BICONSTITUENT POLYPROPYLENE/POLYETHYLENE BONDED FIBERS

Abstract: Heat-bonded articles are prepared by employing as the heat-bondable material, biconstituent PP/PE fibers which comprise polypropylene as one phase and polyethylene as another phase. The biconstituent fibers provide improved tenacity and hand when compared to polypropylene alone. The biconstituent fibers of PP/PE having co-continuous zones exhibit a stronger heat-bond than PP alone and exhibit it over a much broader heat-bonding temperature range than PP alone. Favorable shrinkage characteristics are obtained.

Description: BICONSTITUENT POLYPROPYLENE/POLYETHYLENE BONDED FIBERS Blends consisting of polypropylene and polyethylene are spun into fibers having improved bonding properties and lower shrinkage. Polypropylene (PP) fibers and filaments are items of commerce and have been used in making products such as ropes, non-woven fabrics, and woven fabrics. In conformity with commonly accepted vernacular or jargon of the fiber and filament industry, the following definitions apply to the terms used in this disclosure: A "monofilament" (a.k.a. monofil) refers to an individual strand of denier greater than 15, usually greater than 30. A "fine denier fiber or filament" refers to a strand of denier less than about 15. A "multi-filament" (a.k.a. multifil) refers to simultaneously formed fine denier filaments spun as a bundle of fibers, generally containing at least 3, preferably at least about 15 to 100 fibers and can be several hundred or several thousand. "Staple fibers" refer to fine denier strands which have been formed at, or cut to, staple lengths of generally about 1 to about 8 inches. An "extruded strand" refers to an extrudate formed by passing polymer through a forming-orifice, such as a die. A "fibril" refers to a superfine discrete filament embedded in a more or less continuous matrix. Whereas it is known that virtually any thermoplastic polymer can be extruded as a coarse strand or monofilament, many of these, such as polyethylene and some ethylene copolymers, have not generally been found to be suitable for the making of fine denier fibers or multi-filaments at feasibly high production speeds. Practitioners are aware that it is easier to make a coarse monofilament yarn of 15 denier than to make a multi-filament yarn of 15 denier, especially where high¬ speed spinning is needed to obtain economical production rates. It is also recognized that the mechanical and thermal conditions experienced by a bundle of filaments, whether in spinning staple fibers or in multi-filaments yarns, are very different to those in spinning monofilaments. The fact that a given man-made polymer can be extruded as a monofilament, does not necessarily herald its use in fine denier or multi-filament spinning. Whereas an extruded monofilament which has been cooled can usually be cold-drawn (stretched) to a finer denier size, even if it does not have sufficient melt-strength to be melt-drawn without breaking, it is apparent that a polymer needs to have an appreciable melt-strength to be melt-drawn to fine denier sizes. Low density polyethylene (LDPE) is prepared by polymerizing ethylene using a free-radical initiator, e.g. peroxide, at elevated pressures and temperatures, having densities in the range, generally, of about 0.910 to 0.935 gms/cc. The LDPE, sometimes called "I.C.I.- type" polyethylene is a branched (i.e. non-linear) polymer, due to the presence of short-chains of polymerized ethylene units pendent from the main polymer backbone. Some of the older art refers to these as high pressure polyethylene (HPPE). High density polyethylene (HDPE) is prepared using a coordination catalyst, such as a "Ziegler-type" or "Natta-type" or a "Phillips-type" chromium oxide compound. These have densities generally in the range of about 0.94 to about 0.98 gms/cc and are called "linear" polymers due to the substantial absence of short polymer chains pendent from the main polymer backbone. Linear low density polyethylene (LLDPE) is prepared by copolymerizing ethylene with at least one α- olefin alkylene of C3 to Ci2» especially at least one of C4 to C8, using a coordination catalyst such as is used in making HDPE. LLDPE is "linear", but has alkyl groups of the α-olefin pendent from the polymer chain. These pendent alkyl groups usually cause the density to be in about the same density range (0.88 to 0.94 gms/cc) as the LDPE; thus the name "linear low density polyethylene" or LLDPE is used in the industry in referring to these linear low density copolymers of ethylene. Polypropylene (PP) is known to exist as atactic (largely amorphous), syndiotactic (largely crystal- -4- line), and isotactic (also largely crystalline), some of which can be processed into fine denier fibers. It is preferable, in the present invention, to use the largely crystalline types of PP grades, sometimes referred to as constant