Patent Application
20130337714
This disclosure generally relates to fabrics (e.g., webs or web laminates) made of thermoplastic fibers or filaments. In particular, this disclosure relates to nonwoven fabrics such as such as those produced by melt spinning thermoplastic material.
The term “nonwoven fabric”, as used herein, means a web of individual fibers, filaments, or threads that are positioned and oriented in a random manner (i.e., without an identifiable pattern). Examples of nonwoven fabrics include meltblown webs, spunbond webs, carded webs, air-laid webs, wet-laid webs and spunlaced webs and composite webs comprising two or more nonwoven layers.
The term “spunbonding”, as used herein, means a process in which filaments are formed by extruding molten thermoplastic polymer material from a plurality of fine capillaries of a spinneret, with the diameter of the extruded filaments then being rapidly reduced by drawing. Spunbond nonwoven fabrics or webs are formed by laying spunbond filaments randomly on a collecting surface such as a foraminous screen or belt. Spunbond webs can be bonded by methods known in the art such as hot-roll calendering, through air bonding (generally applicable to multiple component spunbond webs), or passing the web through a saturated-steam chamber at an elevated pressure.
Spunbond nonwoven fabrics formed from continuous bicomponent fibers are known in the art. The term “bicomponent fiber” as used herein refers to any fiber or filament (i.e., continuous or discontinuous) that is composed of two distinct polymers which have been spun together to form a single filament or fiber. Preferably each bicomponent fiber is made from two distinct polymers arranged in distinct substantially constantly positioned zones across the cross section of the bicomponent fiber and extending substantially continuously along the length of the fiber. Continuous bicomponent fibers are fibers produced by extruding two polymers from the same spinneret with both polymers contained within the same filament. Depending on the arrangement and relative quantities of the two polymers, the structure of a bicomponent fiber can be classified as core and sheath, side by side, tipped, microdenier, mixed fibers, etc.
A sheath-core bicomponent fiber comprises a core made of one thermoplastic material and a sheath made of a different thermoplastic material. The core can be concentric or eccentric relative to the sheath and can have the same or a different shape compared to that of the sheath. The sheath-core structure is employed when it is desirable for the surface of the fiber to have the property of the sheath such as luster, dyeability or stability, while the core may contribute to strength, reduced cost and the like.
Nonwoven webs can be thermally bonded using methods known in the art, including point or pattern bonding. Point or pattern bonding typically comprises the application of heat and pressure at discrete areas of the web, e.g., by passing the web through a nip formed by a patterned roll and a smooth roll or by two patterned rolls. One or both of the rolls can be heated to thermally bond the nonwoven web at distinct points, lines, areas, etc. A nonwoven fabric or web can be thermally point bonded at a plurality of spaced thermal bond points. As used herein, the term “thermal pattern bonding” refers to a process that involves passing a nonwoven fabric or web through a nip formed by a heated engraved roll and a cooperating heated smooth anvil roll. Several roll configurations (e.g., the single pass, double pass, S-wrap and three-stack idler roll configurations) are well known in the art.
Nonwoven fabrics are useful for a wide variety of applications such as surgical blankets, diapers, feminine hygiene products, towels, recreational or protective fabrics and geotextiles. In many of these applications, it is necessary for one or both surfaces of the nonwoven fabric to be abrasion resistant.
Various methods of enhancing abrasion resistance of nonwoven fabric are known. In one known method, the nonwoven fabric is passed through a nip formed by two calender rolls. Following this calendering operation, the thickness of the calendered fabric is lower than the thickness of the uncalendered fabric. Another method uses a thermal point bond calendering system (the primary bonding mechanism) with a bonding area greater than about 22%. This results in a fabric with higher stiffness. Yet another prior art method utilizes binders. This results in fabric with higher stiffness and affects the capillary action of the fabric.
Known thermoplastic, bicomponent spunbond nonwovens are either soft/silky/drapeable with very poor abrasion resistance or have good abrasion resistance without the characteristics of softness, silkiness or drapeability. Thickness is usually a good measure of drapeability. That is, for a given basis weight, the thinner the spunbond nonwoven fabric, the more compact it is, which translates to reduced drapeability.
