The present invention relates to water-based paste or ink compositions and, in particular, to water-based printable inks, such as decorative glass or ceramic printing inks or water-based pastes for use in the electronics industry such as thick film conductor pastes, thick film resistor pastes, dielectric pastes, die attach adhesive compositions and solder or sealing glass pastes. The transient carrier vehicles which are commonly used in the preparation of pastes for the ceramics and electronics industries are typically based on volatile organic liquids with or without the inclusion therein of a thickener/binder, a rheological modifier to enhance thixotropy or pseudoplasticity, a surfactant and other common additives. Examples of organic liquids which are commonly used are aliphatic alcohols and their esters, terpenes such as pine oil and terpineol, solutions of resins such as the polymethacrylates of lower alcohols and solutions of ethyl cellulose in solvents such as pine oil, glycol ethers and the like.

There is a continuing attempt for the level of volatile organic liquids used in the chemical industry to be reduced and it would thus be particularly advantageous if such paste or printable ink compositions could be produced which are water-based.

We have now developed such a system and, accordingly, the present invention provides a water-based paste or ink composition which comprises at least one powdered or flaked material and a transient carrier vehicle which is an aqueous medium including therein at least one associative thickener (as hereinafter defined) , the amounts of the powdered or flaked material, the vehicle and any optional

additives included in the composition being chosen so that the composition has an appropriate rheology for it to be dispensed, for example by syringe, screen printing, stencilling or K bar application, and the transient carrier vehicle being removable from the composition by heating or pyrolysis to leave no or substantially no residues.

The powdered or flaked material for use in the present invention will depend upon the particular composition. It will typically be a glass, ceramic, metal or metal oxide powder, or a mixture thereof, or precursors for these materials, for example organo etallic precursors thereof. For example, for a ceramic printing ink the powdered material may be a powdered coloured ceramic pigment and/or glass; for thick film conductor pastes the powdered material may be an admixture of particles of a noble metal or noble metal alloy and a glass binder, or a mixture of nickel or copper or their alloys and a glass binder; for thick film resistor pastes the powdered material may be a mixture of particles of a conductive material and a glass binder; for dielectric pastes the powdered material is typically a ceramic dielectric material; for die attach adhesive compositions the powdered material may be a mixture of a metal and a glass frit; and for an adhesive "solder glass" paste, the powdered material may be a single glass or a mixture of glass and one or more ceramic materials chosen to modify the thermal expansion. It will be understood that the compositions of the present invention may include therein other optional additives, such as organic compounds or waxes, provided that the additives will burn out to leave no or substantially no residue on heating or pyrolysis.

The aqueous paste or ink composition may also include therein at least one dispersant (as hereinafter defined) .

By the term "dispersant" as used herein is meant a compound or composition which acts to maintain the particles in a suspension in a dispersed state in which they do not associate with other particles to form aggregates. A dispersant acts by adsorbing on the particle surface, so altering the particle surface chemistry for minimum interparticle interaction. The chemistry of dispersants (surfactants) is discussed in Surfactants ed. Th F Tadros, Academic Press, London 1984 and Industrial Applications of Surfactants, ed. D.R. Karsa, Special Publication No. 59, Royal Society of Chemistry, London 1987. By the term "associative thickener" as used herein, is meant hydrophobically modified polymers, for example, polyacrylate or polyurethane copolymers, which form aggregates or micelles in aqueous media and which exhibit shear thinning behaviour in aqueous media, i.e. have a higher viscosity at low shear rates than the viscosity at high shear rates. The chemistry of associative thickeners is discussed in Polymers in Aqueous Media: Performance through Association, ed. J.E. Glass, Advances in Chemistry Series No. 223, American Chemical Society 1989.

The hydrophobically modified polyacrylate associative thickeners used in the present invention are alkali swellable materials which are substantially insoluble in water at low pH, but which exhibit thickening on swelling or dissolution in aqueous media at higher degrees of ionization and which are stable at a pH of above 7. The alkali swellable associative polymeric thickeners are generally terpolymers which comprise a carboxylic monomer, a hydrophobic monomer and an associative monomer. Currently available

materials generally have a molecular weight in the range of from 1000 to 10,000. A particular class of such associative thickeners for use in the present invention are the hydrophobically modified acrylate polymers sold under the Trade Name Rheovis CR, CRX, CR2 and CR3 (from Allied Colloids) . Mixtures of associative thickeners may also be used, if desired. The carboxylic functionality on the carboxylic monomer is provided by carboxylic acid or anhydride groups, examples of these groups being those derived from acrylic acid and methacrylic acid.

