GB2570784A

GB2570784A - Adhesive for gluing abradable felts

Info

Publication number: GB2570784A
Application number: GB1820390.1A
Authority: GB
Inventors: Fernandes Nicolas; Crabos Fabrice; Banks Richard
Original assignee: Safran Helicopter Engines SAS; Indestructible Paint Ltd
Current assignee: Safran Helicopter Engines SAS; Indestructible Paint Ltd
Priority date: 2017-12-15
Filing date: 2018-12-14
Publication date: 2019-08-07
Anticipated expiration: 2038-12-14
Also published as: GB2570784B; FR3075217A1; FR3075217B1; GB201820390D0

Abstract

A method of fabricating an adhesive for gluing abradable felts comprises a first step in which aluminium powder is mixed with zinc oxide powder to form a first component and a second step in which the first component is mixed with a second component comprising a silicate so as to obtain the adhesive. The silicate may be a silicate of potassium, sodium, or calcium and may be mixed with water. The first step may comprise mixing 94-98 wt.% aluminium powder with 2-6 wt.% zinc oxide powder. The second step may comprise mixing 35-45 wt.% first component with 55-65 wt.% second component. Typically, the method does not use hexavalent chromium. An adhesive so formed that consists of aluminium powder, zinc oxide powder, and silicate is disclosed. A two-component kit is also claimed, which comprises the first component and the second component separately, wherein the components are intended to be mixed together to produce the adhesive.

Description

ADHESIVE FOR GLUING ABRADABLE FELTS

DESCRIPTION

TECHNICAL FIELD

The invention relates in general to an adhesive for gluing abradable felts, and more precisely for gluing abradable felts that does not contain any components that have an adverse effect on health and the environment and that is stable during storage. The invention also relates to a method for fabrication of this adhesive, and a kit to fabricate it.

STATE OF PRIOR ART

In the aeronautical field, leak tightness performances of labyrinth seals have been continuously improving due to a better understanding of the labyrinth system and abradable felts. Under the operating conditions of a gas turbine engine, abradable felts are used to the extent that their partial separation is frequently observed.

An optimal attachment capable of accommodating all operating cycles guarantees the integrity of abradable felts on their support. Apart from fabrication operations such as soldering, heat projection, abradable felts are often mounted on supports in the engine using an adhesive (or cement) to glue them. This specially designed adhesive is adapted for this function, capable of withstanding the aggressive environment in the engine, e.g. the high temperature, high stresses, vibrations, etc.

US3918925 A (United Tech Corp) discloses an example of such an adhesive, called SermeTel 481 (or SermaBond 481). This known adhesive comprises two components intended to be mixed to fabricate the adhesive for gluing abradable felts. It comprises a powder of aluminium (Al), hexavalent chromium (CrVI) in solution in ion form (CrCU² ) and silicate (SiC>4²“) in solution in ion form, in a first mix, and separately zinc oxide (ZnO) powder, intended to be mixed together before using the adhesive for gluing.

After mixing the two components to fabricate the adhesive, and after leaving to stand for half a day to mature, the adhesive is ready for use. For gluing an abradable felt on a support, the parts are cleaned, adhesive is applied on each of the faces to be glued, the abradable felt is then placed on the support followed by a polymerisation step and machining if necessary.

The adhesive is supplied as a product in the two-component state because this makes its supply easier than an adhesive in individual components. It avoids additional errors during weighing, it can save time during mixing because the two principal components are already homogeneous, and has fewer storage constraints. At the same time, in the two-component state, it lasts longer than an adhesive with all components mixed together.

However, the use of products (including adhesives) containing hexavalent chromium is not recommended (recently prohibited by the REACH regulations, EU Directives) due to its negative effect on health and the environment. Consequently, there is a need for an adhesive with no hexavalent chromium for abradable felts that complies with environmental criteria of the aeronautical industry.

PRESENTATION OF THE INVENTION

This invention thus discloses a method of fabricating an adhesive for gluing abradable felts, the method comprising the supply of an aluminium powder, the supply of a zinc oxide powder, the supply of a silicate, and a mixing step, the mixing step including a first mixing step during which the aluminium powder is mixed with zinc oxide powder to form a first component, then a second mixing step during which the first component is mixed with a second component comprising the silicate, whereby the adhesive is obtained.

Preferably, the first step includes mixing 94% to 98% by mass of aluminium powder with 2% to 6% by mass of zinc oxide powder.

Also preferably, the second step includes mixing 35% to 45% by mass of the first component with 55% to 65% by mass of the second component.

Advantageously, the silicate is a potassium silicate, a sodium silicate or a calcium silicate.

Preferably, the method does not include the supply of hexavalent chromium.

Preferably, the method includes the supply of water and the mixing step also includes a third mixing step before the second mixing step, during which the silicate is mixed with water to form the second component.

Preferably, the method also includes an adhesive application step, then a polymerisation step including drying and oven drying/curing at high temperature, i.e. between 340°C and 500°C.

The invention also discloses an adhesive that can be fabricated using the fabrication method, the adhesive consisting of aluminium powder, zinc oxide powder and silicate.

The invention also discloses a two-component kit for fabrication of the adhesive for gluing abradable felts, the first component comprising aluminium powder and zinc oxide powder, and separately the second component comprising silicate, the two components intended to be mixed together to form the adhesive.

Preferably, the first component contains 94% to 98% by mass of aluminium powder with 2% to 6% by mass of zinc oxide powder.

Preferably, the first component is 35% to 45% by mass and the second component is 55% to 65% by mass.

Preferably, the kit does not contain any hexavalent chromium.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

In the adhesive fabrication method according to a preferred embodiment of the invention, the aluminium (Al) powder in metal form is mixed with zinc oxide (ZnO) powder during a first mixing step to form the first component, Component A. Proportionally, ideally 95% by mass of aluminium powder is mixed with 5% by mass of zinc oxide powder.

The aluminium powder is preferably supplied with D50 size grading: 6pm ± 2pm and the zinc powder is preferably supplied with D50 size grading: 10pm ± 5pm. Component A will keep for about 2 years if not exposed to air. This size grading optimises the physicochemical properties of the mixture with the objective of improving the mechanical and chemical aspects of gluing.

The second component, Component B, comprises silicate (SiCU² )- Water can optionally be present in Component B, added principally to facilitate mixing of the two components and also application of the adhesive by adjusting its viscosity. The presence of water does not lead to any chemical reaction. It can be eliminated. When water is present, Component B is composed of silicate (SiC>4²“) in solution in ion form and water, therefore an aqueous solution of silicate. The silicate in this case is ideally a potassium silicate, but it will be understood that another silicate, such as sodium silicate or calcium silicate, or any combination of them, can be used.

Component B may be composed of 60% to 100% by mass of silicate and 0% to 40% by mass is water, knowing that a minimum water content of 3% by mass is preferred. However, in the adhesive fabrication method according to the preferred embodiment, 80% by mass is composed of silicate and 20% by mass is water. Component B will keep for about 1 year if not exposed to air.

This component B is then mixed during a second mixing step with Component

A. Proportionally, 40% by mass of Component A is mixed with 60% by mass of Component

B. If water is to be present in Component B, it can be supplied, and during a third mixing step, before the second mixing step, be mixed with silicate to form Component B.

Therefore globally, the adhesive is composed of 38% by mass of aluminium, 2% by mass of zinc oxide, 48% by mass of silicate and 12% by mass of water. Crucially, it does not contain any hexavalent chromium (CrVI). It will keep for about three days if it is not exposed to air, which is at least as long as the known adhesive.

A chemical reaction is set up when the second mixing step takes place, particularly when the silicate interacts with the aluminium (Al) powder. At the same time, the zinc oxide (ZnO) powder acts as reaction catalyst. The adhesive becomes a cement partially insoluble in water. Once well mixed, the adhesive obtained is ready for immediate application on abradable felts and their supports.

Its new composition of the components enables immediate use after mixing and with no maturing time. Therefore it is very efficient for gluing abradable felts using this adhesive, unlike known adhesives. The inventors surprisingly found a method of making an adhesive in the same range as the known adhesive, using essentially the same components except for hexavalent chromium (although in different proportions), that complies with EU Directives but is also good quality.

The first step before gluing an abradable felt to a support, for example to a labyrinth envelope, is to clean the felt and the labyrinth envelope, especially their faces to be glued. Once the adhesive has been applied to the corresponding faces, the abradable felt is placed on the labyrinth envelope to be glued together. A polymerisation step is then done on the adhesive inserted between the labyrinth envelope and the abradable felt; firstly drying at ambient temperature for between 1 hour and 12 hours, followed by drying at 80°C ± 5°C to be held without interruption for between 4 hours and 20 hours, then oven drying/curing (polymerisation or cross-linking) between 340°C and 500°C for 30 minutes.

During this polymerisation step, the adhesive (that is in the form of hydrated cement) hardens, thus gluing the abradable felt to the labyrinth envelope. If necessary, machining may be done to terminate fabrication. It will be noted that apart from the method to fabricate adhesive, the method of gluing abradable felts to a support (cleaning of parts, application of the adhesive, polymerisation and machining) is very similar to existing methods.

An optimal attachment is obtained in which all functioning cycles can be accommodated, capable of withstanding the severe environment in the gas turbine engine, that guarantees the integrity of abradable felts on their support. In this step, the adhesive after evacuation of water (20% of Component B) globally consists of 43.2% by mass of aluminium, 2.3% by mass of zinc oxide, 54.5% by mass of silicate. Obviously, the hexavalent chromium that can have adverse effects on health and the environment, is no longer present in the mixture, neither in Component A nor in B. In fact, no hexavalent chromium is used during the method. Therefore the resulting adhesive contains no hexavalent chromium.

A two-component kit for fabrication of the adhesive according to one particularly preferred embodiment of the invention comprises component A and component B separately, as described above, the two components intended to be mixed together to fabricate the adhesive. The kit makes it possible to implement the fabrication method.

