Hot Melt Adhesive for Elastic Banding

In the garment industry, vulcanized rubber in sheet or thread form is typically used for elastic banding purposes. Traditionally, the vulcanized rubber has been sewn, woven, or bonded to the garment or discrete unit of material. Crosslinked synthetic rubbers can be used in place of vulcanized natural rubber for this purpose.

Sewing or weaving or similar attachment techniques are not well suited to modern, high-production processes and may even be inconvenient or cumbersome to use in making homemade garments. A more efficient technique for attaching an elastic band involves the use of a heat-activated coating on the elastic or a separate adhesive, which adhesive can be coextensive in length with the elastic or merely applied in spots. Taking his cue from this more efficient approach, the patentee of U.S. Pat. No. 4,081,301 (Buell), issued Mar. 28, 1978 developed a high-production process particularly well suited to providing disposable diapers with elastic leg bands. According to the Buell patent, glue applicators can apply adhesive along the length of continuous bands of elastic which are applied, in a stretched condition, to the continuous web from which the disposable diapers are made. The patent further suggests that the elastic band or ribbon can be coated with a heat-activated adhesive prior to contact with the web. Still another suggestion relates to the use of a heat-sealable elastic ribbon which can be adhered to the web with the aid of a suitable heating means.

Still greater efficiency (with a concurrent simplification of the overall elastic banding process) could theoretically be provided if the adhesion of the elastic band to the substrate did not require either a separate adhesive or a heating means. Unfortunately, there are few guidelines in the prior art for one who would attempt to formulate an adhesive which could itself be an elastic band. It is known that "pressure-sensitive" adhesives do not require heat, solvents, moisture, or the like to form a reasonably strong adhesive bond under normal ambient conditions. It is also known that such adhesives possess a degree of stretchiness, cohesion, and elasticity as well as adhesion characteristics. However, the four-fold balance of adhesion, cohesion, stretchiness, and elasticity is a delicate one, and any substantial increases in the last three of these properties can result in unacceptable losses of adhesion.

Since the invention of pressure-sensitive adhesives, literally decades of research effort have gone into the investigation of the aforementioned four-fold balance and the development of tests for reproducibly measuring the desired properties. For example, the adhesive bond strength of a pressure-sensitive adhesive can be measured by 180.degree. peel resistance tests such as PSTC-1. The adhesive tack can be measured, for example, by probe tack tests such as A.S.T.M. D2979. Cohesion and stretchiness of adhesives can be measured with modern tensile testing equipment.

A particularly unusual problem may be encountered when the adhesive is in a temperature environment which is continuously above normal ambient or room temperature. It has been found that some pressure-sensitive adhesives have their four-fold balance significantly altered when the environment is characterized by a modestly elevated temperature. This finding is of great importance in the case of garments or body-encircling members which are continuously exposed to body temperature (e.g. 37.degree. C.).

Most modern pressure-sensitive adhesives ("PSA's") are applied to a substrate by one of three techniques: coating from an organic-solvent based solution (e.g. solvent casting), coating from a suspension or dispersion such as an aqueous latex, and coating or extruding of a hot melt pressure-sensitive composition. The hot melt technique has a number of advantages; for example, thicker layers of adhesive are readily obtainable, solvent recovery is unnecessary, and drying or "setting" time is minimal or nonexistent. Research activity in the field of hot melt pressure-sensitive adhesives has been very extensive, and even a representative citation of references drawn from this field would be difficult to provide. The following selection of patents and literature is believed to be reasonably representative.

Kirk-Othmer Encyclopedia of Chemical Technology, 2nd Edition, Volume 1, John Wiley & Sons, Inc., New York, N.Y., 1963, pages 381-384.

"SOLPRENE.RTM. 418 in Pressure Sensitive Adhesives", Bulletin 304 of Phillips Chemical Co., a division of Phillips Petroleum Company.

It has now been found that a greatly simplified method for imparting elasticity to a relatively inelastic substrate can be provided if one extrudes a band comprising a suitable viscoelastic hot melt pressure-sensitive adhesive, cools the thus-extruded band to a temperature below its softening point but above its glass transition temperature, and brings the thus-cooled band into contact with the substrate to form a pressure-sensitive adhesive (PSA) bond, typically by means of pressure only, although modest amounts of heat can be used also, if desired. The band of hot melt can be pre-extruded and formed into a convoluted roll of hot melt pressure-sensitive adhesive tape. If the pre-extruded technique is used, a cooling step is ordinarily unnecessary, since the roll will ordinarily be stored and unreeled at ordinary ambient temperatures. Extruding the band of hot melt pressure-sensitive adhesive is by far the most practical approach when applying the elastic band to a continuous non-elastomeric film, membrane, or web substrate which is subsequently cut into portions suitable for use as elastic-banded articles. The convoluted tape approach is very practical when a length of tape is to be applied to an individual article.

In the context of this invention, a suitable hot melt pressure-sensitive adhesive will have viscoelastic behavior because of its glass transition and/or softening point characteristics and particularly because of a microstructure comprising the combination of crystalline domains with rubbery domains. The crystalline domains contribute a pseudo-crosslinked character and greater elasticity and cohesion. High elasticity and elastomeric behavior are typically manifested by a storage modulus (G') which is higher, and a loss tangent (tan .delta.or G"/G') which is lower, than most conventional PSA's. However, the storage modulus values cannot be so high as to preclude viscoelastic behavior in the temperature range of 25.degree.-50.degree. C. A viscoelastic solid, under stress, has some of the properties of a highly viscous liquid (e.g. "creep" or "cold flow") as well as some of the properties of an elastomer. A PSA with suitable "creep" or "cold flow" properties will have some tendency to flow in the temperature range of 25.degree.-50.degree. C., but this tendency should be kept within limits, as manifested by a limited range of loss modulus (G") values.

The following are considered to be illustrative values for the G", G', and tan .delta. (tan .delta.=G"/G') of a suitable PSA.

