Releasing film

A releasing film comprising: PA1 a base film produced from a polyethylene naphthalate, and PA1 a releasing layer formed on one or both sides of the base film, wherein the polyethylene naphthalate contains naphthalenedicarboxylic acid as the main acid component and ethylene glycol as the main glycol component and further contains a manganese compound, a phosphorus compound and an antimony compound in Mn, P and Sb amounts satisfying all of the following formulas (1) to (3): EQU 0.7.ltoreq.Mn.ltoreq.1.6 (1) EQU 0.5.ltoreq.Mn/P.ltoreq.1.2 (2) EQU 0.7.ltoreq.Sb.ltoreq.2.2 (3) (wherein Mn is moles of manganese element per 10.sup.6 g of acid component; P is moles of phosphorus element per 10.sup.6 g of acid component; and Sb is moles of antimony element per 10.sup.6 g of acid component), and the base film has a surface center line average roughness (Ra) of 2 to 50 nm.

DETAILED DESCRIPTION OF THE INVENTION
 1. Technical Field Pertinent to the Invention
 The present invention relates to a releasing film. More particularly, the
 present invention relates to a releasing film superior in thermal
 stability, surface smoothness and surface flatness, which is useful as a
 carrier film used in production of a resin sheet, a resin coating or the
 like from a resin solution, or as a carrier film used in production of a
 ceramic sheet or the like from a ceramic slurry, or as a protective film
 for adhesive layer such as adhesive tape or the like. The present
 invention further relates to a releasing film which allows uniform coating
 of even aqueous adhesive coating fluid, aqueous resin solution or aqueous
 slurry and which can be smoothly transferred in production of adhesive
 coating, resin coating or sheet, or the like.
 2. Prior Art
 Releasing films are in use as a carrier film in producing a resin sheet, a
 resin coating, a ceramic sheet or the like.
 That is, a resin sheet is produced, for example, by coating (casting), on a
 carrier film, a solution of a resin obtained by polyvinyl chloride or the
 like, removing the solvent contained in the solution, by heating, and
 peeling and removing the carrier film; and is used in applications such as
 marking sheet and the like.
 A resin coating is produced, for example, by dissolving a resin such as an
 adhesive in a solvent, coating the solution on a carrier film, and
 removing the solvent by heating.
 A ceramic sheet is produced, for example, by dispersing a ceramic powder, a
 binder, etc. in a solvent, coating the resulting slurry on a carrier film,
 removing the solvent by heating, and peeling and removing the carrier
 film.
 In recent years, production of such a resin sheet, a resin coating, a
 ceramic sheet as aforementioned using a releasing film has become
 frequent. When a ceramic sheet or a resin sheet is used in applications
 requiring high performances, such as electronic parts, optical parts and
 the like, the sheet must have a uniform thickness and high surface
 properties, all of which are superior to conventional levels. A ceramic
 sheet, when used in production of a ceramic electronic part exemplified as
 a condenser, is often laminated; for obtaining a condenser of higher
 electrostatic capacity, the ceramic sheet is becoming thinner and coming
 to be used in multilayer lamination.
 As IC packages or circuit substrates have become more complex, ceramic
 substrates are moving toward higher functions brought about by multilayer,
 higher performances, and smaller size and lighter weight, as in laminated
 condensers. In forming a multilayer circuit, throughholes are formed for
 layer-to-layer connection by wiring; therefore, accurate production of
 ceramic sheet and accurate positioning of throughholes are necessary. This
 is true also for resin substrates.
 Hence, such a resin sheet, a ceramic sheet or the like must have a uniform
 thickness, high accuracy and a smooth surface. In order to produce such a
 sheet, the facility used for coating a coating fluid such as resin
 solution, adhesive, ceramic slurry or the like is required to be of high
 accuracy type, and the carrier film used for production of the above sheet
 is also required to have surface smoothness of resin sheet after peeling,
 flatness free from sagging or curling habit, dimensional stability after
 heat treatment, and good processability in punching, cutting, etc.
 As the base film for the carrier film, there are used various films, for
 example, an olefin-based film (e.g. OPP) and a biaxially oriented
 polyethylene terephthalate (hereinafter abbreviated to PET in some cases)
 film. For example, a PET film has, in most cases, a roughened surface by
 adding a filler or the like for improved windability. Therefore, when a
 PET film containing a filler and resultantly having a rough surface is
 used as a carrier film for production of a resin sheet, a ceramic sheet or
 the like, pin holes may generate when a resin solution or the like has
 been coated on the carrier film, making it impossible to produce a uniform
 thin sheet and giving a faulty product, in some cases. When such a sheet
 is laminated, voids appear easily at the interface of two adjacent layers,
 in some cases.
 When the filler or the like is restricted strictly in the size or the
 addition amount, the base film comes to have too small a surface
 roughness. As a result, in the wind-up roll for the film, the film-to-film
 contact area becomes large, abnormal peeling, etc. arise owing to the
 blocking, and film slipperiness becomes low; further, in the step of
 producing a resin sheet or the like, the base film has a problem in
 transferability, in some cases.
 A releasing film, after a resin solution or a ceramic slurry has been
 coated thereon, is heated for removal of the solvent contained in the
 solution or the slurry. The heating temperature is, in many cases, close
 to or higher than the glass transition temperature (Tg) of the base film
 used in the releasing film. As a result, the releasing film causes
 dimensional change and thermal deformation such as wrinkles and the
 produced resin sheet or the like has thickness non-uniformity and poor
 surface smoothness and is deteriorated in quality. It is feared that with
 a shortened heating time and an increased heating temperature for improved
 productivity of the resin sheet or the like, the above quality
 deterioration appears as a bigger problem. Therefore, a base film of high
 heat resistance is desired.
 A releasing film, before a resin solution or a ceramic slurry is coated
 thereon, is often wound in a roll form; when the roll is unwound for
 coating the solution or slurry thereon, the unwound releasing film shows
 sagging or curling at times. Thereby, the resin sheet coated on the
 unwound releasing film may have inferior surface flatness. Further, the
 prepared base film has residual internal stress, depending upon the
 production conditions thereof; when a resin solution or the like is coated
 on the releasing film, the residual internal stress is rapidly relaxed,
 sagging appears locally, and the resin sheet coated has inferior surface
 flatness.
 A releasing film generally has thereon a layer of a silicone resin, a
 fluororesin or an aliphatic wax in order to allow the film to have
 releasability. A silicone resin is particularly preferred because it can
 be easily peeled when released, can be applied on the releasing film in a
 small layer thickness, and is inexpensive. The silicone resin has a small
 surface energy (the surface tension (.gamma.S) of the silicone resin is
 about 19 to 21 dyne/cm) and, therefore, almost uniform coating is possible
 when there is applied, on the silicone resin, an adhesive coating fluid or
 a resin solution which are dispersed or dissolved in an organic solvent;
 however, when there is applied, on the silicon resin, an aqueous adhesive
 coating fluid or an aqueous resin solution, the applied fluid or solution
 may be scattered in drops (a state of cissing), because water has a large
 surface tension (.gamma.L) which is about 73 dyne/cm. In order to
 alleviate this problem, there are taken a method of using a coating fluid
 (a resin solution or a slurry) of higher viscosity and a method of adding
 a surfactant or the like to a coating fluid to reduce the surface tension
 of the coating fluid. However, the method of using a coating fluid of
 higher viscosity has a problem in that the leveling in application of the
 coating fluid is difficult, the thickness of the film formed tends to be
 nonuniform, and it is difficult to obtain a resin sheet, a resin coating
 or the like in a thin layer. In case of adding a surfactant invites
 problems, for example, depending upon the kind and amount of the
 surfactant added, the sheet obtained has a low strength and it is
 impossible to obtain a sheet of stable quality.
