THERMOPLASTIC ELASTIC ADHESIVE

Abstract: Hot melt pressure sensitive adhesive prepared from (a) a rubbery block copolymer containing a rubbery polyisoprene midblock portion and a plurality of crystalline poly(vinylarene) endblocks; (b) a tackifying resin compatible with the midblock portion and (c) poly(alkyloxazoline); and (d) an antioxidant. These adhesives are characterized by exceptional elastic memory, high tensile strength and excellent peel adhesion and, as such, are especially suited for use in elasticized bands for disposable items.

Description: THERMOPLASTIC ELASTIC ADHESIVE Elasticized leg and waist bands have gained increasing popularity in the areas of disposable applications, such as in disposable diapers, incontinent pads and hospital gowns. A commercially viable approach for use on high speed production equipment was developed by Buell and disclosed in U.S. Pat. No. 4,081,301 issued March 28, 1978. According to this method, continuous bands of elastic, maintained in a stretched position, are glued or heat sealed onto the continuous web from which the disposable diapers are made. Recognizing that greater efficiency could be achieved if the adhesion of the elastic band to the substrate did not require a separ­ ate adhesive or heating means, attempts have been made to prepare ad­ hesives which exhibit the required degree of elasticity without sacri­ ficing the necessary adhesive and cohesive properties. In U.S. Pat. No. 4,259,220 issued March 31, 1981, Bunnelle et al. proposed viscoelastic hot melt pressure sensitive compositions prepared from a rubbery block copolymer containing a rubbery polyisoprene midblock portion and a plurality of crystalline poly(vinylarene) endblocks; and two different resins, one of which is a tackifying resin compatible with the midblock and the other a reinforcing resin for the endblock portion of the copolymer. The present invention is directed to thermoplastic hot melt adhe­ sive compositions which function as elastic bands for disposable items and particularly for leg or waist band closures on disposable diapers. In particular, the present invention discloses a hot melt pressure sensitive adhesive prepared from (a) a rubbery block copolymer containing a rubbery polyisoprene midblock portion and a plurality of crystalline poly(vinylarene) endblocks; (b) a tackifying resin compatible with the midblock portion; (c) poly(alkyloxazoline), and (d) an antioxidant. The resulting adhesives are characterized by exceptional elastic memory, high tensile strength and, in particular, are by excellent peel adhesion to the polyethylene and nonwoven substrates generally used for disposable applications. While not wishing to be bound by theory, it is believed that these properties are achieved from the inherent adhesive strength contributed by the high molecular weight poly(alkyloxazoline) polymer to the entire hot melt adhesive system rather than be a mere reinforcement of any portion of the block copolymer. The primary component of the adhesive compositions used in the present invention are block copolymers having the general configuration: A-B-A or A-B-A-B-A-B- wherein the polymer blocks A are non-elastomeric polymer blocks which, as homopolymers have glass transition temperatures above 20°C., while the elastomeric polymer blocks B are isoprene. Further, they may be linear or branched. Typical branched structures contain an elastomeric portion with at least three branches which can radiate out from a cen­ tral hub or can be otherwise coupled together. The non-elastomeric blocks which make up approximately 17 to 75%, by weight of the block copolymer may comprise homopolymers or copoly­ mers of vinyl monomers such as vinyl arenes, vinyl pyridines, vinyl halides and vinyl carboxylates, as well as acrylic monomers such as acrylonitrile, ethylacrylonitrile, esters of acrylic acids, etc. Mono­ vinyl aromatic hydrocarbons include particularly those of the benzene series such as styrene, vinyl toluene, vinyl xylene, ethyl vinyl ben­ zene as well as dicyclic monovinyl compounds such as vinyl naphthalene and the like. Other non-elastomeric polymer blocks may be derived from alpha olefins, alkylene oxides, acetals, urethanes, etc. Styrene is preferred. Typical of the rubbery block copolymers useful herein are the poly­ styrene-polyisoprene-polystyrene types containing at least 17% styrene which may be prepared using methods taught, for example, in U.S. Pat. Nos. 3,239,478 issued March 8, 1966 to Harlen; 3,427,269 issued Feb. 11, 1969 to Davis et al.; 3,700,633 issued Oct. 24, 1972 to Wald et al.; 3,753,936 issued Aug. 21, 1973 to Marrs; and 3,932,327 issued Jan. 13, 1976 to Naylor. Alternatively, they may be obtained from Shell Chemical Co. for example under the tradename Kraton D1111 or from Phillips under the