GB2037343A - Gasket Material and Method of Manufacture Thereof - Google Patents

Abstract

A soft sheet gasket material comprises 50 to 75% by weight monoclinic mica (Muscovite) and 2 to 20% cellulosic fibers. The material may contain 10 to 35% of binders, fillers, pigments, curatives and processing agents. A process for the preparation of the gasket material is provided whereby the constituents are mixed by the beater addition process and the material then formed on paper-making equipment. The gasket material of the invention has properties comparable to those of asbestos-containing materials heretofore used without the health problems associated with such materials.

Description

SPECIFICATION Gasket Material and Method for the Manufacture Thereof This invention relates to gasket material and the method of manufacture thereof. More particularly, this invention relates to soft sheet gasket material which is composed primarily of common monoclinic mica (Muscovite) and cellulose fibers, the gasket material being free of asbestos fibers.

For many years, gasket materials for many important uses have contained asbestos fibers. Asbestos fibers have been uniquely suited for gasket materials because asbestos fibers impart to the gasket material critically important performance and structural features, such as heat resistance, good sealability and good mechanical properties such as compressability, creep resistance and tensile strength. Asbestos fibers have also been desirable not only because of their structural and performance features, but also because of their low cost.

As the result of recent concerns about the health hazards of asbestos fibers, concerted efforts have been made to produce asbestos-free gasket materials. However, the highly desirable objective has not been achieved merely by substituting other fibers for asbestos fibers. Regardless of cost considerations, no substitute has yet been found which can replace asbestos fibers and still produce gasket materials of comparable characteristics. Thus especially for some critical applications, it has been necessary up to now to continue to use asbestos fibers in many gasket materials.

In accordance with the present invention, there is provided a soft sheet gasket material comprising, by weight 50% to 75% monoclinic mica particles, and 2% to 20% cellulose fibers. Quite surprisingly and unexpectedly, this gasket material composed primarily of common monoclinic mica (Muscovite) and cellulose fibers is capable of providing performance and structural characteristics essentially equal to or better than asbestos containing gasket materials, and the gasket material of this invention is, in general, the functional equivalent of and can be substituted for asbestos containing gasket materials, at least at temperatures up to about 2050C.

The gasket materials of the present invention may be formed by adding the materials in predetermined order in the beater addition process and then forming the gasket material on conventional papermaking machinery, such as a cylinder machine or a Fourdrinier machine, heretofore used to make asbestos containing gasket materials. The gasket material may then be cut to form gaskets of desired shape. Since there are no asbestos fibers, the health hazards previously of concern in forming, handling and cutting asbestos containing material are eliminated.

The soft sheet gasket material of the present invention is preferably composed of the following constituent materials (it being understood in all discussions of weight percentages that reference is to percents of solid materials in the aqueous slurry prior to formation or the percents of solid material in the final product):: Table 1 % (By weight of Material solid materialJ Muscovite (monoclinic mica, H2KAI3(SiO4)3) 50-75 Cellulose fiber 220% (preferably 515%) Binder, filler and others 1035% With regard to the general composition set forth in Table 1, it is critically important that this weight percentage of cellulose fibers not exceed 20%, and preferably not exceed 1 5%, and the weight percentage of mica should not be less than 50%, and preferably not less than 55%. If these maximum and minimum limitations are not maintained, the gasket material will not have performance, physical and economic features equivalent to asbestos containing gasket materials.

One of the most critical performance parameters of soft sheet gasket material (and gaskets formed therefrom) is its ability to perform in high temperature environments, or stated in another way, its ability to resist degradation after exposure to elevated temperatures. This high temperature characteristic must, however, be coupled with good structural and mechanical properties. Cellulose fibers are known to have some good mechanical properties (such as tensile strength) but are also known to have very poor high temperature properties. Therefore, in searching for substitutes for asbestos fibers, cellulose fibers have heretofore been rejected as a possible substitute because it was thought that they would put a very severe upper temperature limitation of no greater than 1 50 C on the gasket material.However, it has been discovered that by limiting the amount of cellulose fibers in a composition with a large amount of monoclinic mica, a gasket material is produced which has the desirable mechanical and structural features of the cellulose fibers while still having high temperature characteristics. The upper limit of cellulose fibers is 20% (by weight) and preferably not more than 15%. When the weight percentage of cellulose fibers is limited in this way, in a composition with monoclinic mica, it has been determined that the resulting gasket material will function effectively in temperature environments up to 2050C. Thus, from the standpoint of high temperature use the soft sheet gasket material of the present invention is comparable to asbestos containing gasket materials.

Also, the resulting gasket material reflects the desirable mechanical features of the cellulose fibers, such as good tensile strength.

