CROSS REFERENCE TO RELATED APPLICATIONS
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
This application claims priority to U.S. provisional application serial No. 60/254,747 filed on Dec. 11, 2000, which is incorporated by reference herein in its entirety.
- REFERENCE TO MICROFICHE APPENDIX
- FIELD OF THE INVENTION
- BACKGROUND OF THE INVENTION
This invention relates to nonwoven fabrics formed from polyolefin polymers and methods of making the fabrics.
Fabrics made from fibers include both woven and nonwoven fabrics. Nonwoven fabrics are used for sanitary and medical uses including hospital gowns, diaper linings, and sanitary wipes. Many processes for producing bonded nonwoven fabrics exist. For example, one can apply heat and pressure for bonding at limited areas of a nonwoven web by passing it through the nip between heated calender rolls either or both of which may have patterns of lands and depressions on their surfaces. During such a bonding process, depending on the types of fibers making up the nonwoven web, the bonded regions may be formed autogenously, i.e., the fibers of the web are melt fused at least in the pattern areas, or with the addition of an adhesive. The advantages of thermally bonded nonwoven fabrics include low energy costs and speed of production.
Nonwoven fabrics also can be made by a number of other methods, e.g., spunlacing or hydrodynamically entangling (as disclosed in U.S. Pat. No. 3,485,706 and U.S. Pat. No. 4,939,016); by carding and thermally bonding staple fibers; by spunbonding continuous fibers in one continuous operation; or by melt blowing fibers into fabric and subsequently calendering or thermally bonding the resultant web.
Various properties of nonwoven fabrics determine the suitability of nonwoven fabrics for different applications. Nonwoven fabrics can be engineered to have different combinations of properties to suit different needs. Variable properties of nonwoven fabrics include liquid handling properties such as wettability, distribution, and absorbency, strength properties such as tensile strength and tear strength, softness properties, durability properties such as abrasion resistance, and aesthetic properties.
Polypropylene has been the primary polymer for nonwovens because of its cost, high strength, and processability. However, polypropylene nonwovens generally do not have a soft, cotton-like feel. As such, polyethylene nonwovens have gained interest. Polyethylenes produce softer fabrics but may have relatively low tensile strength and abrasion resistance.
Although nonwoven fabric properties such as liquid handling properties, strength properties, softness properties and durability properties, are normally of primary importance in designing nonwoven fabrics, the appearance and feel of nonwoven fabrics are often critical to the success of a nonwoven fabric product. The appearance and feel of nonwoven fabrics is particularly important for nonwoven fabrics which form exposed portions of products. For example, it is often desirable that the outer covers of nonwoven fabric products have a cloth-like feel and a pleasing decorative design.
- SUMMARY OF THE INVENTION
Despite the advances in the art described above, there is still a need for improved nonwoven fabrics and methods of their manufacture. In particular, there is a need for nonwoven fabrics with improved: tensile strength, elongation, abrasion resistance and softness as defined by fabric flexural rigidity.
Embodiments of the invention meet the above need by one or more of the following aspects of the invention. In one aspect, the invention relates to a method for producing a nonwoven fabric with increased tensile strength, elongation, abrasion resistance, flexural rigidity, and/or softness. The method comprises passing a fiber web through a pair of rollers to obtain a thermally bonded fabric with a high percentage of bond areas. The high percentage of bond areas is formed by an engraved pattern on at least one of the rollers. The engraved pattern has a high percentage of bond point areas and/or wide bond point angles.
In some embodiments, the percentage of bond areas of the fabric is at least about 16 percent, at least about 20 percent, or at least about 24 percent. The bond point angel is about 20° or higher, about 35° or higher, about 37° or higher, about 42° or higher, or about 46° or higher. The engrave pattern has at least about 1.55×105 bond points per square meter, at least about 2.31×105 bond points per square, at least about 3.1×105 bond points per square meter, at least about 3.44×105 bond points per square meter, at least about 4.6×105 bond points per square meter, or at least about 4.65×105 bond points per square meter. The fiber web may comprise polyethylene, which may be a homopolymer of ethylene or a copolymer of ethylene and a comonomer. The polyethylene may be obtained in the presence of a single site catalyst, such as a metallocene catalyst or a constrained geometry catalyst.
In another aspect, the invention relates to a non-woven fabric made by the method described herein. The non-woven fabric comprises a polymer and is characterized by a high percentage of bond area and a high abrasion resistance. In some embodiments, the polymer is polyethylene, which may be a homopolymer of ethylene or a copolymer of ethylene and a comonomer. The polyethylene may be obtained in the presence of a single site catalyst, such as a metallocene catalyst or a constrained geometry catalyst. In other embodiments, the percentage of bond areas of the fabric is at least about 16 percent, at least about 20 percent, or at least about 24 percent.
BRIEF DESCRIPTION OF THE DRAWINGS
Various aspects of the invention and advantages provided by the embodiments of the invention are apparent with the following description.
FIG. 1 is a simplified diagram of a process for producing fabrics for use in embodiments of the invention.
FIG. 2A is a fragmentary elevation view of the embossing roll illustrating one arrangement of the bond points.
FIG. 2B is a simplified view of a nonwoven fabric produced from the process of FIG. 1 and the engraved roll of FIG. 2A.
FIGS. 3A-3I are schematics of bond patterns for use in embodiments of the invention on an arbitrary scale.
FIGS. 4A-4I are micrographs of nonwoven fabrics produced from the bond patterns in FIGS. 3A-3I for PE1 resin used in Example 1.
FIG. 5 is a graph of normalized peak loads vs. temperature for fabrics produced from the bond patterns in FIGS. 3A-3I for PE1 resin.
FIG. 6 is a graph of percent elongation vs. temperature for fabrics produced from the bond patterns in FIGS. 3A-3I for PE2 resin used in Example 1.
FIG. 7 is a graph of typical stress-strain curves for three fabrics produced in Example 1.
FIG. 8 is a graph of abrasion resistance vs. temperature for fabrics produced from the bond patterns in FIGS. 3A-3I for PE1 resin.
FIG. 9 is a graph of flexural rigidity vs. temperature for fabrics produced from the bond patterns in FIGS. 3A-3I for PE1 resin.
FIGS. 10A-10I are scanning electron microscope micrographs, at 80× magnification, of bond points of nonwoven fabrics produced from bond patterns in FIGS. 3A-3I for PE1 resin.
FIGS. 11A-11C are scanning electron microscope micrographs of tensile test fracture sites of nonwoven fabrics produced from bond patterns in FIGS. 3A-3I for various resins.
DESCRIPTION OF EMBODIMENTS OF THE INVENTION
FIGS. 12A-12B are scanning electron microscope micrographs of abraded bond sites of nonwoven fabrics produced from bond patterns in FIGS. 3A-3I for various resins.
Embodiments of the invention provide a method for producing a non-woven fabric by thermal bonding. The fabric has a high percentage of bond areas which are produced by passing a fiber web through a pair of rolls, with at least one of the rolls having an engraved pattern with a high percentage of bond point areas along with wide bond point angles. The term “nonwoven” as used herein means a web or fabric having a structure of individual fibers or threads which are randomly interlaid, but not in an identifiable manner as is the case for a knitted fabric. The term “bonding” as used herein refers to the application of force or pressure (separate from or in addition to that required or used to draw fibers to less than or equal to 50 denier) to fuse molten or softened fibers together. In some embodiments, the bond strength is greater than or equal to about 1,500 grams results. The term “thermal bonding” is used herein refers to the reheating of fibers and the application of force or pressure (separate from or in addition to that required or used to draw fibers to less than or equal to 50 denier) to effect the melting (or softening) and fusing of fibers. In some embodiments, the bond strength is greater than or equal to about 2,000 grams results. Operations that draw and fuse fibers together in a single or simultaneous operation or prior to any take-up roll (for example, a godet), for example, spunbonding, are not considered to be a thermal bonding operation.
A thermal bonding process for producing a non-woven fabric is illustrated in FIG. 1. Such a process or variations thereof is described, for example, in the following U.S. Pat. Nos.: 5,888,438; 5,851,935; 5,733,646; 5,654,088; 55,629,080; 5,494,736; 4,770,925; 4,635,073; 4,631,933; 4,564,553; 4,315,965, which are incorporated by reference herein in their entirety. All such disclosed processes may be utilized in embodiments of the invention with or without modifications.
Referring to FIG. 1, a web forming system 10, such as a carding system, is employed to initially form a fibrous web 12. The fibers are aligned predominantly in the machine direction of web formation, as indicated by arrow 13. Alternatively, a spunbond system could be used to produce more random orientation of the fibers. The web 12 may be directed through a preheating station 14. The preheated web is then passed to the pressure nip of a bonding station provided by opposed rolls 20 and 22. The roll 20 is a metal engraved roll and is heated to a temperature near the melting point of the fibers. The backup roll (i.e., smooth roll) 22 is heated in a controlled manner to a temperature near the melting point of the fibers, preferably below the stick point of such fibers. In some embodiments, the engraved pattern comprises circles, although other shapes, such as ovals, squares, and rectangulars, may be used.
The engraved roll as illustrated in FIG. 2A contains areas, bond points, that are in intimate, compressed contact with a flat roll. These areas induce melting and create bond areas. The size of these areas determines the number of fibers bonded at a single point and also the total area of the fabric that contains non-fibrous integrity. The number of fibers connected at one bond point can influence its overall strength, but also can contribute to its overall stiffness. There are three factors of an engraved pattern that effect the overall properties of a nonwoven fabric. They include bond area, bond point or side-wall angle, and the concentration of bond points usually stated as points per square unit area.
The engraved pattern on the roll is produced via bond points. These points extend from the engraved roll and when in contact with the flat roll, produce a bonded area. Generally the bond points produce a pattern on the nonwoven fabric, such as seen in FIG. 2B. The bond points of an engraved pattern are generally expressed in terms of bond points per square area. In a preferred embodiment, the engraved pattern has about 1.55×105 bond points per square meter (100 bond points per square inch), preferably about 2.31×105 bond points per square meter (149 bond points per square inch), more preferably about 3.10×105 bond points per square meter (200 bond points per square inch), or about 3.44×105 bond points per square meter (222 bond points per square inch), or about 4.60×105 bond points per square meter (297 bond points per square inch), or about 4.65×105 bond points per square meter (300 bond points per square inch). Higher bond points per square meter such as 5.42×105, 6.20×105, 7.75×105, 9.30×105, or more, (per square inch, such as 350, 400, 500, 600, or more) also may be feasible.
