MALCOM JEFFREY D
TRAFICANTE MICHAEL S
| EP0629720A2 | 1994-12-21 |
Background of the Invention Spunbond processing involves the direct extrusion of a polymer to a nonwoven fabric and has been described in numerous patents, for examples see U. S. Pat. Nos. 4,812,112 and 4,820,142. The term nonwoven fabric, or web, generally refers to a web having a structure of individual fibers or threads which are interlaid, but not in a regular or identifiable manner such as in a knitted fabric.
Polypropylene polymer has been used to make spunbond fabric. The polypropylene typically used in spunbond processing has a melt flow rate (MFR) of about 35dg/min. and a narrow molecular weight distribution. Polypropylene with these polymer characteristics is preferred by those skilled in the art because it will result in the optimum processability for spunbond material obtained therefrom.
In general, as the melt flow rate of the polypropylene decreases, or as molecular weight distribution increases, spinning continuity will decrease. For fiber processing in general, as the melt flow rate of the polypropylene decreases, fiber tensile strength will decrease. In making spunbond fabric, the physical properties of the resultant fibers are dependent upon molecular weight of the polymer, therefore as the molecular weight of the polypropylene increases, the strength of the resultant fiber increases.
Because of the above-listed factors, polypropylene of relatively narrow molecular weight distribution and relatively high melt flow typically is used in spunbond processes to produce fabric for essentially all applications. The basis weight is used to effect changes in the physical properties of the fabric. Basis weight refers to the weight of a unit area of fabric. To produce a high strength fabric, a high basis weight is used. To produce a low strength fabric, a low basis weight is used. There are limitations, however, on the combinations of basis weight and fabric properties achievable if polypropylene is used in spunbond processes. As an alternative to changing the basis weight, a spunbond nonwovens producer can substitute another polymer, such as polyester, for polypropylene to change the nonwoven fabric strength.
A disadvantage of substituting polymers is the time lost due to the transition from one polymer to the next. This lost time can be longer than that required to change the basis weight. If the polymer substitution is carried out on. a commercial line, the transition time can result in increased cost.
Because polypropylene for spunbond processes tends to be of a relatively narrow range of melt flow rates (and thus of a narrow molecular weight range) for a given basis weight of spunbond fabric, the
tabnc strength is tixed by the melt flow rate. Thus for a process optimized for a given polymer melt flow, physical strength can only be further increased by increasing fabric basis weight.
Therefore a need exists in the art for a method of utilizing low melt flow rate polypropylene to produce high strength spunbond fabric.
Summary of the Invention The present invention provides a method of making spunbond fabric comprising, forming discrete fibers by continuously extruding a polypropylene polymer having a melt flow rate of 4 to 12 through openings in a spinneret, the openings having a diameter of at least 0.6 mm; drawing the discrete fibers by airflow to a reduced diameter; and depositing the drawn discrete fibers onto a support to form the fabric.
The present invention further provides a spunbond fabric produced by forming discrete fibers by continuously extruding a polypropylene polymer having a melt flow rate of 4 to 12 through openings in a spinneret, the openings having a diameter of at least 0.6 mm; drawing the discrete fibers by airflow to a reduced diameter; and depositing the drawn discrete fibers onto a support to form the fabric.
The present invention yet further provides a method of improving spunbond fabric strength comprising, adding 100 ppm to 2500 ppm of an additive selected from the group consisting of hindered phenols and amine oxides to a polypropylene polymer having a melt flow rate of 4 to 12; adding 100 ppm to 2500 ppm of an organophosphite to the polypropylene polymer; adding 50 ppm to 750 ppm of an acid acceptor to the polypropylene polymer; forming discrete fibers by continuously extruding the polypropylene polymer through openings in a spinneret, the openings having a diameter of at least 0.6 mm; drawing the discrete fibers by airflow to a reduced diameter; and depositing the drawn discrete fibers onto a support to form the fabric with an improved tensile strength.
The present invention still further provides a spunbond fabric with improved strength produced by adding 100 ppm to 2500 ppm of an additive selected from the group consisting of hindered phenols and amine oxides to a polypropylene polymer having a melt flow rate of 4 to 12; adding 100 ppm to 2500 ppm of an organophosphite to the polypropylene polymer; adding 50 ppm to 750 ppm of an acid acceptor to the polypropylene polymer; forming discrete fibers by continuously extruding the polypropylene polymer through openings in a spinneret, the openings having a diameter of at least 0.6 mm ; drawing the discrete fibers by airflow in the direction of extrusion to a reduced diameter ; and depositing the drawn discrete fibers onto a support to form the fabric.
