Patent Publication Number: US-2004044324-A1

Title: Shaped elastic ear

Description:
BACKGROUND AND FIELD OF THE INVENTION  
       [0001] The present invention relates to an elastic attachment element for use on a disposable article, particularly it relates to an elastic ear portion for use on a disposable article providing elasticity and closure functions.  
       [0002] Disposable absorbent articles such as diapers, training pants or adult incontinent materials are well-known and are provided in a wide variety of configurations. A common configuration makes use of a closure element located on a tab or an ear on one end of the, e.g., diaper or disposable absorbent article which attaches to the opposite end of the disposable absorbent article to provide for adjustable fit and optionally also elasticity. Generally, the attachment element is a rectangular tab, which is attached to one or both corners of the diaper at the back portion, which then attaches to an attachment zone at the front portion of the disposable absorbent article. Generally, one end of the rectangular tab is permanently attached to the diaper with the opposite end of the tab provided with a repositionable fastener element such as a pressure-sensitive adhesive or a mechanical fastener. This repositionable fastener element is mated to a suitable material (from which it can be removably attached) on the opposite front face of the diaper or disposable absorbent article. It is also known that this rectangular fastening tab can be provided with an extensible elastic zone, for example, as disclosed in U.S. Pat. No. 3,800,796. The disposable absorbent article can also be rectangular in shape as disclosed in the &#39;796 patent. However, more commonly in today&#39;s diapers and like disposable absorbent articles, the diaper is in an hourglass-type shape, as disclosed in U.S. Pat. No. 5,368,584. This hourglass shape creates a more form fitting engagement with the waist, leg and the hip area of the wearer. The wider portions of the hourglass shape are at the ends of the disposable article and are commonly referred to as the ear portions. These ear portions can be nonelastic (or inelastic) or provided with an elastic as disclosed in the &#39;584 patent. These ear portions engage the waist at the top of the ear and the hips and top portions of the leg of the wearer at the bottom of the ear. U.S. Pat. No. 5,496,298 proposes an alternative construction where the diaper chassis is substantially rectangular and the hourglass shape is formed by an attached ear, which ear is entirely elastic. This is described as providing form-fitting engagement around the waist and leg area of the wearer. The elastomeric ear is provided in a specific shape where the elastomeric ear tapers from a proximal edge to a distal edge. The taper is substantially continuous and in a preferred embodiment the taper is in an arc-type shape. However, this construction requires the use of a large elastic element, which does not necessarily provide the preferred elastic forces around the waist area. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0003]FIG. 1 illustrates a nonelastic ear element, having the shape of the invention elastic ear element.  
     [0004]FIG. 2 illustrates a first embodiment of elasticized ear elements according to the present invention.  
     [0005]FIG. 3 is a side view of the FIG. 2 embodiment.  
     [0006]FIG. 4 illustrates an elasticized ear of a comparative example of the invention.  
     [0007]FIG. 5 is an alternative embodiment of the invention of elasticized ear.  
     [0008]FIG. 6 is a further alternative embodiment of an elasticized ear in accordance with the invention.  
     [0009]FIG. 7 is a graph of elastic properties of the embodiments depicted in FIGS. 2, 4,  5  and  6 , as also shown in Example 1, Table 1.  
     [0010]FIG. 8 is a graph of elastic properties of the embodiments depicted in FIGS. 2, 4,  5  and  6 , as also shown in Example 2, Table 2. 
    
    
     DETAILED DESCRIPTION  
     [0011] The present invention relates to an attachable ear element  1  which can be attached to the side of a disposable absorbent article for use in closure of the absorbent article. Referring to the Figures, wherein the like numerals represents like elements, FIG. 1 illustrates an ear shape used in the present invention having a free end  2  generally provided with a fastening element  20  and an attached end  3 , which is generally attached to the disposable absorbent article. The top edge  4  of the ear element generally defines a portion of the waist opening of the absorbent article when the ear element is being used. The bottom edge  5  of the ear element forms or faces the leg engaging area of the absorbent article. The ear element has two zones. A rectangular zone  7 , which generally includes a fastener zone  9  for provision of a fastening element  20 , such as a mechanical fastening element such as a hook and/or loop or an adhesive fastening element. Generally, an adhesive fastening element is a suitable pressure-sensitive adhesive or cohesive material. The rectangular zone  7  is also provided at least in part with an elastic region  15  as shown in FIG. 2. The ear element  1  also has a tapered zone  6 , which tapered zone is preferably at least in part nonelastic. A tapered zone  6  generally has a tapered edge  21  on the bottom edge  5  of the ear element. Opposite the tapered edge  21  is a top edge  22  of the tapered zone, which preferably is a flat edge contiguous with the top edge  24  of the rectangular zone  7 . Rectangular zone  7  has a top edge  24  and a bottom edge  23 . The ear element shown in FIG. 1 further is provided with an attachment zone  8  for permanent attachment to the side edge of the disposable absorbent article. This attachment zone can be attached to the disposable absorbent article by use of a conventional attachment method such as ultrasonic, point heat bonding, adhesives or the like. Preferably this attachment zone  8  is nonelastic in its entirety.  