The bond point is made up of a bond point angle and bond area. Referring to FIGS. 3A-I, various bond point patterns of different bond point angles and bond areas are shown. Bond point angle refers to the angle at which the bond point extends from the engraved roll. The bond point angle is about 20 degrees or higher, preferably about 35 degrees or higher, more preferably about 37 degrees or higher, most preferably about 42 degrees or higher, and still most preferably about 46 degrees or higher. FIG. 3A is for bond pattern 1 having a 46° angle, 20 percent bond area, 3.44×105 pts/m2 (222 pts/in2), base width of 1.7×10−3 m (0.067 inch), base height of 4.32×10−4 m (0.017 inch), and a point width of 7.62×10−4 m (0.03 inch). FIG. 3B is for bond pattern 2 having a 20° angle, 16 percent bond area, 3.44×105 pts/m2 (222 pts/in2), base width of 1.7×10−3 m (0.067 inch), base height of 4.32×10−4 m (0.017 inch), and a point width of 6.86×10−4 m (0.027 inch). FIG. 3C is for bond pattern 3 having a 20° angle, 24 percent bond area, 3.44×105 pts/m2 (222 pts/in2), base width of 1.7×10−3 m (0.067 inch), base height of 4.32×10−4 m (0.017 inch), and a point width of 8.38×10−4 m (0.033 inch). FIG. 3D is for bond pattern 4 having a 20° angle, 20 percent bond area, 2.31×105 pts/m2 (149 pts/in2), base width of 1.7×10−3 m (0.067 inch), base height of 4.32×10−4 m (0.017 inch), and a point width of 9.30×10−4 m (0.0366 inch). FIG. 3E is for bond pattern 5 having a 20° angle, 20 percent bond area, 4.60×105 pts/m2 (297 pts/in2), base width of 1.7×10−3 m (0.067 inch), base height of 4.32×10−4 m (0.017 inch), and a point width of 6.60×10−4 m (0.026 inch). FIG. 3F is for bond pattern 6 having a 42° angle, 16 percent bond area, 3.44×105 pts/m2 (222 pts/in2), base width of 1.7×10−3 m (0.067 inch), base height of 4.32×10−4 m (0.017 inch), and a point width of 6.86×10−4 m (0.027 inch). FIG. 3G is for bond pattern 7 having a 37° angle, 24 percent bond area, 3.44×105 pts/m2 (222 pts/in2), base width of 1.7×10−3 m (0.067 inch), base height of 4.32×10−4 m (0.017 inch), and a point width of 8.38×10−4 m (0.033 inch). FIG. 3H is for bond pattern 8 having a 46° angle, 20 percent bond area, 2.31×105 pts/m2 (149 pts/in2), base width of 1.7×10−3 m (0.067 inch), base height of 4.32×10−4 m (0.017 inch), and a point width of 9.3×10−4 m (0.0366 inch). FIG. 3I is for bond pattern 9 having a 35° angle, 20 percent bond area, 4.60×105 pts/m2 (297 pts/in2), base width of 1.7×10−3 m (0.067 inch), base height of 4.32×10−4 m (0.017 inch), and a point width of 6.60×10−4 m (0.026 inch).
Bonded areas and unbonded areas make up the nonwoven fabric. Bonded areas may be defined as the percentage of the surface area of the nonwoven fabric that is covered by a bond produced by the bond point The bond area in embodiments of the invention is preferably at least 16 percent, more preferably at least 20 percent and most preferably at least 24 percent, 30 percent, 35 percent, 40 percent, 45 percent, 50 percent or more.
Fiber and Nonwoven Fabric Fabrication
A web forming system generally includes processes for producing fibers which can be thermally bonded to form fabrics include dry laid, wet laid, and polymer laid or any other processes. In some embodiments, the fibers are produced by spunbond, meltblown or carded staple processes. These processes are further described in the following United States Patents, which are hereby incorporated by reference in their entirety: U.S. Pat. Nos. 3,338,992; 3,341,394; 3,276,944; 3,502,538; 3,978,185; and 4,644,045. In general, the spunbond process uses a high powered vacuum chamber to increase the velocity of the fibers in order to decrease the fiber's diameters to produce a continuous fiber. The meltblown process blows air down from above and uses surface forces to drag the fibers to higher velocities to produce very low denier non-continuous fibers.
Conventional spunbond processes are described in U.S. Pat. Nos. 3,825,379; 4,813,864; 4,405,297; 4,208,366; and 4,334,340 all of which are incorporated by reference herein in their entirety. The spunbonding process is one which is well known in the art of fabric production. Generally, continuous fibers are extruded, laid on an endless belt, and then bonded to each other, and often times to a second layer such as a melt blown layer, often by a heated calendar roll, or addition of a binder. An overview of spunbonding may be obtained from L. C. Wadsworth and B. C. Goswami, Nonwoven Fabrics: “Spunbonded and Melt Blown Processes” proceedings Eight Annual Nonwovens Workshop, Jul. 30-Aug. 3, 1990, sponsored by The Textiles and Nonwovens Development Center (hereinafter “TANDEC”), University of Tennessee, Knoxville, Tenn.
The term “meltblown” is used herein to refer to fibers formed by extruding a molten thermoplastic polymer composition through a plurality of fine, usually circular, die capillaries as molten threads or filaments into converging high velocity gas streams (e.g. air) which function to attenuate the threads or filaments to reduced diameters. Thereafter, the filaments or threads are carried by the high velocity gas streams and deposited on a collecting surface to form a web of randomly dispersed meltblown fibers with average diameters generally smaller than 10 microns.
The term “spunbond” is used herein to refer to fibers formed by extruding a molten thermoplastic polymer composition as filaments through a plurality of fine, usually circular, die capillaries of a spinneret with the diameter of the extruded filaments then being rapidly reduced and thereafter depositing the filaments onto a collecting surface to form a web of randomly dispersed spunbond fibers with average diameters generally between about 7 and about 30 microns.
Nonwovens can be produced by numerous methods. Most methods include substantially the same basic procedures: (1) material selection; (2) web formation; (3) web consolidation; and (4) web finishing. Material selection provides the properties suitable for the application. The web is formed from fibers of the selected materials. The web is then bonded to form a fabric and the fabric is finished to produce the final product for cutting and folding.
The diameter of the fiber affects properties of the fabric including strength and flexural rigidity. Fiber diameter can be measured and reported in a variety of fashions. Generally, fiber diameter is measured in denier per filament. Denier is a textile term which is defined as the grams of the fiber per 9000 meters of that fiber's length. Monofilament generally refers to an extruded strand having a denier per filament greater than 15, usually greater than 30. Fine denier fiber generally refers to fiber having a denier of about 15 or less. Microdenier (i.e., microfiber) generally refers to fiber having a diameter not greater than about 100 micrometers. For the fibers disclosed herein, the diameter can be widely varied, with little impact upon the fiber's elasticity. However, the fiber denier can be adjusted to suit the capabilities of the finished article and as such, would preferably be: from about 0.5 to about 30 denier/filament for melt blown; from about 1 to about 30 denier/filament for spunbond; and from about 1 to about 20,000 denier/filament for continuous wound filament. One can convert the fiber diameter in denier to meter according to the equation:
Other fiber properties that influence the fabric's final properties include the fiber's orientation, crystallinity, diameter and cooling rates. The strength of the bond is a limiting factor in nonwoven fabric strength. Lower fiber orientation allows for greater amounts of melting during bonding, causing stronger bonding regions. In addition, high amounts of orientation induced by drawing a polymer causes high amounts of shrinkage during thermal bonding making processability difficult.
Crystalline portions of a fiber are particularly of interest to the thermal bonding process due to the melting that occurs. The degree of melting and flow significantly impacts the bond strength. Less stable crystals melt first; followed by the more stable or oriented crystals if enough heat is transferred to the polymer. Because of the short duration of heat transfer to the bond area, only a fraction of the crystals melt.
After the web has been loosely formed, the individual fibers need to be bonded together. Web consolidation provides strength and rigidity to the fabric. Ways to consolidate the web include mechanical, chemical, and thermal bonding. Mechanical consolidation is accomplished by entangling fibers at various points in the web, including needle punching, stitch bonding, spunlacing, or any other mechanical consolidation process. Chemical bonding involves spraying or saturating the web with an adhesive such as latex. Thermal bonding of the web is a common bonding technique and include point-calender, ultrasonic and radian-heat bonding. In some embodiments, point-calender bonding is used and comprises passing the web through two heated rolls that are in intimate contact. One roll is male-patterned engraved and the other is a flat roll. The fibers melt and flow over one another. Upon cooling, the fabric is formed.
As a web of fibers is pulled into calender there are many thermomechanical processes of different magnitudes that occur. These processes include: conductive heat transfer; heat of deformation; flow of melted polymer; diffusion; and the Clapeyron effect.
Conductive heat transfer is transported across the steel roll, fabric interface. The amount of heat transferred by conduction is proportional to the temperature of the steel rolls and the amount of time the web spends under the bond pin (roll speed). Also adding heat to the system is the heat of deformation. Due to high pressures between the steel rolls, the web is formed into a different shape very quickly and mechanical work is done on the system. This mechanical work is transferred to heat. These two forms of heat raise the temperature of the web between the rolls and is highest under the bond pin. An equation assuming that all of the mechanical work transfers to heat is given by:
[F(s)ds]=VρC p ΔT+fΔH f χΔV
where F(s)ds is the force exerted on the web over a distance ds, a is the fraction of mechanical work converted to heat, V is the volume of the web, χ is the crystallinity, and f is the fraction of crystals that melt. The first term on the right side is the amount of heat used to increase the temperature and the second term describes the amount of heat that melts the polymer crystals.
