BACKGROUND OF THE INVENTION Cellulose filter papers are well known for many applications, such as automotive and heavy-duty oil and air filtration. Generally, they are produced by dispersing cellulose fibers, such as wood pulps, in water and filtering the fiber suspension through the continuous screen or wire of a paper machine.

Conventionally, the resulting paper is dried once, then saturated with an aqueous or non-aqueous solution or suspension of polymer or resin, and finally dried again.

Alternatively, while still wet, the freshly formed paper is saturated with the aqueous or non-aqueous solution or suspension of polymer or resin, and then dried only once. The dried, saturated paper may be then pleated, dimpled, or otherwise formed into a filter configuration, and possibly given a thermal cure to develop its final properties.

In order to perform adequately in automotive and heavy-duty (truck) oil and air filtration applications, the saturant must confer high Mullen burst strength, tensile strength, and particularly stiffness, under cured, wet, or elevated temperature conditions. In addition, since wood pulp is typically much less expensive than the polymers and resins comprising the saturant, it is economically desirable to achieve these mechanical properties using the lowest possible saturant content in the finished paper.

One convenient method of characterizing stiffness at a range of elevated temperatures is through the use of a Dynamic Mechanical Analyzer in bending beam mode, e.g. the "DMA-7" from Perkin Elmer. Such an instrument can measure the "in phase" bending modulus (elastic stiffness), and the "out of phase" loss modulus of the paper as the temperature is scanned from about room temperature to about 200"C.

Previously known saturants typically have been low molecular weight formaldehyde resins of phenol, urea or melamine, dissolved in organic solvent or water. Other saturants have included aqueous dispersions of high molecular weight polymers such as polyvinyl acetate, polyvinyl chloride, or polyacrylic esters dispersed, but not dissolved, in water, or combinations of such polymers.

U.S. Pat. No. 4,461,858 (Adelman), for instance, discloses an acidic colloid- al system for use in cellulose pulp slurries that includes a stable aqueous polyvinyl- alcohol/melamine-formaldehyde resin interaction product that comprises a polyvinyl alcohol polymer and a cationic melamine-formaldehyde resin colloid in a polyvinyl alcohol/melamine formaldehyde resin acid colloid. U.S. Pat. No. 4,324,833 (Yau) discloses an acidic aqueous phenolic resin-urea wet process mat binder comprising an aqueous solution of a partially methylated melamine-formaldehyde resin and a polyvinyl alcohol, the aqueous solution having a pH within the range of from about 3.5 to about 6.5.

SUMMARY OF THE INVENTION The invention is directed to a filter paper made with a saturant comprising a hydrolyzed polyvinyl alcohol and a formaldehyde resin, wherein the polyvinyl alcohol and the formaldehyde resin are substantially or completely dissolved in the system. The saturant is applied to a filter base paper, such as a cellulose base paper, such that the saturant forms about 5% to 20% of the filter paper in the dried condition.

Preferably, the saturant used with the invention is an aqueous solution of a fully hydrolyzed high molecular weight polyvinyl alcohol and a water-soluble, low molecular weight, methylol-containing resin. The methylol-containing resin is preferably a formaldehyde resin of phenol, urea or melamine. The formaldehyde resin acts as a crosslinking agent for the polyvinyl alcohol. In one embodiment, the system includes a solution of polyvinyl alcohol and a formaldehyde resin of phenol having a pH within the range of from about 7.5 to about 10.5. An inorganic salt can be used as a catalyst to assist the crosslinking of urea and melamine resins.

The invention is also directed to methods for making and using the water- based saturant.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of the present invention and the attendant advantages and features thereof will be more readily understood by reference to the following detailed description when it is considered in conjunction with the accompanying drawings, wherein: FIG. 1 is a graph comparing the stiffness of cellulose paper made in accordance with the present invention, containing phenolic and polyvinyl alcohol resins, and cellulose paper made with either resin alone; FIG. 2 is a graph comparing the stiffness of cellulose paper made in accordance with various embodiments of the present invention; FIG. 3 is a graph comparing the stiffness of cellulose paper made in accordance with various embodiments of the present invention; and FIG. 4 is graph comparing the stiffness of cellulose paper made in accordance with various embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION The invention is directed to a filter paper formed with a saturant that comprises of an aqueous solution of a hydrolyzed polyvinyl alcohol and a formalde- hyde resin, where the formaldehyde resin and the polyvinyl alcohol are completely or substantially dissolved.

The polyvinyl alcohol is present in a sufficient amount to enable the system to function as a saturant in cellulose papermaking operations. Preferably, the polyvinyl alcohol is present in the dried saturant composition in an amount that ranges from about 25 % to 95 % by weight. One of ordinary skill in the art will appreciate that other ranges are within the scope of the invention.

