HOT MELT ADHESIVE COMPOSITION WITHOUT ADHESIVES, METHOD FOR USING THE SAME AND ARTICLE MANUFACTURED WITH THE SAME.

Abstract

A hot-melt adhesive composition, without bonding agents, comprises a single-site catalyzed, amorphous, high-molecular-weight propylene copolymer, preferably with a weight-average molecular weight (Mw) greater than 80,000 g / mol; a single-site catalyzed, semicrystalline, low-molecular-weight propylene copolymer, preferably with an Mw less than 60,000 g / mol; and polyisobutene. The composition may optionally include a plasticizer, a butene-rich copolymer, and an antioxidant. The hot-melt adhesive is particularly useful for bonding substrates that expand or elongate significantly during use. Such uses include bonding superabsorbent polymers to a substrate or bonding two substrates, at least one of which has superabsorbent polymers, such as the layers of an absorbent core in a disposable hygiene product, such as a diaper.The composition, which does not contain an adhesive agent, demonstrates adequate tensile strength, tension retention, and thermal stability, making it well-suited for stabilizing expanded species such as superabsorbent polymers.

Description

HOT MELT ADHESIVE COMPOSITION WITHOUT ADHESIVES, METHOD FOR USING THE SAME AND ARTICLE MANUFACTURED WITH THE SAME FIELD OF INVENTION This invention relates to hot-melt adhesive compositions without an adhesive agent suitable for use on disposable hygiene articles, such as diapers and training pants. The adhesive is particularly suitable for applications requiring tear strength, stress retention, and thermal stability. BACKGROUND OF THE INVENTION Hot melt adhesives typically exist as a solid mass at room temperature and can be converted into a fluid liquid by applying heat. These adhesives are particularly useful in the manufacture of a variety of typically disposable items where bonding multiple substrates is often required. Specific applications include disposable diapers, hospital compresses, feminine sanitary napkins, panty liners, surgical drapes, and adult incontinence briefs, collectively known as disposable hygiene products or articles. Additionally, the adhesives can be used to make preformed absorbent cores that are then inserted into a disposable or reusable product. Other diverse applications have involved paper products, packaging materials, automotive headliners, household 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 called the primary substrate. A second substrate, often called the secondary substrate, is immediately placed in contact with the first and pressed against it. The adhesive solidifies upon cooling to form a strong bond. The main advantage of hot melt adhesives is the absence of a liquid vehicle, as would be the case with water-based or solvent-based adhesives, thus eliminating the costly process associated with the disposal of potentially harmful and / or odorous solvents. For many applications, hot melt adhesives are often extruded directly onto a substrate as a thin film or bead using piston or gear pump equipment. The die temperature must be maintained well above the adhesive's melting point to allow the hot melt material to flow smoothly through the application nozzle. Most applications, particularly those in food packaging and the manufacture of nonwoven disposable hygiene products, often involve bonding delicate, heat-sensitive substrates such as thin-gauge plastic films. This imposes an upper limit on the coating temperature for hot melt adhesive applications.Today's commercial hot melt adhesives are typically formulated to have a coating temperature below 200°C to prevent burning or distortion of the substrate. In this case, the adhesive is usually sprayed from a nozzle at a certain distance above the substrate or material of interest, such as a substrate containing superabsorbent polymers. Several indirect or non-contact coating methods have also been developed, whereby a hot melt adhesive can be spray-coated onto a substrate at a distance using compressed air. These non-contact coating techniques include conventional spiral spraying, Signature™, Control Coat™, UFD™, and various forms of melt blowing methods.The indirect method, however, requires that the viscosity of the adhesives be low enough, generally in the range of 2,000 to 30,000 mPas, often in the range of 2,000 to 15,000 mPa s, at the application temperature to obtain an acceptable coating pattern. Hot melt adhesives are traditionally composed of polymers, plasticizers, bonding resins (also referred to herein as bonding agents), and optional additives such as waxes and antioxidants. Bonding agents have long been considered a necessary component of hot melt adhesives. Traditionally, the role of bonding agents with styrene block copolymer systems is to increase adhesion by raising the glass transition temperature of the adhesive system. Bonding agents also help suppress the viscosity of the final formulation. However, bonding agents can be considered negative because they can contribute odor and volatile organic compounds (VOCs) to the adhesive formulation. Therefore, it is desirable to develop a hot melt adhesive free of bonding resins.An adhesive of this type would ideally be polyolefin-based, as such adhesives withstand high temperatures well and are generally perceived to have less odor than others, such as adhesives based on styrene block copolymer. Efforts have been made to develop hot melt adhesives without bonding agents. For example, U.S. Patent No. 9,139,755 describes a hot melt adhesive composition comprising approximately 10 to 90 wt% of an amorphous polyolefin copolymer; approximately 50 to 70 wt% of 1-butene; approximately 10 to 90 wt% of a heterophasic polypropylene copolymer composition comprising propene and a comonomer comprising ethylene, 1-hexene, or 1-octene and having an amorphous character and crystalline blocks; and approximately 0.1 to 30 wt% of a polyisobutylene plasticizer made with an AICI3; wherein the adhesive provides cohesive strength from the heterophasic polypropylene copolymer and adhesive strength from the amorphous polyolefin copolymer. Furthermore, U.S. Patent No. 8,623,480 describes a hot melt adhesive composition comprising at least 55% by weight of a first polymer consisting of a non-functionalized amorphous polyalphaolefin polymer comprising more than approximately 50% by weight of 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 free of bonding agents. International Patent Application No. WO 2013 / 039261 discloses a hot melt adhesive composition comprising: (A) a propylene homopolymer having a melting point of 100 °C or lower that can be obtained by polymerizing propylene using a metallocene catalyst; and (B) an ethylene-based copolymer. BRIEF DESCRIPTION OF THE INVENTION There is still a need in the technology for an adhesive that has low tack and tensile strength, tension retention and thermal stability high enough to withstand significant expansion, such as the expansion of a superabsorbent polymer when wet, and at the same time has a viscosity suitable at the desired application temperature. Therefore, it would be advantageous to provide a hot-melt adhesive that overcomes the shortcomings of the adhesives of the previously mentioned art. In particular, it is desired to manufacture a hot-melt adhesive that does not contain bonding agents. It is desirable that such an adhesive be polyolefin-based because such adhesives can handle high temperatures well and are generally perceived as having a lower odor. Such an adhesive would have sufficiently high tensile strength and tensile retention to withstand the expansion of a superabsorbent polymer when wet or swollen / expanded. Such an adhesive would be particularly suitable as a microfiber adhesive, responsible for containing superabsorbent polymers (SAPs) within a hygiene article, such as a diaper, either alone or in conjunction with another adhesive and / or cellulose fibers.An adhesive that fulfills this function is known to provide core stabilization. The adhesive is not required to have a high degree of adhesion, but it must have very good elongation to contain the SAP as it swells / expands. A challenge in manufacturing an adhesive without bonding agents is developing a formulation with good thermal stability and sufficient strength to meet mechanical properties such as tensile strength and tensile retention. In view of the shortcomings of the prior art, the present invention provides a hot-melt adhesive composition comprising a single-site catalyzed, essentially amorphous, high-molecular-weight propylene copolymer; a single-site catalyzed, semicrystalline, low-molecular-weight propylene copolymer; and polyisobutene, wherein the adhesive contains no bonding agents. Such a composition provides both high tensile strength and excellent tensile retention while exhibiting exceptional thermal stability. According to one embodiment of the present invention, a method for 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 matching a secondary substrate to the first substrate by bringing the secondary substrate into contact with the adhesive composition. According to 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 bonded together by the hot-melt adhesive composition of the present invention, and the adhesive composition bonds the superabsorbent polymers within the absorbent core. According to another embodiment of the present invention, a disposable hygiene article comprises the absorbent core of the invention. DETAILED DESCRIPTION OF THE INVENTION According to one embodiment of the present invention, a