rheology ("CR"), which are suitable for spinning fibers, especially fine denier fibers. It was found that improvements are made in polypropylene fibers if the polypropylene is first blended with 20 percent to 90 percent by weight of a polyethylene, especially a linear low density ethylene copolymer (LLDPE) containing, generally, 3 percent to 20 percent of at least one α-olefin alkylene of 3 to 12 carbon atoms. It was also found that certain polyethylenes (more specifically LLDPE's) can be blended in a molten state with polypropylene in all proportions and then melt spun into fine denier fibers, some of which offer improved properties over polyethylene and polypropylene alone. In accordance with this invention heat-bonded articles having excellent bond strength when bonded over a wide range of temperatures are prepared from fibers comprising a dynamically-mixed melt-spun blend of polypropylene (PP) and polyethylene (PE), said blend comprising a PP/PE ratio in the range of 0.6 to 1.5, said fiber having a substantially co-continuous domains morphology. The heat-bonded articles, including those wherein the above fibers are used alone or are blended with other fibers or other materials, can take a number of shapes and sizes including, e.g., various non-woven fabrics, composites and other items in which bonding into a unit is accomplished using the above-described fibers. It will be understood that the present invention is not limited to only neat PP and PE, but also includes polymers containing additives that are often used in such polymers, such as, stabilizers, dyes, colorants, pigments, wetting agents, water-proofing agents, soil-proofing agents, and the like, so long as the additives have no substantial detrimental effect of the fiber-making ability of the polymers. Considering that most fibers produced on a commercial scale for ordinary application in fabrics and the like are drawn as fibers in the presence of air while they are hot, and considering that the surface area to volume ratio of fine fibers is quite high, then it will be understood that an antioxidant is often used to avoid, or at least reduce, oxidation of the polymer during the fiber-making process. Useful and novel fibers, especially fine denier fibers, are prepared from blends of polypropylene (PP) and polyethylene (PE), especially linear low density ethylene copolymer (LLDPE) which have been melt blended in an intensive mixer just ahead of the melt spinning of the fibers when using ratios of PP and PE which result in co-continuous zones in the resulting fiber, said co- continuous zones being microscopically detectable in the sectioned fibers when cooled. Generally, these co- continuous zones are produced when the ratio of PP/PE is in the range of 0.6 to 1.5, especially in the range of 0.8 to 1.2, most especially in the range of 0.9 to 1.1. Such fibers have unexpectedly been found to exhibit appreciably stronger fiber-to-fiber bonds over a wide temperature range employed when heat bonding, as compared with PP alone. The tenacity and softness of the fibers is improved over that of the polypropylene or the polyethylene alone. The polyethylene for use in this invention may be LDPE or HDPE, but is preferably LLDPE. The molecular weight of the polyethylene should be in the moderately high range, as indicated by a melt index, M.I., (a.k.a. melt flow rate, M.F.R.) value in the range of 12 to 120, preferably 20 to 100, most preferably 50 + 20 gms/10 min. as measured by ASTM D-1238(E) (190°C/2.16 Kg). Regarding the use of preferred LLDPE, it is preferred that the comonomer α-olefin alkylenes in the upper end of the C3 to C12 range be used, especially 1- octene. Butene (C4) is preferred over propylene (C3) but is not as preferred as 1-octene. Mixtures of the alkylene comonomers may be used, such as butene/octene or hexene/octene in preparing the ethylene/alkylene copolymers. The density of the LLDPE is dependent on the amount of, and the molecular size (i.e. the number of carbons in the alkylene molecule) of, the alkylene incorporated into the copolymer. The more alkylene comonomer used, the lower the density; also ' , the larger the alkylene comonomer, the lower the density. Preferably an amount of alkylene comonomer is used which results in a density in the range of 0.88 to 0.94, most preferably 0.92 to 0.93 gms/cc. An ethylene/octene copolymer having a density of about 0.925 gms/cc, an octene content in the range of 10 to 15 percent and a M.F.R. at or near 50 gms/10 min. is very effective for the purposes of this invention. The method of melt-mixing is important due to generally acknowledged immiscibility of the PP and PE. An intensive mixer-extruder is required which causes, in the blender, on the one hand, molten PE to be dispersed in the molten PP and the dispersion maintained until the mixture, as an extrudate, is expelled from the extruder. On the other hand, molten PP is dispersed in molten PE when the amount of PE exceeds the amount of PP. The following chart is provided as a means for describing the results believed to be obtained for the various ratio ranges of PP/PE, when using PE (esp. LLDPE) having an M.F.R. in the range of about 12 to about 120 gms./10 min., and a crystalline PP, where the melt viscosity and melt strength are such that reasonably good melt-compatibility and miscibility are achieved by use of the high-intensity mixer-extruder: Approx. Range of Ratio of PP/PE General Results One May Obtain* 4.0 - 1.5 Mostly PE fibrils dispersed in PP continuous matrix. 