Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
TRANSPARENT S~EETS AND CONTAINERS FORMED
FROM POLYCARBONATE - POLYESTER BLENDS
AND FORMATION THE~EOF
BACKGROUND OF THE INVENTION
_
Field of the Invention
_ . .
This invention relates to transparent sheets formed
from blends of polycarbonate and polyethylene terephthalate
resins, a process for their production and containers formed
therefrom.
Discussion of the Prior Art
=
Polyethylene terephthalate (sometimes referred to
as "PET") resins can be employed to prepare transparent film
and sheet. Usually the resin is extruded into an amorphous
flat sheet, which is then biaxially stretched and thereafter
heat set to impart a desired de~ree of crystallization to the
sheet. Such biaxially oriented and crystallized products are
strong and clear but cannot readily be formed into containers
since the process of biaxially stretching removes most of the
extensibility of the sheet~ If amorphous PET sheet is
produced by rapid cooling of the molten sheet, a clear and
transparent product may be obtained which is formable into
containers. However, these containers soften at too low a
temperature to permit their use in hot-filled food packaging
applications where the filling may typically be at a temper-
ature of about 150 to 180F. or greater which facilitates rapid
filling of viscous products as well as destroying bacteria. On
the other hand t if PET sheet is produced by slow cooling of the
molten resin, the product obtained is partially crystallized,
milky and brittle and hence unsuited for container fabrication.
Although it is known that lower intrinsic viscosity
PET resins ma~ be modi~ied by the addition of polycarbonates as is
described in USP 3,218,372 to Okamura et al issued in :L965, in
order to increase the hardness, strength and electrica.L properties
~~
~f the molding material, such mixtures introduce additional prob-
lems. For example, polycarbonate resins employed herein are
s~nsitive to decomposition at extrusion temperatures in the pre-
sence of other polymeric materials, such as E~ET, and tend to form
bubbles of gas which are believed to be main]y carbon dioxide~
The presence of these bubbles destroy the va]ue of the sheet for
thermally ~ormed containers since holes develop and the optical
properties are diminished. In addition, no prior process is known
to the inventors which permits the extrusion oE such blends into
highly clear sheets having uniform transparency and low haze. This
additional problem evidently arises from the wide dissimilarity of
flow characteristics between the two resins of the Okamura et al.
patent so that intimate mixing to obtain the very high degree of
uniformity needed in transparent sheet is very difficult to achieve
and in practice non-uniformities of various types such as localized
surface roughness, flow streaks and other defects become readily
evident. It would be desirable if such defects of PET and PET-
polycarbonate blends were overcome to provide a practical process
for extruding high clarity sheets which permits hot-filling to be
used when clear containers are made from the sheet.
In our U.S.P. 3,956,229 (1976) there is described film
and sheet formed from blends o~ 60 to 85 parts of PET having an
intrinsic viscosity of at least about 0.90 and 40 to 15 parts of
a polycarbonate resin. Such film or sheet, which has a degree of
crystallinity in the range of about 20 to ~0%, is essentially non-
oriented and may be thermoformed into cook-in-trays and like
articles. The film or sheet disclosed therein is formed by blend-
ing the polymers, extruding the blend at a temperature above about
500F~ onto a moving support and cooling the support to a surface
temperature of about 225 to 380F~ Although such film and sheet
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.ave requisite strength and toughness to be utilized for cook in-
tray applications, such sheets have a very high degree of haziness
and consequently would not be suitable for applications wherein
a clear sheet is desired. In U.S.P. 3,975,355 (1976) of two of
the present inventors (Bollen and Amin), there are described film
or sheet similar to that of our copending application but which
also includes about 5 to 20 parts by weight of a non-acidic silica
filler, such as novaculite. However, such film or sheet likewise
has a degree of haziness which precludes its utili~ation in hot-
filled applications, wherein a clear and transparent sheet is
required.
SUMMARq OF HE IN~ENTION
In accordance with this invention, there is provided a
sheet suitable for use in hot filling of foods and being formed
from a uniform blend of from about 80 to 97~ by weight of PET
having an intrinsic viscosity of above about 0.9 and a melt
viscosity at 525F. of above about 10,000 poises and correspond-
ingly from about 20 to 3% by weight of a polycarbonate resin
having an intrinsic viscosity of about 0.4 to 0.6 and melt
viscosity at 500F. of less than 50,000 poises; said sheet having
a haze value as determined by ASTM D-1003 of less than about
2% and being essentially amorphous and non-oriented. There is
also provided containers which may be thermoformed from such
sheet at temperatures in the range of about 210 to 280F.
