More particularly, the present invention relates to hot-melt adhesive compositions for the manufacture of sanitary articles such as infant and adult diapers, sanitary napkins, incontinent pads, bed pads, feminine pads, panty shields, and the like, comprising at least one element of non-woven material, and also to block copolymers to be used therein.

Background Art [002] It is generally known in the art, as taught in WO 9102039, EP 0532831A and EP 0802251A, that block copolymers comprising terminal poly (styrene) blocks and one or more central poly (isoprene) blocks, more particularly triblock copolymers, are used in hot melt adhesive compositions for the manufacture of disposable articles. More specifically triblock copolymers having a poly (styrene) content of from 25 to 35 wt% and having a total apparent molecular weight of from 140,000 to 145,000 (e. g., KRATON D-1165 block copolymer) are used in hot-melt adhesive compositions in the diaper industry in two manners : 1. as assembly adhesive to glue the diaper poly (ethylene) main frame and 2. as an attachment adhesive to glue the elastic attachments which are used as waist and leg bands.

[003] Sch adhesives must be lightly colored, transparent, low in odor, sprayable at 149°C (300°F), show good adhesion to poly (olefin) films and not strike through the non-woven back sheet. They must also be relatively inexpensive.

[004] Elastic attachment adhesive is used to adhere elastic threads to polyethylene and non-woven films in order to form an elastic waist or leg band. Multiple strands of elastic threads are elongated, commonly 300%, and coated with adhesive composition at a temperature of 149pic (300°F) just prior to being pinched inside of the poly (olefin) and non-woven webs. Like construction adhesives, they are usually sprayed in a spiral pattern. When the ends of the elastic are cut later in the process, the composite contracts causing the non-woven and poly (olefin) films to pucker to form an elastic band.

[005] Although block copolymers such as KRATON D-1165 block copolymer show an acceptable hot-melt viscosity/temperature profile and have a sufficiently low viscosity at the usually applied hot-melt temperatures of up to 160°C (320°F) thereby enabling efficient processing, there is still an economic need for more block copolymers which demonstrate processing efficiency, in combination with other attractive properties noted hereinbefore.

[006] It will be appreciated that a relatively low hot-melt viscosity facilitates a high production speed in a diaper manufacturing line without the production of significant amounts of off spec product. Problems which normally cause such off spec product may be, for example, adhesive viscosity degradation, adhesive discoloration, damage to the polyethylene film or formation or char. Accordingly, developmental efforts in diaper manufacture continue today.

[007] Therefore, it is an object of the present invention to provide improved adhesive compositions which show a reasonable viscosity at temperatures in the range of from 138-160°C (280-320°F) in order to avoid the hereinbefore mentioned problems while allowing sprayability.

[008] Another object of the present invention is the formation of sanitary articles comprising at least one non-woven layer and manufactured using said improved hot- melt adhesive compositions.

[009] As a result of extensive research and experimentation it has now surprisingly been found that radial block copolymers comprising terminal, predominantly poly (styrene) blocks and central (I/B) blocks, afford a good sprayable hot-melt viscosity at the tem- peratures preferably applied in the diaper industry, i. e. in the range of from 138-160°C (280-320°F). In addition, these radial polymers afford excellent color stability, superior adhesive performance and better viscosity stability compared to conventional adhesive compositions based on S-I-S block copolymer.

[010] Moreover, hot melt adhesive compositions comprising said S- (I/B) blocks containing block copolymers have been found to show an increased cohesion.

Disclosure of Invention [011] The present invention relates to a hot melt adhesive composition used in non-woven assemblies, comprising: [012] (a) 100 parts by weight of a block copolymer of the formula [S- (I/B)] X, wherein S n represents a predominantly poly (styrene) block and (1/B) represents a polymer block obtained by at random copolymerization of a mixture of predominantly isoprene and butadiene in a weight ratio in the range of from 70: 30 to 30: 70, wherein n is an integer in the range of from 3 to 5, preferably from 3 to 4, and wherein X is the remainder of a coupling agent, wherein the block copolymer has a poly (styrene) content of from 28 to 50 wt%, a coupling efficiency of from 50 to 100 % and a melt flow index measured at 200°C/5 kg, ranging from 1.0 to 12 g/10 min, [013] (b) from 250 to 300 parts by weight of a tackifying resin, [014] (c) from 50 to 150 parts by weight of a plasticizer, and [015] (d) optionally from 0 to 3 parts by weight of stabilizers and/or antioxidmts.

