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[0001] This application claims priority from U.S. Provisional Patent Application Serial No. 60/463,870, filed on Apr. 18, 2003, entitled “Low Density Nonwoven Glass Fiber Web,” which is expressly incorporated by reference herein.
[0002] The present invention relates to filter media formed of or containing nonwoven glass fiber webs, and in particular to methods of forming nonwoven glass fiber webs having enhanced filtration performance characteristics.
[0003] Glass fiber mats are used for a variety of purposes, including, for example, in battery separators, air and water filters, vacuum bags, automobile air conditioning filters, and indoor air cleaner filters. The two most common methods for producing glass fiber mats are dry laid processing and wet laid processing. In a dry laid process the glass fibers are chopped and dispersed in air that is blown onto a conveyor, and a binder is then applied to form a mat. Such a process is typically more suitable for the production of highly porous mats having bundles of glass fibers. In a wet laid process, a slurry is formed containing the glass fibers, and optionally other chemical agents such as dispersants, viscosity modifiers, and defoaming agents. Glass fibers are anionic by nature, and thus the acidic slurry is used to remove any charge present on the fibers to disperse the fibers from the glass wool conglomeration. As a result, the frictional contact between the fibers is increased and the processability of the fibers is improved. The fibers from the slurry are then collected on a screen which allows a substantial portion of the water from the slurry to be drained. The resulting mat is then dried to yield a nonwoven mat formed of glass fibers.
[0004] A major objective of wet laid nonwoven manufacturing is to produce materials with textile-fabric characteristics, primarily flexibility and strength, at speeds approaching those associated with papermaking. Current processes, however, tend to affect filtration properties, producing nonwoven webs that have a high density with high resistance.
[0005] Accordingly, there is a need for improved glass fiber webs, filter media containing glass fiber webs, and methods for making the same.
[0006] In general, the present invention provides nonwoven filter media formed of or containing glass fiber webs, and methods for making the same. In one embodiment, a nonwoven filter media is formed of or contains at least one glass wool fiber web and has a gamma value of at least about 14, a surface area of at least about 1.2 m
[0007] In another embodiment, a filter media is provided having a support layer, and a filtration layer including glass wool fibers having a diameter in the range of about 0.1μ to 3.5μ. The filter media preferably has a gamma value of at least about 14, a surface area of at least about 1.2 m
[0008] In other aspects of the present invention, a method of making a filter media formed of or containing one or more nonwoven glass fiber webs is provided. The method includes the steps of preparing a slurry having a pH in the range of about 1 to 12, and more preferably in the range of about 2 to 4, which contains glass wool fibers, chopped glass fibers, water, and an acidic agent, subsequently adding a neutral or alkaline pH adjusting agent to the slurry to adjust the pH to the range of about 6 to 10, and removing the water from the slurry to form a wet laid glass fiber web having a gamma value of at least about 14.
[0009] The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
[0010]
[0011]
[0012]
[0013]
[0014]
[0015]
[0016]
[0017]
[0018]
[0019] The present invention relates to nonwoven glass fiber webs, filter media formed of or containing nonwoven glass fibers webs, and methods of making the same using a wet laid processing technique. The filter media can be used in a variety of applications, including as battery separators, air and water filters, vacuum bag filters, cabin air filters, and indoor air cleaner filters. The filter media are particularly effective, however, for use as pleated filters in clean room environments. The present invention is particularly advantageous in that it has been discovered that adjusting the pH during wet laid processing will produce a glass fiber web having improved filtration properties. In particular, neutralizing the pH of a slurry containing mainly glass wool fibers unexpectedly yields a non-electret, glass filter media that has a gamma value of at least about 14, which is a significant improvement over non-electret, glass filter media currently on the market which have been shown to have a gamma value that does not exceed 13. Moreover, the adjusted pH unexpectedly produces a filter media having an improved surface area, which is preferably at least about 1.2 m
[0020] The nonwoven glass fiber webs prepared according to the present invention can contain glass wool fibers, or more preferably a combination of glass wool fibers, chopped glass fibers, and optionally other polymeric fibers. The resulting glass fiber web can be used alone, or it can be combined with additional fiber webs, to form a filter media. By way of non-limiting example, suitable fiber webs that can be combined with the glass fiber include polymeric and/or metallic expanded mesh.