Further in accordance with this invention, there is provided a
process of forming such sheet which comprises uniformly blending
a composition of about 80 to 97% ~ET having an intrinsic
viscosity of above about 0.9 and a melt viscosity at 525~F. of
above about 10,000 poises with about 20 to 3% by weight of a
polycarbonate resin having an intrinsic viscosity of 0.4 to 0.6
and a melt viscosity at 500F. of less than about 50,000 poises,
extruding said blend at a temperature between about ~90 to 530F.
whereby a sheet is formed and cooling said sheet by contact with
at least one cooling surface maintained at a surface temperature
in the range of about 50 to 160F. Eor a period of time not ex-
ceeding about 15 seconds~ whereby an essentially amorphous and
non-oriented sheet is obtained.
It has been found that the selection of the PET and
polycarbonate resins are critical as is the extrusion tempera-
tures and cooling rate. The PET resins employed herein impart anincreased deformation resistance to containers formed from the
blended sheet as well as improved uniformity of optical proper-
ties. Moreover, in order to avoid formation of gas bubbles due
to polymer decomposition, the blend must be extruded at tempera-
tures below about 530F. and above about 490F. In addition, the
temperatures utilized to form containers from the sheet must be
in the range of about 210 to about 280F.; the lower limit relates
to the inability to form containers of precise dimensions while
above the upper limit, the containers become excessably hazy and
loose transparency.
DESCRIPTION OF THE PREFER~ED EMBQDIMENT
The polyethylene terephthalate (hereinafter "~ET')
employed herein is a polymer having an intrinsic viscosity of
at least 0.90, the intrinsic viscosity being measured in a
mixed solvent of 60 parts by weight phenol and 40 parts by
weight tetrachloroethane at 25C. Preferably, the intrinsic
viscosity is in the range of about 0.9 to 1.2, more preferably
about 0.9 to 1Ø The PET resin has a melt viscosity measured
at 525F. of above about 10,000 poises, preferably between about
10,000 to 30,000 poises. The polycarbonate resin employed
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~ erein may be any polycarbonate, such as the reaction product oE
phosgene or a carbonic acid diester, such as diphenol carbonate,
with bisphenol A, i.e., poly (4, 4'-isopropylidene diphenylene
carbonate). The polycarbonate has an intrinsic viscosity in
the range of about 0.4 to 0.6 as measured in dioxane solvent at
30C. Preferably, the polycarbonate has an intrinsic viscosity
in the range of about 0.4 to 0.5. The polycarbonate resin has
a melt viscosity at 500F. of less than 50~000 poises and
preferably less than about 30,000 poises; most preferably, the
polycarbonate has a melt viscosity of about 5,000 to 30,000
poises~ The intrinsic viscosity and melt ~iscosity referred to
herein are the viscosities measured before blending the two
polymers.
As referred to above, blends of from about 80 to 97% by
weight of PET and correspondingly from about 20 to 3% by weight
of polycarbonate are employed herein. It has been found that
below about 3~ by weight polycarbonate, serious distortion of con-
tainers thermoformed therefrom is evidenced during hot filling
with food whereas above about 20% by weight polycarbonate, the
containers are no longer transparent. Preferably, the amount
of polycarbonate in the blend ranges from about 5 to 10~ by
weight. It is preferred to physically blend the two resins in
pellet or powder form at about ambient temperatures. ~ny suit-
able mixing equipment may be employed which provide a uniform
blend, such as drum tumblers, ribbon blenders and the like.
It has been found that if the polymers are blended in their
melted state as suggested in the aforesaid Okamura et al.
patent, degradation of the polycarbonate resin occurs which
results in gas bubbles being formed in the sheet. It has
also been found desirable to dry the mixture to a moisture
level below about 0.02~ by weight water since high
--5~
.~loisture levels may result in rapid hydrolytic decomposition of
both resins. Such decomposition introduces further problems
i~ obtaining uniform mixing of the resins as well as results in
the formation of undesirable gas bubbles.
The blended mixture is thereafter extruded into a sheet
at temperatures in the range of about 490 to 530F. ~s used
herein, the term "sheet" is intended to mean thin cast, extruded
or otherwise formed products which have a thickness up to about
50 mils or more and preferably about 5 to 25 mils and most prefer-
ably about 10 to 20 mils. As such, the term '~sheet" includes"films" (i.e., structures having thickness of below 10 mils~ and
"sheets" (i.e., structures having thickness above 10 mils) as both
terms are used in the plastic film industry. The extrusion tem-
peratures refer to temperatures in the extruder die. Any suitable
melt extrusion apparatus can be employed to extrude the sheet.
