GB1576601A

GB1576601A - Sealing elements of expanded graphite

Info

Publication number: GB1576601A
Application number: GB4728/78A
Authority: GB
Original assignee: Klinger AG
Current assignee: Klinger AG
Priority date: 1977-02-07
Filing date: 1978-02-06
Publication date: 1980-10-08
Also published as: ATA80377A; IT1093636B; IT7820031A0; AT347549B; DE2803952A1; FR2379737A1; SE7801414L; ES467148A1; JPS53112293A

Description

(54) SEALING ELEMENTS OF EXPANDED GRAPHITE (71) We, KLINGER AG, of 10, Baarerstrasse Zug, Switzerland, a body corporate organized according to the laws of Switzerland, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to sealing elements made from graphite, in particular, expanded graphite.

The use of graphite as a lubricating filler in elastomeric material is well known, for example in the manufacture of low friction parts (East German Patent Specification No. 118437 moulded flexible packages made from polymeric materials (US Specification No. 3271351, or as an ingredient in a liquid sealing composition (German Auslegeschrift No. 1013376 and UK Patent Specification No. 441877).

In all of the above, the graphite merely assists in providing lubrication and does not provide the sealing action.

In addition, graphite in expanded form, which is then processed to form mouldings, slabs, sheets or strips, has long been known as a material for making sealing elements. It has good sealing properties with respect to stationary and moving surfaces, and also has the required chemical and thermal resistance necessary for general applications.

However, the material has not become established in general use for making sealing elements, firstly, because there are still functional disadvantages associated with its use in comparison with known materials. With a known sealing element made from expanded graphite, whether employed in a static assembly or as a closure member of a valve or the like, the great disadvantage lies in the inadequate strength of the material, this being because the graphite particles in relation to moving metal surfaces frequently have a great tendency to adhere to the metal - particularly in the case of dry media - than to adhere to one another. The lack of strength, particularly in respect of bending, is also a disadvantage when such a known sealing element is used in a static assembly.

In particular such a sealing element, if constructed of expanded graphite tends to be made in very thin form for reasons of cost, and this may result in damage during operation under the conventional handling of such a sealing element.

A second reason which precludes general use of this material for making sealing elements is that the costs of manufacturing expanded graphite are extremely high; nor is it possible to add filler material~-in-lar-ge q.uantities in order to reduce the cost, because then the strength of the resulting material is even~les-sxthan that of expanded graphite alone.

We have found that in a sealing element of the invention the expanded graphite particles can be bound together more efficiently by including therein an elastomeric bonding agent (thus increasing the strength of a sealing element made therefrom) and this can be achieved without lossof-the advantageous sealing properties associated with expanded graphite, thus enabling the resultant sealingbelement-to be capable of a wide range of uses.

Such a sealing element can be made from a composition containing expanded graphite but which composition may additionally contain large quantities of filler and hence enable vast reductions in costs. This sealing element need not require the presence of asbestos. thus avoiding problems of pollution.

The present invention provides a sealing element which consists of a compressed solid composition, which composition comprises expanded graphite and fine particles of an elastomeric, preferably vulcanized, bonding agent and, if required, fine particles of a filler.

Thus, by including, in the composition of the sealing element fine particles of an elastomeric bonding agent, the bonding between the expanded graphite particles can be increased without reducing the sealing properties below a practically acceptable value, even at temperatures of about 500 C, i.e. near the limits of use of expanded graphite in oxidizing media. This is completely surprising, because mixtures of equal proportions of elastomers and asbestos as are frequently used for static and dynamic purposes give very much greater leakage at these temperatures.

A sealing element of this kind can be made to a higher strength than a sealing element consisting of expanded graphite without the presence of an elastomeric bonding agent, and.

in addition, much higher sealability can be obtained for a given proportion of elastomeric bonding agent that in the case of a sealing element consisting of asbestos having the same proportion of elastomer. It should be pointed out that, for example, asbestos having a proportion of 10% elastomer has at least 100 times the leakage of expanded graphite also having a 10% proportion of elastomer.

The proportion, which will always constitute only a fraction of the total weight of the material, is governed only by the requirements made of the sealing element in respect of its mechanical properties at elevated temperature. Preferably the proportion of elastomer is from 1 to 15%. more preferably 2-10% and is particularly advantageously not more than 7.5% by weight of the bonding agent in the case of non-confined seals to be subjected to relatively high mechanical stresses. At temperatures of up to at least l 50 C. a sealing element embodying the invention can be provided with a higher density and with greater strength than a sealing element of pure expanded graphite alone; the strength can be increased further by vulcanization. although this results in a slight impairment of sealing properties.