The present invention yet still further provides a composition comprising, polypropylene polymer having a melt flow rate of 4-12; 100 ppm to 2500 ppm of one member selected from the group consisting of octadecyl 3,5-bis (1, 1-dimethylethyl)-4-hydroxybenzene propanoate, tris (3,5-di-tert-butyl-4- hydroxybenzyl) isocyanurate, bis (hydrogenated rape-oil alkyl) methyl, amine oxides, R', R'=CI4-C24, and dialkyl methyl amine oxide; 100 ppm to 2500 ppm of one member selected from the group consisting of tris (2,4-di-tert-buty (phenyl) phosphite, bis (2,4-dicumylphenyl) pentaerythritol diphosphite, and trisnonylphenyl phosphite; and 50 ppm to 750 ppm of one member selected from the group consisting of zinc oxide, calcium stearate, zinc stearate and synthetic hydrotalcite DHT-4A
[MgAOH), 3C03 3. 5H20].
These and other advantages and benefits will be apparent from the Detailed Description of the Invention herein below.
Brief Description of the Figure The present invention will be described for the purposes of illustration, but not limitation, in conjunction with the following figure, wherein: Figure 1 is a bond curve for 36 MFR polypropylene at 0.6 mm and 8MFR polypropylene at 0.6 mm and 1.5 mm spinneret openings.
Detailed Description of the Invention As used herein, the term"polymer"generally includes, but is not limited to, homopolymers, copolymers, such as, for example, block, graft, random and alternating copolymers, terpolymers, etc., and blends and modifications thereof. Furthermore, unless otherwise specifically limited, the term"polymer" includes all possible geometrical configurations of the material. These configurations include, but are not limited to, isotactic, syndiotactic, and random symmetries.
The basis weight of nonwoven fabrics is usually expressed in ounces of material per square yard (osy) or grams per square meter (gsm) and fiber diameters are usually expressed in microns. (Note that to convert from osy to gsm, multiply osy by 33.91.) The present invention uses polypropylene that those skilled in the art would consider atypical for spunbond processes to significantly improve spunbond fabric strength, without causing a loss in production rate or throughput. The invention uses polypropylene of a relatively low melt flow rate (no more than 12 dg/min.) and broad molecular weight distribution as compared to conventional processes which use high melt flow rate, narrow molecular weight distribution material. The present invention provides for a fabric tensile strength increase of at least 16%, in the machine direction, over spunbond fabrics made from conventional polypropylene which is approximately 35 melt flow rate.
Polypropylene based resins having a melt flow rate (MFR) in the range of 4-12 can be prepared either directly via polymerization or by visbreaking a polymer having a lower MFR. The visbreaking process is well know to those in the art and increases the MFR of the resulting resin relative-to the starting resin and narrows the molecular weight distribution of the resulting resin.
The fibers and nonwoven fabrics described herein were prepared on a 1 m wide Reicofil pilot spunbond line according to the conditions given in the following examples. All polymers were spun under typical process conditions for making polypropylene spunbond materials. Briefly, the spunbond fabrics are made by continuously extruding a polymer through a spinneret to form discrete fibers. These fibers are then drawn down to a reduced diameter, without breaking, by airflow. The drawn fibers are deposited onto a support, typically a conveyor belt, to form the nonwoven fabric. A particularly preferred apparatus for carrying out the methods of the present invention is made by Reifenhauser and is described
in U. S. Pat. Nos. 4,812,112 and 4,820,142, the contents of which are incorporated in their entirety herein by reference.
The grab tensile strength of the spunbond fabrics of the present invention in the machine direction (MD) and cross direction (CD) was measured by test methods ASTM D-1682 and ASTM D-1776. The melt flow rate of the polypropylene polymers was measured by ASTM D-1238 (2. 16 kg, 230°C).
The Inventors have found that certain additives, preferably a hindered phenol or amine oxide, along with an organophosphite and an acid acceptor can be added to low melt flow rate polypropylene to produce fabrics with superior tensile strength to those produced from the conventional high melt flow rate polypropylene. The additives may be incorporated into the polypropylene polymer in any manner conventionally practiced in the art, for example by dry blending the additives directly with polypropylene polymer pellets or by blending the additives with molten polymer in a screw extruder.