 A resin sheet, a resin coating, a ceramic sheet, etc., which are obtained
 using a releasing film, are required to have a smaller thickness and a
 uniform and flat surface. The presence, on the surface of the releasing
 film, of foreign matter and/or large projections is not preferred because
 it gives a resin sheet having defects such as pin holes and the like.
 Therefore, the releasing film is required to have a surface condition of
 high degree.
 In applying, onto a releasing film, a coating fluid such as resin solution
 or the like, a given tension is generally applied to the releasing film in
 its transfer direction to make uniform the surface of the releasing film;
 then, the coating fluid is applied, followed by a drying step, etc. The
 method for controlling the tension applied includes a method of
 controlling the tension by a balance between the speed of unwind roll and
 the speed of wind-up roll, and a method of, after unwinding, controlling
 the tension by suction via a vacuum roll. In order to obtain a uniform and
 flat surface during application of coating fluid and drying, a method is
 particularly useful which comprises nipping the releasing film between a
 metal roll and a rubber roll and controlling the tension applied to the
 film being driven. At that time, ordinarily, the rubber roll is allowed to
 contact with the releasing layer side of the releasing film and the metal
 roll (to which mirror-surface plating of chromium is applied in many
 cases) is allowed to contact with the other side of the releasing film. If
 there is a difference in slipperiness between the rubber roll-releasing
 layer side interface and the metal roll-other side interface, the rotation
 of rolls and the follow-up action of releasing film relative to driving
 become insufficient; as a result, the releasing film comes to have surface
 flaws (e.g. scratch) or surface chipping, a coating fluid is applied with
 the chip being present on the surface of the releasing film, and the
 adhesive coating, ceramic sheet or the like obtained has defects.
 Problem to be Solved by the Invention
 The first object of the present invention is to provide a releasing film
 used for production of a resin sheet, a resin coating, a ceramic sheet or
 the like, which releasing film has a uniform and flat surface and shows
 small thermal deformation when heat-treated.
 The second object of the present invention is to provide a releasing film
 used for production of a resin sheet, a resin coating, a ceramic sheet or
 the like, which releasing film is superior in processability after
 production of said sheet or film, such as cuttability, punchability or the
 like.
 The other object of the present invention is to provide a releasing film
 which has a surface of releasing layer giving no cissing (having good
 wettability) even when an aqueous coating fluid has been coated thereon,
 which enables release (has good releasability) of an adhesive coating, a
 resin sheet, a ceramic sheet or the like, all produced thereon, at a
 proper force, which enables production of said film or sheet in a small
 thickness and a flat surface, and which is superior in transferability in
 production of said film or sheet.
 Means for Solving the Problem
 According to the study by the present inventor, the above objects of the
 present invention can be achieved by a releasing film comprising:
 a base film produced from a polyethylene naphthalate, and
 a releasing layer formed on one or both sides of the base film, wherein the
 polyethylene naphthalate contains naphthalenedicarboxylic acid as the main
 acid component and ethylene glycol as the main glycol component and
 further contains a manganese compound, a phosphorus compound and an
 antimony compound in Mn, P and Sb amounts satisfying all of the following
 formulas (1) to (3):
EQU 0.7.ltoreq.Mn.ltoreq.1.6 (1)
EQU 0.5.ltoreq.Mn/P.ltoreq.1.2 (2)
 0.7.ltoreq.Sb.ltoreq.2.2 (3)
 (wherein Mn is moles of manganese element per 10.sup.6 g of acid component;
 P is moles of phosphorus element per 10.sup.6 g of acid component; and Sb
 is moles of antimony element per 10.sup.6 g of acid component), and the
 base film has a surface center line average roughness (Ra) of 2 to 50 nm.
 The releasing film of the present invention is described below in more
 detail.
 The base film used in the releasing film of the present invention is a film
 produced from a polyethylene naphthalate, of which constituting polymer is
 a polyethylene naphthalate obtained by subjecting, to polycondensation,
 naphthalenedicarboxylic acid as a main acid component and ethylene glycol
 as a main glycol component, and is hereinafter abbreviated to "PEN" in
 some cases.
 As the naphthalenedicarboxylic acid, there can be mentioned, for example,
 2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid and
 1,5-naphthalenedicarboxylic acid. Of these, 2,6-naphthalenedicarboxylic
 acid is preferred.
 Use of a main acid component other than naphthalenedicarboxylic acid is not
 preferred because, when the polymer obtained therefrom is molded into, for
 example, a film, wound up round a roll, and unwound from the roll, the
 film has residual curling habit and no satisfactory surface smoothness.
 In the PEN polymer, other carboxylic acid components can be used as a
 comonomer in an amount of 20 mole % or less relative to the amount of
 total acid components, as long as the heat distortion resistance of
 releasing film is not impaired. Examples of the other carboxylic acid
 components are other aromatic dicarboxylic acids (e.g. terephthalic acid,
 isophthalic acid, diphenylethanedicarboxylic acid, diphenyldicarboxylic
 acid, diphenyl ether dicarboxylic acid, diphenylsulfonedicarboxylic acid,
 diphenylketonedicarboxylic acid and anthracenedicarboxylic acid),
 aliphatic dicarboxylic acids (e.g. adipic acid, succinic acid, sebacic
 acid and dodecanedicarboxylic acid), alicyclic dicarboxylic acids (e.g.
 cyclohexane-1,4-dicarboxylic acid and 1,3-adamantanedicarboxylic acid),
 and aliphatic oxyacids (e.g. hydroxybenzoic acid and
 .omega.-hydroxycaproic acid).
 As glycol components other than ethylene glycol, constituting the PEN film,
 there can be mentioned, for example, aliphatic glycols (e.g. C.sub.3-10
 polymethylene glycols such as trimethylene glycol, tetramethylene glycol,
 pentamethylene glycol, hexamethylene glycol, decamethylene glycol and the
 like; and cyclohexanedimethanol), aromatic diols (e.g. hydroquinone,
 resorcin and 2,2-bis(4-hydroxyphenyl)propane), and poly(oxy)alkylene
 glycols (e.g. polyethylene glycol and polytetramethylene glycol). The
 amount of these other glycol components is preferably 20 mole % or less
 relative to the amount of total glycol components.
 Also in the PEN polymer, glycol components other than ethylene glycol, for
 example, compounds of three or higher functionalities such as glycerine,
 pentaerythritol, trimellitic acid, pyromellitic acid and the like may be
 used as a comonomer in a very small amount as long as a substantially
 linear polymer is obtained, the heat distortion resistance of releasing
 film is not impaired, and the effects of the present invention are not
 impaired.
 Further, in the PEN polymer, part or all of the terminal hydroxyl groups
 and/or carboxyl groups may be blocked with a monofunctional compound such
 as benzoic acid, methoxypolyalkylene glycol or the like, in order to
 improve the hydrolysis resistance of PEN polymer.
 Most preferably, the PEN polymer of the present invention contains
 ethylene-2,6-naphthalate units in an amount of at least 80 mole %,
 preferably at least 90 mole % relative to the total repeating units.
 Desirably, the PEN polymer contains diethylene glycol units in a small
 proportion.
 In the PEN polymer, the content of the diethylene glycol units is
 preferably 0.4 to 3% by weight, particularly preferably 0.8 to 2% by
 weight. When the content of the diethylene glycol units is less than 0.4%
 by weight, the crystallization of polymer is not suppressed and a large
 energy is required for melting. Therefore, unmolten polymer remains in
 produced film and the film surface may have large protrusions. Meanwhile,
 when the content of the diethylene glycol units is more than 3% by weight,
 the film produced from the PEN polymer is low in strength, for example,
 Young' modulus and inferior in durability, which is not preferred.
 The PEN polymer of the present invention can be produced by a per se known
 process for polyester production. However, it is preferred to produce by
 ester interchange, that is, by reacting a lower alkyl ester of
 naphthalenedicarboxylic acid with ethylene glycol. As the lower alkyl
 ester of naphthalenedicarboxylic acid, there can be mentioned, for
 example, a dimethyl ester, a diethyl ester and a dipropyl ester. A
 dimethyl ester is particularly preferred.
