Hot melt adhesive formulations for fibrous substrates

Abstract

The novel hot melt adhesive formulation disclosed herein exhibits excellent adhesion to both woven and non-woven fibrous substrates. Furthermore, even when those fibrous substrates are made up of highly hydrophilic fibers, such as cotton or other similar plant fibers, even during use, such as within sanitary absorbent articles, the hydrophilic fibers contact moisture and absorb substantial amounts of moisture or other aqueous liquids, such as urine or blood, resulting in associated swelling of the fibers, the hydrophilic fibers will not fall off. Wherein the associated swelling can significantly weaken even destroy and mechanically break the adhesive bond that has been formed under dry conditions, the adhesive still being able to maintain an unexpected high bond on the substrate.

Description

Technical Field

[0001] The present invention relates to a hot melt adhesive formulation which is capable of firmly adhering to fibrous substrates, including both woven and nonwoven substrates. The adhesive particularly exhibits excellent adhesion properties and is resistant to forces that would weaken or break the adhesive bond, even when the fibers of the substrate are highly hydrophilic and thus absorb large amounts of water and swell in an intrinsic manner in the presence of aqueous liquids.

[0002] The hot melt adhesives of the present invention are particularly suitable for the construction and manufacture of sanitary absorbent articles, which generally comprise at least one fibrous substrate, which is a woven or nonwoven substrate, and whose fibers may be natural or synthetic. Background Art

[0003] In sanitary absorbent products, the above-mentioned woven or nonwoven fiber substrates (whether made of natural fibers or synthetic fibers) are often assembled into more complex structures by adhesives. Such structures are called "laminated materials" and are made of two or more layers bonded to each other by glue.

[0004] However, in a few cases, as disclosed in EP0924328A1, a multilayer structure having at least one layer of woven or non-woven fiber substrate can also be prepared by locally melting the fiber substrate at selected points, and the local melting can be achieved, for example, by applying ultrasound or the like. However, it has been shown that this technology is only applicable to synthetic fibers made of thermoplastic polymers, and is not applicable to natural fibers whose main component is cellulose. In addition, partial local melting of at least one fiber substrate will also produce at least two further problems. First, a continuous structure and a non-porous area will be formed at the thermoplastic fiber melting point. As a result, in those areas, the fiber substrate at least partially loses the inherent porosity that is usually possessed by all fiber substrates, and with this porosity, it at least partially loses the advantages of air permeability and liquid permeability brought about thereby. The second major disadvantage is that local melting will form a hard polymer melting area, thereby increasing the rigidity of the entire laminate in an inherent manner. This rigidity may be acceptable in certain industrial applications, such as nonwoven laminates used as carpet substrates or laminates used in battery separators, but it is completely unacceptable for products such as sanitary absorbent products where high flexibility of the overall structure is a basic characteristic that is of great concern to users.

[0005] Therefore, in the preparation of multi-layer laminates, at least one of which is a fiber substrate, the use of adhesive bonding has become a preferred solution in many fields, especially in the hygiene / medical field. In addition, it is believed that in this field and other similar applications, the amount of adhesive used must be reduced as much as possible. This is not only due to significant economic considerations, but also because only by applying the adhesive in a discontinuous manner (i.e., in a limited amount) can the porosity of the fiber substrate be maintained while avoiding the increase in the rigidity of the laminate caused by excessive continuous adhesive layers.

[0006] Some prior art documents, such as US 4,069,822 and US 4,147,580, seem to focus on exploring the best process to achieve a small amount of hot melt adhesive coating (to avoid rigidification of the fiber substrate and complete closure of natural pores) and ensure a sufficiently strong bonding. For example, according to these patents, this goal can be achieved by preferentially bonding free cilia protruding from the surface of woven / non-woven fibers. Regardless of the controversy over the effectiveness of the process itself, these patents do not involve the chemical composition characteristics and physical performance indicators of the adhesive used. This obviously constitutes a defect of the prior art, because the strength of the formed bonding depends on the physical and chemical properties of the adhesive used, rather than the coating process used.

[0007] US 4,849,049 also discloses similar technology, which still focuses only on the coating process, again ignoring the requirements for the chemical and physical properties of the adhesive used on the fiber substrate.

[0008] US 5,360,504 also adopts a similar concept for substrate bonding, even though the substrate is not a fiber substrate but a polyolefin sponge in another special example, it shares the characteristics of extremely high porosity with the fiber substrate, and thus also requires the hot melt adhesive to at least partially physically penetrate into the pores of the substrate to achieve a sufficiently strong bond. In this case, in addition to disclosing a specific coating process, the patent US 5,360,504 only lists the melt viscosity of the thermoplastic adhesive (generally required to be less than 800,000,000 mPa·s) as the only parameter of the adhesive that can be used to ensure the application effect disclosed by the inventor. This solution is questionable in many aspects: in particular, it should be noted that it can be observed that the extremely high melt viscosity (neither the test method nor the temperature range is specified - it may be between 140°F (60°C) and 330°F (165.5°C)) usually occurs in hot melt adhesives, most likely measured according to ASTM D 3236-88 standard (i.e., Brookfield viscosity method). Therefore, the measurement condition is likely to be at a non-zero shear rate, which, in fact (as will be explained below), cannot truly reflect the penetration condition of the molten adhesive inside the porous substrate.

[0009] Finally, other prior art, such as US 5,626,912, uses reactive adhesives to reinforce structures made of fabrics and other fiber substrates; this is because reactive adhesives are applied in the form of monomer mixtures or low-viscosity oligomers, which then undergo polymerization at the application site, and have the advantages of very low initial viscosity (easy to penetrate the pores of the fiber substrate) and can form a high peel strength adhesive layer after polymerization. However, it is clear that reactive adhesives are not suitable in many cases, both because the need for polymerization after application will complicate the production line process (for example, significantly reduce the production line speed), and because many fields (such as the hygiene / medical field, the pharmaceutical field, food contact materials, etc.) tend to avoid the use of reactive adhesives because even after polymerization, they may still have residual unreacted monomers with high toxicity / carcinogenicity.

[0010] It can be seen that the prior art has not yet provided a satisfactory solution for the preparation of a bonded structure (laminated material) containing at least one layer of woven or non-woven fiber substrate using a non-reactive hot melt adhesive. It is particularly noteworthy that when the fiber substrate is composed of highly hydrophilic fibers (such as cotton or other plant fibers), the prior art has not considered the effect of absorbing a large amount of water or other aqueous liquids (such as urine or blood) during use on the bonding. The root of this technical problem is that water absorption (as described in detail below) will have an extremely adverse or even destructive effect on most bonding interfaces - this is due to both the chemical and physical effects caused by the liquid water itself, and the mechanical effect caused by the violent swelling of the fiber after absorbing water, which will lead to the rupture of the existing bonding. Summary of the invention

[0011] The technical problem to be solved by the present invention is to provide a hot melt adhesive formulation that can firmly adhere to woven and nonwoven fiber substrates. The new adhesive exhibits unexpectedly strong adhesion in the dry state and can maintain a strong adhesive bond when the fiber substrate absorbs a large amount of water or other aqueous liquids (such as blood or urine) during use due to the high hydrophilicity of the fibers (such as cotton and other natural fibers) causing the fibers to swell significantly during use - this swelling action mechanically destroys the bonding interface formed under dry conditions.

[0012] Therefore, the hot melt adhesive of the present invention is particularly suitable for manufacturing sanitary absorbent products comprising at least one layer of woven or nonwoven fiber substrates, and the fiber substrate can be composed of natural or synthetic fibers. In particular, as mentioned above, even when the highly hydrophilic natural fiber (such as cotton and similar plant fibers) absorbs a large amount of water or aqueous liquids such as urine and blood during the use of the sanitary absorbent product, the adhesive can still form and maintain an abnormally strong bonding bond. In fact, under the condition that the above-mentioned vast majority of adhesives cannot maintain strong adhesion on the surface of the wet fiber, even on the water-saturated fiber, the hot melt adhesive of the present invention unexpectedly maintains strong adhesion and keeps bonding in an excellent manner. As will be described in detail later, in this way, it can resist the chemical / physical elimination and destructive effect of water molecules on the intermolecular bonding force between the adhesive and the substrate interface, and can resist the extremely strong mechanical action produced by the fiber water absorption swelling, which is usually enough to break the adhesive itself, thereby physically destroying the bonding bond.

