EP0543948A1 - A microwave active coating material including a polymeric resin - Google Patents

Abstract

Coating material active under the effect of microwaves comprising electroconductive elements as well as a bonding system. The bonding system comprises a solvent as well as a polymer resin chosen from the group consisting of nitrocellulose, ethyl cellulose, polyvinyl butyral, poly (vinyl methyl ether / maleic anhydride) co-polymer or its derivatives, and polyvinylpyrrolidone. The coating material is capable of constituting a coating configured on a substrate to form a plurality of discrete electrically conductive elements. The discrete electrically conductive elements have a maximum dimension which is sufficiently small to prevent ignition. In addition, the elements form a network acting as an agent modifying the microwave field. The modifying agent can constitute a dielectric substrate comprising a plurality of discrete surface areas coated with an electroconductive coating material in order to form discrete electroconductive elements, the latter being arranged on a given surface in a predetermined network. The elements are preferably elongated and the network constitutes a plurality of rows of elements in linear alignment which are parallel, and the elements of the network in adjacent rows are staggered. Said electroconductive elements are preferably formed by printing an ink-like electroconductive coating material on the dielectric substrate.

Description

A MICROWAVE ACTIVE COATING MATERIAL INCLUDING A POLYMERIC RESIN

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to microwave susceptible coating materials, and more particularly, to such coating materials useful for creating patterned microwave field modifiers.

2. Description of the Prior Art

Microwave ovens possess the ability to heat, cook or bake items, particularly foodstuffs, extremely rapidly. Unfortunately, microwave heating also has its disadvantages. For example, microwave heating alone in today's microwave ovens often fails to achieve such desirable results as evenness, uniformity, browning, crispening, and reproducibility. Contemporary approaches to achieving these and other desirable results with microwave ovens include the use of microwave field modifying devices such as microwave susceptors and/or microwave shields.

Microwave susceptors and shields, like other materials and constructions have some degree of microwave reflectance (R), absorbance (A) and transmittance (T); or collectively RAT properties. RAT properties are measured in terms of percentage of microwave energy reflected by (R), absorbed by (A), and transmitted through (T) a material or construction. Thus, the aggregate of the R, A and T values will total 100%.

Generically, a microwave shield is relatively opaque to microwave energy. In terms of RAT, a shield will have a relatively low T value. Microwave shields are exemplified by such highly electrically conductive materials as aluminum foil. Although shields are generally thought of as non-heating elements, a shield could also be a susceptor, i.e., heat appreciably, and visa-versa. Thus, a shield is an element with relatively low transmittance regardless of its tendency to generate heat. Generically, microwave susceptors are devices which, when disposed in a microwave energy field such as exists in a microwave ven, respond by generating a significant amount of heat. The susceptor absorbs a portion of the microwave energy and converts it directly to heat which is useful, for example, to crispen or brown foodstuffs. Thus, microwave susceptors generally have a relatively high microwave absorbance value. In addition to high absorbance, susceptors include a mechanism to convert the absorbed microwave energy to heat. For example, heat may result from microwave induced, intramolecular or inter olecular action; or from induced electrical currents which result in so called I-squared-R losses in electrically conductive devices; or from dielectric heating of dielectric material disposed between electrically conductive particles, elements or areas which type of heating is hereinafter alternatively referred to as fringe field heating or capacitive heating.

As noted, microwave susceptors and shields, and other materials and constructions, have an effect on the microwave power distribution within a microwave oven. That is, they interact with the microwave energy within the oven through their RAT properties and cause the microwave energy field to be modified. Accordingly, devices and constructions which act to modify the microwave field or microwave energy power distribution within a microwave oven are referred to herein collectively as microwave field modifiers.

U.S. Patent 4,914,266 discloses a microwave susceptor which is created by coating a coating material. The coating material includes carbon or graphite and an ink vehicle. The ink vehicle comprises a resin and a solvent. The resins include polymeric resins soluble in alcohol but insoluble in water such as nitrocellulose, cellulose acetate, methyl cellulose, ethyl cellulose and cellulose acetate butyrate. The solvent is alcohol which comprehends allyl, amyl , benzyl, butyl, cetyl, isopropyl and propyl alcohols.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention a coating material is provided which is microwave active when dry. The coating material includes electrically conductive particles and a binder system. The binder system comprehends a solvent and polymeric resin selected from the group consisting of nitrocellulose; ethyl cellulose; polyvinylbutyral ; poly (methyl vinyl ether/maleic anhydride) copolymer or derivatives thereof; and polyvinyl- pyrrolidone. The coating material is. preferably susceptible of being coated on a substrate by means of a high speed pattern coating process. The coating material preferably dries forming discrete electrically conductive elements having a conductivity, expressed in terms of surface resistivity, of less than about 100 ohms per square.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims which particularly point out and distinctly claim the invention, it is believed the present invention will be better understood from the following description of a preferred embodiment taken in conjunction with the accompanying drawings, in which like reference numerals identify identical elements and wherein;

Figure 1 is a perspective view of a preferred embodiment of a microwave field modifier which can be made using the coating material of the present invention; and

Figure 2 is an enlarged scale, fragmentary portion of the microwave field modifier shown in Figure 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT The coating material of the present invention generally includes a binder system, which comprehends selected polymeric resin materials and a solvent, and electrically conductive particles. In addition, the coating material may include various other components.