hot-melt adhesive composition comprises (a) a single-site catalyzed, amorphous, high-molecular-weight propylene copolymer; (b) a single-site catalyzed, semi-crystalline, low-molecular-weight propylene copolymer; and (c) polyisobutene, wherein the composition does not contain bonding agents. The adhesive composition may optionally comprise antioxidant(s), optionally plasticizer(s), and an additional butene-rich polymer with a density of less than 0.908 g / cm³. Such a composition provides both high tensile strength and excellent tensile strength while exhibiting exceptional thermal stability.Accordingly, this adhesive is particularly suitable for applications that require the ability to elongate greatly during use (e.g., 350% or more) and offer high tension retention (such as 30% or more at 300% elongation after 10 minutes) and exceptional thermal stability (such as 90% 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 (SSC) systems differ from conventional catalysts (such as Ziegler-Natta catalysts) in at least one significant way: they have only one active transition metal site for each catalyst molecule, and therefore the activity at this metal site is identical for all catalyst molecules. One type of SSC catalyst now widely used on an industrial scale is a metallocene catalyst system consisting of a catalyst and a cocatalyst or activator. The catalyst is a transition metal complex having a metal atom sandwiched between two cyclic organic ligands; the ligands may be identical or different cyclopentadiene derivatives.The cocatalyst can be any compound capable of activating the metallocene catalyst by converting a metallocene complex into a catalytically active species, and an example of such a compound is alumoxane, preferably methylalumoxane, which has an average degree of oligomerization of 4 to 30. For the purposes of this invention, other ionic or neutral activators can be used, including, among others, various organoboron compounds such as tri(n-butyl)ammonium tetrakis(pentafluorophenyl borate), dimethylanilinium tetrakis(pentafluorophenyl borate), or triphyll tetrakis(pentafluorophenyl borate). Another type of SSC catalyst is the constrained geometry catalyst (CGC). As used here, CGC refers to a subclass of SSC catalytic system known as a restricted geometry catalyst. Unlike metallocenes, the restricted geometry catalyst (CGC) is characterized by having only one cyclic ligand attached to one of the other ligands at the same metal center, such that the angle at this metal between the centroid of the system and the additional ligand is smaller than in comparable unbridged complexes. More specifically, the term CGC is used for loop-bridged cyclopentadienyl amido complexes, although the definition extends far beyond this class of compounds. Therefore, the term CGC is widely used to refer to other more or less related ligand systems that may or may not be isobal and / or isoelectronic with the loop-bridged cyclopentadienyl amido ligand system. Furthermore, the term is frequently used for related complexes with long, non-strain-inducing loop bridges.Like metallocene-based catalytic systems (CGCs), suitable CGCs can be activated methylaluminoxane (MAC) cocatalysts, perfluorinated boranes, and triphyll borates. However, CGC-based catalytic systems exhibit the incorporation of higher alpha-olefins to a much greater degree than comparable metallocene-based systems. Non-metallocene-based SSCs, also called post-metallocene single-site catalysts, are known for olefin polymerization. Typical post-metallocene catalysts feature bulky, neutral alpha-dumine ligands. However, these post-metallocene catalysts are most often used for the polymerization of ethylene to produce plastomers and elastomers. They are rarely used for the polymerization of alpha-olefins, such as propylene.Single-site catalytic systems for olefin polymerization are well known to experts in the technique and are discussed extensively in two symposia entitled Stereoselective Polymerization with Single-Site Catalysts edited by Lisa S. Baugh and Jo Ann M. Canich and published by CRC Press (2008), and Polyolefins: 50 Years after Ziegler and Natta II: Polyolefins by Metallocenes and Other Single-Site Catalysts edited by Walter Kaminsky and published by Springer Heidelberg (2013). The advancement of SSC catalytic systems discussed earlier in this paper has made it practical to produce propylene-based polymers and copolymers with 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 intentionally engineered to have a narrow molecular weight distribution, statistically random comonomer incorporation, a high fraction of atactic chain sequences, and shorter crystallizable isotactic or syndiotactic chain sequences. Macroscopically, these polymers may exhibit a relatively lower melting point, lower enthalpy of fusion, lower crystallinity, and lower density, and behave more like elastomers than like polypropylene manufactured with conventional catalysts.These polymers can have various weight-average molecular weight (Mw) ranges, such as from 1,000 g / mol to 1,000,000 g / mol; melting point ranges, such as from 20 °C to 150 °C, which is well below the 170 °C melting point of PP; enthalpy of fusion ranges, such as from 0 J / g to 100 J / g; and density ranges, such as from 0.85 g / cc to 0.90 g / cc. Some of these polymers are well-suited for hot-melt adhesive applications. Both the first and second propylene copolymers are composed primarily of propylene units. This means they comprise at least 50 percent propylene by weight. The two propylene copolymers can be polymers of propylene and the same comonomer or different comonomers. Preferably, the high-molecular-weight propylene copolymer is a copolymer of propylene and a comonomer selected from the group consisting of ethylene and a C4-C12 alkylene, preferably ethylene. Similarly, the low-molecular-weight propylene copolymer is a copolymer of propylene and a comonomer selected from the group consisting of ethylene and a C4-C12 alkylene, preferably ethylene. The first and second propylene copolymers can have the same or different comonomer content. Preferably, the first and second propylene copolymers have an ethylene content of between 5% and 25%, more preferably between 7% and 25%. 20%, even more preferably between 9% and 17%, and more preferably between 10% and approximately 15%. Preferably, the high molecular weight propylene copolymer has a weight average molecular weight of at least approximately 80,000 g / mol, preferably at least approximately 95,000 g / mol, and most preferably at least approximately 105,000 g / mol. Preferably, the weight average molecular weight of the high molecular weight propylene copolymer is at most approximately 250,000 g / mol, preferably at most approximately 200,000 g / mol, and most preferably at most approximately 175,000 g / mol. Where upper and lower limits of the ranges of a property or concentration of a constituent or of the adhesive or otherwise are stated herein, it is understood that the present invention encompasses any range extending from any lower limit to any upper limit, as well as any range extending from any lower limit or any range extending from any upper limit.Therefore, embodiments of the present invention include, as examples, a high molecular weight propylene copolymer having a weight average molecular weight of at least approximately 80,000 g / mol to a maximum of approximately 250,000 g / mol; a high molecular weight propylene copolymer having a weight average molecular weight of at least approximately 95,000 g / mol to a maximum of approximately 250,000 g / mol; and a high molecular weight propylene copolymer having a weight average molecular weight of at least approximately 80,000 g / mol. Preferably, the weight average molecular weight of the low molecular weight propylene copolymer is at most approximately 60,000 g / mol, preferably at most approximately 40,000 g / mol, and most preferably at most approximately 30,000 g / mol.Preferably, the weight average molecular weight of the low molecular weight propylene copolymer is at least approximately 10,000 g / mol, preferably at least approximately 15,000 g / mol, and most preferably at least approximately 17,500 g / mol. The weight average molecular weight of the two propylene copolymers described herein is characterized using high-temperature size exclusion chromatography (SEO) with polypropylene reference standards. Preferably, the molecular weights of the two propylene copolymers are significantly offset from each other. For example, in one 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 more preferably at least four times the molecular weight of the low molecular weight polypropylene copolymer. The polydispersity indices (PDIs) of the propylene copolymers can vary widely and may be equal or different. The polydispersity indices of the two propylene polymers are preferably at most 5, preferably at most 4, and most preferably at most 3. The polydispersity indices of the two propylene polymers are preferably at least approximately 1.5, preferably at least approximately 1.8, and most preferably at least approximately 2. These PDI values ​​are generally characteristic of polymers manufactured using single-site catalysts, such as metallocene catalysts. Preferably, the propylene copolymers of the present invention are prepared using metallocene catalysts.The polydispersity indices (PDI) of propylene copolymers are determined by dividing the weight average molecular weight by the number average molecular weight (Mw / Mn), with each Mw and Mn value being determined using