EP0535451A1

EP0535451A1 - Grease-absorbent microwave cooking pad and package

Info

Publication number: EP0535451A1
Application number: EP92115872A
Authority: EP
Inventors: Gary A. c/o Minnesota Mining and Isakson; Pierre H. c/o Minnesota Mining and LePere; Daniel E. C/O Minnesota Mining And Meyer; James R. c/o Minnesota Mining and Nelson
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1991-09-30
Filing date: 1992-09-17
Publication date: 1993-04-07
Also published as: JPH05280753A; CA2077100A1

Abstract

A pad (14) for use in the cooking of food (12) placed thereon in a microwave oven is disclosed. The pad (14) is specifically designed for use in connection with food (12), (e.g., bacon strips) that contains a substantial amount of solidified grease which melts when the food is cooked by microwave radiation. The pad (14) is formed from an entangled web formed from at least one generally hydrophobic and grease absorbing multilayer microfiber. The web is formed by combining at least two streams of flowable polymer materials (e.g., polypropylene and polyethylene terephthalate) in a layered, combined flowstream, extruding the combined flowstream through a die having at least one orifice, attenuating the extruding flowstream with a high velocity gaseous stream to form fiber, and collecting the fiber on a collective surface so as to form the entangled web. A pad (14) so formed is capable of holding the amount of grease in the food (12) placed thereon when the grease is heated by cooking the food (12) in a microwave oven.

Description

BACKGROUND OF THE INVENTION

This invention relates to a grease-absorbent pad for use in the microwave oven cooking of food that contains a large amount of solidified grease and water, and to a package with such a pad and such food sealed therein for cooking purposes.
Foods, particularly precooked and cured foods such as bacon, sausage, ham, or bologna, that contain a large amount of water and solidified grease can cause problems when cooked in a microwave oven. Water in such foods is vaporized by contact with the heated melting grease as the food cooks, causing tiny explosions that can splatter portions of the grease around the oven. In addition, the solidified grease melts when the food is cooked by microwave radiation. one attempt to address those concerns has been to place the food on a pan that collects the melted grease, and to cover the food with several layers of paper towels to restrict splattering.
It is known to place a liquid absorbent pad within a package for absorbing food by-products such as moisture and grease exuded from food during cooking in a microwave oven. Such pads must not only sufficiently absorb the quantity of food by-products produced during cooking, but must also withstand the elevated temperatures required to adequately cook the precooked or cured foods without degradation.
However, conventional absorbent pads absorb both water and various greases from the food. This is undesirable in that if part of the absorbent capacity of the pad is occupied by moisture, insufficient capacity may remain for grease. Alternatively, the capacity of the pad must be increased by increasing the size and weight of the pad, at additional expense.
It is also desirable in many cases for water exuded from a food in the form of steam during cooking to be maintained in the close proximity to the food to evenly distribute heat within the package and to reduce the cooking time for the food. An additional problem occurs during extended storage and transportation of a package containing the food having substantial amounts of water and grease. A pad that absorbs water as well as grease will tend to gradually absorb water from the food. Thus, a subsequent measurement may show that the weight of the food has been reduced compared to the weight at the time the package was sealed.
One attempt to address these concerns is presented in Larson U.S. Patent 4,865,854, which discloses a microwave oven food package for use in cooking food containing a substantial amount of water and solidified grease. The package includes a pad adjacent the food which is formed from microwave radiation transparent and generally hydrophobic grease-absorbing microfibers which are capable of holding the amount of grease in the food when it is melted. The package further includes a vapor-tight microwave radiation transparent enclosure surrounding the pad and food that has a steam vent which opens as the food is cooked. This patent teaches producing the pad for this package from blown microfibers (BMF) made in accordance with the teachings of U.S. Patent Nos. 4,103,058 and 4,042,740.
A pad formed as disclosed in the Larson patent discussed above, while having, its own utility, has proved to be relatively expensive for commercial microwave food packaging purposes. Such a pad has been made by melt-blown microfibers formed into an entangled web. Polymer pellets were dry blended together to form a 50/50 mixture of polypropylene and poly 4-methylpentene-1 (a relatively expensive polymer). Polypropylene and poly 4-methylpentene-1 can be dry blended and extruded to a usable product.
It is desired to develop a pad for use in a microwave oven package which has an increased grease absorbency, thus reducing the pad size or thickness requirements and hence lowering the pad weight, as well as lowering production costs and material expenses. In some applications, it is also desirable to bond a scrim or cover sheet of a separate material to the pad on its food adjacent surface, and a pad is sought which will readily maintain such a bond. In addition, it has been desired to develop grease-absorbent pads for microwave food packages from blends of polymer materials which were previously unblendable in conventional extrusion techniques to produce a satisfactorily blended microfiber and finished pad product for commercial applications, yet attaining increased absorbency and bonding characteristics.

