GB2335870A

GB2335870A - Recording sheet

Info

Publication number: GB2335870A
Application number: GB9721674A
Authority: GB
Inventors: Cornell Chappell Jr; Junaid Ahmed Siddiqui; Quing Lian
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1997-10-27
Filing date: 1997-10-27
Publication date: 1999-10-06
Also published as: GB9721674D0; GB2335870A8

Abstract

A recording sheet has a polymeric film substrate and a coating layer containing 5 to 95% by weight of at least one filler, and 5 to 95% by weight of at least one binder. The recording sheet can be prepared by applying a coating layer composition having a total solids content of greater than 25% by weight to a film web travelling at a line speed greater than 20 metres/min. Preferably the composition is applied between longitudinal and transverse stretching stages of biaxial orientation of the film. The recording sheet is particularly suitable for use in ink-jet printing.

Description

MTW50470 2335870 1 Recording Sheet This invention relates to a recording

sheet and to a method for producing such a sheet, and in particular a sheet suitable for use in ink-jet printing.

1 There are many printing applications such as conventional offset, gravure, flexographic and other printing roll applications as well as ink-jet, electrostatic or dielectric with liquid toners, dye diffusion thermal transfer, wax thermal transfer printing, and other so-called non-impact printing methods, where imaging marking material is substantially in a liquid state when applied to the printing support material. Typically, the dwell time during which the printing delivery device is applying the marking to a specific area on the printing support material is much less than a second, often of the order of a few milliseconds or less. Thus, the printing support material must very rapidly accept the imaging marking material and provide the desired high resolution, high density images whilst achieving a rapid drying of any volatile liquids in the marking material or ink.

As the image quality of non-impact printers evolves from low resolution monochrome text or line printing, to high resolution colour photorealistic pictorial imaging and as the throughput printing speeds of these printers simultaneously increases significantly, then increasingly higher demands are being placed on the printing support material for high resolution high density images combined with very rapid receptivity and drying of the imaging marking material without undesirable image spreading or image off-set due to slow drying. At the same time, many printing applications require a white opaque, highly reflective or glossy support material in order to achieve the highest visual impact of the printed material. Depending on the type of printing, there may be other critical requirements for the printing support material, such as durability, resistance to curl, tear resistance, and water resistance.

Flexible plastic or polymeric films, such as polyester film, have a large number of performance advantages over paper substrates for many printing and other applications. These advantages include dimensional stability, tear resistance, and resistance to cud at various relative humidities. When the printing application involves absorbing liquids in large amounts, such as in ink-jet printing, paper substrates, which are inherently porous, have advantages over polymeric films, which are typically non-porous and do not absorb significant amounts of liquids. To overcome the disadvantages of polymeric films for applications involving the absorption of liquids, two approaches have been taken. In the first approach, polymeric films have been produced that are microporous throughout the bulk of the film, and thus provide liquid absorbing properties similar to that of paper substrates. This approach is described in MTW50470 2 US-A-5605750, and references therein, the disclosures of which are incorporated herein by reference. This approach involves the extrusion of a highly pigmented thermoplastic film containing a plasticizer or oily material which is subsequently extracted with solvents to produce a microporous, liquid absorbing polymeric film.

Polymeric films made by this approach have disadvantages due to the cost of the extra materials and process steps, and to the use of large amounts of solvents.

In the second approach, which is more commonly used, one or more coatings are applied to the polymeric film to provide the liquid absorbing properties. To meet the critical requirements for high resolution colour imaging and rapid receptivity and drying of the marking material, microporosity of the receptive surface and outer coating layer of the printing support material has been recognised as a beneficial approach.

Typically, these coatings are thick with dry thicknesses ranging from 5 to 25 microns.

For example, US-A-4542059, US-A-5102717 and US-A-5104730, the disclosures of which are incorporated herein by reference, disclose microporous surface zones for an is inkable sheet. In addition to the liquid absorbing coating, it is common to need to apply an additional outer coating over the liquid absorbing coating to provide other properties in addition to liquid absorption. Examples of these other properties include binding of the colorant, durability, and high resolution image quality for dye and pigments in various printing applications, such as ink-jet printing. The relatively thick liquid absorbing coatings of this approach are expensive due to the amount of materials and the higher drying requirements for thick, often water-based coatings, which greatly reduce production coating line speeds. The liquid absorbing coating layers have been traditionally applied to the polymeric film after the production of the film has been completed, ie "off-line", which results in an increase in the number of process steps required to produce the coated film. There is a need to be able to apply the coating layer during the film making process, ie "in-line", in order to simplify and improve the efficiency of the production process. In addition, if a further outer coating layer is required, then an additional costly pass through the coating equipment will be necessary.

Although paper substrates are porous and absorb liquids, their surfaces are not typically smooth enough to meet additional requirements for a white, highly reflective or glossy support material. To overcome this deficiency, white-coatings or resin extrusions, usually containing high levels of a white inorganic or organic pigment, are applied to the paper support material as an additional coating under the imaging receptive coating. For example, US-A-5141599 and US-A-5372884, the disclosures of which are incorporated herein by reference, disclose resin-coated paper substrates to MTW50470 3 achieve white, glossy support materials. The use of these coatings is expensive since they represent an additional coating layer, often requiring a separate pass through a coating machine. Also, these coatings typically interfere with the critical property of porosity of the imaging support material, since the white opaque and reflective coating layer is essentially non-porous.

We have now devised a recording sheet which is particulady suitable for use as an ink-jet recording sheet, which reduces andfor substantially overcomes at least one of the aforementioned problems.

Accordingly, the present invention provides a recording sheet comprising a polymeric film substrate having on at least one surface thereof, a coating layer comprising in the range from 5 to 95% by weight of at least one filler, and 5 to 95% by weight of at least one binder.

The invention further provides a method of producing a recording sheet which comprises forming a polymeric film substrate, and applying a coating composition to at least one surface of the substrate, the coating composition having a total solids content of greater than 25% by weight, and comprising in the range from 5 to 95% by weight of at least one filler, and 5 to 95% by weight of at least one binder, both relative to the total solids of the composition.

