Stock material or miscellaneous articles – Hollow or container type article – Glass – ceramic – or sintered – fused – fired – or calcined metal...
Reexamination Certificate
1996-12-05
2002-04-23
Dye, Rena L. (Department: 1772)
Stock material or miscellaneous articles
Hollow or container type article
Glass, ceramic, or sintered, fused, fired, or calcined metal...
C428S035200, C428S035700, C428S315900, C428S331000, C428S520000, C428S522000, C426S118000, C426S418000, C426S419000, C264S204000
Reexamination Certificate
active
06376032
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to gas-permeable membranes and their use in packaging, especially the packaging of fresh produce.
2. Introduction to the Invention
Fresh cut fruit and vegetables, and other respring biological materials, consume oxygen (O
2
) and produce carbon dioxide (CO
2
), at rates which depend upon temperature and the stage of their development. Their storage stability depends on the relative and absolute concentrations of O
2
and CO
2
in the atmosphere surrounding them, and on temperature. Ideally, a respiring material should be stored in a container whose permeability to O
2
and CO
2
is correlated with (i) the atmosphere outside the package, (ii) the rates at which the material consumes O
2
and produces CO
2
, and (iii) the temperature, to produce and atmosphere within the container having O
2
and CO
2
concentrations equal to the optimum values for preservation of the material. The permeability to water vapor may also be significant. This is the principle behind the technology of controlled atmosphere packaging (CAP) and modified atmosphere packaging (MAP), as discussed, for example, in U.S. Pat. No. 4,734,324 (Hill), U.S. Pat. No. 4,830,863 (Jones), U.S. Pat. No. 4,842,875 (Anderson), U.S. Pat. No. 4,879,078 (Antoon), U.S. Pat. No. 4,910,032 (Antoon), U.S. Pat. No. 4,923,703 (Antoon), U.S. Pat. No. 5,045,331 (Antoon), U.S. Pat. No. 5,160,768 (Antoon) and U.S. Pat. No. 5,254,354 (Stewart), and European Patent Applications Nos. 0,351,115 and 0,351,116 (Courtaulds). The disclosures of each of these publictions is incorporated herein by reference.
The preferred packaging atmosphere depends on the stored material. For example, some materials, e.g. broccoli, are best stored in an atmosphere containing 1-2% O
2
and 5-10% CO
2
. For other materials, an atmosphere containing 1-2% O
2
and 12-30% CO
2
, e.g. about 15% CO
2
, is preferred. Thus, CO
2
concentrations of 10 to 30% slow the respiration rate of some fruit and rduce the activity of some decay-causing organisms; for example, a CO
2
concentration of 20% delays grey mold decay in rasberries and extends their shelf life.
SUMMARY OF THE INVENTION
Although much research has been crried out, known packaging techniques have many shortcomings for respiring biological materials. We have discovered, in accordance with this invention, that by forming thin polymeric coatings on microporous films, it is possible to create gas-permeable membranes which have novel and desirable combinations of O
2
permeabilikty, change in O
2
permeability with temperature, and ratio of CO
2
permeability to O
2
permeability. Improved results can be obtained using a wide range of microporous base flims and coating polymers. However, a particular advantage of the present invention is that it makes it possible to design packages which are tailored to the requirements of particular respiring materials. As further discussed below, the gas-permeable membranes of this invention are generally used as control sections which provide the sole, or at least the principal, pathway for gases to enter or leave a sealed container containing a respiring material.
In describing the invention below, the following abberviations, definitions, and methods of meassurement are used.
OTR
is O
2
permeability.
