Photographic Elements Containing Reflective or Diffusely Transmissive Supports

The present invention is directed to photographic elements having supports such as film supports having an oriented polymer continuous phase and polymer microbeads dispersed therein which are at least partially bordered by voids. The supports have desirable properties of texture, whiteness in the absence of colorants, diffuse transmission of light and generally good physical properties such as tensile properties.

Blends of linear polyesters with other incompatible materials of organic or inorganic nature to form microvoided structures are well-known in the art. U.S. Pat. No. 3,154,461 discloses, for example, linear polyesters blended with, for example, calcium carbonate. U.S. Pat. No. 3,944,699 discloses blends of linear polyesters with 3 to 27% of organic material such as ethylene or propylene polymer. U.S. Pat. No. 3,640,944 also discloses the use of poly(ethylene terephthalate) blended with 8% organic material such as polysulfone or poly(4-methyl-1-pentene). U.S. Pat. No. 4,377,616 discloses a blend of polypropylene to serve as the matrix with a small percentage Of another and incompatible organic material, nylon, to initiate microvoiding in the polypropylene matrix. U.K. Patent Specification 1,563,591 discloses linear polyester polymers for making opaque thermoplastic film support in which have been blended finely divided particles of barium sulfate together with a void-promoting polyolefin, such as polyethylene, polypropylene or poly-4-methyl-1-pentene.

The above-mentioned patents show that it is known to use incompatible blends to form films having paper-like characteristics after such blends have been extruded into films and the films have been quenched, biaxially oriented and heat set. The minor component of the blend, due to its incompatibility with the major component of the blend, upon melt extrusion into film forms generally spherical particles each of which initiates a microvoid in the resulting matrix formed by the major component. The melting points of the void initiating particles, in the use of organic materials, should be above the glass transition temperature of the major component of the blend and particularly at the temperature of biaxial orientation.

As indicated in U.S. Pat. No. 4,377,616, spherical particles initiate voids of unusual regularity and orientation in a stratified relationship throughout the matrix material after biaxial orientation of the extruded film. Each void tends to be of like shape, not necessarily of like size since the size depends upon the size of the particle. Ideally, each void assumes a shape defined by two opposed and edge contacting concave disks. In other words, the voids tend to have a lens-like or biconvex shape. The voids are oriented so that the two major dimensions are aligned in correspondence with the direction of orientation of the film structure. One major dimension is aligned with machine direction orientation, a second major dimension is aligned with the transverse direction orientation, and a minor dimension approximately corresponds to the cross-section dimension of the void-initiating particle.

The voids generally tend to be closed cells, and thus there is virtually no path open from one side of a biaxially oriented film to the other side, through which liquid or gas can traverse. The term "void" is used herein to mean devoid of solid matter, although it is likely the "voids" contain a gas.

Upon biaxial orientation of the resulting extruded film, the film becomes white and opaque, the opacity resulting from light being scattered from the walls of the microvoids. The transmission of light through the film becomes lessened with increased number and with increased size of the microvoids relative to the size of a particle within each microvoid. Also, upon biaxial orientation, a matte finish on the surface of the film results, as discussed in U.S. Pat. No. 3,154,461. The particles adjacent the surfaces of the film tend to be incompressible and thus form projections without rupturing the surface. Such matte finishes enable the film to be written upon with pencil or with inks, crayons, and the like.

U.S. Pat. No. 3,944,699 also indicates that the extrusion, quenching and stretching of the film may be effected by any process which is known in the art for producing oriented film, such as by a flat film process or a bubble or tubular process. The flat film process involves extruding the blend through a slit dye and rapidly quenching the extruded web upon a chilled casting drum so that the polyester component of the film is quenched into the amorphous state. The quenched film is then biaxially oriented by stretching in mutually perpendicular directions at a temperature above the glass transition temperature of the polyester. The film may be stretched in one direction and then in a second direction or may be simultaneously stretched in both directions. After the film has been stretched it is heat set by heating to a temperature sufficient to crystallize the polyester while restraining the film against retraction in both directions of stretching.

Paper is essentially a non-woven sheet of more or less randomly arrayed fibers. The key properties of these structures are opacity, texture, strength, and stability. Natural polymers are generally weaker and less stable. A serious problem, for example, is brightness reversion or fading of papers and fibers.

Although there are many ways to produce opaque media, this invention is concerned with creating opacity by stretching or orienting plastic materials to induce microvoids which scatter light, preferably white and ultraviolet light. A large body of prior art deals with this technique, wherein a plurality of inorganic solid particles are used as the dispersed phase, around which the microvoids form. Some significant problems associated with this approach are: (1) agglomeration and particle size control, (2) abrasive wear of extrusion equipment, guides, and cutters, (3) high specific gravity of these solids, (4) poor void nucleation around the solid particles due to the low thermal contraction of solids relative to liquids and polymer wetting and adhesion to the solid surfaces, (5) cost of these materials on a volume basis, and (6) handling and processing problems in general.

Of particular interest is U.S. Pat. No. 4,770,931 which is directed to articles comprising a continuous polyester phase having dispersed therein microbeads of cellulose acetate which are at least partially bordered by void space, and U.S. Pat. No. 4,900,654 which is a continuation in part thereof directed towards the use of polyester/cellulose acetate films as photographic element supports. The present invention is unexpected, however, in that while the geometries are similar, the cross-linked beads disclosed herein cavitate more efficiently generating higher void fractions and improved properties per weight of added beads. Also, the compositions of this invention have superior thermal and chemical stability, when compared with the prior art, especially the cellulose esters. Also of particular interest is U.S. Pat. No. 4,320,207 which discloses oriented polyester film containing pulverized cross-linked polymers.

In one aspect, this invention is directed to a photographic element comprised of a reflective or diffusely transmissive support and, coated on the support, at least one radiation-sensitive silver halide emulsion layer. The photographic element is characterized in that the support is comprised of a continuous oriented polymer matrix having dispersed therein microbeads of a cross-linked polymer coated with a slip agent which are at least partially bordered by void space, the microbeads being present in an amount of 5-50% by weight based on the weight of the oriented polymer, the void space occupying 2-60% by volume of the support.

The invention can be better appreciated by reference to the following detailed description considered in conjunction with the drawings, in which FIG. 1 is a schematic diagram of a photographic element according to the invention being imagewise exposed;

FIG. 2 is a schematic diagram of a photographic element according to the invention being back illuminated;

FIG. 3 Is a perspective view in section illustrating a preferred embodiment of a support satisfying the requirements of the invention;

FIG. 4 is a perspective view in section illustrating an alternate construction of a support satisfying the requirements of the invention;

FIGS. 5 and 7 are enlarged sectional views illustrating a microbead of a cross-linked polymer entrapped in a void in an oriented polymer continuous phase;

FIG. 6 is a sectional view taken along section line 6--6 in FIG. 5; and

FIG. 8 is a graphical view illustrating the change in size of microvoids surrounding microbeads as a function of the stretch ratio.