[016] Other aspects of the present invention are formed by disposable articles such as infant and adult diapers, sanitary napkins, incontinent pads, bed pads, feminine pads, panty shields, comprising at least one non-woven element and assembled using the hereinbefore specified hot melt adhesive compositions, and by the specific block copolymers to be used therein.

[017] The adhesive compositions of the present invention are useful for applications such as disposable diapers, feminine protection articles, incontinent pads, bed pads, surgical drapes and gowns, and similar articles. The inventive radial, mixed-midblock polymers have been shown to have good spayability, even at surprisingly high viscosities. Other features compared to convention s-based non-wovens adhesives include improved viscosity stability, improved color stability when heat-aged and good creep resistance for elastic fiber attachment formulations.

Mode for the Invention [018] The main components used in the adhesive compositions of the present invention are a block copolymer (component (a) ), a tackifying resin (component (b) ), a plasticizer (component (c) ) and optionally, stabilizers or antioxidants (component (d)).

[019] The block copolymer of the present invention is represented by a structure of the general formula [S- (I/B)] X, optionally mixed with a diblock copolymer S- (I/B) when n the coupling efficiency is less than 100%, wherein S represents a predominantly poly (styrene) block, (I/B) represents a block of a randomly copolymerized mixture of predominantly isoprene and butadiene, wherein the weight ratio of isoprene to butadiene is in the range of from 70: 30 to 30: 70, (or in a mole/mole ratio of from 1.1/0. 55 to 0. 45/1. 3) and wherein n is an integer in the range of from 3 to 5, preferably from3 or 4, and X is the remainder of a coupling agent to be further specified hereinafter.

[020] Preferred weight ratios of isoprene to butadiene are in the range of from 60: 40 to 40: 60 (or in a molar ratio of from 0.89/0. 75 to 0.75/0. 89). More preferably, the weight ratios of isoprene to butadiene are in the range of from 55: 45 to 45: 55.

[021] As used herein, the terms"predominantly poly (styrene)" and"mixtures of pre- dominantly isoprene and butadiene"mean that in addition to the main monomer, i. e. styrene on the one hand and on the other hand isoprene and butadiene, one or more other minor co-monomers may be present in the starting monomer in small amounts, i. e. up to 5 wt% of co-polymerizable monomer. However, preferably substantially pure (co) monomers may be used for the preparation of the respective blocks.

[022] Examples of minor co-monomers, used in addition to styrene, include, but are not limited to alpha-methylstyrene, p-methylstyrene, o-methylstyrene, p-tert-butylstyrene, dimethylstyrene, vinyltoluene, vinylxylene, diphenylethylene and vinyl naphthalene or mixtures thereof.

[023] The mixed polymer midblock (I/B) is made of butadiene and isoprene as copolymerizing monomers, although it may also contain minor amounts of other co- monomers, e. g. up to 5 wt% of a co-polymerizable monomer such as styrene (based on the weight of the total block), but mixtures of substantially pure isoprene and butadiene are preferred.

[024] In the block copolymers according to the present invention, the poly (styrene) content (which may include optional co-monomers) is in the range of from 28 to 50 wt%, preferably from 28 to 35 wt%, even more preferably from 29 to 33 wt%, based on the total block copolymer. The proportion of bound butadiene is from 18 to 80 wt%, preferably from 40 to 70 wt% in total. The proportion of bound isoprene is from 15 to 70 wt%, preferably from 30 to 70 wt%. These amounts of bound monomers (plus co-polymerizable monomers, if any) add up to 100 wt%.

[025] The polymer blocks S preferably have a true molecular weight in the range from 9,500 to 25,000, more preferably from 11,000 to 16,000.

[026] The block copolymers to be applied in the adhesive compositions according to the present invention preferably have a weight average molecular weight (Mw, expressed in terms of polystyrene) ranging from 250,000 to 500,000, more preferably from 150,000 to 230,000, as determined by gel permeation chromatography (GPC, Analogous to the method described in ASTM D5293-97).

