Patent Application
20230193533
The present disclosure relates to articles comprised of polyesters/poly (methyl methacrylate) polymer blends. In particular, the invention relates to nonwoven filters comprised of polylactic acid/poly (methyl methacrylate) polymer blends.
Conventional filters have been made from many polymers such as polypropylene, polyethylene, polyester, polyamide, polyvinyl chloride, and polymethyl methylacrylate. The filters typically are made by a melt blown process forming a nonwoven fabric having substantially nonuniform length and diameter fibers with globules of pooled polymer causing a less than desired pressure drop across the filter for many applications.
Conventional filters lack an electrostatic charge and filter by impingement, impactions and diffusion. Electrostatic charge has been applied to improve the filtration performance of conventional filters. The charge causes the filter media to electrostatically attract small air borne particles and enhance the attachment of the particles to the media surface, thus improving efficiency of the filter.
Unfortunately, the electrostatic charge is known to dissipate with time and with the application of heat lowering the performance of the filters. To remedy the problem, charge enhancing additives such as fatty acid stearates or amides have been added to the polymers used to make the filters (see U.S. Pat. Publ. US20060079145). These, however, may affect the processing and fibers formed as well as potentially cause allergic reactions for applications that contact skin.
Accordingly, it would be desirable to provide a polymer and filter that overcomes one or more problems of the filtration art such as described above and, in particular, to filter applications that may come in contact with human skin (e.g., masks).
An aspect of the invention is the discovery that particular blends of PLA/PMMA may be melt blended, sheared and rapidly cooled to form articles such as nonwoven fabrics that display excellent ability to be charged enabling excellent filtration of particulates as well as retain the surface charge and maintain filtration efficiency. Thus, this aspect of the invention is a polymeric composition of poly(methylmethacrylate) and polylactic acid having a surface charge potential of at least about 50 electron volts in the absence of any other charge enhancing component or any other additive. In particular, the composition surprisingly retains at least 90% of said charge even when exposed to high heat (70° C.) for an hour to 24 hours.
Another aspect of the invention is that PMMA/PLA blends may be formed into particularly useful non-woven fabrics useful for filtration applications. The melt blown fabrics with long uniform diameter fibers without the defects observed in commercial polypropylene melt blown fabrics made for similar uses. This aspect is a method of producing a non-woven material comprising: providing a feed mixture comprising polymethyl methacrylate and polylactic acid to a melt blown extrusion device; liquefying the feed mixture in the extrusion device to form a liquified feed mixture; blowing the liquified feed mixture through at least one nozzle to form a spray; depositing the spray on a surface; e) reducing a temperature of the spray such that the spray solidifies on the surface to form a sheet of non-woven material.
Another aspect of the invention is a method of forming a polymeric composition comprising:(i) melting and blending polylactic acid and poly(methyl methacrylate) to form a melt blend, (ii) extruding the melt blend through a die to form a shaped article and (iii) cooling the shaped article at a cooling rate through Tg of the polylactic acid such that a microstructure is formed that may be charged to surface potential of at least 50, 100, 200, 500 or 1000 electron volts. This aspect may be performed in a manner using the method in the previous aspect.
A further aspect of the invention is a filter comprising at least one layer of a nonwoven fabric comprised of fibers of poly(methylmethacrylate) and polylactic acid that have a diameter of about 0.1 micrometer to 15 micrometers. In a particular embodiment the filter fibers are from about 0.5 or 1 micrometer to 15 micrometers with an average diameter of about 2 to 6 micrometers.
The disclosed non-limiting embodiments are discussed in relation to the drawings appended hereto and forming part hereof, wherein like numerals indicate like elements, and in which:
Non-woven materials useful as filters or masks may be produced from numerous materials, which may include one or more of the following: polymers, polymer blends, shape memory materials, shape memory polymers, and shape memory polymer blends.
Nonwoven fabrics useful to make filters and masks may be made by melt blowing. A blended polymer composition comprising polymethyl methacrylate (PMMA) and polylactic acid (PLA) may be melt blown. The blend of PMMA (30% w/w) and PLA (70% w/w) is fed to a melt blown extrusion machine, such as the one shown in
The PMMA/PLA blend is then extruded and blown in a system such as the system in
The polymeric composition typically is comprised of about 15% to about 85% PMMA with the balance being PLA. Desirably the composition is comprised of at most about 50% of PMMA. The PLA may be any form of PLA such as those formed using L-lactide, D-lactide, or combination thereof. Desirably, the amount of L-lactide is at least 50%, 60%, 70%, 80% or 90% to 98% or 100% (100% may include trace amounts of D-lactide) by weight of the monomer used to make the PLA. Other polyesters such as those known in the art that readily form crystalline or semi-crystalline polymers may also be used.