1.5 - 1.2 Mostly co-continuous domains of PP and PE with some PE fibrils. 1.2 - 0.8 Nearly all co-continuous domains of lamellar structure. 0.8 - 0.6 Mostly co-continuous domains of PP and PE with some PP fibrils. 0.6 - 0.1 Mostly PP fibrils dispersed in PE continuous matrix. *0bviously the results in or around the central ratio ranges are overlapping and are ambiguous in that some of the results obtained are from both sides of the overlap. Polymer blends of PP and PE prepared in such a mixer are found to be useful, strong, and can be extruded into products where the immiscibility is not a -8- problem. As the so-formed extrudate of a mixture which contains more PP than PE is spun and drawn into fibers, the molten PE globules become extended into fibrils within the polypropylene matrix. An important, novel feature of the fibers is that the fibrils of PE are diverse in their orientation in the PP matrix. A larger fraction of PE particles is found close to the periphery of the cross-section of the fibers, and the remaining PE particles are spread in the inner portions of the fiber. The size of the PE particles is smallest at the periphery of the fiber's cross-section and a gradual increase in size is evidenced toward the center of the fiber. The frequency of small particles at the periphery is highest, and it decreases toward the center where the PE particles are largest, but spread apart more. The PE fibrils near the periphery of the fiber's cross-section are diverse in the direction in which they are oriented or splayed, whereas close to the center of the fiber the orientation is mostly coaxial with the fiber. For the purpose of being concise, these fibers will be referred to herein as blends consisting of PP as a continuous phase, and containing omni-directionally splayed PE fibrils as a dispersed phase. Microscopic examination reveals that the PE fibrils, when viewed in a cross-section of the biconstituent fiber, are more heavily populated near the outer surface than in the middle. The shape of each PE fibril in the cross- section is dependent on whether one is viewing a PE fibril sliced at right angles to the axis of the PE fibril at that point or at a slant to the axis of the PE fibril at that point. An oval or elongate shaped section indicates a PE fibril cut at an angle. An elongate shaped section indicates a PE fibril which has skewed from axial alignment to a transverse position. The mixer for preparing the molten blend of PP/PE is a dynamic mixer, especially one which provides 3-dimensional mixing. Insufficient mixing will cause non-homogeneous dispersion of PE in PP resulting in fibers of inconsistent properties, and tenacities lower than that of the corresponding PP fibers alone. A 3- dimensional mixer suitable for use in the present invention is disclosed in a publication titled "Polypropylene—Fibers and Filament Yarn With Higher Tenacity", presented at International Man-Made Fibres Congress, September 25-27, 1985, Dornbirn/Austria, by Dr. Ing. Klaus Schafer of Barmag, Barmer Maschinen- Fabrik, West Germany. The distribution of PE fibrils in a PP matrix are studied by using the following method: The fibers are prepared for transverse sectioning by being attached to strips of adhesive tape and embedded in epoxy resin. The epoxy blocks are trimmed and faced with a glass knife on a Sorvall MT-6000 microtome. The blocks are soaked in a mixture of 0.2 gm ruthenium chloride dissolved in 10 ml of 5.25 percent by weight aqueous sodium hypochlorite for 3 hours. This stains the ends of the fibers with ruthenium to a depth of about 30 microns. The blocks are rinsed well and remounted on the microtome. Transverse sections of fibers in epoxy are microtomed using a diamond knife, floated onto a water trough, and collected onto copper TEM grids. The grids are examined at 100 KV accelerating voltage on a JE0L 100C transmission electron microscope (TEM). Sections taken from the first few microns, as well as approximately 20 microns from the end are examined in the TEM at magnifications of 250X to 66,OOOX. The polyethylene component in the samples are preferentially stained by the ruthenium. Fiber sections microtomed near the end of the epoxy block may be overstained, whereas sections taken about 20 microns away from the end of the fibers