There is a need for a method of making a nonwoven fabric having enhanced abrasion resistance without adversely impacting drapeability, capillary action and/or feel of the fabric.
The subject matter of this disclosure are methods for improving abrasion resistance on at least one side of a nonwoven fabric made of thermoplastic material while maintaining high degrees of drapeability and air permeability. Such fabric is useful for medical applications. Nonwoven materials are often used in hospital operating rooms for various applications (e.g., patient drapes, operating staff gowns). If pills or loose fiber from a nonwoven material are formed by the movements of members of the surgical team (gloved hands moving back and forth, etc.), and these enter the patient's wound, they can form emboli in the cardiovascular system with severe consequences to the patient.
In accordance with some embodiments, a glazing method can be used to manufacture spunbond webs of bicomponent fibers (e.g., sheath/core fibers having a sheath made of thermoplastic material having a melting point which is lower than the melting point of the thermoplastic material of the core). In accordance with one embodiment, the glazing method for improving abrasion resistance uses a heated smooth roll to melt the lower-melting-point portion of bicomponent fibers as the spunbond web passes over the heated smooth roll. Because there is no external pressure exerted in a nip by an opposing second roller, as in calendering, the outer surface of the web which does not contact the heated smooth roll remains essentially unchanged and the nonwoven fabric exhibits no compression as a result of the glazing process. The surface temperature of the heated smooth roll and the dwell time (which is dependent on roll diameter, wrap angle and line speed) are controlled for the purpose of surface treating one side of the nonwoven fabric to improve abrasion resistance while allowing the air permeability and web thickness to remain essentially unchanged. The process can repeated in order to glaze the opposite side of the fabric.
In accordance with an alternative embodiment, at least one side of a pattern bonded nonwoven web can be glazed by wrapping the web against the circumferential surface of a heated smooth roll as the web exits the nip formed by that heated smooth roll and an opposing engraved roll.
The glazing methods disclosed herein can be applied to many different thermoplastic bicomponent spunmelt fabrics, including but not limited to spunbond fabrics and SMS (spunbond-meltblown-spunbond) laminates. These glazing methods are best applied to spunmelt fabrics having a basis weight less than 40 gsm and a bond area of less than 22%. In one application, a spunbond fabric made of polyethylene/polyester sheath/core filaments was glazed, resulting in a surface having enhanced abrasion resistance.
An evaluation of the beneficial effects of the glazing processes disclosed herein included measuring the abrasion resistance of samples of untreated, glazed and calendered spunbond nonwoven fabric. The abrasion resistance was measured in two ways for each fabric sample: (1) using a Taber abrasion tester, a Taber abrasion resistance roping method was used to measure the number of cycles to failure (which is a subjective visual test); and (2) after abrading each fabric sample for 40 cycles using the same Taber abrasion tester, an average Taber shaving weight loss was calculated using a process involving weighing/shaving/re-weighing of the abraded samples.
Since the glazing method of improving abrasion resistance does not rely on compression of the web by applying heat and pressure, the glazed fabric maintains a high thickness to basis weight ratio along with good abrasion resistance properties. The abrasion-resistant nonwoven fabrics disclosed herein comprise thermoplastic filaments and have a thickness (in microns, μm) to basis weight (gsm or g/m2) ratio of at least 5, wherein at least one side of the nonwoven fabric comprises thermoplastic filaments which are at least partially flattened, the one side having an average weight loss not greater than 0.62% when subjected to Taber shaving. In accordance with one embodiment, another side of the fabric has no flattened or partially flattened thermoplastic filaments.
In particular, spunbond webs are disclosed which comprise bicomponent thermoplastic filaments and have a thickness to basis weight ratio of at least 5 μm/gsm, wherein at least one side of the spunbond web comprises bicomponent thermoplastic filaments which are at least partially flattened, the one side having an average weight loss not greater than 0.62% when subjected to Taber shaving.
