Adhesives for preparing a multilayer laminate featuring an ink-bearing surface bonded to a second surface

An adhesive composition that includes the reaction product of (a) a polyester-based polyurethane; (b) an epoxy resin; (c) an epoxy-functional silane; and (d) an isocyanate-functional compound, and multilayer laminates in which this adhesive composition is used to bond two polymeric substrates together where at least one of the substrates has an ink-bearing surface.

BACKGROUND OF THE INVENTION
 This invention relates to adhesives for use in multilayer laminates in
 which at least one of the layers includes an ink-bearing surface.
 There has been an interest in printing images such as photographic images
 onto plastic substrates. It would be particularly desirable to use liquid
 toner-based electrophotographic printing for this purpose because this
 printing technique produces high quality images.
 Once the image has been printed onto the surface of the plastic substrate,
 it is necessary to apply a protective film over the printed ink-bearing
 surface. The bond strength between the protective film and the printed
 surface must be sufficient to resist delamination under typical use
 conditions.
 SUMMARY OF THE INVENTION
 In one aspect, the invention features an adhesive composition that includes
 the reaction product of: (a) a polyester-based polyurethane; (b) an epoxy
 resin; (c) an epoxy-functional silane; and (d) an isocyanate-functional
 compound. The adhesive compositions are useful for preparing multilayer
 laminates in which a major surface of a first polymeric substrate is
 bonded to a major surface of a second polymeric substrate through the
 adhesive composition. One or both of those major surfaces includes an
 ink-bearing image.
 The adhesive compositions exhibit good adhesion (as measured by the 180
 degree peel strength) to both ink-bearing polymeric substrates and non-ink
 bearing polymeric substrates. The compositions are particularly useful
 where the ink is an electrophotographic ink. Examples of suitable
 electrophotographic inks include polymers having a Tg no greater than
 about 30.degree. C., while in other embodiments the ink includes a polymer
 having a Tg greater than about 30.degree. C.
 One example of a suitable ink is derived from gel organosol-containing,
 liquid toner compositions described, e.g., in Baker et al., U.S. Pat. No.
 5,652,282 and Baker et al., U.S. Pat. No. 5,698,616, each of which is
 hereby incorporated by reference. These toners include (a) a carrier
 liquid (e.g., an aliphatic hydrocarbon carrier liquid having a
 Kauri-Butanol number less than 30) and (b) a (co)polymeric steric
 stabilizer having a molecular weight greater than or equal to 50,000
 Daltons and a polydispersity less than 15 covalently bonded to a
 thermoplastic (co)polymeric core that is insoluble in the carrier liquid.
 The core preferably has a Tg no greater than about 30.degree. C. The toner
 may further include a colorant and a charge director. Also suitable are
 non-gel organosol-containing liquid toner compositions described, for
 example, in Baker et al., U.S. Pat. No. 5,886,067, hereby incorporated by
 reference.
 Another example of a suitable ink is derived from liquid toners described
 in Landa et al., U.S. Pat. No. 4,794,651; Landa et al., U.S. Pat. No.
 4,842,974; Landa et al., U.S. Pat. No. 5,047,306; Landa et al., U.S. Pat.
 No. 5,047,307; Landa et al., U.S. Pat. No. 5,192,638; Landa et al., U.S.
 Pat. No. 5,208,130; Landa et al., U.S. Pat. No. 5,225,306; Landa et al.,
 U.S. Pat. No. 5,264,313; Landa et al., U.S. Pat. No. 5,266,435; Landa et
 al., U.S. Pat. No. 5,286,593; Landa et al., U.S. Pat. No. 5,346,796; Landa
 et al., U.S. Pat. No. 5,407,771 and Landa, WO92/17823 published Oct. 15,
 1992 entitled "Polymer Blends," each of which is hereby incorporated by
 reference.
 The compositions are also useful with inks such as ink jet inks and
 lithographic inks. Any of these inks may be used alone or in combination
 with each other. For example, the ink-bearing surface may include
 electrophotographically printed areas featuring an electrophotographic ink
 and offset printed areas featuring a lithographic ink. Preferably,
 however, all the printed areas of the laminate are electrophotographically
 printed areas featuring an electrophotographic ink.