The polyvinyl alcohol is sufficiently hydrolyzed so that when it is combined with the formaldehyde resin and water, the formaldehyde resin crosslinks with the polyvinyl alcohol. Preferably, the polyvinyl alcohol is hydrolyzed at least about

98%. One of ordinary skill in the art, however, will appreciate that other ranges are within the scope of the invention. Suitable polyvinyl alcohols include AisolX 103, 107,125 and 165, available from Air Products Inc., and ElvanolX 70-06, 71- 30 and 90-50, available from duPont de Nemours & Co.

The molecular weight of the polyvinyl alcohol is generally in the range of 10,000 to 190,000. In one embodiment, the molecular weight of the polyvinyl alcohol ranges from about 30,000 to about 50,000. One of ordinary skill in the art will appreciate that other polyvinyl alcohols having other molecular weight ranges may be used as well.

The methylol containing formaldehyde resin of phenol, urea or melamine preferably is water soluble and has the functionality to act as a crosslinking or curing agent for polyvinyl alcohol resin. Preferably, the crosslinking resin has an average degree of polymerization of 1 to 3. Suitable formaldehyde resins of phenol include Arofene 72155, Arotap 8095 and Arofene DR 343, all available from Ashland Chemical Co. An exemplary formaldehyde resin of melamine includes Parez 707, available from Cytec Industries. The formaldehyde resin is ordinarily present in a dry saturant composition in an amount from about 75 % to 5% by weight, but other ranges are within the scope of the invention. Preferably, the formaldehyde resin is a resin of phenol.

Suitable urea and melamine resins may also be used. Exemplary ureas and melamines include urea glyoxal formaldehyde condensate (BERSET 2300, available from Bercen, Inc. of Cranston, Rhode Island), and melamine formaldehyde resins such as BERSET 2003 (available from Bercen, Inc. of Cranston, Rhode Island), SEQUA MEL-80 (available from Sequa Chemicals, Inc. of Chester, South Carolina), AURAMEL 484, AURAMEL M-75, and AURAREZ 963 (available from Sybron-Tanatex, Inc).

Optionally, an acid salt can be used as a catalyst to assist the crosslinking of urea and formaldehyde resins. Suitable acid salts are those formed from a weak base and a strong acid which, upon hydrolysis, yield a strong acid. Exemplary acid salts useful as catalysts in this invention include magnesium chloride, ammonium sulfate, ammonium nitrate, ammonium chloride, and aluminum chloride. A particularly preferred acid salt is magnesium chloride. Such an acid salt is generally

used at a concentration in the range of about 2.5 to 7.5 wt. % solids in the saturant.

The saturant generally has a pH that is sufficiently high to maintain all or substantially all of the formaldehyde resin soluble. The pH of the saturant can be adjusted with a base to be within the range of 7.5 to 10.5. Suitable bases include a fixed base such as potassium or sodium hydroxide, or a fugitive base such as ammonium hydroxide. Other pH ranges are within the scope of the invention.

The water-based saturant of the invention may be made with conventional preparation techniques. Normally, the polyvinyl alcohol is dissolved in water at 185"F and cooled with additional water. The pH is adjusted, and the crosslinking resin is then added.

In use, the saturant imparts improved stiffness and strength to cellulose filter paper under a variety of papermaking conditions. For instance, the saturant imparts improved stiffness and strength during normal conditions and/or wet conditions when applied to cellulose filter papers at lower than normal add on levels. The saturant may be applied at a concentration of 5 % to 25 % by weight in water in order to attain a binder content of 5% to 20% by weight in the dried finished paper.

In a preferred saturant that is suitable for making cellulose filter paper, the polyvinyl alcohol is present in an amount ranging from about 50% to 75% on a dry weight basis, and the formaldehyde resin of phenol is present in an amount from about 50% to 25% on a dry weight basis. A particularly preferred composition is 65% polyvinyl alcohol and 35 % phenol formaldehyde resin, on a dry weight basis.

The preferred binder content is approximately 10% by weight polymer in the dried finished paper.

This saturant may be applied to the paper in a variety of ways, including size press, gravure roll, and other techniques known to one of ordinary skill in the art.

The preferred method of application is by size press.

The invention is further described in the following illustrative examples.

One of ordinary skill in the art will further appreciate that minor modifications may be made to the invention described herein without departing from its intended scope.

All references noted herein are expressly incorporated by reference in their entirety.

EXAMPLE 1 In this Example, saturants of the invention were prepared by first dissolving polyvinyl alcohol in water at 185OF, cooling with additional water, adjusting the pH and then adding formaldehyde resin. Solutions of the resins and polymers in Table 1 were prepared at various concentrations in water and used to saturate a standard all-cellulose base paper in a laboratory size press. The resulting saturated sheets were dried on photo dryers, and the resulting resin content in the finished sheets was determined by the weight gain of the sheet.

Table 1 lists the measured resin content of each of the samples.

TABLE 1 Solution Resin Content of Finished Sheet (wt. %) 1 2 A Arofene 72155 Phenol Formaldehyde 9.2% 20.8% B Airvol 125 Polyvinyl alcohol 4.5% 10.1% C 50/50 Airvol 125/Ashland 72155 4.5% 11.8% The resulting sheets were tested in a Perkin Elmer DMA-7 Dynamic Mechanical Analyzer in the bending beam mode using 20 mm x 5 mm samples cut in the machine direction of the paper. The actual span of the beam was 15 mm.

The samples were run twice in the temperature range from about 25 to 240"C. The sheets were run the first time to characterize the cure rate of the material, and the second time to characterize the fully cured mechanical properties of the resin saturated sheet. The results are shown in Figure 1.

EXAMPLE 2 Solutions were prepared as specified in Table 2 and used to saturate a standard all cellulose base paper in a laboratory size press. The resulting saturated sheets were dried on photo dryers, and the resulting resin content in the finished sheets was determined by the weight gain of the sheet. Table 2 lists the measured resin pickup of each of the samples. The resulting sheets in the DMA-7 were tested again according to Example 1, and the results are shown in Figure 2. Figure 2 shows that solution D5, having the highest PVOH/PhOH ratio of the solutions (75:25) performed best.

Thereafter, the sheets were cured. After curing the sheets 10 minutes at 300OF, the MD Tensile Strength, Mullen Burst Strength, and MD Stiffness of the resulting sheets were measured before and after a 5 minute soak in a 2.5% solution of a commercial liquid cleanser. The dry and wet physical properties are listed in Table 2.

TABLE 2 Ingredient D1 D2 D3 D4 D5 Airvol 165 Polyvinyl Alcohol 16 32 24 12 36 (grams) Arofene 72155 30 60 45 67.5 22.8 Phenol Formaldehyde (grams) KOH (grams) .5 1.0 .75 1.12 .38 Water (grams) 754 708 731 755 742 PVOH/PhOH ratio 50/50 50/50 50/50 25/75 75/25 Resin Pickup (%) 5.0 13.1 8.4 8.1 9.3 Dry Dry MD Tensile (lb/in) 29.1 33.8 36.6 19.2 40.2 Wet MD Tensile (lblin) 14.8 19.3 19.2 13 19.2 Retention (So) 50.9 57.1 52.3 67.7 47.8 Dry MD Mullen (psi) 34.4 52.9 43.8 30.1 65.2 Wet MD Mullen (psi) 26.4 47.0 32.6 29.2 50.8 Retention (%) 76.7 88.8 74.5 97.2 77.9 Dry Stiffness (mg) 3111 3055 3600 2211 3267 Wet Stiffness (mg) 955 1244 1144 1100 897 Retention (%) 30.7 40.7 31.8 49.8 27.4 EXAMPLE 3 Solutions were prepared as specified in Table 3 and used to saturate a standard all-cellulose base paper in a laboratory size press. The resulting saturated sheets were dried on photo dryers, and the resulting resin content in the finished sheets was determined by the weight gain of the sheet. Table 3 lists the measured resin pickup of each of the samples. The resulting sheets in the DMA-7 were tested again according to Example 2, and the results are shown in Figure 3.

The sheets were then cured for 10 minutes at 3000F. The MD Tensile Strength, Mullen Burst Strength, and MD Stiffness of the resulting sheets were measured again before and after a 5 minute soak in a 2.5% solution of a commercial liquid cleanser. The dry and wet physical properties are also listed in Table 3.

TABLE 3 Ingredient El E2 E3 F1 F2 F3 Airvol 165 SF 24 31 38 ---- Polyvinyl Alcohol (grams) Elvanol 90-50 ---- ---- ---- 24 31 38 Polyvinyl Alcohol (grams) Arofene 72155 45 29 18 45 29 18 Phenol Formaldehyde (grams) KOH .7 .45 .3 .7 .45 .3 Water 730 740 744 730 740 744 PVOH/PhOH ratio 50/50 65/35 80/20 50/50 65/35 80/20 Resin Pickup (%) 6.6 7.3 7.5 6.5 6.9 8.1 Dry MD Tensile (lb/in) 24.3 29.7 30.4 22.5 27.9 24.5 WetMDTensile(lblin) 11.3 14.2 13.3 10.1 11.8 10.6 Retention (%) 46.5 47.8 43.8 44.9 42.3 43.3 Dry MD Mullen (psi) 39.7 73.7 60.7 45.3 36.3 48.0 Wet MD Mullen (psi) 32.0 41.3 49.7 21.3 29.7 33.0 Retention (%) 80.6 56.0 81.9 47.0 81.8 68.8 Dry Stiffness (mg) 2400 2700 2767 2033 3633 2900 Wet Stiffness (mg) 1033 1167 833 1333 1333 867 Retention (%) 43.0 43.2 30.1 65.6 36.7 29.9

