SPECIFICATION

TECHNICAL FIELD

(001 ) The present invention relates to a nonwoven substrate having high surface uniformity, and its related method of manufacture, for a wide range of industrial uses, such as a substrate for membranes used in reverse osmosis filtration, microfiltration and ultrafiltration.

BACKGROUND OF INVENTION

(002) Current methods for producing a nonwoven substrate suitable for use as membrane support in filtration applications have employed a variety of web forming techniques. For example, U.S. Published Patent Application 2005/0032451 describes the use of any of wet-laid, carded, melt-blown, and spunbond technologies with thermal or chemical bonding to produce an ultrathin, high porosity, nonwoven fabric for use as a carrier substrate for battery separators. This reference is particularly directed to ultrathin carrier substrates having a web thickness up to about 30 μm (microns) for use as carrier material for separator diaphragms in electrochemical cells, etc. A support material disclosed in U. S Pat. No. 4,454,176 by Buckfelder is prepared by preparing a woven web from a yarn of polyester that is then dried, heat set and calendered. Other support fabrics suggested by Buckfelder include a calendered spunbonded polyester fabric and a resin bonded fabric. US Pat. Nos., 6,156,680, 6,919,026 describe a wet- laid paper making process to make highly uniform substrates. US Pat. No. 4,728,394 describes a two layer nonwoven substrate produced by heat laminating a low density nonwoven substrate formed by a dry laid nonwoven carding process to a high density nonwoven substrate formed by a wet-laid paper making process.

(003) Reverse osmosis (RO) membranes are typically produced by coating or casting a polymeric film or membrane onto a thin and highly uniform nonwoven substrate. . With the coating of extremely thin membranes, continuity of the membrane on the supporting surface is difficult to obtain. Moreover, the lack of proper adhesion between the supporting substrate and the membrane can lead to delamination in use, which results in the formation of blisters between the supporting substrate and the membrane. In general, the wet-laid paper making process is largely used to produce the nonwoven substrate as membrane support due to the high web uniformity and adequate mechanical properties desired during processing. Typically, the wet-laid paper is passed through a calendering step to reduce the thickness of the substrate to the desired level and also to give a smooth surface finish. Although wet-laid technology produces a good uniformity web, there are some inherent limitations associated with the paper making process which result in very high defect levels, which lead to adhesion failures between the membrane and substrate.

(004) A typical paper making process uses an enormous quantity of water in the entire process that has to be filtered through several stages before the water reaches the formation zone. In the formation zone, called the headbox, less than 1 % (typically less than 0.5%) of synthetic fibers are mixed with greater than 99% of water with the help of retention chemicals. This high amount of water is then removed in several stages of dewatering using vacuum, and the final sheet is dried of all moisture by contacting the sheet over a series of heated drums. Even with the best of controls and filtration systems in the process, the levels of surface defects are very difficult to manage.

(005) Nonwoven fabrics produced through wet-laid technology often are characterized by the presence of fibers bundles or lumps, non-uniform light/heavy spots, inconsistent air permeability, dirt, bugs, and lay-flat issues. The sources of such defects, besides the fibers, can arise at various stages of the wet-forming process described above. All the above defects are further accentuated during the post calendering process, either on line or offline, as required to achieve the final physical properties of the fabrics. As product quality requirements can be extremely stringent, there are strong demands imposed on this process, which result in lowering production speeds, high amount of waste, and therefore high production costs. These problems in the wet-laid process can be partially overcome by investing in capital equipment for improved web forming, and adding cleaning equipment to minimize defect levels, as well as process control equipment. Some manufacturers have installed on-line calendering to minimize web-handling requirements and reduce cost. However, while cleaning of process water and process control measures have helped, the inherent problems of bundles or lumps of fibers are not completely eliminated, and continue to result in high waste and high costs making it difficult to compete in the marketplace.

(006) Other nonwoven technologies, such as carding, air laying, spunbond, meltblown to produce nonwoven substrates for reverse osmosis applications have had limited success due to various reasons such as non-uniformity, inadequate mechanical strength, defects and cost. Amongst these, while the carding technology has shown more potential, it has been largely unsuccessful in meeting all the requirements. The carding process is discussed hereunder.

