Patent Application
20140079897
The present invention relates to hot melt adhesives with wide service temperature ranges, making these adhesives particularly well suited for case and carton sealing applications that are exposed to extreme temperatures.
Hot melt adhesives are applied to a substrate while in a molten state and cooled to harden the adhesive layer. The use of hot melt adhesives is widely used in packaging industry to seal containers, e.g. cardboard cases, trays and cartons. Many types of packaging applications require the use of an adhesive that is both heat resistant and cold tolerant. However, conventional hot melt adhesives exhibit strong bond strength at either the high or the low end temperatures, but not both. Polypropylene based systems have been utilized to improve heat resistance, but polypropylenes provide inadequate cold resistance because they become brittle at around 0° C. During transportation and storage, and depending on the season and location, the sealed containers are exposed to temperatures of from about −20° F. to about 140° F. Hence, there is a need for versatile hot melt adhesives with both good heat and cold resistances. The current invention addresses this need.
The invention provides a hot melt adhesive with wide service temperature ranges, methods of using the adhesive to bond substrates together, to close/seal cases and cartons and the like, and to articles of manufacture comprising the adhesive.
One aspect of the invention provides a hot melt adhesive comprising a metallocene catalyzed polyethylene copolymer, less than 10 wt % of a functionalized metallocene catalyzed polyethylene copolymer, greater than 22 wt % of a wax, a tackifier and optionally additives. The resultant hot melt adhesive has both good heat resistance and cold resistance performances.
Another aspect of the invention provides a method of sealing and/or making or forming a case, a carton, a tray, a box or a bag. The method comprises use of a hot melt adhesive wherein the adhesive comprises a metallocene catalyzed polyethylene copolymer and less than 10 wt % of a functionalized metallocene catalyzed polyethylene copolymer.
Yet another aspect of the invention is directed to an article of manufacture comprising a hot melt adhesive comprising a metallocene catalyzed polyethylene copolymer and less than 10 wt % of a functionalized metallocene catalyzed polyethylene copolymer. The article of manufacture is a carton, case, tray or bag used for packaging products. The article may comprise cardboard, paperboard or other substrate that has been adhered by such hot melt adhesives. In another embodiment, the adhesive is pre-applied to the article, e.g., carton, case, tray or bag during the manufacture thereof, and prior to packaging of a product.
The invention also provides a process for bonding a substrate to a similar or dissimilar substrate comprising applying to at least one substrate a molten hot melt adhesive composition and bonding said substrates together, wherein the hot melt adhesive comprises a metallocene catalyzed polyethylene copolymer and less than 10 wt % of a functionalized metallocene catalyzed polyethylene copolymer.
All documents cited herein are incorporated in their entireties by reference.
Weight percent (wt %) are calculated based on the total weight of the adhesive, unless otherwise noted.
The use of a wide service temperature range hot melt adhesives have high heat resistance and good cold resistance is important in when packaged goods are transported and/or stored under conditions where exposure to extreme temperatures is likely.
Conventionally, metallocene catalyzed polypropylene has been used to improve heat resistance in hot melt adhesives; and metallocene catalyzed polyethylene has been used to improve cold resistance in hot melt adhesives. Generally, high softening point tackifiers and less than 20 wt % of high melting point waxes are formulated with the metallocene catalyzed polyethylene copolymer to impart high heat resistance; however, this can lead to poor low temperature performances. Conversely, low softening point tackifiers and low melting point waxes are combined with the polymer to impart good low temperature performance; however, the resultant adhesive has poor high temperature performances.
It has been discovered that hot melt adhesives having an excellent balance of high and low temperature performance can be obtained using a combination of metallocene catalyzed polyethylene copolymer and a low level of functionalized metallocene catalyzed copolymer. A hot melt adhesive composition comprising metallocene catalyzed polyethylene copolymer and less than 10 wt % of a functionalized metallocene catalyzed polyethylene copolymer, as the base adhesive polymer, extends the hot and cold temperature performances of the adhesives, regardless of the softening temperatures of the tackifiers and melting point of the waxes.