[0021] Glass wool fibers are a specific type of fiber that are prepared by blowing or spinning molten glass through small holes. Unlike typical chopped glass fibers, glass wool fibers have a very small diameter, which typically ranges from about 0.1μ up to 4μ or 5μ. In an exemplary embodiment, the nonwoven glass webs are formed from glass wool fibers having a fiber diameter in the range of about 0.1μ to 4.5μ, and more preferably in the range of about 0.3μ to 4.5μ. In an exemplary embodiment, the glass wool fibers have a fiber diameter of about 0.69μ and/or 4.5μ. The glass wool fibers tend to vary significantly in length, and thus no specific length is required. In an exemplary embodiment, however, the aspect ratio (length to diameter ratio) (l/d) of the glass wool fibers is preferably generally in the range of about 100 to 10,000, more preferably in the range of about 200 to 2500, and most preferably in the range of about 300 to 600. A person having ordinary skill in the art will appreciate that a variety of different glass wool fibers can be used to form a nonwoven glass fiber web according to the present invention.
[0022] As indicated above, the nonwoven glass webs can also include chopped glass fibers that can be combined with the glass wool fibers during processing. The chopped glass fibers can be present in any amount, but are preferably present at about 1% to 30% by weight in a web containing about 70% to 99% by weight glass wool fibers. The chopped glass fibers preferably have a fiber diameter in the range of about 4μ to 30μ, and more preferably about 5μ to 12μ, and a length in the range of about 0.125 inch to 1 inch. In an exemplary embodiment, the nonwoven glass webs contain glass wool fibers having a diameter of about 0.69μ and/or 4.5μ, and chopped glass fibers having a diameter in the range of about 5μ to 7μ.
[0023] The nonwoven glass fiber webs and filter media containing the nonwoven glass fiber webs are formed using a wet laid processing technique, which involves preparing a slurry containing glass wool fibers, chopped glass fibers, and water. The fibers are suspended uniformly in the slurry at very low concentrations in the range of about 0.01 to 0.5% by weight of fiber. Since glass wool fibers are anionic by nature, an acidic agent can be added to the slurry to form a slurry having a pH in the range of about 2 to 4, and most preferably about 3. The pH can, however, be adjusted to a range of about 1 to 12. The acidic agent is also effective to remove the charge on the fibers, thereby improving dispersion of the fibers and facilitating processing of the web. While virtually any acidic agent can be used, sulfuric acid is an exemplary pH reducing agent. Other suitable acidic agents include, for example, hydrochloric acid, formic acid, citric acid, and other mineral and organic acids.
[0024] Once the slurry is prepared and the pH is adjusted to about 3, a neutral or alkaline pH adjusting agent is added to the slurry to adjust the pH to a pH in the range of about 6 to 12, and more preferably in the range of about 7 to 10. It has been discovered that this additional step of adding a neutral or alkaline pH adjusting agent to the slurry unexpectedly produces a nonwoven glass web having improved filtration properties, as will be discussed in more detail below. Virtually any neutral or alkaline agent can be used to adjust the pH of the slurry. In an exemplary embodiment, ammonium hydroxide is added to the slurry to adjust the pH to about 7. Other suitable alkaline pH adjusting agents include, for example, metal hydroxides, such as potassium hydroxide and sodium hydroxide, calcium bicarbonate, and buffer solutions. After adjusting the pH, the fibers can be collected on a screen and dried to form a nonwoven glass web having one or more layers of glass fibers.
[0025] Wet laid nonwoven glass webs are typically prepared using a papermaking process, which includes a hydropulper, a former or headbox, a dryer, and optionally a converter. As shown in
[0026] In another embodiment, the nonwoven glass webs can be combined with one or more additional fiber layers to form a filter media. The filter media can include any number of layers, can be formed from a variety of fibers, and can be prepared using a variety of manufacturing methods. By way of non-limiting example, the filter media can be laminated or otherwise attached to an organic or metal backing.
[0027] In an exemplary embodiment, the filter media includes a support layer and one or more layers of a nonwoven glass web deposited onto the support layer to form a filtration layer. The filtration layer(s) are preferably prepared as described above at a pH of at least about 6. The support layer is effective to provide structural integrity to the filter media, and is preferably a wet laid glass fiber web. In an exemplary embodiment, the support layer is formed from chopped glass fibers having a fiber diameter in the range of about 4μ to 30μ, and more preferably about 5μ to 12μ, and the filtration layer(s) are formed from a combination of glass wool fibers having a diameter in the range of about 0.1μ to 4.5μ, and more preferably about 0.3μ to 4.5μ, and chopped glass fibers having a diameter in the range of about 4μ to 30μ, and more preferably about 5μ to 12μ. In an exemplary embodiment, the support layer is formed from chopped glass fibers having a fiber diameter in the range of about 3μ to 7μ, and the filtration layer(s) are formed from a combination of glass wool fibers having a diameter in the range of about 0.69μ and/or 4.5μ and chopped glass fibers having a diameter in the range of about 5μ to 7μ. By way of non-limiting example,
[0028] The filter media can be prepared using a variety of techniques, but preferably the support layer is prepared in a slurry and passed through a first headbox where the fibers are collected on a screen. The fibers then travel on the screen to a second headbox, which contains the glass wool/chopped glass fiber combination having a pH of at least about 6. The glass wool/chopped glass fiber mixture is deposited onto the support layer, and the two-layer structure is then dried to form a filter media. The web can optionally be passed through one or more headboxes to add additional layers to the web.