(007) The main objective of carding is to separate small tufts into individual fibers, to begin the process of parallelization and to deliver the fibers in the form of a web. The principle of carding is the mechanical action in which the fibers are held by one surface while the other surface combs the fibers causing individual fiber separation. At its center is a large rotating metallic cylinder covered with card clothing. The card clothing is comprised of needles, wires, or fine metallic teeth embedded in a heavy cloth or in a metallic foundation. Alternating rollers and stripper rolls in a roller-top card may cover the top of the cylinder. The fibers are fed by a chute or hopper and condensed into the form of a lap or batting. The needles of the two opposing surfaces of the cylinder and flats or the rollers are inclined in opposite directions and move at different speeds. The main cylinder moves faster than the flats and, due to the opposing needles and difference in speeds, the fiber clumps are pulled and teased apart. In the roller-top card the separation occurs between the worker roller and the cylinder. The stripping roller strips the larger tufts and deposits them back on the cylinder. The fibers are aligned in the machine direction and form a coherent web/veil below the surface of the needles of the main cylinder. The web/veil is doffed from the surface of cylinder by a doffer roller and deposited on a moving belt.

(008) Conventional dry laid techniques, such as carding and air-laying, frequently produces a more open web with "hole regions" of the web where the deposited fibers do not have uniform surface coverage. Apart from the difference in the nature of the process between paper making and carding, there are differences in the fiber lengths employed in these two processes. As described earlier, the paper making process typically uses fiber lengths around 6 mm; the carding process uses fiber lengths greater than 30 mm. As a result, the web produced from a typical carding machine is more open in comparison to a web produced by the paper making process. The open and non-uniform web produced through a conventional card is also not desired for membrane substrate applications, as it will affect both the adhesion and performance of the membrane.

(009) On the other hand, nonwoven substrates produced by spunbond or meltblown technologies, where the polymer is directly converted into the fabric often may have problems of poor uniformity resulting in a highly anisotropic web, which leads to non- homogeneity in final application and poor performances. While spunmelt technology has advanced with recent development of fine denier spunbond/meltblown technology providing improved web uniformity, the fine denier technology still produces an anisotropic web at relatively high costs making them unacceptable to be used as nonwoven substrates

SUMMARY OF INVENTION

(0010) The present invention provides a method of manufacturing and resulting nonwoven substrate having high surface uniformity in the web and other desirable physical properties with minimal surface defects and contamination level at lower production costs. The invention includes drylaying a fiber web, comprising a judicious combination of fibers using one or more carding units to lay a series of thin layers or "veils" forming the web, and then consolidating the web by thermal bonding. The carding units are fed by blends of fibers comprising matrix fibers and binder fibers selected to obtain products having high uniformity and the physical properties desired for the intended application.

(0011 ) This invention solves the prior problems of high defects and waste associated with the conventional wet-laid paper making technology and the problems of open, non uniform webs associated with conventional carding technology. The multiple-carding, multiple-veil approach combined with the right fiber composition can achieve similar web uniformity and physical properties to the popularly used wet-laid media, without the associated issues of defects from fiber bundles, lumps and contamination. Further the energy consumption associated with this process is much lower than the paper making and the spunmelt processes, making this a much desired process to be used in the industry. The use of a series of carding units increases web uniformity through layering of thin layers or veils which overlap each other to cover up any "hole regions" of the web that are characteristic of the carding process if used in only a single stage. Thermal bond equipment is used as consolidation method to obtain a consistent nonwoven fabric/web. An online preconsolidation step may be included as an option.

(0012) In a preferred embodiment, 1 to 6 carding units are used in series feeding a fiber blend comprising matrix fibers and binding fibers, wherein the binding fibers have a lower melting temperature than the matrix fibers. The binding fibers are selected to melt at the temperature that they are exposed to during the calendering process. In accordance with the preferred embodiment of the fiber furnish, the preferred matrix fibers are polyolefin (PP) or polyester (PET) fibers have a diameter in the range of 0.5 - 7 denier and fiber length in the range of 30 - 100 mm. The lower binder binding fibers could be polypropylene/polyethylene (PP/PE) bicomponent fibers or polyester/polyethylene (PET/PE) bicomponent fibers or polyester/copolyester (PET/coPET) fibers or undrawn PET fibers having a diameter in the range of 0.5 - 7 denier and fiber length in the range of 30 - 100 mm. Fiber blends used in this invention can comprise up to 80% matrix fibers, the remainder being binding (melting) fibers.

(0013) The combination of 1 to 6 carding units with the appropriate fiber blends produce a highly uniform, defect free web which is then consolidated by thermal bonding using a calendar under controlled temperature and pressure conditions, where the lower temperature binding fibers melt and bind the web with the desired mechanical properties. The calendering step may be performed on-line or off-line with or without an intermediate preconsolidation step. The resulting substrate has a basis weight preferably in the range of 30 - 300 gsm, web thickness in the range of 50 - 635 μm (microns), and web air permeability in the range of 8 - 500 l/m2/sec.