Comparative Example 1 A polymer having a MFR of 36 dg/min. was prepared by visbreaking a polypropylene resin having an MFR of 1.0 dg/min. Additives used were (a) 1000 ppm of a hindered phenol, octadecyl 3,5r bis (l, l-dimethylethyl)-4-hydroxybenzene propanoate, commercially available as IrganoxX 1076 from Ciba Specialty Chemicals Corp., (b) 500 ppm of an organophosphite, available from Ciba Specialty Chemicals as Irganfost) 168, and (c) 350 ppm of an acid acceptor, calcium stearate. The melt temperature for the resin was 230°C, the throughput was 0.67 g/hole/min. The spinneret openings had a diameter of 0.6 mm. The fibers were drawn down by airflow with a cooling temperature of 22. 8°C and the suction fan operating at 2200 rpm. As can be seen by reference to Table I, the spunbond fabric produced from this polypropylene polymer had a maximum tensile strength in the machine direction (MDM) of 1575 g and a maximum tensile strength in the cross direction, i. e., the direction perpendicular to the machine direction, (CDMAx) of 884 g. These values are typical for the conventional polypropylene spunbond material made by many manufacturers.
Example 2 A polymer having a MFR of 8 dg/min. was prepared by visbreaking a polypropylene resin having an MFR of 1.8 dg/min. Additives used were the same, and in the same amounts as given in Comparative Example 1. The melt temperature for the resin was 230°C. The throughput was 0.43 g/hole/min. The spinneret openings had a diameter of 0.6 mm. The cooling air temperature was 22.8°C and the suction fan operated at 1730 rpm. As can be seen by reference to Table I, the spunbond fabric produced from this polypropylene polymer had a MDMAX of 2440 g and a CDMAX of 1479 g. These values are superior to the results obtained by the conventional process demonstrated in Comparative Example 1.
Table I TENSILE STRENGTH (g) Example Spinneret No. MFR Openings MDMAx CDMAx Diameter (mm) Comparative 36 0. 6 1575 884 Example1 Ex. 2 8 0. 6 2440 1479
The Inventors also investigated whether the diameter of the openings in the spinneret had an effect on the tensile strength of the resultant fabric and were surprised to discover that a larger spinneret diameter would produce fibers that when drawn down to the same diameter as conventional fabric fibers (about 20 microns) are stronger. The Inventors draw down the diameter of the fibers produced by the present invention from 1/10 to 1/1000, that of the original fiber exiting the spinneret, without any loss in fabric strength.
Comparative Example 3 A polymer having a MFR of 36 dg/min. was prepared by visbreaking a polypropylene resin having an MFR of 1.0 dg/min. Additives used were (a) 1000 ppm Irganox 1076, (b) 500 ppm Irganfos 168, and (c) 350 ppm calcium stearate. The melt temperature for the resin was 230°C. The throughput was 0.67 g/hole/min. The spinneret openings had a diameter of 1.5 mm. The cooling air temperature was 22.8°C and the suction fan operated at 2200 rpm. As can be seen by reference to Table II, the spunbond fabric produced from this polypropylene polymer had a MDMA, of 2050 g and a CDMA, of 1215 g. These values show an improvement over those seen with the conventional polypropylene spunbond material made in Comparative Example 1, thereby demonstrating that the spinneret opening diameter does have an effect on the strength of the spunbond fabric.
Example 4 A polymer having a MFR of 12 dg/min. was prepared by visbreaking a polypropylene resin having an MFR of 3.6 dg/min. Additives used were the same, and in the same amounts as given in Comparative Example 3. The melt temperature for the resin was 230°C. The throughput was 0.67 g/hole/min. The spinneret openings had a diameter of 1.5 mm. The cooling air temperature was 22.8°C and the suction fan operated at 2100 rpm. As can be seen by reference to Table If, the spunbond fabric produced from this polypropylene polymer had a MDMAX of 2370 g and a CDMAX of 1570 g.
