Patent Description:
Hot melt adhesives typically exist as a solid mass at ambient temperature and can be converted to a flowable liquid by the application of heat. These adhesives are particularly useful in manufacturing a variety of, typically, disposable goods where bonding of various substrates is often necessary. Specific applications include disposable diapers, hospital pads, feminine sanitary napkins, panty shields, surgical drapes and adult incontinent briefs, collectively known as disposable hygiene products or articles. Additionally, the adhesives may be used to make pre-formed, absorbent cores that are later inserted into a disposable or re-useable product. Other diversified applications have involved paper products, packaging materials, automotive headliners, appliances, tapes and labels. In most of these applications, the hot melt adhesive is heated to its molten state and then applied to a substrate, often named as the primary substrate. A second substrate, often named as the secondary substrate, is then immediately brought into contact with and compressed against the first. The adhesive solidifies on cooling to form a strong bond. The major advantage of hot melt adhesives is the absence of a liquid carrier, as would be in the case of water or solvent-based adhesives, thereby eliminating the costly process associated with solvent removal and potentially harmful and/or odorous solvents.

For many applications, hot melt adhesives are often extruded directly onto a substrate in the form of a thin film or a bead by using piston or gear pump equipment. The temperature of the die must be maintained well above the melting point of the adhesive to allow the molten hot melt material to flow through the application nozzle smoothly. For most applications, particularly those encountered in food packaging and disposable nonwovens hygienic article manufacturing, bonding of delicate and heat sensitive substrates, such as thin gauge plastic films, is often involved. This imposes an upper limit on coating temperature for hot melt adhesive applications. Today's commercial hot melts are typically formulated to have coating temperature below <NUM> to avoid substrate burning or distortion. In this case, the adhesive is most typically sprayed from a nozzle at a set distance above the substrate or material of interest, such as a substrate containing super absorbent polymers. Several indirect or noncontact coating methods, through which a hot melt adhesive can be spray-coated with the aid of compressed air onto a substrate from a distance, have also been developed. These noncontact coating techniques include conventional spiral spray, Signature™, Control Coat™, UFD™, and various forms of melt-blown methods. The indirect method, however, requires that the viscosity of the adhesives must be sufficiently low, usually in the range of <NUM>,<NUM> to <NUM>,<NUM> mPa·s, often in the range of <NUM>,<NUM> to <NUM>,<NUM> mPa·s, at the application temperature in order to obtain an acceptable coating pattern.

Hot melt adhesives are traditionally comprised of polymers, plasticizers, tackifying resins (also referred to herein as "tackifiers"), and optional additives such as waxes and anti-oxidants. Tackifiers have long been considered a necessary component of hot melt adhesives. Traditionally, the role of tackifiers with styrene-block-copolymer systems is to increase tack by increasing the glass transition temperature of the adhesive system. Tackifiers additionally help suppress the viscosity of the final formulation. However, tackifiers can be viewed negatively because they can contribute odor and volatile organic compounds (VOCs) to the adhesive formulation. Therefore, it is desired to make a hot melt adhesive free from tackifying resins. Such an adhesive would be desirably polyolefin-based since such adhesives handle high temperature well and are generally perceived as having lower odor than others, such as styrene block copolymer-based adhesives.

Efforts have been made to develop tackifier-free hot melt adhesives. For example, <CIT> discloses a hot melt adhesive composition comprising about <NUM> to <NUM> wt. % of an amorphous polyolefin copolymer; about <NUM> to <NUM> wt. % <NUM>-butene; about <NUM> to <NUM> wt. % of a heterophase polypropylene copolymer composition comprising propene and a comonomer comprising ethylene, <NUM>-hexene or <NUM>-octene and having amorphous character and crystalline blocks; and about <NUM> to <NUM> wt. % of a polyisobutylene plasticizer made with an AlCl<NUM>; wherein the adhesive provides cohesive strength from the heterophase polypropylene copolymer and adhesive strength from the amorphous polyolefin copolymer.

In addition, <CIT> discloses a hot melt adhesive composition comprising at least <NUM>% by weight of a first polymer consisting of a non-functionalized amorphous poly alpha olefin polymer comprising greater than about <NUM>% by weight polypropylene; a second polymer selected from the group consisting of polypropylene homopolymers, propylene copolymers, and combinations thereof; a functionalized polypropylene wax; and polyethylene wax. The hot melt adhesive composition is preferably tackifier-free.

International Patent Application No. <CIT> discloses a hot melt adhesive composition including: (A) a propylene homopolymer having a melting point of <NUM> or lower which is obtainable by polymerizing propylene using a metallocene catalyst; and (B) an ethylene-based copolymer.

There still exists a need in the art for an adhesive which has low tack and a sufficiently high elongation at break, stress retention, and thermal stability to be able to withstand significant swelling, such as the swelling of a superabsorbent polymer when wet, all the while still having a suitable viscosity at the desired application temperature.

Therefore, it would be advantageous to provide a hot melt adhesive that will overcome the shortcomings of the prior art adhesives mentioned above. In particular, it is desired to make a hot melt adhesive which does not contain any tackifiers. Such an adhesive is desirably polyolefin-based because such adhesives can handle high temperatures well and are generally perceived as having lower odor. Such an adhesive would have a sufficiently high elongation at break and stress retention to withstand the swelling of a superabsorbent polymer when wet or swollen/expanded. Such an adhesive would be particularly well suited as a micro-fiberized adhesive, responsible for containing superabsorbent polymers (SAP) within a hygiene article, such as a diaper, either alone or in conjunction with another adhesive and/or cellulose fibers. An adhesive serving this function is known to be providing core stabilization. The adhesive would not be required to have a high degree of tack, but is required to have very good elongation to contain the SAP as it swells/expands. A challenge in making a tackifier-free adhesive is developing a formulation with good thermal stability and "strong" enough to meet mechanical properties, such as elongation at break and stress retention.