The polymeric film substrate may be formed from any suitable film-forming, polymeric material. Thermoplastics materials are preferred, and include a cellulose ester, eg cellulose diacetate, cellulose triacetate, polystyrene, a polymer and copolymer of vinyl chloride, homopolymer or copolymer of a 1-olefin, such as ethylene, propylene and but-l-ene, a polyamide, a polycarbonate, more preferably a polyester, and particularly 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 aliphatic glycols, eg ethylene glycol, 1,3-propanediol, 1,4-butanediol, neopentyl glycol and 1,4cyclohexanedimethanol. A polyethylene terephthalate andlor 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 1250C, and preferably heat MTW50470 4 set, typically at a temperature in the range from 150 to 2500C, for example as described in GB-A-838708.

The substrate may also comprise a polyaryiether or thio analogue thereof, particularly a polyaryletherketone, polyarylethersu [phone, polyaryletheretherketone, polyaryletherethersulphone, or a copolymer or thioanalogue thereof. Examples of these polymers are disclosed in EP-A-1879, EP-A-184458 and US-A-4008203. Blends of these polymers may also be employed. A poly p-phenylene sulphide film is also suitable.

Suitable thermoset resin substrate materials include additionpolymerisation resins, such as acrylics, vinyls, bis-maleimides and unsaturated polyesters, formaldehyde condensate resins such as condensates with urea, melamine or phenols, cyanate resins, isocyanate resins, epoxy resins, functionalised polyesters, polyamides or polyimides.

The polymeric film substrate is a film capable of independent existence in the absence of a supporting base.

A preferred polymeric film substrate may be unoriented or preferably oriented, for example uniaxially odented, or more 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 a polymeric film, for example a tubular or a flat film process.

In a tubular process simultaneous biaxial orientation may be effected by extruding a thermoplastics polymeric 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 polymer is extruded through a slot die and rapidly quenched upon a chilled casting surface (drum) to ensure that the polymer 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 polymer. 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 conventionally 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 polymer, for example a polyester is usually stretched so MTW50470 that the dimension of the oriented polyester film is from 2.5 to 4.5 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 polymer but below the melting temperature thereof, to induce crystallisation of the polymer.

In one embodiment of the invention the polymeric film is transparent, exhibiting high optical clarity and low haze, preferably having a wide angle haze, being measured according to the standard ASTM D 1003-61, of <8%, more preferably <6%, particulady <5%, and especially <3%, preferably for a 100 pm thick film. The aforementioned optical characteristics can be suitably achieved by having little or no particulate additive present in the substrate. The substrate may contain relatively small quantities of filler material, for example in the range from 5 to 3000 ppm, preferably 50 to 2000 ppm, and more preferably 100 to 1000 ppm. Suitable fillers include inorganic materials such as silica, china clay, calcium carbonate, and organic materials such as silicone resin particles. Spherical monodisperse fillers may be employed. The substrate may contain filler due to the normal practice of using reclaimed film in the film manufacturing process.

However, in a preferred embodiment of the invention the polymeric film is opaque, which is defined as a film exhibiting a Transmission Optical Density (Sakura Densitometer; type PDA 65; transmission mode) of greater than 0.75, more preferably in the range from 1.0 to 1.75, and particularly 1.2 to 1.5, preferably for a 100 pm thick film. The polymeric film is conveniently rendered opaque by incorporating into the synthetic polymer of the substrate layer, an effective amount of an opacifying agent.

Suitable opacifying agents include a particulate inorganic filler, an incompatible resin filler, or a mixture of two or more such fillers.

Particulate inorganic fillers suitable for generating an opaque film substrate include conventional inorganic pigments and fillers, and particularly metal or metalloid oxides, such as alumina, silica and titania, and alkaline metal salts, such as the carbonates and sulphates of calcium and barium. Suitable inorganic fillers may be homogeneous and consist essentially of a single filler material or compound, such as titanium dioxide or barium sulphate alone. Alternatively, at least a proportion of the filler may be heterogeneous, the primary filler material being associated with an additional modifying component. For example, the primary filler particle may be treated with a surface modifier, such as a pigment, soap, surfactant coupling agent or MTW50470 6 other modifier to promote or alter the degree to which the filler is compatible with the substrate polymer.

Suitable particulate inorganic fillers may be of the non-voiding or voiding type, ie by voiding is meant comprises a cellular structure containing at least a proportion of discrete, closed cells. Barium sulphate is an example of a filler which results in the formation of voids. Titanium dioxide may be of the voiding or non-voiding type, dependant upon the particular type of titanium dioxide employed. The titanium dioxide particles may be of anatase or rutile crystal form. The titanium dioxide particles preferably comprise a major portion of anatase, more preferably at least 60%, particularly at least 80%, and especially approximately 100% by weight of anatase.

The particles can be prepared by standard procedures, such as using the chloride process or preferably by the sulphate process. In a preferred embodiment of the invention, the film substrate comprises barium sulphate particles.

The amount of inorganic filler incorporated into the film substrate desirably should be not less than 2% nor exceed 40% by weight, based on the weight of the substrate polymer. Particularly satisfactory levels of opacity are achieved when the concentration of filler, suitably barium sulphate, is preferably in the range from 5 to 30, more preferably 10 to 25%, and particularly 16 to 20% by weight, based on the weight of the substrate polymer.

The inorganic filler particles preferably have a volume distributed median particle diameter (equivalent spherical diameter corresponding to 50% of the volume of all the particles, read on the cumulative distribution curve relating volume % to the diameter of the particles - often referred to as the M(v,0.5)" value) in the range from 0.2 to 2 pm, more preferably 0.4 to 1.5 pm, particularly 0.6 to 1.2 pm, and especially 0.7 to 0.9 pm.

The film substrate optionally comprises an Incompatible resin" by which is meant a resin which either does not melt, or which is substantially immiscible with the substrate polymer, at the highest temperature encountered during extrusion and fabrication of the layer. Such resins include polyamides and olefin polymers, particularly a homo- or co-polymer of a mono-alpha-olefin containing up to 6 carbon atoms in its molecule, for incorporation into polyester films, or polyesters of the kind hereinbefore described for incorporation into polyolefin films.