SUMMARY OF THE INVENTION

The present invention provides a grease-absorbent pad for use such as in a package for food containing a substantial amount of water and solidified grease that is to be cooked in a microwave oven. The pad is formed from an entangled web of generally hydrophobic and grease-absorbing multi-layer microfibers. The web is prepared by combining at least two streams of flowable materials in a layered, combined flowstream, extruding the combined flowstream through a die having at least one orifice, attenuating the extruded flowstream with a high velocity gaseous stream to form fibers and collecting the fibers on a collective surface so as to form the entangled web. This pad is capable of holding the amount of grease in the food when the grease is melted by cooking the food in a microwave oven.
In one embodiment, the pad is used in combination with a microwave food cooking package. The package prevents splattering of the grease onto the inside of the microwave oven, collects the grease during the cooking process, does not require special handling to preclude spilling the collected grease after the substance or food has been cooked, and is easy to manufacture. According to the present invention, a package for use in a microwave oven includes foods, and in particular, precooked or cured foods containing a substantial amount of water and solidified grease (e.g., bacon, sausage., ham, or bologna); a pad adjacent the food comprising an entangled web of generally hydrophobic and grease absorbing multi-layer microfibers, formed as described above; and a vapor-tight microwave radiation transparent enclosure surrounding said pad and said food.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described with reference to the accompanying drawings wherein:

FIG. 1 is a perspective view of a microwave food package according to the present invention.
FIG. 2 is an enlarged fragmentary sectional view of the package shown in FIG. 1.
FIG. 3 is an enlarged fragmentary view of the package of FIG. 1 while food therein is being cooked in a microwave oven.
FIG. 4 is a perspective view of the package of FIG. 1 being opened.
FIG. 5 is an enlarged fragmentary sectional view of a food cooking pad with food thereon of the present invention.
FIG. 6 is a schematic representation of an apparatus useful for producing a nonwoven web of longitudinally layered melt-blown microfibers to define a pad useful in cooking food in a microwave oven.
FIG. 7 is an electron micrograph cross section of blended fibers of polypropylene (PP) and polymethylpentene (TPX), blended by a "dry-blend" technique of the prior art.
FIGS. 8 and 9 are scanning electron micrograph cross sections multi-layer blended fibers of polypropylene (PP) and polyethylene terephthalate (PET), blended by the web formation process described herein.

It is understood that the drawing figures herein are provided for illustrative purposes only and are not drawn to scale, nor should they be construed to limit the intended scope and purpose of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The Inventive Package and Web Structures

FIGS. 1-4 show a package of food according to the present invention that can be cooked in a microwave oven, with the package being generally designated by reference numeral 10. As best seen in FIGS. 1 and 2, the package 10 includes food (e.g., strips of bacon) 12 containing water and a substantial amount of solidified grease, and a pad 14 adjacent the food 12. The pad 14 is formed from a microwave radiation transparent generally hydrophobic grease-absorbing material which is capable of at least absorbing the amount of grease in the food 12 when that grease is liquefied. Preferably, the pad comprises coextruded multi-layer blown microfibers, made in accordance with the web formation process described herein.
A generally rectangular vapor-tight microwave radiation transparent enclosure 16 surrounds the pad 14 and food 12 and comprises top and bottom rectangular sheets 17 and 18 of polymeric film fastened together as by heat sealing to provide a vapor-tight seal 19 around their peripheries, with the pad 14 and food 12 therebetween. Suitable means may be provided for venting the enclosure 16 to facilitate cooking the food 12 within the enclosure 16 in a microwave oven. In one embodiment, as shown, the means for venting comprises a layer of microwave radiation absorbable material in the form of a piece of metal vapor coated film 20 adhered by a suitable adhesive to the top sheet 17 of the polymeric film forming the enclosure 16. The vapor coated film 20 and a portion of the top sheet 17 adjacent thereto will be softened by heating of the metal vapor coating to cause rupture of that top sheet 17 of film and vapor coated film 20 due to steam or vapor pressure within the enclosure 16 and/or different amounts of shrinking of the films 17 and 20 during cooking of the food 12 by microwave energy. As illustrated in FIG. 3, the top sheet 17 of film and the vapor coated film 20 will thus allow excess steam or vapor pressure within the enclosure 16 to escape, while retaining sufficient steam or vapor within the enclosure 16 to enhance cooking of the food 12. In another embodiment, the means for venting is a weakened portion of the heat seal between a portion of the periphery of the enclosure, which is ruptured (in a controlled manner) by the buildup of steam or vapor pressure within the enclosure when the food is cooked, thereby regulating the cooking of the food and pressure within the enclosure.
The package 10 also has an arrangement for affording easy manual opening of the enclosure 16 to facilitate removal of the cooked food 12. A portion of the seal 19 between the face-to-face layers of the polymeric film adjacent one edge 24 or corner area of the package 10 is spaced a substantial distance (i.e., over 3 cm. and preferably about 6 cm.) from that edge 24 and is adapted to be peeled apart by manually pulling apart the top and bottom sheets 17 and 18 of the film adjacent the edge 24. This opening can occur without compressing the package 10 so that hot vapors will not be forced from within the package 10 through the vents formed at the vapor coated film 20 as the package is opened.
Preferably, the pad material is selected to have about the same surface area as the food which is supported on the pad. Accordingly, the pad can so completely absorb or otherwise hold all of the grease contained in that food (after the food is cooked and removed from the enclosure) that the enclosure and pad therein will not drip grease even when the opening through which the food was removed is lowermost on the enclosure. Pads which can hold in the range of at least 1 to 2 grams of grease per sq/in. of surface area have been found suitable for packaging conventional bacon strips. FIG. 2 illustrates a pad selected to be about the same size and surface area as the food thereon. During cooking, some food products shrink in size (e.g., bacon strips). FIG. 5 illustrates a pad 14a of reduced surface area size relative to the food 12a placed thereon. The pad 14a may be smaller in surface area than the food 12a thereon, so long as it is sufficient in absorbency of grease to absorb all liquefied grease during cooking of the food 12a. To compensate for a smaller surface area, such a pad may need to be thicker.
Although in one preferred embodiment the pad 14 is designed for use within an enclosure 16 to form a package 10 such as illustrated in FIGS. 1-4, it is also contemplated that such a generally hydrophobic and oleophilic pad may be used alone in a microwave oven for food cooking purposes. As such, the food is placed upon the pad, with no preformed, vapor-tight enclosure thereabout. The pad and food may then be retained within a pan or container (e.g., represented by reference numeral 26 in FIG. 5) for holding the liquefied grease as the food is cooked and for handling the food once cooked. The container may or may not further include a lid or cover (e.g., as represented by reference numeral 28 in FIG. 5) to contain food splattering during the cooking process.