The binder system is used to bind the electrically conductive particles together in contacting relation. The binder system also preferably functions to bind the coating material to the dielectric substrate. Included in the binder system is a solvent and a polymeric resin selected from the group consisting of nitrocellulose; ethyl cellulose; polyvinylbutyral; poly (methyl vinyl ether/maleic anhydride) copolymer or derivatives thereof; and poly inylpyrrolidone. a. Ethyl CelluΗse and Nitrocellulose

The polymeric resin may be nirocellulose or ethyl cellulose. The nitrocellulose or ethyl cellulose resin is dissolved in a solvent to form solutions of preferably from about 8% to about 30% by weight resin in solvent; more preferably from about 10% to about 25%; and from about 15% to about 20% is most preferred. The higher percentages are preferred for particulate carrying properties but the lower percentages are favored for achieving a favorable printing viscosity.

Typical solvents of ethyl cellulose and nitrocellulose include alcohols. "Alcohol" is commonly used to mean ethyl alcohol (or ethanol) but also may include ally!, amyl, benzyl, butyl, cetyl , methyl, n-butyl, isobutyl, isopropyl, and propyl alcohols. Nitrocellulose is also soluble in toluene, xylene and n-butyl acetate, ethyl acetate, n-propyl acetate, isopropyl acetate, and cellosolve acetate.

A nitrocellulose is available from General Printing, Ink Division, Sun Chemical Corporation, Cleveland, Ohio as a 40% solution of 18-25 cps RS Nitrocellulose. This solution includes 17% isopropyl alcohol, 23% ethyl acetate and 20% n-propyl acetate by weight. An ethyl cellulose is available from Hercules Inc., Wilmington, Delaware under the name Ethyl Cellulose N-4.

The nitrogen percentage of the nitrocellulose is preferably from about 10% to about 14%, and more preferably from about 11% to about 13%. The ethoxy content of the ethylcellulose is preferably from about 47% to about 48%. b. Polyvinylbutyral

The polymeric resin may be polyvinylbutyral. Polyvinylbutyral is FDA approved for such things as a component in can coatings and paperboard coatings for food packages. Polyvinylbutyral may be obtained from Hoechst-Celanese of Somerville, NJ under the trade name Mowital* B. It is available in several molecular weights. The preferred types are coded B-20H, B-30H and B-30T by Hoechst-Celanese. Mowital® B polyvinylbutyral resins refer to a class of polymer resins supplied by Hoechst-Celanese Corporation with the chemical name of poly (2-propyl-m-dioxane-4,6-diylene), They are comprised of polyvinyl butyral, polyvinyl acetate, and polyvinyl alcohol groups. The structural formula is shown below.

POLYVINYLBUTYRAL poly(2-propyl-m-dioxane-4,6-diylene)

The molecular weight of this compound (i.e., values of m, n, and o) should be selected such that the coating material will have a viscosity in the range identified hereinafter. The molecular weight is more preferably from about 30,000 to about 60,000 and even more preferably from about 50,000 to about 60,000.

The preferred grades used are Mowital B20H, B30H, and B30T. Both the B20H and B30H have 18-21% polyvinyl alcohol, 75-77% polyvinyl butyral, and 3% polyvinyl acetate. One difference between B20H and B30H is the viscosity of the resins in solution, with B30H having a higher viscosity than that of the B20H resin. The B30T has a higher polyvinyl alcohol content (24-27%), and a lower polyvinyl butyral content (69-71%).

Mowital® B20H and B30H polyvinylbutyral resins have FDA approval for direct and indirect food contact. The materials do not thermally decompose below 232^*C (450^*F).

Typical solvents for polyvinylbutyral include alcohols. "Alcohol" is commonly used to mean ethyl alcohol (or ethanol) but also may include allyl, amyl , benzyl, butyl, cetyl , isobutyl, isopropyl, and propyl alcohols. Polyvinylbutyral is also soluble in glycol ethers (methoxypropanol, butylglycol, and methoxybutanol), and cyclohexanone. Polyvinylbutyral is slightly soluble in methyl & ethyl acetate, acetone, and methylethyl ketone.

