Stock material or miscellaneous articles – Composite – Of silicon containing
Reexamination Certificate
1999-12-20
2002-10-29
Thibodeau, Paul (Department: 1773)
Stock material or miscellaneous articles
Composite
Of silicon containing
C428S451000, C428S500000, C428S515000, C428S516000
Reexamination Certificate
active
06472077
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a block-resistant film. More precisely, the invention relates to a multilayer film having a core layer and a block-resistant layer which inhibits blocking to a functional layer of the film which is printable or sealable or treatable for sealing or printing.
BACKGROUND OF THE INVENTION
Blocking is the unwanted adhesion between layers of plastic film that may occur under pressure, usually during storage or use. It is known that blocking can be prevented with the use of antiblocking agents which are added to the composition which makes-up the surface layer of the film. Known antiblocking additives for plastic packaging film include synthetic waxes. In U.S. Pat. No. 4,692,379, various antiblocking agents for a heat sealable outer skin of a multilayer film are specifically described, including silica, clay, talc and glass.
Sealable coatings are used on flexible packaging films so that the films can be sealed with the application of pressure, with or without out exposure to elevated temperatures. These so called “cold seal” coatings can pose blocking problems. A typical cold seal coating is a natural or synthetic rubber latex combined with a soft polymer which tends to be tacky at room temperature and causes blocking. The rubber component permits sealing with slight pressure and without using heat. The cold seal coating is usually applied to a plastic film as it is wound into a roll. Since the cold seal coatings are tacky, it is important that the backside of the film which contacts the cold seal coating upon winding does not stick (block) to the cold seal coating so that the film can be easily unwound for use on packaging equipment.
One approach for reduced blocking between the cold seal coating and the backside of the film has been to formulate a cold seal coating which is nonblocking to certain surfaces including polypropylene, such a cold seal formulation is described in U.S. Pat. No. 5,616,400.
Another approach uses a cold seal release material on the layer opposite the cold seal surface. See U.S. Pat. Nos. 5,482,780; 5,489,473 and 5,466,734.
In U.S. Pat. No. 4,692,379, a film is described which has an upper heat sealable layer formed from an ethylene-propylene-containing copolymer or terpolymer and an antiblocking agent, the lower heat sealable layer is formed from an ethylene-propylene-containing copolymer or terpolymer and antiblocking agent and a quantity of silicone oil such that the coefficient of friction-reducing amount of the silicone oil will be present on an exposed surface of the upper heat sealable layer following mutual contact of the upper and lower surfaces. The silicone oil additive is described as having a viscosity of from about 350 to about 100,000 centistokes with about 10,000 to about 30,000 centistokes being preferred. An advantage of the invention as described in the '379 patent is that the silicone is present on the exposed surface of the lower layer in discrete microglobules which, to some extent, transfer to the upper surface upon contact. The silicone on the surfaces of the film facilitates machinability.
An attempt was made to produce a block-resistant functional film, typically a film having a printing function or sealing function, with silicone oil in a surface layer as an antiblocking agent. It was found that the silicone oil was detrimental to the printing or sealing function.
Copending U.S. application Ser. No. 09/026,454, U.S. Pat. No. 6,074,762 filed Feb. 19, 1998, describes a multilayer film having a core layer and a block-resistant layer which inhibits blocking to a functional layer of the film which is printable or sealable or treatable for sealing or printing. The block-resistant layer includes polydialkylsiloxane, preferably in small quantities. When the film is wound into a roll, polydialkylsiloxane deposits silicone onto the functional layer but the amount of silicone deposited is not substantially detrimental to the printing function or the sealing function.
SUMMARY OF THE INVENTION
The invention is directed to a block-resistant film, comprising:
(a) a core layer of a thermoplastic polymer, the core layer having a first side and a second side,
(b) a functional layer which is printable or sealable or treatable for printing or sealing on the first side of the core layer, and
(c) a block-resistant layer on the second side of the core layer comprising a thermoplastic polymer and an amount of a polydialkylsiloxane, based upon the entire weight of the block-resistant layer, sufficient to inhibit blocking of the block-resistant layer to the functional layer when they are in contact, wherein the surface of said block-resistant layer is flame treated or corona treated.
It is a feature of the invention to have a block-resistant layer which comprises a thermoplastic polymer and an amount of a polydialkylsiloxane sufficient to inhibit blocking of the block-resistant layer to the functional layer which happens when the film is wound into a roll or stacked so that the functional layer is in contact with the block-resistant layer.
It is an advantage of the invention that when the film is wound into a roll or stacked for storage in such a manner that the functional layer is in contact with the block-resistant layer of the film, the block-resistant layer, made with the polydialkylsiloxane described herein, deposits less silicone onto the functional layer than the silicone oil described in U.S. Pat. No. 4,692,379 such that with the instant invention the printing or sealing function is not substantially impaired.
DETAILED DESCRIPTION OF THE INVENTION
The block-resistant film of the instant invention is resistant to the undesirable adhesion between an exposed surface of the first side of the film to an exposed surface of a second side of the film which adhesion develops under pressure, typically, during storage or use.
The block-resistant layer of the instant invention is made with a particular polydialkylsiloxane additive. The polydialkylsiloxane additive is especially selected because it inhibits blocking and does not tend, in an amount detrimental to the functional properties of the film, to appear on the surface of the block-resistant layer or the functional layer. The appearance of the polydialkysiloxane may be determined by measuring the amount of silicon on the film surface by Electron Spectroscopy for Chemical Analysis (ESCA). It has been found that when a functional layer which is printable or sealable or treated for printing or sealing, contacts a layer which is compounded with a polydialkylsiloxane, of the kind described in U.S. Pat. No. 4,692,379, that polydialkylsiloxane (silicone oil) appears on the surface of both the functional layer and the block-resistant layer, after contact of the functional layer to the block-resistant layer (by measuring the amount of silicone), to an extent which is considered detrimental to the printability or sealability of the functional layer.
The core layer comprises a film-forming thermoplastic polymer which has properties suitable for extrusion or coextrusion followed by biaxial orientation in the machine and transverse directions under elevated temperature so as to form a film. Although, preferably, the thermoplastic polymer of the core layer is a propylene homopolymer, it can be any polymer made from a 2 to 4 carbon atom olefin, such as ethylene or butene-1 or a polymer made predominantly of propylene with a minor amounts of another olefin, usually a 2 to 4 carbon atom olefin.
The layer which is block-resistant and the functional layer may be the same or different. The block-resistant layer and the functional layer comprise a film-forming polymer having properties suitable for extrusion and uniaxial or biaxial orientation (by stretching the extrudate in the machine direction and/or transverse direction under elevated temperatures) and for forming skin layers on the outer surfaces of the core layer. Such layers may comprise a thermoplastic polymer composed predominantly of an olefinic polymer such as polypropylene or polyethylene. A blend of polymers may
Cretekos George F.
Wagner, Jr. John R.
Exxon Mobil Oil Corporation
Jackson Monique R.
James Rick F.
Thibodeau Paul
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