The PLA or polyester may have any Mw to realize a blend with the PMMA that has the desired melt flow rate. Typically, the PLA has a weight average molecular weight (Mw) of about 10 kDa to 500 kDa. The melt flow rate of the PLA or polyester may be any useful to form the articles herein such as the nonwoven fabrics. Typically, the MFR is one when combined with the PMMA realizes the desired fiber diameters and lengths as described herein. Generally, the MFRs may be from 25, 50, 60 or 70 to 90, 100, 125 or 150 grams (210° C./10 min, 2.16 kg). Examples of suitable PLAs are available under the tradename INGEO Biopolymer 625F and 3260HP from NatureWorks LLC and LUMINY L105 from Total Corbion PLA.
The PMMA may be any suitable PMMA such as those known in the art and may have a Mw that varies over a wide range such as from 10 kDa to 3 MDa so long as the PMMA exhibits rheological behavior allowing for the rapid shear and quenching while still realizing the desired degree of crystallinity and charge capability of the PLA/PMMA blend. The PMMA may be made with a small percentage (e.g., less than about 5%, 2%, or 1%) of comonomers such as those commonly used in the art (e.g., methyl acrylate, butyl acrylate and the like) to improve one or more properties such as impact strength or heat stability. The melt flow rate of the PMMA may be any useful MFR to form the articles herein such as the nonwoven fabrics. Typically, the MFR of the PMMA is one, when combined with the PLA, that realizes the desired fiber diameters and lengths as described herein. Generally, the MFR of the PMMA may be from 1, 2, 5, or 10 to 100, 50, 40 or 30 grams (230° C./10 min, 3.8 kg). Examples of PMMAs that may be useful include those available under the tradename designation CA41 from PLASKOLITE and those available under the tradename PLEXIGLAS VM, VS and VSUVT from Arkema.
It has been discovered, surprisingly, that the polymeric composition that is formed under sufficient shear and cooling through the crystallization melt temperature, in the absence of charge enhancing additives, allows for the charging of the composition to high surface potentials. This is believed, without being limiting in any way, to be due to the microstructure that is formed which may be comprised of smaller crystalline grains with greater defects and grain boundaries than previous compositions, for example, of melt blown nonwoven fibers (exposed to adequate shear and cooling). The surface charge of the polymeric composition may be at least 50, 100, 200, 500 or even 1000 electron volts. The surface charge may be measured by commercially available devices such as described in US5401446, col 8, lines 6 to 21.
Surprisingly, the PLA crystallizes when blended with amorphous (PMMA) even when sheared at high rates and rapidly cooled through the melt crystallization temperature of the PLA or polyester. The degree of crystallinity by volume is from the amount of PLA that crystallizes during the method described herein. Typically, the amount of PLA that crystallizes is from about 1%, 2%, 5% or 10% to about 50%, 40% or 30%. Thus, the degree of crystallinity of the blend when 50% of the PLA crystallizes in a 70% PLA/30% PMMA blend would give about 35% degree of crystallinity for the blend.
The charging of the surface may be accomplished by any known method of charging filters and the like such as described in U.S. Pat. and Pat. Publ.: US2740184; US4215682; US4375718; US4588537; US4592815; US4904174; 5122048; US5401446; and US2006/0079145. The charging voltage may be any voltage useful to impart the charge to the filter. Illustratively, the charging voltage may be from about 2 kV to 50 kV.
In an embodiment, the article is a filter comprising at least one layer of a nonwoven fabric comprised of fibers of poly(methylmethacrylate) and polylactic acid that have a diameter of about 1 micrometer to 15 micrometers and average diameter of about 2 to 6 micrometers. The filter desirably has at least about 90%, 95%, 99% or more by number of the fibers being longer than 500 micrometers, 1 millimeter or 2 millimeters. The filter nonwoven fabric surprisingly may have a low fabric weight and even when a higher fabric weight is used the filter may have a low pressure drop. The fabric weight may be for example 25, 50, or 100 to 300 or 200 grams/m2. The filter typically has pressure drop of at most about 5 mm Hg (666 Pa) 4 mm Hg (533 Pa), 2 mm Hg (266 Pa) or 1 mm Hg (133 Pa) even, for example, when the fabric weight is greater than 50 to 300 grams/m2 or multiple layers are used (layer being a separate discreet fabric sheet for example). One or more layers may be used to make the filter. The filter typically also may have an efficiency of at least 95% or 98%. The filter because of the microstructure as described herein retains the surface charge potential even when heated. For example, the surface potential, typically decays at most about 10% after exposure to 70° C. for one hour with essentially no loss in efficiency or less than 5%, 3%, 2%, or 1% loss in efficiency after such heating.
