CA2066065A1 - Microwave oven susceptor - Google Patents

Abstract

MICROWAVE OVEN SUSCEPTOR
Abstract Paperboard packaging material for use in the manufacture of cartons for browning and crisping food in a microwave oven is prepared on a printing press using an aqueous susceptor-ink composition comprising graphite or conductive carbon black dispersed in a solution of sodium silicate. The susceptor-ink composition is printed on the packaging material in a pattern corresponding to the location of the food intended to be packaged and the food contact surface is overprinted with an FDA approved food contact coating. The use of sodium silicate as the binder for the susceptor-ink composition allows the composition to be printed by gravure or flexographic printing processes and provides hereto unavailable flame retardant properties to the final packaging material.

Description

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,l l Ba-ck~round~ ~Eav-~n~lpn The pr~sent invention relates to paperb~ard ! packaging material and packages constructed therefrom, and ¦more particularly to an ~mprovement in applicant'~ prior U.S.
¦ Pat~nt No. 4,914,266 relating to suscep~or packaging for ¦heating and browning foods in a microwave oven.
One of the problems a~ociated with the use of microwave energy for cooking is that it fails to brown and llcrisp those ~oods which are normally expected t~ have such a j quality~ Applicant's prior U.S. Patent No. 4,914,2G6, ;lentitled "Press Applied Susceptor for Controlled Microwave Heating", addressed that problem and provided a susceptor . packaging material that was useful f~r the intended purpose.
: According to applicant's prior patent, i~ was discovered that both carbon and graphite (i.e., conductive carb~n materials), were availa~le in particle sizes which could ~e readily dispersed into 6~1vent based inks. Moreover, it was found that inks incorporating these mat~rial~ could be printed on coated or uncDated paperb~ard in any prescribed pattern with known printing methods and equipment, including sravure and ¦iflexography. Furthermore, by ~verprinting the de~ired l patterns, or by varying the concentrati~n ~f the carbon ¦.~usceptor material within the prescr~bed pattern, the concentration of ~usceptor material applied in the prescribed pattern could be controlled to obtain uselul ti _ -torperature . , .

~l i profiles for the most effectiv~! browning of dif2erent ~inds of ¦food. The packaging material manufactured could then be converted intv any desired package shape using conventional ~ethods. The per~ormance of ~uch packages was found to be comparable to m~talli~ed aluminum ~uscepkor~ in the generaSion of heat for browning during microwave cooking. ~Iowever, the ¦susceptor-ink compositions disclo~ed in applicant's prlor ¦patent had a tendency to char or burn at ~levated ~emperatures ¦when no food was in the packages much like some of the ¦!susceptor packaging that use metallized aluminum. Also, the susceptor-ink composltions proved to be difficult to keep in llplace on the surface of some paperboard, thus causing j potential contamination problem~. Accordingly the present 'invention was developed to overcome those problem~ and to ! provide susceptor pac~aging material of improved quality.
~ Lary of ~nvention ; Carbon and graphite are conductive carbon materials that are available in particle sizes which may be readily ,'dispersed into solvent based inks. Inks incorporating these I materials can be printed on coated or uncoated paperboard ,llpackaging material, and praferably polyester coated paperboard 'IRubstrates, to make ~usceptor packaging materials use~ul in a ~¦micxowave oYen.

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2I~I;~:J~j5 ' Now, how~ver, according to the improvement in the ,ipresent invention, it ha~ b~en discovered that the ~ame or !l6ubstantially the ~ame conductive carbon ~usceptor materials ¦Imay be dispérsed in an aqueous vehicle to produce a ~usceptor-¦ink composi ion which i~ ~urpri~ingly capable of being printed ! using conventional printing techniques such as ~ravure or !Iflexography. Moreover, the use herein of ~n aqueous ~olution li of sodium 6ilicate as the binder ln the ink v~hicle of the I susceptor-ink composition also provides flame retar~ant characteristics to the printed packaging material not available with the prior art products.
Printinq inks typically consist essentially of two components, the colorant, an insolubl~ solid or dye, and a : vehicle, a liquid in which the colorant is ~uspended or dissolved. The combined components form a fluid oapable of being printed on a printing press. The vehicle has a secondary function, namely to dry and thus bind the colorant firmly to the printed substrate. Thus, a typical ink formulation is designed to print by a given process, dry under specified conditions, adhere to a given substrate, and p~rform ,. I
: , 6pecific end use applications.

