Optimized Polymer Rheology for High Extensibility Spunbond Filaments

This application claims priority to and the benefit of U.S. Provisional Patent Application No. 63/649,763, entitled Optimized Polymer Rheology for High Extensibility Spunbond Filaments and filed May 20, 2024, the contents of which are incorporated herein by reference in their entirety.

The present disclosure relates to nonwovens fabric and the process of producing fine fibers and nonwoven fabrics, and in particular to processes for enhancement of nonwoven fabrics properties via reduction of fiber denier.

Nonwovens are routinely used in a large variety of sectors including hygiene, medical and agriculture. In hygiene applications, nonwovens are used to manufacture different products such as baby diapers and adult incontinence articles. Nonwoven fabrics offer different properties to the items, for example, breathability or high liquid barrier. These fabric properties can be modified or customized based on the fabric configuration, processing conditions and/or raw materials composition. Varieties of additives are incorporated during the fabric processing to confer or enhance specific properties to the material. For example, slip agents are used to modify haptic properties, and pigments and opacifiers are used to modify coloration. The main developmental aims in the nonwovens industry include the reduction of the amount of raw material used to produce the fabrics, the premiumization of the materials produced and the use of new/alternative materials.

Objects of the present invention include the following:

It is shown in the current disclosure that extensive spunbond fiber draw, including draw at the sub-denier range, even at high throughputs, above 220 kg/h/m, and more specifically, above 0.5 gr/hole/min, can be achieved by fine tuning the rheology of the base material. The fine-tuned rheology enables a stable process at fast fiber extensibilities, above 7000 Pa cabin pressure. Nonwoven material with enhanced properties, including mechanical strength, homogeneity, softness and barrier properties is produced with this technology. Furthermore, controlled rheology enables the use of very low MFR base polypropylene in both spunbond and meltblown applications.

The present invention relates to the field of nonwovens. More particularly, this disclosure relates to the modification of nonwoven material by the inclusion of additives. Specifically, additives that trigger polymer backbone cleavages, including oxidative catalysts or visbreaking additives, are used as melt-in aids during the nonwoven production process. In the case of visbreaking additives, low dosing enables the fine tuning of the resin rheology enhancing the material cohesion and opening the processability window. This makes possible the generation of ultra-fine fibers during the spinning process. Further, the optimization of the polymer rheology stabilizes the spinning process, mitigating typical process problems such as dripping or generation of married fibers. Base polymer, for example polypropylene, is introduced in the extruder with a low dosage of visbreaking additive, or any other chemistry capable of breaking backbone chains, and is subjected to high extensibility conditions, e.g., cabin pressures and processing temperatures. This process forms fine fibers well under 1.5 denier in the case of polypropylene. Fine fibers boost many desirable nonwovens properties including, for example, material strength, air permeability and softness, to name a few. This engineered material can be used in articles for different fields such as hygiene, medical or automotive.

In terms of nonwoven material, it is desirable to achieve material weight reduction while maintaining critical properties of the material. It is further desirable to enhance material properties. Additionally, it is desirable to adjust the rheology of polymers used to form nonwoven material to adapt the polymers to high-speed continuous filament production lines for both spunbond and SMS configurations. Oxidative aids can be used to adjust the molecular weight distribution of polymers.

The term “polymer” as used herein refers to any plastic material, including polyolefins, that is processed into a spunbond nonwoven material. Within this chemistry family, visbreaking additives are commonly used to reduce the molecular weight of primary resins. For example, during the development of meltblown technology, visbreaking additives are conventionally used to reduce the viscosity (by reducing the molecular weight) of spunbond grade polyolefins and condition them for the meltblown low viscosity requirements. Visbreaking technologies, including peroxide chemistries, have been profusely used with polypropylene. This technology is based on reactive chemistries. Reactive chemistries promote scission of the polyolefin backbone, thereby generating lower molecular weight material.

It is well known that resin rheology has an important role in the nonwoven material processability as well as on the final nonwoven properties. Fiber grade polypropylene with a more or less controlled rheology is mandatory to produce spunbond or meltblown nonwoven material. Resin rheology is related to a series of material properties including raw material melt flow index/rate (MFI/MFR), molecular weight (MW), branching, crystallinity, and molecular weight distribution (MWD). For example, it is known that metallocene polypropylene, having a narrower MWD than Ziegler-Natta based polypropylene, improves molecular orientation during fiber attenuation (fiber draw), thereby producing stronger nonwoven material. Further, it is believed (as pointed out in PCT Patent Application Publication No. WO1997040225A1) that the melted polymer swells when it experiences the lower pressure as it exits the high-pressure die. It is described in WO1997040225A1 that the initial swell can be reduced by eliminating high molecular weight molecules, resulting in the production of finer fibers. Further, lower molecular weight molecules (xylene solubles), with lubricant properties, are also believed to assist in the drawing process of the fiber.