     [0012] It has been discovered that by providing a rectangular zone within an elastic ear element and that the rectangular zone  7  be provided with substantially all the elastic material that a more efficient utilization of elastic material is realized. Specifically there is provided a higher net force per unit area of elastic material while also providing uniform elastic properties over a wide extension range allowing the tabs to be functional over a wide range of user sizes. Generally with the invention attachable elastic ear elements the percentage of the area of the elastic material forming an elastic region in the rectangular zone to the total area of the elastic material forming an elastic region is greater than 25%, preferably greater than 50% and in a most preferred embodiment is 100%. The width  26  of the rectangular zone is generally from 20 to 150 mm and preferably 30 to 100. The end width  25  of the tapered zone  6  is preferably 30 to 180 mm and most preferably 50 to 150 mm where the ratio of the rectangular zone width to the end width of the tapered zone is preferably 1:1.2 to 1:6 and most preferably 1:1.4 to 1:5. The tapered zone  6  preferably has a tapered edge  21 , which tapers in a linear or nonlinear manner and can be arcuate, undulating or the like. If the taper is nonlinear it is preferred that the taper be arcuate and have an inwardly tapered arc as disclosed in U.S. Pat. No. 5,496,298, where the taper defines an arc of a circle generally having a radius of from 25 mm to 150 mm and preferably from 50 mm to 90 mm. The tapered zone length  27  will generally be from 10 to 100 mm, preferably 20 to 80 mm whereas the rectangular zone is preferably 10 to 100 mm, most preferably 20 to 80 mm. The ratio of the rectangular zone length to tapered zone length is generally from about 10:1 to 1:10, preferably 2:1 to 1:2. The tapered zone tapered edge  21  preferably tapers continuously into the rectangular zone bottom edge  23  without any rapid discontinuities at transition zone  29 . A discontinuity is a rapid change in the width of the ear, over a short distance, such as a change in width of about 5 mm or more over a distance of 3 mm or less. Preferably the width of the tapered zone  6  at the transition zone  29  is identical to the width  26  of the rectangular zone  7  at the transition zone  29 .  
     [0013] The elastic material can be any suitable elastic sheet material such as a film elastic or nonwoven elastic or a nonwoven/film elastic laminate. A film elastic can be a blend or a multilayer film elastic with inelastic phases or layers. If there are inelastic phases or layers, the layer thicknesses and the amount of the nonelastic phase needs to be such that the elastic material is still substantially elastic, generally having tensile properties sufficient to maintain a seal to prevent leakage and maintain fit but not cause skin irritation or red marking.  
     [0014] The elastic material can be joined (such as by adhesives, point bonding or other known techniques) to an inelastic substrate web, which preferably is a fibrous layer, which substrate web can be expanded when the elastic material to which the substrate is secured, is stretched. Since the substrate web itself is inelastic, the expandability of the layer is achieved by securing the substrate web to the elastic material such that the length of the substrate web is longer than the length of the elastic in its relaxed state in the area containing the elastic material. Accordingly, it will be possible to expand the substrate web upon stretching of the elastic until the elastic material is stretched to a length equal to the length of the attached substrate web. Stretching beyond this limit will require substantial increase in the stretch force because it would require deformation of the substrate web. The substrate web is generally coextensive with the elastic material and will preferably have an expandability of at least 30% and preferably at least 75%, that is to say that the elastic laminate can be stretched by at least 30% and preferably at least 75% of its length in the relaxed state. Typically, the expandability of the substrate web is between 50 and 400% and most preferably between 75% and 200%.  
