CROSS-REFERENCE TO RELATED APPLICATION

[001] This application claims priority to Japanese Patent Application No. 2010-147983 filed June 29, 2010, the contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

[002] The present invention relates to a hot melt adhesive. The present invention further relates to a hot melt adhesive ideal for affixing absorbent articles, such as sanitary napkins to clothing, e.g., panties.

BACKGROUND OF THE INVENTION

[003] Absorbent articles are products affixed to clothing, such as underwear or shirts, for the purpose of absorbing bodily liquids produced by the body such as menstrual blood, vaginal discharge, urine, or sweat, etc., and are used for the purpose of preventing unpleasant and unsanitary conditions caused when bodily fluids adhere to clothing. Commonly known absorbent articles include, for example, feminine sanitary napkins, urine absorbent liners, maternity disposable panties, nursing pads, underarm perspiration pads, disposable paper diapers, pet sheets, hospital gowns and surgical white gowns. Accordingly, adhesives have been developed for positioning such absorbent articles in the aforementioned clothing, such that their position does not slip.

[004] Many absorbent articles have a general structure with an absorbent body between a moisture-permeable top sheet that contacts the skin and an impermeable back sheet that contacts the underwear. Adhesives for positioning are generally coated onto a peelable material and then transferred to the back sheet. The transferred adhesive for positioning is caused to contact the underwear, and thereby the absorbent product is held in place against the underwear.

[005] If the adhesive used for positioning has insufficient capability, the absorbent product is not held in place against the underwear and bodily fluids may adhere to clothing. Accordingly, in recent years there has been increased demand for improved capability in adhesives used for positioning.

[006] Japanese Publication 2008-2974041 , for example, discloses an adhesive used for positioning that is a hot melt adhesive using a hydrogenated type block copolymer such as styrene-ethylene / butylene-styrene block copolymer (SEBS). However, hot melt adhesives, according to the above reference, do not have sufficient peel strength, and therefore slippage of absorbent products from underwear may occur. In order to completely prevent slippage of absorbent products, it is necessary to coat the back sheet with more hot melt adhesive.

[007] Japanese Publication 2010-506005, for example, discloses a non-hydrogenated type hot melt adhesive containing styrene/butadiene/styrene block copolymer (SBS) that is useful in diapers and feminine napkins. However, when such non-hydrogenated type hot melt adhesives are used with absorbent articles, adhesive may remain on underwear surfaces when the absorbent product is peeled away from the underwear. Note also that adhesive remaining on underwear surfaces is referred to as "adhesive residue."

[008] Japanese Publication 2001-517488, for example, covers feminine napkins affixed directly to the body by means of a nigh molecular weight polymer hot melt adhesive. Such hot melt adhesives are not preferable because the high molecular weight polymer content causes high viscosity and because coating patterns may be uneven. Increasing the coating temperature in order to lower the viscosity of the hot melt adhesive causes the hot melt adhesive do degrade and reduces peel strength.

[009] In recent years, consumers have demanded higher capability from feminine sanitary napkins and other absorbent articles, and will not accept even slight amounts of adhesive residue on underwear. Furthermore, absorbent articlesthat adhere to underwear, such as sanitary napkins, are required not only to leave no adhesive residue on underwear when peeled off but also to be held in position once they adhere to the underwear.

[0010] In general, when peel strength of hot melt adhesive used as an adhesive for positioning is increased in order to hold adhesive products firmly to underwear, the adhesive becomes more likely to remain on the underwear when the absorbent articleis peeled off from the underwear. In other words, adhesive residue is more likely to occur.

[0011] As a result, in recent years, the development of a hot melt adhesive that will hold sanitary napkins in place in underwear without moving position and will not leave adhesive residue on underwear when sanitary napkins are peeled off has become urgent. Also, the hot melt adhesive itself is required to be superior in coatability so that it is easy to handle in manufacturing adhesive products such as sanitary napkins. SUMMARY OF THE INVENTION

[0012] The present invention has the purpose of providing a hot melt adhesive that will hold absorbent articles in place in underwear without moving position and will not leave adhesive residue on underwear when absorbent products are peeled off and also has superior coatability. Another purpose is to provide absorbent articles, such as feminine sanitary napkins, where the adhesive residue does not remain on the underwear even when the hot melt adhesive is directly applied to a back sheet, the surface that does not contact the skin, or the hot melt adhesive is coated onto a peelable type based material and then transferred from the peelable material to a backsheet. The purpose is to provide absorbent articles such as feminine sanitary napkins whereby the adhesive residue does not remain on underwear.

[0013] It has been discovered that by combining thermally reversible block copolymer and a- methylstyrene resin so that the reserve modulus of elasticity of the blended components is within a specified range, a superior hot melt adhesive is obtained for use as an adhesive in positioning absorbent products, having superior coatability and peel strength, and virtually no adhesive residue.

[0014] Specifically, the present invention according to one summary, provides a hot melt adhesive including (A) a thermally reversible block copolymer being a polymer of a vinyl aromatic hydrocarbon and a conjugate diene compound and (B) a-methylstyrene resin, wherein the residual modulus of elasticity G' at 50°C and 10 Rad/s is between 8.0 x 10 ³ Pa and 5.0 x 10 ⁴ Pa.