When the temperature reaches its melting point, the high pressure under the pins causes the melt to flow outside to an area of lower pressure. Also while in the molten state, the polymer self-diffuses. Upon exiting the calender, the melt solidifies and mechanically locks the fibers at the bond point. These two phenomena fuse together several fibers at a bond point and turn the web into a fabric. The diffusion penetration distance for polymers during the bonding process is almost negligible. The penetration distance is given by:
where R is the penetration distance, t is the time, and D is the self-diffusion coefficient. In general, most polymers have a diffusion coefficient of a magnitude of 10−15 and spend 10 to 40 milliseconds under the bond pins. Using these rough numbers it is calculated that the penetration distance is only between 45 Å and 100 Å. Considering most fibers used in thermal bonding are about 20 microns in diameter, the fibers only diffuse 0.00000225 percent of their total diameter. Therefore, the mechanical interlocking of the polymer melt around the fibers in the bonding area is likely to be the dominating force holding the fibers together at the bond point.
The increased pressure under the bond pins leads to an increase in melting temperature otherwise known as the Clapeyron Effect. The effect of pressure increases the melting point of polypropylene 38 K/kbar or 0.38° C./Mpa. Using a typical pressure under the bond pin, polypropylene's melting point increases about 10° C. Polyethylene's melting temperature increases only about 5° C. under typical bonding pressures.
Several factors of the point-bond hot calendering process affect final fabric properties including temperature, pressure, speed, roll diameter and engraved pattern. The choice of temperature is mainly a function of the material, but it should be noted that the total energy transfer to the web is a function of temperature, pressure, roll diameter, and line speed. If the temperature is chosen too low, then the web is under-bonded and the fabric strength tends to be weak. If the roll temperature is too high then the web is over-bonded and the resulting fabric is too stiff or the web completely melts and sticks to the roll.
The effect of pressure applied to the fabric is small, but not negligible. At low pressures, the bonding of the web is poor and strength is, therefore, poor. As pressure increases, the fabric strength is a function of both bonding temperature and pressure. At very high pressure, though, the fabric strength reaches a maximum and then begins to decrease with increasing pressure. Below this pressure the strength increases continuously up to the melting point of the polymer.
The speed and diameter of the bond roll affect the total time of heat transfer to the web. Larger bond roll diameters allow for more intimate contact with the heated rolls than do smaller rolls. Hence there is more heat transferred to the web. In the same manner, slow spinning rolls have more contact time then fast spinning rolls.
The amount of time a fabric spends in the nip (intimate contact region) may be expressed as:
R=radius of bond roll
V=velocity of bond roll
where CO is the original web thickness, CN is the thickness between the bond rolls, and CR is the thickness after compression in the bond roll.
The shape of the fiber is not limited and can be any suitable shape. For example, typical fiber have a circular cross sectional shape, but sometimes fibers have different shapes, such as a trilobal shape, or a flat (i.e., “ribbon” like) shape.
After a thermally bonded fabric has eluded from the bond pins, cooling and solidifying of the bond regions occurs. The quench rate of the fabric and more specifically the bonding region may have an impact on the final fabric properties.
Important fabric properties include strength, elongation, peak load, abrasion and flexural rigidity. The strength or tenacity and elongation of a nonwoven fabric is important to both post production processes and the consumer. The more strength and elasticity a fabric has, the faster it can be combined with other materials into a final consumer product. Another property of a nonwoven fabric is its ability to resist abrasion. When an abrasive surface is applied to a nonwoven fabric, fibers are pulled from the surface and cause fuzz or pilling to form on the surface. As such, high abrasion resistance is desirable for nonwoven fabrics. Still another important property of a material that is worn by humans and placed against the skin is its stiffness. This property can be measured by flexural rigidity or handfeel evaluations.
Any fiber-forming polymers, especially those which can be thermally bonded, may be used in embodiments of the invention. For example, suitable polymers include, but are not limited to, α-olefin homopolymers and interpolymers comprising polypropylene, propylene/C4-C20 α-olefin copolymers, polyethylene, and ethylene/C3-C20 α-olefin copolymers, the interpolymers can be either heterogeneous ethylene/α-olefin interpolymers or homogeneous ethylene/α-olefin interpolymers, including the substantially linear ethylene/α-olefin interpolymers. Also included are aliphatic α-olefins having from 2 to 20 carbon atoms and containing polar groups. Suitable aliphatic α-olefin monomers which introduce polar groups into the polymer include, for example, ethylenically unsaturated nitriles such as acrylonitrile, methacrylonitrile, ethacrylonitrile, etc.; ethylenically unsaturated anhydrides such as maleic anhydride; ethylenically unsaturated amides such as acrylamide, methacrylamide etc.; ethylenically unsaturated carboxylic acids (both mono- and difunctional) such as acrylic acid and methacrylic acid, etc.; esters (especially lower, e.g. C1-C6, alkyl esters) of ethylenically unsaturated carboxylic acids such as methyl methacrylate, ethyl acrylate, hydroxyethylacrylate, n-butyl acrylate or methacrylate, 2-ethyl-hexylacrylate, or ethylene-vinyl acetate copolymers etc.; ethylenically unsaturated dicarboxylic acid imides such as N-alkyl or N-aryl maleimides such as N-phenyl maleimide, etc. Preferably such monomers containing polar groups are acrylic acid, vinyl acetate, maleic anhydride and acrylonitrile. Halogen groups which can be included in the polymers from aliphatic α-olefin monomers include fluorine, chlorine and bromine; preferably such polymers are chlorinated polyethylenes (CPEs). Polymers, such as polyester and nylon, also may be used.
Heterogeneous interpolymers are differentiated from the homogeneous interpolymers in that in the latter, substantially all of the interpolymer molecules have the same ethylene/comonomer ratio within that interpolymer, whereas heterogeneous interpolymers are those in which the interpolymer molecules do not have the same ethylene/comonomer ratio. The term “broad composition distribution” used herein describes the comonomer distribution for heterogeneous interpolymers and means that the heterogeneous interpolymers have a “linear” fraction and that the heterogeneous interpolymers have multiple melting peaks (i.e., exhibit at least two distinct melting peaks) by DSC. The heterogeneous interpolymers have a degree of branching less than or equal to 2 methyls/1000 carbons in about 10 percent (by weight) or more, preferably more than about 15 percent (by weight), and especially more than about 20 percent (by weight). The heterogeneous interpolymers also have a degree of branching equal to or greater than 25 methyls/1000 carbons in about 25 percent or less (by weight), preferably less than about 15 percent (by weight), and especially less than about 10 percent (by weight).
The heterogeneous polymer component can be an α-olefin homopolymer preferably polyethylene or polypropylene, or, preferably, an interpolymer of ethylene with at least one C3-C20 α-olefin and/or C4-C18 dienes. Heterogeneous copolymers of ethylene, and propylene, 1-butene, 1-hexene, 4-methyl-1-pentene and 1-octene are especially preferred.
Linear low density polyethylene (LLDPE) is produced in either a solution or a fluid bed process. The polymerization is catalytic. Ziegler Natta and single-site metallocene catalyst systems have been used to produce LLDPE. The resulting polymers are characterized by an essentially linear backbone. Density is controlled by the level of comonomer incorporation into the otherwise linear polymer backbone. Various alpha-olefins are typically copolymerized with ethylene in producing LLDPE. The alpha-olefins which preferably have four to eight carbon atoms, are present in the polymer in an amount up to about 10 percent by weight. The most typical comonomers are butene, hexene, 4-methyl-1-pentene, and octene. The comonomer influences the density of the polymer. Density ranges for LLDPE are relatively broad, typically from 0.87-0.95 g/cc (ASTM D-792).
Linear low density polyethylene melt index is also controlled by the introduction of a chain terminator, such as hydrogen or a hydrogen donator. The melt index, measured according to ASTM D-1238 Condition 190° C./2.16 kg (formerly known as “Condition E” and also known as “I2”), for a linear low density polyethylene can range broadly from about 0.1 to about 150 g/10 min. For purposes of the present invention, the LLDPE should have a melt index of greater than 10, and preferably 15 or greater for spunbonded filaments. Particularly preferred are LLDPE polymers having a density of 0.90 to 0.945 g/cc and a melt index of greater than 25.
Examples of suitable commercially available linear low density polyethylene polymers include the linear low density polyethylene polymers available from Dow Chemical Company, such as the ASPUNTM series of fibergrade resins, Dow LLDPE 2500 (55 MI, 0.923 density), Dow LLDPE Type 6808A (36MI, 0.940 density), and the EXACTTM series of linear low density polyethylene polymers from Exxon Chemical Company, such as EXACT™ 2003 (31 MI, density 0.921).
The homogeneous polymer component can be an α-olefin homopolymer preferably polyethylene or polypropylene, or, preferably, an interpolymer of ethylene with at least one C3-C20 α-olefin and/or C4-Cl8 dienes. Homogeneous copolymers of ethylene, and propylene, 1-butene, 1-hexene, 4-methyl-1-pentene and 1-octene are especially preferred.
The relatively recent introduction of metallocene-based catalysts for ethylene/α-olefin polymerization has resulted in the production of new ethylene interpolymers known as homogeneous interpolymers.
The homogeneous interpolymers useful for forming fibers described herein have homogeneous branching distributions. That is, the polymers are those in which the comonomer is randomly distributed within a given interpolymer molecule and wherein substantially all of the interpolymer molecules have the same ethylene/comonomer ratio within that interpolymer. The homogeneity of the polymers is typically described by the SCBDI (Short Chain Branch Distribution Index) or CDBI (Composition Distribution Branch Index) and is defined as the weight percent of the polymer molecules having a comonomer content within 50 percent of the median total molar comonomer content. The CDBI of a polymer is readily calculated from data obtained from techniques known in the art, such as, for example, temperature rising elution fractionation (abbreviated herein as “TREF”) as described, for example, in Wild et al, Journal of Polymer Science, Poly. Phys. Ed., Vol. 20, p. 441 (1982), in U.S. Pat. No. 4,798,081, or as is described in U.S. Pat. No. 5,008,204, the disclosure of which is incorporated herein by reference. The technique for calculating CDBI is described in U.S. Pat. No. 5,322,728 and in U.S. Pat. No. 5,246,783 or in U.S. Pat. No. 5,089,321 the disclosures of all of which are incorporated herein by reference. The SCBDI or CDBI for the homogeneous interpolymers used in the present invention is preferably greater than about 30 percent, especially greater than about 50 percent, 70 percent or 90 percent.