are more likely to be properly stained. Scratches made by the microtome knife across the face of the section may also contain artifacts of the stain, but a skilled operator can distinguish the artifacts from the stained PE. The diameter of PE fibrils near the center of the PP fiber have been found to be, typically, on the order of 350 to 500 angstrom, whereas the diameter of the more populace fibrils near the periphery edge of the PP fiber have been found to be, typically, on the order of 100 to 200 angstrom. This is in reference to those which appear under high magnification to be of circular cross-section rather than oval or elongate. At less than 20 percent polyethylene in the polypropylene one obtains better "hand" than with polypropylene alone, but without obtaining a significant increase in tenacity and without obtaining a dimensionally stable fiber. By the term "dimensionally stable" it is meant that upon storing a measured fiber for several months and then remeasuring the tenacity, one does not encounter a significant change in the tenacity. A change in tenacity indicates that stress relaxation has occurred and that fiber shrinkage has taken place. In many applications, such as in non-woven fabrics, such shrinkage is considered undesirable. By using 20 percent to 45 percent polyethylene in the polypropylene one obtains increased tenacity as well as obtaining better "hand" than with polypropylene - 1 1 - alone. By using between 25 percent to 35 percent, especially 28 percent to 32 percent, of polyethylene in the polypropylene one also obtains a substantially dimensionally stable fiber. A substantially dimensionally stable fiber is one which undergoes very little, if any, change in tenacity during storage. A ratio of polypropylene/polyethylene of about 70/30 is especially beneficial in obtaining a dimensionally stable fiber. By using 50 percent to 90 percent polyethylene in the blend, a reduction in tenacity may be observed, but the "hand" is noticeably softer than polypropylene alone. A greater draw ratio gives a higher tenacity than a lower draw ratio. Thus, for a given PP/PE ratio, a draw ratio of, say 3.0 may yield a tenacity greater than PP alone, but a draw ratio of, say 2.0 may not give a greater tenacity than PP alone. In order to establish a nominal base point for making comparisons, several commercially available PP's are spun into fine denier fibers and the results are averaged. The average denier size is found to be 2.1, the average elongation is found to be 208 percent and the average tenacity at the break point is 2.26 gm/denier. Similarly, to establish a nominal base point, several LLDPE samples are spun into fine denier fibers and the results are averaged. The average denier size is found to be 2.84, the average elongation is found to be 141 percent, and the average tenacity at the break point is 2.23 gm/denier. The following examples illustrate particular embodiments, of the invention. Biconstituent PP/PE fibers prepared as described above and heat-bonded at temperatures sufficient to melt the polymers, or at least soften them enough for bonding, exhibit heat-bonding ranges over a surprisingly wide range of temperatures, and the bond strength obtained when heat-bonded over a wide range is unexpectedly high. 0 EXAMPLE 1 (Heat-bonded fibers) This example illustrates the broad temperature range over which strong bonds are obtained by using the 5 biconstituent PP/PE fibers as compared with PP alone. The fabric samples are of 1 ounce/yard 2 (about 33«9gm/m 2 ) weight and are made using a heated flat top calendar roll and a heated, embossed bottom calendar roll. The top calendar roll temperatures are maintained 0 about 4°F (about 2°C) lower than the bottom calendar roll temperatures. Cutting the 4" X 1" (10 x 2.54 cm) strips in the machine direction is done in such a way that the most uniform portions of the fabrics are used before c pulling them apart on an Instron tensile tester. The force to cause failure is measured as gram-force. Each datapoint is the average of 8 sample, and a standard deviation if observed in the range of 5 percent to 15 percent. 0 Commercially available LLDPE (26.5 MFR and 0.940 g/cc density, 1-octene comonomer) is blended with equal parts of commercially available PP (CR fiber grade) and extruded at 2X stretch ratio as continuous biconstituent fine fibers using an intensive mixer extruder. The 50/50 PP/PE biconstituent fibers are made into staple fibers used in making non-woven fabrics at a variety of embossed roll temperatures. An example of a neat PP (without PE) is included as a "Control" for comparison. Table IV below demonstrates the MD strip tensile strength (gram-force) needed to tear the non- woven fabric. The temperatures in the table are the embossed roll temperatures, adjusted to the nearest whole number. TABLE I PE/PP Ratioι in Temp. Biconstituent Fibers Control (°C) Approx. 