Glazing methods in accordance with various embodiments are disclosed in detail hereinafter. One method of enhancing the abrasion resistance of a surface of a nonwoven fabric disclosed herein comprises applying heat and pressure on one surface of a portion of a nonwoven fabric, while not applying any heat or pressure on the other surface of the portion of the nonwoven fabric. The heat and pressure are applied by a circumferential surface of a heated smooth roll, the aforementioned portion of the nonwoven fabric being wrapped around and in contact with a portion of the circumferential surface which subtends a wrap angle. The wrap angle is in a range of 25 to 85 degrees inclusive. The surface temperature of the heated smooth roll is in a range of 290 to 330° F., preferably 300 to 330° F.
Another aspect is a method of enhancing the abrasion resistance of a surface of a nonwoven fabric, comprising: (a) supporting a nonwoven fabric in a position whereat a portion thereof is wrapped around and in contact with a portion of a circumferential surface of a heated smooth roll which subtends a wrap angle; and (b) advancing the nonwoven fabric in a tensioned state to maintain some portion thereof in wrapped contact with some portion of the circumferential surface of the heated smooth roll that subtends the wrap angle, wherein the length of the surface of the wrapped portion of the nonwoven fabric is a function of a diameter of the circumferential surface of the heated smooth roll and the wrap angle. Optionally, while one portion of the nonwoven fabric is in wrapped contact with the heated smooth roll, another portion of the nonwoven fabric is wrapped around and in contact with a portion of a circumferential surface of at least one movable guide roll, the wrap angle of the one portion being adjustable by changing the position of the movable guide roll relative to the position of the heated smooth roll.
A further aspect is a method for fabricating a pattern bonded nonwoven web having a surface with enhanced abrasion resistance, comprising: (a) randomly depositing extruded filaments on a moving carrier belt or screen to form a nonwoven web; (b) forming discrete thermally bonded areas in the nonwoven web by passing the nonwoven web through a nip formed by a patterned roll and a heated smooth roll, the nip continuously forming discrete thermally bonded areas in the nonwoven web in a pattern as the nonwoven web passes therethrough; and (c) glazing a surface of the pattern bonded nonwoven web by wrapping the nonwoven web around a portion of a circumferential surface of the heated smooth roll. A terminal portion of the wrapped portion of the nonwoven web is disposed in the nip.
Other aspects of the invention are disclosed below.
Reference will hereinafter be made to the drawings in which similar elements in different drawings bear the same reference numerals.
Various embodiments of apparatus for enhancing the abrasion resistance of at least one surface of a nonwoven fabric produced by a well-known spunbond nonwoven process will be described later with reference to
The filaments 2 are randomly deposited onto a moving carrier belt 18 that is driven to circulate over a set of rollers 20 by a conventional drive source (not shown), thereby forming a loose web 24 of randomly deposited filaments. Optionally, a suitable suction means 22 can be placed under the carrier belt 18 to assist in the deposit of filaments 2. It should be noted that while a single spinneret assembly and single-layer filament web is shown, it is possible to provide additional spinning assemblies in-line to form a heavier web or a multi-layer nonwoven fabric.
Still referring to
In accordance with the embodiments disclosed hereinafter, the thermally pattern bonded spunbond fabric is further treated to enhance the abrasion resistance on one or both surfaces thereof.
After a pattern bonded spunbond fabric has been produced, e.g., by the process depicted in
Optionally, after the spunbond fabric has been glazed on one side, it could be glazed on the other side by repeating the process shown in
In accordance with an alternative embodiment shown in
Optionally, after the spunbond fabric has been glazed on one side, it could be glazed on the other side by passing the glazed fabric under tension around a second heated smooth roll (not shown in
In accordance with a further alternative embodiment, a method for fabricating a pattern bonded nonwoven web having a surface with enhanced abrasion resistance is provided which comprises: (a) randomly depositing extruded filaments on a moving carrier belt or screen to form a nonwoven web; (b) forming discrete thermally bonded areas in the nonwoven web by passing the nonwoven web through a nip formed by a patterned roll and a heated smooth roll, the nip continuously forming discrete thermally bonded areas in the nonwoven web in a pattern as the nonwoven web passes therethrough; and (c) glazing a surface of the pattern bonded nonwoven web by wrapping the nonwoven web around a portion of a circumferential surface of the heated smooth roll, while a terminal portion of the wrapped portion of the nonwoven web is disposed in the nip.
