U.S. Pat. No. 3,755,499 discloses a synthetic sheet for writing purposes which consists essentially of a linear polyester selected from the group consisting of polyethylene terephthalate, polyethylene isophthalate, and copolymers of ethylene terephthalate and ethylene isophthalate, and a high polymer having a higher glass transition point than that of said linear polyester at a mixing ration of from 7 to 35% by weight of the latter with respect to the polymer mixture, which high polymer is selected from the group consisting of a polymethylmethacrylate, a copolymer of acrylonitrile an styrene, a copolymer of acrylonitrile, butadiene and styrene, said synthetic sheet having a very finely coarsened surface due to said high polymer which is uniformly dispersed in said linear polyester to constitute the nuclei for the irregular surface thereof. Both simultaneous and sequential stretching of these mixed polymer materials are disclosed usually at 85 to 95° C. with stretching ratios from 2 to 3.5 times the original length, the sheet being adjusted for writability and opacity in conformity with its eventual use. The object of the invention of U.S. Pat. No. 3,755,499 is stated to be the provision of a synthetic sheet for writing and similar other purposes having improved surface condition, opacity, and other requisite properties. U.S. Pat. No. 3,755,499 further discloses that the thermoplastic resin to be mixed may or may not have compatibility with the linear polyester, provided that it can be substantially uniformly mixed with and dispersed in the linear polyester at the time of forming, that the formed film, regardless of whether it is transparent or not, may produce a uniform mat surface upon being stretched and the film thus obtained is heat-shrinkable, acceptable in its writing properties, and possesses adequate opacity and that in order to further improve stability in the film size at a high temperature, it may be heat-treated at a temperature above the stretching temperature of the linear polyester and below the melting point of both mixing thermoplastic resin and the linear polyester. U.S. Pat. No. 3,755,499 fails to disclose the influence of addition of an inorganic opacifying pigment or the simultaneous addition of poly(4-methyl-1-pentene).
U.S. Pat. No. 4,128,689 discloses a process for preparing thermoplastic sheets or webs, which process comprises the steps of: (i) extruding a foamable thermoplastic polymer mixture through the die of a screw extruder to produce a foamed extrudate in sheet or web form, the foamable thermoplastic polymer mixture containing at least a first and a second thermoplastic polymer, the first thermoplastic polymer being substantially crystalline and having a higher melting point than, and being substantially immiscible with, the second thermoplastic polymer, and the temperature of extrusion being equal to or greater than the melting point of the first thermoplastic polymer; (ii) stretching the foamed extrudate from step (i) in the direction of extrusion as it leaves the die to rupture most of the cells of the foamed extrudate and to elongate the walls of the collapsed cells in the direction of stretch; (iii) compressing the stretched extrudate from step (ii) while it remains plastic; and (iv) cooling and foamed, stretched and compressed extrudate from step (iii). Furthermore, U.S. Pat. No. 4,128,689 discloses that the first thermoplastic polymer is preferably selected from high density polyethylene, polypropylene, polybutene-1, poly 4-methylpentene-1, polyethylene terephthalate, nylon 6, nylon 66 and nylon 11 and the second thermoplastic polymer is preferably a non-crystalline thermoplastic polymer preferably selected from cellulose acetate, cellulose propionate, cellulose acetate butyrate, ethyl cellulose, polystyrene, styrene-acrylonitrile copolymers, polycarbonates, styrene and methyl styrene copolymers and phenylene oxide polymers.
U.S. Pat. No. 4,243,769 discloses a method for providing a grossly homogeneous, permanently miscible mixture of polymers which has properties not evident in a simple blend of the polymers and which does not separate spontaneously into the component polymers, which comprises uniformly mixing (a) a polymer component containing a nitrile functionality with (b) a polymer component containing hydroxyl or esterified hydroxyl functional groups condensable with nitrites, said polymer components (a) and (b) tending to spontaneously separate from a simple blend thereof, in the presence of from about 0.001 to 8 percent by weight of the mixture of polymers and acid of an acid compatibilizing agent and for a period sufficient to provide the aforesaid permanently miscible mixture of polymers which, at ambient temperature, is in the form of a solid.