Metallocene catalyzed polyethylene are obtained through polymerizing ethylene monomer with alpha-olefin (e.g. butene, hexene, octene) using metallocene catalytic system. Metallocene catalyzed polyethylene copolymers are commercially available from Exxon Mobil Corporation (under the trade name Exact) or Dow Chemical (under the trade name Affinity polymer). Metallocene catalysts are known as single site catalysts. These catalysts have only one reactive site and can produce copolymers of ethylene and other alpha-olefins that have only short chain branches, thereby forming an essentially linear copolymer having a very narrow molecular weight distribution. The catalyst also provides excellent uniform comonomer incorporation into the polymer chain. However, it is the ethylene copolymer itself, not the method of producing the ethylene copolymer, that is critical to the present adhesive. A polymer catalyzed by a single-site metallocene catalyst has a very narrow molecular weight distribution, having a polydispersity index of less than about 2.5. This is distinguishable from a polymer catalyzed by a multi-site Ziegler type catalyst which typically has polydispersity index of about 3 to 6, and a polymer catalyzed by a peroxide catalyst which typically has a polydispersity index of about 2.8 to 4.6. The metallocene catalyzed polyethylene of the invention has a very narrow molecular weight distribution with a polydispersity index of less than 2.5, preferably less than 2.3, more preferably 2.0 or less. The polydispersity index is calculated as the weight average molecular weight (MW), as measured by gel permeation chromatography, divided by the number average molecular weight (MN), as also measured by gel permeation chromatography.
The other comonomer present in the metallocene catalyzed polyethylene copolymer has 3 to 12 carbons. 1-Butene, 1-hexene and 1-octene are the most preferred comonomers. The amount of the other comonomer in the ethylene copolymer varies from about 8 to 30 weight percent. The amount of this other alpha olefin is preferably about 15 to 25 weight percent, with a weight percent of about 21 to 23 being most preferred.
Suitable metallocene catalyzed polyethylenes have a melt index ranging from about 200 to about 2,000 g/10 min, 190° C., 2.16 kg, preferably from about 500 to about 1750 cP, measured in accordance with ASTM D1238. Useful metallocene catalyzed polyethylene include ethylene copolymers with a 20% crystallinity and a Brookfield viscosity of 13,000 cP at 350° F.
The metallocene polyethylene copolymer will typically be used in amounts of from about 20 wt % to about 60 wt %, more preferably from about 25 wt % to about 50 wt % as the base adhesive polymer component.
A functional group is grafted onto a metallocene polyethylene copolymer to form a functionalized metallocene catalyzed polyethylene copolymer. This process may be performed by mixing metallocene catalyzed polyethylene copolymer with a functionalizing component in a reactor or in an extruder. A skilled artisan understands that various functional group may be reacted with metallocene catalyzed polyethylene copolymer to result in a functionalized metallocene catalyzed polyethylene copolymer. Functional groups that can be used in the practice of the invention include acrylic acid, acetate, sulfonate, maleic anhydride, fumaric acid, citraconic anhydride and others. Useful functionalized metallocene catalyzed polyethylene copolymer for the adhesives include, acrylic acid functionalized metallocene catalyzed polyethylene copolymer, acetate functionalized metallocene catalyzed polyethylene copolymer, sulfonate functionalized metallocene catalyzed polyethylene copolymer, maleic anhydride functionalized metallocene catalyzed polyethylene copolymer, and the like. In one embodiment the functionalized metallocene catalyzed polyethylene copolymer is a maleic anhydride modified metallocene catalyzed polyethylene copolymer. In another embodiment the functionalized metallocene catalyzed polyethylene copolymer is a fumaric acid modified metallocene catalyzed polyethylene copolymer. In a further embodiment the functionalized metallocene catalyzed polyethylene copolymer is a citraconic anhydride modified metallocene catalyzed polyethylene copolymer.
