Thermoformable laminated packaging material

A rigid thermoformable packaging material is provided which includes an outer layer of a nonfoamed oriented polystyrene and an inner layer of a functional polymer resin. The laminated packaging material can be produced by either thermal lamination or coextrusion. The packaging material is readily thermoformed into containers for medical or dental products or foods and is heat sealable, tough, resistant to chemicals, and resistant to cracking and tearing during handling by thermoforming equipment.

BACKGROUND OF THE INVENTION 
This invention relates to a rigid thermoformable packaging material and 
containers made therefrom. More specifically, it relates to a composite 
material having a layer of oriented polystyrene laminated to a functional 
polymer resin layer which serves as a barrier to moisture, odors, gases, 
chemicals and the like and/or improves the toughness and resistance to 
cracking and tearing of the material. 
A wide variety of polymer resins are presently used to package medical and 
dental products as well as foods. These polymers are formed into sheets, 
films, and containers and include polystyrene, polyolefins, polycarbonate, 
acrylonitriles, polyvinyl chloride, polyethylene terephthalate polyesters, 
and copolymers such as styrene-butadiene and 
acrylonitrile-butadiene-styrene-polymethylmethacrylate. All of these 
resins have certain properties which make them desirable for these end 
uses. 
However, when used as monolayer packaging materials, each group of resins 
has certain drawbacks. For example, while polyolefins have good moisture 
barrier properties, they are difficult to thermoform. Polystyrene, on the 
other hand, is readily thermoformable but has poor barrier properties. 
Poor resistance to some chemicals, and tends to be brittle. 
It is known to laminate a plurality of layers of dissimilar polymer resins 
in an attempt to obtain materials having a combination of the desirable 
properties of the individual resins. For example, composite multilayer 
films of polystyrene and polyolefins have been made in attempts to take 
advantage of the stiffness and rigidity of polystyrene and strength, 
durability and barrier properties of the polyolefin, Erb. U.S. Pat. No. 
3,589,976, teaches a composite film of a coextruded polystyrene core layer 
and polyolefin surface layers utilized as packaging materials for foods. 
Likewise, Japanese published applications Nos. 53-034,881; 55-144,158; and 
55-163.164 generally teach composite films of polystyrene and polyolefins 
used as packaging materials. 
However, for materials designed for use in packaging medical or dental 
products, or for certain food applications, the polymer resin or laminate 
structure must possess a multiplicity of attributes. Not only must the 
packaging material be rigid, but it also must resist cracking and tearing 
brought about by clamping equipment used on commercial thermoforming, 
filling, and sealing systems. Additionally, the material must be heat 
sealable and yet have clean peel characteristics upon opening. Finally, 
the packaging material desirably has good light transmission 
characteristics is resistant to chemical attack and is stable to radiation 
sterilization procedures. Heretofore, none of the commonly utilized 
packaging materials used in the art have possessed these desirable 
combinations of properties. 
Accordingly, the need still exists in the art for a cost effective 
packaging material possessing all of the desirable attributes set forth 
above for use in the packaging of medical or dental products as well as 
certain foods. 
SUMMARY OF THE INVENTION 
The present invention meets that need by providing a rigid thermoformable 
packaging material and a container made therefrom for use in packaging 
medical and dental products and certain foods. 
According to one aspect of the present invention, a rigid thermoformable 
packaging material is provided which includes an outer layer of a 
nonfoamed oriented polystyrene and an inner layer of a functional polymer 
resin laminated thereto selected from the group consisting of low density 
polyethylene high density polyethylene and nylon. This functional layer 
may be selected to give the laminated packaging material barrier 
properties, chemical resistance, toughness, or other desired 
characteristics. The outer layer of oriented polystyrene resin has a 
thickness of between about 1 to about 30 mils, while the inner polymer 
layer has a thickness of between about 0.5 to about 10 mils. The oriented 
polystyrene makes up from about 78% to about 96.5% by weight of the 
overall mass of the laminate. 
The packaging material of the present invention may be readily thermoformed 
into a variety of rigid containers for medical, dental, and food products. 
The combination of layers in the laminate structure provide a container 
which has a high flexural modulus, toughness, and resistance to tearing 
and cracking during handling by automated form, fill, and seal equipment. 
Additionally, the packaging material is heat sealable to a variety of lid 
stocks. 