EXAMPLE 4 Solutions were prepared as specified in Table 4 and used to saturate a standard all-cellulose base paper in a laboratory size press. The resulting saturated sheets were dried on photo dryers, and the resulting resin content in the finished sheets was determined by the weight gain of the sheet. Table 4 lists the measured resin pickup of each of the samples. The resulting sheets in the DMA-7 were tested again according to Example 1, and the results are shown in Figure 4. Again, after curing the sheets 10 minutes at 300OF, the MD Tensile Strength, Mullen Burst Strength, and MD Stiffness of the resulting sheets were measured before and after a 5 minute soak in a 2.5% solution of a commercial liquid cleanser. The dry and wet physical properties are also listed in Table 4.

TABLE 4 Ingredient Gl G2 G3 G4 Airvol 107 (grams) 24 ---- 24 Elvanol 70-06 Polyvinyl ---- 24 ---- 24 Alcohol (grams) Arofene 72155 45 45 18 Phenol Formaldehyde (grams) Parez 707 ---- ---- 30 30 Melamine Formaldehyde (grams) Water 731 731 746 746 Resin Pickup (%) 8.0 7.5 7.3 7.2 Dry MD Tensile (lb/in) 21.4 19.8 21.4 22.2 Wet MD Tensile (lb/in) 9.9 10.4 6.4 6.6 Retention (%) 46.3 52.8 29.7 29.6 Dry MD Mullen (psi) 35.0 33.0 36.5 29.5 Wet MD Mullen (psi) 33.5 23.5 17.5 13.5 Retention (%) 95.7 71.2 47.9 45.8 Dry Stiffness (mg) 2500 2100 2050 1950 Wet Stiffness (mg) 1100 950 700 500 Retention (%) / 44.0 45.2 34.1 25.6

EXAMPLE 5 Solutions were prepared as specified in Table 5 with various urea and melamine formaldehyde resins. The prepared solutions were then used to saturate a standard all cellulose base paper in a laboratory size press. All solutions included Berchem 3009 magnesium chloride which functions as a catalyst for a cross linking reaction involving the formaldehyde urea and melamine resins. The resulting saturated sheets were dried on photo dryers, and the resulting resin content in a finished "dry" sheet was determined by the weight gain of the sheet. The sheets were cured for 10 minutes at 3000F, and tested for MD tensile strength, MD stiffness and Mullen burst strength. The sheets were also soaked for one minute in a 2.5 % solution of a commercial liquid cleanser; the resulting "wet" sheets were then retested for MD tensile strength, MD stiffness and Mullen burst strength.

These physical properties of the "dry" and "wet" sheets are also included in Table 5.

TABLE 5 Ingredient H1 H2 H3 H4 H5 H6 Airvol 107 Polyvinyl 48.8 48.8 48.8 48.8 48.8 48.8 Alcohol (grams) Berchem 3009 5.08 5.08 5.08 5.08 5.08 5.08 (grams) Berset 2300 (grams) 27.7 ---- ---- ---- Berset 2003 (grams) ---- 15.2 ---- ---- Sequa MEL-80 ---- ---- 15.2 ---- (grams) Auramel 484 ---- ---- ---- 19.0 (grams) Auramel M-75 ---- ---- ---- ---- 15.2 (grams) Aurarez 963 (grams) ---- ---- ---- ---- ---- 19.4 Water (grams) 718.5 730.9 730.9 727.1 730.9 726.8 Resin Content 10% 10% 10% 10% 10% 10% Dry MD Tensile 43.3 33.7 32.3 40.2 31.4 35.8 (lb/in) Wet MD Tensile 9.6 14.2 14.8 12.5 13.0 12.4 (lb/in) Retention (%) 46.5 47.8 43.8 44.9 42.3 43.3 Dry MD Stiffness 5567 5400 4900 5133 4800 4500 (mg) Wet MD Stiffness 1020 1673 1453 1613 1660 1293 (mg) Retention (%) 18.3 31.0 29.6 31.4 34.6 28.7 Dry Mullen (psi) 49.0 28.3 31.0 31.3 26.7 29.0 Wet Mullen (psi) 20.0 20.0 34.0 20.3 21.0 21.3 Retention (%) 40.8 70.7 ---- 64.8 78.6 73.4