The functional groups in the functionalized metallocene catalyzed ethylene copolymer are typically distributed randomly throughout the copolymer. Particularly preferred embodiments of the adhesive of the invention will comprise a functionalized metallocene catalyzed polyethylene copolymer comprising from about 0.3 to about 8 wt %, more particularly about 0.5 to about 5 wt %, of the functional group, based on the weight of the metallocene polyethylene catalyzed copolymer.
Functionalized metallocene catalyzed polyethylene copolymer suitable for the invention will have molecular weight greater than 2,000 Daltons. The functionalized metallocene catalyzed polyethylene copolymer is different than a functionalized modified wax. A skilled artisan understands that functionalized modified wax typically has molecular weight less than 2,000 Daltons, whereas the copolymers have a molecular weight greater than 2,000 Daltons. The crystallinity of the functionalized ethylene copolymer ranges from 10 to 30%. The viscosity of the functionalized metallocene catalyzed polyethylene copolymer range from 10,000 cP to 20,000 cP at 350° F. Suitable functionalized metallocene catalyzed polyethylenes have a melt index ranging from about 200 to about 2,000 g/10 min, 190° C., 2.16 kg, preferably from about 500 to about 1750, measured in accordance with ASTM D1238. One exemplary functionalized metallocene catalyzed polyethylene is AFFINITY™ GA 1000R from Dow®.
The functionalized metallocene polyethylene copolymer will typically be used in amounts of from about 0.5 wt % to 10 wt %, more preferably from about 1 wt % to about 8 wt %, or about 7 wt %, or about 6 wt %, or about 5 wt % of the adhesive. The use of more than 10 wt % may lead to phase separations and incompatibility with the other components of the adhesive.
The adhesive compositions of this invention are preferably tackified. The tackifier component will usually be present in an amount of from about 10 wt % to about 60 wt %, more preferably from about 20 wt % to about 50 wt %, even more preferably from about 20 wt % to about 45 wt %. The tackifying resins typically will have Ring and Ball softening points, as determined by ASTM method E28, of between about 70° C. and 150° C., more typically between about 80° C. and 110° C., and even lower than 100° C., 99° C., 98° C., 97° C., 96° C., and 95° C. Mixtures of two or more tackifying resins may be desirable for some formulations.
In some embodiments, the tackifiers are synthetic hydrocarbon resins. Included synthetic hydrocarbon resins are aliphatic or cycloaliphatic hydrocarbons, aromatic hydrocarbons, aromatically modified aliphatic or cycloaliphatic hydrocarbons and mixtures thereof. Also included are hydrogenated versions of the above mentioned synthetic hydrocarbon resins.
Non-limiting examples include aliphatic olefin derived resins such as those available from Cray Valley under the trade name Wingtack™ Extra and the Escorez™ 2203L series from Exxon. A common C5 hydrocarbon derived tackifier resin in this class is a diene-olefin copolymer of piperylene and 2-methyl-2-butene having a softening point of above 80° C. This resin is available commercially under the trade name Wingtack 95.
Also useful are C9 aromatic modified C5 hydrocarbon derived tackifiers. Such tackifiers are available from Sartomer and Cray Valley under the trade name Norsolene and from Rutgers series of TK aromatic hydrocarbon resins. Norsolene M1090 is a low molecular weight thermoplastic hydrocarbon polymer having a Ring and Ball softening point of 95-110.degree. C. and is commercially available from Cray Valley.
Waxes suitable for use in the present invention include paraffin waxes, microcrystalline waxes, polyethylene waxes, polypropylene waxes, by-product polyethylene waxes, and Fischer-Tropsch waxes. High density low molecular weight polyethylene waxes, by-product polyethylene waxes and Fischer-Tropsch waxes are conventionally referred to in the art as synthetic high melting point waxes. Callista® 122, 158, 144, 435, and 152 available from Shell Lubricants, Houston, Tex.; Paraflint® C-80 and Paraflint® H-1, H-4 and H-8, Fischer-Tropsch waxes available from Sasol-SA/Moore&Munger, Shelton, Conn. are also preferred waxes for use in the practice of the invention.
