Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1998-09-02
2001-03-27
Teskin, Fred (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S474000, C524S476000, C524S484000, C524S490000, C524S570000
Reexamination Certificate
active
06207748
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to hot melt adhesives based on elastomeric ethylene copolymers. In particular, the invention relates to hot melt adhesives especially comprising ethylene/alpha-olefin copolymers wherein the alpha-olefin can have from 3-20 carbon atoms such as ethylene/butene-1, ethylene/hexene-1, ethylene/octene-1, and ethylene/propylene copolymers. These ethylene copolymers with prescribed range of comonomer levels can be prepared by polymerization of the suitable olefins in the presence of supported or unsupported metallocene-alumoxane catalysts systems.
2. Description of the Prior Art
Hot melt adhesives are mixtures of polymer and adjuvants, usually tackifying resin, wax, or low Tg materials such as oils and low molecular weight polymers. The polymer contributes cohesive strength to the final formulation. It is well known that highly crystalline polymers show very poor adhesive performance at temperatures below room temperature due to brittleness, hence, somewhat elastic polymers are most useful in hot melt adhesives. In addition, the presence of the comonomer improves adhesion of the final hot melt adhesive formulation to metal and polar substrates such as aluminum and glass.
Copolymers of ethylene and a mono-olefinically unsaturated polar comonomer such as vinyl acetate, methyl acrylate, acrylic acid and the likes, have been reported as useful polymers to formulate into hot melt adhesives. These hot melt adhesives generally are lacking in elevated temperature performance. These hot melt adhesives, although useful with polar substrates, are less useful with non-polar substrates.
Hot melt adhesives prepared with EVA (ethylene/vinyl acetate copolymer) containing high levels of vinyl acetate are generally ineffective in laminating non-polar plastic surfaces to the same plastic surfaces or to other surfaces. For example, polypropylene is widely used for product assembly in automobile industry due to low cost and solvent resistance. The polypropylene parts need a hot melt adhesive to bond to other polymer parts or bond to other substrates such as steel, aluminum, glass, etc. Besides excellent adhesion, another highly desirable requirement of the hot melt composition is its elevated temperature properties. Besides product assembly applications, polypropylene is widely used in film and in cast sheet form for packaging articles and for packaging food and such packaging requires closure which can resist mechanical breakage.
U.S. Pat. No. 4,072,735 discloses the preparation of a hot melt pressure sensitive adhesive which comprises ethylene-propylene rubber, tackifying resin, polybutene, and, optionally, a crystalline polypropylene. The addition of the crystalline polypropylene is required in order to obtain a useful adhesive at elevated temperatures.
U.S. Pat. No. 4,568,713 discloses a hot melt adhesive system comprising butene-1/ethylene copolymer (containing 5.5-10.0 wt. % ethylene), an aliphatic
on-polar tackifier, an antioxidizing agent and, optionally, microcrystalline wax. The patent discloses utility for long open time and the need for good cold metal bonding to steel and aluminum.
JP-62-129303 discloses the preparation of ethylene/alpha-olefin copolymers using metallocene-alumoxane catalysts. While the products obtained are olefin copolymers having narrow molecular weight distributions and relatively low softening points, these copolymers are waxes characterized by only slight surface stickiness or slight adhesiveness, and are therefore used as pigment dispersing agents and toners.
JP-61-236804 discloses preparing narrow molecular weight distribution ethylene/alpha-olefin copolymers with metallocene-alumoxane catalysts, however, these ethylene/alpha-olefin copolymer products are also waxes that are used as pigment dispersants and resin processing aids. No adhesive characteristics are taught for these products.
JP-62-121709 pertains to the copolymerization of ethylene with an alpha-olefin in the presence of a catalyst comprising a zirconium hydride compound such as a bis(cyclopentadienyl) zirconium monochlorohydride and an alumoxane to obtain a copolymer which is narrow in both molecular weight distribution and composition distribution. However, while the copolymer is excellent in transparency, impact resistance and heat sealability, it is also characterized by surface non-stickiness.
There is an apparent need in the art to obtain hot melt adhesives having improved elevated temperature properties, and better low temperature flexibility.
SUMMARY OF THE INVENTION
It is an objective of this invention to provide hot melt adhesives based on ethylene copolymers which provide good adhesive performance on plastic surfaces, in particular polypropylene and polyethylene. It is another objective of this invention to provide a hot melt adhesive which does not need the addition of an elastomer to modify final performance such as increased high temperature resistance and improved adhesion on non-polar surfaces.
It has been surprisingly found that ethylene/alpha-olefin copolymers having certain molecular weight ranges and certain comonomer level ranges can be utilized in formulating an adhesive formulation absent an elastomeric resin when the copolymers are prepared in the presence of a catalyst system comprising a Group IVB transition metal-cyclopentadienyl containing catalyst such as, for example, either supported or unsupported metallocene-alumoxane catalysts. The hot melt adhesive formulations comprise one of these copolymers, tackifier(s), and optionally, wax or oil. In the hot melt adhesive applications, the weight average molecular weight (M
w
) values of the copolymer will range from about 20,000 to about 100,000, preferably from 40,000 to 80,000, and the comonomer level will range from about 6 wt. % to about 30 wt. %, preferably from 10 wt. % to 20 wt. %.
The ethylene-co-alpha olefin polymers of the present invention can be produced in accordance with any known polymerization process, including a slurry polymerization, gas phase polymerization, and high pressure polymerization process.
A slurry polymerization process generally uses super-atmospheric pressures and temperatures in the range of 40-110° C. In a slurry polymerization, a suspension of solid, particulate polymer is formed in a liquid polymerization medium to which ethylene and comonomers and often hydrogen along with catalyst are added. The liquid employed in the polymerization medium can be an alkane, cycloalkane, or an aromatic hydrocarbon such as toluene, ethylbenzene or xylene. The medium employed should be liquid under the conditions of polymerization and relatively inert. Preferably, hexane or toluene is employed.
In modification, the polymer components of the present invention may be formed by gas-phase polymerization. A gas-phase process utilizes super-atmospheric pressure and temperatures in the range of about 50°-120° C. Gas phase polymerization can be performed in a stirred or fluidized bed of catalyst and product particles in a pressure vessel adapted to permit the separation of product particles from unreacted gases. Thermostated ethylene, comonomer, hydrogen and an inert diluent gas such as nitrogen can be introduced or recirculated so as to maintain the particles at a temperature of 50°-120° C. Triethylaluminum may be added as needed as a scavenger of water, oxygen, and other adventitious impurities. Polymer product can be withdrawn continuously or semi-continuously at a rate such as to maintain a constant product inventory in the reactor. After polymerization and deactivation of the catalyst, the product polymer can be recovered by any suitable means. In commercial practice, the polymer product can be recovered directly from the gas phase reactor, freed of residual monomer with a nitrogen purge, and used without further deactivation or catalyst removal.
The polyethylene copolymers of the present invention can also be produced in accordance with a high pressure process by polymerizing ethylene in combination with other monomers such as butene-1, h
Hughes Vince Leo
Mehta Aspy Keki
Tse Mun Fu
Bell Catherine L.
Exxon Chemical Patents Inc.
Kurtzman Myron B.
Prodnuk Stephen D.
Teskin Fred
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