[027] The block copolymers have a melt flow index measured at 200°C/5 kg ranging from 1. 0 to 12 g/10 min.

[028] The block copolymers to be applied in the adhesive compositions according to the present invention each preferably contain 1,2-vinyl bonds and/or 3,4-vinyl bonds in a proportion in the range of from 4 to 10 wt%, based on the weight of the conjugated diene, (or in the range of from 0.08 to 0.70 mole/mole %).

[029] Preferably the (1/B) block has a vinyl content in the polymerized butadiene in the range of from 7 to 12 wt% and a vinyl content in the polymerized isoprene in the range of from 4 to 8 wt%. Even more preferably the (I/B) block has a vinyl content in the polymerized butadiene in the range of from 7 to 9 wt% and a vinyl content in the polymerized isoprene in the range of from 4 to 6 vit%.

[030] Said block copolymers to be applied as main component (a) in the adhesive composition, have a randomly copolymerized block (I/B), which means that the mixed midblock shows no significant single homopolymer block formation. Such block copolymers can be prepared by a variety of methods such as that described in WO 02057386.

[031] The block copolymers according to the present invention can be made e. g. by coupling living diblock copolymer prepared by anionic polymerization with a coupling agent. The coupling agent, can be tin coupling agents such as methyltin trichloride, tin tetrachloride; halogenated silicon coupling agents such as silicon tetrachloride and silicon tetrabromide; alkoxysilanes such as tetramethoxysilane; and halogenated alkanes such as trichloroethane, trichloropropane and tribromopropane. Slicon tetrachloride, silicon tetrabromide, tetramethoxysilane or other tetra (alkoxy) silanes are preferred.

[032] The main block copolymer in component (a) may hence comprise a mixture of the coupled polymer according to the general formulae [S- (I/B)] X and of the intermediate n diblock S- (I/B), e. g. in a weight ratio of from 100/0 to 20/80, and preferably from 100/0 to 80/20.

[033] The main block copolymer component (a) can be made by mere adaptation of common processes used for the preparation of [S-B] X type block copolymers and/or n [S-1] X type block copolymers, using a mixture of butadiene/isoprene instead. The n important point in the preparation of the block copolymers to be used according to the present invention is to avoid homopolymer diene block formation to ensure the desired (I/B) ratio.

[034] The tackifying resins to be used in the hot-melt adhesive compositions of the present invention can be selected from a great variety of resins known to be applicable in hot-melt adhesives. The resins can be selected from modified aliphatic hydrocarbon resins such as modified C5 hydrocarbon resins (C5/C9 resins), styrenated terpene resins, fully or partially hydrogenated C9 hydrocarbon resins, hydrogenated cy- cloalophatic hydrocarbon resins, hydrogenated aromatic modified cycloaliphatic hy- drocarbon resins, and mixtures thereof.

[035] Preferable examples of resins to be used as component (b) include, but are not limited to: water white hydrocarbon resins of the ESCOREZ resin series, such as ESCOREZ 5600, ESSOREZ 5400, ESCOREZ 5300 and the like, or hydrocarbon resins of the REGALITE resin series such as REGALITE 1090, 7100 and S-5100 resins and the like, or the ARKON resin series, like ARKON P and M resin..

[036] Preferred solid tackifying resins will have Ring and Ball softening points in the range of from 90°C to 105°C and will have an aromatic H-NMR content from 0 to 30%, preferably from 0 to 12 %.

[037] In the hot-melt adhesive compositions of the present invention, component (b) is present in an amount from 250 to 300 parts by weight of tackifing resin per 100 parts by weight of block copolymer component (a). In the preferred adhesive compositions, component (b) is applied in amounts of from 250 to 280 parts by weight of tackifying resin per 100 parts by weight of block copolymer component (a).

[038] Suitable plasticizers include predominantly plasticizing oils that are paraffinic or naphthenic in character (carbon aromatic distribution < 5%, preferably < 2%, more preferably 0% as determined according to DIN 51378) and a glass transition temperature lower than-55°C as measured by Differential Scanning Calorimetry.