The bonding agent is advantageously a rubber. preferably a nitrile rubber. this being because rubber has the necessary chamical resistance for general industrial use in addition to having good sealing properties. The amount of filler added again depends - in combination with the proportion of bonding agent - on the required mechanical properties of the sealing element. but for general use in industry will advantageously be a maximum of 50% by weight and preferably between 30% and 50% by weight. A sealing element of this kind can thus be produced much less expensively than known graphite sealing elements without appreciable deterioration of the sealing properties and can also be made economically in relatively large thicknesses. Fillers which may be used are heavy spar. talcum bentonite (montmorillonite).

plastcr of Paris. lime. or graphite and the ground waste of pressed expanded graphite; the filler is. howevcr. advantageously kaolin. this being because it can be made with reproducible properties and because it can be obtained cheaply. The particle size is advantageously such as to give a maximum residue of 5% on an 0.25 mm mesh screen, One sealing element embodying the invention is particularly advantageously made in the form of an essentially flat sealing element in which is incorporated at least one essentially flat reinforcing insert, preferably in the form of a wire screen. It should be pointed out that it is known to provide flat sealing elements of expanded graphite with reinforcing inserts, but the sealing element embodying the invention can be made to greater strength and with a reduced production cost.

These advantages are particularly manifest if two layers of the compressed expanded graphite composition. each of a thickness of at least 0.05 mm. but a maximum of 30% of the screen thickness, are provided one at each of the sealing surfaces above the reinforcing wire screen.

The invention further provides a solid composition as hereinbefore defined which is capable of compression to form a said sealing element and still further provides a process for the production of a sealing element according to the invention. which process includes the steps of admixing the expanded graphite with the fine particles of bonding agent and. if desired, fine particles of any of vulcanizing agents. expanded graphite residues and fillers and then compressing the resulting mixture (which compression may be effected either by a prcssing or a rolling operation) and. if desired, vulcanizing the resultant product. The admixing of the fine particles of elastomeric bonding agent with the expanded graphite can be effected in various ways. and processes embodying the invention may be carried out very advantageously in respect of quality of distribution and cost. In one process embodying the invention the expanded graphite in an aqueous suspension is admixed with the bonding agent (and. if desired. a vulcanization agent) by precipitation of an elastomer latex added to the aqueous suspension. the resultant suspended composition then being dried to produce the fine powder form. For the production of a flat sealing material. a smilarly very advantageous process is one in which the expanded graphite is admixed with the bonding agent (and. if desired. a vulcanizing agent) by precipitation of an added elastomer latex in such a manner that a slurry is formed.A wet layer is formed from this slurry which, if desired, is provided with added fillers by running the slurry on to a supporting screen which allows the water to pass through it as in paper manufacture, the damp layer then being dried, consolidated by compression and, if desired, vulcanized. As a result, a completely homogenous sealing material of excellent quality, can be produced to any dimensions.

Sealing elements embodying the invention and processes for the production thereof will now be described in more detail with reference to the following Examples.

Example I 90 g of expanded graphite (bulk density about 23 g/ 1) were added to 9.9 1 of water with agitation and a mixture of 10 g (solids weight) of the commerical NBR -Latex Breon 1562 ("Breon" is a Registered Trade Mark) to which vulcanizing agents had been added, was slowly added. After about 5 minutes' agitation, 24 ml of a 5% aqueous potash alum (KA1 (SO4)2) solution was slowly added and agitation was continued until complete coagulation had occurred. The solution was then filtered and the residue carefully dried at about 100"C in a drying chamber. 37.5 g of the resultant powder was homogeneously mixed with 14.4 g ordinary expanded graphite and 48.1 g kaolin Airflo V8 (Airflow is a Registered Trade Mark of a kaolin marketed by Messrs. Watts, Blake Co., Ltd).

37.5 g of this mixture now containing 3.75% by weight of rubber and 48.1% of kaolin was then introduced in batches into an annular press mould for a 70 x 50 mm diameter ring, and pre-pressed by means of a punch. The mould was then subjected to a pressure of 12.5 tonnes (66 N/mm2) in a press, the working pressure being uniformly increased to full load within a period of 30 seconds. After a pressing time of 60 seconds, the pressed ring of 1.2 mm thickness was removed and vulcanized in a drying chamber for 15 minutes at 1600C.