[0013] The adhesive of the present invention achieves its novel and unexpected characteristics through the technical solution contained in claim 1, that is, it has strong adhesion to the fiber substrate and can maintain excellent peeling resistance even if the fiber is soaked with water during use. Specifically, the selected and formed hot melt adhesive has the following characteristics:

[0014] - zero shear viscosity not exceeding 10,000 mPa·s, preferably not exceeding 6,500 mPa·s;

[0015] - The melting enthalpy after aging for five days at room temperature ("aging conditions" is defined below) does not exceed 30 J / g;

[0016] - The inflection point temperature in the Tan Delta temperature spectrum measured in the cooling mode at the initial time point as described below, the inflection point temperature near the set point is not higher than 95°C.

[0017] In addition, the peel strength of the adhesive under dry conditions is required to be no less than 2.0N per 50mm width; and under wet conditions after absorbing water, the deviation between its peel strength and the corresponding dry peel strength does not exceed 80%. The peel strengths under dry and wet conditions are measured according to the method described below, after aging for five days at room temperature.

[0018] In addition, the hot melt adhesive formulation preferably also has the following properties:

[0019] - The first crossover temperature of the rheological modulus at the initial time point (usually represented by the symbol Tx) is not higher than 75°C;

[0020] - The yield stress in the stress-elongation curve is not less than 0.1 MPa after aging at room temperature for five days, measured at 37°C as described below;

[0021] - Toughness value, i.e. the area enclosed by the stress-elongation curve, which directly characterizes the specific energy required to mechanically destroy the adhesive, measured at 37°C as described below, and after aging at room temperature for five days, not less than 0.5 MJ / m 3 .

[0022] The following will explain in detail how the above-mentioned physical and chemical characteristics enable the adhesive of the present invention to produce unexpectedly excellent performance.

[0023] Therefore, the technical problem to be solved by the present invention can be solved in the best and unexpected way by the following solutions: a hot melt adhesive formulation having the characteristics of claim 1) and claims 2) to 21); a bonding structure having the characteristics of claims 22) to 24); and a product having the characteristics of claims 25) to 31). The remaining dependent claims describe preferred embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 - Identification of the inflection point of the experimental curve of Tan Delta changing with temperature in Comparative Example 1.

[0025] Figure 2 - Identification of the inflection point determined by the second derivative of the Tan Delta temperature function in Comparative Example 1 being zero. DETAILED DESCRIPTION

[0026] definition

[0027] The terms "comprising" or "including" used in this article are open-ended expressions used to indicate the existence of the content after the term, but do not exclude the existence of other ingredients or features, such as elements, steps, components, etc. known in the art or disclosed in this article.

[0028] The term "polymer" adopts the definition in the document "How to determine whether a substance is a polymer and related registration procedures" of the European Chemicals Agency (ECHA) in December 2017. Accordingly, the present invention defines "polymer" as: any chemical substance containing more than 50% by weight of "polymer molecules"; wherein "polymer molecules" refer to molecules containing at least three basic units (monomer units or more complex units) and bonded to a fourth unit, which may be the same as or different from the first unit. Therefore, the polymer molecule contains at least four basic units in total, which may be monomer units or more complex units (such as basic units composed of two or more monomers in condensation polymers). The term "polymer" covers polymer molecules composed of a single type of basic unit / monomer (homopolymer) and multiple different types (copolymers).

[0029] Similarly, the term "oligomer" as used herein refers to a chemical substance containing more than 50% by weight of "oligomer molecules"; wherein an "oligomer molecule" comprises a structure of less than three basic units (monomer units or more complex units) bonded to another unit, wherein the other unit may be the same as or different from the previous units. Furthermore, the term "oligomer molecule" also covers oligomer molecules composed of a single type of basic unit / monomer (homo-oligomer) and multiple different types (co-oligomer).

[0030] Specifically, the term "homopolymer" adopts the definition in the "Compilation of Basic Terms in Polymer Science" published by the International Union of Pure and Applied Chemistry (IUPAC) in "Pure and Applied Chemistry" Volume 68, Issue 12 (1996). Therefore, the expression "copolymer" refers to a polymer synthesized from a single type of monomer. According to the same document, the expression "copolymer" in the present invention (unless otherwise specified) refers not only to polymers in chemical compositions using two different monomers, but also to polymers in chemical compositions using three, four, five or more different monomers. According to the above expression, when it is necessary to emphasize the number of different comonomers constituting a specific copolymer, expressions such as "binary copolymer", "ternary copolymer" and "quadruple copolymer" may also be used.

[0031] The expression "rheological modulus first crossover temperature", often represented by the symbol Tx, is sometimes also called "rheological solidification point" or "rheological solidification temperature", which refers to the highest temperature at which the two moduli first intersect (Tan Delta value is 1) in the temperature range above room temperature (i.e., above 23°C) in the rheological curve for measuring the variation of viscosity modulus G", elastic modulus G' and their ratio Tan Delta with temperature. When the rheological curve is measured in a cooling mode at the initial time point, it can accurately simulate the phenomenon that occurs between the adhesive and the substrate as the adhesive slowly cools and solidifies naturally during the actual process of hot melt adhesive application from the molten state and the creation of an adhesive bond. In particular, the "rheological modulus first crossover temperature", or the equivalent expression "rheological solidification point", characterizes the temperature at which the molten hot melt adhesive begins to form a final solid adhesive bond after being applied from the molten state to the substrate.

[0032] The term "cross modulus" and the corresponding symbol Gc in the above rheological curve refer to the absolute value of the elastic modulus G' and the viscous modulus G" of a specific material at the rheological curing temperature Tx (by definition, the two moduli have the same value at this point), and the unit is Pa or MPa.

[0033] Zero shear viscosity is defined in rheology as the asymptotic constant value (plateau value) to which the apparent viscosity of a liquid polymer system (such as a molten polymer or a polymer dissolved in a solvent) tends in the viscosity-shear rate relationship curve when the shear rate approaches zero. For the purposes of the present invention, the liquid polymer system, in which the zero shear viscosity is measured, is a molten thermoplastic adhesive at a temperature of 160°C. This parameter is clearly defined in the article "On the Determination of Zero Shear Viscosity of Polymer Melts" published by MTShow in Volume 61 of Polymer Engineering and Science (February 2021). As is well known, this rheological parameter is proportional to the weight-average molecular weight Mw of the test polymer; its measurement and calculation method is based on the teaching of "Obtaining Zero Shear Viscosity of Polymer Melts Using Different Rheological Test Methods" published by J.Lauger and M.Bernzen in Volume 8 of Annual Transactions of the Nordic Rheology Society (2000), pp. 159-162. More specifically, among the various rheological methods for calculating and measuring the physical parameters described in the document, the present invention adopts the "flow curve under low shear rate" method to measure the zero shear viscosity and express it in mPa·s.

[0034] In contrast, the dynamic viscosity of a molten adhesive at a specific temperature and non-zero shear rate, also known in the industry as the Brookfield viscosity, is determined by ASTM 3236-88 and is again expressed in mPa·s.

[0035] The melting enthalpy, crystallization enthalpy and glass transition temperature of a certain material are determined by differential scanning calorimetry (DSC). All DSC tests, including crystallization cycles during cooling and melting during heating, are performed according to the ASTM D3417-99 standard method. The cycle is carried out at +180°C to -70°C (or reverse), and the heating and cooling rates are all 10°C / minute. When the DSC melting or crystallization curve of a certain material has two or more peaks (even including partially overlapping peaks), it is defined as the "peak melting temperature" of the material or the "peak crystallization temperature" of the material, where the peak temperature of the peak has the largest area, that is, has the maximum enthalpy. The total melting enthalpy or crystallization enthalpy value is calculated as the sum of all melting peaks or crystallization peaks detected in the DSC test. The melting enthalpy and crystallization enthalpy are expressed in J / g, and their values ​​are obtained by integrating the total area of ​​a single peak or peak group appearing in a specific DSC cycle (obtained by integration).

[0036] "Room temperature", unless otherwise specified, refers to a temperature of 23°C; "room temperature conditions" refers to controlled temperature and relative humidity environmental conditions, i.e., 23°C and 50% relative humidity.

[0037] Unless otherwise stated, the rheological parameters described in the present invention, such as the elastic modulus G' of a certain adhesive, are measured by rheological testing at a frequency of 1 Hz, at a heating or cooling rate of 3°C / min between +170°C and -20°C (or vice versa).