Example 5 A polymer having a MFR of 8 dg/min. was prepared by visbreaking a polypropylene resin having an MFR of 1.8 dg/min. Additives used were the same, and in the same amounts as given in Comparative
Example 3. The melt temperature for the resin was 245°C. The throughput was 0.66 g/hole/min.. The spinneret openings had a diameter of 1.5 mm. The cooling air temperature was 22.2°C and the suction fan operated at 2100 rpm. As can be seen by reference to Table II, the spunbond fabric produced from this polypropylene polymer had a MDMAx of 2870 g and a CDMAX of 1600 g. Figure 1 shows a bond curve of the fabric made in this example, the fabric made in example 2 and the fabric made in comparative example 1 demonstrating both the effect of increasing the opening of the spinneret upon fabric strength and the effect of reducing the melt flow rate of the polypropylene polymer.
Example 6 A polymer having a MFR of 4 dg/min. was prepared by visbreaking a polypropylene resin having an MFR of 0.8 dg/min. Additives used were the same, and in the same amounts as given in Comparative Example 3. The melt temperature for the resin was 270°C. The throughput was 0.46 g/hole/min. The spinneret openings had a diameter of 1.5 mm. The cooling air temperature was 21 °C and the suction fan operated at 1821 rpm. As can be seen by reference to Table II, the spunbond fabric produced from this polypropylene polymer had a MDmAx of 2540 g and a CDMAX of 1912 g.
Table II TENSILE STRENGTH (g) Example Spinneret No. MFR Openings MDMAx CDMAx Diameter (mm) Comparative 36 1. 5 2050 1215 Example 3 Ex. 4 12 1. 5 2370 1570 Ex. 5 1. 5 2870 1600 Ex. 6 4 1. 5 2540 1912 Tables m and IV show the amount of improved tensile strength in fabrics made by the present invention by comparing the maximum tensile strength in the machine direction, MDMAX to the tensile strength in the machine direction of the conventional process, MDo.. The same comparison is made for the maximum tensile strength in the cross direction, CDMAvc to CDo. As can be appreciated from Tables III (spinneret openings diameter of 0.6 mm) and IV (spinneret openings diameter of 1.5 mm), the present invention provides spunbond fabrics with much better tensile strength in both the machine and cross directions compared to conventional processes. The combination of larger spinneret opening and lower MFR polypropylene produces superior spunbond fabrics at the same processing conditions as are currently used for 35 MFR polypropylene. Those skilled in the art will appreciate that the present invention will produce a greater fabric strength relative to 35 MFR polypropylene so that at a given basis weight, the spunbond nonwoven fabric of the present invention will be stronger.
Table in Example MFR Spinneret MDMAx/MDo CDMAx/CDo Openings Diameter (mm) Comparative 36 0. 6 1. 00 1 00 Example 1 Ex.2 8 0.6 1.19 1.12 Table IV MFR Spinneret MDMA/MDo CDMAx/CDo Openings Diameter (mm) Comparative 36 1.5 1.00 1.00 Example 3 Ex. 4 12 1. 5 1. 16 1. 29 Ex. 5 8 1.5 1.40 1.32 Ex. 6 1. 5 1. 24 1. 57
Although the present invention has been described using octadecyl 3,5-bis (l, 1-dimethylethyl)-4- hydroxybenzene propanoate, as the first additive, other additives selected from hindered phenols and amine oxides which are useful in the present invention include but are not limited to: tris (3,5-di-tert- butyl-4-hydroxybenzyl) isocyanurate, commercially available as Ciba Irganox# 3114; bis (hydrogenated rape-oil alkyl) methyl, amine oxides, R', R'=CI4-C24 commercially available as GE Specialty Chemicals GENOXrM EP; and dialkyl methyl amine oxide commercially available as Ciba FS042. In addition to tris (2,4-di-tert-buty (phenyl) phosphite, other organophosphites useful in the present invention include but are not limited to: bis (2,4-dicumylphenyl) pentaerythritol diphosphite, commercially available as Dover Chemicals DoverphosX S-9228; and trisnonylphenyl phosphite commercially available as Dover Chemicals Doverphos# HiPure 4HR. Acid acceptors useful in the present invention include but are not limited to: zinc oxide, calcium stearate, zinc stearate and synthetic hydrotalcite DHT-4A [Mg4-5Al2(OH)13CO33.5H2O].
The foregoing illustrations of embodiments of the present invention are offered for the purposes of illustration and not limitation. It will be readily apparent to those skilled in the art that the embodiments described herein may be modified or revised in various ways without departing from the spirit and scope of the invention. The scope of the invention is to be measured by the appended claims.
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