This ring had excellent strength and was completely resistant to the stresses resulting from installation by ordinary assembly staff. The deformation properties, which are important for the sealing function, were determined by means of a rig which subjected the ring to a pressure of 50 N/mm2 hydraulically first at room temperature and then at 300"C, independently of the decrease in thickness of the ring.A deformation of about 17% was found in the cold state (cold deformation), and this is equivalent to the magnitude of the cold deformation of a pure expanded graphite ring of the same dimensions under the same stress (see Example 39. In comparison with the cold deformation values of a rubber-bonded asbestos (It-) material, for which this value was about 9 % given the same thickness and good qualitites (It 400 according to DIN 3754), these values are very high, this being of advantage because this cold deformation ensures good adaptation of the sealing material to the co-acting surfaces.After subsequent heating to 300"C, the pressure of 50 N/mm2 being maintained, there was an additional hot deformation of about 2.0 %. This value (which should be as small as possibe in comparison with the cold deformation) is somewhat poorer than in the case of pure graphite, with which even a slight increase in thickness can be detected, but it is still much better than a good It material of the same thickness, where the value is about 4 to 5%.

The sealing behaviour of this sealing material embodying the invention was also excellent.

When the ring was tested for its sealing behaviour with respect to 40 bar nitrogen at a surface pressure of 20 N/mm2, the leakage was 14 ml per minute, and this is approximately equal to the seal efficiency of It 400 according to DIN 3754. Testing of the same ring after heating for 1 hour at 5000C gave a value of only 57 ml nitrogen per minute, while in the parallel test with It material the leakage at only 20 bar nitrogen was about 660 ml/min and at 40 bar the sealing properties were such that the ring was unusable in practice.

The above results and the results with other percentages of rubber and kaolin will be apparent from the following Table, the cold deformation remaining substantially unchanged.

Rubber Kaolin Hot deformation N2 leakage before after heating to heating 500or mi/min ml/min 2.5 48.8 1.0 14 60 3.75 48.1 2.1 14 64 5 31.7 2.4 6.4 68 7.5 30.8 4.0 6 70 Example 2 4 g of the mixture produced according to Example 1 and containing 3.75% by weight of rubber, 48.1% of kaolin and expanded graphite were well distributed in a rectangular 125 x 75 mm press mould having rounded corners, and a wire screen of the same size and a thickness of 0.52 mm was placed above it, whereupon another 4 g of the mixture was distributed.After the punch has been applied, the sandwich was pressed for 3 seconds at a working pressure of 36.7 tonnes (40 N/mm2), removed from the mould, and vulcanized for 15 minutes at 1600C. The final thickness was 0.68 mm; a layer of sealing material in a thickness of 0.08 mm was thus present above the reinforcing wire screen in each case, and this is equivalent to about 15% of the screen thickness. Under the test conditions indicated in Example 1 for a 50 x 70 mm diameter ring, the cold deformation was about 8% and the hot deformation was - 0.6%(thickness increase). The leakage under the test conditions given in Example 1 was 19 ml/ min N2 in the cold state, and 23.9 ml/min N2 after heating for 1 hour at 500"C. These values compared very favourable with those given in Example 1.

Example 3 Expanded graphite containing a proportion of 3.75% of rubber (made by the method indicated in Example 1) was pressed into the form of a ring as explained in Example 1. The following measured values were obtained: (The test values in brackets denote those obtained by comparison with a ring made in the same way but without the rubber content).

Cold thickness decrease: 19% (16No) Hot thickness decrease: 1.0% (-2%) N2 leakage: 0.15 ml/min (1.0 ml/min) N2 leakage after heating for 1 hour at 5000C: 2 ml/min (1 ml/min).

A sealing material made in this way was bent without difficulty around a 20mm diameter mandrel, while a material of the same thickness made from pure graphite under the same conditions could be bent around only a 35 mm diameter mandrel without breaking.

In a modification of the production process described in Example 1, it is of course possible, instead of producing a dry powder after coagulation of the latex, to separate it from the water and dry the damp slurry in a flat form in the same way as in paper production. The material can then be consolidated and, if required, vulcanized whereupon sealing rings can be produced, for example, by stamping them out of the resulting slab.

