Low orientation thermoplastic sheet products and processes

Inherently low orientation sheet is produced by extruding a thermoplastic material in its melt phase onto the surface of a chill roller and thereafter solidifying the sheet at a relatively slow chill rate. Importantly, the just extruded thermoplastic polymeric sheet (i.e., the melt) is urged into continuous surface-to-surface contact with the chill roll by a curtain of air impinging upon the sheet at a velocity sufficient to ensure such continuous surface-to-surface contact between the sheet and the chill roll, but insufficient to impart substantial stress to the sheet. In such a manner, therefore, the resulting sheet will exhibit a low inherent internal stress level as evidenced by an unusually low shrink rate (ASTM D-1204) of less than about 18%. The sheets of the present invention are thus especially characterized by such unusually low shrink rates, and hence unusually low inherent internal stress levels which find particular utility as containers for the packaging industry--especially containers produced by melt-phase deep-draw thermoforming techniques.

FIELD OF THE INVENTION 
The present invention relates generally to the manufacture of low 
orientation sheets, and to the sheet products formed thereby. In preferred 
forms, the present invention is embodied in sheet products having at least 
one layer formed of a low orientation thermoplastic material. The sheet 
products of the invention may thus be in the form of monolayer or 
multilayer sheets. The low orientation sheet products of the present 
invention find particular utility as containers, for example, in the 
packaging industry. 
BACKGROUND AND SUMMARY OF THE INVENTION 
Monolayer and multilayer coextruded thermoplastic sheet products which 
provide both gas and moisture barriers and which are thereby useful as 
containers in the packaging industry are well known. Conventional 
processes for producing containers from such sheets generally include 
extruding sheets of plastic material, cutting blanks or billets from such 
sheets, heating the material to a desired temperature range, and pressure 
forming the material into containers. 
Recently, processes have been developed which form deep drawn containers 
which are capable of withstanding the pressure and temperature conditions 
of a retort chamber. See in this regard, U.S. Pat. Nos. 4,836,764; 
4,997,691 and 5,091,231 each issued to Keith Parkinson (the entire content 
of each being expressly incorporated hereinto by reference). In general, 
the processes disclosed in the Parkinson '764, '691 and '231 patents 
involves heating a central region of a disc or billet of thermoplastic 
sheet material to a temperature at or above its melt temperature while 
simultaneously keeping an annular region surrounding the central 
melt-phase region at a temperature to maintain the material in its solid 
phase. Raising the temperature of the central region of the billet so that 
it is in its melt-phase allows the central region to be deep drawn (i.e., 
drawn to a ratio of about 3:1 or greater of container lengthwise dimension 
to diameter (for circular cross-section containers) or average widthwise 
dimension (for rectangular cross-section containers). 
Problems are encountered however if the billets from which such deep drawn 
containers are made are highly oriented (i.e. having an ASTM D-1204 shrink 
rate of greater than 20%). For example, if billets formed of highly 
oriented thermoplastic sheet are employed in the processes disclosed in 
the Parkinson '764, '691 and '231 patents cited above, there exists the 
possibility that the billets will shrink during the heating process prior 
to thermoforming to an extent that the billets can no longer be carried by 
their respective support structures. As a result, such highly oriented 
billets can, and do, fall from their respective support structures while 
being heated in the oven thereby causing problems during production. 
It would therefore be desirable if sheets products could be made that had 
an inherently low orientation (i.e., an ASTM D-1204 shrink rate of less 
than about 18%) which could then be employed as the source from which 
billets could be formed for use in the above-cited Parkinson -764, '691 
and '231 processes. It is toward fulfilling such a need that the present 
invention is directed. 
Broadly, the present invention is embodied in processes whereby 
thermoplastic sheets may be made having minimal internal stresses 
(orientation), and to the resulting relatively low orientation sheets made 
therefrom. In preferred forms, the present invention is embodied in sheet 
products having at least one layer formed of a low orientation 
thermoplastic material. The sheet products of the invention may thus be in 
the form of monolayer or multilayer sheets. If multilayer sheets are 
produced, one of the layers may be formed of a thermoplastic material 
which forms an inherent barrier to oxygen transport therethrough. Such 
multilayer sheets are therefore particularly useful in the packaging 
industry--i.e., since containers can be made therefrom which are both 
retortable and which provide excellent oxygen barrier properties. 
