slow blue-sensitive layers, two interlayers, fast and slow green-sensitive layers, an interlayer, fast and slow red-sensitive layers, an antihalation layer, and a portion of the PET support. The overall width of the figure corresponds to a total thickness of 53 jim
colloidal silver. In the UV layer (second from right) one sees evidence of two kinds of particles. The small black particles are Lippmann emulsion particles that serve to filter and adsorb surface-active compounds that accumulate in seasoned developer baths. The apparent voids appearing as ellipsoidal shapes are holes left by UV-filter dye dispersion particles. The size of the silver halide crystals in the fast emulsion layers is larger than the slow-layer silver halide. This is particularly the case in the blue record. The rightmost interlayer between the blue and green records contains colloidal silver of very fine particle size. This yellow colloidal silver is used to absorb blue light that passes through the blue record. The voids evident in this layer are due to particles of oxidized developer scavenger which are incorporated to retard
"cross-talk" between the blue and green records. The interlayer between the green and red records contains a Lippmann emulsion. This very small-sized emulsion serves to adsorb development inhibitors released as coupling-off groups by certain of the couplers, and to keep such inhibitors from crossing over to other records..
Insufficient contrast exists to clearly image the coupler dispersion particles, but many of these are visible as apparent voids in the green and red records. Almost all of them are less than 1 um in diameter, and most are smaller than 0.5 um.
2.3 Development
Silver halide development, whether in black and white or in colour elements, occurs by an autocat-alytic redox process. For example, in the case of
development with hydroquinone, aminophenols and paraphenylenediamines, a two-equivalent reduction of silver halide occurs (as illustrated in the top part of Figure 5.4 for paraphenylenediamine development).
The characterization of this equivalency carries over to the dye-formation chemistry, where the equivalency of image-dye formation is expressed in terms of the amount of silver halide that must be reduced. The cou-plers illustrated in Figure 5.4 exemplify so-called two-equivalent and four-two-equivalent couplers. Because they have coupling-off groups, only one QDI is required to form an image dye from a two-equivalent coupler.
One QDI molecule attacks and reacts with the cou-pler to produce the dye. An example of such a two-equivalent coupling is given by the cyan dye-forming class of couplers in the middle of the bottom part of Figure 5.4. When the coupler does not have a coupling-off group, a leuco-dye is formed instead. This leuco-dye must undergo a two-electron oxidation in order to con-vert to the final dye molecule, and this oxidation is typically effected by another QDI molecule. Such cou-plers are therefore known as four-equivalent coucou-plers.
The acetanilide and pyrazolone coupling species shown in Figure 5.4 are examples of four-equivalent couplers.
More recently, efforts in research have been directed towards developing one-equivalent and less-than-equivalent couplers. A simple method of producing one-equivalent couplers is to incorporate a dye molecule as the coupling-off group. Thus, after coupling, there exist two image-dye molecules. Such efforts are particularly useful in dye-diffusion transfer elements.
2.4 Coupling stoichiometry and reactivity
Although the stoichiometry between silver development and image-dye formation is well defined, achieving near-stoichiometric yields can be difficult or impossi-ble because of system inefficiencies in the utilization of QDI. One can view QDI as the messenger that trans-forms the information in the developed silver image to information in the image dye. QDI that does not react to form an image dye in the record of choice is, in a yield sense, wasted. QDI can diffuse out of the emulsion layer in which it was to form dye. This species reacts with hydroxide and sulfite, present in most developer solu-tions, to form other by-products. Hydroxide is present in developer solutions to activate the couplers (typical pH in the range 9-12) and sulfite is present in many developer solutions as a preservative against aerial oxi-dation of PPD. If we define Y as the molar dye yield, i.e.
the number of moles of image dye produced per mole
of silver halide reduced or developed, one can invoke a steady-state approximation in oxidized developer (gen-eration and consumption) and derive this yield in terms of the dye-forming rates, Rc, and competition rates, Rx, and the respective equivalencies, ec and ex. This relationship is given in terms of the Weaver-Bertucci equation (7), as follows:
Y = + V M (51)
This equation shows that the yield obtained depends on the ratio of the coupling rates to the system com-petition (loss) rates, and the general relationships are illustrated in Figure 5.6. We see from this figure, for either two-equivalent or four-equivalent couplings, that stoichiometric yields, respectively, of 0.5 or 0.25 are only obtained when the ratio or coupling-to-loss rates are of the order of 10 or higher. When this ratio is of order unity, the heterogeneous reactivity of the coupler dispersion comes into play. Various factors can affect the coupler dispersion reactivity. First, it has been very well established that ionized couplers are much more reactive in the coupling reaction than are unionized couplers. The coupling reaction is, by and large, a surface reaction and occurs at the continuous phase-coupler particle inter-face. At a given pH, a certain fraction of the coupler will be ionized, and activated, in analogy to the ionization of fatty acids in monolayers. The observed coupling rates will vary widely, depending on the intrinsic bimolecular reactivity of a particular coupler type, just as has been
RJR0
Figure 5.6. Graphical representation of the Weaver-Bertucci equation, illustrating how the molar dye yield, Y, varies as a function of the ratio of the system competition rate, Rx, to the coupling (dye-forming) rate Rc. Four different curves corresponding to different combinations of two-equivalent and four-equivalent coupling (ec) and competition (ex) are illustrated
Y
6/<c/> (1/jj.m)
Figure 5.7. Coupling reactivity as a function of sur-face-to-volume ratio (proportional to the specific surface area) for the coupler M prepared as an NS dispersion. This coupling reactivity is expressed as PCZA in units of g CZA per litre. Both the coupler M and CZA (citrazinic acid) are four-equivalent.
The specific surface-to-volume ratio for monodisperse spheres is 6/d. The average diameters < d > for the various dispersions were measured by disc centrifugation with turbidity detection.
(Data by courtesy of Dr Andrew Sierakowski)
modelled in homogeneous physical organic chemistry.
However, another important contribution to coupling reactivity is the specific surface area of the coupler dis-persion particles. Thus, for certain classes of couplers, the specific surface area increases with decreasing par-ticle diameter, while the coupling reactivity increases as the particle size decreases. Some example data are illustrated in Figure 5.7, where decreasing size yields increased reactivities for "NS" dispersions (i.e. those formulated without (coupler) solvents - see Section 4.1 below) of the coupler M:
in Figure 5.7 of the coupler M dispersions, PCZA, is defined (8) in terms of the amount of added competi-tor, in this case CZA (citrazinic acid), that is required in the developer in order to decrease the molar dye yield by a factor of 50% over that obtained in the absence of added competitor. Both the coupler M and CZA are four-equivalent with respect to their utilization of QDI, so in this case ex = ec = 4. The linearity illustrated in Figure 5.7 shows that interfacial (specific surface) area (particle size) is a kinetically limiting parameter in the heterogeneous coupling of M.