data from the same analytical method (i.e., using a high-temperature size exclusion chromatograph (SEC) using a polypropylene reference standard). The crystallinities of the two propylene copolymers have been found to be important for achieving certain purposes of the present invention. According to the invention, the high molecular weight propylene copolymer is essentially amorphous, meaning it has relatively small or no crystallinity. Considering crystallinity in terms of the heat of fusion of the polymer, the high molecular weight propylene copolymer preferably has a heat of fusion of at most approximately 20 J / g, preferably at most approximately 15 J / g, and more preferably at most approximately 10 J / g. Preferably, the high molecular weight propylene copolymer preferably has a heat of fusion of at least approximately 0 J / g, preferably at least approximately 2 J / g, and most preferably at least approximately 5 J / g.According to embodiments of the invention, the low molecular weight propylene copolymer preferably has a heat of fusion of at least approximately 20 J / g, preferably at least approximately 25 J / g, and most preferably at least approximately 30 J / g. Preferably, the low molecular weight propylene copolymer preferably has a heat of fusion of at most approximately 100 J / g, preferably at most approximately 75 J / g, and most preferably at least approximately 50 J / g. The test method used to determine these heat of fusion values ​​is ASTM E793-01, Standard Test Method for Enthalpies of Fusion and Crystallization by Differential Scanning Calorimetry. Preferably, the heats of fusion of the two propylene copolymers significantly offset each other. For example, in one 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 more preferably at least four times the heat of fusion of the high molecular weight polypropylene copolymer. The melting temperatures, also called melting points, of the two propylene copolymers can vary over a wide range and can be the same or different. Preferably, the melting temperature of the low molecular weight propylene copolymer is between approximately 35 °C and 100 °C. More preferably, the melting temperature of the low molecular weight propylene copolymer is between approximately 40 °C and approximately 90 °C, even more preferably between approximately 50 °C and approximately 80 °C, and most preferably between approximately 55 °C and approximately 75 °C. Preferably, the melting temperature of the high molecular weight propylene copolymer is between approximately 70 °C and approximately 130 °C.More preferably, the melting temperature of the high molecular weight propylene copolymer is between approximately 75 °C and approximately 125 °C, even more preferably between approximately 85 °C and approximately 115 °C, and most preferably between approximately 90 °C and approximately 110 °C. The melting temperature, as described herein, is measured by differential scanning calorimetry (DSC) according to ASTM E-794-01, except with a modification to the test in which a scan temperature of 20 °C per minute was used instead of 10 °C per minute (the “DSC melting point”). The glass transition temperatures of the two propylene copolymers can also vary over a wide range and may be the same or different. Preferably, the glass transition temperatures of each copolymer are between approximately -45 °C and approximately -5 °C. More preferably, the glass transition temperatures are between -35 °C and -15 °C, even more preferably between approximately -32 °C and -20 °C, and most preferably between approximately -30 °C and -22 °C. The glass transition temperature, as described herein, is measured using differential scanning calorimetry (DSC) in accordance with ASTM E-794-01, except with a modification to the test in which a scan temperature of 20 °C per minute was used instead of 10 °C per minute. The melt flow index of high molecular weight propylene copolymers can also vary widely. A high molecular weight propylene copolymer can have a melt flow index from approximately 1 g / 10 min to approximately 75 g / 10 min, preferably from approximately 2 g / 10 min to approximately 50 g / 10 min, and most preferably from approximately 5 g / 10 min to approximately 25 g / 10 min. A low molecular weight propylene copolymer may have a melt flow index that is higher than that of a high molecular weight propylene copolymer, or it may be too difficult to measure a melt flow index.In one embodiment of the invention, a low molecular weight propylene copolymer is selected having a viscosity of between approximately 500 mPa s (500 cP) and approximately 2,500 mPa s (2,500 cP), preferably between approximately 750 mPa s (750 cP) and approximately 2,000 mPa s (2,000 cP), and most preferably between approximately 1,000 mPa s (1,000 cP) and 1,500 mPa-s (1,500 cP), all at 190°C. As used herein, the melt flow rate of the two propylene copolymers is determined in accordance with ASTM D 1238 using a weight of 2.16 kilograms at 190°C. Typical high molecular weight propylene copolymers suitable for use in the present invention include certain grades of VISTAMAXX™ propylene copolymers marketed by ExxonMobil, including VISTAMAXX 6202 and 6502; and certain grades of VERSIFY™ propylene copolymers commercially available from The Dow Chemical Company, including VERSIFY 3000. Typical low molecular weight propylene copolymers suitable for use in the present invention include certain grades of VISTAMAXX™ propylene copolymers commercially available from ExxonMobil, including VISTAMAXX 8880. 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 repeating unit [C(CH3)2CH2]-. These materials may or may not contain some degree of unsaturation. They generally have low number-average molecular weights (950-4,500 g / mol) and may be low-density or relatively viscous liquids at room temperature. According to the embodiments of the present invention, the polyisobutene has a number-average molecular weight of at least approximately 750 g / mol, preferably at least approximately 900 g / mol, more preferably at least approximately 1,500 g / mol, and most preferably at least approximately 2,000 g / mol.Preferably, the polyisobutene has a number-average molecular weight of at most 7,500 g / mol, preferably at most 6,000 g / mol, and most preferably at most 4,500 g / mol. They typically also possess a narrow polydispersity index (Mw / Mn < 4), although this is not critical to the present invention. Typical polyisobutenes suitable for use with the present invention include INDOPOL® H-1900 or INDOPOL® H-100, commercially available from Ineos Capital Ltd. While not linked to any specific theory, it is believed that polyisobutene serves to enhance the cohesive strength of the adhesive and also to plasticize propylene copolymers. The hot melt adhesive composition of the present invention optionally comprises a 1-butene-based copolymer. As used herein, the term 1-butene-based copolymer means that the copolymer comprises more than 50 mol% butene and is made of at least one other monomer besides butene. In one embodiment, the selected 1-butene-based copolymer can have a wide range of crystallinity (and a wide range of heat of fusion values). Preferably, the 1-butene-based copolymer comprises a butene-ethylene copolymer. In general, the weight average molecular weight of the 1-butene-based copolymer is preferably between 15,000 and 160,000 g / mol, and most preferably between 30,000 and 150,000 g / mol.As used herein, when referring to the weight average molecular weight of the 1-butene-based copolymer, the weight average molecular weight is determined by gel permeation chromatography using polypropylene standards. Preferably, the 1-butene-based copolymer has a density of at most approximately 0.907 g / cm³, preferably at most approximately 0.905 g / cm³, more preferably at most approximately 0.900 g / cm³, and most preferably at most approximately 0.895 g / cm³. The hot melt adhesive composition of the present invention optionally comprises a plasticizer. The plasticizer may be any known compatible plasticizer and is preferably 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 that may be used are materials with relatively high boiling points that contain only a small proportion of aromatic hydrocarbons. In this regard, the aromatic hydrocarbons should preferably be less than 30% and more particularly less than 15% of the oil, as measured by the fraction of aromatic carbon atoms. More preferably, the oil may be essentially non-aromatic.The plasticizers useful in the present invention can be any number of different plasticizers, but mineral oil and other plasticizers with an average molecular weight of less than 5,000 daltons are particularly suitable. As will be seen, plasticizers have normally been used to reduce the viscosity of the overall adhesive composition without substantially decreasing the adhesive strength and / or operating temperature, as well as to extend the open time and improve the adhesive's flexibility. The embodiments of the present invention are an adhesive based on mixtures of high molecular weight propylene copolymer and low molecular weight propylene copolymer in which the ratio of high molecular weight propylene copolymer and low molecular weight propylene copolymer is between approximately 1:1 and approximately 5:1, preferably between approximately 3:2 and approximately 4:1, more preferably between approximately 2:1 and approximately 7:2 and most preferably approximately 3:1. The quantities of the various constituents can vary over a wide range, depending on the desired application temperature and other conditions, as well as the desired performance characteristics of the adhesive. According to embodiments of the invention, the adhesive comprises: the high molecular weight propylene copolymer in an amount of between approximately 10% and approximately 60% by weight, preferably between approximately 20% and approximately 50% by weight, and more preferably between approximately 25% and approximately 40% by weight, based on the total weight of the composition; The low molecular weight propylene copolymer is present in an amount of between approximately 3% and approximately 40% by weight, preferably between approximately 4% and approximately 30% by weight, and more preferably between approximately 5% and approximately 20% by weight, based on the total weight of the composition; and the polyisobutene is present in an amount of between approximately 5% and approximately 50% by weight, preferably between approximately 10% and approximately 40% by weight, and more preferably between approximately 15% and approximately 30% by weight, based on the total weight of the composition The invention includes any combination of any range of one constituent with any range or a limited amount of another constituent, or both constituents. When present, the 1-butene-based copolymer may be present in an amount of between approximately 5% and approximately 40% by weight, preferably between approximately 7% and approximately 30% by weight, and most preferably between approximately 10% and approximately 25% by weight, based on the total weight of the composition. Furthermore, when present, the plasticizer may be present in an amount of between approximately 5% and approximately 60% by weight, preferably between approximately 10% and approximately 50% by weight, and most preferably between approximately 15% and approximately 40% by weight, based on the total weight of the composition. The present invention may optionally include an antioxidant, also called a stabilizer. If included, the antioxidant may be present in an amount of approximately 0.1% to approximately 3% by weight of the total adhesive composition. Preferably, approximately 0.2% to 2% of an antioxidant is incorporated into the composition. The antioxidants that are useful in the hot melt adhesive compositions of the present invention are incorporated to help protect the aforementioned polymers, and thus the total adhesive system, from the effects of thermal and oxidative degradation that normally occurs during the manufacture and application of the adhesive, as well as from ordinary exposure of the final product to the environment. Applicable antioxidants include high molecular weight hindered phenols and multifunctional phenols, such as sulfur- and phosphorus-containing phenols.Sterically hindered phenols are well known to those skilled in the art and can be characterized as phenolic compounds that also contain spherically bulky radicals very close to their phenolic hydroxyl group. In particular, tertiary butyl groups are generally substituted on the benzene ring at at least one of the ortho positions with respect to the phenolic hydroxyl group. The presence of these sterically substituted bulky radicals in the vicinity of the hydroxyl group serves to retard its stretching frequency and, consequently, its reactivity; this steric hindrance thus provides 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-hydroxpionate n-octadecyl-3(3,5-di-tert-butyl-4-hydroxyphenyl); 4,4'-methylenebis(4-methyl-6-tert-butylphenol); 2,6-di-tert-butylphenol; 6-(4-hydroxy¡-phenoxy¡)-2,4-bis(n-octylth¡o)-1,3,5-triazine; di-n-octadecyl-3,5-di-tert-butyl-4-hydroxybenzylphosphonate; 2-(n-octylthio)ethyl-3,5-di-tert-butyl4-hydroxybenzoate; and sorbitol hexa-3(3,5-di-tert-butyl-4-hydroxyl-phenyl) propionate. The performance of these antioxidants can be further enhanced by using, along with them; (1) synergists such as, for example, thiodipropionate esters and phosphites; and (2) chelating and metal-deactivating agents such as, for example, ethylenediaminetetraacetic acid, formulations thereof, and disalicyl propylenedimine. It should be understood that other auxiliary additives may be incorporated into the adhesive composition of the present invention to modify particular physical properties. These may include, for example, materials such as inert colorants, e.g., titanium dioxide, fillers, fluorescent agents, UV absorbers, surfactants, other types of polymers, etc. Typical fillers include talc, calcium carbonate, silica clay, mica, wollastonite, feldspar, aluminum silicate, alumina, hydrated alumina, glass microspheres, ceramic microspheres, thermoplastic microspheres, barite, and wood flour. Surfactants are particularly important in nonwoven disposable hygiene products because they can drastically reduce the surface tension, e.g., of the adhesive applied to the diaper core, allowing for faster 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, less than 1% by weight, more preferably less than 0.5% by weight based on the total weight of the composition, and most preferably no wax at all. According to embodiments of the invention, a hot melt adhesive composition essentially consists of, or comprises, a single-site catalyzed, essentially amorphous, high molecular weight propylene copolymer; a single-site catalyzed, semicrystalline, 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 contains no bonding agents. Being 'bonding agent-free' means the composition contains no bonding agents or only a minimal number of bonding agents known in the art for use in hot melt adhesives, such as those described in U.S. Patent No. 10,011,744, incorporated herein by reference.Classes of such bonding agents include: aliphatic and cycloaliphatic petroleum hydrocarbon resins; aromatic petroleum hydrocarbon resins and hydrogenated derivatives thereof; aliphatic / aromatic petroleum hydrocarbon resins and their hydrogenated derivatives; aromatic modified cycloaliphatic resins and their hydrogenated derivatives; polyterpene resins having a softening point of approximately 10°C to approximately 140°C; copolymers and terpolymers of natural terpenes; natural and modified rosin; glycerol and pentaerythritol esters of natural and modified rosin; and phenol-modified terpene resins. According to one embodiment of the invention, the adhesive composition does not contain an amorphous polyalpha-like structure.According to another embodiment of the invention, the adhesive composition comprises less than 40% by weight, preferably less than 25% by weight, more preferably less than 10% by weight, even more preferably less than 5% by weight, and most preferably less than 1% by weight of an amorphous polyalphaolefin. According to another embodiment of the invention, the adhesive composition does not contain styrene. The flow characteristics and viscosity of the adhesive composition can be adjusted within certain parameters in ways known to experts in the field. The desired viscosity at a given temperature will depend on the application conditions, including the application method, the desired flow rate, the line speed, and the system used, among other factors. According to the embodiments of the invention, the viscosity of the composition is at most approximately 17,500 milliPascals per second (mPas) (17,500 centipoises (cP)) at 177 °C (350 °F), preferably at most approximately 15,000 milliPascals per second (mPa s) (15,000 centipoises (cP)) at 177 °C (350 °F), and most preferably at most approximately 12,500 milliPascals per second (mPa s) (12,500 centipoises (cP)) at 177 °C (350 °F).According to the embodiments of the invention, the viscosity of the composition is at least approximately 1,000 milliPascals per second (mPa-s) (1,000 centipoise (cP)) at 177 °C (350 °F), preferably at least approximately 2,500 milliPascals per second (mPa-s) (2,500 centipoise (cP)) at 177 °C (350 °F), and most preferably at least approximately 5,000 milliPascals per second (mPa-s) (5,000 centipoise (cP)) at 177 °C (350 °F). The viscosity of the adhesive is measured according to ASTM D3236 using head 27. The adhesives according to the embodiments of the present invention exhibit excellent mechanical properties (e.g., high tensile strength and high tensile strength) and thermal stability. These properties make the adhesives of the invention useful for hygiene, construction, and packaging applications, and particularly suitable for stabilizing absorbent cores containing superabsorbent polymers, which expand significantly upon exposure to moisture. The hot-melt composition of the present invention is further characterized by having a tensile strength of at least 350%, preferably at least 400%, more preferably at least 450%, and most preferably at least 475%, when measured at a tensile strength rate of 5.08 cm / min (27 min). The inventive formulation also meets additional mechanical properties such as tensile strength retained at 300% elongation after ten minutes.Preferably, the retained tensile strength at 300% elongation after ten minutes is at least 30%, preferably at least 40%, more preferably at least 45%, and most preferably at least 50%. Furthermore, the embodiments of the present invention demonstrate excellent yield strength, such as at least approximately 0.4 MPa, preferably at least approximately 0.5 MPa, and most preferably at least approximately 0.6 MPa. The hot-melt adhesive composition also has exceptional thermal stability, such that the retained viscosity at elevated temperatures is greater than 85% for at least three days, preferably greater than 90% for at least three days, and most preferably