[0038] In view of the fact that some properties of some materials used in the present invention, such as the amount and morphology of the crystal phase (and thus the mechanical properties and melting enthalpy), will change with the passage of time after the molten state solidifies, the present invention distinguishes such time-varying properties into "initial time point properties" and "aged properties" after certain hours or days, usually referring to the properties five days after the molten state solidifies. Therefore, a certain property at the "initial time", (e.g., initial melting enthalpy) is also called "initial property" or "property under initial conditions", which refers to the property measured at 23°C (unless other test temperatures are specified) and 50% relative humidity no more than 120 minutes after the material solidifies from the molten state. And "five-day aged properties" or "performance under aged conditions" refers to the properties measured at 23°C (unless other test temperatures are specified) and 50% relative humidity five days after the test material solidifies from the molten state. During the five days of aging, the test material was kept in a climate chamber at 23°C and 50% relative humidity.

[0039] The tensile properties of a material, also called "tensile mechanical properties", are the stress-elongation at break curve of an adhesive formulation and its derived parameters, such as peak stress, yield stress, stress at break. Elongation at break and toughness values ​​are determined as described below at 37°C and 50% relative humidity. In particular, in the stress-elongation curve of a material, the yield stress is defined as the stress value at the yield point, which is the point of the curve from which the material ceases to have an "elastic" behavior (i.e. the applied stress is proportional to the recorded elongation) and begins to have a "plastic" deformation. In practice: the yield point is the point at which the stress-elongation curve ceases to have a straight-line form and bends to show a curved form.

[0040] The toughness value of a material is numerically represented by the total area enclosed by the stress-elongation curve, which is calculated by integration. It is expressed as the specific energy required to mechanically destroy a material, and the unit of measurement is MJ / m 3 .

[0041] The rheological properties, tensile properties and stress-elongation curves were measured using the Ares G2 rotational rheometer produced by TA Instruments. The rheometer is equipped with an accessory tool called an extension viscosity fixture (EVF) for tensile property testing and stress-elongation curve measurement. The rheometer is also equipped with a temperature control chamber (FCO) that can be tested in the range of -50°C to +250°C.

[0042] In the stress-elongation curve test, the sample is extruded onto silicone paper at 160°C through a laboratory coater of a hot melt adhesive to form a continuous adhesive strip with a width of 50mm and a thickness of 0.2mm. The extruded adhesive is aged in a climate chamber at 23°C and 50% relative humidity for five days before being tested. After five days of aging, a 100mm long test sample is cut from the continuous adhesive strip of the aged adhesive. The sample is tested at a temperature of 37°C and a stretching roller speed frequency of 1 rev / sec using an EVF tool. This parameter setting is intended to simulate the actual working environment of the adhesive in sanitary absorbent products to the greatest extent possible: at a body temperature of 37°C, when cotton fibers rapidly absorb body fluids such as urine and menstrual blood, they swell rapidly, thereby exerting a strong tensile effect on the adhesive attached to the fibers at a high tensioning rate equivalent to 1 rev / sec.

[0043] The noun "compatibility" and the adjective "compatible" refer to the blending state of the components of the hot melt adhesive formulation of the present invention, especially the blending of two or more polymers, and the definition thereof follows the IUPAC Chemical Terminology (Second Edition, 1997). That is, when the blend exhibits uniform physical properties macroscopically, it is considered "compatible" regardless of whether it is a "miscible" blend (showing only one glass transition temperature Tg) or an "immiscible" blend (showing two or more Tgs). In particular, the present invention regards blends that do not show visible separation of two or more layers after being kept in a molten state at 170°C for 72 hours as "compatible".

[0044] The "polydispersity index" or "molecular weight distribution index" or "PDI" characterizes the molecular weight distribution measurement of a particular polymer. It is defined as the ratio of the weight average molecular weight Mw to the number average molecular weight Mn: PDI = Mw / Mn. A larger PDI value indicates a wider molecular weight distribution curve and vice versa. Even for compatible polymer blends, the average Mw, average Mn and overall "polydispersity index" can be defined with reference to the case of a single polymer. Mw, Mn and their ratio Mw / Mn = PDI can be determined by gel permeation chromatography (GPC).

[0045] The "open time" of an adhesive, especially for hot melt adhesives, refers to the time interval from the time the adhesive is applied to the first substrate in a molten state to the time when it contacts the second substrate under moderate pressure to form a sufficiently strong bond to meet the requirements of use. Obviously, too short an open time will make it difficult to apply the adhesive and form a strong bonding interface. The open time of hot melt adhesives is determined according to the ASTM D 4497-94 test method. The test conditions for hot melt adhesives are as follows:

[0046] - Adhesive film coating temperature: 170°C

[0047] - Adhesive film thickness: 1mm

[0048] "Ring and ball softening point" or "ring and ball softening temperature" means the softening temperature of a material as determined by ASTM D 36-95.

[0049] For wax materials, the softening point (also called "dropping point" or "drop melting point") is determined according to ASTM D 3954-94.

[0050] The "needle penetration" of an adhesive is a measure of its softness, usually measured in units of 0.1 millimeters (dmm) according to ASTM D1321-04.

[0051] Total bond strength or "peel strength" is defined as: the average force per unit width required to separate two substrates bonded by the tested adhesive. It is measured by separation test at a controlled and constant speed and a controlled and constant peeling angle. It is measured by ASTM D 1876-01 method, using a 180° peeling angle to separate two substrates at a separation speed of 150 mm / min, i.e., the actual moving speed of the tester is 300 mm / min. The two substrates used are 15 g / m2 basic weight polyethylene films provided by Poligof (Italy), which are glued to a 35 g / m2 basic weight pure cotton rotary mesh nonwoven fabric (HyDry Cotton) made of cotton fibers provided by Glatfelter (USA). After the tested adhesive is melted at 160°C, it is applied to the cotton substrate with a basic weight of 8 g / m2 by spraying or slit coating, and immediately contacted and pressed with the polyethylene film. The bonded laminate is then aged for five days in a climate box at 23°C and 50% relative humidity. After aging, the samples were tested for their peel strength. This measurement records the strength required to separate two bonded substrates over a width of 50 mm at 23°C and 50% relative humidity according to the ASTM method recommended above. The peel strength measured according to the above method is defined as the "dry peel strength" or the peel strength under "dry conditions".

[0052] However, since the peel strength may change significantly in the presence of liquid water, as will be better explained below, the present invention distinguishes between the peel strength under dry conditions, or dry peel strength, and the peel strength "after water absorption", also known as "wet peel strength" or peel strength "under wet conditions". In fact, liquid water can cause a drastic change in the bond strength when the fiber substrate consists of highly hydrophilic fibers, such as cotton fibers and other similar plant fibers, wherein such fibers can absorb a large amount of water during use and thus swell significantly.

[0053] To determine the peel strength "after water absorption" or "wet peel strength", the following method was used: at room temperature, i.e., 23°C and 50% relative humidity, take laminate samples identical to those tested for dry peel strength. On each sample, 5 ml of distilled water was applied evenly on the surface of the cotton nonwoven using a calibrated syringe. Allow to stand for 300 seconds to ensure that the cotton has fully absorbed all the water and that its fibers are fully swollen. At this point, the peel strength "after water absorption" or "wet peel strength" was determined according to the recommended ASTM D 1876-01 method under the same conditions as above.

[0054] "Hygiene absorbent products" refers to devices and / or methods that include the following disposable absorbent / non-absorbent items: adult incontinence care pants and liners, baby diapers and bibs, training pants, infant care wipes, feminine hygiene pads, interlabial pads, sanitary pads, pessaries, sanitary napkins, tampons and inserters, wound dressing products, nursing absorbent pads, cleansing wipes and other similar products.

[0055] Other non-routine parameters will be defined in detail later in conjunction with their specific test methods.

[0056] Preferred embodiments of the hot melt adhesive of the present invention and detailed description of the main components and properties

[0057] The hot melt adhesive formulations of the present invention exhibit excellent adhesion properties to both woven and nonwoven fiber substrates. In particular, these adhesives can maintain unexpectedly high bond strengths even when the fiber substrates are composed of highly hydrophilic fibers, such as cotton and similar plant fibers, and when the hydrophilic fibers are exposed to and absorb large amounts of water or other aqueous liquids, such as urine or blood, during use.