Some frequency-dependence of these values can be observed, but extreme temperature dependence is not desirable. For example, a loss of more than 50.times.10.sup.4 dynes/cm.sup.2 in G' at 50.degree. C., as compared to 25.degree. C. indicates the likelihood of "heat set" or inadequate elastomeric behavior at moderately elevated temperatures. Permanent deformation due to elongation should not exceed about 1.5 times the original length of a sample of the PSA (i.e. a permanent increase in length equal to 50% of the original length) throughout the 25.degree.-50.degree. C. range, using the dead load creep test described subsequently.

FIG. 1 is a schematic illustration of a typical apparatus and typical method steps used in a preferred embodiment of this invention.

FIG. 2 is a similar schematic illustration of another embodiment of this invention.

When used in this application, the following terms have the indicated meanings.

"Pressure-sensitive adhesive" denotes those adhesives which bond almost instantaneously when contact pressure is applied to force the mating surfaces together. Such adhesives have rather high cohesive strength, such that, if the adhesive is peeled away from a smooth surface to which it has adhered, no apparent offsetting occurs and no appreciable residue remains on the smooth surface. True pressure-sensitive adhesives (PSA's) need not be in a liquid or molten state in order to have adhesive properties. Similarly, moisture, solvents, heat, or the like are not needed to activate a PSA. Some PSA's have aggressive tack or "quick stick" at room temperature and tend to bond instantly upon contact. The PSA's of this invention, on the other hand, typically require light pressure to form a bond of appreciable strength. In other words, the tack is relatively non-aggressive and ordinarily would not be sensed until some pressure were applied to the surface of the PSA, causing it to "cold flow" in the manner of most viscoelastic materials. Thus, a PSA of this invention has a rheology which permits sufficient flow under pressure to form a strong adhesive bond while nevertheless maintaining a high level of cohesion, stretchiness, and elasticity.

"Hot melt" refers to thermoplastic solids with reasonably stable properties in the molten state, which are easily melted at modestly elevated temperatures (e.g. temperatures above 65.degree. C.) and/or easily extruded, and which can be melted and resolidified a number of times without excessive degradation of the thermoplastic properties. A "hot melt pressure-sensitive adhesive" or "hot melt PSA" refers to a hot melt adhesive using PSA characteristics at temperatures below the softening point and above the glass transition temperature of the hot melt.

"Softening point" refers to a specific temperature or range of temperatures which can be determined by any of the standard softening point tests such as the ring and ball ("R & B") test. Accordingly, the term "softening point" includes and subsumes "softening range".

"Elastomer" and "elastomeric" refer to a material which, in the form of an unsupported film or layer can be elongated to at least 100% of its original length and which will return with force to substantially its original length when permitted to contract spontaneously. Thus, this invention contemplates as "elastomers" those materials which would be defined as "elastomeric" by the American Society for Testing and Materials (A.S.T.M.). "Non-elastomeric" materials are those which exhibit some degree of significant deformation or "set" when elongated 100% of their original length or less; that is, such non-elastomeric materials typically do not provide the elongation-resistant forces of an elastomer.

"Essentially hydrocarbon resin" refers to a resin in the molecular weight range of a few hundred up to several thousand (e.g. 8,000) which is obtained or synthesized from rather basic hydrocarbonaceous materials such as petroleum, coal tar, turpentine, olefins and other unsaturated simple hydrocarbons, and the like. In the context of this invention, an "essentially hydrocarbon resin" need not be a hydrocarbon in the strictest sense of the term and may contain oxygen, nitrogen, or sulfur, e.g. as hetero-atoms or as atoms of functional groups. Thus, an "essentially hydrocarbon resin" can be made from a monomer such as coumarone (also known as benzofuran). And, in industrial practice, coumarone-indene resins are typically referred to as "hydrocarbon resins".

The terms "loss tangent" (tan .delta. or G"/G'), "storage modulus" (G'), "loss compliance" (J") and "storage compliance" (J') are defined according to established principles of dynamic mechanics. These rheological quantities are measured on samples approximately 2.5 mm in thickness placed between 25 cm parallel plate fixtures of a Rheometrics Mechanical Spectrometer (RMS). The sample was allowed to equilibrate with the test temperature (e.g. 25.degree. C. or 50.degree. C.). A minicomputer accurately governs the application of a 5% peak-to-peak shear strain to the sample. The frequency of the application can be accurately controlled to a fraction of a Hertz (Hz). The values of the complex modulus (G*) and loss tangent are calculated by the computer from geometry factors, peak-to-peak amplitude of the torque signal, and phase lag of the torque output wave. The definition of loss tangent and the relationship between G*, G', and G" provide two equations in two unknowns which can be solved by the computer to provide G" and G', since G* and loss tangent are both known values calculated as described previously. The value for J' is given by the reciprocal of G' divided by the expression 1+tan.sup.2 .delta.; the value of J" is given by the reciprocal of G" divided by 1+(tan.sup.2 .delta.).sup.-1. For any of these values, the frequency in Hz (e.g. 0.25 Hz or 0.01 Hz) must be specified. Other instruments for measuring these rheological properties over a range of frequencies are known, e.g. the "RHEOVIBRON".

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.degree. C. or 25.degree. C., 40.degree. or 41.degree. C., and 49.degree. or 50.degree. C. A sample of known length is suspended vertically in a chamber maintained at the test temperature and a weight (e.g. 1500 grams) 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/cm.sup.2. 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 (L.sub.2) is compared to the original length (L.sub.1) and the "dead load creep" (permanent deformation) is determined according to the formula (L.sub.2 -L.sub.1)/L.sub.1 .times.100%.