[0015] One form of the present invention provides a hot melt adhesive whereby (A) thermally reversible block copolymer is triblock copolymer of weighted mean molecular weight 7.5 x 10 ⁴ to 1.0 x 10 ⁵ .

[0016] At 50°C and 10 Rad/s, residual modulus of elasticity G' is preferably 1.0 x 10 ⁴ Pa to 2.0 x 10 ⁴ Pa.

[0017] As a preferable embodiment of the present invention, a hot melt adhesive is provided including at least one type whereby the triblock copolymer is selected from styrene-butadiene- styrene block copolymer (SBS) and styrene-isoprene-styrene block copolymer (SIS).

[0018] As another preferred embodiments of the present invention, a hot melt adhesive is provided wherein the glass transfer temperature is not greater than 10°C. [0019] As a further preferable embodiment of the present invention, a hot melt adhesive is provided for the purpose of manufacturing absorbent articles.

[0020] The present invention according to other summaries provides absorbent articles coated with the aforementioned hot melt adhesives.

[0021] In the present invention, "absorbent products" refers to absorbent products in general, but more specifically refers to goods affixed to clothing for the purpose of absorbing bodily fluids arising from the body, for the purpose of preventing unpleasant or unhygienic conditions arising when bodily fluids adhere to clothing, and specifically including products such as feminine sanitary napkins, urine absorbent liners, maternity disposable panties, nursing pads, underarm perspiration pads, disposable paper diapers, pet sheets, hospital gowns and surgical white gowns etc.

[0022] Among the aforementioned absorbent products, the present invention preferably provides so-called insertable absorbent products, being absorbent products used by being placed inside underwear. Such absorbent products in particular include absorbent material placed between a top sheet with a surface that contacts the skin and a back sheet with a surface that does not contact the skin (or contacts the underwear), whereby in a sanitary napkin the back sheet is preferably a polyolefin film.

[0023] A hot melt adhesive according to the present invention is a hot melt adhesive including (A) a thermally reversible block copolymer being a polymer of a vinyl aromatic hydrocarbon and a conjugate diene compound and (B) a-methylstyrene resin, whereby residual modulus of elasticity G' at 50°C and 10 ad/s is between 8.0 x 10 ³ Pa and 5.0 x 10 ⁴ Pa, and therefore has superior peel strength and no adhesive remains on clothing, and also has excellent coatability. As a result, the hot melt adhesive is able to enhance productive efficiency in absorbent articles, such as sanitary napkins, and the like.

[0024] As the absorbent product is manufactured, a hot melt adhesive according to the present invention is coated onto a peel[able] base material, then transferred to the back sheet. When the absorbent article is stuck onto underwear, the coagulating strength of the hot melt adhesive is high, so that the bond between the back sheet and hot melt adhesive is also high. As a result, when a sanitary napkin or other absorbent product using a hot melt adhesive according to the present invention is peeled away from underwear, the hot melt adhesive does not peel away from the back sheet, and thus no adhesive residue remains on the underwear. Also, because the peel strength with respect to the underwear remains high, the sanitary napkin or other absorbent product attached to the underwear does not slip out of its place.

[0025] Because coatability is even more preferable when (A) the thermally reversible block copolymer includes a tri-block copolymer with weighted mean molecular weight 7.5 x 10 ⁴ to 1.0 x 10 ⁵ , it is possible to easily attach absorbent articles with textured materials onto a surface with no unevenness. Because the coating temperature does not have to be set high, the hot melt adhesive does not degrade easily and has excellent peel strength. As a result the adhesive article can be held in position in the underwear.

[0026] Absorbent articles can be easily attached even to materials with uneven textured surfaces when the hot meld adhesive has residual elasticity G' at 50°C and 10 Rad/s of 1.0 x 10 ⁴ Pa to 2.0 x 10 ⁴ Pa, and as a result it is possible to securely position the absorbent product even if the surface of the underwear of other clothing is not flat or smooth. Also, this prevents adhesive residue when the absorbent article is peeled away from underwear.

[0027] A hot melt adhesive according to the present invention will have even more superior peel strength and therefore can be securely positioned in underwear or other clothing when the triple block copolymer contains at least one type selected from styrene-butadiene-styrene block copolymer (SBS) or styrene-isoprene-styrene block copolymer (SIS). The enhancement of peel strength of the hot melt adhesive is due to the fact that non-hydrogenated block copolymers, e.g., SBS and SIS, are superior in strength to hydrogenated block copolymers, e.g., SEBS.

[0028] When a hot melt adhesive according to the present invention has a glass transition temperature of no greater than 10°C, the adhesive product sticks more easily to clothing without the hot melt adhesive becoming too hard. A hot melt adhesive according to the present invention is even better suited for use in adhesive products in cold regions in particular.

[0029] Because a hot melt adhesive according to the present invention has superior tear strength without adhesive residue, as well as superior coatability, it is better suited as an adhesive for the purpose of manufacturing absorbent products.