[0058] Achieving strong adhesion of completely dry fiber substrates alone is a technical challenge in itself, for several reasons that have been partially revealed by the analysis of the prior art. The difficulties in forming a strong adhesive bond on a dry fiber substrate mainly arise from, for example, uneven distribution of the adhesive due to uneven fiber surface, reduced contact area with the binder, and thus the appearance of interfiber pores, the presence of free cilia that are not firmly fixed to the remaining fibers and are easily pulled off, etc.

[0059] The adhesion becomes more difficult when the fiber substrate is in a wet state due to the strong hydrophilicity of the fibers, or even worse, absorbs a large amount of water during use. In fact, it is well known in the field of adhesive technology that obtaining good adhesion on a wet substrate is one of the most difficult technical effects to achieve unless extremely special adhesives are used, such as reactive hydrogels based on water and acrylic monomers and crosslinked by ultraviolet light. However, such special adhesives are not suitable for the target field of the present invention for many reasons: including high cost, special packaging to prevent dehydration, complex production process, and especially the possible toxicity / carcinogenicity of unreacted monomers.

[0060] With regard to hot melt adhesives, the industry generally recognizes that the presence of liquid water, wet substrates, and substrates that can absorb large amounts of water and swell significantly due to their hydrophilic nature are the most detrimental factors affecting the ability of most hot melt adhesives to achieve and maintain strong adhesion.

[0061] Moisture has a very negative impact on adhesion through many different mechanisms and phenomena: it prevents the formation of a strong bond on wetted substrates and destroys strong bonds that have already formed in the dry state. This destructive effect is more significant when the already bonded dry substrates come into contact with moisture, especially when they absorb large amounts of water due to their hydrophilic nature.

[0062] First, water, a highly polar liquid substance, radically alters the fundamental forces that are the essence of all adhesive bonds at the molecular level. In fact, even the presence of a layer of water on the substrate surface, even a very thin layer, like a water film of only a few molecules thick, completely alters the physicochemical and physical phenomena of the adhesive bond between the hot melt adhesive and the substrate due to the strong polarity of the liquid water. In terms of physical phenomena, this liquid water film, even a very thin film of water of only a few molecules thick, forms what is known in the science of bonding as a "weak boundary layer", i.e. a layer that mechanically fails and breaks immediately under small stresses, leading to failure and breakage of the entire adhesive bond and the entire macrostructure. In addition, from a physicochemical point of view, the strong polarity of water alters and destroys all the fundamental attractive forces between the adhesive and the substrate, including van der Waals forces, dipole effects and hydrogen bonding forces. This extremely negative effect on adhesion caused by liquid water on the substrate surface is the primary reason why it is difficult, and in most cases impossible, to adhere to wet substrates using standard thermoplastic adhesives. This is also the fundamental mechanism by which the strong adhesive bond formed between the adhesive and the substrate in the dry state is significantly weakened or even completely destroyed after the bonded structure comes into contact with liquid water. This mechanism is applicable to fibrous substrates regardless of whether the substrate is composed of hydrophobic or hydrophilic fibers.

[0063] However, when the substrate consists of hydrophilic fibers, in particular strongly hydrophilic natural fibers such as cotton, an additional negative effect occurs which has an even more pronounced destructive effect on the strong adhesive bond between adhesive and substrate, which bond was previously formed in the dry state.

[0064] When natural fibers, such as cotton or similar plant fibers, or artificial fibers obtained from plant materials, such as rayon, come into contact with liquid water, they are able to absorb large amounts of water, especially when these fibers (such as common textile fibers) have been subjected to special treatments to improve their properties, such as mercerization.

[0065] For example, under the above conditions, cotton can absorb nearly 12 times its own dry weight of liquid water (about 1200% dry weight). This data is published in the article "Analysis of Water Absorption Properties of Different Natural Fibers" in the Journal of Textile Science and Technology, Volume 7, Issue 4, November 2021; most other cellulose fibers also exhibit similar properties.

[0066] In addition to the negative impact of the aforementioned wet surface on bonding, the violent swelling of hydrophilic fibers such as cotton due to absorbing a large amount of water has a more significant negative impact on bonding durability.

[0067] It has been calculated that this volumetric swelling can reach several times the initial dry volume of the same fiber, and because the swelling force is a "hydraulic force", its strength is extremely high. Therefore, in addition to the negative effect on the bond strength, the huge swelling of hydrophilic fibers such as cotton will also produce further destructive mechanical effects on the adhesive itself. In this way, when the fiber contacts liquid water, the extremely strong hydraulic force generated by its strong swelling can cause the adhesive to actually break and shatter.

[0068] The hot melt adhesives described in the present invention are firstly capable of producing extremely strong adhesion to dry fiber substrates, including woven and non-woven structures, whether made of hydrophobic fibers (such as synthetic polymer fibers), or hydrophilic fibers (such as natural fibers, such as cotton and similar plant fibers, or man-made fibers obtained from plant materials, such as rayon, lyocell, etc.).

[0069] Furthermore, in a completely surprising and unexpected manner, the hot melt adhesive is able to form and maintain extremely strong adhesive bonds on the above-mentioned fibrous substrates even when said substrates are wet, and even when said substrates (when they are highly hydrophilic) come into contact with large amounts of liquid water or body fluids such as urine, blood, etc. during use and undergo significant swelling due to the absorption of water.

[0070] Therefore, the hot melt adhesive of the present invention is particularly suitable for manufacturing absorbent sanitary products, which in most cases contain at least one fiber substrate, which is not woven or non-woven fiber substrate, and the fibers thereof can be natural or synthetic fibers, which are hydrophobic or hydrophilic. In particular, when the woven / non-woven fiber substrate is composed of highly hydrophilic natural or artificial fibers, such as cotton and other similar cellulose fibers, and when they can absorb a large amount of aqueous liquids such as urine, menstrual blood, etc. during use, the present adhesive exhibits an excellent effect beyond expectation.

[0071] Although not limited to any theoretical explanation, we reasonably speculate that all these unexpected positive phenomena can be explained by the following mechanisms.

[0072] First, the following reasonable assumptions can be made about the excellent adhesion performance of dry fiber substrates. As known to those skilled in the art of bonding technology, the overall bonding strength between the adhesive and the substrate is derived from the superposition of at least two basic contributing factors: the aforementioned intermolecular attractive forces (van der Waals forces, dipole forces, hydrogen bonding forces), and in addition, the mechanical interlocking effect between the substrate and the adhesive when the substrate has unevenness and roughness on its surface, or even some pores. For fiber substrates, due to their very high porosity and the presence of a large number of voids between fibers, a good mechanical interlock can be formed between the adhesive and the fiber substrate when the adhesive can penetrate, at least partially penetrate, into the pores / spaces between the fibers, and even cover individual fibers to varying degrees. This phenomenon is particularly important for establishing a strong adhesive bond on a completely dry fiber substrate. The adhesive of the present invention is able to penetrate, at least partially penetrate, into the spaces between the fibers, thereby forming a particularly effective mechanical interlock with them, and even in the dry state, it is possible to obtain excellent bonding strength, this characteristic is also due to the following two specially selected performance parameters.

[0073] First, the zero shear viscosity of the adhesive of the present invention at 160°C, where the temperature of 160°C is selected because it is a typical average temperature for hot melt adhesive processing applications, does not exceed 10,000 mPa·s, preferably does not exceed 6,500 mPa·s. Intuitively, a relatively low viscosity, in the molten state and at the application temperature, is conducive to the physical penetration of the molten adhesive into the pores of a porous substrate or a substrate with considerable space, such as a typical woven or non-woven fiber substrate. However, the inventors of the present invention have found that the dynamic melt viscosity commonly used in the field of hot melt adhesives, that is, the "Brookfield viscosity" measured at a non-zero shear rate according to the ASTM D-3236-88 method, is not suitable in this case. There is a conceptual error in this cognition because the shear rate of the molten adhesive drops to zero instantaneously when it is extruded from the die head and applied to the fiber substrate. Therefore, the physical property of the molten adhesive that truly controls the ability of the adhesive to more or less penetrate the pores of the fiber substrate is actually its zero shear viscosity. As is well known, this rheological property is defined as a constant asymptotic value (plateau value) in mPa·s, which is the viscosity of a molten polymer system as the applied shear rate approaches zero, in an experimental curve reporting the apparent viscosity of a molten polymer system (in this case, the 160°C molten adhesive) as a function of the applied shear rate.