     [0015] The substrate web is preferably secured to the elastic material in intervals, i.e., when viewed in the longitudinal direction, portions of the substrate web that are connected to the elastic are alternated with portions of the substrate web that are not connected to the elastic material layer. This may be achieved by corrugating the fibrous layer to form arcuate portions and anchor portions therein and then extruding a molten thermoplastic material thereon that forms the elastic film when cooled as disclosed in U.S. Pat. No. 6,159,584. Alternatively, the elastic material and the substrate material may be of equal lengths when laminated or bonded. The laminate can be stretched to permanently deform the substrate in a process such as ringrolling as taught in EP 704 196. A nonwoven substrate web used in connection with the present invention can also be a necked or reversibly necked nonwoven web as described in U.S. Pat. Nos. 4,965,122; 4,981,747; 5,114,781; 5,116,662; and 5,226,992. In these embodiments the nonwoven web is elongated in a direction perpendicular to the desired direction of extensibility. When the nonwoven web is set in this elongated condition, it will have stretch and recovery properties in the direction of extensibility.  
     [0016] The elastic material comprises an elastomeric material that exhibits elastomeric properties at ambient conditions, i.e., the material will substantially resume its original shape after being stretched. Preferably, the elastic material will sustain only a small permanent set following deformation and relaxation, which set is preferably less than 30% and more preferably less than 20% of the original 50% to 500% stretch. The elastomeric material can be either pure elastomers or blends with an elastomeric phase or content that will still exhibit substantial elastomeric properties at room temperature. Suitable elastomeric thermoplastic polymers include block copolymers or the like. These block copolymers are described in, for example, U.S. Pat. Nos. 3,265,765; 3,562,356; 3,700,633; 4,116,917 and 4,156,673. Particularly useful are styrene/isoprene, styrene/butadiene or ethylenebutylene/styrene block copolymers. These blocks may be arranged in any order including linear, radial, branched or star block copolymers. Other useful elastomeric polymers can include elastomeric polyurethanes, elastomeric ethylene copolymers such as ethylene vinyl acetates, ethylene/propylene copolymer elastomers or ethylene/propylene/diene terpolymer elastomers. Blends of these elastomers with each other or with modifying elastomers are also contemplated.  
     [0017] Viscosity reducing polymers and plasticizers can also be blended with the elastomers such as low molecular weight polyethylene and polypropylene polymers and copolymers, or tackifying resins such as Wingtack™, aliphatic hydrocarbon tackifiers available from Goodyear Chemical Company. Examples of tackifiers include aliphatic or aromatic hydrocarbon liquid tackifiers, polyterpene resin tackifiers, and hydrogenated tackifying resins. Aliphatic hydrocarbon resins are preferred.  
     [0018] Additives such as dyes, pigments, antioxidants, antistatic agents, bonding aids, antiblocking agents, slip agents, heat stabilizers, photostabilizers, foaming agents, glass bubbles, reinforcing fiber, starch and metal salts for degradability or microfibers can also be blended into the elastomeric composition for making the elastic material.  
     [0019] In a preferred embodiment of the present invention, the elastic material is a film comprising an elastomeric core layer of elastomeric material provided on one or both major surfaces with a skin layer. Such a multilayer film can conveniently be produced through co-extrusion. The use of a skin layer is particularly useful on the side of the film where the film is being attached to a substrate layer or another material by adhesives because the skin layer may function as a barrier layer to migration of tackifiers and other low molecular weight species into the elastic core layer and also creates a more stable surface for attachment, particularly when the skin layer is an inelastic material.  
     [0020] The multilayer film can be stretched beyond an elastic limit of these skin layers. The skin layers are generally nontacky materials or blends formed of any semicrystalline or amorphous polymer(s) which are less elastomeric than the elastic core layer, are preferably generally inelastic and will undergo relatively more permanent deformation than the elastic core layer at the percentage that the elastic film is stretched. Preferred elastomeric materials for the core layer are olefinic elastomers, e.g. ethylene-propylene elastomers, ethylene propylene diene polymer elastomers, metallocene polyolefin elastomers or ethylene vinyl acetate elastomers, or styrene/isoprene, butadiene or ethylene-butylene/styrene (SIS, SBS, SEBS) block copolymers, or polyurethanes or blends. Generally, the elastomeric materials used can also be blended with nonelastomeric materials in a weight percent range of 0-70%, preferably 5-50%. High percentages of elastomeric materials in the skin layer(s) generally require use of antiblock and/or slip agents to reduce the surface tack and roll unwind force. Preferably, these skin layers are polyolefinic and are formed predominately of polymers such as polyethylene, polypropylene, polybutylene, polyethylene-polypropylene copolymer, however, these skin layers may also be wholly or partly polyamide, such as nylon, polyester, such as polyethylene terephthalate, or the like, and suitable blends thereof. Generally, the skin layer material following the stretching and recovery of the coextruded elastic is in contact with the elastic core layer material in at least one of three suitable modes; first, continuous contact between the elastic core layer and the microtextured skin layer; second, continuous contact between the layers with cohesive failure of the core layer material under the microtextured skin folds; and third, adhesive failure of the skin layer to the core layer under the microtextured folds with intermittent skin layer to core layer contact at the microtexture fold valleys. Generally, in the context of the present invention, all three forms of skin-to-core contact are acceptable. However, preferably the skin and core layers are in substantially continuous contact so as to minimize the possibility of delamination of the skin layer(s) from the elastic core layer.  