Example 4 A stock solution was prepared from 5.0 g of rubber Europrene 1500, 0.37 g of vulcanizing agent and 175.0 g of toluene; 80 g of this stock solution were mixed with 20 g of expanded graphite and 420 g of toluene by agitation for 15 minutes, poured into a filtration vessle where the toluene was suction-filtered and the expanded graphite mixed with the rubber was left.The latter was dried for 4 hours at 900C and taking into account the rubber left in the suction filtered toluene the expanded graphite had a rubber content of about 10%. 8.5 g of this expanded graphite mixed with rubber was thoroughly mixed with 3.5 g of pure expanded graphite and 12 g of bentonite and, as stated in Example 1, pressed into the form of a 50 x 70 mm diameter ring of lmm thickness; the working pressures may, of course differ from those given in Example 1, but the same pressures were chosen in all the examples in order to provide a satisifactory comparison. These rings containing about 3.5% of rubber had fully adequate strength for normal storage and assembly.Testing showed a cold deformation of 16 % and a hot deformation of 1.5 % and a cold leakage of 11 ml/ min, while after heating for 1 hour at 5000C the leakage was 45 ml/min, this being an excellent result in comparison with those achieved using conventional seals previously employed in industry.

WHAT WE CLAIM IS: 1. A sealing element consisting of a compressed solid composition, which composition comprises expanded graphite and fine particles of an elastomeric bonding agent.

2. A sealing element according to claim 1 wherein the composition further comprises fine particles of a filler.

3. A sealing element according to claim 2 wherein the said filler is present in an amount of not more than 50 % by weight based on the weight of the composition.

4. A sealing element according to claim 3 wherein the filler is present in an amount of from 30 to 50% by weight based on the weight of the composition.

5. A sealing element according to any one of claims 2 to 4 wherein the said filler is kaolin.

6. A sealing element according to claim 5 wherein at least 95 of the said fine particles of the said kaolin are of a size less than 0.25 mm.

7. A sealing element according to any one of the preceding claims wherein the elas tomeriebonding agent is present in an amount of not more than 7.5 % by weight based on the weight of the said composition.

8. A sealing element according to any one of the preceding claims wherein the said

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Claims

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Example 2

4 g of the mixture produced according to Example 1 and containing 3.75% by weight of rubber, 48.1% of kaolin and expanded graphite were well distributed in a rectangular 125 x 75 mm press mould having rounded corners, and a wire screen of the same size and a thickness of 0.52 mm was placed above it, whereupon another 4 g of the mixture was distributed. After the punch has been applied, the sandwich was pressed for 3 seconds at a working pressure of 36.7 tonnes (40 N/mm2), removed from the mould, and vulcanized for

15 minutes at 1600C. The final thickness was 0.68 mm; a layer of sealing material in a thickness of 0.08 mm was thus present above the reinforcing wire screen in each case, and this is equivalent to about 15% of the screen thickness.Under the test conditions indicated in Example 1 for a 50 x 70 mm diameter ring, the cold deformation was about 8% and the hot deformation was - 0.6%(thickness increase). The leakage under the test conditions given in Example 1 was 19 ml/ min N2 in the cold state, and 23.9 ml/min N2 after heating for 1 hour at 500"C. These values compared very favourable with those given in Example 1.

Example 3 Expanded graphite containing a proportion of 3.75% of rubber (made by the method indicated in Example 1) was pressed into the form of a ring as explained in Example 1. The following measured values were obtained: (The test values in brackets denote those obtained by comparison with a ring made in the same way but without the rubber content).

Cold thickness decrease: 19% (16No) Hot thickness decrease: 1.0% (-2%) N2 leakage: 0.15 ml/min (1.0 ml/min) N2 leakage after heating for 1 hour at 5000C: 2 ml/min (1 ml/min).

A sealing material made in this way was bent without difficulty around a 20mm diameter mandrel, while a material of the same thickness made from pure graphite under the same conditions could be bent around only a 35 mm diameter mandrel without breaking.

In a modification of the production process described in Example 1, it is of course possible, instead of producing a dry powder after coagulation of the latex, to separate it from the water and dry the damp slurry in a flat form in the same way as in paper production. The material can then be consolidated and, if required, vulcanized whereupon sealing rings can be produced, for example, by stamping them out of the resulting slab.

