For many years, food products such as fresh poultry were shipped in bulk from the food processors to supermarkets where the bulk pieces were divided into small quantities and repackaged for retailing. For example, fresh poultry was cut up, placed on cardboard or plastic trays, and covered with stretch film secured to the tray by tack welding.
For improved efficiency the current trend is to perform the retail packaging operation in the food processing plant and ship the small retail packages from such plants to the retailers. There is also a trend towards evacuating and hermetically sealing the food retail packages in the central food processing plants, due to the longer storage time between retail packaging and consumer use. Such packaging increases the shelf life of the food packages. Also, there is a need for increased abuse resistant food retail packages because of the more frequent handling, impact and abrasion inherent in the above-described food processing plant retail packaging system.
This retail packaging can be accomplished in a variety of ways. A number of systems are available to food processors for wrapping and heat sealing poultry-containing trays with plastic overwrap film. One packaging system that uses a continuous belt sealer is the Ossid.TM. 500, produced by the Ossid Corporation. (Ossid.TM. is a trademark of the Ossid Corporation, North Carolina, USA.) Generally, a poultry-containing tray is moved by a series of conveyors and rolls while a film is fed from a dispenser and mechanically pulled over the tray top. The film edges are wrapped around the tray, pressed against the tray bottom, overlapped and sealed against it. Finally, the tray is moved forward and the opposite ends cut. In the Ossid.TM. 500 system both cut portions of the film, the flaps, are pulled under the tray and shaped under the tray.
Sealing is done with heat and pressure. In impulse sealing systems, the contact temperature ranges from about 400.degree. to 800.degree. F. with a high applied pressure due to the narrow sealing surface. In a bar heating sealing system, the temperature ranges from 250.degree. to 400.degree. F. with a similar pressure as above. The heat contact time is longer than with the impulse system. The Ossid 500 employs a continuous belt sealer that consists of a release belt that is drawn over a heated platen. The resultant platen seal has characteristics that vary significantly from other commercial sealing processes. In the platen heating-sealing system, the temperature is at about 250.degree. to 400.degree. F., using a low pressure of about 0.1 to 2.0 psi, with the heat contact time being 2 to 4 seconds.
The wrapped tray is passed to the belt or platen which applies heat that bonds the film layers together and, upon cooling, the seal is fixed. The heat bonded film is then immediately cooled to a temperature below about 200.degree. F.
The machinability of the film, or the way that it operates on the machine, is an important characteristic of the film used.
The most commonly used film in the trayed food overwrap market is polyvinyl chloride (PVC). This thermoplastic polymer has been used because of satisfactory elongation and elastic memory. But in some instances the package becomes leaky during shipment and thereby unacceptable to the customer. This is because the PVC material in overwrapped trays is tack welded to itself and not hermetically sealed. This problem cannot be solved by hermetically sealing because of a very narrow sealing temperature range between which the seal is effective at the low temperature end and "burn through" or melting of the PVC at the high temperature end. This range is so limited as to be impractical with many commercially employed heat sealing systems.
Another limitation of PVC tray overwrap material is its poor resistance to physical abuse. The PVC material tends to tear along the edges of the overwrapped tray if rubbed during transit by another tray or an enclosing carton.
One characteristic of PVC is that although it contracts, it is not generally "heat-shrinkable", i.e., it is a material that tends not to return to its original unstretched (unextended) dimension when heated to its softening point. The terms "orientation" or "oriented" are used to describe the manufacture of heat-shrinkable films, wherein resin material is heated to its flow or melting point and extruded through a die into either tubular or sheet form. After cooling, the relatively thick extrudate is reheated to a temperature range suitable to orient or align the crystallites and/or molecules of the material. The orientation temperature range for a given material or materials is understood by those skilled in the art to be in a range which revises the intermolecular configuration of the material by physical alignment of the crystallites and/or molecules of the material to improve certain mechanical properties of the film such as shrink tension as, for example, measured in accordance with ASTM D-2838-81. When the stretching force is applied in one direction, uniaxial orientation results. When the stretching force is simultaneously applied in two directions, biaxial orientation results.
