CA2048342A1 - Microwave active coating material including polyvinylpyrrolidone - Google Patents

Abstract

A MICROWAVE ACTIVE COATING MATERIAL INCLUDING
POLYVINYLPYRROLIDONE

ABSTRACT OF THE INVENTION
A microwave active coating material is provided which includes electrically conductive elements and a binder system.
The binder system includes a polyvinylpyrrolidone resin and a solvent. The coating material is capable of pattern coating a substrate to form a plurality of discrete electrically conductive elements. The discrete electrically conductive elements have a maximum dimension sufficiently small to obviate arcing. In addition, the elements form an array which operates as a microwave field modifier. The modifier may constitute a dielectric substrate having a plurality of discrete surface areas which are coated with an electrically conductive coating material to form discrete electrically conductive elements, and which discrete elements on a given surface are disposed in a predetermined array.
Preferably the elements are elongate; and, preferably, the array constitutes a plurality of rows of linearly aligned elements which rows are in parallel relation, and in which the elements in adjacent rows are in staggered relation. The electrically conductive elements are preferably formed by printing an electrically conductive ink-like coating material on the dielectric substrate.

Description

2~83~2 A MICROWAVE ACTIVE COATING MATERIAL INCLUDING
POLYVINYLPYRROLIDONE

BACKGROUND OF THE INVENTION
1. Field of the Invention The present invention relates to microwave susceptible coating materials, and more particularly, to such coating S materials useful for creating patterned microwave field modifiers.
2. DescriDtion 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 2~83~2 microwave oven, 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 intermolecular action;
or from induced electrical currents which result in so called I-squared-R losses in electrically conductive devicesi or from d'ielectric 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 ~icrowave 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 ~icrowave 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.
SUMM~RY 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 a polyvinylpyrrolidone resin. The coating material is preferably 2~483~2 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 conducttvity, expressed in terms of surface resistivity, of less than about IOO 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(s) 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 I.

DESCRIPTION OF THE PREFERRED EMBODIMENT
The coating material of the present invention generally includes a binder system, which comprehends a polyvinylpyrrolidone resin material 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 su~strate. Included in the binder system is a polyvinylpyrrolidone resin and a solvent.
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.

2048~42 \

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), lactam~ (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.
Electrically conductive particles which may be used to make coating materials in accordance with the present invention preferably include carbon particles and graphite particles; and even more preferably include pure metallic particles such as 2~8342 nickel, aluminum, iron, copper, silver and tin, some metallic oxides such as tin oxide, and 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-l which may be purchased from the Novamet Company, Wyckoff, N.J. The Novamet Nickel Flake HCA-l 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 (except carbon and graphite) preferably constitute from about 40Y, 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 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 . , .

20483~2 printing or rotogravure printing. Achieving the desired viscosity may require the addition of resint 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 variaus 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 is preferably from about 2:1 to about 50:1 and even more preferably from about 4:1 to about 20:1. The carbon and graphite particle to resin ratio 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 20~83~2 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 4cm and even more preferably from about lcm 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.

2~483~2 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 ma'terial 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. Exemplary additives include phthalic acid, pyromellitic acid, salicylic acid and benzoic acid and mixtures thereof. While not intending to be bound it is believed that surface adsorption of the acidic additives by the conductive particles 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.

.

20483~2 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 2g 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 (OS/L)(100).
Examples of polyvinylpyrrolidone based coating materials f the present invention which may be used to print a microwave field modifier similar to the one described above are as follows:
xamDle 1 Th~s 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% (10X 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 ~0~83~2 inches, length L of .787 inches, width W of .035 inches and overlap of 31%.
ExamDle 2 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 (20)

1. A coating material which is microwave active when dry comprising electrically conductive particles and a binder system including a solvent and a polyvinylpyrrolidone resin.

2. A microwave active coating material according to Claim 1 wherein said binder system comprises from about 5% to about 30% by weight resin in solvent.

3. A microwave active coating material according to Claim 2 wherein said binder system comprises from about 8% to about 15% by weight resin in solvent.

4. A microwave active coating material according to Claim 1 wherein said electrically conductive particles comprise from about 40% to about 80% by weight of said coating material.

5. A microwave active coating material according to Claim 2 wherein said electrically conductive particles comprise from about 40% to about 80% by weight of said coating material.

6. A microwave active coating material according to Claim 1 wherein said electrically conductive particles are carbon or graphite particles and comprise from about 10% to about 50% by weight of said coating material.

7. A microwave active coating material according to Claim 1 wherein the weight ratio of said conductive particles to said binder resin is from about 2:1 to about 50:1.

8. A microwave active coating material according to Claim 1 wherein said conductive particles are carbon or graphite and the weight ratio of said conductive particles to said binder resin is from about 0.4:1 to about 13:1.

9. A coating material which is microwave active when dry comprising electrically conductive particles and a binder system including a solvent and a polyvinylpyrrolidone resin, said coating material having a viscosity of from about 100 cps to about 175 cps.

10. A microwave active coating material according to Claim 9 wherein said binder system comprises from about 10% to about 25%
by weight resin in solvent.

11. A microwave active coating material according to Claim 1 wherein said conductive particles are selected from the group consisting of pure metallic particles, some metallic oxides, metal alloy particles, and carbon and graphite particles.

12. A microwave active coating material according to Claim 9 wherein said conductive particles are less than about 25 microns in size.

13. A microwave active coating material according to Claim 9 wherein said conductive particles have a flat shape.

14. A microwave active coating material according to Claim 11 wherein said conductive particles have a dendritic shape.

15. A microwave active coating material according to Claim 12 wherein said conductive particles are nickel flakes having a flat dendritic shape.

16. A microwave active coating material according to Claim 1 further comprising an acidic material additive selected from the group consisting of phthalic acid, pyromellitic acid, salicylic acid and benzoic acid or mixtures thereof.

17. A coating material which is microwave active when dry comprising electrically conductive particles and a binder system including a polyvinylpyrrolidone resin and sufficient solvent to enable application of said coating material by a high speed pattern coating process and subsequent drying in situ.

18. A coating material according to Claim 14 wherein said high speed coating process is rotogravure printing.

19. A coating material according to Claim 14 wherein said high speed coating process is rotary screen printing.

20. A coating material according to Claim 14 wherein said dried coating has an electrical conductivity as measured in terms of resistivity of less than about 100 ohms per square.