A disposable plate or tray includes a substantially planar bottom portion, a sidewall and a flange. A profile extending from the center to the outer edge of the article is characterized by a plurality of direction changes having a radius of curvature to diameter ratio of at least about 0.02.
In accordance with the present invention, there are provided disposable service articles without a curled or severely downturned flange, which features are undesirable in terms of aesthetic qualities and brittleness.
SUMMARY OF THE INVENTION
The invention is described below with reference to the attached figures which show preferred shapes and dimensions.
The plates of the present invention include in a preferred embodiment a plastic plate with a four-radius profile which balances the need for increased rigidity (strength) and rim stiffness (sturdiness) per given material weight/cost. The ergonomic rim profile provides for ease of holding and carrying, consumer friendly shape denoting Permanentware qualities, without the negative side effect of brittleness encountered when using high modulus/stiffness construction materials. The four-radius disposable plastic plate design has a curvilinear rim surface onto which patterning can be applied for visual, tactile and strength purposes. The plate design is strong but not brittle during use even with the high modulus/stiffness mica filled polypropylene (PP) plastic material.
Plates produced with other shapes were rigid but often failed by brittle cracking in the flange and downturn areas. The stresses generated in the flange and downturn areas by deflection of the product during use apparently exceeded the highly filled material strength resulting in failure. It is possible that imperfections on the product's trimmed edge may contribute to brittle cracking by providing failure initiation points for the notch sensitive, highly filled materials preferably used in accordance with the invention and set forth in the attached Appendix.
It was discovered that plastic plates described in this invention disclosure still had exceptional strength per material weight, but also significantly reduced brittle cracking with the highly filled nonhomogeneous materials. The four-radius design, for example, would not build up the high stress levels during deflection even with trimmed edge may imperfections and was less prone to brittle cracking.
Plates having a circular configuration as illustrated employ the four-radius plastic plate design. The plastic articles of manufacture may also be square or rectangular in shape having angular comers, such as found in a tray. Further, additional plastic shapes such as triangular, multi-sided, polyhexal, etc. are contemplated including compartmented trays and oval platters.
It will be appreciated that a salient feature of the inventive articles is the smooth profile as described herein. In general, the transitions between the center, sidewall and flange of the plate are kept free of sharp bends or curves so that mechanical stresses are not concentrated beyond the ability of the material to withstand them. In addition to being operative to avoid undesirable stress regions, the profile is flowing in appearance and provides a pleasing, ergonomic hand feel.
In general, the invention is directed to disposable food contact articles formed of a polyolefin, mineral-filled sheet and have a characteristic diameter as well as a substantially planar central portion, a sidewall portion and a flange portion. For a circular article such as a plate, the characteristic diameter is simply the diameter of the plate as the term is ordinarily employed, i.e., the distance through the center between opposing outer edges of the flange. For non-circular articles, the characteristic diameter is the average distance through the center between opposing outer edges of the flange of the article. Thus, for a rectangular article the characteristic diameter is the average of the shorter side and the longer side, for an oval article the characteristic diameter is the average of the minor axis length and major axis length of the oval and so forth.
The inventive articles are characterized by a smooth profile wherein direction changes are accomplished by way of a plurality of arcuate portions, each of which has a radius of curvature. A particularly preferred embodiment is a four radius plate as described herein, characterized in that the ratio of the length of each radius of curvature to the diameter is at least about 0.02. A ratio of at least about 0.03 is preferred with a ratio of at least about 0.035 being still more preferred. Various details will become more understood by reference to the drawings and detailed description which follows.
BRIEF DESCRIPTION OF DRAWINGS
The invention is described in detail below with reference to the various drawings. In the drawings:
FIG. 1 is a view in perspective of a plate constructed in accordance with the present invention;
FIG. 2 is a view in cross-section and elevation of the plate of FIG. 1 illustrating the profile of the plate; and
FIG. 3 is a schematic diagram illustrating the profile of the plate of FIGS. 1 and 2.
DETAILED DESCRIPTION
The invention is described in detail below with reference to the figures. Such description is for purposes of illustration only and is not limitative of the invention in any way.
Illustrated in FIGS. 1 through 3, there is a plate 180 which includes a planar center 182 which, in turn, includes an outer peripheral surface 184. This center region 182 may have a slight convex crown to improve plate stability during use. The planar center 182 forms a bottom for the plate 180. An outwardly projecting sidewall 186 includes a first rim portion 188 which is joined to the outer peripheral surface 184 of the planar center 182. A second rim portion 190 is joined to the first rim portion 188. The first rim portion 188 and the second rim portion 190 form the outwardly projecting sidewall 186 which forms the sidewall of the plate 180. A rim 192 includes a third rim portion 194 which is joined to the second rim portion 190 of the outwardly projecting sidewall 186. A fourth rim portion 196 is joined to the third rim portion 194. The fourth rim portion 196 forms the outer edge of the plate 180.
FIG. 3 illustrates a partial cross-sectional view of a plate, diameter D, according to the present invention. The plate 180 defines a center line 204. A base or bottom-forming portion 200 extends from the center line 204 to an outer peripheral surface 202.
From the center line 204 a predetermined distance X12 extends toward the outer peripheral surface forming portion 202. A distance Y12 extends a predetermined distance from the base or bottom-forming portion 200 upwardly therefrom. A radius R12 extends from the intersection point of the distance X12 and Y12 to form a first rim portion 206 of the outwardly projecting sidewall 205. The first rim portion 206 is defined by an arc A12 which extends from a substantially vertical line defined at the outer peripheral surface 202 to a fixed point 210. The arc A12 may be approximately 60.degree..
