Transparent sheets and containers formed from polycarbonate-polyester blends and formation thereof

A sheet formed from a uniform blend of from about 80 to 97% by weight of polyethylene terephthalate having an intrinsic viscosity of above about 0.9 and a melt viscosity at 525.degree. F. of above about 10,000 poises and correspondingly from about 20 to 3% by weight of a polycarbonate resin having an intrinsic viscosity of about 0.4 to 0.6 and a melt viscosity at 500.degree. F. 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. Also disclosed is a process for forming a sheet which comprises uniformly blending the composition, extruding the composition into a sheet at a temperature between about 490.degree. to 530.degree. F. and rapidly cooling the sheet by contact with at least one cooling surface maintained at a surface temperature in the range of about 50.degree. to 160.degree. F. for a period of time not exceeding about 15 seconds. Containers may be formed from such sheets by a thermoforming step at temperatures in the range of about 210.degree. to 280.degree. F.

BACKGROUND OF THE INVENTION 
1. 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. 
2. 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 degree 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 temperature of about 150.degree. to 
180.degree. F. or greater which facilitates rapid filling of viscous 
products as well as destroying bacteria. On the other hand, 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 may be 
modified by the addition of polycarbonates as is described in U.S. Pat. 
No. 3,218,372 to Okamura et al issued in 1965, in order to increase the 
hardness, strength and electrical properties of the molding material, such 
mixtures introduce additional problems. For example, polycarbonate resins 
employed herein are sensitive to decomposition at extrusion temperatures 
in the presence of other polymeric materials, such as PET, and tend to 
form bubbles of gas which are believed to be mainly carbon dioxide. The 
presence of these bubbles destroy the value of the sheet for thermally 
formed containers since holes develop and the optical properties are 
diminished. In addition, no prior process is known to the inventors which 
permits the extrusion of 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. Pat. No. 3,956,229 (1976) there is described film and sheet 
formed from blends of 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 40%, is essentially non-oriented and may be thermoformed into 
cook-in-trays and like articles. The film or sheet disclosed therein is 
formed by blending the polymers, extruding the blend at a temperature 
above about 500.degree. F. onto a moving support and cooling the support 
to a surface temperature of about 225.degree. to 380.degree. F. Although 
such film and sheet have 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. Pat. No. 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 utilization in hot-filled applications, wherein a clear and 
transparent sheet is required. 
SUMMARY OF THE INVENTION 
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 525.degree. F. of above about 10,000 
poises and correspondingly 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 500.degree. F. 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.degree. to 280.degree. F. 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% PET 
having an intrinsic viscosity of above about 0.9 and a melt viscosity at 
525.degree. 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 500.degree. F. of less than about 50,000 poises, 
extruding said blend at a temperature between about 490.degree. to 
530.degree. F. 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.degree. to 160.degree. F. for a period of time not 
exceeding 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 temperatures and cooling rate. The PET 
resins employed herein impart an increased deformation resistance to 
containers formed from the blended sheet as well as improved uniformity of 
optical properties. Moreover, in order to avoid formation of gas bubbles 
due to polymer decomposition, the blend must be extruded at temperatures 
below about 530.degree. F. and above about 490.degree. F. In addition, the 
temperatures utilized to form containers from the sheet must be in the 
range of about 210.degree. to about 280.degree. F.; 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 PREFERRED EMBODIMENT 
The polyethylene terephthalate (hereinafter "PET") 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 25.degree. C. Preferably, the 
intrinsic viscosity is in the range of about 0.9 to 1.2, more preferably 
about 0.9 to 1.0. The PET resin has a melt viscosity measured at 
525.degree. F. of above about 10,000 poises, preferably between about 
10,000 to 30,000 poises. The polycarbonate resin employed herein may be 
any polycarbonate, such as the reaction product of 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 30.degree. C. 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 500.degree. F. 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 viscosity 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 containers 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. Any suitable 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 
moisture levels may result in rapid hydrolytic decomposition of both 
resins. Such decomposition introduces further problems in 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.degree. to 530.degree. F. As 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 preferably 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 
temperatures 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.degree. to 160.degree. F., 
and preferably in the range of about 80.degree. to 120.degree. F. The 
sheet is in contact with the cooling surfaces for a period of time not 
exceeding about 15 seconds, preferably not exceeding 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 75 feet per minute. On the other 
hand, for sheet of 15 mil thickness (other parameters being the same), the 
contact time may be in the range of about 1.5 to 7.5 seconds, for example. 