COTR
CO
2
permeability. OTR and COTR values are given in ml/m
2
.atm.24 hrs, with the equivalent in cc/100 inch
2
.atm.24 hrs given in parentheses. OTR and COTR were measured using a permeability cell (supplied by Millipore) in which a mixture of O
2
, CO
2
and helium is applied to the sample, using a pressure of 0.7 kg/cm
2
(10 psi) except where otherwise noted, and the gases passing through the sample were analyzed for O
2
and CO
2
by a gas chromatograph. The cell could be placed in a water bath to control the temperature. The abbreviation P
10
is used to denote the ration of the oxygen permeability at a first temperaturre T
1
° C. to teh oxygen permeablility at a second temperature T
2
, where T
2
is (T
1
−10)° C., T
1
being 10° C. and T
2
being 0° C. unless otherwise noted. The abbreviation R is used to denote the ratio of CO
2
permeability to O
2
permeability, both permeabilities being measured at 20° C. unless otherwise noted. Pore sizes given in this specification are measured by mercury porosimetry or an equivalent procedure. Parts and percentages are by weight, temperatures are in degrees Centigrade, and molecular weights are weight average molecular weights expressed in Daltons. For crystalline polymers, the abbreviation T
o
is used to denote the onset of melting, the abbreviation T
p
is used to denote the crystalline melting point, and the abbreviation &Dgr;H is used to denote the heat of fusion. T
o
, T
p
and &Dgr;H are measured by means of a differential scanning calorimeter (DSC) at a rate of 10° C./minute and on the second heating cycle. T
o
and T
p
are measured in the conventional way well known to those skilled in the art. Thus T
p
is the temperature at the peak of the DSC curve, and T
o
is the temperature at the intersection of the baseline of the DSC peak and the onset line, the onset line being defined as the tangent to the steepest part of the DSC curve below T
p
.
Typically, a microporous film has an R ratio of about 1, and OTR and COTR values which (i) are very high, (ii) do not change much with the thickness of the film, and (iii) do not change much with temperature (leading to P
10
ratios of about 1). A continuous polymeric layer, on the other hand, typically has an R ratio substantially greater than 1 (generally 2 to 6, depending on the polymer itself), and has OTR and COTR values which (i) are relatively low, (ii) are inversely proportional to the thickness of the layer, and (iii) change substantially with temperature (leading to P
10
ratios substantially graeter than 1, generally at least 1.3). At practical thicknesses, such continuous polymeric layers have OTR and COTR values which are undesirably low.
We have discovered that when a membrane is prepared by coating a thin layer of a polymer onto a suitable microporous film, it has permeability characteristics which depend on both the coating polymer and the microporous film. We do not know exactly why this is so, and theresults achieved by this invention do not depend upon any theory of its operation. However, we believe that the coating polymer effectively blocks most, but not all, of the pores of the microporous film (with the smaller pores being preferentially blocked); and that as a result, the permeability of the membrane results in part from gases which pass through the unblocked pores and in part from gases which pass through the coating polymer. In any event, the invention makes it possible to prepare novel membranes having very desirable permeability characteristics, and to achieve controlled variation of those characteristics. For example, the invention makes it possible to prepare membranes having an OTR greater than 775,000 (50,000), e.g. 1,550,000 (100,000) to 3,875,000 (250,000), or even higher, e.g. up to 7,750,000 (500,000) or mnore, a P
10
ratio of at least 1.3, e.g. at least 2.6, and an R ratio of at least 1.5, e.g. at least 2.0.
The microporous film and the coating polymer must be selected and correlated to produce a membrane having particular properties, but those skilled in the art will have no difficulty, having regard to the disclosure in this specification and their owin knowledge, in achieving a wide range of usefuil results.
The size and distribution of the pores in the microporous film are important factors. If the pores are too small, the coating polymer tends to form a continuous layer which is either too thin to be durable under routine handling, or too thick to have an acceptable OTR. If the pores are too large, the coating polymer may be unable to bridge over them, so that the coating polymer plays little or no part in determining the permeability characteristics of the membrane. This may happen even if the average pore size is relatively low, if the pores ha
Clarke Raymond
McClary Bradley
Schultz Donald A.
Stewart Ray F.
Yoon Valentine Y.
Dye Rena L.
Landec Corporation
Sheldon Jeffrey G.
Sheldon & Mak
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