The sheet is extruded through the extruder onto one or
more cooling surfaces, preferably rotating or moving support(s~,
which are cooled to a surface temperature in the range of about 50
to 160F., and preferably in the range of about 80 to 120~. The
; 20 sheet is in contact with the cooling sur~aces for a period of time
not exceeding about 15 seconds, preferably not exceedlng about 10
seconds, in order to cool the sheet into an essentially amorphous
structure. The minimum contact time is that sufficient to cool
the sheet and may be in the range of about 0.04 seconds and is
preferably in the range of about 1 second. The contact time is
dependent upon the thickness and width of the sheet, the speed
of the sheet and the temperature and size of the cooling surface.
For example, using a three 16 inch diameter roll system with an
S-wrap (described below), the contact time may be in the range
of about 3 to 15 seconds for sheet of 25 mil thickness and 45
inches width and with a sheet speed in the range of about 15 to
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; feet per minute. On the other hand, for sheet of 15 mi:L thick-
ness (other parameters being the same~, the contact time may be
in the range of about 1.5 to 7.5 seconds, for example.
Preerably, the sheet is extruded directly into a stack oE
three chill rolls rotating at substantially the same speed. For
example, the blend may be charged to a screw extruder wherein
the blend is melted and additional mixing occurs and the sheet
exists through a flat die head into the nip formed by a pair of
rotating casting or cooling rolls which may be of any conven-
tional type. For instance, chromium plated rolls provided withnecessary internal cooling means ~water or organic solvent) may
be employed. The sheet is carries over a generally S-shape
over the bottom of the two rollers that form the nip and thence
around a third roller in contact with the second roller. The
third roller serves to further cool down the sheet. As is well
understood by those skilled in the art, the rate of extrusion,
the width of the extruder die orifice and the speed of the
casting rolls may be varied widely and determine the thickness of
the sheet. Alternatively, the sheet may be cast directly onto
a single casting roll provided with cooling means or between the
nip of a pair of cooling rolls rotating at substantially the same
speed and without utilizing a third roll in contact therewith.
In any case, following extrusion, the sheet may be further cooled
down prior to collecting the same by passing the sheet over one
or more 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 rolls move
or rotate at substantially at the same linear speed as the
casting rolls so that the sheet is not subjected to a drawing or
stretching operation which would orient the same. The sheet
is collected utilizing conventional apparatus such as a
--7--
winding roll or the like.
The sheet of this invention is essentially non-oriented,
that is, has a machine direction minimum elongation at break of
at least about 200~, preferably at least about 300~. The sheet is
essentially amorphous, that is, the PET portion of the sheet is
essentially non-crystallized and has a de~ree of crystallinity of
less than about 5%. The crystallinity referred to is that obtained
by the density method as described in "Engineering Desiyn for
Plastic", E. Baer, Reinhold Publishing Company, 1964, pages 9~-99.
The sheet has a very low haze level (as determined by ASTM D-1003)
of less than about 2%, preferably less than about 1~ and has
excellent uniformity of transparency.
It has been found that the sheet produced in accordance
with this invention is eminently suitable for forming high clarity
containers useful in hot-filling applications such as packaging of
jellies, syrups, sauces and other food products which are heated
in the range of about 150 to 180F. or higher during the filling
operation. Such containers evidence little if any distortion
during filling and retain their high clarity.
In order to further describe the present invention,
the following non-limiting examples are given.
Example 1 (Comparative)
A sheet of 10 mil thickness was prepared from PET
resin having an intrinsic viscosity of 0.95 and a melt viscosity
of 525F. of about 13,000 poises. A 3 1/2 inch extruder was
used with extrusion temperatures maintained in the range of
about 495 to 520F. The molten polymer was passed through a
34 inch wide slit die which was located about 2 inches
~rom the nip formed by a pair of rotating water chilled rolls
which were maintained at surface temperatures in the range of
`:
d5 to 115F. The sheet was cast upon such rolls and was then
further cooled with a roll held to a surface temperature of
65F. and thereafter collected on a winding roll. Small cups
measuring 1-3/4 by 1-1/4 by 1/2 inches were thermofor~ed from the
PET sheet at temperatures at the range of 240 to 250F. and
thereafter filled with hot jelly which was at a temperature of
175F. and an aluminum foil cover was adhered to the container.