 The PEN polymer used in the present invention has an inherent viscosity as
 measured at 35.degree. C. in an o-chlorophenol solution, of preferably
 0.40 to 0.90 dl/g, particularly preferably 0.50 to 0.85 dl/g. When the
 inherent viscosity is in the above range, a polyester film superior in
 heat resistance is obtained easily and the extrudability and film
 formability of molten PEN polymer are good.
 To the PEN polymer of the present invention can be added inorganic or
 organic fine particles having an average particle diameter of 0.01 to 20
 .mu.m, preferably 0.1 to 5 .mu.m as long as the effects of the PEN polymer
 are not adversely affected, in order to allow the film produced therefrom,
 to have slipperiness and good windability.
 Such fine particles are added preferably in an amount of, for example,
 0.001 to 10% by weight so that the base film can have a surface center
 line average roughness (Ra) of 2 to 50 nm, preferably 6 to 30 nm. A
 particularly preferably amount of the fine particles is 0.01 to 3% by
 weight.
 As preferred specific examples of the fine particles, there can be
 mentioned inorganic fine particles of silicon dioxide, anhydrous silicon,
 hydrated silicon, aluminum oxide, kaolin, calcium carbonate, titanium
 oxide, aluminum silicate (including calcined product, hydrate, etc.),
 lithium benzoate, barium sulfate, double salts of these compounds
 (including hydrates), carbon black, glass powder, clay (including kaolin,
 bentonite, terra abla, etc.), and so forth; and organic fine particles of
 crosslinked acrylic resin, crosslinked polystyrene resin, melamine resin,
 crosslinked silicone resin, polyamideimide resin, etc.
 The fine particles are preferred to be inactive particles so that neither
 interaction nor resultant property change takes place and the effects of
 the present invention can be maintained. The fine particles may consist of
 one kind or two or more kinds. The PEN polymer, as compared with PET
 polymer, has a rigid molecular chain and, when made into a film, has high
 stiffness. Therefore, the PEN polymer gives good slipperiness and shows
 sufficient windability even when the fine particles are added in a smaller
 amount than in PET polymer.
 To add the fine particles to the PEN polymer, there may also be used a
 method of forming, on at least one side of the base film, a thin layer
 containing the fine particles. The layer formation can be made, for
 example, by forming such a layer at the time of film formation, or by
 co-extrusion using, for example, a plurality of extruders or a feed block
 and a multi-manifold die. To the PEN polymer can be added, besides the
 fine particles, additives such as stabilizer, ultraviolet absorber,
 coloring agent, flame retardant, antistatic agent, light stabilizer,
 antioxidant and the like, as long as the surface flatness and thermal
 stability of film are not impaired. Also, other thermoplastic resin may be
 added in a small amount (for example, 20% by weight or less, particularly
 10% by weight or less).
 In producing the PEN polymer in the present invention, a manganese
 compound, preferably a manganese compound soluble in the reaction system
 is added as an ester interchange catalyst, to naphthalenedicarboxylic
 acid, preferably a lower alkyl ester thereof and ethylene glycol, to
 conduct ester interchange.
 The manganese compound is added in an amount of 0.7 to 1.6 moles in terms
 of manganese element (Mn) amount per 10.sup.6 g of total acid components
 (the unit of the amount added is hereinafter is referred to as mole(s)
 element/ton). The amount added is preferably 1.0 to 1.5 moles element/ton.
 When the amount of manganese compound added is more than 1.6 moles
 element/ton, the residual catalyst comes out as particles and, when a film
 is produced, the film is inferior in surface flatness; as a result, the
 film has poor transparency in some cases. Further, when a releasing layer
 is formed on the film and, in particular, an addition reaction type
 silicone resin or the like is coated on the film, the above-mentioned
 particles become a catalyst poison to the curing of the silicone, which
 invites insufficient curing of the silicone in some cases. Meanwhile, when
 the addition amount is less than 0.7 mole element/ton, ester interchange
 takes place insufficiently and successive polymerization is slow, which is
 not preferred.
 There is no particular restriction as to the kind of the manganese compound
 used in the present invention. However, the compound is preferably an
 oxide, a chloride, a carbonate, a carboxylate or the like, particularly
 preferably an acetate, i.e. manganese acetate.
 In producing the PEN polymer in the present invention, a phosphorus
 compound is added when the ester interchange is substantially complete, in
 order to deactivate part of the ester interchange catalyst used.
 The molar ratio (Mn/P) of the ester interchange catalyst, i.e. the
 manganese compound to the phosphorus compound is required to be 0.5 to
 1.2, preferably 0.6 to 1.1. When the molar ratio is less than 0.5, the
 residual catalyst comes out as particles and, when a film is produced, the
 film is inferior in surface flatness. Further, when a releasing layer is
 formed on the film and, in particular, an addition reaction type silicone
 resin or the like is coated on the film, the above-mentioned particles
 become a catalyst poison to the curing of the silicone, which may invite
 insufficient curing of the silicone. Meanwhile, a molar ratio more than
 1.2 is not preferred because the activity of the manganese compound not
 sufficiently deactivated by the phosphorus compound deteriorates the
 thermal stability of PEN polymer, deteriorating the color and surface
 flatness of the film produced.
 As the phosphorus compound used in the present invention, there can be
 mentioned trimethyl phosphate, triethyl phosphate, tri-n-butyl phosphate,
 phosphoric acid, etc. Trimethyl phosphate is preferred particularly.
 In producing the PEN polymer in the present invention, the phosphorus
 compound is added and then the reaction product is subjected to
 polycondensation to obtain a PEN polymer.
 At that time, to the reaction product is added an antimony compound as a
 main catalyst for polycondensation. The antimony compound may be added
 before the start of the ester interchange.
 The amount of the antimony compound added is required to be 0.7 to 2.2
 moles element/ton, preferably 0.7 to 2.0 moles element/ton. An addition
 amount of less than 0.7 mole element/ton is not preferred because the
 polycondensation does not proceed sufficiently, resulting in low
 productivity. Meanwhile, an addition amount of more than 2.2 moles
 element/ton is not preferred because, when film formation is conducted for
 a long time, an antimony compound-based precipitate appears and becomes a
 flaw of the film produced and further because, when a releasing layer is
 formed on the film and, in particular, an addition reaction type silicone
 resin or the like is coated on the film, the above-mentioned precipitate
 becomes a catalyst poison to the curing of the silicone, making
 insufficient the curing of the silicone.
 The antimony compound used in the present invention is preferably an oxide,
 a chloride, a carbonate, a carboxylate or the like. An acetate, i.e.
 antimony acetate is particularly preferred because this compound, as
 compared with other compounds, generates a smaller amount of particles in
 the produced polymer and gives a film of improved transparency.
 There is no particular restriction as to the method for film formation from
 the PEN polymer, and the film formation from the PEN polymer can be
 conducted by a per se known method. Examples of the method for film
 formation from the PEN polymer are mentioned below.
 The biaxially oriented PEN film used in the present invention can be
 produced by a known method such as successive biaxial stretching method,
 simultaneous biaxial stretching method or the like. In, for example,
 successive biaxial stretching method, a PEN polymer is sufficiently dried;
 then, the polymer is melt-extruded at a temperature of Tm to
 (Tm+70).degree. C. (Tm is the melting point of the PEN polymer) to produce
 an unstretched film; successively, the unstretched film is stretched 2 to
 6 times in the film-winding direction at a temperature of (Tg-10) to
 (Tg+50).degree. C. (Tg is the glass transition temperature of the PEN
 polymer); the resulting film is stretched 2 to 6 times in a direction
 perpendicular to the above direction at a temperature of Tg to
 (Tg+50).degree. C.; the stretched film is thermoset for 5 seconds to 1
 minute at a temperature of (Tg+60) to (Tg +140).degree. C.; thereby, film
 formation is completed. The thermosetting may be conducted under tension
 or under restricted shrinkage. In the melt extrusion, electrostatic
 adhesion is used preferably. The stretching conditions in the winding
 direction and the direction perpendicular thereto are preferably selected
 so that the properties of the biaxially oriented PEN film obtained are
 approximately equal in the longitudinal direction and the transverse
 direction.