~ The formulation of a printing ink and its phy~ical Ij appearance are determined greatly by the method by which the , image is produced. Inks which are printed by the flexographic and gravure printing prrcesses are oharacterized by their .

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i extremely fluid nature. trhey are generally described as ~ liquid inks. Both inking ~ystems involve only a ehort tim~
¦~and distance between the ink supply and the metered film of ink applied to the sub~trate. q~hu6, hiyh:ly volatile ~olvents can be used and must flexographic and gravure inks are fast ¦drying. Thus, water ba~ed ink~ are r~rely used in the gravure Iand flexographic proce6ses. When u~e~, ~uch u~e is generally ¦~ confined to ab~orbent 6ubstrate~ ~ince for non-absorbent ¦l substrates the use of water based inks requires ~evere ~ curtailment in press speeds because of difficulty in drying.
Nevertheless, with the present invention, a water based ink was found to be surprisingly successful primarily because of , the use of sodium silicate in the susceptor-ink formulation.
Sodium silicates are compounds of silica (Sio2) and . soda ash (Na20) and are readily available as an aqueous solutisn. The properties of such solutions are dependent upon the ratio of silica to soda ash. The weight ratio of silica to soda ash ~aries from about 4:1 to 1.5:1. Such solutions when used as the vehicle in th~ susceptor ink formulation of j; the present invention are belieYed to be successful because solutions containing ~ilicates dry by absorption ~f water. It is known that ~odium ~ilicate will retain from 10-30~ moisture ' depending upon how it i6 dried. Thi~ per~its the ~odium silicate containing susceptor-ink Xormulation o~ t~e present invention tD be used on a typical gravure or fl-xDgraphic li ~l press, ~nce all of the water in the ~olution need not be Il driven off during drying. More!over, the boun~ water in the il otherwise ~ried ink formulatlon applied yields ~nother j~dvantage inherent in the use of ~odium silicate, name~y, ¦lmprovad fire xetardant properties. The use of ~odium Isilicate as a fire ratardant additive to paper and paperboard ¦is known, but the presen~ invention i~ ~he fir~t known ,applicatlon of ~odium ~ cate as a fire retardant material to ¦Ipackaging mater1al used ~n microwa~e ovens.
Furthermore, because the present invention relies on a printing process for applying the susceptor-ink formulation to the paperboard packaging material, the concentration of .
susceptor and thus the rate of heating and temperature of the susceptor can be controlled ~y overprinting the packaging material in the desired pattern for accommodating a specific food product. As disclosed in applicant'6 prior patent, this enables the package designer to obtain useful time-temperature profiles for the most effectiv~ brow~ing of differ~nt kinds of food in the same package. By appl~ing the ~usceptor material only in the parts of the food package which contacts the j ~urface of the food, and by varying the amount o~ 6usceptor , material applied to those ar~as, it i~ possible to reach and ,Imaintain elevated tempera~ures at those locations long enough I to achieve th~ cri~ping and browning desired without i overheating other parts of the package. Moreo~er, , rJ~