[0030] Because absorbent products according to the present invention are coated with the aforementioned hot melt adhesive, they are held in position when placed inside garments and do not slip, and also do not leave adhesive residue when peeled from garments. In addition, because the hot melt adhesive has superior coatability, it can be applied evenly by coating at lower temperatures. Thus, the absorbent products can be manufactured easily even when tack sheets are polyolefin film, which is sensitive to heat, or when using base materials with textured surfaces.

DETAILED DESCRIPTION OF THE INVENTION

[0031] A hot melt adhesive according to the present invention comprises (A) a thermally reversible block copolymer being a polymer of a vinyl aromatic hydrocarbon and a conjugate diene compound and (B) a-methylstyrene resin, whereby residual modulus of elasticity G' at 50°C and 10 Rad/s is between 8.0 x 10 ³ Pa and 5.0 x 10 ⁴ Pa.

[0032] "Residual modulus of elasticity G'" is a characteristic value equivalent to elasticity. In the present invention, residual modulus of elasticity G' is correlated to the quality of hot melt adhesive dispersion on a back sheet in a short period of time. In conceptual terms, the use of an absorbent product involves heating of the hot melt adhesive by means of body heat, then application of low-frequency stress from addition of weight and motion. Residual elasticity G' at 50°C of the hot melt adhesive is chosen as an indicator for in consideration of thermal conversion of body heat and frequency during use of the absorbent article.

[0033] "(A) thermally reversible block copolymer" in the present invention refers to a block copolymer obtained as a block copolymer of vinyl aromatic hydrocarbon and a conjugated diene compound. These are not specifically limited so long as a hot melt adhesive according to the purpose of the present invention is obtained.

[0034] A "vinyl aromatic hydrocarbon" as used above refers to an aromatic hydrocarbon compound having a vinyl radical, specifically a styrene, o-methylstyrene, p-methylstyrene, p- tert-butylstyrene, 1,3-dimethylstyrene, a-methylstyrene, vinyl naphthalene, or vinyl anthracene etc. In particular, a styrene is preferable. The vinyl aromatic hydrocarbons may be used singly or in combinations.

[0035] "Conjugated diene compound" refers to a diolefin compound having at least one conjugated double bond. "Conjugated diene compounds" specifically include, for example, 1,3- butadiene, 20methyl-l,3-butadiene (or isoprene), 2,3-dimethyl-l ,3-butadiene, 1,3 -butadiene, 1 ,3- hexadiene, andin particular, 1 ,3 -butadiene, and 2-methyl-l,3-butadiene are preferable. The conjugated diene compounds may be used singly or in combinations.

[0036] The (A) thermally reversible clock copolymer according to the present invention may be either a non-hydrogenated type or a hydrogenated type. [0037] The"(A) non-hydrogenated thermally reversible block copolymer" may include, specifically, a block copolymer whereby hydrogen is not added to blocks based on the conjugate diene compound of the block copolymer, after the vinyl aromatic hydrocarbon and conjugated diene compound are block copolymerized.

[0038] Also, the "(A) hydrogenated thermally reversible block copolymer" may include, specifically, a block copolymer whereby hydrogen is added to some or all blocks based on the conjugate diene compound of the block copolymer, after the vinyl aromatic hydrocarbon and conjugated diene compound are block copolymerized.

[0039] The degree to which hydrogen is added to the "(A) hydrogenated thermally reversible block copolymer" is indicated by the "hydrogenation ratio." The "hydrogenation ratio" of the "(A) hydrogenated thermally reversible block copolymer" is the proportion of double bonds after hydrogenation that have been converted to saturated hydrocarbon bonds by hydrogenation, with respect to all aliphatic double bonds included in blocks based on conjugated diene copolymers before hydrogenation. The "hydrogenation ratio" can be measured by means of an infrared spectrophotometer or nuclear magnetic residence spectrometer.

[0040] The "(A) non-hydrogenated thermally reversible block copolymer" may include, specifically, styrene-isoprene-styrene block copolymer ("SIS"), or styrene-butadiene-styrene block copolymer ("SBS"). "(A) hydrogenated thermally reversible block copolymer" may include, specifically, hydrogenated styrene-isoprene-styrene block copolymer (also referred to as styrene-ethylene/propylene-styrene block copolymer "SEPS") as well as hydrogenated styrene- butadiene-styrene block copolymer (also referred to as styrene-ethylene/butylene-styrene block copolymer "SEBS").

[0041] In the present invention, the (A) thermally reversible block copolymer preferably includes both SBS and SIS, or either SBS or SIS. By including SBS and/or SIS, the hot melt adhesive according to the present invention improves peel strength and provides absorbent products with superior bondability for holding position.

[0042] In the present invention, the (A) thermally reversible block copolymer preferably includes tri-block copolymer with weighted mean molecular weight (Mw) 7.5 x 10 ⁴ to 1.0 x 10 ⁵ . By means of a Mw within the range, the hot melt adhesive according to the present invention has superior coatability, and also superior peel strength. [0043] The weighted mean molecular weight is measured by means of gel permeation chromatography (GPC). More specifically, weighted mean molecular weight is the value measured using the GPC devices and GPC measurement methods stated below.