tradename Solprene 423. If desired, a portion of these high styrene containing copolymers can be replaced by those of lower styrene contents as, for example, Kraton 1107 or Kraton 1117 in order to adjust the melt indeces for use on various manufacturing equipment. In formulating adhesives for use herein, the block copolymer should be used in an amount of 35 to 75%, preferably 40 to 60%, by weight of the adhesive. The tackifying resins which are present in the hot melt adhesive used herein serve to extend the adhesive properties of the block copoly­ mer. As contemplated, the term "tackifying resin" comprise on any tackifying resin compatible with the isoprene midblock and includes: (1) polyterpene resins having a softening point, as determined by ASTM method E28 58T, of from 60° to 140°C. the latter polyterpene resins generally resulting from the polymerization of terpene hydrocarbons, such as the bicyclic mono-terpene known as pinene in the presence of Fridel-Crafts catalysts at moderately low temperatures. (2) phenolic-modified terpene resins, for example, the resin product resulting from the condensation in an acidic medium, of a bicyclic terpene and a phenol; (3) aliphatic petroleum hydrocarbon resins having a Ball and Ring softening point of from 60° to 140°C., the latter resins resulting from the polymerization of monomers consisting primarily of olefins and diolefins; and (4) hydrogenated copolymers of alpha-methyl styrene and styrene having a softening point of 78 to 125°C. Especially preferred are resins which are polymerized from a stream of aliphatic petroleum derivatives in the form of dienes and mono­ olefins having 5 or 6 carbon atoms generally in accordance with the teachings of U.S. Pat. No. 3,577,398 issued May 4, 1971 to Pace et al. The resulting solid hydrocarbon resin consists essentially of polymerized structures derived from these aliphatic dienes and mono­ olefins of 5 or 6 carbon atoms and since the dienes are more reactive, at least 40 percent by weight and preferably a major proportion of said structures are derived from the dienes. In this type of resin, the dienes are piperylene and/or isoprene. However, in some formulations, the percentage of isoprene is extremely low. In the solid resin of this embodiment the molecular weight may range between 900 and 1300 with the mean number average molecular weight being about 1100. This solid resin also has a softening point in the neighborhood of 100°C. In one preferred form, i.e., Wingtack 95 offered by Goodyear Chemical Company, the softening point is 95°C. These tackifying resins are present in an amount of 10 to 45, preferably 20 to 40%, by weight of the adhesive. While any of the poly (alkyloxazoline) compounds disclosed in U.S. Pat. No. 4,474,928 issued Oct. 1, 1984 to Hoenig et al may be used herein, the most readily available is poly (2-ethyloxazoline), a compound consisting primarily of repeating units (I) which is prepared by the ring opening polymerization of 2-ethyl-2-oxazoline (II). The ring-opening polymerization of 2-ethyl-2-oxazoline is generally conducted in the presence of a cationic polymerization catalyst at a reaction temperature of 0°C.-200°C. Typical catalysts include strong mineral acids, organic sulfonic acids and their esters, acidic salts such as ammonium sulfate, Lewis acids such as aluminum tri­ chloride, stannous tetrachloride, borom trifluoride and organic diazoalumfluoroborates, dialkyl sulfates and other like catalysts. This ring-opening polymerization is further described by Tomalla et al. J. Polymer Science, 4,2253 (1966); Bassiri et al. Polymer Letters, 5,871 (1967); Seeliger, Ger. 1,206,585; Jones and Roth, U.S. Pat. No. 3,640,909 issued Feb. 8, 1973; and U.S. Pat. No. 3,483,141 issued Dec. 9, 1969 to Litt et al. The pre-hydrolyzed polymer thereby obtained are linear, N-acylated polyethylamines having a molecular structure consisting essentially of repeating units (I). These polymers can be used as such or the partially hydrolyzed (deacylated) by contact with a strong acid, such as HCl, followed by contact with a base, such as NaOH. Alternatively, this polymer is available from Dow Chemical Corp. under the name PEOx. Typically, the poly(alkyloxazoline) has a molecular weight within the range of 1,000 to 1,000,000. In the present invention, it is preferable to use poly(2-ethyloxazoline) having a molecular weight within the range of 50,000 to 500,000 with the lower molecular weight range being especially preferred. The poly(alkyloxazoline) is used in the adhesive in an amount of 5 to 25%, preferably 10 to 20% by weight. Antioxidants for use herein include high molecular weight hindered phenols and multifunctional phenols such as sulfur and phosphorous-con­ taining phenols. Hindered phenols are well known to those skilled in the art and