The Web Formation Process

As mentioned, the pad is formed from blown microfibers produced by a process which, in part, uses the apparatus discussed, for example, in Wente, Van A., "Superfine Thermoplastic Fibers, " Industrial Engineering Chemistry, Vol. 48, pp 1342-1346 and in Wente, Van A. et al., "Manufacture of Superfine Organic Fibers," Report No. 4364 of the Naval Research Laboratories, published May 25, 1954, and U.S. Pat. Nos. 3,849,241 (Butin et al.), 3,825,379 (Lohkamp et al.), 4,818,463 (Buehning), 4,986,743 (Buehning), 4,295,809 (Mikami et al.) or, 4,375,718 (Wadsworth et al.). These apparatuses and methods are useful in the invention process in the portion shown schematically as die 40 in Fig. 6, which could be of any of these conventional designs.
Each microfiber is formed from two or more separate polymer material components. The polymeric components are introduced into the die cavity 42 of die 40 from a separate splitter, splitter region or combining manifold 30, and into the, e.g., splitter from extruders, such as 32 and 34. Gear pumps and/or purgeblocks can also be used to finely control the polymer flow rate. In the splitter or combining manifold, the separate polymeric component flow streams are formed into a single layered flowstream. However, preferably, the separate flowstreams are kept out of direct contact for as long a period as possible prior to reaching the die 40. The separate polymeric flowstreams from the extruder(s) can also be split in the splitter (30). The split or separate flowstreams are combined only immediately prior to reaching the die, or die orifices. This minimizes the possibility of flow instabilities generating in the separate flowstreams after being combined in the single layered flowstream, which tends to result in non-uniform and discontinuous longitudinal layers in the multi-layered microfibers. Flow instabilities can also have adverse effects on nonwoven web properties such as strength, temperature stability, or other desirable properties obtainable with the invention process.
The separate flowstreams are preferably established into laminar flowstreams along closely parallel flowpaths. The flowstreams are then preferably combined so that at the point of combination, the individual flows are laminar, and the flowpaths are substantially parallel to each other and the flowpath of the resultant combined layered flowstream. This again minimizes turbulence and lateral flow instabilities of the separate flowstreams in and after the combining process. It has been found that a suitable splitter, for the above described step of combining separate flowstreams, is one such as is disclosed, for example, in U.S. Pat. No. 3,557,265, which describes a manifold that forms two or three polymeric components into a multi-layered rectilinear melt flow. The polymer flowstreams from separate extruders are fed into plenums and then to one of the three available series of ports or orifices. Each series of ports is in fluid communication with one of the plenums. Each stream is thus split into a plurality of separated flowstreams by one of the series of ports, each with a height-to-width ratio of from about 0.01 to 1. The separated flowstreams, from each of the three plenum chambers, are then simultaneously coextruded by the three series of ports into a single channel in an interlacing manner to provide a multi-layered flowstream. The combined, multi-layered flowstream in the channel is then transformed (e.g., in a coathanger transition piece), so that each layer extruded from the manifold orifices has a substantially smaller height-to-width ratio to provide a layered combined flowstream at the die orifices with an overall height of about 50 mils or less, preferably 15-30 mils or less. Other suitable devices for providing a multi-layer flowstream are such as disclosed in U.S. Patents Nos. 3,924,990 (Schrenk); 3,687,589 (Schrenk) 3,759,647 (Schrenk et al.) or 4,197,069 (Cloeren), all of which, except Cloeren, disclose manifolds for bringing together diverse polymeric flowstreams into a single, multi-layer flowstream that is ordinarily sent through a coat hanger transition piece or neck-down zone prior to the film die outlet. The Cloeren arrangement has separate flow channels in the die cavity. Each flow channel is provided with a back-pressure cavity and a flow-restriction cavity, in successive order, each preferably defined by an adjustable vane. The adjustable vane arrangement permits minute adjustments of the relative layer thicknesses in the combined multi-layered flowstream. The multi-layer polymer flowstream from this arrangement need not necessarily be transformed to the appropriate length/width ratio, as this can be done by the vanes, and the combined flowstream can be fed directly into the die cavity 42.
The multi-layer polymer flowstream is normally fed into the die cavity 42 as an integral flow. However, it is possible to keep the layer flowstreams separate in the die cavity 42 by use of separator plates that would allow the separate polymer flowstreams to combine immediately prior to reaching the die orifices.
From the die cavity 42, the multi-layer polymer flowstream is extruded through an array of side-by-side orifices 41. As discussed above, prior to this extrusion, the feed can be formed into the appropriate profile in the cavity 42, suitably by use of a conventional coathanger transition piece. Air slots 48, or the like, are disposed on either side of the row of orifices 41 for directing uniform heated air at high velocity at the extruded layered melt streams. The air temperature is generally about that of the melt stream, although preferably 20-30°C higher than the melt temperature. This hot, high-velocity air draws out and attenuates the extruded polymeric material, which will generally solidify after traveling a relatively short distance from the die 40. The solidified or partially solidified fibers are then formed into a web by known methods and collected on a collector surface 49, such as rotating drum 50. The collecting surface can be a solid or perforated surface in the form of a drum (as shown), or a flat surface, a moving belt, or the like. If a perforated surface is used, the backside of the collecting surface can be exposed to a vacuum or low-pressure region to assist in the deposition of fibers, such as is disclosed in U.S. Pat. No. 4,103,058 (Humlicek). This low-pressure region allows one to form webs with pillowed low-density regions. The collector distance can generally be from 3 to 50 inches from the die face. With closer placement of the collector, the fibers are collected when they have more velocity and are more likely to have residue tackiness from incomplete cooling. This is particularly true for inherently more tacky thermoplastic materials, such as thermoplastic elastomeric materials. Moving the collector closer to the die face, e.g., 3 to 12 inches, will result in stronger inter-fiber bonding and a less lofty web. Moving the collector back (e.g., 20 inches) will generally tend to yield a loftier and less coherent web.
The temperature of the polymers in the splitter region is generally about the temperature of the higher melting point component as it exits its extruder. The splitter region or manifold is typically integral with the die and is kept at the same temperature. The temperature of the separate polymer flowstreams can also be controlled to bring the polymers closer to a more suitable relative viscosity. When the separate polymer flowstreams converge, they should generally have an apparent viscosity of from 150 to 800 poise, preferably from 200 to 400 poise (as measured by a capillary rheometer). The relative viscosities of the separate polymeric flowstreams to be converged should generally be fairly well matched. Empirically, this can be determined by varying the temperature of the melt and observing the crossweb properties of the collected web. The more uniform the crossweb properties, the better the viscosity match. The overall viscosity of the layered combined polymeric flowstream(s) at the die face should be from 150 to 800 poise. The differences in relative viscosities are preferably generally the same as when the polymeric flowstreams are first combined. The apparent viscosities of the polymeric flowstream(s) can be adjusted at this point by varying the temperatures as per U.S. Pat. No. 3,849,241.
The size of the polymeric fibers formed depends to a large extent on the velocity and temperature of the attenuating airstream, the orifice diameter, the temperature of the melt stream, and the overall flow rate per orifice. At high air volume rates, the fibers formed have an average fiber diameter of less than about 10 micrometers, however, there is an increased difficulty in obtaining webs having uniform properties as the air flow rate increases. At more moderate air flow rates, the polymers have larger average diameters, however, with an increasing tendency for the fibers to entwine into formations called "ropes". This is dependent on the polymer flow rates, of course, with polymer flow rates in the range of 0.05 to 0.5 cjm/min/orifice generally being suitable. Coarser fibers, e.g., up to 25 micrometers or more, can be used in certain circumstances such as large pore or coarse webs.
The multi-layer microfibers formed by this process can be admixed with other fibers or particulates prior to being collected. For example, sorbent particulate matter or fibers can be incorporated into the coherent web of blown multi-layered fibers as discussed in U.S. Pat. Nos. 3,971,373 or 4,429,001. In these patents, two separate streams of melt-blown fibers are established with the streams intersecting prior to collection of the fibers. The particulates, or fibers,, are entrained into an airstream, and this particulate laden airstream is then directed at the intersection point of the two microfiber streams other methods of incorporating particulates or fibers, such as staple fibers, bulking fibers or binding fibers, can be used with the invention method of forming melt-blown microfiber webs, such as is disclosed, for example, in U.S. Pat. Nos. 4,118,531, 4,429,001 or 4,755,178, where particles or fibers are delivered into a single stream of melt-blown fibers.
Other materials such as surfactants or binders can be incorporated into the web before, during or after its collection, such as by use of a spray jet. if applied before collection, the material is sprayed on the stream of microfibers, with or without added fibers or particles, traveling to the collection surface.
The inventive web formation process and microfibers formed thereby also forms the basis for the following patent applications, all filed on the same date as this application:

