WO2004003093A1

WO2004003093A1 - Coating compositions

Info

Publication number: WO2004003093A1
Application number: PCT/EP2003/006742
Authority: WO
Inventors: Richard Southward
Original assignee: Ucb, S.A.
Priority date: 2002-07-01
Filing date: 2003-06-26
Publication date: 2004-01-08
Also published as: AU2003242760A1; GB0215154D0

Abstract

There is described an aqueous coating composition for a plastic film which comprises (a) from 40% to 60% (by dry weight) of a binder resin (such as a PVdC:/ EEA copolymer; vinylidene chloride / vinyl chloride copolymer; vinyl acetate / ethylene copolymer; and/or a vinyl acetate / ethylene / vinyl chloride terpolymer); and (b) from 60% to 40% (by dry weight) of ink receptive filler particles (such as PVC, precipitated silica, calcium carbonate, sodium aluminosilicate and/or sodium magnesium aluminosilicate) and has a mean particle size of 10 nanometres (0.01 microns) to about 5 microns. Preferably the filler particles are at least partially soluble in the binder resin. Films (such as primed BOPP) which are top coated with such compositions are printable with solvent piezo ink-jettable inks.

Description

COATING COMPOSITIONS

The present invention relates to a coating composition for a plastic film. More specifically, the invention relates to a coating composition which improves the physical characteristics of a plastic film. Even more specifically, the invention relates to a coating composition which renders a plastic film receptive to inks. Particularly, the invention concerns a coating composition designed to render a plastic film printable with an ink-jet printing ink such as an non-aqueous ink applied by a piezo ink-jet printer. Such a coat may usefully comprise a binder resin with compatible ink absorbing particles

Polyolefin films are very useful as protective packaging because they resist tearing and water penetration. However, a significant drawback to their use is the difficulty encountered in printing the film, with certain types of ink. Typically, polyolefin films have low surface energies which make them non-receptive to certain inks which tend to exhibit bad ink wetting properties; i.e. they fail to form a complete coating on the film; the ink forms into discrete beads and streaks on the film. Additionally, certain inks tend to dry slowly, smear easily and rub off once dry. These are serious problems particularly when plastic films are used for shipping articles which are often exposed to moisture and rough handling. To overcome these difficulties printed matter is usually applied to a paper label which is then attached to the wrapping but this is inconvenient and presents recycling problems because paper recycling is usually incompatible with plastic recycling.

Thus it is well known to coat polymeric films to improve ink reception. Some prior art ink receptive coatings are described below.

The following top coats are described generally without particular reference for use with ink jet printing.

US 3,489,597 discloses that plastic surfaces are made ink receptive by coating with a composition containing a copolymer of vinyl acetate and vinylpyrrolidone. No specific reference to polyolefins is made.

FR 2,212,371 includes a disclosure of the use of coatings composed of: either acrylate or methacrylate copolymers containing carboxyl groups or copolymers of vinyl chloride or vinyl acetate as ink receptive layers on plastic surfaces. This reference teaches away from combining these two types of coating. FR 2,352,667 relates to providing a printable layer to receive an ink inscription. The layer can contain talc or colloidal silica with a binder which can be a copolymer of (meth)acrylate with carboxy groups or a copolymer of vinyl chloride or vinyl acetate

US 4,085,245 describes a transparency for coloured photocopies which includes a layer which includes as components a mixture of an acrylic polymer and a copolymer of vinyl acetate and vinyl chloride. Photocopying is an electroreprographic process using toner which is very different from ink jet printing with an ink-jettable ink and so the properties require for film printability in each case are very different. Therefore this reference would be considered irrelevant to someone interested in substrates for ink-jet printing.

EP 0,315,063 (Hitachi Maxwell) describes a substrate coated with an indicia-receiving layer comprising a pigment and at least one binder resin selected from the group consisting of vinyl chloride-vinyl acetate copolymers and polyurethane.

EP 0,524,635 (Mitsubishi Paper) discloses a sheet with an ink receiving layer of starch particles, modified starch particles and ethylene-vinyl acetate copolymer resins (10 to 40 mol % ethylene) and a cationic dye fixing agent (0.2 to 40 mEq / m² of cation charge per unit area). The opposite side of the sheet also has a back-coat layer with an inorganic pigment of mean aspect ratio 5 to 90 and mean particle size 0.1 to 25 nm.