A distance X22 extends from the center line 204 to a predetermined point. A distance Y22 extends from the base or bottom-forming portion 200 of the plate 180 downwardly a predetermined distance. A radius R22 extends from the intersection of the lines X22 and Y22 to form a second rim portion 208 of the sidewall 205. The radius R22 extends from the first fixed point 210 to the second fixed point 212 through an arc A22. The arc A22 may be approximately 4.degree..
A distance X32 extends from the center line 204 to a predetermined distance. A distance Y32 extends from the base or bottom-forming section 200 of the plate 180 to project upwardly a predetermined distance. A radius R32 extends from the intersection of the lines X32 and Y32 to form the third rim portion 214 of the rim 216. The radius R32 extends from the second fixed point 212 to a third fixed point 218. An arc A32 is formed between the second fixed point 212 and the third fixed point 218 to extend a predetermined distance. The arc A32 may be approximately 55.degree..
A distance X42 extends a predetermined distance from the center line 204. Similarly, a distance Y42 extends from the base or bottom-forming section 200 of the plate 180 to project upwardly. A radius R42 extends from the intersection of the lines X42 and Y42 to form a fourth rim portion 217 of the rim 216. An arc A42 is formed between the third fixed point 218 and a fourth fixed point 220 at diameter D from the center line. The arc A42 may be approximately 60.degree.. A section 220 forms the outer edge of the plate.
The article made according to the present invention may have any particular size as desired by the user so long as the relative profile dimensions are maintained. More specifically, square or rectangular with rounded comers, triangular, multi-sided, polyhexyl and similar shapes may be made having the profile described above, including compartmented trays and plates. In various embodiments of the present invention the container may be a 9-inch or 11-inch plate with profile coordinates as illustrated in FIGS. 1 through 3 having the dimensions, angles, or relative dimensions enumerated in Tables 1 through 3.
TABLE 1
Dimensions and Angles For 9" Plate
DIMENSION and ANGLES VALUE (inches or degrees)
R12 0.537
X12 3.156
Y12 0.537
R22 2.057
X22 5.402
Y22 0.760
R32 0.564
X32 4.167
Y32 0.079
R42 0.385
X42 4.167
Y42 0.258
A12 60.00.degree.
A22 4.19.degree.
A32 55.81.degree.
A42 60.00.degree.
D 9.00
BOTTOM CONVEX CROWN 0.06
TABLE 2
Dimensions and Angles For 11' PLATE
DIMENSION/ANGLES VALUE (inches or degrees)
R12 0.656
X12 3.857
Y12 0.656
R22 2.514
X22 6.602
Y22 0.929
R32 0.689
X32 5.093
Y32 0.097
R42 0.470
X42 5.093
Y42 0.315
A12 60.00.degree.
A22 4.19.degree.
A32 55.81.degree.
A42 60.00.degree.
D 11.00
BOTTOM CONVEX CROWN 0.06
TABLE 3
Dimensions For 9 and 11 INCH PLATE
DIMENSION
RATIO OR VALUES (Dimensionless or degrees)
ANGLE PREFERRED MINIMUM MAXIMUM
R12/D 0.060 0.045 0.075
X12/D 0.351 0.280 0.420
Y12/D 0.060 0.045 0.075
R22/D 0.228 0.180 0.275
X22/D 0.600 0.480 0.720
Y22/D 0.084 0.065 0.100
R32/D 0.063 0.050 0.075
X32/D 0.463 0.370 0.555
Y32/D 0.009 0.007 0.011
R42/D 0.043 0.034 0.052
X42/D 0.463 0.370 0.555
Y42/D 0.029 0.023 0.035
A12 60.00.degree. 55.00.degree. 75.00.degree.
A22 4.19.degree. 1.00.degree. 10.00.degree.
A32 55.81.degree. 45.00.degree. 75.00.degree.
A42 60.00.degree. 45.00.degree. 75.00.degree.
Salient features of the plate illustrated in FIGS. 1 through 3 generally include a substantially planar center portion (which may be crowned as noted above and illustrated throughout the various figures) with four adjacent rim portions extending outwardly therefrom, each rim portion defining a radius of curvature as set forth above and further noted below. The first rim portion extends outwardly from the planar center portion and is convex upwardly as shown. There is defined by the plate a first arc A12 with a first radius of curvature R12 wherein the arc has a length S1. A second rim portion is joined to the first rim portion and is downwardly convex, subtending a second arc A22, with a radius of curvature R22 and a length S2. A third, downwardly convex, rim portion is joined to the second rim portion and subtends an arc A32. There is defined a third radius of curvature R32 and a third arc length S3. A tangent to the third arc at the upper portion thereof is substantially parallel to the planer center portion as shown in FIG. 20. A fourth rim portion is joined to the third rim portion, which is also downwardly convex. The fourth rim portion subtends a fourth arc A42 with a length S4, with a radius of curvature R42.
The length of the second arc, S2 is generally less the length of the fourth arc S4, which, in turn, is less than the length S1 of the first arc A12. The radius of curvature R42 of the fourth arc is less than the radius of curvature R32 of the third rim portion, which in turn, is less than radius of curvature R22 of the second rim portion. The angle of the first arc, A12 is generally greater that about 55 degrees, while, the angle of the third arc, A32 is generally greater than about 45 degrees as is set forth in the foregoing tables. The angle of the fourth arc A42 is generally less than about 75 degrees and more preferably is about 60 degrees.