[0044] The GPC equipment used was a TS -GEL Super Multipore HZ-M (commercial name) mounted on an HLC-8220 (commercial name) GPC device manufactured by Tosoh Corporation. Samples were dissolved in tetrahydrofuran and the solvent tetrahydrofuran was flowed at a rate of 0.35 ml/min at a column temperature of 40°C. Mw was derived by calculating molecular weight using a standard curve created using polystyrene with monodispersed molecular weight as a control substance. Di-block and tri-block were separated by chromatogram waveform processing, before the weighted average molecular weight (Mw) of the tri-block was calculated.

[0045] The present invention may use (A) thermally reversible block copolymers in the form of commercially available products. Examples include Tufprene T125, Tuftec L518X, or Tuftec HI 053 manufactured by Asahi Kasei Chemical Corporation; TR2000 manufactured by JSR Corporation; Tipol4202 manufactured by TSRC Corporation; Kraton Dl 162PT or G1650M manufactured by Kraton Polymers.

[0046] These commercially available products may be used either singly or in combination. Of these commercially available products, Tufprene T125, and Kraton Dl 162PT are particularly preferable as the (A) thermally reversible block copolymers in the present invention.

[0047] In the present invention, the (B) a-methylstyrene resin may be a-methylstyrene polymer or styrene- a-methylstyrene copolymer. In forms of the present invention, (b) a-methylstyrene resin is more preferably a styrene- α-methylstyrene copolymer, and more preferably has a softening point of 85° to 120°C (measured according to environmental method JIS K2207). Specifically, examples include Kristalex 3085, Kristalex 3100, Kristalex 1 120, or Kristalex 5140 manufactured by Eastman Chemical Company, and FTR-2120 manufactured by Mitsui

Chemicals Inc.

[0048] The (B) α-methylstyrene resin is preferably blended in 5 to 40 parts by weight per 100 parts of by weight of total weight of (A) and (B), more preferably 5 to 20 parts by weight, and especially preferably 10 to 20 parts by weight.

[0049] The hot melt adhesive by means of the present invention may further include tackifier resin. The "tackifier resin" is normally used in hot melt adhesives, and is not specifically limited so long as a hot melt adhesive according to the purpose of the present invention can be provided. [0050] The tackifier resins may include for example natural rosin, denatured rosin, hydrogenated rosin, glycerol ester of natural rosin, glycerol ester of denatured rosin, pentaerithritol ester of natural rosin, pentaerithritol ester of denatured rosin, pentaerithritol ester of hydrogenated rosin, natural terpene copolymer, three-dimensional polymer of natural terpene, hydrogenated derivatives of natural terpene copolymer, polyterpene resin, hydrogenated derivatives of phenolic denatured terpene resin, aliphatic petroleum hydrocarbon resin, hydrogenated derivatives of aliphatic petroleum hydrocarbon resin, aromatic petroleum hydrocarbon resin, hydrogenated derivatives of aromatic petroleum hydrocarbon resin, cycloaliphatic petroleum hydrocarbon resin, or hydrogenated derivatives of cycloaliphatic petroleum hydrocarbon resin. The above tackifier resins may be used singly or in combination. Any liquid type tackifier resin may be used as long as it is colorless to light yellow in color and effectively odorless and has good thermal stability. In consideration of the overall characteristics thereof, the hydrogenated derivatives of resins, and the like, are preferable as tackifier resins.

[0051] Commercially available tackifier resin products may be used. Such commercially available products include, for example, Marcaclear-H manufactured by Maruzen

Petrochemical, Clearon K100 manufactured by Yasuhara Chemical Co., Ltd., Arkon Ml 00 manufactured by Arakawa Chemical Industries, Ltd., Aimab SI 00 manufactured by Idemitsu petrochemical Co., Ltd., Clearon 4090, and Clearon K4100 manufactured by Yasuhara Chemical Co., Ltd., ECR 5380, ECR179EX (commercial name), ECR 5400, and ECR 5600 manufactured by Exxon Mobil, or Ligalite R7100 manufactured by Eastman Chemical

Company. These commercially available tackifier resins may be used singly or in combinations.

[0052] In addition, a hot melt adhesive according to the present invention may contain plasticizers. The "plasticizers" are added for the purpose of lowering the melt viscosity of hot melt adhesives, and to contribute softening, and enhance wetting of the subject material, and are not specifically limited as long as they are mutually soluble with the block copolymers, and provide a hot melt adhesive according to the purpose of the present invention.

[0053] The plasticizers may be, for example, paraffinic oils, naphthenic oils or aromatic oils. In particular paraffinic oils and/or naphthenic oils are preferable, and paraffinic oils, being colorless and odorless, are particularly preferable.

[0054] The plasticizers may be commercially available, including for example White Oil Broom 350 manufactured by Kukdong Oil & Chemical Co., Diana Fresia S-32, Diana Process Oil PW- 90, Daphne Oil KP-68, or manufactured by Idemitsu Kosan Co., Ltd., Enerper M1930 manufactured by BP Chemicals, Ltd., Kaydol manufactured by Crompton Ltd., Primol 352 manufactured by Exxon Inc., or Process Oil NS-100 manufactured by Idemitsu Kosan Co., Ltd. These may be used singly or in combination.