A photographic element 10 satisfying the requirements of the invention is shown in FIG. 1. The photographic element is comprised of a support 12 and an imaging unit 14 coated on the support. The imaging unit is comprised of at least one radiation-sensitive silver halide emulsion layer. In the simplest form of the invention the imaging unit consists of a single silver halide emulsion layer. Black-and-white photographic elements, for example, often contain a single silver halide emulsion layer. Alternatively, the imaging unit can be comprised of a plurality of silver halide emulsion layers. For example, color photographic elements typically contain blue, green, and red color forming layer units. Each of these color forming layer units contains at least one silver halide emulsion layer. In the most elaborate commonly employed form each color forming layer unit contains three separate silver halide emulsion layers of similar spectral sensitivity within a single color forming layer unit, but differing in speed.

As shown in FIG. 1 the photographic element is being imagewise exposed to radiation capable of producing a latent image in the silver halide emulsion layer or layers of the imaging unit. Exposing radiation, indicated by arrows 16a and 16b, strikes the imaging unit. Part of the incident radiation is absorbed in the imaging unit, with the remainder penetrating the imaging unit and striking the support, as indicated by arrows 18a and 18b.

Because of the highly reflective nature of the support chosen for the photographic element, a very high percentage of the exposing radiation striking the support is reflected, as indicated by arrows 20a and 20b. The reflected radiation traverses the imaging unit a second time, where, again, it is in part absorbed. The unabsorbed radiation is indicated by arrows 22a and 22b.

An important point to notice is that the reflective support allows exposing radiation to penetrate the imaging unit twice, almost doubling the opportunity for its absorption. This is in direct contrast to a photographic element having a transparent or absorptive support, which relies almost entirely on radiation producing a latent image in the imaging unit based on a single penetration of the imaging unit. Thus, with the same imaging unit a photographic element with a reflective support exhibits a higher photographic speed than a photographic element with a transmissive or absorptive support.

The photographic elements of this invention exhibit higher effective speeds than photographic elements with comparable imaging units and conventional reflective supports, since the reflective supports which satisfy the requirements of this invention reflect a larger percentage of exposing radiation that impinges upon them.

The reflective properties of the microvoided supports of the invention also make them particularly suitable for use as intensifying screen supports for use with silver halide radiographic elements. The polymer microbeads and voids surrounding them form reflective lenslets which provide an advantageous balance of imaging speed and sharpness as set forth in U.S. Pat. No. 4,912,333 of Roberts et al., the disclosure of which is incorporated by reference.

A portion of the exposing radiation incident on the support 12 is neither reflected nor absorbed, but is transmitted through the support. This portion of the radiation is not shown in FIG. 1, since it is possible to reduce transmitted radiation to a negligibly small portion of the total radiation while concurrently increasing reflected radiation by increasing the thickness of the support.

However, it is another significant advantage of the photographic elements of this invention that they can be constructed with supports that are diffusely transmissive. This is illustrated by reference to FIG. 2. In FIG. 2 the photographic element 10 is shown after it has been imagewise exposed and processed to produce an image. To view the image formed in the imaging unit 14 one of two different modes of viewing can be undertaken. The first of these is reflection viewing, in which ambient light penetrates the imaging unit and is reflected by the support back toward the viewer's eyes. The support reflects uniformly, with the perceived image being a function of the modulation of the ambient light that occurs during its initial and reflection passages through the imaging unit.

The second mode of viewing is transmission viewing. As shown in FIG. 2, an illumination source 30 directs light, indicated by arrows 32, to the support 12. While a portion of this light, not shown, is reflected by the support, a significant portion of the incident light enters the support and is diffusely transmitted. Diffuse transmission differs from specular transmission in that the light becomes scattered during the course of penetrating the support. Stated another way, even with the imaging unit exhibiting zero density or being entirely omitted, one viewing the support 12 from above (as shown) could not see the light source, but could see transmitted light, indicated by arrows 34, emanating from the support. With no image present in the imaging unit, the illuminated support appears uniformly white. When the imaging unit contains an image, this is superimposed on the illuminated white background provided by the support, and the image is readily viewed.

If the support were specularly transmissive (i.e., transparent), viewed from above (as shown), the viewer could see the illumination source as well as any image present in the imaging unit. This Is visually objectionable, since the two images, the illumination source and the image in the imaging unit are superimposed. By providing a photographic element with a diffusely transmissive support a superior support for transmission viewing is provided.

In accordance with the present invention, supports are provided which have unique properties such as texture, opacity, low density, whiteness, etc. The supports are especially useful when in the form of film or sheets.

An important aspect of this invention is that during melt processing the orientable polymer does not react chemically or physically with the microbead polymer and/or its coating in such a way as to cause one or more of the following to occur to a significant or unacceptable degree: (a) alteration of the crystallization kinetics of the matrix polymer making it difficult to orient, (b) destruction of the matrix polymer, (c) destruction of the microbeads, (d) adhesion of the microbeads to the matrix polymer, or(e) generation of undesirable reaction products, such as toxic or high-color moieties.

FIG. 3 illustrates a support 60 which has been biaxially oriented (biaxially stretched, i.e., stretched in both the longitudinal (X) and transverse (Y) directions), as indicated by the arrows. The support 60 is illustrated in section, showing microbeads of polymer 62 contained within circular voids 64 in the polymer continuous matrix 66. The voids 64 surrounding the microbeads 62 are theoretically doughnut-shaped, but are often of irregular shape. Often, a line drawn perpendicular to and through the article will penetrate several voids and possibly some microbeads.

FIG. 4 also illustrates a shaped article in the form of a support 70 which has been unidirectionally oriented (stretched in one direction only, as indicated by the arrow). Microbeads of polymer 72 are contained between microvoids 74 and 74'. The microvoids in this instance form at opposite sides of the microbeads as the sheet is stretched. Thus, if the stretching is done in the machine direction (X) as indicated by the arrow, the voids will form on the leading and trailing sides of the microbeads. This is because of the unidirectional orientation as opposed to the bidirectional orientation of the sheet shown in FIG. 3. This is the only difference between FIG. 3 and 4. Note particularly the bumpy texture of the surfaces.

FIGS. 5 and 6 are sectional views illustrating enlargement of a section of a support according to this invention, microbead 80 being entrapped within polymer continuous matrix 82 and encircled by void 84. These structure result from the support being stretched in the X and Y directions.

FIG. 7 is a view similar to FIG. 5, except illustrating in enlarged form microbead 90 entrapped in polymer continuous matrix 92, having formed on opposite sides thereof microvoids 94 and 94', which are formed as the shaped article is stretched in the direction of the arrow X.

FIG. 8 is an enlargement illustrating the manner in which microvoids are formed in the polymer continuous matrix as the support is stretched or oriented. The formation of the microvoids 100 and 100' around microbeads 102 is illustrated on a stretch ratio scale as the support is stretched up to 4 times its original dimension. For example, as the support is stretched 4 times its original dimension in the X direction (4.times.), the voids 100 and 100' would extend to the points 104 and 104' respectively.