After cooling, the cups were visually examined and found to be
unacceptably distorted. That is, there was substantial shrinkage
in localized areas of the cup so that the jelly was forced out
over the lip of the cup. Similar tests using PET sheets produced
from resin of 0.7 intrinsic viscosity (melt viscosity of about
5,000 poises at 525F.) were also conducted and the extent of
the distortion developed after hot filling was even greater than
that for the sheet produced from the higher viscosity resin.
Example_2
Sheet of about 10 mil thickness was extruded from a
blend of 0.95 intrinsic viscosity PET resin and three different
types of poly(4,4l-isopropylidene diphenylene carbonate) resins
having intrinsic viscosities ranging from 0.45 to 0.57 as shown
in Table 1. Blends were produced at polycarbonate percentages of
10% by weight and the two resins were physically mixed in pellet
form in a drum tumbler at ambient (i.e., 75F.) temperature
and dried at 250F. prior to being passed directly into the
extruder. A 3 1/2 inch extruder was used with barrel temperatures
of 490 to 550F. and die temperatures of 500 to 540F. The
extruder screw was operated at 32 to 34 rpm and dies of 34 inches
and 43 inches in width were used. The die head was located about
2 inches from the nip formed by a pair of chromium plated rolls
of 16 inch diameter of a three stack roll which were internally
water cooled to surface temperatures of 100F. The sheet was
_g_
passed over the second roll and then around a third cooling roll
of similar construction which was maintained at the same surface
temperature. The contact time of the sheet against the three
rolls was about 3.5 seconds for the narrower sheet and about 4.5
seconds for the wider sheet. Table 1 lists t:he viscosity
characteristics of the polycarbonate resins tested.
TABLE 1
Type A B C
Intrinsic Viscosity 0.45 0.51 0.57
Melt Viscosity, Poises
at 500F. 18,000 27,000 44,000
Under similar mixing conditions in the extruder,
type C resin tended to provide sheets having the least uniformity
of optical properties. Type ~ resin was somewhat improved over
type C, whereas type A gave exceptionally uniform clarity and
transparency. In each case, the sheet was essentially amorphous
as indicated by density measurements (a crystallity level of less
than about 5%). It was further discovered that unless the extru-
der temperatures were closely held below about 530F., the sheet
contained bubbles of gas and was unacceptable for thermoforming
of containers, regardless of the uniformity of transparency.
Additional samples of sheet were prepared from several
blends of ~.95 I.V~ PET, and 5 to 20% of polycarbonate resins
(types A and C from Table 1). The sheets (10 mils) were extruded
under the conditions of Example 2 and were all essentially amor-
phous as indicated by density measurements (crystallinity of less
than about 5%). Table 2 gives the results of the tests on optical
properties and distortion of resistance as measured by a Vicat
type test.
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~6~2~
\
TABLE 2
Sample 1 2 3 4 5 6 7
Polycarbonate Type A A A C C C
% Polycarbonate 5 7 10 lO 20 30 0
~ Haze <2 <2 <2<2 <2 >5 <2
Uniformity of
Optical Properties VG VG VG F M M VG
Vicat Distortion,
mm at 190F. 3.5-4.5 - - 5-5.5 2.3-3.5 - 5.8-6.0
In the above Table, the Vicat test is a distortion test
showing the relative softness of the sheet and measures the probe
penetration level. A modified Vicat test was performed by mounting
a 3x3 inch sample of the sheet in a frame. The sample was immersed
in a glycerine bath which was heated at a rate of 2C per minute.
A vertically mounted steel rod free to move in a supporting collar
and having a l/8 inch diameter tip was placed against the sheet,
the rod being weighted to a total weight of about 390 grams. As
the heating progressed, the depth of penetration of the rod into
the sheet was measured using a cathetometer. Under uniformity of
optical properties, VG means very good, F means fair and M marginal.
This was determined visually using as a comparison standard a sheet
extruded from 100% polyester resin which was rated VG (Sample 7).
As can be seen from Table 2, levels of polycarbonate
greater than about 20% gave higher haze content, whereas the
distortion leveI was signifanctly diminished at l90~F. when 5 to
20% polycarbonate was present compared to sheet formed from lO0
PET and cooled using chilled rollers maintained at 75 to 160F.