Typically, the length S1 of arc A12 is equivalent to the length S3 of arc A32 and R12 of the first rim portion is equivalent in length to the radius of curvature R32 of the third rim portion.
Generally speaking, the height of the center of curvature of the first arc (that is the origin of ray R12) above the central planar portion is substantially less than, perhaps twenty five percent or so less than, the distance that the center of curvature of the second rim portion (the origin of ray R22) is below the central planar portion. In other words, the length Y12 is about 0.75 times or less the length Y22.
So also, the horizontal displacement of the center of curvature of the second rim portion from the center of curvature of the first rim portion is at least about twice the length of the first radius of curvature R12. The height of the center of curvature of the third rim portion above the central planar portion is generally less than the height of the center of curvature of the fourth rim portion above the plane of the central planar portion. The horizontal displacement of the center of curvature of the second rim portion is generally outwardly disposed from the center of curvature of the third and fourth rim portions.
A further noteworthy feature of the plate of FIGS. 1 through 3 is that the height of the center of curvature of the third rim portion above the planar central portion is less than about 0.3 times the radius of curvature R42 of the fourth rim portion; while the height of the center of curvature of the fourth rim portion above the plane of the central portion is at least about 0.4 times the first radius of curvature R12. The plates are preferably made from mineral-filled polyolefin sheet such as polyethylene or polypropylene mineral-filled sheet. As will be appreciated from the foregoing data tables as well as from the drawings and discussion above, the ratio of the fourth radius of curvature to the diameter of the plate is preferably at least about 0.03, while the ratio of the third radius of curvature to the diameter of the plate is preferably at least about 0.050. The ratio of the second radius of curvature to the diameter of the plate is preferably at least about 0.2 and the ratio of the length of the first radius of curvature to the diameter of the plate is preferably at least about 0.045.
Preferred materials are plastics or filled plastics. Typically, in filled plastics the primary mineral filler is mica, talc, kaolin, bentonite, wollastonite, milled glass fiber, glass beads (solid or hollow), silica, or silicon carbide whiskers or mixtures thereof. We have discovered that when polypropylene is melt-compounded with acidic-type minerals the resulting mixture has a higher odor index (offensive odors) that would disqualify them from use in food service products. Acidic type fillers such as mica; natural clay minerals such as kaolinite, bentonite, attapulgite, montmorillonite, clarite, or fuller's earth; and silica are particularly detrimental in generating odor compounds when processed under high shear and high temperature conditions experienced during twin screw compounding. We have found that changing the compounding process and adding a basic or other odor supressing compound or component to the primary acidic filler allows the production of low odor index compounds. The reason for this effect is unknown since the fundamental cause of the degradation in polypropylene may be due, in part, to catalysis effects caused by impurities in the mineral as well as its acidic or basic nature. In this regard, the addition of CaCO.sub.3 to talc is beneficial whereas, it may be unnecessary when wollastonite is used as the primary filler.
The preferred primary fillers are mica, talc, kaolin, bentonite, milled glass fibers, and wollastonite or mixtures thereof. Of these, milled glass fibers and wollastonite are basic in nature and may not necessarily require the addition of a secondary basic component. An odor suppressing compound is also preferably included. As noted above, suitable mineral fillers include mica, talc, kaolin, bentonite, wollastonite, milled glass fiber, glass beads (hollow or solid), silica whiskers, silicon carbide whiskers and mixtures thereof as well as the mineral fillers recited herein, whereas the basic organic or inorganic compound is generally the reaction product of an alkali metal or alkaline earth element with carbonates, phosphates, carboxylic acids as well as alkali metal and alkaline earth element oxides, hydroxides, or silicates and basic metal oxides including mixtures of silicon dioxide with one or more of the following oxides: magnesium oxide, calcium oxide, barium oxide, and mixtures of the foregoing. More specifically, the basic organic or inorganic compound may be selected from the group consisting of: calcium carbonate, sodium carbonate, potassium carbonate, barium carbonate, aluminum oxide, sodium silicate, sodium borosilicate, magnesium oxide, strontium oxide, barium oxide, zeolites, sodium citrate, potassium citrate, calcium stearate, potassium stearate, sodium phosphate, potassium phosphate, magnesium phosphate, mixtures of silicon dioxide with one or more of the following oxides: magnesium oxide, calcium oxide, barium oxide, and mixtures of one or more of the above. Furthermore, hydroxides of the metals and alkaline earth elements recited above may be utilized.
Where a basic inorganic odor suppressing compound is chosen, generally such compound is selected from the group consisting of calcium carbonate, sodium carbonate, potassium carbonate, barium carbonate, aluminum oxide, sodium silicate, sodium borosilicate, magnesium oxide, strontium oxide, barium oxide, zeolites, sodium phosphate, potassium phosphate, magnesium phosphate, mixtures of silicon dioxide with one or more of the following oxides: magnesium oxide, calcium oxide, barium oxide, and mixtures of one or more of the basic inorganic compounds set forth above. The amount of a basic inorganic compound is generally from about 2 to 20 weight percent, but is usually from about 5 to about 15 weight percent of the article. Most preferably the basic inorganic compound selected is calcium carbonate; typically present from about 5 to about weight percent.
Where an organic compound is chosen, it is typically selected from the group consisting of sodium stearate, calcium stearate, potassium stearate, sodium citrate, potassium citrate, and mixtures of these where the amount of such compound is from about 0.5 to about 2.5 weight percent of the article. Typically, microwaveable articles produced in accordance with the present invention exhibit an odor index of less than about 0.75; preferably less than about 0.6; with a practical lower limit being 0.1 or so.