[0074] The inventors have found that excellent bond strength to a fibrous substrate can be obtained when the zero shear viscosity of the hot melt adhesive at 160° C. does not exceed 10,000 mPa·s, preferably does not exceed 6,500 mPa·s. For the present invention, this property is measured and calculated according to the "Flow Curve at Low Shear Rate" method described in "Obtaining Zero Shear Viscosity of Polymer Melts Using Different Rheological Test Methods" by J. Lauger and M. Bernzen, Annual Transactions of the Nordic Rheology Society, Vol. 8 (2000), pp. 159-162.

[0075] In addition, the inventors have found that in addition to the need to control the relatively low value of the zero shear viscosity at 160°C, at least two other rheological parameters of the molten adhesive also play a key role in ensuring excellent dry adhesion properties, which promotes the penetration of the adhesive into the pores and spaces between the fibers, thereby forming a strong mechanical interlock between the adhesive and the fiber substrate. The two supplementary rheological parameters are:

[0076] - A defined rheological modulus "first crossover temperature" Tx, also called "rheological curing temperature" or "rheological curing point";

[0077] - An additional rheological parameter called "inflection temperature in the Tan Delta temperature spectrum located near the solidification point".

[0078] The rheological modulus parameter Tx or "first crossover temperature" has been explained above. This temperature, when measured at an initial time point with a frequency of 1 Hz, a temperature drop range of +170°C to -20°C, and a sufficiently small cooling rate of 3°C / minute, can accurately simulate the phenomenon of the hot melt adhesive being applied from the molten state, as the adhesive slowly cools and solidifies naturally to form an adhesive bond between the adhesive and the substrate. Specifically, the Tx value at the initial time point identifies the point at which the molten adhesive extruded and coated on the substrate begins to "rheologically" solidify during the spontaneous cooling process: that is, above the value of the temperature Tx, the viscosity modulus G" of the molten adhesive is numerically greater than the elastic modulus G', so that the adhesive can spontaneously flow and fully penetrate into the pores and spaces of the fiber substrate. In contrast, below the temperature Tx, the G' of the adhesive is greater than its G", and the adhesive loses its flow and penetration ability. The inventors have found that in order to achieve excellent adhesion to the fiber substrate, the Tx value of the hot melt adhesive of the present invention is preferably not higher than 75°C.

[0079] However, the inventors unexpectedly discovered that the so-called "inflection point temperature in the Tan Delta temperature spectrum near the solidification point" has a more critical influence on ensuring strong adhesion of the adhesive to the fiber substrate than the Tx value, and this parameter is located near the fixed point. Like Tx, this parameter is also measured by rheological testing at the initial time point: at a cooling rate of 3°C / min, a strain frequency of 1Hz, and a temperature range of +170°C to -20°C. Specifically, the inflection point temperature of the Tan Delta diagram is determined as follows: in the above rheological spectrum, a curve of Tan Delta versus temperature is plotted, and the temperature range covers the area near Tx, that is, the temperature of Tan Delta = 1, and the temperature at which the adhesive is cooled and solidified from its molten state. The selected temperature range is between the temperature at which the TanDelta value is equal to 0.5 and the Tan Delta value is equal to 10. Within this temperature range and the range of the Tan Delta value, the Tan Delta-temperature curve will have an inflection point, that is, at this point the curvature changes its sign and direction. This inflection point in the curve of Tan Delta and the corresponding temperature can be directly identified by visual inspection. As an alternative to the visual determination of the inflection point, the following method can also be used: fit the Tan Delta-temperature experimental points with a cubic equation so that its corresponding determination coefficient "R square" is not less than 0.95. From the equation, calculate the point by calculating the point where the second derivative of the function is zero and changes sign. The temperature corresponding to this point is the inflection point temperature in the arc. If for all the rheological tests above, the TA Instruments ARES G2 rotational rheometer is used as recommended in this article, the device is equipped with TRIOS software. The software can automatically calculate the fitted Tan Delta experimental points; and the software can calculate the first and second derivatives of the function. Therefore, the temperature corresponding to the Tan Delta inflection point is immediately identified by the temperature when the second derivative of the Tan Delta-temperature arc is zero.

[0080] This inflection point temperature is called "the inflection point temperature near the solidification point in the Tan Delta temperature spectrum" because its corresponding point is located in the temperature range near Tx (Tan Delta = 1), more precisely, between the temperature corresponding to Tan Delta = 0.5 and the temperature corresponding to Tan Delta = 10, and the temperature range above room temperature. Unexpectedly, the inflection point temperature, whose value does not necessarily coincide with Tx, is usually higher than Tx, and is a more sensitive and more important parameter than Tx to determine the adhesion performance of the hot melt adhesive disclosed in the present invention to the fiber substrate. Although not limited to any theory, it can be inferred as follows: near this inflection point, the TanDelta value will change dramatically, and when the temperature changes by only about 10°C, the Tan Delta value can reach an order of magnitude or even more (for example, ten times or more). At the same time, the elastic modulus G' also shows a corresponding and similar rapid rise. The drastic change in the values ​​of Tan Delta and G' within a limited temperature range near the TanDelta inflection point is usually significantly greater than the changes in Tan Delta and G' observed near the rheological solidification temperature Tx. This can be explained by the fact that even above Tx, the molten adhesive has already begun to partially solidify, and its viscosity and semi-solid properties make it difficult to effectively penetrate, even partially penetrate, the pores of the fiber substrate, so that at slightly above Tx, they can no longer provide a good adhesion to the substrate. Based on this, the inventors found that in order to achieve excellent adhesion to the fiber substrate, the hot melt adhesive of the present invention must not exceed 95°C in the inflection point temperature near the solidification point in the Tan Delta temperature spectrum measured at the initial time point, 1 Hz frequency, 3°C / min cooling rate, and within the range of +170°C to -20°C.

[0081] For the sake of clarity, Figure 1 ) and 2) respectively show the identification of the inflection point of the Tan Delta experimental curve in Comparative Example 1 ( Figure 1 ), and the method of determining the inflection point by the second derivative of the Tan Delta function being zero ( Figure 2 ).

[0082] The above-mentioned physicochemical and rheological parameters ensure, for example, excellent bonding strength to a fiber substrate in a multilayer composite material when the laminate has been completely dried.

[0083] However, these parameters alone are not sufficient to provide the adhesive with the properties required to maintain good adhesion when the substrate and laminate are exposed to liquid water or aqueous fluids, i.e., during use of sanitary absorbent products containing fibrous substrates or laminates bonded to the adhesive. This is particularly true when the fibers of the substrate are natural fibers such as cotton, which are highly hydrophilic and can absorb a surprising amount of water and swell several times in volume. In fact, as mentioned above, liquid water not only weakens the intermolecular forces on which the bonding depends, but also has a substantial mechanical stripping effect on the adhesive coated on the swollen fibers when the fibers swell, and can even cause the adhesive to physically break and fragment.

[0084] In these cases, the inventors have found that if the hot melt adhesive of the present invention meets the following two supplementary parameter requirements in addition to the aforementioned physical, chemical and rheological parameters, it can still maintain unexpectedly excellent adhesion properties even for highly hydrophilic fibers such as cotton when they come into contact with a large amount of liquid water or aqueous liquid and swell violently due to water absorption. Specifically:

[0085] -The yield stress after five days of aging is not less than 0.1MPa;

[0086] - The toughness value measured after five days of aging is not less than 0.5MJ / m 3 This toughness value is numerically characterized by the total area enclosed by the stress-elongation curve in the stress-fracture elongation experiment of the tested adhesive, and as mentioned above, it expresses the specific energy required to physically destroy the adhesive.

[0087] As mentioned above, the tensile properties and stress-elongation at break curve of the hot melt adhesive of the present invention are measured at 37°C according to the method described above.

[0088] Although not being bound by any theory, we believe that the findings of the present invention can be reasonably explained as follows: in addition to the parameters mentioned above that ensure excellent adhesive and mechanical bonding of the dry fiber substrate, sufficiently high yield stress and toughness values ​​can ensure that the adhesive bond still has excellent mechanical resistance and durability when the substrate is wetted and the fibers swell significantly due to water absorption.