Those products are commercially available from the Royal Dutch Shell Group of companies, such as SkIELLFLEX, CATENEX, and ONDINS 0 oils. Other oils 'include KAYDOL oil from Witco, TUFFLO oils from Arco, N^YPLAST from NYNAS or CALSOL 5555 from CALUMET LUBRICANTS. Other plasticizers include compatible liquid tackifying resins like REGALREZ R-1018 or WlNGTACK 10.

[039] Other plasticizers may also be added, such as olefin oligomers; low molecular weight polymers (< 30,000 g/mol) such as liquid polybutene, liquid polyisoprene copolymers, liquid styrene/isoprene copolymers or liquid hydrogenated styrene/ conjugated diene copolymers; vegetable oils and their derivatives ; or paraffin and mi- crocrystalline waxes.

[040] The hot-melt adhesive composition according to the present invention comprises a plasticizer in a weight proportion of from 10 to 30 wt%, relative to the weight of the complete composition and of from 50 to 150 parts by weight of plasticizer per 100 parts by weight of block copolymer constituent (a). Preferably the hot-melt adhesive composition comprises from 100 to 125 parts by weight of plasticizer per 100 parts by weight of block copolymer.

[041] It will be appreciated that each block copolymer of component (a) may be pre- blended with a small amount of plasticizer by the manufacturer of said copolymer.

[042] A variety of other rubber components nay be incorporated into the adhesive com- positions according to the present invention. It is also known in the art that various other components can be added to modify the tack, the odour, the color of the adhesives.

[043] Antioxidants and other stabilizing ingredients such as endblock compatible resins like ENDEX 160 can also be added to protect the adhesive from degradation induced by heat, light and processing or during storage. Several types of antioxidants can be used, either primary antioxidants like hindered phenols or secondary antioxidants like phosphite derivatives or blends thereof.

[044] Examples of commercially available antioxidants include IRGANOX 565 from Gba-Geigy (2. 4-bis- (n-octylthio)-6- (4-hydroxy-3, 5-di-tertiary-butyl anilino)-1, 3,5-triazine), IRGANOX 1010 from Ciba-Geigy (tetrakis-ethylene- (3, 5-di-tertiary-butyl-4-hydroxy-hydrocinnamate) methane) and POLYGARD HR from Uniroyal (tris- (2, 4-di-tertiary-butyl-phenyl) phosphite). Other antioxidants developed to protect the gelling of the polybutadiene segments can also be used, such as SUMILIZER GS from Simitomo (2 [1- (2-hydroxy-3, 5-di-ter-pentylphenyl) ethyl)]-4, 6-di-tert-pentylphenylacrylate) ; SUMILIZER T-PD from Samitomo (pentaerythrythyltetrakis (3-do decylthio- propionate) ); or mixtures thereof.

[045] In the hot-melt adhesive compositions of the present invention, one or more stabilizers and/or antioxidants are optionally present. Accordingly, the one or more stabilizers and/or antioxidants are present in an amount from 0 to 3 parts by weight of stabililzers and/or antioxidants per 100 parts by weight of block copolymer component (a). In the preferred adhesive compositions, component (d) is applied in amounts of from 0.5 to 3, even more preferably from 0.5 to 2.5, parts by weight of component (d) per 100 parts by weight of block copolymer component (a).

[046] No particular limitation is imposed on the preparation process of the adhesive composition. Therefore, any process may be used, such as a mechanically mixing process making use of rolls, a Banbury mixer or a Dalton kneader, a hot-melt process characterized in that heating and mixing are conducted by using a melting kettle equipped with a stirrer, such as a turbo-mixer, a high shear Z-blade mixer or a single- or twin-screw extruder, or a solvent process in which the compounding components are poured in a suitable solvent and stirred, thereby obtaining an intimate solution of the pressure sensitive adhesive composition.

[047] Hot-melt adhesive compositions according to the present invention are efficiently applied as assembly adhesive composition or as attachment adhesive composition for assembling sanitary articles, in particular diapers, comprising a polyethylene framework and at least one non-woven lining element.