Turning now to the Drawing, FIG. 1 illustrates the use of a pre-tensioned, cooled band of hot melt PSA to bond two continuous substrates into an assembly having gathers all along the bond line. A hot melt reservoir 11 extrudes an elongated extrudate (referred to herein as a "band") by forcing the hot melt PSA material in reservoir 11 through an extrusion die 12. The hot melt PSA band 13 comes into contact with chill rolls 15 and 16 almost immediately after the extrusion step, so that band 13 will be cooled to a temperature below its softening point, e.g. to a normal ambient temperature such as 20.degree.-25.degree. C. (Typically, the hot melt PSA will be formulated to have a glass transition temperature below normal ambient temperatures.) Chill rolls 15 and 16, in addition to cooling band 13, also advance it toward tensioning rolls 17 and 18. Accordingly, the portions 23 and 33 of band 13 which are on either side of tensioning rolls 17 and 18 will be under tension and will be in an essentially elongated state. Tensioning rolls 17 and 18 advance portion 33 of band 13 to nip rolls 27 and 28. Substrates 22 and 23 are all the while being continuously unreeled from storage rolls 21 and 31, so that the pretensioned, cooled band of hot melt PSA 33 and substrates 22 and 32 all enter the nip provided by rolls 27 and 28 to be formed into the composite or assembled product 40 (i.e. the banded substrates). Although nip rolls 27 and 28 can be heated to a moderately elevated temperature, in the preferred embodiment of this invention, the modest pressure provided by the nip rolls is all that is needed to adhesively bond substrate 22 to substrate 32 with the hot melt PSA band 33. Since the nip rolls 27 and 28 provide only line contact with the composite or assembly comprising substrate 22 and 32 and adhesive 33, it is difficult to express the pressure applied by these rolls in conventional terms such as Kg/m.sup.2 or the like. Light pressure on the order of tens or hundreds of grams per square centimeter can be sufficient; however, there is almost no upper limit on the pressure applied by nip rolls 27 and 28 so long as the rolls themselves or the substrates 22 and 32 are not damaged. The hot melt PSA in band 13 can be formulated to take into account the amount of pressure available at nip rolls 27 and 28.

Since band 13 is a viscoelastic hot melt PSA which meets the definition of an elastomer, it will tend to contract spontaneously with force if the tension applied to banded substrates 40 is less than the tension on portion 33 of band 13. For example, additional rolls or conveying devices (not shown) can be used on banded substrates 40 merely to move the composite product along toward a cutting station and not exert any significant tension upon the banded substrates. In such a situation, the portion 43 of band 13 on the exit side of nip rolls 27 and 28 will spontaneously cause the formation of gathers 41 all along the line of the adhesive bond between band 43 and substrates 22 and 32.

The banded substrate product 40 can be cut into individual elastic-banded articles such as disposable diapers by techniques known in the art.

In the embodiment of the invention shown in FIG. 2, pre-tensioning of the hot melt PSA band 113 is not required. As in FIG. 1, band 113 is extruded from reservoir 111 through extrusion die 112. A single substrate 132 is continuously unreeled from storage roll 131. Substrate 132 is taken up by vacuum chill roll 127 at a point which permits pre-gathering of the substrate. Teeth-like projections 129 on the surface of vacuum chill roll 127 create the flutes or gathers 141 in substrate 132. The two chill rolls 127 and 128 serve to cool band 113 in a manner analogous to the action of chill rolls 15 and 16 of FIG. 1; in addition, these chill rolls apply light pressure to the composite of the band 113 and the pre-gathered substrate 132, so that the pressure-sensitive adhesive bond between band 113 and substate 132 is formed without permanently flattening out gathers 141. This result occurs because, in the composite emerging from the exit side of rolls 127 and 128, band 113 tends to be bonded only to the peaks of gathers 141.

The foregoing methods are particularly well suited to high-volume production techniques using continuous substrates. For low-volume or batch production, the elongation-resistant gathers can be imparted to portions of the substrate by means of a pressure-sensitive adhesive tape. In this technique, the desired length of tape is simply unreeled from a convoluted roll of hot melt pressure-sensitive adhesive, which adhesive has been applied to a flexible continuous backing. For good results in this technique, the pressure-sensitive adhesive tape should have an extruded layer at least 50 micrometers (.mu.M) in thickness, more typically at least 75 or 100 .mu.m in thickness. If the flexible continuous backing is relatively inelastic as compared to the hot melt PSA layer of the tape, the backing should have release characteristics, so that it can be delaminated from the tape structure in the manner of a transfer tape. The cohesive, self-supporting PSA layer removed from the tape can be elongated and applied to, for example, a garment in the stretched condition. Gathers will be introduced into the garment all along the lines of the adhesive bond after pressure-sensitive bonding has been completed. A modest amount of pressure can be applied by machine or by hand to insure a strong adhesive bond. A non-sticking coated roller or the like can be used to apply the pressure, particularly superior non-stick properties being obtained with fluoropolymer or, less preferably, silicone coatings.

Delamination of the tape is not necessary if the backing of the pressure-sensitive adhesive tape is itself highly elastomeric. Such a tape structure can be provided, for example, by coextruding the backing and the PSA through a single extrusion die. The resulting stretchy tape can be rolled upon itself or convoluted, particularly if the exposed side of the elastomeric backing is treated so as to be essentially non-sticky. Special sizings and the like which prevent the formation of a PSA bond are known in the art and can be employed for this purpose.

Viscoelastic hot melt pressure-sensitive adhesives (PSA's) of this invention preferably comprise a rubbery block copolymer and at least two different types of resins which associate with different parts of the rubbery block copolymer molecule. The resin which associates with the crystalline vinyl arene end blocks of the block copolymer tends to increase the size of these crystalline domains, thereby, it is believed, decreasing the temperature dependence of the elastic aspect of the viscoelastic behavior of the PSA. (However, this invention is not bound by any theory.) The adhesive character of the PSA is believed to be dependent in part upon the high-viscosity liquid character of the PSA within the 25.degree.-50.degree. C. temperature range. A characteristic of a viscous liquid is that it will yield to stress, and at least some strain (in an oscillating stress situation) will be up to 90.degree. out of phase with the stress. By contrast, in a perfectly elastic solid the stress and strain would always be in phase. The previously given broad ranges of G", G', and loss tangent (and particularly the preferred and optimum ranges given subsequently) are believed to indicate a highly effective balance of viscoelastic properties in the 25.degree.-50.degree. C. range, whereby at least some elasticity is provided (note the loss tangent<1 and the G' of 65.times.10.sup.4 dynes/cm.sup.2 or more), but in combination with some ability to flow or "wet out" a substrate (note the loss tangent>0.03, the G'<200.times.10.sup.4 dynes/cm.sup.2, and G"<100.times.10.sup.4 dynes/cm.sup.2), without resulting in a viscosity so low as to permit excessive "creep" or cold flow (note the G">5.times.10.sup.4).