(1) Novel Material and Material Properties From Multi-Layer Blown Microfibers;
(2) Stretchable Nonwoven Webs Based on Multi-Layer Blown Microfiber;
(3) Improved Modulus Nonwoven Webs Based on Multi-Layer Blown Microfibers;
(4) High Temperature Stable Nonwoven Webs Based on Multi-Layer Blown Microfibers;
(5) Film Materials Based on Multi-Layer Blown Microfibers; and
(6) Wipe Materials Based on Multi-Layer Blown Microfibers; and all of which are incorporated by reference.

The microfiber formation process described provides webs having unique properties and characteristics when compared to webs formed from a homogeneous polymer melt, of a single polymer or blends of polymers (compatible or incompatible). For example, FIG. 7 illustrates an electron micrograph preparation of a 50/50 blend of polypropylene (PP) and polymethylpentene (TPX). These polymers were "dry blended" together in pellet form, prior to extrusion. In other words, the polymers were blended and then forced through a single extrusion orifice in a conventional dry-blend extrusion process. The extruded fiber was formed into a web as described above on a collector.
To form the illustration of FIG. 7, samples of the blown microfiber were first stained with a solution having 0.2 grams of RuCl₃:H₃O powder dissolved in 10 ml. of 5.25 aqueous sodium hypochlorite. Each sample was soaked in this solution for two to two and one-half hours at room temperature (about 20°C). Each sample was then removed, rinsed with deionized water and air dried on filter paper for 24 hours. Each sample was then embedded into "Scotch-Cast" brand electrical resin No. 5 available from the Minnesota Mining and Manufacturing Company, using embedding molds for microtomy, and the resin was cured for 24 hours at room temperature. Thin sections, approximately 0. 1 micrometer thick were cut from the sample with a diamond knife on a Reichert Ultracut E/F D-4 cryoultramicrotome at a temperature between -45°C to - 50°C. Each section was then picked up on a carbon-saturated grid and brought to room temperature before examining with a JEOL 100 CX transmission electron microscope operated at 100 kV. The dark portions in the electron photomicrograph of FIG. 7 is the stained polymethylpentene, which can be seen to be randomly distributed in the fiber. This results in a fiber having nonuniform strength, blending and bonding characteristics, as can be readily appreciated by viewing FIG. 7.
As long as the viscosities of the particular polymers are suitably matched, it is possible to form generally uniform multi-layered microfibers from two (or more) polymers which otherwise may be incompatible (e.g., polypropylene and polyethylene terephthalate). It is thus possible to obtain microfiber nonwoven webs having certain desired characteristics which would otherwise not be obtainable from these otherwise incompatible polymers used individually. For example, a blown microfiber pad of 100 percent polyethylene terephthalate (PET) heated to 350°F will shrink excessively, and a blown microfiber pad of 100 percent polypropylene (PP) heated to 350°F will show visible melting. However, a blown microfiber pad formed by the microfiber formation process described herein which is a 50/50 coextrusion of polyethylene terephthalate and polypropylene heated to 350°F shows no visible signs of melting and no perceptible shrinking. The addition of any amount of a high temperature stable polymer (e.g., PET) improves the desired properties of the bonding polymer (e.g., PP) in such a blown microfiber pad of coextruded microfibers. A combined flowstream of such polymers, with one of the polymers constituting between 20 to 80 percent by weight of the flowstream, results in a usable web product, although a preferred composition would have one of the polymers constituting between 40 to 60 percent of the flowstream.
Surprisingly, the overall web properties of these novel webs formed from multi-layered microfiber webs are generally unlike the web properties of homogeneous webs formed of any of the component materials. In fact, the multi-layered microfibers frequently provide completely novel web properties and/or ranges of properties not obtainable with any of the component polymer materials. For example, fiber and web modulus can be controlled within wide ranges for given combinations of polymers by varying, independently, the relative ratios of the polymers, the layer order in the microfibers, the number of layers, the collector distance and other process variables. The web formation process thus allows precise control of such properties as web modulus and absorbency by varying one or all of these variables.
In forming a web which is grease absorbent and suitable for microwave cooking of food, it is necessary that the web microfibers be temperature stable at the temperature levels required for cooking foods by microwave radiation. The microfibers in the inventive heat stable melt-blown web are formed from a combination of at least two distinct layer types. The first layer type comprises a heat-stable melt-blowable material which is used in combination with a second layer type of a relatively non heat-stable but comparably good web-forming layer material.
The relatively heat-stable material can be any heat-stable (a high melting point polymer) polymeric material capable of being melt-blown. These materials are generally highly crystalline and have a high melting point. However, a problem with these materials is that they exhibit a relatively low degree of self-bonding. Self-bonding refers to the ability of the individual fibers to bond to each other when collected on a collecting surface from the melt-blowing die. These heat-stable materials as such form low-strength webs generally lacking the integrity required for most typical applications of melt-blown web products unless post-embossed.