ll l because of the pre~ence of the sodium silicate in the susceptor-ink formulation, the possibility of the package catching fire because o~ overheating is ~ubstantially reduced.
I D _ailed ~)escription 'I The pra-~ent invention is diracted to the u~e of graphite or a conductive carbon black ~usceptor material in the manufacture of 6usceptor packaging material for the microwave oven. The susceptor material i~ dispersed in an ink vehicle compri~ing an aqueous solution of ~odium silicate to ~ produce a ~usceptor-ink composition which is printed on a suitable paperboard ~ubstrate using conventional printing technology. The printed susceptor is then overprinted with a barrier coating suitable to provide an FDA approved food c~ntact surface. An example of such a barrier coating i5 Adcote 33R2AH, a polyester m~terial supplied by Morton International, Chicago, Illinois. The susceptor-ink composition is preferably printed on the paperboard substrate using a rot~gravure or flexographic printing process. The concentration of the susceptor composition printed on the ~ubstrate is varied to correspond with the location and type 1~ of food in the package made with the ~u~ceptor packaging i material.
¦I FIGURE 1 illustrates in cross section the structure of the composite 6usceptor packaging material according to the pre~ent invention. Reference character 11 represents the 6ub-'.' I

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strate on which the susceptor-ink ~omposition i8 printed.
When applied to an uncoated 6ubstrate 11, the ~ilicate containing ~usceptor~in~ composltion oE the prese~t invention '¦ tends to soak into the 6ubstrate. A clay coated ~ub~trate presents less problems than uncoat~d papQrboard because the silicate containing ~usceptor-ink composition readily bonds to Il the clay coated surface~ ~owever, because the ~ilic~te ¦ ~ontaining susceptor-ink compo~ition 1~ essentially retained ~ on the surface of 6ubstrates containing a polymeric coating, and particularly the polyester coated paperboard normally used for microwave packaging, primers or additives must ba used to bond the silicate containing susceptor-ink composition to the polymer surface. Silanes with an organic component to bond to the polymeric surface and an inorganic component to bond to the silicate containing susceptor-ink compositio~ are useful for this purpose. In any event, as the holdout of the substrate increases, the amount of ~usceptor-ink composition needed to achieve microwave browning decreasPs.
Reference character 12 in FIGURE 1 designates the susceptor-ink composition printed on the ~ubstrate 11. This composition comprises an ~que~us 601ution of sodium ~ilicate into which there is dispersed a conductive carbon material ~ ~uch as graphite or carbon blank. Sodium ~ilicate, unlike the i solvent binders used in conventional gravure and flexographic printing inks provides th~ the~mal stability nec~ssary for the ,5 i I high temp~ratures achieved with ~usceptor packaging mat~rials.
I The ~odium silicate containing l~usceptor ink composikion of ¦ the present invention is surprisingly useful for printing by ¦igravure and ~lexography~ The thermal ~tability and useful jlprinting characteristics of the suscep~or-ink compo~ition Illdisclosed herein i~ ~elieved to be attributable ~o the lcharacteristics of ~odium ~ilicate coating~ which are dried or llset by absorp~ion o~ water ~o that, ev~n after drying, they l still retain cQnsiderable bound water on the order o~ about 10-30% by weight.
Sodium ~ilicat~ solutions are mixtures of silica (sio2) and soda ash (Na2O) dissolv~d in water. The preferred ratio for the present invention is a~ut 3.22 with about 61.7%
water. A grade of silicate that ha~ proven to be particularly useful in the present invention is Occidental Chemical's Grade 40-clear, which is prefiltered to remove lar~e particles.
Particulate graphite is available in a wide range of particle sizes, shapes and purities. Particle sizes less than about 10 microns are preferred for application with a gravure printing press. Obviously, the highest purity product available is ¦' preferred for making food packagin~. Superior Graphite's 1 Grade 5539 i~ an example o~ a graphite pro~uct useful in the ,, preEent invention. It ha~ a partiole 6ize of about 5 microns, I it is 6pherical in shape, and it has a purity of 99 . 8% carbon .
,I Grade 5539 graphlte may be dispersed in a 60dium ~ilicate l l.
; 5 .1 ,solution ~t a ratio of 1:1 (graphite to silicate), achieving a total ~olids of about 40%. A useful 6usceptor-ink composition llfor the present invention may c~ompri~e a ratio of one part ¦jSuperior Graphite Gra~e 5539 to three parts Occidental Chemical ~odium silicate Gra~e 40-clear, ad~usted to a total ~olids of ~bout 40~. This composit~on has a vi~cosity o~ ¦