In view of the shortcomings of the prior art, the present invention provides a hot melt adhesive composition comprising an essentially amorphous, single site-catalyzed, high molecular weight propylene copolymer; a semicrystalline, single site-catalyzed, low molecular weight propylene copolymer; polyisobutene; and a <NUM>-butene-based copolymer polymer, wherein the <NUM>-butene-based copolymer has a density of at most <NUM>/cm<NUM>, wherein the adhesive is tackifier-free, the high molecular weight propylene copolymer has a weight average molecular weight of at least about <NUM>,<NUM>/mol, preferably at least about <NUM>,<NUM>/mol, and most preferably at least about <NUM>,<NUM>/mol and has a heat of fusion of at most about <NUM> J/g, preferably at most about <NUM> J/g, and most preferably at most about <NUM> J/g; and the low molecular weight propylene copolymer has a weight average molecular weight of at most about <NUM>,<NUM>/mol, preferably at most about <NUM>,<NUM>/mol, and most preferably at most about <NUM>,<NUM>/mol and has a heat of fusion of at least about <NUM> J/g, preferably at least about <NUM> J/g, and most preferably at least about <NUM> J/g. Such a composition provides both high elongation at break and stress retention while having exceptional thermal stability.

In accordance with an embodiment of the present invention, a method of making a laminate comprises the steps of: applying the hot melt adhesive composition of the invention in a molten state to a primary substrate; and mating a secondary substrate to the first substrate by contacting the secondary substrate with the adhesive composition.

In accordance with another embodiment of the present invention, an absorbent core comprises a first layer and a second layer, wherein at least one of the first layer and the second layer comprises superabsorbent polymers, and the first layer and the second layer are adhered to each other by a hot melt adhesive composition of the present invention and the adhesive composition adheres the superabsorbent polymers within the absorbent core.

In accordance with another embodiment of the present invention, a disposable hygiene article comprises the absorbent core of the invention.

In accordance with an embodiment of the present invention, a hot melt adhesive composition comprises (a) an amorphous, single site-catalyzed, high molecular weight propylene copolymer; (b) a semicrystalline, single site-catalyzed, low molecular weight propylene copolymer; (c) polyisobutene; and (d) a <NUM>-butene-based copolymer polymer, wherein the <NUM>-butene-based copolymer has a density of at most <NUM>/cm<NUM>, wherein the composition is tackifier-free, the high molecular weight propylene copolymer has a weight average molecular weight of at least about <NUM>,<NUM>/mol, preferably at least about <NUM>,<NUM>/mol, and most preferably at least about <NUM>,<NUM>/mol and has a heat of fusion of at most about <NUM> J/g, preferably at most about <NUM> J/g, and most preferably at most about <NUM> J/g; and the low molecular weight propylene copolymer has a weight average molecular weight of at most about <NUM>,<NUM>/mol, preferably at most about <NUM>,<NUM>/mol, and most preferably at most about <NUM>,<NUM>/mol and has a heat of fusion of at least about <NUM> J/g, preferably at least about <NUM> J/g, and most preferably at least about <NUM> J/g. The adhesive composition may optionally comprise antioxidant(s), and optionally plasticizer(s). Such a composition provides both high elongation at break and stress retention while having exceptional thermal stability. According, such an adhesive is particularly well-suited to applications requiring the ability to elongate in use to a great extent (e.g., such as by <NUM>% or more) and offer a high stress retention (such as <NUM>% or more at <NUM>% elongation after <NUM> minutes), and exceptional thermal stability (such as <NUM>% viscosity retained at elevated temperatures after three days).

Both the first and second propylene copolymers used in this invention are single site-catalyzed. Single site catalyst systems (SSC) differ from the conventional catalysts (such as Ziegler-Natta catalysts) in at least one significant way; they have only a single active transition metal site for each catalyst molecule and the activity at this metal site is therefore identical for all the catalyst molecules. One type of SSC catalyst that has now been widely used on industrial scale is a metallocene catalyst system consisting of a catalyst and a co-catalyst or activator. The catalyst is a transition metal complex having a metal atom situated between two cyclic organic ligands; the ligands may be the same or different derivatives of cyclopentadiene. The co-catalyst can be any compound capable of activating the metallocene catalyst by converting a metallocene complex to a catalytically active species, and an example of such compound is alumoxane, preferably methylalumoxane having an average degree of oligomerization of from <NUM> - <NUM>. For the purpose of this invention, other neutral or ionic activators can be used, including, but not limited to, various organic boron compounds such as tri(n-butyl)ammonium tetrakis(pentafluorophenyl borate, dimethylanilinium tetrakis(pentafluorophenyl borate, or trityl tetrakis(pentafluorophenyl borate. Another type of SSC catalyst is the constrained geometry catalyst (CGC).

As used herein, CGC refers to a sub-class of SSC catalyst system known as constrained geometry catalyst. Different from metallocenes, the constrained geometry catalyst (CGC) is characterized by having only one cyclic ligand linked to one of the other ligands on the same metal center in such a way that the angle at this metal between the centroid of the pi-system and the additional ligand is smaller than in comparable unbridged complexes. More specifically, the term CGC is used for ansa-bridged cyclopentadienyl amido complexes, though the definition goes far beyond this class of compounds. Hence, the term CGC is broadly used to refer to other more or less related ligand systems that may or may not be isolobal and/or isoelectronic with the ansa-bridged cyclopentadienyl amido ligand system. Furthermore, the term is frequently used for related complexes with long ansa-bridges that induce no strain.

Like metallocenes, suitable CGCs may be activated methylaluminoxane (MAO), perfluorinated boranes and trityl borates co-catalysts. The catalytic systems based on CGCs, however, display incorporation of higher alpha-olefins to a much larger extent than comparable metallocene based systems. Non-metallocene based SSCs, also referred to as post-metallocene, single-site catalysts for olefin polymerization are also known. Typical post-metallocene catalysts feature bulky, neutral, alpha-diimine ligands. These post-metallocene catalysts, however, are more frequently used for polymerization of ethylene to produce plastomers and elastomers. They are rarely used for polymerization of alpha-olefins, such as propylene. Single-site catalyst systems for olefin polymerization are well-known to those skilled in the art and are extensively discussed in two symposia entitled <NPL>), and <NPL>).

