lOS~84 FILMS AND SHEETS OF POLY~S~I'OL"CAKIW~A~ DLII~105 BACKGROUND OF THE INVENTION
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Field of the Invention This invention relates to films and sheets formed - from blends of polyethylene terephthalate and polycarbonate resins useful as cook-in-trays.
Discussion of the Prior Art Frozen foods, which can be cooked in the tray in which they are packaged, are a standard commercial item. These so-called "convenience" foods are usually packaged in a tray or similar structure formed of aluminum and are intended for home as well as industrial uses. Such trays, however, are somewhat aesthetically unpleasing. Additionally, the price of aluminum has been increasing to very high levels. Consequently, efforts have been expended to find a ~ubstitute for aluminum as the material used in forming cook-in-trays and the like. Recently there has been marketed polysulfone trays which are generally ; thermoformed from a sheet of polysulfone resin. Although material costs and resultant phys1cal properties compare favorably with aluminum trays, relatively long cycle times are required to pro-duce such trays on conventional thermoforming machines.
It i~ known that various thermoplastic resin~ can be blended to generally take advantage of the separate properties of each re~in. ~or example, U.S. Patent 3,218,372 issued in 1965 to Oka~ura et al. di~closes a molding material formed from a blend of a polyalkylene terephthalate (e.g. polyethylene tereph-thalate) having an intrinsic viscosity of 0.5 - 0.85 and a polycarbonate havlng an intrin~ic vi~cosity of 0.46 - 1.2. Blend ratios di~clo~ed ~n #aid patent range, in parts by weight, from 95 - 5 polycarbonate and from 5 - 95 polyalkylene terephthalate.
It i~ there~n ~isclo~d t:hat coMp~ltlon~ contalning 95 - 70 poly-1~5S~84 carbonate and 5-30 polyalkylene terephthalate possess low melt viscosities compared with polycarbonate alone so as to facilitate molding operations, whereas compositions containing 5 - 70 parts polycarbonate and 95 - 30 parts polyalkylene terephthalate have enhanced hardness and tensile strength over polyalkylene tereph-thalate alone.
Summary of the Invention This invention provides a film or sheet having a thickness of 1 to 50 mils, capable of being thermoformed into a shaped article, formed from a blend, in parts by weight percent, of about 60 to 85 parts of a polyethylene terephthalate having an intrinsic viscosity of at least about 0.90, about 15 to 40 parts of a polycarbonate and preferably also containing about 5 to 20 parts of a non-acidic silica filler, the polyethylene terephthalate portion of the film or sheet having a degree of crystallinity in the range of about 20 to 40~. The ilm is essentially non-oriented.
This invention also provides film- and sheet- orming compositions which comprise about 60 to 85 parts by weight of a polyethylene terephthalate having an intrinsic viscosity of at least about 0.90, about 15 to 40 parts by weight of a polycarbon-ate and about 5 to 20 parts by weight of a non-acidic silica filler.
In addition, this invention also provides, in one embodiment, a composition comprising a polycarbonate and about - 20 to 40 percent by weight of a non-acidic silica filler, such as novaculite, and in another embodiment, also including about 5 to 20 percent by weight of the total composition of a pigment, such as titanium dioxide.
The film or sheet may be molded by conventional mold-ing techniques, such as thermoforming, into shaped articles suchas cook-in-trays which possess a high degree of toughness and im-pact resistance and exhibit an improved resistance to distortion at elevated temperatures.
Description of the Preferred Embodiment As mentioned above, the film or sheet of this invention - is formed from a specific blend of a polyethylene terephthalate - and a polycarbonate and preferably a non-acidic silica filler.
- The terms "film" and "sheet" are intended to mean thin cast, ex-truded or otherwi~e formed products. In general, the term ~film~
denotes thin structures having a thickness of up to about 10 mils whereas the term "sheet" denotes thin structures having a thickness of lO mils or above. The film or sheet of this in-vention have a thickness in the range of 1 to 50 mils, preferably 5 to 25 mils and more preferably 10 to 20 mils.
The polyethylene terephthalate (hereinafter ~PET~) em-ployed herein i8 a polymer having an instrinsic viscosity of at least about 0.90, the intrinsic vi~co~ity being measured in a mixed ~olvent of 60 part~ by weight phenol and 40 parts by weight tetrachloroethane at 25C, Preerably, the intrinsic 30 viwo~ity i~ ~n the range o about 0.9 to 1.2, more preferably about 0.9 to 1Ø 5uch P~T polymers melt ln their cry~tallized 10551~34 state at about 490 to 525F. The polycarbonate employed hereinmay be any polycarbonate such as the reaction product of phosgene or a carbonic acid diester with bisphenol A, i.e., poly(4,4'-isopropylidene diphenylene carbonate). The polycarbonate may have an intrinsic viscosity in the range of about 0.4 to 1.2 as - measured in dioxane solvent at 30C. Such polycarbonates are essentially non-crystalline and soften at about 275-350~.