[0089] In addition to all the aforementioned parameters, the adhesive has a sufficiently low crystallinity, which is reflected in a melting enthalpy of no more than 30 J / g after aging for five days, which has a positive impact on both aspects: it is conducive to the formation of an excellent initial adhesive bond under dry conditions, and it enables the adhesive to withstand the violent swelling of the fiber substrate after contact with liquid water. In fact, it is known that for good initial viscosity, low-crystallinity materials are more "sticky" than crystalline materials; and in order to make the adhesive bond more durable under the action of liquid water and swollen fibers, it can be reasonably assumed that low-crystallinity materials are less brittle and are more likely to undergo plastic deformation rather than fracture when subjected to tensile mechanical stress. It can therefore be considered that the reason why the adhesive with the characteristics of the present invention can resist the destructive effect of fiber swelling on the adhesive bond is not only because the adhesive has a sufficiently high toughness value and yield stress, but also because of the non-brittle characteristics imparted by the low crystallinity, which prevents it from breaking into multiple pieces under the mechanical action of the fiber absorbing water and expanding its volume multiple times.

[0090] Typical ingredients of the adhesive formulation of the present invention

[0091] The adhesive formulation of the present invention comprises a series of chemical components as described in detail below, which have the aforementioned physicochemical parameter characteristics.

[0092] polymer

[0093] The formulation of the invention comprises at least one thermoplastic polymer as a main component. In particular, it comprises at least one polymer, which may be a homopolymer or a copolymer, or comprises a blend of two or more polymers, which again may be homopolymers or copolymers, and are compatible with each other, according to the definition as described above. The polymer or the polymer blend may have a variety of chemical properties. Particularly suitable homopolymers and copolymers for the present invention are, for example: homopolymers of C2-C12 olefins or C4-C12 dienes, and copolymers of the same olefins and dienes; copolymers between C2 and C12 olefins and vinyl and acrylic monomers, such as ethylene-vinyl acetate copolymers, ethylene-methyl acrylate copolymers, ethylene-acrylic acid copolymers, etc.; styrene block copolymers, including non-hydrogenated and fully hydrogenated forms; and their blends.

[0094] Of the above polymers, particularly preferred are, for example, homopolymers and copolymers of C2-C8 olefins, and fully hydrogenated styrenic block copolymers, such as styrene-ethylene-butylene-styrene, styrene-ethylene-propylene-styrene, and blends thereof. Trademarks of industrial polymers suitable for use in the present invention are: polyolefin copolymers sold under the trademark of the same name by Rextac; C2 / C3 / C4 copolymers sold under the trademark Vestoplast by Evonik; C4 homopolymers and C2 / C4 copolymers sold under the trademark Koattro by LyondellBasell; C2 / C3 copolymers sold under the trademark Vistamaxx by ExxonMobil; polyolefin copolymers sold under the trademark Eastoflex by Synthomer; polyolefin copolymers sold under the trademark Licocene by Clariant; and C3 homopolymers sold under the trademark L-MODU by Idemitsu.

[0095] The hot melt adhesive of the present invention comprises 5% to 99.5% by weight of a polymer component, which may be a single homopolymer or copolymer, or a blend of two or more mutually compatible polymers, each of which may be a homopolymer or copolymer.

[0096] Tackifier

[0097] In one embodiment of the present invention, the hot melt adhesive formulation comprises at least one tackifier having a ring and ball softening point between 5°C and 160°C.

[0098] Generally speaking, the tackifier in the formulation of the present invention can be selected from: aliphatic hydrocarbon tackifiers and partially or completely hydrogenated derivatives thereof; aromatic hydrocarbon tackifiers and partially or completely hydrogenated derivatives thereof; aliphatic / aromatic hydrocarbon tackifiers and partially or completely hydrogenated derivatives thereof; modified polyterpenes and terpene tackifiers and partially or completely hydrogenated derivatives thereof; rosin and its esters and partially or completely hydrogenated derivatives thereof; and mixtures thereof. Among them, partially or completely hydrogenated hydrocarbon tackifiers, including aliphatic, aromatic and aliphatic-aromatic, are particularly preferred. Further studies have found that the ring and ball softening point of the tackifier resin contained in the adhesive formulation of the present invention is more preferably 70°C to 135°C, preferably 80°C to 130°C, and more preferably 85°C to 125°C.

[0099] High purity tackifying resins are particularly preferred, which contain very low amounts of volatile impurities and residual monomers, such as xylene, toluene, hexene, vinyltoluene, indene, etc., which can cause odor in the final product and reduce the thermal stability of the resin. The above volatile compounds are detected by headspace gas ion chromatography: 3g of tackifier sample is heated at 190°C for 30 minutes, with a headspace volume of 20ml. The present invention particularly prefers tackifying resins with a volatile impurity content of no more than 5ppm (parts per million), more preferably no more than 2ppm, and most preferably no more than 1ppm. Industrial examples of such high purity low volatile tackifying resins include products sold under the UltraPure trademark by Synthomer (USA).

[0100] In an embodiment of the present invention, when the hot melt adhesive formulation contains at least one tackifying resin, its usage is 0 to 80 weight % relative to the total weight of the adhesive, which may be at least one tackifying resin or a mixture of two or more tackifying resins, preferably 3 to 75 weight %, more preferably 5 to 70 weight %.

[0101] Plasticizers

[0102] In another embodiment of the present invention, the hot melt adhesive formulation further comprises a plasticizer that is liquid at room temperature, typically 23° C., or a mixture of two or more plasticizers that are liquid at room temperature. The plasticizer can further reduce the melt viscosity of the adhesive formulation and enhance its initial tack.

[0103] Plasticizers suitable for the present invention include, for example, paraffinic and naphthenic mineral oils and mixtures thereof; paraffinic hydrocarbons and naphthenic hydrocarbons and mixtures thereof that are liquid at room temperature; polyolefin oligomers and copolymers thereof that are liquid at room temperature, such as oligomers of ethylene, propylene, butylene, isobutylene and copolymers thereof and mixtures thereof; ester plasticizers that are liquid at room temperature, such as phthalates, benzoates, sebacates; natural and synthetic fats; vegetable oils; and mixtures thereof. Among the suitable plasticizers mentioned: paraffinic or naphthenic mineral oils and mixtures thereof; polyisobutylene; and polyalphaolefin synthetic oligomers that are liquid at room temperature, referred to as PAO, are particularly preferred. The synthetic oligomer plasticizers that can be used in the present invention are synthesized from C2-C20 olefins and generally have a number average molecular weight (Mn) of 150 to 15,000 g / mol, preferably 200 to 10,000 g / mol, and more preferably 400 to 6,000 g / mol. Such PAO plasticizers are fully saturated and may have a linear or branched form that is essentially a paraffin structure. Liquid polyalphaolefin plasticizers suitable for use in the adhesive formulation of the present invention are produced and sold by ExxonMobil (trade names SpectraSyn, Elevast), Ineos (trade name Durasyn), Chevron Phillips (trade name Synfluid), and other companies.

[0104] In an embodiment of the present invention, when the hot melt adhesive formulation contains at least one liquid plasticizer, relative to the total weight of the adhesive, it may be at least one plasticizer that is liquid at room temperature, or a mixture of two or more plasticizers that are liquid at room temperature, and its usage amount is 0 to 40% by weight), preferably 0 to 30% by weight, and more preferably 0 to 15% by weight.

[0105] Wax components

[0106] In another embodiment of the present invention, the hot melt adhesive formulation contains at least one wax or a mixture of two or more waxes. The wax may be a natural or synthetic wax, and its drop melting point ranges from 40°C to 170°C as measured by the ASTM D 127-87 method. However, since waxy substances, even in small amounts, tend to significantly increase the inflection temperature of the Tan Delta temperature spectrum and the Tx value of the adhesive formulation, the present invention preferably uses waxes whose drop melting point temperature is not too high. Specifically: if the formulation contains only a single wax component, the drop melting point measured by the ASTM D 127-87 method should not be higher than 125°C. On the contrary, if the formulation contains a mixture of two or more waxes, then preferably, at least 25% by weight of at least one wax component in the mixture has a drop melting point of no more than 125°C relative to the total weight of all waxes in the formulation, which is also measured by the ASTM D 127-87 method.