The advancement of SSC catalyst systems herein discussed above has made it practical to produce propylene based polymers and copolymers having various chain microstructures and specific stereochemistry. Depending on the choice of catalyst and reaction conditions, specific types of propylene polymers and copolymers, for example, can be purposely made to have narrow molecular weight distribution, statistically random comonomer incorporation, high fraction of atactic chain sequences, and shorter crystallizable isotactic or syndiotactic chain sequences. Macroscopically, the polymers may exhibit relatively lower melting point, lower enthalpy of melting, lower crystallinity, and lower density and behave more similar to elastomers than to polypropylene made by conventional catalysts. Such polymers may have various weight average molecular weight (Mw) ranges, such as from <NUM>,<NUM>/mol to <NUM>,<NUM>,<NUM>/mol; a range of melting points, such as between <NUM> to <NUM> which is well below the melting point <NUM> of iPP; a range of enthalpies of melting, such as between <NUM> J/g and <NUM> J/g; and a range of densities, such as between <NUM>/cc and <NUM>/cc. Some of these polymers are well suited for hot melt adhesive applications.

Both the first and second propylene copolymers are comprised primarily of propylene units. This means that they comprise at least <NUM> weight percent of propylene. The two propylene copolymers can be polymers of propylene and the same co-mononer or different co-monomers. Preferably, the high molecular weight propylene copolymer is a copolymer of propylene and a co-monomer selected from the group consisting of ethylene and a C<NUM> - C<NUM> alkylene, preferably ethylene. Similarly, the low molecular weight propylene copolymer is a copolymer of propylene and a co-monomer selected from the group consisting of ethylene and a C<NUM> - C<NUM> alkylene, preferably ethylene. The first and second propylene copolymers may have the same or different co-monomer content. Preferably, the first and second propylene copolymers have an ethylene content of between about <NUM>% and about <NUM>%, more preferably between about <NUM>% and about <NUM>%, even more preferably between about <NUM>% and about <NUM>%, and most preferably between about <NUM>% and about <NUM>%.

The high molecular weight propylene copolymer has a weight average molecular weight of at least about <NUM>,<NUM>/mol, preferably at least about <NUM>,<NUM>/mol, and most preferably at least about <NUM>,<NUM>/mol. Preferably, the weight average molecular weight of the high molecular weight propylene copolymer is at most about <NUM>,<NUM>/mol, preferably at most about <NUM>,<NUM>/mol, and most preferably at most about <NUM>,<NUM>/mol. When upper and lower limits of ranges of a property or concentration of a constituent or of the adhesive or otherwise are set forth herein, any range extending from any lower limit to any upper limit is presumed to be contemplated by the present invention, as well as any range extending from any lower limit or any range extending from any upper limit. Therefore, the embodiments of the present invention include a high molecular weight propylene copolymer having a weight average molecular weight of at least about <NUM>,<NUM>/mol to at most about <NUM>,<NUM>/mol; a high molecular weight propylene copolymer having a weight average molecular weight of at least about <NUM>,<NUM>/mol to at most about <NUM>,<NUM>/mol; and a high molecular weight propylene copolymer having a weight average molecular weight of at least about <NUM>,<NUM>/mol, as examples. The weight average molecular weight of the low molecular weight propylene copolymer is at most about <NUM>,<NUM>/mol, preferably at most about <NUM>,<NUM>/mol, and most preferably at most about <NUM>,<NUM>/mol. Preferably, the weight average molecular weight of the low molecular weight propylene copolymer is at least about <NUM>,<NUM>/mol, preferably at least about <NUM>,<NUM>/mol, and most preferably at least about <NUM>,<NUM>/mol. Weight average molecular weight of the two propylene copolymers set forth herein is characterized using a high temperature size exclusion chromatograph (SEC) using polypropylene reference standards.

Preferably, the molecular weights of the two propylene copolymers are significantly offset from one another. For example, in an embodiment of the invention, the molecular weight of the high molecular weight propylene copolymer is at least two times, preferably at least three times, and most preferably at least four times the molecular weight of the low molecular weight polypropylene copolymer.

The polydispersity indices (PDI) of the propylene copolymers may vary over a wide range and may be the same or different. The polydispersity indices of the two propylene polymers is preferably at most about <NUM>, preferably at most about <NUM>, and most preferably at most about <NUM>. The polydispersity indices of the two propylene polymers is preferably at least about <NUM>, preferably at least about <NUM>, and most preferably at least about <NUM>. These PDI values are generally characteristic of polymers that have been made using single site catalysts, such as metallocene catalysts. Preferably, the propylene copolymers of the present invention are made by using metallocene catalysts. The polydispersity indices (PDI) of the propylene copolymers are determined by dividing weight average molecular weight by number average molecular weight (Mw/Mn), with each value of Mw and Mn being determined by using data from the same analytical method (i.e., using a high temperature size exclusion chromatograph (SEC) using a polypropylene reference standard).

It has been found that the crystallinities of the two propylene copolymers are important to achieve certain purposes of the present invention. According to the invention, the high molecular weight propylene copolymer is essentially amorphous, meaning that it has relatively little, or no, crystallinity. Viewing crystallinity in terms of the polymer's heat of fusion, the high molecular weight propylene copolymer has a heat of fusion of at most about <NUM> J/g, preferably at most about <NUM> J/g, and most preferably at most about <NUM> J/g. Preferably, the high molecular weight propylene copolymer preferably has a heat of fusion of at least about <NUM> J/g, preferably at least about <NUM> J/g, and most preferably at least about <NUM> J/g. The low molecular weight propylene copolymer has a heat of fusion of at least about <NUM> J/g, preferably at least about <NUM> J/g, and most preferably at least about <NUM> J/g. Preferably, the low molecular weight propylene copolymer preferably has a heat of fusion of at most about <NUM> J/g, preferably at most about <NUM> J/g, and most preferably at least about <NUM> J/g. The test method used to determine these heat of fusion values is ASTM E793-<NUM>, "Standard Test Method for Enthalpies of Fusion and Crystallization by Differential Scanning Calorimetry.

Preferably, the heats of fusion of the two propylene copolymers are significantly offset from one another. For example, in an embodiment of the invention, the heat of fusion of the low molecular weight propylene copolymer is at least two times, preferably at least three times, and most preferably at least four times the heat of fusion of the high molecular weight polypropylene copolymer.

