Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
2000-05-05
2002-08-20
Chen, Vivian (Department: 1773)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C428S212000, C428S323000, C428S480000, C264S173160, C430S523000, C430S531000, C430S533000
Reexamination Certificate
active
06436219
ABSTRACT:
This invention relates to a polyester film, and in particular to an opaque polyester film which is suitable for use as a photographic film.
The development of instant photography has enabled the immediate production of a finished photograph. Instant photography has become a powerful media for both professional and amateur photographers alike. Instant photographs have been used in a wide range of applications, for example for identification purposes, such as passports, for industrial and business applications, and in medical and scientific fields.
In instant photography, two types of sheet are used, a photosensitive sheet, and an image-receiving or print sheet in which a positive image is formed. A reagent system is used which is released from a sealed pod, spreads to form a thin layer between the two sheets, and acts to produce both an image in the photosensitive layer(s) of the photosensitive sheet, and a final positive image in the image-receiving layer of the image-receiving sheet. A series of complementary positive and negative images may be formed, one or more of which can serve as a starting point for a transfer process resulting in the final positive image in the image-receiving layer.
There are two main types of instant photographic systems, known as peel-apart and integral type. In the peel-apart system, the photosensitive sheet and image-receiving sheet are laminated together as reagent is spread at the start of processing. The sheets are then peeled apart to terminate processing and enable viewing of the image. In the integral system, the photosensitive sheet and image-receiving sheet are permanently secured as a single unit. The image-receiving layers are located on the inner surfaces of the two sheets, at least one of which is transparent. The image is usually viewed through the transparent sheet against a reflective white pigment layer within the film unit.
In the peel-apart system, resin-coated paper has generally been used as the image-receiving sheet. The paper generally has a grey or black layer which prevents the unwanted transmission of light, and a white layer for reflecting the formed image. However, the natural fibre content of paper can result in the final image appearing grainy or of uneven colour distribution. Thus, there is a requirement for an image-receiving sheet having improved image quality. In addition, paper has relatively poor tensile properties, which means that relatively thick paper must be used, which limits the number of sheets which can be included in a single photographic film cassette for an instant camera.
Polyester film has been used in a wide range of photographic applications. White polyester film has been used for producing traditional photographic prints. Unfortunately, existing white polyester films are of relatively low opacity such that some light can pass through the film. The opacity of a film can be increased by increasing the concentration of the whitening or opacifying agent, ie filler, present in the film, or by increasing the thickness of the film. However, there are practical limits on how much filler can be incorporated into a film, and increasing film thickness will correspondingly increase the cost of the film and may make it unsuitable for the particular intended application.
We have now devised a polyester film which reduces or overcomes at least one of the aforementioned problems.
Accordingly, the present invention provides a polyester film comprising an opaque first layer having an optical density greater than 2.0, and a white second layer.
The invention also provides a method of producing a polyester film which comprises forming an opaque first layer having an optical density greater than 2.0, and applying on one surface thereof, a white second layer.
The invention further provides a use of a polyester film comprising an opaque first layer having an optical density greater than 2.0, and a white second layer, in instant photography.
The invention still further provides a photographic sheet comprising a polyester film comprising an opaque first layer having an optical density greater than 2.0, and a white second layer.
The opaque first layer and/or white second layer of a polyester film according to the invention may be formed from any film-forming polyester material, such as a synthetic linear polyester which may be obtained by condensing one or more dicarboxylic acids or their lower alkyl (up to 6 carbon atoms) diesters, eg terephthalic acid, isophthalic acid, phthalic acid, 2,5-, 2,6- or 2,7-naphthalenedicarboxylic acid, succinic acid, sebacic acid, adipic acid, azelaic acid, 4,4′-diphenyidicarboxylic acid, hexahydro-terephthalic acid or 1,2-bis-p-carboxyphenoxyethane (optionally with a monocarboxylic acid, such as pivalic acid) with one or more glycols, particularly an aliphatic glycol, eg ethylene glycol, 1,3-propanediol, 1,4-butanediol, neopentyl glycol and 1,4-cyclohexanedimethanol. A polyethylene terephthalate or polyethylene naphthalate film is preferred. A polyethylene terephthalate film is particularly preferred, especially such a film which has been biaxially oriented by sequential stretching in two mutually perpendicular directions, typically at a temperature in the range from 70 to 125° C., and preferably heat set, typically at a temperature in the range from 150 to 250° C., for example as described in GB-A-838,708.
In a preferred embodiment of the invention the opaque first layer and white second layer of the polyester film comprise the same polyester, more preferably polyethylene terephthalate or polyethylene naphthalate, and particularly polyethylene terephthalate. It is also preferred that the first layer and/or second layer comprise crystalline and/or semi-crystalline polyester material.
The opaque first layer and/or white second layer of a polyester film according to the invention may be uniaxially oriented, but are preferably biaxially oriented by drawing in two mutually perpendicular directions in the plane of the film to achieve a satisfactory combination of mechanical and physical properties. Formation of the film may be effected by any process known in the art for producing an oriented polyester film, for example a tubular or flat film process.
In a tubular process simultaneous biaxial orientation may be effected by extruding a thermoplastics polyester tube which is subsequently quenched, reheated and then expanded by internal gas pressure to induce transverse orientation, and withdrawn at a rate which will induce longitudinal orientation.
In the preferred flat film, process a film-forming polyester is extruded through a slot die and rapidly quenched upon a chilled casting drum to ensure that the polyester is quenched to the amorphous state. Orientation is then effected by stretching the quenched extrudate in at least one direction at a temperature above the glass transition temperature of the polyester. Sequential orientation may be effected by stretching a flat, quenched extrudate firstly in one direction, usually the longitudinal direction, ie the forward direction through the film stretching machine, and then in the transverse direction. Forward stretching of the extrudate is conveniently effected over a set of rotating rolls or between two pairs of nip rolls, transverse stretching then being effected in a stenter apparatus. Stretching is effected to an extent determined by the nature of the film-forming polyester, for example polyethylene terephthalate is usually stretched so that the dimension of the oriented film is from 2 to 5, more preferably 2.5 to 4.5 times its original dimension in the, or each direction of stretching.
A stretched film may be, and preferably is, dimensionally stabilised by heat-setting under dimensional restraint at a temperature above the glass transition temperature of the film-forming polyester but below the melting temperature thereof, to induce crystallisation of the polyester.
Formation of a polyester film according to the invention may be effected by conventional techniques, for example by laminating together a preformed opaque first layer, and
Francis John
Higgins David Edward
Chen Vivian
DuPont Teijin Films U.S. Limited Partner
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