The intrinsic viscosities referred to herein are the viscosities measured before blending the two polymers.
It has now been found that while such compositions are in general suitable for use as cooking containers, in some ca~es the trays exhibit an initial distortion while in the oven, which detrzcts from their aesthetic appearance. This distortion is believed to be caused by a slight initial softening and subse-quent shrinkage of the trays as further crystallization of the polyethylene terephthalate occurs. Thi~ problem is overcome by the addition of non-acidic silica fillers.
Silica fillers which can be incorporated herein are silica materials which are "non-acidic" in an aqueous dispersion. By ~non-acidic~, it is meant that the pH of such dispersion is not less than about 6 and preferably is in the range of about 6 to 10. It has been found that silica fillers that are acidic are not acceptable in the present compo~itions since they degrade either or both of the polymers.
An e~pecially preferred silica filler which may be employed herein i~ novaculite. Novaculi~e i~ a micro-cry~talline form of a-~uartz which is found in useable ~uantities in and around the Devonlan-Mississippian deposit~ of Hot Spring~, Arkansas in the Un~ted ~ta~es. Under the petrographic microscope, the grains of ~uartz ~re seen to posses~ ~mooth, very ~lightly curved ~urface~. Large particle~ are clu~ters of cry~tals which are ~551~4 easily broken down into smaller grains. The particle ~hape of novaculite is believed to be unique among all other forms of quartz. Particles are generally square or rectangular in outline, and in three-dimensional aspect might be designated as pseudo-cubic or rhombohedronic. Novaculite is closely related to chert and - flint, although mineralogical inspection reveals significant differ-ences in crystalline form, since fine-sized particle~ of chert or flint, or most other forms of fine quartz, possess irregular, - jagged outlines and edges. The particle size of the novaculite useful in this invention can range up to a maximum of about 100~, preferably less than about 25~. There is no minimum particle size although, in fact, particles less than 1~ in size are comparatively rare. Most preferred particle sizes range from about 3 to 12~.
Other silica fillers which may be employed herein include diatomaceous earth. As pointed out above, such ~ilica containing materials are non-acidic in an aqueous dispersion. To be u~eful in the present invention, a filler must be compatible with both polymers and must of course meet applicable regulation~ governing product~
i which contact food.
An example of silica fillers which are unsatisfactory for use herein are certain pure grades of silicon dioxide formed by hydrolysis of silicon tetrachloride; ~uch materials are sufficiently acidic to cause breakdown of the polycarbonate molecule (as i8 evidenced by bubble formation in the sheet or film). Additionally, other common fillers such as calcium carbonate likewise degrade either or both polymers.
The PBT, polycarbonate and silica filler are blended to provlde a preferably un~form blend of about 60 to 85 part~ by weight PeT, about 15 to 40 part~ by weight polycarbonate and preferably 3~ about 5 ~o 20 part~ by w~ight ~ilica. Preferred compo~ltlon~ com-prl~e 65 to 80 ~oarts by welght PeT, about 20 to 35 part~ by we$ght . -5-polycarbonate and about 5 to 15 parts by weight silica. Blends containing less than about 15 parts by weight polycarbonate result in films and sheets which have inadequate impact and fracture resistance. Blends containing less than about 60 parts by weight PET have relatively poor heat resistance and poor thermoformability as well as present problems in obtaining uniform optical properties.
Blends containing less than about 5 parts by weight silica do not exhibit improved heat distortion resistance whereas blends of more than about 20 parts by weight silica result in brittle films and sheets and poor uniformity.
The two polymers may be blended together using any conventional blending apparatus. They may be blended in the solid or melted state; preferably the PET and polycarbonate are dry blended in pellet form at about room temperature. The duration of the mixing is primarily dependent upon the desired degree of blend uniformity. Blending may be performed prior to or in the extruder. The silica may be added in any conventional manner and may suitably be added to either ~he PBT or polycarbonate separately or after the polymers are blended.
Conventional additives may be added during the mixing operation. Buch additives include pigments, such as titanium dioxide and carbon black, stabilizers, other fillers such as glasses, carbonates and alumina and reinforcing agents such as glass fibers.