The melting temperatures, also referred to as the melting points, of the two propylene copolymers may vary over a wide range and may be the same or different. Preferably, the melting temperature of the low molecular weight propylene copolymer is between about <NUM> and <NUM>. More preferably, the melting temperature of the low molecular weight propylene copolymer is between about <NUM> and about <NUM>, even more preferably between about <NUM> and about <NUM>, and most preferably between about <NUM> and about <NUM>. Preferably, the melting temperature of the high molecular weight propylene copolymer is between about <NUM> and about <NUM>. More preferably, the melting temperature of the high molecular weight propylene copolymer is between about <NUM> and about <NUM>, even more preferably between about <NUM> and about <NUM>, and most preferably between about <NUM> and about <NUM>. The melting temperature as described herein is measured using Differential Scanning Calorimetry (DSC) according to ASTM E-<NUM>-<NUM> except with one modification to the test in that a scanning temperature of <NUM> per minute instead of <NUM> per minute was used (the "DSC melting point").

The glass transition temperatures of the two propylene copolymers may also vary over a wide range and may be the same or different. Preferably, the glass transition temperatures of the each copolymer are between about -<NUM> and about -<NUM>. More preferably, the glass transition temperatures are between -<NUM> and -<NUM>, even more preferably between about -<NUM> and -<NUM>, and most preferably between about -<NUM> and -<NUM>. The glass transition temperature as described herein is measured using Differential Scanning Calorimetry (DSC) according to ASTM E-<NUM>-<NUM> except with one modification to the test in that a scanning temperature of <NUM> per minute instead of <NUM> per minute was used.

The melt flow rate of the high molecular weight propylene copolymers may also vary over a wide range. The high molecular weight propylene copolymer may have a melt flow rate of from about <NUM>/<NUM> to about <NUM>/<NUM>, preferably from about <NUM>/<NUM> to about <NUM>/<NUM>, and most preferably from about <NUM>/<NUM> to about <NUM>/<NUM>. The low molecular weight propylene copolymer may have a melt flow rate, which would be higher than that of the high molecular weight propylene copolymer, or it may be too difficult to measure a melt flow rate. In one embodiment of the invention, a low molecular weight propylene copolymer is selected that has a viscosity of between about <NUM> cP to about <NUM>,<NUM> cP, preferably between about <NUM> to about <NUM>,<NUM> cP, and most preferably between about <NUM>,<NUM> to <NUM>,<NUM> cP, all at <NUM>. As used herein, the melt flow rate of the two propylene copolymers is determined according to ASTM D <NUM> using a <NUM> kilogram weight at <NUM>.

Typical high molecular weight propylene copolymers suitable for use in the present invention include certain grades of VISTAMAXX™ propylene copolymers commercially available from ExxonMobile, including VISTAMAXX <NUM> and <NUM>; and certain grades of VERSIFY™ propylene copolymers commercially available from The Dow Chemical Company, including VERSIFY <NUM>. Typical low molecular weight propylene copolymers suitable for use in the present invention include certain grades of VISTAMAXX™ propylene copolymers commercially available from ExxonMobile, including VISTAMAXX <NUM>.

In addition to two propylene copolymers, the adhesive composition of the present invention comprises polyisobutene. Herein, polyisobutene (PIB) refers to homopolymers or oligomers made from isobutene monomers and possessing the repeat unit of - [C(CH<NUM>)<NUM>CH<NUM>]-. These materials may or may not contain some degree of unsaturation. Generally, they possess low number average molecular weights (<NUM> - <NUM>,<NUM>/mol) and can be thin or relatively viscous liquids at room temperature. According to embodiments of the present invention, the polyisobutene has a number average molecular weight of at least about <NUM>/mol, preferably at least about <NUM>/mol, more preferably at least about <NUM>,<NUM>/mol, and most preferably at least about <NUM>,<NUM>/mol. Preferably, the polyisobutene has a number average molecular weight of at most about <NUM>,<NUM>/mol, preferably at most about <NUM>,<NUM>/mol, and most preferably at most about <NUM>,<NUM>/mol. They also typically possess a narrow polydispersity index (Mw/Mn < <NUM>) though this is not critical to the present invention. Typical polyisobutenes suitable for use with the present invention include INDOPOL® H-<NUM> or INDOPOL® H-<NUM>, commercially available from Ineos Capital Ltd. Without being bound to any theory, it is believe that the polyisobutene serves to improve the cohesive strength of the adhesive and also serves to plasticize the propylene copolymers.

The hot melt adhesive composition of the present invention comprises a <NUM>-butene-based copolymer. As used herein, the term "<NUM>-butene-based copolymer" means that the copolymer comprises greater than <NUM> mol % butene and is made of at least one other monomer in addition to butene. In one embodiment, the <NUM>-butene-based copolymer selected may have a broad range of crystallinity (and a broad range of heat of fusion values). Preferably, the <NUM>-butene-based copolymer comprises a butene-ethylene copolymer. In general, the weight average molecular weight of the <NUM>-butene-based copolymer is preferably between about <NUM>,<NUM> and about <NUM>,<NUM>/mol, and most preferably between about <NUM>,<NUM> and about <NUM>,<NUM>/mol. As used herein, when referring to the weight average molecular weight of the <NUM>-butene-based copolymer, the weight average molecular weight is determined by gel permeation chromatography using polypropylene standards. The <NUM>-butene-based copolymer has a density of at most <NUM>/cm<NUM>, preferably at most about <NUM>/cm<NUM>, more preferably at most about <NUM>/cm<NUM>, and most preferably at most about <NUM>/cm<NUM>.

The hot melt adhesive composition of the present invention optionally comprises a plasticizer. The plasticizer may be any known compatible plasticizer and preferably is selected from the group consisting of mineral oil and synthetic poly-alphaolefin oils. A suitable plasticizer may also be selected from olefin oligomers and low molecular weight polymers, as well as vegetable and animal oils and derivatives thereof. The petroleum derived oils which may be employed are relatively high boiling materials containing only a minor proportion of aromatic hydrocarbons. In this regard, the aromatic hydrocarbons should preferably be less than <NUM>% and more particularly less than <NUM>% of the oil, as measured by the fraction of aromatic carbon atoms. More preferably, the oil may be essentially non-aromatic. The plasticizers that find usefulness in the present invention can be any number of different plasticizers but mineral oil and other plasticizers having an average molecular weight less than <NUM>,<NUM> daltons are particularly advantageous. As will be appreciated, plasticizers have typically been used to lower the viscosity of the overall adhesive composition without substantially decreasing the adhesive strength and/or the service temperature of the adhesive as well as to extend the open time and to improve flexibility of the adhesive.