[0029] A person having ordinary skill in the art will appreciate that the glass fibers used according to the present invention, as well as the compositions of these glass components, can be varied to achieve optimal performance depending on the intended use. The nonwoven glass webs are not intended to be limited to webs formed from only glass wool fibers, but can include a variety of other fiber types in addition to the glass wool fibers and the chopped glass fibers disclosed herein. Preferably, however, the nonwoven glass webs contain a majority of glass wool fibers. The nonwoven glass webs can also include a variety of other ingredients, such as additives, surfactants, coupling agents, crosslinking agents, etc. In one embodiment, the nonwoven glass webs contain a binding agent. The binder coats the fibers and is used to adhere the fibers to each other to facilitate adhesion between the fibers. In general, the binder, if present in the nonwoven web, is in the range of about 2% to 10% by weight, preferably in the range of about 3% to 9% by weight, and most preferably in the range of about 4% to 7% of the total composite weight.
[0030] As previously stated, it has been discovered that neutralizing the pH of the glass fiber slurry during the wet laid process unexpectedly improves filtration efficiency of the resulting filter media. In general, filter performance is evaluated by different criteria. It is desirable that filters, or filter media, be characterized by low penetration across the filter of contaminants to be filtered. At the same time, however, there should exist a relatively low pressure drop, or resistance, across the filter. Penetration, often expressed as a percentage, is defined as follows:
[0031] where C is the particle concentration after passage through the filter and C
[0032] Because it is desirable for effective filters to maintain values as low as possible for both penetration and pressure drop across the filter, filters are rated according to a value termed gamma value (γ). Steeper slopes, or higher gamma values, are indicative of better filter performance. Gamma value is expressed according to the following formula
[0033] The pressure drop across the filter is typically a few mm of H
[0034]
[0035]
[0036] A slurry was prepared containing 50 lbs. of Evanite 706× fiber having an average fiber diameter of about 0.69μ, 30 lbs. of Evanite 712× fiber having an average fiber diameter of about 4.2μ, 3 lbs. of Owens-Corning Chopped Glass fiber DE having an average fiber length of about 0.25 inches, and 3 lbs. of Owens-Corning Chopped Glass fiber DE having an average fiber length of about 0.5 inches. The slurry contained water and sulfuric acid sufficient to yield a fiber concentration of 0.75% by weight. The headbox pH was varied between 2.3 and 3.8, and the samples of web were collected at a headbox pH of 3.8, 3.6 and 2.3. The experiment was repeated containing the same fiber formulation combined with water and ammonium hydroxide to vary the headbox pH between 4.3 and 10.3. Samples were collected at a headbox pH of 10.4, 9.6, 9.2, 8.4, 7.0, 6.0 and 4.2. The properties of each sample were tested and are shown in the chart below. All tests were conducted at an air velocity of 5.33 cm/sec with a DOP particle size of 0.3 microns.
TABLE 1 Ream Specific Caliper Apparent Surface DOP Resistance Weight Resistance (mm @ Density Area Gamma pH (%) (mmH (lbs) (mm/lb) 10 kpa) (g/cc) (sq.m/g) (TDA100P) 2.3 0.297088 22.31 42.8 0.5213 0.331 0.21 n/a 13.115 3.6 0.4116 20.88 44.4 0.4703 0.358 0.202 n/a 13.57 3.8 0.173885 24.26 50 0.4853 0.403 0.202 n/a 13.283 4.2 0.181495 23.54 47.1 0.4997 0.380 0.202 1.2517 13.42 6.0 0.02003 29.39 47.9 0.6135 0.425 0.184 1.4488 14.475 7.0 0.004853 32.64 48.8 0.6689 0.433 0.1845 1.5174 15.083 8.0 0.001248 37.23 49.9 0.7461 0.462 0.176 n/a 15.03 8.4 0.001883 35.90 49.7 0.7224 0.461 0.176 1.5596 16.037 9.2 0.002668 34.74 48.8 0.7119 0.473 0.168 n/a 16.055 9.6 0.001552 36.79 51.5 0.7143 0.510 0.164 1.9726 15.615 10.4 0.00291 34.36 48.2 0.7128 0.451 0.174 n/a 16.76
[0037] As shown in Table 1 and in
[0038] Adjusting the pH during processing of glass fibers webs also significantly improves the specific resistance of the web. Specific resistance is tested by blowing air through the web at a particular velocity and measuring the pressure drop across the web. A fiber web having a high specific resistance is preferred, as the filter media is more effective to capture small particles.