[048] The following advantages of the formulated hot-melt adhesive compositions according to the present invention have been found: [049] improved initial and heat-aged color; [050] more resistance to viscosity changes; [051] improved creep properties in elastic fiber adhesion; and [052] good sprayability, even at high viscosities.

[053] The following examples are provided to illustrate the present invention. The examples are not intended to limit the scope of the present invention and they should not be so interpreted.

[054] EXAMPLES [055] The polystyrene content was determined by 1H-NMR.

[056] The hot-melt viscosity was measured with a rotational viscometer following ASTM D-3236-78. The viscosity was measured by the torque required to rotate a spindle at constant speed while immersed in a fluid. The sample was placed in a RVTDV-II equipped with a Brookfield Thermocell and the hot-melt viscosity was measured in a temperature range of l lt0°C to 160°C. The results were expressed in Pascal. second (Pas).

[057] Synthesis of the block copolymers A to D [058] Cyclohexane, styrene, butadiene and isoprene were purified by activated aluminium oxide and stored at 4°C under a nitrogen atmosphere. Prior to synthesis, a monomer mixture of butadiene and isoprene (at the weight/weight ratio given in Table 1) was prepared and stored under nitrogen at 4°C. This mixture was used as such.

[059] An autoclave, equipped with a helical stirrer was charged with cyclohexane and the content was heated to 50 to 60°C. As initiator, sec-BuLi was dosed immediately followed by styrene monomer, which was allowed to polymerize to completion. The reaction temperature was increased to 70°C, at which temperature a butadiene/isoprene monomer mixture (B/I) was dosed and reacted. The resulting diblock was coupled with a coupling agent dosed at a stoichiometry typically less than 1/2 mole per mole of active polymer lithium chain-ends. This excess was optionally scavenged with sec- BuLi and followed by addition of ethanol to terminate the polymerization. The reaction mixture was cooled to 40°C, transported to a blending vessel and a sta- bilization package was added (comprising IRGANOX 565 and IRGANOX 168 0.08/0. 35 phr as a cyclohexane solution) and stirred at room temperature. Dry rubber was obtained by steam coagulation finishing, followed by drying in an oven.

[060] Table 1 lists the amounts in which the components (a) and (b) have been used. The polymers were analyzed by GPC and the results thereof are included in Table 2.

[061] Polymers A, B and C were coupled with GPTS (3-GLYCI- DOXYPROPYL) TNMETHOXY3LANE) and have an average branching of 3.2 arms. Polymer D was coupled with EPON 826 and has a linear structure.

[062] These polymers can also be polymerized with the hereinbefore described sequential/reinitiation technology.

[063] Table 1 Polymer A B) * C* D Cyclohexane (1) 262.4 320.8 320.8 77 Initiator (mmol) 680.0 998.7 998.7 20.5 styrene (gram) 9100 15230 15230 638 B/I (ratio) 60/40 1 1 1 B/I (gram) 21000 35220 35220 1490 EPON 826 (gram) 0.24 GPTS (gram) 40.2 38.25 23.98 added to added to 126.4 kg 120.7 kg cement cement Ratio GPTS/Li 0.25 0.29 0.23 [064] *Polymers B and C were made from a masterbatch of diblock then separated to make radial polymers of different coupling efficiency.

[065] Table 2 Polymer A B C D GPC Mw Polystyrene* 103 13. 3 15.0 15.4 14.8 GPC Mw *103 152.7 200.3 190.1 134 Coupling efficiency % 75 78 66 81 Polystyrene content wt% 29.3 29.4 29.4 31 Bd/Ip ratio wt/wt 60/40 50/50 50/50 50/50 Vinyl in Bd wt % 10 9 9 9 vinyl in Ip wt% 6 6 6 6 [066] Ruther components used in the tested adhesive compositions are as follows: [067] VECTOR 4211 from DEXCO is a linear styrene-isoprene-styrene block copolymer with 30% polystyrene content, a melt flow rate of 13 g/lOmin and virtually no diblock content since the polymer was made sequentially.

[068] Several tackifiers were used in the examples of the invention. ESCOREZ 5600 is a hydrocarbon resin from EXXON MOBIL CHEMICAL. ESCOREZ 5400 is a hy- drocarbon resin from EXXON MOBIL CHEMICAL. CALSOL 5555 is a naphthenic oil from CALUMET LUBRICANTS. ENDEX 160 is a styrene block compatible resin from EASTMAN CHEMICAL. IRGANOX 1010 is an antioxidant from CIBA.