Embodiments of the present invention are an adhesive based on mixtures of the high molecular weight propylene copolymer and the low molecular weight propylene copolymer in which the ratio of the high molecular weight propylene copolymer and the low molecular weight propylene copolymer is between about <NUM>:<NUM> and about <NUM>:<NUM>, preferably between about <NUM>:<NUM> and about <NUM>:<NUM>, more preferably between about <NUM>:<NUM> and about <NUM>:<NUM> and most preferably about <NUM>:<NUM>.

The amounts of the various constituents may vary over a wide range, depending on the desired application temperature and other conditions and the desired performance characteristics of the adhesive. According to embodiments of the invention, the adhesive comprises:.

The invention includes any combination of any range of one constituent with any range or an unlimited amount of another constituent or both other constituents. The <NUM>-butene-based copolymer may be present in an amount of between about <NUM>% and about <NUM>% by weight, preferably between about <NUM>% and about <NUM>% by weight, and most preferably between about <NUM>% and about <NUM>% by weight, based on the total weight of the composition. Also, when present, the plasticizer may be present in an amount of between about <NUM>% and about <NUM>% by weight, preferably between about <NUM>% and about <NUM>% by weight, and most preferably between about <NUM>% and about <NUM>% by weight, based on the total weight of the composition.

The present invention may optionally include an antioxidant, also referred to as a stabilizer. If included, the antioxidant may be present in an amount of from about <NUM>% to about <NUM>% by weight of the total adhesive composition. Preferably, from about <NUM>% to <NUM>% of an antioxidant is incorporated into the composition. The antioxidants which are useful in the hot melt adhesive compositions of the present invention are incorporated to help protect the polymers noted above, and thereby the total adhesive system, from the effects of thermal and oxidative degradation which normally occurs during the manufacture and application of the adhesive as well as in the ordinary exposure of the final product to the ambient environment. Among the applicable antioxidants are high molecular weight hindered phenols and multifunction phenols, such as sulfur and phosphorous-containing phenols. Hindered phenols are well-known to those skilled in the art and may be characterized as phenolic compounds that 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 steric hindrance thus provides the phenolic compound with its stabilizing properties. Representative hindered phenols include: <NUM>,<NUM>,<NUM>-trimethyl-<NUM>,<NUM>,<NUM>-tris(<NUM>-<NUM>-di-tert-butyl-<NUM>-hydroxybenzyl) benzene; pentaerythirtol tetrakis-<NUM>(<NUM>,<NUM>-di-tert-butyl-<NUM>-hydroxyphenyl) propionate; n-octadecyl-<NUM>(<NUM>,<NUM>-di-tert-butyl-<NUM>-hydroxyphenyl) propionate; <NUM>,<NUM>'-methylenebis(<NUM>-methyl-<NUM>-tert butylphenol); <NUM>,<NUM>-di-tert-butylphenol; <NUM>-(<NUM>-hydroxyphenoxy)-<NUM>,<NUM>-bis(n-octylthio)-<NUM>,<NUM>,<NUM>-triazine; <NUM>,<NUM>,<NUM>-tris(<NUM>-hydroxy-<NUM>,<NUM>-di-tert-butyl-phenoxy)-<NUM>,<NUM>,<NUM>-triazine; di-n-octadecyl-<NUM>,<NUM>-di-tert-butyl-<NUM>-hydroxybenzylphosphonate; <NUM>-(n-octylthio)ethyl-<NUM>,<NUM>-di-tert-butyl-<NUM>-hydroxybenzoate; and sorbitol hexa-<NUM>(<NUM>,<NUM>-di-tert-butyl-<NUM>-hydroxy-phenyl) propionate.

The performance of these antioxidants may be further enhanced by utilizing, in conjunction therewith; (<NUM>) synergists such as, for example, thiodipropionate esters and phosphites; and (<NUM>) chelating agents and metal deactivators as, for example, ethylenediaminetetraacitic acid, slats thereof, and disalicylalpropylenediimine.

It should be understood that other auxiliary additives may be incorporated into the adhesive composition of the present invention in order to modify particular physical properties. These may include, for example, such materials as inert colorants e.g. titanium dioxide, fillers, fluorescent agents, UV absorbers, surfactants, other types of polymers, etc. Typical fillers include talc, calcium carbonate, clay silica, mica, wollastonite, feldspar, aluminum silicate, alumina, hydrated alumina, glass microspheres, ceramic microspheres, thermoplastic microspheres, baryte and wood flour. Surfactants are particularly important in hygienic disposable nonwoven products because they can dramatically reduce the surface tension, for example, of the adhesive applied to diaper core, thereby permitting quicker transport and subsequent absorption of urine by the core.

According to embodiments of the invention, waxes are included in the adhesive composition. Such waxes could include low molecular weight waxes, petroleum waxes such as paraffin wax, synthetic waxes, and polyolefin waxes. Preferably, the adhesive composition contains substantially no wax, such as less than <NUM>% by weight, more preferably less than <NUM>% by weight based on the total weight of the composition, and most preferably no wax.

According to embodiments of the invention, a hot melt adhesive composition consists essentially of, or consists of, an essentially amorphous, single site-catalyzed, high molecular weight propylene copolymer; a semicrystalline, single site-catalyzed, low molecular weight propylene copolymer; and polyisobutene and optionally a butene-rich copolymer and optionally, one or more of the other optional constituents mentioned herein, wherein the adhesive is tackifier-free. By being 'tackifier-free,' the composition contains no tackifiers. Classes of such tackifiers include: aliphatic and cycloaliphatic petroleum hydrocarbon resins; aromatic petroleum hydrocarbon resins and hydrogenated derivatives thereof; aliphatic/aromatic petroleum derived hydrocarbon resins and the hydrogenated derivatives thereof; aromatic modified cycloaliphatic resins and the hydrogenated derivatives thereof; polyterpene resins having a softening point of from about <NUM> to about <NUM>; copolymers and terpolymers of natural terpenes; natural and modified rosin; glycerol and pentaerythritol esters of natural and modified rosin; and phenolic-modified terpene resins. According to an embodiment of the invention, the adhesive composition does not contain an amorphous polyalpha olefin. According to another embodiment of the invention, the adhesive composition comprises less than <NUM>% by weight, preferably less than <NUM>% by weight, more preferably less than <NUM>% by weight, still more preferably less than <NUM>% by weight, and most preferably less than <NUM>% by weight of an amorphous polyalpha olefin. According to another embodiment of the invention, the adhesive composition does not contain any styrene.