 When the temperature of thermosetting is lower than (Tg+60).degree. C., the
 film produced, when stored in a wound state, may come to have curling
 habit or may cause interlaminar delamination. Meanwhile, when the
 temperature of thermosetting is higher than (Tg+140).degree. C., the film
 produced is excessively crystallized and accordingly causes whitening, and
 is insufficient in transparency and mechanical strengths.
 Also in simultaneous biaxial stretching, there can be used the same
 stretching temperature, draw ratio and thermosetting temperature, etc. as
 in the above successive biaxial stretching.
 The base film preferably has an elastic modulus of 1.0.times.10.sup.11 to
 1.7.times.10.sup.11 dyne/cm.sup.2 in both the film-winding direction and a
 direction perpendicular thereto. When the elastic modulus is smaller than
 1.0.times.10.sup.11 dyne/cm.sup.2, fin, warpage, etc. tend to appear in
 punching such as cutting or the like. When the elastic modulus is larger
 than 1.7.times.10.sup.11 dyne/cm.sup.2, cutting proceeds more than
 required and no intended cutting is attained.
 The base film of the present invention preferably has a density of 1.360 to
 1.370 g/cm.sup.3. When the film has a density of smaller than 1.360
 g/cm.sup.3, the molecules oriented by stretching are not crystallized
 sufficiently, the film is inferior in dimensional stability, and, when
 treated at 200.degree. C. for 10 minutes in free length, the film shows a
 thermal shrinkage as absolute value of larger than 1.0% in at least one
 direction. When the film has a density of larger than 1.370 g/cm.sup.3,
 the film is fragile owing to the crystallization and is inferior in impact
 resistance.
 Further, the base film of the present invention has a surface center line
 average roughness (Ra) of preferably 2 to 50 nm, particularly preferably 6
 to 30 nm. When the surface center line average roughness (Ra) is less than
 2 nm, the film has inferior slipperiness and poor windability. When the
 surface center line average roughness (Ra) is more than 50 nm, the film
 has too large a surface roughness and, when the film is used as a
 releasing film, the resin sheet or the like formed thereon by casting has
 a rough surface and is unable to have a smooth surface.
 The base film used in the present invention has no particular restriction
 as to the thickness. However, the thickness is preferably 5 to 500 .mu.m,
 particularly preferably 10 to 200 .mu.m.
 It is advantageous that the base film used in the present invention has a
 refractive index of 1.49 to 1.53, preferably 1.495 to 1.520 in the
 thickness direction. When the refractive index in the thickness direction
 is smaller than 1.49, the film may cause delamination or may have inferior
 flexing resistance, which tends to cause fluffing, etc. in processing such
 as cutting, punching, perforation and the like. When the refractive index
 in the thickness direction is larger than 1.53, the film has high
 thickness non-uniformity and tends to generate surface wrinkles.
 It is desirable that the base film used in the present invention has a
 plane orientation coefficient (NS) of 0.23 to 0.27. When the plane
 orientation coefficient is smaller than 0.23, the film has poor thickness
 non-uniformity, and the resin or ceramic sheet formed thereon by coating a
 resin solution, a ceramic slurry or the like may have thickness
 non-uniformity. When the plane orientation coefficient is larger than
 0.27, the film easily causes delamination, the scars generated at the
 surface or edge by scratching tend to become uneven scars, and the
 delaminated parts and the scars look whitish and conspicuous.
 The base film used in the present invention preferably has a sum of the
 tearing propagation resistances in longitudinal direction (film-winding
 direction) and in direction perpendicular thereto, of 4 N/mm or less. A
 sum of the tearing propagation resistances, of larger than 4 N/mm is not
 preferred because the punchability of film is low. A sum of the tearing
 propagation resistances, of 4 N/mm or less can be obtained by setting the
 production conditions of film, particularly the draw ratio of film at a
 relatively high level in the above-mentioned range.
 The releasing film of the present invention preferably has a dimensional
 change as absolute value of 0.3% or less in both of the longitudinal
 direction of base film (film-winding direction) and a direction
 perpendicular thereto, at 120.degree. C. under a stress of 150
 gf/mm.sup.2. When the dimensional change as absolute value is larger than
 0.3%, the releasing film is deformed when a ceramic slurry or a resin
 solution is coated thereon and the solvent in the slurry or the solution
 is removed by heating, and a ceramic or resin sheet having a flat surface
 is not be obtained in some cases.
 In the present invention, on at least one side of the base film is formed a
 releasing layer. As the component constituting the releasing layer, there
 can be mentioned a silicone resin, a fluororesin and an aliphatic wax. The
 releasing layer has a thickness of 0.02 to 50 .mu.m, preferably 0.04 to 10
 .mu.m.
 To the component constituting the releasing layer can be added various
 known additives as long as the objects of the present invention are not
 hindered. As the additives, there can be mentioned, for example, an
 ultraviolet absorber, a pigment, an antifoaming agent and an antistatic
 agent.
 Formation of the releasing layer can be conducted by coating, on a base
 film, a solution containing the components to constitute the releasing
 layer, followed by heating and drying to form a coating film. The heating
 conditions are preferably 80 to 160.degree. C. for 10 to 120 seconds,
 particularly preferably 120 to 150.degree. C. for 20 to 60 seconds. The
 coating can be conducted by a known method, and preferred are, for
 example, roll coater coating and blade coater coating.
 In the releasing film of the present invention, it is preferred that an
 adhesive layer is formed between the base film and the releasing layer in
 order to obtain higher adhesivity. As the component constituting the
 adhesive layer, a silane coupling agent is preferred when the releasing
 layer is, for example, a silicone resin layer. The silane coupling agent
 is preferably one represented by a general formula of Y--Si--X.sub.3.
 Here, Y is a functional group represented by amino group, epoxy group,
 vinyl group, methacrylic group, mercapto group or the like; and X is a
 hydrolyzable functional group represented by alkoxy group. The adhesive
 layer has a thickness of preferably 0.01 to 5 .mu.m, particularly
 preferably 0.02 to 2 .mu.m. When the thickness of the adhesive layer is in
 the above range, the adhesivity between the base film and the releasing
 layer is good and the base film on which the adhesive layer has been
 formed hardly causes blocking; therefore, the releasing film has
 substantially no handling problem.
 A study by the present inventors revealed that when, in a releasing film
 produced by forming a releasing layer on one or both sides of a base film,
 there is no large difference between the dynamic friction coefficient of
 the releasing layer side of the releasing film to a rubber surface and the
 dynamic friction coefficient of the other side of the releasing film to a
 metal surface, that is, when the difference of the above two dynamic
 friction coefficients is in a certain range, the releasing film is
 superior in slipperiness and processability, productions of various sheets
 can be smoothly transferred thereon, thereby making possible production of
 a sheet having a smooth surface.