In general essence, inherently low orientation sheet is produced by 
extruding a thermoplastic material in its melt phase onto the surface of a 
chill roller and thereafter solidifying the sheet at a relatively slow 
chill rate (e.g., less than about 60.degree. F./min, typically between 
about 40 to about 60.degree. F./min, most preferably between about 50 to 
about 55.degree. F./min). Importantly, the just extruded thermoplastic 
polymeric sheet is urged into continuous surface-to-surface contact with 
the chill roll by a curtain of air impinging upon the sheet at a velocity 
sufficient to ensure such continuous surface-to-surface contact between 
the sheet and the chill roll, but insufficient to impart substantial 
stress to the sheet. The term "continuous surface-to-surface" contact and 
like terms, are meant to convey that the sheet-like melt extruded from the 
die has an entirety of one surface between opposed widthwise points of the 
melt in contact with a chill roll such that no gaps or spaces exist 
between the melt and the chill roll along a line joining such widthwise 
points. "Discontinuous contact" therefore is meant to refer to gaps or 
spaces that exist between the surfaces of the melt and the chill roll that 
are present along a line joining opposed widthwise points of the melt. 
In such a manner, therefore, the resulting sheet will exhibit a low 
inherent internal stress level as evidenced by an unusually low shrink 
rate (ASTM D-1204) of less than about 18%, preferably between about 5% to 
about 16%, and more preferably between about 5% to about 10%. By way of 
example, low density polyethylene sheets having shrink rates of between 
about 15% to about 16% have been successfully produced and employed to 
form deep drawn containers. The sheets of the present invention are thus 
especially characterized by such unusually low shrink rates, and hence 
unusually low inherent internal stress levels, which find particular 
utility as containers for the packaging industry--especially containers 
produced by melt-phase deep-draw thermoforming techniques. 
These and other aspects and advantages of the present invention will become 
more clear after careful consideration is given to the detailed 
description of the preferred exemplary embodiments of this invention which 
follow.

DETAILED DESCRIPTION OF THE INVENTION 
Accompanying FIG. 1 depicts in schematic fashion an exemplary sheet 
processing system 10 that may be employed in the practice of the present 
invention. Specifically, a conventional autoflex die 12 (e.g., Extrusion 
Die Inc.) having a die slit of controllable thickness receives the melt 
flows via input lines 14-1, 16-1 and 18-1 from conventional screw 
extruders 14, 16 and 18, respectively. The multiple extruders 14, 16 and 
18 most preferably supply the die 12 with respectively different 
thermoplastic melt flows so as to form a multilayer composite sheet 
structure. However, the present invention is equally applicable to 
monolayer sheet structures in which case only one of the extruders 14, 16 
or 18 needs to be used. 
The output of the die 12 is a coextruded, sheet-like melt 20 which is 
solidified by passage in contact with chill rolls 22-1, 22-2 to form sheet 
product 24. The sheet product 24 may then be wound into a suitable package 
26. The widthwise dimensions of the melt 20, and hence the resulting sheet 
product 24, is not critical and may be selected depending upon the end use 
of the sheet 24 (e.g., the equipment which may be employed to process the 
sheet downstream). Thus, for example, standard sheet widths of 18 inches, 
24 inches and 30 inches or greater may be employed in the practice of this 
invention. 
The thickness dimension of the final sheet product 24 is substantially the 
same as the thickness dimension of the melt 20. In other words, 
substantially no draw-down of the melt 20 occurs between the chill rolls 
22-1 and 22-2 (i.e., the chill rolls 22-1 and 22-2 are operated at 
substantially the same speed). 
For a given extrusion rate of die 12, the chill rolls 22-1 and 22-2 will be 
rotated at the proper speed so that minimal stress is induced to the melt 
20. The thickness dimension of the final sheet product 24 is controlled by 
a feed-back control loop to the die 12 from thickness sensors 28. In this 
regard, the thickness sensors 28 will monitor the thickness dimension of 
the final sheet product and, in response to sensing a thickness that 
deviates from a predetermined set point, will command the die 12 to make 
relatively minor (but meaningful) thickness corrections of the die slit. 
Important to the present invention, the system 10 includes an air knife 30 
positioned downstream of the die 12 and oriented parallel to the roll axis 
of the chill roll 22-1. The air knife 30 extends the entire widthwise 
dimension of the melt 20. The air knife 30 thus directs a curtain of air 
perpendicularly against the melt 20 immediately downstream of the die 12. 
As noted previously, the air curtain created by the air knife 30 causes 
the melt 20 to be brought into continuous surface-to-surface contact with 
the chill roll 22-1. That is to say, the air knife 30 urges the entire 
widthwise extent of the melt 20 into contact with the surface of the chill 
roll 22-1 so that no air pockets or like areas of non-contact between the 
chill roll 22-1 and the melt 20 are present. As a result, substantially 
even cooling, and hence solidification, of the thermoplastic material 
ensues so as to achieve the final sheet product 24. 