The flow characteristics and viscosity of the adhesive composition can be adjusted within certain parameters in ways known to one of ordinary skill in the art. The desired viscosity at a certain temperature will depend on the application conditions, including the manner of application, the desired flow upon application, the line speed, and the system used in such application, among other factors. According to embodiments of the invention, the viscosity of the composition is at most about <NUM>,<NUM> centipoise (cP) at <NUM> (<NUM>°F), preferably at most about <NUM>,<NUM> centipoise (cP) at <NUM> (<NUM>°F), and most preferably at most about <NUM>,<NUM> centipoise (cP) at <NUM> (<NUM>°F). According to embodiments of the invention, the viscosity of the composition is at least about <NUM>,<NUM> centipoise (cP) at <NUM> (<NUM>°F), preferably equal at least about <NUM>,<NUM> centipoise (cP) at <NUM> (<NUM>°F), and most preferably at least about <NUM>,<NUM> centipoise (cP) at <NUM> (<NUM>°F). The viscosity of the adhesive is measured according to ASTM D3236 with spindle <NUM>.

Adhesives according to embodiments of the present invention exhibit excellent mechanical properties (e.g., high elongation at break and high stress retained) and thermal stability. Such properties make adhesives of the invention useful for hygiene, construction, and packaging applications, and especially suitable for core stabilization of absorbent cores containing superabsorbent polymers, which swell significantly upon being insulted with moisture. The hot melt composition of the present invention is further characterized by having elongation at break of at least about <NUM>%, preferably at least about <NUM>%, more preferably at least about <NUM>%, and most preferably at least about <NUM>%, when measured at a pull rate of <NUM>/min (<NUM>"/min). The inventive formulation further meets additional mechanical properties such as stress retained at <NUM>% elongation after ten minutes. Preferably the stressed retained at <NUM>% elongation after ten minutes is at least <NUM>%, preferably at least about <NUM>%, more preferably at least about <NUM>%, and most preferably at least about <NUM>%. In addition, embodiments of the present invention demonstrate excellent stress at yield, such as at least about <NUM> MPa, preferably at least about <NUM> MPa, and most preferably at least about <NUM> MPa. The hot melt adhesive composition further has exceptional thermal stability such that the viscosity retained at elevated temperatures is greater than about <NUM>% over at least about three days, preferably greater than about <NUM>% over at least about three days, and most preferably greater than about <NUM>% over at least about five days. The methodologies for determining elongation at break, stress retained, stress at yield, and thermal stability (as reflected by viscosity retention) are set forth in more detail in the Examples section below.

The hot melt adhesive composition of the present invention may be formulated by using any of the mixing techniques known in the art. A representative example of the mixing procedure involves placing all the components in a jacketed mixing kettle equipped with a rotor, and thereafter raising the temperature of the mixture to a range from <NUM> to <NUM> to melt the contents. Any of the constituents may be pre-blended or added individually to the mixing kettle. For example, the polymers can be a preformed mixture or blend or can be added to the mixing kettle individually. It should be understood that the precise temperature to be used in this step would depend on the melting points of the particular ingredients. The mixing is allowed to continue until a consistent and uniform mixture is formed. The content of the kettle is protected with inert gas such as carbon dioxide or nitrogen during the entire mixing process. Without violating the spirit of the present invention, various additions and variations can be made to the procedure to produce the hot melt composition, such as, for example, applying vacuum to facilitate the removal of entrapped air. Other equipment useful for formulating the composition of the present invention includes, but not limited to, single or twin screw extruders or other variations of extrusion machinery, kneaders, intensive mixers, Ross™ mixers, and the like. The hot melt adhesive is then cooled to room temperature and formed into chubs with a protective skin formed thereon or into pellets for shipment and use.

The adhesive composition of the present invention may be used as a general purpose hot melt adhesive in a number of applications such as, for example, in disposable nonwoven hygiene articles, paper converting, flexible packaging, wood working, carton and case sealing, labeling and other assembly applications. Particularly preferred applications include nonwoven disposable diaper and feminine sanitary napkin construction, re-useable hygiene products that incorporate pre-formed cores, diaper and adult incontinent brief elastic attachment, diaper and napkin core stabilization, diaper backsheet lamination, industrial filter material conversion, surgical gown and surgical drape assembly, etc. Compositions of the present invention are especially suitable for use in core stabilization of absorbent cores having superabsorbent polymers for disposable hygiene articles, such as diapers, feminine care pads, and adult incontinence products.

The resulting hot melt adhesives may be applied to substrates using a variety application techniques. Examples includes hot melt glue gun, hot melt slot-die coating, hot melt wheel coating, hot melt roller coating, melt blown coating, spiral spray, contact or noncontact strand coatings branded as Signature™, Control Coat™, UFD™, and the like. In a preferred embodiment, the hot melt adhesive is indirectly sprayed onto the substrates.

In an embodiment of the invention, a method of making a laminate comprises the steps of: (<NUM>) applying the hot melt adhesive composition of the invention in a molten state to a primary substrate; and (<NUM>) mating a secondary substrate to the primary substrate by contacting the secondary substrate with the adhesive composition. In an embodiment of the invention, the first substrate comprises a first layer (such as a bottom layer) of an absorbent core and the secondary substrate comprises a second layer (such as a top layer) of the absorbent core, wherein at least one of the first layer or the second layer have superabsorbent polymers associated therewith. The first substrate and secondary substrate may be a single continuous material, but folded over, so that the two folds form the first and secondary substrate.

Any suitable absorbent core having superabsorbent polymers may be used in connection with the present invention. Suitable absorbent cores are described in <CIT>; <CIT>; and <CIT>. As described in <CIT>, the absorbent core structure typically includes absorbent polymer material, such as hydrogel-forming polymer material, also referred to as absorbent gelling material, AGM, or super-absorbent polymer, SAP. This absorbent polymer material ensures that large amounts of bodily fluids, e.g. urine, can be absorbed by the absorbent article during its use and be locked away, thus providing low rewet and good skin dryness. Thinner absorbent core structures can be made by reducing or eliminating the traditional use of cellulose or cellulosic fibers in the absorbent core structure. To maintain the mechanical stability of these absorbent core structures, a fiberized net structure, which in some cases may be an adhesive, may be added to stabilize the absorbent polymer material. The absorbent core may also have additional adhesives, either to assist the fiberized net structure adhesive and/or to bond other core materials to each other and/or to other article components. The superabsorbent polymer material may be deposited on or associated with the first and second substrates and a fiberized net structure covers the superabsorbent polymer material on the respective first and second substrates.