 When the releasing film of the present invention has a releasing layer at
 both sides, it is advantageous that the dynamic friction coefficient
 (.mu.dR) of one releasing layer side of the releasing film to a rubber
 surface and the dynamic friction coefficient (.mu.dM) of the other
 releasing layer side of the releasing film to a metal surface satisfy the
 following formula (4):
EQU -0.5.ltoreq.(.mu.dR-.mu.dM).ltoreq.0.5 (4)
 When the releasing film of the present invention has a releasing layer at
 one side, it is advantageous that the dynamic friction coefficient
 (.mu.dR) of the releasing layer side of the releasing film to a rubber
 surface and the dynamic friction coefficient (.mu.dM) of the other side of
 the releasing film to a metal surface satisfy the following formula (4):
EQU -0.5.ltoreq.(.mu.dR-.mu.dM).ltoreq.0.5 (4)
 The dynamic friction coefficients to a rubber surface and a metal surface
 are, as described later, the dynamic friction coefficients of the present
 releasing film to a rubber surface and a metal surface, each having a
 particular surface and made of a particular material, and indicate the
 slipperiness of the releasing film. Specifically, the formula (4)
 indicates that when the releasing film is transferred while being nipped
 between a rubber roll and a metal roll, the dynamic friction coefficients
 of the releasing film at the rubber surface and the metal surface have a
 small difference between them. That is, the formula (4) indicates that the
 difference of the two dynamic friction coefficients (.mu.dR-.mu.dM) is in
 a range of -0.5 to +0.5. The difference is more preferably in a range of
 -0.4 to +0.4.
 A study by the present invention revealed that, in order to achieve the
 above dynamic friction coefficient at the releasing layer surface of the
 releasing film, it is desirable to use a silicone resin as the component
 constituting the releasing layer and further to mix, into the silicone
 resin, a particular proportion of a melamine resin and an alkyd resin
 and/or an acrylic resin.
 That is, it is desirable that the releasing layer of the present releasing
 film is composed mainly of a resin composition comprising 1 to 50 parts by
 weight of a silicone resin on the basis of 100 parts by weight of a resin
 mixture consisting of a melamine resin and an alkyd resin and/or an
 acrylic resin.
 When the resin constituting the releasing layer consists of a silicone
 resin alone, it is known to use, as the silicone resin, a reactive
 silicone, for example, an addition reaction type or condensation reaction
 type silicone in order to reduce the transferability of silicone as low as
 possible. In the slipperiness (friction coefficient) between releasing
 layer and rubber, good slipperiness (low friction coefficient) is obtained
 when such a silicone resin has a small thickness; however, when the
 thickness exceeds a certain level, the slipperiness tends to become bad
 (high friction coefficient) suddenly. This is due to the small hardness of
 the silicone resin per se and, to improve the slipperiness, a resin of
 high hardness can be used in the releasing layer.
 When a resin solution or the like is coated on the releasing layer made of
 a silicone resin of releasing film, cissing appears at times because the
 silicone resin has a low surface energy and poor wettability. Therefore,
 it is particularly preferred that the releasing layer is made of a resin
 mixture consisting of a silicone resin with a melamine resin and an alkyd
 resin and/or an acrylic resin, because high wettability and releasability
 are achieved.
 In the resin mixture in the releasing layer of the present invention, at
 least part of the alkyd resin and the acrylic resin forms a copolymer with
 the silicone resin when they are blended, and the melamine resin has a
 crosslinking reaction mainly with the alkyd resin to give a hard releasing
 layer.
 The alkyd resin is obtained by modifying a condensation product between a
 polybasic acid exemplified by a phthalic anhydride as an acid component
 and a polyhydric alcohol exemplified by a glycerine or ethylene glycol as
 a glycol component, with a fatty acid such as drying oil or the like. The
 fatty acid can be exemplified by castor oil, soybean oil and linseed oil;
 however, various fatty acids of any combination may also be used.
 With respect to the resin mixture in the releasing layer, the silicone
 resin can be added, for example, during or after production of the alkyd
 resin, to graft the silicone resin to the alkyd resin, that is, give rise
 to graft copolymerization.
 The acrylic resin is added in order to allow the releasing layer to have
 higher toughness and good surface wettability. As the acrylic resin, there
 can be used, for example, a polyacrylic acid, a polymethacrylic acid and a
 polymethyl methacrylate.
 The mixing ratio of the alkyd resin and the acrylic resin can be varied
 depending upon the intended purpose because a different mixing ratio gives
 different properties. However, it is preferred to mix at a ratio of 100
 parts by weight of the alkyd resin and 50 to 300 parts by weight of the
 acrylic resin.
 As the melamine resin, there can be used, for example, a methylated
 melamine resin, a butylated melamine resin and a methylated urea melamine
 resin. The mixing ratio of the melamine resin to the alkyd resin and the
 acrylic resin is preferably 10 to 200 parts by weight of the melamine
 resin to 100 parts by weight of the total of the alkyd resin and the
 acrylic resin. By using this mixing ratio, the releasing film can be
 allowed to have a small difference as mentioned above, in the dynamic
 friction coefficients. It is possible to use an acid catalyst exemplified
 by a sodium p-toluenesulfonate as a catalyst for crosslinking reaction
 between the melamine resin and the alkyd resin.
 The silicone resin in the releasing layer is a polymer having a basic
 skeleton of polydimethylsiloxane. It preferably has a phenyl group, an
 alkyl group or the like at the terminal(s) or side chain(s) so as to have
 improved compatibility with the alkyd resin, etc. As specific examples of
 such a silicone resin, there can be mentioned a polyphenylpolysiloxane and
 a hydroxyl-substituted diphenylpolysiloxane. The amount of the silicone
 resin used is preferably 1 to 50 parts by weight, more preferably 1 to 30
 parts, particularly preferably 5 to 10 parts by weight relative to 100
 parts by weight of the total of the alkyd resin, the melamine resin and
 the acrylic resin.
 In the present invention, the releasing layer is formed on at least one
 side of the base film. The releasing layer can be formed, for example, by
 coating, on the base film, a solution containing the above-mentioned
 components constituting the releasing layer, followed by heating/drying
 and curing to form a coating film. The method for coating can be any per
 se known method. There can be mentioned, for example, roll coating, blade
 coating and bar coating. The heating/drying conditions are preferably 80
 to 160.degree. C. and 10 to 120 seconds, particularly preferably 120 to
 150.degree. C. and 20 to 60 seconds. The thickness of releasing layer
 after drying is preferably 0.02 to 50 .mu.m, particularly preferably 0.04
 to 10 .mu.m.
 In the releasing film of the present invention, the center line average
 roughness (Ra) of the releasing layer surface is preferably 6 to 30 nm,
 more preferably 20 nm or less, particularly preferably 10 nm or less. A
 center line average roughness (Ra) exceeding 30 nm is not preferred
 because the surface state of the releasing film is transferred onto a
 resin sheet or the like, causing pin holes, etc.
 Thus, according to the present invention, there are also provided the
 following releasing films (A) and (B). (A) A releasing film comprising a
 base film produced from a polyethylene naphthalate and a releasing layer
 formed on both sides of the base film, wherein the dynamic friction
 coefficient (.mu.dR) of one releasing layer side of the releasing film to
 a rubber surface and the dynamic friction coefficient (.mu.dM) of the
 other releasing layer side of the releasing film to a metal surface
 satisfy the following formula (4):
EQU -0.5.ltoreq.(.mu.dR-.mu.dM).ltoreq.0.5 (4)
 (B) A releasing film comprising a base film produced from a polyethylene
 naphthalate and a releasing layer formed on one side of the base film,
 wherein the dynamic friction coefficient (.mu.dR) of the releasing layer
 side of the releasing film to a rubber surface and the dynamic friction
 coefficient (.mu.dM) of the other side of the releasing film to a metal
 surface satisfy the following formula (4):
EQU -0.5.ltoreq.(.mu.dR-.mu.dM).ltoreq.0.5 (4)
 In the releasing films (A) and (B), the base film may be a biaxially
 oriented film produced from a polyethylene naphthalate and its thickness
 is, as mentioned previously, preferably 5 to 500 .mu.m, particularly
 preferably 10 to 200 .mu.m.
 In order for the releasing films (A) and (B) to have the above-mentioned
 dynamic friction coefficients to a rubber surface and a metal surface, it
 is desirable that their releasing layers are composed mainly of a resin
 composition comprising 1 to 50 parts by weight of a silicone resin on the
 basis of 100 parts by weight of a resin mixture consisting of a melamine
 resin and an alkyd resin and/or an acrylic resin.
