Stock material or miscellaneous articles – Structurally defined web or sheet – Including components having same physical characteristic in...
Reexamination Certificate
2002-11-05
2004-09-07
Tarazano, D. Lawrence (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Including components having same physical characteristic in...
C428S515000, C428S516000, C428S520000, C428S910000
Reexamination Certificate
active
06787220
ABSTRACT:
This invention relates to multilayer heat shrinkable film.
This invention has particular but not exclusive application to packaging for foods such as meat, poultry, and cheese and for illustrative purposes reference will be made to such application. However, it is to be understood that this invention could be used in other applications, such as packaging for curable putties and sealants, other foodstuffs such as tofu and the like.
Heat shrinkable films for perishables, such as meat, generally have a heat sealable inner layer and an outer abuse layer. The inner and outer layers are generally composed of the same resin, since resins of different melting points generally do not have the same shrinkage. This is to ensure that the shrink of the inner and outer layer is balanced. If the shrink of the inner and outer layers is not balanced, during the biorientation step, the secondary bubble is not stable and the film tends to curl inward or outward. In other words, if a high shrinkage resin in the inner layer is combined with a low shrink outer layer, or vice versa, the resultant film will curl or roll up and will not be able to be fabricated into bags.
In these types of films, the composition of the outer layer is chosen to be the same as the inner layer in order to achieve adequate seal strength and to prevent curl. However, an abuse outer layer having the same composition as the inner layer, leads to problems of burn-through and the need for irradiation. When heat sealing a film to make a bag, the seal area often becomes extruded during sealing. This elongation of the product results in thinning of the film and in an extreme situation severing of the thinned film. This is known as burn-through. One solution to this burn-through problem is to irradiate the film prior to manufacture of the bag.
Irradiation of a multilayer film causes the polymers in the film to cross-link. However, not all cross-linked thermoplastic films are easy to melt, which causes a reduction in heat sealability at conventional temperatures, pressures and times when using typical heat sealing equipment, thereby resulting in seals in bags made from the film. The equipment to irradiate the film is also highly expensive, costly to operate, increases the product cost, and usually requires an added step in the production operation.
International Publication No. WO00/32393 describes an attempt to overcome the problems of the above prior art films. There is disclosed a multilayer heat shrinkable film having an outer layer composed of a blend of thermoplastic polymers to provide a higher average melting point material than the inner layer, but having comparable shrink to prevent curling. The high melting point outer layer prevents burn-through. This allows a multilayer heat shrinkable film to be produced with excellent heat sealability while maintaining a high puncture resistant, high temperature outer layer to obtain a broad heat sealing range without the need to cross-link either the inner or outer layer. However, the film lacks the necessary clarity, gloss and haze requirements for a film for many products.
Accordingly, this invention in one aspect resides broadly in one aspect of the invention in a heat shrinkable multilayer film including:
a heat sealable inner layer of a thermoplastic polymer or a thermoplastic polymer blend;
an abuse layer of a thermoplastic polymer blend having a melting point at least about 20° C. higher than said inner layer, said abuse layer blend being selected to have substantially balanced shrinkage relative to said inner layer, and
a thermoplastic, optically clear outer layer adjacent said abuse layer, wherein said outer layer has a lower Vicat softening point as determined by ASTM D-1525 than said abuse layer.
The Vicat softening point (VSP) in accordance with the ASTM D-1525 is the temperature at which a flat ended needle of 1 mm
2
circular cross section will penetrate a thermoplastic specimen to a depth of 1 mm under a specified load and temperature. The test specimen is moulded or cut from a sheet with a specified minimum thickness and width. The specimen is placed on support bars and heated at a selected uniform rate of temperature rise. The needle is lowered so that it rests on the surface of the specimen. When the needle penetrates 1 mm the temperature is recorded, being the Vicat softening point. The data obtained from this test is useful in comparing the heat softening qualities of thermoplastic materials. Generally, thermoplastic products with a higher Vicat softening point tend to be harder to penetrate, whereas a lower Vicat softening point indicates a softer product.
Not being bound by theory, it is thought that films having an outer abuse layer with a higher melting point than the heat sealable inner layer, have less than desirable optical properties as the outer layer does not shrink as much as the inner layer. This may be so even if the inner and outer layers have compatible shrink such that the film does not curl. Since the outer layer is adhered to the inner layer, whether directly or via other layers of the film, it becomes wrinkled. This wrinkling causes the light to refract and makes the film appear hazy or white. When a softer material is added over the abuse layer it fills in the voids created by the wrinkles and allows the light to pass cleanly through the film. Since the light is not refracted the film appears clear.
The heat sealable inner layer may include very lower density polyethylene (VLDPE), linear low density polyethylene (LLDPE) ethylene vinyl acetate copolymer (EVA), ethylene &agr;-olefin plastomer copolymer (plastomer) or a blend on two or more thereof. Suitably, the LLDPE is produced using metallocene single site catalysts.
Suitable polymer blends for the heat sealable layer may include:
a blend of VLDPE, EVA, and plastomer, and
a blend of LLDPE, EVA, and plastomer.
The polymer blend may have a melting point in the range of about 94° C. to 96° C. Each of the aforementioned three component blends may include between about 20% to about 33% of VLDPE or LLDPE, between about 33% to about 60% of EVA, and between about 15% to about 30% of plastomer of the total blend. The EVA may contain between about 3% and about 18% vinyl acetate based on the weight of the copolymer.
The heat sealing inner layer may include other suitable polymers and copolymers such as polypropylene, ethylene-propylene copolymer or an ionomer. It is to be understood, however, that the thermoplastic polymers mentioned herein are not intended to be an exhaustive list, but merely exemplary.
Thermoplastic polymers that are suitable for use in the abuse layer are recognized by those skilled in the art. The thermoplastic polymers may be selected to enable a broad heat sealing range while preventing burn-through during impulse heat sealing. Suitably polymers that may be included in the abuse layer are polypropylene (PP), polybutylene (PB), plastomer and EVA. Examples of suitable polymer blends for the abuse layer may include:
a blend of PP and PB,
a blend of PP, PB and EVA, and
a blend of PP, PB and plastomer.
The abuse layer polymer blend may have a melting point in the range of about 115° C. to 150° C., more suitably in range of about 123° C. to 136° C. The VSP of the abuse layer may be in greater than 115° C., more suitably about 119° C. to 120° C.
The abuse layer may include other thermoplastic materials, such as, for example, an ionomer, VLPDE, LLDPE or blends of these materials.
The optically clear outer layer may be a thermoplastic polymer or polymer blend. Suitable resins for use in the abuse layer may include PP, VLDPE, LLDPE, EVA, plastomer or suitable blends thereof. The LLDPE may be Ziegler-Natter catalyzed polymer. However, more suitably, the LLDPE is produced using metallocene single site catalysts.
Suitable polymers or polymer blends for the optically clear layer may include:
a blend of PP and plastomer;
a blend of VLDPE and plastomer;
a blend of LLDPE and plastomer;
a blend of VLDPE, EVA, and plastomer,
a blend of LLDPE, EVA, and plastomer, and
a blend of VLDPE, plastomer and
Ginossatis Stamatis
Wallace Rodney L.
Corless Peter F.
Edwards & Angell LLP
O'Day Christine C.
Tarazano D. Lawrence
Vector Europe NV
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