     [0021] Generally, the overall elastic core layer to skin layer(s) thickness ratio of the elastic film will be at least 1.5, preferably at least 5.0 but less than 1000 and most preferably from 5.0 to 200. Generally, the overall caliper of the multilayer elastic film is preferably 25 to 200 microns. The addition of the skin layer materials generally tends to reinforce the elastic film material. The skin layers can be sufficiently thin and/or soft so that little or no reinforcement of the elastomeric core layer occurs and the film is elastic in its initial elongation as well as its second and subsequent elongations at suitably low stress elongation forces and low hysteresis loss levels when the elastic is cycled in use (e.g. by dimensional changes caused by breathing).  
     [0022] The substrate web may be a woven material, nonwoven material, knit material, paper, film, or any other continuous media. The substrate web may have a wide variety of properties, such as extensibility, elasticity, flexibility, conformability, breathability, porosity, stiffness, etc. Further, the substrates may include pleats, corrugations or other deformations from a flat planar sheet configuration. The substrate web is preferably a fibrous layer and typically a nonwoven web of thermoplastic polymer fibers. Suitable processes for making the nonwoven web include, but are not limited to, airlaying, spunbond, spunlace, bonded melt blown webs and bonded carded web formation processes.  
     [0023] Spunbond nonwoven webs are made by extruding a molten thermoplastic, as filaments from a series of fine die orifices in a spinneret. The diameter of the extruded filaments is rapidly reduced under tension by, for example, non-eductive or eductive fluid-drawing or other known spunbond mechanisms, such as described in U.S. Pat. Nos. 4,340,653; 3,692,618; 3,338,992 and 3,341,394; 3,276,944; 3,502,538; 3,502,763; and  3 , 542 , 615 . The spunbond web is preferably bonded.  
     [0024] The nonwoven web layer also may be made from bonded carded webs. Carded webs are made from separated staple fibers, which fibers are sent through a combing or carding unit which separates and aligns the staple fibers in the machine direction so as to form a generally machine direction-oriented fibrous nonwoven web. However, randomizers can be used to reduce this machine direction orientation. Once the carded web has been formed, it is then bonded by one or more of several bonding methods to give it suitable tensile properties. One bonding method is powder bonding wherein a powdered adhesive is distributed through the web and then activated, usually by heating the web and adhesive with hot air. Another bonding method is pattern bonding wherein heated calender rolls or ultrasonic bonding equipment are used to bond the fibers together, usually in a localized bond pattern though the web can be bonded across its entire surface if so desired. Generally, the more the fibers of a web are bonded together, the greater the nonwoven web tensile properties.  
     [0025] Airlaying is another process by which fibrous nonwoven webs useful in the present invention can be made. In the airlaying process, bundles of small fibers usually having lengths ranging between about 6 to about 19 millimeters are separated and entrained in an air supply and then deposited onto a forming screen, often with the assistance of a vacuum supply. The randomly deposited fibers are then bonded to one another using, for example, hot air or a spray adhesive.  
     [0026] Alternatively, known melt-blown webs or spunlace nonwoven webs or the like can be used to form the fibrous layer of the elastic laminate. Melt blown webs are formed by extrusion of thermoplastic polymers from multiple die orifices, which polymer melt streams are immediately attenuated by hot high velocity air or steam along two faces of the die immediately at the location where the polymer exits from the die orifices. The resulting fibers are entangled into a coherent web in the resulting turbulent airstream prior to collection on a collecting surface. Generally, to provide sufficient integrity and strength for the present invention, melt blown webs must be farther bonded such as through air bonding, heat or ultrasonic bonding as described above.  
