John Texter
Strider Research Corporation, Rochester, New York, USA
Handbook of Applied Surface and Colloid Chemistry. Edited by Krister Holmberg ISBN 0471 490830 © 2001 John Wiley & Sons, Ltd
Figure 5.1. Generic multilayer photographic film element shown in cross-section, illustrating the key morphological com-ponents: overcoats, emulsion layers, subbing layer, support and antistat layer. The layer thicknesses are not drawn proportion-ately to scale. A typical emulsion layer may be 3 - 6 \im thick, a typical overcoat or interlayer may be 1-2 um thick, and the support is typically of the order of 100 um in thickness
paper coatings is discussed elsewhere in this volume by Tiberg et al. (see Chapter 7). Transparent supports are important for colour negative, movie and slide films.
The main materials used for such supports are cellulose triacetate (CTA), poly(ethylene terephthalate) (PET) and poly(ethylene naphthalate) (PEN). The thickness of these supports ranges from 60 to 200 um. CTA supports are typically of the order of 130 um, PET supports are about 100 um thick, and PEN supports are about 80-90 jam in thickness. PEN was recently introduced to allow thinner, more tightly wound roles of film for cartridge storage. Its mechanical properties give it superior performance in comparison to PET.
A subbing layer is usually coated in order to provide good wetting and good adhesion between the support and the layer coated upon it. Good adhesion is required in order to make sure that the multilayer element will maintain its mechanical integrity throughout all of the physical and chemical processes it is put through.
The composition of subbing layers varies with the composition of the layers to be joined. Prior to applying a subbing layer, the support may be physically or chemically treated to improve wetting and adhesion.
For example, corona discharge may be applied in order to increase the surface energy of the support, thereby improving wettability. Solvents for the support material may be applied to provide limited swelling.
Such swelling will promote polymeric entanglement of the polymers applied in the subbing layer, thereby improving the adhesive interaction.
Gelatin is often used in subbing layer composi-tions, since it is the most prevalent binder used in the emulsion and overcoat layers. Gelatin in the subbing
layer promotes adhesion of an overcoated layer con-taining gelatin. This adhesion is promoted further by chemical cross-linking of the gelatin, between various amine and carboxyl sites in the latter. Cross-linking of interpenetrating gelatin strands from the subbing and overcoated layers results in covalently based adhe-sion. Good binding to polyester supports is promoted by subbing layer polymers that contain chlorine, such as poly(vinyl and poly(vinylidene chloride)-containing copolymers (1). Using copolymers that inter-act strongly with the support provides good adhesion on one side of the subbing layer-support interface. Hav-ing subbHav-ing layer copolymers which also contain blocks or components that strongly adhere to the emulsion layer is also important. Monomer components contain-ing carboxylic acid functionalities or maleic anhydride groups provide chemical sites with which amine groups, from overcoated gelatin layers, can chemically cross-link. Various mixtures of gelatin, poly(alkyl acrylate)s, vinyl acrylate copolymers and polyurethanes can be used as the components of such subbing layers, wherein the glass transition temperature and stress accommodation properties can be varied.
1.2 Antistat layers
An antistat layer is a conductive layer prepared in order to dissipate electrostatic charge that builds up during manufacture (e.g. slitting operations), exposure (e.g. amateur roll film and, movie film) and viewing (e.g. motion picture film) processes. Unwanted discharge can cause fog and other defects in unexposed elements and can also cause sparks during projection. Since it is useful to dissipate electrostatic charge both before and after development processing, it is important to have layers formulated with materials that will withstand the harsh chemical treatments attendant to development processing of photographic papers and films. Antistat layers have been formulated with a wide variety of organic and inorganic conductive materials (2). Conduc-tive materials that dissolve in aqueous processing solu-tions must be protected with a water-resistant overlayer in certain circumstances. Ionically conductive polymers, typically containing monomers having quaternary nitro-gen groups and halide counterions, have been used in antistat layers for a long time. They suffer, however, in that their conductivities vary appreciably with rela-tive humidity. Antistat layers comprising carbon black are very effective in providing electronic conductiv-ity and insensitivconductiv-ity to humidconductiv-ity. The requisite removal of the carbon black from transparent print supports Emulsion
layers
Support Overcoat
Overcoat
Subbing layer Antistat layer
(slide film and movie print film) demands fairly expen-sive processing steps. Colloidal tin oxide and vana-dium pentoxide have been successfully used to provide electronic conductivity in antistat layers. Nanoparticles (80 A diameter) of antimony-doped tin oxide can pro-vide humidity-insensitive conductivity. Vanadium pen-toxide is typically nanoscopic in two dimensions, and provides excellent electronic conductivity, although it requires protection from aqueous processing solutions.