greater than 90% for at least five days.The methodologies for determining tensile strength, retained stress, yield strength, and thermal stability (as reflected in viscosity retention) are discussed in more detail in the Examples section below. The hot melt adhesive composition of the present invention can be formulated using any of the mixing techniques known in the art. A representative example of the mixing procedure consists of placing all the components in a lined mixing vessel equipped with a rotor, and then raising the temperature of the mixture to a range of 150°C to 200°C to melt the contents. Any of the constituents can be premixed or added individually to the mixing vessel. For example, the polymers can be a preformed blend or combination, or they can be added to the mixing vessel individually. It should be understood that the precise temperature to be used in this step will depend on the melting points of the particular ingredients. Mixing is allowed to continue until a consistent and uniform mixture is formed.The contents of the container are protected with an inert gas such as carbon dioxide or nitrogen throughout the mixing process. Without violating the spirit of the present invention, various additions and variations to the procedure for producing the hot melt composition may be made, such as applying a vacuum to facilitate the removal of trapped air. Other useful equipment for formulating the composition of the present invention includes, but is not limited to, single- or double-head 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 shaped into cubes with a protective skin formed on top or into granules for transport and use. The adhesive composition of the present invention can be used as a general-purpose hot melt adhesive in a variety of applications, such as, for example, in non-woven disposable hygiene products, paper converting, flexible packaging, woodworking, sealing boxes and cartons, labeling, and other assembly applications. Particularly preferred applications include the construction of non-woven disposable diapers and feminine sanitary napkins, reusable hygiene products incorporating pre-formed cores, elastic bonding of diapers and incontinence briefs, stabilization of the core of diapers and napkins, coating of the backsheet of diapers, conversion of industrial filter material, surgical gowns and assembly of surgical drapes, etc.The compositions of the present invention are especially suitable for use in stabilizing absorbent cores having superabsorbent polymers for disposable hygiene articles, such as diapers, feminine sanitary napkins, and adult incontinence products. The resulting hot melt adhesives can be applied to substrates using a variety of application techniques. Examples include hot melt glue gun, hot melt groove matrix coating, hot melt wheel coating, hot melt roller coating, melt blow coating, spiral spraying, and contact or non-contact filament coatings with the Signature™, Control Coat™, UFD™, and similar brands. In a preferred embodiment, the hot melt adhesive is sprayed indirectly onto the substrates. In one embodiment of the invention, a method for making a laminate comprises the steps of: (1) applying the hot-melt adhesive composition of the invention in a molten state to a primary substrate; and (2) bonding a secondary substrate to the primary substrate by contacting the secondary substrate with the adhesive composition. In one embodiment of the invention, the first substrate comprises a first layer (as a bottom layer) of an absorbent core, and the secondary substrate comprises a second layer (as a top layer) of the absorbent core, wherein at least one of the first or second layers has associated superabsorbent polymers. The first substrate and the secondary substrate may be a single continuous, folded material, such that the two folds form the first and secondary substrates. Any suitable absorbent core containing superabsorbent polymers may be used in connection with the present invention. Suitable absorbent cores are described in U.S. Patent Applications Nos. 2017 / 0209616, 2017 / 0165133, and 2016 / 0270987, all of which are incorporated herein by reference. As described in U.S. Patent Application No. 2017 / 0165133, the structure of the absorbent core typically includes absorbent polymeric material, such as hydrogel-forming polymeric material, also called absorbent gelling material (AGM) or superabsorbent polymer (SAP). This absorbent polymeric material ensures that large quantities of body fluids, e.g., urine, can be absorbed by the absorbent article during use and retained, thereby providing low rewetting and good skin drying.Thinner absorbent core structures can be manufactured by reducing or eliminating the traditional use of cellulose or cellulosic fibers in the core structure. To maintain the mechanical stability of these absorbent core structures, a fibrous network structure, which in some cases may be an adhesive, can be added to stabilize the absorbent polymer material. The absorbent core may also have additional adhesives, either to assist the fibrous network structure adhesive and / or to bond other core materials to each other and / or to other components of the article. The superabsorbent polymer material can be deposited on or associated with the first and second substrates, and a fibrous network structure covers the superabsorbent polymer material on the respective first and second substrates. In embodiments of the invention, the fibrous network structure comprises an adhesive composition of the present invention. The fibrous network structure may contain other materials, such as other adhesives or cellulose fibers. In another embodiment of the invention, the only adhesive used in the fibrous network structure is an adhesive composition of the present invention. In other embodiments of the invention, the fibrous network structure consists solely of one or more adhesive compositions of the present invention.In one embodiment of the invention, the first and second absorbent layers are combined such that at least a portion of the fibrous network structure of the first absorbent layer comes into contact with at least a portion of the fibrous network structure of the second absorbent layer, and wherein the adhesive of the present invention used in the fibrous network structure serves to bond the 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 with them. In other embodiments of the invention, only one of the layers has superabsorbent polymers associated with it. In another embodiment of the method for manufacturing a laminate of the invention, the primary substrate is a first layer of an absorbent core, and the secondary substrate is a superabsorbent polymer. The superabsorbent polymer can be deposited onto the first layer before the application of the adhesive composition. In this embodiment, the adhesive can form a fibrous network on and around the superabsorbent polymer and can also adhere to the first layer. In another embodiment, a second layer is formed in the same manner, and then the two layers are bonded together, before the adhesive cools, to form an absorbent core. ASPECTS OF THE INVENTION 1. A hot melt adhesive composition comprising: (a) a high molecular weight, single-site catalyzed, essentially amorphous propylene copolymer; (b) a low molecular weight, single-site catalyzed, semicrystalline propylene copolymer; and (c) polyisobutene; where the composition is free of bonding agents. 2. The composition of aspect 1, where: The high molecular weight propylene copolymer has a weight average molecular weight of at least approximately 80,000 g / mol, preferably at least approximately 95,000 g / mol, and most preferably at least approximately 105,000 g / mol and has a heat of fusion of at most approximately 20 J / g, preferably at most approximately 15 J / g, and most preferably at most approximately 10 J / g; and the low molecular weight propylene copolymer has a weight average molecular weight of at most approximately 60,000 g / mol, preferably at most approximately 40,000 g / mol, and most preferably at most approximately 30,000 g / mol and has a heat of fusion of at least approximately 20 J / g, preferably at least approximately 25 J / g, and most preferably at least approximately 30 J / g. 3. The composition of aspects 1 or 2, where: The high molecular weight propylene copolymer has a polydispersity index of less than approximately 5, preferably less than approximately 4 and most preferably less than approximately 3; and the low molecular weight propylene copolymer has a polydispersity index of less than approximately 5, preferably less than approximately 4 and most preferably less than approximately 3. 4. The composition of any of aspects 1-3, where: The high molecular weight propylene copolymer is a copolymer of propylene and a comonomer selected from the group consisting of ethylene and a C4-C12 alkylene, preferably ethylene; and the low molecular weight propylene copolymer is a copolymer of propylene and a comonomer selected from the group consisting of ethylene and a C4-C12 alkylene, preferably ethylene. 