The amount of incompatible resin, preferably polyolefin, incorporated into the film substrate is preferably in the range from 1 to 15%, more preferably 3 to 10%, and particularly 5 to 8% by weight, based on the weight of the substrate polymer.

MTW50470 7 Incorporation of the opacifying agent, preferably inorganic filter, into the substrate layer polymer may be effected by conventional techniques, for example by mixing with the monomeric reactants from which the polymer is derived, by dry blending with the polymer in granular or chip form prior to formation of a film therefrom, or by using masterbatching technology.

The coating layer of a recording sheet according to the invention can function as a liquid absorbing layer, resulting in rapid absorption of applied imaging marking material, such as ink. The coating or ink-absorbent layer can permit rapid drying of an applied ink pattern, and is desirably such that an aqueous-ethylene glycol (50:50 wlw) based ink, or similar composition, applied to the surface of a sheet from an ink-jet printer will resist off-setting when the inked surface is placed in contact with the surface of a paper sheet within 180, more preferably within 120, particularly within 60, and especially within 30 seconds, of application of the ink.

The amount of filler present in the coating layer composition, and consequently in the final coating layer, is suitably in the range from 5 to 95%, preferably 15 to 90%, more preferably 30 to 85%, particularly 40 to 80%, and especially 50 to 75% by weight, relative to the total solids of the composition.

The filler component of the coating layer suitably comprises inorganic filler, preferably particulate inorganic materials such as silica, alumina, titanium dioxide andlor metal oxides. Silica, and particularly alumina are preferred inorganic fillers.

The filler particles, preferably alumina, suitably have a volume distributed median particle diameter (equivalent spherical diameter corresponding to 50% of the volume of all the particles, read on the cumulative distribution curve relating volume % to the diameter of the particles - often referred to as the M(v,0.5)" value) in the range from 2 to 50 pm, preferably 5 to 35 pm, more preferably 10 to 20 pm, particularly 13 to 17 pm, and especially 14 to 16 pm.

The size distribution of the filler particles can also be an important parameter in obtaining a coating layer having the required surface properties. The filler particles suitably have a particle size distribution ratio D,1D21 (where D,, and D21, respectively, are the particle diameter of 75% and 25% of a volume based cumulative particle size distribution curve) value of from 1.1 to 10, preferably 1.5 to 7, more preferably 2 to 6, and particularly 4 to 5.

The filler particles in the coating layer preferably exhibit an average aspect ratio d,:d, (where d, and d2, respectively, are the maximum and minimum dimensions of the particle) of from 1 to 5:

1, more preferably 1.05 to 2: 1, and particularly 1.1 to 1.5: 1. The aspect ratio can be determined by measuring the cl, and d2value of a filler MTW50470 8 particle selected from a photographic image obtained by using a scanning electron microscope. An average aspect ratio can be obtained by calculating the mean value of 100 typical filler particles.

In a particularly preferred embodiment of the invention, the filler particles have a BET specific surface area, measured as described herein, of greater than 100, more preferably in the range from 150 to 700, particularly 200 to 500, and especially 300 to 400 m21g.

The filler particles preferably have a pore volume, measured as described herein, in the range from 0.1 to 3.0, more preferably 0.2 to 2.0, and particularly 0.4 to 1.0 CM31g.

The filler particles preferably have a Mobs hardness in the range from 2 to 8, more preferably 5 to 7.5, and particularly 6 to 7 units.

In a preferred embodiment of the invention, the coating layer composition, and consequently the final coating layer, comprises a second, preferably inorganic, filler is having an average particle size less than that of the aforementioned filler, hereinafter referred to as the first filler.

The second filler preferably has a volume distributed median particle diameter in the range from 1 to 15 pm, more preferably 2 to 10 pm, particularly 4 to 8 pm, and especially 5 to 7 pm, with the proviso that the particle diameter is less than that of the first filler. In a preferred embodiment, the volume distributed median particle diameter of the second filler is less than that of the first filler by an amount in the range from 0.5 to 30 pm, more preferably 1 to 12 pm, particularly 5 to 15 pm, and especially 8 to 12 pm.

The second filler preferably has a Mobs hardness in the range from 2 to 7, more preferably 2 to 5, and particularly 2 to 3 units, with the proviso that the Mobs hardness is less than that of the first filler, preferably by an amount in the range from 1 to 6, more preferably 3 to 5 units.

The second filler may be any of the filler materials mentioned herein, but is preferably silica, particularly in the form of a silica gel, which can be prepared by the reaction of sodium silicate solution with sulphuric acid.

The coating layer suitably comprises a mixture of both first and second fillers, preferably alumina and silica particles, more preferably present in a weight ratio of first filler to second filler in the range from 0.1 to 10: 1, particularly 0.2 to 5 A, and especially 2 to 3:

The amount of binder present in the coating layer composition, and consequently in the final coating layer, is suitably in the range from 5 to 95%, MTW50470 9 preferably 10 to 85%, more preferably 15 to 70%, particularly 20 to 60%, and especially 25 to 50% by weight, relative to the total solids of the composition.

The coating layer composition may comprise one or more binders selected from hydrophilic binders, hydrophobic binders, andlor mbdures thereof. Suitable binders are oligomeric, preferably polymeric materials, and include cellulosics, such as nitrocellulose, ethylcellulose and hydroxyethylcel lu lose; gelatines; vinyls, such as polyvinyl acetate, polyvinylchloride, and copolymers of vinyl chloride and vinyl acetate; acrylics, such as polyacrylic acid and acrylic copolymers; polyesters, such as sulphonated polyesters, polyurethanes and polyvinyl pyrrol idones.