Example 4 A stock solution was prepared from 5.0 g of rubber Europrene 1500, 0.37 g of vulcanizing agent and 175.0 g of toluene; 80 g of this stock solution were mixed with 20 g of expanded graphite and 420 g of toluene by agitation for 15 minutes, poured into a filtration vessle where the toluene was suction-filtered and the expanded graphite mixed with the rubber was left.The latter was dried for 4 hours at 900C and taking into account the rubber left in the suction filtered toluene the expanded graphite had a rubber content of about 10%. 8.5 g of this expanded graphite mixed with rubber was thoroughly mixed with 3.5 g of pure expanded graphite and 12 g of bentonite and, as stated in Example 1, pressed into the form of a 50 x 70 mm diameter ring of lmm thickness; the working pressures may, of course differ from those given in Example 1, but the same pressures were chosen in all the examples in order to provide a satisifactory comparison. These rings containing about 3.5% of rubber had fully adequate strength for normal storage and assembly.Testing showed a cold deformation of 16 % and a hot deformation of 1.5 % and a cold leakage of 11 ml/ min, while after heating for 1 hour at 5000C the leakage was 45 ml/min, this being an excellent result in comparison with those achieved using conventional seals previously employed in industry.

WHAT WE CLAIM IS: 1. A sealing element consisting of a compressed solid composition, which composition comprises expanded graphite and fine particles of an elastomeric bonding agent.
2. A sealing element according to claim 1 wherein the composition further comprises fine particles of a filler.
3. A sealing element according to claim 2 wherein the said filler is present in an amount of not more than 50 % by weight based on the weight of the composition.
4. A sealing element according to claim 3 wherein the filler is present in an amount of from 30 to 50% by weight based on the weight of the composition.
5. A sealing element according to any one of claims 2 to 4 wherein the said filler is kaolin.
6. A sealing element according to claim 5 wherein at least 95 of the said fine particles of the said kaolin are of a size less than 0.25 mm.
7. A sealing element according to any one of the preceding claims wherein the elas tomeriebonding agent is present in an amount of not more than 7.5 % by weight based on the weight of the said composition.
8. A sealing element according to any one of the preceding claims wherein the said

elastomeric bonding agent is an unvulcanized rubber and wherein the composition includes a vulcanizing agent.
9. A sealing element according to any one of claims 1 to 7 wherein the elastomeric bonding agent is a vulcanized rubber.
10. A sealing element according to claim 9 wherein the rubber is a nitrile rubber.
11. A sealing element according to any one of the preceding claims which is an essentially flat configuration and which has, incorporated therein, an essentially flat reinforcing insert.
12. A sealing element according to claim 11 wherein the reinforcing insert is a wire screen.
13. A sealing element according to claim 12 wherein the wire screen is incorporated between two layers of said compressed solid composition which said layers are each of a thickness of at least 0.05 mm but of a maximum thickness of 30% of the thickness of the screen.
14. A sealing element according to any one of the preceding claims substantially as herein described and exemplified.
15. A process for the production of a sealing element as defined in claim 1, which process includes the steps of admixing the said expanded graphite with the bonding agent and optionally with a filler and or vulcanizing agent to form the said solid composition and compressing the said composition to form the said sealing element.
16. A process according to claim 15 wherein the said admixing of the expanded graphite and the said elastomeric bonding agent includes the steps of forming, in an aqueous medium a suspension containing at least a portion of the expanded graphite and adding to the aqueous medium a latex of the said elastomeric bonding agent and, when the elastomeric bonding agent is an unvulcanized rubber bonding agent, optionally adding a vulcanizing agent, causing the elastomeric bonding agent in the aqueous medium to precipitate as said fine particles thereof and thereby produce a suspended composition' and thereafer removing the suspended composition from the aqueous medium for form a solid composition and drying the solid composition.
17. A process according to claim 16 wherein the said aqueous medium contains only a portion of the expanded graphite, a further portion thereof being admixed with the said solid composition and optionally with a said filler after the said drying.
18. A process according to claim 16, wherein the said suspended composition takes the form of a slurry, the said step of removing the said suspended composition from the aqueous medium being effected by filtering off the said slurry on to a supporting screen to form a slab of the said solid composition.
19. A process according to any one of claims 15 to 18 substantially as herein described and examplified.
20. A sealing element whenever prepared by a process according to any one of claims 15 to 19.
21. A solid composition capable of compression to form a sealing element which composition contains expanded graphite and fine particles of an elastomeric bonding agent, which solid composition optionally contains fine particles of at least one further additive selected from a filler and, when the elastomeric bonding agent is an unvulcanized rubber, a vulcanizing agent.