There are two aspects of the present invention. First, the microbeads are preferably cross-linked to given them resiliency and elasticity. Second, the microbeads are coated with a "slip agent" to permit easier sliding with respect to the matrix polymer to thereby result in more microvoiding. Although both aspects are believed to be unique and result in improved results to an extent, it is preferred that the microbeads be both cross-linked and coated with a slip agent. Thus, the microbeads will be often described herein as being both cross-linked and coated with a slip agent.

The present invention provides supports comprising a continuous thermoplastic polymer phase having dispersed therein microbeads of polymer which are at least partially bordered by voids, the microbeads of polymer having a size of about 0.1-50 microns, preferably about 2-20 microns, and being present in an amount of about 5-50% by weight based on the weight of continuous phase polymer, the voids occupying about 2-60% by volume of the support. The matrix polymer containing the generally spherical polymer microbeads which, according to one aspect of the invention, are cross-linked to the extent of having a resiliency or elasticity at orientation temperatures of the matrix polymer such that a generally spherical shape of the cross-linked polymer is maintained after orientation of the matrix polymer. The composition of the support when consisting only of the polymer continuous phase and microbeads bordered by voids, is characterized by having a specific gravity of less than 1.20, preferably about 0.3-1.0, preferably a Kubelka-Munk R value (infinite thickness) of about 0.90 to about 1.0, and preferably the following Kubelka-Munk values when formed into a 3 mil thick film:

wherein the opacity values indicate that the article is opaque, the SX values indicate a large amount of light scattering through the thickness of the article, the KX values indicate a low amount of light absorption through the thickness of the article, and the Ti values indicate a low level amount of internal transmittance of the thickness of the article. The R (infinite thickness) values indicate a large amount of light reflectance. Obviously, the Kubelka-Munk values which are dependent on thickness of the article must be specified at a certain thickness. Although the supports themselves may be very thin, e.g., less than 1 mil or they may be thicker, e.g., 20 mils, the Kubelka-Munk values, except for R infinity, are specified at 3 mils and in the absence of any additives which would effect optical properties. Thus, to determine whether supports have the optical properties called for, the oriented polymer containing microbeads at least partially bordered by voids, without additives, should be formed in a 3 mils thick film for determination of Kubelka-Munk values.

The supports according to this invention are useful, for example, when in the forms of sheets or films. In the absence of additives or colorants, they are very white. The supports are very resistant to wear, moisture, oil, tearing, etc.

The supports are preferably in the form of a paper-like sheet having a thickness of about 0.10-20 mils. Preferably, the support is made by biaxial orientation using procedures well known in the art.

The products of this invention are environmentally desirable products. They are long lasting, durable, and recyclable. Also, upon incineration, less than 1% ash and no undesirables such as chlorine, cyanides, etc. are found. Finally, if they are put into landfills, they will not make toxic gases or liquids, and they will not threaten the quality of our air or ground water.

The continuous phase polymer may be any article-forming polymer such as a polyester capable of being cast into a film or sheet, spun into fibers, extruded into rods or extrusion, blow-molded into containers such as bottles, etc. The polyesters should have a glass transition temperature between about 50.degree. C. and about 150.degree. C., preferably about 60.degree.-100.degree. C., should be orientable, and have an I.V..of at least 0.50, preferably 0.6 to 0.9. Suitable polyesters include those produced from aromatic, aliphatic or cycloaliphatic dicarboxylic acids of 4-20 carbon atoms and aliphatic or alicyclic glycols having from 2-24 carbon atoms. Examples of suitable dicarboxylic acids include terephthalic, isophthalic, phthalic, naphthalene dicarboxylic acid, succinic, glutaric, adipic, azelaic, sebacic, fumaric, maleic, itaconic, 1,4-cyclohexanedicarboxylic, sodiosulfoisophthalic and mixtures thereof. Examples of suitable glycols include ethylene glycol, propylene glycol, butanediol, pentanediol, hexanediol, 1,4-cyclohexanedimethanol, diethylene glycol, other polyethylene glycols and mixtures thereof. Such polyesters are well known in the art and may be produced by well-known techniques, e.g., those described in U.S. Pat. Nos. 2,465,319 and 2,901,466. Preferred continuous matrix polymers are those having repeat units from terephthalic acid or naphthalene dicarboxylic acid and at least one glycol selected from ethylene glycol, 1,4-butanediol and 1,4-cyclohexanedimethanol. Poly(ethylene terephthalate), which may be modified by small amounts of other monomers, is especially preferred. Polypropylene is also useful. Other suitable polyesters include liquid crystal copolyesters formed by the inclusion of a suitable amount of a co-acid component such as stilbene dicarboxylic acid. Examples of such liquid crystal copolyesters are those disclosed in U.S. Pat. Nos. 4,420,607, 4,459,402 and 4,468,510.

Suitable cross-linked polymers for the microbeads are polymerizable organic materials which are members selected from the group consisting of an alkenyl aromatic compound having the general formula ##STR1## wherein Ar represents an aromatic hydrocarbon radical, or an aromatic halohydrocarbon radical of the benzene series and R is hydrogen or the methyl radical; acrylate-type monomers including monomers of the formula ##STR2## wherein R is selected from the group consisting of hydrogen and an alkyl radical containing from about 1 to 12 carbon atoms and R' is selected from the group consisting of hydrogen and methyl; copolymers of vinyl chloride and vinylidene chloride, acrylonitrile and vinyl chloride, vinyl bromide, vinyl esters having the formula ##STR3## wherein R is an alkyl radical containing from 2 to 18 carbon atoms; acrylic acid, methacrylic acid, itaconic acid, citraconic acid, maleic acid, fumaric acid, oleic acid, vinylbenzoic acid; the synthetic polyester resins which are prepared by reacting terephthalic acid and dialkyl terephthalics or ester-forming derivatives thereof, with a glycol of the series HO(CH.sub.2).sub.n OH, wherein n is a whole number within the range of 2-10 and having reactive olefinic linkages within the polymer molecule, the hereinabove described polyesters which include copolymerized therein up to 20 percent by weight of a second acid or ester thereof having reactive olefinic unsaturation and mixtures thereof, and a cross-linking agent selected from the group consisting of divinylbenzene, diethylene glycol dimethacrylate, oiallyl fumarate, diallyl phthalate and mixtures thereof.

Examples of typical monomers for making the cross-linked polymer include styrene, butyl acrylate, acrylamide, acrylonitrile, methyl methacrylate, ethylene glycol dimethacrylate, vinyl pyridine, vinyl acetate, methyl acrylate, vinylbenzyl chloride, vinylidene chloride, acrylic acid, divinylbenzene, arrylamidomethylpropane sulfonic acid, vinyl toluene, etc. Preferably, the cross-linked polymer is polystyrene or poly(methyl methacrylate). Most preferably, it is polystyrene and the cross-linking agent is divinylbenzene.