In a preferred embodiment of the invention, the coating layer composition comprises at least one binder, more preferably a hydrophilic binder, which functions as a viscosity reducer. The viscosity reducer has the effect of reducing the viscosity of the coating layer composition, preferably by at least 10%, more preferably in the range from 25 to 95%, particularly 40 to 90%, and especially 60 to 80%, compared to the viscosity of the composition without the viscosity reducer. The viscosity of the final coating composition is preferably in the range from 50 to 500, more preferably 100 to 300, particularly 130 to 250, and especially 150 to 200 centipoise. The presence of the viscosity reducer enables the coating layer composition to be applied to the surface of the polymeric film at a very high solids content, as hereinafter described.

The amount of viscosity reducer present in the coating layer composition, and consequently in the final coating layer, is suitably in the range from 0 to 30%, preferably 1 to 25%, more preferably 3 to 20%, particularly 5 to 20%, and especially 6 to 18% by weight, relative to the total solids of the composition.

The viscosity reducer is preferably a relatively low molecular weight species, more preferably having a weight average molecular weight in the range from 500 to 100,000, particularly 1,000 to 50,000, more particularly 2,500 to 25,000 and especially 5,000 to 10,000. The viscosity reducer is preferably a copolymer containing carboxylic andlor other acid groups, suitably having an acid number (mg KOH/g) of greater than 10, preferably in the range from 50 to 800, more preferably 100 to 500, particulady 200 to 400, and especially 300 to 350. A preferred viscosity reducer is an acrylic polymer, particularly a copolymer of acrylic or methacrylic acid with other acrylic monomers.

The coating layer composition preferably contains a hydrophilic, more preferably thermoplastic, binder suitably having a glass transition temperature (Tg) of greater than OOC, preferably in the range from 10 to 800C, more preferably 20 to 700C, and particularly 25 to 550C. The hydrophilic binder is preferably hydrophilic to the extent that it can be dispersed in water with heat and high shear mixing to form a MTW50470 dispersion which is then stable at room temperature without the need for any dispersing agents.

The chemical composition of the hydrophilic binder component of the coating layer may vary over a relatively wide range of materials, and may be any one or more of the binder materials described herein, with the proviso that it exhibits sufficient hydrophilicity. The hydrophilic binder is preferably a polyester, particularly a sulphonated polyester. Suitable sulphonated polyesters may be formed from sulphonate containing polycarboxylic acids, including the ammonium and alkali metal, particularly sodium, salts of 4-sulphophthalic acid, 5-sulpho-isophthalic acid and sulphoterephthalic acid, or the acid anhydrides or lower alkyl (up to 10 carbon atoms) esters thereof. Such acids, or derivatives, are available as alkali metal salts, particularly the sodium sulphonate salt, and are conveniently incorporated in salt form into the polyester resin. Unsulphonated polycarboxylic acid components of the polyester resin include phthalic acid, isophthalic acid, terephthalic acid, cyclohexane-1,4-dicarboxyiic acid, adipic acid, sebacic acid, trimellitic acid and itaconic acid, or the acid anhydrides or lower alkyl (up to 10 carbon atoms) esters thereof.

Mixtures of two or more thereof, particularly those containing a predominant amount (>50 mole %) of isophthalic acid may also be employed. Suitable polyhydric alcohols for incorporation into the polyester resin include aliphatic and cycloaliphatic alkylene glycols, such as ethylene glycol, 1,2-propylene glycol, neopentyl glycol, cyclohexane- 1,44 methanol and 1,3-propane diol, and particularly aliphatic alkylene-oxy-glycois, such as diethylene glycol. The polyester resin may additionally comprise at least one aliphatic or cycloaliphatic dicarboxylic acid, such as cyclohexane-1,4-dicarboxylic acid, adipic acid, sebacic acid, trimellitic acid or itaconic acid, or polyester-forming equivalents thereof. If desired, the polyester resin may be modified by the inclusion therein of one or more monohydric alcohols, such as ethylene glycol monobutyi ether, benzyl alcohol and cyclohexanol.

The amount of hydrophilic binder present in the coating layer composition, and consequently in the final coating layer, is preferably in the range from 0 to 40%, more preferably 1 to 35%, particularly 5 to 30%, and especially 10 to 25% by weight, relative to the total solids of the composition.

The hydrophilic binder preferably has a number average molecular weight in the range from 500 to 100,000, more preferably 1,000 to 50,000, particularly 2,500 to 25,000, and especially 5,000 to 10,000.

In addition, the coating layer composition preferably contains a hydrophobic binder. The chemical composition of the hydrophobic binder component of the coating MTW50470 11 layer may vary over a relatively wide range of materials, and may be any one or more of the binder materials described herein. The hydrophobic binder is preferably an acrylic resin which can be prepared as an aqueous emulsion.

r By "acrylic resin" is meant a resin which comprises at least one acrylicandlor methacrylic component. The acrylic resin preferably comprises at least one monomer derived from an ester of acrylic acid andlor an ester of methacrylic acid, andlor derivatives thereof. In a preferred embodiment of the invention, the acrylic resin comprises in the range from 50 to 100 mole %, more preferably 70 to 100 mole %, and particularly 80 to 100 mole % of at least one monomer derived from an ester of acrylic acid andlor an ester of methacrylic acid, andlor derivatives thereof. A preferred acrylic resin comprises an alkyl ester of acrylic andlor methacrylic acid where the alkyl group contains up to ten carbon atoms such as methyl, ethyl, n-propyl, isopropyl, n-butyi, isobutyl, terbutyl, hexyi, 2-ethylhexyl, heptyl, and n-octyi. Polymers derived from an alkyi acrylate, for example ethyl acrylate and butyl acrylate, together with an alkyl methacrylate are preferred. Polymers comprising ethyl acrylate and methyl methacryiate are particularly preferred. The acrylate monomer is preferably present in the acrylic resin in a proportion in the range 30 to 65 mole %, and the methacrylate monomer is preferably present in a proportion in the range of 20 to 60 mole %.