EP 0 436 178A2 discloses a polymeric shaped article characterized in that said article is comprised of a continuous oriented polymer matrix having dispersed therein microbeads of a cross-linked polymer which are at least partially bordered by void space, said microbeads being present in an amount of 5-50% by weight based on the weight of said oriented polymer, said void space occupying 2-60% by volume of said article.
EP-A 0 654 503 discloses a shaped article prepared from a polymer blend of 50 to 97 wt. % of a linear polyester and 3 to 50 wt. % of a polymer containing styrene e.g. a graft polymer of acrylonitrile, butadiene and styrene (ABS), a styrene-acrylonitrile copolymer or a high impact polystyrene (HIPS), wherein the percentages relate to the sum of the polyester and the polymer containing styrene. EP-A 0 654 503 further discloses that the support material according to the invention may contain further additives, for example pigments, in particular TiO2, BaSO4, CaCO3, optical whiteners or blue dyes, which further increase covering power and improve sharpness, in particular 0.5 to 10 wt. %, related to the total weight of the constituent used, preferably 2 to 10, preferably 3.5 to 6.5 wt. % of TiO2 pigment, preferably of the anatase type, are added.
U.S. Pat. No. 6,096,684 discloses A porous polyester film having an apparent specific gravity of less than 1.3, which comprises a polyester film having many voids [layer (A)] comprising a polyester and a thermoplastic resin incompatible with said polyester, and a polyester film having many fine voids [layer (B)] comprising a polyester and inorganic fine particles having an average particle size of less than 1 μm, the two layers being bonded to each other, wherein the layer (B) has a porosity of not less than 20% by volume, and a thickness of 1-20 μm that is less than 30% of the thickness of the porous film. U.S. Pat. No. 6,096,684 further discloses that preferably the porous polyester film wherein the thermoplastic resin incompatible with polyester in layer (A) includes polystyrene resin, polymethylpentene resin and polypropylene resin, and a polystyrene resin content (X wt %), a polymethylpentene resin content (Y wt %) and a polypropylene resin content (Z wt %) satisfy the following equations: 0.01≦X/(Y+Z)≦1; Z/Y≦1; 5≦X+Y+Z≦30.
U.S. Pat. No. 6,703,193 discloses an image recording element comprising a microvoided layer comprising a continuous phase polyester matrix having dispersed therein crosslinked organic microbeads and non-crosslinked polymer particles that are immiscible with the polyester matrix of said microvoided layer. U.S. Pat. No. 6,703,193 further discloses that if only non-crosslinked polymer particles that are immiscible with the polyester matrix are used in the microvoided layer of a silver halide display media the raw material and manufacturing cost is low, as a compounding step is not required, but the image sharpness is very poor due to the relatively large voids that result. Thus although the use of immiscible polymer particles as voiding agents in imaging median is attractive from a cost standpoint, the quality with respect to sharpness is prohibitively inferior. U.S. Pat. No. 6,703,193 also discloses that it has been unexpectedly discovered that by mixing both the crosslinked organic microbeads and the non-crosslinked polymer particles that are immiscible with polyester into the polyester matrix of the microvoided layer the deficiencies of the void initiators when used singularly are synergistically overcome, especially with respect to image quality and manufacturability. The combination of crosslinked organic beads and non-crosslinked polymer particles immiscible in a polyester matrix enjoys the quality, with respect to sharpness of microbead-voided media, without the expected degradation associated with the addition of a material with poor sharpness quality, with significant cost reductions and manufacturing time and effort reductions resulting from the need to use less costly raw material which in turn lowers the time and effort needed to compound microbeads with matrix polymer. U.S. Pat. No. 6,703,193 also discloses that the voided layer may contain white pigments which are known to improve the photographic responses such as whiteness or sharpness such as titanium dioxide, barium sulphate, clay, calcium carbonate or silica; and that addenda may be added to the layers to change the color of the imaging element. U.S. Pat. No. 6,703,193 fails to disclose the influence of image-wise heating on the opaque microvoided films disclosed therein.
The prior art non-transparent microvoided biaxially stretched film has suffered from insufficient opacity together with a lack of dimensional stability or sufficient dimensional stability and insufficient opacity or sufficient opacity and poor mechanical properties such as tear strength after longitudinal stretching and after biaxial stretching. Moreover, for particular applications the whiteness of the non-transparent microvoided biaxially stretched film was insufficient.