Typical examples of such heat-stable materials include polyesters such as polyethylene terephthalate, polyolefins such as poly 4-methyl-1-pentene or a polyallylene sulfide such as poly(phenylene sulfide). Such materials exhibit relatively high individual fiber strength, yet exceedingly low interfiber bonding, and as such form generally low-strength webs even at relatively close collector to die distances. Generally, these materials are characterized as melt-blowable polymers having glass transition points above room temperature or melt temperature of greater than 180°C, and preferably greater than 150°C. Preferably, the heat-stable polymers can produce webs that are stable at temperatures above about 130°C, more preferably above 150°C.
The second layer material used in the inventive microfibers and web is generally a material exhibiting significantly higher self-bonding characteristics at melt blowing conditions. Typically these materials will exhibit a softening or melting temperature approximately 30°C below that of the high modulus material, but preferably within 150°C of the high modulus material melting point. Too large a difference in melting points can render the polymers difficult to coextrude. Generally, the self-bonding component will have a glass transition temperature below room temperature, preferably below about 15°C. The preferred materials will be amorphous or semicrystalline materials exhibiting relatively good bonding characteristics at melt-blowing conditions. Suitable materials include polyolefins such as polypropylene. The materials comprising the relatively high-bonding layer material can also include conventional additives.
By using relatively low levels (e.g., <50%) of the relatively heat-stable material in combination with the second layer material, as defined herein, the mechanical performance characteristics of the relatively heat-stable material can be obtained. The web will also exhibit the desirable characteristics of the second layer material at lower temperatures.
The heat-stable webs formed of the above described multi-layer microfibers have relatively high strength properties over an extended temperature range.
Fiber and web modulus is further controllable within wide ranges for given combinations of polymers by varying, independently, the relative ratios of the polymers, the layer order in the microfibers, the number of layers, the collector distance and other process variables. The invention thus allows precise control of web strength by varying one or all of these variables.
At least a portion of the high-bonding component is preferably at the fiber surface. The heat-stable layer stabilizes the bonding component layer while the bonding component material at the surface provides interfiber bonding. Theoretically, the relative volume percent of the individual layers can vary widely, for example, from 1 to 99 volume percent for each individual layer component. The preferred amount of the individual layer components will depend upon the relative amount of modulus desired with the individual high-temperature web and the desired high temperature performance required. Generally, the outside layers will contribute significantly to the surface properties forming at the web without significantly modifying the bulk fiber properties, such as tensile strength and modulus behavior when used at a relatively low-volume percent. In this manner, the relatively high-bonding materials can be used as thin outer layers to contribute to web properties without significantly affecting the bulk fiber properties.
With the web formation process described herein, the web properties are further altered by variations in the number of layers employed at a given relative volume percent and layer arrangement. As described above, variation in the number of layers, at least at a low number of layers, has a tendency to significantly vary the relative . proportion of each polymer (assuming two polymeric materials) at the microfiber surface. This (assuming alternating layers of two polymeric materials) translates into variation of those web properties to which the microfiber surface properties significantly contribute. Thus, web properties can change depending on what polymer or composition comprises the outside layer(s) . However, as the number of layers increases, this variation in web properties based on surface area effects diminishes. At higher-layer numbers, the relative thicknesses of the individual fiber layers will tend to decrease, significantly decreasing the surface area effect of any individual layer. Preferably, the melt-blown microfibers have average diameters of less than 20 micrometers.
The number of layers obtainable with the process described is theoretically unlimited. Practically, the manufacture of a manifold, or the like, capable of splitting and/or combining multiple polymer streams into a very highly layered arrangement would be prohibitively complicated and expensive. Additionally, in order to obtain a flowstream of suitable dimensions for feeding to the die orifices, forming and then maintaining layering through a suitable transition piece can become difficult. A practical limit of 1,000 layers is contemplated, at which point the processing problems would likely outweigh any potential added property benefits.
The webs formed can be of any suitable thickness for the desired end use. However, generally a thickness from 0.01 to 5 centimeters is suitable for most applications. Further, for some applications, the web can be a layer in a composite multi-layer structure. For example, another lay can be a nonwoven web. The other layers can be attached to a layer of the inventive melt-blown web by conventional techniques such as heat bonding. Suitable materials for such layers include other high temperature stable materials such as polyesters or polycarbonates. Webs, or composite structures including webs formed by this process can be further processed after collection or assembly such as by calendering or point embossing to increase web strength, provide a patterned surface, and fuse fibers at contact points in a web structure or the like; orientation to provide increased web strength; needle punching; heat or molding operations; coating, such as with adhesives to provide a tape structure; or the like.