As shown below in connection with microwaveability testing, and summarized in Table 11, competing commercial polystyrene type plates cannot withstand the high temperatures generated in the microwave oven during food contact and either significantly warp or deform when the aforementioned food products were heated on them. Under the usual microwaving conditions with high grease content foods, the prior art plates tend to deform and flow to the point where parts of the plate become adhered to the inside of the microwave oven. For disposable plates and containers, having suitable food contact olfactory properties, appearance and feel are important attributes. Another significant property of the containers and plates of this invention is their cut resistance. These rigid articles of manufacture are of sufficient toughness to be resistant to cutting by serrated polystyrene flatware. In normal usage they are also resistant to cutting by regular metal flatware.
Whereas any microwaveable article may be produced in accordance with the invention, most typically the article is a bowl or a plate suitable for serving food at a meal. Preferred articles are thermoformed and include a micronodular food contact surface. Micronodular food contact surfaces are produced by thermoforming a sheet into the article which has been extruded optionally with at least one matte roll and by vacuum thermoforming the sheet by applying vacuum opposite to the surface where the micronodular surface is desired. Most typically the micronodular surface will have a surface gloss of less than about 35 at 75.degree. as measured by TAPPI method T-480-OM 92. Articles also will typically have a Parker Roughness Value of at least about 12 microns.
While any suitable polypropylene polymer may be used, the polypropylene polymers are preferably selected from the group consisting of isotactic polypropylene, and copolymers of propylene and ethylene wherein the ethylene moiety is less than about 10% of the units making up the polymer, and mixtures thereof. Generally, such polymers have a melt flow index from about 0.3 to about 4, but most preferably the polymer is isotactic polypropylene with a melt-flow index of about 1.5. In particularly preferred embodiments, the melt-compounded composition from which the resultant extruded sheet is formed into articles further includes a polyethylene component and titanium dioxide. The polyethylene component may be any suitable polyethylene such as HDPE, LDPE, MDPE, LLDPE or mixtures thereof.
The various polyethylene polymers referred to herein are described at length in the Encyclopedia of polymer Science & Engineering (2d Ed.), Vol. 6; pp: 383-522, Wiley 1986; the disclosure of which is incorporated herein by reference. HDPE refers to high density polyethylene which is substantially linear and has a density of generally greater that 0.94 up to about 0.97 g/cc. LDPE refers to low density polyethylene which is characterized by relatively long chain branching and a density of about 0.912 to about 0.925 g/cc. LLDPE or linear low density polyethylene is characterized by short chain branching and a density of from about 0.92 to about 0.94 g/cc. Finally, intermediate density polyethylene (MDPE) is characterized by relatively low branching and a density of from about 0.925 to about 0.94 g/cc. Unless otherwise indicated these terms have the above meaning throughout the description which follows.
The microwaveable articles according to the invention typically exhibit melting points from about 250 to about 330.degree. F. and include mica or other primary fillers in amounts from about 20 to about 35 weight percent. Most preferably mica is present at about 30 weight percent, and calcium carbonate is present from about 8 to about 12 weight percent.
It has been found that C8 and C9 organic ketones correlate well with or are associated with undesirable odors in polypropylene/mica compositions. Accordingly, it is preferred that articles in accordance with the invention are substantially free from volatile C8 and C9 organic ketones. In order to avoid undesirable odors, articles in accordance with the invention are preferably prepared from a melt-compounded polyolefin mica composition which is prepared at a process melt temperature of less than about 425.degree. F.; with below about 400.degree. F. being even more preferred. Optionally, the melt processed polyolefin/mineral composition is melt-compounded in a nitrogen atmosphere.
In another aspect of the invention, there is provided a thermoformed, mineral-filled polypropylene food contact article formed from a melt-compounded composition comprising from about 40 to about 90 percent by weight of a polypropylene polymer, from about 10 to about 50 percent by weight of a primary mineral filler and an effective odor-reducing amount of a basic organic or inorganic compound operative to impart an odor index of less than about 0.75 to said melt-compounded composition.
Preferably the inventive articles are prepared from a melt-compounded polyolefin/mica composition prepared by way of a low temperature compounding process.
A preferred low temperature compounding process used for producing mineral-filled polypropylene melt-compounded compositions with an odor index of less than about 0.75 including a basic odor suppressing agent in accordance with the invention with from about 40 to about 90 percent by weight of a polypropylene polymer includes the sequential steps of: (a) preheating a polypropylene polymer while maintaining the polymer below a maximum temperature of about 370.degree. F. and preferably below 350.degree. F. and more preferably below a maximum of about 260.degree. F.; but suitably above about 240.degree. F.; followed by; (b) admixing mineral filler to said preheated polymer in an amount from about 10 to about 50 percent weight based on the combined weight of the resin and primary filler and maintaining the mixture below about 425.degree. F.; followed by, (c) extruding the mixture. Polymer may be melted exclusively through the application of shear, or the shear may be supplemented through heating by infrared radiation or ordinary heating coils or performed externally to the mixing chamber. Preferably, the basic odor suppressing agent is added simultaneously with the mineral filler. It is desirable to keep the duration of the step of admixing mineral filler and a basic odor suppressant agent to the mixture relatively short so as not to generate compounds which cause odor and to preserve the particle size and aspect ratio of the mineral filler. Accordingly, the step of admixing the mineral filler should be no more than about five minutes with the duration of the admixing step of less than about three minutes being even more preferred. Any suitable means may be used to carry out the sequential process in accordance with the invention, however, the process is normally carried out in a batch mode in a mixing chamber provided with a pair of rotating rotors in an apparatus referred to in the industry as a Banbury type mixer. One may choose to use a twin screw extruder or a Buss kneader to practice the inventive process if so desired, provided that appropriate elements are used to minimize shear heating.