Here, we show a methodology that benefits from the oxidative chemistries, including visbreaking additives and oxidative catalysis, to adjust resin rheology and enhance material properties. Visbreaking additives or oxidative catalysis, not excluding other additives capable of chain scission, e.g., reducing the molecular weight of the polymer, can be used to fine tune and optimize the raw material rheology for spunbond and meltblown material. It is important to reduce the high molecular weight species, correlated with Mz, in order to increase the resin MFR and reduce the shear stresses during fiber draw as well as to narrow the molecular weight distribution. This molecular monodispersity, narrow molecular weight distribution, favors crystallinity induced during fiber draw, which is known to improve fiber strength. Further, maintaining high molecular weight average (Mw) enhances intermolecular forces and therefore further enhances fiber strength. Without being bound by theory, we believe that this fine-tuned rheology, high strength fiber (high crystallinity and Mw) together with the resulting lower shear stresses at high MFR during fiber draw, widens the material stability window and enables the generation of highly extensible filaments enabling finer fibers production and subsequent nonwovens with enhanced properties. Further, we show this technology can achieve fabric basis weight reduction by 10% or greater and production of 5-6 gsm spunbond/SMS fabrics on existing commercial continuous filament spinning lines without significant capital upgrades. Additionally, some polypropylene grades, specifically typical recycled polypropylene grades, are not suitable for high-speed fiber/filament spinning due to their low MFR (less than 10). The exact MFR of the recycled polypropylene grade is dependent on the source raw material and in most cases the MFR is around 5 due to the wide use of injection molding polypropylene and/or packaging waste for recycling. Visbreaking technology described here enables the adjustment of low MFR recycled polypropylene to adapt it for its use in both spunbond and meltblown technologies.

Melted polymers have complex viscoelastic qualities highly dependent on their rheology. The Deborah number (De=t/t, where tis the material relaxation time and tis the experimental time) characterizes the fluidity of a material. For high De, the material exhibits a more elastic solid-like behavior, whereas for low De, the material shows a more viscous liquid-like nature. White & Ide (Journal of applied polymer science, 22, 11, 3057 (1978)) described a series of melt spinning experiments with polymers spanning a wide range of viscosities. They observed that highly elastic material was difficult to spin due to ductile failures (high De, solid-like behavior). High viscosity polymers showed dripping instabilities, attributed most likely to be capillarity driven (low De, liquid-like behavior). Intermediate viscosity polymers showed both indefinite (on the lower viscosity end) and almost indefinite elongations at low extensions rate but exhibiting cohesion failure (solid-like behavior) at higher extension rates (on the higher viscosity end). Analogously to White & Ide's work, we observe: A. For the high end of intermediate viscosities. Solid-like elastic failures when spinning pristine polypropylene only at high extension rates (high cabin pressures) resulting in A.1. Spring-like behavior upon downstream fracture of partially solidified filaments at higher working temperatures. This condition generates coils of filament material millimeter scale with characteristic lengths orders of magnitude above the filament diameter (<500 microns), indicating that the fracture is not liquid-like capillarity-driven and A.2. More upstream fractures (fully melted fibers) at lower working temperatures. This condition generates centimeter scale drips/blobs 2 orders of magnitude larger than the diameter of the filament (indicating again a non-capillarity driven phenomenon) B. For the low end of intermediate viscosities, an extremely stable processing region at both low and high extensibility rates (low and high cabin pressures) that enables extended fiber elongations well below 1 denier with no observed instabilities. This is enabled by fine tuning the polymer rheology (optimized oxidative chemistries levels) as demonstrated in. C. For the even lower end of intermediate viscosities (high oxidative chemistries levels) but still high enough, material still does not exhibit liquid-like behaviors (this is not described in White & Ide). However, the process is unstable even at low extensibility rates (low cabin pressures), resulting on millimeter scale non-spherical drips/blobs, inconsistent with the mostly spherical particles generated upon capillarity driven liquid-like instabilities. Without being bound by theory, it is believed that these defects are possibly due to melted fibers coalescence, most likely triggered by some degree of air turbulence always present in the cabin chamber coupled with the small inertia of the very fine fibers produced at this lower viscosity condition.