Other monomers which are suitable for use in the preparation of the acrylic resin, which may be preferably copolymerised as optional additional monomers together with esters of acrylic acid andlor methacrylic acid, andlor derivatives thereof, include acrylonitrile, methacrylonitrile, halo-substituted acrylonitrile, halo-substituted methacrylonitrile, acrylic acid, methacrylic acid, acrylamide, methacrylamide, N-methylol acryiamide, Wethanol acrylamide, N-propanol acrylamide, N-methacrylamide, Wethanol methacrylamide, N-methyl acrylamide, N- tertiary butyl acrylamide, hydroxyethyl methacrylate, glycidyl acrylate, glycidyl methacrylate, dimethylamino ethyl methacrylate, itaconic acid, itaconic anhydride and half esters of itaconic acid. Other optional monomers include vinyl esters such as vinyl acetate, vinyl chloracetate and vinyl benzoate, vinyl phosphonic acid and esters and half esters thereof, vinyl pyridine, vinyl chloride, vinylidene chloride, maleic acid, maleic anhydride, styrene and derivatives of styrene such as chloro styrene, hydroxy styrene and alkylated styrenes, wherein the alkyl group contains from one to ten carbon atoms.

The amount of hydrophobic binder present in the coating layer composition, and consequently in the final coating layer, is suitably in the range from 0 to 40%, preferably 0 to 30%, more preferably 1 to 25%, particularly 3 to 20%, and especially 5 to 15% by weight, relative to the total solids of the composition.

MTW50470 12 The hydrophobic binder preferably has a minimum film forming temperature in the range from 0 to 600C, more preferably 10 to 40%, and particularly 20 to WC.

In one embodiment of the invention, the coating layer composition additionally comprises a, preferably low molecular weight, cross-linking agent. The cross-linking agent is suitably an organic material, preferably a monomeric andlor oligomeric species, and particularly monomedc, prior to formation of the coating layer. The molecular weight of the cross-linking agent is preferably less than 2000, more preferably less than 1500, especially less than 1000, and particularly in the range from 250 to 500. Suitable cross-linking agents may comprise alkyd resins, amine derivatives such as hexamethoxymethyl melamine, andlor condensation products of an amine, eg melamine, diazine, urea, cyclic ethylene urea, cyclic propylene urea, thiourea, cyclic ethylene thiourea, aziridines, alkyl melamines, aryl melamines, benzo guanamines, guanamines, alkyl guanamines and aryl guanamines, with an aldehyde, eg formaldehyde. A preferred cross-linking agent is the condensation product of is melamine with formaldehyde. The condensation product may optionally be alkoxylated. A catalyst is also preferably employed to facilitate cross- linking action of the cross-linking agent. Preferred catalysts for cross-linking melamine formaldehyde include ammonium chloride, ammonium nitrate, ammonium thiocyanate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, para toluene sulphonic acid, sulphuric acid, maleic acid stabilised by reaction Wth a base, ammonium para toluene sulphonate and morpholinium para toluene sulphonate.

The cross-linking agent preferably exhibits at least trifunctionality (ie three functional groups) to promote inter-molecular cross-linking with the functional groups present in the binder(s), and to improve adhesion of the coating layer to the surface of the underlying layer.

The amount of cross-linking agent present in the coating layer composition is preferably in the range from 0.1 to 50, more preferably 0. 15 to 25, particularly 0.2 to 10, and especially 0.25 to 5 weight %, relative to the total solids of the composition.

A surfactant may, if desired, be incorporated into the coating layer composition.

Suitable surfactants include a non-ionic surfactant, such as a fluorocarbon, a cationic surfactant, such as a quaternary ammonium salt, or an anionic surfactant. Additionally a viscosity control agent, or a humectant, such as glycerol, may be employed.

The ratio of filler(s) to binder(s) present in the coating layer composition, and consequently in the final coating layer, is preferably in the range from 0.5 to 4: 1, more preferably 1 to 3.5: 1, particularly 1.2 to 3.2: 1, and especially1.5 to 3: 1 by weight.

MTW50470 13 In a preferred embodiment of the invention, the coating layer composition has a total solids content of greater than 25% by weight, preferably in the range from 30 to 80%, more preferably 35 to 70%, particularly 40 to 55%, and especially 42 to 48% by weight.

The thickness of the dry coating layer is preferably in the range from 5 to 50, more preferably 6 to 40, particularly 10 to 35 pm, and especially 15 to 30 pm.

The coat-weight of the dry coating layer is preferably in the range from 0.5 to 25, more preferably 1 to 20, particularly 5 to 15, and especially 6 to 10 gm-2.

The surface of the coating layer preferably exhibits a 60' gloss value, measured as herein described, in the range from 5 to 85%, more preferably 10 to 75%, particularly 15 to 60%, and especially 20 to 40%.

The surface of the coating layer preferably exhibits a root mean square surface roughness (Rq), measured as herein described, in the range from 0. 1 to 10 pm, more preferably 0.25 to 7.5 pm, particularly 0.5 to 5.0 pm, and especially 1.0 to 3.0 pm.

The surface of the coating layer preferably exhibits a peak to valley surface roughness (Ry), measured as herein described, in the range from 0.1 to 50 pm, more preferably 0.5 to 40 pm, particularly 1.0 to 30 pm, and especially 1.0 to 20 pm.

The coating layer suitably exhibits a liquid absorption, measured as herein described, of greater than 10%, preferably in the range from 30 to 95%, more preferably 40 to 90%, particularly 50 to 85%, and especially 60 to 80%. By "liquid absorption" is meant the % increase in the weight of the coating material when saturated with a 1:1 w/w mixture of water and ethylene glycol.

The coating layer composition is conveniently applied to the polymeric film substrate by a conventional coating technique, for example by deposition from a dispersion of the filler(s) and binder(s) in a volatile medium, such as an aqueous or organic solvent medium. An aqueous medium is particularly preferred.

The coating layer may be applied to the polymeric film substrate after the production of the film has been completed, ie "off-line", but is preferably applied during the film making process, ie "in-line", in order to simplify and improve the efficiency of the production process.