Thermoforming is typically conducted at a sheet temperature of from about 260.degree. to about 310.degree. F., and more preferably at a temperature of from about 280.degree. to about 300.degree. F.
There is provided in a still further aspect of the invention a crack-resistant, thermoformed food contact article having a wall thickness ranging from about 10 to about 80 mils consisting essentially of from about 40 to about 90 weight percent of a polypropylene polymer, from about 10 to about 50 percent by weight of a mineral filler, from about 1 to about 15 percent by weight polyethylene, from about 0.1 to about 5 weight percent titanium dioxide and optionally including a basic organic or inorganic compound. The basic compound is, generally speaking, the reaction product of an alkali metal or alkaline earth element with carbonates, phosphates, carboxylic acids as well as alkali metal and alkaline earth element oxides, hydroxides, or silicates and basic metal oxides, including mixtures of silicon dioxide with one or more of the following oxides: magnesium oxide, calcium oxide, barium oxide, and mixtures thereof. A particularly preferred article is where the basic organic or inorganic compound is calcium carbonate which is present in an amount of from about 5 to about 20 weight percent.
Polyethylene is more typically present from about 2.5 to about 15 weight percent, preferably from about 4 to about 5 weight percent of the crack resistant article. Titanium dioxide is included in various amounts, from about 0.1 to about 3 percent by weight being typical; from about 0.25 to 2 percent titanium dioxide may be included. Preferably, titanium dioxide is included in at least 0.5 percent by weight.
The caliper, or wall thickness, of the articles is usually from about 0.05 to about 0.050 inches or from about 10 mils to 50 mils. A caliper of from about 15 to 25 mils is most typically employed.
While any suitable polypropylene polymer may be employed, the most preferred polymer is isotactic polypropylene having a melt index in the range of from about 0.3 to 4, with a melt index of about 1.5 being typical. The polyethylene employed may be HDPE, LLDPE, LDPE or MDPE, mixtures thereof or a polyethylene with bimodal molecular weight distribution. Polypropylene is sometimes referred to hereafter as "PP".
The inventive compositions from which the crack resistant articles are made do not include coupling agents such as maleic anhydride containing polypropylene as further described herein, but may optionally include other components which do not alter the basic and novel characteristics of the crack-resistant plates. For example, nucleants such as sodium benzoate in amounts detrimental to crack resistance are to be avoided.
In a still further aspect of the invention there is provided a method of making a microwaveable mineral-filled polypropylene food contact article comprising preparing a melt-compounded composition comprising from about 40 to about 90 percent by weight of a polypropylene polymer and from about 10 to about 50 percent by weight of a mineral filler and optionally an effective amount of an odor-reducing compound. The melt-compounded composition exhibits a relative aroma index, relative to a corresponding composition consisting essentially of polypropylene and mica of less than about 0.75. The composition is extruded into a sheet and formed into a suitable food contact article. Preferably, the article consists essentially of polymer and mineral filler and excludes such components as excess anti-oxidants and the like.
Suitably the basic inorganic or organic compounds are selected from the group consisting of calcium carbonate, sodium carbonate, potassium carbonate, barium carbonate, aluminum oxide, sodium silicate, sodium borosilicate, magnesium oxide, strontium oxide, barium oxide, zeolites, sodium phosphate, potassium phosphate, magnesium phosphate, mixtures of silicon dioxide with one or more of the following oxides: magnesium oxide, calcium oxide, barium oxide, and mixtures of these or other basic inorganic or organic compounds such as sodium stearate, calcium stearate, potassium stearate, sodium citrate, potassium citrate, and mixtures of these basic organic compounds.
The function of the basic inorganic compound or organic compound is to minimize the formation of odor-causing compounds in the mineral-filled polyolefin composition and thus provide products with food contact compatible olfactory properties for consumer use. In this connection, the amount of the basic inorganic compound or organic compound added is controlled to be sufficient to reduce formation of decomposition products to sufficiently low levels to provide containers and plates with suitable food contact compatible olfactory properties. Suitably 5 to 15 weight percent of the container comprises the basic inorganic compound, advantageously about 8 to 12 percent. When the basic organic compounds are used, lower quantities are required, suitably from about 0.5 to 2.5 weight percent, advantageously 1.0 to 1.5 percent. Coupling agents and pigments may be utilized. Maleic anhydride and acrylic modified polypropylenes are suitable coupling agents for some embodiments.
Advantageously, the sheet is formed by an extrusion process utilizing a compounded polymer/mica basic inorganic compound or basic organic compound mixtures. The final extrusion process renders a sheet with excellent thermal properties, cut resistance, and food contact compatible olfactory properties.
Mica is easily cleaved into thin, relatively regular, flexible yet strong sheets (leaf-like flakes) with thickness in the range of half a micron and aspect ratio as high as 300. Mica is much softer than other inorganic fillers (wollastonite, glass) yet only slightly harder than talc. Mica has a slippery tactile feel and low abrasiveness relative to other common inorganic fillers.
The reinforcement effect at 40 weight percent mica is equivalent to that of 30 weight percent glass fiber. Hard inorganic fibrous fillers such as glass (various lengths) and wollastonite (acicular structures) have drawbacks in some respects such as abrasiveness and are prone to fracture degradation during conventional melt processing. Other fibrous (organic) fillers are derived from wood and vegetable sources and are not suitable for use in the manufacture of the containers of this invention since the organic fillers, when used in substantial amounts, tend to degrade during processing and they are also moisture sensitive.
In some applications it may be preferred to treat the mineral and/or basic inorganic compounds prior to using them in the inventive articles. A suitable compound for this treatment is amino-silane; sometimes referred to as a "coupling" agent.
Suitable basic inorganic and organic compounds used in the process include: calcium carbonate, sodium carbonate, sodium hydroxide, potassium carbonate, barium carbonate, aluminum oxide, sodium silicate, sodium borosilicate, magnesium oxide, strontium oxide, barium oxide, zeolites, sodium phosphate, potassium phosphate, magnesium phosphate, mixtures of silicon dioxide with one or more of the following oxides: magnesium oxide, calcium oxide, barium oxide, and mixtures of these or other basic inorganic or organic compounds such as sodium stearate, calcium stearate, potassium stearate, sodium citrate, potassium citrate, and mixtures. of these basic compounds.
In the case where microwaveability is desired for a plastic disposable food contact article, the not so perfect solution has been the use of relatively expensive high heat modified polystyrene based or heat resistant materials (e.g., unfilled PPO and SMA engineering resins), where PPO refers to polyphenylene oxide and SMA refers to styrene-maleic anhydride copolymer.
Mica or another mineral filler and the basic inorganic compound or the basic organic compound filled polypropylene is compounded by pre-blending the polypropylene in pellet or flake form with mica powder and the basic inorganic compound or the basic organic compound powder and other additives (color concentrates, pigments, antioxidants, lubricants, nucleating agents, antistatic agents, etc.). This mixture is conveyed into the feed section addition point of a twin screw compounding extruder, or compounded in a Banbury-type mixer to provide a melt-processed polyolefin composition. Alternatively, the components are advantageously fed separately into the same or different points of addition, using combinations of volumetric and/or gravimetric (i.e., loss in weight type) feeders as further described herein.
For white pigmentation, titanium dioxide is preferred due to combination of brightness, and opacity, as well as stability during processing and final use. Surface treatment may be optionally used to further enhance wetting, dispersion, compatibility with matrix resins whereas the titanium dioxide forms may be of the rutile or anatase type. Alternate white pigments may also consist of calcined clay or blends of calcined clay with titanium dioxide. For black pigmentation, carbon black is preferred due to a combination of desirable characteristics such as blackness, and dispersibility, the latter of which can be carefully controlled by choice of particle size and surface chemistry. Carbon black is amorphous carbon in finely divided form which is made by either the incomplete combustion of natural gas (channel black) or by reduction of liquid hydrocarbons in refractory chambers (furnace black).
A twin screw extruder provides sufficient mixing action to effectively cause the wetting and dispersion of the filler into the polymer matrix. The twin screw extruder may be of the co-rotating or counter-rotating type, where each type is equipped with different screw flight elements which are appropriate for the feed, mixing, and melt metering zones. The discharge zone normally consists of a strand die where the exiting molten material strands are quenched in a circulating water bath followed by knife cutting into pellets. In a particularly preferred embodiment, a Banbury-type mixer is used for compounding the resin, mica and basic compound as further described herein.
Low molecular weight additives such as waxes, fluorinated polymers, and other specialty lubricants are suitably used as process aids to reduce the melt viscosity and improve throughput. Polyethylene resin may also be added to the blend. Other additives may include nucleating agents and antistatic agents. Antioxidants may be added in small amounts, generally less than one weight percent, to minimize shear and thermal degradation of the polypropylene during the extrusion and forming processes as well as to promote the chemical stability of the sheet prior to and during final article use. Suitable antioxidants are advantageously selected from the group of phenolics and phosphites and blends thereof. These are produced by Ciba-Geigy and General Electric Corporation. Plastic sheet extrusion equipment is suitable for the manufacture of multilayered or single layered mica or other mineral filler and the basic inorganic or organic compound filled sheets of a polyolefin selected from the group consisting of polypropylene, polypropylene/polyethylene copolymer or blend, and mixtures of these. Melt strength of the sheets is improved when mica is used as a filler since geometry of the mineral in the form of high aspect ratio flakes serves to provide "inter-particle connectivity" or physical cross-linking. The food contact compatible olfactory properties are enhanced when in addition to the mica, basic inorganic compounds or organic compounds such as calcium carbonate, sodium carbonate, potassium carbonate, barium carbonate, aluminum oxide, sodium silicate, sodium borosilicate, magnesium oxide, strontium oxide, barium oxide, zeolites, sodium phosphate, potassium phosphate, magnesium phosphate, mixtures of silicon dioxide with one or more of the following oxides: magnesium oxide, calcium oxide, barium oxide, and mixtures of these or other basic inorganic or organic compounds such as sodium stearate, calcium stearate, potassium stearate, sodium citrate, potassium citrate, and mixtures of these are mixed with mica or other mineral filler and the polyolefin to produce the containers of this invention. Exemplary inorganic materials which may also be employed as a primary mineral filler include talc, barium sulfate, calcium sulfate, magnesium sulfate, clays, glass, dolomite, alumina, ceramics, calcium carbide, silica and so on. Many of these materials are enumerated in the Encyclopedia of Materials Science and Engineering, Vol. # 3, pp. 1745-1759, MIT Press, Cambridge, Mass. (1986), the disclosure of which is incorporated herein by reference.