In accordance with a method of forming a nonwoven material in accordance with an exemplary embodiment of the present invention, a polymer is fed into a spunbond extruder. A certain percentage of an oxidative enhancing additive, between 10 and 1000 ppm in one embodiment, between 50 and 500 ppm in another embodiment, between 100 and 300 ppm in yet another embodiment is added to the polymer to adjust its rheology. MFR of the rheology-adjusted polymer is between 100 and 140MFR combined with Mw above 150 kDa and 110 kDa respectively in one embodiment, above 140 MFR combined with Mw above 110 kDa in another embodiment and MFR between 70 and 100 combined with Mw above 180 kDa and 150 kDa respectively.

MFR of the rheology-adjusted polymer was measured using the following method:

In accordance with a method of forming a nonwoven material in accordance with another exemplary embodiment of the present invention, a spunbond grade polymer is fed into the meltblown extruder with initial MFR between 5 and 20MFR in one embodiment, above 20MFR in another embodiment and below 5 MFR in yet another embodiment. A certain percentage of an oxidative enhancing additive, between 100 and 5000 ppm in one embodiment, between 500 and 3000 ppm in another embodiment, between 1000 and 2000 ppm in yet another embodiment is added to the polymer to adjust its rheology. MFR of the rheology-adjusted polymer is between 1000 and 2500MFR in one embodiment, above 2500 MFR in another embodiment and MFR between 500 and 1000 in yet another embodiment.

An oxidative-enhancing additive as used herein refers to any component that helps cleave the backbone of the polymer into smaller molecular weight parts of the whole molecule. Exemplary oxidative-enhancing additives include but are not limited to visbreaking additives (peroxide and peroxide-free), organo-metallic salts, and metal oxides. The polymer/additive blend melts in the temperature-controlled extruder. The temperature in the extruder has an important effect on the final molecular weight distribution of the polymer. During this extruding process, the rpm of the extruder usually experiences a boost due to the reduction in viscosity of the blend and the need to maintain the working pressure constant. The throughput is not negatively affected for either spunbond or meltblown by this process and can be maintained high, from 0.3 to 0.8 gr/hole/min in one embodiment, from 0.4 to 0.7 gr/hole/min in another embodiment and from 0.5 to 0.6 gr/hole/min in yet another embodiment in the case of spunbond spinning and between 15 and 50 kg/h/m in one embodiment and above 50 kg/h/m in another embodiment. The filaments are drawn into fine fibers as they exit the die into the cabin chamber. In the spunbond case, the adjusted rheology of the optimized polymer enables the use of high drawn speeds, e.g., high cabin pressures, between 7000 and 9000 Pa in one embodiment, between 9000 and 11000 Pa in another embodiment and between 11000 and 15000 Pa in yet another embodiment, or even higher cabin pressures in spunbond spinning. Common spinning additives such as pigments, slip additives and polymer modifiers including but not limited to Exxon's Vistamaxx series are compatible with the herein described technology. Furthermore, different fiber configurations including but not limited to bicomponent core/sheet also show compatibility with the herein described technology.

Process stability is extremely high, common defects, such as drops and married fibers in spunbond, and shots in meltblown, have not been observed. The fibers are laid onto a moving belt. The unbonded fibers may be bonded using any type of bonding technology, including calendaring and hydroentanglement, to produce the final nonwoven material, which exhibits enhanced properties such as mechanical strength, air permeability and liquid barrier.

Example 1.shows MWD of a control 35MFR Zn-PP and a peroxide-based visbreaking adjusted-rheology sample. The addition of a small amount of visbreaking additive, below 500 ppm, shifted the MWD slightly toward the low MW, reducing the material viscosity by depleting entanglement-prone high MW populations, and boosting lubricant properties with lower MW populations. MFR increased from 35 to between 100 and 200g/10 min (230C/2.16).

Example 2.show a top-view and cross section of a ziegler-natta polypropylene homopolymer control sample. Cabin pressure was maximized to just below triggering instabilities. Average fiber diameter was 16.5 μm (1.75 denier).show top-view and cross section of ziegler-natta polypropylene homopolymer with a low dosage between 100 and 300 ppm of a peroxide-based visbreaking additive with MFR in the 100 to 200 range. This opened the process stability window enabling very high cabin pressures and intense fiber draw. The spunbond process ranged between 7000 to 15000 Pa cabin pressure, achieving average fiber diameter down to 11.4 μm (0.83 denier).