The polyvinylbutyral resin is dissolved in a solvent to form solutions of preferably from about 8% to about 30% by weight resin in solvent; more preferably from about 10% to about 25%; and from about 15% to about 20% is most preferred. The higher percentages are preferred for particulate carrying properties but the lower percentages are favored for achieving a favorable printing viscosity. c. Poly (Methyl Vinyl Ether/Maleic Anhydride) Co-Polv er

The binder resin may be a poly (methyl vinyl ether/maleic anhydride) co-polymer resin or a derivative thereof. Variation in the side group substitutions determine the nature and type of the polymer. Preferred poly (methyl vinyl ether/maleic anhydride) co-polymer resins or derivatives thereof are the Gantrez^® resins available from the GAF Corporation of Wayne, New Jersey. Gantrez^® is a registered trademark of the GAF Corporation. Gantrez® currently comes in three basic forms; Gantrez^® AN, Gantrez^® ES and Gantrez^® S.

The Gantrez^® AN series resins are referred to as the anhydride resins and are supplied as dry powders.

Typical solvents for the Gantrez^® AN series resins are water, alcohols ( ethanol , ethanol, propanol, isopropanol, butanol), phenols, pyridine, acetic acid, aldehydes & ketones, (benzaldehyde, formaldehyde, acetone, cyclohexanone, methyl ethyl ketone, mesityl oxide, diacetone alcohol), lactams (2-pyrrolidone, N-methyl-2-pyrrolidone, N-vinyl-2-pyrrolidone, and butyrolactone), and lower aliphatic esters (methyl and ethyl acetate). The chemical structure for Gantrez^® AN resins is shown below.

GANTREZ^® AN - SERIES (ANHYDRIDE POLYMER FORM)

The molecular weight of this compound (i.e., value of n) should be selected such that the coating material will have a viscosity in the range identified hereinafter. The molecular weight for the Gantrez^® AN series resins is more preferably from about 190,000 to about 2,200,000 and even more preferably from about 900,000 to about 1,150,000.

The Gantrez^® AN series are basically the starting point for the other series resins, based on the solvent system used. Thus, the other Gantrez^® resins are derivatives of the AN series. When dissolved in water, the anhydride linkage is cleaved so as to form the highly polar hydrolyzed form of the resin. The chemical structure for the Gantrez® S series resins is shown below.

GANTREZ^® S - SERIES (HYDROLYZED POLYMER FORM)

The molecular weight of this compound (i.e., value of n) should likewise be selected such that the coating material will have a viscosity in the range identified hereinafter. The molecular weight for the Gantrez® AN series resins is more preferably from about 200,000 to about 2,000,000.

This hydr yzed form of the polyms^ is the Gantrez^® S series of resins. The Gantrez^® AN and S series resins have FDA approval as adhesives. Gantrez^® S series resins are also supplied as powders.

The Gantrez^® ES series resins are half esters formed from alcohols. One way to form the Gantrez^® ES series resins is to use the Gantrez^® AN series resins with alcohol as a solvent. The functional group of the alcohol used for dissolving the polymer forms an ester with the side chains of the polymer. The chemical structure is shown below.

GANTREZ^® ES - SERIES (ETHYL HALF ESTER IN ETHYL ALCOHOL)

The molecular weight of this compound (i.e., value of n) should likewise be selected such that the coating material will have a viscosity in the range identified hereinafter. The molecular weight for the Gantrez® ES series resins is more preferably from about 100,000 to about 150,000.

Based on the type of alcohol used, the ethyl side group will vary. A preferred ES resin is Gantrez^® ES-225 which is the mono-ethyl ester of the poly (methyl vinyl ether/maleic anhydride) co-polymer resin. The chemical name for the ES-225 polymer is 2-Butenedioic acid monoethyl ester polymer with Methoxyethene.

Gantrez^® ES resins are supplied as alcohol solutions with 50% resin solids in the appropriate alcohol. For example, Gantrez^® ES-335 is supplied in isopropanol, and is the isopropyl ester version of the same polymer. The ES resins are soluble in alcohols (variations will change the polymer), ethylene glycol, butyl carbitol, acetone, cyclohexanone, dioxane, and tetrahydrofuran, and are insoluble or swell in diethyl ether, chloroform, carbon tetrachloride, and toluene.

