Method for Dispersing Glass Fibers for the Preparation of Glass Filter Media

This invention relates to methods of forming aerosol filters and more particularly to aerosol filters which are formed with a uniform mat of a Beta fiber reinforced glass paper filter media.

Heretofore highly efficient filters have been made which are capable of retaining over 99% of particulate matter having a diameter of less than 0.3 micron with a small pressure drop in gas flow rates. However an all glass filter paper typically contains a variety of sizes of micro fibers. The relative amounts of these different sizes determines the aerosol filtration and air resistance properties. There are also quantities of coarser fibers made from chopped yarn which add mechanical strength to the sheet. An example of this chopped yarn is A-10 glass, rather coarse and stiff, they are 1/4 and 1/2 inch long and about 15 microns in diameter. Use of these large diameter stiff fibers tend to puncture the media when folding to accommodate the formation of pleated filters, and poor dispersion severely affects the mechanical strength during these forming operations.

A small diameter long, length glass reinforcing fiber (Beta fiber, made by Owens Corning) is used in a standard type glass micro fiber formulation to construct high efficiency particulate aerosol filters. Using such glass fibers, requires a unique method which functions to prevent the basic glass fibers from roping or tangling into a mat. Thus, high efficient uniform mats may be formed which may be folded without damage to the fibers.

The drawing illustrates the theory of what happens chemically to hold the fibers apart.

Glass filter media is made by a formulation consisting of different size and types of glass fibers and yarns. The following method is used to obtain good dispersion of Beta Fibers in the construction of high efficiency particulate aerosol filters. The method comprises removing the starch type sizing or surface coating from the Beta Glass Fibers made by Owens Corning. The Beta Glass Fibers 0.05 grams each, of 1/4 inch and 1/2 inch having a diameter of from 3-4 microns are placed into a high temperature oven having a temperature of from about 325.degree.c to about 375.degree.C for high temperature exposure for a period of about 16 hours. The applied temperature causes the degradation of the existing coating of a starch type sizing, thereby exposing a restored surface of glass. This restored surface will be highly susceptible to the solution of acid (HCL) and Silanox which is the next step in the process. These heat cleaned Beta Glass Fibers are then added to 21/2 liters of a 2.5 to 3.0 pH hydrochloric acid solution in a beaker to which has been added 0.02 grams of Silanox, a trade mark of the Cabot Corporation. Silanox is a hydrophobic fumed silicon dioxide treated with a silane. The heat treated Beta Glass Fiber, acid, Silanox forms a slurry which is subjected to a sonic probe power of 3 amps D.C. for a time period of about 6 minutes. The sonic probe disperses any clumps or bundles of Beta Fiber into individual fibers and helps to open the fibers so that they will become coated with the Silanox within the slurry. The Silanox conditions the Beta Fibers so that they are held apart thus preventing roping or tangling. The operation of Silanox to prevent roping or tangling is not fully understood; however, the theory is believed to be as shown by the drawing. After having subjected the Beta Glass Fiber slurry to the sonic probe power, other glass fibers such as 0.356 grams of A grade (2.1 micron), and 1.168 grams of AAA Grade 0.6 micron, are added to the acid-Silanox solution and mixing by further use of the sonic probe is carried on for from about 5 to about 6 minutes depending on the solids concentration of the slurry. Once the glass fiber-solution has been mixed, it is poured into a hand sheet mold for forming the filter. The valve control for the hand sheet mold is opened to permit the solution to drain through the bottom. The filter is formed on the bottom of the hand sheet mold. The tube of the mold is opened and the filter is removed by use of a roller to which the filter clings as the roller is rolled along over the filter. The filter is then placed on a sheet for final drying. The use and operation of the hand sheet mold for making small filters is well known in the art. The different type of glass fibers mix uniformly and settle on the bottom of the hand sheet mold in such a manner as to form a highly improved filter. Filters formed by the above method have a thickness of 17 to 20 mils; weight of 64.32 sq. ft. per pound with a tensile strength of 1.4 lbs. per inch of width. Such filters, used in a gas mask, filters 99.7% of 0.3 micron particulate matter at a flow rate of 16 liters/minute.

The cleaning, acid-Silanox mixture treatment of the Beta Glass Fibers conditions the Beta Glass Fibers such that the fibers are held apart thus preventing roping or tangling into a mat. The reaction between the Beta Glass Fibers and the Silanox is not understood; however, the treatment is known to enhance dispersion of clumps of Beta Fiber into individual strands resulting in a highly improved filter media.

In carrying out the above method, a typical Glass Filter media contains by weight the following table formulation:

Using terms well known in the art, the concentration of the above glass fibers in 1000 milliliters of water is 4.5 grams of solids comprising the above percentages by weight.

The hand sheet mold may be used for making small sized filters. Filters of much larger size such as for room air filters in hospitals and other clean places which require a high percentage of particulate filtering may be made by other machines such as a Fourdrinier paper machine. A Beta Glass Fiber mat is placed in a pulp beater with the beater plate spaced about 1/2 inch from the beater so that the beater will not chop or damage the glass fibers. The mat is beat for about 15 minutes so that the glass fibers are spaced and broken down to lengths of from 1/4 to 1/2 in length. About 7 liters of 2.5 to 3.0 pH hydrochloric acid solution containing 0.46 grams of Silanox, is added to the glass fibers and mixed for about 6 minutes. The Silanox solution forms a spiniferous surface that attaches to the restored surface of the glass fibers (see drawing). The restored surface of the glass fiber has a number of reactive Si-O-Si sites tht can form bonds between the silanol groups of the glass. Some Si-O-Si bonds must form between unadsorbed SiOH groups of adjacent adsorbed coupling molecules. The remainder of the glass fibers are added to the slurry acid solution and mixed for from about 10 to 15 minutes. The slurry is then diluted with water and added to the storage tank of the Fourdrinier paper machine. The diluted slurry is then pumped to the head tank from which it is deposited onto the moving drain screen where the glass filter media takes its final sheet form as the liquid mix drains away.

Glass fiber filters made in accordance with each of the methods above without Silanox in the acid solution were not efficient because the Beta Glass Fibers were not uniform due to roping or tangling into a mat. The small amount of Silanox added to the acid solution attaches itself to the Beta Glass Fibers and holds the fibers apart to prevent roping or tangling. It has been determined that the percent by weight of Silanox necessary to prevent tangling and roping is the same for corresponding percent amounts of Beta Glass Fibers used.

Obviously many modifications and variations of the present invention are possible in light of the above teachings. It is therefore understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.