More recently, electronically conducting polymers and latexes have been introduced. These materials are very competitive with the nanoparticulate semiconductors in their efficacy.
1.3 Overcoats
The overcoat layers are thin layers, typically less than 1 jim in thickness, that can provide both chemical (e.g.
water barrier properties to protect a water-sensitive anti-stat layer material) and mechanical (e.g. abrasion and wear resistance) protection to the undercoated layers.
They also often provide lubrication and may contain wax dispersions or other lubricants. To guard against blocking or ferrotyping, where adhesion between touch-ing surfaces causes one or more layers to strip off when elements are stacked or rolled, relatively large beads, known as matte beads, are often coated in these over-coat layers. Such beads are typically formulated from polystyrene or polymethacrylate and are up to several um in diameter. They are typically coated at a relatively low number concentration and do not materially affect the gloss or light scattering from the materials. They serve to prevent strong interactions between adjacent planar surfaces by providing point contacts that prevent large surface area contacts. Matte beads are typically prepared by using suspension polymerization methods, sometimes following limited coalescence (see Section 6 below).
1.4 Emulsion layers
Finally we address the emulsion layers that contain light-sensitive silver halide. Some of these other layers, as dis-cussed above, contain particulates such as matte beads in overcoats, nanoparticulate semiconductors in antistat layers, copolymeric latexes in subbing layers, etc. Such layers are intrinsically composite multiphase layers, and this is also the case for the light-sensitive emulsion lay-ers. In addition to microcrystals or nanocrystals of silver halide, present to capture light and to form developable
latent image, much of the image dye-forming chemistry (for colour-negative films and papers and for colour pos-itive films, papers and transparencies) is incorporated as nanoparticles. This is especially the case for disper-sions of couplers that react with oxidized developer to form cyan, magenta or yellow image dyes. Other impor-tant chemicals are also often incorporated in particulate form. Dyes and organic pigments are used to manage light transmission and reflection through such elements (to control speed and to prevent halation). Such materi-als are often prepared as nanoparticulate dispersions and incorporated in thin interlayers between thicker light-sensitive layers containing silver halide.
The silver-halide-containing layers may be of differ-ent types, depending on whether the silver halide therein is "slow", "fast" or "something in between". The layers that generate the bulk of the colour density are typi-cally termed "slow" layers, and these layers are thickest because they contain a larger stoichiometric amount of dye-forming chemistry, namely silver halide and coupler dispersion. "Fast" layers are typically much thinner and contain much less dye-forming coupler. These combina-tions of slow and fast layers may be coated in several different formats or layer orders. The most common sit-uation is to have the slow red-sensitive layer coated closest to the support. If the element is a print mate-rial (e.g. colour paper and motion picture print film), there probably will be no distinguishable fast layers. A fast red-sensitive layer will then typically be coated on top of the slow layer. There may or may not be an interlayer between these two layers - typically, there is not. An interlayer often will be coated above the fast red-sensitive layer. This interlayer may contain oxidized developer scavenger to reduce chemical "cross-talk"
between different colour records during development and/or a magenta filter dye to prevent green light from reaching the red-sensitive record. Slow and fast green-sensitive layers are then coated in sequence, often with another interlayer coated above these. Such an inter-layer may also contain oxidized developer scavenger and a yellow filter dye dispersion. This yellow dispersion absorbs blue light, and keeps blue light from expos-ing the underlyexpos-ing green- and red-sensitive layers, all of which are intrinsically blue-sensitive. An alternative coating order used in some products is to coat the slow layers in sequence, i.e. red-sensitive, green-sensitive and blue-sensitive, and then coat the fast layers in the same sequence above the slow layers. Some products con-tain slow, medium and fast layers, and there are many permutations possible. However, those articulated here illustrate the main function of some of the auxiliary particulate materials, such as filter dye dispersions and
oxidized developer scavengers. The composite nature and prevalent use of nanoparticulate materials in pho-tographic elements will be discussed further below.