The homogeneous interpolymers used in this invention essentially lack a measurable “high density” fraction as measured by the TREF technique (i.e., the homogeneous ethylene/α-olefin interpolymers do not contain a polymer fraction with a degree of branching less than or equal to 2 methyls/1000 carbons). The homogeneous interpolymers also do not contain any highly short chain branched fraction (i.e., they do not contain a polymer fraction with a degree of branching equal to or more than 30 methyls/1000 carbons).
The substantially linear ethylene/α-olefin polymers and interpolymers are also homogeneous interpolymers but are further herein defined as in U.S. Pat. No. 5,272,236, and in U.S. Pat. No. 5,272,872, the entire contents of which are incorporated by reference. Such polymers are unique however due to their excellent processability and unique rheological properties and high melt elasticity and resistance to melt fracture. These polymers can be successfully prepared in a continuous polymerization process using the constrained geometry metallocene catalyst systems.
The term “substantially linear” ethylene/α-olefin interpolymer means that the polymer backbone is substituted with about 0.01 long chain branches/1000 carbons to about 3 long chain branches/1000 carbons, more preferably from about 0.01 long chain branches/1000 carbons to about 1 long chain branches/1000 carbons, and especially from about 0.05 long chain branches/1000 carbons to about 1 long chain branches/1000 carbons.
Long chain branching is defined herein as a chain length of at least one carbon more than two carbons less than the total number of carbons in the comonomer, for example, the long chain branch of an ethylene/octene substantially linear ethylene interpolymer is at least seven (7) carbons in length (i.e., 8 carbons less 2 equals 6 carbons plus one equals seven carbons long chain branch length). The long chain branch can be as long as about the same length as the length of the polymer back-bone. Long chain branching is determined by using 13C nuclear magnetic resonance (NMR) spectroscopy and is quantified using the method of Randall (Rev. Macromol. Chem. Phys., C29 (2&3), p. 285-297), the disclosure of which is incorporated herein by reference. Long chain branching, of course, is to be distinguished from short chain branches which result solely from incorporation of the comonomer, so for example the short chain branch of an ethylene/octene substantially linear polymer is six carbons in length, while the long chain branch for that same polymer is at least seven carbons in length.
Additional suitable polymers are disclosed in the following U.S. Pat. No.: 6,316,549; 6,281,289; 6,248,851; 6,194,532; 6,190,768; 6,140,442; 6,037,048; 5,603,888; 5,185,199, and 5,133,917, all of which are incorporated by reference herein in their entirety.
Examples of commercial fiber-forming polyethylene include ASPUN™ 6806A (melt index: 105.0 g/10min.; density: 0.930 g/cc), ASPUN™ 6842A (melt index: 30.0 g/10 min.; density: 0.955 g/cc), ASPUN™ 681A (melt index: 27.0 g/10min.; density: 0.941 g/cc), ASPUN™ 6830A (melt index: 18.0 g/10min.; density: 0.930 g/cc), ASPUN™ 6831A (melt index: 150.0 g/10min.; density: 0.930 g/cc), and ASPUN™ 8635A (melt index: 17.0 g/10min.; density: 0.950 g/cc), all available from The Dow Chemical Company, Midland, Mich. These linear low density polyethylene may be blended with a homogeneous substantially linear ethylene polymer, such as AFFINITY™ resin from The Dow Chemical Company.
Examples of commercial fiber-forming polypropylene include homopolypropylene designated as 5A10 (melt flow rate: 1.4 g/10min.; flexural modulus: 1585 MPa (230,000 psi)); 5A28 (melt flow rate: 3.0 g/10min.; flexural modulus:: 1585 MPa (230,000 psi)); 5A66V (melt flow rate: 4.6 g/10min.; flexural modulus: 1654 MPa (240,000 psi)); 5E17V (melt flow rate: 20.0 g/10min.; flexural modulus: 1344 MPa (195,000 psi)); 5E40 (melt flow rate: 9.6 g/10min.; flexural modulus: 1378 MPa (200,000 psi)); NRD5-1258 (melt flow rate: 100.0 g/10min.; flexural modulus: 1318 MPa (191,300 psi)); NRD5-1465 (melt flow rate: 20.0 g/10min.; flexural modulus: 1344 MPa (195,000 psi)); NRD5-1502 (melt flow rate: 1.6 g/10min.; flexural modulus: 1347 MPa (195,500 psi)); NRD5-1569 (melt flow rate: 4.2 g/10min.; flexural modulus: 1378 MPa (200,000 psi)); NRD5-1602 (melt flow rate: 40.0 g/10min.; flexural modulus: 1172 MPa (170,000 psi)); SRD5-1572 (melt flow rate: 38.0 g/10min.; flexural modulus: 1298 MPa (188,400 psi)); SRD5-1258 (melt flow rate: 25.0 g/10min.), and INSPIRE™ resin (melt flow rates ranging from 1.8 to about 25 g/10min.), all available from The Dow Chemical Company. The melt flow rate is measured according to ASTM D 1238 (230° C./2.16 kg), and flexural modulus according to ASTM D 790A. It should be understood that resins from other companies, such as Exxon, Bassel, Mitsui, etc., also may be used.
Additives such as antioxidants (e.g., hindered phenolics such as IRGANOX™ 1010 or IRGANOX™ 1076 supplied by Ciba Geigy), phosphites (e.g., IRGAFOS™ 168 also supplied by Ciba Geigy), cling additives (e.g., PIB), pigments, colorants, fillers, and the like, can also be included in the fiber materials disclosed herein.
Similarly, the polymers disclosed herein can be admixed with other polymers to modify characteristics such as elasticity, processability, strength, thermal bonding, or adhesion, to the extent that such modification does not adversely affect the desired properties.
Some useful materials for modifying the polymers include, other substantially linear ethylene polymers as well as other polyolefins, such as high pressure low density ethylene homopolymer (LDPE), ethylene-vinyl acetate copolymer (EVA), ethylene-carboxylic acid copolymers, ethylene acrylate copolymers, polybutylene (PB), ethylene/.alpha.-olefin polymers which includes high density polyethylene (HDPE), medium density polyethylene, polypropylene, ethylene-propylene interpolymers, ultra low density polyethylene (ULDPE), as well as graft-modified polymers involving, for example, anhydrides and/or dienes, or mixtures thereof.
Still other polymers suitable for modifying the polymers include synthetic and natural elastomers and rubbers which are known to exhibit varying degrees of elasticity. AB and ABA block or graft copolymers (where A is a thermoplastic endblock such as, for example, a styrenic moiety and B is an elastomeric midblock derived, for example, from conjugated dienes or lower alkenes), chlorinated elastomers and rubbers, ethylene propylene diene monomer (EDPM) rubbers, ethylene-propylene rubbers, and the like and mixtures thereof are examples of known prior art elastic materials contemplated as suitable for modifying the elastic materials disclosed herein.
Polypropylene can be blended with a lower melting polymer such as polyethylene to increase the strength in the bond region. In the same way LLDPE can be blended with a low melting/low density polyethylene to produce the same results.
The initial chemical structure of the polymer used to produce nonwovens has an affect on the fabric properties. A polymer's chemical structure impacts the polymer's density/crystallinity, viscosity, and molecular weight distribution. Also, addition of two or more polymers to make a blend can have a significant impact on the nonwoven properties. Fabric strength increases with increasing molecular weight distribution. The increase in MWD decreases the orientation of the fibers in the spinning process, causing greater melting during calendering.
The nonwoven fabrics in accordance with embodiments of the invention have utility in a variety of applications. Suitable applications include, but are not limited to, disposable personal hygiene products (e.g. training pants, diapers, absorbent underpants, incontinence products, feminine hygiene items and the like), disposable garments (e.g. industrial apparel, coveralls, head coverings, underpants, pants, shirts, gloves, socks and the like) and infection control/clean room products (e.g. surgical gowns and drapes, face masks, head coverings, surgical caps and hood, shoe coverings, boot slippers, wound dressings, bandages, sterilization wraps, wipers, lab coats, coverall, pants, aprons, jackets, bedding items and sheets). The nonwoven fabrics also may be used in manners taught in the following U.S. Pat. Nos.: 6,316,687; 6,314,959; 6,309,736; 6,286,145; 6,281,289; 6,280,573; 6,248,851; 6,238,767; 6,197,322; 6,194,532; 6,194,517; 6,176,952; 6,146,568; 6,140,442; 6,093,665; 6,028,016; 5,919,177; 5,912,194; 5,900,306; 5,830,810; and 5,798,167, all of which are incorporated by reference herein in their entirety.
The following examples exemplify some embodiments of the invention. They do not limit the invention as otherwise described and claimed herein. All numbers in the examples are approximate values. In the following examples, various nonwoven fabrics were characterized by a number of methods. Performance data of these fabrics were also obtained. Most of the methods or tests were performed in accordance with an ASTM standard, if applicable, or known procedures.
Preparation of Polymer Blends
A HAAKE twin screw extruder was used to produce polymer blends. The extruder has the following characteristics:
6 heating zones with temperatures of 110° C., 120° C., 130° C., 135° C., 135° C., 135° C. respectively.
Two 18 mm diameter screws.
Melt temperature=146° C.