In the method of forming a fibrous article of the polymeric composition (e.g., melt blowing process) it is desirable for the PMMA/PLA blend to have a viscosity at 200° C. that allows for the production of fibers having a diameter of 1 to 15 micrometers with an average diameter of about 2 or 3 to 8, 7 or 6 micrometers. Typically, the MFR of the polymer blend of PLA and PMMA is 20 or 30 to 90 or 80 grams (210° C./10 min, 2.16 kg). The diameter may be determined by measuring the diameter of a number of fibers (~100) on one or more micrographs of the nonwoven fabric utilizing known image analysis techniques or by hand.
It has been discovered that ultra-high efficiency, the fabric, typically non-woven, may be comprised of fiber diameters all less than 1 micrometer to about 0.05 micrometer, with the lengths as described above. For both this embodiment and the embodiment described in the prior paragraph, the diameter along the length of the fibers display surprisingly good uniformity, which is readily apparent in
When forming a shaped article such as a filter element, the PLA/PMMA are melt blended and extruded through a die, typically forming a fibrous sheet or fabric, which may be woven or nonwoven. The shear rate through the die should be sufficient to realize the desired microstructure and surface charging potential. Typically, the shear rate is at least 10, 100, 250, 500 or 750 1/s to 5000, 4000 or 3500 1/s. Likewise, cooling rate through crystallization melt temperature of the polylactic acid or polyester (typically from about 140° C. to 180° C.) is sufficiently rapid such that the desired microstructure is formed allowing for the surface charge potential to be realized upon charging of the filter element formed by the process. Typically, the cooling rate is 10, 50, 100, 1000° C./min to 5000° C./sec, 4000° C./sec or 3500° C./sec, which is typically from about the melt temperature of the polymer blend to about 100° C.
Illustratively, when forming a melt blown nonwoven fabric, the melt pump temperature is above the melt point of the PLA/PMMA polymers to melt blend them and typically is from about 175 to 250° C. The die temperature is generally slightly higher than the melt pump temperature (i.e., 10% or 20% higher). For example, if the melt pump temperature is 200° C. then the die temperature may be from about 220 to 250° C. To ensure a desired cooling rate to realize the desired microstructure the air temperature from the exiting of the die to impingment on the collection rolling is typically set to at least about 1.3, 1.5, 1.8 or even 1.9 times greater than the melt pump temperature or die temperature, with it being understood that the air cools somewhat upon being blown from the blowing orifice until impingement upon the fiber exiting the die.
Surprisingly, the non-woven fabric comprised of PMMA/PLA) may have an efficiency of 98-99% with only a 2 mm Hg (millimeters of mercury) pressure drop, having the fiber structure described here, which may be made by the melt blown process described herein. Filtering efficiency and pressure drop were measured using a TSI 8130A instrument. An air flow rate of 32 L/min was used for measurements with a salt (NaCl) diameter of 0.1 µm or 0.2 µm. The fabric filter contained 30% w/w PMMA and 70% w/w PLA. The filter material was electrically charged and had a circular shape with a 111 mm diameter. The filter material had a density of 100 grams per square meter (gsm). The PMMA/PLA sheet was thermoformable and exhibited shape memory. That is, the PMMA/PLA sheet can be placed over a fixture and heated to an elevated temperature in order to mold the PMMA/PLA sheet into a desired shape. It may be desirable to heat the PMMA/PLA sheet to a temperature that is approximately equal to, equal to, or above a transition temperature. The transition temperature (to heat to) may be the transition temperature of a polymer that is present in the polymer blend. The transition temperature (to heat to) may be the transition temperature of the polymer with the lowest transition temperature (compared to the other polymers in the polymer blend).