The associative monomer normally comprises a long chain hydrophilic segment which is terminated with a hydrophobe. The hydrophilic segment preferably comprises a polyethoxy or poly(ethoxy/propoxy) chain situated between ethylenic unsaturation at one end of the molecule and a terminal hydrophobe at the other end of the molecule. Preferably there are at least 40 oxyethylene units or 40 oxyethylene/oxypropylene units between the surfactant hydrophobe and the ethylenic unsaturation. The associative monomers are generally made by coupling the hydroxyl end group of a conventional non-ionic surfactant to a monomer containing ethylenic unsaturation. The hydrophobe on the associative monomer is the primary interactive component in the associative thickener. The hydrophobic monomer as such can be omitted from the polymer if the associative monomer is able to impart the hydrophilic-hydrophobic balance to the thickener necessary for pH-dependent solubility. The copolymer backbone of the associative thickener is predominantly hydrophilic since it contains sufficient carboxyl groups or anhydride groups to render the thickener water soluble at a high pH.

The mechanism by which the associative thickener acts is by the non-specific hydrophobic association of

water-insoluble groups in water-soluble polymers. The terminal hydrophobes on the ethoxylate or ethoxylate/propoxylate side chains are repelled from the hydrophilic portions of the molecule and are attracted to other terminal hydrophobes. This intermolecular association of terminal hydrophobes of associative thickeners in aqueous solution may be regarded as micellization in a continuous network and the driving force for association is the increase in entropy resulting from the loss of water around the hydrophobes and the minimization of water-hydrophobe contacts.

An example of the hydrophobically modified polyurethane based associative thickener for use in the present invention is a non-ionic modified polyether urea polyurethane produced and sold by Rheox Inc. under the trade name Rheolate 200 Series, for example, Rheolate 244 or Rheolate 255. These copolymers are the reaction product of a diisocyanate with a diol and a hydrophobic capping agent. The associative thickener is generally included in the compositions in an amount of from lpp to 10.0% by weight based on the total weight of solids, preferably 0.01 to 2.0% by weight based on the total weight of solids. The associative polymeric thickeners are synthetic and are resistant to microbial degradation. The associative thickeners and dispersant, if present, burn out completely at temperatures of below 500°C leaving little or no residues.

Examples of dispersants which may be used in the present invention are any appropriate cationic anionic, non-ionic or amphoteric surfactants or mixtures thereof which act under the prevailing conditions as a dispersant. The selection criteria and mode of use are as described in the literature. An example of a non-ionic surfactant for use in the

present invention is CT111, an acetylenic diol based material from Air Products. Examples of anionic surfactants for use in the present invention are naphthalene sulfonic acid, or a derivative or salt thereof, poly(naphthalene sulfonate-co-formaldehyde) , and Dispex N40, a poly(acrylic acid) -sodium salt from Allied Colloids. An example of a cationic surfactants for use in the present invention is cetyltrimethylammonium bromide. An example of an amphoteric surfactant for use in the present invention is Mirataine JCHA, a sodium alkylamino propionate from Rhone Poulenc. The dispersant is preferably included in the paste or ink formulation in an amount of from 1 ppm to 5% by weight based on the total weight of solids, more preferably 0.1 to 2% by weight based on the total weight of solids.

The amount of the powdered or flaked material in the compositions of the present invention will generally be in the range of from 25 to 95% by weight based on the total weight of the composition, more preferably in the range of from 40 to 90% by weight based on the total weight of the composition.

The amount of the vehicle will generally be in the range of from 75 to 5% by weight based on the total weight of the composition, more preferably in the range of from 60 to 10% by weight based on the total weight of the composition.