Die Pressure=2.64×106 Pa (383 psi)
Torque=3.44×107 Pa (5000 psi)
Preparation of Polymer Fibers
Fibers were produced by extruding the polymer using a one inch diameter extruder which feeds a gear pump. The gear pump pushes the material through a spin pack containing a 40 micrometer (average pore size) sintered flat metal filter and a 108 hole spinneret. The spinneret holes have a diameter of 400 micrometers and a land length (i.e., length/diameter or L/D) of 4/1. The gear pump is operated such that about 0.3 grams of polymer are extruded through each hole of the spinneret per minute. Melt temperature of the polymer varies depending upon the molecular weight of the polymer being spun. Generally the higher the molecular weight, the higher the melt temperature. Quench air (slightly above room temperature (about 24° C.) is used to help the melt spun fibers cool. The quench air is located just below the spinneret and blows air across the fiber line as it is extruded. The quench air flow rate is low enough so that it can barely be felt by hand in the fiber area below the spinneret. The fibers are collected on godet rolls having a diameter of about 0.152 m (6 inches). The godet roll's speed is adjustable, but for the experiments demonstrated herein, the godet's speed is about 1500 revolutions/minute. The godet rolls are located about 3 meters below the spinneret die. Immediately following the spinning process, all fibers are cut into fibers of 0.0381 m (1.5 inches) in length.
Preparation of Nonwoven Fabrics
Nonwoven fabric samples were produced on a laboratory calender equipped with a hardened, chromed engraved steel roll according to the procedures described herein. An engraved pattern contains a 20 percent total bonding area and 3.44×105 bonding points per square meter (222 bonding points per square inch). FIGS. 3A-3I schematically show various bonding patterns along with their dimensions that were used in embodiments of the invention.
For each pattern design the following procedure was followed. All fibers were 3 denier. The fibers were then fed into a carding machine. The fibers were pulled into the RotorRing by a vacuum and passed through a series of needles. The fibers were then neatly arranged for future carding by a high speed centrifuge. This process was repeated for each sample. Next, the fibers were distributed evenly on a steel tray of dimensions 10 cm by 40 cm a paper feed card encases the front end of the fiber web. This produces a web with a basis weight of 33 g/m2 or 1 oz/yd2. The fiber web was placed between the moving, heated calender rolls where the web was thermally bonded into a nonwoven fabric. The starting bond roll conditions were as follows:
Top (engraved) roll temperature—from about 110° C. (230° F.) to about 121.1° C. (250° F.), which is the temperature described in the figures and tables.
Bottom (smooth) roll temperature—from about 110° C. (230° F.) to about 121.1° C. (250° F.), which is about 4° C. higher than the top roll to avoid sticking to the top roll.
Hydraulic pressure—from about 4.82×106 Pa (700 psi) to about 1.03×107 Pa (1500 psi).
Roll Speed/dial setting=from about 3 to about 5 m/min.
The fabrics produced contain mostly machine direction alignment. There is very little cross direction alignment of the fibers. Characterization of fabrics and fiber orientation were conducted using the following technique:
1. Optical micrographs were obtained from randomly selected fabrics from this experiment. Both the top of the fabric and the bottom of the fabric were photographed at 40× magnification. Optical micrographs were also obtained from commercial Spunbond PP fabric made at TANDEC in the same manner.
2. The micrographs were transferred to Scion Imaging software and divided into four quarters.
3. The angles of the fibers in each quarter of the micrographs were measured with the machine direction being vertical (0°) and the cross direction being horizontal (90°).
Once all fibers were measured, the following equation was used to quantify the orientation:
θ is the angle of the fiber and Fp is the orientation parameter is which a value of 0 corresponds to random orientation and a value of 1 corresponds to perfect alignment in one direction.
The tensile strength of each fabric sample was investigated using an Instron 4501 tensile tester. Line grip jaws were used to fasten the fabric to the Instron. The “Standard Test Method for Breaking Force and Elongation of Textile Fabrics” (ASTM D 5035-90) was used with one exception. The strips were not cut into 0.152 m (6 inch) strips but into 0.101 m (4 inch) strips.
A standard abrasion procedure was developed comprising the following steps using a Taber Abraser model 503 (Rotary Platform-Double-Head Method) with an 8 compartment sample holder:
1. The fabric was cut into 0.0762×0.0762 m (3×3 inch) pieces and labeled.
2. An adhesive backing was applied to the edges of the abraded surface to prevent tearing at the edges.
3. Samples were weighed individually to 4 decimal places.
4. The samples were placed in the sample holder making sure not to cause any wrinkles or loose areas. The samples were arranged with the machine direction pointing at the center of the sample holder and the engraved pattern side facing up.
5. The fabric samples were abraded for a determined amount of cycles (100) using CO2 rubber abrasion wheels. Masking tape made by American Tape was applied to the abraded surface and then removed in a steady, but quick motion.
6. The fabric was again weighed and recorded.
Any samples that tore or completely degraded during abrasion were thrown out and deleted from further testing.
Flexural rigidity was measured according to the design specifications of ASTM method D 1388-64. A leveling bubble was placed on the horizontal platform before measurements were taken to ensure consistency. The length of overhang and the basis weight of the fabric was then used to calculate flexural rigidity. Although the cantilever test is a way to easily measure the stiffness of all fabrics, it is important to be able to correlate the results with consumer opinion. The feel of a fabric in a persons hand may have different properties than that found in a mechanical test. In addition, the surface of the fabric should have a soft feel to the touch as well.
All handfeel evaluations were conducted by a panel of 12 who were chosen to make evaluations on the graininess and stiffness of the fabrics. All panelists followed the following procedure:
1. Each panelist was given 4 anchor samples and their corresponding number ranging from 1 for the least grainy or least stiff to 15 for the most stiff or most grainy. The anchors and their corresponding numbers are given in Table 10.
2. The panelist laid the sample flat on the table with the embossed side of the fabric face up. Their wrist lay on the table top and their index and middle fingers moved across the entire surface of the sample. This process was repeated in all four directions of the sample. Their evaluation rating for graininess was recorded.
3. The panelist laid the sample flat on a table with their dominant hand on top of the sample. Their fingers were positioned so the fingers are pointed toward the top of the sample. The sample was gathered with fingers moving toward their palm while the opposite hand guided the sample into a cupped hand. The sample is squeezed and released repeatedly.
4. Their evaluation rating was recorded for stiffness.
- Example 1
All samples were evaluated before a number rating was given for each. Due to the availability of panelists only a selected set of samples were tested.
Polyethylene (PE) polymers were obtained from The Dow Chemical Company. The polyethylene polymers have varying density and melt indices. A polypropylene (PP) polymer also was obtained from The Dow Chemical Company. The properties of the polymers are given in Table 1.
|TABLE 1 |
|Polymers Used in Experiments |
| || ||Melt Index || |
|Polymer Grade ||Density (g/cc) ||(g/10 minutes) ||Melt Point (° C.) |
|PE1 ||0.955 ||29 ||131 |
|PE2 ||0.941 ||27 ||125 |
|PE3 ||0.950 ||17 ||129 |
|PE4 ||0.870 || 1 || 55 |
|PP1 ||0.910 ||35 ||165 |
Polyethylene that is representative of PE1 include ASPUN™ 6842A available from The Dow Chemical Company, Midland, Mich. Polyethylene that is representative of PE2 include ASPUN™ 6811 available from The Dow Chemical Company, Midland, Mich. Polyethylene that is representative of PE3 include ASPUN™ 6835A available from The Dow Chemical Company, Midland, Mich. Polyethylene that is representative of PE4 include AFFINITY™ EG8100 available from The Dow Chemical Company, Midland, Mich. Polypropylene that is representative of PP1 include H500-35 available from The Dow Chemical Company, Midland, Mich. Four samples were formulated from the polyethylene polymers. Three homopolymers and a 95 percent/5 percent blend of PE1 and PE4 were tested. Compounding for the blend was as described above. 4.75 kg of PE1 pellets were combined with 0.25 kg of PE4 and placed in the hopper of the twin screw extruder. After exiting the extruder, the polymer is pulled through a cooling bath maintained at 5° C. The solid polymer is then fed into a Berlyn Clay Group chipper where it is cut into pellets. The polymer was purged for 15 minutes and pellets were collected for 100 minutes.
Fibers were produced using the spinning conditions given in Table 2 and the process described above.
|TABLE 2 |
|Spinning Conditions for Various Fibers |
| || || || || ||Predicted ||Total |
| || ||Extruder ||Godet ||Godet ||Fiber ||Mass of |
| || ||Temp. ||Speed ||Speed ||Diameter ||Sample |
|Fiber ||Polymer ||(° C.) ||(rpm) ||(m/min) ||(microns) ||(g) |
|1 ||PE1 ||190 ||1800 ||900 ||21 ||540 |
|2 ||PE2 ||190 ||1800 ||900 ||21 ||180 |
|3 ||PE3 ||190 ||1800 ||900 ||21 ||180 |
|4 ||PE1 + PE4 ||190 ||1800 ||900 ||21 ||180 |
|5 ||PP1 ||230 ||1800 ||900 ||21 ||180 |
Fabrics were produced from the above described processes using the fibers produced in Table 1 and were coded in the following manner. A series of three numbers was assigned to each sample. The first number indicated the polymer used. The second number indicated the bond pattern number and the third number indicated the bonding temperature. Refer to Table 2 for reference to the polymer number and FIGS. 3A-3I for reference to bond pattern numbers. For convenience, this labeling system is used to identify samples.