Example 4
Sheet of 10 mil thickness containing 5 and 7~ poly-
carbonate resin (Type A of Table l) was produced under conditions
of Example 2. Containers 8x6xl-l/2 inches were formed on a
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~hermtrol pressure-type thermoforming machine in which the sheet
was heated to temperatures in the range of to 210 to 280F. using
the heating of 1/4 second, 1/2 second and 1 second dwell times.
With 1/4 second heating time, temperatures of at least 270F.
were required with the 5% blend while at longer heating times,
temperatures as low as 210F. could be used to produce highly
transparent trays. With the 7% blend at heating times of 1/2
second to 1 second, containers with best optical properties
as judged visually were obtained at 270 to 280F. No distortion
was seen in containers which were heated to 170 to 190F. in
water.
Example 5
Containers formed in a manner similar to that of
Example 4 from the sheet of Sample 4 of Table 2 were hot filled
with jelly which was at a temperature of 175F. ~here was no
visual distortion or spillover of the jelly after the containers
were cooled.
Example 6
Samples of amorphous sheet were prepared from PET resins
having intrinsic viscosities of about 0.7 and 0.95 (samples 1 & 2)
and from mixtures of PET and polycarbonate containing 5%, 7%, 10%,
and 20%, by weight (Samples 3,4,5, & 6 respectively). Containers
of about 150 ml~ in volume were made by thermoforming these samples
on a Thermtrol pressure former at temperatures of 195 to 280F.
using a 3 second cycle which included 1 second heating, 1 second
cooling and 1 second cutting. The containers were tested for
heat resistance by the two following methods.
Method A-Hot Fill Test ~ the containers were filled with hot
water at various temperatures allowing 10 minutes after filling
before the samples were examined for any evidence of shrinkage
or distortion. The water temperature was steadily raised
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Intil the temperature at which visible distortion or shrinkage
of the container took place was reached. Table 3 lists these
maximum temperatures for the various samples.
Method B-Water Inversion Test - the thermoformed containers
were immersed in water at various temperatures with 10 minute
exposure each temperature. The temperature was steadily
increased until the maximum temperature at which the container
resisted distortion or shrinkage was determined. Table 3
lists the maximum temperatures.
TABLE 3
I.V. of Maximum Temperature ~F
Sample No. Poly_arbonate (%) PET Resin Method A Method B
_
1 0 0.7 145 13~
2 0 0.95 151 138
3 5 " 158 145
4 7 " 161 147
1O(a) ~I 164 151
6 20~b) ., 164 151
(a) some haze developed on forming the container
(b~ considerable haze developed on forming the container
It can be seen that the maximum distortion temperatures
for the 5~ polycarbonate sample was 7F higher under both tests
than a 100~ PET sheet. As the percent polycarbonate increased
to 10%, an increase in the distortion temperature occured,
although some haze was present in containers thermoformed from
such sheet. No increase in maximum distortion temperature was
noted with the 20% polycarbonate sample as opposed to the 10%
sample. Table 3 also shows that the use of a PET resin having
an intrinsic viscosity of 0.95 has improved maximum distortion
temperatures over a PET resin having an intrinsic viscosity of
0.7
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Example 2 was followed except that the sheet had a thick-
ness of about 16 mil and contained 3% by weight of a polycarbonate
resin having an intrinsic viscosity of 0.49 and a melt viscosity
at 500F. of 1~,000 cps. The resins were predried to a moisture
content below 0.01%. The extrusion temperature was about 570F.
and an extrusion die of 47 inches in width was used with the die
head located about 3 inches from the nip. The third cooling roll
was maintained at a surface temperature of about 80F.
Cheese and cracker containers were thermoformed from the
sheet using a commercial double sided contact heating thermo$ormer
at a temperature at 310F. for about 1/3 second contact time. The
containers included two compartments, the cheese compartment
measuring 1 inch deep x 1-1/2 inches wide x 1-5/16 inches long and
the cracker compartment measuring 1 inch deep x 1-3/8 inches
wide x 2-3/~ inches long. The containers were ~illed with melted
cheese at 163F. and crackers and a 2 mil thick overlayer of a
biaxially oriented PET sheet provided with a heat seal layer was
placed on top o the containers.
No distortion of the containers was observed. In com-
parison, in similar containers which were made from a sheet
containing 100~ of the PET, unacceptable distortion at the corners
of the containers was observed a~ter filling.
It will be understood that variations and modifications
of the present invention may be made without departing from
the scope of the invention. It is also to be understood that
invention is not io interpreted as limited to the specific
embodiment disclosed herein, but only in accordance with the
appended claims when read in light of the foregoing disclosure.
