Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1999-10-28
2001-05-15
Nutter, Nathan M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S240000
Reexamination Certificate
active
06232402
ABSTRACT:
This invention relates to polyolefin films having certain unique properties. Specifically, it relates to clear polyolefin films based on polyethylene and polypropylene that exhibit an increased oxygen transmission property as compared to conventional polypropylene films known to the art.
BACKGROUND INFORMATION
It is known that, for many applications, particularly packaging applications, polypropylene is less than totally satisfactory due to a relatively high permeability to oxygen. Typically, a 0.7 mil (70 gauge) biaxially oriented film of polypropylene exhibits an oxygen transmission rate (OTR) of about 125 cc/100 sq. in./day/atmosphere. Such an OTR is too great for such applications as packaging of potato chips, most chocolate candy, processed meats and cheese and other packaging applications where an excess of oxygen can lead to premature spoilage of the packaged product.
A great amount of effort has been expended in recent years to decrease the oxygen transmission characteristics of polypropylene. The best results to date have been obtained by metallization of the films.
There are, however, some applications in which a greater and controllable oxygen transmission rate is desired. This is particularly the case when fresh fruits, vegetables and flowers are to be packaged as these products continue to respire after they are packaged. Absence or insufficient levels of oxygen, which occurs as the oxygen initially present in the package is consumed, leads to premature senescence and spoilage of the products. At the same time, respiration leads to a build-up of moisture in the package, which can also lead to spoilage of the product if the moisture cannot escape from the package. It is thus desirable to provide polypropylene packaging wherein the oxygen content can be replenished as necessary and from which the moisture can escape.
Conventional polypropylene films of a thickness required for most applications, whether monolayer or composite, have oxygen and moisture vapor permeability values that are not sufficient to allow the optimum oxygen and moisture levels to be maintained in a sealed package containing products of the type mentioned.
It is known to prepare microporous films based on an opaque polymer mixture comprised of about 45% to 55% homopolypropylene and 55% to 45% of a copolymer of propylene and ethylene containing about 2% to 5% ethylene by weight, which films have greater oxygen permeability than films made of plain polypropylene. This polymer mixture is blended into a mixture comprised of about 40 to 60% of the polymer mixture and 60 to 40% of certain inorganic filler materials and is subjected to biaxial orientation. Anderson, U.S. Pat. No. 4,842,875, teaches the use of such films in the preparation of controlled atmosphere containers for use with fresh vegetables, fruits and flowers.
Microporous films known to the art as described above function reasonably well for preparing films having increased and, to a degree, controllable oxygen and moisture vapor permeability. However, the voiding pigments employed in the prior art are of a particle size large enough and are employed in concentrations great enough to result in formation of voids of such a size that the resultant films are almost totally opaque.
It is also known to prepare polyolefin films of high oxygen transmission rate for some applications by laminating a polyethylene film of about one mil thickness to a polypropylene film of about 0.4 to 0.5 mil thickness. The polypropylene film acts as a stiffener to give the polyethylene sufficient stiffness that it can be used as a produce wrapper. The polyethylene, which has an inherently high OTR, then acts as a high OTR sealant layer.
In work leading to this invention, it has been found that, by blending low density polyethylene and polypropylene within specific blending ratios, a film can be prepared that has a sufficiently high oxygen transmission rate (OTR) that it can be used for packaging fresh fruits, flowers and vegetables that require a continuous replenishment of the oxygen supply in the closed package. Films having the desired properties can be prepared from blends consisting essentially of about 61 to 85% by weight of isotactic polypropylene and 39 to 15% by weight of low density polyethylene. Such films also exhibit a sufficiently great stiffness to permit their use in this application.
Films prepared with the blends just described, when cast and drawn to a typical packaging film thickness, i.e. 0.7 to 0.8 mil (70 to 80 gauge), exhibit several properties that make them desirable for the packaging of fresh produce items. They are of a stiffness required of packaging materials generally, i.e. a tensile modulus sufficient to allow the films to be handled in conventional film conversion equipment. In addition, such films exhibit an OTR greater than 200 cc/100 sq. in./day/atmosphere; a level at which the packaged product can continue to respire for a reasonable time after it has been harvested, thus assuring a reasonable shelf life for the product.
While films prepared with polypropylene and low density polyethylene in the ratios expressed in the previous paragraph exhibit the OTR and stiffness required for the contemplated application, they are not so clear and haze-free as is desired for prepackaging of produce for the retail market. For this application, a clear film is required in order to display the product most attractively to the shopper in, e.g. a supermarket display or the like. A clear film, for the purposes of this invention, is defined as one having a haze value less than about 4% as determined by the Gardner Haze Meter.
Based on the above, it can be said that, in general, a film product for use in packaging of fresh fruits, vegetables and flowers for the retail market must have sufficient stiffness to allow it to be handled on conventional film conversion equipment, sufficient OTR to allow the contents to continue respiring for a reasonable time after they are packaged and be sufficiently haze-free to be attractively displayed to a purchaser in the marketplace.
It is the object of the invention to provide polyolefin films based on polypropylene and low density polyethylene that have the properties required to be useful in modified atmosphere packaging applications for packaging fresh vegetables, fruits and flowers.
BRIEF DESCRIPTION OF THE INVENTION
In accordance with this invention, there are provided films having the desired higher oxygen transmission rate (OTR) required for the packaging of fresh produce items, said films being prepared with blends of polypropylene and low density polyethylene, which films also contain an olefin heteropolymer of at least two &agr;-monoolefins.
Specifically, the invention is a biaxially oriented film comprised of a polyolefin mixture consisting essentially of
a) about 75 to 92% by weight of isotactic polypropylene;
b) about 5 to 15% by weight of low density polyethylene having a density less than about 0.935; and
c) about 3 to 10% by weight of an olefin heteropolymer containing propylene and at least one other &agr;-olefin of 2 to 4 carbon atoms, said heteropolymer having a melting point less than that of polypropylene and a density no greater than about 0.95 g/cc.
Films prepared with polymer compositions within the specified limits are found to have properties that meet specifications for commercial use in the packaging of fresh fruits, vegetables and flowers. Specifically, these films exhibit an OTR of at least 200 cc/100 sq.in./day/atmosphere, a modulus of at least about 195,000 psi, preferably at least 200,000 psi and a haze value no greater than about 4%.
DETAILED DESCRIPTION OF THE INVENTION
In the following discussion, gas permeability properties of the films of the invention will be discussed in terms of their OTR. Carbon dioxide transmission rate is also an important parameter of these films affecting their utility as packaging materials for fresh produce items since carbon dioxide is also generated as the product continues to respire. Carbon dioxide transmission rates can be estimated using the g
Caesar Rivise Bernstein Cohen & Pokotilow Ltd.
Nutter Nathan M.
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