[0107] The waxes suitable for the present invention are, for example: synthetic hydrocarbon waxes, such as paraffin wax, especially waxes synthesized from C2-C10 olefins and mixtures thereof; C12-C40 hydrocarbon waxes, including carboxylic acid or alcohol modified types; ethylene-maleic anhydride or propylene-maleic anhydride copolymer waxes; microcrystalline waxes; Fischer-Tropsch waxes; C12-C40 fatty acid ester waxes; natural waxes, such as beeswax, carnauba wax, montan wax, etc.; and mixtures thereof.

[0108] In an embodiment of the present invention, when the hot melt adhesive formulation contains at least one wax or a mixture of two or more waxes, the amount used is 0 to 15% by weight of the wax or the mixture of waxes, preferably 0 to 10% by weight, and more preferably 0 to 5% by weight, relative to the total weight of the hot melt adhesive.

[0109] Other auxiliary components

[0110] The hot melt adhesive formulation of the present invention may further comprise 0.01% to 10% by weight of at least one stabilizer, such as antioxidants, light stabilizers, anti-ultraviolet stabilizers and mixtures thereof, and may optionally comprise up to 15% by weight of auxiliary components, such as mineral fillers, pigments, dyes, fragrances, surfactants, antistatic agents.

[0111] Example

[0112] The following examples further illustrate the present invention, which are for illustrative purposes only and should not be construed as limiting the scope or implementation of the present invention. Unless otherwise specified, the parts and percentages are by weight.

[0113] According to the embodiment of the present invention

[0114] Example 1

[0115] The following components were blended in a molten state at 170° C. to prepare the hot melt adhesive formulation of the present invention.

[0116]

[0117] The adhesive formulation according to this embodiment 1 of the present invention has the following characteristic parameters: 160°C zero shear viscosity is 2,390 mPa·s; the inflection point temperature near the curing point in the Tan Delta temperature spectrum is 85.3°C; the first crossover temperature Tx of the rheological modulus at the initial time point is 53.5°C; the melting enthalpy after aging for five days is 15.8 J / g; measured by the aforementioned method, the 37°C yield stress after aging for five days is 0.27 MPa; measured by the same method, the 37°C toughness value after aging for five days is 2.55 MJ / m 3 .

[0118] The adhesive exhibits excellent adhesion to the fiber substrate both in the dry and wet state. In fact, in the test described previously for the cotton peel strength, the dry peel strength of the hot melt adhesive of Example 1 was as high as 7.38 N / 50 mm; its wet peel strength remained at an excellent level, which was 2.16 N / 50 mm. Therefore, comparing its peel strength under dry and wet conditions, it lost 70.7% of its initial dry strength after the cotton absorbed water, and still retained 29.3%, which is a result far beyond expectations. In fact, this adhesive strength reduction is completely acceptable, and the absolutely high value of the wet peel strength ensures optimal resistance even in the presence of large amounts of liquid water and other aqueous liquids.

[0119] The comparative data of the dry peel strength and the corresponding wet peel strength of all the embodiments (including the present invention and comparative examples) are also summarized in the following Table 1.

[0120] The adhesive formulation of Example 1 of the present invention also has the following properties: a Brookfield viscosity of 2,000 mPa·s at 170° C. and a ring and ball softening point of 98.9° C.

[0121] Example 2

[0122] The following components were blended in a molten state at 170° C. to prepare the hot melt adhesive formulation of the present invention.

[0123]

[0124] The adhesive formulation of Example 2 of the present invention has the following characteristic parameters: 160°C zero shear viscosity is 2,950 mPa·s; the inflection point temperature near the curing point in the Tan Delta temperature spectrum is 71.6°C; the first crossover temperature Tx of the rheological modulus at the initial time point is 58.7°C; the melting enthalpy after five days of aging is 16.6 J / g; measured by the aforementioned method, the 37°C yield stress after five days of aging is 0.35 MPa; measured by the same method, the 37°C toughness value after five days of aging is 1.17 MJ / m 3 .

[0125] The adhesive also exhibits excellent adhesion to the fiber substrate: in the above peel strength test, its dry peel strength is as high as 7.44N / 50mm. In addition, the adhesive still maintains the excellent adhesive strength retention rate in the wet state. After the cotton substrate is made to absorb water according to the above method, its wet peel strength still maintains a high level, reaching 2.95N / 50mm, which is only 60.3% lower than the corresponding dry value (see Table 1).

[0126] In addition, the adhesive has the following properties: Brookfield viscosity at 170°C is 1,660 mPa·s and ring and ball softening point is 107.3°C.

[0127] Example 3

[0128] The following components were blended in a molten state at 170° C. to prepare the hot melt adhesive formulation of the present invention.

[0129]

[0130] The adhesive formulation of Example 3 of the present invention has the following characteristic parameters: 160°C zero shear viscosity is 3,240 mPa·s; the inflection point temperature near the curing point in the Tan Delta temperature spectrum is 73.3°C; the first crossover temperature Tx of the rheological modulus at the initial time point is 54.6°C; the melting enthalpy after aging for five days is 13.2 J / g; measured by the aforementioned method, the 37°C yield stress after aging for five days is 0.77 MPa; measured by the same method, the 37°C toughness value after aging for five days is 8.4 MJ / m 3 .

[0131] The formulation of Example 3 also has excellent adhesion to the fiber substrate: in the above peel strength test, its dry peel strength is as high as 9.13N / 50mm. In addition, the adhesive still maintains excellent adhesive strength in the wet state. After the cotton substrate is made to absorb water according to the above method, the wet peel strength still reaches an excellent level of 3.05N / 50mm, which is 66.7% lower than the dry value (see Table 1).

[0132] The Example 3 formulation also has the following properties: a Brookfield viscosity of 1,900 mPa·s at 170° C. and a Ring and Ball softening point of 87.4° C.

[0133] Comparative Examples

[0134] Comparative Example 1

[0135] The following components were blended in a molten state at 170° C. to prepare the hot melt adhesive formulation of the present invention.

[0136]

[0137] The adhesive formulation of Comparative Example 1 is substantially the same as that of Inventive Example 1, but contains only one wax component having a drop melting point higher than 125° C. as measured by ASTM D127-87.

[0138] Therefore, the inflection point temperature near the curing point in its Tan Delta temperature spectrum is too high, reaching 100.8° C. Although its dry peel strength is comparable to that of Example 1, specifically 7.37 N / 50 mm, the wet peel strength drops sharply to an unacceptable 1.1 N / 50 mm after contacting and absorbing water, a decrease of 85.1% from the corresponding initial dry value.

[0139] The formulation of Comparative Example 1 also has the following characteristics: 160°C zero shear viscosity is 3,040 mPa·s; the first crossover temperature Tx of the rheological modulus at the initial time point is too high, at 76.7°C; the melting enthalpy after five days of aging is 17.0 J / g; the yield stress at 37°C after five days of aging measured by the aforementioned method is 0.32 MPa; the toughness value at 37°C measured by the same method after aging for 5 days is 1.45 MJ / m 3 ; Brookfield viscosity at 170°C is 1,830 mPa·s; Ring and Ball softening point is 100.7°C.

[0140] Comparative Example 2

[0141] The following components were blended in a molten state at 170°C to prepare a hot melt adhesive formulation.

[0142]

[0143]

[0144] Unlike the previous Comparative Example 1, the hot melt adhesive formulation of Comparative Example 2 has too high a melting enthalpy. In fact, it reaches 31.2 J / g after five days of aging. It also exhibits a relatively high zero shear viscosity, with a zero shear viscosity of 6,550 mPa·s at 160°C at the initial time point and a Brookfield viscosity of 5,300 mPa·s at 170°C. In addition, the inflection point temperature in its Tan Delta temperature spectrum, which is located near the solidification point, is 59.5°C.

[0145] Despite its higher zero shear viscosity at 160°C, the comparative formulation still exhibited good adhesion in the dry state, with a dry peel strength of 7.1 N / 50 mm. However, its wet peel strength dropped to an unacceptably low value of 0.55 N / 50 mm, losing 92.3% of its initial dry value after contact with and absorption of water.

[0146] Other characteristics of the formula include: the first crossover temperature Tx of the rheological modulus at the initial time point is 60.2°C; the yield stress at 37°C after five days of aging is 2.4MPa as measured by the aforementioned method; the toughness value at 37°C after five days of aging is 1.9MJ / m as measured by the same method 3 ; The ring and ball softening point is 94.5℃.