 The adhesive compositions exhibit good adhesion to a number of polymeric
 substrates. In one preferred embodiment, the adhesive is used to bond a
 rigid core layer to a flexible overlay film, where at least one of these
 materials includes an ink-bearing surface. Preferably, at least one of the
 substrates is substantially transparent to permit viewing of the printed
 image on the ink-bearing surface.
 Examples of suitable polymeric substrates are selected from the group
 consisting of polyvinyl chloride, vinyl chloride-vinyl acetate copolymers,
 polyesters, polyolefins, polycarbonates, and combinations thereof. Also
 suitable are microporous substrates such as Teslin.TM. films commercially
 available from PPG, Inc. of Pittsburgh, Pa. One preferred construction
 includes a polyvinyl chloride substrate adhesively bonded to a
 substantially transparent polyester overlay film.
 The adhesive compositions are also useful for preparing laminated articles
 having more than two substrates. For example, the article may include a
 core substrate having a pair of opposed major surfaces, each of which is
 bonded to a separate overlay film. Such constructions are particularly
 useful for articles having printed images on two different surfaces.
 In a second aspect, the invention features a lamination process for
 preparing the above-described articles. The ink-bearing image preferably
 is performed according to an electrophotographic imaging process that
 includes:
 (i) charging the surface of an electrophotographic photoreceptor;
 (ii) imagewise exposing the charged surface of the photoreceptor to
 radiation to dissipate charge in selected areas and thereby form a latent
 image on the photoreceptor surface;
 (iii) contacting the latent image with a toner to form a toned image; and
 (iv) transferring the toned image to the major surface of the first
 polymeric substrate.
 The toner preferably is a liquid toner. Preferred liquid toners, in turn,
 include a film-forming polymer. In some embodiments, the film-forming
 polymer has a Tg no greater than about 30.degree. C., while in other
 embodiments the film-forming polymer has a Tg greater than about
 30.degree. C. Following lamination, the article may subjected to a number
 of operations, including slitting, cutting, hole punching and drilling,
 foil stamping, sewing and grommeting, foil stamping, perforation, folding,
 surface texturing, and the like.
 Other features and advantages of the invention will be apparent from the
 following description of the preferred embodiments thereof, and from the
 claims.
 DETAILED DESCRIPTION
 The invention will now be described further by way of the following
 examples.
 The black, positive-acting, film-forming, electrophotographic ink used in
 the examples was prepared at an organosol/pigment ratio of 6 following the
 procedure described in Example 40 of U.S. Pat. No. 5,652,282 modified as
 follows.
 The gel organosol prepared according to the procedure of Example 22 of U.S.
 Pat. No. 5,652,282 was mixed using a Silverson mixer (Model L2R, Silverson
 Machines, Ltd.) operated at the lowest speed setting. After mixing for
 five minutes, 1912 g of the homogenized organosol at 16.14% (w/w) solids
 in NOR 12 were combined with 1031 g of NOR 12 (Exxon Chemical Co.,
 Houston, Tex.), 51 g of MONARCH 120 carbon black (Cabot Corp., Billerica,
 Mass.), and 6.08 g of Zirconium HEX-CEM (OMG Chemical Company, Cleveland,
 Ohio) in a 4.0 liter polyethylene container. This mixture was then milled
 in ten vertical bead mills, each having a capacity of 0.5 liter (Model
 6TSG-1/4, Aimex Co. Ltd., Tokyo, Japan) by placing 300 g of millbase and
 390 g of 1.3 mm diameter Potters glass beads (Potters Industries, Inc.,
 Parsippany, N.J.) in each mill. Each mill was operated at 2,000 rpm for
 1.5 hours without cooling water circulating through the cooling jacket of
 the milling chamber.