1.5 Coating methods
These thin layers may be coated and prepared by using a variety of different coating technologies. While the coating industry as a whole uses dip coating and doctor blades for high volumes of coating manufacturing, the uniformity and low-defect-level requirements and the multiple number of coated layers present in a particular element necessitate more sophisticated coating processes than those generally available in other industries. For the coating of multilayer elements out of aqueous melts, extrusion, slide hopper and curtain coating processes are predominant in the photographic industry. Extru-sion coating, also known as ("Ex") "X-hopper" coating, involves pumping a melt composition out of an "extru-sion" slot in near-contact with a moving substrate (web).
This mode of coating usually is carried out one layer at a time. After coating, the applied film is typically chill set and then dried. At a given coating speed, there typically will be an upper and lower limit to the wet coating thickness that can be applied. If too much melt is extruded at a given web speed, the resulting thin film will not be uniform. If too little melt is extruded, break-lines will develop. For a given (linear) coating speed, melts having greater viscosity can typically be coated in thicker layers. Slide hopper coating has revolutionized the manufacture of multilayer films and papers. Up to 15 layers have been successfully coated simultaneously by using slide hopper coating, although three to five layers is a more usually encountered situation. A given melt is pumped out of an extrusion slot on to a hopper chute that is gravity fed onto a moving web. Each succeed-ing (overcoatsucceed-ing) layer is pumped out and gravity fed on to the lower layer. Curtain coating involves a sim-ilar cascading of multilayers, but instead of crossing a narrow gap onto a moving web, the array of contigu-ous layers falls as a curtain onto a moving web. This approach offers faster coating speeds. Whether coating one or multiple layers, the surface energy of the sub-strate and each succeeding layer must be tuned to insure wetting of each interface by the overcoating layer melt.
1.6 Black and white photography
The image-wise exposure, processing and viewing of a negative-working black and white element are illustrated
Figure 5.2. Schematic of simple black and white photographic element illustrating "negative" development. A region of film that is exposed produces silver halide microcrystals containing a latent image speck. During development, this speck catalyses the complete reduction of the silver halide crystal to silver metal. Silver halide crystals without latent image are not reduced to silver during the limited development process time
in Figure 5.2. Microcrystals or nanocrystals of silver halide (generally AgCl or AgBr with some iodide dop-ing) provide light sensitivity in photographic elements.
The nature and formation of silver halide microcrys-tals are discussed in greater detail below. These crysmicrocrys-tals are the primary particulate component of the emulsion layer in black and white elements and are known as photographic emulsions. This usage of "emulsions" is, as a term of art, restricted to photographic technol-ogy, and does not refer to the more usually understood meaning of a dispersion of one immiscible liquid in another. Such crystals can be chemically and/or spec-trally sensitized (as discussed below) to respond to very narrow or to very broad segments of the visible (or near-infrared) spectrum. Light exposure of the emulsion layer results in the formation of a latent image. The structure of latent image specks has not been unequiv-ocally identified, but much indirect evidence suggests such specks are charged silver clusters, comprising neu-tral silver atoms and charged silver cations (3). This latent image is typically located at or on the surface of a silver halide particle and is chemically amplified by chemical processing or development which produces an image density in developed silver metal in response to the degree of light (photo) exposure (Figure 5.3).