The amount of such additi~es may of course vary; it has been found, for example, that a composition including about 1 to 5~ by weight of titanium dioxlde may be used to form a very aesthetically attractive cook-in-tray.
In one preferred embodiment, the silica flller together with optional additlves may be blended into one of ~he polymers, preferably the polycarbonate, and then blended w$th the other polymer which may contaln the remaindsr, 1 any, of the flller or additives. For example, a blended composition may be prepared which comprises polycarbonate and about 20 to 40, more preferably 25 to 35, percent by weight of the non-aci~ic silica filler, preferably novaculite. This filled polycarbonate composition may be blended with an amount of unfilled or partially silica-filled PET to obtain the desired blended composition. In another embodiment, if titanium dioxide pigment is utilized, the polycar-bonate composition may also include about 5 to 20, preferably 5 to 10, percent by weight of such pigment. The pigmented composition may then be blended with the requisite amount of unfilled or filled PET.
Since both PET and polycarbonate polymers are hygro-scopic, preferably the blended mixture is dried at elevated temperatures (e.g., above 212F) for a sufficient period of time.
For example, the blend may be dried at 200 to 300F for about - 1 to 18 hours in a circulating hot air oven.
The blend is formed into a film or sheet by extrusion of a molten mixture. Preferably, the blend is charged to a screw extruder wherein the blend is melted and additional mixing occurs and the film or sheet exits through a flat die head. Con-ventional film or sheet extruders may be utilized for this purpose.
Preferred extrusion temperatures are above about 500~, preferably 510 to 600~. The extrudate exiting the die head is passed onto a moving support such as a rotating casting or cooling roll which serves to cool the molten layer into a coherent film or sheet.
Conventional casting rolls may be employed for this purpose, such as chromium plated rolls. As is well understood by those skilled in the art, the rate of extrusion, the width of the extruder die orifice and the gpeed of the moving ~upport may be varied w~dely and determine the thicknesg of the fllm.
- The surface temperature of the rotatlng support i8 ,, 1055~t~4 maintained in the range of about 225 to 380F, preferably 250 to 360F by providing the support with heating means.
This can readily be accomplished, for example, by providing a heat transfer fluid within the interior of the casting roll or the like in a conventional manner.
Following extrusion onto the moving support, the film or sheet may be further cooled down prior to collecting the same by passing the film or sheet over one or more additional moving supports, such as additional rolls, in a manner generally employed for extrusion of films and sheets. Such additional rolls may be heated or unheated. However, any such additional moving supports move or rotate at substantially the same linear speed as the first moving support so that the film or sheet is not subjected to a drawing or stretching operation which would orient the same.
The film or sheet is collected using conventional apparatus, such as a winding roll or the like.
The film or sheet of this invention is essentially non-oriented, that is, shrinks less than 5% in the machine and tran~verse directions after 10 minutes at temperature~ above about 400P, and is partially crystallized. As discussed above, the P~T portion of the film or sheet is partially crystallized, with a crystallinity in the range of about 20 to 40%. The crystallinity referred to is that obtained by the well-known density method as described in ~Engineering Design for Plastics~, E. Baer, Reinhold Publishing Co., 1964, pp. 98-99. This partial crystallinity results from utilizing the above disclosed range of surface temperatures on the support onto which the film or sheet i8 extruded (e.g., ca~ing roll). Por example, cry~tallinities in the range of 35-40~ can be ob~ained u~ing a ca~ting roll temperature of about 350P; 25-30% at 300P and 20-25~ at 250P.
It ha~ been del:ermlned that when the cry~tall~nlty of the blended film or sheet is below about 20~ the thermal re~i~t-ance thereof is unacceptable. That is, articles molded from such - film or sheet, such as cook-in-trays distort when s~jected to elevated temperatures. For instance, cook-in-trays thermoformed from the blended film or sheet have low heat resi~tance when ex-posed to temperatures above 300F, e.g., cooking temperatures.
Such trays become distorted and shrink in the oven, with a minimum result of an unaesthetically appealing product and the possibility of spill-over of the contained food during cooking. When the crystallinity level is above about 40~, articles formed from the film or sheet have low impact and fracture resistance. This is especially a problem with cook-in-trays which are designed to be frozen with the contained food; inadvertent dropping of such trays may result in their fracture if the crystallinity level is above about 40%.
It has been additionally found that films and sheets produced from a PE~ polymer having an intrinsic viscosity of at least about 0.90 have increased toughness as compared to films and sheets produced from a PET polymer having an intrinsic viscosity below about 0.90. Additionally, the higher intrinsic viscosity provides improved proce~sibility of the film and sheets in terms of easier extrusion and better control of thickness.