[069] The following hot-melt adhesive compositions, comprising the ingredients as ,, _ specified hereinafter were composed as listed in Tables 3 and 5. Moreover, test results based on these formulations are presented in Tables 3-5.

[070] Table 3 Viscosity and Peel Adhesion for Hot-Melt Adhesive For- mulations Benchm Fl F2 F3 F4 ark* Polymer D Linear 20 Polymer A 20 Polymer B 20 Polymer C 20 ESOC) REZ 5600 56.5 56.5 56.5 56.5 CALSOL 5550 23 23 23 23 IRGANOX 1010 0.5 0.5 0.5 0.5 Viscosity @ 149°C (300°F) (cps) 1480 3000 4790 5710 2960 Spiral Peel Peak (grams) 249 293 246 285 259 Spiral Peel Average (grams) 148 154 121 136 134 [071] *Benchmark Formulation 20 phr SB polymer (Dexco VECTORS 4211) 55.6 phr ESCOREZ 5400 9 phr ENDEX 160 15 oil 0.4 IRGANOX 1010 [072] To test thermal stability 300 grams of each adhesive was placed into a glass jar and conditioned at 300°F for 96 hours. At 24 hour intervals, 10.5 grams was removed from each and checked for Gardner color and Brookfield viscosity at 300°F.

[073] Table 4 Thermal Stability Date Benchmar F1 F2 F3 F4 k Initial Viscosity @ 149°C (300°F) (cps) 1480 3000 4790 5710 2960 24 hours 1390 2520 4140 4940 2475 48 hours 1160 1970 3170 4210 1660 72 hours 985 1580 2810 3010 1130 96 hours 910 1410 2150 2570 970 % change-38-53-55-55-67 Initial Gardner Color 2 1 1 1 3 24 hours 3 2 1 2 3 48 hours 4 3 2 2 4 72 hours 5 4 2 3 5 96 hours 6 5 3+ 4+ 5 [074] To test elatic attachment 2000 gram batches of each adhesive were prepared in a high shear sigrm blade mixer set at 163°C (325°F). The adhesives were then transferred to a melter set at 149°C (300°F) and applied with a Nordson spiral spray system onto three strands of Lycra thread using the method described in Werenicz U. S.

Patent No. 4,842, 666. After equilibrating to room temperature, the laminions were then stretched to 95% of full extension and fastened to a rigid piece of cardboard. The ends of the elastic were then cut through the polyethylene film and the test board placed in an incubator set at 38°C (100°F). After a period of four hours, the test board was removed and the percent creep calculated using the formula: Initial length minus final length divided by initial length times 100.

[075] Table 5 Elastic Attachment Viscosity and % Creep Data F1 F2 F3 F4 Comparison* Polymer D Linear 23 Polymer A 23 Polymer B 23 Polymer C 23 ESCOREZ 5400 56.5 56.5 56.5 56.5 CALSOL 5550 13 13 13 13 ENDE 160 7 7 7 7 IRGANOX 1010 0.5 0.5 0.5 0.5 Brookfield viscosity @ 149°C (300°F) (cps) 4940 9350 13900 13900 5725 % creep 16.8 13.3 11.6 13 12.3 LYCRA XA-940 (% 300 300 300 300 300 stretch) [076] *Comparison Formulation 20 phr as polymer (Dexco VECTOR 4211) 55.6 phr ESCOREZ 5400 9 phr ENDEX 160 15 oil 0.4 IRGANOX 1010 [077] Compared to the S radial polymer comparison, both the linear and the radial mixed mid-block polymers of the present invention had excellent resistance to viscosity degradation. The radial polymers showed superior color stability which is an important property in non-woven article manufacturing. For elastic attachment to LYCRA XA-940, the radial polymers A-C showed superior creep properties compared to the linear versions. One unanticipated result was that even at very high viscosity, the radial mixed-midblock polymers had excellent sprayability. This is an important feature of the radial mixed-midblock polymers and allows a wider formulation window.