 The resin mixture preferably consists of 10 to 200 parts by weight of a
 melamine resin on the basis of 100 parts by weight of an alkyd resin
 and/or an acrylic resin.
 The silicone resin, melamine resin, alkyd resin and acrylic resin
 constituting the above releasing layers can be the same as mentioned
 previously, and their proportions are preferred to be also the same as
 mentioned previously. The thickness of the releasing layers is preferably
 0.02 to 50 .mu.m as mentioned previously.
 Effects of the Invention
 According to the present invention, there can be provided a releasing film
 which is superior in thermal stability and surface smoothness, which is
 low in deformation caused by the heat during production of a resin sheet
 or the like thereon, and which can produce a sheet having good
 processability such as cuttability.
 The releasing film of the present invention is useful as a protective film
 for an adhesive coating produced from an adhesive solution, or as a
 carrier film for a resin sheet, a resin coating, a ceramic sheet or the
 like, each produced from a resin solution, a slurry or the like; can allow
 coating thereon of even an adhesive solution, a resin solution or a slurry
 each of aqueous type; can be transferred smoothly in production thereon of
 an adhesive coating, a resin coating, a resin sheet or the like; and can
 be used advantageously in producing obtained various film or sheet having
 a smooth surface.
 EXAMPLES
 The present invention is described in more detail below by way of Examples.
 Each characteristic value was obtained by the following measurement
 methods.
 (1) Glass transition temperature (Tg)
 Glass transition peak temperature was measured using a differential thermal
 analyzer (DSC 2100, a product of Du Pont), at a temperature elevation rate
 of 20.degree. C./min.
 (2) Surface center line average roughness (Ra)
 The surface of a film was scanned using a stylus type surface roughness
 tester (Surfcorder 30C, a product of Kosaka Kenkyusho K.K.) under the
 conditions of stylus radius =2 .mu.m and stylus pressure =30 mg to measure
 the displacements of the film surface and prepare a surface roughness
 curve. The cut-off value was 80 .mu.m. From the surface roughness curve, a
 length (L) measured in the direction of its center line was extracted, and
 when the surface roughness curve was expressed as (Y=f(x)) for which a
 scanning length was taken as X axis and the surface displacement was taken
 as Y axis, the surface center line average roughness (Ra) of the film was
 calculated using the following formula:
 ##EQU1##
 (3) Inherent viscosity of polymer
 Measured at 35.degree. C. in an o-chlorophenol solution of polymer (polymer
 concentration: % by weight).
 (4) Thermal stability of base film
 A PEN polymer was calculated for the heat deterioration index from the
 inherent viscosities of the polymer before and after production of base
 film, using the following formula to evaluate thermal stability.
 Heat deterioration index=([.eta.o]/([.eta.x])-1 wherein [.eta.o] is the
 inherent viscosity of the polymer before production of base film, and
 [.eta.x] is the inherent viscosity of the polymer after production of base
 film.
 Standard for Evaluating of Thermal Stability
 .largecircle.: Heat deterioration index .ltoreq.0.05
 .DELTA.: 0.05 &lt;heat deterioration index .ltoreq.0.10
 X: 0.10 &lt;heat deterioration index
 (5) Surface smoothness of releasing film
 A base film was coated, for formation of a releasing layer thereon, with an
 addition reaction type silicone, followed by heating at 140.degree. C. for
 1 min. The resulting releasing film was cut into a size of 10 cm.times.20
 cm. The cut releasing film was placed on a flat floor in free length. The
 10 cm.times.10 cm portion of the film was covered with a metal sheet and
 the film edge not covered was observed for rise. The surface smoothness of
 the releasing film was evaluated according to the following standard.
 Standard for Evaluating
 .largecircle.: The rise (height) of the edge is less than 2 mm.
 .DELTA.: The rise (height) of the edge is 2 mm to less than 8 mm.
 X: The rise (height) of the edge is 8 mm or more.
 (6) Processability
 A releasing film was coated with an aqueous solution of a polyvinyl alcohol
 resin as a model coating in an as-dried thickness of 30 .mu.m, followed by
 drying to evaporate the water contained in the coated solution. Then, the
 resulting material was punched using a metal die of 15 cm.times.15 cm, and
 the punched material was observed for fin of edge, bending of corner, and
 occurrence of cracking. The processability of the releasing film was
 evaluated according to the following standard.
 .largecircle.: There is neither fin nor bending.
 .DELTA.: Very small fin or film bending appears, and resin film shows
 slight stretching. Or, very slight interlaminar delamination appears.
 X: Releasing film shows interlaminar delamination, fin, or cracking; or,
 resin film does not cut well.
 (7) Refractive index (nz) in thickness direction, and plane orientation
 coefficient (NS)
 Refractive indices of base film in film-winding direction, direction
 perpendicular thereto and thickness direction were measured at 25.degree.
 C. using Na D-line, using an Abbe's refractometer (a product of K.K.
 Atago). Plane orientation coefficient is represented by the following
 formula: Plane orientation coefficient (NS)=(nMD+nTD)/2-nz wherein nMD is
 the refractive index of base film in film-winding direction; nTD is the
 refractive index of the base film in direction perpendicular thereto; and
 nz is the refractive index of the base film in thickness direction.
 (8) Tearing propagation resistance
 An Elmendorf tear tester (a product of K.K. Toyoseiki Seisakusho) was used.
 A rectangular film sample of 52 mm (X) .times.65 mm (Y) was collected; at
 the center of the X side, a crack of 13 mm in length in the Y direction
 was formed; and a force required for tearing the remaining 52 mm was
 measured. The force was divided by the thickness of film to take the
 quotient as the tearing propagation resistance of the film sample. This
 measurement was conducted in the film-winding direction (the film
 longitudinal direction) and a direction perpendicular thereto.
 (9) Resistance to heat distortion
 A rectangular releasing film cut out to length of 30 mm or more in
 measurement direction and to width of 4 mm was fitted to the jig of a TMA
 (TMA/SS120 C, a heat stress strain tester, a product of Seiko Instruments
 Inc.) so that the distance between chucks became 10 mm. While a stress of
 150 gf/mm.sup.2 was applied to the film, the film was heated from room
 temperature at a temperature elevation rate of 5.degree. C./min. When the
 film temperature reached 120.degree. C., the film was measured for
 dimensional changes in stress direction and direction perpendicular
 thereto. The dimensional change of the film was determined using the
 following formula. Dimensional change rate (absolute
 value)=.vertline.dimensional change/distance between
 chucks.vertline..times.100
 (10) Friction coefficient
 A releasing film sample having a releasing layer on one side was placed on
 a rubber sheet or a metal sheet (a metal having a hard chromium-plated
 surface). The rubber sheet or the metal sheet was placed on a fixed glass
 plate to be pulled by a roll at a constant speed of 150 mm/min. A detector
 for pulling force was fixed to one end at the upper side of the releasing
 film (contrary end in the direction for pulling force at the lower side of
 the film) to detect a pulling force between the releasing film and the
 rubber sheet or the metal sheet. On the film was placed a load of 200 g
 having, at the lower side, a surface area of 10 cm.times.5 cm. Dynamic
 friction coefficient (.mu.d) is a non-dimensional value obtained by
 dividing the puling force (unit: g) after the rubber sheet or the metal
 sheet has begun to move, by 200 g (the weight of the load). The dynamic
 friction coefficient between the releasing layer of the releasing film and
 the rubber sheet is expressed as .mu.dR, and the dynamic friction
 coefficient between the releasing layer of the releasing film and the
 metal sheet is expressed as .mu.dM.
 As the metal sheet was used a ferro sheet S-grade having a surface
 roughness of 11 nm. As the rubber sheet was used a neoprene rubber having
 a surface roughness of 740 nm.