Since the film or sheet of the invention is essentially non-oriented, it may be drawn, molded or shaped to a high degree with ~hort cycle times despite the product being partially crystallized, Such shaped articles as trays, cups, bowls, and the like can be readily formed from such film or sheet using convent~onal shaping equipment. Por example, film or sheet in roll form may be rapidly thermoformed on conventional therms-forming machine~ into cook-in-trays and the like. A typlcal commercial thermoforming machlne ~often~ the fllm or sheet and _g_ forces it by pressure into the desired shape. Typical thermo-forming temperatures useful in forming shaped articles from the film or sheet of this invention may range from about 250 to 500F, with a thermoforming cycle of about 1 to 10 seconds followed by a cooling cycle of about 1 to 10 seconds. The re-sultant shaped articles exhibit a high toughness and high heat resistance. Cook-in containers so produced are essentially distortion-free at temperatures above 300F to as high as 400F or greater.
The following non-limiting examples are given to further illustrate the present invention:
Example 1 Various blends were prepared of PET resin having an intrinsic viscosity of 0.95 (measured in a 60:40 solution of phenol and tetrachloroethane at 25C.) and polycarbonate (PC) resin having an intrinsic visco~ity of 0.56 (measured in dioxane at 30C.) as ~hown in Table I. The blends al o contained about 3% by weight titanium dioxide pigment. The blend~ were prepared by tumbling dry pellets of the resin and pigment in a 250 lb, capacity drum for 15 minute~ at 25 rpm. The blend wa~ extruded using a 2-1/2 inch diameter extruder and mixing type screw through a 30" wide die onto a steel ca~ting roll heated to a desired tem-perature. The resulting sheet was 17 mils thick. Typical extru-sion conditions were: extruder temperature, feed zone to front zone, 490 to 510P; die connector temperature 525F, die te~pera-ture 515~, ~crew spread 60 - 75 rpm.
The casting roll temperature for each blend was varied between a~out 225P to 380P. The level of cooling imparted by the ca~ting roll wa~ found to be critical in determining the cry~tallin~y of the P~T fraction ~n the extruded film. Thu~
a temperature of 200P gave an e~en~ially non-cry~tallLne film wherea~ the temperature range 250P to 350~ g~ve be~t re~ult~
for the dimensional stability of formed containers from the sheet.
Under these conditions partially crystallized film were obtained e.g., with a blend of type 1 in Table I, cooled with the casting roll at 350F, the PET fraction was about 33~ crystallized in the sheet. Above 350F the dimensional stability was very high but some decrease in toughness occurred.
Trays were prepared from the blended sheets formed on the casting roll at 350F using a commercial thermoforming machine.
The thermoforming cycle was g seconds heating time at 400F and 3 seconds cooling time. The trays measured 5-1/4" x 4" x 1" deep and had a 1/4" rim around the top.
Example 2 The trays formed in Example 1 were tested for resistance to distortion and shrinkage in an oven as follows: Three fish sticks measuring 3-3/4" by 3/4" diameter were placed in each tray.
The trays were heated in a 350F oven for 1/2 hour and allowed to cool to 75 - 80P. The volume of the tray before the oven te~t was determined by measuring the volume of water required to fill - the tray just to its rim. Similarly the volume of trays after heating at 350P was determined and the change in volume calculated.
Additional tests were made on trays from the six blends heated to 400F for 1/2 hour. The results are shown in Table I.
Table I
PET:PC Reduction in Volume Blend No Wei~ht Ratio at Oven Temp.
3soop 400~
1 80:17 13.7 15
2 75:22 12.4 13.7
3 70:27 12.4 17.4
4 65:32 20 26.2 ~0 5 60:37 21 27.5 6 58:38 -- 29.5 Over the range PET:PC of 80:17 to 58:38 the volume decreases on heating ranged from about 13 to a high of 30%, the latter volume reduction considered to be borderline for use as a cook-in-tray.
Example 3 Sheet was extruded from non-pigmented samples of peT:pc of the weight ratios shown in Table II. Low temperature impact strength of the sheet was measured at -10F using the Dart Drop Method of ASTM D-1709. Drop weights for 50% failure of the sample were determined. The samples were extruded using conditions simi-lar to those for Example 1 using a casting roll temperature of 350F which produced crystallinity levels in the PET fraction of the sheet of about 35 to 40~.
Table II
F50 Failure Level Weight Ratio PET:PC (Grams) 90:10 490 80:20 565 70:30 765 60:40 1100 50:50 >1750 The results in Table II indicate acceptable impact resistance forall of the samples except the 90:10 sample.
