Stock material or miscellaneous articles – Structurally defined web or sheet – Including aperture
Reexamination Certificate
1994-06-08
2001-10-02
Watkins, III, William P. (Department: 1772)
Stock material or miscellaneous articles
Structurally defined web or sheet
Including aperture
C428S131000, C428S220000, C428S315500, C428S536000, C428S516000, C428S347000, C428S338000, C428S339000, C428S523000, C428S913000, C426S415000, C426S419000, C426S326000, C426S316000, C426S410000, C426S118000, C426S395000, C053S396000, C053S141000, C053S432000, C053S469000, C053S449000
Reexamination Certificate
active
06296923
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention concerns polymeric films, and in particular polymeric films for the storage or packaging of plant materials.
2. Description of the Prior Art
During storage, plant materials continue to respire even when the materials have been removed from the plant on which they were growing or when the plant material has been dug out of the ground. Thus fruit and vegetables, for example, continue to place demands on the surrounding atmosphere during storage, and deterioration of the quality of the plant materials occurs through water loss and surrounding levels of oxygen and carbon dioxide which do not favour their remaining fresh.
The freshness of fruit and vegetables can be prolonged by packaging, and this can have the added advantage of reducing damage when the fresh produce is displayed on a supermarket shelf. However, there are problems with the use of many packaging materials as the atmosphere within the package changes as respiration proceeds. This can be a particular problem with plant materials which undergo a climacteric stage during ripening, when a sharp rise in the rate of respiration occurs. Thus, while polymeric films, e.g. polyolefin films, can improve the shelf life of fruit and vegetables, a point can come during their storage when deterioration is accelerated by the changes in the atmosphere within the package.
Various proposals have been made for overcoming the problems with storing plant materials in packages made from polymeric films. British Patent Specifications 1106265 and 1134667, for example, describe control of the atmosphere within a package so that the oxygen content is less than that of normal air while the carbon dioxide content is greater than that of normal air, this being effected by the use of imperforate polyethylene sheet of a thickness that it is permeable to oxygen and carbon dioxide and of an area sufficient to allow the sealed-in produce to establish and maintain a controlled atmosphere within the package. Although oxygen and carbon dioxide levels are controlled by this method, the water content of the atmosphere is not and this can lead to undesirable water levels which can increase deterioration of the packaged materials.
Films with very high water permeability are proposed in Japanese Patent Publication 62.148247, 50 to 300 holes per square centimetre being made in the film, each hole being from 50 to 300 microns in diameter. These films are proposed for wrapping cut flowers where the water vapour permeability has to be sufficient to remove condensed water droplets.
Other proposals include the use of as and water-vapour impermeable films which have permeable windows let into them, the windows being made of more permeable materials. Alternatively, composite containers have been proposed in which one side of the container is made from an impervious plastics film and another side is made from a microporous film.
SUMMARY OF THE INVENTION
According to the present invention there is provided a polymeric film for the storage or packaging of plant material, the film being perforate and having a water vapour permeability of not more than 800 g m
−2
day
−1
and an oxygen permeability of not more than 200000 cm
3
m
−2
day
−1
atmosphere
−1
, both permeabilities being measured at 25° C. with a relative humidity of 75 percent.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Films of the present invention have the advantage of providing packages with the desired degree of oxygen permeability to give good storage life to plant materials stored in them while at the same time enabling the water permeability of the packages to be controlled to a desired level, this being achieved without the necessity for special windows or a number of films for the one package. Films of the present invention can be produced which are generally stiffer than conventional cling film, thus enabling them to be used more readily on horizontal or vertical form fill seal packaging machinery. Furthermore, they will usually be clear as the perforations used are very small.
The water vapour permeability of the films of the present invention can be selected by the type of polymer used for the film. Examples of polymers which can be used include regenerated cellulose, homo and copolymers of polyolefins, e.g. with vinyl acetate or methyl acrylate, polyesters and polyamides. The films can furthermore be laminates and/or can include one or more layers, e.g. a heat sealable layer. Films of regenerated cellulose can be used to achieve water vapour permeability over a wide range, typically up to 800 g m
−2
day
−1
measured at 25° C. and 75 percent relative humidity for a film 24 microns thick. Lower permeabilities can be achieved by the use of a thicker film, but it is generally preferred to apply a coating to the film when it is desired to reduce its permeability to water vapour. Suitable materials for the purpose are known in the art. Thus water vapour permeabilities of 100-800 g m
−2
day
−1
can be achieved, and if desired lower values, e.g. down to 80 g m
−2
day
−1
, or even lower, e.g. as little as 10 g m
−2
day
−1
can be achieved. When a coating is present, the permeability will usually be less than 500 g m
−2
day day
−1
.
Polyolefins can also be used to make films of the present invention, the inherent water vapour permeability of films of such materials tending to be substantially less than that of uncoated regenerated cellulose films of the same thickness. Polyethylene films 30 microns thick typically have water vapour permeabilities of about 4 g m
−2
day
−1
, while polypropylene films of the same thickness typically have water vapour permeabilities of 1-2 g m
−2
day
−1
.
The water vapour permeability of the film will be selected to suit the respiration requirements of the plant material to be packaged, and therefore there are no overall preferences for water vapour permeability other than that the permeability be selected to optimise the storage life of the packaged plant material.
The oxygen permeability of films of the present invention is not more than 200000 cm
3
m
−2
day
−1
atmosphere
−1
as measured at 25° C. and 75 percent relative humidity. As with water vapour permeability, different plant materials require films with different oxygen permeabilities, and permeabilities of not more than 100000, e.g. less than 50000 cm
3
m
−2
day
−1
atmosphere
−1
are often preferred. Lower oxygen permeabilities still can be achieved, for example less than 10000 cm
3
m
−2
day
−1
atmosphere
−1
. The oxygen permeability will, however, be greater than that inherent for the material of the film, and typically it should be at least 900 cm
3
m
−2
day
−1
atmosphere
−1
greater than that of the material of the film. This usually means at least 3500 cm
3
m
−2
day
−1
atmosphere
−1
.
The oxygen permeability of films of the present invention is achieved by perforations in the film. The size of the perforations affects the oxygen permeability of the film, and they are preferably from 20 to 100 microns, more preferably 40 to 60 microns and advantageously about 50 microns mean diameter. If the perforations are too large, control of oxygen permeability is not possible, and if the holes are too small, large numbers of holes are required which in particular adds to the cost of the film. Typically it is preferred to have up to 1000 perforations in the film per square metre of film surface, but as few as 10 perforations or even less can be used. These sizes (20 to 100 microns, preferably 40 to 60 microns) and numbers (100 to 1000 per m
2
of film surface) represent a percentage of perforations in the range of 3.14×10
−7
to 7.85×10
−4
%, preferably 1.26×10
−6
to 2.83×10
−4
% of the total film surface. This is very significantly lower than the frequency of perforations in th
Bacon & Thomas PLLC
Sidlaw Flexible Packaging Limited
Watkins III William P.
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