 (11) Wettability of aqueous coating fluid
 The following components were mixed in a ball mill and dispersed so as to
 obtain a grade of 7 or more as measured using a Hegmann grinding gauge.

Composition of ceramic powder-dispersed slurry
 a. Ceramic powder (barium titanate) 100 parts by weight
 b. Water-soluble acrylic emulsion 9-13 parts by weight
 c. Water-soluble polyurethane resin 1 part by weight
 d. Ammonium polycarboxylate 1 part by weight
 e. Water 10-20 parts by weight
 f. Ammonia 1 part by weight
 The ceramic powder-dispersed slurry obtained was coated on the releasing
 layer side of a releasing film using an straight-edge applicator having
 gaps of 1 mil, followed by drying at 110.degree. C. for 2 minutes; then,
 the state of cissing at the edge portions of the film was observed. The
 wettability of the aqueous coating fluid was evaluated according to the
 following standard.
 A: No cissing is observed (good wettability).
 B: Slight cissing is observed (moderately good wettability)
 C: Cissing is observed (bad wettability).
 (12) Releasability of ceramic sheet
 The ceramic sheet formed on a releasing film in the same manner as in the
 above item (11) was released from the releasing film, and the condition of
 release was observed. The releasability of the ceramic sheet was evaluated
 according to the following standard.
 A: Easily peelable (good peelability).
 B: The releasing strength is large, and rapid pulling gives rise to rupture
 of ceramic sheet (moderately good peelability).
 C: The peeling strength is too large, and peeling gives rupture of ceramic
 sheet.
 D: The ceramic sheet has an inferior surface owing to the rough surface of
 the releasing film and ruptures (inferior sheet surface).
 E: Sheeting is impossible owing to cissing.
 (13) Residual adhesivity percentage
 A polyester adhesive tape (Nitto 31B) was stuck on a cold rolled stainless
 steel plate (SUS 304) specified by JIS G 4305 and measured for peeling
 strength. This peeling strength was taken as base adhesivity (f.sub.o).
 The polyester adhesive tape separately laminated on the releasing layer of
 a releasing film was subjected to contact-bonding by the use of a pressure
 roll of 2 kg and allowed to stand for 30 minutes; then, the adhesive tape
 was peeled. This peeled adhesive tape was stuck on the above stainless
 steel plate and measured for peeling strength. This peeling strength was
 taken as residual adhesivity (f). A residual adhesivity percentage was
 determined from the base adhesivity (f.sub.o) and the residual adhesivity
 (f), using the following formula. Residual adhesivity percentage
 (%)=(f/f.sub.o).times.100
 Incidentally, the preferred range of the residual adhesivity percentage of
 releasing film is 85% or above. A residual adhesivity percentage of below
 85% is not preferred because, when, for example, the releasing film is
 stored in a rolled state, the component constituting the releasing layer
 is transferred on the adjacent surface of the film (back transferring) in
 some cases, thereby making inferior the wettability, peelability and the
 like of the releasing layer.

EXAMPLE 1
 To 100 parts by weight of dimethyl 2,6-naphthalenedicarboxylate and 60
 parts by weight of ethylene glycol was added manganese acetate
 tetrahydrate as an ester interchange catalyst in an amount shown in Table
 1, to conduct ester interchange. Thereto was added trimethyl phosphate in
 an amount shown in Table 1, to complete the ester interchange. Further,
 antimony trioxide was added in an amount shown in Table 1, after which
 polycondensation was conducted at a high temperature under high vacuum to
 obtain a PEN polymer having an inherent viscosity of 0.62. The PEN polymer
 contained 1.0% by weight of diethylene glycol (DEG) units.
 To the PEN polymer were added 0.15% by weight of truly spherical silica
 particles having an average particle diameter of 0.3 .mu.m, and they were
 melted in an extruder. The molten polymer was extruded from the die of the
 extruder onto a rotary cooling drum kept at 40.degree. C., and was
 electrostatically adhered thereto and rapidly cooled to obtain an
 unstretched film. The unstretched film was stretched 3.6 times in the
 longitudinal direction and subsequently 3.6 times in the transverse
 direction, and thermoset at 230.degree. C. to obtain a biaxially oriented
 PEN film having a thickness of 50 .mu.m.
 On one side of the biaxially oriented PEN film was coated a coating fluid
 prepared as mentioned below, in an amount of 6 g/m.sup.2 (wet), and the
 coated fluid was heat-dried and cured at 140.degree. C. for 1 minute to
 produce a releasing film having a releasing layer of 0.1 .mu.m in
 thickness. The properties of the releasing film are shown in Table 1.
 The coating solution was prepared by dissolving a curing silicone of
 addition-reaction type comprising a vinyl group-containing
 polydimethylsiloxane and dimethylhydrogensilane, in a mixed solvent of
 methyl ethyl ketone, methyl isobutyl ketone and toluene, adding a silicone
 resin thereto so as to become an amount of 10% by weight based on the
 solid content of the curing silicone to obtain a solution having the total
 solid content of 2%, and adding a platinum catalyst to the resulting
 solution.
 EXAMPLE 2
 Comparative Examples 1 to 2
 Releasing films were produced in the same manner as in Example 1 except
 that the addition amounts of manganese acetate, trimethyl phosphate and
 antimony trioxide were changed as shown in Table 1 and the content of
 truly spherical silica particles was changed to 0.40% by weight. The
 properties of the films are shown in Table 1.
 TABLE 1
 Comparative
 Comparative
 Example 1 Example 2 Example 1
 Example 2
 Catalyst amount Mn (moles) 1.20 1.57 1.86
 1.44
 P (moles) 1.60 2.09 2.48
 3.00
 Sb (moles) 0.84 0.84 1.24
 2.40
 Catalyst ratio (Mn/P) 0.75 0.75 0.75
 0.48
 Glass transition temperature (Tg) (.degree. C.) 120 121 121
 120
 Surface center line average roughness (nm) 12 28 36
 52
 Thermal stability of base film .largecircle. .largecircle. .DELTA.
 X
 Surface smoothness of releasing film .largecircle. .largecircle. .DELTA.
 X
 Processability .largecircle. .largecircle. .DELTA.
 .DELTA.
 As is clear from Table 1, the releasing films of the present invention
 shown in Examples were superior in thermal stability of base film, were
 superior in dimensional stability and surface smoothness after heat
 treatment in formation of releasing layer, and were good in
 processability. The releasing films of Comparative Examples 1 and 2 were
 inferior in thermal stability of the film after forming and consequently
 in flatness and transparency, were inferior in surface smoothness of
 releasing film obtained, were large in thickness non-uniformity of resin
 solution coated thereon, caused interlaminar delamination during cutting,
 and were insufficient.
 EXAMPLES 3 and 4
 Comparative Examples 3 and 4
 A PEN polymer having an inherent viscosity shown in Table 2, containing
 0.15% by weight of truly spherical silica particles having an average
 particle diameter of 0.3 .mu.m was melted in an extruder. The molten
 polymer was extruded from the die of the extruder onto a rotary cooling
 drum kept at 40.degree. C., and was electrostatically adhered thereto and
 rapidly cooled to obtain an unstretched film. The unstretched film was
 subjected to successive biaxial stretching first in the longitudinal
 (machine axial) direction and then in the transverse (width) direction,
 under the conditions shown in Table 2. The stretched film was thermoset
 under the temperature condition shown in Table 2, to obtain a biaxially
 oriented PEN film having a thickness of 50 .mu.m. The Mn, P and Sb
 contents in the PEN polymer were about the same as in Example 1.
 On one side of the biaxially oriented PEN film was coated a coating fluid
 prepared as mentioned below, in an amount of 6 g/m.sup.2 (wet), and the
 coated fluid was heat-dried and cured at 140.degree. C. for 1 minute to
 produce various releasing films each having a releasing layer of 0.1 .mu.m
 in thickness. The properties of the releasing films are shown in Table 2.