Advantageously, thermoforming a PMMA/PLA polymer blend results in a predefined shape without deforming the fibers. The amount of PMMA in a PMMA/PLA blend may be from 15 to 85% w/w to produce a PMMA/PLA blend that exhibits shape memory properties. Likewise, the amount of PLA in a PMMA/PLA blend may be from 15 to 85% w/w to produce a PMMA/PLA blend that exhibits shape memory. PMMA/PLA blends having between 15-85% w/w PMMA and having between 15-85% PLA may be used to produce a PMMA/PLA blend that exhibits shape memory properties. The shape memory effect and the moldability of a PMMA/PLA blend may be increased by adding an elastomeric additive, such as an impact modifier. The elastomeric additive, such as an impact modifier, may be added at a concentration of 0 to 25% w/w of the total weight of the mixture. A temperature range of 80 to 85° C. may be used in the heating step to form the polymer blend around a fixture to give it a permanent shape. Alternatively, the polymer blend may be heated and formed to a user’s face.
Many materials that are used for producing surgical masks experience deformation of the polymer fibers when heated near their thermoformable temperature. Therefore, many thermoformed masks today have a layer of material that is thermoformable to provide the thermoformability of the product. This inherently results in a multi-layer structure, as opposed to the single layer structure as described herein.
A material that is a moldable thermoform, such as PMMA/PLA, enables a mask to be produced that forms a better seal with the user’s face, compared to today’s surgical masks. A better fit could be offered to consumers (mask wearers) by offering different sizes and shapes of masks. For example, a face mask that can be thermoformed without altering or deforming the polymer fibers could be molded into small, medium, large, etc. sizes and could be offered in different shapes, such as circular, oval, square, rectangular, etc. to better fit users faces. Custom surgical masks could also be produced by making a mold of an individual’s face, fitting the moldable thermoform mask over the mold, and heating the mask material on the mold to give the mask a permanent shape.
It has been discovered that the PLA/PMMA compositions are particularly useful to form thermoformed fabric shapes such as thermoformed masks without deforming the fibers. Typically, a temperature of about 50 to about 100° C. may be used to thermoform a shaped fabric such as a filter comprising PMMA and PLA. A heated (and softened) mask can be pressed to a user’s face and as cools it forms to the contours of the user’s face. The thermoformed mask results in a face mask having a tight seal to the user’s face. At least in part, because the fibers do not deform when thermoforming, the shape may be altered and refitted or shaped for another use or the like (reverse thermoformability). Other polymers or polymer blends that exhibit shape-memory and that are capable of filtering may also be used in the disclosed embodiments.
A thermoformable filter material, such as a PMMA/PLA blend described herein, may further include a backing layer to provide strength and support to the filter material. The backing layer may be a polymeric material that includes an adhesive on one side to adhere to the filter material.
Support 1102 provides structure to the protective face mask 1100 and assists in holding the mask to a user’s face. Support 1102 may be a polymer that comprises a rubber elastomer. Support 1102 may be over molded onto the thermoformable material 1101. Alternatively, support 1102 may be a functional sealant that adheres to the thermoformable material 1101 and provides rigidity to the mask when cured. Support 1102 may be a functional sealant that comprises or consists of silicone.
Support strap 1103 may be an elastic strap and functions to hold the support 1102 and the thermoformable material 1101 to a user’s face. Alternatively, support strap 1103 may be made of a non-elastomeric material. Support strap(s) 1103 may be slid through apertures (holes/openings) 1104 that are present in the support 1102, as shown in
Nonwoven fabric filters are made by melt blowing through a die having a 200 micrometer diameter orifice. The composition is 70% by weight PLA (INGEO Biopolymer 625F) and 30% by weight PMMA (PLASKOLITE CA41), unless otherwise noted in Table 1. The melt extruder temperature is from about 230 to 250° C., the die temperature was set at 200° C. and the blowing air set temperature is 380° C. and air pressure is 40 pounds per square inch. The collector distance is as noted in Table 1. The filters were tested with salt particles in a similar manner as described in US20060079145 (para. 28) except that the salt size was 0.1 and 0.2 micrometer as shown in Table 1 and as described herein. The Comparative Examples are made in the same way except that a charge enhancing additive was added or the fabric was made with polypropylene as also noted in Table 1.
From Table 1, it is readily apparent that the filters of the invention when charged have a greater efficiency for a given pressure drop compared to polypropylene typically used for such filters. (see Examples 6 and 7 versus Comparative Examples 5 and 6). Likewise, it is quite evident that the filters of the present invention have a much lower pressure drop for heavier weight fabrics allowing for more robust breathable filters. Surprisingly, the filters and polymeric composition of the present invention do not require a charge enhancing additive like typical polypropylene filters (see Comparative Examples 1-3). This may be due to the additive deleteriously affecting the desired microstructure.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2021/038889 | 6/24/2021 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63116512 | Nov 2020 | US | |
| 63044637 | Jun 2020 | US |