[0055] The hot melt adhesive according to the present invention may contain a variety of other additives as necessary. Examples of such additives may include stabilizers, waxes, or fine particulate fillers, and the like.

[0056] "Stabilizers" are substances blended in for the purpose of enhancing the stability of hot melt adhesives as well as to prevent reduction of molecular weight of the hot melt adhesive due to heat, or to [promote] gelling, add color, or prevent odor and the like, and are not specifically limited so long as the purpose of the hot melt adhesive according to the present invention is achieved. "Stabilizers" may also be, for example, anti-oxidants or ultraviolet ray absorbents.

[0057] "Ultraviolet ray absorbents" are used to improve the durability of hot melt adhesives with respect to light. "Anti-oxidants" are used to prevent degradation of hot melt adhesives due to oxidation. Anti-oxidants and ultraviolet ray absorbents are normally used in disposable products and may be used without limitation as long as disposable products having the purposes described below are obtained.

[0058] Anti-oxidants may include, for example, phenolic anti-oxidants, sulfur-related antioxidants, or phosphorus-related anti-oxidants. Ultraviolet ray absorbents may include, for example, benzotriazol related ultraviolet ray absorbents, or benzophenone related ultraviolet ray absorbents. Lactone stabilizers may also be added. These may be used either singly or in combinations. Examples of commercially available anti-oxidants include the products listed below.

[0059] Specific examples include Sumilizer-GM, Sumilizer-TPD or Sumilizer-TPS

manufactured by Sumitomo Chemical Co., Ltd., Irganox 1010, Irganox HP2225FF, Irgaphos 168 or Irganox 1520, or Chinubin-P manufactured by Ciba Specialty Chemicals Co., Ltd., JF77 manufactured by Johoku Chemical Co., Ltd., and Tominox TT (commercial name) manufactured by ABI Corporation. These stabilizers may be used either singly or in combination.

[0060] "Waxes" may be waxes generally used in hot melt adhesives, and are not specifically limited as long as a hot melt adhesive according to the purpose of the present invention is obtained. Specifically, these may include for example synthetic waxes such as Fischer-Tropsh wax or polyolefin wax (e.g., polyethylene wax, polypropylene wax), petroleum waxes such as paraffin wax or microcrystalline wax, or natural waxes such as castor wax.

[0061] The hot melt adhesive according to the present invention may further contain particulate fillers. Any fine particulate filler in general use may be used without limitation so long as a hot melt adhesive according to the purpose of the present invention is obtained. "Fine particulate fillers" may include for example, mica, calcium carbonate, kaolin, talc, titanium oxide, diatom earth, urea resins, styrene particles, baked clay, starch, and the like. The form thereof is preferably spherical, and the dimensions (diameter if spherical) are not specifically limited.

[0062] The hot melt adhesive according to the present invention may by manufactured by blending (A) and (B), as well as other additives as necessary, using commonly known methods of manufacture of hot melt adhesives. For example, manufacturing may be by blending the aforementioned components in the prescribed quantities, and melting them by heating. The order of addition of each component, the method of heating etc. is not specifically limited as long as a hot melt adhesive according to the purpose is obtained.

[0063] The hot melt adhesive according to the present invention should preferably have residual elasticity G' at 50°C and 10 Rad/s of 8.0 x 10 ³ Pa to 5.0 x 10 ⁴ Pa, and particularly preferably from 1.0 x 10 ⁴ Pa to 2.0 x 10 ⁴ Pa. By setting residual elasticity G' within the aforementioned range, the hot melt adhesive will have superior peel strength and resistant to creating adhesive residue, and further will be easy to coat onto uneven base material surfaces.

[0064] If residual elasticity G' is less than 8.0 x 10 ³ Pa, the diffusion of hot melt adhesive will be excessive, peel strength will be excessively increased, and adhesive reside will occur. On the other hand, if residual elasticity G' exceeds 5.0 x 10 ⁴ , the diffusion of hot melt adhesive onto base material will be insufficient, and therefore sufficient bonding strength will not be obtained. In addition, melt viscosity will be high, and thus coatability will be reduced.

[0065] In the present invention, a dynamic viscometer was used in temperature sweep mode with velocity fixed at 10 Rad/s, scanning from -25°C to 150°C at a rate of temperature increase of 5°C per minute, and the scan value at 50° was used as residual elasticity G' of the present invention. Residual elasticity G' is a characteristic value corresponding to elasticity, and lost elasticity G" is [also] known as a characteristic value corresponding to viscosity. When G' is measured by means of a dynamic viscometer, G" is also measured, so that it is possible to obtain the loss tangent (tan δ) expressed by the ratio of lost elasticity G" to residual elasticity G' (G'VG') at the same time.