The coating layer composition may be applied "in-line" to an already oriented film substrate, for example by post-stenter coating, but application of the coating medium is preferably effected before or during any stretching operation. In particular, it is preferred that the coating layer composition should be applied to the film between the two stages (longitudinal and transverse) of a biaxial stretching operation, ie by "inter-draw coating". Such a sequence of stretching and coating is especially preferred MTW50470 14 is for the production of linear polyester films, such as polyethylene terephthalate films, which are preferably firstly stretched in the longitudinal direction over a series of rotating rollers, coated with the coating layer composition and then stretched transversely in a stenter oven, preferably followed by heat-setting. A coating layer applied by 9nter-draw coating" surprisingly exhibits increased liquid absorption, preferably greater than 1 %, more preferably in the range from 2 to 20%, particularly 4 to 15%, and especially 5 to 10%, compared to the liquid absorption of a coating layer applied to an oriented film.

Curing of the applied coating layer is preferably effected by conventional drying techniques, for example by suspending the coated substrate in a hot air oven maintained at an appropriate temperature. In a particularly preferred embodiment of the invention, a heating or curing temperature in the range from 135 to 2500C, more preferably 150 to 2300C, particularly 180 to 2250C, and. especially 200 to 220% is employed.

A surprising feature of the present invention is that coating layers of the required thickness, particularly for use in ink-jet printing, can be applied at high line speeds. Thus, the coating layer composition is suitably applied to a film web travelling at a line speed of greater than 20 metres min-1, preferably in the range from 20 to 120, more preferably 25 to 100, particularly 30 to 65, and especially 40 to 50 metres min-l.

The coating layer composition is suitably applied to the polymeric film substrate at a wet thickness of greater 5 pm, preferably in the range from 10 to 100 pm, more preferably 15 to 85 pm, particularly 25 to 75 pm, and especially 30 to 60 pm. The applied wet coat weight is preferably in the range from 5 to 150, more preferably 10 to 130, particularly 25 to 100, and especially 30 to 80 grTy'.

In a preferred embodiment of the invention, the recording sheet additionally comprises a primer layer which functions to improve the adhesion of the coating layer to the substrate. The chemical composition of the primer layer may vary over a wide range, and may comprise polymeric materials such as an acrylic resin, polyurethane, polyester, andlor copolymers thereof.

The primer layer, or polymeric material thereof, preferably has a melting point less than that of the polymeric material of the substrate layer. The primer layer suitably has a melting point in the range from 2 to 1000C, preferably 5 to 650C, more preferably to 600C, particularly 30 to 55OC, and especially 400C to 500C less than the melting point of the polymeric material of the substrate layer. The melting point of the primer layer is preferably in the range from 180 to 2500C, more preferably 200 to 240%, particularly 205 to 230%, and especially 210 to 2200C.

MTW50470 The primer layer preferably comprises a polyester, particularly a copolyester resin derived from one or more dibasic aromatic carboxylic acids, such as terephthalic acid, isophthalic acid and hexahydroterephthalic acid, and one or more glycols, such as ethylene glycol, diethylene glycol, triethylene glycol and neopentyl glycol. Typical copolyesters which provide satisfactory properties are those of ethylene terephthaiate and ethylene isophthalate, especially in the molar ratios of from 50 to 90 mole % ethylene terephthalate and correspondingly from 50 to 10 mole % ethylene isophthalate. Preferred copolyesters comprise from 65 to 85 mole % ethylene terephthalate and from 35 to 15 mole % ethylene isophthalate, and especially a copolyester of about 82 mole % ethylene terephthalate and about 18 mole % ethylene isophthalate.

Formation of the primer layer on the polymeric film substrate may be effected by conventional techniques, for example by casting the polymer onto a preformed substrate layer. Conveniently, however, formation of a composite film (substrate and primer layer) is effected by coextrusion, either by simultaneous coextrusion of the respective film-forming layers through independent orifices of a multi- orifice die, and thereafter uniting the still molten layers, or, preferably, by single- channel coextrusion in which molten streams of the respective polymers are first united within a channel leading to a die manifold, and thereafter extruded together from the die orifice under conditions of streamline flow without intermixing thereby to produce a composite film.

A coextruded film is preferably stretched to effect molecular orientation of the substrate, and preferably heat-set. Generally, the conditions applied for stretching the substrate layer will induce partial crystallisation of the primer layer polymer and it is therefore preferred to heat set under dimensional restraint at a temperature selected to develop the desired morphology of the primer layer. Thus, by effecting heat-setting at a temperature below the crystalline melting temperature of the primer layer polymer and permitting or causing the composite to cool, the primer layer polymer will remain essentially crystalline. However, by heat-setting at a temperature greater than the crystalline melting temperature of the primer layer polymer, the latter will be rendered essentially amorphous. Heat-setting of a composite film comprising a polyester substrate and a copolyester primer layer is conveniently effected at a temperature within a range of from 175 to 20WC to yield a substantially crystalline primer layer, or from 200 to 2500C to yield an essentially amorphous primer layer. An essentially amorphous primer layer is preferred.

MTW50470 16 Primer layers may be disposed on one or both sides of the polymeric film substrate. The primer layer preferably has a thickness of up to 50 pm, more preferably in the range from 0,5 to 35 pm, particularly 1 to 20 pm, and especially 5 to 15 pm.

The recording sheet may also comprise one or more additional, inkpermeable super-coat layers which can be applied to the surface of the coating layer during or after manufacture of the polymeric film substrate. The super-coat layer may be applied in order to increase the rate of absorption of ink and total absorptive capacity of the recording sheet; or to increase the gloss of the recording sheet; or to control the imaging properties of the recording sheet such as dot gain, bleed, colour density, etc., or to enhance the dye andlor pigment binding properties of the recording sheet; or to provide resistance to mechanical damage, smudging, ultra-violet light, water, environmental damage, etc.

The super-coat layer may comprise hydrophilic organic polymers andlor hydrophilic inorganic polymers. Suitable hydrophilic organic polymers include polyvinyl alcohol and copolymers thereof, polyvinyl pyrrolidone and copolymers thereof, cationic polymers, cellulosic polymers, gelatine and combinations thereof. Suitable hydrophilic inorganic polymers include so] gel polymers such as alumina boehmite, alumina, silica, and transition metal oxides, preferably having an average pore radius between 0.001 and 0.08 pm and particularly between 0.02 and 0.06 pm.

