WO1993006168A1

WO1993006168A1 - Compatible polypropylene/poly(1-butene) blends and fibers made therefrom

Info

Publication number: WO1993006168A1
Application number: PCT/US1992/007841
Authority: WO
Inventors: Kenneth William Bartz; Joseph Calvin Floyd; Prasadarao Meka; Ferdinand Christian Stehling
Original assignee: Exxon Chemical Patents Inc.
Priority date: 1991-09-16
Filing date: 1992-09-15
Publication date: 1993-04-01

Abstract

Compatible blends of polypropylene and poly(1-butene) are disclosed. The blend preferably contains from about 55 to about 90 percent by weight of crystalline polypropylene, from about 10 to about 45 percent by weight of poly(1-butene) dispersed in a matrix of the polypropylene, and an optional compatibilization enhancing amount of an ethylene/α-olefin plastomer having a weight average molecular weight between about 5,000 to about 50,000, a density from about 0.865 to about 0.90 g/cm3, and a melt index of at least about 50 dg/min. The blend is useful in the formation of melt spun and melt blown fibers. Also disclosed are spun bonded-melt blown-spun bonded fabrics made from the blends.

Description

COMPATIBLE POLYPROPYLENE/POLY(1-BUTENE) BLENDS AND FIBERS

MADE THEREFROM

Field of the Invention

This invention pertains to compatible blends of polypropylene and poly(l-butene) suitable for use in applications such as, for example, fibers used in nonwoven fabrics.

Background of the Invention

There is a great demand for polyolefin fibers which can be used in applications such as inner cover stock for disposable diapers and sanitary napkins. In such applications, the fibers are formed into nonwoven fabrics which have specific property requirements, including soft hand (comfortable touch to the skin) , light-weightness and high tensile strength. The fibers can be bonded together to form a nonwoven fabric by several conventional techniques. The needle punch method, for example, interlaces fibers to bond them into a fabric. In addition, fiber binding has been achieved by depositing a solution of adhesive agent on webs of the fibers, but this requires additional processing and energy to remove the solvent from the adhesive agent. Another approach has been the use of binder fibers having a lower melting point than the primary, bulk fibers in the fabric. The binder fibers are heated to fuse to the bulk fibers and produce the nonwoven fabric.

Various attempts have been made in the prior art to employ polyethylene in the manufacture of fibers. Fibers containing polyethylene and polypropylene have been used to manufacture nonwoven fabrics. Polypropylene fibers are known for their high strength and good processability, but suffer from a lack of softness (poor hand) . Polyethylene, on the other hand, is known for its good hand, but has poor strength and processability. Blending the polyethylene and polypropylene to form fibers having a good balance of properties has been a long sought goal, i.e. a polyolefin . with the hand of polyethylene, but having the strength and processability characteristics of polypropylene. However, problems have been encountered in the manufacture of polyolefin fibers containing both polyethylene and polypropylene. Also, low density polyethylene (LDPE) and high density polyethylene (HDPE) have been used as bicomponent fiber- forming polymers but are not popular because nonwoven fabrics produced using these polyethylenes have unsatisfactory rigid hand and do not feel soft. Polymer blends including linear low density polyethylene (LLDPE) and polypropylene are generally immiscible and incompatible. This results in properties of polymer mixtures which are typically inferior to the blend components. Biconstituent fibers containing them generally have a "bicomponent" morphology, i.e. the polyethylene and polypropylene are present in the fibers in co-continuous phases (side-by-side or sheath/core) rather than a dispersion of fibrils of one constituent in a matrix of the other. This has in turn led to various processing problems which are generally addressed by the judicious selection of polyethylene and polypropylene having a specific density and melt index or melt flow ratio. U. S. Patent 4,874,666 teaches biconstituent fibers produced by melt spinning a blend comprising more than_50 weight percent of a linear low density polyethylene (LLDPE) having a melt index (MI) of 25-100 dg/min and heat of fusion below 25 cal/g, and less than 50 weight percent of crystalline polypropylene having a melt flow rate (MFR) below 20 dg/min. It is stated that these fibers can be produced at relatively high spinning rates. However, it is taught that if the MI of the LLDPE is below 25, fibers cannot be^" made by high speed spinning, and if the MI of the LLDPE is higher than 100, its viscosity does not match the polypropylene so that a uniform blend cannot be obtained during melt spinning and a serious defect will take place in that the filaments being extruded will frequently break as they emerge from the spinnerette. It is also taught that the LLDPE must have the low heat of fusion in order to obtain a uniform blend. Similarly, it is taught that a crystalline polypropylene cannot have an MFR exceeding 20 or uniform blending with the LLDPE cannot be obtained by any of the known commonly employed spinning apparatus, and as a result, great difficulty is involved in spinning the blend at high speed. It is also taught that the LLDPE in the spun fibers is a continuous phase and the polypropylene is a dispersed phase, and that too great a difference in the melt viscosities between the LLDPE and polypropylene results in the dispersed polypropylene particle size being too large for smooth high-speed spinning.