Paraffin waxes that can be used in the practice of the invention include Pacemaker® 30, 32, 35, 37, 40, 42, 45 & 53 available from Citgo Petroleum, Co., Houston, Tex.; Okerin® 236 TP available from Astor Wax Corporation, Doraville, Ga.; Penreco® 4913 available from Pennzoil Products Co., Houston, Tex.; R-7152 Paraffin Wax available from Moore & Munger, Shelton, Conn.; and Paraffin Wax 1297 available from International Waxes, Ltd. in Ontario, Canada; R-2540 available from Moore and Munger; and other paraffinic waxes such as those available from CP Hall under the product designations 1230, 1236, 1240, 1245, 1246, 1255, 1260, & 1262, available from CP Hall (Stow, Ohio).
The microcrystalline waxes useful here are those having 50 percent by weight or more cyclo or branched alkanes with a length of between 30 and 100 carbons. They are generally less crystalline than paraffin and polyethylene waxes, and have melting points of greater than about 70° C. Examples include Victory® Amber Wax, a 70° C. melting point wax available from Petrolite Corp. located in Tulsa, Okla.; Bareco® ES-796 Amber Wax, a 70° C. melt point wax available from Bareco in Chicago, Ill.; Okerin® 177, an 80° C. melt point wax available from Astor Wax Corp.; Besquare® 175 and 195 Amber Waxes and 80° C. and 90° C. melt point microcrystalline waxes both available from Petrolite Corp. in Tulsa, Okla.; Indramic® 91, a 90° C. melt point wax available from Industrial Raw Materials located in Smethport, Pa.; and Petrowax® 9508 Light, a 90° C. melt point wax available from Petrowax Pa., Inc. located in New York, N.Y.
Exemplary high density low molecular weight polyethylene waxes falling within this category include ethylene homopolymers available from Petrolite, Inc. (Tulsa, Okla.) as Polywax 500, Polywax™ 1500 and Polywax™ 2000. Polywax™ 2000 has a molecular weight of approximately 2000, an Mw/Mn of approximately 1.0, a density at 16° C. of about 0.97 g/cm3 and a melting point of approximately 126° C.
Wax will typically be present in the formulations of the adhesive in amounts greater than 20 wt %, preferably greater than about 22 wt %, about 23 wt %, about 24 wt %, and most preferably greater than about 25 wt % to about 40 wt %, based on the total weight of the adhesive. It is widely understood that large amounts of waxes, typically greater than 20 wt % negatively impacts the low temperature resistance of the adhesives. Surprisingly, the use of greater than 20 wt % of wax in the adhesive improves the low temperature resistance of an adhesive formed with a metallocene catalyzed polyethylene copolymer and a functionalized metallocene catalyzed polyethylene copolymer.
Preferred waxes have a melt temperature between 120° F. and 250° F., more preferably between 150° F. and 230° F., and most preferable between 180° F. and 220° F. The melting temperature of the wax can be measured by various means known in the art, but melting temperature values as reported herein are determined by a DSC: the wax was heated at a rate of 10° C./min to about 20° C. above its melting temperature and held isotherm for at about 3 minutes, then quench to −50° C. at a rate of 100° C./min and then heated again at a rate of 10° C./min, and the highest peak of the second heating curve of the DSC was deemed as the DSC melting temperature.