A portion of the manufacturing process described in the preceding paragraph is shown in
Using the foregoing methods, thermoplastic nonwoven fabrics having enhanced abrasion resistance and satisfactory drapeability, capillary action and/or feel of the fabric can be produced. These methods are preferably applied to pattern bonded nonwoven fabrics having a basis weight less than 40 gsm and a bond area of less than 22% of the total area of the fabric. Testing has shown that these nonwoven fabrics have a thickness to basis weight ratio of at least 5 μm/gsm. In the event that only one side of the fabric is glazed, then that glazed surface comprises thermoplastic filaments which are at least partially flattened and has an average weight loss not greater than 0.62% when subjected to Taber shaving, while the other side of the nonwoven fabric has no thermoplastic filaments which are at least partially flattened. In addition, the glazed side does not fail a Taber abrasion resistance roping test method before 13 cycles. In the event that both sides of the fabric are glazed, then each glazed surface has the aforementioned properties.
In contrast,
The data graphically depicted in
The thermoplastic materials and glazing parameters used for the glazed samples are listed in Table 2, which also identifies the thermoplastic materials used for control sample No. 25716. The thermoplastic materials and calendering parameters used for the calendered samples are listed in Table 3, which also identifies the thermoplastic materials used for the 36-gsm control sample No. 257290.
As seen in Tables 2 and 3, there are some differences between the samples. The differences revolve around the type of PE used and the ratio of sheath (PE) to core (PET). Two different polyethylenes were used: Alathon 4620 is a high-density PE and Alathon 6018 is a higher-density PE. Experiments revealed that both of these polyethylenes (unglazed) have poor abrasion resistance (surface phenomenon). After glazing the abrasion resistance improves for both versions. The control sample No. 25716 is a commercially available spunbond fabric and has a PE to PET ratio of 40/60, whereas all other samples listed have a 48/52 ratio (PE to PET). The control sample 25730 (36 gsm) is another commercially available spunbond grade, which is the same grade used for all the listed calendered samples (the only difference being no post calendering). The differences between the two control samples are the PE used, the PE/PET ratio and the basis weight. The glazing parameters listed in Table 2 are different for different glazing samples (part of the design of the experiments).
As previously noted, the weight loss percentages listed in Table 1 were derived using the Taber Shaving Weight Loss test method. This test method is designed to quantitatively evaluate the abrasion resistance of spunbond nonwovens and composites (i.e., laminates). In accordance with this method, a specimen is prepared, attached to the Taber abrasion apparatus, and abraded using two wheels comprised of abrasive particles which scuff the test sample as it rotates. Each rotation is a cycle. One abrading wheel rubs the specimen outward, i.e., toward the periphery and the other rubs it inward, i.e., toward the center. The wheels traverse a complete circle (cycle) on the specimen surface for a total of 40 cycles. This allows for evaluation of abrasion resistance at all angles relative to the weave or grain of the material. The fiber that is lifted creates an appearance of a fluffy ring on the specimen at the point of contact with the abrasive wheels. (As used herein, the term “fluffiness” means the fuzzy appearance of the fiber after abrasion caused by fibers lifting off of the web.) The sample is weighed after abrasion (Wt1), the loose material shaved off, and then the sample is re-weighed (Wt2). The Taber Shaving Weight Loss is then calculated as the difference between the weight of the sample after being subjected to 40 Taber abrasion cycles (Wt1) and the weight of the same abraded sample after shaving (Wt2), divided by weight Wt1 and then multiplied by 100:
The Taber Shaving Weight Loss was measured for 32 replicates taken from each fabric sample and then an average Taber Shaving Weight Loss was calculated based on the 32 measured values to arrive at a single data point for each fabric sample.
The apparatus used to preform the Taber Shaving Weight Loss measurements includes the following: (1) a Taber Model 503 Abraser; (2) CS-10 (part #125320) medium abrasive wheels (with a recommended shelf life of 4 years); (3) S-11 refacing discs (for refacing the CS-10 abrasive wheels; (4) a sample cutter for producing a 5¼-inch test piece; (5) an Oster model 76 shaver with 000 blade attached; and (6) a weighing scale.