may be characterized as phenolic compounds which also con­ tain bulky radicals in close proximity to the phenolic hydroxyl group thereof. In particular, tertiary butyl groups generally are substi­ tuted onto the benzene ring in at least one of the ortho positions relative to the phenolic hydroxy group. The presence of these radicals in the vicinity of the hydroxyl group serves to retard its stretching frequency and correspondingly, its reactivity. This steric hindrance thus provides the phenolic compound with its stabilizing properties. Representative hindered phenols include: 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzylbenzene; pentaerythrityl tetrakis-3(3,5-di-tert-butyl-4-hydroxy­ phenyl)-propionate, n-octadecyl-3(3,5-di-tert-butyl-4-hydroxyphenyl)pro­ pionate, 4,4'-methylenebis (2,6-di-tert-butylphenol); 2,2'-methylenebis (4-methyl-6-tert-butylphenol); 4,4'-thiobis(6-tert-butyl-o-cresol); 2,6-di-tert-butylphenol; 6-(4-hydroxyphenoxy)-2,4-bis(n-octylthio)-1, 3,5 triazine; 2,4,6-tris(4-hydroxy-3,5-di-tert-butylphenoxy)-1,3,5-­ triazine; di-n-octadecyl-3,5-di-tert-butyl-4-hydroxy-benzylphosphonate; 2-(n-octylthio)3,5-di-tert-butyl-4-hydroxy-benzoate, and sorbitol hexa-[3-(,5-di-tert-butyl-4-hydroxy-phenyl)propionate.] The anti­ oxidant and is generally used at levels of 0.2 to 2% by weight. The performance of these antioxidants may be further enhanced by utilizing, in conjunction therewith (1) synergists such, for example, as thiodipropionate esters and phosphites; and (2) chelating agents and metal deactivators such, for example, as ethylenediamine tetraace­ tic acid. Optional additives may be incorporated in minor amounts, generally less than 3% by weight, into the hot melt compositions in order to modify certain properties thereof. Among theses additives may be in­ cluded colorants such as tianium dioxide; fillers such as talc and clay, etc., hydrocarbon process oils; etc. These hot melt adhesive compositions may be formulated using tech­ niques known in the art. An exemplary procedure involve placing the tackifying resin, poly (alkyloxazoline), stabilizer and any optional additives whose presence may be desired in a jacketed mixing kettle, preferably in a jacketed heavy duty mixer of the Baker Perkins or Day type, which is equipped with rotors and thereupon raising the temperature to a range of from 120° to 175°C., the precise temperature utilized depending on the melting point of the particular tackifying resin. When the resin has melted, stirring is initiated and the block polymer, is added, the addition of the polymer being extended over a prolonged period in order to avoid the formation of lumps. Mixing and heating are continued until a smooth, homogeneous mass is obtained. In the examples that follow, the adhesive prepared were subjected t the tests described below: Tensile Strength : Tensile strength values for the elastic of this invention are used as a measure of cohesive strength. A sample of known thickness is elongated at room temperature using an Instron and the tensile strength at 100% elongation is recorded. Samples having tensile strengths of at least about 35 psi (2.5 kg/sq cm) at 100% elongation provide adequate cohesive strength. Dead Load Deformation : The term "dead load deformation" or dead load creep" refers to a measurement of "cold flow" or permanent deformation at one or more fixed test temperatures, e.g., 23°C or 25°C., 40 or 41°C and 49 or 50°C. A sample of known length is suspended vertically in a chamber maintained at the test temperature and a mass is attached to the lower (free) end of the sample. The sample is cut to a size such that the force per unit area is 1500 g/cm². After approximately 3 hours at the test temperature, the sample is removed, the weight is detached, and the sample is allowed to relax under the influence of its own inherent elastomeric forces. The length of the relaxed sample (L₂) is compared to the original length (L₁) and the "dead load creep" (permanent deformation) is determined according to the formula (L₂-L₁)/L₁X100%. Values of less than 20% deformation are considered adequate. T-peel : Peel adhesion values are determined 24 hours after formation of the bond between cold-rolled steel plates and the self-adhering elastic. The bond is formed at room temperature by applying a 0.5" X 20 mil X 3" elastic/adhesive strip onto the steel, backing it with masking tape, and sealing at room temperature using 20 psi (1.4 kg/sq. cm) pressure for 2 seconds. The bond is tested using an Instron. Values of at least about 4 pounds (1.8 kg) per linear inch indicate acceptable tack levels for the more stringent leg banding applications, however lower levels are acceptable for lower tension applications such as waist banding. Polyethylene film and Non Woven peel : A 20 mil film of