40/60 50/50 60/40 0/100 122 2854 124 3450 3523 127 4061 3521 129 4230 3373 131 4310 3847 133 4402 4113 136 4475 4031 138 4593 1865 140 3626 4422 2696 142 3943 4629 3686 144 4146 4272 3903 147 4029 4219 3528 149 3809 4180 __.__ 3498 Table I clearly shows that the mid-ranges of the ratios of the PE/PP biconstituent fibers not only produce stronger fiber bonds, but also provide lower effective bonding temperatures, a wider effective bonding range of temperatures, and softer fabrics. Advantages are found in range including and between the 40/60 to 60/40 ratios. Having found this phenomenon, one may extrapolate the range of ratios a little beyond each end of the mid-range. Thus a PP/PE ratio range of about 1.5 (i.e. about 60/40) to about 0.6 (i.e. about 40/60) is operable, with a ratio somewhere around 50/50 being most preferable. EXAMPLE 2 (Heat-bonded fibers) Similarly to Example 1 above, additional data is collected for LLDPE (12 MFR, 1-octene, 0.935 density) in Table II, LLDPE (98 MFR, 1-octene, 0.936 density) in Table III, and LLDPE (25 MFR, propene, 0.955 density) in Table IV. These tables show the improved results obtained when operating within the range of 40/60 to 60/40 PE/PP ratio. TABLE TT ] PE/PP Ratio of Temp. Biconsti Ltuent Fibers (°C) Approx. 30/70 40/60 50/50 60/40 70/30 118 __— —.-•__ 120 2743 122 ———— 3382 3046 124 ———— __— 3332 2913 127 ———— 3538 3593 2883 129 3671 3589 3196 131 4047 3688 —_—_ 133 3724 136 1609 4090 ____ 138 1825 2568 4156 mm , mm 140 1991 3140 4169 —___ 142 2645 3310 4155 ____ 144 3173 3415 4738 »««_ TABLE III PE/PP Ratioι in Biconstituent Fibers Temp. (°C) Approx. 30/70 50/50 70/30 118 2387 120 2480 122 2714 124 2905 127 2842 129 _—__ 131 ____ 133 3491 136 1134 3299 — 138 1283 3642 — 140 1585 3578 _ 142 2078 3413 ——__ -17- TABLE IV PE/PF ' Ratio in Biconstj Ltuent Fibers Temp. (°C) Approx. 30/70 40/60 50/50 60/40 70/30 124 2896 2753 127 3659 2892 129 3796 3854 3156 131 4008 3774 3201 133 4066 3924 3149 136 4049 138 4032 140 2961 4049 142 3363 3947 4331 144 3557 4147 4579 147 4060 4223 4333 ____ _—__ EXAMPLE 3 (Heat shrinkage) Fibers of PP/LLDPE of various ratios between the range 60/40 to 40/60, are tested in comparison with PP fibers alone and LLDPE alone, by being subjected to boiling water for 5 minutes and the shrinkage measured. It is found that in this range there is little or no increase in shrinkage when compared with PP. Thus the benefits of adding LLDPE to PP are not substantially compromised by the greater tendency of the neat LLDPE fibers to undergo shrinkage in boiling water. The heat-bonding capabilities of these biconstituent PP/PE fibers are useful in blends with other fibers, both natural and synthetic, especially -18- when staple fibers are blended and then heat-bonded at temperatures favorable for the particular blend being employed. Also, the heat-bondable PP/PE fibers can be employed as the bonding agent when in admixture with, or place between, other materials which are not thermoplastic in or near melt or softening point of the PP/PE biconstituent. Other materials, such as cellulosic fibers, metal fibers, mineral fibers, wood fibers, high melting synthetic fibers, and other particulate material can be mixed with, and thermally bonded into a unit by, the PP/PE biconstituent fibers.

Claims: CLAIMS: 1. A heat-bonded article comprising a plurality of heat-bonded fibers, said fibers comprising a melt spun blend of polypropylene (PP) and polyethylene (PE), said blend comprising a PP/PE ratio in the range of 0.6 to 1.5, said fibers comprising a substantial amount of the PP and PE as co-continuous zones. 2. The article of Claim 1 wherein the polyethylene is LLDPE. 3. The article of Claim 1 wherein the polyethylene is LLDPE having a melt flow rate of 12 to 120 gms/10 min. 4. The article of Claim 1 wherein the polyethylene is a copolymer of ethylene/1-octene wherein the 1-octene comprises about 3 to about 30 percent by weight of the copolymer. 5. The article of Claim 1 wherein the ratio of PP/PE is in the range of 0.8 to 1.2. 6. The article of Claim 1 wherein the ratio of PP/PE is in the range of 0.9 to 1.1. 7. The article of Claim 1 wherein the PP/PE fibers are of a size less than a denier of about 30. 8. The article of Claim 1 wherein the polyethylene is LLDPE having a density in the range of about 0.92 to about 0.94 gms/cc. 9. The article of Claim 1 wherein the polyethylene is a copolymer of ethylene and 1-octene. 10. A heat-bonded article comprising a heat- bonded mixture of (A) biconstituent PP/PE fibers having a PP to PE ratio in the range of about 1.5 to about 0.6, said fibers comprising a substantial amount of the PP and PE as co-continuous domains morphology, and (B) at least one other particulate material.

Inventors: JEZIC ZDRAVKO, JEZIC, ZDRAVKO
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1990
Main classification: D01F8
IPC codes: C08L23, C09J123, D01D5, D01F8, D04H1