For each nonwoven fabric sample, 32 replicates or test pieces were cut from the fabric sample. The weight loss following 40 cycles of Taber abrasion and shaving was measured for each of the 32 replicates and then an average and a standard deviation were calculated for each set of 32 weight loss values.
The test procedure for determining the Taber Shaving Weight Loss of an individual test piece was as follows:
(1) Make sure that the CS-10 abrasive wheels have been refaced. Wheels can be refaced as often as required, down to the minimum usable diameter of 1¾ inches as indicated on the wheel label. If the wheels are new, the CS-10 wheels should be refaced using S-11 refacing discs. Two refacings (using two separate discs) of 50 cycles each are recommended to ensure contact of abrading faces with the specimen surface. If the CS-10 wheels have been previously used, they should be refaced after 100 cycles. Use one S-11 refacing disc for 25 cycles. Press the START button to begin refacing the CS-10 wheels. Press the stop button after 25 cycles. Discard the S-11 refacing disc after one use (regardless of whether it has been used for 25 or 50 cycles).
(2) Each wheel arm is pre-loaded for 250 grams of pressure.
(3) A stud is available on the back of the abrading arm. The purpose of this stud is to hold an abrading wheel the same size as a counterweight to compensate for the weight of the working wheel. In this test method, do not use counterweight wheels.
(4) Cut a specimen having an outer diameter of 5% inches using the appropriate die. Then cut a small hole in the center of this sample. This hole should fit over the screw of the Taber tester.
(5) Place the specimen (test side upward) on the rubber mat of the specimen holder and secure the specimen in place.
(6) Adjust the hold down ring to fit firmly along the sample, keeping it wrinkle free.
(7) Lower both wheel mounting assemblies.
(8) Reset the cycle counter to zero and press Start to start the abrasion cycles
(9) Run the Taber abraser until the sample has been subjected to 40 abrasion cycles. Press Stop at the end of forty (40) cycles.
For the purpose of data acquisition, each abraded sample should be marked with a sample identifier and then the weight of the abraded sample should be measured in grams to at least four decimal places. [This pre-shaving sample weight is designated as Wt1 in Eq. (1).] Then one side of the sample (e.g., the glazed side for glazed samples) should be shaved in the area where the sample was abraded. The person performing the shaving operation should verify that the shaver blade is clean and free of any loose fibers and dust. Then any loose or raised fibers in the abraded area should be shaved off using an Oster model 76 clipper having a 000 clipper blade attached thereto. The tester should make sure that the leading edge of the clipper blade is parallel, to the sample, and avoid digging into the sample.
After the abraded surface of each sample has been shaved, that sample should be re-weighed, again to four decimal places. This post-shaving sample weight is designated as Wt2 in Eq. (1). The Taber Shaving Weight Loss (%) can now be calculated by plugging the pre- and post-shaving sample weight Wt1 and Wt2 into Eq. (1).
As previously noted, the Taber abrasion cycles listed in Table 1 were derived using the Taber abrasion roping method. This is a subjective test method that is designed to provide a performance rating in cycles, wherein the sample is run to failure and the point of failure is noted in cycles. The nature of the test first causes the sample to fluff in a circular pattern and continued cycles cause this fluff to pill into a rope-like formation and collect along the inner circumference of the abraded area. The failure point is defined when roping is seen along a total of 80% of the inner circumference. The sample preparation, test equipment and calibration of the equipment for the Taber abrasion roping method are the same as with the Taber Shaving Weight Loss (%) method.
The test procedure for the Taber abrasion roping method is as follows: Run the Taber tester for 3 continuous cycles. Stop the instrument at the completion of three cycles. Check for roping. Continue the testing, one cycle at a time until 80% roping along the inner circumference is observed. Note the number of cycles it takes to get to 80% roping as the failure point. Repeat the test for a total of four replicates. The Taber abrasion roping cycle performance is the average of these four samples.
While various embodiments have been described, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the teachings herein. In addition, many modifications may be made to adapt a particular situation to those teachings without departing from the essential scope thereof. Therefore it is intended that the scope of the claims set forth hereinafter not be limited to the disclosed embodiments.
As used in the claims, the phrase “in a range” includes the endpoints of that range, and the term “average weight loss” refers to an average weight loss which is calculated based on measurements of not less than 32 replicates.