[0039]
[0040]
[0041]
[0042] Accordingly,
[0043] A slurry was prepared containing 50 lbs. of Evanite 706× fiber having an average fiber diameter of about 0.69μ, 30 lbs. of Evanite 712× fiber having an average fiber diameter of about 4.2μ, 6 lbs. of Owens-Corning Chopped Glass fiber DE having an average fiber length of about 0.25 inches, and 1.7 lbs. of 1.7 denier Polyester fiber having an average fiber length of about 0.25 inches. The slurry was prepared in a hydropulper with a fiber concentration of about 2.7% and a pH at about 9. The slurry was then transferred to a storage tank containing additional water. The fiber concentration in the storage tank was about 0.7%, and the pH was about 9.0. The slurry was then fed from the storage tank into the headbox to yield a fiber concentration less than about 0.2%. The headbox was maintained at a pH of 9.2 and a sample of web was collected. The headbox pH was then adjusted to 3.6 and 3.4 by the addition of sulfuric acid, and samples of web were collected at a headbox pH of 3.6 and 3.4. The experiment was repeated containing the same fiber formulation combined with water and ammonium hydroxide to vary the headbox pH between 4.1 and 8.2. Samples were collected at a headbox pH of 4.1, 6.6, 7.4, and 8.2. The properties of each sample were tested and are shown in the chart below. The apparent density was determined according to the following formula:
TABLE 2 Basis Caliper Apparent DOP Resistance Gamma Weight (mm @ Density pH (%) (mmH (TDA 100P) (lbs) 10 Kpa) (g/cc) 9 0.0003 34 16.24 47.28 0.4708 0.162 9 0.0003 32.9 16.79 n/a n/a n/a 9.1 0.0026 29 15.84 45.59 0.4295 0.172 9.1 0.0023 29.5 15.72 n/a n/a n/a 9.2 0 36.4 n/a 50.58 0.4748 0.173 9.2 0 36.1 n/a n/a n/a n/a 9.3 0.0078 25.5 16.1 n/a n/a n/a 9.3 0.0079 25.8 15.9 n/a n/a n/a 3.6 0.38 17.2 14.07 36.53 0.3 0.197 3.6 0.4 17.2 13.94 n/a n/a n/a 3.4 0.33 17.3 14.34 34.11 0.274 0.202 3.4 0.36 17.4 14.07 n/a n/a n/a 4.1 0.17 19 14.57 37.60 0.3133 0.194 4.1 0.16 19.2 14.56 n/a n/a n/a 5 0.085 20.4 15.05 36.63 0.3283 0.181 5 0.078 20.7 15.01 n/a n/a n/a 6 0.012 25.4 15.44 37.89 0.355 0.173 6 0.011 25.3 15.64 n/a n/a n/a 6.9 0.0022 29.2 15.95 39.81 0.3715 0.174 6.9 0.0017 29.2 16.33 n/a n/a n/a 7.4 0.0013 29.1 16.79 39.18 0.3735 0.17 7.4 0.002 29.1 16.15 n/a n/a n/a 8.2 0.0027 28.8 15.86 41.12 0.3965 0.168 8.2 0.0027 28.6 15.97 n/a n/a n/a
[0044] A two layer fiber web was prepared. The first layer was formed from a slurry containing 10 lbs. of Evanite 706× fiber having an average fiber diameter of about 0.69%, 50 lbs. of Evanite 712× fiber having an average fiber diameter of about 4.2μ, 12.5 lbs. of Owens-Corning Chopped Glass fiber DE having an average fiber length of about 0.25 inches, and 2.1 lbs. of cellulose pulp. The cellulose pulp was refined to a Canadian standard freeness number of less than 200. The first layer was collected from a headbox at a pH of about 3.2 to 3.3. The second layer was formed from a slurry containing 50 lbs. of Evanite 706× fiber having an average fiber diameter of about 0.69μ, 6.25 lbs. of Owens-Corning Chopped Glass fiber DE having an average fiber length of about 0.25 inches, and 0.56 lbs. of cellulose pulp. The second layer was collected on the first layer from a headbox at a pH of about 7.0. Each layer had a basis weight in the range of about 34 to 37 g/mTABLE 3 Basis Weight 74 g/m Thickness 0.371 mm Apparent Density 0.199 g/c.c. DOP penetration 0.010% Resistance 26.1 mm Gamma(TDA 100P) 15.3 Stiffness - Machine Direction 286 mg (TAPPI T-410 om-98)
[0045] One of ordinary skill in the art will appreciate further features and advantages of the invention based on the above-described embodiments. Accordingly, the invention is not to be limited by what has been particularly shown and described, except as indicated by the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety.