FIELD OF THE DISCLOSURE

This disclosure relates to an oriented polypropylene film with improved blocking resistance. More specifically, this disclosure relates to a cold seal release film with an outer release layer that provides blocking and scuff resistance.

BACKGROUND

Cold seal release films are most commonly used for the outside of a multilayered packaging structure needed to be sealed under low temperature region. The cold seal release film should not form a cohesion with a cold seal adhesive layer coated onto the opposite side of the multilayered laminated packaging structure when the film is wound into a roll. As such, blocking is the unwanted adhesion between the cold seal adhesive layer and the release layer under the conditions of pressure and aging.

U.S. Pat. No. 4,925,728 describes a method of producing a fine particle-pigmented BOPP release film comprising a core layer of polypropylene homopolymer and two top release layers consisting of 98.5 to 99.6 wt % of polypropylene homopolymer and 0.4 to 1.5 wt % of polydiorganosiloxane.

Patent Application WO 1995015850A1 discloses a release label stock comprising a substrate of oriented high crystalline polypropylene and a release coating material which is an electron beam curable silicone-acrylate release material.

U.S. Pat. No. 6,465,107 describes a method of producing silicone containing LDPE resins by chemical grafting reaction and a method of incorporating a silicon-containing LDPE resin into a laminate as an outer layer by coextrusion.

U.S. Pat. No. 7,105,233 describes a method of using organo-modified polydialkylsiloxane to greatly reduce the migration of silicone oils by incorporating large alkyl groups into siloxane backbone.

U.S. Pat. Nos. 6,074,762; 6,472,077; and 6,682,822 describe a type of the outer layer of polymers based on homopolymer PP, HCPP, and HDPE, comprising essentially homopolymer or a blend of homopolymers of ethylene or propylene with silicone oils and ultra high molecular weight silicone (silicone gum) at an amount of 0.05 to 10 wt % in the release layer. Silicone oil transfer and contamination are still a problem resulting from masterbatches of silicone oils with lower viscosity.

U.S. Pat. Nos. 5,981,047; 5,798,174; 5,489,473; and 5,792, 549 describe a type of cold seal release film with butene-based release materials essentially consisting of polybutene homopolymers and a blend of random copolymers or terpolymers of ethylene, butene, or propylene monomers. The release layer of the films has poor scuff resistance.

U.S. Pat. No. 5,840,419 describes a method of making printable BOPP films with two outer layers comprising ethylene-propylene copolymer and partially cross-linked polydimethylsiloxane.

SUMMARY

Applicants have discovered an economical method to improve blocking resistance of cold seal release films using stiff high crystalline polymers and avoid using migratory additives and soft release materials in the outer release layer. Specifically, none of the references cited above demonstrated that a combination of high crystalline polypropylene and low ethylene containing polybutene-1 can result in excellent release properties to a cold seal adhesive layer. In addition, partially crosslinked polydialkylsiloxane can efficiently lower and stabilize the coefficient of friction (“COF”) of the outer release layer, greatly avoiding silicone oil, and silicone moieties contamination or transfer.

Blocking can be prevented by using anti-blocking agents and special thermoplastic polymers which can reduce the tendency of forming cohesion with cold seal adhesives—either natural rubber latex or synthetic rubber latex, which are rubbery materials with glass transition temperatures (Tg) lower than 0° C. Some polyolefin thermoplastic polymers and additives demonstrate blocking resistance to cold seal adhesives or pressure sensitive adhesives when they are incorporated into an outer layer of a film with a direct contact to a cold seal adhesive layer. The special polyolefin thermoplastic polymers can include high crystalline polypropylene (“HCPP”, high crystallinity, isotactic content 95% or greater, Tg=0˜15° C.) homopolymers, high density polyethylene (“HDPE”, high crystallinity, Tg=−125° C.) homopolymers, and butene-1 containing polymers (Tg=−18° C.). The special additives can include crosslinked silicones and silicone-grafted polyolefins which have a low tendency to transfer to the opposite side of the film.

Silicones also known as polydialkylsiloxanes (most commonly used are polydimethylsiloxane (“PDMS”)) have a unique chemical structure that has a backbone of silicon-oxygen (Si—O) linkages. Polydimethylsiloxane has a low glass transition temperature (Tg=−125° C.) and can be used as slip and release agents. Commercially available are silicone fluids (silicone oil and gum types of silicones) with a viscosity of from 0.65 to 2,500,000 cSt, partially cross-linked particles, fully cross-linked particles, and grafted silicone containing polyolefin polymers. Crosslinking of the polymers can significantly increase glass transition temperature. Both non-crosslinked silicone fluids and cross-linked silicones not only can provide good release properties but can also reduce the coefficient of friction of the surface layer. The difference in physical properties (especially shape and hardness) of those crosslinked and partially crosslinked silicones can show very different slip and release performance as they are used in the outer layer of a film. The drawbacks of these liquid-like silicones can include the usually undesirable cross-contamination resulting from low molecular weight silicone moieties or silicone oils transferred to the opposite side of a film.

In some embodiments, a cold seal release film includes a core layer comprising polypropylene; a first outer layer on a side of the core layer comprising polyolefin resin; and a second outer layer on a side of the core layer opposite the first outer layer comprising high crystalline polypropylene and 0.75-2.5 wt. % partially crosslinked polydialkylsiloxane. In some embodiments, the high crystalline polypropylene has less than 3 wt. % of xylene solubles and has a melt flow rate of 2-4 g/10 min. In some embodiments, the second outer layer comprises polybutene-1. In some embodiments, the second outer layer comprises 2.5-40 wt. % polybutene-1. In some embodiments, the polybutene-1 is low ethylene-containing polybutene-1. In some embodiment, the low ethylene-containing polybutene-1 has an ethylene content of less than 2 mol %. In some embodiments, the low ethylene-containing polybutene-1 has a melt flow rate of 2 to 6 g/10 min. In some embodiments, the second outer layer comprises spherical anti-blocking agents. In some embodiments, the second outer layer comprises 1000-5000 ppm spherical anti-blocking agents. In some embodiments, the spherical anti-blocking agents comprise at least one of crosslinked silicone polymers and synthetic SiO2. In some embodiments, the partially crosslinked polydialkylsiloxane is partially crosslinked polydimethylsiloxane. In some embodiments, the second outer layer has a dynamic coefficient of friction of 0.20-0.35. In some embodiments, the polypropylene in the core layer is high crystalline polypropylene. In some embodiments, the core layer comprises 2.5-25 wt. % hydrogenated hydrocarbon resins. In some embodiments, the core layer comprises 1-1000 ppm of antistatic additives. In some embodiments, the polyolefin resin comprises ethylene homopolymer, propylene homopolymer, ethylene or propylene-based copolymers and terpolymers, or blends thereof. In some embodiments, a side of the second outer layer opposite the core layer is discharge-treated using high densities of energy flux. In some embodiments, a side of the first outer layer opposite the core layer is discharge-treated.

In some embodiments, a method of forming a cold seal release film includes coextruding a laminate comprising: a core layer comprising polypropylene; a first outer layer on a side of the core layer comprising polyolefin resin; and a second outer layer on a side of the core layer opposite the first outer layer comprising high crystalline polypropylene and 0.75-2.5 wt. % partially crosslinked polydialkylsiloxane; and biaxially orienting the coextruded laminate. In some embodiments, the high crystalline polypropylene has less than 3 wt. % of xylene solubles and has a melt flow rate of 2-4 g/10 min. In some embodiments, the second outer layer comprises polybutene-1. In some embodiments, the second outer layer comprises 2.5-40 wt. % polybutene-1. In some embodiments, the second outer layer comprises 2.5-25 wt. % polybutene-1. In some embodiments, the polybutene-1 is low ethylene-containing polybutene-1. In some embodiments, the low ethylene-containing polybutene-1 has an ethylene content of less than 2 mol %. In some embodiments, the low ethylene-containing polybutene-1 has a melt flow rate of 2 to 6 g/10 min. In some embodiments, the second outer layer comprises spherical anti-blocking agents. In some embodiments, the second outer layer comprises 1000-5000 ppm spherical anti-blocking agents. In some embodiments, the spherical anti-blocking agents comprise at least one of crosslinked silicone polymers and synthetic SiO2. In some embodiments, the partially crosslinked polydialkylsiloxane is partially crosslinked polydimethylsiloxane. In some embodiments, the second outer layer has a dynamic coefficient of friction of 0.20-0.35. In some embodiments, the polypropylene in the core layer is high crystalline polypropylene. In some embodiments, the core layer comprises 2.5-25 wt. % hydrogenated hydrocarbon resins. In some embodiments, the core layer comprises 1-1000 ppm of antistatic additives. In some embodiments, the polyolefin resin comprises ethylene homopolymer, propylene homopolymer, ethylene or propylene-based copolymers and terpolymers, or blends thereof. In some embodiments, the method further comprises discharge-treating a side of the second outer layer opposite the core layer using high densities of energy flux. In some embodiments, the method further comprises discharge-treating a side of the first outer layer opposite the core layer.

In some embodiments, the release layer of the coextruded release film comprises a blend of 75 to 99.8 wt % high crystalline polypropylene (HCPP, isotactic content≥95%) and 1 to 25 wt % low ethylene containing butene-1 polymers, and 0.2 to 1.5 wt % crosslinked silicone polymer particles or synthetic silica (SiO2) particles. In some embodiments, the release layer of the coextruded release film comprises a blend of 1 to 99.8 wt % high crystalline polypropylene (HCPP, isotactic content≥95%) and 1 to 25 wt % low ethylene containing butene-1 polymers (PB-1), and 0.2 to 0.5 wt % crosslinked silicone polymer particles or 0.1 to 3 wt % partially crosslinked polydialkylsiloxane particles. In some embodiments, the release layer of the coextruded cold seal release film comprises a blend of 0 to 99.8 wt % high crystalline polypropylene (HCPP, isotactic content≥95%), 0 to 99.8 wt % high density polyethylene (HDPE), and 0 to 40 wt % low ethylene containing PB-1 polymers, 0 to 40 wt % the random copolymers of ethylene, propylene or butene-1, and 0.2 to 5.0 wt % crosslinked silicone polymer particles or partially crosslinked polydialkylsiloxane particles. In some embodiments, the release layer of the coextruded cold seal release film comprises a blend of 60 to 99.8 wt % high density polyethylene (HDPE) and 1 to 40 wt % low ethylene containing PB-1 polymers, and 0.2 to 0.5 wt % crosslinked silicone polymer particles, and 0.1 to 3 wt % partially crosslinked polydialkylsiloxane particles.

Additional advantages will be readily apparent to those skilled in the art from the following detailed description. The examples and descriptions herein are to be regarded as illustrative in nature and not restrictive.

All publications, including patent documents, scientific articles and databases, referred to in this application are incorporated by reference in their entirety for all purposes to the same extent as if each individual publication were individually incorporated by reference. If a definition set forth herein is contrary to or otherwise inconsistent with a definition set forth in the patents, applications, published applications and other publications that are herein incorporated by reference, the definition set forth herein prevails over the definition that is incorporated herein by reference.