Poly(1-butene) , a flexible thermoplastic also known in the art as polybutylene, has been used for various pipe and film applications requiring good flexibility and resistance to creep, environmental stress cracking, chemicals and abrasion. A problem which limits the commercial use of polybutylene has been that polybutylene crystallizes from the melt in a metastable tetragonal form, but changes irreversibly to the stable twinned hexagonal form after 5 to 7 days at room, temperature. This crystalline structure change can induce undesirable stresses in the polybutylene after molding. As far as applicant is aware, however, poly(1-butene) has not been generally used in polymer blends for fiber manufacture.

Summary of the Invention

The present invention provides a polypropylene/poly(1-butene) blend especially useful for the production of fibers and nonwovens. It has been discovered that poly(l-butene) can be dispersed in a generally continuous matrix of polypropylene (PP) . Furthermore, fibers made from the blend have good hand and strength properties. Blends of polypropylene and poly(1-butene) are generally compatible. As a result, blend properties may be obtained which are intermediate to those of the blend components. The blend results in the formation of relatively small particles of the poly(l-butene) dispersed through the polypropylene matrix phase which facilitate processability of the blend into melt spun or melt blown biconstituent fibers having a good balance of strength and hand. Use of a compatibilizer such as a low molecular weight plastomer to enhance compatibilization of the polypropylene and polybutene-1 is optional.

Broadly, the present invention provides a polypropylene/poly(1-butene) blend of crystalline polypropylene and poly(l-butene) . The polypropylene preferably comprises from about 55 to about 90 percent by weight of the blend. The poly(l-butene) preferably comprises from about 10 to about 45 percent by weight of the blend. The poly(1-butene) is dispersed in a matrix of the polypropylene. The blend can contain an optional compatibilization enhancing plastomer. The plastomer is an ethylene/α-olefin copolymer having a weight average molecular weight between about 5000 and about 50,000, a density of from about 0.865 g/cm³ to about 0.90 g/cm³, and a melt flow rate of at least about 50 dg/min.

In another aspect, the present invention provides fibers made from the polypropylene/poly(1-butene) blend. Melt spun fibers are preferably prepared from the blend wherein the polypropylene has a melt flow rate from about 20 to about 50 dg/min, preferably at least about 35 dg/min. Melt blown fibers are preferably prepared from the blend wherein the polypropylene has a melt flow rate of from about 400 to about 1000 dg/min. In either case, the polypropylene is preferably of controlled rheology and has M_w/M_n less than about 4, especially from about 1.5 to about 2.5. The poly(l-butene) preferably has a density from about 0.89 to about 0.92 g/cm³, and a melt index from about 1 to about 100 dg/min.

In a further aspect of the invention, there is provided a nonwoven fabric made from melt spun and/or melt blown fibers of the polypropylene/poly(1-butene) blen .

Detailed Description of the Invention

The blend of the present invention includes crystalline polypropylene and poly(l-butene) . The primary constituent is polypropylene, preferably in an amount of from about 55 to about 90 percent by weight of the blend, more preferably from about 60 percent to about 70 percent by weight. If insufficient polypropylene is employed, the strength characteristics of the blend are adversely affected. If too much polypropylene is employed, the properties of the blend imparted by the presence of the poly(1-butene) , i.e. improved hand, are not achieved. The polypropylene is generally crystalline, for example, isotactic. The polypropylene is generally prepared by conventional controlled rheological treatment of a high molecular weight polypropylene (which is made by polymerizing propylene in the presence of a Ziegler Natta catalyst under temperatures/conditions well known in the art) with peroxide or another free-radical initiator to provide a polypropylene having a lower molecular weight and a narrow molecular weight distribution. The polypropylene preferably has M_w/M_n less than about 4, and especially from about 1.5 to about 2.5. The melt flow rate (MFR) of the polypropylene depends on the intended application of the blend. For example, where the blend is to be melt spun into fiber, the MFR of the polypropylene should be at least 20 dg/min, preferably at least about 35 dg/min. For melt blown fiber which generally requires a lower melt viscosity, the polypropylene should have an MFR in the range from about 400 to about 1000 dg/min. As used herein, polypropylene MFR is determined in accordance with ASTM D-1238, condition L. Such polypropylene is well known in the art and is commercially available. The poly(l-butene) which is used in the blend and fiber of the present invention is a high polymer of 1- butene. The poly(l-butene) generally has a density in the range from about 0.89 to about 0.92 g/cm³, and a MFR from about 1 to about 100 dg/min. As used herein, the MFR of poly(l-butene) is determined in accordance with ASTM D-1238, condition L. Poly(1-butene) is generally prepared by conventional Ziegler polymerization techniques known in the art. Poly(l-butene) having an MFR of 2.1 dg/min and a density of 0.91 g/cm³ may be obtained commercially from Shell Chemical Co. under the designation 600 SA.

The poly(l-butene) constituent should be present in the blend in an amount sufficient to obtain the desired properties, for example, improved hand, without seriously detracting from the desirable properties of the polypropylene, for example, strength and processability. The poly(l-butene) preferably comprises from about 10 to about 45 percent by weight of the blend, more preferably from about 20 to about 30 percent by weight.

The optional compatibilizer is desirably a low molecular weight ethylene/α-olefin copolymer which has properties generally intermediate those of thermoplastic materials and elasto eric materials. Thus this copolymer is called a "plastomer." The plastomers used in the blend and fiber of this invention comprise ethylene and at least one C₃-C2₀ α—olefin most preferably 1-butene, preferably a C -C₈ α-olefin, polymerized in a linear fashion using a single site metallocene catalyst such as disclosed in U. S. Patents to Welborn, 4,808,561 and 4,897,455, U. S. Patent to Turner, 4,752,597, and European Patent to Welborn, EP 129,368, all of which are herein incorporated by reference. The α-olefin is present at from about 0.01^"to about 10.0 weight percent, preferably about 4 to about 6 percent by weight. In general the plastomer has a density in the range of 0.865 to 0.90 g/cm³. The plastomer generally has M_w in the range of from about 5000 to about 50,000, preferably from about 20,000 to about 30,000. The melt index of the plastomer is generally above about 50 dg/min, preferably from about 50 to about 200 dg/min, as determined in accordance with ASTM D-1238, condition E. The plastomer is used in an amount sufficient to enhance the compatibilization of the polypropylene/poly(1-butene) blend, i.e. to facilitate dispersion of the poly(l- butene) in the polypropylene. An excessive amount of the plastomer is preferably avoided so that the desirable strength properties of the polymer are not adversely affected thereby. Preferably, the plastomer is used in an amount of up to about 10 percent by weight, more preferably from about 5 to about 10 percent by weight.

The blend may also contain relatively minor amounts of conventional polyolefin additives such as colorants, pigments, UV stabilizers, antioxidants, heat stabilizers and the like which do not significantly impair the desirable features of the blend. However, the blend should be essentially free of additives which adversely affect the compatibility of the blend components, and particularly such components which adversely affect the ability to form the blend into fiber.

The blend constituents may be blended together in any order using conventional blending equipment, such as, for example, roll mills, Banbury mixer, Brabender, extruder and the like. A mixing extruder is preferably used in order to achieve good dispersion of the compatible poly(1-butene) particles in a continuous polypropylene matrix. In an unoriented state, i.e. before fiber formation or other mechanical drawing, the blend is characterized by a dispersion of relatively fine particles of poly(1-butene) suspended in the polypropylene. Of course, when the blend is oriented as in fiber formation, or other mechanical drawing techniques, the particles tend to become more ellipsoid and/or fibrile than spherical. The spherical poly(l- butene) particles generally have a particle size less than about 30 μm, preferably from about 1 to. about 5 μra. This is in sharp contrast to the prior art blends of polyethylene and polypropylene which result in relatively large particles of the dispersed phase, and in extreme cases, even cocontinuous phases,- which adversely affect fiber formation.

It has also been observed that the volume of dispersed polybutylene particles seen in scanning electron micrographs (SEMs) of the blend is considerably less than the bulk volume of polybutylene in the blend. For example, in a blend of polypropylene/polybutylene in a 60/40 weight ratio, polybutylene particles accounted for just 10 percent of the blend volume. Although the present invention is not constrained or limited by theory, the low polybutylene particle volume can be a result of excellent compatibility, partial miscibility and/or cocrystallization of the polypropylene and polybutylene.

The blend of the present invention may be formed into fiber using conventional fiber formation equipment, such as, for example, equipment commonly employed for melt spinning or to form melt blown fiber, or the like. In melt spinning, either monofilaments or fine denier fibers, a higher melt strength is generally required, and the polypropylene preferably has an MFR of from about 20 to about 50 dg/min. A target MFR for the polypropylene of about 35 dg/min is usually suitable. Typical melt spinning equipment includes a mixing extruder which feeds a spinning pump which supplies polymer to mechanical filters and a spinnerette with a plurality of extrusion holes therein. The filament or filaments formed from the spinnerette are taken up on a take up roll after the polyolefin has solidified to form fibers. If desired, the fiber may be subjected to further drawing or stretching, either heated or cold, and also to texturizing, such as, for example, air jet texturing, steam jet texturing, stuffing box treatment, cutting or crimping into staples, and the like. In the case of melt blown fiber, the blend is generally fed to an extrusion die along with a high pressure source of air or other inert gas in such a fashion as to cause the melt to fragment at the die orifice and to be drawn by the passage of the air into short fiber which solidifies before it is deposited and taken up as a mat or web on a screen or roll which may be optionally heated. Melt blown fiber formation generally requires low melt viscosity material, and for this reason, it is desirable to use a polypropylene in melt blown fiber formation which has an MFR in the range from about 400 to about 1000 dg/min.

In a preferred embodiment, the blend of the present invention is used to form nonwoven fabric. The fiber is bonded using conventional techniques, such as, for example, needle punch, adhesive binder, binder fibers, embossed hot roll calendaring and the like. In a particularly preferred embodiment, the fiber of the present invention is used to form a fabric having opposite outer layers of melt spun fiber bonded to an inner layer of melt blown fiber disposed between the outer melt spun layers. Typically, each outer layer is from about 5 to about 10 times thicker than the inner layer. The melt spun fiber prepared from the present invention is preferably used as one or both outer layers, and the melt blown fiber of the present invention for the inner melt blown fiber layer, although it is possible, if desired, to use a different material for one or both of the spun bonded layers or a different melt blown fiber for the inner melt blown fiber layer. Conventional heated calendaring equipment can be used, for example, to bond the outer melt spun fiber layers to the intermediate melt blown fiber layer by heating the composite layered structure sufficiently to at least partially melt the inner layer which melts more easily than the outer layers. As is known, insufficient heating may not adequately bond the fibers, whereas excessive heating may result in complete melting of the inner and/or outer layer and void formation. Upon cooling, the inner melt blown layer fuses to the fiber in the adjacent outer layers and bonds the outer layers together.