[0066] A hot melt adhesive according to the present invention should preferably have a glass transition temperature (Tg) of no greater than 10°C, more preferably from—8° to 10°C, and most preferably from -5°C to 5°C. When the hot melt adhesive has a glass transition temperature (Tg) within this range, the absorbent article will stay firmly in place even in cold regions without becoming too hard. If the hot melt adhesive Tg is high in comparison with the temperature of the environment in which the absorbent article is used, the hot melt adhesive will become glassy in state and the hot melt adhesive will have difficulty in keeping the absorbent product in place on the underwear. In consideration of use of absorbent products in cold regions, the aforementioned ranges for hot melt adhesive Tg are preferable.

[0067] Glass transition temperature (Tg) in terms of the present invention refers to the temperature indicated by the highest peak obtained from the loss tangent (tan δ) measured at the same time as the measurement of residual elasticity G' with frequency fixed at 10 Rad/s on the aforementioned dynamic viscometer, plotted with respect to temperature.

[0068] As a more preferable form of the present invention, the hot melt adhesive should have viscosity (or melt viscosity) at 130°C of no greater than 15000 mPa-s, and especially preferably not greater than 6000 mPa ^» s. The viscosity at which hot meld adhesive can be uniformly coated is up to 15000 mPa*s, and the viscosity at which uniform coating can be easily applied is up to 6000 mPa ^» s. If viscosity exceeds 15000 mPa ^» s, coating becomes difficult. In this Detailed Statement, viscosity (or melt viscosity) at 130°C represents the value measured by a Brookfield viscometer using a spindle number 27.

[0069] When coating a hot melt adhesive onto a sanitary napkin, with a back sheet of polyolefin film, and the viscosity of the adhesive at 130°C is less than 6000 mPa ^» s the hot melt adhesive can be easily coated directly onto the polyolefin film. Because hot melt adhesive can easily be uniformly coated onto a film at 130°C, sanitary napkins can be held firmly in place on underwear without the hot melt adhesive degrading, and when peeled off from the underwear, no adhesive residue will remain on the underwear.

[0070] A hot melt adhesive according to the present invention can be used in a wide variety of uses such as paper fabrication, book binding, disposable products etc., but is particularly effective with absorbent products. "Absorbent products" includes so-called hygienic products, without particular limitation. Specific examples include, for example, feminine sanitary napkins, urine absorbent liners, maternity disposable panties, nursing pads, underarm perspiration pads, disposable paper diapers, pet sheets, hospital gowns and surgical white gowns, and the like.

[0071] Absorbent articles are composed of at least one type of material from a group including woven cloth, nonwoven cloth, rubber, plastic, paper, and polyolefin film, coated with a hot melt adhesive according to the present invention. Note that of the polyolefin films, polyethylene film is preferable for reasons including durability and cost.

|0072] A hot melt adhesive according the present invention is ideally suited for sanitary napkins. In general, sanitary napkins have an absorptive material between a top sheet and a back sheet, wherein the top sheet contacts the skin, and the back sheet contacts underwear, and the like. A hot melt adhesive according to the present invention is particularly effective when the back sheet, the surface that does not contact the skin, is a polyolefin film.

[0073] By increasing the peel strength between the polyolefin coated with hot melt adhesive and the underwear, the sanitary napkin becomes less likely to slip from the underwear. In the present invention, either by applying hot melt adhesive directly to the polyolefin film, or by coating hot melt adhesive onto a peel-away film and then transferring it to polyolefin film, not only is slippage from underwear prevented, but the occurrence of adhesive residue on the underwear is also prevented.

[0074] In manufacturing lines for absorbent articles, in general hot melt adhesive is coated onto a variety of materials for disposable products (such as peel-away paper, tissue, cotton, nonwoven cloth, polyolefin film, and the like). In coating, the hot melt adhesive may be sprayed from a variety of spray devices.

[0075] Methods of coating hot melt adhesive are not specifically limited so long as the desired absorbent articles are obtained. Such coating methods are broadly classified into contact coating and noncontact coating. "Contact coating" refers to coating methods whereby in coating with hot melt adhesive, the spray device comes into contact with the material or film, and "noncontact coating" refers to coating methods whereby in coating with hot melt adhesive, the spray device does not come into contact with the material or film. Contact coating methods include, for example, slot coater coating or roll coater coating, and noncontact coating methods include, for example, spiral coating that can be applied in a spiral shape, omega coating or controlled seam coating that can be applied in wave shapes, slot spray coating or curtain spray coating (applied in planar shapes), and dot coating (applied in dot shapes).

[0076] Hot melt adhesive is applied by the aforementioned coating methods and the absorbent article is manufactured. Absorbent articles include, for example, feminine sanitary napkins, urine absorbent liners, maternity disposable panties, nursing pads, underarm perspiration pads, disposable paper diapers, pet sheets, hospital gowns and surgical white gowns etc., but the effect of the present invention is most fully realized with sanitary napkins having a back sheet of polyolefin film.

|0077] The principal forms of the present invention is as follows:

1. A hot melt adhesive comprising:

(A) a thermally reversible block copolymer that comprises a polymer of a vinyl aromatic hydrocarbon and a conjugate diene compound, and;

(B) ct-methylstyrene resin;

2. wherein the residual modulus of elasticity G' at 50°C and 10 Rad/s of the adhesive is from 8.0 x 10 ³ Pa to 5.0 x 10 ⁴ Pa.