 A portion of the 12% (w/w) solids toner concentrate thus formed was diluted
 to approximately 3% (w/w). This dilute toner sample exhibited the
 following properties, as determined using the test methods described in
 U.S. Pat. No. 5,652,282:
 Number Mean Particle Size: 0.261 micron
 Bulk Conductivity: 149 picoMhos/cm
 Percent Free Phase Conductivity: 5%
 Dynamic Mobility: 0.0402 micron-cm/[volt-second]
 This 3% toner was tested on the toner plating apparatus described in U.S.
 Pat. No. 5,652,282. The reflection optical density (ROD) was greater than
 1.47 at plating voltages greater than 400 volts.

EXAMPLES
 Example 1
 An adhesive coating solution was prepared by adding methyl ethyl ketone
 (201 g), Epon 1007F epoxy resin (143 g of a 20 wt. % solution in methyl
 ethyl ketone, available from Shell Chemical Co.), epoxycyclohexylethyl
 trimethoxy silane (14.4 g, available from Aldrich Chemical Co.), and
 Vestanat T1890E isocyanate (215 g of a 20 wt. % solution in methyl ethyl
 ketone, available from Creanova) to 1428 g of a solution of Desmocoll 8634
 polyurethane resin (15 wt. % in methyl ethyl ketone, available from Bayer
 Chemical Co.). The solution was applied to Mellinex 454 polyester film
 (0.92 mil, available from Dupont) at a wet coating coverage of
 approximately 375 g/square meter and dried to give an adhesive-coated
 film. The dry thickness of the adhesive layer was 1.0 mil.
 The adhesive portion of the film was area printed using a liquid
 toner-based, black, positive-acting, film-forming, electrophotographic ink
 (prepared as described above) to a net optical density of 1.6. The net
 optical density is equal to the white light optical density minus the
 white light optical density of unprinted film, measured in reflectance
 mode with a Macbeth densitometer. The net optical density corresponded to
 an ink net optical density of 1.3 for a paper substrate printed under
 identical conditions.
 After printing, the adhesive-coated film was laminated to a white polyvinyl
 chloride substrate. Lamination took place between two heated rollers (roll
 surface temperature=135-138EC) at a rate of 0.4 inches/second to give a
 laminated, printed article. The article was cut into strips measuring one
 inch wide and the 180 degree peel force required to cause delamination was
 measured 15 minutes after the lamination step using an Instron Tester
 (Model 5542). The crosshead speed was 12 inches/minute. The peel force was
 determined to be 4.5 pounds/inch (7.9 N/cm).
 In a separate experiment, unprinted adhesive film was laminated to an
 identical white polyvinyl chloride substrate in an identical manner. The
 peel force was determined to be approximately 12 pounds/inch (21 N/cm),
 which resulted in tearing of the polyester film.
 Example 2
 A laminated article was prepared following the procedure of Example 1
 except that lamination took place 30 minutes after printing. The peel
 strength was determined to be 1.9 pounds/inch (3.4 N/cm). The experiment
 was then repeated except that subsequent to the printing step, the printed
 polyester film was heated at 100.degree. C. for 2 minutes and then
 laminated after a total of thirty minutes had elapsed since the printing
 step. The peel force, measured approximately five minutes after the
 lamination, was 2.4 pounds/inch (4.2 N/cm).
 Example 3
 The procedure of Example 1 was followed except that the adhesive was
 prepared by combining 15 parts Desmocoll 530, 85 parts methyl ethyl
 ketone, 3 parts Vestanat T1890E isocyanate, 1 part epoxycyclohexylethyl
 trimethoxy silane, and 2 parts Epon 1007F epoxy resin. The resulting
 article exhibited a peel force, measured as described in Example 1, of 4.0
 pounds/inch (7.0 N/cm). The failure mode was ink splitting.
 In a separate experiment, unprinted adhesive-coated polyester film was
 laminated to an identical white polyvinyl chloride substrate in an
 identical manner. The resulting article was stored for two weeks at
 60.degree. C. and 100% relative humidity, after which the peel strength
 was measured and determined to be 10 pounds/inch (17.6 N/cm).
 Other embodiments are within the following claims.