Processes well known in the art yield non-uniformly sized particles, characterized by broad particle size distributions. The resulting beads can be classified by screening to produce beads spanning the range of the original distribution of sizes. Other processes such as suspension polymerization, limited coalescence, directly yield very uniformly sized particles. Suitable slip agents or lubricants include colloidal silica, colloidal alumina, and metal oxides such as tin oxide and aluminum oxide. The preferred slip agents are colloidal silica and alumina, most preferably, silica. The cross-linked polymer having a coating of slip agent may be prepared by procedures well known in the art. For example, conventional suspension polymerization processes wherein the slip agent is added to the suspension is preferred. As the slip agent, colloidal silica is preferred.

It is preferred to use the "limited coalescance" technique for producing the coated, cross-linked polymer microbeads. This process is described in detail in U.S. Pat. No. 3,615,972, incorporated herein by reference. Preparation of the coated microbeads for use in the present invention does not utilize a blowing agent as described in this patent, however.

The following general procedure may be utilized in a limited coalescence technique:

1. The polymerizable liquid is dispersed within an aqueous nonsolvent liquid medium to form a dispersion of droplets having sizes not larger than the size desired for the polymer globules, whereupon

2. The dispersion is allowed to rest and to reside with only mild or no agitation for a time during which a limited coalescence of the dispersed droplets takes place with the formation of a lesser number of larger droplets, such coalescence being limited due to the composition of the suspending medium, the size of the dispersed droplets thereby becoming remarkably uniform and of a desired magnitude, and

3. The uniform droplet dispersion is then stabilized by addition of thickening agents to the aqueous suspending medium, whereby the uniform-sized dispersed droplets are further protected against coalescence and are also retarded from concentrating in the dispersion due to difference in density of the disperse phase and continuous phase, and

4. The polymerizable liquid or oil phase in such stabilized dispersion is subjected to polymerization conditions and polymerized, whereby globules of polymer are obtained having spheroidal shape and remarkably uniform and desired size, which size is predetermined principally by the composition of the initial aqueous liquid suspending medium.

The diameter of the droplets of polymerizable liquid, and hence the diameter of the beads of polymer, can be varied predictably, by deliberate variation of the composition of the aqueous liquid dispersion, within the range of from about one-half of a micron or less to about 0.5 centimeter. For any specific operation, the range of diameters of the droplets of liquid, and hence of polymer beads, has a factor in the order of three or less as contrasted to factors of 10 or more for diameters of droplets and beads prepared by usual suspension polymerization methods employing critical agitation procedures. Since the bead size, e.g., diameter, in the present method is determined principally by the composition of the aqueous dispersion, the mechanical conditions, such as the degree of agitation, the size and design of the apparatus used, and the scale of operation, are not highly critical. Furthermore, by employing the same composition, the operations can be repeated, or the scale of operations can be changed, and substantially the same results can be obtained.

The present method is carried out by dispersing one part by volume of a polymerizable liquid into at least 0.5, preferably from 0.5 to about 10 or more, parts by volume of a nonsolvent aqueous medium comprising water and at least the first of the following ingredients:

1. A water-dispersible, water-insoluble solid colloid, the particles of which, in aqueous dispersion, have dimensions in the order of from about 0.008 to about 50 microns, which particles tend to gather at the liquid-liquid interface or are caused to do so by the presence of

2. A water-soluble "promotor" that affects the "hydrophilic-hydrophobic balance" of the solid colloid particles; and/or

3. An electrolyte; and/or

4. Colloid-active modifiers such as peptizing agents, surface-active agents and the like; and, usually,

5. A water-soluble, monomer-insoluble inhibitor of polymerization.

The water-dispersible, water-insoluble solid colloids can be inorganic materials such as metal salts or hydroxides or clays, or can be organic materials such as raw starches, sulfonated cross-linked organic high polymers, resinous polymers and the like.

The solid colloidal material must be insoluble but dispersible in water and both insoluble and nondispersible in, but wettable by, the polymerizable liquid. The solid colloids must be much more hydrophilic than oleophilic so as to remain dispersed wholly within the aqueous liquid. The solid colloids employed for limited coalescence are ones having particles that, in the aqueous liquid, retain a relatively rigid and discrete shape and size within the limits stated. The particles may be greatly swollen and extensively hydrated, provided that the swollen particle retains a definite shape, in which case the effective size is approximately that of the swollen particle. The particles can be essentially single molecules, as in the case of extremely high molecular weight cross-linked resins, or can be aggregates of many molecules. Materials that disperse in water to form true or colloidal solutions in which the particles have a size below the range stated or in which the particles are so diffuse as to lack a discernible shape and dimension are not suitable as stabilizers for limited coalescence. The amount of solid colloid that is employed is usually such as corresponds to from about 0.01 to about 10 or more grams per 100 cubic centimeters of the polymerizable liquid.

In order to function as a stabilizer for the limited coalescence of the polymerizable liquid droplets, it is essential that the solid colloid must tend to collect with the aqueous liquid at the liquid-liquid interface, i.e., on the surface of the oil droplets. (The term "oil" is occasionally used herein as generic to liquids that are insoluble in water.) In many instances, it is desirable to add a "promoter" material to the aqueous composition to drive the particles of the solid colloid to the liquid-liquid interface. This phenomenon is well known in the emulsion art, and is here applied to solid colloidal particles, as a expanded of adjusting the "hydrophilic-hydrophobic balance."

Usually, the promoters are organic materials that have an affinity for the solid colloid and also for the oil droplets and that are capable of making the solid colloid more oleophilic. The affinity for the oil surface is usually due to some organic portion of the promoter molecule while affinity for the solid colloid is usually due to opposite electrical charges. For example, positively charged complex metal salts or hydroxides, such as aluminum hydroxide, can be promoted by the presence of negatively charged organic promoters such as water-soluble sulfonated polystyrenes, alignates and carboxymethylcellulose. Negatively charged colloids, such as Bentonite, are promoted by positively charged promoters such as tetramethyl ammonium hydroxide or chloride or water-soluble complex resinous amine condensation products such as the water-soluble condensation products of diethanolamine and adipic acid, the water-soluble condensation products of ethylene oxide, urea and formaldehyde, and polyethylenimine. Amphoteric materials such as proteinaceous materials like gelatin, glue, casein, albumin, glutin and the like, are effective promoters for a wide variety of colloidal solids. Nonionic materials like methoxy-cellulose are also effective in some instances. Usually, the promoter need be used only to the extent of a few parts per million of aqueous medium although larger proportions can often be tolerated. In some instances, ionic materials normally classed as emulsifiers, such as soaps, long chain sulfates and sulfonates and the long chain quaternary ammonium compounds, can also be used as promoters for the solid colloids, but care must be taken to avoid causing the formation of stable colloidal emulsions of the polymerizable liquid and the aqueous liquid medium.