5. The composition of any of aspects 1-4, wherein polyisobutene has a number-average molecular weight of at least approximately 750 g / mol, preferably at least approximately 900 g / mol, more preferably at least approximately 1,500 g / mol, and most preferably at least approximately 2,000 g / mol. 6. The composition of any of aspects 1-5, wherein the high molecular weight propylene copolymer is present in an amount of between approximately 10% and approximately 60% by weight, preferably between approximately 20% and approximately 50% by weight, and more preferably between approximately 25% and approximately 40% by weight, based on the total weight of the composition; The low molecular weight propylene copolymer is present in an amount of between approximately 3% and approximately 40% by weight, preferably between approximately 4% and approximately 30% by weight, and more preferably between approximately 5% and approximately 20% by weight, based on the total weight of the composition; and the polyisobutene is present in an amount of between approximately 5% and approximately 50% by weight, preferably between approximately 10% and approximately 40% by weight, and more preferably between approximately 15% and approximately 30% by weight, based on the total weight of the composition 7. The composition of any of aspects 1-6 further comprising a 1-butene-based copolymer polymer. 8. The composition of aspect 7, wherein the 1-butene-based copolymer comprises a butene-ethylene copolymer. 9. The composition of any of aspects 7 or 8, wherein the 1-butene-based copolymer has a density of at most approximately 0.907 g / cm3, preferably at most approximately 0.905 g / cm3, more preferably at most approximately 0.900 g / cm3, and most preferably at most approximately 0.895 g / cm3. 10. The composition of any of aspects 7-9, wherein the 1-butene-based copolymer is present in an amount of between approximately 5% and approximately 40% by weight, preferably between approximately 7% and approximately 30% by weight, and more preferably between approximately 10% and approximately 25% by weight, based on the total weight of the composition. 11. The composition of any of aspects 1-10 which further comprises a plasticizer. 12. The composition of aspect 11, wherein the plasticizer is selected from the group consisting of a mineral oil and a synthetic polyalphaolefin oil and mixtures thereof. 13. The composition of aspects 11 or 12, wherein the plasticizer is present in an amount of between approximately 5% and approximately 60% by weight, preferably between approximately 10% and approximately 50% by weight, and more preferably between approximately 15% and approximately 40% by weight, based on the total weight of the composition. 14. The composition of any of aspects 1-13, wherein the composition does not contain an amorphous polyalpha olefin. 15. The composition of any of aspects 1-14, where the composition does not contain wax. 16. The composition of any of aspects 1-15, wherein the molecular weight of the high molecular weight propylene copolymer is at least two times, preferably at least three times and more preferably at least four times the molecular weight of the low molecular weight propylene copolymer. 17. The composition of any of aspects 1-16, wherein the heat of fusion of the low molecular weight propylene copolymer is at least two times, preferably at least three times, and more preferably at least four times the heat of fusion of the high molecular weight propylene copolymer. 18. The composition of any of aspects 1-17, wherein the viscosity of the composition is at most approximately 17,500 milliPascals per second (mPa s) (17,500 centipoises (cP)) at 177 °C (350 °F), preferably at most approximately zirZQcn / zznz / q / υιλι 15,000 milliPascals per second (mPa-s) (15,000 centipoises (cP)) at 177 °C (350 °F), and most preferably at most approximately 12,500 milliPascals per second (mPa s) (12,500 centipoises (cP)) at 177 °C (350 T). 19. The composition of any of aspects 1-18 which further comprises an antioxidant. 20. The composition of any of aspects 1-19, wherein the composition has a breaking strength of at least approximately 350%, preferably at least approximately 400%, and most preferably at least approximately 450%. 21. The composition of any of aspects 1-20, wherein the composition has a yield strength of at least approximately 0.4 MPa, preferably at least approximately 0.5 MPa, and more preferably at least approximately 0.6 MPa. 22. The composition of any of aspects 1-21, wherein the composition has a retained tension of at least 30%, preferably at least approximately 40%, and most preferably at least approximately 45%. 23. The composition of any of aspects 1-22, wherein the composition has a viscosity retained at 177 °C greater than approximately 90% for at least three days, preferably greater than approximately 95% for at least three days, and most preferably greater than approximately 90% for at least five days. 24. A method for making a laminate comprising the steps of: Apply the hot melt adhesive composition of any of aspects 1 to 23 in a molten state to a primary substrate; and match a secondary substrate to the primary substrate by bringing the secondary substrate into contact with the adhesive composition. 25. The method of aspect 24, wherein the main substrate comprises a first layer of an absorbent core and the secondary substrate comprises a second layer of the absorbent core, wherein at least the first layer or the second layer has associated superabsorbent polymers. 26. An absorbent core comprising a first layer and a second layer, wherein at least the first layer and the second layer comprise superabsorbent polymers, and the first layer and the second layer are bonded together by a hot-melt adhesive composition of any of aspects 1 to 23, and the adhesive composition bonds the superabsorbent polymers within the absorbent core. 27. A disposable hygiene article comprising an absorbent core of the appearance 26. EXAMPLES The invention is further illustrated by the examples below. To prepare the hot melt adhesive, all components were weighed into an unlined aluminum can and heated to 177°C under a layer of nitrogen (5 scfh). A double-blade impeller in an overhead mixer was inserted into the aluminum can and stirred at 25 rpm until the polymers were sufficiently agitated. The speed was then increased to 50 rpm until the mixture was homogeneous and at a constant temperature. The formulation was considered complete when the mixture appeared homogeneous and no lumps of polymer were visible. The formulation was then used to test viscosity, strength properties, and / or to fabricate laminates for final performance testing. The extreme compatibility of the system allowed all components to be mixed together without compromising the overall mixing time.This compatibility and stability created a very heat-resistant system that was able to maintain viscosity when stored at 177°C for several days. The ingredients listed below and in Tables 1 to 3 were used to manufacture the adhesives. The numerical values ​​listed for a given raw material are expressed as a percentage by weight and must equal 100%. Calsol 5500 is a naphthenic process oil available from Calumet Specialty Products. Indopol H-1900 is a polyisobutene oligomer available with a number-average molecular weight of 2,500 g / mol as determined by the supplier (Ineos Capital Ltd) by gel permeation chromatography (GPC), and all references to the number-average molecular weight of polyisobutene made herein are based on the same method. Indopol H-100 is a polyisobutene oligomer available with a number average molecular weight of 910 g / mol as determined by the supplier (Ineos Capital Ltd) by gel permeation chromatography (GPC). Irgafos 168 is a tris(2,4-di-tert-butylphenyl) phosphate available from BASF Chemicals and is used as an antioxidant. Irganox 1010 is pentaerythritoltetrakis (3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate) available from BASF Corp and is used as an antioxidant Koattro PB M 1200M is a 1-butene copolymer with a density of 0.908 g / cm3 (tested by the supplier according to ISO 1183-1) available from LyondelIBasell Industries Holdings. Koattro PB M 1500M is a 1-butene copolymer with a density of 0.890 g / cm3 (tested by the supplier according to ISO 1183-1) available from LyondelIBasell Industries Holdings. Vistamaxx 6502, obtained from ExxonMobil Chemical Company, Houston, TX, is an essentially amorphous poly(propylene-co-ethylene) containing approximately 13% by weight of ethylene comonomer and has a weight average molecular weight (Mw) greater than 80,000 g / mol and a density (reported by the supplier using ASTM D1505) of 0.865 g / cm3. Vistamaxx 8380, obtained from ExxonMobil Chemical Company, Houston, TX, is a low-viscosity, low-molecular-weight propylene-ethylene random copolymer. It contains approximately 12 wt% ethylene comonomer, has a weight-average molecular weight (Mw) of less than 60,000 g / mol, and a supplier-reported density of 0.864 g / cm³. Vistamaxx 8880, obtained from Exxonmobil Chemical Company, Houston, TX, is a low molecular weight semicrystalline propylene copolymer consisting of approximately 6% by weight of ethylene comonomer and has a weight average molecular weight (Mw) of less than 60,000 g / mol and a density (reported by the supplier) of 0.879 g / cm3. Viscosity was measured according to ASTM D3236 using a 27 nozzle at 163, 177, and 190 °C. The nozzle speed was adjusted so that the percentage torque was between 45% and 90%. The viscosity had to be low enough to produce fiber diameters between 10 and 60 µm when the adhesive was sprayed at 5 g / m² using a Nordson Signature© Low Flow nozzle. The dogbones for strength testing were fabricated by pouring molten adhesive into silicone molds so that the overall length of the dogbone was 8.89 cm x 2.54 cm (3.5” x 1.0”) with a thickness, flush with the mold, of 0.31 cm (0.125”). The test area of ​​the dogbone was 1.27 cm x 1.27 cm (0.5” x 0.5”). A hot spatula was used to scrape off any excess adhesive from the