     [0027] The invention elastic ear element is attached as a separate element and generally joined directly to the disposable absorbent article however it could be joined to an intermediate functional or nonfunctional substrate. The elastic ear element may be joined to the article at any suitable location, however it is preferable joined to the absorbent article at a top edge so that the top edge of the ear element is close to the top edge of the absorbent article.  
     [0028] The substrate web to which the elastic material is joined, on one or both sides, preferably forms both the rectangular zone and the tapered zone. In this way the elastic forces are directly translated from the rectangular zone to the tapered zone without weak areas or hard bands formed by bonding of separate webs to form the different zones. This substrate web can generally have a Gurley stiffness of 1-1000 and is preferably formed of a nonwoven, woven or film, or a continuous laminate thereof.  
     [0029] Further, the substrate web could be reinforced and/or weakened at certain locations to help provide desired flexibility and/or stiffness at certain locations. Methods of weakening the material include scoring, cutting, thinning, bending, heat treating, chemical treating and the like. Methods of reinforcing include heat or chemical treating the material, adding material, and increasing the thickness and the like.  
     [0030] A web can be made extensible by skip slitting as is disclosed in, e.g., International Publication No. WO 96/10481. If the substrate web is desired to be extensible in the area attached to the elastic material, the slits are discontinuous and are generally cut on the web prior to the web being attached to the elastic material. Although more difficult, it is also possible to create slits in the substrate web after the substrate web is laminated to the elastic material. At least a portion of the slits in the nonelastic web should be generally perpendicular (or have a substantial perpendicular vector) to the intended direction of extensibility or elasticity (the at least first direction) of the elastic layer. By generally perpendicular it is meant that the angle between the longitudinal axis of the chosen slit or slits and the direction of extensibility is between 60 and 120 degrees. A sufficient number of the described slits are generally perpendicular such that the overall laminate is elastic. The provision of slits in two directions is advantageous when the elastic laminate is intended to be elastic in at least two different directions.  
     Test Methods  
     [0031] Stress-Strain  
     [0032] To measure the stress-strain tensile properties of the webs, a cyclic stress-strain tensile test was performed using an INSTRON Model 1122 constant rate of extension tensile tester. The samples were cut with a razor blade into the shapes shown in FIGS. 2, 4,  5  and  6 . The dimensions of the rectangular elastic zone were 36 mm×55 mm with the 36 mm dimension being the dimension that was stretched as the sample was elongated. Both the top and bottom crossheads of the tensile tester were equipped with flat jaws. The crosshead separation was set at 50.8 mm. The sample was clamped into the jaws and the crossheads were separated at a rate of 508 mm/minute. The crossheads were stopped for 1 second after an extension of 50 mm (139% elongation; based on the length of the elastic zone) was achieved and then returned to the starting point (50.8 mm jaw separation or 0% elongation). After again pausing for 1 second, a second extension or upload to a 50 mm extension (139% elongation) was performed. After pausing for one second, the crossheads were returned to the starting point to conclude the test. The force in grams for the first pull (upload) was read from the resultant stress-strain chart at 12.5 mm extension (35% elongation), 25 mm extension (69% elongation), 37.5 mm extension (104% elongation) and 50 mm extension (139% elongation). The force for the second pull (upload) was read at 25 mm and 50 mm extension. The force was divided by the actual area of elastic in the cut sample to obtain a ‘normalized’ tensile in grams/mm 2 .  
     EXAMPLES  
     Example 1  
     [0033] A continuous coextrusion was carried out to prepare a three layer laminate consisting of two outer inelastic skin layers of polypropylene; (5E57 available from Union Carbide Corporation, a subsidiary of the Dow Chemical Company, Danbury, Conn.) and a core layer of block copolymer rubber, (KRATON D1114PX, available from Kraton Polymers, Belpre, Ohio). The laminate had an overall thickness of 92 microns (3.6 mils) with skin/core/skin thicknesses of 9/74/9 microns. The laminate was then stretched in the transverse direction at a ratio of about 5:1 and was then annealed by passing over a patterned heated roll (106 degrees Celsius). This resulted in alternating parallel longitudinal bands of inelastic and elastic regions running in the machine direction of the film. The elastic regions had a width of 100 mm and the inelastic regions had a width of 44 mm as measured when the elastic laminate was held in the stretched state.  