Mineral fillers are sometimes referred to by their chemical names. Kaolins, for example, are hydrous alumino silicates, while feldspar is an anhydrous alkali, alumino silicate. Bentonite is usually an aluminum silicate clay and talc is hydrated mangesium silicate. Glass, or fillers based on silicon dioxide may be natural or synthetic silicas. Wollastonite is a calcium metasilicate whereas mica is a potassium alumino silicate. Mineral fillers are further discussed below. As noted above, clays may be employed as a primary filler. The two most common of which are kaolin and bentonite. Kaolin refers generally to minerals including kaolinite which is a hydrated aluminum silicate (Al.sub.2 O.sub.3.2SiO.sub.2.2H.sub.2 O) and is the major clay mineral component in the rock kaolin. Kaolin is also a group name for the minerals kaolinite, macrite, dickite and halloysite. Bentonite refers to hydrated sodium, calcium, iron, magnesium, and aluminum silicates known as montmorillonites which are also sometimes referred to as smectites. A large number of siliceous materials may also be employed as a primary filler. These materials include diatomite, perlite, pumice, pyrophillite, silica, and talc. These minerals typically consist of an alkali metal oxide or alkaline earth element oxide, and silicon dioxide together with a minor amount of water and other elements. Talc, for example, includes from about 25% to about 35% MgO, 35-60% SiO.sub.2 and about 5% H.sub.2 O. These materials are further described below. Diatomite or kieselguhr is a sedimentary material formed by centuries of life cycles of aquatic diatoms, a simple plant in the algae family with an opaline silica cell wall. Thousands of species of diatoms have flourished and continue to do so in both marine and lacustrine environments. Fossilized skeletal remains of diatoms in commercial quantities are found in many parts of the world. Perlite is believed to result from hydration of volcanic glass or obsidian. Generally, hydration is about 2-5%; this water content is important to the expansibility of the perlite, influencing melting point and supplying expansion steam.
The rapid expansion of dissolved gases in silica lavas during volcanic eruptions produces the light density pumice or pumicite. The finer pumicite particles are transported by wind away from the source volcano, whereas pumice accumulates closer to the vent.
The hydrous aluminum silicate, pyrophilite, is formed by hydrothermal metomorphism of acid tuffs or braccias.
Silica sand is frequently obtained from the weathering of quartz-containing rock. Decomposition and disintegration of the rock with decomposition of other minerals leaves a primary quartz sand that has been concentrated by water movement. Induration of sands to sandstone results in another source for silica sand. Amorphous silica, or more properly cryptocrystalline or microcrystalline silica, is formed by the slow leaching of siliceous limestone or calcareous chert. Talc is formed by the metamorphic (hydrothermal) alteration of magnesium silicates such as serpentine, pyroxene or dolomite.
The siliceous fillers are generally inert in most applications as shown by pH values in the range from about 6-10.
Sulfate minerals, such as gypsum and barite may likewise be employed as a primary filler. Gypsum is the name given to the mineral that consists of hydrous calcium sulfate (CaSO.sub.4 2H.sub.2 O), and also to the sedimentary rock that consist primarily of this mineral. In its pure state, gypsum contains 32.6% lime (CaO), 46.5% sulfur trioxide (SO.sub.3), and 20.9% water. Single crystals and rock masses that approach this theoretical purity are generally colorless to white, but in practice, the presence of impurities such as clay, dolomite, silica and iron imparts a gray brown, red or pink color to the rock.
There are three common varieties of gypsum: selenite, which occurs as transparent or translucent crystals or plates; satin spar, which occurs as thin veins (typically white) of fibrous gypsum crystals; and alabaster, which is compact, fine-grained gypsum that has a smooth, even-textured appearance. Most deposits or rock gypsum that are suitable for industrial purposes are aggregates of fine to coarse gypsum crystals that have intergrown to produce a thick, massive sedimentary rock unit that is 90-98% gypsum. Alabaster is highly prized because of its uniformly fine particle size, but the more common deposits of rock gypsum consisting of coarser-grained selenite can generally be crushed and ground to produce a suitable filler and coating material.
Gypsum has a hardness of 2 on the Mohs scale, and can be scratched with the fingernail. Large rock masses are easily crushed and ground to a fine powder. The specific gravity of gypsum is about 2.31 and the refractive index is about 1.53. Gypsum is slightly soluble in water but it is an inert substance that resists chemical change. The oil-absorption capacity of gypsum is fairly low (0.17-0.25 cm.sup.3 g.sup.-1).
Raw or crude gypsum is one of the forms used as fillers and coatings, but for some purposes calcined or deadbumed gypsum is desired. In calcining, the gypsum is heated to abut 120-160.degree. C. to drive off free water and partially remove the water of crystallization. The calcined material or stucco, has a chemical composition of CaSO.sub.4.1/2H.sub.2 O, and it readily takes up water. Calcination at higher temperatures (500-725.degree. C.) results in a product called deadbumed gypsum, which has a composition of CaSO.sub.4.
Anhydrite, a sulfate mineral and rock that is closely associated with gypsum in nature and has minor uses as a filler, in anhydrous calcium sulfate (CaSO.sub.4) containing 41.2% CsO and 58.8% SO.sub.3. It is typically fine grained (like alabaster), and occurs in thick, massive sedimentary rock units. Anhydrite usually is white or bluish gray when pure, but it may be discolored by impurities. Anhydrite has a hardness of 3.5, a specific gravity of 2.98, and a refractive index of 1.57-1.61.