!, about 20 6econds ~#2 Zahn Cup), and is readily printed by the gravure printing process.
I'Reference character 13 in FIGURE 1 represents the food contact overprint applied to the ~usceptor~ink composition. Layer 13 serves to protect the susceptor-ink composition from moi~ture as well as to protect the food product fro~ any possible contamination from by-products in ', the ~usceptor-ink composi~ion layer 12 or ~ubstrake layer 11.
Layer 13 may comprise a material selected from the group consisting of acrylics, polyesters and silicvnes. An example of an overprint suitable for the present invention is AVCOTE
33R2AH from Morton International. Suitable acrylic coatings, for example, Acrylic Latex 26315, axe ~vailable ~rom B.F~
Goodrich, and suitable silicones are available fxom Dow Corning Corporation ~SYL-OFF ~eries 7146, 7044 and 7600).
1~ ExamPle I
!~ Susceptor-ink compositions prepared ~rom an aqueous jlsolution of ~odium sil;cate as the binder were printed on clay !~coated paperboard at graphite concentrations of from about 25%
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to 75~. ~sing these composi~i~nS; it was possible to brown and crisp micl-owave pizza with a compositi~n containing a6 l~ttle a 25~ graphite having a coslducti~rity of about 0.001 mhos/~q.
jjThere was no arcing or charring tn the ranye of conductiviti~s ¦l tested. Other compositions were found to be u~eful with ¦I conductivities ranging from about 0.0001 mhO5/S~. ko about ¦l 0.006 mhos/sq.
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~I Susc~ptor-ink compositions were prPpared ~sing three grades of graphite, differing in particle size and ~hape, a~d two grades of sodium silicate (filtered ~nd unfilter~d). All compositions w~re prepared using 25 parts graphite to 75 parts sodium ~ilicate solution at 40% 601ids. Th~ compositions were applied to a clay coated paperboard substrate at three different thicknesses, 2 mils, 4 mils and 6 mils. Conduc-ti~ity measurements made of these compositions ~howed a direct increase in conductivity with increasing ~ilm thickness. Oven tests demonstrated useful browning and crisping of microwave pizza over the conductive rangs from ab~ut 0.0008 mho~/sq.
to about 0.0033 mhos/~q. Representative graphite components ,~ were MICRO-250 Asbury Graphite, Asbury, New ~ersey (0.5 micron spherical particle~: Grad~ 5539 - Superior Graphite, Chicago, Illinois (5 micron ~pherical particle); and, Grade 703~ -1, Superior Graphite, Chicago, Illinoi6 (5 micron flake I part~cle~. Silicate solutions included 40~clear ~Occidental , . 10 ~:~?~
i Il Chemical) and N-Grade (Philade].phia ~uartz). MICRo 250 graphite, with a partiole ~ize o 0.5 micron was difficult to disperse into the silicate solution and gave inconsi~tent results in the oven test.
~ e A susceptor-ink composition consii~tir-g of 2 graphite and 75~ aqueous 60dium silicate ~olution at 40~
601ids was applied to clay coated paperboard using a 100 lines-per-inch gravure cylinder on a Geiger pres~. The viscosity of ~he susceptor-ink composition was 20 ~econds, #2 Zahn cup. Up to six bumps on the press were made to determine the coat weight necessary for browning microwave pizza~ Based on this trial it was found that at least four bumps on the press were necessary to achieve a conductivity of about 0.00~3 mhos/sq., and achieve satisfactory browning of the pizza crust.
Example IV
Up to six bumps of the susceptor-ink composition of Example III was applied to clay coated paperboard with a lOo lines-per-inch gravure cylinder and then overc~ated with a polyester food contact coatlng, one or twv bumps, using a 150 lines-per-lnch gravure cylinder. The overcoat consi~ted of Adcote 33R2~H polyester. Thi~ combination prov~ded good microwave oven ~rowning without any risk of charring. The overcoat als~ prevented th~ ~usceptor-ink composition from jl !
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. . l jbeing rubbed off the ~urface of the paperboard substrate both before and after cooking. The oYercoat meets existing FDA
" guidelines for direct food contact provided all of the solvent ~ drlven Dff during drying. I
¦l It will thus be ~een that the present invention discloses an improvement in microwave susceptor packaging that may be practiced on a printing press. The improvement yields ,ladditlonal benefits not addressed by the prior art. Although l the process and product has been described somewhat ~specifically, lt is to be understood that vaxious modifications and alternatives are available without departing from the spirlt and scope of the appended claims.