(0014) The invention method can produce a nonwoven substrate product having high uniformity and homogeneity and avoids problems with fibers bundles or lumps, inconsistent air permeability, dirt, bugs and lay-flat issues. These desired physical properties are also obtainable at a lower cost than conventional substrate forming methods. The multi-stage carding process completely circumvents the problems of defects associated with wet-laid technology, and solves the problem with holes coverage in dry laid technology. Also as compared to spunbond or meltblown technologies, the multi-stage dry-laid process with judicious use of preferred fiber blends forms a more uniform and more isotropic web.

(0015) Other objects, features, and advantages of the present invention will be explained in the following detailed description of the invention having reference to the appended drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a preferred embodiment of a method of manufacturing a nonwoven substrate using multi-stage carding units in accordance with the present invention.

FIG. 2 shows a common defect in the carding process of "hole regions" in a fiber web when laid down by a single card stage.

FIG. 3 shows the high surface uniformity obtained in the fiber web using multistage carding units in the present invention for layering of multiple thin layers or veils on each other.

FIG. 4 shows a typical patchiness in the fiber web when using the spunbond process.

FIG. 5 shows a common lump defect in the fiber web when using the wet-laid process.

FIG. 6 shows a preferred embodiment of a double-doffer card used to produce the substrates described in examples A to C in the application.

FIG. 7 shows process conditions used to produce the substrates of examples A to C. FIG. 8 shows a preferred embodiment of a single-doffer card used to produce the substrates describes in the examples D through G.

FIG. 9 shows process conditions used to produce the substrates of examples D through F.

FIG. 10 shows process conditions used to produce the substrates of example G.

DETAILED DESCRIPTION OF INVENTION

(0016) Preferred embodiment of the method of manufacturing and resulting nonwoven product of the present invention are described in detail below. However, it is to be understood that the principles of the invention disclosed herein are equally applicable to other analogous embodiments, methods, products, and types of application.

(0017) FIG. 1 illustrates a preferred method of manufacturing a nonwoven substrate in accordance with the present invention. Bales of matrix and binder fibers are opened/blended in fibers blend and opening systems 10 and then passed through one or more fibers coarse/fine openers units 11 a, 11 b, for mixing the fibers together for good homogeneity. The fiber mix is then fed to a card section 12 having multiple carding stages, i.e., Stages 1 , 2, ... n, for carding and laying down a fiber web 12a in a series of thin layers or "veils" on each other. The layered fiber web 12a may be pre- bonded partially through a preconsolidation unit 13 (such as an air thru-oven or a calender) as an option. It is then thermally bonded through a calender stack comprising a series of calendering rolls 14 and output as a nonwoven substrate product.

(0018) As shown in FIG. 2, a common defect in the carding process is the forming of "hole regions" where the carded fibers have partial or non-uniform distribution in the web when laid down by a single card stage. In the present invention, multiple carding units are used to obtain a high surface uniformity in the fiber web 12a, as shown in FIG. 3, through the layering of multiple thin layers or veils on each other that cover up any "hole regions" that any one layer may have. The high surface uniformity of the multi- carded fiber web 12a is also markedly better as compared to the typical patchiness of the spunbond process, as shown in FIG. 4 (see light and dark patches), and defects such as lumps in the wet-laid process, as shown in FIG. 5.

(0019) One or more carding units may be used as needed for increasing web uniformity and making up the desired weight and thickness of the resulting substrate product. For any desired end product, the number of carding units may be selected by factoring in the number of doffers in each carding stage and the minimum weight of each veil to be carded. Each carding unit can deliver a number of veils. Preferably, the number of carding units is from 1 to 6. The most preferred number of veils is in the range 3-6. The max/min grammage of a carded veil depends on the design of the card unit itself. Similar card equipment can have different components (clothings, number of doffers or workers/strippers design, etc.) giving different laying capabilities (grammage or fibers usage, etc.). The preferred grammage range for each veil is between 12 and 25 gsm (depending on how many cards/doffers are used). The line speed, in order to maximize the web uniformity, needs to be quite low compared to a classical carding process.