The mica in the gasket material of the present invention contributes the thermal properties that make the gasket material thermally equivalent to asbestos containing gasket materials. To achieve desired temperature performance and an economic material, the mica should be at least 50% of the gasket material. The mica also contributes to the mechanical, structural and performance features of the gasket material. The sheet structure of asbestos containing gasket material is formed primarily by an intertwining of the asbestos fibers. By way of a major distinction, mica is a "platey" material rather than a fibrous material; and the gasl < et sheet structure of the present material is formed primarily by alignment and overlapping of the mica plates to form the sheet structure (which is, of course, held together by the binder and fibrous materials).The overlapping of the mica plates forms, in effect, a labyrinth seal structure through the gasket material to enhance the sealability characteristics of the gasket material. This plate structure also resists deformation under compressive loading, thereby imparting dimensional stability to the gasket material.

It is also highly desirable that Muscovite mica of relatively uniform particle size be used in forming the gasket material of the present invention. To that end water ground or dry ground monoclinic mica may be used to insure relatively uniform mica particle size.

The use of water ground or dry ground mica of uniform particle size is desirable not only for the labyrinth seal structure, but also for manufacturing and processing considerations. Material loss during processing is minimized by the use of particles of discrete and uniform size, since there are no "fines" which are lost or settled out during processing. Also, it has been observed that the use of Muscovite mica of uniform particle size improves "freeness" of the stock-i.e. it improves the separation and draining of water from the stock when the stock is deposited on the cylinder or wire of a papermaking machine. Since the formed gasket material contains less water, the amount of energy required to dry this final sheet material is reduced.

Thus, the use of dry ground or water ground mica of uniform particle size is considered to be desirable in the present invention. The use of materials such as vermiculate, a designation for a group of hydrated micaceous materials of indefinite chemical nature and nonuniform particle size, is not considered acceptable for the purposes of the present invention. Indeed, the use of materials such as vermiculate would present a further disadvantage in that substantial additional energy and processing time would be required to dry sheets using such material.

Several examples of the gasket material of the present invention and the method of manufacture thereof were made to illustrate this invention. In each example, the constituent materials were mixed in an aqueous slurry by the beater addition process. The cellulose is first beaten or refined in an aqueous slurry to a freeness of 250 CSF, and the other materials are added to this aqueous slurry in the order in which they are listed. The final slurry is adjusted by addition of water to form a slurry having 4% solids, and the slurry is agitated until deposited on the papermaking machinery. The materials were then pumped to the headbox of a papermaking machine of the cylinder type and formed into sheets 101 .6x457.2 mm and 0.889 mm thick. The sheets were then dried at 680C for about one hour. The sheets were then calendered by being passed through calendering rolls to a nominal density of 1.60 g/cc. The sheets were then subjected to various tests to yield the results listed in Tables 2 and 3.

Examples 1 and 2 Example I Example 2 Material Weight % Weight % Cellulose fibers 10 20 325 mesh mica (Muscovite) 70 60 Pigments 8 curatives (1) 4 4 Phenolic resins 4 4 Nitrile rubber latex 1 2 12 (1) Sulfur, carbon black, zinc oxide 8 antioxidant.

Sheets of soft gasket material made in accordance with the composition of Examples 1 and 2 yielded the following properties: Table 2 Example 1 Example 2 Density (g/cc) 1.67 1.60 Compressibility (%) (1) 12.1 12.3 Recovery (%) 49.4 47.4 CD tensile strength Kg/cm2 (2) 180 214 Creep relaxation (%) (3) 30.6 24.2 Sealability (ml/hr) (4) 2.3 2.5 (1) under 352 kg/cm2 load (ASTM F36) (2) Cross machine, pull to fracture (ASTM D282) (3) Deformation at 211 kg/cm2 at 1000C for 22 hrs (ASTM F38, Method B) (4) Migration of fluid under 762 mm Hg (ASTM F37) The values listed for the various propertes in Table 2 are very similar to values for those properties with asbestos containing gasket materials.

Examples 3, 4 and 5 Three sheets of soft gasket material were formed in accordance with the present invention having three different latex binders to correspond with the latex binders of three asbestos reinforced soft gasket materials, R1, R2, R3, commercially available from Rogers Corporation, the applicants of the present invention. This was done for the purpose of making as direct a comparison as possible between gasket materials of the present invention and prior art asbestos containing gasket materials.The compositions, by weight%, of Examples 3, 4 and 5 and the three prior art asbestos reinforced gasket materials were as follows: Example 3 Example 4 Example 5 R 1 R2 R3 Weight % Weight % Weight % % % Cellulose 14.8 13.9 15.1 10.6 10.0 10.8 Mica 64.5 60.6 65.8 0 0 0 Asbestos 0 0 0 68.7 64.5 70.1 Pigments a curatives(1) 3.4 4.5 3.7 3.4 4.5 3.7 Surfactants a coagulants (2) 1.5 0.5 0.5 1.5 0.5 0.5 Phenolic resin 0 1.9 2.7 0 1.9 2.7 Neoprene latex 1 5.8 0 0 1 5.8 0 0 Nitrine rubber latex 0 18.6 0 0 18.6 0 Acrylic latex 0 0 12.2 0 0 12.2 (1) sulfur, zinc oxide, nickel butyl carbamate, antioxidant, dithiocarbamate accellerator, carbon black (2) Formic acid, cationic polymeric retention aid Sheets of soft gasket material made in accordance with the compositions of Examples 3, 4 and 5, and sheets of gasket material made in accordance with the composition of prior art asbestos reinforced composition R1, R2 and R3 yielded the following properties: Table 3 Example 3 Example 4 Example 5 R1 R2 R3 Density (g/cc) 1.61 1.46 1.55 1.57 1.30 1.33 Compressibility (%) (1) 18.2 19.1 15.3 14.4 25.5 20.5 Recovery (%) 47.6 37.8 48.8 52.1 35.4 46.9 CD tensile strength kg/cm2(2) 126 180 154 195 170 189 Creep relaxation (%)(3) 31.0 36.8 37.0 37.8 51.5 44.6 Sealability (ml/hr) (4) 8.7 3.4 7.7 1.0 1.1 2.5 (1) under 352 kg/cm2 load (ASTM F36) (2) Cross machine, pull to fracture (ASTM D282) (3) Deformation at 211 kg/cm2 at 1000C for 22 hrs (ASTM F38, Method B) (4) Migration of fluid under 762 mm Hg. (ASTM F37) In addition, three fluid resistance tests were made to determine resistance to ASTM Fuel B, ASTM Oil 3 and water, the test results being shown in Tables, 4, 5 and 6 respectively. Each fluid resistance test was made for 22 hours at room temperature.