It will be appreciated that other polymeric additives may, if desired, be added to the adhesive formulation. The adhesives of the present invention may also contain a stabilizer or antioxidant. These compounds are added to protect the adhesive from degradation caused by reaction with oxygen induced by such things as heat, light, or residual catalyst from the raw materials such as the tackifying resin. Among the applicable stabilizers or antioxidants included herein are high molecular weight hindered phenols and multifunctional phenols such as sulfur and phosphorous-containing phenol. Hindered phenols are well known to those skilled in the art and may be characterized as phenolic compounds which also contain sterically bulky radicals in close proximity to the phenolic hydroxyl group thereof. In particular, tertiary butyl groups generally are substituted onto the benzene ring in at least one of the ortho positions relative to the phenolic hydroxyl group. The presence of these sterically bulky substituted radicals in the vicinity of the hydroxyl group serves to retard its stretching frequency, and correspondingly, its reactivity; this hindrance thus providing the phenolic compound with its stabilizing properties. Representative hindered phenols include; 1,3,5-trimethyl-2,4,6-tris-(3,5-di-tert-butyl-4-hydroxybenzyl)-benzene; pentaerythrityl tetrakis-3(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate; n-octadecyl-3(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate; 4,4′-methylenebis (2,6-tert-butyl-phenol); 4,4′-thiobis (6-tert-butyl-o-cresol); 2,6-di-tertbutylphenol; 6-(4-hydroxyphenoxy)-2,4-bis(n-octyl-thio)-1,3,5 triazine; di-n-octylthio)ethyl 3,5-di-tert-butyl-4-hydroxy-benzoate; and sorbitol hexa[3-(3,5-di-tert-butyl-4-hydroxy-phenyl)-propionate].
The performance of these antioxidants may be further enhanced by utilizing, in conjunction therewith, known synergists such as, for example, thiodipropionate esters and phosphites. Distearylthiodipropionate is particularly useful. These stabilizers, if used, are generally present in amounts of about 0.1 to 1.5 weight percent, preferably 0.25 to 1.0 weight percent.
Such antioxidants are commercially available from BASF and include Irganox® 565, 1010 and 1076 which are hindered phenols. These are primary antioxidants which act as radical scavengers and may be used alone or in combination with other antioxidants such as phosphite antioxidants like Irgafos® 168 available from BASF. Phosphite catalysts are considered secondary catalysts and are not generally used alone. These are primarily used as peroxide decomposers. Other available catalysts are Cyanox® LTDP available from Cytec Industries in Stamford, Conn., and Ethanox® 1330 available from Albemarle Corp. in Baton Rouge, La. Many such antioxidants are available either to be used alone or in combination with other such antioxidants. These compounds are added to the hot melts in small amounts and have no effect on other physical properties. Other compounds that could be added that also do not affect physical properties are pigments which add color, or fluorescing agents, to mention only a couple. Additives like these are known to those skilled in the art. Depending on the contemplated end uses of the adhesives, other additives such as plasticizers, pigments, dyestuffs and fillers conventionally added to hot melt adhesives may be included. In addition, small amounts of additional tackifiers and/or waxes such as microcrystalline waxes, hydrogenated castor oil and vinyl acetate modified synthetic waxes may also be incorporated in minor amounts, i.e., up to about 10% by weight, into the formulations of the present invention.
The adhesive compositions of the present invention are prepared by blending the components in the melt at a temperature of above about 275° F., typically at about 300° F. until a homogeneous blend is obtained. Various methods of blending are known in the art and any method that produces a homogeneous blend is satisfactory. For example, a Cowles stirrer provides effective mixing for preparing these compositions.
The adhesive compositions of the present invention typically have a viscosity range of about 500 cP (centipoise) to about 1500 cP at 350° F.
The hot melt adhesives of the present invention are particularly useful in case sealing applications where exceptionally high heat resistance in addition to cold resistance is important, i.e., in hot filled packaging applications; e.g. sealing and closing operations for cartons, cases, or trays used in packaging molten cheese, ice creams, yogurt or freshly baked goods which are subsequently subjected to refrigeration or freezing, and for corrugated cases, which are often subjected to high stresses and adverse environmental conditions during shipping and storage.