 The coating solution was prepared by dissolving a curing silicone of
 addition-reaction type comprising a vinyl group-containing
 polydimethylsiloxane and dimethylhydrogensilane, in a mixed solvent of
 methyl ethyl ketone, methyl isobutyl ketone and toluene, adding a silicone
 resin thereto so as to become an amount of 10% by weight based on the
 solid content of the curing silicone to obtain a solution having the total
 solid content of 2%, and adding a platinum catalyst to the resulting
 solution.
 TABLE 2
 Comparative
 Comparative
 Example 3 Example 4 Example 3
 Example 4
 Inherent viscosity of polymer (dl/g) 0.62 0.60 0.62
 0.66
 Draw ratio Longitudinal direction (times) 3.5 3.6 2.8
 4.3
 Transverse direction (times) 3.6 3.7 3.0
 4.4
 Thermosetting temperature (.degree. C.) 260 250 250
 200
 Refractive index (nz) in thickness direction 1.51 1.495 1.535
 1.485
 Plane orientation coefficient (NS) 0.24 0.26 0.225
 0.275
 Resistance to Longitudinal direction (%) 0.1 0.05 0.25
 0.05
 heat distortion Transverse direction (%) 0.15 0.1 0.35
 0.05
 Tearing propagation resistance (N/mm) 2.5 3.0 4.5
 1.8
 (longitudinal direction + transverse direction)
 Processability .largecircle. .largecircle. .DELTA.
 X
 As is clear from Table 2, the releasing films of the present invention
 shown in Examples were superior in thermal stability of base film, were
 superior in dimensional stability and surface smoothness after heat
 treatment in formation of releasing layer, and were good in
 processability. The releasing films of Comparative Examples 3 and 4 were
 inferior in thermal stability of the film after forming and consequently
 in flatness, were inferior in surface smoothness of releasing film
 obtained, were large in thickness non-uniformity of resin solution coated
 thereon, caused interlaminar delamination during cutting, and were
 insufficient.
 EXAMPLE 5
 A polyethylene-2,6-naphthalenedicarboxylate (hereinafter abbreviated to PEN
 in some cases) having an inherent viscosity of 0.60, containing 0.25% by
 weight of truly spherical silica particles having an average particle
 diameter of 0.3 .mu.m was melted in an extruder. The molten polymer was
 extruded from the die of the extruder onto a rotary cooling drum kept at
 400.degree. C., and was electrostatically adhered thereto and rapidly
 cooled to obtain an unstretched film. The unstretched film was stretched
 3.7 times in the longitudinal direction and successively 3.7 times in the
 transverse direction, and thermoset at 230.degree. C. The biaxially
 oriented PEN film obtained was subjected to a relaxation treatment at
 120.degree. C. for 1 minute under a tension of 8 kgf/cm.sup.2, to obtain a
 biaxially oriented PEN film having a thickness of 50 .mu.m.
 Next, there were mixed 100 parts by weight of an alkyd resin (a linseed
 oil-modified alkyd resin), 100 parts by weight of an acrylic resin (a
 polyethyl acrylate), 100 parts by weight of a melamine resin (a butylated
 melamine resin) and 75 parts by weight of a silicone resin (a
 hydroxyl-substituted diphenyl polysiloxane) (25 parts by weight of the
 silicone resin corresponded to 100 parts by weight of a resin mixture
 consisting of the alkyd resin, the acrylic resin and the melamine resin).
 The resulting resin composition was dissolved in a mixed solvent
 consisting of methyl ethyl ketone, methyl isobutyl ketone and toluene, to
 prepare a solution having a total solid content of 3% by weight. To the
 solution was added an acid catalyst (p-toluenesulfonic acid) as an
 accelerator for curing reaction, whereby a coating fluid was prepared.
 The coating fluid was coated on one side of the previously-obtained PEN
 film in an amount of 8 g/m.sup.2 (wet), followed by heat-drying at
 150.degree. C. for 1 minute to cure the coating film, to obtain a
 releasing film having a releasing layer of 0.3 .mu.m in thickness. The
 properties of the releasing film are shown in Table 3.
 Comparative Example 5
 A PET polymer having an inherent viscosity of 0.62, containing 0.1% by
 weight of truly spherical silica particles having an average particle
 diameter of 0.12 .mu.m was melted in an extruder. The molten polymer was
 extruded from the die of the extruder onto a rotary cooling drum kept at
 40.degree. C., and was electrostatically adhered thereto and rapidly
 cooled to obtain an unstretched film. Then, the unstretched film was
 stretched 3.6 times in the longitudinal direction and successively 3.9
 times in the transverse direction, and thermoset at 220.degree. C. The
 biaxially oriented PET film obtained was subjected to a relaxation
 treatment at 120.degree. C. for 1 minute under a tension of 8
 kgf/cm.sup.2, to obtain a biaxially oriented PET film having a thickness
 of 50 .mu.m.
 There was prepared, as a coating fluid, a solution having a total solid
 content of 5% by weight, by dissolving a curing silicone of addition
 reaction type (KS 847(H), a product of Shin-Etsu Chemical Co., Ltd.)
 obtained by adding a platinum catalyst to a mixed solution of a
 polydimethylsiloxane and dimethylhydrogensilane, in a mixed solvent
 consisting of methyl ethyl ketone, methyl isobutyl ketone and toluene.
 This solution was coated on one side of the above-prepared biaxially
 oriented PET film after relaxation treatment, in an amount of 10 g/m.sup.2
 (wet), followed by heat-drying at 150.degree. C. for 1 minute to cure the
 coating film, to produce a releasing film having a releasing layer of 0.6
 .mu.m in thickness. The properties of the releasing film are shown in
 Table 3.
 Comparative Example 6
 A PET polymer having an inherent viscosity of 0.62, containing 0.5% by
 weight of truly spherical silica particles having an average particle
 diameter of 0.4 .mu.m was melted in an extruder. The molten polymer was
 extruded from the die of the extruder onto a rotary cooling drum kept at
 40.degree. C., and was electrostatically adhered thereto and rapidly
 cooled to obtain an unstretched film. Then, the unstretched film was
 stretched 3.6 times in the longitudinal direction and successively 3.9
 times in the transverse direction, and thermoset at 220.degree. C. The
 biaxially oriented PET film obtained was subjected to a relaxation
 treatment at 120.degree. C. for 1 minute under a tension of 8
 kgf/cm.sup.2, to obtain a biaxially oriented PET film having a thickness
 of 50 .mu.m.
 A solution having a total solid content of 3% by weight was prepared by
 dissolving a resin composition consisting of 100 parts by weight of an
 alkyd resin (a coconut oil-modified alkyd resin) and 40 parts by weight of
 a melamine resin (a butylated melamine resin), in a mixed solvent
 consisting of methyl ethyl ketone, methyl isobutyl ketone and toluene.
 This solution was coated on one side of the above-prepared biaxially
 oriented PET film in an amount of 8 g/m.sup.2 (wet), followed by
 heat-drying at 150.degree. C. for 1 minute to cure the coating film, to
 produce a releasing film having a releasing layer of 0.3 .mu.m in
 thickness. The properties of the releasing film are shown in Table 3.
 TABLE 3
 Slipperiness (dynamic Properties of releasing layer
 friction coefficient) Ra Residual
 .mu.dR .mu.dM (.mu.dR - .mu.dM) (nm) Wettability Releasability
 adhesivity (%)
 Example 5 0.35 0.25 +0.10 7 A A 93
 Comparative 4.00 0.10 +3.90 9 C D 86
 Example 5
 Comparative 0.40 0.10 +0.30 40 A C 94
 Example 6
 As is clear from Table 3, the releasing film of the present invention shown
 in Example 5 has good slipperiness, has good surface wettability although
 small in surface roughness, that is, is superior in cissing against
 coating fluid or the like as well as in sheet releasability, and enables
 formation thereon of a thin resin coating of smooth surface.