Preferably, the sheet is extruded directly into a stack of 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 
conventional type. For instance, chromium plated rolls provided with 
necessary 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 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 
degree of crystallinity of less than about 5%. The crystallinity referred 
to is that obtained by the density method as described in "Engineering 
Design for Plastic", E. Baer, Reinhold Publishing Company, 1964, pages 
98-99. 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.degree. to 
180.degree. F. 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 525.degree. F. of about 13,000 
poises. A 31/2 inch extruder was used with extrusion temperatures 
maintained in the range of about 495.degree. to 520.degree. F. The molten 
polymer was passed through a 34 inch wide slit die which was located about 
2 inches from the nip formed by a pair of rotating water chilled rolls 
which were maintained at surface temperatures in the range of 85.degree. 
to 115.degree. F. The sheet was cast upon such rolls and was then further 
cooled with a roll held to a surface temperature of 65.degree. F. and 
thereafter collected on a winding roll. Small cups measuring 13/4 by 11/4 
by 1/2 inches were thermoformed from the PET sheet at temperatures at the 
range of 240 to 250.degree. F. and thereafter filled with hot jelly which 
was at a temperature of 175.degree. F. 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 
525.degree. F.) 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 ml thickness was extruded from a blend of 0.95 intrinsic 
viscosity PET resin and three different types of poly(4,4'-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., 75.degree. F.) temperature 
and dried at 250.degree. F. prior to being passed directly into the 
extruder. A 31/2 inch extruder was used with barrel temperatures of 
490.degree. to 550.degree. F. and die temperatures of 500.degree. to 
540.degree. F. 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 100.degree. F. The sheet was 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 the 
viscosity characteristics of the polycarbonate resins tested. 
TABLE1 
______________________________________ 
Type A B C 
______________________________________ 
Intrinsic Viscosity 
0.45 0.51 0.57 
Melt Viscosity, Poises 
at 500.degree. F. 
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 B 
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 extruder 
temperatures were closely held below about 530.degree. F., the sheet 
contained bubbles of gas and was unacceptable for thermoforming of 
containers, regardless of the uniformity of transparency. 
EXAMPLE 3 
Additional samples of sheet were prepared from several blends of 0.95 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 amorphous 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. 
TABLE 2 
______________________________________ 
Sample 1 2 3 4 5 6 7 
______________________________________ 
Polycarbonate Type 
A A A C C C -- 
% Polycarbonate 
5 7 10 10 20 30 0 
% haze &lt;2 &lt;2 &lt;2 &lt;2 &lt;2 &gt;5 &lt;2 
Uniformity of 
Optical Properties 
VG VG VG F M M VG 
Vicat Distortion 
3.5- -- -- 5- 2.3- -- 5.0- 
mm at 190.degree. of F. 
4.5 5.5 3.5 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 3.times.3 inch sample of 
the sheet in a frame. The sample was immersed in a glycerine bath which 
was heated at a rate of 2.degree. C. per minute. A vertically mounted 
steel rod free to move in a supporting collar and having a 1/8 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 level was significantly 
diminished at 190.degree. F. when 5 to 20% polycarbonate was present 
compared to sheet formed from 100% PET and cooled using chilled rollers 
maintained at 75.degree. to 160.degree. F. 
EXAMPLE 4 
Sheet of 10 mil thickness containing 5 and 7% polycarbonate resin (Type A 
of Table 1) was produced under conditions of Example 2. Containers 
8.times.6.times.11/2 inches were formed on a Thermtrol pressure-type 
thermoforming machine in which the sheet was heated to temperatures in the 
range of to 210.degree. to 280.degree. F. 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 270.degree. F. were required with the 5% blend 
while at longer heating times, temperatures as low as 210.degree. F. 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.degree. to 280.degree. F. No 
distortion was seen in containers which were heated to 170.degree. to 
190.degree. F. 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 175.degree. F. There 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.degree. to 280.degree. F. 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 
until 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 
______________________________________ 
Poly- 
car- 
bon- I.V. of Maximum Temperature .degree.F. 
Sample No. 
ate % PET Resin Method A 
Method B 
______________________________________ 
1 0 0.7 145 134 
2 0 0.95 151 138 
3 5 " 158 145 
4 7 " 161 147 
5 10.sup.(a) 
" 164 151 
6 20.sup.(b) 
" 164 151 
______________________________________ 
.sup.(a) some haze developed on forming the container 
.sup.(b) considerable haze developed on forming the container 
It can be seen that the maximum distortion temperatures for the 5% 
polycarbonate sample was 7.degree. F. 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 tthat 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. 
EXAMPLE 7 
Example 2 was followed except that the sheet had a thickness 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 500.degree. F. of 
18,000 cps. The resins were predried to a moisture content below 0.01%. 
The extrusion temperature was about 570.degree. F. 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 80.degree. F. 
Cheese and cracker containers were thermoformed from the sheet using a 
commercial double sided contact heating thermoformer at a temperature at 
310.degree. F. for about 1/3 second contact time. The containers included 
two compartments, the cheese compartment measuring 1 inch deep.times.11/2 
inches wide.times.1-5/16 inches long and the cracker compartment measuring 
1 inch deep.times.13/8 inches wide.times.23/4 inches long. The containers 
were filled with melted cheese at 163.degree. F. and crackers and a 2 mil 
thick overlayer of a biaxially oriented PET sheet provided with a heat 
seal layer was placed on top of the containers. 
No distortion of the containers was observed. In comparison, in similar 
containers which were made from a sheet containing 100% of the PET, 
unacceptable distortion at the corners of the containers was observed 
after 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 to 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.