In embodiments of the invention, the fiberized net structure comprises an adhesive composition of the present invention. The fiberized net structure may contain other materials, such as other adhesives or cellulose fibers. In another embodiment of the invention, the sole adhesive used in the fiberized net structure is an adhesive composition of the present invention. In still other embodiments of the invention, the fiberized net structure consists solely of one or more adhesive compositions of the present invention. In an embodiment of the invention, the first and second absorbent layers are combined together such that at least a portion of the fiberized net structure of the first absorbent layer contacts at least a portion of the fiberized net structure of the second absorbent layer and wherein an adhesive of the present invention used in the fiberized net structure serves to adhere to two layers together to form the absorbent core. In embodiments of the invention, both the first and second layers of the absorbent core have superabsorbent polymers associated therewith. In other embodiments of the invention, only one of the layers has superabsorbent polymers associated therewith.

In another embodiment of the method of making a laminate of the invention, the primary substrate is a first layer of an absorbent core and the secondary substrate is a super-absorbent polymer. The super-absorbent polymer may be deposited on the first layer before the application of the adhesive composition. In this embodiment, the adhesive may form a fiberized net over and around the superabsorbent polymer and may also adhere to the first layer. In a further embodiment, a second layer is formed in the same way and then the two layers are mated, before the adhesive is cooled, to form an absorbent core.

The invention is further illustrated by way of the examples which are set forth below. To prepare the hot melt adhesive, all components were weighed into an unlined aluminum pint can and heated to <NUM> while under a nitrogen blanket (<NUM> scfh). A double-blade impellor in an over-head mixer was lowered into the aluminum can and agitated at <NUM> rpm until the polymers were moving sufficiently. Then, the speed was increased to <NUM> rpm until the mixture was homogenous and at a constant temperature. The formulation was deemed complete when the mix appeared homogenous and no clumps from polymer were visible. The formulation was then used to test viscosity, tensile properties, and/or to make laminates for end performance testing. The extreme compatibility of the system allowed for all components to go in together without compromising total mix time. This compatibility and stability created a very heat resistant system that was capable of maintaining viscosity when aged at <NUM> for several days.

The ingredients listed below and in Tables <NUM> through <NUM> were used to make the adhesives. The numeric values listed for a given raw material are in weight percent and should equal to <NUM>%.

Calsol <NUM> is a naphthenic process oil available from Calumet Specialty Products.

Indopol H-<NUM> is a polyisobutene oligomer available with a number average molecular weight of <NUM>,<NUM>/mol as determined by the supplier (Ineos Capital Ltd) using gel permeation chromatography (GPC), and all references to the number average molecular weight of the polyisobutene made herein are based on the same method.

Indopol H-<NUM> is a polyisobutene oligomer available with a number average molecular weight of <NUM>/mol as determined by the supplier (Ineos Capital Ltd) using gel permeation chromatography (GPC).

Irgafos <NUM> is a tris(<NUM>,<NUM>-di-tert-butylphenyl) phosphate available from BASF Chemicals and is used as an antioxidant.

Irganox <NUM> is pentaerythritoltetrakis(<NUM>-(<NUM>,<NUM>-di-tert-butyl-<NUM>-hydroxyphenyl)propionate) available from BASF Corp. and is used as an antioxidant
Koattro PB M <NUM> is a <NUM>-butene copolymer with a density of <NUM>/cm3 (tested by supplier according to ISO <NUM>-<NUM>) available from LyondellBasell Industries Holdings.

Koattro PB M <NUM> is a <NUM>-butene copolymer with a density of <NUM>/cm3 (tested by supplier according to ISO <NUM>-<NUM>) available from LyondellBasell Industries Holdings.

Vistamaxx <NUM>, obtained from ExxonMobil Chemical Company, Houston, TX, is an essentially amorphous poly(propylene-co-ethylene) containing about <NUM> wt% of ethylene comonomer and having a weight average molecular weight (Mw) greater than <NUM>,<NUM>/mol and a density (reported by supplier using ASTM D1505) of <NUM>/cm<NUM>.

Vistamaxx <NUM>, obtained from Exxonmobil Chemical Company, Houston, TX, is a low molecular weight, low viscosity random propylene-ethylene copolymer. It has about <NUM> wt% of ethylene comonomer and a weight average molecular weight (Mw) less than <NUM>,<NUM>/mol and a density (reported by supplier) of <NUM>/cm<NUM>.

Vistamaxx <NUM>, obtained from Exxonmobil Chemical Company, Houston, TX, is a semicrystalline low molecular weight propylene copolymer consisting of about <NUM> wt% by weight of ethylene comonomer and having a weight average molecular weight (Mw) of less than <NUM>,<NUM>/mol and a density (reported by supplier) of <NUM>/cm<NUM>.

Viscosity was measured according to ASTM D3236 with Spindle <NUM> at <NUM>, <NUM>, and <NUM>. The spindle speed was adjusted so the percent torque was between <NUM>% and <NUM>%. The viscosity was to be low enough to yield fiber diameters between <NUM> - <NUM> when the adhesive is sprayed at <NUM> gsm with a Nordson Signature© Low Flow nozzle.