The compositions of the present invention may additionally include therein one or more additives such as antifoamers, defoa ers, wetting and levelling agents and on occasions other water soluble polymers which also act as thickeners and binders. It is also frequently desirable to add a humectant such as glycerol to the compositions. The above optional ingredients are used to optimise the chemical and physical properties of the paste and/or the transient

film, coating or layer formed by the paste prior to the heating/pyrolysis process step in forming the finished article.

It will be understood that the amount of the powdered or flaked material, the vehicle and any optional additives included in the composition are chosen so that the composition has an appropriate rheology for it to be printed or dispensed by the chosen application method, e.g screen printing, stencilling, K bar application or using a syringe. The main advantage of the vehicle system used in formulating the paste and ink compositions of the present invention is that it is a simple water-based system free of volatile organic compounds which is Theologically stable (e.g. does not exhibit any adverse reactions with glass frits such as boron gelling) and also has good burn out characteristics. The present invention also includes within its scope a method for the preparation of the aqueous printable paste or ink compositions as defined herein which comprises mechanically mixing at least one powdered or flaked material with an aqueous medium containing at least one associative polymeric thickener, or mixing at least one powdered or flaked material with an aqueous medium and thereafter adding at least one associative polymeric thickener to the mixture. The method of mixing may be any of those currently used, for example, triple roll milling.

The systems are designed to have a high viscosity at low shear and low viscosity at high shear - that is to shear thin. The precise viscosities required will depend, for example, on the particulate materials present, the printing or dispensing equipment to be used, the application speed and desired shelf life of the product. The grades and types of associative

thickener being selected and/or blended accordingly to provide optimum performance.

The present invention also includes within the scope a method for the formation of a coating, film, joint, adhesive bond or decoration from a powdered or flaked material which method comprises using as a transient carrier for the powdered or flaked material an aqueous medium comprising an associative thickener (as hereinbefore defined) and removing the transient carrier vehicle by heating or pyrolysis to leave no or substantially no residues.

The present invention furthermore includes within its scope an electric circuit produced using a paste or ink composition of the invention, or an electronic component mounted using a paste or ink composition of the invention; a glass or ceramic article decorated using a paste or ink composition of the invention; or an item joined using a paste or ink composition of the invention.

The present invention will be further described with reference to the following non-limiting Examples.

EXAMPLE 1

A blue glass-ceramic pigment powder was blended with different weights of a 1% solution of an associative polymeric thickener, Rheovis CR, in order to form three inks A, B and C.

wt of solids wt of solution Description

A 2 . 5g 1.5g gel-like

B 2 . 5g 1.75g quite gel-like

C 2 . 5g 2. Og very fluid

The three inks were screen printed onto standard printing paper, a glass surface and directly onto a glazed tile using a 200 mesh screen (75-77μ) . Each of the inks printed satisfactorily, the ink C being slightly easier to print than ink B, which itself was easier to print than ink A.

The tiles printed with inks A, B and C were fired up to 850°C using a ramp rate of 13°C/min, a dwell time of 10 minutes and a cooling rate of 13°C/min. The fired coatings each showed good adherence, good gloss and good uniformity.

EXAMPLE 2

The procedure of Example 1 was repeated using a dielectric lead magnesium niobium titanate (PMNT) . In order to reduce frothing, two drops of a dispersant were added to the composition.

wt of wt of Dispersant Description solids solution

D 2.5g 2. Og Synperonic Thickened

LF/RA 280 ink-smooth print

2.5g 2. Og Dow Corning Bubbles

65 eliminated- smooth

F 2.5g 2. Og DP6 Thickened ink

To test adherence ink D was screen printed directly onto an alumina plate using the same conditions as in Example l. The plate was fired up

to 1180°C. The fired surface was smooth, crack free and had good adherence.

EXAMPLE 3

A blue glass-ceramic pigment powder was blended with different weights of a 10% solution of a urethane associative thickener Rheolate 244 in order to form 3 inks.

wt of wt of Dispersant Description solids solution

G 2.5g 2.0 - very thin

H 2.5g 1.75g - thin but easy to print, but poor wetting

I 2.5g 2. Og 2 drops of easy to Dow Corning print,

65 good wetting

The three inks were screen printed directly onto a glazed ceramic tile using a 200 mesh (75-77μ) screen. Each of the inks printed satisfactorily, ink I giving the best print.