The packaging material of the present invention may be produced by a 
thermal lamination process or may be coextruded. In the thermal lamination 
process, a layer of a nonfoamed oriented polystyrene having the thickness 
described above is joined together with a second layer of a functional 
polymer resin, the second layer of polymer resin having an adhesive layer 
thereon. The adhesive layer preferably comprises a copolymer of ethylene 
with vinyl acetate or ethylene with acrylic acid. 
After the layers are joined, they are heated to a temperature sufficient to 
activate the adhesive layer and bond the oriented polystyrene layer to the 
second polymer resin layer over substantially the entire surfaces thereof. 
The temperature is controlled to insure that stress relaxation of the 
oriented polystyrene layer does not occur as an unoriented polystyrene 
would be excessively brittle and tend to crack and tear, even when 
laminated to another polymer resin. 
An alternate procedure for forming the packaging material of the present 
invention comprises coextruding a laminate structure which includes a 
first layer of a nonfoamed polystyrene, a second layer of a functional 
polymer resin selected from the group consisting of low density 
polyethylene, high density polyethylene, polypropylene and nylon, and an 
adhesive layer sandwiched between the first and second layers. The 
adhesive layer preferably comprises a copolymer of ethylene with vinyl 
acetate or ethylene with acrylic acid. 
The coextruded laminate is then stretched causing at least the polystyrene 
layer to become oriented. Preferably, this stretching occurs in both the 
machine direction and cross-direction to produce a biaxially oriented 
material. 
Accordingly, it is an object of the present invention to provide a rigid 
thermoformable packaging material and method of manufacture, as well as a 
container made therefrom, which is cost effective, chemically resistant, 
tough, and resistant to tearing or cracking when handled by thermoforming 
equipment. This and other objects and advantages of the invention will 
become apparent from the following detailed description, the accompanying 
drawings, and the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIGS. 1 and 2, there is illustrated a container or tray 10 
into which medical or dental products or a food may be packed. The wall 12 
and rim 14 of container 10 comprise the laminated packaging material of 
the present invention. Container 10 may be formed by conventional 
thermoforming equipment as described in further detail below. A lid 16, 
which may be heat sealed in a conventional manner to rim 14 of container 
10 is also illustrated. Lid 16 may be made of the laminated packaging 
material of the present invention or may be a different monolayer or 
multilayer polymer resin. For example, suitable heat sealable lid stock 
may be a spun-bound polyolefin sold by Dupont under the designation Tyvek 
(trademark). 
As best shown in FIG. 2, the rigid thermoformable packaging material of the 
present invention includes an outer layer 22 of a nonfoamed oriented 
polystyrene. By "oriented", it is meant that the polystyrene has undergone 
stretching in at least one axial direction, and preferably has been 
biaxially stretched to produce a tougher resin which will be more 
resistant to tearing and cracking. Use of an oriented polystyrene is 
critical to the practice of the present invention as unoriented 
polystyrene resin is excessively brittle and tends to crack and tear 
during handling by the clamping systems utilized by conventional 
thermoforming systems. 
The thickness of oriented polystyrene layer 22 is between about 1 to about 
30 mils (0.025 to 0.75 mm), and preferably about 8 to about 12 mils (0.2 
to 0.3 mm). 
The laminate also includes an inner layer 24 of a functional polymer resin 
selected from the group consisting of low density polyethylene, high 
density polyethylene, polypropylene and nylon. The functional polymer 
resin layer has a thickness of between about 0.5 to about 10 mils (0.012 
to 0.25 mm). The particular polymer resin utilized depends on the 
particular properties or characteristics desired in the laminated 
packaging material. 
For example, where the material is to be utilized to contain medical or 
dental products, the inner layer is preferably a polyolefin such as low 
density polyethylene. The polyethylene is resistant to chemicals and 
protects the outer polystyrene layer from chemical attack. For instance, 
the plasticizer used in many medical products made of polyvinylchloride 
will attack polystyrene. Polyethylene will also provide the necessary 
toughness (elongation) to enable the composite material to withstand 
normal handling and clamping operations during thermoforming. Finally, the 
polyethylene has been found to be unaffected by radiation sterilization 
procedures utilized for medical and dental products. 
The inner and outer layers of the packaging material are bonded together 
across substantially their entire surfaces by an adhesive layer 26 which 
preferably comprises a copolymer of ethylene with vinyl acetate or 
ethylene with acrylic acid. Preferably, the inner layer 24 and adhesive 
layer 26 are coextruded together initially to form a two layer laminate. 
Inner layer 24 and outer layer 22 can then be thermally laminated together 
as described in further detail below. 