It is also contemplated that the blend of the present invention can be used as one component of a bicomponent fiber wherein the fiber includes a second component in a side-by-side or sheath-core configuration. For example, the polypropylene/poly(1-butene) blend and polyethylene terephthalate (PET) can be formed into a side-by-side or sheath-core bicomponent fiber by using equipment and techniques known for formation of polypropylene/PET bicomponent fibers.

The present invention is illustrated by the examples which follow.

Example l

Polypropylene and poly(l-butene) were blended together on a Brabender and formed into pressed film and fiber for evaluation. The polypropylene was prepared from a 1.0 MFR polypropylene by peroxide treatment to obtain a controlled rheology polypropylene of 35 MFR. The poly(1-butene) had a density of 0.91 g/cm³ and a 2.1 MFR. The Brabender was operated at between 170°C and 200°C for 5-10 minutes with a mixing head speed of 60-80 rp . The blend was pressed into films using a Carver press at about 100 psi pressure and 170°C-20Θ°C temperature for 1-4 minutes. The composition of the Example 1 blend is summarized in Table 1 below. Low voltage scanning electron micrographs (SEM) of the pressed film revealed a dispersed morphology wherein the poly(l-butene) was dispersed in a continuous phase of the polypropylene. The visible poly(1-butene) particles where in the 1-2 micron size range but accounted for less than 10 % of the total volume. The film had a stress at break of 4857 psi, a strain at break of 593 percent, a modulus of 56,940 psi and impact strength of 111 lbs/in. The physical properties are summarized in Table 2 below. The blend was also formed into a fiber on an apparatus similar to a melt indexer having a single holed die and a take-up reel. The blend extrudate was drawn on the take-up reel at increasing speeds until the formed fiber broke away from the die. The die temperature was between 180°C and 250°C. The fiber exhibited a compliance of 2.68, could be spun at a rate of 440 feet/min, and had a melt strength of 5.7 g. The fiber formation and morphology are summarized in Table 3 below.

Example 2

The equipment and procedures of Example 1 were used to prepare a pressed film and fiber from the same blend, but also including a plastomer. The plastomer was an ethylene-butene copolymer containing 0.8 weight percent butene, had a 120 MI and a 0.89 g/cm³ density. The blend composition is given in Table 1. A low voltage SEM of the pressed film revealed a dispersed morphology wherein the poly(l-butene) was dispersed in a continuous phase of the polypropylene. The poly(1-butene) particles where in the 1-5 micron size range but accounted for less than 20 percent of the total volume. The film had a stress at break of 4281 psi, a strain at break of 544 percent, a modulus of 50,300 psi and effect strength of 94 lbs/in. The physical properties are summarized in Table 2. The presence of the plastomer had little impact on the blend. The fiber exhibited a compliance of 4.39, could be spun at a rate of 630 feet/min and had a melt strength of 14.2 g. The morphology and fiber forming tests are summarized in Table 3 below.

Example 3

A pressed film was prepared according to Example 2 except that the controlled rheology polypropylene (PP) used had a 400 MFR. The composition is given in Table 1.

A low voltage SEM of the pressed film revealed a dispersed morphology wherein the poly(1-butene) was dispersed in a continuous phase of the polypropylene. The poly(1-butene) particles where in the 1-4 micron size range. The film had a stress at break of 2830 psi, a strain at break of 10 percent, a modulus of 72,000 psi and impact strength of 2 lbs/in. The physical properties are summarized in Table 2. The morphology is included in Table 3.

A melt blown fiber is prepared from the blend using conventional polypropylene melt blowing equipment. The melt blown fiber has suitable properties for use in nonwoven fabrics.

Comparative Example A

A polypropylene film and fiber is made according to the procedures and techniques of Example 1 for comparison to the compatible polypropylene/poly(1-butene) blends of Examples 1-3. The filament has greater strength properties but a poorer hand. The composition, typical physical properties and typical spinning and morphological characteristics are summarized in Tables 1, 2 and 3.