Desired Web Characteristics for Microwave Cooking

The web of the present invention, which forms the pad for use in microwave cooking as described herein, must be grease absorbent (oleophilic) and generally hydrophobic. Since the pad is placed in direct contact with food, it must be of neutral food grade material which will not leach any components into the food. In addition, it must not dehydrate the food by wicking moisture therefrom during storage together in a sealed enclosure. The pad must further be formed of materials which can withstand the high temperatures necessary to properly cook the food by microwave radiation exposure. It is also desirable, of course, to make the pad as grease absorbent as possible to thereby reduce the size, thickness and weight of the pad, which in turn lowers the pad material requirements and pad expense. In some applications, it is desirable to bond the pad to another material, and thus forming the pad of a material with good bonding characteristics is necessary.
The blown microfibers (BMF) which comprise the pad can be formed from multi-layer blends of polymer materials. In a preferred embodiment, the temperature stable polymer layer(s) in each microfiber consists of polyethylene terephthalate (PET), which has a relatively high melting temperature (above 4601,F) . The other layer(s) of the microfiber are preferably formed of polypropylene (PP). Polypropylene (PP) has excellent grease absorbent and hydrophobic characteristics for use in the multi-layer microfiber blend. Polypropylene alone a blown microfiber web exhibits some melting for microwave applications in which fatty foods such as bacon are cooked. A multi-layer microfiber web of polypropylene (PP) and polyethylene terephthalate (PET) formed from the web formation process described herein has demonstrated improved absorbency over previously attainable web structures (e.g., dry blended microfiber webs of 50/50 polypropylene and poly 4-methylpentene-1) and exhibits no significant melting at those temperatures necessary for microwave food cooking. This is believed to be the result of the "increased strength" of the coextruded web structure of the present invention over the extruded type prior web structure.
Polypropylene and polyethylene terephthalate are such dissimilar polymers that it is difficult to form a usable melt-blown microfiber by dry blending the polymer pellets and extruding. Upon extrusion of a dry blend of these polymers the polyethylene terephthalate forms excessive "shot" or "sand" in the polypropylene meltstream.
For the high temperature or temperature stable component, the material must have a high crystalline melting point above 200°C, such as polybutylene terephthalate (PBT), or polycyclohexane terephthalate (PCT) or a material with a high glass transition temperature above 60°C (such as polycarbonate (PC). In addition to the polyesters (PET, PBT, and PCT) mentioned above, other families of high temperature materials are also applicable. Polyamides (nylon 6 or Nylon 66), polyolefins (such as poly 4-methylpentene-1 (PMP)) or polyphenylene sulfide (PPS) may suffice. Of course, this list is not inclusive--a person skilled in the art will recognize that other materials or properties will be fully applicable to this application.
Surface treatment of the web is necessary to eliminate food sticking after cooking. One means to accomplish such surface treatment is to hot-roll calender the web using a predetermined gap for a particular web thickness. Another means would be to flame treat the surface of the web. A further means of achieving a nonstick web cook surface is to attach a scrim on the food-side surface of the web. The scrim may also be calendered as it is attached to the absorbent coextruded web. The use of PET in the blended microfibers also provides a basis for heat bonding a PET scrim to the web, if desired.
The multi-layer blown microfibers can be made with two layers, three layers or other numbers of layers within each fiber. The choice of the number of layers is determined by the end use. When the microfiber web is exposed to heat subsequent to the formation of the web, such as in embossing or calendering, the bonding component can "film over" the high temperature component. In other words, the bonding component (PP) can melt and coat the high temperature component (PET) with a thin film of polypropylene. A two-layer microfiber construction (one layer PP and one layer PET) will result in a web where there is less chance for filming over of the PET during calendering and more PET is exposed for lamination of a PET scrim to the web. A three-layer microfiber construction (one layer PET and two layers PP) results in less exposed PET and thus more filming over of the polypropylene during calendering. This results in a less fuzzy surface for the calendered side of the web, which may be preferred for some applications.
The following examples are provided to illustrate presently contemplated preferred embodiments and the best mode for practicing the invention, but are not intended to be limiting thereof.