Virtually any sheet-formable thermoplastics material may be employed in the 
practice of this invention, including (but not limited to) polymers and 
copolymers of polyolefins (such as polypropylenes, polyethylenes and the 
like), polyesters, polyamides (e.g., nylons), polyphenylene oxides, 
polyphenylene sulfides, polyvinyl chlorides, acrylonitriles and the like. 
Blends of such thermoplastics materials may also be employed in any 
desired blend ratio to suit desired end use applications. Particularly 
preferred for the ultimate fabrication of containers for use in the 
packaging industry are food grade polyolefins, with polyethylenes being 
particularly preferred. One particularly preferred thermoplastics material 
due to its compatibility with packaging products is low density 
polyethylene (LDPE). The preferred LDPE will have a melt index of less 
than about 2.5 g/10 min (typically about 1.9 g/10 min.), a density of less 
than about 0.950 g/cm.sup.3 (typically about 0.922 g/cm.sup.3) and a 
melting point of less than about 250.degree. F. (typically about 
230.degree. F.). One preferred LDPE is ESCORENE.RTM. low density 
polyethylene commercially available from Exxon Chemical Company. 
Accompanying FIG. 2 shows one embodiment of an exemplary multilayer final 
sheet product 24 that may be produced in accordance with the present 
invention. In this regard, the sheet product 24 includes a core layer 24-1 
of a thermoplastics material sandwiched between outer layers 24-2 and 
24-3. Optionally, intermediate layers 24-4, 24-5 may be interposed between 
the outer layers 24-2 and 24-3 and the core layer 24-1, respectively, so 
as to impart desired physical properties to the resulting sheet product 
24. 
When the sheet product 24 is employed to later form containers, the outer 
layers 24-2 and 24-3 may be a food grade polyolefin, for example, a low 
density polyethylene. The core layer 24-1 may be formed of a 
thermoplastics material having oxygen barrier properties. By sandwiching 
an oxygen barrier layer 24-1 between outer layers 24-2, 24-3 formed of 
food grade LDPE, a sheet product 24 may be made which exhibits excellent 
strength properties while also providing enhanced oxygen barrier 
properties that are desirable of containers. By the term "oxygen barrier" 
is meant a thermoplastics material that exhibits an oxygen permeability 
according to ASTM D-1434 of less than about 0.010, preferably less than 
about 0.005, and most preferably less than 0.003 cc.mil/100 in.sup.2 /24 
hrs./atm. at 65% relative humidity (RH) and 68.degree. F. Exemplary 
classes of oxygen barrier thermoplastics materials include ethylene vinyl 
alcohol (EVOH) copolymers (e.g., commercially available from Eval Company 
of America under the registered trademark EVAL.RTM.), polyvinylidene 
chlorides (e.g., commercially available from Dow Chemical under the 
registered trademark SARAN.RTM.), oriented and non-oriented polyamides 
(e.g., nylon 6), and oriented polyesters. The selection of any particular 
oxygen barrier material is dependent upon a number of factors, including 
the desired container properties and its end use application. 
The core layer 24-1 may include blends of thermoplastic materials such as 
those described previously. In addition, the core layer 24-1 may be 
composed partly or entirely of recycled thermoplastics material. One 
particularly convenient source of recycled thermoplastics material is the 
sheet web that remains as scrap after billets have been cut therefrom 
according to the processing techniques described in the above-cited 
Parkinson '764, '691 and '231 patents. Such scrap sheet web may be ground 
into suitable nominal particle sizes as feed to that one of the extruders 
14, 16 or 18 which coextrudes the core layer 24-1 with the outer layers 
24-2, 24-3. If present, the recycled thermoplastics material should be 
blended with a compatible virgin thermoplastics material in an amount of 
less than about 25 vol. %, typically less than about 20 vol. %, based on 
the total weight of the blend of virgin and recycled thermoplastics 
materials. 
The intermediate layers 24-4 and 24-5 may be formed of virtually any 
thermoplastics material chosen to enhance particular physical properties 
of the resulting sheet product 24. In the context of preparing 
continuously cast sheet as described above, the intermediate layers 24-4 
and 24-5 are most preferably a thermoplastics material which contributes 
to the melt strength of the resulting multilayer sheet product 24. One 
particularly preferred thermoplastics material for such purpose is 
metallocene linear low density polyethylene (MLLDPE) having a melt index 
of less than about 4.0 g/10 min. (typically about 3.4 g/10 min.), a 
density of less than about 0.925 g/cm.sup.3 (typically about 0.917 
g/cm.sup.3) and a melting point of less than about 245.degree. F. 
(typically about 239.degree. F.). One particularly preferred MLLDPE 
material is commercially available from Exxon Chemical Company as 
EXCEED.TM. 357C32 polyethylene. 