EP 1 ,125,169 (= WO 99/28791 ) (3M) describes a multiple-layer imaging medium (sheet) comprising: (a) a backing layer of polypropylene, (b) an intermediate layer of ethylene α (alpha)-olefin polymer (10 to 30 % w/w α (alpha)-olefin, density < 0.910 g / cm³, polydispersity < 3.5) and ethylene vinyl acetate (EVA) (vinyl acetate from 9 to 45% w/w, including unmodified, acid modified and/or anhydride modified EVA) and (c) a toner receptor layer of polymer(s) formed from ethylene monomer, optional anhydride monomer(s) and a monomer from vinyl acetate, vinyl acrylate and/or vinyl carboxylic acids. These sheets are designed for use with electrophotographic printing processes which have very different requirements to those substrates to be ink-jet printed. For example the sheets as described in this document are constrained by various other parameters given therein which are irrelevant or non optimal for ink-jet printing.

EP 0228835 (3M) describes a receptor film for thermal mass transfer printing. The film is transparent with a haze value < 15% and comprises a transparent backing with transparent image receptive layer thereon of a wax-compatible material (softening between 30° to 90°C), the layer with a coefficient of static friction < 0.50 and having critical surface tension > 31 mN / m and > the donor material on the donor sheet used in the thermal mass transfer. The receptive layer is sufficiently anchored to the backing to allow the image receptive layer to remain on the backing during the transfer printing.

US 4686549 describes a receptor sheet for thermal mass transfer printing which comprises an image receptive layer of an optional wax with a polymeric material such as polycaprolactones, chlorinated polyolefins, blends of chlorinated polyolefin and polymethyl methacrylate, block copolymers of styrene-ethylene / butylene-styrene, and copolymers of ethylene and vinyl acetate. Example VIII is a comparative example without wax using 20% w/w of an ethylene vinyl acetate copolymer ("Elvax" 310) in toluene as an image receptive layer. This image receptive layer had a coefficient of static friction against aluminium of 1.50 and a softening temperature of 88°C. Film haze was less than 4%.

The preceding references illustrate that for non-ink-jet printing processes such as electroreprography and/or thermal mass transfer the chemistry of any printable top coat cannot be considered in isolation as other physical parameters limit optimal image quality on the substrate and these very different for (and tailored towards) each printing method. Thus references describing printable coatings for substrates printed by thermal mass transfer would not be considered relevant to the problem of providing improved ink-jet printable substrates.

EP 0507409 (= US 5,215,814) (Arkwright) describes a fast drying printing film for use in offset lithography and similar printing applications comprising a transparent, translucent or opaque film substrate having an ink receptive essentially transparent polymeric layer on at least one side of the substrate, said ink receptive layer containing one or more polymers or copolymers, at least one of said polymers or copolymers being soluble or swellable in an aliphatic hydrocarbon solvent, said ink receptive layer having a solvent absorptivity of Isopar G of from 14% to 45% by weight with respect to the weight of the ink receptive layer, a Sheffield surface roughness value of less than 140 cc of air/minute and an offset dry time of less than about two hours. This again illustrates the previous point about the variation in properties required of substrates used in different printing methods

Nevertheless there have been some attempts to make a universal substrate printable by many methods.

EP 0778156 (Oce USA Inc) describes a multi-purpose imageable sheet stated to be useful for multiple printing methods including manual drafting, ink jet printing and electrophotography. The sheet comprises a surface coating formed from an aqueous- dispersion of a pigment and a cross-sinkable polymer and cross-linker therefor where the coating has a matrix hardness of at least 4B to 9H, a pencil abrasivity of from 0.001 to 0.010 gram, a surface resistivity of from 1 x 10⁸ to 1 x 10¹⁵ Ω(ohms) / m², and an ink contact angle of from 27° to 120°. Notwithstanding this reference it would be appreciated that such substrates will inevitably lead to compromise in print quality for each method and if improved print properties are desired it would be better to optimise the substrate to the specific printing method and/or ink to be used.

The following documents disclose substrates design to be printed by various ink-jet printing processes.