In view of the limitations of PVC as a stretch film food tray overwrap material, there have been prior efforts to identify a heat-shrinkable thermoplastic film having an improved combination of elongation, elastic memory, heat sealability and puncture resistance. However, most heat-shrinkable thermoplastic film packaging materials suitable for food contact have relatively poor elasticity or elastic memory. Thus, when a food wrapped in such a material shrinks from moisture loss, the film does not shrink, resulting in a loose package that is unacceptable.
One PVC replacement film for use as a trayed food overwrap material is described in U.S. Pat. Nos. 5,272,016 and 5,279,872, issued to D. J. Ralph ("Ralph"), incorporated herein by reference to the extent pertinent. The Ralph film is the biaxially oriented heat-shrinkable multilayer stretch type, comprising at least a first outer layer, a second outer layer, and a core layer between the first and second outer layers. The outer layers each comprise a blend of between about 20 and about 35 wt. % ("wt. %") ethylene .alpha.-olefin plastomer copolymer of density below about 0.90 g/cm.sup.3, and between about 65 and about 80 wt. % very low density polyethylene ("VLDPE"). The core layer comprises ethylene .alpha.-olefin copolymer having a higher melting point than the melting point(s) of either of the first or second outer layers. By way of example, the core layer may be polypropylene or a polyolefin. Embodiments of the latter include VLDPE, linear low density polyethylene ("LLDPE"), and blends of two different density VLDPE's or VLDPE and LLDPE.
The Ralph-type film in nonirradiated form has been demonstrated to be suitable as a PVC replacement for food tray overwrapping and sealing by the impulse and bar heating-sealing systems. However, it has substantial limitations when used in platen sealing systems.
When the heat seal between the film layers forming the end flaps is not complete, the package is defective. Incomplete seals represent potential air/fluid leaks and loss of food quality and/or desired appearance in the retail market. As seen from the above, the number of overlapping film layers to be sealed together may vary substantially, generally from six to at least twenty. This means that the required maximum seal temperature must be relatively high, because as the number of film layers increases, the temperature at a given residence time required for a complete seal increases. However, "burn through" places an upper limit on the sealing temperature. Burn through means the temperature at which any hole or penetration of the film occurs, resulting from melting and/or shrinking of the film during sealing.
When the Ralph-type film with a 100% VLDPE core layer was tested for use on platen sealed poultry-containing trays, the sealing range was too narrow as complete sealing could not be obtained without burn through. In an attempt to overcome this problem, LLDPE was added to the core layer making the core a VLDPE-LLDPE blend. This formulation provided a broad enough platen seal range on an Ossid 500 system under ideal conditions, but under production conditions the sealing range was still too narrow. To broaden the heat sealing range of the VLDPE-LLDPE blend core layer Ralph-type film, the film was irradiated at a dose of 8 MR after biaxial orientation.
Although irradiation of the VLDPE-LLDPE blend core layer Ralph-type film provided the needed wide heat sealing range, a new and unexpected problem developed. The film had poor machinability. The slide characteristics of the film were such that the film could not be smoothly transported through and over the various belts and rollers which are essential to film movement through the wrapping machine. At the same time, it should be recognized that the film cannot be so frictionless that it cannot be secured by the clamps as, for example, chain grips, which grab the film edges and pull it taut over the top of the tray.
During testing, another facet of the machinability problem with the irradiated Ralph-type film, flap pull back, was discovered. Flap pull back refers to the tendency for the folded under flap to be pulled away from the tray bottom surface by movement across the rollers prior to entering the platen sealer-cooler. This problem appears to be related to the frictional properties of the film, possibly the ratio of the friction between the flap folding rollers on the platen sealing-cooling system and the film-to-film slip.
An improved polyolefin-type heat-shrinkable oriented stretch film suitable for use as a platen heat sealed overwrap material for a food tray with a wide sealing range and good machinability is needed. This film should also be characterized by good elongation, good elastic memory, puncture resistance and abrasion resistance.
An improved method for wrapping and platen-sealing a polyolefin stretch-shrink film as an overwrap on a food-containing tray is also needed.
Also needed is an improved evacuated hermetically sealed food-containing tray overwrapped by a heat shrunk polyolefin film.