The poly (methyl vinyl ether/maleic anhydride) co-polymer resins or derivatives thereof are dissolved in a solvent (Gantrez ES series are diluted from 50%) to form solutions of preferably from about 8% to about 30% by weight resin in solvent; more preferably from about 10% to about 25%; and from about 15% to about 20% is most preferred. The higher percentages are preferred for particulate carrying properties but the lower percentages are favored for achieving a favorable printing viscosity. d. PolvvinylDyrrolidone The binder resin may be polyvinylpyrrolidone. Polyvinylpyrrolidone is the common name for a material with the chemical name of poly (N-vinyl-2-pyrrolidone). Polyvinylpyrrolidone may be obtained from GAF Corporation, Wayne, New Jersey, and Sigma Chemicals, St Louis, Missouri. The structural formula is shown below.

POLYVINYLPYRROLIDONE poly(N-vinyl-2-pyrrolidone) The molecular weight of this compound (i.e., value of n) should be selected such that the coating material will have a viscosity in the range identified hereinafter. The molecular weight is even more preferably from about 10,000 to about 500,000 and even more preferably from about 300,000 to about 500,000.

Typical solvents for polyvinylpyrrolidone include water, alcohols (all), ether alcohols (glycol ethers, diethylene glycol, triethylene glycol, hexamethylene glycol, polyethylene glycol), some chlorinated hydrocarbons (methylene dichloride, ethylene dichloride, chloroform), lactams (2-pyrrolidone N-methyl-2-pyrrolidone, N-vinyl-2-pyrrolidone), amines (butylamine, cyclohexylamine, aniline, ethylenediamine, pyrridine), and nitroparaffins (nitromethane, nitroethane).

The polyvinylpyrrolidone resin is dissolved in a solvent to form solutions of preferably from about 5% to about 30% by weight resin in solvent; more preferably from about 8% to about 15%. The higher percentages are preferred for particulate carrying properties but the lower percentages are favored for achieving a favorable printing viscosity. e. Other Components

Electrically conductive particles which may be used to make coating materials using any of the resins identified above in accordance with the present invention preferably include carbon particles and graphite particles; and even more preferably include pure metallic particles such as nickel, iron, copper, silver and tin, some metallic oxides such as tin oxide, metal alloy particles such as aluminum iron alloys. Coating materials made with the more preferred particles are more conductive and therefore more reflective. Furthermore, the conductive particles preferably have irregular shapes; even more preferably are also relatively flat; and most preferably are also of differing shapes and sizes; all of which promote electrical contact between the elements. In addition, the conductive particles are preferably less than 25 microns in size and more preferably less than 10 microns in size; i.e., their maximum dimension.

A preferred conductive particle which has been found successful is Nickel Flake HCA-1 which may be purchased from the Nova et Company, Wyckoff, N.J. The Nova et Nickel Flake HCA-1 is a dendritic particle formed of spheroids which have been connected together and smashed flat. Thus, a preferred conductive particle has a flattened dendritic shape. A second preferred particle is graphite which may be purchased from Cabot Corporation, Waltham, Mass. as Carbon Black Regal® 99R. This particle is also relatively flat and has a size of about .36 nanometers.

The conductive particles preferably constitute from about 40% to about 80% by weight of the coating material; and even more preferably from about 55% to about 65% by weight of the coating material. Alternatively, the carbon or graphite preferably constitute from about 10% to about 50% by weight of the coating material; and even more preferably from about 13% to about 23% of the coating material by weight. Although the preferred quantities of the carbon or graphite systems are controlled more by performance characteristics, the preferred quantities for the other particles mentioned above are controlled more by viscosity limitations.

The viscosity of the coating material is preferably from about 50 cps to about 7000 cps. For rotogravure printing the viscosity is preferably from about 100 cps to about 175 cps as measured with a #3 zahn cup. In any event the viscosity should be such that the coating material is suitable for the chosen coating process used; be it painting, spraying, printing, silkscreen printing or rotogravure printing. Achieving the desired viscosity may require the addition of resin, solvents or other additives after the initial mixing of the coating material as is commonly done in printing processes. The percentages given above as percent by weight of the resin, solvent and conductive particles are believed broad enough to cover these situations.

Viscosity, however, can sometimes be drastically affected by the addition of small quantities of various additional components. In these situations the conductive particle to resin ratio will generally remain the same. Thus, preferred ranges can also be expressed in terms of a conductive particle:resin ratio by weight. The conductive particle (except for carbon and graphite):resin ratio for all the resins described above except nitrocellulose and ethyl cellulose is preferably from about 2:1 to about 50:1; and even more preferably, from about 4:1 to about 20:1. For nitrocellulose and ethyl cellulose the conductive particle (except for carbon and graphite) :resin ratio is preferably from about 2:1 to about 20:1; and even more preferably, from about 4:1 to about 10:1. The carbon and graphite particle to resin ratio for all :ne resins described above except nitrocellulose and ethyl cellulose is preferably from about 0.4:1 to about 13:1; and more preferably, from about 0.7:1 to about 4:1.