An effect similar to that of organic promoters is often obtained with small amounts of electrolytes, e.g., water-soluble, ionizable alkalies, acids and salts, particularly those having polyvalent ions. These are especially useful when the excessive hydrophilic or insufficient oleophilic characteristic of the colloid is attributable to excessive hydration of the colloid structure. For example, a suitably cross-linked sulfonated polymer of styrene is tremendously swollen and hydrated in water. Although the molecular structure contains benzene rings which should confer on the colloid some affinity for the oil phase in the dispersion, the great degree of hydration causes the colloidal particles to be enveloped in a cloud of associated water. The addition of a soluble, ionizable polyvalent cationic compound, such as an aluminum or calcium salt, to the aqueous composition causes extensive shrinking of the swollen colloid with exudation of a part of the associated water and exposure of the organic portion of the colloid particle, thereby making the colloid more oleophilic.

The solid colloidal particles whose hydrophilic-hydrophobic balance is such that the particles tend to gather in the aqueous phase at the oil-water interface, gather on the surface of the oil droplets and function as protective agents in the phenomenon of limited coalescence.

Other agents that can be employed in an already known manner to effect modification of the colloidal properties of the aqueous composition are those materials known in the art as peptizing agents, flocculating and deflocculating agents, sensitizers, surface active agents and the like. It is sometimes desirable to add to the aqueous liquid a few parts per million of a water-soluble, oil-insoluble inhibitor of polymerization effective to prevent the polymerization of monomer molecules that might diffuse into the aqueous liquid or that might be absorbed by colloid micelles and that, if allowed to polymerize in the aqueous phase, would tend to make emulsion-type polymer dispersions instead of, or in addition to, the desired bead or pearl polymers.

The aqueous medium containing the water-dispersible solid colloid is then admixed with the liquid polymerizable material in such a way as to disperse the liquid polymerizable material as small droplets within the aqueous medium. This dispersion can be accomplished by any usual means, e.g., by mechanical stirrers or shakers, by pumping through jets, by impingement, or by other procedures causing subdivision of the polymerizable material into droplets in a continuous aqueous medium.

The degree of dispersion, e.g., by agitation is not critical except that the size of the dispersed liquid droplets must be no larger, and is preferably much smaller, than the stable droplet size expected and desired in the stable dispersion. When such condition has been attained, the resulting dispersion is allowed to rest with only mild, gentle movement, if any, and preferably without agitation. Under such quiescent conditions, the dispersed liquid phase undergoes a limited degree of coalescence.

"Limited coalescence" is a phenomenon wherein droplets of liquid dispersed in certain aqueous suspending media coalesce, with formation of a lesser number of larger droplets, until the growing droplets reach a certain critical and limiting size, whereupon coalescence substantially ceases. The resulting droplets of dispersed liquid, which can be as large as 0.3 and sometimes 0.5 centimeter in diameter, are quite stable as regards further coalescence and are remarkably uniform in size. If such a large droplet dispersion be vigorously agitated, the droplets are fragmented into smaller droplets. The fragmented droplets, upon quiescent standing, again coalesce to the same limited degree and form the same uniform-sized, large droplet, stable dispersion. Thus, a dispersion resulting from the limited coalescence comprises droplets of substantially uniform diameter that are stable in respect to further coalescence.

The principles underlying this phenomenon have now been adapted to cause the occurrence of limited coalescence in a deliberate and predictable manner in the preparation of dispersions of polymerizable liquids in the form of droplets of uniform and desired size.

In the phenomenon of limited coalescence, the small particles of solid colloid tend to collect with the aqueous liquid at the liquid-liquid interface, i.e., on the surface of the oil droplets. It is thought that droplets which are substantially covered by such solid colloid are stable to coalescence while droplets which are not so covered are not stable. In a given dispersion of a polymerizable liquid the total surface area of the droplets is a function of the total volume of the liquid and the diameter of the droplets. Similarly, the total surface area barely coverable by the solid colloid, e.g., in a layer one particle thick, is a function of the amount of the colloid and the dimensions of the particles thereof. In the dispersion as initially prepared, e.g., by agitation, the total surface area of the polymerizable liquid droplets is greater than can be covered by the solid colloid. Under quiescent conditions, the unstable droplets begin to coalesce. The coalescence results in a decrease in the number of oil droplets and a decrease in the total surface area thereof up to a point at which the amount of colloidal solid is barely sufficient substantially to cover the total surface of the oil droplets, whereupon coalescence substantially ceases.

If the solid colloidal particles do not have nearly identical dimensions, the average effective dimension can be estimated by statistical methods. For example, the average effective diameter of spherical particles can be computed as the square root of the average of the squares of the actual diameters of the particles in a representative sample.

It is usually beneficial to treat the uniform droplet suspension prepared as described above to render the suspension stable against congregation of the oil droplets.

This further stabilization is accomplished by gently admixing with the uniform droplet dispersion an agent capable of greatly increasing the viscosity of the aqueous liquid. For this purpose, there may be used any water-soluble or water-dispersible thickening agent that is insoluble in the oil droplets and that does not remove the layer of solid colloidal particles covering the surface of the oil droplets at the oil-water interface. Examples of suitable thickening agents are sulfonated polystyrene (water-dispersible, thickening grade), hydrophilic clays such as Bentonite, digested starch, natural gums, carboxy-substituted cellulose ethers and the like. Often the thickening agent is selected and employed in such quantities as to form a thixotropic gel in which are suspended the uniform-sized droplets of the oil. In other words, the thickened liquid generally should be non-Newtonian in its fluid behavior, i.e., of such a nature as to prevent rapid movement of the dispersed droplets within the aqueous liquid by the action of gravitational force due to the difference in density of the phases. The stress exerted on the surrounding medium by a suspended droplet is not sufficient to cause rapid movement of the droplet within such non-Newtonian media. Usually, the thickener agents are employed in such proportions relative to the aqueous liquid that the apparent viscosity of the thickened aqueous liquid is in the order of at least 500 centipoises (usually determined by means of a Brookfield viscosimeter using the No. 2 spindle at 30 r.p.m.). The thickening agent is preferably prepared as a separate concentrated aqueous composition that is then carefully blended with the oil droplet dispersion.

The resulting thickened dispersion is capable of being handled, e.g., passed through pipes, and can be subjected to polymerization conditions substantially without mechanical change in the size or shape of the dispersed oil droplets.

The resulting dispersions are particularly well suited for use in continuous polymerization procedures that can be carried out in coils, tubes and elongated vessels adapted for continuously introducing the thickened dispersions into one end and for continuously withdrawing the mass of polymer beads from the other end. The polymerization step is also practiced in batch manner.

The order of the addition of the constituents to the polymerization usually is not critical, but beneficially it is more convenient to add to a vessel the water, dispersing agent, and incorporated the oil-soluble catalyst to the monomer mixture, and subsequently add with agitation the monomer phase to the water phase.