adhesive is made using a Carver press at 350°F under 8,000 lbs (3200 kg) pressure with a residence time of approximately 15 sec. Then 3 X 1/2" (7.6 X 1.27 cm) samples are cut and backed with masking tape. The samples are bonded to non-woven or 2 mil polyethylene film backed with tape using 2 passes with a 4-1/2 lb. (2 kg) roller. The bond is peeled immediately using an Instron at 12" (30.5 cm) min. Results are recorded in gram per linear inch (grams per centimeter). The following examples will further illustrate the embodiments of the invention. In these examples all parts are by weight and all temperatures in degrees Celsius unless otherwise indicated. Example 1 Adhesives were prepared in a Sigma mixer heated to 170°C by blend­ ing until homogeneous the components in the amounts shown in Table I. The adhesives were then tested as described above with the results also shown in Table I. (1) Molecular weight approximately 50,000 (2) A C₅ terpene resin from Goodyear Chemical Company (3) A terpene-phenolic tackifying resin from Reichhold Chemical (4) A hydrogenated styrene-alpha-methyl-styrene resin from Hercules Chemical (5) A hydrogenated dicyclopentadiene resin from Exxon Chemical * Not tested Example 2 Other adhesives were prepared using different amounts of the components as well as blends of the rubber copolymers. Compositions and tests results are shown in Table II. In a similar manner, other thermoplastic hot melt adhesives may be prepared using other poly (alkyloxazolines) including, for example, poly(2-ethyloxazolines) of different molecular weights as well as those poly(alkyloxazolines) where the alkyl group is hydrogen, methyl, propyl, pentyl, cyclohexyl, dodecyl, octadecyl, as well as the various halogenated or ethylenically unsaturated derivatives thereof, such as poly(2-trichloromethyl-2-oxazoline), poly(2-isopropenyl-2-oxazoline), etc.

Claims: 1. A thermoplastic hot melt adhesive composition comprising: a) from 35 to 75% by weight of a rubbery block copolymer containing a rubbery polyisoprene midblock portion and a plurality of crystaline poly (vinylarene) endblocks wherein the polyvinylarene portion consti­ tutes from 17 to 75% by weight of the copolymers; b) from 10 to 45% by weight of a tackifying resin selected from the group consisting of polyterpene resins having a softening point, as determined by ASTM method E28 58T, of from about 60° to 140°C; phenolic -modified terpene resins; aliphatic petroleum hydrocarbon resins having a Ball and Ring softening point from about 60° to 140°C; and hydro­ genated copolymers of styrene and alpha-methyl styrene having a softening point of from 78 to 125°C; c) from 5 to 25% by weight of poly(alkyloxazoline); and d) from 0.2 to 2% by weight of an antioxidant. 2. A method for imparting elongation resistant gathers to portions of a generally non-elastomeric film, or web substrate which comprises contac­ ting a surface of the substrate with a band of a self-adhering elastic composition comprising: a) from 35 to 75% by weight of a rubbery block copolymer containing a rubbery polyisoprene midblock portion and a plurality of crystaline poly (vinylarene) endblocks wherein the polyvinylarene portion constitutes from 17 to 75% by weight of the copolymers; b) from 10 to 45% by weight of a tackifying resin selected from the group consisting of polyterpene resins having a softening point, as determined by ASTM method E28 58T, of from about 60° to 140°C; phenolic­ -modified terpene resins; aliphatic petroleum hydrocarbon resins having a Ball and Ring softening point of from about 60° to 140°C; and hydrogenated copolymers of styrene and alpha-methyl styrene having a softening point of from 78 to 125°C; c) from 5 to 25% by weight of poly(alkyloxazoline); and d) from 0.2 to 2% by weight of an antioxidant. 3. Claims 1 and 2 wherein the block copolymer is a polystyrene-­ polyisoprene-polystyrene copolymer. 4. Claims 1 and 2 wherein the polystyrene portion comprises at least 22% by weight of the block copolymer. 5. Claims 1 and 2 wherein the poly(alkyloxazoline) is poly(2-ethyl­ oxazoline). 6. Claims 1 and 2 wherein the block copolymer is present in an amount of 40 to 60% by weight of the adhesive. 7. Claims 1 and 2 wherein the tackifying resin is present in an amount of 20 to 40% by weight of the adhesive. 8. Claims 1 and 2 wherein the tackifying resin is a synthetic poly­ terpene resin having a softening point of 60 to 140°C. 9. Claims 1 and 2 wherein the poly(alkyloxazoline) is present in an amount of 10 to 20% by weight of the adhesive. 10. Claim 5 wherein the poly(2-ethyloxazoline) has a molecular weight of about 50,000.

Inventors: SALERNO, CATHERINE E.
Applicants: NATIONAL STARCH AND CHEMICAL INVESTMENT HOLDING CORPORATION
Application year: 1986
Main classification: C09J109
IPC codes: C09J153, C08L53