The latent image catalyses development of the rest of the crystal to elemental silver, at a rate faster than the rate at which crystals without latent image will be reduced to silver metal. This catalytic discrimination provides the basis for modern silver halide photographic development, and the effective degree of amplification obtainable (e.g. 108) has not been matched by other competing technologies. When the amount of image developed is directly proportional to the amount of exposure, the photographic element is called "negative"
working. When this proportionality is an inverse one, the element is called "positive" working, and a den-sity versus logE plot would appear as the dotted line
Exposure
Development
log Exposure
Figure 5.3. Relationship between exposure (E) and developed silver density (D). The continuous curve illustrates the basic sensitometry obtained in negative-working systems. Increasing exposure results in increasing developed (optical) density (elemental silver or image dye in the case of colour systems).
The dotted curve illustrates positive-working sensitometry, where increasing exposure results in decreasing developed density
shown in Figure 5.3. Either type of response can be obtained, depending on the details of the chemistry incorporated in the emulsion layer. A more thorough discussion of such options is beyond the scope of this present chapter, although further information is readily accessible (4). The negative development of black and white elements is typically carried out with aqueous alkaline hydroquinone solutions. The partially ionized hydroquinone chemically reduces silver halide to silver metal. Other popular black and white reducing agents include aminophenols, catechols, pyrogallols and ascor-bic acid.
1.7 Colour negative photography
Amateur colour negative photographic elements have one or more emulsion layers for each of three regions of the visible spectrum, i.e. red, green and blue (5, 6). Water-insoluble organic couplers are incorporated as submicron diameter dispersions in each of these layers.
Typically, red-sensitive layers contain cyan dye-forming couplers (e.g. phenols and naphthols), green-sensitive layers contain magenta dye-forming couplers (e.g. pyra-zolones and pyrazolotriazoles), and blue-sensitive layers contain yellow dye-forming couplers (e.g. benzoylac-etanilides and pivaloylacbenzoylac-etanilides). Such layers typi-cally contain three phases, i.e. silver halide, coupler and a continuous gelatin (as binder) phase. Spectral sensi-tivity is imparted by spectral sensitizing dyes that are adsorbed on to the silver halide particle surfaces (dis-cussed at greater length below). This method of subtrac-tive colour generation produces (negasubtrac-tive) dye densities
in proportion to the amount of "minus" exposure. For example, the amount of yellow image dye produced is in proportion to the amount of blue light converted to latent image, the amount of magenta image dye pro-duced is in proportion to the amount of green light converted to latent image, and similarly, the amount of cyan dye produced is in proportion to the amount of red light converted to latent image. A schematic of a typical colour negative film element is similar to that depicted in Figure 5.1, except that there are typ-ically additional interlayers above, between and below the various light-sensitive emulsion layers. For example, in colour negative print materials, such as papers and colour transparencies, a UV filter dye layer is often included right below the overcoat layer. Such a UV layer typically comprises a UV filter dye dispersed in partic-ulate form and protects the resulting image dyes from UV-induced photodegradation.
A schematic basis for colour development chem-istry is illustrated in Figure 5.4 (6). Silver halide is reduced by colour developer to metallic silver. Para-phenylenediamines (PPDs) are preferred colour devel-opers, but aminophenols may also be used. The PPD undergoes a two-electron oxidation process and pro-duces two silver atoms and one quinonediimine, (QDI) molecule. This two-equivalency is illustrated in the top part of Figure 5.4. Surrounding the silver halide micro-crystals in a given layer are much smaller coupler dispersion particles. The coupler in these particles is partially ionized under dilute alkaline development con-ditions (pH ~ 10-11) and reacts with QDI to form indoaniline image dyes. The formation of some of these image dyes is illustrated in Figure 5.4. The cou-pler dispersion particles are typically of the order of 100-500 nm in diameter, and surface chemistry is key to their formation, as well as affecting their coupling kinetics.
1.8 Other types of photographic elements
There are many other important types of photographic elements (7) which space limitations do not allow us to discuss at length here. For example, colour separation photography allows one to capture the colour informa-tion in a scene by exposing three different black and white films, one to each of the principle scene colors, i.e. red, green and blue. Each of these exposures can then be put through a black and white development, chemically fogged, and then "colour developed" using aqueous diffusible couplers to produce the appropriate subtractive colour. The resulting three colour images
Density
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.