Typically, high melting point waxes and/or high softening point tackifiers are commonly used to boost high temperature performance of the adhesive; however, adhesion of the adhesive at low temperature suffers. Conversely, the use of low melting point waxes and/or low softening point tackifiers are often used to improve the low temperature adhesion performance of the adhesive; however, adhesion suffers at high temperatures. High temperature adhesion was measured at 130° F. and above and low temperature adhesion was measured at 0° F. and below. The adhesive of the present invention, with the addition of less than 10 wt % functionalize metallocene catalyzed polyethylene improves both high and low temperature adhesion of the adhesive, regardless of the melting and softening temperatures of tackifiers and waxes. The adhesive of the present invention surprisingly extends the service temperature range into both the low and high temperatures. The adhesive widens the temperature range of the hot melt adhesive from about −20° F. to about 140° F. Such novel hot melt adhesive offer versatility for packaging application for it streamline the adhesives to fewer type of adhesive while providing the performance requirement of extreme service temperature ranges.
The hot melt adhesives of the invention also find use in packaging, converting, cigarette manufacture, bookbinding, bag ending and in nonwoven markets. The adhesives find particular use as case, carton, and tray forming adhesives, and as sealing adhesives, including heat sealing applications, for example in the packaging of cereals, cracker and beer products. Encompassed by the invention are containers, e.g., cartons, cases, boxes, bags, trays and the like, wherein the adhesive is applied by the manufacturer thereof prior to shipment to the packager. Following packaging, the container is heat sealed. The adhesive is also particularly useful in the manufacture of nonwoven articles. The adhesives may be used as construction adhesives, as positioning adhesives, and in elastic attachment applications in the manufacture of, e.g., diapers, feminine hygiene pads (which include conventional sanitary napkins and panty liners) and the like.
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.
Many modifications and variations of this invention can be made without departing from its spirit and scope, as will be apparent to those skilled in the art. The specific embodiments described herein are offered by way of example only, and the invention is to be limited only by the terms of the appended claims, along with the full scope of equivalents to which such claims are entitled.
The following examples are provided for illustrative purposes only. All parts in the formulation are by weight.
All examples were formulated by combining the components listed in Table 1 in a metal vessel and mixing with a stainless steel mixing blade at 350° F. until homogeneous adhesive mixtures was formed.
The adhesives were then applied with a compressed bead width of ½″ between two kraft paper substrates at the designated application temperatures.
Cloud point of an adhesive is measured as the temperature at which the adhesive begins to solidify. It was measured as follows. The adhesive was heated to its specified application temperature. A thermometer was immersed into the adhesive and allowed to equilibrate. The thermometer was then removed and immediately twirled to retain a blob of adhesive on the bulb of the thermometer. The cloud point was recorded as the temperature at which the blob started to turn cloudy.
The viscosity was measured at 350° F. with a Brookfield viscometer with spindle #27.
Heat stress is defined as being the temperature at which a stressed bond fails. Heat stress test was conducted by forming a composite construction of adhesive (½″ compressed) between two pieces of corrugated paperboard of specific dimensions. At least three test samples were prepared. The test samples were conditioned at room temperature for 24 hours. The adhesive bead forming this composite was then placed under approximately 100 grams of cantilever stress for 24 hours at specific temperatures. The highest temperature at which the adhesive passed the heat stress was recorded
Fiber tear test was determined as follows. A ½″ wide bead of adhesive was applied at specified application temperature to a 2″×3″ piece of double fluted corrugate board, and was immediately brought in contact with a second piece of corrugated to form a bond. A 200 gram weight was immediately placed on the top of the bond for 10 seconds to provide compression. The specimens prepared were conditioned at room temperature for 24 hours and further conditioned at specified temperatures for 24 hours. The bonds were separated by hand and the resulting fiber tear was recorded (higher values indicated better adhesion). Fiber tear was calculated as the amount of fiber left on the surface of the adhesive, which indicates failure within the substrate and not at the interface between the adhesive and the substrate. Three specimens were tested to obtain the average percent fiber tear.
The adhesive of Comparative Example 1, with high softening point tackifiers and high melting point waxes, exhibited strong bond strength at high temperatures (135° F. and above); however, the adhesion was poor at low temperature. On the other hand, Comparative Example 2, with low softening point tackifiers resulted in poor high and low temperature performances. The addition of the functionalized metallocene catalyzed polyethylene in the adhesive significantly improved both high and low temperature performance.