FIG. 3A is for bond pattern 1 having a 46° angle, 20 percent bond area, 3.44×105 pts/m2 (222 pts/in2), base width of 1.7×10−3 m (0.067 inch), base height of 4.32×10−4 m (0.017 inch), and a point width of 7.62×10−4 m (0.03 inch). FIG. 3B is for bond pattern 2 having a 20°angle, 16 percent bond area, 3.44×105 pts/m2 (222 pts/in2), base width of 1.7×10−3 m (0.067 inch), base height of 4.32×10−4 m (0.017 inch), and a point width of 6.86×10−4 m (0.027 inch). FIG. 3C is for bond pattern 3 having a 20° angle, 24 percent bond area, 3.44×105 pts/m2 (222 pts/in2), base width of 1.7×10−3 m (0.067 inch), base height of 4.32×10−4 m (0.017 inch), and a point width of 8.38×10−4 m (0.033 inch). FIG. 3D is for bond pattern 4 having a 20° angle, 20 percent bond area, 2.31×105 pts/m2 (149 pts/in2), base width of 1.7×10−3 m (0.067 inch), base height of 4.32×10−4 m (0.017 inch), and a point width of 9.30×10−4 m (0.0366 inch). FIG. 3E is for bond pattern 5 having a 20° angle, 20 percent bond area, 4.60×105 pts/m2 (297 pts/in2), base width of 1.7×10−3 m (0.067 inch), base height of 4.32×10−4 m (0.017 inch), and a point width of 6.60×10−4 m (0.026 inch). FIG. 3F is for bond pattern 6 having a 42° angle, 16 percent bond area, 3.44×105 pts/m2 (222 pts/in2), base width of 1.7×10−3 m (0.067 inch), base height of 4.32×10−4 m (0.017 inch), and a point width of 6.86×10−4 m (0.027 inch). FIG. 3G is for bond pattern 7 having a 37° angle, 24 percent bond area, 3.44×105 pts/m2 (222 pts/in2), base width of 1.7×10−3 m (0.067 inch), base height of 4.32×10−4 m (0.017 inch), and a point width of 8.38×10−4 m (0.033 inch). FIG. 3H is for bond pattern 8 having a 46° angle, 20 percent bond area, 2.31×105 pts/m2 (149 pts/in2), base width of 1.7×10−3 m (0.067 inch), base height of 4.32×10−4 m (0.017 inch), and a point width of 9.3×10−4 m (0.0366 inch). FIG. 3I is for bond pattern 9 having a 35° angle, 20 percent bond area, 4.60×105 pts/m2 (297 pts/in2), base width of 1.7×10−3 m (0.067 inch), base height of 4.32×10−4 m (0.017 inch), and a point width of 6.60×10−4 m (0.026 inch).
Next, pieces of fabric were cut for tensile testing, abrasion testing, and a cantilever test. All samples were cut from the center due to inconsistency in the fiber web and processing temperature at the edges.
The fabrics were visually evaluated. Temperature, pressure and resin choice had no effect on the visual appearance of the fabric. Bond roll pattern had a noticeable effect on the fabric's visual property. FIGS. 4A-4I are micrographs at 20× magnification of nonwovens produced from resin 6824A at 119.4° C. (247° F.) which show the differences in the fabrics visually. The dark diamond areas are the bond areas of the fabrics, while the lighter areas are unbonded fibers.
A comparison of FIGS. 4A, 4F, 4
H and 4
I to FIGS. 4B, 4C, 4
D, and 4
E show that a side wall angle of 20° produces a smaller bond area than that of patterns that contain a larger side wall angle. Measurements of the bond site areas of the fabric are given in Table 3. The data show a greater percent bond area than the roll pattern that produced the fabric in bond patterns 1, 6, 7 and 8. This is due to the melt flow of polymer from under the bond pin and also the increased heat transfer due to compaction of fibers in the void areas between the bond pins. The fibers contain less free space and heat transfer via conduction is higher. All patterns containing 20° side wall angles show a fabric percent bond area less than that of the roll pattern. Shrinkage of the polymer fibers is a possible cause. During the spinning process of the fibers, the fibers are solidified under tension in an oriented state. When the fibers are exposed to the higher temperatures under the bond pins, the polymer molecules relax back or shrink to a more stable state.
|TABLE 3 |
|Measured Bond Areas of Nonwoven Samples |
| || ||Temperature ||Average percent |
|Resin ||Pattern ||° C. (° F.) ||bond area |
|PE1 ||1 ||119.4 (247) ||33.7 |
|PE1 ||2 ||119.4 (247) ||16.5 |
|PE1 ||3 ||119.4 (247) ||30.8 |
|PE1 ||4 ||119.4 (247) ||19.0 |
|PE1 ||5 ||119.4 (247) ||20.7 |
|PE1 ||6 ||119.4 (247) ||17.7 |
|PE1 ||7 ||119.4 (247) ||31.8 |
|PE1 ||8 ||119.4 (247) ||24.7 |
|PE1 ||9 ||119.4 (247) ||24.1 |
|PE2 ||1 ||116.1 (241) ||31.1 |
|PE2 ||2 ||116.1 (241) ||14.0 |
|PE2 ||3 ||116.1 (241) ||20.7 |
|PE2 ||4 ||116.1 (241) ||17.6 |
|PE2 ||5 ||116.1 (241) ||17.5 |
|PE2 ||6 ||116.1 (241) ||17.1 |
|PE2 ||7 ||116.1 (241) ||28.6 |
|PE2 ||8 ||116.1 (241) ||22.2 |
|PE2 ||9 ||116.1 (241) ||23.1 |
|PE3 ||1 ||119.4 (247) ||33.0 |
|PE3 ||2 ||119.4 (247) ||13.8 |
|PE3 ||3 ||119.4 (247) ||23.5 |
|PE3 ||4 ||119.4 (247) ||15.5 |
|PE3 ||5 ||119.4 (247) ||17.3 |
|PE3 ||6 ||119.4 (247) ||16.3 |
|PE3 ||7 ||119.4 (247) ||28.4 |
|PE3 ||8 ||119.4 (247) ||23.2 |
|PE3 ||9 ||119.4 (247) ||19.8 |
|95 percent PE1 + ||1 ||119.4 (247) ||30.8 |
|5 percent PE4 |
|95 percent PE1 + ||2 ||119.4 (247) ||13.4 |
|5 percent PE4 |
|95 percent PE1 + ||3 ||119.4 (247) ||19.0 |
|5 percent PE4 |
|95 percent PE1 + ||4 ||119.4 (247) ||17.0 |
|5 percent PE4 |
|95 percent PE1 + ||5 ||119.4 (247) ||16.5 |
|5 percent PE4 |
|95 percent PE1 + ||6 ||119.4 (247) ||15.3 |
|5 percent PE4 |
|95 percent PE1 + ||7 ||119.4 (247) ||26.8 |
|5 percent PE4 |
|95 percent PE1 + ||8 ||119.4 (247) ||21.8 |
|5 percent PE4 |
|95 percent PE1 + ||9 ||119.4 (247) ||20.0 |
|5 percent PE4 |
The 20° side wall angle patterns also appear to have fibers that are less compacted together or a higher porosity. Bond patterns 4, 5, 7 and 8 have the same percent bond area, but different concentrations of points per square meter. The distance between each bond point is greater for the patterns with the lower concentration of points per square meter.
An analysis of the fabric weight is shown in Table 4. Due to the variation in the carding process and the handling of fiber webs, thin spots appear in the fabric. Variability in thickness can have a strong impact on mechanical properties. The weight of one square inch samples within a fabric has very low variability.
|TABLE 4 |
|Analysis of Fabric Weight Between and Within Samples |
| ||Resin ||Pattern ||Average Weight (g) |
| || |
| ||PE1 ||1 ||0.021 |
| ||PE1 ||2 ||0.023 |
| ||PE1 ||3 ||0.023 |
| ||PE1 ||4 ||0.020 |
| ||PE1 ||5 ||0.019 |
| || |
I are micrographs of varying bond patterns of PE1 resin at 119.4° C. (247° F.) that were also used to evaluate fiber orientation. A study of the figures show most of the fibers arranged in one direction (up and down). This is the machine direction (MD) of the fibers. Most spunbond and meltdown fabrics contain more of a random arrangement of fibers so that the fabric contains cross direction (CD) strength as well as machine direction strength. An evaluation of randomly selected fabrics showed that fabric orientation (fp
) values for a commercial spunbond fabric were much lower than that of the samples produced and tested in these examples. The commercial spunbond fabric was made from polypropylene at TANDEC. The results are given in Table 5. The fp
values at the bottom of the fabric are higher than that of the top meaning the fibers are more aligned in the machine direction on the bottom. The bond pins on the top pushes the fibers into a more random state while the bottom fibers bonded against a flat roll maintain the alignment of the web.
|TABLE 5 |
|Data Collected from Orientation of Fibers Measurement |
| ||Fabric ||fp (1) ||fp (2) ||fp (3) ||Average fp |
| || |
| ||PE (Top) ||0.56 ||0.54 ||0.53 ||0.54 |
| ||PE (Bottom) ||0.7 ||0.82 ||0.69 ||0.74 |
| ||Spunbond PP ||0.22 ||0.19 ||0.25 ||0.22 |
| ||Spunbond PP ||0.16 ||0.18 ||0.22 ||0.19 |
| || |
Tables 6 through 9 show various properties of the nonwoven fabrics for each polymer fiber tested using various bond patterns at various temperatures. A series of three numbers was assigned to each sample. The first number indicates the polymer used. The second number indicates the bond pattern number and the third number indicates the bonding temperature in ° F. Refer to Table 2 for reference to the polymer number and FIGS. 3
I for reference to bond pattern numbers. For convenience, this labeling system is used to identify samples. For example, 1-1-116.1 stands for fabric made of PE1 resin using bond pattern 1 (FIG. 3A) at a bonding temperature of 116.1° C. (241° F.). Tensile properties of all samples were measured for peak load and elongation at break using an Instron 4501 and procedure ASTM D 5035-90 as previously described. Due to variability between fabrics produced at the same conditions 6 tensile samples were tested. The average abrasion (ABR) observed at each of the processing conditions is listed. For flexural rigidity (FR), each fabric was measured for length of overhang along with its basis weight to determine FR according to ASTM D 1388-64 as described above. The average FR measured of each resin with each processing condition is listed.