Dogbones for tensile tests were made by pouring molten adhesive into silicone molds so that the total dogbone length was <NUM> x <NUM> (<NUM>"x <NUM>") with a thickness, when flush with the mold, of <NUM> (<NUM>"). The testing area of the dogbone was <NUM> x <NUM> (<NUM>"x0. A hot spatula was used to scrape away any excess adhesive from the silicone mold so that the thickness of the dogbone was as close to <NUM> (<NUM>") as possible. The sample was allowed to cool to room temperature for at least <NUM> hours before being tested for elongation to break, max stress, and other mechanical tests. The top and bottom of the dogbone were clamped into an Instron tensile tester so that only the <NUM> x <NUM> (<NUM>" x <NUM>") testing area is exposed. The pull rate was <NUM>/min (<NUM>"/min) and was continued until the specimens broke. The elongation at break and max stress were automatically calculated in the BlueHill3 software available with Instron. The elongation at break was recorded as a percentage based on the difference of final length and the initial length divided by the initial length to determine if the adhesive can withstand the stretch from SAP particles swelling. A goal was to have the adhesive maintain about <NUM>% elongation or greater. It was also deemed necessary to have good stress retention over time when held at a given elongation. The elongation value was <NUM>%; meaning a <NUM> (<NUM>") initial test height would stretch (at <NUM>/min (<NUM>"/min)) until the jaws on the Instron were a total distance of <NUM> (<NUM>") apart. The stress or load value is recorded upon reaching <NUM> %. Then the specimen is held at <NUM>% elongation for ten minutes before a final stress or load value is noted. The "Stress Retained at <NUM> % elongation, <NUM> (%)" as found in the below tables refers to the stress after the ten minute hold at <NUM>% elongation divided by the initial stress at <NUM>% elongation times <NUM>, so the value is listed as a percentage of stress maintained. This factor should be at least <NUM>%, preferably at least about <NUM>%, and most preferably at least about <NUM>% or even at least about <NUM>%.

Pattern quality was determined qualitatively and quantitatively. The adhesive was heated to approximately <NUM> and pumped through hoses to a Signature Low Flow Nozzle (Nordson Corp. ) at <NUM> gsm coat-weight with a line speed of <NUM>/min (<NUM> ft/min). Air-flow was adjusted to create the most visually fiberized-looking pattern with minimal agglomerates (those having a size about more than double the average size of droplet) or fly-away adhesive strands (qualitative). On average, air pressure was around <NUM> psi, but this is dependent on the set-up of each piece of equipment and is only intended as a reference. The nozzle head was positioned <NUM> above the primary substrate, which was a <NUM> gsm spunbond nonwoven. The secondary substrate was a release liner so the samples could be better analyzed. Qualitative analysis was done with a microscope to determine the diameter of the fibers produced during the spray application. The goal was to produce fiber diameters less than <NUM> and preferably less than <NUM>. The adhesive's tensile properties are a function of the fiber diameter, so the desired fiber diameter may change depending on the adhesive formulation, the desired properties, and other conditions.

Thermal Stability was determined by pouring approximately <NUM> of adhesive into an <NUM> oz glass jar and loosely securing the lid. The jar was placed in an oven at elevated temperature, namely <NUM>. The jar was removed from the oven each day to look for charring or gelling that would suggest incompatibility. Additionally, a <NUM> slug of adhesive was poured from the jar each day to determine viscosity retention. Viscosity retention is calculated by dividing the viscosity at a given temperature by the initial viscosity before heat aging and multiplying by <NUM>. Preferably, the viscosity retained would be greater than <NUM> % over at least three days. Ideally, the viscosity retained would be greater than <NUM>% over at least three days or even at least five days.

Table <NUM> demonstrates the importance of the polyisobutene (PIB) component and the optional butene-rich polymer. <NUM> shows that the butene-rich copolymer is optional, but preferred to include as it lowers the viscosity, shown in EX. <NUM> and increases the elongation and stress retained. In comparative example one (CE1), the use of only a naphthenic oil results in an adhesive that displays an unacceptably low elongation at break that consequently failed during the hold at <NUM> % elongation. Inventive examples <NUM> and <NUM> containing PIB had an elongation at break greater than <NUM>% and retained greater than about <NUM>% of their original stress after <NUM> minutes at <NUM>% elongation. Example <NUM> containing the lower molecular weight PIB resulted in about <NUM>% stress retained at <NUM>% elongation after <NUM> minutes and <NUM>% elongation at break. All examples of the invention had a maximum stress of at least <NUM> MPa. Furthermore, Examples <NUM> and <NUM> desirably produced an average fiber diameter of <NUM> when 5gsm was applied at <NUM>.

Table <NUM> demonstrates the role of the high molecular weight propylene copolymer component. Comparative example three (CE3) replaces the high molecular weight component with an additional low molecular weight component. The loss of a high molecular weight component results in a drastic decrease in elongation at break and a complete loss of stress retained at <NUM>% elongation after <NUM> minutes.

Comparative example four (CE4) contains a butene-rich copolymer with a specific gravity of <NUM>/cm<NUM>. The resulting formulation becomes partially incompatible resulting in a quick decrease in elongation at break and complete loss of stress retained at <NUM>% elongation after ten minutes.

Table <NUM> demonstrates the exceptional thermal stability via viscosity retention when aged in an oven at <NUM>. Ideally, hot melt adhesives are heated only for the duration of time needed, but occasionally they can remain in the molten state for several hours to several days. For example, if a production line is having issues, the adhesive could be retained in a molten state before the line is running again. Additionally, some manufacturers do not produce products over weekends and occasionally may leave the melt tank on to avoid having to wait for adhesive to re-melt upon re-starting a line. Therefore, it is advantageous to have an adhesive with exceptional thermal stability to allow for these circumstances. Preferably, the hot melt composition would have a retained viscosity at elevated temperatures greater than about <NUM>% over at least about three days, preferably the greater than about <NUM>% over at least about three days, and most preferably greater than about <NUM>% over at least about five days.

Claim 1:
A hot melt adhesive composition comprising:
(a) an essentially amorphous, single site-catalyzed, high molecular weight propylene copolymer;
(b) a semicrystalline, single site-catalyzed, low molecular weight propylene copolymer;
(c) polyisobutene; and
(d) a <NUM>-butene-based copolymer polymer, wherein the <NUM>-butene-based copolymer has a density of at most <NUM>/cm<NUM>;
wherein:
the composition is tackifier-free,
the high molecular weight propylene copolymer has a weight average molecular weight of at least about <NUM>,<NUM>/mol, preferably at least about <NUM>,<NUM>/mol, and most preferably at least about <NUM>,<NUM>/mol and has a heat of fusion of at most about <NUM> J/g, preferably at most about <NUM> J/g, and most preferably at most about <NUM> J/g; and
the low molecular weight propylene copolymer has a weight average molecular weight of at most about <NUM>,<NUM>/mol, preferably at most about <NUM>,<NUM>/mol, and most preferably at most about <NUM>,<NUM>/mol and has a heat of fusion of at least about <NUM> J/g, preferably at least about <NUM> J/g, and most preferably at least about <NUM> J/g.