In the known two-component adhesive, the hexavalent chromium acts as a reaction inhibitor between the aluminium powder and the silicate. Without this compound, the mixture of the aluminium powder and the silicate quickly becomes unusable. Therefore the presence of such a reaction inhibitor is essential to maintain stability between the aluminium powder and the silicate during storage. Since the aluminium powder and the silicate are not kept together in the invention, there is no longer any need for hexavalent chromium.

Therefore, a technician who needs to glue an abradable felt to a support can bring a kit to the site. He then takes the two components from the kit, each of which is already present in the necessary proportions as indicated above. Typically, each of the two components is supplied in a respective container. An adhesive is obtained for gluing abradable felts by mixing the two components of this kit.

Ideally, the kit is supplied with proportions of the two components as indicated above. In other words, when the entire quantity of the two components have been mixed together, globally, the adhesive is composed of 38% by mass of aluminium, 2% by mass of zinc oxide, 48% by mass of silicate and 12% by mass of water. This further facilitates fabrication of the adhesive because all the technician needs to do to obtain the preferred adhesive is to mix the entire content of the two components without weighing anything.

Compared with a three-component adhesive, the two-component kit prevents additional errors during weighing, can save time during mixing and has fewer storage constraints. It is ready to use immediately after mixing, and it especially contains no hexavalent chromium (CrVI).

This inorganic adhesive is particularly suitable for gluing abradable felts, such as gluing an abradable felt to a support for a labyrinth seal. It is also suitable for high temperature applications.

By changing the order of the mixing step, and keeping the components separate, the presence of hexavalent chromium is no longer necessary. An adhesive is obtained that contains no hexavalent chromium, that is in the same range or even better than the known adhesive. Consequently, the adhesive is less dangerous to health and to the environment and always remains compatible (chemically and physically) for the same function. It also enables an increase in mechanical strength, since its tensile strength is higher during the tests. This new adhesive is still capable of withstanding the aggressive environment in the engine.

The adhesive according to the invention is very different from the known adhesive. The skilled person, hoping to eliminate hexavalent chromium, will try a mixture of aluminium and silicate as in the known adhesive, but using an agent other than hexavalent chromium necessary to inhibit the reaction to be avoided between aluminium and silicate. Otherwise, he will use a commercially available solution specific to high temperature applications as a substitute for the known adhesive that contains hexavalent chromium. He will not arrive at the method or the kit according to the invention.

It should be noted that the term adhesive was applied just after mixing the components, after it has hardened (adhesive final state) and even in the kit before mixing, for obvious practical reasons. However, the state of the adhesive will be understood from the context.

Obviously, although this adhesive is appropriate for gluing abradable felts, it is not limited to this function and can also be used for gluing other parts, for example instruments such as sensors on engine blades, etc. It can be used in all circumstances in which the known adhesive is used.

Claims

1. Method of fabricating an adhesive for gluing abradable felts, the method comprising:

supply of an aluminium powder, supply of a zinc oxide powder, supply of a silicate, a mixing step, characterised in that the mixing step comprises:

a first mixing step in which the aluminium powder is mixed with zinc oxide powder to form a first component, then a second mixing step during which the first component is mixed with a second component including silicate, whereby the adhesive is obtained.

2. Method of fabricating an adhesive accordingto claim 1, characterised inthat the first step includes mixing of 94% to 98% by mass of aluminium powder with 2% to 6% by mass of zinc oxide powder.

3. Method of fabricating an adhesive according to claim 1 or 2, characterised in that the second step includes mixing of 35% to 45% by mass of the first component with 55% to 65% by mass of the second component.

4. Method of fabricating an adhesive according to any one of the preceding claims, characterised in that the silicate is a potassium silicate, a sodium silicate or a calcium silicate.

5. Method of fabricating an adhesive according to any one of the preceding claims, characterised in that the method does not include the supply of hexavalent chromium.

6. Method of fabricating an adhesive according to any one of the preceding claims, characterised in that the method includes the supply of water and the mixing step also includes a third mixing step before the second mixing step, during which the silicate is mixed with water to form the second component.

7. Method of fabricating an adhesive according to any one of the preceding claims, characterised in that it also includes an adhesive application step, then a polymerisation step including drying and oven drying/curing at high temperature.

8. An adhesive that can be fabricated using the fabrication method according to any one of the preceding claims, characterised in that the adhesive consists of aluminium powder, zinc oxide powder and silicate.

9. A two-component kit for fabrication of adhesive for gluing abradable felts, characterised in that the first component comprises aluminium powder and zinc oxide powder, and separately, the second component comprises a silicate, the two components intended to be mixed together to fabricate the adhesive.

10. A kit according to claim 9, characterised in that the first component includes 94% to 98% by mass of aluminium powder and 2% to 6% by mass of zinc oxide powder.

11. A kit according to claim 9 or 10, characterised in that the first component is 35% to 45% by mass and the second component is 55% to 65% by mass.

12. A kit according to any one of the preceding claims, characterised in that the kit does not include any hexavalent chromium.