The super-coat layer may also comprise hydrophobic organic polymers, non-film forming organic polymers, white pigments and colloidal pigments.

The recording sheet may also comprise a protective top-coat layer which can be applied to the surface of the coating layer or super-coat layer.

The recording sheet of the invention may conveniently contain any of the agents conventionally employed in the manufacture of polymeric films. Thus, agents such as dyes, pigments, lubricants, anti-oxidants, antistatic agents, surface active agents, gloss-improvers, prodegradants, fire-retardants, and ultra-violet light stabilisers may be incorporated in the substrate andlor primer layer andlor coating layer, andior super-coat layer, andlor top-coat layer, as appropriate.

The recording sheet may vary in thickness depending on the intended application, but sheets preferably have a total thickness in the range from 5 to 350, more preferably 25 to 250, and particularly 100 to 175 pm.

In this specification the following test methods have been used:

(i) Volume distributed median particle diameter, and particle size distribution ratio D,,1D,, of the filler particles were measured using a Microtrak 11 (Leeds & Northrup Co.) particle size analyser.

MTW50470 17 BET specific surface area and pore volume of the filler particles were measured using an Autoscan 60 Mercury Porosimeter (Quantachrome Co.).

(iii) Viscosity of the coating layer composition was measured by a Brookfield DW 1+ (Brookfield Engineering Lab) viscometer.

(iv) 600 gloss value of the surface of the recording sheet was measured using a Glossgard 11 (Gardner/Neotec Instrument Division) glossmeter.

(v) The root mean square roughness (Rq) and peak to valley roughness (Ry) of the coating layer surface were measured using a Talysurf 6 surface profilometer.

(vi) Adhesion of the coating layer to the surface of the underlying film was measured by scodng a cross-hatch pattern onto the surface of the coating layer, applying adhesive tape to the entire scored area and pulling the tape quickly off the film. The area of coating remaining on the film was expressed as a percentage of the total area of the cross-hatch pattem.

(vii) Ink-jet ink drying time was measured by the following method. A vertical stdpe of solid green colour was phnted onto a 85' x 1 '1" sample using a Deskjet 855Cxi (Hewlett Packard) ink-jet printer. The length and width of the test image was previously adjusted so that the phnt completed in 3 minutes. As soon as the phirited sample was ejected from the pirinter it was placed in contact with a sheet of Tidal DP (Hammermill) office paper and manual pressure applied to ensure close contact. The paper sheet was removed and examined for offset of ink from the test sample. The dry time is approximated by the proportion of the length of the test stripe which shows offset of ink onto the paper, eg if offset occurred along 113 of the length of the test stripe, the drying time was recorded as 1 minute.

(viii) Liquid absorption of the coating layer was measured using the following procedure. A 2 cm x 2 cm sample of the coated film was first weighed accurately The sample was then immersed in the test fluid for 5 minutes, removed from the test fluid and the surface blotted dry using an absorbent paper towel, and quickly re-weighed (W). The sharp edge of a scalpel or similar instrument was used to scrape all the coating off the film and the sample re-weighed again (W). The weight of the coating on the film sample was calculated as W, - W, and the amount of test fluid absorbed in this coating amount is W2 - W, To express the liquid absorption as a percentage, the quotient (W2 - W) 1 (W, - W) was multiplied by 100.

The invention is illustrated by reference to the accompanying drawings in is which:

Figure 1 is a schematic sectional elevation, not to scale, of a recording sheet having a coating layer bonded to a substrate layer.

MTW50470 18 Figure 2 is a similar schematic elevation of a recording sheet having an additional primer layer between the substrate and coating layer.

Figure 3 is a similar schematic elevation of a recording sheet having an additional super-coat layer bonded to the coating layer.

Figure 4 is a similar schematic elevation of a recording sheet having an additional top-coat layer bonded to the super-coat layer.

Referring to Figure 1 of the drawings, the film comprises a polymeric substrate layer (1) having a coating layer (2) bonded to one surface (3) of the substrate.

The film of Figure 2 additionally comprises a primer layer (4) between the substrate (1) and coating layer (2).

The film of Figure 3 additionally comprises a super-coat layer (5) bonded to the surface (6) of the coating layer (2).

The film of Figure 4 additionally comprises a top-coat layer (7) bonded to the surface (8) of the super-coat layer (5).

The invention is further illustrated by reference to the following examples.

Example 1

A polyethylene terephthalate film was melt extruded, cast onto a cooled rotating metal drum having a polished surface and stretched to 3.6 times its original dimension in the direction of extrusion at a temperature of about 900C. The monoaxially oriented film was coated on one side only with an aqueous coating layer composition comprising the following ingredients:

APA-ETA420 5.7 wt% (alumina, supplied by Condea) Syloid 74 6.7 wt% (silica, supplied by W. R. Grace) AQ35D 34.0 wt% (30% w/w aqueous dispersion of sulphonated copolyester, supplied by Eastman Chemical) CL-204 (43% w/w aqueous dispersion of acrylic ester emulsion, supplied by Rohm & Haas) 28.8 wt% MTW50470 19 Acrysol 1-3000 (49% w/w aqueous dispersion of acrylic binder, supplied by Rohm & Haas) Demineralised water 1.4 WM 23.4 wt% Total Dry Solids 37.0% The coated film was passed into a stenter oven, where the film was stretched in the sideways direction to approximately 3.8 times its original dimensions. The coated biaxially stretched film was heat set at a temperature of about 2200C by conventional means. The resulting film comprised a polyethylene terephthalate substrate layer of 125 pm thickness, and a dry coating layer of 9 pm thickness, having a coat weight of 1.37 gM-2. The surface of the coating layer exhibited the following properties; (a) Root mean square roughness (Rq) = 1.8 pm (b) Peak to valley roughness (Ry) = 18.1 pm (c) Liquid Absorption = 35.0% A super-coat layer comprising a 4:1 mixture of polyvinyl alcohol (Gohsenol N-300, Nippon Gohsei) and polyvinyl pyrrolidone (Luviskol K90, BAS17) at 20% w/w total solids in water was applied to the surface of the dry coating layer at 4 pm wet coat weight by Meyer rod and dried at 11 OOC for 4 minutes. The surface of the super-coat layer exhibited the following properties; (d) 600 gloss = 20.9% (e) Drying Time = 2.5 min The recording sheet was suitable for ink-jet printing.