The tiles printed with inks G, H and I were fired up to 850°C using a ramp rate of 13°C/min, a dwell time of 10 minutes and a cooling rate of 13°C/min. The fired coatings each showed good adherence, good glass, good colour and good uniformity.

EXAMPLE 4

A blue glass-ceramic pigment powder was blended with different weights of a 10% solution of a urethane associative thickener Rheolate 255 in order to form 3 inks.

wt of wt of Dispersant Description solids solution

J 2.5g 1.5g - thick

K 2. Og 2. Og - quite thick gel-printable but moderate wetting.

L 2.5g 2. Og 2 drops of smooth

Dow Corning uniform 65 surface

The three inks were screen printed directly onto a glazed tile using a 200 mesh (75-77μ) screen. Each of the inks printed satisfactorily.

The tiles printed with inks J, K and L were fired up to 850°C using a ramp rate of 13°C/min, a dwell time of 10 minutes and a cooling rate of 13°C/min. The fired coatings each showed good adherence, good gloss, good colour and good uniformity.

EXAMPLE 5

1.5 parts of a 1% solution of Rheovis CR3 in water was added to 12.5 parts of a TV sealing glass powder (Corning Glass No. 7590) . After mixing for a

period of five minutes this was dispensed onto an alumina substrate and fired to test the burn out characteristics of the paste. The firing conditions were a ramp rate of 10°C per minute up to a temperature of 450°C followed by a hold time of 45 minutes.

On cooling the sealing glass was found to have melted and vitrified in an acceptable manner with no indication of carbon residue or reduction of the PbO in the glass to Pb metal.

EXAMPLE 6

5g of a 1% solution of Rheovis CRX in water was added to 40g of a TV sealing glass powder (Corning Glass No.7590) . After mixing to ensure uniformity the mixture was extruded through a syringe onto a microscope slide, a second slide was placed on top and the assembly was fired according to the regime detailed in Example 5 in order to test the burn out charactisticε of the paste.

On cooling, the sealing glass was found to have form a strong bond between the microscope slides.

EXAMPLE 7

2.5g of a 1% solution of Rheovis CRX in water was added to 8g Metz Ag powder No. 3000-1 together with 2 drops of Surfynol 104 (Air Products) as an antifoa er/defoamer and 5 drops of glycerol as a humectant. The ingredients were throughly mixed together and the paste so formed possessed suitable rheological properties for screen printing onto a dielectric substrate.

EXAMPLE 8

1.5g of a 1% solution of Rheovis CRX was added to 3.8g of lead zinc borate powdered glass described as glass A in EP-A-0317694 and lg of Metz Ag powder No. 3000-1. The ingredients were throughly mixed and then used as a die attach paste by application by syringe to a chip holder. A silicon die was pressed firmly onto the die attach paste and the assembly was fired at 5°C per minute to 180°C, then at 20°C per minute to 435°C, maintained at this temperature for 10 minutes, removed from the furnace and cooled to room temperature.

EXAMPLE 9

a) 18.5g of Alcowax OG (Allied Colloids) was dispersed in 100 ml of deionised water with heating to 90°C using a high shear Silverson mixer to form a wax emulsion. b) 0.25 ml of an adhesion promoter Dow Corning

"additive 21" was added to 19 ml water and 0.25 ml of a 1% solution of Rheovis CRX, 0.5 ml of a viscosity point additive HA12P (Rohm & Haas) and 9g of powdered cyan enamel were added thereto. The enamel was dispersed with mixing and the pH of the mixture adjusted to pH 9.5 to 10.5 by the addition of 1M NaOH solution, c) 13.5 ml of the wax emulsion prepared in step (a) was added to the enamel dispersion prepared in step (b) and the pH of the mixture readjusted to 9.5 to 10.5 by the addition of 1M NaOH solution. The resulting ink paste was coated by a standard K-bar coating technique onto a 3 to lOμm thick

polyester film and a glazed tile to form thin even coatings. The tile was fired at 13°C/minute to 850°C, held for 15 minutes and furnace cooled, yielding a good adherent coating indicating no burn out problems.