|TABLE 6 |
|Data for Resin PE1 |
| ||Avg. || || || |
| ||percent ||Normalized Avg. ||Avg. Abrasion ||Avg. FR |
|Sample ||Elongation ||Peak Load (g) ||(mg/cm2) ||(mg*cm) |
|1-1-116.1 ||43.17 ||1974 ||0.76 ||29.1 |
|1-1-117.7 ||52.20 ||2026 ||0.71 ||33.8 |
|1-1-119.4 ||70.00 ||2161 ||0.55 ||45.9 |
|1-2-116.1 ||16.77 ||920 ||1.02 ||17.6 |
|1-2-117.7 ||17.40 ||882 ||1.01 ||20.3 |
|1-2-119.4 ||31.61 ||1186 ||0.83 ||22.0 |
|1-2-116.1 ||17.00 ||927 ||0.77 ||30.4 |
|1-3-117.7 ||20.06 ||1032 ||0.71 ||28.8 |
|1-3-119.4 ||27.81 ||1330 ||0.53 ||33.6 |
|1-4-116.1 ||20.73 ||956 ||1.08 ||17.8 |
|1-4-117.7 ||19.66 ||915 ||0.92 ||19.6 |
|1-4-119.4 ||27.64 ||1141 ||0.64 ||19.6 |
|1-5-116.1 ||9369 ||806 ||0.99 ||17.9 |
|1-5-117.7 || || ||0.89 ||25.3 |
|1-5-119.4 ||19.37 ||1073 ||0.66 ||32.4 |
|1-6-116.1 ||32.08 ||1253 ||0.93 ||28.4 |
|1-6-117.7 ||49.86 ||1393 ||0.89 ||38.4 |
|1-6-119.4 ||76.48 ||1619 ||0.75 ||50.8 |
|1-7-116.1 ||41.15 ||1511 ||0.72 ||48.0 |
|1-7-117.7 ||52.51 ||1821 ||0.66 ||51.4 |
|1-7-119.4 ||93.13 ||2149 ||0.54 ||70.1 |
|1-8-116.1 ||48.27 ||1517 ||0.97 ||41.4 |
|1-8-117.7 ||75.35 ||1666 ||0.83 ||46.2 |
|1-8-119.4 ||70.34 ||1865 ||0.61 ||56.0 |
|1-9-116.1 ||24.08 ||1193 ||0.94 ||45.7 |
|1-9-117.7 ||28.04 ||1335 ||0.85 ||55.6 |
|1-9-119.4 ||53.75 ||1493 ||0.64 ||85.2 |
|TABLE 7 |
|Data for Resin PE2 |
| ||Avg. || || || |
| ||percent ||Normalized Avg. ||Avg. Abrasion ||Avg. FR |
|Sample ||Elongation ||Peak Load (g) ||(mg/cm2) ||(mg*cm) |
|2-1-112.7 ||24.62 ||1646 ||0.94 ||31.8 |
|2-1-114.4 ||31.54 ||1912 ||0.80 ||43.3 |
|2-1-116.1 ||45.24 ||2075 ||0.68 ||66.2 |
|2-2-112.7 ||49.30 ||1228 ||1.20 ||30.1 |
|2-2-114.4 ||60.96 ||1336 ||0.92 ||36.7 |
|2-2-116.1 ||36.26 ||1188 ||0.75 ||50.3 |
|2-3-112.7 ||63.42 ||1370 ||1.00 ||35.9 |
|2-3-114.4 ||62.79 ||1544 ||0.74 ||41.8 |
|2-3-116.1 ||33.12 ||1336 ||0.59 ||55.1 |
|2-4-112.7 ||81.15 ||1482 ||1.06 ||20.2 |
|2-4-114.4 ||90.96 ||1525 ||0.78 ||24.2 |
|2-4-116.1 ||39.15 ||1316 ||0.65 ||35.9 |
|2-5-112.7 ||54.37 ||1409 ||0.97 ||35.0 |
|2-5-114.4 ||64.09 ||1508 ||0.84 ||40.1 |
|2-5-116.1 ||19.75 ||1344 ||0.61 ||51.5 |
|2-6-112.7 ||74.57 ||1530 ||1.06 ||39.6 |
|2-6-114.4 ||56.29 ||1438 ||0.93 ||52.8 |
|2-6-116.1 ||38.05 ||1057 ||0.75 ||57.4 |
|2-7-112.7 ||68.12 ||1583 ||0.96 ||43.0 |
|2-7-114.4 ||64.24 ||1743 ||0.78 ||54.3 |
|2-7-116.1 ||50.48 ||1858 ||0.58 ||72.5 |
|2-8-112.7 ||95.53 ||1594 ||1.04 ||35.5 |
|2-8-114.4 ||91.61 ||1617 ||0.75 ||40.3 |
|2-8-116.1 ||33.78 ||1122 ||0.65 ||48.6 |
|2-9-112.7 ||60.33 ||1685 ||0.95 ||54.6 |
|2-9-114.4 ||78.11 ||1705 ||0.83 ||54.1 |
|2-9-116.1 ||73.58 ||1950 ||0.61 ||60.8 |
|TABLE 8 |
|Data for Resin PE3 |
| ||Avg. || || || |
| ||percent ||Normalized Avg. ||Avg. Abrasion ||Avg. FR |
|Sample ||Elongation ||Peak Load (g) ||(mg/cm2) ||(mg* cm) |
|3-1-116.1 ||17.74 ||1447 ||0.92 ||40.0 |
|3-1-117.7 ||21.50 ||1702 ||0.61 ||41.0 |
|3-1-119.4 ||27.82 ||1919 ||0.55 ||46.3 |
|3-2-116.1 ||13.53 ||1242 ||1.09 ||41.4 |
|3-2-117.7 ||23.23 ||1785 ||0.97 ||40.1 |
|3-2-119.4 ||32.40 ||1992 ||0.79 ||46.0 |
|3-2-116.1 ||21.65 ||1922 ||0.89 ||36.8 |
|3-3-117.7 ||28.69 ||2021 ||0.62 ||44.8 |
|3-3-119.4 ||40.03 ||2274 ||0.56 ||44.9 |
|3-4-116.1 ||22.66 ||1721 ||1.06 ||27.7 |
|3-4-117.7 ||26.83 ||1845 ||0.89 ||38.7 |
|3-4-119.4 ||38.57 ||2035 ||0.69 ||42.1 |
|3-5-116.1 ||12.33 ||1248 ||1.05 ||28.8 |
|3-5-117.7 ||16.31 ||1582 ||0.87 ||35.4 |
|3-5-119.4 ||28.89 ||1975 ||0.70 ||40.3 |
|3-6-116.1 ||18.79 ||1138 ||1.03 ||60.4 |
|3-6-117.7 ||28.29 ||1677 ||0.88 ||88.4 |
|3-6-119.4 ||41.52 ||1980 ||0.80 ||98.0 |
|3-7-116.1 ||24.87 ||1597 ||0.94 ||82.9 |
|3-7-117.7 ||41.28 ||1879 ||0.66 ||90.1 |
|3-7-119.4 ||51.97 ||2376 ||0.55 ||125.5 |
|3-8-116.1 ||26.63 ||1255 ||0.97 ||74.1 |
|3-8-117.7 ||43.24 ||1806 ||0.81 ||79.9 |
|3-8-119.4 ||36.78 ||2017 ||0.68 ||88.4 |
|3-9-116.1 ||16.56 ||904 ||0.90 ||80.7 |
|3-9-117.7 ||16.83 ||1279 ||0.84 ||103.7 |
|3-9-119.4 ||20.26 ||1456 ||0.65 ||116.4 |
|TABLE 9 |
|Data for Resin Containing 95 percent PE1 and 5 percent PE4 |
| ||Avg. || || || |
| ||percent ||Normalized Avg. ||Avg. Abrasion ||Avg.FR |
|Sample ||Elongation ||Peak Load (g) ||(mg/cm2) ||(mg*cm) |
|4-1-116.1 ||54.03 ||2065 ||0.96 ||22.9 |
|4-1-117.7 ||81.32 ||2288 ||0.75 ||35.8 |
|4-1-119.4 ||31.72 ||1988 ||0.50 ||39.0 |
|4-2-116.1 ||20.23 ||1322 ||1.09 ||32.6 |
|4-2-117.7 ||33.20 ||1659 ||1.00 ||42.0 |
|4-2-119.4 ||33.48 ||1676 ||0.72 ||53.4 |
|4-2-116.1 ||27.46 ||1485 ||0.95 ||35.3 |
|4-3-117.7 ||36.27 ||1735 ||0.71 ||32.6 |
|4-3-119.4 ||51.98 ||2192 ||0.53 ||49.0 |
|4-4-116.1 ||27.59 ||1452 ||1.33 ||26.5 |
|4-4-117.7 ||39.67 ||1756 ||1.05 ||30.3 |
|4-4-119.4 ||42.27 ||1928 ||0.77 ||29.4 |
|4-5-116.1 ||19.75 ||1344 ||1.28 ||31.0 |
|4-5-117.7 ||34.79 ||1800 ||1.03 ||47.7 |
|4-5-119.4 ||41.19 ||2017 ||0.70 ||48.7 |
|4-6-116.1 ||34.41 ||1590 ||0.97 ||56.9 |
|4-6-117.7 ||60.42 ||1812 ||0.84 ||71.0 |
|4-6-119.4 ||28.85 ||1589 ||0.63 ||91.0 |
|4-7-116.1 ||49.89 ||1920 ||0.93 ||67.9 |
|4-7-117.7 ||75.67 ||2241 ||0.73 ||82.0 |
|4-7-119.4 ||32.57 ||1861 ||0.48 ||102.7 |
|4-8-116.1 ||54.02 ||1862 ||0.99 ||46.5 |
|4-8-117.7 ||45.77 ||2076 ||0.85 ||62.4 |
|4-8-119.4 ||46.92 ||1884 ||0.64 ||77.9 |
|4-9-116.1 ||29.05 ||1362 ||1.03 ||67.7 |
|4-9-117.7 ||53.70 ||1737 ||0.85 ||80.7 |
|4-9-119.4 ||57.83 ||1862 ||0.58 ||109.6 |
Peak load values ranged from 800 g to as high as 2400 g. These values are much less than a typical PP sample. Using pattern 2 at 136.6° C. (278° F.), PP1 produces a peak load of 4875 g. In general, the normalized peak load increases with an increase in temperature, bond area, and bond angle. For resin PE2 and the blend of 95 percent PE1 and 5 percent PE4 the peak load decreased when increasing the temperature from 114.4° C. (238° F.) to 116.1° C. (241° F.) for PE2 and from 117.7° C. (244° F.) to 119.4° C. (247° F.) for the blend. This may be contributed to a change in the fracture mechanism. The pairwise comparison of samples shows that a 24 percent bond area has a higher peak load than the 16 percent bond area. As shown earlier, the bond angle has a dramatic effect on the actual bond area on the samples. FIG. 5 is a graph of normalized peak load versus temperature of PE2 resin at different temperatures and using various bond patterns. The peak load was linearly normalized to a basis weight of 33 g/m2(1 oz/yd2) because peak load is a strong function of basis weight.