TEST PROCEDURES

Tensile Strength

Tensile strength data on a blown microfiber web having multi-layer BMF microfibers was obtained using an Instron Tensile Tester (Model 1122) with a 10.48 cm (2 in.) jaw gap and a crosshead speed of 25.4 cm/min. (10 in./min.). Web samples were 2.54 cm (in.) in width and samples are taken both in the machine direction (MD) and in the transverse direction (TD) of the web. Each sample was stretched until failure, with the break force measured at failure.

Mineral Oil Absorption

This test indicates the oil absorbency of a grease absorbing pad. The pad, measuring 397 square centimeters (61.5 square inches), is weighed, soaked in mineral oil at room temperature (about 20°C) for 30 seconds, agitated in the oil, and left for another 30 seconds. The pad is then hung for 2 minutes to allow excess oil to drip out of the pad and weighed to determine the amount, in grams, of mineral oil absorbed and held by the pad.

Maximum Grease Absorption Under Cooking Conditions

This test is used to determine the maximum amount of grease that is absorbed by a pad by cooking 12 slices of bacon. A preweighed pad measuring 15.2 cm by 26 cm (6 inches by 10.25 inches) is placed in a microwave cooking package as described by FIGS. 1-3.
The bacon is placed on top of the pad and the bacon in the package is cooked in a 700 watt microwave oven for 7.5 minutes. The pad is then removed from the package and hung until dripping of grease substantially stops (about 30 to 45 seconds). The pad is then weighed (as compared to the pad's precooking weight) and the amount of grease absorbed is determined.

Example 1

A polypropylene/polyethylene terephthalate BMF web of the present invention was prepared using a meltblowing process similar to that described, for example, in Wente, Van A., "Superfine Thermoplastic Fibers," in Industrial Engineering Chemistry, Vol. 48, pages 1342 et seq (1956), or in Report No. 4364 of the Naval Research Laboratories, published May 25, 1954, entitled "Manufacture of Superfine Organic Fibers" by Wente, Van A.; Boone, C.D.; and Fluharty, E.L., except that the BMF apparatus utilized two extruders, each of which was equipped with a gear pump to control the polymer melt flow, each pump feeding a three-layer feedblock assembly similar to that described in U.S. Pat. Nos. 3,480,502 (Chisholm et al.), 3,487,505 (Schrenk) or 4,197,069 (Cheren), which was connected to a melt-blowing die having circular smooth surfaced orifices (10/cm) with a 5:1 length to diameter ratio.
The first extruder delivered a melt stream of polypropylene (PP) resin (PP 386OX, available from Fina Oil & Chem. Co.), to a gear pump which feeds a three-layer feedblock assembly which was heated to about 300°C. The second extruder, which was maintained at about 300°C, delivered a melt stream of a polyethylene terephthalate resin (PET, having an intrinsic viscosity of 0.60, and a melting point of about 257°C prepared as described in U.S. Pat. No. 4,939,008, col. 2, line 6, to col. 3, line 20) for a second gear pump which also feeds the feedback assembly. The polymer melt streams were merged in three layers on exiting the feedblock. The gear pumps were adjusted so that a 50/50 weight percent PP/PET polymer melt was delivered to the feedblock assembly and a 0.11 kg/hr/cm die width (0.6 lb/hr/in.) polymer throughput rate was maintained at the BMF die. The primary air temperature was maintained at approximately 305°C and at a pressure suitable to produce uniform web width with a 0.76 mm gap width. Webs were collected at a collector to BMF die distance of 50.8 cm (20 in.). The resulting BMF web, comprising three-layer microfibers having an average fiber diameter less than 10 micrometers, had a thickness of about 0.150 in., and a basis weight of about 200 cJm/m2.
An electron micrograph preparation illustrating the laminar and uniform blending of the PP and PET in the blown microfibers made by the process described above is shown in FIG. 8 herein. The electron micrograph of FIG. 8 was prepared in the same manner as described above with respect to FIG. 7. The dark portion of each microfiber section shown in FIG. 8 is PET polyester, while the lighter portion represents the polypropylene.
The surface of the blown microfiber web is smoothed by calendering the web with one surface of a calendering roll heated to a temperature of about 260°F (127°C) while the other roll is kept at ambient or room temperature. These rolls are illustrated in FIG. 6 as calender roll 52 and heated calender roll 54. A gap of 0.050 inch (1.27 mm) between two 10 inch diameter (25.4 cm) calender rolls 52 and 54 produces an acceptable surface on a web that is about 0.150 inch (3.8 mm) thick. The web is advanced through the calender rolls 52 and 54 at a speed of 8 feet per minute (2.4 meters per minute).
The web was then tested for mineral oil absorption, and cooking twelve slices of bacon. No visible melting was observed in the pad after the cooking tests. The test results for oil absorption and tensile strength are summarized in Table 1.