Some other components which may be used as constituents of coating materials in accordance with the present invention include emulsifying agents, acids and liquid materials which will chemically unite with the other constituents of the coating material to cause the coating material to solidify after being applied in a fluidized state. In applications of coating materials which require some flexibility, the coating materials may further comprise plasticizer material. Additionally, anti-settling agents or other constituents may be included in coating material formulations.

The coating material may be used to create a particularly preferred microwave field modifier such as is indicated generally as 20 in Figure 1. The microwave field modifier basically includes a substrate 22 and an array formed from a plurality of discrete electrically conductive elements 24 disposed thereon. The discrete electrically conductive elements 24 are formed from pattern coating the coating material 26 to areas of the substrate 22. Among the advantages of this structure are significant cost and equipment savings relative to current thin film susceptors. It is even more preferable if the coating material is applied to the substrate 22 by printing and most preferably by rotogravure printing. Printing offers advantages such as cost and efficiency savings over other coating processes and rotogravure printing equipment is generally currently available to carton manufacturers.

The conductive elements 24 have relatively low surface resistance: preferably one-hundred (100) ohms or less per square; more preferably ten (10) ohms or less per square; still more preferably three (3) ohms or less per square; even more preferably one (1) ohm or less per square; and most preferably one-tenth (0.1) ohm or less per square. The maximum dimension of the elements 24 is preferably less than about four centimeters and even more preferably from about 1cm to about 3cm. The elements 24 preferably have an elongate portion and/or preferably are staggered relative to each other side to side as described hereinafter.

As used herein, elongate has its ordinary meaning: i.e., having a form notably long in comparison to its width. Additionally, the elongate elements 24 described herein are preferably substantially straight albeit it is not intended to thereby exclude serpentine, wavy, and curved shapes. Also, the elongate elements 24 preferably have radiused ends as shown to ' lessen the propensity for electrical arcing.

Staggered relation, as used herein, is intended to include but not be limited to shapes such as elongate, square, and rectangular elements 24 which are in side by side relation but which have their ends offset from one another. The offset need not be necessarily uniform throughout the array.

Achieving a dried coating having conductivity in the desired ranges identified above is aided by certain features. For example, putting down a sufficient quantity of coating material is important. The more coating material, i.e., the thicker the coating material, the more conductive. Coating material thickness is preferably from about 0.0001 inches to about 0.003 inches and even more preferably from about 0.0005 inches to about 0.002 inches.

Additionally, an undercoating placed on the substrate prior to printing increases conductivity. Likewise, an overcoating placed over the elements increases conductivity. Conductivity is further increased by using both an undercoating and an overcoating. If binder of the undercoating and/or overcoating uses the same solvent as the binder of the coating material conductivity is increased even further. Consequently, an undercoating or overcoating is used, more preferably an undercoating and an overcoating is used, even more preferably an undercoating or overcoating uses the same solvent as the coating material and most preferably an undercoating and overcoating uses the same solvent as the binder of the coating material.

Furthermore, increasing the acidity of the binder seems to have a beneficial effect on conductivity. The more acidic the binder the greater the conductivity. Thus, it may be beneficial to add acidic binder additives to the coating material, such as acid complex forming additives. At least for polyvinylpyrrolidone, exemplary additives include phthalic acid, pyromellitic acid, salicylic acid and benzoic acid and mixtures thereof. While not intending to be bound it is believed the surface chemistry effects of adsorption may be providing this benefit. The adsorption may allow for closer contact of the metal particles with each other giving rise to better conductivity. Also, it is possible salts are being formed with the oxide making the metal more free for electrical conduction.

Figure 1 illustrates a preferred microwave field modifier, indicated generally as 20, which may be printed using the coating material of the present invention. The substrate 22 of Figure 1 is twenty point cartonboard such as is commonly converted into such things as cartons for packaging microwaveable food products: i.e., packages which are suitable for being placed in a microwave oven to heat, cook or bake the contents of the package without removing the contents from the carton. Other exemplary substrate 22 materials include cartonboard, coated cartonboard, thermoplastic film, thermoplastic nonwovens, thermoset plastics, or ceramic.

Disposed on the substrate 22 is an array formed from a plurality of discrete electrically conductive elements 24. In Figure 1, the array extends over the entire top surface 28 of the substrate 22 except for a perimetric zone 29 which is devoid of coating material 26. The perimetric zone 29 acts to insulate the edges of modifier 20 so as to substantially obviate arcing between the electrically conductive elements 24 and any metallic material disposed adjacent the modifier 20. Furthermore, although the array of Figure 1 extends over the entire surface of the substrate 22, the modifier may be limited to one or more zones of the substrate 22.