Although this invention is not bound by any theory, it is believed that enlargement of crystalline or vinyl arene domains will increase the storage modulus (G') and decrease the dead load creep, but some PSA behavior (e.g. peel strength) may be lost. Conversely, enlargement of midblock domains and/or increased tackifying of these domains may increase PSA behavior but also increase dead load creep and excessively decrease loss modulus (G") and storage modulus (G') data. The ideal PSA for this invention appears to provide a balance of high-viscosity liquid and elastic solid behavior, resulting in low dead load creep, good PSA properties, and properly balanced rheological data. The G', G", loss tangent, J' and J" data can be considered to be "parameters" of the ultimate properties and dead load creep and PSA characteristics.

A tackifying resin with aliphatic character and relatively minimal aromatic character can associate with the midblock of the block copolymer and, if properly selected, help provide this viscoelastic balance in hot melt PSA behavior. Thus, a suitable viscoelastic hot melt PSA will typically comprise:

(1) a rubbery block copolymer which includes a rubbery midblock portion and which is terminated with crystalline vinyl arene blocks,

(2) 20-150 parts by weight, per 100 parts by weight of said rubbery block copolymer, of a tackifying resin generally compatible with and generally associated with the midblock portion of the block copolymer, and

(3) 10-150 parts by weight, per 100 parts by weight of the rubbery block copolymer, of an aromatic, essentially hydrocarbon resin having a glass transition temperature and a softening point above those of the tackifying resin and the end blocks of the block copolymer, which aromatic resin is generally compatible with and generally associated with the aforementioned end-blocks.

The hot melt pressure-sensitive adhesive will typically have a ball and ring softening point within the range of 65.degree. to 240.degree. C. It will exhibit elastomeric behavior above its glass transition temperature and particularly for extended periods at body temperature (e.g. 37.degree. C.).

In addition to the block copolymer and resins, the hot melt PSA can contain the usual antioxidants or stabilizers and essentially inert ingredients which do not have a significant effect upon the properties of the combination of the rubbery block copolymer and the resins. For example, minor amounts of fillers and pigments can be included in the hot melt PSA, typically in amounts less than 5% by weight of the total hot melt PSA composition. Substantially inert extenders can also be included in the composition, e.g. the typical hydrocarbon process oils. The amount of process oil will typically also be kept below 5 weight-% of the compositions, since large amounts of oil can detract from the elastic recovery characteristics of the PSA.

Typical antioxidants useful in PSA's of this invention include the pentaerithritol phosphite ester type (e.g. di[stearyl] pentaerithritol diphosphite), the hindered phenol or polyphenol type, and the like. Typical hindered phenol-type antioxidants include those in which a phenolic (i.e. hydroxyphenyl or hydroxybenzyl) group or groups is or are substituted on a short hydrocarbon chain, and the hydroxy group of the phenolic substituent is hindered by nearby or adjacent alkyl groups substituted on the phenol nucleus. Such structures can be obtained, for example, by alkylating or styrenating hydroxyphenyl compounds such as phenols and cresols.

Typical pigments useful in formulating PSA's of this invention include titanium dioxide, typically having a particle size in the sub-micrometer range, and similar finely divided materials. Fillers may tend to be a bit coarser in particle size, though still typically smaller than 40 .mu.M (minus 325 U.S. mesh), e.g. finely ground calcium salts or silicates.

The following description of the preferred ingredients of hot melt PSA's of this invention will concentrate on the block copolymers and the resins.

A number of rubbery block copolymers can be tackified to produce hot melt PSA compositions, as can be seen from the disclosures in the aforementioned U.S. Pat. Nos. 3,686,107, 3,736,281, 3,827,999, 3,935,338, 3,954,692, and 4,089,824. Additional disclosures of this type can be found in British Pat. No. 1,405,786 and trade literature of Phillips Petroleum Company and Shell Chemical Company. The block copolymers used in this invention are rubbery, i.e. elastomeric. Though these copolymers are thermoplastic in the sense that they can be melted, formed, and resolidified several times with little or no change in physical properties (assuming a minimum of oxidative degradation), they exhibit some of the characteristics of cross-linked or vulcanized rubber. The apparent cross-linked character is provided by the aforementioned crystalline domains provided by vinyl arene terminal blocks or end blocks. The block copolymers also include a rubbery midblock portion which can be either linear or branched. In typical examples of a branched midblock, the midblock portion contains at least three branches which can be radiating out from a central hub or can be otherwise coupled together.

One way of synthesizing such rubbery block copolymers is to begin with the polymerization of the vinyl arene blocks which provide the end blocks. Once the vinyl arene blocks have been formed, they can be linked to elastomeric blocks, which elastomeric blocks are typically obtained by polymerizing unsaturated hydrocarbons, e.g. dienes such as butadiene, isoprene, and dienes of higher hydrocarbons. When an end block A is joined to an elastomer block B, an A-B block copolymer unit is formed, which unit can be coupled by various techniques or with various coupling agents to provide a structure such as A-B-A, which may in reality be two A-B blocks joined together in a tail/tail arrangement. By a similar technique, a radial block copolymer can be formed having the formula (A-B).sub.n X, wherein X is the hub or central, polyfunctional coupling agent and n is a number greater than 2. (If n were 2, the polymer would be A-B-X-B-A, which is equivalent to the A-B-A structure described previously and is essentially linear.) Using the coupling agent technique, the functionality of X determines the number of A-B branches.