(0020) In a preferred embodiment, 1 to 6 carding units are used in series feeding a fiber blend comprising matrix fibers and binding fibers, wherein the binding fibers have a lower melting temperature than the matrix fibers. The binding fibers are selected to melt at the temperature that they are exposed to during the calendering process. In accordance with the preferred embodiment of the fiber furnish, the preferred matrix fibers are polyolefin (PP) or polyester (PET) fibers have a diameter in the range of 0.5 - 7 denierand fiber length in the range of 30 - 100 mm. The lower binder binding fibers could be polypropylene/polyethylene (PP/PE) bicomponent fibers or polyester/polyethylene (PET/PE) bicomponent fibers or polyester/copolyester (PET/coPET) fibers or undrawn PET fibers or a combination thereof, having a fiber diameter in the range of 0.5 - 7 denier and fiber length in the range of 30 - 100 mm. Fiber blends used in this invention can comprise up to 80% matrix fibers, the remainder being binding (melting) fibers. In highly preferred blends, the matrix fibers are in the range of about 50 - 75%.

(0021 ) The preferred method of manufacturing dry-laid multi-card nonwoven substrate products was tested with different types of fibers. The most common fibers used were PP and PET matrix fibers with binder fibers as the meltable binder part, with up to 80% matrix fibers (in different mixes) and the remaining being binding fibers. The types of fibers were used in denier range of 0.8 - 3.8 denier and around 38 mm length. The preferred resulting web has a basis weight in the range of 30 - 300 gsm, web thickness in the range of 50 - 635 μm (microns), and web air permeability in the range of 8 - 500 l/m2/sec .

(0022) In the calendering stage 14, the fiber web is subjected to heat and pressure to melt the binding fibers to consolidate the nonwoven fabric/web. The calender unit/stack comprises at least two rolls that may have smooth surfaces and made of a selection of steel, composite rubber or any other suitable material. In the case of polyolefin binder fibers, calendering temperatures of typically 100 degrees C to 160 degrees C are used as a function of the particular olefin fiber or fiber component. For polyester melting/binder fibers, the calendering temperatures are typically 100 degrees C to 240 degrees C. The calender conditions are adapted to the melting and softening behavior of the polymers used in each particular case. Prior to calendering under heat and pressure, the fiber web may be pre-bonded using any known method of nonwoven pre- bonding.

(0023) FIG. 6 shows a preferred embodiment of a double-doffer card used to produce the substrates described in examples A to C below. FIG. 7 shows process conditions used to produce the substrates of examples A to C. FIG. 8 shows a preferred embodiment of a single-doffer card used to produce the substrates describes in the examples D through G. FIG. 9 shows process conditions used to produce the substrates of examples D through F. FIG. 10 shows process conditions used to produce the substrates of example G. Visual comparisons were done to test surface uniformity, freedom from defects (bumps, uneven air permeability, debris, lay-flat issues) by counting the number of non-acceptable defects present in 5000 linear meters of web. For reducing fibers defects, it was important to ensure the proper condition of the fiber opening systems. In addition, proper cards clothing needs to be used to avoid melting of fibers during carding. The parameters and results of Samples A-G of the preferred nonwoven substrate and comparison examples are outlined below. Examples:

Dry-Laid Multi-Card Nonwoven Sample A

Number of carding units used: 2 Number of veils used: 4

Fiber mix:

Matrix fibers: 75 %, type: PET, 1 ,6 denier. 38 mm length

Binder fibers: 25 %, type: PET, 3.8 denier. 38 mm length

Weight of each veil: 17.5 gsm Pre-consolidation: _X_ yβs, no

Calendaring type: _X_ smooth, point bonded, pattern bonded

Nonwoven substrate product: 102 μm thickness, 70 final gsm

Uniformity: high, _X_ good, OK, poor

Defect#/5Km: high, _X_ good, OK, poor

Dry-Laid Multi-Card Nonwoven Sample B

Number of carding units used: 2

Number of veils used: 4

Fiber mix: Matrix fibers: 50 %, type: PET, 1.6 denier. 38 mm length

Binder fibers: 50 %, type: PP/PE, 1.7 denier. 38 mm length

Weight of each veil: 16 gsm

Pre-consolidation: _X_ yes, no

Calendaring type: _X_ smooth, point bonded, pattern bonded Nonwoven substrate product: 152 μm thickness, 64 final gsm

Uniformity: high, _X_ good, OK, poor

Defect#/5Km: high, _X_ good, OK, poor

Dry-Laid Multi-Card Nonwoven Sample C Number of carding units used: 2

Number of veils used: 4

Fiber mix:

Matrix fibers: 50 %, type: PP, 1.7 denier. 38 mm length

Binder fibers: 50 %, type: PP/PE, 1.7 denier. 38 mm length Weight of each veil: 13.5 gsm Pre-consolidation: _X_ yes, no

Calendaring type: _X_ smooth, point bonded, pattern bonded

Nonwoven substrate product: 152 μm thickness, 54 final gsm

Uniformity: high, _X_ good, OK, poor Defect#/5Km: high, _X_ good, OK, poor

Dry-Laid Multi-Card Nonwoven Sample D

Number of carding units used: 5

Number of veils used: 5 Fiber mix:

Matrix fibers: 50 %, type: PP, 2.2 denier. 38 mm length

Binder fibers: 50 %, type: PP/PE, 2.2 denier. 38 mm length

Weight of each veil: 14 gsm

Pre-consolidation: _X_ yes, no Calendaring type: _X_ smooth, point bonded, pattern bonded

Nonwoven substrate product: 185 μm thickness, 70 final gsm

Uniformity: high, _X_ good, OK, poor

Defect#/5Km: high, _X_ good, OK, poor

Dry-Laid Multi-Card Nonwoven Sample E

Number of carding units used: 5

Number of veils used: 5

Fiber mix:

Matrix fibers: 50 %, type: PP, 1.7 denier. 38 mm length Binder fibers: 50 %, type: PP/PE, 2.2 denier. 38 mm length

Weight of each veil: 14 gsm

Pre-consolidation: _X_ yes, no

Calendaring type: _X_ smooth, point bonded, pattern bonded

Nonwoven substrate product: 160μm thickness, 70 final gsm Uniformity: high, _X_ good, OK, poor

Defect#/5Km: high, _X_ good, OK, poor

Dry-Laid Multi-Card Nonwoven Sample F Number of carding units used: 4 Number of veils used: 4 Fiber mix:

Matrix fibers : 75 %, type: PET, 1 ,6 denier. 38 mm length

Binder fibers: 25 %, type: PET, 3.8 denier. 38 mm length

Weight of each veil: 14 gsm Pre-consolidation: _X_ yes, no

Calendaring type: _X_ smooth, point bonded, pattern bonded

Nonwoven substrate product: 107μm thickness, 70 final gsm

Uniformity: high, _X_ good, OK, poor

Defect#/5Km: high, _X_ good, OK, poor

Dry-Laid Multi-Card Nonwoven Sample G

Number of carding units used: 9

Number of veils used: 9 Fiber mix:

Matrix fibers : 75 %, type: PET, 1 ,6 denier. 38 mm length

Binder fibers: 25 %, type: PET, 3.8 denier. 38 mm length

Weight of each veil: 9 gsm

Pre-consolidation: _X_ yes, no Calendaring type: _X_ smooth, point bonded, pattern bonded

Nonwoven substrate product: 4.2mils = 107μm thickness, 78 final gsm

Uniformity: _X_ high, good, OK, poor

Defect#/5Km: high, _X_ good, OK, poor

Comparative Example: Wet-Laid

Fiber web:

Matrix fibers: 70%, type: PET, mix of 0.5+1.5 denier. 5 mm length Binder fibers: 30 %, type: PET, 1.1 denier, 5 mm length Pre-consolidation: yes, _X _ no Calendaring type: X smooth, point bonded, pattern bonded

Nonwoven substrate product: 102 μm thickness, 67 final gsm

Uniformity: _X_ high, good, OK, poor

Defect#/5Km: high, good, OK, _X_ poor

Comparative Example: Spunbond Spunbond type: SS

Fiber web:

Matrix polymer: 75 %, type: PP, 2 denier.

Binder polymer: 25%, type: PE, 2 denier. Pre-consolidation: _X _ yes, no

Calendaring type: _X _ smooth, point bonded, pattern bonded

Nonwoven substrate product: 130 μm thickness, 60 final gsm

Uniformity: high, good, OK, _X_ poor

Defect#/5Km: high, good, OK, _X_ poor

Products for Commercial or Industrial Use

(0024) Dry-laid multi-card nonwoven substrate products produced in accordance with the present invention may be used for a wide range of commercial or industrial uses. For example, it may be used as substrate materials for filtration in a range of applications, such as micro and ultrafiltration, nanofiltration and reverse osmosis.

(0025) It is to be understood that many modifications and variations may be devised given the above description of the principles of the invention. It is intended that all such modifications and variations be considered as within the spirit and scope of this invention, as defined in the following claims.