Example 3.shows a comparison of both the mechanical strength in the machine (MD) and cross directions (CD) of the web of visbroken samples. For this example, the spundbond process ran between 9000 Pa and 11000 Pa cabin pressure with a low dosage of a peroxide-based visbreaking additive between 100 and 300 ppm. Results are compared with the control run at its maximum stable cabin pressure. Results are shown at several web basis weights ranging from 26 to just above 31 gsm. A material strength gain of around 50% was observed in the machine direction compared to the control samples, whereas a close to 20% increase was observed in the cross direction.

Example 4. This is an example of how MWD affects material strength.shows a comparison of both the mechanical strength in the machine (MD) and cross directions (CD) of a nonwoven produced with organo-metallic salt additive against a control sample. In this example, the spundbond process ran between 2500 Pa and 3500 Pa cabin pressure with a low dosage of an organo-metallic salt additive between 20 and 500 ppm. Results are compared with the control run at the same cabin pressure. Even though fiber diameter observed is similar, lower Mz (see table below) enabled higher crystallinity and fiber strength. Results are shown at several web basis weights ranging from 10 to 40 gsm. Material strength gains between 20 and 40% were observed in the machine direction compared to the control samples, and from 35 to 50% increase was observed in the cross direction. In both cases the difference increased with material basis weight.

Example 5.shows a comparison of the air permeability (AP) of several peroxide-based visbroken samples ran between 7000 to 9000 Pa cabin pressure in one embodiment, between 9000 and 11000 Pa in another embodiment and between 11000 and 15000 Pa in yet another embodiment as well as the control, ran at its maximum stable cabin pressure, at different basis weights ranging from 14 to 31 gsm. A favorable decrease of AP was observed with increasing cabin pressure attributable to the finer fibers produced, with reductions of 20% between 7000 to 9000 Pa cabin pressure, 25% between 9000 and 11000 Pa cabin pressure and 30% between 11000 and 15000 Pa cabin pressure.

Example 6. Table 2 shows fiber diameter and denier for both control and visbroken samples processed at different cabin pressures. As expected, a reduction in fiber diameter/denier was observed with increasing cabin pressure. Also, visbroken fibers showed a reduction of fiber diameter/denier compared to the control processed at the same pressure, most likely due to its lower viscosity and higher tendency to be drawn (without being bound by theory).

Example 7.shows a comparison of the air permeability of a 30 gsm peroxide-based visbroken spunbond fine fiber sample with Exxon's Vistamaxx 7020BF between 2 and 25 wt %, ran between 7000 to 9000 Pa cabin pressure, compared to a 30 gsm control, ran at a typical 3000 Pa cabin pressure. Average 1.0 denier was achieved with this configuration. A favorable decrease of AP was observed with increasing cabin pressure attributable to the finer fibers produced, with reductions of 30% for the fine fiber plus Vistamaxx sample compared to the control sample.shows a 25% increase in both MD and CD mechanical strength on the fine fiber plus Vistamaxx sample compared to the control sample.shows a 25% improvement on the drapeability of the fine fiber sample with Vistamaxx 7020BF in both MD and CD directions as measured via handle-o-meter (HOM) compared to the control sample.

Example 8.shows a comparison of the air permeability of two 30 gsm peroxide-based visbroken spunbond bicomponent core/sheath PP/PE fine fiber samples with 70/30 and 80/20 PP/PE ratios. Visbreaking agent was added just on the PP core and the material was ran between 7000 to 9000 Pa cabin pressure. These samples are compared to a 30 gsm bicomponent core/sheath PP/PE 70/30 control, ran at a typical 3000 Pa cabin pressure. Average 1.1 denier with minimum of 0.65 denier was achieved with this configuration. A favorable decrease of AP was observed with increasing cabin pressure attributable to the finer fibers produced, with reductions of 35% and of 40% for the 70/30 and the 80/20 PP/PE ratio fine fiber samples, respectively, compared to the control sample.shows a 150% and 80% increase in MD and CD mechanical strength, respectively, on the 70/30 PP/PE ratio fine fiber sample and a 140% and 60% increase in MD and CD mechanical strength, respectively, on the 80/20 PP/PE ratio fine fiber sample compared to the control sample.

While in the foregoing specification a detailed description of specific embodiments of the invention was set forth, it will be understood that many of the details herein given may be varied considerably by those skilled in the art without departing from the spirit and scope of the invention.