3. The hot melt adhesive according to Claim 1 wherein the (A) thermally reversible block copolymer contains a tri-block copolymer having a weighted average molecular weight of from 7.5 x 10 ⁴ to 1.0 x 10 ^s .

4. The hot melt adhesive according to Claims 1 -2, wherein the residual elasticity G' of the adhesive is from 1.0 x 10 ⁴ Pa to 2.0 x 10 ⁴ Pa.

5. The hot melt adhesive according to Claims 1 -3, wherein the tri-block copolymer consists of a styrene-butadiene-styrene block copolymers (SBS), styrene-isoprene- styrene block copolymers (SIS), and mixtures thereof.

6. The hot melt adhesive according to Claim 1 -4, wherein theglass transition

temperature of the adhesive is less than 10°C.

7. An absorbent article comprising the hot melt adhesive of Claims 1-5. 8. The absorbent articles of Claim 6 which is a sanitary napkin.

EXAMPLES

[0078] The present invention is described in further detail in the following examples. The examples are for the purpose of describing the present invention, and do not limit the present invention in any way.

[0079] The components for the purpose of blending the hot melt adhesives are indicated as follows:

(A)a thermally reversible block copolymer that comprises a copolymer of a vinyl aromatic hydrocarbon and a conjugated diene compound

(Al) Styrene-butadiene-styrene block copolymer, tri-block weighted average molecular weight 8.5 x 10 ⁴ (Tufprene T125 manufactured by Asahi Kasei Chemical Corporation) (A2) Styrene-butadiene-styrene block copolymer, tri-block weighted average molecular weight 9.4 x 10 ⁴ (TR2000 manufactured by JSR Corporation)

(A3) Styrene-isoprene-styrene block copolymer, tri-block weighted average molecular weight 8.1 x 10 ⁴ (Kraton Dl 162PT manufactured by raton Polymers)

(A4) Styrene-butadiene-styrene block copolymer, tri-block weighted average molecular weight 10.6 x 10 ⁴ (Asaprene T420 manufactured by Asahi Kasei Chemical Corporation) (A5) Styrene-butadiene-styrene block copolymer, tri-block weighted average molecular weight 12.9 x 10 ⁴ (Asaprene T438 manufactured by Asahi Kasei Chemical Corporation) (A6) Styrene-isoprene-styrene block copolymer, tri-block weighted average molecular weight 30.8 x 10 ⁴ (Quintac 3520 manufactured by Zeon Japan)

[0080] (B) a-methylstyrene resin

(Bl) Styrene- a-methylstyrene copolymer (Kristalex 3100 manufactured by Eastman Chemical Company)

(B2) Styrene- a-methylstyrene copolymer (Kristalex 3085 manufactured by Eastman Chemical Company)

(B3) Styrene- α-methylstyrene copolymer (FTR 2120 manufactured by Mitsui Chemicals Inc.)

[0081] (C) Tackifier resin

(CI) Hydrogenated derivative of aromatic petroleum hydrocarbon resin (Arkon Ml 00 manufactured by Arakawa Chemical Industries, Ltd.) (C2) Hydrogenated derivative of cycloaliphatic petroleum hydrocarbon resin (ECR 179EX manufactured by Exxon Mobil)

(C3) Hydrogenated derivative of cycloaliphatic petroleum hydrocarbon resin (ECR 5400 manufactured by Exxon Mobil

[0082] (D) Plasticizer

(Dl) Paraffinic oil (Diana Fresia S-32 manufactured by Idemitsu osan Co., Ltd.) (D2) Paraffinic oil (White Oil Broom 350 manufactured by ukdong Oil & Chemical Co.)

(D3) Naphthenic oil (NS-100 manufactured by Idemitsu Kosan Co., Ltd.)

[0083] (E) Anti-oxidants

(El) Hindered phenolic anti-oxidant Irganox 1010 manufactured by Ciba Specialty Chemicals Co., Ltd.)

(E2) Benzodriazol related ultraviolet absorbent (JFF77 manufactured by Johoku

Chemical Co., Ltd.)

[0084] The weighted average molecular weight of tri-blocks included in (A) thermally reversible block copolymer was analyzed using gel permeation chromatography (TSK-GEL Super

Multipore HZ-M mounted on an HLC-8220 GPC device manufactured by Tosoh Corporation). Samples were dissolved in tetrahydrofuran, and the solvent tetrahydrofuran was flowed at a rate of 0.35 ml/min at a column temperature of 40°C. Mw was derived by calculating molecular weight using a standard curve created using polystyrene with monodispersed molecular weight as a control substance. For samples having di-blocks the di-block and tri-block were separated by chromatogram waveform processing, before the weighted average molecular weight (Mw) of the tri-block was calculated.

[0085] Hot melt adhesives according to examples 1 through 9 and comparative examples 1 through 5 were manufactured by blending components (A) through (E) were blended in the proportions shown in Tables 1 through 3, melt blended for two hours at approximately 140°C using an all-purpose agitator. Values relating to composition (blend) of hot melt adhesives shown in Tables 1 through 3 are all in percentage by weight.

