Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
lOS~84
FILMS AND SHEETS OF POLY~S~I'OL"CAKIW~A~ DLII~105
BACKGROUND OF THE INVENTION
.
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
1055184
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
-3-
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
--4--
~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$ght
. -5-
1055184
polycarbonate and about 5 to 15 parts by weight silica. Blends
containing less than about 15 parts by weight polycarbonate result
in films and sheets which have inadequate impact and fracture
resistance. Blends containing less than about 60 parts by weight
PET have relatively poor heat resistance and poor thermoformability
as well as present problems in obtaining uniform optical properties.
Blends containing less than about 5 parts by weight silica do not
exhibit improved heat distortion resistance whereas blends of more
than about 20 parts by weight silica result in brittle films and
sheets and poor uniformity.
The two polymers may be blended together using any
conventional blending apparatus. They may be blended in the
solid or melted state; preferably the PET and polycarbonate
are dry blended in pellet form at about room temperature. The
duration of the mixing is primarily dependent upon the desired
degree of blend uniformity. Blending may be performed prior to
or in the extruder. The silica may be added in any conventional
manner and may suitably be added to either ~he PBT or polycarbonate
separately or after the polymers are blended.
Conventional additives may be added during the mixing
operation. Buch additives include pigments, such as titanium
dioxide and carbon black, stabilizers, other fillers such as glasses,
carbonates and alumina and reinforcing agents such as glass fibers.
The amount of such additi~es may of course vary; it has been found,
for example, that a composition including about 1 to 5~ by weight
of titanium dioxlde may be used to form a very aesthetically
attractive cook-in-tray.
In one preferred embodiment, the silica flller together
with optional additlves may be blended into one of ~he polymers,
preferably the polycarbonate, and then blended w$th the other
polymer which may contaln the remaindsr, 1 any, of the flller
--6--
1055184
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
--8--
1055184
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_
1055184
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~
-10-
1055184
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
-11-
1055184
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.
-12-
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
350P 375P 400P
1 5.5 7 10
2 12.4 12.~ 13.-~
-14-
1055184
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.
.,
-15-
