Package comprising low application temperature hot melt...

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

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C156S334000, C206S813000, C524S275000, C524S277000, C524S487000, C524S488000, C524S579000, C525S210000, C525S211000, C525S216000, C525S247000, C526S248000

Reexamination Certificate

active

06319979

ABSTRACT:

The subject invention pertains to hot melt adhesives. In particular, the subject invention pertains to hot melt adhesives comprising at least one first ethylene polymer, and optionally at least one wax and/or tackifier. In particular, this invention relates to low viscosity hot melt adhesives comprising at least one homogeneous linear or substantially linear interpolymer of ethylene with at least one C
3
-C
20
&agr;-olefin, further characterized by each said interpolymer having a polydispersity less than 2.5. In preferred embodiments, the inventive hot melt adhesives may be applied at application temperatures of less than 150° C. These hot melt adhesives are particularly useful for case and carton sealing, and for tray forming applications in the packaging industry. Also disclosed is a dual reactor process for the preparation of such hot melt adhesives. Also disclosed are preferred hot melt adhesives for the bonding of cardboard or paperboard, as well as the resultant packaging articles comprising cardboard or paperboard which have been adhered by such hot melt adhesives.
Hot melt adhesives are used widely in the packaging industry for such applications as case and carton sealing, tray forming and box forming. The substrates to be bonded include virgin and recycled kraft, high and low density kraft, chipboard and various types of treated and coated kraft and chipboard. Composite materials are also used for packaging applications such as for the packaging of alcoholic beverages, These composite materials may include chipboard laminated to an aluminum foil which is further laminated to film materials such as polyethylene, mylar, polypropylene, polyvinylidene chloride, ethylene vinyl acetate and various other types of films. Additionally, these film materials also may be bonded directly to chipboard or kraft. The aforementioned substrates by no means represent an exhaustive list, as a tremendous variety of substrates, especially composite materials, find utility in the packaging industry.
Hot melt adhesives for packaging are generally extruded in bead form onto a substrate using piston pump or gear pump extrusion equipment. Hot melt application equipment is available from several suppliers including Nordson, ITW and Slautterback. Wheel applicators are also commonly used for applying hot melt adhesives, but are used less frequently than extrusion equipment.
Hot melts are required to have sufficient adhesion to substrates to firmly hold the package together, and in many cases, end users of hot melt adhesives require full fiber tearing bonds, meaning substantially all the fiber is removed from the substrate along the entire length of the adhesive application when the bond is separated by hand. Generally, in order to get full fiber tearing bonds, hot melts need to be applied at temperatures of 175° C. or greater. This increases the open time of the adhesive and lowers the viscosity for better penetration into the substrate. Open time refers to the amount of time that the adhesive can form a bond to the substrate.
In addition to bonding requirements, customers are demanding higher performance in other areas, such as thermal stability. Good thermal stability means that the product will not darken in the glue pot with prolonged exposure to high temperatures, will not produce char, skin or gel, and will not exhibit a substantial viscosity change over time. High application temperatures, along with exposure to oxygen, can increase the degradation of the hot melt adhesives. This problem is most commonly alleviated with the use of antioxidants such as Irganox® 565, 1010 and 1076, which are hindered phenolic antioxidants produced by Ciba-Geigy, located in Hawthorne, N.Y.
Another way to reduce char, skin, gel formation, discoloration and viscosity changes, is to lower the application temperature of the hot melt adhesive. In addition to improving thermal stability, lowering the application temperature also reduces the risk of severe burns to hot melt equipment operators, decreases the amount of electricity required to heat the adhesives which can result in energy cost savings, decreases maintenance costs, and reduces the amount of odors due to volatiles coming from the adhesives. Decreasing the odor and fumes coming from the adhesive is particularly appealing to customers, and to the employees who work in plants utilizing hot melt adhesives on a regular basis.
Hot melt adhesives are typically applied at temperatures of 175° C. For the aforementioned reasons, it is desirable to apply hot melt adhesives at temperatures of less than 155° C., and preferably from 135° C. to 150° C. Hot melt adhesives intended for application temperatures of less than 155° C., based on polymers such as ethylene n-butyl acrylate, ethylene vinyl acetate, and polyethylene, are known. Such adhesives generally employ lower melting point raw materials such as tackifying resins and waxes which tend to sacrifice important physical characteristics of the adhesive such as heat resistance. Lower application temperatures can also reduce the open time resulting in less penetration into the substrates and therefore poorer bondability of the hot melt adhesives.
Hot melt adhesives known in the art generally comprise three components: a polymer, a tackifier, and a wax. Each component may comprise a blend of two or more components, that is, the polymer component may comprise a blend of two different polymers. The polymer provides strength to the adhesive bond. The tackifier provides tack to the adhesive which serves to secure the items to be bonded while the adhesive sets, and reduces the viscosity of the system making the adhesive easier to apply to the substrate. The wax shortens the open/close times and reduces the viscosity of the system. Certain hot melt adhesives known in the art further comprise oil to reduce the viscosity of the system. Hot melt adhesives based on previously used polymers include ethylene vinyl acetate copolymers (EVA), atactic polypropylene (APP), low density polyethylene (LDPE), and homogeneous linear ethylene/&agr;-olefin copolymers. Prior art hot melt adhesives typically employed large levels of tackifier to reduce the viscosity of the system to levels which enabled its facile application to the substrate, e.g., to viscosities less than about 5000 centipoise (50 grams/(cm.second)). However, the use of such tackifiers poses disadvantages, as tackifiers have the tendency to corrode equipment, are malodorous, and impede recyclability of paperboard bearing them.
Industry would find advantage in hot melt adhesives exhibiting good adhesion to both coated and uncoated paperboard, which adhesives exhibit good low temperature adhesion and/or high shear adhesion failure temperatures (SAFT). Industry would find particular advantage in hot melt adhesives which exhibit such properties, and, additionally, which minimize the use of tackifiers.
U.S. Pat. No. 5,041,482 to Ornsteen et al. issued Aug. 20, 1991 discloses glue stick adhesives for use in glue guns that can be applied at application temperatures in the range of 82° C. to 138° C., and preferably less than 121° C. Ornsteen discloses ethylene vinyl acetate, polyethylene and polypropylene polymers having melt indices in excess of 750 g/10 minutes. The exemplified glue stick adhesive compositions are high in viscosity, and thus unsuitable for piston pump or gear pump extrusion application equipment used widely in the packaging industry.
U.S. Pat. No. 5,373,049 to Ornstern et al. issued Dec. 13, 1994 teaches cool melt adhesives, again based on polyethylene, polypropylene and ethylene vinyl acetate.
U.S. Pat. No. 5,550,472 to Liedermooy et al. issued Mar.19, 1996 teaches a hot melt adhesive designed for low application temperature based on ethylene n-butyl acrylate copolymers having a melt index of at least 600 g/10 min., a terpene phenolic tackifying resin and a low melting point synthetic Fischer-Tropsch wax. It is suggested that small amounts, up to 20 percent by weight, of other polymeric additives such as typically known ethylene vinyl acetate, ethylene methyl

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