Table 4 Fluid resistance: Fuel 8--22 hrs.-Room temperature.

Example 3 Example 4 Example 5 R 1 R2 R3 Thickness change (%) 25.8 11.2 10.3 29.3 1 6.9 11.7 Weight change (%) 24.6 20.7 22.4 25.2 29.2 30.4 Compressibility (%) Ruptured 28.1 20.0 Ruptd 33.7 27.9 Table 5 Fluid resistance: Oil 3-22 hrs.-Room Temperature Example 3 Example 4 Example 5 R 1 R2 R3 Thickness change (%) 17.7 4.1 5.2 22.0 7.9 5.8 Weight change (%) 26.8 17.9 20.5 26.1 25.0 29.8 Compressibility(%) Ruptured 22.1 16.4 28.1 26.9 23.8 Table 6 Fluid resistance:: Water-22 hrs.-Room Temperature Example 3 Example 4 Example 5 R1 R2 R3 Thickness change (%) 13.6 11.2 33.0 12.9 10.9 26.5 Weight change (%) 9.2 5.7 51.1 21.1 12.2 56.7 Compressibility (%) 22.6 24.2 32.1 25.6 27.5 37.1 The data reported in Tables 3-6 show that the compositions of the present invention are the structural and functional equivalent of prior art asbestos containing gasket materials. In interpreting this data on sealability in Table 3, it is to be noted that any result in the range of from 1-10 ml/hr is considered to be equivalent.

In addition, thermal stabiiity tests were conducted on Example 5, since acrylic latex is the preferred organic material for use in high temperature applications, i.e. applications where there is a steady state environment on the order of 2050C. This temperature stability test consisted of exposing material from Example 5 to a temperature of 2050C for 40 hours and then measuring compressibility and recovery. After heat exposure, the compressibility was 15.2% (compared to 15.3% as shown in Table 3 before heat exposure), and recovery was 48.7% (compared to 48.8% as shown in Table 3 before heat exposure). Thus there was essentially no degradation of the material.

Claims (13)

Claims

1. A soft sheet gasket material comprising, by weight 50% to 75% monoclinic mica particles, and 2% to 20% cellulose fibers.

2. A soft sheet gasket material as claimed in claim 1 wherein the amount of cellulose fibers is from 5% to 15%.

3. A soft sheet gasket material as claimed in claim 1 or 2, including 10% to 35% binders, fillers, pigments, curatives and processing agents.

4. A soft sheet gasket material as claimed in any of claims 1 to 3, wherein said monoclinic mica is of relatively uniform particle size.

5. The process of forming a soft sheet gasket material including the steps of forming an aqueous slurry having, by weight of solid material, 50% to 75% monoclinic mica particles and 2% to 20% cellulose fibers, and forming sheets of material from said slurry on papermaking machinery.

6. A process as claimed in claim 5, wherein the aqueous slurry contains 5 to 15% cellulose fibers.

7. A process as claimed in either of claims 5 and 6, including the further steps of drying said sheets of material, and calendering said dried sheets of material.

8. A process as claimed in any of claims 5, 6 and 7 including the further step of adding to said slurry from 10% to 35% binders, fillers, pigments, curatives and processing agents.

9. A process as claimed in any of claims 5 to 8 wherein said step of forming an aqueous slurry includes forming a slurry by the beater addition process.

10. A process as claimed in any of claims 5 to 9 wherein said cellulose fibers are first beaten or refined to a predetermined freeness, in an aqueous slurry, and said mica particles are thereafter added to said slurry.

11. A process as claimed in any of claims 5 to 10 wherein said monoclinic mica is in the form of particles of uniform particle size.

12. A process as claimed'in claim 5 substantially as hereinbefore described in any of the foregoing examples.

13. A material as claimed in claim 1, substantially as hereinbefore described in any of the foregoing examples.