GB 2,050,866 describes a recording sheet for inkjet printing formed by applying a layer of a water soluble polymer to, a support. The coating formed has a water absorption of not more than 30 gm/m². A long list of suitable polymers is provided which includes a passing reference to vinyl acetate maleic anhydride copolymer.

US 4,904,519 (3M) describes polymer sheets coated with a transparent ink receptive layer of cross-linked (2 to 8 % w/w of borate, titante dichromate or aldehyde cross-linker) hydrolysed (80% to 95%) copolymer from vinyl ester co-monomer (30% to 70% w/w of vinyl acetate, vinyl propionate or vinyl stearate) and vinyl amide co-monomer (70% to 30% w/w of N-vinyl pyrrolidone or vinyl acetamide). These sheets are designed to be printed by pen plotters or ink jet printers that using either water- or solvent based dye inks, although in fact water based systems are prefer. The ink receptive coating is stated to be resistant to fingerprinting and inhibits clogging of pen tips. This reference does not provide a solution to the problem of printing with solvent based pigmentary ink-jet inks.

US 6,113,679 (3M) relates to piezo inkjet inks and methods for making and using same and includes a method which describes certain polymeric films as the ink receiving substrate including acrylic-containing films, poly(vinyl chloride)-containing films, multi-layered films having an image reception layer comprising an acid- or (acid / acrylate) modified ethylene vinyl acetate resin. This reference does not disclose coated polyolefin films as a substrate.

EP 0,914, 258 (= US 5,721 ,086) (3M) describes an image receptor medium comprising a outer surface layer of an acid- or acid / acrylate-modified ethylene vinyl acetate (EVA) resin. The image receptor medium may further include optional layers of primer, adhesive and/or an inkjet layer.

EP 1 ,122,083 (Mitsubishi Paper Mills) discloses an ink jet recording material for non- aqueous ink, which comprises an ink-absorbing layer containing at least a pigment on a support, the ink-absorbing layer being coated or impregnated with a polymer soluble or swellable in a petroleum system high boiling point solvent, wherein at least 30 % w/w of the pigment is calcium carbonate, kaolin or barium sulphate.

None of the prior art printable polymeric films are entirely satisfactory. It is an object of the present invention to provide ink-jet printable substrates which solve some or all or the deficiencies in the prior art, preferably by having improved printablity to solvent based pigmentary inks applied thereto by a peizo inkjet printer. It is a more preferred object of the invention that the printable top coat exhibits improved compatibility with underlying film coats such as those to modify the barrier properties of the film.

WO 03/016045 (A very) is a co-pending patent application published 27 February 03 after the priority date of the present application. It describes an ink receptive coating of the combination of a filler; a binder of a vinyl alcohol, vinyl acetate and/or vinyl chloride polymer and also a cationic polymer or polyethylene imine.

Surprisingly the applicant has discovered that including certain particles in a top coat formulation leads to an unexpected improvement in the printability thereon.

Therefore broadly in accordance with the present invention there is provided a polymeric film coated on its outer surface with a formulation comprising a) from about 40% to 60% by dry weight of the coat of a binder resin and b) from about 60% to about 40% of compatible filler particles optionally at least partialy soluble in said binder resin and having a mean particle size from about 10 nanometres (0.01 microns) to about 10 microns.

Preferably the formulation of the invention is substantially free of cationic polymer or polyethylene imine.

The coating is designed to improve printability of the film especially with solvent based ink- jettable inks.

Preferably the binder resin is filled with the compatible particles which are partially or wholly soluble in the binder resin and/or the ink solvent so that the particles are compatible with the coat and absorb the ink.

Preferably the dry coating formulation is dispersible in an aqueous media and is applied to the primed film as an aqueous formulation. Preferably the binder resin comprises a vinyl (co)polymer such as a vinylidene halide polymer.

More preferably the binder resin comprises co and/or ter polymers obtained and/or 5 obtainable from monomers of vinyl halide, vinyl acetate, ethylene, acrylonitrile, (meth)acrylic acid and/or acrylate.