silicone mold so that the dogbone thickness was as close as possible to 0.31 cm (0.125”). The specimen was allowed to cool to room temperature for at least 12 hours before being subjected to elongation at break, tensile strength, and other mechanical tests. The top and bottom of the dogbone were clamped in an Instron tensile tester so that only the 1.27 cm x 1.27 cm (0.5” x 0.5”) test area was exposed. The tensile speed was 5.08 cm / min (27 min) and continued until the samples broke.Tensile strength and ultimate tensile strength are automatically calculated in the BlueHIII3 software available with Instron. Tensile strength was recorded as a percentage based on the difference between the final and initial lengths divided by the initial length to determine if the adhesive could withstand the stretching caused by the swelling of the SAP particles. One objective was for the adhesive to maintain an elongation of approximately 475% or more. Good tensile retention over time at a given elongation was also considered necessary. The elongation value was 300%, meaning that an initial test height of 0.5 inches would stretch (at 5.08 cm / min (27 min)) until the Instron jaws were a total distance of 5.08 cm (2 inches) apart. The tensile or load value is recorded at 300%.The sample is then held at 300% elongation for ten minutes before a final stress or load value is recorded. The "Tension Held at 300% Elongation, 10 min (%)" as shown in the following tables refers to the stress after ten minutes of holding at 300% elongation divided by the initial stress at 300% elongation multiplied by 100, so the value is shown as a percentage of sustained stress. This factor should be at least 30%, preferably at least approximately 40%, and most preferably at least approximately 45% or even at least approximately 50%. Pattern quality was determined both qualitatively and quantitatively. The adhesive was heated to approximately 190 °C and pumped through hoses to a Signature Low Flow nozzle (Nordson Corp.) with a layer weight of 5 g / m² and a linear velocity of 60.96 m / min (200 ft / min). The airflow was adjusted to create the most visually fibrous pattern with minimal clumping (those approximately more than twice the average droplet size) or loose adhesive strands (qualitative). On average, the air pressure was approximately 1.40 kg / cm² (20 psi), but this depends on the individual equipment settings and is intended only as a reference. The nozzle head was positioned 20 mm above the primary substrate, which was a 33 g / m² spunbond nonwoven material. The secondary substrate was a non-stick coating to facilitate sample analysis.Qualitative analysis was performed using a microscope to determine the diameter of the fibers produced during the spray application. The objective was to produce fiber diameters less than 60 µm and preferably less than 30 µm. The adhesive's strength properties are a function of the fiber diameter; therefore, the desired fiber diameter may vary depending on the adhesive formulation, the desired properties, and other conditions. Thermal stability was determined by pouring approximately 150 g of adhesive into an 8 oz (227.3 ml) glass jar and loosely securing the lid. The jar was placed in an oven at a high temperature, specifically 177 °C. The jar was removed from the oven daily to check for charring or gelation, which would suggest incompatibility. In addition, a 10 g piece 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 100. Preferably, the retained viscosity should be greater than 85% for at least three days. Ideally, the retained viscosity would be greater than 90% for at least three days or even at least five days. Table 1. Importance of polyisobutene (PIB) Ex.1 Ex.2 Ex.3 EC.1 Indopol H-1900, % by weight 20.27 24.35 - - Indopol H-100, % by weight - - 20.27 - Calsol 5550, % by weight 22.15 26.62 22.15 42.42 Vistamaxx 6502, % by weight 29.17 35.05 29.17 29.17 Vistamaxx 8880, % by weight 10.97 13.18 10.97 10.97 Koattro PB 1500M, % by weight 16.64 - 16.64 16.64 IRGAFOS 168, % by weight 0.53 0.53 0.53 0.53 IRGANOX 1010, % Weight 0.27 0.27 0.27 0.27 Viscosity at 177 °C (350 °F) (mPa s and cP) 11300 13300 9638 7325 Viscosity at 190.55 °C (375 °F) (mPa-s and cP) 8300 - 6950 5313 Tensile strength (%) 669 536 463 393 Tensile strength (MPa) 0.68 0.74 0.61 0.55 Tensile strength at 300% elongation, 10 min (%) 54.1 49.6 34.7 0.0 Table 1 demonstrates the importance of the polyisobutene (PIB) component and the optional butene-rich polymer. Example (Ex.) 2 shows that the butene-rich copolymer is optional, but its inclusion is preferred because it reduces viscosity, as shown in Ex. 1, and increases elongation and retained stress. In comparative example one (EC1), the use of only a naphthenic oil resulted in an adhesive exhibiting unacceptably low tensile strength, which consequently failed during retention at 300% elongation. Inventive examples 1 and 2, which contained PIB, had a tensile strength exceeding 525% and retained more than approximately 50% of their original stress after 10 minutes at 300% elongation. Example 3 containing the lowest molecular weight PIB resulted in a retained stress of approximately 35% with an elongation of 300% after 10 minutes and an elongation at break of 463%.All examples of the invention had a maximum tensile strength of at least 0.61 MPa. Furthermore, examples 1 and 2 desirablely produced an average fiber diameter of 17 µm when 5 g / m² were applied at 190°C. Table 2. Importance of the high molecular weight propylene copolymer compound zirZQcn / zznz / q / υιλι EC. 3 Indopol H-1900, wt. % 20.27 Calsol 5550, wt. % 22.15 Koattro PB 1500M, wt. % 16.64 Irgafos 168, wt. % 0.53 Irganox 1010, wt. % 0.27 Vistamaxx 6502, wt. % - Vistamaxx 8880, wt. % 10.97 Vistamaxx 8380, wt. % 29.17 Viscosity at 177 °C (350 °F) (mPa sy cP) 1063 Viscosity at 190.55 °C (375 °F) (mPa sy cP) 793 Tensile strength (%) 115 Tensile strength (MPa) 0.18 Tensile strength at 300% elongation, 10 min (%) 0.0 Table 2 demonstrates the role of the high-molecular-weight propylene copolymer component. Comparative example three (EC3) 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 tensile strength and a complete loss of retained tension at 300% elongation after 10 minutes. Table 3. Impact of the third polymer having a specific gravity that is too high EC. 4 Calsol 5550, wt. % 22.18 Indopol H-1900, wt. % 20.29 Vistamaxx 6502, wt. % 29.21 Vistamaxx 8880, wt. % 10.98 KOATTRO PB M 1200M, wt. % 16.66 Irgafos 168, wt. % 0.53 Irganox 1010, wt. % 0.27 Viscosity at 162.77 °C (325 °F) (mPa sy cP) 23650 Tensile strength (%) 380 Tensile strength (MPa) 0.85 Tensile strength at 300% elongation, 10 min (%) 0.0 Comparative example four (EC4) contains a butene-rich copolymer with a specific gravity of 0.908 g / cm3. The resulting formulation becomes partially incompatible, resulting in a rapid decrease in tensile strength and a complete loss of retained tension at 300% elongation after ten minutes. Table 4. Thermal stability: viscosity retention at 177 °C Duration at 177 °C (days) EX. 1 (%) EC. 5* (%) 1 94.6 86.3 2 94.5 70.7 3 94.3 51.8 4 93.8 34.8 5 92.6 24.9 *CE. 5 is a commercially available styrene block copolymer from Bostik Inc. Table 4 demonstrates the exceptional thermal stability through viscosity retention when stored in an oven at 177°C. Ideally, hot melt adhesives are heated only as long as necessary, but occasionally they may remain molten for several hours or even days. For example, if a production line experiences problems, the adhesive might remain molten until the line is back online. Furthermore, some manufacturers do not produce goods on weekends and may occasionally leave the melt tank running to avoid waiting for the adhesive to remelt when restarting a line. Therefore, it is advantageous to have an adhesive with exceptional thermal stability to accommodate these circumstances.Preferably, the hot melt composition would have a viscosity retained at elevated temperatures greater than 85% for at least three days, preferably greater than 90% for at least three days, and more preferably greater than 90% for at least five days. When a range of values ​​is provided, it is understood that each intermediate value, and any combination or subcombination of intermediate values, between the upper and lower limits of that range and any other stated or intermediate value within that stated range, is encompassed within the stated range of values. Furthermore, the invention includes a range of a constituent that is the lower limit of a first range and an upper limit of a second range of that constituent. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by a person skilled in the art to which this invention pertains. All publications and patents specifically mentioned herein are incorporated by reference in their entirety for all purposes, including the description and disclosure of chemicals, instruments, statistical analyses, and methodologies reported in the publications that could be used in connection with the invention. All references cited in this specification are to be taken as indicative of the level of expertise in the art. Nothing contained herein should be construed as an admission that the invention is not entitled to prior disclosure by virtue of a prior invention. Although illustrated and described herein with reference to certain specific embodiments, the present invention is not intended to be limited to the details shown. Rather, various modifications to the details may be made within the scope and range of equivalents of the claims and without departing from the spirit of the invention.