FIG. 6 is a graph of percent elongation versus temperature for resin PE2 at different temperatures using various bond patterns. The elongation of PE nonwovens ranged from 10 percent to as high as 95 percent. Using pattern 2 at 136.6° C. (278° F.), the elongation of PP only reached 31 percent and 37 percent was the highest value reached at any process condition. A decrease in the concentration of bond points increases the elongation significantly. In fact, resin PE2 almost doubled its elongation at 114.4° C. (238° F.) with a decrease in concentration of bond points from 4.60×105 pts./m2 to 2.31×105 pts./m2 (297 pts./in2 to 149 pts./in.2). The exception to this is resin of 95 percent PE1 and 5 percent PE4 which does not show a large difference in elongation with decreasing concentration of bond points. This can be explained by the highly elastic property of PE4 that could be more significant than the effect of the bond pattern. Temperature control is important. A 1.6° C. (3° F.) difference in temperature can have as much as a 100 percent decrease in elongation.
Three examples of typical stress-strain curves for resin PE1 are given in FIG. 7. The samples were manufactured using PE1 resin using bond pattern 3 at temperatures of 116.1° C. (241° F.), 117.7° C. (244° F.), and 119.4° C. (247° F.). As the temperature increases so does the peak load. At the highest temperature of 119.4° C. (247° F.), the elongation of the fabric decreases. Also the initial modulus of the fabric produced at 119.4° C. (247° F.) is higher than those produced at lower temperatures. This is typical of all the fabric samples.
FIG. 8 is a typical graph of abrasion versus temperature for resin PE1. In general, the data show that elongation is a function of all processing variables. An increase in bond area and bond angle which are interrelated increases the elongation of the fabric. In general the abrasion resistance is mostly a function of temperature, although significant differences can be seen between bond patterns. This may be explained by its fracture mechanism. As the surface is abraded, the fibers are pulled from the bond points. Because of the fracture mechanism for abrasion, the amount of fuzz on the surface depends on bond strength more than the size of the bond. The values for abrasion ranged from 0.48 mg/cm2 to greater than 1 mg/cm2. A PP sample using bond pattern 2 at 136.6° C. (278° F.) has an abrasion value of 0.15 mg/cm2, over 3 times less than that of PE.
A plot of flexural rigidity (“FR”) vs. temperature is shown as FIG. 9 for resin PE2. This is a typical plot and represents the trends found in the other resins. A high length of overhang indicates a stiff fabric. Also, a high basis weight contributes to an increase in stiffness since the fabric supports a larger weight as it hangs over the edge. An average of the fabric's overhang with the engraved roll side facing up and facing down was considered the total overhang for an individual piece of fabric. An average of each was taken. This is thought to better represent the overall stiffness of the fabric since the fabric bends in both directions during wear. Four measurements were taken for each sample in this manner.
It is observed that bond patterns 6-9 with larger bond angles have higher values than patterns 2-5 with 20° bond angles. The flexural rigidity of all the PE samples ranged from the low twenties to a high of 125 mg*cm for sample 3-7-119.4. These values are relatively low, considering a typical PP fabric has a FR value of over 200 mg*cm. Resin PE2 showed the least stiffness when compared to other resins of the same processing conditions. This is likely due to the low density of the polymer. The highest FR values were obtained by PE3 and can be attributed to a higher polymer density. The addition of PE4 to PE1 produced a higher FR values. It is likely due to an increase in melting in the bond area and/or shrinkage of the fibers and fabric. Concerning the bond pattern, it is shown that low bond areas, low side wall angles, and low bond point concentrations produce the lowest values of FR. It should be noted that low bond areas, side wall angles, and bond point concentrations can affect other properties, i.e., abrasion. Therefore, due to PE's low modulus, the FR value may not be as important as other properties.
The effect that bond roll patterns have on the stiffness (ST) of the fabric and the graininess (GR) of the surface was evaluated by the handfeel test. 12 panelists rated the two properties on a scale of 1 to 15. Anchors (used as a baseline) were provided as listed in Table 10. Resin PE1 processed at 119.4° C. (247° F.) on each bond pattern were used as samples. Table 11 summarizes the averages of the two hand ratings for each bond pattern.
|TABLE 10 |
|Anchor Materials and their Corresponding Value |
|Test Type ||Anchor Material ||Anchor Number |
|Grainy ||Bleached mercerized cotton poplin ||2.1 |
|Grainy ||Army carded cotton sateen ||4.9 |
| ||bleached |
|Grainy ||Cotton momie fabric ||9.5 |
|Grainy ||Cotton duck greige ||13.6 |
|Stiffness ||Polyester/cotton 50/50 single knit ||1.3 |
|Stiffness ||Bleached mercerized cotton print ||4.7 |
| ||cloth |
|Stiffness ||Bleached mercerized cotton poplin ||8.5 |
|Stiffness ||Cotton organdy ||14.0 |
|TABLE 11 |
|Data Collected for Hand Survey |
| ||Sample ||Stiffness ||Grainy |
| || |
| ||1-1-119.4 ||2.5 ||5.6 |
| ||1-2-119.4 ||0.9 ||2.9 |
| ||1-3-119.4 ||1.8 ||3.9 |
| ||1-4-119.4 ||1.6 ||4.0 |
| ||1-5-119.4 ||1.1 ||2.8 |
| ||1-6-119.4 ||1.7 ||3.5 |
| ||1-7-119.4 ||3.0 ||5.4 |
| ||1-8-119.4 ||1.5 ||4.0 |
| ||1-9-119.4 ||2.5 ||4.9 |
| ||5-2-140 ||5.3 ||6.4 |
| || |
Scanning Electron Microscopy (SEM) was used to analyze the effects of processing conditions on the nonwoven surface, bond perimeter, cross section and failure mechanism. It has been shown that processing conditions effect the feel and the strength of the fabric. This section discusses the relationship between the fabric surface and its properties and also identifies the fracture mechanisms as a function of processing conditions.
Arial views and cross-section views were obtained using the following procedure:
1. The cross-section of the fabric was cut by placing it between two pieces of paper and placing the sample into liquid nitrogen for about 1 min, followed by cutting with a razor blade perpendicular to the machine direction.
2. The sample was placed on a stage with conductive tape and the edges were lined with conductive graphite paint.
3. A Denton Vacuum Hi-Res 100 high-resolution chromium sputtering system was used to coat the fabric with 100-120 Angstroms thick film.
4. The sample was placed in the sample compartment and the compartment was evacuated to 1.3×10−5 Pa (10−7 torr).
5. 5 kEV was used out of the 20 kEV available due to problems with charge buildup on the fabric surface.
6. Micrographs were obtained at various magnifications.
7. Scion imaging software was used to view and measure the micrograph images.
All tested samples were micrographed at a low magnification of between 60× and 100× focusing on the bond point. Since there was no noticeable surface difference between temperatures, all pictures were taken of samples at 1.6° C. (3° F.) below their stick point. This temperature is 119.4° C. (247° F.) for all samples except for resin PE2 which was at 119.4° C. ( 247° F.). All nine bond patterns made from resin PE1 are shown in FIGS. 10A-10J. All bond points contain a large flat surface in the middle that raises up toward the edge. Patterns 1, 6, 7 and 8 all contain a large side wall angle. The effect of this is not nearly as noticeable with the PE1 resin as it is in the other resins, probably due to its high melt index. The patterns with the small side wall angle produce a bond that contains a smaller flat area and, geometrically, a more rounded bond point. Because the shape of the bond point produced with a 20 degree side wall angle is rounded and covers less surface area as shown previously, then the space between each bond point is larger. This larger space gives the fabric its softer feel due to the increase in exposure area of the fibers. This correlates well with the handfeel evaluation data. Conversely, the small bond point surface coverage produces less entangled fibers and decreased the fabric strength. This was seen previously in tensile data.
One effect of processing conditions on the nonwoven fabric is its failure mechanism during destructive testing such as tensile and abrasion testing. Three types of failure can occur. The fibers can pull out of the bond sight, break at the bond perimeter, or break away from the bond. SEM micrographs were also used to identify the failure mechanism for selected nonwoven samples. FIGS. 11A-C shows examples of the failure mechanisms during tensile failure. Notice that most processing conditions cause the polyethylene fabric to fail by the fibers pulling away from a weak bond point. In some cases at higher temperatures it was evident that the bonds were strong enough to cause fiber breakage at the bond perimeter. The addition of 5 percent the PE4 resin to the PE1 resin increased the bond strength enough at 119.4° C. (247° F.) to cause some fibers to break at the bond perimeter. There was evidence of two fracture mechanisms at this point including fibers pulling away from the bond and fibers breaking at the perimeter.
An analysis of failure mechanisms caused by abrasion showed no sign of failure by breaking at the bond perimeter. FIGS. 12A-B show two examples of a fractured bond point caused by abrasion. The thin ribbon-like strips are remnants of the previously thermally bonded point. Even those samples that failed in tensile tests by brittle fiber failure at the bond perimeter did not show the same fracture mechanism. After abrasion the fabric failed by destruction of the bond point. This phenomena may explain why abrasion resistance does not reach a peak value and then decrease as the processing temperature is increased as does the tenacity and elongation. The abrasion resistance is dependent only on the bond strength.
As demonstrated above, embodiments of the invention provide a nonwoven fabric which has relatively increased tensile strength, elongation, abrasion resistance, flexural rigidity, and/or softness. Additional characteristics and advantages provided by embodiments of the invention are apparent to those skilled in the art.
While the invention has been described with reference to a limited number of embodiments, variations and modifications therefrom exist. For example, the fabric composition need not be a mixture within the compositions given above. It can comprise any amount of components, so long as the properties desired in the fabric composition are met. It should be noted that the application of the fabric composition is not limited to sanitary articles; it can be used in any environment which requires a thermally bonded nonwoven fabric. The appended claims intend to cover all such variations and modifications as falling within the scope of the invention.