Comparative Example B

A poly(l-butene) film is made according to the procedures and techniques of Example 1 for comparison to the compatible polypropylene/poly(1-butene) blends of Examples 1-3. The composition, typical physical properties and typical morphological characteristics are summarized in Tables 1, 2 and 3. TABLE 1

1. 35 MFR; 2.5 M_w/M_n.

2. 2.1 MFR; 0.91 g/cm³; obtained . from Shell Chemical Company under designation 600 SA.

3. 120 MI; 0.89 g/cm³; 0.8 wt % butene-1.

4. 400 MFR; 3.7 M_w/M_n.

TABLE 2

TABLE 3

*Dispersed particles less than 10% volume. ** Dispersed particles less than 20% volume. N/A = Data not available.

From the foregoing, it is seen that compatible blends of polypropylene and poly(l-butene) wherein polypropylene is the primary constituent can be prepared with or without the use of a plastomer compatibilizer. Blends prepared have the necessary properties for easy fiber formation, and have surprisingly good mechanical properties. However, the foregoing teachings are intended only to illustrate and explain the invention and the best mode contemplated, and are not intended to limit the invention. Variations and modifications will occur to those skilled in the art in view of the foregoing. It is intended that all such variations and modifications which fall within the scope or spirit of appended claims be embraced thereby.

Claims

Claims;

1. A polypropylene/poly(1-butene) blend, comprising: a matrix phase of from about 55 to about 90 percent by weight of the blend of crystalline polypropylene, and from about 10 to about 45 percent by weight of the blend of poly(l-butene) dispersed therein.

2. The blend of claim 1, wherein said polypropylene is isotactic.

3. The blend of claim 1, wherein said polypropylene has a melt flow rate greater than 20 dg/min.

4. The blend of claim 1, wherein said polypropylene has a melt flow irate of from about 400 to about 1000 dg/min.

5. The blend of claim 1, wherein said polypropylene has M_w/M_n less than about 3.

6. The blend of claim 1, wherein said poly(1-butene) has a density from about 0.89 to about. 0.92 g/cm³ and a melt index from about l to about 100 dg/min.

7. The blend of claim 1, further comprising a compatibilizing amount of an ethylene/α-olefin plastomer having a weight average molecular weight between abut 5000 and about 50,000, and a density from about 0.865 to about 0.90 g/cm³.

8. The blend of claim 8, wherein said plastomer comprises up to about 10 percent by weight of said blend.

9. Fiber melt spun from the blend of claim 1.

10. The fiber of claim 9, wherein said polypropylene has a melt flow rate from about 20 to about 50 dg/min.

11. Fiber melt blown from the blend of claim 1.

12. The fiber of claim 11, wherein said polypropylene has a melt flow rate from about 400 to about 1000 dg/min.

13. A nonwoven fabric, comprising fiber melt spun from the blend of claim 1.

14. The nonwoven fabric of claim 13, wherein said polypropylene has a melt flow rate greater than 20 dg/min.

15. The nonwoven fabric of claim 13, further comprising a compatibilizing amount of an ethylene/α-olefin plastomer having a weight average molecular weight between about 5000 and about 50,000, and a density from about 0.865 to about 0.90 g/cm³.

16. A nonwoven fabric comprising fiber melt blown from the polypropylene/poly(1-butene) blend of claim 1.

17. The nonwoven fabric of claim 16, wherein said polypropylene has a melt flow rate from about 400 to about 1000 dg/min.

18. The nonwoven fabric of claim 16, further comprising a compatibilizing amount of an ethylene/α-olefin plastomer having a weight average molecular weight between about 5000 and about 50,000, a density from about 0.865 to about 0.90 g/cm³.

19. A nonwoven fabric, comprising: opposite outer layers of melt spun fiber bonded to an inner layer of melt blown fiber disposed between said outer layers; wherein at least one outer layer comprises fiber melt spun from the blend of claim 1.

20. A nonwoven fabric, comprising: opposite outer layers of melt spun fiber bonded to an inner layer of melt blown fiber disposed between said outer layers; wherein said melt blown fiber comprises the blend of claim 1.