The following is an example illustrating a procedure for preparing the cross-linked polymeric microbeads coated with slip agent. In this example, the polymer is polystyrene cross-linked with divinylbenzene. The microbeads have a coating of silica. The microbeads are prepared by a procedure in which monomer droplets containing an initiator are sized and heated to give solid polymer spheres of the same size as the monomer droplets. A water phase is prepared by combining 7 liters of distilled water, 1.5 g potassium dichromate (polymerization inhibitor for the aqueous phase), 250 g polymethylaminoethanol adipate (promoter), and 350 g LUDOX (a colloidal suspension containing 50% silica sold by DuPont). A monomer phase is prepared by combining 3317 g styrene, 1421 g divinylbenzene (55% active crosslinking agent; other 45% is ethyl vinyl benzene which forms part of the styrene polymer chain) and 45 g VAZO 52 (a monomer-soluble initiator sold by DuPont). The mixture is passed through a homogenizer to obtain 5 micron droplets. The suspension is heated overnight at 52.degree. C. to give 4.3 kg of generally spherical microbeads having an average diameter of about 5 microns with narrow size distribution (about 2-10 microns size distribution). The mol proportion of styrene and ethyl vinyl benzene to divinylbenzene is about 6.1%. The concentration of divinylbenzene can be adjusted up or down to result in about 2.5-50% (preferably 10-40%) crosslinking by the active cross-linker. Of course, monomers other than styrene and divinylbenzene can be used in similar suspension polymerization processes known in the art. Also, other initiators and promoters may be used as known in the art. Also, slip agents other than silica may also be used. For example, a number of LUDOX colloidal silicas are available from DuPont. LEPANDIN colloidal alumina is available from Degussa. NALCOAG colloidal silicas are available from Nalco and tin oxide and titanium oxide are also available from Nalco. Normally, for the polymer to have suitable physical properties such as resiliency, the polymer is crosslinked. In the case of styrene crosslinked with divinylbenzene, the polymer is about 2.5-50% cross-linked, preferably about 20-40% cross-linked. By percent cross-linked, it is meant the mol % of crosslinking agent based on the amount of primary monomer. Such limited crosslinking produces microbeads which are sufficiently coherent to remain intact during orientation of the continuous polymer. Beads of such crosslinking are also resilient, so that when they are deformed flattened) during orientation by pressure from the matrix polymer on opposite sides of the microbeads, they subsequently resume their normal spherical shape to produce the largest possible voids around the microbeads to thereby produce articles with less density.

The microbeads are referred to herein as having a coating of a "slip agent". By this term it is meant that the friction at the surface of the microbeads is greatly reduced. Actually, it is believed this is caused by the silica acting as miniature ball bearings at the surface. Slip agent may be formed on the surface of the microbeads during their formation by including it in the suspension polymerization mix.

Microbead size is regulated by the ratio of silica to monomer. For example, the following ratios produce the indicated size microbead:

The microbeads of cross-linked polymer range in size from about 0.1-50 microns, and are present in an amount of about 5-50% by weight based on the weight of the polyester. Microbeads of polystyrene should have a Tg of at least 20.degree. C. higher than the Tg of the continuous matrix polymer and are hard compared to the continuous matrix polymer.

Elasticity and resiliency of the microbeads generally results in increased voiding, and it is preferred to have the Tg of the microbeads as high above that of the matrix polymer as possible to avoid $ deformation during orientation. It is not believed that there is a practical advantage to cross-linking above the point of resiliency and elasticity of the microbeads.

The microbeads of cross-linked polymer are at least partially bordered by voids. The void space in the supports should occupy about 2-60%, preferably about 30-50%, by volume of the shaped article. Depending on the manner in which the supports are made, the voids may completely encircle the microbeads, e.g., a void may be in the shape of a doughnut (or flattened doughnut) encircling a micro-bead, or the voids may only partially border the microbeads, e.g., a pair of voids may border a microbead on opposite sides.

The invention does not require but permits the use or addition of a plurality of organic and inorganic materials such as fillers, pigments, antiblocks, anti-stats, plasticizers, dyes, stabilizers, nucleating agents, optical brighteners, etc. These materials may be incorporated into the matrix phase, into the dispersed phases, or may exist as separate dispersed phases.

During stretching the voids assume characteristic shapes from the balanced biaxial orientation of paperlike films to the uniaxial orientation of microvoided/satinlike fibers. Balanced microvoids are largely circular in the plane of orientation while fiber microvoids are elongated in the direction of the fiber axis. The size of the microvoids and the ultimate physical properties depend upon the degree and balance of the orientation, temperature and rate of stretching, crystallization kinetics, the size distribution of the microbeads, and the like.

The supports according to this invention are prepared by:

(a) forming a mixture of molten continuous matrixpolymer and cross-linked polymer wherein the cross-linked polymer is a multiplicity of microbeads uniformly dispersed throughout the matrix polymer, the matrix polymer being as described hereinbefore, the cross-linked polymer microbeads being as described hereinbefore,

(b) forming a shaped article from the mixture by extrusion, casting or molding,

(c) orienting the article by stretching to form microbeads of cross-linked polymer uniformly distributed throughout the article and voids at least partially bordering the microbeads on sides thereof in the direction, or directions of orientation.

The mixture may be formed by forming a melt of the matrix polymer and mixing therein the cross-linked polymer. The cross-linked polymer may be in the form of solid or semi-solid microbeads. Due to the incompatibility between the matrix polymer and cross-linked polymer, there is no attraction or adhesion between them, and they become uniformly dispersed in the matrix polymer upon mixing.

When the microbeads have become uniformly dispersed in the matrix polymer, a shaped article is formed by processes such as extrusion, casting or molding. Examples of extrusion or casting would be extruding or casting a film or sheet, and an example of molding would be injection or reheat blow-molding a bottle. Such forming methods are well known in the art. If sheets or film material are cast or extruded, it is important that such article be oriented by stretching, at least in one direction. Methods of unilaterally or bilaterally orienting sheet or film material are well known in the art. Basically, such methods comprise stretching the sheet or film at least in the machine or longitudinal direction after it is cast or extruded an amount of about 1.5-10 times its original dimension. Such sheet or film may also be stretched in the transverse or cross-machine direction by apparatus and methods well known in the art, in amounts of generally 1.5-10 (usually 3-4 for polyesters and 6-10 for polypropylene) times the original dimension. Such apparatus and methods are well known in the art and are described in such U.S. Pat. No. 3,903,234, incorporated herein by reference.

The voids, or void spaces, referred to herein surrounding the microbeads are formed as the continuous matrix polymer is stretched at a temperature above the Tg of the matrix polymer. The microbeads of cross-linked polymer are relatively hard compared to the continuous matrix polymer. Also, due to the incompatibility and immiscibility between the microbead and the matrix polymer, the continuous matrix polymer slides over the microbeads as it is stretched, causing voids to be formed at the sides in the direction or directions of stretch, which voids elongate as the matrix polymer continues to be stretched. Thus, the final size and shape of the voids depends on the direction(s) and amount of Stretching. If stretching is only in one direction, microvoids will form at the sides of the microbeads in the direction of stretching. If stretching is in two direction (bidirectional stretching), in effect such stretching has vector components extending radially from any given position to result in a doughnut-shaped void surrounding each microbead.