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Claims (7)

1. A susceptor food package for use in a microwave oven comprising:
(a) a food package formed from a dielectric substrate;
(b) a microwave susceptor material in the form of a susceptor-ink composition printed on the food package only in the areas of the food package which contact the surface of the food, said susceptor-ink composition comprising an aqueous solution of sodium silicate having dispersed therein a conductive carbon pigment in sufficient quantity to achieve a surface resistivity of between about 0.0001 - 0.006 mho/sq.; and, (c) a barrier coating applied over the printed susceptor-ink composition to provide a food contact surface for the packaged food.

2. The susceptor food package of claim 1 wherein the amount of susceptor-ink composition applied to the food contact areas is varied depending upon the food product to achieve crisping and browning of the food product without overheating other parts of the package when the package is exposed to microwave radiation.

3. A susceptor for crisping and browning food in a microwave oven comprising:
(a) a microwave transparent dielectric substrate;
(b) a microwave susceptor-ink composition printed on one surface of said dielectric substrate, said susceptor-ink composition comprising a conductive carbon pigment dispersed in an aqueous solution of sodium silicate in sufficient quantity to achieve a surface resistivity of from about 0.0001 - 0.006 mho/sq.; and, (c) a food contact coating applied over said susceptor-ink composition.

4. The susceptor of claim 3 wherein the susceptor-ink composition comprises from about 25-75 parts pigment and from about 75-25 parts sodium silicate at 40-55% solids and has a viscosity of from about 20-50 seconds measured by a No. 2 Zahn Cup.

5. Susceptor packaging material for food products capable of the generation of heat by absorption of microwave energy comprising:
(a) a microwave transparent dielectric substrate;
(b) a microwave susceptor-ink composition printed on one surface of said substrate in a pattern corresponding to the location of food intended to be packaged with said packaging material and in varying concentration so as to control the rate of heating and temperature reached when exposed to microwave radiation, said susceptor-ink composition comprising a conductive carbon pigment dispersed in a printable aqueous solution of sodium silicate, to achieve surface resistivities in the range of from about 0.0001 - 0.006 mho/sq.; and, (c) a food contact coating applied over said susceptor-ink composition.

6. The method of making susceptor packaging material on a printing press for use in the manufacture of packages for a microwave oven comprising:
(a) selecting a dielectric paperboard substrate for the susceptor packaging material having an inner surface and an outer surface, said substrate permitting the passage of microwave energy therethrough;
(b) preparing a susceptor-ink composition for printing on the inner surface of said dielectric substrate comprising from about 25-75 parts conductive carbon pigment dispersed in an aqueous ink vehicle consisting essentially of from about 75-25 parts sodium silicate binder, said susceptor-ink composition having a solids content of from about 40-55% and a viscosity of from about 20-50 seconds measured by a No. 2 Zahn cup;
(c) printing the susceptor-ink composition of step (b) onto the inner surface of said dielectric substrate in a preselected pattern corresponding to the location and type of food to be packaged with said packaging material;
(d) overprinting the inner surface of said dielectric substrate containing the susceptor-ink composition with a food contact barrier coating to provide a food contact surface for the food to be packaged with said packaging material.

7. The method of claim 6 wherein the sodium silicate is a combination binder material and flame retardant for said susceptor packaging material when the sodium silicate is present in the susceptor-ink composition in an amount of at least about 25% by weight of the susceptor ink composition.