[0147] Table 1

[0148]

Claims

1. A hot melt adhesive formulation, characterized in that: have: The zero shear viscosity at -160°C is no more than 10,000 mPa·s, preferably no more than 6,500 mPa·s; - The melting enthalpy after aging for five days does not exceed 30 J / g; - the inflection point temperature of the Tan Delta temperature spectrum measured in the cooling mode at the initial time point according to the method described herein does not exceed 95°C, wherein the inflection point is located near the solidification point; The hot melt adhesive formulation is further characterized by: - The dry peel strength after aging for five days is not less than 2.0N / 50mm width; - The wet peel strength after water absorption and after aging for five days does not deviate from the corresponding dry peel strength by more than 80%; The peel strength is measured according to the method described in this article.

2. The hot melt adhesive formulation according to claim 1 comprises at least one polymer, which may be a homopolymer or a copolymer, or a mixture of two or more mutually compatible polymers, wherein two or more polymers may be homopolymers or copolymers.

3. The hot melt adhesive formulation according to any one of the preceding claims 1 and 2, characterized in that The polymer or polymer mixture is selected from: homopolymers of C2-C12 olefins or C4-C12 dienes; copolymers of the same olefin and diene; copolymers of C2-C12 olefins and vinyl or acrylic monomers; styrene block copolymers in non-hydrogenated and fully hydrogenated states; and mixtures thereof.

4. The hot melt adhesive formulation according to any one of the preceding claims 1 to 3, characterized in that The polymer or mixture of mutually compatible polymers comprises 5% to 99.5% by weight of the total adhesive formulation.

5. The hot melt adhesive formulation according to any one of the preceding claims 1 to 4, characterized in that The first crossover temperature of the rheological modulus measured at the initial time point in the cooling mode according to the method described herein does not exceed 75°C.

6. The hot melt adhesive formulation according to any one of the preceding claims 1 to 5, characterized in that The yield stress after aging for five days is not less than 0.1 MPa when measured at 37°C according to the method described in this article.

7. A hot melt adhesive formulation according to any one of the preceding claims 1 to 6, characterised in that According to the method described in this article, the toughness value after aging for five days is not less than 0.5MJ / m 3 .

8. A hot melt adhesive formulation according to any one of the preceding claims 1 to 7, characterised in that It comprises at least one tackifier or a mixture of tackifiers having a ring and ball softening temperature of 5°C to 160°C, preferably 70°C to 135°C.

9. The hot melt adhesive formulation according to claim 8, characterized in that The tackifier or tackifier mixture is selected from: aliphatic hydrocarbon tackifiers and partially or completely hydrogenated derivatives thereof; aromatic hydrocarbon tackifiers and partially or completely hydrogenated derivatives thereof; aliphatic / aromatic hydrocarbon tackifiers and partially or completely hydrogenated derivatives thereof; modified polyterpenes and terpene tackifiers and partially or completely hydrogenated derivatives thereof; rosin and its esters and partially or completely hydrogenated derivatives thereof; and mixtures thereof.

10. The hot melt adhesive formulation according to any one of the preceding claims 8 to 9, characterized in that The tackifier or tackifier mixture accounts for 0 to 80% by weight of the total weight of the adhesive formulation, preferably 3 to 75%, more preferably 5 to 70%.

11. Hot melt adhesive formulation according to any one of the preceding claims 1 to 10, comprising at least one plasticizer or plasticizer mixture which is liquid at room temperature.

12. The hot melt adhesive formulation according to claim 11, characterized in that The plasticizer or plasticizer mixture that is liquid at room temperature is selected from: paraffin-based mineral oil and cycloalkane mineral oil and mixtures thereof; paraffin hydrocarbons and cycloalkanes that are liquid at room temperature and mixtures thereof; polyolefin oligomers and copolymers that are liquid at room temperature, such as oligomers of ethylene, propylene, butylene, and isobutylene and copolymers thereof and mixtures thereof; ester plasticizers that are liquid at room temperature, such as phthalates, benzoates, and sebacates; natural and synthetic fats; vegetable oils; and mixtures thereof.

13. Hot melt adhesive formulation according to claims 11 and 12, characterized in that The plasticizer or plasticizer mixture that is liquid at room temperature is composed of a poly-α-olefin oligomer synthesized from C2-C20 olefins or a mixture thereof, and has a number average molecular weight Mn of 150 to 15,000 g / mol.

14. Hot melt adhesive formulation according to any one of the preceding claims 11 to 13, characterized in that The plasticizer or plasticizer mixture that is liquid at room temperature accounts for 0 to 40% by weight of the total weight of the adhesive formulation, preferably 0 to 30%, more preferably 0 to 15%.

15. Hot melt adhesive formulation according to any one of the preceding claims 1 to 14, comprising at least one wax or wax mixture having a drop melting point in the range of 40 to 170°C as determined by ASTM D 127-87.

16. The hot melt adhesive formulation according to claim 15, comprising a single wax component and having a drop melting point not higher than 125°C as measured by ASTM D127-87.

17. The hot melt adhesive formulation according to claim 15, comprising a mixture of two or more waxes, wherein at least 25% by weight of one or more waxes in the wax mixture relative to the sum of all waxes has a drop melting point of not higher than 125°C as measured by ASTM D 127-87.

18. Hot melt adhesive formulation according to any one of the preceding claims 15 to 17, characterized in that The wax or wax mixture is selected from: paraffin wax, especially wax synthesized from C2-C10 olefins and mixtures thereof; C12-C40 hydrocarbon wax, including carboxylic acid or alcohol modified types; ethylene-maleic anhydride or propylene-maleic anhydride copolymer wax; microcrystalline wax; Fischer-Tropsch wax; C12-C40 fatty acid ester wax; natural wax, such as beeswax, carnauba wax, montan wax, etc.; and mixtures thereof.

19. A hot melt adhesive formulation according to any one of the preceding claims 15 to 18, characterised in that The wax or wax mixture accounts for 0 to 15% by weight of the total weight of the adhesive formulation, preferably 0 to 10%, more preferably 0 to 5%.

20. Hot melt adhesive formulation according to any one of the preceding claims 1 to 19, characterized in that The Brookfield viscosity at 170° C. measured according to ASTM D3236-88 is not more than 15,000 mPa·s, preferably not more than 10,000 mPa·s, and more preferably not more than 7,000 mPa·s.

21. A hot melt adhesive formulation according to any one of the preceding claims 1 to 20, characterised in that The ring and ball softening temperature thereof is not more than 135°C, preferably not more than 125°C.

22. A bonding structure comprising: - a first substrate; - a second substrate; - According to any one of the hot melt adhesive formulations of claim 1 to 21, when 0.5 to 50 g / m 2 When the first substrate is coated with a basic weight of , the first substrate is bonded to the second substrate so that the dry peel strength of the bonded structure measured by the method described herein is greater than 0.25N / 50mm width.

23. The bonding structure according to claim 22, characterized in that: At least one bonding substrate is a fibrous substrate, which is a woven or nonwoven structure.

24. The bonding structure according to claim 23, characterized in that: The at least one woven or nonwoven fibrous substrate comprises at least 50% by weight of natural or man-made cellulosic fibers, such as cotton and the like.

25. Hygienic absorbent article comprising the hot melt adhesive formulation according to any one of the preceding claims 1 to 21.

26. A sanitary absorbent article comprising a bonded structure according to any one of the preceding claims 22 to 24.

27. The sanitary absorbent article according to claim 25 or 26, characterized in that The product is absorbent diapers for infants or children, training pants for infants, incontinence care pants for adults or sanitary absorbent products for women.

28. The sanitary absorbent article according to any one of the preceding claims 25 to 27, characterized in that The hot melt adhesive formulation is used for at least one of the following purposes: i) as a general adhesive for the overall structure of the product; ii) bonding elastic components such as elastic threads, tapes, films or elastic panels; iii) to reinforce and ensure that the integrity of the absorbent layer of sanitary absorbent products is maintained even during use; iv) bonding perforated films with two-dimensional or three-dimensional structures.

29. Article comprising a hot melt adhesive formulation according to any one of the preceding claims 1 to 21, characterized in that The product is a medical absorbent pad, a sheet, a surgical laminate or a wound covering and protecting product.

30. An article comprising a hot melt adhesive formulation according to any one of the preceding claims 1 to 21, characterized in that The article is a mat or a component thereof.

31. An article comprising a hot melt adhesive formulation according to any one of the preceding claims 1 to 21, characterized in that The product is a packaging product.

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