Thus, fillers commonly include:
Barium Salt
Barium Ferrite
Barium Sulfate
Carbon/Coke Power
Calcium Fluoride
Calcium Sulfate
Carbon Black
Calcium Carbonate
Ceramic Powder
Chopped Glass
Clay
Continuous Glass
Glass Bead
Glass Fiber
Glass Fabric
Glass Flake
Glass Mat
Graphite Powder
Glass Sphere
Glass Tape
Milled Glass
Mica
Molybdenum Disulfide
Silica
Short Glass
Talc
Whisker
Particulate fillers, besides mica, commonly include:
Glass
Calcium carbonate
Alumina
Beryllium oxide
Magnesium carbonate
Titanium dioxide
Zinc oxide
Zirconia
Hydrated alumina
Antimony oxide
Silica
Silicates
Barium ferrite
Barium sulphate
Molybdenum disulphide
Silicon carbide
Potassium titanate
Clays
Whereas fibrous fillers are commonly:
Whiskers
Glass
Mineral wool
Calcium sulphate
Potassium titanate
Boron
Alumina
Sodium aluminum
Hydroxy carbonate
Suitably the extruded sheet includes coloring agents for aesthetic appeal, preferably titanium dioxide, carbon black, and other opacifying agents in the range of 0.5-8 weight percent based on total composition, preferably 1.5 to 6.5 weight percent. The extruded sheet comprises minor amounts of other additives such as lubricants and antioxidants. These articles of manufacture may be suitably colored with pigments or dyes. Pigments are defined as small insoluble organic or inorganic particles dispersed in the resin medium to promote opacity or translucency. Usual pigments include carbon black, titanium dioxide, zinc oxide, iron oxides, and mixed metal oxides. Dyes are organic and soluble in the plastic, and may be used alone or in combination with pigments to brighten up pigment based colors. All such colorants may be used in a variety of modes which include dry color, conventional color concentrates, liquid color and precolored resin.
Aroma Profile Test Method
The Sensory Analysis Center at Kansas State University has developed a profiling protocol in which a highly trained panel identifies specific odors and rates their intensity. The intensity scale is a 15-point "universal" scale of the type typically chosen for sensory studies, where 1 is barely perceptible or threshold and 15 is extremely strong. If an attribute or odor component is not listed in the tables which follow, it means it is not present and would score a 0. The panel members are selected on the basis of a series of screening tests that include basic taste, odor recognition, taste intensity recognition, taste intensity ranking, and a personal interview to evaluate availability and personality traits. Training, which includes the fundamental sensory principles and all aspects of the profile technique, is done over a 4-12 month period.
The panelists work as a group to arrive at a description of the product. Individual results are compiled by the panel leader and-discussion follows in which disagreements are discussed until a consensus is reached on each component of the profile. Reference materials and more than one session usually are required in order to reach the consensus.
The procedure for resin is to place 40 ml. of resin in a 340 ml. glass brandy snifter, which is covered with a watch glass. Sheet samples are cut into two 2".times.2" sections and placed in the same size brandy snifter. In testing, panelists found that some samples had initial odor components that disappeared rapidly. Therefore an initial impact and a sustained impact were evaluated for each sample. The initial impact was judged. immediately after the watch glass had been removed; the sustained impact was judged 10 seconds after the watch glass had been removed. Typical results are shown in the Table 4 below for Low Odor and High Odor Compositions. "Low" odor formulations were produced using lower melt processing temperatures in compounding and adding 10% calcium carbonate to the formulation.
TABLE 4
High Odor vs. Low Odor Polypropylene Composites:
Effect of Adding 10% CaCO.sub.3
ODOR PROFILE FOR COMPOUNDED RESIN
Consensus Odor Profile on Resin
Resin Impact (Kansas State University Sensory Analysis
Center)
Resin Initial Sustained Petroleum Pungent Musty Scorched Medicinal
Sweet Waxy Soapy
High 9.0 3.5 8.0 4.0 7.0 3.5 3.0
Odor
Low 5.5 2.5 2.5 4.5 1.5
2.0 4.5
Odor
High Odor Resin Low Odor Resin
65.63% Polypropylene 55.63% Polypropylene
30% Mica 30% Mica
2.5% Coupling Agent 10% CaCO.sub.3
1.87% Pigment 2.5% Coupling Agent
1.87% Pigment
High Odor and Low Odor compositions were compounded utilizing the process melt temperatures indicated in the first column of Table 5 and formed into sheets as described above. Thermoformed sheet was evaluated for aroma profile.
TABLE 5
ODOR PROFILE FOR SHEET FORMED
FROM COMPOUNDED RESIN AT TWO TEMPERATURES
Sheet Impact Consensus Odor Profile on Sheet
Resin Initial Sustained Petroleum Pungent Musty Scorched Medicinal
Sweet Waxy Soapy
High Odor 12.0 6.0 10.0 8.0 7.5 4.5 4.0
370.degree. F.
High Odor 11.0 8.0 7.5 7.5 6.0 3.5 2.0
459.degree. F.
Low Odor 5.5 2.0 3.5 4.0
2.0 2.5 2.5
371.degree. F.
Low Odor 5.5 2.0 3.0 3.5
2.0 3.5
460.degree. F.
The foregoing data demonstrates that: when a bas