     [0034] A 15.5 grams/meter 2  (0.46 ounce/yard 2 ) spunbond polypropylene nonwoven web was adhesively bonded to both the top and bottom side of the elastic laminate while it was held in the stretched state. The nonwoven/elastic/nonwoven laminate was then allowed to relax resulting in a laminate having elastic and inelastic zones. The laminate was then cut into the shape as shown in FIGS. 2, 4,  5  and  6 . The samples were cut in such a way that 100% of the elastic region was located entirely in the rectangular portion of the shape only (FIG. 2), 65% of the elastic region was located in the rectangular portion of the shape and 35% was located in the trapezoidal portion (FIG. 5), 30% of the elastic region was located in the rectangular portion of the shape and 70% was located in the trapezoidal portion (FIG. 6) and finally where 100% of the elastic region was located in the trapezoidal portion of the shape (FIG. 4). The samples were tested for stress-strain properties with the results shown in Table 1, and summarized in FIG. 7.  
     Example 2  
     [0035] The three layer elastic material described in Example 1 above (not laminated to the spunbond nonwovens) was tested for stress-strain properties. The shapes and the location of the elastic within the shape were also the same as described in Example 1 with the exception being that the elastic zone was 28 mm×55 mm with the 28 mm dimension being the dimension that was stretched as the sample was elongated. The results are shown in Table 2, and summarized in FIG. 8.  
               TABLE 1                          (Example 1; film/nonwoven laminate)                                                                 Shape as shown in:               (Comparative)   Curve                                                 Area of elastic in   (%)   100%   65%   30%    0%           rectangular zone   (mm 2 )   1980   1320   660   0       Amount of elastic in   (%)    0%   35%   70%   100%       non-rectangular   (mm 2 )   0   700   1523   2577       zone       Total Elastic area   (mm 2 )   1980   2020   2183   2577       Distance into   (mm)   36   24   12   0       rectangular zone       Distance into   (mm)   0   12   24   36       nonrectangular zone       Tensile - first pull at   (grams)   594   620   634   673   a       12.5 mm extension   (g/mm 2 )   0.300   0.307   0.290   0.261       Tensile - first pull at   (grams)   851   889   921   1011   b       25 mm extension   (g/mm 2 )   0.430   0.440   0.422   0.392       Tensile - first pull at   (grams)   1233   1294   1353   1472   c       37.5 mm extension   (g/mm 2 )   0.623   0.641   0.620   0.571       Tensile - first pull at   (grams)   1953   2063   2165   2271   d       50 mm extension   (g/mm 2 )   0.986   1.021   0.992   0.881       Tensile - second   (grams)   337   355   376   391   b′       pull at 25 mm   (g/mm 2 )   0.170   0.176   0.172   0.152       extension       Tensile - second   (grams)   1672   1775   1861   1931   d′       pull at 50 mm   (g/mm 2 )   0.844   0.879   0.853   0.750       extension                  
 
     [0036]               TABLE 2                          (Example 2; elastic film only)                                                                 Shape as shown in:               (Comparative)   Curve                                                 Area of elastic in   (%)   100%   65%   31%    0%           rectangular zone   (mm 2 )   1540   1029   512   0       Amount of elastic in   (%)    0%   33%   67%   100%       non-rectangular   (mm 2 )   0   554   1145   1915       zone       Elastic area   (mm 2 )   1540   1583   1657   1915       Distance into   (mm)   28   9.3   18.7   0       rectangular zone       Distance into   (mm)   0   18.7   9.3   28       nonrectangular zone       Tensile - first pull at   (grams)   358   364   361   415   a       12.5 mm extension   (g/mm 2 )   0.233   0.230   0.218   0.217       Tensile - first pull at   (grams)   429   437   441   501   b       25 mm extension   (g/mm 2 )   0.278   0.276   0.266   0.261       Tensile - first pull at   (grams)   501   509   517   594   c       37.5 mm extension   (g/mm 2 )   0.325   0.332   0.312   0.310       Tensile - first pull at   (grams)   608   622   635   731   d       50 mm extension   (g/mm 2 )   0.395   0.393   0.383   0.382       Tensile - second   (grams)   319   327   325   369   b′       pull at 25 mm   (g/mm 2 )   0.207   0.206   0.196   0.192       extension       Tensile - second   (grams)   567   582   588   677   d′       pull at 50 mm   (g/mm 2 )   0.368   0.368   0.355   0.353       extension