Preferably, each block A has an average molecular weight between 1,000 and 60,000, and each block B has an average molecular weight between 5,000 and 450,000. The total molecular weight of the block copolymer is preferably in excess of 100,000 or 200,000, e.g. 300,000. An extensive discussion of rubbery radial block copolymers can be found in the disclosure of the aforementioned U.S. Pat. No. 4,089,824. As pointed out by the '824 patent, the residual unsaturation of the midblock or diene-containing portion of the block copolymer molecule can be hydrogenated selectively so that the content of olefinic double bonds in the radial block copolymers can be reduced to a residual proportion of less than 5% or even less than 2%. Such hydrogenation tends to reduce sensitivity to oxidative degradation and may have beneficial effects upon elastomeric properties.

Preferred block copolymers used in this invention have styrene end blocks and an isoprene midblock portion. The isoprene typically comprises the major amount of the repeating units in the copolymer and can constitute, for example, 70% by weight or more of the copolymer molecule. The midblock, if branched, can have three or more branches, and good results can be obtained with, for example, four, five, or six branches. The midblock can be hydrogenated, if desired.

Linear or A-B-A type block copolymers (including A-B-A-B-A, etc.) are preferably selected on the basis of end block content, large end blocks being preferred. For S-I-S (styrene-isoprene-styrene) block copolymers, a styrene content in excess of 14% by weight is preferred, e.g. 15-30% by weight. A commercially available example of such a linear polymer is KRATON.RTM. 1111 rubber, an S-I-S polymer which contains about 21.5% styrene units, essentially the balance being isoprene units. Thus, the optimum styrene content for linear S-I-S copolymers appears to be greater than 20% by weight. As a result of the higher styrene content, the polystyrene end blocks have a relatively high molecular weight. Typical properties of KRATON.RTM. 1111 are reported to include a tensile strength of 2900 psi (2.0.times.10.sup.6 Kg/m.sup.2), a 300% modulus of 200 psi (1.4.times.10.sup.5 Kg/m.sup.2), an elongation of 1200% at break, a 10% set at break, and a Shore A hardness of 52; the Brookfield viscosity of a toluene solution is 1300 centipoise at room temperature, less than that of KRATON.RTM. 1107.

A variety of resins with tackifying properties are compatible with polymerized diene polymer blocks, including those diene blocks which have been hydrogenated so as to become virtually identical, chemically and physically, to polymerized mono-olefins (e.g. polyethylene, polypropylene, polybutylene, etc.). These "midblock" tackifying resins tend to associate with the rubbery midblock of the linear or radial block copolymer and thereby tend to extend or build up as well as tackify these rubbery domains. Both natural and synthetic "essentially hydrocarbon resins" can be used as "midblock resins", provided that these resins contain at least some aliphatic character, which character can be provided by the aliphatic portion of rosin acids, repeating isoprene or other diene units (e.g. polymerized 1,3-pentadiene), polymerized cycloaliphatics, and the like.

Although esters of polyhydric alcohols and rosin acids will associate with a rubbery midblock, some of these esters tend to detract from the elastic recovery characteristics of the hot melt PSA and are not preferred. It is generally more preferable to use "essentially hydrocarbon resins", particularly the so-called "terpene" resins, i.e. polymers with repeating C.sub.5 H.sub.8 or C.sub.10 H.sub.16 units. These polymers can be natural or synthetic and can be copolymers (including terpolymers, etc.), since isoprene is an olefin which can be copolymerized with other olefins. Terpene-phenols have also been produced.

All terpene resins do not work with equal effectiveness in this invention, and synthetic terpenes having a softening point (ball and ring method) of about 80.degree. to about 115.degree. C. are preferred, particularly the commercially available resin known as "WINGTACK" 95. This commercially available terpene resin is reported to be derived from a mixed olefin feedstock as a by-product of isoprene or polyisoprene production. According to U.S. Pat. No. 3,935,338 and South African Pat. No. 700,881, "WINGTACK" 95 (trademark of Goodyear Tire and Rubber Company) is a thermoplastic tackifying resin essentially comprising a copolymer of piperylene and 2-methyl-2-butene which results from the cationic polymerization of 60% piperylene, 10% isoprene, 5% cyclopentadiene, 15% 2-methyl-butene, and about 10% dimer. Other tackifying resins of the same general type typically comprise 20-80 weight-% of piperylene and 80-20 weight-% of 2-methyl-butene.

Good elastic memory or elastic recovery characteristics can be obtained with natural hydrocarbon resins such as "PICCOLYTE D-135" (trademark), a natural dipentene terpene resin. However, this resin is not as effective as the "WINGTACK" 95 in providing good adhesive properties, e.g. good PSTC-1 peel.

The naturally occurring terpenes can be classified as monocyclic (dipentene), dicyclic (pinene), or acyclic (micrene). A small amount of cyclic character is not detrimental in the context of this invention. A significant amount of aromatic character in the terpene resin is, however, ordinarily avoided, if such aromatic character is sufficient to interfere with the midblock association properties of the resin.

As explained in British Pat. No. 1,405,786, resins with aromatic character tend to associate with the vinyl arene end blocks. Such "end block" resins include the coumarone-indenes, polystyrene, the polymethylstyrenes, the polyindenes, and other resins containing mono or polycyclic aromatic groups. Such resins are commercially available, e.g. as "PICCOTEX 75" (low molecular weight alpha-methylstyrene-vinyl toluene synthetic copolymer), "PICCOTEX 100" (trademark for higher molecular weight version of "PICCOTEX 75"), "PICCOLASTIC D-150" (trademark for polystyrene resin), and "CUMAR" resins (trademark for coumarone-indenes). It is particularly desirable that the "end block" resin have a glass transition temperature and a softening point above those of the end block and of the "midblock" resin. For example, it would ordinarily not be desirable for the glass transition and for significant heat softening to occur in the 80.degree.-110.degree. C. range; hence, "end block" resins with somewhat higher molecular weights and softening points above 115.degree. C. are typically selected. From the standpoint of strong elastic recovery (both initial and aged) and good adhesive properties, the high softening point coumarone-indene resins appear to be by far the most effective. Such resins with softening points within the range of 140.degree.-160.degree. C. are commercially available.