Example 2

A BMF web was formed according to the procedure of Example 1. During the calendering process, a nonwoven polyester scrim (Reemay 2250 available from Reemay Corporation) was laminated to the BMF web. The nonwoven scrim was positioned between the heated calender roll and the BMF web and the heated calender roll was heated to a temperature of 160°C (320°F) to effect the lamination. No visible melting was observed in the pad after the cooking tests. The test results for oil absorption and tensile strength are shown in Table 1.

Example 3

A BMF web having a basis weight of 200 grams per square meter was prepared according to the procedure of Example 1 except that a two-layer feedblock was used. The resulting two-layer microfibers had an average fiber diameter of less than 10 micrometers and the web thickness was about 3.8 mm. (0.150 inch).
An electron micrograph preparation illustrating the laminar and uniform blending of the PP and PET in the blown microfibers made by the process described in this Example 3 (using a two-layer feedblock) is shown in FIG. 9 herein. The electron micrograph of FIG. 9 was prepared in the same manner as described above with respect to FIG. 7. The dark portions of each microfiber section shown in FIG. 9 is PET polyester, while the lighter portion represents polypropylene.
No visible melting was observed on the web after the cooking tests. The calendered web was tested for oil absorbency and tensile strength, and test results are shown in Table 1.

Example 4

A BMF web was prepared according to the procedure of Example 3 except that the gear pumps were adjusted so that a 75/25 weight percent PP/PET polymer melt was delivered to the feedblock assembly. The resulting BMF web, comprising two-layer microfibers having an average diameter of less than 10 micrometers, had a thickness of 3.8 mm. (0. 150 inch) and a basis weight of about 200 grams per square meter. The web was tested for oil absorbency and tensile strength and the test results are shown in Table 1. No visible melting was observed on the pad after the cooking tests.

Comparative Example C1

A BMF web was formed according to the procedure described in U.S. Patent No. 4,873,101 (incorporated herein by reference). The composition of the microfiber was 50/50 weight percent polypropylene and poly 4-methylpentene-1 (MX-007 TPX brand resin from Mitsui Petrochemicals America, LTD). The web had a basis weight of 200 grams per square meter and was calendered according to the procedure described in Example 1. Test results for oil absorbency and tensile strength are shown in Table 1.

Table 1

Example	Mineral Oil Absorbency gms/397 sq.cm.	Maximum Grease Absorbency gms/397 sq.cm.	Tensile Strength Newtons/Decimeter
			MD	TD
1	110	79	44	139
2	66	77	84	105
3	123	69	26	114
4	108	80	44	124
C1	54	65	16	21

Table 1 shows the superior oil absorbency and tensile strength of the inventive BMF webs made with multi-layer microfibers.

Water Absorbency

The microwave cooking pad should be hydrophobic in nature so that it does not absorb water from the food and dehydrate the food during transportation and storage, and so that the capacity of the pad to absorb grease during cooking is not inhibited by the water produced during cooking of the food. The moisture or water absorbency of the pads were tested by packing four slices of bacon (each weighing about 22.7 grams (0.8 ounces)) in a sealed package with (1) the tared BMF pad of Example 1 measuring 15.2 cm. by 26.0 cm. (6 inches by 10.25 inches) and weighing about 9.6 grams, or (2) a tared paper towel (Wyp All) weighing about 7.0 grams. The sealed packages were constructed by heat sealing together two sheets of 0.005 cm. (0.002 inch) thick "Scotchpak" brand film (sold by Minnesota Mining and Manufacturing Company) measuring 30.5 cm. (12 inches) by 25.4 cm. (10 inches). The sealed packages were held at 4°C (40°F) for four days. After four days the pads were removed and weighed to determine the amount of moisture pick absorption. The BMF pad weighed 11.4 grams and had absorbed about eighteen percent by weight water and grease while the Wyp All pad weighed 11.9 grams and had absorbed about 69% by weight water and grease. The BMF pad was dried at 121°C (250°F) for about one hour and the actual moisture absorption was determined to be about nine percent.
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.

Claims

A pad (14) for use in the cooking of food (12) placed thereon in a microwave oven wherein the food (12) contains a substantial amount of solidified grease which melts when the food is cooked by microwave radiation, the pad (14) comprising:
an entangled web formed from at least one generally hydrophobic and grease absorbing multi-layer microfibers, said web being prepared by combining at least two streams of flowable materials in a layered, combined flowstream, extruding the combined flowstream through a die having at least one orifice, attenuating the extruded flowstream with a high velocity gaseous stream to form a fiber and collecting the fiber on a collective surface so as to form the entangled web, with the pad being capable of holding the amount of grease in the food when the grease is melted by cooking the food in a microwave oven.
The pad (14) of claim 1 wherein the separate streams of flowable materials comprise a stream of polypropylene and a stream of polyester.
The pad (14) of claim 1 wherein the polyester is polyethylene terephthalate.
The pad (14) of claim 1 wherein each microfiber comprises, in transverse cross section, a plurality of alternating laminate layers of the flowable materials.
The pad of claim 1 wherein the pad (14) includes first and second major opposing surfaces with said first major surface having been calendered to form a relatively smooth surface thereon, and with said first major surface adjacent said food (12).
The pad of claim 1 wherein the pad (14) includes first and second major opposing surfaces, and further comprising:
a web formed from a high temperature stable material, the web having first and second major opposing surfaces, the first major surface of said web being bonded to the first major side of said pad, with the second major side of said web adjacent said food.
The pad of claim 1, the pad (14) further including a vapor-tight microwave radiation transparent enclosure (16) that surrounds both the pad (14) and food (12) to be cooked by microwave energy to form a package (10) for use in a microwave oven.