Referring now to the enlarged fragmentary view of Figure 2, the preferred modifier 20 of Figure 1 includes elements 24 which are uniform in size and shape except at some row ends, and have lengths L and widths W, respectively. Thus, the array preferably substantially comprises elements 24 which are uniformly configured. Additionally, the elements 24 are preferably linearly aligned in straight rows parallel to each other. The rows are linearly spaced apart by a distance designated SL; and side by side rows are spaced widthwise a distance designated SW. Further, the rows are in staggered relation so that side by side adjacent elements are linearly offset by a distance designated OS. The degree of stagger, in percent, of such an array is (0S/L)(100).

Examples of ethyl cellulose and nitrocellulose based coating materials of the present invention which may be used to print a microwave field modifier similar to the one described above are as follows:

Example 1

This Example uses a 9.3 percent solution of Nitrocellulose resin. A 9.3 percent solution can be obtained by starting with 4.0 grams of nitrocellulose resin. To this add 4.0 grams of a plasticizer such as Hercolyn D (hydrogenated methyl ester of rosin), available from Hercules Chemical Corp, Wilmington, Delaware. Add 35 grams of 38/28/34 mixture of isopropyl acetate/isopropyl alcohol/n-propyl acetate as a solvent, providing a total of 43 grams of the 9.3 percent solution. To this add 57 grams of Novamet nickel HCA-1 flakes; creating a 57% nickel and 43% (9.3% resin) resin solution coating material. Thus the final solution consists of 57 grams nickel and 43 grams of 9.3% resin solution.

This solution is then screen printed or rotogravure printed (with only minor adjustments to viscosity) in a pattern similar to that of Figure 1. The dimensions of this pattern may be as follows: end gap SL of 0.045 inches; side gap SW of 0.0275 inches; length L of 0.787 inches; width W of 0.035 inches; and a stagger of 31%.

Example 2

This Example uses a 5.4 percent solution of ethyl cellulose. A 5.4 percent solution can be obtained by starting with 2.2 grams of ethyl cellulose resin. To this add 0.5 grams of an anti-settling agent such as Bentone SD2 which is available from National Lead Chemicals, Hightstown, New Jersey. Add 4.4 grams of a modifier such as Uni-Rez 7055 (fumaric-acid modified rosin ester binder), available from Union Camp Corp., Wayne, New Jersey; and 1.8 grams of a plasticizer such as Herculon D (hydrogenated methyl ester of rosin), available from Hercules Chemical Corp, Wilmington, Delaware. Also add 32.1 grams of n-propyl acetate as a solvent providing a 5.4 percent ethyl cellulose solution. To this add 59 grams of Novamet nickel HCA-1 flakes; creating a 59% nickel and 41% (5.4% resin) ethyl cellulose resin solution coating material. Thus the final solution consists of 59 grams nickel and 41 grams of 5.4% resin solution.

This solution is then screen printed or rotogravure printed (with only minor adjustments to viscosity) in a pattern similar to that of Figure 1. The dimensions of this pattern may be as follows: end gap SL of 0.045 inches.; side gap SW of 0.0275 inches; length L of 0.787 inches; width W of 0.035 inches; and a stagger of 31%.

Example 3

Either of the formulations of the Examples above may be rotogravure printed; requiring only minor alterations to solution viscosity through the addition of ethanol or n-propyl acetate solvent. These solutions may be rotogravure printed in a pattern similar to Figure 1. The dimensions of this pattern may be as follows: end gap SL of 0.080 inches; side gap SW of 0.040 inches; length L of 0.787 inches; width W of 0.020 inches; and a stagger of 25%.

Examples of polvvinylbutyral based coating materials of the present invention which may be used to print a microwave field modifier similar to the one described above are as follows:

Example 4

This example uses a ten percent solution of Mowital^® B30H. A ten percent solution can be obtained by dissolving 5 grams of B30H powder in 45 grams of methanol (methyl alcohol). To this add 75 grams of Novamet nickel; creating a 60% nickel and 40% (10% solid resin) resin solution, thus, the final solution consists of 75 grams nickel and 50 grams of 10% resin solution.

This coating material may then be screen printed in a pattern similar to that of Figure 1. The dimensions of this pattern may be as follows: end gap SL of .045 inches, side gap SW 0.275 inches, length L of .787 inches, width W of .035, and overlap of 31% Furthermore, if this coating material is screen printed onto a substrate such as cartonboard which has been pre-coated with a layer of the polyvinylbutyral solution, (i.e., 10% solution in methanol without nickel) by using a doctor blade or a mayer rod the conductivity and the reflectance (R) may be increased.