Considerable skill and knowledge already exist in the PSA art with respect to determining proportions of tackifiers and rubbery block copolymers. However, it has been found that these proportions cannot be selected with blind reliance upon prior experience. Nor can one rely too heavily upon PSTC-1 or probe tack values. Probe tack values and initial (immediate) PSTC-1 values have been found to be unreliable or unreproducible indicators of performance in the context of this invention, whereas PSTC-1 values taken 24 hours after the adhesive bond has been formed are relatively reliable and reproducible. "Dead load deformation" ("creep") and the rheological properties (G', G", loss tangent, J', and J") described previously have also been found to be reliable, reproducible parameters of PSA and elastomeric behavior.

Preferred PSTC-1 values determined 24 hours after formation of the PSA bond at room temperature on the standard steel plate using pressure from the standard 2 Kg roller include values in excess of 1 pound per inch width (1 p.i.w.), i.e. more than 450 grams 180.degree. peel force is required to delaminate a tape/steel plate sample wherein the tape sample is 25.4 mm in width. PSTC-1 values in excess of 1500 g/25.4 mm-sample or even 3000 g/25.4 mm-sample can be obtained in practice. These values are believed to indicate a reasonably permanent or semi-permanent bond between a band of PSA and a polymeric film substrate of the type used in disposable diapers.

Tensile strength values for the PSA at 20.degree.-25.degree. C. can be determined as a measure of cohesive strength. Values in excess of 3.times.10.sup.4 Kg/m.sup.2 (e.g. above 33,000 Kg/m.sup.2) can be obtained in practice. "Dead load deformation" values (1500 g/cm.sup.2 for 3 hours at the test temperature) can be well below 100% and even below ##EQU1## throughout the range of 25.degree. C. to 50.degree. C. It is particularly desirable that this deformation remain roughly constant over this temperature range and preferably show a gain of less than 50% (e.g. less than about 35%) at 49.degree. or 50.degree. C., as compared to the room temperature value.

Preferred and optimum rheological parameters are set forth below.

The temperature-dependence of these values over the 25.degree.-50.degree. C. range is preferably minor, as indicated below.

With these physical properties in mind, proportions of rubbery block copolymer and end block and midblock resins can be selected to provide an effective PSA with good elastic recovery or elastic memory characteristics. The following Table of broad, preferred, and optimum proportions assumes that the rubbery block copolymer is either (1) "SOLPRENE.RTM. 418", trademark of Phillips Chemical Company for a radial isoprene-styrene block copolymer having an approximate molecular weight of 300,000, a specific gravity of 0.92, an inherent viscosity in toluene of 1.16, and an isoprene/styrene ratio of 85/15, or (2) KRATON.RTM. 1111, trademark of Shell Chemical Co. for a polystyrene-polyisoprene-poystyrene (S-I-S) block copolymer containing 21.5% styrene, the balance being isoprene. This copolymer has the previously reported tensile strength, 300% modulus, elongation at break, set at break, and Shore A hardness. The Brookfield viscosity in toluene is 1300 cps. SOLPRENE.RTM. 418 in toluene has a viscosity of 2,900 centipoise at 25.degree. C. Hydrogenated linear block copolymers, including those of the styrene-isoprene-styrene type are available according to U.S. Pat. No. 3,827,999; see also U.S. Pat. No. 4,089,824, which discloses the hydrogenated butadiene analog. Such hydrogenated block copolymers can be used in the context of this invention. Compounds of the "KRATON.RTM. G" series (trademark of Shell Chemical Company) have a saturated or essentially saturated ethylene-butylene midblock and, if used, are preferably used in combination with the KRATON.RTM. 1111 or SOLPRENE.RTM. 418 type of rubbery block copolymer.

The aforementioned Table of proportions is set forth below.

The nature of the block copolymer (linear or branched, amount of vinyl arene units, etc.) can have a slight effect upon the optimum formula. The best balance of viscoelastic properties appears to be obtained with 100 parts by weight linear, high-styrene block copolymer (35-45% by weight of the PSA), 70-78 parts by weight of the synthetic polyterpene, and 45-55 parts by weight of a coumarone-indene resin having a softening point in excess of 145.degree. C. About 0.005-1 part by weight of antioxidant should be added to such a formula.

Similar formulas can provide similar results with branched block copolymers and low-styrene linear block copolymers, but it may be useful to extend the ranges up to 85 phr of polyterpene and up to 65 phr of coumarone-indene resin.

The most effective criteria for the selection of amounts and types of ingredients are believed to be (a) rheological properties such as G', G", loss tangent, J' and J", and (b) dead load creep test results. New resins and new block copolymers are constantly being discovered, and the formulation of suitable PSA's can be attempted with new materials by referring to these criteria.

It will be understood that variations in the aforementioned methods and proportions can be made without departing from the spirit and scope of this invention. For example, if the substrate to be elastic banded is heat resistant, the extruded band need not be cooled very much below its softening point and can be applied directly to the substrate while still relatively hot. However, even for such temperature resistant substrates, maintaining the extruded band at or above its softening point is neither necessary nor desirable. In another variation of the method, the extruded band need not be applied as a line or linear bead, but can be "indexed" with a moving web to provide a series of discrete circular or eliptical bands. Such "indexed" bands can provide a hat-banding effect (e.g. for mass-produced surgical caps), a gathered, banded opening for a plastic bag, a waist band, or the like. In the case of disposable diapers, however, it is not necessary for the band to form a complete circle; the essential equivalent of a circular leg band results when the diaper is pinned or snapped together at the child's hips.

The following non-limiting Examples illustrate the preferred practice of this invention. In these Examples, all parts and percentages are by weight unless otherwise indicated. The following raw materials were used in formulating the hot melt PSA's of the Examples:

"SOLPRENE.RTM. 418": trademark for the radial block copolymer described previously.