It is important to the practice of the present invention that the oriented 
polystyrene outer layer comprise from about 78% to about 96.5% of the 
weight overall of the packaging material. It has been found that a 
composite layered material formed within this range and in accordance with 
the present invention has the necessary structural rigidity, stiffness, 
and high flexural modulus to form a crush-resistant package while 
exhibiting the necessary toughness and chemical resistance required. When 
the composite material contains less than about 78% by weight of the 
oriented polystyrene, the reduced flexural modulus and decreased stiffness 
require the increasing of the thickness of the package to obtain 
sufficient stiffness. This increases materials costs and complicates the 
thermoforming operation. 
On the other hand, when the composite material contains greater than about 
96.5% by weight of oriented polystyrene, it has been found that there is 
an unacceptable level of cracking and tearing of the laminate during 
handling and clamping procedures in the thermoforming operation. 
Additionally, such laminates exhibit reduced chemical resistance. 
Referring now to FIG. 3, the thermal lamination process for forming the 
composite packaging material of the present invention is illustrated. 
Inner layer 22 of oriented polystyrene is unwound from roll 28 and outer 
composite layer 24, 26 (which previously has been formed by, for example, 
a coextrusion process) is unwound from roll 30, and both layers are joined 
together at the nip between heated rolls 32 and 34 respectively. Heated 
rolls 32 and 34 are maintained at a temperature sufficient to activate 
adhesive layer 26 and cause it to bond the inner and outer layers together 
but insufficient to cause stress relaxation of the oriented polystyrene 
layer 22. As the stress relaxation temperature of oriented polystyrene is 
approximately 230.degree. F., the temperature of the heated rolls as well 
as the line speed of the material through the rolls should be adjusted to 
maintain the temperature of the layers below about 230.degree. F. It has 
been found that a suitable temperature range for heated rolls 32 and 34 is 
between about 260.degree. to 280.degree. F. for a line speed of between 
about 50-70 feet per minute. 
Laminated packaging material 29 is then passed around rollers 36 and 38 to 
a conventional vacuum thermoforming apparatus, the details of which are 
well-known in this art. Rollers 36 and 38 may be chilled if desired to 
cool down the laminate. Alternatively, laminate 29 may be wound into 
spools (not shown) and stored. 
After thermoforming and trimming, individual containers (not shown, but 
having a structure as illustrated, for example, in FIG. 1) are then passed 
to a filling and sealing station 42 where medical or dental products or 
food, as the case may be, are placed in the containers and lid 1 is sealed 
thereto. The laminated packaging material of the present invention 
provides an excellent surface for directly heat sealing spun-bound 
polyolefin (Tyvek) lid stock. Typically, such a lid stock requires a 
costly coating before it can be heat sealed to many existing medical 
packaging materials. The polyethylene inner layer 24 of the packaging 
material of the present invention will directly heat seal to such lid 
stock without the need for precoatinq. Moreover, once heat-sealed, such 
lid stock can be cleanly peeled off in total from the material utilized in 
the present invention. This "clean peel" capability eliminates the 
possible contamination of the contents of the container by fragments of 
the lid material. This "clean peel" characteristic also aids in 
maintaining the removed lid intact and any instructions, which are 
typically printed on the inside of the lid, are preserved, 
In an alternate embodiment of the invention which is not shown, a portion 
of rim 14 on container 10 may be extended to form an end flap label. The 
end flap can be heated with sonic welding horns and then folded 90 
degrees. This permits the contents of the containers to be identified 
through information printed on an end flap when the containers are stacked 
on top of one another. 
In an alternate process for forming the packaging material, FIG. 4 
illustrates a coextrusion technique which utilizes in-line stretching and 
orientation of the laminate. As shown, conventional extruders 50, 52, and 
54 serve as sources of supply for the inner, outer, and adhesive layers, 
respectively. Conduits 51, 53 and 55 supply heated resin to coextrusion 
feedblock 56. There, the resins merge together to form under pressure a 
unitary three-layer stream 57 having a generally circular cross-section. 
This stream 57 is then passed into a conventional extrusion die 58 and is 
extruded into a continuous sheet. The sheet is then passed through 
sequential stretching stations 60 and 62 which stretch the sheet in the 
machine direction and in the cross direction, respectively, to produce a 
biaxially oriented polystyrene layer 22 in the composite material. The 
composite material may then be thermoformed, filled, and sealed as 
described above to produce finished, sealed containers for medical or 
dental products or foods. 
Having described the invention in detail and by reference to preferred 
embodiments thereof, it will be apparent that modifications and variations 
are possible without departing from the scope of the invention defined in 
the appended claims.