Example 2

A polymeric film comprising polyethylene terephthalate as the substrate and a primer layer comprising a copolyester of 82 mole % ethylene terephthalate/18 mole % ethylene isophthalate was produced.

The aforementioned polyesters were prepared using a conventional process by direct esterification of ethylene glycol with an acid (ie terephthalic acid in the case of polyethylene terephthalate or a mixture of 82 mole % terephthalic acid and 18 mole % MTW50470 isophthalic acid in the case of the copolyester) followed by polycondensation. After terminating the polycondensation, the polymer was cut into small granules suitable for extrusion.

The polymeric film was produced from the above polyesters by a process of single channel coextrusion wherein streams of polyethylene terephthalate and the copolyester supplied by separate extruders were united in a tube leading to the manifold of an extrusion die and were extruded simultaneously through the die under conditions of streamline flow and without intermixing. The composite film emerging from the extrusion die was quenched immediately upon a water-cooled rotating metal drum having a polished surface and stretched to 3.6 times its original dimension in the direction of extrusion at a temperature of about 900C. The monoaxially odented film was coated on the surface of the primer layer with a coating layer composition as described in Example 1. The resulting film comprised a biaxially oriented and heat-set polyethylene terephthalate substrate, an amorphous copolyester phmer layer and a coating layer. The substrate layer was 112 pm, the primer layer 13 pm, and the dry coat weight of the coating layer was approximately 1.4 gm-1. The surface of the coating layer exhibited the following properties; (a) Root mean square roughness (Rq) = 1.75 pm (b) Peak to valley roughness (Ry) = 17.8 pm (c) Liquid Absorption = 36.5% The surface of the coating layer was coated with a super-coat layer as described in Example 1. The surface of the super-coat layer exhibited the following properties; (d) 600 gloss = 21.0% (e) Drying Time = 2.5 min The recording sheet was suitable for ink-jet phnting.

Example 3

The procedure of Example 2 was repeated except that the coating layer composition was applied to give a dry coat thickness of 32 pm, a dry coatweight of 8 gM-2 and had the following composition:

APA-ETA420 16.2 wt% MTW50470 21 (alumina, supplied by Condea) Syloid 74 (silica, supplied by W. R. Grace) 11.6 WM AQ35D (30% w/w aqueous dispersion of sulphonated copolyester, supplied by Eastman Chemical) CL-204 (43% w/w aqueous dispersion of acrylic ester emulsion, supplied by Rohm & Haas) Acrysol 1-3000 (49% w/w aqueous dispersion of acrylic binder, supplied by Rohm & Haas) Demineralised water Total Dry Solids 14.3 WM 14.0 WM 7.2 wt% 36.7 wt% 38.9% The surface of the coating layer exhibited the following properties; (a) Root mean square roughness (Rq) = 5.6 pm (b) Peak to valley roughness (Ry) = 36.5 pm (c) Liquid Absorption = 69.2% The surface of the coating layer was coated with a super-coat layer as described in Example 1. The surface of the super-coat layer exhibited the following properties; (d) 600 gloss = 25.4% (e) Drying Time = 10 sec The recording sheet was suitable for ink-jet printing.

MTW50470 22 Example 4

The procedure of Example 2 was repeated except that the coating layer composition was applied to give a dry coat thickness of 21 pm, a dry coatweight of 6.3 gM-2 and had the following composition:

APA-ETA420 23.0 WM (alumina, supplied by Condea) AQ35D 15.9 WM (30% w/w aqueous dispersion of sulphonated copolyester, supplied by Eastman Chemical) CL-204 (43% w/w aqueous dispersion of acrylic ester emulsion, supplied by Rohm & Haas) 14.8 wt% Acrysol 1-3000 (49% w/w aqueous dispersion of acrylic binder, supplied by Rohm & Haas) Demineralised water Total Dry Solids 7.7 wt% 38.6 wt% 38.0% The surface of the coating layer exhibited the following properties; (a) Root mean square roughness (Rq) = 2.6 pm (b) Peak to valley roughness (Ry) = 25.7 pm (c) Liquid Absorption = 58.7% The surface of the coating layer was coated with a super-coat layer as described in Example 1. The surface of the super-coat layer exhibited the following properties; (d) 60' gloss = 43.5% (e) Drying Time = 1.5 min MTW50470 23 The recording sheet was suitable for ink-jet printing.

MTW50470 24

Claims

1 A recording sheet comprising a polymeric film substrate having on at least one surface thereof, a coating layer comprising in the range from 5 to 95% by weight of at least one filter, and 5 to 95% by weight of at least one binder.

2. A sheet according to claim 1 wherein the coating layer comprises in the range from 40 to 80% by weight of filler.

3. A sheet according to either one of claims 1 and 2 wherein the filler has a BET specific surface area of greater than 100 m'lg.

4. A sheet according to any one of the preceding claims wherein the coating layer comprises a binder which functions as a viscosity reducer.

5. A sheet according to any one of the preceding claims wherein the coating layer has a liquid absorption of greater than 10%.

6. A sheet according to any one of the preceding claims wherein a supercoat layer is present on the surface of the coating layer.

7. A method of producing a recording sheet which comprises forming a polymeric film substrate, and applying a coating composition to at least one surface of the substrate, the coating composition having a total solids content of greater than 25% by weight, and comprising in the range from 5 to 95% by weight of at least one filler, and 5 to 95% by weight of at least one binder, both relative to the total solids of the composition.

8. A method according to claim 7 wherein the coating layer composition is applied during the polymeric film substrate making process.

9. A method according to either one of claims 7 and 8 wherein the coating layer composition is applied to a film web travelling at a line speed of greater than 20 metres min'.

10. A method according to any one of claims 7 to 9 wherein the applied coating layer is heated to a temperature in the range from 135 to 2500C.