Example 4 ~ ilm of 7 mil thickness was extruded from two P~T resins, resin A of intrinsic viscosity 0.85 and resin B of intrinsic visco-sity 0.95. The resins were melted and extruded on a one inch diame~er extruder through a flat die maintained at 510F. The molten film i~uing from the die was cooled on a rotating steel roll main~ained at 200P. The result~ in Table III ~how the improved ~en~ile ~trength, yleld ~trength and increased ultimate elongation obtained wlth re~in B.
1~55184 Table III
-Resin A Resin B
I.V. 0.85 0.95 Tensile Strength (lbs./in.2) M.D.(l) 6500 ll,000 T.D. * 7/900 Yield Strength (lbs./in.2) .D. 5000 8,300 T.D. * 6,900 -Ultimate Elongation (~) M.D. 300 400 T.D. less than 10 100 (1) M.D. = ~achine Direction T.D. = Transverse Direction *sample snaps as stretching begins The above results demonstrate that films produced from a PET resin having an i.v. of 0.95 has a greater toughness than films produced from a resin having an i.v. of 0.85.
EXAMPLE S
__ _ A blend of 67 parts by weight of PET having an in-trinsic viscosity of 0.95, 19 parts by weight polycarbonate having an intrinsic visco ity of 0.56 (measured in dioxane at 30C), 10 part~ by weight of novaculite (average particle size o~ 5.5 microns) and 4 part~ of a 50:50 titanium dioxide-polycarbonate master batch (the polycar~onate also having an intrinsic vi~cosity of 0.56) was prepared and dried for 3 hours at 250F and then drum tumbled immediately prior to extru~ion. The blend was extruded from a single 2-1/2 inch diameter screw extruder through a 30 inch wide flat sheeting die into a ~teel casting roll maintained at 350F.
The speed of the roll was controlled to provide a sheet of 17 mil thicknes~.
; The physical propertie~ of the sheet were measured and ~re reported in Table IV under Bample 1. A comparatlve 17 mil ~heet ~a~ or~ in the same m~nner except that novaculi~ waa not pr~ent --13~
~ 1~55184 in the blend. Its physical properties are reported in Tabl IV
under Sample 2.
TABLE IV
Property Sample 1 Sample 2 Tensile strength, psi 7000-8000 8000-9500 (machine direction) Tensile modulus, psi 283,000-288,000 250,000-260,000 (machine direction) Oxygen Permeability at 25C, cc. 2/24 hrs/atm. 0.6 1.4 Water Vapor Transmission, 0.23 0.23 g/100 in.2/24 hrs.
at 100F, 90% RH
Shrinkage at 400F, 1.0 2.0 min.~% machine direction Trays were thermoformed from sheet Samples 1 and 2 using a commercial thermoforming machine, with a 4 second heating time and a 3 second cooling time. The trays measured 5-1/2 x 4 x 1 inch deep - and had a 1/2 inch rim around the top.
The trays were tested for recistance to distortion and shrinkage in an oven as follow~: Three fish ~tick~ measuring 3 3/4~ by 3/4" diameter were placed in each tray. The trays were heated in a 350P oven for 1/2 hour and allowed to cool to 75 - 80P. The volume of the tray before the oven test was determined by measuring the volume of water required to fill the tray ju~t to its rim. Similarly the volume of trays after heating at 350P was determined and the change in volume calculated.
Additional tests were made on trays formed from Samples 1 and 2 and heated to 375P and 400P for 1/2 hour. The re~ults are shown in Table 2.
TABL~ V
Sam~le Volume Reduction 1 5.5 7 10 2 12.4 12.~ 13.-~
As can be seen from Table IV, the addition of 10% by weight novaculite does not significantly adversely affect the physical properties of the sheet and in some cases improved properties are noticed. Table V, however, demonstrates a significant and unexpected decrease in the volume reduction in trays when 10% novaculite is incorporated into the sheet.
At 350F, which is a usual cooking temperature, the volume reduction was decreased by over 50~ (from 12.4 to 5.5%) and large decreases were also measured at 375 and 400F oven temperatures. Accordingly, trays incorporating the present fillers exhibit a much superior appearance with a greatly reduced distortion over trays not incorporating such fillers.
The distortion of the comparative trays is particularly noticed in the rim area.
It is to be understood that variations and modifica-tions of the present invention may be made without departing from the ~cope of the invention. It is also understood that the invention is not to be interpreted as limited to the spe-cific embodiment disclosed herein, but only in accordance with the appended claims when read in light of the foregoing disclosure.
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