Most preferably the binder resin comprises ethylene acrylic acid (EEA) copolymers and/or vinyl chloride / vinyl acrylic / ethylene terpolymers, for example those identical terpolymers 0 available commercially from Air Products under the trade designations CEF 19 (unplasticised) and/or CEF 18 (plasticised). Other suitable copolymer and terpolymer binder resins include Ethylene-Vinyl Chloride (E/VC) copolymer; Ethylene-Vinyl Acetate (E/VA) copolymer; and Ethylene-VC-VA terpolymers and others as described in the applicant's co- pending application GB 0207089.4 filed 26 March 2002 (entitled "Printable film" - applicant's 5 ref. 31.19 GB) the contents of which are hereby incorporated herein by reference, including the definitions of general terms therein

Vinylidene halide polymer (preferably vinylidene chloride or PVdC) may be a homo polymer or may also to be useful as a binder resin herein be a copolymer comprising other

.0 ethylenically unsaturated acid copolymers obtained and/or obtainable by polymerising monomers selected from acrylic acid, ethyl acrylate and methyl methacrylate. Other ethylenically unsaturated acid copolymers suitable for use in the present invention are those described in WO 00/63309 (Paramelt) the contents of which are hereby incorporated by reference.

.5

Preferably the filler particles comprise soluble organic particles, more preferably hard acrylic spheres (e.g. those available commercially under the trade names Diakon or Paraloid) microcrystalline cellulose (e.g. that of mean particle size 10 micron available commercially from Rettenmeier under the trade names Arbocel BE 600/10) and/or polyvinyl halide (such

)0 as PVC, for example the PVC latex available commercially from Solvin under the trade designation GA072. Conveniently the filler particles are PVC.

The filler particles may also additionally comprise inorganic absorptive filler particles such as silica (whether colloidal, precipitated, gel or fumed silica), optionally of a particle size S5 from about 0.02 micron to about 5 micron.

Preferably the filler particles have a mean particle size from about 10 nanometres (0.01 microns) to about 5 microns; more preferably from about 100 nanometres (0.1 microns) to about 4 microns, most preferably from about 200 nanometres (0.2 microns) to about 3 microns .

In a particularly advantageous embodiment of the invention the top coat comprises PVC particles within a PVdC / EEA binder resin (preferably where the EEA is non acidified). Such a top coat has been found to unexpectedly improve ink-jet printing of film with solvent based inks by for example speeding absorption of ink into the coat ("strike-in") reducing dot spread and improving print resolution. The top coat also formulated to be compatible with an underlying primer coating and is applied from an aqueous dispersion.

Optionally other conventional additives may be incorporated into the composition. Suitable additives are emulsifiers, anti-foaming agents, coalescing agents, dispersing agents, wetting agents, anti-settling agents, thickeners, flatting agents, stabilisers, etc. Other suitable additives will be well-known to those skilled in the art.

Suitable substrates, which can be used in this invention, are any plastic films suitable for the desired end use application. Polyolefin films are preferred, especially oriented polypropylene films, and still more preferred is a multi-layer oriented polypropylene film as disclosed in the EP 0202812.

The (polyolefin) film is preferably primed with any suitable method or composition for example polyurethane, silane, epoxy, PEI and the like.

The thickness of the substrate is not critical and may vary largely. Most often, films having a thickness comprised between 10 and 100 microns, preferably between 50 and 75 microns are used for label or tapes applications; thinner films may be used for packaging applications (for example between 10 and 50 microns). According to the present invention,

2 the coating may be applied at a coat weight of at least 3 g/m and preferably at more than

5 g/m² on the substrate by the method of roll coating, blade coating, spray coating, air knife coating, brush coating, rod bar coating, reverse gravure, etc. on the substrate and then dried, for example, in a hot air oven. The present invention relates therefore also to a film coated with the composition according to the invention.

Applications of the coated film of the invention include ink-jet printable facestocks for tapes, labels and self-adhesive materials as well as ink-jet printable packaging films or sheets. Generally, the invention relates to ink-jet printable synthetic paper.

Further aspects of and preferred features of the present invention are described in the claims.

The invention will now be illustrated by the following non limiting examples.

Primed film A white pigmented, 90 micron bi-axially oriented polypropylene film (available commercially from UCB Films under the trade designation WG90) was first coated by an offset process with a conventional aqueous primer coating and then dried at 120°C to give a final dry coat weight of 0.02 g/m². The primed film was used in the following examples.