If the first sample is coated with the same 10% solution such as to produce an overcoating of the susceptor coating, the change in RAT properties may be similar to those of the second sample which has an undercoating.

If the sample with the undercoating of polyvinylbutyral is now overcoated to produce a printed sample- with nickel which has both an undercoating and an overcoating, the conductivity and the reflectivity (R) may be further increased.

Example 5

This example uses a twenty percent solution of Mowital^® B30H. A twenty percent solution can be obtained by dissolving 10 grams of B30H powder in 50 grams of methanol (methyl alcohol). To this add 75 grams of Novamet nickel; creating a 60% nickel and 40% (20% solid resin) resin solution. Thus, the final solution consists of 75 grams nickel and 50 grams of 10% resin solution.

This coating material may then be screen printed in a pattern similar to that of Figure 1. The dimensions of this pattern may be as follows: end gap SL of .045 inches, side gap SW of 0.275 inches, length L of .787 inches, width W of .035 inches, and overlap of 31%.

Example 6

This example uses the formulation described in Example 1 above in the application of gravure printing. This formulation has been found suitable for rotogravure printing with only minor alterations in solution viscosity through addition of ethanol or ethyl acetate solvent. The formulation may be printed by means of rotogravure printing in a pattern similar to that of Figure 1. The dimensions of this pattern may be as follows: end gap SL of .080 inches, side gap SW of .040 inches, length L of .787 inches, width W of .020 inches, and overlap of 25%.

Examples of coating materials of the present invention based on polv (methyl vinyl ether/maleic anhydride) co-polvmer resin or derivatives thereof which may be used to print a microwave field modifier similar to the one described above are as follows:

Example 7 This Example uses a twenty percent solution of Gantrez ES-225. A twenty percent solution can be obtained by starting with 20 grams of material as supplied (50% resin and 50% ethanol solvent) This provides 10 grams resin and 10 grams solvent. Add 30 grams of ethanol solvent to the solution; creating a 20% resin and 80% solvent solution. This provides 10 grams resin and 40 grams solvent or 50 grams of total solution. To this add 75 grams of Novamet nickel; creating a 60% nickel and 40% (20% resin) resin solution coating material. Thus the final solution consists of 75 grams nickel and 50 grams of 20% resin solution.

This solution is then screen printed or rotogravure printed (with only minor adjustments to viscosity) in a pattern similar to that of Figure 1. The dimensions of this pattern may be as follows: end gap SL of 0.045 inches; side gap SW of 0.0275 inches; length L of 0.787 inches; width W of 0.035 inches; and a stagger of 31%.

Example 8

This Example uses a ten percent solution Gantrez AN-139. A ten percent solution can be obtained by dissolving 5 grams of AN-139 powder in 45 grams of water. 75 grams of Novamet nickel flakes is added the resin solution; creating a 60% nickel and 40% (10% resin) resin solution coating material. Thus the final solution consists of 75 grams nickel and 50 grams of the 10% resin solution.

This solution is screen printed in a pattern similar to that of Figure 1. The dimensions of this,pattern may be as follows: end gap SL of 0.045 inches; side gap SW of 0.0275 inches; length L of 0.787 inches; width W of 0.035 inches; and a stagger of 31%.

Examples of polyvinylpyrrolidone based coating materials of the present invention which may be used to print a microwave field modifier similar to the one described above are as follows:

Example 9

This example uses a ten percent solution of polyvinylpyrrolidone. A ten percent solution can be obtained by dissolving 5 grams of polyvinylpyrrolidone powder in 45 grams of methanol (methyl alcohol). To this add 75 grams of Novamet nickel; creating a 60% nickel and 40% (10% solid resin) resin solution. Thus, the final solution consists of 75 grams nickel and 50 grams of 10% resin solution.

This solution may then be screen printed in a pattern similar to that of Figure 1. The dimensions of this pattern may be as follows: end gap SL of .045 inches, side gap SW 0.275 inches, length L of .787 inches, width W of .035 inches and overlap of 31%.

Example 10

This example uses a thirty percent solution of polyvinylpyrrolidone. A thirty percent solution can be obtained by dissolving 18 grams of polyvinylpyrrolidone powder in 42 grams of methanol (methyl alcohol). To this add 40 grams of 325 mesh graphite from J.T. Baker; creating a 40% graphite and 60% (30% solid resin) resin solution. Thus, the final solution consists of 40 grams graphite and 60 grams of 30% resin solution.

This solution may then be screen printed in a pattern of staggered square elements. The dimensions of this pattern may be as follows: end gap SL of .06mm, side gap SW of 0.06mm, length L of 7.0mm, width W of 7.0mm, and overlap of 31%.