Claims

1. A hot melt adhesive composition characterized in that it comprises: (a) a single-site catalyzed, essentially amorphous, high molecular weight propylene copolymer; (b) a single-site catalyzed, semicrystalline, low molecular weight propylene copolymer; and (c) polyisobutene; wherein the composition is free of bonding agents.

2. The composition according to claim 1, characterized in that: the high molecular weight propylene copolymer has a weight average molecular weight of at least approximately 80,000 g / mol, preferably at least approximately 95,000 g / mol, and most preferably at least approximately 105,000 g / mol and has a heat of fusion of at most approximately 20 J / g, preferably at most approximately 15 J / g, and most preferably at most approximately 10 J / g; and the low molecular weight propylene copolymer has a weight average molecular weight of at most approximately 60,000 g / mol, preferably at most approximately 40,000 g / mol, and most preferably at most approximately 30,000 g / mol and has a heat of fusion of at least approximately 20 J / g, preferably at least approximately 25 J / g, and most preferably at least approximately 30 J / g.

3. The composition according to claim 1, characterized in that: the high molecular weight propylene copolymer has a polydispersity index of less than approximately 5, preferably less than approximately 4 and most preferably less than approximately 3; and the low molecular weight propylene copolymer has a polydispersity index of less than approximately 5, preferably less than approximately 4 and most preferably less than approximately 3.

4. The composition according to claim 1, characterized in that: the high molecular weight propylene copolymer is a copolymer of propylene and a comonomer selected from the group consisting of ethylene and a C4-C12 alkylene, preferably ethylene; and the low molecular weight propylene copolymer is a copolymer of propylene and a comonomer selected from the group consisting of ethylene and a C4-C12 alkylene, preferably ethylene.

5. The composition according to claim 1, characterized in that the polyisobutene has a number average molecular weight of at least approximately 750 g / mol, preferably at least approximately 900 g / mol, more preferably at least approximately 1,500 g / mol and most preferably at least approximately 2,000 g / mol.

6. The composition according to claim 1, characterized in that the high molecular weight propylene copolymer is present in an amount of between approximately 10% and approximately 60% by weight, preferably between approximately 20% and approximately 50% by weight, and more preferably between approximately 25% and approximately 40% by weight, based on the total weight of the composition; the low molecular weight propylene copolymer is present in an amount of between approximately 3% and approximately 40% by weight, preferably between approximately 4% and approximately 30% by weight, and more preferably between approximately 5% and approximately 20% by weight, based on the total weight of the composition;and polyisobutene is present in an amount of between approximately 5% and approximately 50% by weight, preferably between approximately 10% and approximately 40% by weight, and more preferably between approximately 15% and approximately 30% by weight, based on the total weight of the composition; 7. The composition according to claim 1 characterized in that it further comprises a 1-butene-based copolymer polymer.

8. The composition according to claim 7, characterized in that the 1-butene-based copolymer comprises a butene-ethylene copolymer.

9. The composition according to claim 7, characterized in that the 1-butene-based copolymer has a density of at most approximately 0.907 g / cm3, preferably at most approximately 0.905 g / cm3, more preferably at most approximately 0.900 g / cm3, and most preferably at most approximately 0.895 g / cm3.

10. The composition according to claim 1, characterized in that the 1-butene-based copolymer is present in an amount of between approximately 5% and approximately 40% by weight, preferably between approximately 7% and approximately 30% by weight, and most preferably between approximately 10% and approximately 25% by weight, based on the total weight of the composition.

11. The composition according to claim 1 characterized in that it further comprises a plasticizer.

12. The composition according to claim 11, characterized in that the plasticizer is selected from the group consisting of a mineral oil and a synthetic polyalphaolefin oil and mixtures thereof.

13. The composition according to claim 11, characterized in that the plasticizer is present in an amount of between approximately 5% and approximately 60% by weight, preferably between approximately 10% and approximately 50% by weight, and more preferably between approximately 15% and approximately 40% by weight, based on the total weight of the composition.

14. The composition according to claim 1, characterized in that the composition does not contain an amorphous polyalpha olefin.

15. The composition according to claim 1, characterized in that the composition does not contain wax.

16. The composition according to claim 1, characterized in that the molecular weight of the high molecular weight propylene copolymer is at least two times, preferably at least three times and more preferably at least four times the molecular weight of the low molecular weight propylene copolymer.

17. The composition according to claim 1, characterized in that the heat of fusion of the low molecular weight propylene copolymer is at least two times, preferably at least three times and more preferably at least four times the heat of fusion of the high molecular weight propylene copolymer.

18. The composition according to claim 1, characterized in that the viscosity of the composition is at most approximately 17,500 milliPascals per second (mPa s) (17,500 centipoises (cP)) at 177 °C (350 °F), preferably at most approximately 15,000 milliPascals per second (mPa-s) (15,000 centipoises (cP)) at 177 °C (350 °F), and most preferably at most approximately 12,500 milliPascals per second (mPa-s) (12,500 centipoises (cP)) at 177 °C (350 °F).

19. The composition according to claim 1, characterized in that it further comprises an antioxidant.

20. The composition according to claim 1, characterized in that the composition has a breaking strength of at least approximately 350%, preferably at least approximately 400% and most preferably at least approximately 450%.

21. The composition according to claim 1, characterized in that the composition has a yield strength of at least approximately 0.4 MPa, preferably at least approximately 0.5 MPa and most preferably at least approximately 0.6 MPa.

22. The composition according to claim 1, characterized in that the composition has a retained tension of at least 30%, preferably at least approximately 40% and most preferably at least approximately 45%.

23. The composition according to claim 1, characterized in that the composition has a viscosity retained at 177 °C greater than 90% for at least three days, preferably greater than approximately 95% for at least three days, and most preferably greater than approximately 90% for at least five days.

24. A method for making a laminate characterized in that it comprises the steps of: applying the hot melt adhesive composition according to any of claims 1 to 23 in a molten state to a primary substrate; and bonding a secondary substrate to the primary substrate by bringing the secondary substrate into contact with the adhesive composition.

25. The method according to claim 24, characterized in that the primary substrate comprises a first layer of an absorbent core and the secondary substrate comprises a second layer of the absorbent core, wherein at least one of the first layer or the second layer has superabsorbent polymers associated therewith.

26. An absorbent core characterized in that it comprises a first layer and a second layer, wherein at least the first layer and the second layer comprise superabsorbent polymers, and the first layer and the second layer are bonded together by a hot-melt adhesive composition according to any one of claims 1 to 23, and the adhesive composition bonds the superabsorbent polymers within the absorbent core.

27. A disposable hygiene article characterized in that it comprises the absorbent core according to claim 26.