Example 1

A dry top coat formulation was prepared from a conventional PVdC binder resin (also comprising some methyl acrylate, acrylonitrile and acrylic acid) . Optionally (Examples 1 to 2) the PVdC was mixed with same weight of PVC particles with an average particle size of 3 micron (such as those made by suspension polymerisation and available commercially from Norsk Hydro under the trade name Pevikon PE 702). The top coat dry formulation was dispersed in water (at 40 % w/w solids) and applied to the primed film by a direct, reverse- gravure process. The topcoat was then dried in an oven at 120° C to give a final dry coat weight of 7 g/m².

Example 2

A film was prepared identically to Example 1 except the topcoat was applied twice to effectively double the coat weight to 14 g/m².

Comp A A film was prepared identically to Example 1 except the PVC particles were omitted to provide a comparison. The single top coat had a coat weight of 7 g/m².

Comp B

A film was prepared identically to Example 2 except the PVC particles were omitted to provide a comparison. The double top coat had a coat weight of 14 g/m².

Each of the films above were tested as follows:

Strikinα-in test "Striking -in" generally denotes the relative penetration of a liquid into a material and used with reference to printing, it refers to the absorption of the ink vehicle into the printed substrate as ink "dries" by means of such absorption (strike in). Thus a test for "strike-in" is designed to determine the speed of the absorption of ink. The test is performed as follows.

The test is executed under standard conditions (23.0°C ± 1.0 ° and 50.0% RH ± 2.0%) to minimise any influence of temperature and relative humidity on the properties of the substrate.

A test strip (typically 35 x 270 mm) of the substrate of interest is printed using an aluminium disc. The ink used for the test is a conventional striking-in ink (8% carbon black, 90% mineral oil, 2% resin, wax, colouring agent such as "Penetration Ink 404.003.02" available commercially from IGT Reprotest b.v., Amsterdam). After 30 seconds the printed strip is brought into contact with a blank set-off strip (IGT coated paper, code Ka, 404.005.04). When a printed test strip that is not yet dry is brought into contact with the blank piece of paper, part of the ink that has not been absorbed on the test substrate will smear (or set-off) on the blank strip. Thus by measuring the density of the ink transferred to the set-off strip the degree of "striking-in" of the ink in the original test sample can be determined. The set- off strip is left for 24 hours to dry before the ink density is measured as described below.

The ink density of the set-off sample is measured using a conventional Macbeth Densiometer (Model TR927) and is reported as the average of 5 readings of contrast density calibrated against 100% white and 100% black. This is a ratio of the density of the set-off strip to that of a 100% black standard so it is dimensionless.

The smaller the number, the lower the contrast density of the set-off strip and therefore the more ink that was absorbed onto the original sample film (i.e. the better the top coat absorbency) . A top coat with rapid absorption of ink therein (i.e. low strike-in) is desirable for an ink-jet print as this reduces dot spread and improves print resolution.

Table 1 ("Striking-in" results)

Ex 1 2 Comp A Comp B

Single Double Single Double 0.090 0.103 0.610 0.853

0.090 0.100 0.750 0.750

0.090 0.103 0.713 0.760

0.090 0.102 0.691 0.788

The data shows that films of the present invention (Examples 1 & 2) which comprise PVC particles in the top coat typically exhibit about a seven times faster strike-in rate (~ 0.1 compared to 0.75) than the same top coats without these particles. Thus the particle filled systems of the present invention produce an unexpectedly increased speed of ink absorption onto the surface which may also reduce ink dot spread and improving resolution of an ink jet print thereon.

No benefits or detriments are seen for the strike-in rate when doubling the coat thickness so it is possible to use high top coat weights for printing where high ink loads are used. The binder resin was found to have good adhesion to a primed film (it is believed due to the acrylic acid) and to have good compatibility with the solvents used in non-aqueous ink-jet printing inks (it is believed due to the acrylonitrile (AN) and methyl acrylate (MA) monomers) and thus helps the printability of the OPP film. The PVC particles used in Examples 1 and 2 were compatible with the binder and soluble in the ink solvent so they absorbed solvent to rapidly dry the printed ink resulting in a top coating with improved properties.