"SOLPRENE.RTM. 423": another trademark for essentially the same radial block copolymer in a pellet form.

"KRATON.RTM. 1111": trademark of Shell Chemical Co. for the polystyrene-polyisoprene-polystyrene copolymer described previously.

"KRATON.RTM. 1107": trademark of Shell Chemical Co. for polystyrene-polyisoprene-polystyrene linear block copolymer having a styrene/isoprene ratio of 14/86.

"WINGTACK 95": trademark for synthetic polyterpene resin described previously.

"KRYSTALEX.RTM. 3100": trademark of Hercules Inc. for low molecular weight thermoplastic hydrocarbon resin of the alpha-methylstyrene type having a ring and ball softening point of 97.degree.-103.degree. C., an acid number less than 1.0, a bromine number which is typically about 2, a specific gravity at 25.degree. C. of 1.06, and a melt viscosity of 10,000 centipoise (cps) at 128.degree. C., 1,000 cps at 152.degree. C., and 100 cps at 190.degree. C. The softening point substantially below 115.degree. C. (typically not more than 103.degree. C.) indicates a spectrum of molecular weights, with a significant number of resin molecules having molecular weights well below those of the relatively pure, narrow-spectrum coumarone-indene resins which are commercially available, e.g. as the "CUMAR" (trademark) series described subsequently. (It has been found that the higher molecular weight, higher softening point, narrow-spectrum aromatic hydrocarbon resins are preferred for use as "endblock" association with the rubbery block copolymers described previously.)

"CUMAR LX-509": trademark of Neville Chemical Company for coumarone-indene resin having a softening point (by the ring and ball technique of A.S.T.M. E-28) of at least about 155.degree. C., a specific gravity at 25/15.6.degree. C. of 1.114, and an average molecular weight (by osmometry) of 1,120.

"EASTMAN.RTM. Resin H-100": trademark of Eastman Kodak Company for a hydrocarbon resin produced from petroleum feedstock by polymerization, followed by hydrogenation. This particular hydrocarbon resin has an acid number less than 0.1, a density at 23.degree. C. of 1.04 g/cm.sup.3, a Brookfield viscosity at 190.degree. C. of 200 centipoise, a bromine number of 11.1, and a ring and ball softening point (A.S.T.M. E-28) which is reported to be 100.degree. C. and in any event is below 115.degree. C.

"IRGANOX 1010": trademark of Ciba-Geigy for an antioxidant and thermostabilizer of the hindered phenol type.

"SOLPRENE.RTM. 420": trademark for a branched, teleblock copolymer having polystyrene terminal blocks and a structure essentially similar to "SOLPRENE.RTM. 423", except for a lower molecular weight.

"KRATON.RTM. 1102": trademark for S-B-S (styrene-butadiene-styrene) block copolymer having a styrene/butadiene ratio of 28/72, a Brookfield viscosity in toluene solution (25 weight-%) of 1200 centipoise at 25.degree. C., a specific gravity of 0.94, a Shore A hardness of 62, a set at break of 10%, an elongation of 880% (A.S.T.M. method D412 with a tensile tester jaw separation speed of 25.4 cm/min.), a 300% modulus of 281,200 Kg/m.sup.2, and a tensile strength (same A.S.T.M. method as the elongation determination) of 3.23.times.10.sup.6 Kg/m.sup.2 determined on typical films cast from a toluene solution.

"WESTON.RTM. 618": trademark of Borg Warner Corporation for an antioxidant described in U.S. Pat. Nos. 3,047,608 and 3,205,269, i.e. an antioxidant which is reported to be di(stearyl) pentaerythritol diphosphite.

For convenience of sample preparation, the antioxidants and pigments were sometimes omitted from the exemplary formulations which follow. Since incompatibility between resins and various portions of the rubbery block copolymer tend to be minimized in the molten state, the order of addition of ingredients is not usually critical. It is generally preferred to begin with one of the relatively larger components such as the rubbery block copolymer and add the tackifiers and other resins to it, e.g. adding the synthetic terpene "midblock" resin next, followed by the "endblock" resin. Samples can be prepared by blending in a solvent medium; however, the data obtained from such samples is believed to be less reliable as compared to samples formulated in the molten state. Samples can be solvent-cast to films ranging from 100 to 200 micrometers for test purposes, even though the industrial practice of this invention involves extrusion of the hot melt PSA.

The following ingredients were blended in a heated mixer in the indicated amounts.

A purpose of this Example was to evaluate the effect upon rheology and room-temperature 180.degree. peel strength when the radial block copolymer/resin blend was provided with large crystalline (vinyl arene) domains and minimally tackified or plasticized rubbery (elastomeric) domains, in this case no "midblock" resin. According to the scientific and patent literature, the coumarone-indene resin probably became associated with the polystyrene end blocks of the radial elastomeric block copolymer.

The following formula is a rubbery block copolymer/tackifier resin blend which would have low tack and peel and would have significant elastomeric behavior at normal ambient temperatures. The formula theoretically contains no "endblock" resin.

This formula was similar to that of Example B, except that an S-I-(polystyrene-polyisoprene-) radial block copolymer was blended with a different tackifier.

This formula had both "endblock" resin and "midblock" resin in addition to the S-I-S block copolymer; however, more than 80% by weight of the "endblock" resin was a relatively low molecular weight, low softening point material. The block copolymer was also a relatively lower molecular weight material. Room temperature performance of the formula would be expected to include poor PSA properties and some elastomeric behavior, but at higher temperatures (e.g. 37.degree. C.), performance would be unpredictable.

This formula appeared to conform to all of the criteria of a formula of this invention, except that the amount of "endblock" resin was relatively high and the amount of polyterpene tackifier was relatively low.

The formulas for Examples 1 through 5 are set forth in the following Table.