Although particular embodiments of the present invention have been shown and described, modification may be made to the coating material without departing from the teachings of the present invention. Accordingly, the present invention comprises all embodiments within the scope of the appended claims.

Claims

CLAIMS :

1. A coating material which is microwave active when dry comprising electrically conductive particles and a binder system characterized in that the binder system includes a solvent and a polymeric resin selected from the group consisting of ethyl cellulose; nitrocellulose; polyvinylbutyral; poly (methyl vinyl ether/maleic anhidride) co-polymer or derivatives thereof; and polyvinylpyrrolidone, and wherein the binder system for polyvinylpyrrolidone preferably comprises from about 5% to about 30% by weight resin in solvent, and more preferably, from about 8% to about 15% by weight resin in solvent; and for the remaining polymeric resins, the binder system preferably comprises from about 8% to about 30% by weight resin in solvent, more preferably from about 10% to about 25% by weight resin in solvent, and most preferably from about 15% to about 20% by weight resin in solvent.

2. A coating material which is microwave active when dry comprising electrically conductive particles and a binder system characterized in that the binder system includes a solvent and a polymeric resin selected from the group consisting of ethyl cellulose; nitrocellulose; polyvinylbutyral; poly (methyl vinyl ether/maleic anhidride) co-polymer or derivatives thereof; and polyvinylpyrrolidone, and the coating material having a viscosity of preferably from about 50 cps to about 7000 cps, and more preferably from about 100 cps to about 175 cps.

3. A coating material which is microwave active when dry comprising electrically conductive particles and a binder system characterized in that the binder system includes a polymeric resin selected from the group consisting of nitrocellulose; ethylcellulose; polyvinylbutyral; poly (methyl vinyl ether/maleic anhidride) co-polymer or derivatives thereof; and polyvinylpyrrolidone, and sufficient solvent to enable application of the coating material by a high speed pattern coating process and subsequent drying in situ. -Al'

4. A coating material according to Claim 3 wherein the high speed coating process is rotogravure printing or rotary screen printing.

5. A microwave active coating material according to any one of Claims 2-4 further characterized in that a coating material which is microwave active when dry comprising electrically conductive particles and a binder system characterized in that the binder system includes a solvent and a polymeric resin selected from the group consisting of ethyl cellulose; nitrocellulose; polyvinylbutyral; poly (methyl vinyl ether/maleic anhidride) co-polymer or derivatives thereof; and polyvinylpyrrolidone, and wherein the binder system for polyvinylpyrrolidone preferably comprises from about 5% to about 30% by weight resin in solvent, and more preferably, from about 8% to about 1% by weight resin in solvent; and for the remaining polymeric resins, the binder system preferably comprises from about 8% to about 30% by weight resin in solvent, more preferably from about 10% to about 25% by weight resin in solvent, and most preferably from about 15% to about 20% by weight resin in solvent.

6. A microwave active coating material according to any one of Claims 1-5 wherein the electrically conductive particles preferably comprise from about 40% to about 80% by weight of the coating material; and more preferably, comprise from about 55% to about 65% by weight of the coating material.

7. A microwave active coating material according to any one of Claims 1-6 wherein the electrically conductive particles are graphite or carbon and preferably comprise from about 10% to about 50% by weight of the coating material; and more preferably, comprise from about 13% to about 23% by weight of the coating material.

8. A microwave active coating material according to any one of Claims 1-7 wherein the weight ratio of the conductive particles to the resin for a binder system including nitrocellulose or ethyl cellulose is preferably from about about 2:1 to about 20:1; and more -53- preferably from about 4:1 to about 10:1; and wherein the weight ratio of the conductive particles to the resin for a binder system including the other polymeric resins is preferably from about 2:1 to about 50:1; and more preferably, from about 4:1 to about 20:1.

9. A microwave active coating material according to any one of Claims 1-8 wherein the conductive particles are carbon or graphite and the weight ratio of the particles to the resin is preferably from about 0.4:1 to about 13:1; and more preferably, from about 0.7:1 to about 4:1.

10. A microwave active coating material according to any one of Claims 1-9 further characterized by an acidic material additive.

11. A microwave active coating material according to any one of Claims 1-10 wherein the conductive particles are less than about 25 microns in size.

12. A microwave active coating material according to any one of Claims 1-11 wherein the dried coating has an electrical conductivity expressed in terms of surface resistivity of less than about 100 ohms per square; preferably, less than about 13 ohms per square; more preferably less than about 3 ohms per square; even more preferably, less than about 1.0 ohms per square; and most preferably, less than about 0.1 ohms per square.

13. A microwave active coating material according to any one of Claims 1-12 wherein the dried coating material has a thickness of from about 0.001 to about 0.003; and preferably, from about 0.0005 to about 0.002.