[0086] Residual elasticity (G') and glass transition temperature (Tg) of hot melt adhesives were measured by the following methods. <Residual elasticity (G'>

[0087] Dynamic elasticity was measured using a dynamic viscometer (Rheometer AR-G2 manufactured by T.A. Instruments Japan). Hot melt adhesive was warmed in the container of the device, forming a disk-shape with diameter 25 mm and thickness 1500 μη . Measurement was performed in temperature sweep mode using stainless steel parallel plates, with the angular velocity fixed at 10 rad/s, scanning from -25°C to 150°C at a rate of temperature increase of 5°C per minute. The scan value was read at 50° ±1°C and used as residual elasticity G' of the present invention.

<Glass transition temperature (Tg)>

[0088] Using the aforementioned viscosity measurement, the loss tangent Tan δ (G'VG'), the ratio of lost elasticity (G") to residual elasticity (G'), was measured in the temperature range not exceeding the softening point. Loss tangent Tan δ was potted with respect to temperature, and the temperature at the top of the resulting peaks was read and used as the glass transition temperature.

[0089] Each of the hot melt adhesives according to the examples and comparative examples was read and evaluated for coatability, retention strength, and adhesive residue. An overview of each of the evaluations is presented below.

<Coatability (melt viscosity at 130°C>

[0090] The viscosity of each hot melt adhesive at 130°C was measured using method B as described in JAI-1991. Measurement was made using a Brookfield viscometer using a spindle number 27.

[0091] In general if the melt viscosity of a hot melt adhesive is less than 6000 mPa ^» s, it can easily be uniformly spread on the base film, and if between 6000 mPa ^» s and 15000 mPa ^» s, it can be uniformly coated. If the melt viscosity exceeds 15000 mPa ^» s, coating is difficult.

[0092] Further, the temperature at which direct coating is enabled on polyethylene film without shrinkage or breakage due to heat is generally not more than 130°C.

[0093] Accordingly, the coatability of hot melt adhesives was evaluated as follows.

OO: Melt viscosity at 130°C less than 6000 mPa*s O: Melt viscosity at 130°C between 6000 mPa ^» s and 15000 mPa ^» s

X: Melt viscosity at 130°C greater than 15000 mPa ^» s

<Retention strength (peel strength)>

[0094] Each hot melt adhesive was coated onto PET film of thickness 50μηι, forming an adhesive layer 50μηι thick, and this was formed into 25mm widths and used for samples.

[0095] At the same time silk for JIS color fastness testing (according to JIS L0803) was cut into 30 x 60mm pieces in the direction of the weave, and used as material for laminating.

[0096] Test samples and fastening strips were acclimated for 30 minutes in a 23°C environment, then laminated using a 600g roller at a speed of 150mm min. Immediately after lamination, 180° peel testing was performed at a speed of 300 mm/min using a vise-type pull testing machine. At least three samples were tested of each hot melt adhesive (preferred embodiments and comparison embodiments), and the average values were determined and used as peel strength values. Peel strength was evaluated according to the following standard.

OO: Peel strength exceeds 500g/25mm

O: Peel strength between 300 and 500 g/25mm

X: Peel strength less than 300g/25mm

<Adhesive residue>

[0097] Immediately after measurement of peel strength, the peel surface of the laminated material was felt by hand and the presence or absence of adhesive residue confirmed. Silk was used as the material for evaluation of adhesive residue because silk was the material on which adhesive residue was most conspicuous. Adhesive residue was evaluated according to the following standard.

OO: (no adhesive residue): No stickiness felt by hand

O: (slight adhesive residue): Slight stickiness felt by hand

X: (obvious adhesive residue): Obvious stickiness felt by hand. Table 1

Table 2

Table 3

[0098] As shown in Tables 1 and 2, hot melt adhesives according to Examples 1 through 9 contain component (A) (requirement 1 ), and also contain component (B) (requirement 2), and in addition have residual elasticity G' at 50°C and 10 Rad/s within the range from 8.0 10 ³ Pa to 5.0 x 10 ⁴ Pa (requirement 3). Examples 1 through 9 thus constituted have superior coatability (low viscosity) and holding power (high peel strength), and also do not cause adhesive residue.

[0099] In comparison, as shown in Table 3, hot melt adhesives according to Comparative Examples 1 through 5 do not satisfy at least some of the aforementioned requirements 1 through 3. Because Comparative Examples 1 through 5 do not satisfy at least some of requirements 1 through 3, they are deficient and show an evaluation of "X" with respect to either coatability, holding power, or adhesive residue.

[00100] As can be seen from the results for viscosity at 130°C, peel testing, and adhesive residue testing, hot melt adhesives, by satisfying all requirements 1 through 3, are superior with respect to coatability (low viscosity), holding power (high peel strength), and also able to prevent occurrence of adhesive residue, and thus are exceptionally useful as adhesives for positioning of absorbent articles.

[00101] The present invention provides hot melt adhesives as well as absorbent articles wherein the hot melt adhesive is applied. The absorbent articles are particularly useful as feminine sanitary napkins where the backsheet, the surface not contacting the skin, is made of polyolefin.