Photothermographic imaging systems based on silver halide and silver carboxylates have been utilized for a wide variety of imaging applications for many years. We have been investigating the morphology of silver particles formed in the image area of photothermographic imaging systems in order to understand the forces that affect their morphology. Because the size, shape, morphology and silver particle distribution within an image area affects the overall optical properties of the imaging material, control of these properties should lead to improved photographic responses of the materials. We have found in this investigation that there are multiple factors that influence the overall morphology of the silver particles, and show how morphological properties are changed by different conditions, e.g., the nature of the materials used, including the toners (succinimide, phthalimide, phthalazine), phenolic developers, and related components.
Klosterboer and Rutledge developed a mathematical model for photothermographic systems consisting of silver halide grains, silver carboxylates, development chemistry and binder. The Klosterboer-Rutledge (K-R) model incorporates several simplifying assumptions, the most limiting of which is the assumption that the spheres of influence do not overlap. This assumption effectively restricts the validity of the model to low grain-number densities. The model has now been modified to remove this restriction resulting in a more general model that resolves the discrepancy between the Klosterboer-Rutledge model and the observed dependence of DMAX, contrast and speed on grain-number density. The Klosterboer-Rutledge model also treats the spheres of influence as spheres with perfectly sharp boundaries. The generalized model was derived with an arbitrarily shaped region of influence thereby extending the validity to more realistic model films. In addition, other simplifying assumptions were addressed to complete the generalization of the K-R model.
The structural investigation is reported of the reactions between components utilized in photothermographic imaging systems, including toners, developers and silver with potential ligands such as triphenylphosphine, tribromomethylsulfonylbenzothiazole and 2-mercapto-benzimidazole. Depending on the complexing agent and silver counterion, various forms of silver complexes can be isolated and characterized. This report demonstrates the interactions, and characterizes the new compounds formed between various components in the photothermographic materials with emphasis on the ormulation containing silver carboxylate, the toner system comprising phthalazine and phthalic acid and phenolic type developers.
Silver behenate is used in thermographic materials as the image silver reservoir. To obtain an imaging material with optimal sensitometric and physical properties, it is important to obtain fundamental insight into and basic knowledge of silver behenate. A method for determining the relative crystallinity has been developed based on powder x-ray diffraction (XRD). In addition, the thermal behavior of silver behenate crystals has been studied using XRD. The image silver crystal size, silver behenate crystallinity and phase transitions were determined at different temperatures.
Spectroscopic measurements of photothermographic systems at development temperatures show how Ag particles form during the course of the development process. In standard systems, development follows autocatalytic kinetics. Systems containing infectious developers exhibit more unusual kinetics which do not generally fit simple mathematical models. In standard photothermographic systems, Ag particles have a median diameter of approximately 0.065 μm. The median diameter is reduced to 0.028 μm and the particles are generally more spherical in the presence of an infectious developer. Intermediate sizes and morphologies are possible with low levels of infectious development. Particle size and morphology correlates to light absorption and Ag covering power. Standard photothermographic development proceeds sequentially, while infectious development occurs in parallel.
Various properties of polyvinylbutyral (PVB), the most widely used polymer binder in photothermographic imaging materials, have been identified and characterized. These properties, including its solution and coating behavior, its ability to disperse silver behenate, as well as being a reducing agent for it, and its compatibility with behenic acid can all be credited for its widespread use as a binder for photothermographic recording materials. The reducing character of PVB is particularly important, although it was not found to be a property common to all sources of PVB, and is strongly dependent upon manufacturing method and purity. It is concluded that it is the combination of reducing character of PVB, together with its strong affinity for behenic acid, plasticizing the matrix, that makes PVB the binder of choice for photothermographic recording materials. This model enables targeted screening of alternative polymers to be carried out for future generations of photothermographic recording materials.
Analysis of patents, publications and our own experimental data concerning thermally developed photographic materials based on silver organic salts (AgO2CxH2x−1, x = 10–22) makes it possible to outline the prospects of their future development as well as the trends in the scientific activity devoted to optimizing these materials. The study of the competing processes of the catalytic thermal development of silver carboxylates, in conjunction with the developing agents, and catalytic thermal degradation of silver carboxylates in the absence of developers, has revealed that radical reactions play a significant role in these processes. Because the formation of radicals adversely affect the stability of thermally developed materials we have found that it is very important to use developing agents that are inhibitors of radical processes. We have studied the effect of the composition and ingredient ratios in the photosensitive films on the sensitivity of thermally developed photographic papers (TDPP), both spectrally and chemically sensitized. The correlation between structure and the developing ability of reducing agents used in TDPP is established. The sensitivity of TDPP can be increased both through the modification of the light-sensitive composition with benzotriazolidophosphate and through increasing the efficiency of sulfur sensitization with amines. Also, boric acid is found to be an efficient crosslinking agent for the polymer binder in TDPP, polyvinylbutyral. In addition, high stability of TDPP can be achieved by using a reducing agent encapsulated within a polyvinylacetate caplet.
The sensitometric effects of pre-exposure heating in TDPM of the Dry Silver type were investigated. Loss of developable density with increasing time of pre-exposure heating was shown to involve the toners which are essential to selective, image-wise TDPM development. Pre-exposure heating was found not to affect the latent image forming system (silver halide-silver carboxylate interface) in TDPM. Our observations support previously proposed interpretations of the mechanism of TDPM development. Preexposure heating may thus lead to either (a) loss of toner, e.g., by volatilization or air oxidation, and/or (b) side reactions between toner and silver carboxylate leading to formation of less reactive species. From a practical point of view these results provide mechanisms for both the storage instability of the TDPM sensitometric responses, as well as for inefficiency in latent image amplification.