Examples 3 to 14 The topcoat coating formulations were prepared by the method described for Examples 1 and 2 to give a (dry) composition with the proportion of ingredients (by weight %) are given in Tables 2 and 3. Each coating was diluted to give a solids content between 35-55% (depending upon formulation and coat weight specified in the table). The topcoats were applied onto primed film by a direct reverse gravure process, then dried in an oven at 120^eC. The coated films were tested as previously described.

Table 2

Ingredient / Example 3 A 5. 6. 7 8. 9

Airflex EN1020 90 85 80 80 80 - Airflex EPN865 . . . . . 80 -

Airflex CEF19 . . . . . . 80

Sipernat 500LS 10 15 20 20

Sipernat 310 _{: : :} 20 20 20

Coat Weight (g/m²) 8.5 8.5 8.5 11 1 1 14 18 Striking In 0.45 0.43 0.38 0.48 0.38 0.45 0.10

Print Penetration (mm^-1) 10.87 11.24 11.49 12.66 12.05 12.82 21.74 Table 3

Inαredient / Example 10 11 12 13 14

Airflex EN1020 74.9 44.9 34.9 34.9 34.9

Sipernat 500LS 20 - - - -

Snowcal 30 - 50 - - -

Zeolex 123 - - 60 - -

Hydrex P - - 60 -

Vinnolit - - - - 60

Iso-propyl Alcohol 5 5 5 5 5

Foamaster DF198L 0.1 0.1 0.1 0.1 0.1

Coat Weight (g/m²) 11.2 11.8 8.6 10.75 17.0

Striking In 0.61 0.57 0.0 0.0 0.0

Print penetration (mm^"1) 10.99 8.85 13.51 15.15 30.30

The ingredients identified by trade name In Tables 2 and 3 are:

Binders Trade Name Material Supplier

Airflex E 1020 VAc/E/ copolymer Air Products

Airflex EPN865 VAc/E copolymer Air Products

Airflex CEF19 VAc/E/VC terpolymer Air Products

Fillers

Trade Name Material Supplier

Sipernat 500LS Precipitated silica Degussa

Sipernat 310 Precipitated silica Degussa

Snowcal 30 Calcium carbonate OMYA

Zeolex 123 Sodium aluminosilicate Huber

Hydrex P Sodium magnesium aluminosilicate Huber Vinnolit E 5/65C PV/VAc copolymer Vinnolit

Claims

1. Aqueous coating composition for a plastic film which comprises: (a) from 40% to 60% (by dry weight) of a binder resin comprising

(b) from 60% to 40% (by dry weight) of ink receptive filler particles having a mean particle size of 10 nanometres (0.01 microns) to about 5 microns

2 Composition as claimed in claim 1 , where the filler particles are at least partially soluble in the binder resin.

3. Composition as claimed in claim 2, where the binder resin comprises a vinyl (co)polymer.

4. Composition as claimed in either preceding claim, where the binder resin comprises a vinylidene halide and/or vinyl acetate polymer with monomers of vinyl halide, vinyl acetate, ethylene, acrylonitrile, (meth)acrylic acid and/or acrylate .

5. Composition as claimed in any preceding claim, where the binder resin comprises: a vinylidene chloride / vinyl chloride copolymer; a vinyl acetate / ethylene copolymer; and/or a vinyl acetate / ethylene / vinyl chloride terpolymer.

6. Composition as clamed in any preceding claim, characterised in that the ink receptive filler particles comprise soluble organic particles, selected from hard acrylic spheres, microcrystalline cellulose; polyvinyl halide; and/or polyvinyl vinyl acetate copolymer.

7. Composition as clamed in claim 5, characterised in that the ink receptive particles comprise PVC.

8. Composition as clamed in any of claims 1 to 4, characterised in that the ink receptive filler particles comprise inorganic particles, selected precipitated silica, calcium carbonate, sodium aluminosilicate and/or sodium magnesium aluminosilicate,

9. Process for imparting an ink-jet printable character to a plastic film comprising the step of coating a plastic film with a composition according to any preceding claim.

10. Ink-jet printable plastic substrate selected from synthetic paper, facestock for labels, tapes or self-adhesive materials and coated with a composition according to anyone of claims 1 to 8.