The preferred preform stretching operation simultaneously opens the microvoids and orients the matrix material. The final product properties depend on and can be controlled by stretching time-temperature relationships and on the type and degree of stretch. For maximum opacity and texture, the stretching is done just above the glass transition temperature of the matrix polymer. When stretching is done in the neighborhood of the higher glass transition temperature, both phases may stretch together and opacity decreases. In the former case, the materials are pulled apart, a mechanical anticompatibilization process. Two examples are high-speed melt spinning of fibers and melt blowing of fibers and films to form non-woven/spun-bonded products. In summary, the scope of this invention includes the complete range of forming operations just described.

In general, void formation occurs independent of, and does not require, crystalline orientation of the matrix polymer. Opaque, microvoided films have been made in accordance with the methods of this invention using completely amorphous, non-crystallizing copolyesters as the matrix phase. Crystallizable/orientable (strain hardening) matrix materials are preferred for some properties like tensile strength and barrier. On the other hand, amorphous matrix materials have special utility in other areas like tear resistance and heat sealability. The specific matrix composition can be tailored to meet many product needs. The complete range from crystalline to amorphous matrix polymer is part of the invention.

Other ingredients are often added such as surfactants, emulsifiers, pigments, and the like during the preparation of such microbeads. Due to the nature of these additives, they tend to remain on the surfaces of the microbeads. In other words, they tend to accumulate at the interface between the polymer and the immiscible medium in which the suspension polymerization is carried out. However, due to the nature of such processes, some of these materials can remain within the core of the beads and some in the immiscible medium. For example, processing and formulating may be done to entrap ingredients within the beads. In other cases, the goal may be to concentrate ingredients on the surface of the beads. It is this highly diverse and very controllable set of bead properties that adds to the uniqueness of this invention.

Examples 1-36 which follow describe in considerable detail biaxially oriented films made with polyethylene terephthalate as the matrix polymer. These examples are submitted for a better understanding of the invention. For the examples involving cross-linked microbeads, the preparation steps are as follows:

(1) The microbeads are prepared by conventional aqueous suspension polymerization to give nearly monodisperse bead diameters from 2 to 20 microns and at levels of cross-linking from 5 mol % to 30 mol %. Almost all of these examples employ coated microbeads, with the coating thickness being about 50-100 nm.

(2) After separation and drying, the microbeads are compounded on conventional twin-screw extrusion equipment into the orientable polymer to a level of 25% by weight and pelletized to form a concentrate, suitable for let-down to lower loadings.

(3) The microbead concentrate pellets are mixed with virgin pellets and dried using standard conditions for polyethylene terephthalate, 170-180.degree. C. convection with desiccated air for 4-6 hours.

(4) The dried blends are extruded on conventional single-screw extruders at melt temperatures at about 265.degree.-280.degree. C., standard conditions for the polyethylene terephthalate used.

(5) Films are cast through a standard coat hanger slit die onto a chill roll controlled to a temperature about 50.degree.-60.degree. C. yielding films ranging from 20 to 60 mils (508-1524 microns) thick.

(6) The films are stretched biaxially and simultaneously using a standard laboratory film stretching unit. Unless otherwise specified all samples are stretched at 105.degree. C.

Examples involving cellulose acetate microbeads are included for comparative purposes. The preparation procedure is as follows:

(1) The polyethylene terephthalate pellets are ground through a 2 mm screen and dry-blended with the cellulose acetate powder.

(2) The blends are pan dried in a vacuum oven with dry nitrogen bleed at about 125.degree.-150.degree. C. for 16 hours.

(3) The dried blends are simultaneously extruded and compounded on conventional single-screw extruders using a standard Maddock mixing section in the metering region of the screw. Melt temperatures are kept as low as possible, about 260.degree.-270.degree. C., to minimize thermal degradation of the cellulose acetate.

(4) During the extrusion, molten CA microbeads form "in situ" by a process of shear emulsification and remain uniformly dispersed due to their high immiscibility with the PET. A distribution of particle diameters is produced ranging from about 0.1-10 microns, with the average being about 1-2 microns.

(5) Films are cast through a standard coat hanger slit die onto a chill roll controlled to a temperature about 50.degree.-60.degree. C. yielding films ranging from 20 to 30 mils (508-761 microns) thick.

(6) The films are stretched biaxially and simultaneously using a standard laboratory film stretching unit. These films were stretched at 100.degree., 105.degree., and 110.degree. C.

The materials used in the examples are identified as follows:

PET--polyester having repeat units from terephthalic acid and ethylene glycol; I.V.=0.70

CA--cellulose acetate, viscosity=3.0 seconds, 11.4 poises; acetyl content 39.8%; hydroxyl content 3.5%; melting range=230.degree.-250.degree. C.; Tg=180.degree. C.; number average molecular weight=30,000 (Gel Permeation Chromatography)

PS--polystyrene cross-linked with divinylbenzene to various levels

PMMA--polymethylmethacrylate cross-linked with divinylbenzene to various levels

silica--colloidal silica, SiO.sub.2, mean particle diameter=20-40 nm

alumina--colloidal alumina, Al.sub.2 O.sub.3, mean particle diameter=20-40 nm

The following are specific examples illustrating the preparation of stretch cavitation microvoided articles suitable for use as supports for the photographic elements of this invention. The examples are organized into five groups, and the key information is summarized in a pair of tables for each group. Examples 1 through 14 contain silica-coated, cross-linked PS microbeads (Tables 1 & 2). Here variations of microbead loading, level of cross-linking, and bead diameter are described along with the effect of stretch ratio (Examples 7-10). Example 11 is included to illustrate the invention when the coating has been intentionally removed from the microbeads. The higher the cross-linking level, the more resilient or elastic the microbeads, and the efficiency of the cavitation process is increased.

Examples 15 through 18 contain silica-coated, cross-linked PMMA microbeads (Tables 3 & 4). These samples illustrate the invention with the same silica coating on microbeads of a different cross-linked polymer. The next two sets of samples repeat the demonstration with a different coating material.

Examples 19 through 22 contain alumina-coated, cross-linked PS microbeads (Tables 5 & 6); and Examples 23 through 26 contain alumina-coated, cross-linked PMMA microbeads (Tables 7 & 8).

Examples 27 through 36 (Tables 9 & 10)compare, at equall volume % loading, the performance of the silica-coated, 30% cross-linked, 5 micron PS microbeads of this invention to the performance of in situ-generated. high Tg CA microbeads. These comparisons clearly demonstrate a 20% increase in cavitating efficiency for this specific embodiment of the invention relative to the cited prior art.