The G", G', loss tangent (G"/G'), J", and J' were determined, as described previously, at 25.degree. and 50.degree. C. and at 0.01 and 0.25 Hz. The results for the Comparative Examples and Examples 1-5 are set forth below. All G" and G' data are in 10.sup.4 dynes/cm.sup.2, and all J" and J' data are in 10.sup.-8 cm.sup.2 /dyne. For purposes of comparison, typical data for natural rubber (at 0.25 Hz) are included.

For Example A, the high G' values and the temperature dependency of these values indicate unsuitability, according to the principles of this invention. The high G' may, it is believed, indicate good elastic behavior, but, conversely, poor PSA performance. The rheological parameters were confirmed by 180.degree. peel data (PSTC-1). Even 24 hours after formation of the PSA bond, no PSTC-1 value could be obtained. "Dead load deformation" values were acceptable, ranging from 0% to only 6% throughout the 25.degree.-50.degree. C. range.

For Example B, the temperature dependence of G' also indicates unsuitability, which was reflected in the "dead load deformation" data. These data were as follows: 20% at room temperature, 344% at 37.8.degree. C., and 528% at 43.3.degree. C. Cohesive failure occurred at 48.9.degree. C.

For Example C, G" at 0.01 Hz (25.degree. C.) was marginal, and at 0.25 Hg (50.degree. C.) was very low; G' data at 0.01 Hz were marginal. This analysis of the rheological parameters was confirmed by "dead load deformation" data: 6% at room temperature, 44% at 37.8.degree. C., 92% at 43.3.degree. C., and cohesive failure at 48.9.degree. C. The PSTC-1 values after 24 hours were acceptable (2770 g/25.4 mm-width), indicating the ability to wet out a substrate (but inadequate resistance to heat set).

Example D had poor PSA behavior, and performed poorly in the "dead load deformation" test. It is believed that the high G" and, most important, temperature dependence of G' were significant in these regards.

The high coumarone-indene content of Example E was believed to be reflected in the high G' values. The PSA behavior of Example E was marginal (e.g. 910 g/25.4 mm-width in PSTC-1), but a sample of Example E performed adequately in the "dead load deformation" test.

Examples 1-4 performed well in terms of both (1) PSTC-1 (180.degree. peel) values taken at room temperature after 24 hours PSA bond formation and (2) "dead load deformation". Example 5 showed excellent PSTC-1 performance but nearly failed the "dead load deformation" test. The 24-hour PSTC-1 values for Examples 1, 3, 4, and 5 are set forth below. All samples tested were 25.4 mm in width, and the values are reported in grams per 25.4 mm-width.

Example 1: 2670 g/25.4 mm

Example 3: 2170 g/25.4 mm

Example 4: 2130 g/25.4 mm

Example 5: 3580 g/25.4 mm*

All of the products tested were manufactured using a high shear, double arm mixer heated to 200.degree. C. Inert gas was used throughout processing to minimize degradation.

Samples taken from each batch were pressed between release paper using a Carver laboratory press. The release paper was heated in a forced air oven to 200.degree. C. for 15 minutes to drive out residual moisture. The press was equipped with heated jaws set at 200.degree. C. and was shimmed to yield an adhesive thickness of 50 micrometers.

A dwell time of approximately 5 seconds at 3.5.times.10.sup.6 Kg/m.sup.2 was generally sufficient to form an air-free film.

Dumbbell-shaped specimens were cut from the essentially air-free films using a standard striking die.

Marks were placed on the reduced section of a specimen approximately equidistant from its center and perpendicular to its longitudinal axis. The centers of the marks were 25.0 mm.+-.0.5 mm apart.

The specimens were fastened in a vertical position at test temperature and weights equalling 1500 gms/cm.sup.2 of cross sectional area were attached. The specimens were conditioned in this mode for a period of 3 hours. After 3 hours, the weights were removed and the specimens allowed to retract and equilibrate at 25.degree. C. for 5 minutes. The distance between marks was remeasured and the percent "dead load creep" or elongation (permanent longitudinal deformation or set) was calculated as follows:

Elongation percent=L.sub.2 -L.sub.1 /L.sub.1 .times.100 wherein L.sub.2 =measured distance between marks on the conditioned specimen, and L.sub.1 =original distance between marks.

This procedure is a modification of A.S.T.M. D-412 (tension testing of vulcanized rubber) and the dumbbell-shaped samples were formed using Die C.

The dead load deformation data for Examples 1, 3, 4, and 5 are set forth below.

In the case of Example 5, some temperature dependency of G", G', and loss tangent should be noted at 0.25 Hz, and the deformation data are believed to reflect a similar temperature dependency. Furthermore, G' at 0.01 Hz was considered marginal for this formula.

The data for Examples 1-5 are believed to establish that a variety of relatively high molecular weight, relatively high vinyl-arene elastomeric block copolymers can be used in this invention, e.g. both the radial and linear types. Blends of these various types of block copolymers (e.g. 1/99-99/1) will provide suitable elastomeric bases for admixture with endblock and midblock resins or other PSA-forming materials. Examples 1-5 are also believed to demonstrate the value of a good balance between high softening point "endblock" resin and "midblock" resin in the preferred compositions.

Samples of Examples 1-5 were tested for tensile strength at 500% elongation at room temperature (20.degree.-25.degree. C.). It was found that the tensile strength exceeded 35,000 Kg/m.sup.2 for all samples. For Examples 1-4, this measurement was significantly higher, generally in excess of 70,000 Kg/m.sup.2, indicating good cohesiveness.

To provide a further standard of comparison for the rheological data obtained from Examples 1-5, data regarding untackified, unextended, linear A-B-A block copolymers (e.g. of the KRATON.RTM. type) were obtained for 0.25 Hz/room temperature conditions. These data indicate that G' is typically above 300.times.10.sup.4 dynes/cm.sup.2 and G" is typically above 50.times.10.sup.4 dynes/cm.sup.2 and even, in some cases, above 100.times.10.sup.4 dynes/cm.sup.2. Like vulcanized natural rubber, these block copolymers exhibit excellent elastomeric behavior but, in the absence of tackifying resins, essentially no PSA behavior.