The core layer 24-1 will most preferably comprise between about 2 vol. % to 
about 90 vol. % of the sheet product 24, more preferably between about 5 
vol. % to about 80 vol. %, based on the total volume of the sheet product 
24. The outer layers 24-2 and 24-3 will most preferably collectively 
comprise between about 10 vol. % to about 98 vol. %, more preferably 
between about 20 vol. % to about 90 vol. %, of the sheet product 24, based 
on the total weight of the sheet product 24. The outer layers 24-2, 24-3 
will most preferably be present in the sheet product 24 in symmetrical 
amounts (and thicknesses), but asymmetrical amounts (and thicknesses) 
could also be employed, if desired. The intermediate layers 24-4 and 24-5, 
if present, will most preferably collectively comprise between about 10 to 
about 50 vol. %., typically between about 30 to about 40 vol. % of the 
sheet product 24, based on the total volume of the sheet product 24. 
Although not depicted in FIG. 2, the multilayer sheet product 24 may be 
provided, if needed, with tie layers interposed between adjacent ones of 
the other layers described previously. The tie layers are most preferably 
a thermoplastics material which adhesively bonds one thermoplastic 
material to another and is chosen based on its compatibility and adhesive 
qualities depending on the nature of the thermoplastics materials forming 
the layers between which it is interposed. If used, the tie layers may be 
coextruded with the core, outer and, if present intermediate layers using 
a separate dedicated extruder. One particularly preferred thermoplastic 
material that may be employed as a tie layer is PLEXAR.RTM. PX 380 
modified linear low density polyethylene commercially available from 
Quantum Chemical Company. 
A further understanding of the present invention will be obtained from the 
following non-limiting examples thereof. 
EXAMPLE 1 
The system 10 described above was used to produce a multilayer sheet 
product 24 having a core layer 24-1 formed of EVOH and outer layers 24-2 
and 24-3 comprised of LDPE. The core layer constituted about 5% by volume 
of the sheet product and was about 2 mils thick, while the outer layers 
collectively constituted about 95% by volume of the sheet product with 
each outer layer having a thickness of about 19 mils. 
Extruders 14 and 16 were employed so as to extrude the EVOH and LDPE 
components, respectively, while extruder 18 was employed so as to extrude 
a modified LLDPE thermoplastic material forming the tie layers. The melt 
flows of thermoplastic materials were directed to the die 12 so as to 
coextrude the core layer sandwiched between the outer layers with the tie 
layers interposed between each adjacent layer. 
The melt was extruded from the die 20 at a rate of 14 ft/min and a 
thickness of about 40 mils. The chill rolls 22-1 and 2202 were each 24 
inches in diameter and were rotated at a rate to achieve a chill rate of 
about 53.degree. F./min. A conventional air knife (Welex Corporation, Blue 
Bell, Pa.) was positioned perpendicular to the melt 20 and oriented 
parallel to the central axis of roll 22-1. The air knife had a slit 
opening of about 0.007 inch extending the entire widthwise dimension of 
the melt. The air knife was supplied with 2-3 psi pressurized air which 
exited the air knife slit and impinged directly on the upper surface of 
the melt so as to encourage the melt to come into continuous contact with 
the surface of the chill roll 22-1. 
The sheet produced by this Example 1 was tested for shrink rate according 
to ASTM D-1204 and was found to have a shrink rate of about 16%. Baby 
bottle liners of good quality were thereafter produced from billets cut 
from the sheet using the techniques described in the above-referenced 
'764, '691 and '231 patents. 
EXAMPLE 2 
Example 1 above was repeated except that a monolayer sheet product having a 
sheet thickness of about 40 mils and formed of LDPE was produced. The 
sheet produced by this Example 2 had a shrink rate (ASTM D-1204) of about 
16%. Baby bottle liners of good quality were thereafter produced form 
billets cut from the sheet using the techniques described in the 
above-referenced '764, '691 and '231 patents. 
EXAMPLE 3 (Comparative) 
Example 2 was repeated except that no air knife 30 was employed. The 
resulting sheet product had a low shrink rate of about 13%, but was 
commercially unusable as feedstock to produce deep drawn containers due to 
the severe surface and thickness irregularities that were present by 
virtue of the discontinuous contact between the sheet-like melt and the 
chill roll surface. 
EXAMPLE 4 (Comparative) 
LDPE sheet produced by extruding a sheet-like melt into the nip of an 
opposed pair of chill rolls was obtained and tested for shrink orientation 
according to ASTM D-1204. The sheet was determined to have an unacceptably 
high shrink rate of between about 60-70%. 
While the invention has been described in connection with what is presently 
considered to be the most practical and preferred embodiment, it is to be 
understood that the invention is not to be limited to the disclosed 
embodiment, but on the contrary, is intended to cover various 
modifications and equivalent arrangements included within the spirit and 
scope of the appended claims.