Sheet material comprising a polypropylene matrix having microbeads of cross-linked polystyrene is prepared by mixing the microbeads in molten polypropylene and extruding a sheet. The sheet is oriented by stretching at 140.degree. C. in both directions. The microbeads are 5 microns in diameter and account for 15% by weight of the polypropylene. The microbeads are cross-linked with divinylbenzene and are cross-linked to different degrees, indicated with specific gravity as follows:

A very white sheet having good hand is obtained.

The following examples illustrate the invention with respect to two important orientable polyesters. Due to the higher orientation temperatures involved with these matrix polyesters, microbeads with the higher level of cross-linking (about 40 mol %, i.e., 60 mol polystyrene and 40 mol % divinylbenzene) are used.

A blend is prepared comprising 85 parts by weight poly(1,4-cyclohexylenedimethylene terephthalate) and 15 parts cross-linked polystyrene. The polyester (I.V.=0.61) is ground through a 2 mm screen, dry blended with the microbeads, vacuum-oven dried, and compounded at 290.degree. C. on a laboratory-sized co-rotating, twin-screw extruder. The pellets are then dried and extruded on a single-screw extruder to make 5.5 inch wide film 20 mils thick. The films were then biaxially oriented (3.times.3) at 115.degree. C. The resulting films showed the high degree of whiteness and opacity as well as desirable low density discussed in greater detail in the earlier examples.

In the same manner as the previous example, a blend is prepared comprising 85 parts by weight poly(ethylene naphthalate) and 15 parts cross-linked polystyrene. The polyester (I.V.=0.71) is ground through a 2 mm screen, dry blended with the microbeads, vacuum-oven dried, and compounded at 290.degree. C. on a co-rotating, twin-screw extruder. The pellets are then dried and extruded on a single-screw extruder to make 5.5 inch wide film 20 mils thick. The films are then biaxially oriented (3.times.3) at 145.degree. C. The resulting films showed the high degree of whiteness and opacity as well as desirable low density discussed in greater detail in the earlier examples. Where ratios or parts are given, e.g., 80/20, they are parts by weight, with the polyester weight specified first.

The following applies to Kubelka-Munk values:

SX is the scattering coefficient of the whole thickness of the article and is determined as follows:

wherein:

b=(a.sup.2 -1).sup.1/2

Ar ctgh is the inverse hyperbolic cotangent

a=1/2 R+(Ro-R+Rg)/RoRg

Ro is reflectance with black tile behind sheet

R is reflectance with white tile behind sheet

Rg is reflectance of a white tile=0.89

KX is the absorption coefficient of the whole thickness of the article and is determined as follows:

wherein SX and a are as defined above

R (infinity) is the reflectance of an article if the article was so-thick that additional thickness would not change it and is determined as follows:

wherein a is as defined above

Ti is the internal light transmittance and is determined as follows:

wherein Ro and Rg are as defined above.

In the above formulae, Ro, R and Rg are determined in a conventional manner using a Diano Match-Scan II Spectrophotometer (Milton Roy Co.) using a wavelength of 560 nanometers. Also above, X in the formulae SX and KX is the thickness of the article. A full description of these terms is found in Colors in "Business, Science and Industry" 3rd Edition, by Deane B. Judd & Gunter Wyszecki, published by John Wiley & Sons, N Y. (1975), pages 397-439, which is incorporated herein by reference.

Glass transition temperatures, Tg and melt temperatures, Tm, are determined using a Perkin-Elmer DSC-2 Differential Scanning Calorimeter.

Unless otherwise specified inherent viscosity is measured in a 60/40 parts by weight solution of phenol/tetrachloroethane 25.degree. C. and at a concentration of about 0.5 gram of polymer in 100 ml of the solvent.

Where acids are specified herein in the formation of the polyesters or copolyesters, it should be understood that ester forming derivatives of the acids may be used rather than the acids themselves as is conventional practice. For example, dimethyl isophthalate may be used rather than isophthalic acid.

Unless otherwise specified, all parts, ratios, percentages, etc. are by weight.

The imaging units of the photographic elements of this invention contain one or more radiation-sensitive silver halide emulsion layers. The silver halide emulsion layers can take any convenient conventional form.

In the simplest possible form the photographic elements contain a single silver halide emulsion layer. In the simplest possible form a silver halide emulsion can consist of radiation-sensitive silver halide grains and a vehicle. The silver halide grains can be chosen from among silver bromide, silver chloride, silver iodide, silver chlorobromide, silver chloroiodide, silver bromoiodide, silver chlorobromoiodide, or mixtures thereof. The vehicle can be comprised of a hydrophilic colloid peptizer, such as gelatin or a gelatin derivative.

Suitable imaging units containing one or more silver halide emulsion layers are illustrated by Research Disclosure, Vo. 308, Dec. 1989, Item 308119, the disclosure of which is here incorporated by reference. Research Disclosure is published by Kenneth Mason Publications, Ltd., Emsworth, Hampshire P010 7DD, England. The silver halide grain structures of silver halide emulsions are specifically disclosed by Section I of Item 308119. Vehicles for the emulsions are specifically illustrated by Section IX of Item 308119.

The following conventional photographic element features can be present in the imaging unit, again referring to sections of Item 308119:

Section III.

Chemical sensitizers;

Section IV.

Spectral sensitizers and desensitizers;

Section V.

Brighteners;

Section VI.

Antifoggants and stabilizers;

Section VII.

Color materials;

Section VIII.

Absorbing and scattering materials;

Section X.

Hardeners;

Section XI.

Coating Aids;

Section XII.

Plasticizers and Lubricants;

Section XVI.

Matting agents;

Section XX.

Developing agents;

Section XXI.

Development modifiers;

Section XXII.

Physical development systems;

Section XXIII.

Image transfer systems;

Section XXIV.

Dry development systems;

Section XXV.

Printing and lithography;

Section XXVI.

Printout; and

Section XXVII.

Direct-print.

In addition to the varied forms of imaging units disclosed by Item 308119, cited above, the following are additionally specifically contemplated:

(a) Imaging units containing one or more radiation-sensitive tabular grain silver halide emulsion layers, illustrated by Research Disclosure, Vol. 225, Jan. 1983, Item 22534; Abbott et al U.S. Pat. No. 4,425,426; Daubendiek et al U.S. Pat. Nos. 4,672,027 and 4,693,964; Sowinski et al U.S. Pat. No. 4,656,122; Maskasky U.S. Pat. Nos. 4,173,320 and 4,173,323; and Reeves U.S. Pat. No. 4,435,499.

(b) Element constructions specifically adapted for radiography, illustrated by Research Disclosure, Vol 184, Aug. 1979, Item 18431.

(c) Reflection color print materials, illustrated by Research Disclosure, Vol. 176, Nov. 1979, Item 18716.

The imaging units can be exposed and processed by any convenient conventional technique. Such techniques are illustrated by the items of paragraphs (a), (b), and (c), cited above.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.